Kathy Lu

In this study, montmorillonite (MMT) nanosheets are purified and exfoliated from a crude clay source and further twice‐functionalized with cetritrimethylammonium bromide and [3‐(2‐aminoethylamino)propyl]trimethoxysliane (AEAPTMS) to promote dispersion in the preceramic polymer. Phase profiles and compositions of MMT nanoflakes and MMT‐silicon oxycarbide (SiOC) are characterized with X‐ray diffraction, infrared spectroscopy, and thermogravimetric analysis. The microstructures are examined by scanning and transmission electron microscopy. MMT nanoflakes are randomly dispersed in the SiOC matrix with α‐quartz forming at the MMT‐SiOC interface. Pyrolysis to 1400 °C induced the formation of SiC nanowhiskers that are observed up to 20 µm in length and 200 nm in diameter. After selective etching of SiO₂ domains with HF, pore sizes and specific surface areas of MMT‐SiOC are analyzed with nitrogen adsorption. The study provided a new fundamental understanding of MMT‐SiOC interactions at different pyrolysis temperatures and also led to composites with specific surface areas reaching 120 m² g⁻¹ up to 1200 °C pyrolysis, and between 340 and 772 m² g⁻¹ at 1400 °C pyrolysis and pore size distributions between 2 and 5 nm. The methodology and results presented improve the understanding and viability of 2D nanomaterial‐reinforced ceramic composites and MMT as a precursor for nanostructured SiC.

A significant challenge during the polymer-to-ceramic pyrolysis conversion is to understand the polymer-to-ceramic atomic evolution and correlate the composition changes with the precursor molecular structures, pyrolysis conditions, and resulting ceramic characteristics. In this study, a Reactive Force Field (ReaxFF) simulation approach has been used to simulate silicon oxycarbide (SiOC) ceramic formation from four different polysiloxane precursors. For the first time, we show atomically that pyrolysis time and temperature proportionally impact the new Si-O rich and C rich cluster sizes as well as the composition separation of Si-O from C. Polymer side groups have a more complex effect on the Si-O and C cluster separation and growth, with ethyl group leading to the most Si-O cluster separation and phenyl group leading to the most C cluster separation. We also demonstrate never-before correlations of gas release with polymer molecular structures and functional groups. CH4, C2H6, C2H4, and H2 are preferentially released from the pyrolyzing systems. The sequence is determined by the polymer molecular structures. This work is the first to atomically illustrate the innate correlations between the polymer precursors and pyrolyzed ceramics.

A silicon oxycarbide (SiOC) ceramic composite produced via a high-temperature spark plasma sintering process exhibits excellent mechanical robustness, high electrical conductivity, and low thermal conductivity, useful for various applications.

The carbon content in SiOCN coatings governs the steel oxidation behavior at 800 °C. Carbon-rich coatings form Fe-rich oxides and gaps at the coating/alloy interface, carbon-free ones offer protection, and intermediate ones show partial degradation.

We present unreported mechanistic insight into how water vapor-assisted pyrolysis in the presence of MMT hinders SiO₂ crystallization and carbon removal, demonstrating a reversal of water vapor-induced effects in the presence of MMT nanosheets.

Martensitic phase transformation of zirconia (ZrO₂), when realized from the superelastic effect, has immense potential in energy applications, such as micro‐actuation and high‐energy dissipation/damping. In this study, martensitic transformation in ZrO₂ is studied using a newly developed microscale phase field model, which is scale‐independent and contains an averaged gradient term. The model is implemented using the FEniCS package in Python. With a thorough explanation of the model construction, the mechanics of phase transformation are detailed, and the material behaviors under different stress conditions are discussed. Mesh sensitivity, multivariant evolution, and the effect of particle size are analyzed to understand the martensite evolution in superelastic ZrO₂. This work provides new insight into the martensitic phase transformation behaviors of micron‐sized spherical ZrO₂ particles.

Polymer‐derived SiCN materials have great application potential in high‐temperature environments. In the current work, SiCN ceramics were tailored with Hf incorporation through doping of a molecular precursor in the preceramic polymer. The ceramics remained in a single amorphous phase up to 900°C. Further annealing caused nucleation and crystallization of tetragonal hafnium oxide (t‐HfO₂) within the SiCN matrix, resulting in a unique glass–ceramic nanocomposite of t‐HfO₂–SiCN. The crystal sizes of t‐HfO₂ were in the range of 2.3–5.1 nm. Raman spectroscopy was employed to examine the effect of Hf‐doping on the phase separation and crystallization of sp²‐carbon. Thermal stability in an oxidizing environment was analyzed by thermogravimetry. The incorporation of Hf in the SiCN ceramic system led to improved phase stability and oxidation resistance compared to the undoped SiCN system.

Polysiloxane coatings on yttria stabilized zirconia (YSZ) microspheres of 500 µm were simulated using Multiphase Flow with Interface Exchange-Discrete Element Modelling (MFiX-DEM). Two different coater configurations were developed to study the influence of gas velocity and its distribution on particle dynamics. The presence of the Wurster tube not only enhances the distribution but also increases the overall residence time of the particles. For different Wurster tube positions (normal, 10% and 20% lowered from its initial position), 20% lowered Wurster tube position demonstrated the most effective coating process. The effects of gas inlet pressure on the average gas velocity and the distribution of particles were analyzed. More than 97% of the particles can be retained. The derived results, including average gas velocity, particle retention percentage, and distribution of particles with gas velocity, are being used to guide the experimental work in obtaining defect-free coatings for YSZ microspheres.

This study focuses on the early stage of polymer‐derived SiOC ceramic conversion. We demonstrate that the perceived SiOC phase separation is nonexistent. Instead, SiO₂ and free carbon clusters form first and then carbothermal reduction sets in to induce SiOC formation. Such fundamental understanding is supported by both synchrotron X‐ray diffraction study and reactive force field simulation. This work for the first time unifies the understanding of atomic evolution process of polysiloxane‐based polymer to ceramic conversion.

This study focuses on the pyrolysis and ion irradiation behaviors of polymer‐derived SiFeOC–C–SiC ceramic. The pyrolyzed material is composed of SiO₂ and SiOC (amorphous), carbon (amorphous and turbostratic), and Fe₃Si and β‐SiC (nanocrystalline). Irradiation was carried out at both room temperature and 600°C using 400 keV Kr ions with fluences of 4 × 10¹⁵ and 1 × 10¹⁶ ions cm⁻², respectively. The Fe₃Si and SiC nanocrystals are stable against irradiation up to 3 displacement per atom (dpa) at room temperature and up to 12 dpa at 600°C. The SiOC tetrahedrals show phase separation and minor carbothermal reduction. The high irradiation resistance and the dense, defect‐free amorphous microstructure of SiFeOC–C–SiC after prolonged irradiation demonstrate its great potential for advanced nuclear reactor applications.

Polymer-derived ceramic (PDC) nanocomposites enable access to a large library of functional properties starting from molecular design and incorporating nanofillers. Tailoring preceramic polymer (PCP) chemistry and nanofiller size and morphology can lead to usage of the nanocomposites in complex shapes and coatings with enhanced thermal and mechanical properties. A rational design of targeted nanocomposites requires an understanding of fundamental structure–property–performance relations. Thus, we tailor our discussions of PCP design and nanofiller integration into single source precursors as well as pyrolytic processing for functionalizing PDCs. We also discuss the promises and limitations of advanced characterization techniques such as 4D transmission electron microscopy and pair distribution functions to enable in situ mapping structural evolution. The feedback loop of in situ monitoring sets the foundation for enabling accelerated materials discovery with artificial intelligence. This perspective assesses the recent progress of PDC nanocomposite research nanocomposites and presents scientific and engineering challenges for synthesis, fabrication, processing, and advanced characterization of PDC nanocomposites for enhanced magnetic, electrical, and energy conversion and storage properties.

In this study, silicon oxycarbide (SiOC) is selected as the base polymer to derive a SiOC ceramic (PDC) matrix, and four transition metals M (M = Ni, Mo, Co, and Zr) are individually introduced into the SiOC base to form various SiOC/M systems. SiOC‐Ni, SiOC‐MoC_x, and SiOC‐CoSi_x are obtained by pyrolysis at 1100°C, whereas SiOC‐ZrO_x forms upon pyrolysis at 1400°C. The selected SiOC/M systems encompass four different types of phase separation pathways—pure metal, metal carbide, metal silicide, and metal oxide (SiC‐SiO₂‐C‐Ni, SiC‐SiO₂‐C‐MoC_x, SiC‐SiO₂‐C‐CoSi_x, and SiC‐SiO₂‐C‐ZrO_x). The driving force for crystallization has been analyzed using a Gibbs free energy minimization method and phase fractions of these different PDC systems are calculated based on the lever rule. This work also reveals the energetics related to the quaternary systems and provides guidance to synthesizing metal‐containing PDCs with desired phase contents. In addition, we have examined the broad applicability of the phase content prediction method for a variety of other SiOC/M systems.

Flash pyrolysis, which combines conventional pyrolysis with flash sintering, was first conducted to produce polymer derived SiC‐TiC nanocomposites. Pre‐pyrolysis at 800℃ allows the conversion from titanium isopropoxide (TTIP) modified polysiloxane to an amorphous SiTiOC ceramic. The subsequent application of an electric field gives rise to the formation of turbostratic carbon and creates Joule heating to obtain a sample internal temperature of ~1400℃. The precipitation of β‐SiC, TiC, as well as titanium oxides is realized upon carbothermal reduction of extensively phase separated SiO₂ and TiO₂ with carbon. Increasing TTIP content embodies the nanocomposites with prominent electrical percolation behaviors. The electrical transport of the synthesized ceramics follows an amorphous semiconductor mechanism. High thermal stability in air is guaranteed, thanks to the in‐situ formed TiC nanocrystals and preferentially reduced amorphous carbon. Flash pyrolyzed nanocomposite with a Ti:Si molar ratio of 0.20 exhibits the highest electrical conductivity (0.696 S/cm) and minimum mass change (~2%) at 1000℃, serving as a competitive candidate for electro‐discharge machining (EDM) applications or self‐standing conducting devices that must withstand high temperature conditions.

Effects of Fe and POSS on the phase formation of SiOC between 1100 °C and 1500 °C were studied. Fe induces higher SiO₂and SiC contents. Phase contents are calculated based on a modified Gibbs free energy minimization method.

In this study, novel ferromagnetic Ni‐containing silicon oxycarbide (SiOC–Ni) was successfully fabricated from a base polysiloxane (PSO) with the addition of nickel 2,4‐pentanedionate. The resultant SiOC–Ni nanocomposite consists of in situ formed Ni nanocrystallites with a small amount of NiO uniformly dispersed in the amorphous SiOC matrix, and the corresponding nanocrystallite size increases with the increase of the pyrolysis temperature. The formation of nickel silicides (Ni_xSi_y) is completely suppressed by the effect of water vapor during the pyrolysis. The fundamental phase evolution process and mechanisms are explained. In an argon atmosphere, the SiOC–Ni materials pyrolyzed at 900°C are stable up to 1000°C with less than 6 wt% weight loss; they exhibit desirable electrical conductivity up to ~900°C with the highest electrical conductivity at ~247 S/m. This series of SiOC–Ni materials also demonstrates exciting ferromagnetic behaviors. Their new semiconducting behavior with soft ferromagnetism presents promising application potentials for magnetic sensors, transformers, actuators, etc.

Photothermal self-healing efficiency increased with Au nanoparticle contents and particle agglomeration deteriorated the efficiency.

This study uses a Monte Carlo simulation method to evaluate the agglomeration behaviors of different ZnO nanoparticle and polymethacrylate hybrid suspensions in an organic solvent. Interaction energies are primarily from the steric layer on the particle surfaces. The effects of nanoparticle size, steric layer thickness, particle content, and total solids loading are evaluated based on the average agglomerate size and agglomeration rate. Smaller particle, thicker steric layer, lower particle content, and higher solids loading are desirable factors to stabilize a suspension. Agglomerate size and agglomeration rate increase continuously with increasing particle content in the hybrids from 1 to 50 vol%. A drastic transition from an unstable suspension to a stable suspension occurs when the total solids loading of the suspension increases to greater than 10 vol%. This work provides important guidance for co‐dispersing nanoparticles and dissolved polymer chains in organic solvents.

In this work, ZnO and ZrO₂ ridges with 2 μm size are created based on a centrifuge‐aided micromolding approach and then sintered with different time. Characterization of feature morphology, fidelity, grain size, relative density, and linear shrinkage has been conducted. The densification mechanisms for both ZnO and ZrO₂ are controlled by grain‐boundary diffusion, but their grain growth mechanisms are dominated by gas diffusion and surface diffusion respectively. The sintering behavior for the bulk can be described with a N_g/N_b factor at 36, while for the features, a smaller N_g/N_b factor (15 for ZnO and 8 for ZrO₂) is needed. Attributed to their sintering mechanism difference, the grains in the ZnO features have a faster growth rate than those in the bulk, while the grains in the ZrO₂ features have a similar growth rate to those in the bulk. ZnO has a much faster grain growth behavior, leading to ridge fidelity loss and severe ridge destruction, while ZrO₂ has a much slower grain growth rate, resulting in high ridge fidelity and strong resistance to ridge destruction.

Silicon oxycarbide (SiOC) ceramics with highly adjustable properties and microstructures have many promising applications in batteries, catalysis, gas separation, and supercapacitors. In this study, additive structures on the nucleation and growth of SiO₂ within SiOC ceramics are investigated by adding cyclic tetramethyl‐tetravinylcyclotetrasiloxane (TMTVS) or caged octavinyl‐polyhedral oligomeric silsesquioxane (POSS) to a base polysiloxane (PSO) precursor. The effects of the 2 additives on the polymer‐to‐ceramic transformation and the phase formation within the SiOC are discussed. POSS encourages SiO₂ nucleation and leads to more SiO₂ formation with significantly increased ceramic yield, which subsequently leads to higher specific surface of 1557 m²/g with a larger pore size of ∼1.8 nm for the porous SiOC. High TMTVS content decreases both the specific surface area and pore volume of the resulting porous SiOCs. This study demonstrates a new approach of using Si‐rich additive POSS to increase the SiOC yield while maintaining or even increasing the specific surface area.

Silicon carbide (SiC) is a promising material with excellent chemical and physical performance under irradiation for advanced nuclear applications. The addition of nanostructured ferritic alloy (NFA) has been proven beneficial for the densification of SiC ceramics based on our previous work. To understand their microstructural evolution and irradiation resistance, spark plasma sintered (SPSed) SiC ceramics with and without NFA aid (0 vol% NFA‐100 vol% SiC, 2.5 vol% NFA‐97.5 vol% SiC, and 5 vol% NFA‐95 vol% SiC) were exposed to 5 MeV Si⁺⁺ irradiation. The ion irradiation strongly modifies the surface morphology with isolated sand dune shaped structures, which can be explained by the Bradley‐Harper (B‐H) theory. SRIM simulation for both the pure SiC and NFA‐SiC predicts similar surface damage of ~45 dpa and peak damage of ~790 dpa at ~2.0 μm depth. For the actual samples, the SiC matrix is completely amorphous up to ~2.2 μm thickness (from the surface dune valley to the amorphous layer boundary), which is consistent with the SRIM predicted depth of ~2.3 μm. Reaction product (Fe,Cr)₃Si in the NFA‐SiC samples maintains a crystalline structure with dislocation loops. A defect rate model is applied to understand the fundamental difference in ion irradiation resistance between SiC and (Fe,Cr)₃Si.

Spark plasma sintered pure silicon carbide (SiC) and nanostructured ferritic alloy‐silicon carbide (NFA‐SiC) systems are investigated in a water vapor containing air atmosphere at elevated temperatures up to 1000°C. Both of them exhibit excellent corrosion resistance with a dense amorphous SiO₂ layer as the main oxidation barrier. Crystalline α‐quartz and α‐cristobalite from the oxidation of silicides and SiC, respectively, further benefit the corrosion resistance. For the new NFA‐SiC system, the original graphite and silicide phases can be desirably sustained. The NFA‐SiC materials have promising applications in high temperature moist environments and are especially important for nuclear reactor cladding.

A co-suspension lithographic process is developed to create 250 nm to 1 μm features with 1–20 vol% ZnO solids loading.