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Kinetics of antimony production via carbothermal reduction of Sb2O3–carbon powder–NaCl mixture using microwave and conventional heating was investigated to identify the dominant controlling mechanism. Results of conventional heating revealed the temperature range of conventional carbothermal reduction reaction is 500°C to 800°C, with the average activation energy of each stage being 81.97 kJ/mol (α = 0.1–0.5), 65.17 kJ/mol (α = 0.5–0.75), and 69.86 kJ/mol (α = 0.75–1.0), respectively. In the microwave field, the carbothermal reduction reaction of raw materials can be completed at 600°C to obtain antimony, and the weight loss data of the carbothermal reduction process were recorded for the first time. The above results show that the microwave field enhanced the interfacial chemical effect, accelerated the interfacial diffusion from the metal phase to the oxide phase, and reduced the activation energy of the carbon thermal reduction process to 6.85 kJ/mol. The growth index of antimony grain growth process is estimated to be 4.33, controlled by the surface diffusion. These data provide a reliable theoretical basis for studying the reduction reactions of minerals in microwave fields. [ABSTRACT FROM AUTHOR]

The grain growth behavior during an AC flash event was examined in 8 mol% yttria‐stabilized cubic zirconia (8Y‐CSZ) polycrystals. The effects of current/power densities on the grain growth behavior were investigated in 8Y‐CSZ samples with different specific surface areas at a constant sample temperature and applied field strength. The grain growth rate of flash‐treated 8Y‐CSZ was 300 times faster than that of heat‐treated 8Y‐CSZ at the same sample temperature in the absence of an electric current/field, suggesting that the promoted grain growth cannot be ascribed only to a thermal effect but also to an athermal effect occurring during the AC flash event. Moreover, the grain growth during the flash treatment strongly depends on the applied current/power densities and grain size; in particular, the grain growth showed enhancements with increasing applied current/power densities and for relatively small grain sizes. This result suggests that the grain boundary diffusivity of cations, which are regarded as the rate‐controlling species for grain growth, could be accelerated by tuning the current/power densities during the flash event. The grain growth mechanism was characterized using a grain growth exponent (n) value of 4.8 for the flash treatment at high current/power densities and using a conventional value of n = 3 under normal heat treatment conditions. The dependence of the grain growth behavior on the AC current/power density suggests that because the cation diffusivity is accelerated due to the formation of numerous point defects during the AC flash event, the grain growth mechanism might depend on the current/power densities and differ from that of conventional grain growth. [ABSTRACT FROM AUTHOR]

Sintering phenomena, densification, and grain growth are crucial for proper control of the microstructure for good mechanical strength. Here, materials and processing parameters, the addition of La2O3, and two‐step sintering (TSS) were combined to lead to higher strength in the porous alumina prepared by freeze‐casting. Based on grain growth and densification behaviors with the La2O3 doping, a beneficial thermal profile was designed for the TSS. As a result, higher relative density levels and smaller grain size were obtained compared with the results with conventional sintering (CS): 38.56% and 0.82 µm at 1500°C by CS; 40.78% and 1.78 µm at 1600°C by CS; 41.43% and 0.87 µm by TSS. The microstructural benefits provided ∼1.4 times higher compressive strength (5.46 MPa) from TSS than from the CS sample (3.92 MPa), highlighting the synergetic effect of La2O3 doping and the TSS strategy. [ABSTRACT FROM AUTHOR]

In a previous study (Okamoto & Hiraga, 2022, https://doi.org/10.1029/2022jb024638), we concluded that diffusion creep and grain growth in polymineralic rocks proceed by a common diffusional mechanism. Here, we built on that finding and estimated lower mantle grain size and viscosity during a single mantle convection cycle dominated by diffusion creep. We approximated the lower mantle as a two‐phase material consisting of bridgmanite + ferropericlase and post‐perovskite + ferropericlase, depending on depth. We used previously reported self‐diffusivities for bridgmanite and post‐perovskite. We predict a bridgmanite grain‐size of tens to hundreds of microns shortly after the phase transition at ∼660 km depth. This size remains relatively constant until the mantle material enters the post‐perovskite zone, which is marked by significant grain growth up to ∼9 mm just prior to upwelling. This size is sufficient to prevent further grain growth until the mantle material reaches the top of the lower mantle. These grain sizes combined with the diffusivities yield viscosities that vary laterally and with depth. At a lateral temperature difference of up to 800 K in the lower mantle, fine‐grained cold downwelling mantle is almost as viscous as, or more likely to be softer than coarse‐grained hot upwelling mantle. The lateral viscosity variations cannot be more than 2 orders of magnitude, and we estimated viscosities of 1018–1020 Pa · s in the upper lower mantle, 5 × 1020–5 × 1022 Pa · s in the lower bridgmanite zone, and 1017–1019 Pa · s in the post‐perovskite zone, which compare well with the values estimated in previous geophysical modeling studies. Plain Language Summary: We estimated lower mantle viscosity during a mantle convection cycle by assuming that the mantle flows by diffusion creep. Because diffusion creep is a grain‐size sensitive mechanism, grain size is a key parameter for determining mantle viscosity. We recently concluded that diffusion creep and grain growth in polymineralic rocks occur by the same diffusional mechanism. With previously reported self‐diffusivities for bridgmanite and post‐perovskite, we were able to estimate how the grain size of these minerals evolves after the ∼660‐km phase transition in downwelling mantle. Using these grain sizes and diffusivities, we were able to determine the mantle viscosity. We demonstrate that lower mantle viscosity has temporal and spatial variations induced by grain growth. At a lateral temperature difference of up to 800 K in the lower mantle, fine‐grained cold downwelling mantle is almost as viscous as, or more likely to be softer than coarse‐grained hot upwelling mantle. The estimated overall viscosity structure compares well with a one‐dimensional depth structure of the lower mantle viscosity estimated in previous geophysical modeling studies. Key Points: We estimated lower mantle viscosity during a mantle convection cycle by assuming mantle flow with a diffusion creep mechanismOur estimated viscosity structure agrees well with the viscosity structures predicted in previous geophysical modelsTemperature and grain size dependence of diffusion creep suppresses lateral changes in the viscosity of the lower mantle [ABSTRACT FROM AUTHOR]

There is a growing interest in field‐assisted fast ceramic processing and long‐term service of electrochemical devices under harsh operation conditions. These extreme redox conditions can make some conventionally thought slow ions highly mobile in electrochemical materials and devices and greatly accelerate the microstructural evolution. Experimental observations for enhanced slow ion diffusion that controls mass transport include accelerated grain growth in zirconia and ceria under atmospheric and electrochemical reduction above 1000°C and lattice cavitation of high‐voltage Li‐rich layered cathodes of lithium‐ion batteries at room temperature. These observations cannot be explained by altered defect concentrations under defect chemistry arguments. Instead, they support hugely enhanced slow ion mobility. My colleagues and I recently proposed a new mechanism resulted from strong electron–phonon interactions and dynamic electronic relaxations at the saddle point of an ion hopping event, which significantly lowers the ion migration barrier. In this perspective article, the theory and its experimental and computational supports shall be discussed. Such an electronic effect holds even in nominally ionic ceramic materials with wide bandgaps and is expected to be general in many transition metal oxides. It rationalizes a variety of intriguing experimental observations and offers an electronic perspective on the redox enhancement of slow ion kinetics in structural, functional, and energy ceramics. [ABSTRACT FROM AUTHOR]

The role of anisotropic grain boundary energy in grain growth is investigated using textured microstructures that contain a high proportion of special grain boundaries. Textured and untextured Ca‐doped alumina was prepared by slip casting inside and outside a high magnetic field, respectively. At 1600°C, the textured microstructure exhibits faster growth than the untextured microstructure and its population of low‐angle boundaries increases. Atomic force microscopy (AFM) is employed to measure the geometry of thermal grooves to assess the relative grain boundary energy of these systems before and after growth. In the textured microstructure, the grain boundary energy distribution narrows and shifts to a lower average energy. Conversely, the energy distribution broadens for the untextured microstructure as it grows and exhibits abnormal grain growth. Further analysis of the boundary networks neighboring abnormal grains reveals an energy incentive that facilitates their growth. These results suggest that coarsening is not the only dominant grain growth mechanism and that the system can lower its energy effectively by replacing high energy boundaries with those of low energy. The faster growth of lower energy boundaries suggests that isotropic simulations do not adequately account for anisotropic grain growth mechanisms or anisotropic mobility. [ABSTRACT FROM AUTHOR]

β‐silicon nitride (β‐Si3N4) ceramics with an additive oxide system of 1 wt% MgO and 3 wt% Re2O3 (Re = Gd, La, Y, and Yb) were fabricated by liquid phase sintering at 1950°C under nitrogen pressure of 748 kPa. Starting α‐Si3N4 powder with 10 vol% rod‐like β‐Si3N4 seed crystallites and an extended sintering time from 5 to 40 h resulted in the formation of bimodal microstructure composed of fine matrix grains and large grains. The 40 h‐sintered specimens of pseudo ternary β‐Si3N4‐MgO‐Gd2O3 system exhibited enhanced thermal conductivity of 127.2 ± 2.5 W m−1 K−1 associated with a degradation of the fracture strength from 1008.0 ± 38.0 to 491.7 ± 32.0 MPa, which was due to the formation of coarse‐grained aggregates that acted as both fracture origin as well as a thermal conductive pathway. The theoretical thermal conductivity was predicted for the sintered specimens by using equations based on a mean‐field micromechanics model to estimate the effective thermal conductivity of each component of binary composites. The calculation results suggested that the thermal conductivity of the large β‐Si3N4 grains (≥ 2 μm in diameter) was relatively high and estimated to be in the range of 175 to 191 Wm−1 K−1. The improved thermal conductivity of the 40 h‐sintered specimens was further discussed for the series of β‐Si3N4‐MgO‐Re2O3 systems based on the nanostructure characterization results obtained by the high‐resolution transmission electron microscopy and scanning transmission electron microscopy‐energy dispersive X‐ray spectrometry analyses. [ABSTRACT FROM AUTHOR]

Ultrafast high‐temperature sintering (UHS) is a joule heating method with an extremely high heating rate (∼103–104°C/min). In this work, commercial ZrB2 powders were rapidly densified by UHS to >90% relative density within 60 s in vacuum without pressure. Bulk density improved from 75% to 93% of relative density by increasing the sintering duration from 10 to 60 s. The final grain size increases from 2.7 ± 1.1 to 19.0 ± 8.4 μm when the sintering time increased from 10 to 60 s. X‐ray diffraction and energy‐dispersive spectroscopy show crystalline phase and compositional uniformity in ZrB2 after UHS. [ABSTRACT FROM AUTHOR]

The development of high‐performance (K,Na)NbO3 (KNN)‐based lead‐free piezoceramics for next‐generation electronic devices is crucial for achieving environmentally sustainable society. However, despite recent improvements in piezoelectric coefficients, correlating their properties to underlying multiscale structures remains a key issue for high‐performance KNN‐based ceramics with complex phase boundaries. Here, this study proposes a medium‐entropy strategy to design "local polymorphic distortion" in conjunction with the construction of uniformly oversize grains in the newly developed KNN solid‐solution, resulting in a novel large‐size hierarchical domain architecture (≈0.7 µm wide). Such a structure not only facilitates polarization rotation but also ensures a large residual polarization, which significantly improves the piezoelectricity (≈3.2 times) and obtains a giant energy harvesting performance (Wout = 2.44 mW, PD = 35.32 µW mm−3, outperforming most lead‐free piezoceramics). This study confirms the coexistence of multiphase through the atomic‐resolution polarization features and analyzes the domain/phase transition mechanisms using in situ electric field structural characterizations, revealing that the electric field induces highly effective multiscale polarization configuration transitions based on T–O–R sequential phase transitions. This study demonstrates a new strategy for designing high‐performance piezoceramics and facilitates the development of lead‐free piezoceramic materials in energy harvesting applications. [ABSTRACT FROM AUTHOR]

Cd alloying has been theoretically proved to be an effective strategy to suppress Cu‐Zn antisite defects and related defect cluster for improving device performance of pure sulfide kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. However, the potential of Cd alloying has not been fully realized by solely doping without further post heat‐treatment. Here, Cd alloying CZTS (Cu2(Zn,Cd)SnS4, CZCTS) is reported through dimethyl sulfoxide (DMSO) solution and how alloying concentration affects reaction path, grain growth, and electronic properties of the CZCTS absorbers is investigated. This study found that Cd can be incorporated into CZTS through direct phase transformation grain growth, which sufficiently suppresses band tailing. High quality CZCTS absorber films and efficient solar cells are fabricated within a wide range of alloy concentration. A champion CZCTS device with a power conversion efficiency of 12.3% is achieved at 35% Cd concentration without any post heat treatment, improved by over 70% compared to 7.0% of CZTS. This device exhibits a high VOC gain to the Shockley–Queisser (Voc/VocSQ = 59.7%), the lowest VOC deficit achieved in pure sulfide kesterite solar cells. The results demonstrate the importance of the Cd alloying strategy for mitigating band tailing and achieving high efficiency pure sulfide kesterite solar cells. [ABSTRACT FROM AUTHOR]

Clarifying the austenite grain growth law in the thin slab casting and rolling (TSCR) process can provide theoretical guidance for the control of austenite grain in the slab. Starting with the austenite nucleation during solidification process, the growth law of austenite grains is methodically studied throughout the TSCR continuous casting and soaking process. The results show that the austenite growth is not interrupted during the TSCR continuous casting and soaking process. The austenite grain growth in the continuous casting process accounts for more than 70% of the total growth. The growth rate of austenite in the continuous casting cooling process is always faster than that when reheated to this temperature. Compared with the holding temperature and holding time, the final size of austenite grains in the TSCR process slab is most affected by the continuous casting cooling rate. In addition, compared with the traditional process, the growth rate of austenite in TSCR process is faster at the end of soaking. [ABSTRACT FROM AUTHOR]

A cellular automaton finite‐element coupling model is developed to investigate the influences of the solidification structure of a continuous casting slab by the state of the cooling spray and process conditions. The model is validated. The solidification structure is simulated for different cooling spray schemes, superheat degrees, and casting speeds. The grain growth and distribution of the continuous casting slab near the corner area are sensitive to the cooling spray. As the distance of the nozzle from the surface of the continuous casting slab increases, the radii of the columnar and equiaxed grains tend to increase near the corners. The distribution of a columnar‐to‐equiaxed grain transition near the corner area gradually moves toward the core of the slab as the distance of the nozzle from the continuous casting slab decreases. At a casting speed of 1.2 m min−1 and superheat degree of 20 °C, the best nozzle height from the surface of the continuous casting billet is 234.5 mm. The casting speed and superheat significantly affect the grain radius. With the increase in the casting speed and decrease in the degree of superheat, the average grain radius decreases, while the equiaxed grain area increases. [ABSTRACT FROM AUTHOR]

In this paper, we report the synthesis of fully dense and ultrafine‐grained binderless tungsten carbide (WC) by novel dynamic sinter forging. A nearly fully dense sample was obtained at 1500°C and 220 ± 30 MPa@90 min. The hardness and fracture toughness of the sample with forging time of 180 min are 29.6 GPa and 7.3 MPa·m1/2, respectively, much higher than the counterparts prepared by conventional sinter forging and other techniques. The enhanced densification and mechanical properties are attributed to the intragranular plastic deformation accommodated grain‐boundary sliding induced by oscillatory pressure during dynamic sinter forging. The current results suggest a promising approach for the manufacturing of high‐performance binderless WCs. Also, the method is likely applicable to other material systems. [ABSTRACT FROM AUTHOR]

Hybrid chemical vapor deposition (HCVD) is a promising method for the up‐scalable fabrication of perovskite solar cells/modules (PSCs/PSMs). However, the efficiency of the HCVD‐based perovskite solar cells still lags behind the solution‐processed PSCs/PSMs. In this work, the oxygen loss of the electron transport layer of SnO2 in the HCVD process and its negative impact on solar cell device performance are revealed. As the counter‐measure, potassium sulfamate (H2KNO3S) is introduced as the passivation layer to both mitigate the oxygen loss issue of SnO2 and passivate the uncoordinated Pb2+ in the perovskite film. In parallel, N‐methylpyrrolidone (NMP) is used as the solvent to dissolve PbI2 by forming the intermediate phase of PbI2•NMP, which can greatly lower the energy barrier for perovskite nucleation in the HCVD process. The perovskite seed is employed to further modulate the kinetics of perovskite crystal growth and improve the grain size. The resultant solar cells yield a champion power conversion efficiency (PCE) of 21.98% (0.09 cm2) with a stable output performance of 21.15%, and the PCEs of the mini‐modules are 16.16% (22.4 cm2, stable output performance of 14.72%) and 12.12% (91.8 cm2). Furthermore, the unencapsulated small area device shows an outstanding operational stability with a T80 lifetime exceeding 4000 h. [ABSTRACT FROM AUTHOR]

Rising atmospheric CO2 concentration ([eCO2]) increases the yield of wheat mainly by increasing grain number, but effects on single grain weight are variable. It is discussed whether single grain growth is limited by the sink or the source size under a non‐stress environment. This study explores the effect of e[CO2] combined with varying N supply on the source and sink size during grain filling. Source size was defined as the amount of stem reserves per grain (SRG) and the proportion of incident radiation intercepted by the green canopy per grain (fIRG) at anthesis. Data from a 2‐year free‐air CO2 enrichment experiment with wheat with three N levels (on average 38 [Nd], 190 [Nad] and 320 kg N ha−1 [Nex]) and two CO2 levels (393 and 600 ppm) on SRG, fIRG and grain filling rate (GFR) and duration (GFD) were evaluated. SRG ranged from 2.5 to 12.9 mg and fIRG from 4.0 × 0.001% to 6.8 x 0.001%. Rising N supply or e[CO2] decreased SRG and fIRG via their increases in grain number. Accordingly, there was a negative linear relationship between grain number and SRG (r2 ≥ 0.84) or fIRG (r2 ≥ 0.97). Increasing N supply decreased GFR, but increased GFD, and GFR was increased by e[CO2] under Nad and Nex. For GFR and final grain weight, there was a strong positive (r2 ≥ 0.85), and for GFD, a strong negative linear relationship (r2 ≥ 0.76) with fIRG under Nad and Nex. Under these N levels, fIRG supplied the largest share (>86%) for grain growth and thus single grain growth was possibly source limited under Nad and Nex. Under high grain number such as under Nex and e[CO2], there might be a risk for low final grain weight due to the low SRG that is insufficient for buffering assimilate shortage under unfavourable environmental conditions in early grain filling. [ABSTRACT FROM AUTHOR]

Cold sintering process (CSP) is a new method to prepare ceramics under quite low temperature. In this work, two‐step CSP under different pressures was employed to prepare (K0.5Na0.5)NbO3 (KNN) ceramics. The density of KNN green pellets can be raised by enhancing the pressure of second‐step CSP. Energy‐dispersive spectroscopy reveals the composition segregation of A‐site cations in large grains. The dissolution rate of K+ in an aqueous medium is faster than Na+, and high pressure can accelerate K+ dissolution, resulting in more Na+ in some grains. Besides, the diffusion rate of Na+ in grains is better than K+, which promote the grains growth. Finally, the piezoelectric property is improved even with low ceramic density due to the larger grains, which possess the higher performance composition. This result demonstrates that the pressure and inhomogeneous dissolution of alkali metal ions among CSP play an important role in grain growth and piezoelectric enhancement. [ABSTRACT FROM AUTHOR]

Doping and forming solid solution is an effective approach to tailor densification and grain growth. In this study, submicron Li2(Ti,Zr)O3 solid solution ceramics was successfully fabricated by a modified solid‐state route for the first time. The use of appropriate starting powders can greatly reduce the synthesis temperature, and the preparation of Li2(Ti,Zr)O3 with submicron grain size is possible. The substitution of Ti by Zr will inhibit the phase transition from cubic to monoclinic structure, as well as the grain growth and pore removal. By doping 10‐at.% Zr, the grain size was significantly decreased from several μm to 400 nm at 900°C, which contributes to a high conductivity eight times that of pure Li2TiO3. Moreover, after being heated at 900°C for 10 days, the grain size of Li2(Ti,Zr)O3 only increases to 5 μm; however, the grains of Li2TiO3 grows up to 16 μm with abnormal grain growth. The compositions of Li2(Ti,Zr)O3 are high uniform with no element segregation, indicating the sluggish grain growth rate is caused by the slow diffusion of Zr rather than segregation‐induced solute drag. By adding excess Li and two‐step sintering, the Li‐rich Li2(Ti,Zr)O3 ceramic pebbles with high crush load of 50–60 N and small grain size of 300–500 nm were successfully fabricated. This work demonstrates a simple method for the synthesis of Li2(Ti,Zr)O3, which makes this material more widely accessible for exploration and also help accelerate its engineering application to be an advanced solid breeder. [ABSTRACT FROM AUTHOR]

Although the addition of other phases into TiB2 matrix to form ceramic composites has been widely used to improve the mechanical properties of monolithic TiB2 ceramics, it is still difficult to greatly enhance the flexural strength and fracture toughness simultaneously. In this work, TiB2–TiC–SiC composites were successfully prepared by reactive spark plasma sintering of Ti3SiC2–B4C–Ti powder mixtures. During the sintering process, TiB2 grains grew into an elongated morphology, endowing the composites with integrated high strength and high toughness. The growth mechanism of TiB2 grains was attributed to the evaporation–condensation kinetics induced by the presence of B2O3. These findings can accelerate the exploration of ceramic composites with excellent comprehensive properties. [ABSTRACT FROM AUTHOR]

The typical feature of hot‐rolled sheets exhibits strong inhomogeneity in microstructure and texture along the thickness direction, and the latter can be named as texture gradient. Herein, the recrystallization and grain growth behaviors of the hot‐rolled sheet in a thin‐gauge nonoriented electrical steel are investigated at different normalization temperatures. The origin and the growth behavior of the main recrystallization texture {114}<481> are revealed. The results show that the texture gradient in hot‐rolled sheets leads to the difference in microstructure and texture between the surface layer and the central layer after normalization. The strong {114}<481> recrystallization texture originates in the orientation transition zone and the vicinity of the grain boundaries of deformed α‐fiber grains in the central layer of hot‐rolled sheets. The volume advantage of {114}<481> grains stem from the favorable growth environment caused by the low‐deformation stored energy, low orientation gradient, higher mobility of grain boundary, and size advantage to their surrounding deformation structure. In the final sheet, {100}<021> texture is the most vital texture, but {114}<481> grains exhibit a large volume fraction. As there are only 4–5 grains on average along the thickness direction of the final sheet, the texture gradient along the thickness direction no longer exists. [ABSTRACT FROM AUTHOR]

The origin of nonuniform microstructure and abnormal grain growth (AGG) was investigated in flash sintered 3 mol% yttria‐stabilized zirconia (3YSZ) ceramics. The microstructural homogeneity decreased with increasing direct current (DC) density and with dwell time in a flash state, eventually resulting in AGG in the specimen core, the first observation of AGG in 3YSZ. Abnormal grains up to 100 μm in size emerged when the DC density was ≥160 mA/mm2, and the specimen's density exceeded 99% of theoretical, starting from the cathode and propagating toward the anode. The results are discussed by comparison with established mechanisms and previous experimental evidence concerning AGG in oxides, focusing on the possible effects of the electrochemical reduction at the cathode end of the specimen. [ABSTRACT FROM AUTHOR]

In the present work, Na0.5Bi0.5TiO3 (NBT) ceramics with the addition of excess Bi in two different ways – before calcination and before sintering – are considered, revealing how the excess Bi affects their microstructure and chemical content. Average grain size is seen to decrease, with the grain size distribution becoming less diffused at higher excess Bi concentrations. The reason for such a feature is the shift of the sintering temperature region where the abnormal grain growth starts to contribute towards higher temperatures. The influence of excess Bi is more pronounced in the case if it is added before calcination. It was discovered that a small amount of excess Bi helps to prevent the formation of Bi‐deficient inclusions. While, high concentrations of excess Bi induce the formation of Bi‐rich inclusions – most probably Na0.5Bi4.5Ti4O15. Possible mechanisms of formation of both types of inclusions are discussed in detail. Instead of Bi over‐stoichiometry, elevated Na content and slightly lower O content were detected in the matrix grains of the sintered NBT ceramics prepared with excess Bi. These deviations increase upon increasing the added excess Bi concentration. The presence of another, Na‐rich phase, is assumed, which could not be detected by X‐ray diffraction or by energy‐dispersive X‐ray analysis. [ABSTRACT FROM AUTHOR]

In this study we explored the densification, microstructure evolution, and high‐temperature properties of bulk lanthanum hexaboride. LaB6 bulks were consolidated using spark‐plasma sintering only in the temperature range between 1400°C and 1700°C. We adopted flash spark plasma sintering (SPS) of LaB6 using a direct current heating without a graphite die. We observed a peculiar grain‐size gradient when coarse grains exceeding 300 μm were observed on the top side of the specimen, while the bottom side had a grain size of 15–20 μm. Such large grain was not observed using SPS at 2000°C, suggesting that these might originate from a local overheating. Based on the three‐point flexural tests, it was observed that the toughness and strength of the LaB6 were acceptable at room‐temperature (3.1 ± 0.2 MPa m1/2, 300 ± 20 MPa). However, at 1600°C, these parameters would decrease to 1.3 ± 0.1 MPa m1/2 and 120 ± 40 MPa, respectively. [ABSTRACT FROM AUTHOR]

Grain growth behaviors of niobium free, 0.021 wt. % niobium microalloyed and 0.024 wt. % niobium‐0.0022 wt. % boron microalloyed 20MnCr5 steels are investigated in this paper. Optical microscopy and field‐emission scanning electron microscopy were used to analyze the prior austenite grain. Transmission electron microscopy and energy dispersive spectroscopy were used to investigate the morphologies and compositions of the precipitates, respectively. The results show that adding niobium reduces the size of prior austenite grain. The addition of boron to niobium microalloyed steel, however, causes grain coarsening. Thermodynamic and kinetic calculations were performed to explain the mechanism of grain growth behavior. The results indicate that the addition of niobium and boron causes the difference in solution temperatures of aluminum nitride and niobium carbonitride precipitates. The addition of boron inhibits the precipitation of niobium carbonitride and aluminum nitride and causes precipitates coarsening. The precipitation observation backs up the calculation results. [ABSTRACT FROM AUTHOR]

Sand erosion is a major factor that shortens the service life of aircraft in desert regions. The anti‐erosion performance of titanium alloys may be improved using TiN coatings. However, few studies have employed Ti sputtering on Ti transition layers to improve the erosion resistance of the coatings. Herein, TiN coatings with a sputtering layer between the Ti transition layer and the TiN layer were deposited on a Ti–6Al–4V alloy by filtered cathodic vacuum arc deposition. For comparison, another group of TiN coatings without a Ti sputtering layer but processed using the same deposition parameters were prepared. The effects of the nanoscale sputtering layer on the microstructure, mechanical properties, and anti‐erosion performance of the coatings were investigated using scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, and nanoindentation. The results show that the introduction of a Ti sputtering layer promoted the growth of TiN grains, leading to the formation of finer and denser TiN columnar grains. The preferred orientation of the coatings with the sputtering layer is (111) plane, whereas that of the coatings without the sputtering layer is (200) plane. The TiN coatings containing a sputtering layer exhibit higher hardness, elastic modulus, and H3/E2 ratio, which enhances the anti‐erosion performance of the coatings. The coatings with a sputtering layer exhibited better erosion resistance (erosion rate reduced by 75%) than the coatings without a sputtering layer. The underlying mechanism to understand the effect of the sputtering layer on the erosion resistance was discussed based on variations in the microstructure and mechanical properties of the coatings with and without the Ti sputtering layer. Ti sputtering layer was finally proved to be an effective method to improve the erosion resistance of TiN coatings. [ABSTRACT FROM AUTHOR]

Zirconium carbide (ZrC) ceramic is one of the most important and promising materials with a high melting point. However, its low diffusion coefficient affects its densification behavior, which dramatically limits its engineering application. For polycrystal ceramic materials, sintering by thermal diffusion is the most widely used method for consolidation. But in view of the difficult densification behavior of ZrC, it is necessary to develop new sintering methods together with another driving force. In the study, we reported an ultra‐high‐pressure sintering strategy to fully densify ZrC ceramic under 1.7 GPa at 1600℃. The sintered sample exhibited high density, fine grain size, excellent mechanical properties, and a large number of crystal defects, including dislocation networks and walls, which were similar to those in deformed metals. Its hardness increased to 2057.44 HV0.1 because of its unique microstructure. [ABSTRACT FROM AUTHOR]

This paper reviews investigations of silicon nitride–silicon carbide micro–nanocomposites from the original work of Niihara, who proposed the concept of structural ceramic nanocomposites, to more recent work on strength and creep resistance of these unique materials. Various different raw materials are described that lead to the formation of nanosized SiC within the Si3N4 grains (intragranular) and at grain boundaries (intergranular). The latter exert a pinning effect on the amorphous grain boundary phases in the silicon nitride and also act as nucleation sites for β‐Si3N4, which limits grain growth during sintering. This finer microstructure results in strengths higher than for the monolithic silicon nitride. Intragranular SiC particles enhance strength and fracture toughness as a result of residual compressive thermal stresses within the nanocomposites. High temperature strength and creep resistance are also much higher than for monolithic silicon nitride and a few investigations of these topics are briefly reviewed and the proposed mechanisms are described. Within the context of other studies cited, work on Si3N4–SiC micro–nanocomposites by the current authors describes an aqueous processing route for better dispersion of commercial powders prior to sintering. [ABSTRACT FROM AUTHOR]

The design of commercial‐grade specialty alumina for improved behavior relies on better understanding the complex interplay between high concentrations of intentional dopants and unintentional impurities, which are unavoidably introduced during industrial‐scale processing. In particular, dopants and impurities govern grain boundary structure/composition, grain growth, and bulk material properties. While the effects of single dopants in ultra‐high‐purity ceramics are well understood, the same concepts have not been adequately extended to commercial materials containing numerous impurities, especially when utilizing the latest atomic‐resolution characterization techniques to evaluate grain boundary structure and composition. Therefore, this work investigated the effects of varying co‐doping levels of MgO, CaO, and SiO2 on grain boundary structure and composition in commercial‐grade alumina by applying aberration‐corrected scanning transmission electron microscopy. First, it was observed that grain growth behavior, specifically grain morphology, in doped specialty aluminas was anomalous to ultra‐high‐purity alumina; in the composition range that was investigated in this study, Ca‐doped specialty alumina exhibited equiaxed grains, whereas Si‐doped specialty alumina exhibited elongated grains. Afterward, it was determined that elemental ratios of bulk doping concentrations differed from grain boundary compositions. Grain boundary compositions, as well as grain boundary structure, were therefore determined to be the dominant metrics to predict grain growth behavior. Overall, co‐doping effects on grain boundary structure and composition are the key parameters to consider to control commercial‐grade ceramics and improve material reliability. [ABSTRACT FROM AUTHOR]

The nepheline‐based transparent glass‐ceramics are promising candidates for cover glass applications in electronic displays owing to their superior mechanical properties (than glasses) and ability to be chemically strengthened. However, our poor understanding about the kinetic and thermodynamic drivers controlling their crystallization processes usually results in their opacification and development of large internal stresses. The present work focuses on the development of nepheline‐based nanocrystalline transparent glass‐ceramic designed in the Na2O–Al2O3–SiO2 ternary system nucleated with P2O5. The temporal evolution of the phosphate and nepheline nanocrystal formation has been followed using X‐ray diffraction, scanning/transmission electron microscopy, and energy‐dispersive spectroscopy. The incorporation of P2O5 in the glass structure leads to the phase separation resulting in the crystallization of nanocrystalline Na3PO4 as an intermediate phase; thus, acting as a nucleating site for volume crystallization of nepheline. The optimization of nucleation and growth profile in the designed composition results in the formation of a transparent glass‐ceramic with high optical transmittance (91.5 ± 0.1%). [ABSTRACT FROM AUTHOR]

Cation doping of Y2O3 is an established approach for tailoring densification and grain growth during sintering. However, the segregation of doped cations to the grain boundary and their impact on processing are still not completely understood. Segregation can be driven by electrostatic effects due to charge mismatch with the host lattice or elastic effects induced by ion size mismatch. While segregation is caused by thermodynamics, it impacts diffusion and the kinetics of grain boundaries during densification and microstructure evolution. In this study, we utilize two isovalent dopants (La3+ and Gd3+), that is we focus on the elastic component of segregation. We investigate the densification as well as the grain growth kinetics of both doped and undoped Y2O3 during field‐assisted sintering/spark plasma sintering (FAST/SPS). While Gd3+ is showing no significant effect on densification, La3+ resulted in a strongly reduced sintering activity. Furthermore, the analysis of the grain growth behavior during sintering and on predensified samples revealed a decrease in the grain growth coefficient, with La3+ having the strongest impact. The structure and chemistry at the grain boundary were observed by aberration‐corrected TEM. While no structural change was caused by doping, the chemical analysis showed a strong segregation of La3+ to the grain boundary, which could not be observed for Gd3+. The results indicate that segregated La3+ causes a drastic decrease in grain boundary migration rates through solute drag as well as much slower sintering kinetics, likely caused by a decrease in the grain boundary self‐diffusion due to segregation. This study further underlines the importance of the elastic contribution to cation segregation and establishes a clear relationship to grain growth and sintering kinetics, which are both decreased by segregation. [ABSTRACT FROM AUTHOR]

Metal‐halide perovskite solar cells (PSCs) have had a transformative impact on the renewable energy landscape since they were first demonstrated just over a decade ago. Outstanding improvements in performance have been demonstrated through structural, compositional, and morphological control of devices, with commercialization now being a reality. Here the authors present an aerosol assisted solvent treatment as a universal method to obtain performance and stability enhancements in PSCs, demonstrating their methodology as a convenient, scalable, and reproducible post‐deposition treatment for PSCs. Their results identify improvements in crystallinity and grain size, accompanied by a narrowing in grain size distribution as the underlying physical changes that drive reductions of electronic and ionic defects. These changes lead to prolonged charge‐carrier lifetimes and ultimately increased device efficiencies. The versatility of the process is demonstrated for PSCs with thick (>1 µm) active layers, large‐areas (>1 cm2) and a variety of device architectures and active layer compositions. This simple post‐deposition process is widely transferable across the field of perovskites, thereby improving the future design principles of these materials to develop large‐area, stable, and efficient PSCs. [ABSTRACT FROM AUTHOR]

Eu‐doped MgAl2O4 has been used to evaluate the kinetics of equilibrium grain boundary transformations, otherwise known as complexion transitions, by monitoring abnormal grain growth induced the nucleation of highly mobile complexions. The assumption in prior works was that abnormal grain growth can be charted using time‐temperature‐transformation (TTT) diagrams to reflect the complexion transition nucleation and growth kinetics. A model depending on doping concentration, grain size, and abnormal area fraction has also been recently developed to estimate excess grain boundary coverage, and thereby predict complexion types depending on microstructural descriptors. While useful, the grain boundary excess model has not been validated using atomic‐resolution characterization. This work aimed to directly validate the grain boundary excess model and complexion TTT diagrams by applying aberration‐corrected electron microscopy to characterize grain boundary structures and compositions (i.e., complexions) in Eu‐doped MgAl2O4. Eu doping concentrations of 100 ppm and 500 ppm were produced and bulk samples were annealed at 1600°C for different times. Forty‐five distinct grain boundaries were characterized. Overall, at least five grain boundary complexion types were identified and the grain boundary excess model was validated. Interpretability of the grain boundary excess model and correlations between grain boundary structures and compositions are discussed. [ABSTRACT FROM AUTHOR]

Relationships between the recrystallized grain size and stress are investigated for experimentally deformed water‐added quartz aggregates. For stresses ≥100 MPa there is a variation in the measured recrystallized grain size for a given stress. This variation correlates with a change in the c‐axis fabric in general shear experiments, where samples with larger recrystallized grain sizes for a given stress have dominantly prism c‐axis fabrics and samples with smaller recrystallized grain sizes for a given stress have dominantly basal c‐axis fabrics. The dislocation creep flow law also changes at conditions where these two c‐axis fabrics form (Tokle et al., 2019, https://doi.org/10.1016/j.epsl.2018.10.017). Using the wattmeter model (Austin & Evans, 2007, https://doi.org/10.1130/G23244A.1), different piezometric relationships are quantified for samples that develop prism and basal c‐axis fabrics, respectively. The wattmeter model is sensitive to grain growth kinetics; a new grain growth law for quartz is formulated based on reanalysis of microstructures in samples from previous work. The activation enthalpies and water fugacity exponents for our grain growth law and dislocation creep flow laws are the same within error, suggesting the recrystallized grain size versus stress relationships are nearly independent of temperature and water fugacity, consistent with laboratory observations. The wattmeters successfully predict the recrystallized grain size versus stress relationships of all quartzite samples from experiments with added water. These results support the use and extrapolation of the wattmeter model for both experimental and geologic conditions to investigate the stress state and grain size evolution of quartz rich rocks. Plain Language Summary: Stress is a fundamental rheological parameter in understanding how rocks deform; however, stress is hard to quantify in rocks that flow rather than fracture at geologic conditions. The relationship between the recrystallized grain size and stress is established through rock deformation experiments where both the stress and grain size can be measured. Therefore, the stress can be estimated in naturally deformed rocks by measuring the recrystallized grain size and applying the experimentally calibrated relationship. Here we show new evidence for different recrystallized grain size versus stress relationships in quartz, and that these different relationships correlate with the crystallographic orientations of grains in samples deformed in the shear geometry. To quantify these new observations, we use the wattmeter model, which models the recrystallized grain size of a deforming aggregate based on the balance between the rates of grain growth and grain size reduction, to predict the quartz recrystallized grain size. This model is sensitive to grain growth kinetics; therefore, we formulated a new grain growth law for quartz. The calibrated wattmeters predict the recrystallized grain size versus stress relationships in experimentally deformed wet quartzite samples. The wattmeter model can therefore be used to estimate stress magnitudes in naturally deformed quartz‐rich rocks. Key Points: Different recrystallized grain size versus stress relationships in samples with prism and basal c‐axis fabricsNew grain growth law for wet quartziteNo temperature or water fugacity dependence on the recrystallized grain size within error, consistent with experimental observations [ABSTRACT FROM AUTHOR]

Tin (Sn)‐based perovskites with favorable optoelectronic properties and ideal bandgaps have emerged as promising alternatives to toxic lead (Pb)‐based perovskites for photovoltaic applications. However, it is challenging to obtain high‐quality Sn‐based perovskite films by solution process. Here, liquid‐exfoliated 2D transition‐metal dichalcogenides (i.e., MoS2, WS2, and WSe2) with smooth and defect‐free surfaces are applied as growth templates for spin‐coated FASnI3 perovskite films, leading to van der Waals epitaxial growth of perovskite grains with a growth orientation along (100). The authors find that WSe2 has better energy alignment with FASnI3 than MoS2 and WS2 and results in a cascade band structure in resultant perovskite solar cells (PSCs), which can facilitate hole extraction and suppress interfacial charge recombination in the devices. The WSe2‐modified PSCs show a power conversion efficiency up to 10.47%, which is among the highest efficiency of FASnI3‐based PSCs. The appealing solution phase epitaxial growth of FASnI3 perovskite on 2D WSe2 flakes is expected to find broad applications in optoelectronic devices. [ABSTRACT FROM AUTHOR]

This study aims to understand the effect of the electrical field on microstructure evolution during field‐assisted sintering or spark plasma sintering (FAST/SPS) of 10 mol% gadolinium‐doped ceria (GDC) with experimental and numerical methods. The novelty of this study has been the observation of enhanced grain growth in the region closer to the anode, even under FAST/SPS conditions with electrical fields less than 5 V/cm. The grain growth kinetics, including determination of activation energy and grain‐boundary mobility, were analyzed along the cross section of the samples for different temperatures and dwell periods. With an increase in distance from the anode, reduction in the activation energy for grain growth and grain‐boundary mobility was observed. These observations attributed to the attraction of oxygen ions to the anode region under an electrical field with an increase in defects along the grain boundaries. Thereby an increase in the grain‐boundary mobility and larger grains in that region were observed. A homogenous microstructure was observed in a case where the current did not flow through the sample. Furthermore, a numerical strategy has also been developed to simulate this behavior in addition to heat generation, heat transfer, and densification using Finite Element Methods (FEM) simulations. The simulation results provided an insight into the presence of a potential difference across the cross section of the samples. The simulation results were also in good agreement with the experimental observations. [ABSTRACT FROM AUTHOR]

The effect of pressure on grain‐growth kinetics of olivine was investigated up to 10 GPa at 1773 K under relatively water‐poor conditions. The results are interpreted using a relation dn−d0n=k0texp(−(Ek∗+PVk∗)/RT) to obtain the activation volume Vk∗ = 5.0 ± 1.1 cm3/mol for n = 2 or Vk∗ = 5.2 ± 1.1 cm3/mol for n = 3. The small activation volume means that grain‐growth kinetics in pure olivine aggregates is fast even in the dry deep upper mantle, implying that grain‐size is controlled by the pinning by other phases or by dynamic recrystallization except for the early stage after the phase transformation from wadsleyite in upwelling materials. The present results are applied to seismic wave attenuation that is likely controlled by grain‐boundary processes. The inferred peak in attenuation just below the oceanic lithosphere‐asthenosphere boundary from the NoMelt array is difficult to be explained by the pressure effects assuming the absorption band behavior because such a model requires a much larger activation volume than determined in this work and it also fails to explain high attenuation in the deep asthenosphere. This suggests that either melt accumulation or other processes such as elastically accommodated grain‐boundary sliding (EAGBS) is responsible for the peak in attenuation. The present results are also applied to EAGBS. We suggest that the deep upper mantle is likely to be relaxed by EAGBS, which implies that the shear velocity of the deep upper mantle can be several percent smaller than that inferred from single crystal elasticity. Plain Language Summary: Processes involving grain‐boundaries play an important role in Earth's interior. They include seismic wave attenuation and resultant reduction in seismic wave velocities. These properties are controlled by thermally activated processes, and hence they are expected to be sensitive to pressure. However, direct experimental studies in seismic attenuation are difficult under high‐pressure conditions. In this study, we performed experiments of grain‐growth in olivine (the dominant mineral of the Earth's upper mantle) at various pressures and applied the experimental data to discuss how the propagation of seismic waves in the Earth's mantle is affected by grain boundary‐related processes. Key Points: The activation volume of grain‐growth in olivine aggregates was experimentally determinedA small pressure effect was observed, indicating that grain‐growth is fast even in the deep upper mantle unless pinning is effectiveThe anelasticity in the deep upper mantle is evaluated under the assumption that it has a similar activation volume as grain‐growth [ABSTRACT FROM AUTHOR]

Nanocrystalline materials with superior properties are of great interest. Much is discussed about obtaining nanograins, but little is known about maintaining grain‐size uniformity that is critical for reliability. An especially intriguing question is whether it is possible to achieve a size distribution narrower than what Hillert theoretically predicted for normal grain growth, a possibility suggested—for growth with a higher growth exponent—by the generalized mean‐field theory of Lifshitz, Slyozov, Wagner (LSW), and Hillert but never realized in practice. Following a rationally designed two‐step sintering route, it has been made possible in bulk materials by taking advantage of the large growth exponent in the intermediate sintering stage to form a uniform microstructure despite residual porosity, and freezing the grain growth thereafter while continuing densification to reach full density. The bulk dense Al2O3 ceramic thus obtained has an average grain size of 34 nm and a size distribution much narrower than Hillert's prediction. Bulk Al2O3 with a grain‐size distribution narrower than the particle‐size distribution of starting powders is also demonstrated, as are highly uniform bulk engineering metals (refractory Mo and W‐Re alloy) and complex functional ceramics (BaTiO3‐based alloys with superior dielectric strength and energy capacity). [ABSTRACT FROM AUTHOR]

Si‐Al‐C‐O materials are well‐known as a good precursor material for obtaining a dense, excellent heat‐resistant SiC polycrystalline materials. Although it has been found that the aluminum contained in Si‐Al‐C‐O materials plays an important role in obtaining dense SiC materials, its actual behavior during heat‐treatment processes has not been investigated. In this study, we investigated the behavior of aluminum contained in the SiC crystals, during the densification of SiC crystals produced from the Si‐Al‐C‐O material. A part of the aluminum contained in the SiC crystals plays an important role in creating a thermodynamically stable grain boundary by the diffusional transportation of aluminum to the SiC grain boundaries at temperatures above 1700°C. Subsequently, most of the aluminum disappeared during the heat treatment at higher temperatures (~1900°C); as a consequence, a dense SiC solid solution with uniformly distributed residual aluminum (0.15wt%) was formed without a secondary phase at the grain boundary. [ABSTRACT FROM AUTHOR]

We performed deformation and grain growth experiments on natural olivine aggregates with olivine water contents (COH = 600 ± 300 H/106 Si) similar to upper mantle olivine, at 1000–1200°C and 1,400 ± 100 MPa confining pressure. Our experiments differ from published grain growth studies in that most were (1) conducted on natural olivine cores rather than hot‐pressed aggregates and (2) dynamically recrystallized prior to or during grain growth. We combine our results with similar experiments performed at 1200–1300°C and fit the data to a grain growth relationship, yielding a growth exponent (p) of 3.2, activation energy (EG) 620 ± 145 kJ mol−1 (570 ± 145 kJ mol−1 when accounting for the role of temperature on water content), activation volume (VG) ~5 × 10−6 m3 mol−1, and rate constant (k0) 1.8 × 103 mp s−1. Our EG is within uncertainty of that predicted for dislocation creep of wet olivine (E* = 480 ± 40 kJ mol−1). Grain size in strain rate‐stepping samples adjusted to the olivine piezometer within 1.3–7.9% strain. The active grain boundary migration processes during deformation and dynamic recrystallization affect the kinetics of postdeformation grain growth, as grain boundary migration driven by strain energy density (ρGBM) may delay the onset of grain growth driven by interfacial energy (γGBM). We compared our postdeformation grain growth rates with data from previously published hydrostatic annealing experiments on synthetic olivine. At geologic timescales, the growth rates are much slower than predicted by the existing wet olivine grain growth law. Key Points: Postdeformation grain growth of moderately wet olivine is slower than predicted by the previously published wet olivine grain growth lawGrain boundary migration driven by gradients in strain energy density may delay the onset of grain growth driven by interfacial energyGrain size reduction by dynamic recrystallization can play an important role in the longevity of strain localization in Earth's lithosphere [ABSTRACT FROM AUTHOR]

Magnesium (Mg) alloys are good candidates for applications with requirement of energy saving, taking advantage of their low density. However, the fewer slip systems of the hexagonal‐close‐packed (hcp) structure restrict ductility of Mg alloys. Here, a hybrid nanostructure concept is presented by combining nano‐dual‐phase metallic glass (NDP‐MG) and gradient nanograin structure in Mg alloys to achieve a higher yield strength (230 MPa, 31% improvement compared with the reference base alloy) and larger ductility (20%, threefold higher than the SMAT‐H sample), which breaks the strength–ductility trade‐off dilemma. This hybrid nanostructure is realized by surface mechanical attrition treatment (SMAT) on the surface of a crystalline Mg alloy, and followed by physical vapor deposition of a Mg‐based NDP‐MG. The higher strength is provided by the nanograin layer generated by SMAT. The larger ductility is a synergistic effect of multiple shear bandings and nanocrystallization of the NDP‐MG, inhibition of crack propagation from the SMATed nanograined structure by the NDP‐MG, and strain‐induced grain growth in the SMATed nanograin layer. This hybrid nanostructure design provides a general route to render brittle alloys stronger and ductile, especially in hcp systems. [ABSTRACT FROM AUTHOR]

High‐frequency ferrite ceramics exhibiting high permeability and low magnetic loss have been recently attracting more and more attention owing to the rapid development of modern communication technologies. It is known that the magnetic performance of ferrites is strongly associated with the morphology of their polycrystalline structure. In this work, we focus on the control of grain growth and densification process for planar Co‐Ti doped M‐type barium hexaferrites, BaCoxTixFe12‐xO19, by one‐step sintering (OSS) and two‐step sintering (TSS) techniques. Experimental results indicate that a uniform and fine‐grained microstructure is achieved by the TSS method. More importantly, the ferrites prepared by TSS demonstrate high permeability (µ' = 18‐20), low magnetic loss (tan δm = 0.1‐0.3), and high Snoek's product (>25 GHz) in the frequency range of 100‐400 MHz. By fitting experimental data, we have determined that low magnetic loss is derived from the small damping coefficient of spin rotation in terms of Kittel's theory. Therefore, the TSS technique provides an effective and efficient approach to prepare planar BaM hexaferrite materials for ultra‐high frequency (UHF) communication devices requiring low loss and high Snoek's product. [ABSTRACT FROM AUTHOR]

In this work, MgO‐ZrO2 and MgO‐6YSZ ceramic fibers were prepared with sol‐gel method via electrospinning. Polymorph stability and microstructure evolution of zirconia fibers were fully characterized by X‐ray diffraction, Raman spectra, X‐ray photoelectron spectroscopy, and Scanning electron microscope. The results indicated that tetragonal zirconia for MgO‐ZrO2 was obtained and cubic zirconia could be fully stabilized for MgO‐6YSZ with MgO molar fractions varying from 0.1 to 0.5 at 800°C. Monoclinic phase appeared with MgO molar fractions even up to 0.5 for MgO‐ZrO2 system and partially or fully stabilized zirconia could be achieved for MgO‐6YSZ at 1000°C and 1200°C. Grain size was gradually decreased with increasing of MgO content at 800°C both for MgO‐ZrO2 and MgO‐6YSZ ceramic fibers. The grain size of both systems increased with MgO molar fractions varying from 0.1 to 0.2 and then decreased at higher contents at 1000°C and 1200°C. A discussion on relationship among MgO state and the phase stability and grain size was presented. This work shows surface excess and solid solution of MgO predominantly controlled the phase stability and microstructure evolution of zirconia fibers. [ABSTRACT FROM AUTHOR]

The environmental benefits of fuel cells and electrolyzers have become increasingly recognized in recent years. Fuel cells and electrolyzers that can operate at intermediate temperatures (300–450 °C) require, in principle, neither the precious metal catalysts that are typically used in polymer‐electrolyte‐membrane systems nor the costly heat‐resistant alloys used in balance‐of‐plant components of high‐temperature solid oxide electrochemical cells. These devices require an electrolyte with high ionic conductivity, typically more than 0.01 S cm−1, and high chemical stability. To date, however, high ionic conductivities have been found in chemically unstable materials such as CsH2PO4, In‐doped SnP2O7, BaH2, and LaH3−2xOx. Here, fast and stable proton conduction in 60‐at% Sc‐doped barium zirconate polycrystal, with a total conductivity of 0.01 S cm−1 at 396 °C for 200 h is demonstrated. Heavy doping of Sc in barium zirconate simultaneously enhances the proton concentration, bulk proton diffusivity, specific grain boundary conductivity, and grain growth. An accelerated stability test under a highly concentrated and humidified CO2 stream using in situ X‐ray diffraction shows that the perovskite phase is stable over 240 h at 400 °C under 0.98 atm of CO2. These results show great promises as an electrolyte in solid‐state electrochemical devices operated at intermediate temperatures. [ABSTRACT FROM AUTHOR]

HfB2, Hf0.95Ta0.05B2, and Hf0.95Ti0.05B2 powders were self‐synthesized and the grain sizes were 2.04, 0.36, and 1.15 μm, respectively. The three powders were pressureless sintered from 1700℃ to 2000℃ to study and compare effects of the introduction of TaB2 and TiB2 on the grain growth behavior and kinetics of HfB2. The results revealed that HfB2 showed moderately slow grain growth in the whole process. However, significant grain growth consistently happened in Hf0.95Ti0.05B2 and Hf0.95Ta0.05B2 at 1900℃ or above. Eventually, the grain size of Hf0.95Ti0.05B2 increased to almost the same as HfB2, but Hf0.95Ta0.05B2 still possessed smaller grains due to the finest original powders. The grain growth exponent was determined to be ~3, and the dominant growth rate‐controlling mechanism was volume diffusion. The average activation energy of HfB2, Hf0.95Ta0.05B2, and Hf0.95Ti0.05B2 for grain growth at 1700℃‐2000℃ was 191 ± 34, 678 ± 73, and 321 ± 61 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]

Al2O3‐C foam filters have been successfully used for steel and aluminum melts filtration. Improving the high‐temperature mechanical properties enhances the capability and application of these filters for the filtration of large steel casting parts along with ingot casting or continuous casting of steel for long periods of time. In this study, adding nano‐Al2O3 and/or nano‐TiO2 to the composition of Al2O3‐C filters was investigated. Uniaxially pressed Al2O3‐C samples containing additives in the range from 0 to 1.0 wt% of nano‐oxides were prepared. The physical and mechanical properties, as well as microstructural analysis, were measured after sintering at 1600°C for 5 hours under a petroleum coke bed. The results showed that the presence of nano‐Al2O3 particles in the composition led to an increase in the cold crushing strength (CCS) due to the formation of the Al3CON phase which acts as a chemical bonding between Al2O3 particles and the carbonaceous matrix. On the other hand, the addition of 1.0 wt% of nano‐TiO2 in the composition caused an increase of 50% in the hot modulus of rupture (HMOR), attributed to the formation of columnar Al2O3 grains in the microstructure. [ABSTRACT FROM AUTHOR]

Herein, a liquid‐assisted grain growth (LGG) mechanism for a vacuum‐processed Cu2ZnSn(S1−xSex)4 (CZTSSe) absorber that is enabled by the presence of a liquid phase containing predominantly Cu, Sn, and Se (L‐CTSe) is suggested to explain the large grain size of up to ≈6 µm obtained at low temperatures, such as 480 °C. In this system, LGG plays a key role in achieving a large grain CZTSSe absorber, but the residual L‐CTSe, a key factor in LGG, deteriorates the device performance. L‐CTSe residue can possibly remain when using metal precursors for reasons such as local composition nonuniformity due to the agglomeration of Cu–Sn alloys, uncontrolled Zn volatilization. It is expected that CZTSSe cells with L‐CTSe residues have lower efficiency and fill factor than CZTSSe cells without L‐CTSe due to the role of L‐CTSe as a shunt path and compositional misfit. These tendencies are observed in the statistical results. Thus, when synthesizing CZTSSe using a metal precursor in a two‐step process, it is considered important to design the precursors to remove unwanted L‐CTSe residues and optimization of composition. [ABSTRACT FROM AUTHOR]

Cold sintering is a low‐temperature powder process methodology that enables the densification of ceramics and ceramic‐based composites at significantly reduced times and temperatures. Although the general notion of required pressure for the cold sintering is in the hundreds MPa, some material systems were reasonably demonstrated to be densified in the pressure below 50 MPa, which allows to increase the sample size up to 25 cm2 using a small tabletop laboratory press. Indeed, the pressure requirement has been a major constraint on promoting its application deployments, but this study is intended to propose a path to alleviate that limitation. Five different ceramic and composite systems (three ZnO‐based composites, Li1.5Al0.5Ge1.5(PO4)3, and zeolite Y) with applications in electronic, structural, and energy storage were investigated as a preliminary example of the size scale‐up process. One of the observed challenges of the scale‐up process was to obtain homogeneous microstructure all over the sample as the transient phase evaporation rate may be different upon the localization. In the case of ZnO, the inhomogeneous pellet translucency may pertain to partial anisotropic grain growth within the same sample. [ABSTRACT FROM AUTHOR]

Preparation of 3YSZ/Al2O3‐platelet composites always requires high temperature, long duration, and/or high pressure. Herein, 3YSZ/Al2O3‐platelet composites are prepared at low temperature of 492°C‐645°C in 30 seconds by flash sintering under the electric field of 300‐800 V/cm. The influence of electric field and current limit on the densification and grain growth of composites is investigated. The onset temperature for flash sintering is determined by electric field, which is decreased with increasing the electric field. Under the constant electric field, the current limit has a great effect on the density and grain size of composite. The flash‐sintered 3YSZ/Al2O3‐platelet composites exhibit relatively high hardness and elastic modulus. Both Joule heating and defects generation are proposed to be responsible for the rapid densification in flash sintering. This work demonstrates the feasibility of employing the flash sintering to prepare ceramic composites with fine grain size. [ABSTRACT FROM AUTHOR]

Nondestructive X‐ray diffraction contrast tomography imaging was used to characterize the microstructure evolution in a polycrystalline bulk strontium titanate specimen. Simultaneous acquisition of diffraction and absorption information allows for the reconstruction of shape and orientation of more than 800 grains in the specimen as well as porosity. Three‐dimensional microstructure reconstructions of two coarsening states of the same specimen are presented alongside a detailed exploration of the crystallographic, topological and morphological characteristics of the evolving microstructure. The overall analysis of the 3D structure shows a clear signature of the grain boundary anisotropy, which can be correlated to surface energy anisotropy: the grain boundary plane distribution function shows an excess of ⟨100⟩‐oriented interfaces with respect to a random structure. The results are discussed in the context of interface property anisotropy effects. [ABSTRACT FROM AUTHOR]

In this study, lightweight alumina containing nanoscale intracrystalline pores was fabricated by introducing zirconia sol and alumina sol into α‐Al2O3 micropowder. The effects of zirconia sol on the sintering behavior of lightweight alumina were also investigated. With the introduction of zirconia sol, the grain growth and phase transformation of nano‐alumina are delayed. Therefore, the structure of the nanoscale pores between nanoparticles can be stabilized during heat treatment, and numerous nanoscale intracrystalline pores with diameters in the range of 50‐300 nm are trapped in the alumina grains. Because of the existence of these nanoscale intracrystalline pores, the thermal conductivity of lightweight alumina decreases significantly. Lightweight alumina with the addition of 0.75 wt% zirconia sol exhibits 31% lower thermal conductivity compared to alumina without zirconia sol. [ABSTRACT FROM AUTHOR]

A novel ZrSi2–MgO system was used as sintering additive for fabricating high thermal conductivity silicon nitride ceramics by gas pressure sintering at 1900°C for 12 hours. By keeping the total amount of additives at 7 mol% and adjusting the amount of ZrSi2 in the range of 0‐7 mol%, the effect of ZrSi2 addition on sintering behaviors and thermal conductivity of silicon nitride were investigated. It was found that binary additives ZrSi2–MgO were effective for the densification of Si3N4 ceramics. XRD observations demonstrated that ZrSi2 reacted with native silica on the Si3N4 surface to generate ZrO2 and β‐Si3N4 grains. TEM and in situ dilatometry confirmed that the as formed ZrO2 collaborated with MgO and Si3N4 to form Si–Zr–Mg–O–N liquid phase promoting the densification of Si3N4. Abnormal grain growth was promoted by in situ generated β‐Si3N4 grains. Consequently, compared to ZrO2‐doped materials, the addition of ZrSi2 led to enlarged grains, extremely thin grain boundary film and high contiguity of Si3N4–Si3N4 grains. Ultimately, the thermal conductivity increased by 34.6% from 84.58 to 113.91 W·(m·K)−1 when ZrO2 was substituted by ZrSi2. [ABSTRACT FROM AUTHOR]