Symposium CT08—Mechanochemical Coupling in Chemical Treatment and Materials Degradation—Modeling and Experimentation

Diffusion of atoms, ions and molecules in solid materials may result in local mechanical stress. A review is presented of literature studies in semiconductors materials, high temperature oxidation and electrochemical charging and discharging of electrodes in lithium ion batteries. The main focus is inorganic silicate glasses submitted to alkali ion-exchange below glass transition temperature. Interdiffusion driven ion-exchange in glass results in stress effects that are used in technological applications for glass articles strengthening. These processes are extensively and widely used in screen covers for consumer electronics and structural windows for architectural and transportation applications. Optical effects are also discussed as a consequence of ion interdiffusion and its relationship with residual stress. Underpinning physics of ion exchange in glass is reviewed in respect to both stress build-up and relaxation considering, that ion exchange is a irreversible mass transfer process performed on a non-equilibrium state of matter which spontaneusly relaxes towards a metastable supercooled liquid state.

Novel materials enable solutions to the global challenges that we face today pertaining to energy and the environment. The realization of materials technology that allows for reliable lightweight structures, improved energy storage, improved efficiency of waste/nutrient recovery, etc. critically hinges on our understanding of strongly coupled material behavior under a wide range of conditions. Some natural as well as engineered material systems exhibit an interplay between their chemistry and mechanical response as the microstructures of these materials dynamically evolve as a response to their chemical composition/reaction kinetics. In some cases, even when our understanding of said coupled behavior is advanced, there are limitations to how we can account for them in computational models owing to a need for multiscale transfer of information. Building predictive mechanics models that can accurately capture the origins of material behavior by explicitly incorporating the chemical structure of constituents and reaction kinetics can be computationally prohibitive. This brings up the need for effective upscaling strategies in multiscale, multiphysical models that can preserve traceability across scales.
Here, we explore strategies for upscaling the mechanochemistry captured through all-atom classical and reactive molecular dynamics simulations in a few example materials such as polymer adhesives, engineered nanocomposites, and clay-rich porous media that are used in aerospace, electrochemical, and geological applications.
The polymer curing reaction in a multicomponent aerospace-grade epoxy is modelled based on a cut-off distance-based approach that determines bond formation. The resulting degree of conversion of the polymer and its glass transition temperature are confirmed with experimental data. We also studied the chemical and physical interactions in nanocomposites where carbonaceous nanoparticles are dispersed in a thermoset polymer. The interfacial adhesion (arising from chemical functionalization and non-bonded forces) between the nanoparticles and surrounding polymer is also quantified as a function of the degree of conversion of the polymer and the chemical composition (% wt. of nanoparticle in the nanocomposite). From a large combination of these simulations, we determined the effect of the nanostructure and chemistry of the polymers and nanocomposites on their elastic properties, crack initiation from bond breakage and chain sliding, traction-separation behavior at crack openings and fracture toughness. In the case of clay minerals whose chemical-mechanical coupling has tremendous implications for their role as geological barriers for the subsurface disposal of nuclear waste, we modelled the water and ion diffusion, clay swelling states and ion exchange selectivities for a range of pore fluid chemical compositions and concentrations. The swelling pressure, and therefore, the stress states of clay tactoids (containing multiple layers of clay minerals) was determined as a function of the local chemical composition.
In all the above-mentioned studies, the molecular models provide valuable insight into the dependence of stress states and interfacial load transfer on the chemical composition and the energetics of reactions. We then use a stochastic representative volume element (RVE) approach to march up the length scale while accounting for uncertainties characterized at the molecular scale. With the stochastic RVEs, we can predict the ‘bulk’ stress response and associated damage propagation under a given loading condition. The choice of parameters (and their probability distributions to make a stochastic prediction) to be included in the RVE model depends on the material system and its related performance parameter of interest. For example, in clay minerals, our interest is to predict the swelling and collapse of mineral layers (the swelling pressure & stress state) as a function of water activity and ambient conditions whereas the interest in polymer adhesives is to predict crack nucleation and propagation. Holistic computational models that can provide top-down/bottom-up traceability of chemical-mechanical coupling can be a valuable tool in our arsenal to advance and accelerate the design and certification of these material systems in critical applications.

Thermochemical surface engineering of metals is characterized by a deliberate and targeted modification of the (sub)surface region of metals, with the aim to improve materials performance with respect to fatigue, wear, corrosion and combinations thereof. Generally, thermochemical surface engineering is understood in terms of thermodynamics and diffusion kinetics to describe the evolution of the microstructure under the influence of the chemical modification at elevated temperature. Associated with the change in composition in the surface-adjacent region strains and stresses are introduced. These strains/stresses affect the thermodynamics and the kinetics of the ingress of chemical species and, consequently, the microstructural evolution. This contribution illustrates several of the effects of composition-induced stresses/strains during thermochemical surface engineering with interstitials in metals through examples from activities in the authors’ research group. The presentation includes experimental as well as modelling aspects. The following examples are covered.
1. The dissolution of N and/or C into stainless steels and high entropy alloys (HEAs) at temperatures below 725 K for N and below 825 K for C is associated with the development of a supersaturated solid solution of interstitials in the fcc phase. This supersaturation is “kinetically stabilized” by sluggish decomposition, because of slow diffusion of substitutionally dissolved components. The lattice expansion caused by dissolution of N or C is accommodated elastically for low interstitial contents, leading to strengthening. As solid solution strengthening scales with the cube root of composition squared and biaxial compressive stresses caused by lattice expansion scale linearly with composition, plastic accommodation will occur above a threshold composition. Also, with an increase in N content long range ordering (LRO) of N atoms is encountered. The combination of elasto-plastic accommodation of lattice expansion and LRO leads to anisotropic growth of the developing case.
2. Hcp titanium can dissolve high contents of oxygen and is applied for surface hardening of Ti and its alloys. The thermo-chemical treatment of Ti in CO above the hcp-bcc transition temperature leads in principle to the uptake of both O and C. Since C has a very low solubility as compared to O, hence, primarily O is dissolved, thereby stabilizing a case of hcp-Ti atop bcc-Ti. The conversion of bcc to hcp is associated with the introduction of tensile stresses in the hcp case, leading to crack development in the hard and brittle part of the case close to the surface. Upon cracking, CO ingress leads to the development of a mixed interstitial compound Ti(C,O) along the crack surface, thereby converting this part into an extremely wear resistant Ti-surface, with a hardness of up to 3,000 HV.
3. Oxidizing of ZrCuAl-based bulk metallic glass (BMG) below the glass transition temperature is a potential thermo-chemical surface engineering treatment of BMGs. Within the BMG an internal oxidation zone (IOZ), consisting of nano-crystalline ZrO2, develops. The volume expansion caused by zirconia formation leads to compressive residual stresses in the IOZ. The theoretical stresses are 30 times higher than the experimentally determined stress levels, indicating that stress relaxation has occurred. This is accomplished by shear band formation in the BMG adjacent to the IOZ, which later affects the ZrO2 development of the advancing oxidation front. In addition, the compressive stresses induce outward diffusion of “noble” elements, as for example Ag (if present) and Cu. Surprisingly, segregation of these noble elements at free surfaces, as the outer surface, but also crack surfaces, leads to crystalline metallic regions, which can be considered as self-healing in the case of crack formation. Depending on the oxygen partial pressure, Cu can oxidize upon arrival at the surface.

This paper discussed the experimental magnetic force microscopy (MFM) studies on MTJMSDs. We showed that molecules are much more than simple spin channels between two magnetic electrodes. MFM produced vivid evidence showing that a paramagnetic molecule covalently bonded to the two ferromagnetic electrodes catalyzed a large-scale ordering on ferromagnetic electrodes and impacted several hundred-micron areas near molecular junctions. Transport studies showed that paramagnetic molecule induced long-range impact on ferromagnetic electrodes resulted in several orders of current suppression at room temperature. Future studies with various forms of magnetic tunnel junctions and molecules are in order. The present work provides the details about the efficacy of cost-effective, and mass producible liftoff based molecular device fabrication.

A variety of materials of technological interest change their properties through contact with reactive media. Solid-gas reactions lead to a variety of reaction products on the surfaces and internal interfaces. The observation of nucleation and growth processes in the environment where they occur (in situ) from a chemical-structural perspective is especially challenging for aggressive atmospheres. The talk presents innovative approaches to study corrosion mechanisms using advanced X-ray methods. Using energy dispersive X-ray diffraction and X-ray absorption spectroscopy in different tailor made environmental reaction chambers, valuable insights into high temperature oxidation and sulfidation processes were gained. Fe-based alloys were exposed to hot and reactive atmospheres containing gases like SO₂, H₂O and O₂ at 650°C. During the gas exposure the tailor made reaction chambers were connected to a high energy diffraction end station at the synchrotron. The crystallization and growth of oxide and sulfide reaction products at the alloy surfaces were monitored by collecting full diffraction pattern every minute. Careful examination of shape and intensity of phase-specific reflections enabled to a detailed view on growth kinetics. These studies showed, oxides are the first phases occurring immediately after experimental start. As soon as reactive gas media enter the chamber, the conditions change and different reaction products, such as sulfides start to grow. A comparison of different gas environments applied, illustrated the differences in the type of reaction products. The in situ observation of high temperature material degradation by corrosion made it possible to study the contribution of phases, which are not stable at room temperature. For instance, wuestite (Fe_1-xO), was frequently observed at high temperatures in humid gases on Fe with 2 wt.% and 9 wt.% chromium, but not at room temperature. The strength of the occurrence of this phase additionally explains why, despite a higher Cr content, ferritic alloys with 9 wt.% Cr in a challenging atmosphere prevent the intrinsic formation of protective layers. The in situ observations were supplemented by careful considerations of thermodynamic boundary conditions and detailed post characterization by classical metallographic analysis. Additionally, the structure and chemistry of the dominant oxide layers were evaluated using X-ray absorption near edge structure spectroscopy. The talk will give an overview about chances and challenges for studying high temperature corrosion phenomena by advanced X-ray methods.

Titanium alloys have excellent corrosion resistance, high-temperature strength, low density, and biocompatibility. Therefore, they are increasingly used for aerospace, chemical, and biomedical applications. Moreover, investment casting is a well-established process for manufacturing near-net-shape intricate parts for such applications. However, mass transfer arising from metal-mold reactions is still a major problem that drastically impairs the surface and properties of the castings. This work investigates the interfacial reactions and mass transfer problems that lead to alpha case formation. Improvements based on modeling and experimental validation are discussed, highlighting ceramic oxide refractories namely—zirconia, yttria, calcia, alumina, and novel perovskites particularly calcium zirconate and barium zirconate. It was found that while mold material selection is vital, alloy composition should also be carefully considered in mitigating metal-mold reactions and mass transfer.

The uranium dioxide nuclear fuel of light water reactors is contained in 4 m long tubes with a diameter of about 10 mm and a wall thickness of about 0.7 mm, called cladding tubes. The cladding tubes are made of zirconium alloys, the main reason being the very low cross section of Zr for thermal neutrons. The cladding tubes are subjected to several degradation mechanisms, including water-side corrosion, hydrogen pick-up and irradiation damage, affecting the mechanical integrity, which restricts the usage of the fuel. There are several different Zr alloys that are used for cladding tubes. We have studied Zircaloy-2, which is commonly used in boiling water reactors. It has a composition of Zr-1.5%Sn-0.15%Fe-0.1%Cr-0.05%Ni, and it is known that Fe, Cr and Ni play important roles for the performance of the alloy. These elements have a low solubility and precipitates of Zr₂(Fe,Ni) and Zr(Fe,Cr)₂ form. In order to study both the corrosion behavior and the neutron damage, atom probe tomography (APT) is a very useful technique, combining a sub-nm resolution in 3D with a high elemental sensitivity. By analyzing the oxide and the metal/oxide interface of samples corroded in autoclave it was shown that Fe and Ni were segregated at grain boundaries in the metal and that these segregations were inherited by the inwards growing oxide. Modeling provided support for that Fe present at oxide grain boundaries should reduce hydrogen pick-up, whereas the opposite should be true for Ni. We have also analyzed Zircaloy-2 after in-reactor exposure, for three and nine years, respectively. The neutron irradiation leads to a gradual dissolution of the precipitates as well as formation of dislocation loops. It was found that the dissolved Fe, Cr and Ni segregates to the loops. This is very interesting, although it is not clear how this segregation affects the growth and stability of the loops, or the corrosion and hydrogen pick-up behavior.

Cu nanofoams processed by dealloying were studied as a promising applicant for anode material in lithium-ion batteries. Pack cementation were applied to produce the precursor materials for different times (3, 6, 12 and 15 h). The microstructure of the specimens was studied after the dealloying process. Samples produced with 6 h pack cementation time were also annealed to investigate the influence of heat treatment on the microstructure of Cu nanofoams. The foam structure was examined by scanning electron microscopy (SEM). The phase composition of the materials was determined using X-ray diffraction (XRD). Furthermore, X-ray line profile analysis (XLPA) was utilized for quantitative characterization of the diffraction domain size, the dislocation density and the twin fault probability.

The early stages of radiation damage in metals and alloys are governed by phenomena on the atomic scale. The most elementary are the creation of individual vacancies and interstials (Frenkel pairs) and their motion that leads to either their annihilation or evolution towards larger defect clusters. The detailed mechanisms of the evolution of the radiation damage strongly depend on the defect-matrix-solute-impurity interactions. Experimental identification of the radiation-induced defects and the characterization of their properties is crucial for developing a detailed understanding and physics-based predictive models for radiation damage.
Positron annihilation spectroscopy is a set of methods selectively sensitive to vacancy-type (open volume) defects [1]. It can be used to identify the sizes of the defects from mono-vacancies to clusters of tens of missing atoms, with concentration sensitivity in the range 10 ppb - 100 ppm. The chemical identity of the atoms immediately surrounding the vacancy-type defects can also be identified to a certain extent. Experiments can be performed with near-surface depth resolution up to a few microns, as well as in the bulk of the material, in a temperature range 10 - 1000 K, allowing for various in situ irradiation experiments.
I will review some of the recent positron annihilation experiments dealing with early-stage radiation damage in tungsten and high-entropy alloys (HEAs). The results obtained in tungsten include the direct determination of mono-vacancy and interstitial migration barriers [2] that employ irradiation experiments at cryogenic temperatures for freezing in the radiation-induced defects, and hydrogen-vacancy-cluster interactions at elevated temperatures [3]. In HEAs, the experiments reveal atomic-scale segregation already at the early stages (below 1dpa) of radiation damage [4] and non-trivial effects of C interstitials in the HEA matrix on hydrogen irradiation induced damage [5]. Finally, I will discuss preliminary results obtained in C and N interstitial containing HEAs that show intriguing mono-vacancy migration barrier behavior most likely related to the unusual effects of these interstitials on the distribution of local lattice distortions.
[1] F. Tuomisto and I. Makkonen, Rev. Mod. Phys. 85, 1583 (2013).
[2] J. Heikinheimo et al., APL Mater. 7, 021103 (2019).
[3] M. Zibrov et al., J. Nuclear Mater. 531, 152017 (2020).
[4] F. Tuomisto et al., Acta Mater. 196, 44 (2020).
[5] E. Lu et al., J. Appl. Phys. 127, 025103 (2020).

Active functioning of materials will often trigger, or be accompanied by, one or more deleterious processes, such as oxidation, corrosion, fatigue, and thermal or mechanical effect, which in turn will couple with the active process to collectively affect the materials behavior and functionality. In a typical example, electrochemically driven functioning of a rechargeable battery inevitably induces thermal and mechanical effects that couple with the electrochemical process and collectively govern the performance of the battery. A direct visualization of such a coupling effect appears to be a fundamental challenge. In this presentation, we will discuss the utility of in-situ TEM to directly probe the mechanochemical coupling effect, intending to unravel mechanical effect on ionic transport, oxidation induce stress generation, mechanical effect on lithium dendrite growth, and electrochemical induced crystal deformation. We demonstrate that the intricate coupling of electrochemical, thermal, and mechanical effects will surpass the superposition of individual effects.

Safety is an important concern in Li-ion battery operation and storage. As both energy stored per unit volume and higher current densities have increased in these batteries, energy dissipation in the form of heat has become one of the main issues for proper battery operation. As temperature increases, secondary chemical reactions may be initiated. These reactions release more heat and undesired products, triggering an auto-catalytic feedback process to occur, which is commonly classified into thermal-runaway mechanisms. This can potentially lead to a complete degradation of the battery, and possible explosion or combustion of the battery components.
Several researchers have worked on developing models to predict thermal behavior under conditions with potential thermal runaway outcomes on Li-ion batteries. The first studies proposed a model that emulates the solid electrolyte interface (SEI) decomposition and regeneration reactions on a standard 18650 cylindrical cell. Later works extended these models and included the reactions of cathode decomposition and electrolyte decomposition with potential combustion. Although macroscopic level energy balance helps to predict potential thermal behavior in a battery of multiple layers, the study of all the different transport mechanisms that happen inside a single layer battery is essential for a better understanding of the process taking place when temperature begins to rise. With the aim of improving heat dissipation inside a battery, the electrolyte is a key component to consider as it can potentially handle heat flux easier through convention within the solution.
This work investigates spatially dependent electrolyte conditions in the presence of a thermal gradient to produce an electrolyte-centric thermal runaway model. Non-current conditions are considered for this specific case study, emulating oven-test experiments, or storage conditions. The objective of the study is to model transport mechanisms that can potentially lead to thermal degradation of the battery. Li-ions, momentum, and thermal flux are the main components of the analysis. Numerical simulation techniques are used for this purpose. Findings show that non-uniform temperature distribution triggers free convection mechanisms within the electrolyte solution. According to our results, electrolyte stratification should be appreciated at the microscopic level. A thorough comprehension of the transport processes taking place inside the battery should contribute to mitigate damage induced due to thermal abuse conditions.

When Fe alloys are subjected simultaneously to an applied tensile stress and a corrosive, high-temperature aqueous medium, interplay of hydrogen and oxygen interactions with the alloy microstructure are thought to lead to intergranular stress corrosion cracking (SCC). Using transmission electron microscopy and in situ atom probe tomography we develop atomic scale understanding of this mechanochemical coupling during SCC of model Fe-18Cr-10Ni alloy. Specifically, the structure and composition of oxide layers and elemental partitioning as well as hydrogen segregation at the oxide-metal interface was revealed as a function of prior deformation. These new insights are expected to provide the scientific basis for tailoring the microstructure of metallic alloys used in nuclear and automotive applications to control the impact of coupled extreme environments of corrosion, stress, and temperature.

Stress corrosion and corrosion fatigue cracking of structural materials used in the commercial nuclear power industry must be well understood to ensure the integrity of the reactor plant. While the cracking behaviors are generally recognized to show an Arrhenius temperature dependence, depature from this behavior has been observed at high temperatures. Mechanistic understanding of this behavior would support the development of improved predictive models for crack growth. Advanced characterization of crack tips and corrosion films have provided important insights into these behaviors. Chemical and mechanical signatures of crack retardation behavior will be discussed in the context of potential retardation mechanisms.

Mechanical forces and chemical reactions at materials interfaces are often coupled to each other. This coupling is relevant to fields such as rock friction, micro/nano devices, wafer bonding. In particular, in sliding contacts, compressive and shear stress can activate interfacial chemical bonding reactions, resulting in higher friction and adhesion, higher wear rate, and higher tribofilm and tribo-polymerization production rate. The mechanochemical coupling has been generally described by phenomenological theories, including the Eyring model. In these theories, the activation energy for a chemical reaction is increased or decreased by the mechanical work done on the system , where is the stress acting on the system, and is the activation volume. The physical meaning of activation volume has been under debate for decades, but the magnitude of the activation volume is often considered to be comparable to the dimension of the local deformation of chemical bonds participating in the reaction. However, recent experimental results showed that the range of activation volumes can be quite large even for the same material system. For example, for tribochemical wear of silicon tips, where the wear is due to the chemical bond formation and breaking at the interface, the range of activation volumes can be as large as ~6.7-115 ų. With such a large range, the physical picture of the activation volume corresponding approximately to the dimension of the local volume involved directly in stretching or compressing of the chemical bonds might be oversimplified.
In order to understand the physico-chemical coupling at interfaces, we performed ab initio calculations of stress-induced reactions silica/silica interfaces. Our results show that the mechanochemical response arises not only from the interface region, where the chemical reaction occurs, but there is also a significant (and in some cases dominant) contribution from the deformation of the bulk solid. The relative contributions from each region depend on the stiffness of these regions, and the softer region dominates the response to the mechanical work done on the system. We also demonstrate that the contribution from the bulk region can be large even for stiff materials. This is because the near surface-region of the solid can be significantly softer than the bulk solid. The larger stresses experienced by the near-surface regions of two materials in contact can also lead to a larger mechanochemical response from the near-surface region. Our study provides new insights into the physical origins of the mechanochemical coupling at interfaces and the meaning of the activation volume.

Despite the fundamental importance of mass transport, there is still a lack of knowledge regarding the mechanisms in many materials. In particular, how the magnetic spin structure interacts with migrating defects is unclear. Here, using density functional theory to examine cation interstitial transport in corundum-structure oxides, Al₂O₃, Cr₂O₃, and Fe₂O₃. These oxides have their own technological importance in various applications, such as diagnostic windows in nuclear energy systems, protective surface coatings, catalyst supports, magnetic storage, etc. In this study, we reveal an interplay between the migration of the interstitial and the magnetic structure of the oxide. The magnetic spin configuration impacts the migration energy of the migrating defect but, critically, the defect modifies the spin configuration. Thus, the two aspects change in concert to modify one other. This has profound implications for mass transport in magnetic materials.

The ever-increasing energy demand has driven the search for new oil and gas resources in harsh environments. However, corrosion problems of carbon steels in oil and gas production under harsh operational conditions and during the product transportation considerably impede the productivity and cause safety concerns. Majority of these corrosion problems are related to CO₂ corrosion. In the journey of seeking solutions, a high-phosphorus electroless Ni-P coating has gained growing attentions as an advantageous candidate to carbon steels due to its good balance between the outstanding anti-corrosion performance and reasonable cost.
When Ni and P atoms are co-deposited under the catalytic effect of adsorbed hydrogen atoms in the coating deposition process, H₂ gas bubbles are easily produced and left in the coating to form “micropores” that favor the penetration of aggressive medium and lead to coating degradation. To effectively mitigate electrolyte penetration and coating degradation, we designed a smart Ni-P coating incorporated with nanocapsules that enable the release of the pH-responsive corrosion inhibitor when corrosive medium reaches the micropores.
In this work, the microstructure and chemical compositions of as-fabricated Ni-P coating and nanocapsule-incorporated Ni-P coating were analyzed by SEM, EDS, XPS and AFM, and the corrosion behaviors of the two coatings in the CO₂-saturated NaCl solution were then evaluated by potentiodynamic polarization measurements and EIS. The results show that BTA-loaded nanocapsules are successfully incorporated into Ni-P coating without altering the microstructure and deposition rate, and the presence of nanocapsules significantly reduces the coating porosity by filling in the micropores. As a result, the corrosion resistance of the smart coating notably increases due to the release of BTA originated from the local acidification of the micropores, as compared to the as-deposited coating. A good anti-corrosion stability is further confirmed by few noticeable corrosion pits and no evidence of lateral coating disbondment at the coating/substrate interface after immersion tests. This study offers new insights into developing smart electroless Ni-P coatings by incorporating nanoscale functionalized additives.

The inspiration derives from the concept of structural water in nature that water molecules are bound inside hydrophobic pockets and help to stabilize protein structures. However, water has rarely been found a similar role in material science. By using this water-activated glue, the resulting supramolecular polymeric materials exhibits strong adhesion to surfaces, and can be reused many times without losing its performance. Considering the unique feature of water, this discovery triggered the transition of negative role of water, and we started the water-centered the soft material design.

The formation of hetero-structures between, either piezo-electric LiTaO₃ or LiNbO₃ with Si-based materials exhibits three major issues: the mismatch of crystal structure, of their lattice constants and of their coefficient of thermal expansion (CTE). In particular, the thermal expansion of LiTaO₃ or LiNbO₃ crystals is much larger. It differs from Si and SiO₂ by an order of magnitude.

The present work uses Nano-Bonding^TM [1] to directly bond LiTaO₃ and LiNbO₃ to Si and SiO₂. This is done via Surface Energy Engineering (SEE). In synergy, SEE modifies the surface hydro-affinity and the surface energy to far-from-equilibrium states. These are more likely to react in air at room temperature. H₂O vapor is used to catalyze bonding of LiTaO₃ and LiNbO₃ to Si and SiO₂.

Hydro-affinity and surface energies are mapped across 4-6” wafers via Three Liquid Contact Angle Analysis (3LCAA). Mapping is done in air in a Class 100 Laminar Flood hood. Hydro-affinity is mapped via the water contact angle. Surface Energy is computed using van Oss-Chaudury-Good theory [2], via the mapping of contact angles of two polar liquids (water and glycerin), and non-polar α-bromo-naphthalene. The measured surface energies are found to be close for the (110) orientation for both LiTaO₃ and LiNbO₃. They are 41 ± 2 mJ/m² for LiTaO₃ and 39 ± 2.5 mJ/m² for LiNbO₃ respectively.

The △G_Piezo-Si-H2O of interaction computed from measured surface energies by applying van Oss- Chaudhury-Good theory [2] for ‘as received’ LiTaO₃, with ‘after SEE’ Si and SiO₂ were -0.1 mJ/m² and -16.8 mJ/m² and for ‘as received’ LiNbO₃, with ‘after SEE’ Si and SiO₂ were -13.4 mJ/m² and -28 mJ/m².

Thus △G_Piezo-Si-H2O values are < 0, to bond ‘as received’ hydrophobic LiTaO₃ and LiNbO₃ with ‘after SEE’ hydrophilic Si and SiO₂ in the presence of atmospheric moisture. Experimentally, SEE based on these computations is found to be successful in activating Nano-Bonding™ (at room temperature in 25 % HR) of LiTaO₃ to Si, and LiNbO₃ to SiO₂.

[1] Nano-Bonding^TM, Herbots, et al. U.S. Pat. # 9,018,077 (2015), 9,589,801(2017), pend. (2020)
[2] Van Oss, et.al, (1988). “Interfacial Lifshitz-van der Waals and polar interactions in macroscopic systems”. Chemical reviews, 88(6), 927-941

Ga-doped Li₇La₃Zr₂O₁₂ (LLZO) has been experimentally fabricated with high lithium ionic conductivity and is a promising candidate for the electrolyte in all-solid-state Li-ion batteries. However, the thermodynamic properties of Ga-doped LLZO remain underreported. In this study, Coulomb energy analysis for large disordered crystal with supercell program and density functional theory (DFT) calculations provide insights into the compositional and structural stability of Li_56-3xGa_xLa₃Zr₂O₁₂, where x = 0, 1, 2, 4, 6, 8. The lattice parameters decrease with increased Ga concentration because of the smaller ionic radii of the dopant and the formation of Li vacancies. Relationships between local coordination environment of Ga dopants and Li vacancies were considered, leading to an understanding of the configurational preferences in Ga-doped LLZO and the potential for correlated configurational dependencies of Li mobility. The enthalpy of formation as calculated based on the binary oxide reactants, for Ga-doped LLZO increases from -1540.72 kJ/mol for undoped tetragonal Li₅₆La₂₄Zr₁₆O₉₆ to -1411.70 kJ/mol for Li₃₂Ga₈La₂₄Zr₁₆O₉₆. The trend in enthalpy of formation with respect to the Ga concentration is in agreement with recently reported experimental calorimetry measurements, which are reported for the nominal formula unit Li_7-3xGa_xLa₃Zr₂O₁₂ (where x = 0, 0.25, 0.5, 0.75, and 1). Detailed energetic analysis associated with a stepwise increase in [Ga] were performed to understand the changes in thermodynamic stability with increased Ga content. The relation between Li-ion conductivity and the stability of the Ga-doped LLZO emphasizes that a balance must be achieved between the increase in Ga concentration, which drives the formation of Li vacancies necessary for Li migration.

A computational framework based on multi-phase-field method was developed to study the complex microstructure evolution of metallic materials under the influence of chemical reaction, bulk and surface diffusion, surface passivation, sensitization, applied electrical and/or mechanical loading in aqueous or gas environment. The proposed framework makes use of the Allen-Cahn equation for phase variables, Cahn-Hilliard equation for conserved variables, Nernst-Planck equation and Poisson's equation for electrochemical condition, together with mechanical equilibrium equations. When chemical reactions were involved, the reaction rate was expressed as a function of the chemical (or electrochemical) potentials of reactants and products based on a detailed balance of reactions. The framework was applied to study pitting corrosion of steels with or without the formation of insoluble corrosion products, stress corrosion cracking, corrosion of metal matrix composite under mechanical loading, high temperature metal oxide formation under the influence of high PB ratio, intergranular corrosion of sensitized aluminum alloys, nano-porous structure evolution during chemical dealloying of binary alloys. The characteristics of the model predictions agree well qualitatively, in many cases also quantitatively, with the experimental observations. This work was supported by grants from the Research Grants Council of Hong Kong (PolyU 152140/14E and PolyU 152174/17E).

Precisely evaluating the mechanical properties of polymeric components in a solid–electrolyte interphase (SEI) is crucial for ensuring its mechanical stability on silicon anodes, which significantly expand during lithiation. However, the complex inorganic/organic nanocomposite structure of SEIs hinders the ability to directly measure the elastic moduli of its constituent polymer components. To address this issue, in our prior DFT study, we proposed a theoretical methodology to determine these elastic moduli. First-principles calculations were performed to evaluate the energy barrier for a 1,2-radical shift in different carbons in the corresponding polymers. Using the identified crosslinking sites, the elastic moduli of the crosslinked fluoroethylene carbonate (FEC) and vinylene carbonate (VC) polymers were evaluated, which could not be directly measured on the formed SEI. Our calculations provided evidence of the elastic behavior of polymeric species in the SEI formed by the electrochemical reduction of FEC and VC on the surfaces of silicon anodes. Moreover, these findings suggested a new avenue for attaining mechanically stable SEIs on silicon anodes through the chemo-mechanical design of an inorganic/organic nanocomposite structure based on those mechanical properties. Then, cyclic nanoindentation tests and numerical methods were used to quantify the elasto- and viscoelasticity of SEIs as well as the effects of the stress field in the silicon anode material below the SEI, thus enabling the precise evaluation of the elastic modulus of the SEI. Instrumented nanoindentation was used to quantitatively measure the indentation force response on the heterogeneous surface of composite electrodes. The measured results showed that the mechanical properties of the SEI as determined by the Hertz model resulted in a significantly overestimated elastic modulus. The stress field in underlying Si significantly affected the results, especially for a thinner SEI thickness, and could cause the false appearance of a mechanically bilayered structure of the SEI with a hard, inorganic inner layer and a soft, outer polymeric layer. The elastic modulus of the SEI formed in the FEC electrolyte agreed with first-principles results and supported the recent solid-state NMR results demonstrating the existence of polymeric species in the interfacial region of the FEC-SEI and the silicon anode. In addition, the results corroborated recent transmission electron microscopy–electron energy loss spectroscopy and density functional theory results revealing that the FEC-SEI exhibits a nano-composite structure with inorganic particles distributed in crosslinked poly(FEC).

Magnesium (Mg) alloys have shown great potential as both structural and biomedical materials due to their high strength-to-weight ratio and good biocompatibility. However, poor corrosion resistance limits their further application, while alloying is believed to be one of the most effective strategies to develop corrosion-resistant Mg alloys. Traditional new magnesium alloy development is mainly based on the experiments, which is time-consuming and low-efficient. Recently, high-throughput computational method has become a useful tool to screen promising materials for various applications. In this work, we first collected 27919 (including repeated) Mg intermetallics structures from online databases, from which 332 stable candidates were selected. Then, the equilibrium potential based on Nernst equation were calculated and 50 Mg intermetallics with smallest equilibrium potential difference from that of Mg matrix and hence the lowest thermodynamic driving force of galvanic corrosion were reserved. Additionally, the adsorption energy of hydrogen adatom on the intermetallic surfaces were obtained for the further prediction of exchange current density based on the volcano curve. From the idea of small cathodic exchange current density and thermodynamic driving force of galvanic corrosion, several intermetallics were selected to be the promising phases for corrosion-resistant binary alloys. Our work not only predicts some new corrosion-resistant strengthening phases, but also provides a high-throughput screening strategy for corrosion-resistant alloy design, which can also be extended to screen ternary intermetallics or other alloy systems.

In this talk, a phase-field model (PFM) is introduced to study high temperature oxidation and aqueous corrosion of metals in a mechanico-chemical coupled field. The reaction rate is expressed as a function of the electrochemical potentials of reactants and products and conforms to the generalized Butler-Volmer relationship. The Gibbs free energy is expressed as a sum of chemical potential, interfacial energy, electrostatic potential energy, and mechanical strain energy of the system. And the variation of phase order parameter is governed by the Allen-Cahn type equation, which is derived from the equivalence of reaction rate and phase transformation rate. This governing equation captures the influences of reaction kinetics, elemental concentration, electric potential, and mechanical deformation. Coupled with the generalized Nernst-Planck equation, the Poisson equation, and the mechanical equilibrium equation, the complete set of phase-field equations are implemented. The proposed PFM is applied to study the corrosion mechanisms for the two typical scenarios: (i) the oxide scale roughening induced by high-temperature oxidation, and (ii) the localized corrosion in the wet environment. The numerical results of the first case unravel the roughening mechanism of oxide scale in an inward growth process, which is consistent with experimental results. In addition, it is shown that the initial metal surface morphology also significantly influences the resulted scale roughness, which renders a way to mitigate roughening-induced stress concentration and cracking in some applications. For localized corrosion, we study a complex corrosion process that involves mechano-electrochemical coupling, anodic dissolution, insoluble depositions (IDs) formation, and resulted Galvanic-pitting corrosion. Based on a quantitative investigation into the effects of Cl^- concentration, pH value, mechanical loading, and electric field, we reveal the autocatalytic process of pitting assisted by increasingly aggressive chemical environment and concentrated stress and study how the external electric field can arrest assisted corrosion and prolong service lifetime.

The oxidation of copper surfaces has been studied extensively in literature — from simple oxygen chemisorption structures to the formation of complex surface oxides and thin oxide films. Having an accurate atomistic model for this metal/oxide interface plays a pivotal role in determining interfacial processes in many copper-based technologies, ranging from electronic circuitry wirings to chemical catalysis in carbon dioxide reduction. The "29" and "44" complex surface oxides represent two of the most classical embryonic oxides on Cu(111). Although many attempts have been made to offer a detailed atomistic model of these surface oxides, their atomic structures remain elusive and ambiguous. In this work, we address this open question via state-of-the-art ab initio scanning tunneling microscopy (STM) and spectroscopy (STS) simulations that go beyond the simplistic Tersoff-Hamann's approach where the (functionalized) metal tips are explicitly included, and are corroborated by precise single crystal growth methods (with ultra-low surface roughness) and high-resolution STM/STS experiments. In particular, we reexamine the "29" structure and elucidate a complete atomistic model for the larger "44" surface oxide, thus completing the picture of early oxidation on copper.