Symposium Organizers
Douglas L. Irving North Carolina State University
Susan B. Sinnott University of Florida
Martin H. Mueser Saarland University
Izabela Szlufarska University of Wisconsin-Madison
RR1: Tribology
Session Chairs
Monday PM, November 28, 2011
Commonwealth (Sheraton)
9:15 AM - RR1.1
Water Droplet Freezing: Humidity, Shearing Gas Flow and Surface Energy Effects.
Stefan Jung 1 , Manish Tiwari 1 , N. Vuong Doan 1 , Dimos Poulikakos 1
1 Mechanical and Process Engineering, ETH Zurich, Zurich Switzerland
Show AbstractFreezing of water is a ubiquitous phenomenon and therefore crucial to understand. Ice formation on supercooled surfaces exposed to different environmental conditions affects applications such as power transmission lines and aerodynamic surfaces from the aviation to wind turbine industries. Substrate heating and coatings with anti-icing chemicals have been used with partial success in tackling the frosting problem. Recently, use of superhydrophobic surfaces with hierarchical micro-to-nanoscale roughness has been proposed since their outstanding water repellency imparts them with an additional advantage of being able to minimize ice accretion and adhesion. Therefore, the role of surface roughness of superhydrophobic surfaces, which are inherent in their synthesis, on their icephobic behavior needs critical examination. For example, we show here that a droplet on a polished silicon wafer (a hydrophilic surfaces) with nanometer (nm)-scale roughness (smaller than the size of the first stable ice nuclei) and higher wettability freezes with an order of magnitude longer freezing delays compared to typical superhydrophobic surfaces. However, the low ice adhesion on superhydrophobic surfaces is an advantage. Therefore, herein we report the role of environmental conditions such as shearing gas flow, humidity and presence of frost on freezing of supercooled sessile water droplets on supercooled surfaces (both at -15oC). The experiments were performed in a custom built double layered, low temperature chamber (cooled using nitrogen vapor) fitted with a ventilator fan (to impose shearing gas) and with optical access for visualization using a high speed camera. We show herein that environmental humidity and gas flow have strong influence on freezing of water droplets on supercooled superhydrophobic surfaces, thereby also drastically affecting their icephobic behavior. The gas flow leads to a strong evaporation of the supercooled droplets, which alters the droplet temperature due to evaporative cooling. Through a scaling analysis, the effect of this cooling on nucleation of ice is examined using nucleation theories factoring in both surface roughness and contact angle. The role of shear gas flow on droplet roll-off vis-à-vis freezing dynamics is also analyzed by image processing of the high speed videos recording. Beyond a critical value of the gas velocity, the droplet begins to roll-off on the supercooled drolets. During roll-off, we show that freezing can ensue if the droplet comes in contact of an ice nucleus. It is observed that in dry humidity condition, the frozen droplets can roll-off at critical shear velocity, however, under saturated humidity condition the frozen droplet remains stuck to the surface. These results are interpreted by in terms of droplet state on the superhydrophobic state switching from Cassie state (in dry humidity) to Wenzel state (in saturated humidity).
9:30 AM - **RR1.2
Quantum Chemical Molecular Dynamics Simulation on Tribochemical Reaction Dynamics for Super-Low Friction System.
Momoji Kubo 1 , Nobuki Ozawa 1
1 Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, Sendai, Miyagi, Japan
Show Abstract Diamond-like carbon (DLC) has been expected as a promising material which indicates super-low frictions for reducing carbon dioxide emissions and saving energies. It is experimentally pointed out that the detailed understanding of the tribochemical reactions of the DLC is necessary for fully explaining its super-low friction. Here, classical molecular dynamics simulation is frequently employed to investigate the tribological phenomena on atomic scale. However, the classical molecular dynamics method cannot simulate the chemical reaction dynamics. Therefore, we developed a quantum chemical molecular dynamics simulator for the elucidation of the tribochemical reaction dynamics. In the present study, we applied our quantum chemical molecular dynamics simulator to the investigation on the tribochemical reaction dynamics of the DLC. This simulator is based on our original tight-binding theory and realized over 5,000 times acceleration compared to the first-principles molecular dynamics method. The tribochemical reaction dynamics of the hydrogen-terminated DLC under 1 GPa pressure condition was simulated. Our calculation results indicate that the hydrogen-hydrogen repulsion at the interface leads to the super-low friction of the DLC. Moreover, we observed the formation of H2 molecule at the friction interface by the tribochemical reaction of hydrogen atoms which terminate carbon atoms. The time profile of the friction coefficient of the hydrogen terminated DLC is analyzed. The super-low friction coefficient of 0.07 is realized after the formation of H2 molecule. Then we suggested that the formation of vapor phase at the solid-solid interface is the reason for the super-low friction of the DLC. Moreover, it is very interesting that the distance between the DLC substrates increased just after the formation of H2 molecule. The increment of the DLC substrates distance was found to be another important reason for the super-low friction of the DLC. We also calculated that the tribochemical reaction dynamics of the hydrogen terminated DLC under the higher pressure condition of 10 GPa. At that time, the creation of new C-C bonds at the friction interface was observed. Moreover, the formation of new C-C bonds increased the friction coefficient of the DLC. Therefore, we suggested that the avoidance of the C-C bond formation is key factor to realize the super-low friction of the DLC. Moreover, we also suggested that the experimental fluctuation of the friction coefficient of the DLC is due to the dynamic tribochemical reactions of the hydrogen and carbon atoms in the hydrogen-terminated DLC.
10:00 AM - RR1.3
Surface Forces in Confined Electrolyte Solution up to Saturation.
Rosa Espinosa-Marzal 1 , Manfred Heuberger 2
1 Materials Department, ETH Zurich, Zurich Switzerland, 2 Advanced Fibers, Empa, St Gallen, 9014, Switzerland
Show AbstractIn this work we measure the surface forces across a single-slit pore consisting of two atomically flat mica surfaces submerged in a reservoir of potassium nitrate (KNO3) solution. We use solution concentrations ranging from 0.08 mM to 2.6 M in the extended surface force apparatus (eSFA). Our experimental results are in general agreement with previously reported results, yet the higher resolution of our SFA reveals some new structural insights. We discuss our data in terms of three different concentration regimes that are defined by shifts of the energetics of structures and interactions at the molecular scale, as probed by the eSFA. Below a concentration of 0.3 mM, the surface force fits the classical DLVO theory at all surface separations. A mechanical instability leads to a jump into a mica-mica contact. Above 0.3 mM hydrated ions populate the interface to produce a collective layering transition; a layer of hydrated ions can be expelled in a 5.8±1 Å film thickness transition, which is unprecedented evidence of the existence of water cluster ions, i.e. ions with strongly associated hydration water, in salt solutions. In a concentration range between 1 and 10 mM multiple layers of cluster ions are detected. The ion hydration shell has a soft character, which gives rise to layering transitions of irregular sizes (4±1 Å). There is a tendency to smaller cluster sizes at higher concentrations. The water coordination number of ions at the interface, as inferred from layering transitions, is 5-10x smaller than in the surrounding bulk solution. Ions adjacent to the surface can reversibly be semi-dehydrated and immobilized in a 2.9±0.3 Å film thickness transition by external pressure; once immobilized they prevent intimate surface contact and lead to a reduced mica-mica surface adhesion. Above 20 mM the attractive ion correlation force comes into play as an additional attractive force. It affects both surface force and cluster ion structure. A highly dehydrated ionic condensate at the interface finally undergoes solidification around 100 mM. A 1-3 nm thick ionic solid finally forms at the surface while the bulk salt solution is still unsaturated. The surface force measured in the presence of this solid is weakly attractive and the measured adhesion increases again towards bulk solution saturation. Our results are of particular relevance to the fundamental understanding of saline hydration of charged hydrophilic surfaces, colloidal stabilization, biological systems, electrochemistry, and of the destructive salt weathering of concrete and stone by pore-cracking mechanisms, and, the pressure induced-dissolution at mineral-water interfaces.
10:15 AM - RR1.4
Comparison of the Kinetic Friction of Planar Neutral and Polyelectrolyte Polymer Brushes Using Molecular Dynamics Simulations.
Yangpeng Ou 2 , Jeffrey Sokoloff 1 , Mark Stevens 3
2 Physics Department, Northeastern University, Boston, Massachusetts, United States, 1 Physics, Northeastern University, Boston, Massachusetts, United States, 3 MS 1411, Sandia Nathional Laboratory, Albuquerque, New Mexico, United States
Show AbstractWe have simulated the relative shear motion of both neutral and polyelectrolyte end-grafted polymer brushes using molecular dynamics. The flexible neutral polymer brush is treated as a bead-spring model, and polyelectrolyte brush is treated same way except that each bead is charged and there are counterions present to neutralize the charge. We investigated the friction coefficient, polymer monomer density, and brush penetration for the two kinds of brushes with both the same grafting density and the same normal force under good solvent condition. For the case of equal grafting density, we find that polyelectrolyte brushes had a smaller friction coefficient and monomer penetration than neutral polymer brushes, although the polyelectrolyte brushes supported a much higher normal load than the neutral brushes for the same degree of compression. Charged and neutral brushes with their grafting densities chosen so that they support the same load exhibited approximately the same degree of interpenetration, but the polyelectrolyte brush exhibited a significantly lower friction coefficient. We present evidence that the reason for this is that the extra normal force contribution provided by the counterion osmotic pressure that exists for polyelectrolyte brushes permits polyelectrolyte brushes to support the same load as an identical neutral polymer brush of higher grafting density. Because of the resulting lower monomer density for the charged brushes, fewer monomer collisions take place per unit time, resulting in a lower friction coefficient.
10:30 AM - **RR1.5
Atomic Friction Experiments under Electrochemical Control.
Roland Bennewitz 1
1 , INM Leibniz Institute for New Materials, Saabrucken Germany
Show AbstractElectrochemical methods allow for fast and reversible modification of metal surfaces through deposition and dissolution of metal films, adsorption and desorption of anions, as well as oxidation and reduction. The surface composition and structure undergo dramatic changes in these processes, which should cause significant changes in the friction on the surface. We will report on our friction force microscopy experiments, in which we study atomic friction processes in order to explore the prospects of friction control through electrochemical methods.The resolution of atomic stick-slip events in an electrochemical cell is improved by the development of a dedicated instrument [1]. The lateral force contrast reveals the atomic structure of the Au(111) surface and its herringbone reconstruction. After deposition of one monolayer of Cu by underpotential deposition, the atomic stick-slip changes into a periodicity which indicates frictional response of a CuCl rather than a Cu layer.Wear-less friction on Au(111) surfaces is extremely weak and exhibits almost no load dependence. Upon electrochemical oxidation of the surface, significant friction with linear load dependence is observed. The process is reversible and allows switching repeatedly between high and low friction [2]. More subtle effects are found in the regime of anion adsorption, namely a frictional response with threshold behaviour. The threshold depends on both applied normal load and the electrochemical potential, indicating confinement effects [3].[1] A. Labuda et al., Rev. Sci. Instruments 81, 083701 (2010)[2] A. Labuda et al., Langmuir 27 (2011) 2561[3] F. Hausen et al., Electrochimica Acta (2011) doi:10.1016/j.electacta.2011.03.013
11:30 AM - **RR1.6
Adhesion and Friction on Patterned Surfaces.
Robert McMeeking 1 2 3
1 Department of Mechanical Engineering, University of California, Santa Barbara, California, United States, 2 School of Engineering, University of Aberdeen, Aberdeen United Kingdom, 3 , INM-Leibniz Institute for New Materials, Saarbruecken Germany
Show AbstractPatterned surfaces having small fibrils of various shapes and depressions when fabricated from soft materials such as PDMS are known to have superior adhesion properties compared to unstructured surfaces. Further features of note are that fibrils having flanges or spatula at their ends are more adhesive than those that have square or rounded perimeters at the adhering end. Flat on flat adhesion involving one patterned surface against an unpatterned one is very sensitive to alignment, with the pull-off force falling off dramatically with even slight misalignment of a few degrees. These insights and other evidence indicate that peeling is the dominant mechanism for detachment of such surfaces, whether at the scale of individual fibrils or dimples, or at the macroscopic scale of the entire patterned surface. It is also apparent that the mechanisms of detachment are significantly influenced by the presence of friction on the surface, both when it eliminates sliding completely and when sliding occurs. These phenomena are also relevant to the adhesion utilized by the gecko and exploited by mussels through their byssal attachments. Another feature that influences adhesion is the buckling of fibrils under compression that can eliminate adhesion through rate effects that prohibit reattachment after unbuckling. These phenomena are described through models of the various mechanisms and interactions among them, with the important influences of large strain allowed for. Insights into the resulting adhesion, pull-off forces and the interaction of adhesion and friction are provided.
12:00 PM - RR1.7
Contact of Adhesive Randomly Rough Surfaces.
Lars Pastewka 1 , Mark Robbins 1
1 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractAdhesive contact of rigid, randomly rough surfaces and elastic substrates is studied using molecular statics and continuum simulations. The surfaces are self-affine with Hurst exponent 0.3 to 0.8 and different short λs and long λl wavelength cutoffs. The range and strength of the adhesive potential is also varied. In all cases the effect of adhesion decreases as the ratio λl/λs increases. In particular, the pull-off force decreases and the area of contact becomes linear in the applied load. The results will be discussed in the context of the Maugis-Dugdale [1] theories for individual asperities and the more recent scaling theory of Persson [2].[1] D. Maugis, J. Colloid Interface Sci. 150, 243 (1992)[2] B.N.J. Persson, Phys. Rev. Lett. 74, 75420 (2006)
12:15 PM - RR1.8
Synthesis, Characterization and Tribological Implications of Surface-Tethered Hydrogel Thin Films with Various Vertical Architectures.
Ang Li 1 , Edmondo Benetti 1 , Rosa Espinosa-Marzal 1 , Shivaprakash Ramakrishna 1 , Davide Tranchida 2 , Holger Schoenherr 2 , Nicholas Spencer 1
1 Department of Materials, Laboratory for Surface Science and Technology, Zurich, Zurich, Switzerland, 2 Department of Physical Chemistry I, University of Siegen, Siegen Germany
Show AbstractIn this study, thin films of poly(acrylamide) (PAAm) brushes and hydrogel-brushes with various degrees of crosslinking were grown from iniferter-functionalized silicon substrates by UVLED-initiated photopolymerization and their properties studied by means of a variety of analytical methods. In the case of homogeneous PAAm hydrogel-brushes, both bulk and interfacial properties of the polymer films were found to be strongly influenced by the lateral crosslinking of the grafted polymer chains. In agreement with theoretical expectations, the decrease of polymer-brush mobility with increasing crosslinking density results in a substantial increase of film wettability. The swelling ratio of polymer hydrogel-brushes, as measured by ellipsometry and atomic force microscopy, also confirms the formation of grafted networks and is directly related to the amount of crosslinker in the monomer feed. Young's moduli and friction coefficients of PAAm hydrogels are higher than those of the grafted polymer-brush analogues and can be tuned by varying the crosslinking densities.In the second part of the study, stratified brushes/gel films with various vertical architectures were successfully fabricated by repeatedly reinitiating the dormant species at the polymer chain ends in a monomer solution containing a predefined amount of crosslinker. The fabricated stratified hydrogel thin films differ in both lubricating performance and tribological durability. A three-layer film structure (brush-gel-brush) is proposed, in order to minimize the stress at the iniferter binding sites, to average the load, to strengthen the film and to achieve low friction coefficients. The optimized brush-gel-brush polymer film showed outstanding tribological stability and lubricating performance compared to unstratified polymer brushes or gel analogues. Thus, these stratified water-born polymeric thin films may find promising applications in the fast-developing aqueous-based coatings industry and provide a platform to design films with excellent mechanical and aqueous lubricating properties.
12:30 PM - RR1.9
Comparison of Friction Forces Measured in Adhesive and Non-Adhesive Systems with SFA and AFM.
Jagdeep Singh 1 , Yutao Yang 1 , Marina Ruths 1
1 Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractThe boundary friction of different polyaromatic self-assembled monolayers was measured in single asperity contacts with the surface force apparatus (SFA) and atomic force microscopy (AFM). Measurements were done in ethanol and in dry N2 gas. In ethanol, the adhesion was significantly reduced. This allowed a direct, quantitative comparison of load-dependent friction between surfaces of very different radii. A linear dependence of the friction force on load was obtained, and good agreement was found between friction coefficients measured with the two techniques despite the large differences in contact areas, applied loads, and pressures. The investigations have been extended to measurements in dry N2 gas, where the surfaces adhere due to van der Waals interactions. In the adhesive systems, the friction force shows a non-linear dependence on load and is dependent on the radius of curvature of the interacting surfaces.
12:45 PM - RR1.10
Ultra Low Friction of Multilayer Graphene Studied by Coarse-Grained Molecular Simulation.
Hitoshi Washizu 1 , Seiji Kajita 1 , Mamoru Tohyama 1 , Toshihide Ohmori 1 , Hiroshi Teranishi 2 , Noriaki Nishino 2 , Atsushi Suzuki 2
1 , Toyota Central R&D Labs. Inc., Aichi, Aichi, Japan, 2 , Toyota Motor Corporation, Toyota, Aichi, Japan
Show AbstractCoarse-grained Metropolis Monte Carlo Brownian dynamics (MCBD) simulations are employed to investigate the friction dynamics of a transfer film of multilayered graphene sheets. The mechanism of ultra low friction of some type of layered materials such as graphite is not well explained yet. This is because the transfer film is consisted by a huge amount of atoms which is hard to treat by atom based simulation such as molecular dynamics. MCBD is the numerical solver of the Fokker-Plank diffusion equation and physically equal to the Langevin dynamics. Each circular graphene sheet consist of 400 to 1,000,000 atoms are allowed to move in 3 translational and 1 rotational directions due to thermal motion in 300K. Sheet-sheet interaction energy is calculated by the sum of the pair potential of sp2 carbon. The time scale is related to the diffusion coefficient. The mono- and the multilayer up to 20 layer graphene sheets are confined by two sliders. The upper slider is fixed to the upper solid body and the lower slider is modeled as infinitely wide graphite sheet. The end effects are included to all calculation of interactions of neighboring sheets except the interaction of the lowest sheet and the lower slider, by decreasing the potential energy when two sheets are separated in translational direction. The effect of incommensurate surface interaction is included by adopting smooth energy surface when two sheets are in twisted yaw angle. Then the sliding simulations are done by moving the upper slider in constant velocity. In monolayer case, the friction force showed stick-slip like curve and the average of the force was large. In multilayer case, the friction force did not show the oscillation and the average of the force was very low even in commesurate condition. This is because the whole transfer film obtain the internal degree of freedom in multilayer case and the lowest sheet of the layer are able to follow the equipotential surface of the lower slider. The dependence of sheet size, number of sheets are explained from the point of view of thermostatistical stability.
RR2: Interfaces in Mechanics
Session Chairs
Monday PM, November 28, 2011
Commonwealth (Sheraton)
2:30 PM - **RR2.1
Designing Interfaces for Nano Crystalline Diamond Coatings.
Yue Qi 1
1 Chemical Sciences and Materials Systems Lab, General Motors R&D, Warren, Michigan, United States
Show AbstractTo enable nano-crystalline diamond (NCD) as tool coatings for aluminum machining, first principles calculations were integrated with cohesive zone and growth chemistry models to investigate all three interfaces: the Al/NCD interface, the grain boundaries (GB) inside NCD, and the coating/substrate interface. First, the tribochemical reactions on diamond surfaces was captured by first principles thermodynamics, we predicted the environmental conditions (gas species, pressure and temperature) to achieve fully –H or –OH terminated surface with minimized adhesion with Al. Second, our integrated model revealed that lower substrate temperatures increases the hydrogen content at the surface, which reduces tensile stress during film growth. Third, to enhance the adhesion of the coating, we discovered the anisotropic fracture behavior of interlayer can be utilized to form oriented microcracks, thus providing a new interface toughening mechanism. These predictions were validated with experiments at each step and guided NCD coating design.
3:00 PM - RR2.2
Analytical Model for Nanoscale Ploughing and Wear.
Maneesh Mishra 1 , Izabela Szlufarska 2 1
1 Materials Science Program, University of Wisconsin Madison, Madison, Wisconsin, United States, 2 Materials Science and Engineering, University of Wisconsin Madison, Madison, Wisconsin, United States
Show AbstractAlthough nanoscale friction has been widely studied in the elastic regime, very few studies have focused on friction and wear in the elasto-plastic regime where material removal can have significant contributions to the coefficient of friction. Existing analytical models, which have been developed for macroscale elasto-plastic friction, fail to describe a ploughing contribution to the coefficient of friction in nanometer sized contacts. Two reasons for this breakdown are a large contribution to nanoscale friction from pile up and a reduction of friction due to elastic recovery in the wake of the cutting tip. We have developed a new analytical model of ploughing friction that includes the above two contributions. The model was validated using large scale molecular dynamics simulations of machining on silicon carbide (SiC) and copper (Cu) single crystals using a spherical tip with radius of curvature R=10 nm. We find that the elastic recovery in the wake of the cutting tip is a function of depth of cut and significantly reduces coefficient of friction at small depths of cut (<0.1 R) where deformation is predominantly elastic. At larger depths of cut (0.2-0.5 R), deformation is elasto-plastic and both SiC and Cu deform via dislocation plasticity. Dislocation plasticity leads to formation of a pile up and the coefficient of friction increases as the cutting tip pushes the piled up material in front of it. In addition, we show that once the total coefficient of friction is known, a transition from ploughing to cutting during nanoscale scratching of singe crystals can be accurately predicted by macroscale geometric models.
3:15 PM - RR2.3
Mechanics and Friction of Individual Nanoparticles.
Eric Bucholz 1 , Susan Sinnott 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractIn situ transmission electron microscopy (TEM) experiments can be used to visualize and manipulate nanoparticles, including subjecting them to compression and shearing, which facilitate the study of their responses to compression and shearing. Understanding these responses is critical for optimizing their use as solid lubricants or lubricating additives to base oils in tribological applications. Despite the new insights provided by these experimental studies, it is nonetheless difficult to determine the source of changes in mechanical behavior and specific lubrication mechanisms from the resulting data; therefore, a complementary computational analysis of these systems is needed. Here, classical molecular dynamics simulations are performed to characterize the behavior of individual amorphous carbon (aC) and inorganic fullerene-like (IF) MoS2 nanoparticles at an interface when subjected to externally applied forces. In particular, the response of aC nanoparticles as a function of diameter and normal load are quantified based, in part, on the ratio of sp2:sp3 carbon atoms. The simulations predict that the transition from elastic to plastic deformation is triggered by an increase in the percentage of sp3 carbon atoms with the mechanical response also being independent of nanoparticle size over the range of diameters considered (2 – 5 nm). In the case of IF MoS2 nanoparticles, the simulations reveal how their mechanical and tribological properties depend on the atomic-scale details of the nanoparticle’s structure. Two specific configurations are considered with three nested MoS2 layers each: 1) a curved, ellipsoidal structure and 2) an octahedron. These different structures allow for the characterization of the role of curved and faceted morphologies as well as grain boundaries on the rolling and sliding behavior as well as any lamellar exfoliation of the individual MoS2 nanoparticles. Through the combination of experimental and simulation analysis, a comprehensive picture of the mechanical and tribological properties of nanomaterials is provided which should ultimately aid in the optimization of their use in tribological applications. This work is supported by the Office of Naval Research.
3:30 PM - **RR2.4
Hybrid Modeling Approaches for Characterizing Interfacial Structure and Dynamics.
Donald Brenner 1 , Lipeng Sun 1 , Shijing Lu 1 , Hongli Dang 1 , Mohammed Zikry 2
1 Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractWe have been developing hybrid approaches that combine analytic models, first principles calculations, atomic and continuum modeling to characterize the structure and properties of solid interfaces. This talk will focus on two examples of this work. In the first example a meso-scale defect model for tilt grain boundary energies that is parameterized to first principles calculations is used to predict and understand solute-induced grain boundary stabilization in AlPb and CuZn alloys. In contrast to prior suggestions in the literature, no correlation was apparent between the magnitude of the stabilization energy and the grain boundary energy of the pure systems. This result is attributed to an elastic term in the model that is unchanged by doping. Instead the stabilization energy is a well behaved function of tilt angle with a maximum at 53.13o. This relation occurs because the common elastic term for the pure and doped system, which produces cusps in the energy versus tilt angle relations at low sigma structures, cancels when calculating the stabilization energy. In the second example molecular simulations were used to model the dynamics of dislocations moving at very high strain rates in an Al 2139 alloy as they interact with nanometer-thick platelet precipitates. A unique deformation mechanism was observed that is a combination of precipitate cutting and Orowan looping. The resulting “step-ladder” structure of the sheared precipitates correlates with experimental high resolution transmission electron micrographs of shock-loaded alloys. A new analytic expression for the critical shear stress needed for platelet deformation was developed that is based on an activated reaction kinetics model. The strain rate in this expression appears in a log term so that the very high strain rates inherent to the simulations can be scaled to experimental conditions. This expression reproduces the dependence of the critical shear stress on temperature and platelet size given by the molecular simulations; it also reproduces the large dependence of flow stress on strain rate observed experimentally but not apparent in the high strain rate simulations.
RR3: Defect and Oxygen Dynamics near Interfaces I
Session Chairs
Monday PM, November 28, 2011
Commonwealth (Sheraton)
4:30 PM - RR3.1
Structural Transition of SrTiO3 Asymmetric Tilt Grain Boundaries.
Akihisa Fukumoto 1 , Haksung Lee 1 , Teruyasu Mizoguchi 1 , Yuichi Ikuhara 1 2 , Takahisa Yamamoto 1 2
1 , The University of Tokyo, Tokyo Japan, 2 , Japan Fine Ceramics Center, Nagoya Japan
Show Abstract Grain boundary (GB) in ceramics often plays an important role on obtaining unique properties. For example, nonlinear I-V characteristics of SrTiO3[1] and positive temperature coefficient of resistivity (PTCR) of BaTiO3[2] are originated from GBs and strongly depend on GB atomic structures. Therefore, to predict and control GB structures are essential for the materials design. To understand GB atomic structures, the combination of transmission electron microscopy (TEM) observation and theoretical calculation is a powerful way. So far, a number of related studies have been reported. However, most of them are concerned to symmetric tilt boundaries because such type of GB is easy to be analyzed from a view point of theoretical calculation. In contrast, the understanding of asymmetric GB structures is limited although GBs in practical polycrystalline materials are mostly asymmetric. In this study, structures of the SrTiO3 asymmetric tilt GB annealed in oxidizing or reducing atmospheres were investigated.Bicrystal samples of SrTiO3 [1-10](114)//(110)Σ3 asymmetric tilt boundary which were heat-treated in air or H2 were observed by conventional TEM (CTEM), high-resolution TEM (HRTEM), and Cs-corrected scanning transmission electron microscope (STEM) from the edge-on direction, and further, compositional analysis at the GB was also performed by STEM equipped with energy dispersive X-ray spectrometer (STEM-EDS). In order to understand the structural transition theoretically, GB structures and GB energies were calculated by the first principles projector augmented wave (PAW) method.It is revealed that GB structural transition is induced by changing oxygen partial pressure. The CTEM observation shows that the asymmetric tilt GB is periodically faceted with two kinds of GB planes of which plane indices are (11-2)//(552) and (115)//(11-1) in air-annealed sample. On the other hand, GB planes parallel to the original plane of (114)//(330) appear in H2-annealed sample. The STEM observation reveals that all GB atomic structures consist of 3 types of structural units, of which plane indices correspond to (111)//(11-1) ((111) twin), (112)//(11-2) ((112)twin), and (221)//(001). Using the structural unit model theory, GB energies and their dependence on annealing atmosphere are calculated. With the results, we discuss the structural transition of GBs by considering the stability of GBs that depends on the oxygen partial pressure.[1]T. Yamamoto et al. Journal of Materials Science, 40, 881(2005)[2]B. Hyubrechts et al. Journal of Materials Science, 30, 2463(1995)
4:45 PM - RR3.2
Surfactant Stabilization of Polar Oxide Surfaces: Enabling the Creation of Novel Functional Interfaces.
Benjamin Gaddy 1 , Elizabeth Paisley 1 , Mark Losego 1 , James Tweedie 1 , Ramón Collazo 1 , Zlatko Sitar 1 , Jon-Paul Maria 1 , Douglas Irving 1
1 Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractHeteroepitaxy has the potential to enable the coupling of properties at interfaces between active materials, such as polar semiconductors and complex oxides, and will facilitate rich functionalities including non-linear dielectrics, non-linear magnets, and 2-D conductors. Many of the most interesting interfaces require the integration of dissimilar materials, with different structures, symmetries, and types of bonding. One such interface is the one between CaO, which has a rocksalt structure, and wurtzite GaN. The hexagonal symmetry of (0001) GaN leads to an epitaxial preference for seeding <111> oriented CaO growth, but the high surface free energy of this rocksalt orientation quickly leads to the formation of {100} facets and prevents smooth layer by layer growth. In this talk we will present results from ab-initio thermodynamics calculations. These results demonstrate that a hydrogen surfactant can overcome the preference for {100} faceting common to rocksalt oxides by altering the relative surface free energies, thereby enabling smooth layer by layer growth of <111> CaO on (0001) GaN. We compare our results to experimental growth of rocksalt oxides where water vapor is used instead of molecular oxygen in Molecular Beam Epitaxy. The effect of the hydrogen surfactant on the stability of the surface is examined through analysis of electrostatic effects, local density of states, interplanar relaxations, atomic reconstructions at the surface, electronic relaxations, and surfactant configurations. This work was supported by a U.S. Dept. of Defense NDSEG Fellowship, U.S. Dept. of Education GAANN Fellowship, an NSF Graduate Fellowship, and NSF DMR grant 0547134.
5:00 PM - RR3.3
Investigation of Nucleation and Growth of Hafnium Oxide Deposited by Thermal and Plasma-Enhanced Atomic Layer Deposition Using In Situ Spectroscopic Ellipsometry.
Amir Afshar 1 , Kenneth Cadien 1
1 Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada
Show AbstractThe nucleation and growth of hafnium oxide on silicon during thermal and plasma-enhanced atomic layer deposition (ALD) has been studied. Changes in the thickness of the HfO2 layer after reactant doses and during the purge times were studied using in-situ spectroscopic ellipsometry (SE). Tetrakis(dimethylamino)hafnium (TDMAH) was used as the hafnium precursor, and oxygen plasma and water were used as oxidizing reagents for plasma-enhanced and thermal ALD, respectively. Argon was used as the carrier gas. The substrate temperature was in the range of 100 - 300 °C. The results showed that the apparent thickness of the HfO2 layer (Δt1) increased sharply after the TDMAH dose and then decreased gradually during the purge time. This was attributed to the desorption of TDMAH from the surface. After introducing the oxidizing reagents, the apparent thickness (Δt2) decreased sharply due to replacement of large dimethylamino groups by small OH groups. The apparent thickness remained constant during the second purge time indicating that reaction was completed and the HfO2 layer was stabilized. Δt1 and Δt2 decreased with increasing temperature for both thermal and plasma-enhanced ALD. This is explained by the fact that at higher temperatures the rate of desorption would be higher than the rate of chemisorption resulting in a decreased Δt1 and Δt2. The results also showed that Δt1 was higher for the first cycle when the TDMAH reacts with the native silicon oxide on the substrate. After the first step Δt1 and Δt2 remained constant.
5:15 PM - RR3.4
First Principles Analysis of the Initial Oxidation of Treated Si(001) and Si(111) Surfaces and Its Implications for Protecting Quantum Confined Silicon Nanostructures.
Huashan Li 1 , Zhigang Wu 1 , Mark Lusk 1
1 Physics, Colorado School of Mines, Golden, Colorado, United States
Show AbstractThe photovoltaic properties of quantum confined silicon (Si) nanostructures are extremely sensitive to oxidation because the high surface-to-volume ratio amplifies the influence of defects on electron-hole recombination, and because even small oxide films can change the optical gap, a sensitive function of the size of the underlying pristine structure. Even more extreme, sufficiently small Si nanostructures can completely oxidize before the encroaching oxide layer becomes a barrier to further oxygen attack. Understanding the mechanisms and the detailed processes of the earliest first steps oxygen interaction with functionalized Si surfaces is therefore critical to designing surface treatments that result in robust Si nanostructures. Interestingly, no systematic computational investigations have been carried out to elucidate the role of surface passivants for the extreme curtailing oxidation required to protect quantum confined Si particles and wires. In this study, we use density functional theory combined with transition state analysis to identify the most likely paths for dissociative adsorption of the first O2 molecules on hydrogen, methyl, and siloxane terminated Si (111), Si(001) surfaces as well as Si quantum dots. Specifically, barrier saddle point searching via an iterative application of linear synchronous transition and quadratic synchronous transition searching is supplemented by an eigenvector following method to determine the activation barriers for initial stage oxidation. Our results indicate that oxidation occurs without any energy barrier on H-terminated surface via direct dissociative adsorption, and that the passivant provides a degree of protection only because of its influence on the subsequent hopping of O atoms. On the other hand, O2 molecules are forced to react first with surface ligands on methyl and siloxane terminated surfaces, and the associated intermediate barriers offer more substantial protection that is possible with the H-treatment. In addition, the coverage dependence of adsorption barriers as well as the influence of ligands on the lateral and inward hopping mobilities of O atoms shed light on the role of ligands in preventing the initial stage of oxidation within these more complex systems. The important role of steric shielding leaves nanostructures vulnerable to initial oxygen attack, but it may be possible to close off these avenues by designing systems with very low O mobility from these sites.
5:30 PM - RR3.5
Optimization of the Properties of the Interface of SiNx:H/Si in Crystalline Silicon Solar Cells and Its Effect on Cell Efficiency.
Machteld Lamers 1 , Keith Butler 2 , Ingrid Romijn 1 , John Harding 2 , Arthur Weeber 1
1 , ECN Solar Energy, Petten Netherlands, 2 , University of Sheffield, Sheffield United Kingdom
Show AbstractThe efficiency of a crystalline silicon solar cell depends to a large extent on the passivating qualities of the layers placed on either side of the cell. Passivating layers should reduce the recombination losses at the surfaces of the cells as this will increase the cell output. In standard silicon cell production an Aluminium Back Surface Field (BSF, which is a layer with a higher dopant concentration than the base of the cell) is created on the rear side as a surface passivating layer, while on the front side SiNx:H (hydrogenated silicon nitride) is used. The latter layer acts not merely as a passivating layer, but also as an anti-reflection coating. The properties required for a highly passivating SiNx:H layer do not necessarily correspond to the properties needed to ensure maximum light transmission. Usually, highly passivating SiNx:H layers have a high absorption, which reduces the light transmission and subsequently the short circuit current Jsc of the solar cell. In optimizing the SiNx:H layer used on the front side of solar cells a careful balance is therefore required between the light management on one hand and the passivating properties on the other hand. Passivation itself can subsequently be divided in bulk- and surface defect passivation, and in this case also a careful balance needs to be maintained. In this paper we show experimental data from various SiNx:H layers and its SiNx:H/Si interfaces fabricated with an industrial remote-PECVD system (PECVD is Plasma Enhanced Chemical Vapour Deposition). We show that the gradient and the concentration of Si-N bonds at the interface determine the surface passivating properties. We show that a steep gradient or a very gradual gradient gives low passivating qualities and low open circuit voltage Voc of a solar cell. The optimal gradient can be found in between the extremes. We present molecular dynamics simulations in which the geometries of layers with various gradients are analyzed and the locations of important defective geometries are mapped. Surface passivation is determined by the fixed charge and defect density at the interface. A higher fixed charge and a lower defect density improves the surface passivation. We show that a steep gradient gives less defects, while a more gradual gradient will give a higher fixed charge. The optimal surface passivation is a balance between both the defect density and the fixed charge. This finding of the optimal gradient is supported by the molecular dynamics simulations. We show how this interface can be optimized by varying the deposition properties using the PECVD tool and how this understanding will lead to further increase solar cell efficiency.
5:45 PM - RR3.6
Studying the Effect of Interface Structure on the Electronic and Optical Properties of Silicon/Silicon Nitride Interfaces in Solar Cells Using Molecular Dynamics Simulations.
Keith Butler 1 , Machteld Lamers 2 , John Harding 1 , Arthur Weeber 2
1 Materials Science and Engineering, University of Sheffield, Sheffield United Kingdom, 2 Solar Energy, ECN, Petten Netherlands
Show AbstractOne of the major research areas in high performance solar cell production is the optimization of the so-called passivation interface, between crystalline silicon (c-Si) and silicon nitride (SiNx). This interface is important for two reasons. Firstly it acts an anti-reflective coating (ARC) to trap light within the c-Si wafer. Secondly it serves to passivate defects, which act as charge carrier recombination centres, present at the surface of the c-Si. Unfortunately it is often found that the optimization of one of these properties is achieved at the detriment of the other. Much of the work done on the development of the passivation interface to date has focused on finding the optimal ratio of Si to N (i.e. the value of x in SiNx). Plasma-enhanced chemical vapour deposition (PECVD) has become a popular technique for deposition of the SiNx layer, due to its ability to produce interfaces with the c-Si wafer containing fewer defect states. Under different conditions, either an abrupt interface, or one with a nitrogen gradient can be produced. Recent research has shown that a gradual varying of the nitrogen content across the interface rather than an abrupt interface between SiNx and c-Si can give solar cells with improved optical and electronic properties. We show how molecular dynamics simulations can explain how a gradual structure of the interface can result in a reduction in the density of the defective atom environments which have been implicated as charge traps which hamper solar cell performance. In addition we highlight the effects of varying the stoichiometry of the SiNx layer.We then investigate how the locations of certain defects are related to the width of the layer over which the N gradient spans. Mapping of the defects reveals how the majority of defects are located away from the c-Si surface in the presence of a N gradient. We also find that where the N gradient is too wide the number of defects at the interface increases. These results point clearly towards an optimal interface deign for improved efficiency in silicon based solar cells.
RR4: Poster Session: Interfacial Physics, Chemistry, and Mechanics
Session Chairs
Tuesday AM, November 29, 2011
Exhibition Hall C (Hynes)
9:00 PM - RR4.1
Characterization of Ceramic Powders during Compaction Using Electrical Measurements.
Timothy Pruyn 1 , Rosario Gerhardt 1
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractIn this study we evaluated the electrical response of ceramic compacts during dry pressing as a function of applied pressure. Semiconductive SiC and insulating Al2O3 powders were used for the experiments. In order to determine the influence of porosity in the ceramic powder compacts, a custom made die with an insulating outer sleeve was used to carry out dc and ac measurements. Measurements were performed as a function of loading and unloading compaction pressure. Dc measurements can only detect the combined response from the powders and the porosity. However, from the SiC impedance spectroscopy data, at least two semicircles were observed in the complex impedance plot that allows separation of the two processes. One of these semicircles represents the bulk material property, while the other is likely due to the void space and interfaces between particles. The admittance, modulus and permittivity were also examined and showed behavior highly dependent on these two processes. The impedance behavior of the insulating Al2O3 was more sensitive to the compacted microstructure and humidity and often displayed trends different from the semiconducting SiC.
9:00 PM - RR4.10
Superlattice Vibrations and Debye Temperature of Nanoparticle Supercrystals.
Kaifu Bian 1 , Ananth Kaushik 1 , Paulette Clancy 1 , Detlef Smilgies 2 , Tobias Hanrath 1
1 Chemical Engineering, Cornell University, Ithaca, New York, United States, 2 Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, New York, United States
Show AbstractThe Debye theory of lattice vibrations and specific heat in atomic crystals marks a cornerstone in the development of solid-state physics. Supercrystals comprised of colloidal nanoparticles present an interesting analogy to atomic crystals with the mass of the inorganic core corresponding to the atomic mass, while the complex interactions of the organic ligand shells correspond to the interatomic force. An important, yet unresolved question is whether highly ordered nanoparticle assemblies exhibit superlattice vibrations analogous to phonon modes in atomic crystals. Phonon modes in atomic crystals are commonly approximated by Debye model. Can Debye theory also describe vibrations in nanoparticle superlattices?To answer this question, we investigated supercrystals (SC) comprised of lead sulfide (PbS) nanoparticles using small-angle X-ray scattering (SAXS). We determined the Debye temperature of PbS SCs via two complementary experimental approaches, which probed the SC ordering in response to changes in pressure and temperature. We measured the bulk modulus of PbS SCs from high-pressure SAXS measurements with a diamond anvil cell. In complementary experiments, we probed temperature-dependent scattering intensities and determined the Debye-Waller factor. We will compare our two experimental determinations of the superlattice Debye temperature with results from molecular dynamics simulations of nanocrystal superlattices. Together, these results provide interesting new insights into the vibrational dynamics of nanoparticle superlattices.
9:00 PM - RR4.12
Temperature Evolution of the Cooperative Characteristic Length in Drawn Polymers.
Florian Hamonic 1 , Allisson Saiter 1 , Daniele Prevosto 2 , Eric Dargent 1 , Jean-Marc Saiter 1
1 , LECAP, Institute for Materials Research. – Université de Rouen, Faculté des Sciences, Saint Etienne du Rouvray France, 2 , Italian National Research Council - Institute for Chemical and Physical Processes, Pisa Italy
Show AbstractThe existence of a characteristic length scale of structural relaxation in glass formers has been introduced by Adams and Gibbs [1] with the concept of cooperatively rearranging regions and more recently gained new interest in the framework of the dynamic heterogeneity concept [2]. The characteristic length scale should be connected with the evolution of the structural relaxation time and the glass transition phenomenon. For this reason its investigation is of great relevance for glass formers. In particular, when a polymer is mechanically drawn a certain macromolecular order is induced, which eventually leads to the formation of crystal regions coexisting with amorphous ones if the material has some tendency to crystallize. In such situation chains in the amorphous regions can suffer for constrains due to the presence of the crystalline parts, similar to the case of confined amorphous materials. Such constrains modify the dynamic properties of the polymer, altering the distribution of relaxation time and the glass transition itself.The goal of this work is to follow the evolution of cooperativity along the Arrhenius plot for drawn PET and PETg samples, the former developing crystalline regions the latter remaining almost completely amorphous during drawing. For this goal dielectric spectroscopy and temperature modulated differential scanning calorimetry measurements will be used, which allow studying the CRR size temperature dependence over a wide temperature interval [3]. The comparison of the results for the two materials allows evidencing the effect on structural dynamics of pure drawing (no crystals, PETg) and that of the confinement of amorphous regions within the crystalline ones (PET). Moreover, the interpretation of the results in term of modification of the size of cooperatively rearranging regions will be proposed.[1] G. Adam, J. H. Gibbs, J. Chem. Phys., 43, 139, 1965.[2] L. Berthier, G. Biroli, J.P. Bouchaud, L. Cipelletti, D. El Masri, D. L’Hôte, F. Ladieu, M. PiernoScience, 310, 1797, 2005.[3] A. Saiter, L. Delbreilh, H. Couderc, K. Arabeche, A. Schönhals, J.-M. Saiter Phys.Rev. E 81, 041805, 2010.
9:00 PM - RR4.13
First-Principles Study on the Self-Induced Oscillation for Charged-Particle Pairs Confined in Square Quantum Dots.
Atsushi Tsubaki 1 , Tomoki Tagawa 1 , Kyozaburo Takeda 1
1 Electrical Enginnering and Bioscience, Waseda University, Tokyo Japan
Show AbstractDynamics in charged particles confined in the quantum dot (QD) system is attractive not only for the scientific understanding on the kinetics in Coulomb interaction but also for the technical application in the quantum devices. Here, we theoretically study the time-dependent (TD) features of the two kinds of charged-particle pairs (electron-electron and electron-hole) confined in the square QD system. We computationally solve TD Schroedinger equation under the typical two mean-field approaches of UHF and DFT calculations. In order to deepen our understanding on TD behaviors, we compare results of the electron-hole pair with those of the electron-electron pair and also the DFT results with the UHF ones. The TD UHF calculation demonstrates that the charged-particle pairs vacillate spatially when the individual particles initially have the same irreducible symmetry. The projection analysis reveals that the vacillation changes to be resonative when the confinement is strengthened. The FFT investigation further elucidates that this characteristic vacillation is caused by the mutual transition of charged particles by the self-induced Coulomb interaction. The TD DFT calculation, however, decreases the vacillation frequency Ω of the electron-electron pair whereas it increases the Ω value of the electron-hole pair. This opposite tendency in Ω is caused by the electron correlation (EC) sophisticatedly included in the DFT calculation. The EC for the electron-electron pair works to delocalize the two-electron wave function. Consequently, the confinement is effectively weakened to reduce the resulting Coulomb. Contrary, the wave function for the electron-hole pair originally localizes at the SQD center. The enegetical stabilization due to the EC now enhances this central localization of the electron-hole pair. Accordingly, the confinement is effectively strengthened and the vacillation frequency increases. Thus, the present UHF and DFT calculations demonstrate that electron correlation works oppositely for the charge-particle pairs, in accordance with the Coulomb attractive or repulsive interaction.
9:00 PM - RR4.14
Adsorption and Interaction Forces of Serum Proteins as a Measure of the Biocompatibility of Novel Polymeric Biomaterials.
David Cozzens 1 , Umaprasana Ojha 1 , Marina Ruths 1 , Rudolf Faust 1
1 Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractWe have studied the surface characteristics and biocompatibility of thermoplastic polyurethanes (TPUs) with applications in blood-contacting medical devices. Commercial TPUs as well as novel polyisobutylene-based TPUs with superior oxidative stability were characterized through contact angle measurements, XPS, and AFM imaging. Since thrombogenicity is a key concern with long-term blood-contacting biomaterials, the first step in this process, the adsorption of serum proteins, was quantified by quartz crystal microbalance measurements with dissipation monitoring (QCM-D). The adsorption of fouling fibrinogen or passivating albumin onto thin films of the polymers spin-coated onto the quartz crystal electrode was studied, as well as competitive adsorption from a mixture of these proteins. The interaction forces and adsorption strength of the proteins with the polymer surfaces were measured in buffer solution with the colloidal probe technique. The results indicate comparable adsorbed amounts and similar adhesion strengths of the proteins on the novel materials and on the commercial ones. This suggests that the urethane nature of the surface is of high importance for the protein interactions, and that the novel TPUs may have excellent biocompatibility.
9:00 PM - RR4.2
Hierarchical Textured Architectures for Continuous Dropwise Condensation and Collection of Water Droplets.
Xuemei Chen 1 , Jun Wu 2 , Shuhuai Yao 2 , Zuankai Wang 1
1 Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon Tong Hong Kong, 2 Department of Mechanical Engineering , The Hong Kong University of Science and Technology, Hong Kong Hong Kong
Show AbstractThe miniaturization of electronic devices demands novel thermal management techniques to efficiently dissipate heat over a small area. Two-phase heat transfer devices such as condensers allow for high heat transfer coefficients. In particular, dropwise condensation is able to produce heat transfer coefficients an order of magnitude higher than film condensation. Therefore engineering surfaces that enable dropwise condensation is of paramount importance. Surfaces that enable both efficient droplet nucleation and droplet self-removal (i.e. droplet departure) are essential to accomplish successful dropwise condensation. However it is extremely challenging to design such surfaces. This is because droplet nucleation requires a hydrophilic surface while droplet departure necessitates a super-hydrophobic surface. Here we report that these conflicting requirements can be satisfied using a hierarchical (multiscale) nanograssed micropyramid architecture that yields global superhydrophobicity as well as locally wettable nucleation sites, allowing for ~65% increase in the drop number density and ~450% increase in the drop self-removal volume as compared to a superhydrophobic surface with nanostructures alone. Further we find that synergistic co-operation between the hierarchical structures contributes directly to a continuous process of nucleation, coalescence, departure, and re-nucleation enabling sustained dropwise condensation over prolonged periods. Exploiting such multiscale coupling effects can open up novel and exciting vistas in surface engineering leading to among other applications, optimal condensation surfaces for high performance electronics cooling, heat transfer and water harvesting applications.
9:00 PM - RR4.3
The Almost Universal Occurrence of Dry Friction is Due to Tomlinson Instabilities on Small Length Scales.
Jeffrey Sokoloff 1
1 Physics, Northeastern University, Boston, Massachusetts, United States
Show AbstractThe virtual universal existence of “dry” (i.e. velocity independent in the slow sliding speed limit) friction when two solids slide relative to each other is responsible for a large percentage of the energy loss that occurs in machines and other mechanical devices. It is well known that Tomlinson model instabilities, that have long been considered as the explanation for occurrence of “dry” friction, cannot occur for the micron scale asperities that occur at most interfaces because these asperities are generally too short and fat to exhibit such instabilities. When one considers the fact that smaller length scale asperities exist on the surface of each of the micron scale asperities and still smaller length scale asperities occur on the surface of each of these new asperities, and this continues until one reaches atomic length scales, it is possible to show that Tomlinson model instabilities will always occur for sufficiently small length scale asperities. This provides a likely explanation for the almost universal occurrence of “dry” friction. This hypothesis is substantiated by calculations based on a multiscale contact mechanics model due to Persson, which includes effects of elastic and plastic deformation as well as adhesion on multiple length scales.
9:00 PM - RR4.4
Mass and Point Defect Transport at Solid-Solid Semicoherent Interfaces.
Kedarnath Kolluri 1 , Michael Demkowicz 1
1 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractUsing atomic-scale modeling of several fcc-bcc interfaces, we show that mass and point-defect transport at semicoherent interfaces depends strongly on interface structure. Point defects trap at specific interface sites and migrate from one trap to another when the traps are close enough. What's more, interface structure strongly influences migration of point defects that are not trapped as well. Our findings suggest that constitutive models that currently describe diffusion at solid-state interfaces need revision.This material is based upon work supported as part of the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number 2008LANL1026.
9:00 PM - RR4.5
Relaxation of Polymer Melts in Nanofilms at the Polymer-Solid Interface.
Yongjin Wang 1 , Jianing Sun 2 , Lei Li 1
1 Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 , J.A. Woollam Co, Lincoln, Nebraska, United States
Show AbstractThe relaxation of polymer chains in bulk melt is fast, e.g., on the order of milliseconds to seconds. However, the relaxation in nanometer-thick films on solid substrates could be orders of magnitude slower. The very slow relaxation brings up the concerns on the long-term performance and the lifetime of the thin films. In order to design more robust thin-film materials, it is critical to understand the thermodynamics and the kinetics governing the relaxation. Here we report the experimental results indicating that the polymer-solid interfacial interaction, which does not have a bulk counterpart, plays a key role in both thermodynamic driving force and kinetics of the relaxation. Two Perfluoropolyethers (PFPEs) with the same backbone and different endgroups, one polar and the other non-polar, were coated on a hydrophilic substrate and the relaxation of the polymer films were monitored by contact angle testing. The result shows that the PFPE with polar endgroups relaxes with time and the relaxation time constant, obtained from Kohlrausch-Williams-Watts (KWW) model, is ten orders of magnitude higher than that of bulk polymer. However, the PFPE with non-polar endgroups does not show relaxation behavior. Based on the experimental results, we proposed that the relaxation is driven by the attractive interaction between the polar endgroups of the polymers and the polar sites on the substrate. The very slow kinetics of the relaxation has been attributed to the heterogeneity of the polymer-solid interaction at the interface and the cooperative nature of the molecular motions in the relaxation.
9:00 PM - RR4.6
Chemical Changes Underlying Aging of Silica in Nanoscale Frictional Contacts.
Yun Liu 1 , Izabela Szlufarska 1
1 Materials Science & Engineering Department, University of Wisconsin - Madison, Madison, Wisconsin, United States
Show AbstractFriction and adhesion of silica are important both in naturally occurring phenomena, such as shallow tectonic earthquakes, as well as in engineering applications, including wafer bonding for device applications. Since water is often ubiquitously present in the environment and because of the high reactivity of silica, water can dominate the tribological response of silica-silica interface. Here we focus specifically on aging of silica, which is a fascinating phenomenon that is dramatically accentuated by the presence of humidity and is relevant for understanding of earthquakes. Aging refers to the process by which static friction changes (generally increases) with time when the two surfaces are held still. One of the two main hypotheses for physical mechanisms of aging is that it results from chemical changes at the silica interface, which may include dissolution of silica, reorganization of H-bond network in confined water, and incorporation of water into silica. In this work we will discuss chemical reactions that can lead to aging of silica in aqueous environments. Using ab initio calculations based on the Density Functional Theory and molecular dynamics simulations based on ReaxFF reactive force field we determined rates of relevant reactions and identified processes that underlie the experimentally observed logarithmic dependence of aging on hold time. The effect of the interfacial chemistry on adhesion will also be discussed.
9:00 PM - RR4.7
Defects in a Block Copolymer Photonic Gel.
Yin Fan 2 , Jae-Hwang Lee 1 , Steven Kooi 3 , Alfredo Alexander-Katz 1 , Edwin Thomas 1 3
2 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Institute for Soldier Nanotechnology, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractBy selectively swelling one block in a lamellar forming diblock copolymer, we successfully made a tunable and responsive photonic gel. Applications include sensing, active display etc. It is of interest to study the swelling process for better control over the gel's properties, e.g. rate of response as well as robust behavior. Fast-optical reflectivity spectra during the swelling process show variations among samples made with the same material and the same processing conditions. The real-time swelling behaviors can be categorized into two types: fast response with low final swelling ratio, or slow response and high swelling ratio. We propose two swelling profiles to explain the difference: type A is of a uniform swelling profile and type B a gradient swelling profile. The two types of swelling profile can be further explained with regards to the types of defects and the defect density in the layered structure. Physical models linking the defect structure and the swelling properties are developed.
9:00 PM - RR4.8
Polymer Globule Dynamics in the Presence of Surfaces and Shear Flows.
Charles Sing 1 , Alfredo Alexander-Katz 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractHomopolymer globules have long been considered important as model systems for the study of proteins, with the understanding of globular polymers represents the origins of many theoretical treatments of protein behavior. In particular, recent work has shown that the quaternary structure of the blood protein von Willebrand Factor (vWF) is well represented by these simple models, especially in understanding its response to external fluid flow stimuli. vWF is activated during blood clotting, where it adsorbs readily to surfaces under high shear conditions. Our research seeks to understand this counterintuitive adsorption behavior by investigating through theory and Brownian Dynamics simulation the interaction between a collapsed polymer globule and an attractive surface in the presence of shear flow. These investigations reveal a rich interplay between hydrodynamic, cohesive, and entropic effects that ultimately determine the dynamics of a collapsed homopolymer in shear flow. This allows for a more nuanced understanding of the structural and chemical features governing vWF behavior, and more generally will provide insight into methods to manipulate homopolymer adsorption using fluid flows.
9:00 PM - RR4.9
Resonating UHF Study on Electron Correlation in a Ground State of Two Electrons Confined in 2D Quantum Dot.
Takuma Okunishi 1 , Kyozaburo Takeda 1
1 , Waseda university, Tokyo Japan
Show AbstractSemiconductor quantum dots (QDs) are meaningful system to deepen our inherent understandings on the electronic characteristics. When electrons are confined in such nano-meter sized QDs, the competition with a Coulomb force and a confinement potential is crucial, and the inclusion of the electron correlation is requested in the theoretical consideration. In this study, we reconsider two electrons confined in a 2D square QD, because this system is simple but is critical for fundamental considerations of electron correlation. In order to include the electron correlation, we employ the resonating UHF (res-UHF) method, where the configuration interaction (CI) is taken into account through the multi-reference description of many-electron wave functions. This method is straightforward to approximate a many-body wave function by the superposition of the nonorthogonal Slater solutions. An UHF approach might give an inconsistent ground state due to neglect of an electron correlation. However, res-UHF approach consistently corrects with reverting the symmetry nature in Hamiltonian. This res-UHF approach is also powerful because an inclusion of only 9 UHF scf-solutions is sufficient for describing the singlet ground state within an accuracy of 2% in the total energy. This is because these UHF scf-solutions are nonorthogonal mutually but represent the conceivable electron-spin configurations rationally. We further combine this res-UHF approach with solving the time-dependent (TD) equation, and study how an electron correlation causes a quantum fluctuation in determining the ground system. The TD approach demonstrates that an electron correlation induces a resonative vacillation between two energetically degenerated UHF ground states. The FFT analysis further reveals that the "bonding and anti-bonding" energetically separation between these two UHF ground states determines the vacillation frequency of the quantum fluctuation. The transitions into other UHF scf-states cause a characteristic beating in the electron-correlated quantum fluctuation.
Symposium Organizers
Douglas L. Irving North Carolina State University
Susan B. Sinnott University of Florida
Martin H. Mueser Saarland University
Izabela Szlufarska University of Wisconsin-Madison
RR5: Defect and Oxygen Dynamics near Interfaces II
Session Chairs
Elizabeth Dickey
Blas Uberuaga
Tuesday AM, November 29, 2011
Commonwealth (Sheraton)
9:30 AM - **RR5.1
Point Defect Dynamics at BaTiO3 – Electrode Interfaces.
Elizabeth Dickey 1 , Matthew Burch 1 , Clive Randall 2 , Malay Samantaray 2
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractThe migration of charged point defects under applied voltage bias is a phenomenon that can lead to long-term degradation of oxide devices or which can be utilized to create unique device functionalities. This talk discusses point defects and interfacial reactions in BaTiO3 capacitors as a function of processing history and voltage biasing. The relationship between local microchemistry and electrical transport is studied through a combination of transmission electron microscopy and electrical transport measurements, and it is found that the local microchemistry at the electrode interfaces is particularly important for the device leakage current. Low oxygen activities at this interface can lead to interfacial reactions and/or high concentrations of oxygen vacancies, both of which lower the interfacial resistance.The BaTiO3 - electrode interface becomes increasingly influential on the overall electronic and ionic transport as the dielectric layers are scaled down in thickness. In particular, porosity and interfacial roughness lead to field concentrations, which increase electrode-interface leakage and can accelerate the point defect redistribution kinetics. Scaling laws are studied by finite element modeling in which three-dimensional microstructures obtained by FIB/SEM imaging are used as input models. The scaling behavior and relative contributions of electrode porosity versus electrode roughness will be discussed. Finally, processing strategies for controlling interfacial properties at small layer thicknesses will be presented. This work has been supported by the National Science Foundation Ceramics Program through grant No. 0606352.
10:00 AM - RR5.2
Tuning the Limiting-Thickness of a Thin Oxide Layer on Al (111) with Oxygen Gas Pressure.
Na Cai 1 , Guangwen Zhou 1 , Kathrin Mueller 2 , David Starr 2
1 Department of Mechanical Engineering & Multidisciplinary Program in Materials Science and Engineering , SUNY-Binghamton, Binghamton, New York, United States, 2 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractThe oxidation of metal surfaces is of great importance for a wide range of technological applications including heterogeneous catalysis, electrochemistry, corrosion, gate oxides, and lubrication. Specifically, aluminum oxide films exhibits a variety of unique properties including a large dielectric constant (~10), a large barrier height for electron tunneling (~2eV), high corrosion resistance, good thermal and mechanical stability, and good adhesion to the underlying metal. Amorphous aluminum oxide films, formed by low-temperature oxidation of aluminum, fulfill the requirements for these applications because of their better structural perfections and better interface at the oxide-metal interface. A generic model describing low-temperature oxide film growth is the Cabrera-Mott model. According to this model, an electric field is formed across the oxide film due to the potential difference (called the Mott Potential) between the metal/oxide work function and the oxygen/oxide work function as a result of electron tunneling between the Fermi level of the parent metal substrate and acceptor levels of chemisorbed oxygen at the surface. The self-generated electric field reduces the energy barrier for the migration of ions through the oxide film (the limiting step for mass transport in oxidation), leading to rapid initial oxidation rates at low temperature. As the tunneling current diminishes with increasing oxide film thickness, the oxidation virtually stops at a limiting-thickness. We report an x-ray photoelectron spectroscopy (XPS) study of the oxidation of Al(111) surfaces at room temperature. Our results reveal that the actual value of the self-generated electrostatic potential (designated as the kinetic potential) can deviate significantly from the Mott potential and is tunable by varying the oxygen pressure during oxidation providing control of the limiting thickness of the oxide film. We found that a significantly large oxygen gas pressure (> 1 Torr) is required to form sufficient adsorbed oxygen at the oxide surface to accept the tunneling electrons in order to develop the saturated kinetic potential and therefore the maximum limiting thickness of the oxide film. At lower oxygen pressures, the lower coverage of oxygen anions leads to a kinetic potential of lower magnitude and therefore a thinner limiting-thickness of the oxide film.
10:15 AM - RR5.3
Precursor to the Onset of the Bulk Oxidation of Cu(100).
Liang Li 1 , Xi Mi 2 , Yunfeng Shi 2 , Guangwen Zhou 1
1 Mechanical Engineering, SUNY-Binghamton, Binghamton, New York, United States, 2 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractThe oxidation of metals plays a critical role in many important technological processes, such as corrosion, chemical catalysis, fuel reactions, and thin film processing. The general reaction sequence of the oxidation on a clean metal surface is thought to proceed as oxygen surface chemisorption, oxide nucleation and growth, and then bulk oxide growth. Much is known about the oxygen surface chemisorption, and particularly, the atomic structure of the chemisorbed phases on metal surfaces. However, the atomistic processes leading to the crossover between oxygen chemisorption and the nucleation of bulk oxide phase are still quite unexplored, although this information is critical for developing a complete understanding of the mechanism of metal oxidation. Using density-functional theory, we investigate the energetics of oxygen sub-surface adsorption governing the onset of bulk oxidation of Cu(100) terrace. Our results reveal that the presence of domain boundaries formed from merged (2√2×√2)R45°-O oxygen chemisorbed nanodomains mismatched by a half unit-cell leads to the preferred oxygen adsorption at the sub-surface tetrahedral sites. The resulting Cu-O tetrahedron resembles strikingly that of the bulk oxide phase of Cu2O and facilitates the subsequent oxide growth along the domain boundary. The insights and approach obtained from this study can be extended to understand the transient oxidation of many other metals, where oxygen-chemisorption induced surface restructuring generally occurs, but its effect on initiating the onset of the bulk oxide formation has been hitherto rarely been addressed.
10:30 AM - RR5.4
Passivity Breakdown and Pitting Nucleation of Copper Oxide Films under Aqueous Conditions: Simulations by Reactive Molecular Dynamics.
Byoungseon Jeon 1 , Subramanian Sankaranarayanan 2 , Adri van Duin 3 , Shriram Ramanathan 1
1 SEAS, Harvard University, Cambridge, Massachusetts, United States, 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 3 Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractPassive oxide films formed on metal or alloy surfaces play deterministic role on the surface chemistry and properties such as corrosion, wear resistance and so forth. The chemical and electronic nature of these oxide films are therefore of significance and how they are perturbed in realistic environments containing reactive elements such as halide ions in aqueous media need to be understood. For example, under aqueous conditions, like marine environment or water-based electrolyte, oxide films are degraded by corrosion, yielding passivity breakdown. Once pitting is nucleated, bare metals are contacted by corrosive elements and corrosion is accelerated. Protection and corrosion control is of great importance but understanding the atomistic scale mechanism is not done yet completely.Employing ReaxFF reactive force-fields, we conduct simulations on interaction of technologically relevant metal such as copper with water containing chlorine ions. Under aqueous conditions with chlorine ions, the behavior of copper and its dissolution is analyzed. Chemical thinningof oxide films by chlorine is observed while surface condition plays a key role on initial adsorption of chlorine. Pitting is nucleated when chemical thinning reaches the inner bulk substrate, yielding high stresson metal substrates.
10:45 AM - RR5.5
The Mechanical and Chemical Behavior of the Thin Oxide Skin on a Liquid Metal.
Ju-Hee So 1 , Mohammad Khan 1 , Michael Dickey 1
1 Chemical and Biomolecular Engineering, NC State University, Raleigh, North Carolina, United States
Show AbstractWe describe the mechanical and electrochemical behavior of the thin oxide skin that forms on a liquid metal alloy and discuss ways to modify these properties. The alloy, eutectic gallium indium, is useful for moldable microelectrodes, stretchable antennas, soft diodes and flexible solar cells. The ability to micromold the metal for these applications is enabled by the mechanical properties of a thin oxide skin that forms spontaneously on its surface. The oxide skin is elastic and yields under a critical stress, at which point the metal flows. We studied the mechanical modulus of the oxide skin under different environments using a rheometer with a parallel-plate geometry. The modulus of the skin can be tuned based on the chemical environment surrounding the metal and the physical hysteresis of processing. We demonstrate that water and acid can lower the modulus of the skin relative to that in air. Depositing materials on the skin offers a way to increase the modulus of the oxide skin. We found that certain polymers deposit spontaneously from solution on to the oxide skin. The modulus increases with time as it is exposed to the polymer solution (on the time scale of hours) and then levels off at a value that is two orders of magnitude higher than the native oxide skin. The kinetics of the deposition process can be correlated with the change of modulus. This technique offers a new, in situ method of studying film deposition kinetics and measuring thin film mechanics. We use electrochemistry to detect the physical breakage of the oxide skin since the exposed metal spontaneously oxidizes and generates current. Electrochemical oxidation or reduction of the skin that forms on the metal can also be used to change the thickness of the native oxide skin. These studies provide new tools to tune the modulus of the moldable liquid metal and engineer the forces under which the metal will flow.
11:30 AM - **RR5.6
The Competing Effects of Grain Boundaries on the Radiation Damage Response of Cu.
Blas Uberuaga 1
1 , Los Alamos Natl Lab, Los Alamos, New Mexico, United States
Show AbstractIt is well accepted that grain boundaries serve as effective sinks for radiation-induced defects (interstitials and vacancies). However, fundamental insight into the atomic-scale origin of this behavior is still lacking. The promise of engineering materials with interfaces to meet the radiation tolerance demands of future nuclear applications requires that the origin of this enhanced radiation tolerance be understood. We use molecular dynamics, temperature accelerated dynamics, and molecular statics to study radiation damage phenomena near a variety of grain boundaries in Cu over three different temporal regimes: the short-time damage production phase of a collision cascade, the longer-time scales over which defect annihilation and aggregation occur, and the thermodynamic-limiting behavior of the system. We find that both the production and the subsequent annealing of the radiation-induced defects are modified significantly by the presence of the grain boundary. In particular, we identify a new mechanism by which interstitials efficiently annihilate vacancies, promoting enhanced defect recombination. We compare to previous experimental results and identify three regimes over which different thermally activated processes are active, resulting in different responses, both better and worse than large-grained counterparts, of the material to irradiation. Our results show that nanostructured materials have a very sensitive response to irradiation and offer new insights into the design of radiation tolerant materials.
12:00 PM - RR5.7
Dynamic Propensity at Interfaces in a Radiation Environment.
Walid Mohamed 1 , Xiaojun Mei 1 , Jacob Eapen 1
1 Nuclear Engineering, NC State University, Raleigh, North Carolina, United States
Show AbstractRadiation tolerance is generally associated with the ability of a material to withstand the deleterious effects of radiation-induced phenomena such as amorphization, point-defect clustering, vacancy induced cavitation, swelling and precipitation of new phases. Following a collision cascade, the freely migrating point defects escape the cascade zone and contribute to the long-time, ‘extended’ defect structures in conventional materials. The accumulation of defects is regarded as a competition between defect formation by cascades, thermal fluctuations, interaction between the defects, and the transport or migration of defects.Several experimental studies and simulations show that nanocrystalline materials and interfaces, in general, can exhibit radiation tolerant properties. Transport of point defects to grain boundaries and interfaces (followed by absorption) is considered to be a central mechanism for radiation tolerance in such materials. The essential role of the interfaces is to act as an impediment to the growth of ‘extended’ defects. Using molecular dynamics simulations, we analyze the dynamical heterogeneity in nanocrystalline Cu following radiation. We quantify the radiation dynamics through directional diffusivities and dynamical propensity which is defined as the displacement of an atom averaged over an isoconfigurational ensemble. These measures allow a quantitative measure of migration of atoms following radiation and the dynamics at grain boundary interfaces that can potentially influence radiation tolerance in materials.
12:15 PM - RR5.8
Simultaneous Reduction of Radiation-Induced Defect Concentrations and Fluxes Using Interfaces with Precisely Controlled Sink Strengths.
Michael Demkowicz 1 , Richard Hoagland 2 , Blas Uberuaga 2 , Amit Misra 3
1 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 MST-8: Structure-Property Relations Group, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 Center for Integrated Nanotechnologies , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractWe use a reaction-diffusion model to demonstrate that solid-state interfaces in polycrystalline composites may simultaneously reduce the concentration and flux of radiation-induced defects, given a sufficiently high interface area per unit volume. The effect of interfaces on defect concentration is shown to be extremely sensitive to interface sink strength, η. These findings open an opportunity for mitigating both radiation-induced segregation, which depends on defect flux to interfaces, and radiation-induced swelling and hardening, which depend on defect concentration.This material is based upon work supported as part of the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number 2008LANL1026.
12:30 PM - RR5.9
The Effect of the Water-Silica Interface on Enhanced Hydronium Ion Formation and Proton Transport in Mesoporous Silica.
Glenn Lockwood 1 , Stephen Garofalini 1
1 Materials Science & Engineering, Rutgers, the State University of New Jersey, Piscataway, New Jersey, United States
Show AbstractElectrochemical studies have shown enhanced proton transport in mesoporous silica containing water, but modeling the kinetic processes underpinning this phenomenon requires molecular simulations which can capture the structural complexity of mesoporous silica as well as the chemical fidelity required to reproduce the reactive and dissociative properties of water molecules at interfaces. To this end, we have employed a dissociative water potential that matches many properties of bulk and nanoconfined water as well as the chemisorptive behavior of water at the silica surface, and we have simulated the enhanced formation of hydronium ions at these surfaces and shown behavior consistent with ab initio molecular dynamics simulations.In addition to showing the formation of surface silanol (SiOH) sites where protons are strongly bound, our simulations have revealed additional weakly binding proton adsorption sites on the silica surface. These sites contribute to enhanced proton transport beyond that observed in the nanoconfined water phase alone. The silica surface also affects the transport properties of molecular water, and the magnitude of this effect in nanoconfined geometries has been found to be dependent on both distance from the interface and the curvature of the confining silica surface. Thus, the enhanced proton conduction in hydrated mesoporous silica is affected by surface chemistry, topology, and curvature, and the details of our findings will be discussed.
12:45 PM - RR5.10
Thermodynamics, Mechanics, and Dynamics Properties of Water and Ions Confined in Nanopores: Application to Cement Hydrates.
Patrick Bonnaud 1 , Krystyn Van Vliet 1 , Benoît Coasne 4 , Roland Pellenq 2 3
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 4 Institut Charles Gerhardt Montpellier, CNRS and Université Montpellier, Montpellier France, 2 Civil and Environmental Engineering, MIT, Cambridge, Massachusetts, United States, 3 CINaM, CNRS and Aix-Marseille Université, Marseille France
Show AbstractHydrated counter-ions in a suspension or paste of charges colloids play a central role in the overall system’s stability and thermodynamics. For instance Ca2+ ions in a cement paste that can be seen at early ages as an aqueous colloidal system are inherent part of the structure. They compensate the substrate charge (due to local chemical/substitution defects in cement hydrate ribbon-shaped particles also named as C-S-H with C = CaO; S = SiO2; H = H2O). C-S-H particles combine to each other to form brick-shaped particles. Cement paste is a disordered assembly of these particles which are maintained together thanks to electrostatic forces. Because of the different scales involved in the formation of cement pastes, this material is a multiscale porous material with a wide range of pore sizes. Depending on the ambient conditions, the cement paste can encounter mechanical damage due to the presence of the confined fluid (e.g. cryo-suction effects in freezing conditions which are the consequence of thermodynamic equilibrium between solid-like and liquid-like water inside the pore). In this work, the thermodynamics, mechanics and dynamics properties of the electrolyte (Ca2+ ions and water) in the pore are studied to understand such effects and the influence of ions using classical simulation techniques (Grand Canonical Monte Carlo and Molecular Dynamics). Several models of pores are considered to study the role of chemistry and surface roughness: silica (hydroxylated and with Ca charge-compensating ions) and cement. The substrates were atomistically described and three pore sizes were considered (1, 2 and 4 nm) at various temperatures ranging from 175 K to 300 K.The adsorption/desorption isotherms of water at room temperature in the different pore sizes were first estimated. A very hydrophilic behaviour is observed in all studied systems where water can adsorb and for the larger pores, condensate into liquid at pressure lower than the bulk saturating pressure while it is in the gas phase in the outside Grand Canonical reservoir. Solvation pressures inside these pores are computed via the virial expression on a series of GCMC configurations. A drop of the solvation pressure at the capillary condensation is observed for silica system with hydroxylated surfaces when a different behaviour arises for systems with a charged surface compensated by calcium counterions and C-S-H. In all these hydrophilic and super hydrophilic mesopore models, confined water is strongly perturbed in the vicinity of the pore surface even at low temperatures. Water behaves like in the bulk in the core region of the pores. Finally, the MD trajectories show that the stronger the confinement, the slower the dynamics of water molecules and ionic species. Surface chemistry and surface roughness play an important role in the slowing down of the electrolytic film. Characteristic residence times were derived for water molecules in the adsorbed layer located close to the pore.
RR6: Dynamics in Porous Media
Session Chairs
Martin Mueser
Izabela Szlufarska
Tuesday PM, November 29, 2011
Commonwealth (Sheraton)
2:30 PM - RR6.1
Electrophoretic and Electroosmotic Flow through Carbon Nanotube Membranes as Chemical Pumps.
Ji Wu 1 , Bruce Hinds 1
1 Chemical and Materials Engineering, Univ. of Kentucky, Lexington, Kentucky, United States
Show AbstractCarbon nanotubes have three key attributes that make them of great interest for novel membrane applications 1) atomically flat graphite surface allows for ideal fluid slip boundary conditions and extremely fast flow rates 2) the cutting process to open CNTs inherently places functional chemistry at CNT core entrance for chemical selectivity and 3) CNT are electrically conductive allowing for electrochemical reactions and application of electric fields gradients at CNT tips. In general, the transport mechanisms through CNT membrane are a) ionic diffusion is near bulk expectation with no enhancement from CNT b) gas flow is enhanced by ~1-2 order of magnitude due to specular reflection off of flat graphitic surface c) and pressure driven flux of a variety of solvents (H2O, hexane, decane ethanol, methanol) are 4-5 orders of magnitude higher than conventional Newtonian flow [Nature 2005, 438, 44] due to atomically flat graphite planes inducing nearly ideal slip conditions. Nearly all applications require chemical selectivity in what is allowed to pass across the membrane. However the act of placing selective functional chemistry at pore entrance or along the core of CNTs, dramatically/completely eliminates the enhanced flow effects by eliminating the near perfect slip boundary condition[ACS Nano 2011 5 3867]. Needed is a mechanism to pump chemicals through the pore where selective chemistry is. This is routinely achieved in protein channels where permeates are accelerated through regions of precise functionality. The CNT membrane, with tips functionalized with charged molecules, is a nearly ideal platform to induce electro-osmotic flow with high charge density at pore entrance and a nearly frictionless surface for the propagation of plug flow. Through diazonium electrochemical modification we have successfully bound anionic surface charge to CNT tips and along CNT cores. High electro-osmotic flows of 0.16 cm/s-V at are seen by the pumping of neutral caffeine molecules. Improvements in electroosmotic power efficiency of 25-112 fold are seen in CNTs compared to conventional nanoporous materials with atomically rough interfaces [RCS Nanoscale 2011 DOI: 10.1039/c1nr10303b]. Use of the electro-osmotic phenomenon for responsive/programmed transdermal drug delivery devices is discussed with the voltage gated delivery of clonidine and nicotine across CNT membrane at therapeutically useful fluxes [Proc. Nat. Acad. Sci. 2010 107(26) 11698-11702]. In small diameter SWCNTs ion mobilities are seen to be ~6 fold enhanced due to induced electroosmotic flow.
2:45 PM - RR6.2
Charge Transport in Confined Ionic Liquids.
Joshua Sangoro 2 1 , Ciprian Iacob 2 1 , Friedrich Kremer 2 1
2 Institute of Experimental Physics I, University of Leipzig, Leipzig Germany, 1 Molecular Physics, University of Leipzig, Leipzig Germany
Show AbstractCharge transport and glassy dynamics in several classes of ionic liquids confined in nanoporous silica membranes with mean pore diameters ranging between 4 nm and 15 nm are investigated in a wide frequency and temperature range by a combination of Broadband Dielectric Spectroscopy (BDS), Fourier Transform Infrared spectroscopy (FTIR), and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). Remarkable enhancement of the ionic mobility by more than two orders of magnitude is traced down to changes in the conformation of the ions as a result of the density reduction accompanying two-dimensional confinement. In addition, it becomes possible to determine the resulting change in density of ionic liquids in nanopores from dielectric spectra in quantitative agreement with recent molecular dynamics simulations. For some ionic liquids, slower ionic mobility is observed. This trend is fully reversed upon silanization as proved by FTIR spectra, proving the significant role of guest molecule-host membrane interaction. Technological implications of the observed confinement effects on transport properties of ionic liquids will be discussed.
3:00 PM - RR6.3
Controlling Wettability of Nanoporous Materials - A Novel Technique of Liquid Actuation.
Yu Qiao 2 1 , Weiyi Lu 1 , Taewan Kim 2
2 Program of Mater. Sci. Eng., UCSD, La Jolla, California, United States, 1 Dept of Struct. Eng., UCSD, La Jolla, California, United States
Show AbstractWith its ultra-large specific surface area, a nanoporous material is an ideal, yet relatively unexplored, platform for liquid actuation, with potential performance gains typical of disruptive technologies. Our preliminary results indicate dramatically improved performance not attainable by conventional intelligent materials. In a nanostructured solid material, due to the lack of control of solid-solid interaction, how to fully utilize the large interface area usually imposes tough technical challenges. Our recent exploratory study provides a novel way to circumvent this problem: When a liquid phase, with the motion being controlled by voltage or heat, is confined in nanopores, the ultralarge nanopore surface exposed to the liquid can greatly amplify the beneficial surface processes, such as the thermocapillary and the electrocapillary effects. As the solid-liquid interface energy is controlled by temperature or potential difference, the effective wettability at nanopore inner surfaces can vary considerably, which triggers the liquid molecules and ions to "flow" into or out of the nanopores. Since this process can be reversible, the system exhibits a volume memory characteristic. Due to the ultralarge value of surface area, the output energy density is order-of-magnitude higher than that of conventional intelligent materials. The deformability of the system is dominated by the porosity, leading to an effective maximum deformation of 20-40%.
3:15 PM - RR6.4
Comparison between the Phase Transition of Physically Confined Cyclohexane and 1-Decanol Inside Nano Porous Silica.
Samuel Amanuel 1 , Hillary Bauer 1 , Alexandrea Safiq 1 , Jargalsaikhan Dulmaa 1 , Amer Khraisat 1
1 Physics and Astronomy, Union College, Schenectady, New York, United States
Show AbstractThe linear relationship between melting (and freezing) temperatures and inverse physical size is explained qualitatively by the Gibbs-Thompson equation. It is not clear, however, how the other thermodynamic parameter