Jian Luo

Calcite and silica were selected as model systems for investigating the cold sintering of cementitious materials, mimicking negative‐carbon products formed during the mineral carbonation of silicate‐rich feedstocks. Dense calcite and calcite–silica composites were fabricated using NaOH solutions at 200°C under applied pressures of 375–500 MPa, reaching maximum relative densities of ∼95% and ∼92%, respectively. This study first examined how NaOH concentration and applied pressure influence the cold sintering behavior of calcite. In alkaline environments, calcite exhibited enhanced dissolution and reprecipitation, forming nanocrystalline calcite that facilitated cold sintering. Although the solubility of calcite is expected to decrease in alkaline solutions, enhanced cold sintering and grain growth were nevertheless observed and rationalized using a kinetic model that considers potentially enhanced dissolution and reprecipitation rates. During the cold sintering of calcite–silica composites, competitive dissolution between calcite and amorphous silica led to distinct microstructural evolution pathways. While a small amount of silica impeded cold sintering and generated inhomogeneous microstructures, effective densification of the composite was achieved at higher silica contents, resulting in more homogeneous microstructures. Partial crystallization of amorphous silica into α‐quartz was observed, generating nanoscale porosity that was offset by enhanced shrinkage under applied pressure, such that the overall relative density was not significantly affected. These findings provide a mechanistic basis for developing low‐temperature consolidation strategies for cementitious materials.

This article explores the impact of grain boundary structures and compositions on the functional properties of various materials for photovoltaics, batteries, and other energy-related applications. Examples of correlative microscopy studies highlight the potential to discover structure–property relationships at grain boundaries, essential for the design of energy devices to achieve superior performance. A grain boundary transition that promotes grain growth and reduces the boundary resistance in solid electrolytes is given as an example. A key focus will be on transport phenomena at grain boundaries, including mass, thermal, electrical, and ionic transport mechanisms. These transport phenomena are directly correlated with the charge defects that lead to a buildup of electric charges and potential barriers at the grain boundaries. In addition, applied electric fields can also induce boundary transitions that can affect grain boundary transport and other properties. Finally, we demonstrate that these potential barrier heights can be tuned by modulating the chemical composition, structure, and carrier concentration of the grain boundaries. Graphical abstract

Engineering short-range atomic order in amorphous materials offers a promising yet scarcely explored route to high-performance materials. Here, we establish a boron-assisted amorphization strategy using ApolloX, a theory-guided, short-range-order–constrained generative framework that enables the synthesis of multielement materials with tunable boron content and yields FeCoNiMoBOx compositions with promising oxygen evolution reaction (OER) activity. In particular, ApolloX identifies an ensemble of candidate low-energy amorphous configurations for the FeCoNiMoBOx family across systematically varied boron contents. Ab initio molecular dynamics simulations based on these configurations reveal that increasing boron content slows atomic diffusion and suppresses crystallization, with the stabilization of BO3-centered motifs identified as a critical structural descriptor that governs the amorphization propensity. Guided by these predictions, we first synthesize three representative FeCoNiMoBOx compositions with distinct boron contents within the theoretically identified composition window and perform synchrotron-based scattering and electron microscopy to verify compositional fidelity, structural homogeneity, and the targeted structural features, thereby experimentally validating our boron-assisted synthesis strategy. Building on this validation, we further extend the approach to abroader library of multimetal BOx amorphous compositions with diverse metal combinations and boron loadings, confirming the generality and transferability of the proposed method. Overall, these results demonstrate that this boron-assisted amorphization strategy provides a practical means to rationally design compositionally complex amorphous materials with tunable and potentially improved performance.

This Perspective critically assesses recent advances in ultrafast sintering and highlights open scientific questions and emerging technological opportunities. Mechanistic studies of flash sintering indicate that the flash event initiates as a coupled thermal and electrical runaway, while rapid densification is enabled by ultrahigh heating rates and elevated sintering temperatures. Building on this understanding, ultrafast sintering has been realized without passing electric currents through the specimens via multiple approaches, including rapid thermal annealing (using intense infrared heating), ultrafast high-temperature sintering (in which specimens are sandwiched between graphite felt heaters), blacklight sintering (employing blue laser or intense ultraviolet irradiation), atmospheric-pressure plasma sintering, and induction ultrafast sintering (utilizing skin currents in direct induction heating or no current in the specimens in susceptor-heating mode). Reactive ultrafast synthesis and sintering have also been demonstrated. Although several hypotheses have been proposed, the mechanisms governing ultrafast sintering and its kinetics warrant further investigation. In particular, reactive ultrafast synthesis and sintering of compositionally complex ceramics are scientifically intriguing to understand while also presenting technological opportunities. The expanding range of ultrafast sintering methods provides a versatile platform for high-throughput materials discovery, especially in the rapidly growing field of high-entropy and compositionally complex ceramics, which feature vast compositional spaces to explore.

Dense ultrafine-grained (UFG) refractory MoW, MoWNb, and MoWNbTa alloys were fabricated by combining high-energy ball milling (HEBM) and spark plasma sintering (SPS), achieving ~92-96% relative densities and ~70-180 nm grain sizes. The effects of 2 at.% cobalt (Co) addition on sintering behavior and high-temperature grain growth resistance were investigated as a function of compositional complexity. Activated sintering was observed, with 2 at.% Co addition increasing relative densities from ~92-96% to ~96-98%. Isothermal grain growth experiments at 1200 °C and 1300 °C showed that Co doping suppressed the relative grain growth rate, despite a modest initial grain size increase due to Co-activated sintering, with the effect becoming more pronounced in compositionally complex alloys. The observed trend is consistent with the recently proposed high-entropy grain boundary (HEGB) effect. Notably, Mo24.5W24.5Nb24.5Ta24.5Co2 achieved a 96.4% relative density and maintained an ultrafine grain size, increasing only slightly from ~122 nm to ~127 nm after 5 h annealing at 1200 °C. Scanning transmission electron microscopy (STEM and energy-dispersive X-ray spectroscopy (EDS) confirmed strong Co segregation at grain boundaries, accompanied by minor depletion of Ta and W, supporting a recently proposed grain boundary segregation model for high-entropy alloys and HEGBs.