Symposium SF08—Advanced Ceramics and Glasses—From Advanced Manufacturing to Data-Driven Methods for Synthesis and Mechanical Characterization

The field of architected materials has enabled engineered materials with mechanical properties that were previously thought unattainable. For instance, hollow truss-based lattices demonstrate high stiffness- and strength-to-weight ratios but these structures suffer from a bending-dominated response and stress concentrations at the sharp joints, the latter of which result in failure. Such stress concentrations become particularly challenging when working with nominally brittle materials, such as ceramics. As an alternative, aperiodic and asymmetric morphologies attained through spinodal decomposition (spinodal metamaterials) may provide a route to combat these stress concentrations due to their finite (low) curvature and bicontinuous morphologies. Such morphologies could provide a pathway to engineer ceramic materials with unique combinations of properties such as low density, high stiffness, and extreme mechanical resilience.

In this study, we explore the effect of curvature and geometric anisotropy on the stiffness, strength and toughness of spinodal metamaterials. The spinodal morphologies are generated using a computational spinodal decomposition framework where tunable anisotropy is introduced by energetically penalizing preferential directions for interface formation, resulting in a mean-curvature and Gaussian-curvature distribution that is dependent on the direction of the surface normal. Using a two-photon lithography process, we fabricate prototypes of these anisotropic spinodal morphologies possessing nanoscale ceramic coatings and spatially varying curvature distributions and perform in-situ nanomechanical uniaxial compression experiments to determine their curvature-dependent stiffness, strength, and toughness. Complemented by analytical and finite element models of the as-fabricated morphologies, we demonstrate a relation between curvature distributions, the stress distribution, and the mechanical properties of these metamaterials. Our findings present the potential of architected spinodal metamaterials as a route to create new ceramic-based materials with widely tunable properties.

Fabricating inorganic materials with designed three-dimensional nanostructures is an exciting yet challenging area of research and industrial application. We recently developed an approach to 3D print high quality nanostructures of silica with sub-200 nm resolution and with flexible capability of rare earth element doping [1]. The printed SiO₂ can be either amorphous glass or polycrystalline cristobalite controlled by the sintering process. The 3D printed nanostructures demonstrate attractive optical properties. For instance, the fabricated microtoroid optical resonators can reach quality factors (Q) over 10⁴. Moreover, importantly for optical applications, doping and co-doping of rare earth salts such as Er³⁺, Tm³⁺, Yb³⁺, Eu³⁺ and Nd³⁺ can be directly implemented in the printed SiO₂ structures, showing strong photoluminescence at the desired wavelengths. Additionally, mechanical properties of the 3D printed silica and its composite nanostructures have been carefully studied.
On the other hand, nanomaterials such as carbon nanotubes, hexagonal boron nitride (h-BN) and graphene platelets, are promising candidates for reinforcing ceramic matrix composites. To quantitatively study the toughening behavior in two-dimensional (2D) material reinforced ceramics, pull-out experiments were conducted to investigate the properties of the interface between multi-layer h-BN nanosheet and polymer-derived ceramic (PDC) using nanoindentation-assisted micro-mechanical devices integrated with scanning electron microscopy (SEM) [2]. The failure process was monitored in situ with precise quantitative measurements of the relative displacements across the interface that were obtained with digital image correlation (DIC). A micromechanical analysis shows that the interfacial failure in these materials is governed by the interfacial strength at small length scales, rather than the interfacial fracture energy.
[1] X. Wen, B. Zhang, W. Wang, F. Ye, S. Yue, H. Guo, G. Gao, Y. Zhao, Q. Fang, C. Nguyen, X. Zhang, J. Bao, J.T. Robinson, P.M. Ajayan, J. Lou, 3D-printed silica with nanoscale resolution, Nature Materials, Vol. 20, 1506-1511, 2021.
[2] B. Zhang, X. Liu, H. Guo, K. Yang, G. Gao, B.W. Sheldon, H. Gao, J. Lou, Quantitative In-situ Study of Strength-governed Interfacial Failure Between h-BN and Polymer-derived Ceramic, Acta Materialia, Vol. 210, 116832, 2021.

Digital biomaterials are designed through an integrated approach of large-scale computational modeling, material informatics, and artificial intelligence/machine learning to optimize and leverage novel smart material manufacturing for advanced mechanical properties. In this talk we show how we use mechanics to fabricate innovative materials from the molecular scale upwards, with tailored mechanical properties, while sourced from sustainable resources, and breaking the barriers between living and non-living systems. Applied specifically to protein-based materials, this integrated materiomic approach is revolutionizing the way we design and use materials, and has the potential to impact many industries, as we harness data-driven modeling and manufacturing across domains and applications. The talk will cover several case studies focused on the discovery of unifying mechanical principles covering distinct scales and modalities, from spider webs and silk, to collagen, to biomineralized materials, as well as applications to food and agriculture, showing we mechanics can be applied to uncover universal scaling laws that govern mechanical behavior. We further discuss manufacturing methods that enable us to synthesize computationally designed architectures with high precision, spanning multiple scales from the nano to the macro level.

Ceramics exhibit a range of unique properties that make them the ideal candidates for many applications, particularly those involving demanding environments. However, these same properties, such as the high hardness, that make ceramics so interesting also make their shaping challenging. In addition, many high-end applications demand the integration of ceramics in multicomponent composites and devices with a sophisticated degree of structural control. Additive manufacturing is opening new opportunities to address these needs but presents its own set of challenges. In this presentation we introduce embedded printing as an alternative to introduce structures in dense ceramic bodies. This is achieved by printing in a self-healing ceramic gel that enables the printing in its interior followed by subsequent healing to form a dense, defect-free ceramic that encapsulates the printed structure. We will discuss the necessary matching of the properties of the viscoelastic response of gel and ink in order to print structures inside the ceramic that retain their shape and illustrate the technique with a couple of case studies. One is the printing of sacrificial structures to introduce complex microchannel arrangements in a ceramic body. The other is to embed metallic structures designed to enhance fracture resistance by orders of magnitude.

The recent emergence of multicomponent, high-entropy oxides (HEOs) has sparked research into novel ceramics with significant compositional and structural diversity. The wide range of achievable materials presents promising research opportunities, as they may be custom-designed to enhance performance in any of a number of important applications, such as thermal barrier coatings and solid oxide fuel cells. In these arenas and in many others, the extent to which high entropic lattice disorder versus specific chemical short-range order, nanoscale phase segregation, and intrinsic defect/disorder states are achievable remains an open research question. We present detailed investigation of the defects, local atomic order, microstructure, and thermomechanical properties achieved in a family of rare-earth and transition metal-based pyrochlore and fluorite HEOs prepared through compositional tuning and variation in synthesis/processing conditions. A combination of local to long-range electron, X-ray and neutron scattering probes are employed to investigate the complex configurational diversity and associated structure-property trends in the series. Experimentally derived models are supported by Density Functional Theory calculations and Metropolis Monte Carlo simulations. We demonstrate that nanostructured oxides produced through polymeric steric entrapment and other low temperature reaction routes are promising precursors for the synthesis of compositionally complex pyrochlore/fluorite ceramics with specific nanoscale ordering motifs. We also discuss the exquisite level of experimental characterization and theoretical modeling that may be required to understand the phase stability, atomic structure, and respective properties of emerging HEO materials classes, highlighting some remaining challenges.

SiO₂ (silica) is one of the most widely used inorganic materials that demands fabrication methods with nanoscale resolution. Fabricating silica with designed three-dimensional nanostructures is an exciting yet challenging area of research and industrial application. The emerging technology of additive manufacturing (AM) can create fine structures through layer-by-layer deposition to generate complex architectures and simplify fabrication processes. AM of fused silica glass was realized via stereolithography with the resolution of tens of microns. However, the relatively low spatial resolution limits their applications in micro-electronics, MEMS, and micro-photonics. To address the limitation, we propose an approach to 3D print silica nanostructures with sub-200 nm resolution via two-photon polymerization. The technique involves 2PP enabled AM of functionalized colloidal silica nanocomposite ink, followed by pyrolysis and sintering, where the post-processing procedure determines the crystallinity of the structures produced, which can be either amorphous glass or polycrystalline cristobalite. In this technique, making a suitable “ink” containing silica nanoparticles (NPs) and two-photon polymerizable precursor is the dominating factor. The nanocomposite ink has low viscosity, high transparency, and high heat conductivity. The 3D printed nanostructures demonstrate attractive optical properties. The fabricated microtoroid optical resonators can reach quality factors (Q) over 10⁴. Moreover, doping and co-doping of rare earth salts such as Er³⁺, Tm³⁺, Yb³⁺, Eu³⁺ and Nd³⁺ can be directly implemented in the printed SiO₂ structures, showing strong photoluminescence at the desired wavelengths. Especially for Er³⁺, the final printed structures exhibit photoluminescence around 1.55 µm, making the proposed technology a powerful tool for optical telecommunication applications. It is also envisioned that arbitrary 3D structure of crystalline silicon can be fabricated by magnesium reduction of printed crystalline silica, making the dream of 3D printing silicon chips a reality.

Grain growth, a common mechanism in ceramic processing, is difficult to predict because it is influenced by many factors, including porosity, raw powder size and shape, impurities and sintering conditions. To parse out these effects, grain growth studies commonly rely on qualitative comparisons of micrographs because of inadequate physical descriptors of grain size and shape. However, new data techniques can extract high order parameters that better describe microstructural features. In this study, we establish whether a convolutional neural network (CNN) can distinguish two initial microstructures of calcia-doped alumina, which appear similar by traditional grain growth metrics but then evolve differently during grain growth. If a unique signature can be extracted, it can provide mechanistic-insight into grain boundary motion for improved grain growth predictions.
The goal here is to collect a signature that distinguishes two alumina microstructures prepared with the same raw materials, slip casting process, and sintering conditions. However, one sample is slip cast in a magnetic field, which causes the equiaxed particles to rotate and
preferentially align the c-axis of their unit cell, resulting in a bulk crystallographic texture. Recent studies show that the textured microstructure evolves differently from the untextured sample, resulting in a larger grain size and unique grain morphology. Here, we will evaluate the
developed CNN used to identify these microstructures. Briefly, the CNN is trained from at least five micrographs with ~200 grains each from each timestep. The CNN consists of two convolutional layers with an increasing number of filters to learn increasingly abstract features.
The convolutional layers are followed by three fully connected layers with a decreasing number of nodes. The signature is extracted as a scalar quantity from the penultimate layer in the network. We will discuss the accuracy of the CNN to predict the microstructure’s processing
path and its implications regarding how crystallographic orientation influences grain growth. Furthermore, we will demonstrate the need for a new higher order physical descriptor of microstructures.

The variety of local atomic environments found in a glass far exceeds that found in a crystalline material. This makes the task of linking physically important phenomena (e.g. crack nucleation or shear transformation) to local structural risk factors particularly challenging in glassy materials. We present case studies in two simulated materials (silica glass and a binary metallic glass) using unsupervised machine learning techniques (the Gaussian Integral Inner Product Distance with agglomerative clustering and diffusion maps) to extract local structural features, and supervised machine learning to link those features to mechanical behavior. We pinpoint preexisting defects in the as-quenched state as risk factors leading to fracture nucleation in silica, and show how detailed quantification of structural evolution in a metallic glass shear band points the way to improved stateful plastic models. We also consider the comparative merits of human-designed versus machine-learned structural descriptors for glass and look forward to ways to use them synergistically. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525 (SAND2022-8095 A).

Due to their peculiar architecture, Triply Periodic Minimal Surfaces (TPMS) find applications in many engineering domains. The increasing capability of additive manufacturing (AM) technologies allows the printing of microstructured materials with unparalleled spatial resolution and repeatability. This makes the design of TPMS-based material systems and device a feasible approach. Application of TPMS structure in Bone Tissue Engineering scaffolds design is particularly attractive. Indeed, scaffolds made with bioactive materials like glass ceramics and Hydroxyapatite and with specific morphological (such as porosity, pore interconnectivity, micro-feature characteristic size) and mechanical properties can be designed by taking advantage of the TPMS features.
The design of biomechanically reliable BTE scaffolds, possibly following a patient-specific approach, is still a complex task that is based on multidisciplinary input such as biomechanics, transport properties and mechanobiology. Classical optimization approaches may be unsuitable for such a complex approach in which multiple objectives need to be met simultaneously. For this purpose, the use of Machine Learning methodologies can be a suitable approach to support the selection of a design with prescribed multi-disciplinary properties.
In this study, we assessed whether a ML model may support the identification of TMPS architectures by leveraging biomechanical and morphological parameters as design inputs. The ML model was composed by three components. The first and second ones predict the geometrical parameters whereas the third one identifies the architecture type (i.e. diamond, gyroid and IWP). To do so, two regression and one classification problems were designed. Here, we considered three different TPMS architectures fully parametrized by two geometrical parameters. A dataset of TPMS architectures was created by solving the direct problem, i.e. imposing the geometrical parameters and the architecture type. Elastic and strength properties have been obtained through finite element modeling; while post-processing of the simulated micro-CT images of the TPMS structure was used to assess morphological parameters (the main ones are: wall thickness, wall spacing, volume fractions and geometrical anisotropy). Fluid transport properties were estimated by assessing the effective pore connectivity index. A total amount of 38 features were extracted from each of the 1258 designs (352, 476, 430 designs for diamond, gyroid and IWP, respectively).
The pseudo-experimental dataset was partitioned into a training set (60%), a validation set (20%) and a test set (20%). A ML greedy feed-forward feature selection algorithm was used with the aim of selecting the most informative descriptors. Once the selection of the most relevant features was identified the optimal design was obtained by defining the optimal values of the features.
The effectiveness of the predictive capability of the proposed approach was quantified by comparing the optimal structure obtained through the regressions and the actual values of the features for geometry extracted from the validation sets.
The results obtained through this approach have proven the suitability of the proposed ML algorithm to design suitable TPMS scaffold architectures, allowing for patient specificity which prescribes specific biomechanical (stiffness and strength) and transport properties (mainly related to the morphological parameters).

Reducing contaminant particles in the semiconductor etching process has been an important issue for improving the yields of semiconductor production. Recently, the plasma resistance glass (PRG) proposed as a new material to replace the plasma resistant material made of crystalline ceramics. Etching rate of PRG was lower than that of quartz glass, alumina, and sapphire. There is almost no change in the microstructure even after etching, so it is expected to be a suitable material for reducing particulate contaminants. Components to be aware of in the semiconductor process include alkali metals, heavy metals, etc. It can cause a defect of internal voltage on the oxide film, reduced lifetime, and crystal defects. Research has shown that the etching rate was an index of plasma resistance. It depends on the boiling temperature of the fluoride compound formed by a chemical reaction of plasma fluorine radicals. Bismuth was the component remaining after excluding the components to be avoided in the semiconductor process, and at the same time, the boiling temperature of BiF₃ is 900°C. It was expected to contribute to the reduction of the etch rate. However, since bismuth aluminosilicate glass was not well known, information about the clear vitrification range and properties of glasses were lacking. In this study was to investigate the vitrification range of Bi-Al-Si-O, which has not been studied, and to evaluate the plasma resistance. So we intend to investigate the vitrification range, thermal, physical properties, and plasma resistance of the ternary phase diagram.

Soda-lime glass has been used for a long time as a glass container because of its ease of melting, the abundance of raw materials, and cheapness. However, the glass container is damaged due to contact and collision between the containers, in the process of washing, filling, and capping, so the strength of the container is reduced to one-thousandths. HEC (Hot End Coating) and CEC (Cold End Coating) are applied to compensate for the decrease in the strength of glass containers. It is known that the mechanical properties of soda-lime glass can be improved using chemical strengthening. Therefore, in this study, the chemical strengthening was applied under annealing conditions to increase the strength of the container. For chemical strengthening, melted KNO₃ was spray-coated on a glass container. The strengthening temperature was performed at T_g±50°C. for 1 hour and then the cooling rate was 5°C/min until 100°C. After washing the strengthened glass container, mechanical properties and surface changes were confirmed. After that, reliability evaluation was conducted through surface roughness change according to the presence or absence of HEC and chemical strengthening. Reliability was confirmed by measuring the impact strength value.

Ultra-hard and brittle materials have drawn enormous attention as a master mold or itself in manufacturing flexible displays, IT-electronics-semiconductor devices and bio-diagnostic kits when they have holes, channels or other free forms whose dimensions are in micro- and nano-scale. Especially, in machining those hard and brittle ceramic into ultra-fine feature size under 100 μm, errors in size or shape can be occurred by run-out during tool-setting. In this research, we introduce a newly developed ultra-precision machining system with both electro-discharge and mechanical machining modules for brittle ceramic materials using PCD(polycrystalline diamond) and PCBN(polycrystalline boron nitride) tools and suggest optimized machining conditions using a machining signal analyzing system. Force and acoustic emission data were obtained and correlated with resulting morphologies on material surfaces.

Acknowledgement : This work was supported by the Technology Innovation Program (20010984, Development of multiple machining system technology integrated electro discharge and mechanical working for ultrahigh hard material and application machining technology using micro tool having size of 20 μm) funded By the Ministry of Trade, Industry & Energy(MOTIE, Korea) and by the Korea Institute of Machinery and Materials (NK238E).

Quartz crucible for manufacturing silicon ingots has a two-layer structure with inner and outer layer. The inner layer, which is in direct contact with the molten silicon, should minimize silicon contamination due to incorporation of impurities and crucible fragments. Therefore, a high-pure SiO₂ synthetic raw powder had been used. In this study, crucibles were prepared using the three types of high-pure SiO₂ synthetic powder; (1) powder produced by crushing and heat-treating the gel manufactured by the sol-gel method (2) powder manufactured by sol-gel method, crushed and heat-treated (3) powder produced by water glass ion exchange method, crushed and heat-treated. Both the synthetic powder had a purity of 6 N and a particle size of D₅₀ 70 to 160μm, and the microstructure of the powder showed a difference depending on the production method. Therefore, there was a difference in the distribution of bubbles generated in the transparent layer of the crucible. We also confirmed the possible effect on ingot quality as the number of bubbles in the transparent layer increases.

Recently, the development of functional glasses with the phase separation has been extensively researched, and the phase separation process is required to be clarified. However, for glasses without periodic structures, the diffraction patterns generally show broad peaks, and it is difficult to analyze the local structures only by XRD experiments.
In this study, we have developed a tool to analyze local structures by atom-by-atom decomposition of the XRD patterns for atomic configurations obtained by molecular dynamics simulations. We can visualize local structures that contribute significantly to a XRD peak of interest using atom-decomposed XRD patterns. Furthermore, we can analyze the effects of local structures such as coordination number on XRD patterns.

Understanding the structure of glassy materials is a tremendous challenge both experimentally and computationally. Despite decades of scientific research, for instance, the structural origin of the density anomaly in silica glasses is still not well understood. Atomistic simulations based on molecular dynamics (MD) produce atomically resolved structures, but extracting insights about the role of disorder in the density anomaly is challenging. Here, we quantify the topological differences between structural arrangements from MD trajectories using a graph-theoretical approach. Structural differences in silica glasses exhibiting density anomalies are captured using a graph similarity metric. To balance the accuracy and speed of the MD simulations, we utilized force matching potentials to generate the silica glass structures. This approach involves casting all-atom glass configurations as networks, and subsequently applying a graph-similarity metric (D-measure). Calculated D-measure values are then taken as the topological distances between two configurations. By measuring the topological distances of silica glass configurations across a range of temperatures, distinct structural features could be observed at temperatures higher than the fictive temperature. In addition, we compared topological distances between local atomic environments in the glass and crystalline silica phases. This approach suggests that more coesite-like and quartz-like local structures, emerge in silica glasses when the density is at a minimum during the heating process.

High-Entropy Oxides (HEOs) have gathered a lot of attention over the past few years particularly around their formation and properties^1–4. The perovskite lattice (ABO₃) is a particularly exciting opportunity as mixing can be carried out on both the A or B site presenting a wealth of enthalpic and entropic possibilities to achieve a single-phase solid solution with a favourable Gibbs free energy⁴.
Using a combination of computational modelling and experimental studies, we have examined several 3-, 4-, and 5-B element oxides with combinations of (Ga, Y, In), Nb, (Ti, Zr, Sn). We have studied the influence of a range of factors including the ionic size of the sites, the tolerance factor and the ionic size variation which we can then link to the formation of single-phase materials.
Our experiments on Ba-based perovskites demonstrate that single phases can be formed in a range of 4 and 5-B element perovskites with mixed valence charges (e.g. 3+, 4+ and 5+) but mixing was far more challenging with only 3-B element systems. By adjusting the A-site using Sr and Ca we were able to reduce the ionic size penalties and form further 3-B site single phase systems indicating that the size of the A cation also has an effect on the mixing process.
We have coupled the experimental work with a mixture of classical and ab initio simulation work examining the enthalpy of mixing and comparing to the phase formation from experiment. Our DFT simulations have identified the different mixing and hybridisation characteristics of the ions which affects their ability to mix and can run counter to expected ion size behaviour. Our classical simulations have optimised and examined hundreds of different structures which allows us to comment on the ideality of the solid solutions formed and the competition between entropy and enthalpy in the stability of a phase. We are also able to examine different secondary structures and how their enthalpic stability compares.
From our results, we offer thoughts on the differing rules governing the mixing processes in these perovskite phases.

References
(1) Sarkar, A.; Wang, Q.; Schiele, A.; Chellali, M. R.; Bhattacharya, S. S.; Wang, D.; Brezesinski, T.; Hahn, H.; Velasco, L.; Breitung, B. High-Entropy Oxides: Fundamental Aspects and Electrochemical Properties. Adv. Mater. 2019, 31 (26), 1806236. https://doi.org/10.1002/ADMA.201806236.
(2) Sarkar, A.; Velasco, L.; Wang, D.; Wang, Q.; Talasila, G.; de Biasi, L.; Kübel, C.; Brezesinski, T.; Bhattacharya, S. S.; Hahn, H.; Breitung, B. High Entropy Oxides for Reversible Energy Storage. Nat. Commun. 2018, 9 (1). https://doi.org/10.1038/S41467-018-05774-5.
(3) Rost, C. M.; Sachet, E.; Borman, T.; Moballegh, A.; Dickey, E. C.; Hou, D.; Jones, J. L.; Curtarolo, S.; Maria, J. P. Entropy-Stabilized Oxides. Nat. Commun. 2015 61 2015, 6 (1), 1–8. https://doi.org/10.1038/ncomms9485.
(4) Banerjee, R.; Chatterjee, S.; Ranjan, M.; Bhattacharya, T.; Mukherjee, S.; Sourav Jana, S.; Dwivedi, A.; Maiti, T. High-Entropy Perovskites: An Emergent Class of Oxide Thermoelectrics with Ultralow Thermal Conductivity. 2022, 8, 10. https://doi.org/10.1021/acssuschemeng.0c03849.

Glass fibers are extensively employed as the reinforcing material in composites in a variety of fields such as renewable energy and aerospace. Bare E-glass fibers were coated with different concentrations of rutile TiO₂ nanoparticles to improve mechanical strength of fibers. Microscopic and spectroscopic analyses were performed on the fibers to investigate the presence of nanoparticles over the surface of individual filaments. Tensile tests were performed on the coated and uncoated fibers by following standardized preparation and characterization techniques. Results show that nanoparticle-coated fibers are less likely to fail at a given strength compared to uncoated fibers. Moreover, the tensile strength of the fibers has increased between 7.31% and 11.71%, depending on the concentration of TiO₂ nanoparticles in the solutions.

In glass materials, local physical properties vary due to distributions of coordination numbers, interatomic distances and bond angles. Therefore, it is essential to understand local atomic configurations in glass materials. Classical molecular-dynamics simulations have been widely performed. Recently, first-principles simulations based on density functional theory (DFT) have also been applied to glass models. However, by plane-wave based DFT methods, a total energy and a stress tensor can be obtained as quantities integrated or averaged in an entire supercell. To understand on the nature of local structures in glass materials, the analysis of local distribution of energy and stress is important. In this study, we analyzed atomic stresses in oxide glass materials using first-principles local-stress calculations [1] developed by one of our groups.
Generally, in oxide materials, cations show tensile stresses and anions show compressive stresses. For a SiO2 glass model [2], atomic stresses of four-coordinated Si and two-coordinated O have a wide distribution due to the deviation in local atomic environments, but average values are close to those of the quartz-type SiO2 crystal. We found that formation energies of oxygen vacancies depend on sites and have strong correlations with atomic stresses. In a P-Si-Na-O glass model [3], the coordination number of Si is distributed from 4 to 6, and oxygen atoms can be classified into bridging oxygens (BO) and non-bridging oxygens (NBO). Tensile stresses of six-coordinated Si are higher than those of 4-coordinated Si and are close to those of the stishovite SiO2 crystal. We found that compressive stresses of NBO are smaller than those of BO and has a strong correlation with adsorption energies of protons. From these results, the analysis of atomic stresses is effective in predicting defect formation processes.

References: [1] M. Kohyama et al., Mater. Trans. 62, 1 (2021). [2] T. Tamura et al., Phys. Rev. B 69, 195204 (2004). [3] K. Takada et al., Phys. Chem. Chem. Phys. 23, 14580 (2021).

The advanced 3D micro- and nano- structuring of mid-IR material, such as chalcogenide glass, is of high-interest for the fabrication of photonic devices in general, and for spectroscopy applications in particular. Yet, the processing of these materials at microscales remains challenging and limited.
Here, we investigate the use of femtosecond laser for producing arbitrary three-dimensional patterns with varying physical properties in four ternary chalcogenide glass compositions: Ge₂₃Sb₇S₇₀, Ge₂₃As₇S₇₀, Ge₂₃Sb₇Se₇₀, and Ge₂₃As₇Se₇₀.
Interestingly, under specific laser exposure conditions, we observe the occurrence of both, densification and self-organization patterns formation, for which we explore the response to chemical etching. Furthermore, in an attempt to unravel generic laser-induced transformation mechanisms common to the four glass compositions investigated here, we systematically correlate pulse-to-pulse structural changes with material's inner structures evolution, using both, elemental and micro-Raman observations.
These observations emphasize the potential of ultrafast lasers not only for introducing waveguiding properties in chalcogenide glass, but also, for patterning them at the micro-scale.

In the past decade, significant progress has been made in the three-dimensional (3D) structuring of different classes of materials on the microscale. Direct laser writing (DLW) permits unprecedented shape control and resolution among the additive manufacturing approaches. One of the viable routes focuses on the surface functionalization of the 3D structures, e.g., via electroplating^[1] or atomic layer deposition^[2], which can be combined with other processes for the removal of the polymeric core to obtain hollow architectures. Alternatively, solid-beam structures can be formed via tailor-made solutions enabling the fabrication of solid-beam structures composed of materials other than solely organic polymers have been extensively studied. The recent endeavors resulted in several tailor-made solutions extending the range of materials that can be 3D-printed to metals (e.g., Ni^[3]), ceramics (e.g., ZnO,^[4] TiO₂^[5]), and composites (Au nanoparticle-laden polymer^[6]). From the perspective of mechanical stability, such materials often exhibit excellent chemical, mechanical and physicochemical stability when compared with polymeric materials.

In this study, we investigate the additive manufacturing of microscale 3D zirconia structures and the dependence of their mechanical stability on the crystallographic phase. We are using our previously-established method,^[7] we first pattern pre-ceramic structures, which transform into the self-miniaturized ceramic replicas upon the annealing in the air. We then modify the photoresin composition to improve the material performance under extreme stresses. The in situ nanomechanical material performance is assessed in a nanoindentation study for different 3D architectures and crystallographic phases of zirconia. Focused ion beam tomography is conducted better to understand the inner structure of the 3D materials. Besides, the additional focus is dedicated to precisely determining the crystallographic phase (X-ray powder diffraction, Raman Spectroscopy), chemical composition (X-Ray photoelectron spectroscopy), and defects of the materials (Cathodoluminescence) to better understand the opportunities and current limitations of the presented methodology.

The capability of our additive manufacturing methodology is presented with various 3D lattice structures of micro-to-nano feature sizes. Besides the sole focus on improving the material and structure performance under high strains, our methodology can be utilized to fabricate phosphors,^[7] which can in the future find applications in, e.g., 3D light-emitting devices.

Keywords: additive manufacturing, zirconia, direct laser writing, nanoindentation.

References:
[1] G. Williams, M. Hunt, B. Boehm, A. May, M. Taverne, D. Ho, S. Giblin, D. Read, J. Rarity, R. Allenspach, S. Ladak, Nano Res. 2018, 11, 845.
[2] D. Jang, L. R. Meza, F. Greer, J. R. Greer, Nat. Mater. 2013, 12, 893.
[3] A. Vyatskikh, S. Delalande, A. Kudo, X. Zhang, C. M. Portela, J. R. Greer, Nat. Commun. 2018, 9, 593.
[4] D. W. Yee, M. L. Lifson, B. W. Edwards, J. R. Greer, Adv. Mater. 2019, 31, 1901345.
[5] A. Vyatskikh, R. C. Ng, B. Edwards, R. M. Briggs, J. R. Greer, Nano Lett. 2020, 20, 3513.
[6] Q. Hu, X. Z. Sun, C. D. J. Parmenter, M. W. Fay, E. F. Smith, G. A. Rance, Y. He, F. Zhang, Y. Liu, D. Irvine, C. Tuck, R. Hague, R. Wildman, Sci. Rep. 2017, 7, 17150.
[7] J. Winczewski, M. Herrera, C. Cabriel, I. Izeddin, S. Gabel, B. Merle, A. Susarrey Arce, H. Gardeniers, Adv. Opt. Mater. 2022, DOI 10.1002/adom.202102758.

Silicon nitride (SiN_x) film has been widely utilized in passivation coating and dielectric devices due to its chemical stability and excellent insulating properties. Films developed by the plasma-enhance chemical vapor deposition (PECVD) approach exhibit superiority with good uniformity and high deposition rate. However, under a normal PECVD process, residual stress usually builds up along with increasing SiN_x thickness ^[1]. High film stress will lead to SiN_x film cracking to restrict the practical applications. Though there are studies about SiN_x film stress aiming to improve film toughness, limited information has been dug out to build up the relationships between chemical composition, atomic arrangement, and film stress.
In this work, we uncovered the hidden role of high H impurity in PECVD amorphous SiN_x films at the initial growth due to low deposition temperature in normal deposition conditions. Thin and thick SiN_x films grown on silicon (Si) wafers were selected to compare H impurity and bond configuration at different deposition stages. Both X-ray photoelectron spectroscopy (XPS) and Fourier-transfer infrared spectrum (FT-IR) spectrums indicated higher H impurity at the incipient growth as N-H bonds which were much shorter than the Si-N bonds as backbone of SiN_x matrix. Those shorter N-H bonds resulted in a shrinking tendency to induce extra tensile strain at the SiN_x/Si interface. Therefore, at critical thickness, the original SiN_x film exhibited severe cracking initiated from the SiN_x/Si interface and propagated vertically through whole SiN_x film. To avoid high H impurity concentration near SiN_x/Si interface, Si wafer was soaked in reaction chamber with sufficient time to make sure it was under stable deposition temperature. Correspondingly, SiN_x films were more homogenous with consistent atomic spacing. As a result, these long-soaking grown SiN_x films could tolerate stress with 42% fewer cracks film compared to original SiN_x films. In addition, crack length statistical analysis indicated that only ~14% of crack length was longer than 4 cm in long-soaking SiN_x compared to ~52% of crack length above 4 cm in original SiN_x film. This work provides a long-soaking strategy to regulate interfacial tensile strain attributed to unreacted N-H bonds and alleviate cracks in thick SiN_x PECVD coating.
References:
[1] X. Xu*, Q. He, T. Fan, Y. Jiang, L. Huang, T. Ao, C. Ma, Applied Surface Science, 2013, 264, 823-831.

Cemented carbides (WC-Co) are a class of composites that combine the hardness of a metallic carbide (WC) with the toughness of a metallic binder (Co). Applications of WC-Co include metal cutting, mining, construction, and wood and plastic working. The main reason for their consumption in a wide array of applications is their unique combination of high hardness and toughness relative to other hard materials such as ceramics (e.g. aluminium oxide and silicon carbide) and diamond. Cemented carbide remains a growing market with an increase in annual world consumption from 10 t in 1930 to 50000 t in 2008. With the growth in demand for WC-Co, it is beneficial to understand WC-Co more thoroughly.

Fracture and wear affect the performance and lifetimes of tools composed of WC-Co. Cracks in WC-Co has been found to predominantly propagate along WC/WC and WC/Co interfaces at low cobalt contents as is the case in typical commercial grades [1]. Hence, it is useful to gain an understanding of WC boundaries given that the material toughness is related to energy dissipation, which in turn corresponds to crack path. This knowledge can be employed for grain boundary engineering (GBE) of WC-Co tools with superior performance and lifetimes.

In this work an evaluation of the WC grain boundary character distributions (GBCD) for the most abundant boundary type (Σ2) has been analysed for a standard (WC-10wt%Co) and a chromium doped (WC-10wt%Co-1wt%Cr%) WC-Co sample. This evaluation is based on Rohrer et al.’s five parameter GBCD analysis [2]. Statistically, relative boundary populations are determined from GBCD analysis. Since boundary populations are inversely correlated with boundary energies, GBCD analysis will give an indication of relative grain boundary energies. Fracture energies of these boundaries were measured via double cantilever beam (DCB) tests. Relative populations of Σ2 boundaries on each sample was then compared with their respective measured fracture energies.

It was found that the area fraction of Σ2 boundaries increased threefold with chromium doping. Furthermore, it was found that the length fraction of Σ2 boundaries were greater within WC/WC boundaries compared to all possible WC interfaces in the WC-Co microstructure regardless of doping. Fracture energies of 3.57 ± 0.28 Jm^-2 were measured for Σ2 boundaries on the standard sample. In contrast a successful (stable crack growth) DCB test could not be performed on the chromium doped variant. It was hypothesised that the chromium doping altered the boundary chemistry of the Σ2 boundaries as observed from the increased fraction of Σ2 boundaries in the doped sample. This in turn likely increased the Σ2 boundary fracture toughness and hence the reason for the failed DCB tests on the doped sample.

References
[1] K. P. Mingard, H. G. Jones, M. G. Gee, B. Roebuck and J. W. Nunn, International Journal of Refractory Metals and Hard Materials, 36, 136 (2013). Available from: doi: 10.1016/j.ijrmhm.2012.08.006.
[2] G. S. Rohrer, D. M. Saylor, B. E. Dasher, B. L. Adams, A. D. Rollett and P. Wynblatt, Zeitschrift für Metallkunde, 95, 197 (2004). Available from: doi: 10.3139/ijmr-2004-0043.

Nano/micro-structured ceramics combine the unique features of ultrasmall building blocks and their organization into tailored architectures to foster emergent properties up to the macroscale. Many functional applications open up, such as in catalysis, energy and optoelectronics. One of the current main challenges in materials science and engineering is the fabrication of such multiscale materials, and more specifically bridging scales in materials manufacturing. To obtain finely tuned macroscale properties, building blocks assembly and optimization of both material constituents and interfaces are key. This talk will tackle these aspects for some promising examples or nano- and micro architected ceramic-based materials. Insights will be shown on how to tailor macro-mechanical behavior and functional properties via fundamental investigations and a strategic combination of different material processing technologies.

Lightweight composites have become key structural materials for aircrafts and future energy-efficient transportation systems. Nevertheless, the design of composites featuring high strength and high fracture toughness remains an open challenge due to the usual trade-off between these sets of properties in most manmade materials. Taking inspiration from the strong and tough hierarchical architecture of mollusk shells, I designed and fabricated fracture-resistant glass-reinforced composites by combining soft polymer layers with alternating, nacre-like layers that are infiltrated with the same polymer.¹ In my talk, I will highlight the fracture behavior and the toughening mechanisms that govern the high crack growth resistance of these hierarchical glass composites. In particular, I will focus on the effects of plastic deformation and bridging by the polymer phase on the early- and late stages of the fracture process. These effects have been elucidated by using a mechanochromic organic phase, capable of pre-emptively detecting and reporting the evolution of damage through a simple optical readout. The composites signal the presence of damage via a fluorescence color change that arises from the force-activation of mechanophore molecules embedded in the organic matrix.² By optical imaging mechanically loaded composites it is possible to localize damage prior to fracture and to elucidate the key role of plasticity during rupture of the hierarchical composites. Although several examples have shown that polymers can be made damage-reporting using mechanophores, just a handful of examples have targeted the reinforcing phase for the same purpose. To fill this gap, I will also highlight progresses on how high-aspect-ratio glass-reinforcing particles, directly gathered from the upcycling of glass industry residuals, can be engineered to provide optical warnings before and during extended fracture events. With this research, I aim at displaying how multifunctional bioinspired glass composites, capable of combining high strength, high toughness and damage-sensing properties can be fabricated and characterized. I will provide new insights into the interplay of multiscale toughening mechanisms in hierarchical bioinspired architectures and I will offer guidelines for the design of novel multifunctional bioinspired composites.

References
T Magrini, A Senol, R Style, F Bouville, AR Studart, Journal of the Mechanics and Physics of Solids (2022) 159, 104750
T Magrini, D Kiebala, D Grimm, A Nelson, S Schrettl, F Bouville, C Weder, AR Studart, ACS Applied Materials & Interfaces (2021) 13 (23), 27481-27490

Bioinspired microstructures in ceramics have the potential to fulfil combined unusual properties such as local anisotropy with high hardness, stiffness and toughness, but their current fabrication methods remain challenging due to the time-consuming, energy-extensive sintering method. Here, we report a strategy to sinter dense ceramics with bioinspired heterogeneous microstructures that have tunable stiffness and toughness within seconds. This strategy combines a colloidal directed assembly i.e., magnetic assisted slip casting (MASC) with templated grain growth (TGG) using ultrafast high-temperature sintering (UHS). The hierarchical microstructural designs result from the local orientation of alumina microplatelets coated with Fe₃O₄ nanoparticles dispersed in a matrix of alumina nanoparticles. After UHS, the presence of Fe₃O₄ led to ceramics with about 98% density and a toughness (K_I0) of 5.89 MPa.m^0.5 in nacre-like orientations. We further explored periodically oriented and multi-layered microstructured ceramics using different compositions to attain an overall toughness (K_I0) of about 5.73 MPa.m^0.5 combined with enhanced energy dissipation rates of ~200 J. m^-2 that can exhibit high local stiffness and toughness across multiple directions. Our proof-of-concept specimens demonstrate that MASC and UHS can fabricate dense, tough and preferentially textured ceramic in time-saving, cost-effective and energy-efficient ways, opening the design space for achieving ceramic parts with local structural properties.

Compared to metals and polymers, ceramics are great candidates for applications involving extreme environments like high temperatures, radiations, humidity or corrosion. The iono-covalent nature of their bonds make them very strong and stiff materials, although intrinsically brittle. This poor resistance to damage propagation greatly limits the use of ceramics in high-performance domains that cannot tolerate catastrophic failure, such as aeronautics or biomedical applications. For this reason, improving toughness has been a major axis of research in ceramic engineering over the past decades. To this day, the key mechanisms highlighted to improve ceramics’ resistance to crack growth mostly reside in a fine control of the microstructure. This includes reinforcing the bulk matrix with fibers and fillers to bridge the crack behind its tip or adding features that resist progression ahead of the crack tip such as residual compressive stresses and transformation toughening. More recently, other microstructures inspired from biological materials have been investigated, specifically ceramic-based materials such a bone, dentin, or nacre. Indeed, millions of years of evolution have gifted these materials with sophisticated hierarchical structures that enable them to resist fracture propagation despite being primarily composed of brittle ceramics. Among these materials, nacre stands out as having one of the simplest microstructures yet exhibiting many toughening mechanisms. Nacre is one of the main components of many seashells and is composed of 95 vol% calcium carbonate platelets stacked in a brick-and-mortar fashion. This microstructure provides nacre with intricate toughening mechanisms such as deflection, branching, interlocking, shearing, viscoelastic dissipation, all acting at different length scales. Taking inspiration from nacre, many processes and compositions have been developed to produce tough ceramic-based composites. Using the current standards of fracture mechanics, it has been proven that nacre-like composites exhibit higher values of fracture toughness and improved crack-resistance curves compared to bulk ceramics. However, characterizing fracture in such materials that exhibit high degrees of deflection and branching remains a challenge, and all the mechanisms that account for improved damage resistance in nacre-like composites have not been characterized or explained. We have worked on characterizing fracture propagation in ceramic-based nacre-like composites, with the aim of determining the role of microstructure and composition on the mechanical response. To this end, we have developed analytical tools validated by finite element analysis to measure crack resistance curves of highly deflected and branched cracks. We have used it to study and reveal the influence of specific parameters related either to the composition, such as the interphase and addition of residual stresses, or to the microstructure, with the presence of short- and long-range order, on the crack propagation behavior in brick-and-mortar structures.

Bioceramic-based 3-dimensionaly constructed structures and scaffolds are widely used for hard tissue engineering applications and as hard tissue analogs. Robocasting, where a bioceramic powder-binder combination in paste form is used as feed material can be used to develop 3D preforms. The hardening and shape preservation of the printed structures is usually achieved by subsequent polymerization of the organic component. In this study, a novel hybrid printing paste mainly composed of calcium phosphate cement (CaPC) has been developed to achieve in situ hardening during printing process. The cement component is a-tricalcium phosphate(Ca₃(PO4)₂), which readily converts to hardened hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂ or HAp) upon hydration. The hydration product in a calcium-deficient HAp, in that sense it offers higher bioactivity and osteoconductive property as it is resembling chemistry of natural bone mineral better than any other CaP. Parametric printing studies has been performed to to determine optimal formulation of the printing paste. Meanwhile, validation of the processing approach, i.e. cement-conversion kinetics, HAp formation efficiency/extent, shape preservation for 3D-printed structures, mechanical properties has been examined. In addition, a microstructural modification approach to control and alter microporosity adaptable to 3D-printing process by incorporation of an inorganic pore generator has been demonstrated. .

We report glassy carbon ultra-sharp microneedles pyrolyzed from a polymeric precursor structure fabricated via Two-Photon Polymerization lithography (2PP). The glassy carbon microneedles have tip radius of curvature of 1 µm, diameters ranging from 22 µm to 26 µm, and lengths ranging from 106 µm to 126 µm. They are capable of safely penetrating the 1.5 mm wide and 35 µm thick round window membrane in a guinea pig model that serves as a barrier between the middle and inner ear spaces. The precursor needle has a tip diameter of 7.5 µm, a base diameter of 100 µm and a length of 600 µm. To maintain structural integrity during pyrolysis, we show that the exposed surface area to volume ratio of the polymeric precursor needs to be within a certain range. Upon designing the polymeric precursor structure under these constraints, the structures can shrink by up to 81% during pyrolysis while maintaining structural integrity. The pyrolyzed glassy carbon is elastic with a brittle failure mechanism with a Young’s modulus of 9.0 GPa and characteristic strength of 710 MPa. Furthermore, glassy carbon is known to be biocompatible. We show that the pyrolyzed ultra-sharp glassy carbon microneedles perforate the round window membrane safely and can serve to enable safe direct delivery of therapeutics into the inner ear delivery. In addition to glassy carbon microneedles, this pyrolysis process can be adapted to fabricate many other biomedical devices.

The inclusion of piezoelectric ceramics has been in recent years a fast increasing hot topic. Piezoelectric lead-free materials such as BaTiO3 or K0.5Na0.5NbO3 (KNN) have been studied in a wide range of applications such as neural and bone tissue repair, or bacterial/planktonic membranes ^1,2. For many of these applications, porous systems are necessary in order to increase the active surfaces and maximize the interaction with the biological matter.
A promising manufacturing method for porous ceramics is thermoplastic extrusion. This method produces robust cylindrical membranes, also in presence of pores former, and it is also scalable up to industrial levels ³.
In the case of planktonic/bacteria membranes, the device requires high piezoelectricity (depending on the application can be > 100 pC/N), suitable pores size and distribution (1-10 µm), and strong support. However, pores decrease the magnitude of piezoelectric effect both locally and in the overall device.
Here we manufactured tubular porous piezoelectric membranes by thermoplastic extrusion, using graphite and/or polymethyl methacrylate (PMMA) as pore formers and lead-free ceramics as support material. The influence of pore formers on the microstructure, mechanical properties and piezoelectric effect is studied, as well as the sintering temperature and extrusion parameters. The overall device is suitable for biological applications, in particular planktonic/bacteria membrane. Finally, the manufacturing process is scalable and easily integrable in an industrial environment.

(1) Xu, Q. et al., Adv. Mater. 2021, 2008452, 2008452. https://doi.org/10.1002/adma.202008452.
(2) Kumar, S. et al., J. Ind. Eng. Chem. 2021, 97, 95–110. https://doi.org/10.1016/j.jiec.2021.02.016.
(3) Haugen, A. B. et al., J. Eur. Ceram. Soc. 2017, 37 (3), 1039–1047. https://doi.org/10.1016/j.jeurceramsoc.2016.10.001.

Amorphous materials, unlike their crystalline counterparts, have no obvious defects that help facilitate plastic instability, and so methods that predict the locations of plastic events are highly sought after. Two-dimensional model glasses have been used extensively to study amorphous materials and to develop a theoretical basis for determining the location of plastic events. One method that has resulted from such investigations is the Local Yield Stress (LYS) method in which nanoscale regions of the material are probed to quantify the incremental stress necessary to induce plastic rearrangement. The resultant local yield stress depends on the sense of the mechanical load applied on a local region. Higher accuracy predictions can be obtained by sampling many disparate mechanical loads. Doing so allows one to develop a more accurate picture of the “local yield surface,” the limiting stresses that the material can withstand before yielding along all possible mechanical loadings. In three dimensions, the number of distinct mechanical tests needed to accurately sample the local yield surface increases. The resulting dataset is computationally expensive to produce. We seek to use this data to compactly characterize this high-dimensional “surface” and to enable extrapolation along deformations that were not explicitly tested in the original dataset. This can, in principle, be accomplished by describing the local yield surface in terms of a discrete number of Shear Transformation Zones (STZs). By characterizing the number of STZs in a local region we will be able to predict areas of high plasticity more accurately in amorphous materials, eventually leading to the accurate modeling of plastic response and the design of materials with mechanical properties catered to specific needs.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Probing the fracture toughness of brittle materials by measuring the crack-length generated from indentation experiments is often practiced in many areas of materials research. However, when the cracks are too small to be measured accurately, an energy-based approach has recently been proposed by several researchers. In the work presented here, this approach is compared with the well-known crack-length method on six different materials. We found that the energy-based method and crack-length method yield significantly different results for all materials but converge if the energy method is modified with a correction factor using a calibration sample.

Sintering is a thermally active process, in which densification (or pore elimination) and grain growth can occur. In general, the driving force for densification is lowering the surface free energy by replacing solid–gas interfaces, leading to a new solid-solid interface with a lower system’s free energy [1-6]. The optimization of the ceramic’s densification process lies in a better comprehension of the sintering steps and related mechanisms. Many models satisfactorily describe the sintering at a micrometric scale, however, nanometric effects make the direct extrapolation of these models to the nanoscale more complicated [7-9]. By high-resolution transmission electron microscopy, particularly real-time measurements, we followed the pore elimination process in a self-standing ZrO₂ thin film produced by monoclinic ZrO₂ nanoparticles. The absence of carbon membrane and the surface cleaning by removing the capping ligands produced an ideal scenario to perform the experiments. We found a high anisotropic pore elimination, with a gradual and linear decrease in pore area as a function of time. The pore shrinkage occurs with atoms from a rough surface being redistributed on the solid-gas interface, while atoms are attached to a faceted surface. In the final stage, the pore becomes rounded and shrinks until complete elimination. Also, the pore acts as a pin, reducing GB mobility. In summary, these results can contribute to a better understanding of the sintering process in nanoceramics, and to the improvement of kinetic models that better describe the densification process at the nanoscale.

[1] Bordia, R. K.; Kang, S. L.; Olevsky, E. A. J. Am. Ceram. Soc. 2017, 100 (6), 2314– 2352
[2] Gong, Z.; Zhao, W.; Guan, K.; Rao, P.; Zeng, Q.; Liu, J.; Feng, Z. J. Am. Ceram. Soc. 2020, 103 (10), 5900– 5913
[3] Wollmershauser, J. A.; Feigelson, B. N.; Gorzkowski, E. P.; Ellis, C. T.; Goswami, R.; Qadri, S. B.; Tischler, J. G.; Kub, F. J.; Everett, R. K. Acta Mater. 2014, 69, 9– 16
[4] Mitchell, S.; Qin, R.; Zheng, N.; Pérez-Ramírez, J. Nat. Nanotechnol. 2021, 16 (2), 129– 139
[5] Cao, A.; Lu, R.; Veser, G. Phys. Chem. Chem. Phys. 2010, 12 (41), 13499
[6] Rahaman, M. N. Sintering of Ceramics; CRC Press, 2007
[7] Castro, R. H. R.; Gouvêa, D. J. Am. Ceram. Soc. 2016, 99 (4), 1105– 1121
[8] Chen, I.-W.; Wang, X. Sintering of Nanograin Ceramics. In Ceramics Science and Technology, Riedel, R.; Chen, I., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA, 2012; pp 441– 455
[9] Kang, S.-J. L. Grain Boundary Energy and Sintering. In Sintering: Densification, Grain Growth and Microstructure, Kang, S.-J. L., Ed.; Elsevier Butterworth-Heinemann, Oxford, 2004; pp 139– 143.

Bone is a self-healing tissue, but some pathologies, aging, non-physiologic conditions and trauma may cause a critical size bone defect that cannot heal spontaneously. The implant of bioceramic scaffolds is a promising approach for the treatment of these pathologic conditions. Among bioceramics, hydroxyapatite (HAP) is a good candidate for its chemical affinity with the native bone’s mineral component, thus ensuring biocompatibility and promoting osteo-integration. As HAP mechanical property is heavily influenced by its brittle nature, a robust experimental characterization of the bulk material is required for the design of mechanically reliable scaffolds.
Furthermore, recent developments in 3D printing technologies allow for a wide variety of micro-structured devices meeting patient-specific needs. In this work, we deal with HAP material obtained through DLP-stereolithography which is a printing technology allowing for very fine spatial resolution and high printing fidelity. However, as the technological process includes a sintering phase at high temperature, the properties of the obtained material are strongly affected by the intrinsic defects which characterize the final product. For this reason, in this study, the mechanical characterization of the material samples having a simple geometrical shape and obtained through the same process as that used for the scaffold production and having the same characteristic size of the microstructural features of the scaffolds is proposed.
For this purpose, we 3D-printed three different kinds of beams (i.e. simple beams, notched beams and cantilevers) in small scale (approximate size of beams 1mmx0.4mmx10mm). The samples have been subjected to confocal laser imaging, micro-Computed Tomography scanning and mechanical tests. The confocal laser scanning provided the size of the samples having the purpose to complement mechanical tests and to assess printing fidelity; the micro-CT analyses had the purpose to quantify the intrinsic porosity of the material resulting from the sintering process. The mechanical tests were micro-bending test and Berkovich nanoindentation.
The micro-bending tests were used to assess the flexural stiffness and strength, while the nano-indentation tests have been used to verify whether elastic property exhibits a scale effect, expected in case micro-defects occur in the material.
Using the same material and 3D-printing technology, high porous scaffolds were manufactured through DLP stereolithography. Micro-Computed Tomography scans were performed on the scaffolds to obtain the real geometry of the scaffolds. After a proper binarization process, a finite element model was implemented using the real geometry and the mechanical parameters found in the above-described experiments. Micro-CT based finite element analyses on the 3D-printed scaffolds were performed with the aim to determine the macroscopic elastic and strength properties of the scaffolds. The obtained macroscopic properties are validated through comparison with macroscopic experimental data already available.
The experimental and numerical framework presented in this work has shown the capability of in-silico models of 3D-printed scaffolds to predict biomechanically relevant properties, provided that intrinsic properties of the materials are available with specific reference to the manufacturing process parameters and geometrical size of the micro-architecture.

Ceramics and their derived composites which are lightweight and with high strength and high toughness have been long sought after for various engineering applications. In this work, various alumina ceramic based microlattices, including Simple Cubic, Octet Truss and Kelvin Cell have been fabricated using computer-aided design tool and stereolithography 3D printing process, Subsequently, interpenetrating phase composites (IPCs), with the ceramic lattices as backbone and infiltrated with a second phase (e.g., epoxy or aluminum alloy), have been further developed. The mechanical behavior of the ceramic lattices and the IPCs under quasi-static compression tests has been systematically studied by experiments and simulation. Results show that Simple Cubic lattices generally initiate the fractures at the struts in the outermost lattice planes, while the Octet Truss and Kelvin Cell lattices fracture at the (110) diagonal plane. With a second phase infiltrated into the lattices, the resultant composites inherit similar fracture behavior as their corresponding lattices, but in a more gradual manner. The stiffness and compressive strength of the lattices follow the order of Simple Cubic > Octet Truss > Kelvin Cells. The compressive strength of IPCs is generally higher than that of the ceramic lattices. Composites with 50 vol% alumina simple cubic lattices and filled with epoxy exhibit a high compressive strength of 440 MPa and energy absorption of 12 J/g (at 16% strain). The energy absorption is almost 100% higher than that of epoxy and much better than that of the ceramic lattices. The factors controlling the mechanical properties of the lattices and IPCs, including relative density, topology, matrix materials and lattice defects arising from 3D printing, will be discussed and the perspectives on the application of the lattices and IPCs will be given in the presentation.

Electrospinning is a well-developed technique for fabricating one-dimensional (1D) fibres in the form of two-dimensional (2D) nonwoven mats. In junction with sol-gel synthesis, single and multiphase ceramic fibres made of oxides and nitrides have been produced. Such ceramic fibre mats, with a unique combination of large surface area, high porosity, flexibility, show great premises for advanced applications such as flexible electronics and sensors. However, the ability to tune the morphology, porosity and the macroscopic shape of fibre product is still limited. For example, it is challenging to produce highly porous three-dimensional (3D) fibre architectures which are desired in tissue engineering, air filtration, absorbent, etc. Thus, developing strategies to control the micro- and macrostructures of ceramic fibres is the key to facilitate their practical implementation and broaden potential usages.

Here, we report several solution modification strategies to effectively tune the structure of electrospun fibres, allowing customised fibre porosity and surface area. At the microscopic level, inducing liquid-liquid phase separation in spinning solution leads to core-shell fibre structures without the need of employing a complex spinneret. The pores size and shape can be controlled by varying the ratio of alkoxides, solvent, and polymer component. Also, electrospin the solutions with homogeneously dispersed submicron-sized polymer powders gives hybrid fibres. Such powders act as pore-forming templates to render mesopores and macropores inside fibre after calcination. At the macroscopic level, modifying the electrical conductivity of the spinning solution triggers different electrodynamic behaviour of electrified solution jet. The as-spun fibres spontaneously assemble into sponge-like macrostructure with centimetre-scale height, which is in sharp contrast to the conventional electrospun thin nonwovens. After calcination, the resultant ceramic fibre sponges show extremely high porosity (about 99.9%), high specific surface area (up to128 m2/g), excellent flexibility and elasticity. Overall, we introduce novel solution modification strategies towards a new class of electrospun ceramic fibres with unprecedentedly controlled micro- and macrostructures.

Phase Formation of Cubic Silicon Carbide from Reactive Silicon-Carbon Multilayers
Dwarakesh Sudhahar ¹, Deepshikha Shekhawat ^1*, Joachim Döll ² , Jörg Pezoldt ¹*

¹ FG Nanotechnologie, Institut für Mikro- und Nanelektronik and Institut für Mikro- und Nanotechnologien MacroNano^â and Institut für Werkstofftechnik, TU Ilmenau, Postfach 100565, 98684 Ilmenau, Germany; [email protected] (D.S.)
² Zentrum für Mikro- und Nanotechnologien, TU Ilmenau, Gustav-Kirchhoff-Straße 7, 98693 Ilmenau, Germany; [email protected] (J.D.)
* Correspondence: [email protected] (D.S.); [email protected] (J.P.)

Abstract: The scope of this paper is to produce efficient and low-cost silicon carbide multilayers. The method is formed on self-propagating high-temperature synthesis of binary silicon-carbon based reactive multilayers. In order to do so the silicon carbide is fabricated by means of the magnetron sputtering method. The silicon and carbon bilayers were fabricated with two different bilayer thicknesses. The silicon and carbon are deposited in an alternative layer with a total thickness of 1 μm. The entire system is annealed by rapid thermal analysis at different temperatures starting from 500 C to 1100 C and heating uprate. In order to find the phase formation, morphology, and reaction enthalpy the samples were investigated with XRD, SEM and FTIR. From XRD analysis we could find that the formation of the silicon-carbide phase was initiated from 700 C with increasing bilayer thickness the silicon carbide phase formation was partially suppressed by the silicon recrystallization due to resulting lower carbon diffusion into silicon. The transformation process proceeds in a four-step process. From this, it was noted that when compared to low bilayer thickness samples, the formation of the SiC phase is delayed with increasing bilayer thickness and needs a high temperature. The grain size and morphology of the samples were well studied with SEM analysis and the measured average thickness of the Si and the C layers are 53 and 48 nm, respectively.

Keywords: phase transition, reactive materials, reaction enthalpy, X-ray diffraction, differential scanning calorimetry

Well-ordered mesoporous alumina materials derived from block copolymer self-assembly are desirable for many applications such as environmental separation and carbon sequestration. We have developed an improved synthesis protocol mixing block copolymers with stable alumina sol precursors to access various ordered morphologies, including 2D hexagonal cylinders and 3D cubic lattices, as well as 2D lamellar and multiscale hierarchically ordered mesoporous architectures, to our knowledge for the first time. Coupling block copolymer self-assembly with Joule heating provide further enhancements yielding highly crystalline ordered alumina and alumina/carbon structure within second timeframes. Here we will describe the Joule heating-induced crystallization kinetics studies of block copolymer-directed mesoporous alumina and other oxide systems. The X-ray scattering data fits well with established models providing a clearer understanding of crystal growth under such nonequilibrium transient high-temperature conditions.