This study compares reactive and capillary-driven (non-reactive) ultrafast sintering of simple versus compositionally complex silicides via different kinetic pathways. Four systems, VSi2, (Nb0.273V0.343Cr0.384)Si2, (Mo0.186V0.523Ti0.291)Si2, and (Mo0.081Nb0.197V0.25Cr0.3Ti0.172)Si2, were investigated. The compositionally complex silicides (CCS) were designed to achieve an average melting temperature comparable to that of VSi2 while maximizing configurational entropy under this constraint. For each CCS, capillary-driven ultrafast sintering was performed using pre-synthesized single-phase powders and compared with reactive ultrafast sintering starting from mixtures of three or five binary disilicides. Results indicate that reactive ultrafast sintering resulted in higher sinterability than capillary-driven ultrafast sintering, while increasing compositional complexity generally enhanced densification. Compared with capillary-driven sintering, the reactive process initially suppressed sintering but maintained high densification rates for longer duration, resulting in higher final relative densities. The grain size after ultrafast sintering was found to correlate with the final relative density, regardless of the composition or kinetic pathway.

Graded microstructures form in (Si-doped) BaTiO3 under applied electric fields, with the polarity of the gradient reversing as the voltage increases. At 5 V, grain growth is enhanced on the anode side, whereas at 15 V it shifts to the cathode side. Advanced electron microscopy was employed to characterize grain boundaries (GBs) and clarify the underlying mechanisms. At 5 V, anode-side enhancement is attributed to elevated concentrations of cation vacancies, nonstoichiometric GBs, and/or reduced GB space charges. At 15 V, cathode-side reduction, facilitated by SiO2 segregation and an increased specimen temperature from Joule heating at this higher applied voltage, induces a GB disordering transition, forming glass-like intergranular films that enhance GB mobility and reverse the polarity of the graded microstructure. These findings reveal new mechanisms of electric-field-induced GB transitions, demonstrate how electric fields can be exploited to tailor microstructures, and advance the fundamental interfacial science of GB phase-like (complexion) transitions.

Dense ultrafine-grained (UFG) refractory MoW, MoWNb, and MoWNbTa alloys were fabricated by combining high-energy ball milling and spark plasma sintering, achieving relative densities of similar to 92-96% and grain sizes of similar to 70-180 nm. The effects of 2 at.% Co addition on sintering behavior and high-temperature grain-growth resistance were investigated as a function of compositional complexity. Activated sintering was observed, with Co addition increasing relative density from similar to 92-96% to similar to 96-98%. Isothermal grain-growth experiments at 1200 degrees C and 1300 degrees C showed that Co doping suppressed the relative grain-growth rate, despite a modest initial grain-size increase due to Co-enhanced sintering, with the effect becoming more pronounced in compositionally complex alloys. This trend is consistent with the recently proposed high-entropy grain boundary (HEGB) effect. Notably, 2 at.% Co-doped MoWNbTa (Mo24.5W24.5Nb24.5Ta24.5Co2) achieved a relative density of 96.4% and maintained an ultrafine grain size, increasing only slightly from similar to 122 nm to similar to 127 nm after 5 h of annealing at 1200 degrees C. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy 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. These results may guide the compositional design and fabrication of dense refractory complex concentrated or high-entropy alloys with enhanced high-temperature grain-growth stability.

This study proposes and demonstrates induction ultrafast sintering (IUS), which enables rapid densification of refractory and other materials via two contactless modalities: direct IUS (d-IUS), where heating occurs through electromagnetic coupling with the sample, and susceptor IUS (s-IUS), where heating is achieved indirectly via an induction-heated metal case. Ultrahigh heating rates of similar to 75 to >450 degrees C/s and temperatures exceeding 2500 degrees C are readily achieved. Both d-IUS and s-IUS densify molybdenum to high densities within 120 s, with only similar to 1-3 % porosity observed by image analysis. Similarly, 3 mol % yttria-stabilized zirconia (3YSZ) reaches similar to 97 % relative density in 30 s via s-IUS. This study further demonstrates ultrafast reactive sintering of two difficult-to-sinter materials: a refractory compositionally complex alloy-carbide (RCCA-CCC) composite, NbMoTaW-(Nb0.37Mo0.11Ta0.39W0.13)(2)C, using d-IUS, and a compositionally complex silicide (CCS), (Mo1/3Nb1/3Zr1/3)Si-2, using s-IUS. This IUS platform offers a versatile route for high-throughput materials discovery and energy-efficient fabrication of bulk refractory materials.

We propose and demonstrate a novel approach for rapid fabrication of ceramic-ceramic joint assemblies, using sintered Silicon Carbide (SiC) as an initial example. Current Activated Reactive Ultrafast Joining (CARUJ) utilizes resistive heating of carbon-based materials to apply localized heat at or around the joint zone at heating rates of 101-103 degrees C/min. CARUJ is used to fabricate SiC-SiC joints with a Si-SiC interface using minimal pressure (1-2 MPa) at time scales considerably shorter (several minutes opposed to hours) than conventional approaches for similar systems. The reaction of an interfacial precursor based on elemental silicon and carbon leads to in-situ SiC formation to produce a continuous and dense bond in a single step. Measured average joint strengths of roughly 15 MPa are achieved when tested in single lap offset (SLO) compressive shear. Optional additions of refractory metals such as molybdenum can be utilized to introduce secondary inclusions such as MoSi2 within the joint interface. We further demonstrate SiC-SiC joining using Active Brazing Alloys (ABA) and proof of concept joining of tubular geometries. The localized and rapid heat application realizes a versatile material joining technique that could be extended for joining components at the plant site.

Refractory compositionally complex alloy (RCCA) NbMoTaW with a well-dispersed compositionally complex carbide (CCC) reinforcing secondary phase is made through high-energy ball milling (HEBM) and ultrafast high-temperature sintering (UHS). Stearic acid is used as a process control agent (PCA) for HEBM and a carbon source to form a controllable fraction (similar to 2-20 vol%) of CCC in-situ via reactive synthesis. Ultrahigh heating rates of similar to 100 degrees C/s enable sintering of dense (similar to 92-95 % relative densities) bulk compacts of RCCA-CCC composites at similar to 1850 degrees C in similar to 70 s. With the addition of 0.1-0.75 wt% PCA, ultrafast sintering enables the formation of a favorable P3m1 subcarbide CCC phase, (Nb0.37Mo0.11Ta0.39W0.13)(2)C. A thermodynamic model is developed to explain the observed Nb and Ta enrichments in (Nb0.37Mo0.11Ta0.39W0.13)(2)C. An additional rock-salt monocarbide CCC phase forms with more carbon source addition (1 wt% PCA). The well-dispersed and fine-grained CCC precipitates result in grain size refinement and a corresponding increase in hardness.

Activated sintering of refractory metals represents a classical phenomenon in powder metallurgy. This study discovers that Ni addition can enhance sintering of MoNbTaW both above and below the bulk solidus compo-sition at 1800 degrees C, thereby demonstrating the first example of activated sintering of a high-entropy alloy. To probe the underlying mechanism, experiments reveal complete grain boundary (GB) wetting above the bulk solidus composition, which logically infers the stabilization of a liquid-like interfacial phase (GB complexion) in a prewetting region below the bulk solidus composition. Furthermore, bulk CALPHAD (calculation of phase dia-gram) methods have been extended to model GBs in Ni-doped MoNbTaW to compute a GB "phase" diagram to forecast high-temperature GB disordering in the prewetting region and rationalize the observed Ni-activated sintering of MoNbTaW. Only weak GB segregation of Ni is found in furnace-cooled specimens, which suggests GB "drying" during cooling, consistent with the observed nanoscale precipitates along GBs.

The realization of low thermal conductivity at high temperatures (0.11 W m-1 K-1 800 °C) in ambient air in a porous solid thermal insulation material, using stable packed nanoparticles of high-entropy spinel oxide with 8 cations (HESO-8 NPs) with a relatively high packing density of ≈50%, is reported. The high-density HESO-8 NP pellets possess around 1000-fold lower thermal diffusivity than that of air, resulting in much slower heat propagation when subjected to a transient heat flux. The low thermal conductivity and diffusivity are realized by suppressing all three modes of heat transfer, namely solid conduction, gas conduction, and thermal radiation, via stable nanoconstriction and infrared-absorbing nature of the HESO-8 NPs, which are enabled by remarkable microstructural stability against coarsening at high temperatures due to the high entropy. This work can elucidate the design of the next-generation high-temperature thermal insulation materials using high-entropy ceramic nanostructures.

The formation of uniform, nondendritic seeds is essential to realizing dense lithium (Li) metal anodes and long-life batteries. Here, we discover that faceted Li seeds with a hexagonal shape can be uniformly grown on carbon-polymer composite films. Our investigation reveals the critical role of carbon defects in serving as the nucleation sites for their formation. Tuning the density and spatial distribution of defects enables the optimization of conditions for faceted seed growth. Raman spectral results confirm that lithium nucleation indeed starts at the defect sites. The uniformly distributed crystalline seeds facilitate low-porosity Li deposition, effectively reducing Li pulverization during cycling and unlocking the fast-charging ability of Li metal batteries. At a 1 C rate, full cells using LiNi0.8Mn0.1Co0.1O2 cathode (4.5 mA h cm-2) paired with a lithium anode grown on carbon composite films achieve a 313% improvement in cycle life compared to baseline cells. Polymer composites with carbonaceous materials rich in defects are scalable, low-cost substrates for high-rate, high-energy-density batteries.

The fabrication of high-entropy ceramics has recently expanded the pool of ultra-high temperature ceramics (UHTCs). To properly assess the suitability of these new types of ceramics for advanced aerospace applications, it is of vital interest to extend the characterizations beyond ambient conditions. Here, we have studied the thermal and ablation properties of a high-entropy diboride (HEB): (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)B2. The thermal conductivity of the HEB increases as a function of temperature and becomes comparable to that of other UHTCs at high temperatures. The electron dominated thermal conductivity of HEB is also nearly isotropic along different crystallographic orientations. The temperature-dependent volumetric heat capacity of HEB is measured and found to remain in agreement with that of ZrB2. Additionally, both material systems possess nearly the same ablation resistance. The multitudes of characterizations performed in this study establishes the suitability of HEB for high thermal load applications in extreme environments.

The fabrication of high-entropy ceramics has recently expanded the pool of ultra-high temperature ceramics (UHTCs). To properly assess the suitability of these new types of ceramics for advanced aerospace applications, it is of vital interest to extend the characterizations beyond ambient conditions. Here, we have studied the thermal and ablation properties of a high-entropy diboride (HEB): (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)B2. The thermal conductivity of the HEB increases as a function of temperature and becomes comparable to that of other UHTCs at high temperatures. The electron dominated thermal conductivity of HEB is also nearly isotropic along different crystallographic orientations. The temperature-dependent volumetric heat capacity of HEB is measured and found to remain in agreement with that of ZrB2. Additionally, both material systems possess nearly the same ablation resistance. The multitudes of characterizations performed in this study establishes the suitability of HEB for high thermal load applications in extreme environments.

The pronounced compositional inhomogeneity observed in refractory high-entropy alloys (HEAs), as opposed to non-refractory HEAs, has an important influence on their mechanical properties, thereby posing a significant challenge for the development of high-performance refractory HEAs. In this work, by combining transmission electron microscopy imaging, chemical analysis, and nano-hardness tests, we investigate the compositional inhomogeneities in a series of sintered nanostructured HEAs and elucidate their influence on the material's hardness. We reveal that the compositional inhomogeneity of the sintered nanostructured HEAs is temperature- and component-dependent. By correlating the hardness of nanostructured HEAs to the evolving compositional inhomogeneity, our work demonstrates that the compositional inhomogeneity in nanostructured HEAs can be tuned by sintering temperature or alloying towards optimized microstructure and hardness.

Nanocrystalline alloys (nanoalloys) are prone to grain growth. It is known that grain boundary segregation and precipitation can stabilize nanoalloys, but the stabilization becomes less effective at high temperatures and adding grain growth inhibitors often reduces sinterability. Herein, we have simultaneously achieved exceptional high-temperature stability and improved sinterability for a class of TiNbMoTaW-based refractory high-entropy nanoalloys (RHENs). Bulk pellets of RHENs were fabricated through ball milling and spark plasma sintering, achieving 93–96% relative densities with 50–100 nm grain sizes for three compositions. For example, Ti 17.8 Nb 17.8 Mo 17.8 Ta 17.8 W 17.8 Ni 6 Zr 5 sintered at 1300 °C attained ~ 96% relative density with ~ 55 nm mean grain size. Moreover, these RHENs exhibited exceptional stability at 1300 °C. Both Ti 17.8 Nb 17.8 Mo 17.8 Ta 17.8 W 17.8 Ni 6 Zr 5 and Ti 18.8 Nb 18.8 Mo 18.8 Ta 18.8 W 18.8 Ni 6 retained < 150 nm grain sizes after five hours annealing at 1300 °C. Notably, the addition of Ni, a well-known sintering aid for activated sintering of refractory metals such as W and Mo, in high-entropy TiNbMoTaW can promote sintering while maintaining high-temperature stability against rapid grain growth. This may be explained by hypothesized high-entropy grain boundary (HEGB) effects, while we recognize the possible (additional) effects of compositional inhomogeneity and secondary phase (Zener) pinning. These RHENs possess some of the highest temperature stability achieved for nanoalloys and ultrafine-grained metals.

This study demonstrates that 20% of a rare-earth (RE) diboride (ErB2) can be stabilized in a high-entropy transition metal (TM) diboride, despite the dissimilar chemical properties of RE and TM elements and large differences in lattice parameters of ErB2 and typical TMB2. However, the phase formation depends on the fabrication route, which is a noteworthy observation. Specifically, single-phase (Ti0.2Zr0.2Hf0.2Ta0.2Er0.2)B2 is synthesized via reactive spark plasma sintering (SPS) using elemental boron and metal elements. In contract, a specimen made by borocarbothermal reduction of binary oxides and SPS possess significant amounts of two Er-rich secondary phases. Notably, the RE addition in high-entropy TM diboride leads to improved hardness. Aberration-corrected scanning transmission electron microscopy (AC STEM) and energy-dispersive X-ray spectroscopy (EDS) elemental analyses further reveal significant Er segregation at grain boundaries. This work suggests that high-entropy ceramics can have significant solubilities of dissimilar components that may enable new, tunable, and improved properties.

A new type of compositionally complex (medium- or high-entropy) M5Si3 silicides is synthesized. Both (V1/5Cr1/5Nb1/5Ta1/5W1/5)5Si3 and (Ti1/5Zr1/5Nb1/5Mo1/5Hf1/5)5Si3 form single-phase homogenous solid solutions. Notably, (V1/5Cr1/5Nb1/5Ta1/5W1/5)5Si3 forms the hexagonal gamma (D88) phase, while all its five constituent binary silicides, V5Si3, Cr5Si3, Nb5Si3, Ta5Si3, and W5Si3, are stable in the tetragonal alpha (D8l) or beta (D8m) phases. Annealing at 1600C demonstrates that this hexagonal gamma phase is stable. Comparison of the experimental and calculated X-ray diffraction patterns, Rietveld refinements, and analysis of aberration-corrected scanning transmission electron microscopy high-angle annular dark-field images suggest cation ordering, which reduces the configurational entropy. This work expands the field of high-entropy and compositional complex ceramics by not only discovering a new compositional complex silicide phase but also demonstrating the cation ordering and unusual phase stability. These compositionally complex silicides can be combined with refractory high-entropy alloys to make the high-entropy counterparts to the Nb-silicide and Mo-Si-B composites.

Objective. Decoding neural activity has been limited by the lack of tools available to record from large numbers of neurons across multiple cortical regions simultaneously with high temporal fidelity. To this end, we developed the Argo system to record cortical neural activity at high data rates. Approach. Here we demonstrate a massively parallel neural recording system based on platinum-iridium microwire electrode arrays bonded to a CMOS voltage amplifier array. The Argo system is the highest channel count in vivo neural recording system, supporting simultaneous recording from 65 536 channels, sampled at 32 kHz and 12-bit resolution. This system was designed for cortical recordings, compatible with both penetrating and surface microelectrodes. Main results. We validated this system through initial bench testing to determine specific gain and noise characteristics of bonded microwires, followed by in-vivo experiments in both rat and sheep cortex. We recorded spiking activity from 791 neurons in rats and surface local field potential activity from over 30 000 channels in sheep. Significance. These are the largest channel count microwire-based recordings in both rat and sheep. While currently adapted for head-fixed recording, the microwire-CMOS architecture is well suited for clinical translation. Thus, this demonstration helps pave the way for a future high data rate intracortical implant.

Here we demonstrate the Argo System, a massively parallel neural recording system based on platinum-iridium microwire electrode arrays bonded to a CMOS voltage amplifier array. The Argo system is the highest channel count in vivo neural recording system built to date, supporting simultaneous recording from 65,536 channels, sampled at over 32 kHz and 12-bit resolution. This system is designed for cortical recordings, compatible with both penetrating and surface microelectrodes. We have validated this system by recording spiking activity from 791 neurons in rats and cortical surface Local Field Potential (LFP) activity from over 30,000 channels in sheep. While currently adapted for head-fixed recording, the microwire-CMOS architecture is well suited for clinical translation. Thus, this demonstration helps pave the way for a future high data rate intracortical implant.### Competing Interest StatementK.S., A.A.K., A.P.S., T.M.S, Y.N., A.T., P.O., C.L., D.P., K.N., K.M.B., S.S., M.S.H., B.K., M-E.S.H., R.J.E., I.M., D.F., A.M.S., V.G., Y.K., M.S., H.S.S., M.R.A. are current or former compensated employees or consultants of Paradromics, Inc., a brain-computer interface company. P.G., A.B-F, S.V., A.V.K, B.L., B.D. are compensated employees or consultants of Caeleste, CVBA, a circuit design company.