To prevent spontaneous oxidation during the high-temperature synthesis of non-oxide ceramics, an inert atmosphere is conventionally required
. This, however, results in high energy demand and high ...production costs. Here, we present a process for the synthesis and consolidation of oxidation-prone materials, the 'molten salt shielded synthesis/sintering' process (MS
), which uses molten salts as a reaction medium and also to protect the ceramic powders from oxidation during high-temperature processing in air. Synthesis temperatures are also reduced, and the final product is a highly pure, fine and loose powder that does not require additional milling steps. MS
has been used for the synthesis of different ternary transition metal compounds (MAX phases, such as Ti
SiC
, Ti
AlN
, MoAlB
), binary carbides (TiC) and for the sintering of titanium. The availability of high-quality powders, combined with energy and cost savings, may remove one of the bottlenecks for the industrial application of these materials.
Advanced ceramic materials with tailored properties are at the core of established and emerging energy technologies. Applications encompass high‐temperature power generation, energy harvesting, and ...electrochemical conversion and storage. New opportunities for material design, the importance of processing and material integration, and the need for long‐term testing under realistic conditions are highlighted in the present perspective.
Advanced ceramic materials are at the core of established and emerging energy technologies: high‐temperature power generation, energy harvesting, and electrochemical conversion and storage.
Field‐assisted sintering technology/Spark plasma sintering is a low voltage, direct current (DC) pulsed current activated, pressure‐assisted sintering, and synthesis technique, which has been widely ...applied for materials processing in the recent years. After a description of its working principles and historical background, mechanical, thermal, electrical effects in FAST/SPS are presented along with the role of atmosphere. A selection of successful materials development including refractory materials, nanocrystalline functional ceramics, graded, and non‐equilibrium materials is then discussed. Finally, technological aspects (advanced tool concepts, temperature measurement, finite element simulations) are covered.
The processing of novel inorganic materials at the lab scale or the rapid manufacturing of industrial products with higher output and reduced energy costs can be achieved by using versatile electric field‐assisted technologies. This review addresses their historical, technical, and scientific development, highlighting successes but also future research needs.
Rare earth silicate environmental barrier coatings (EBCs) are state of the art for protecting SiC ceramic matrix composites (CMCs) against corrosive media. The interaction of four pure rare earth ...silicate EBC materials Yb2SiO5, Yb2Si2O7, Y2SiO5, Y2Si2O7 and three ytterbium silicate mixtures with molten calcium‐magnesium‐aluminosilicate (CMAS) were studied at high temperature (1400°C). The samples were characterized by SEM and XRD in order to evaluate the recession of the different materials after a reaction time of 8 hours. Additionally, the coefficient of thermal expansion (CTE) was determined to evaluate the suitability of Yb silicate mixtures as EBC materials for SiC CMCs. Results show that monosilicates exhibit a lower recession in contact with CMAS than their disilicate counterparts. The recession of the ytterbium silicates is far lower than the recession of the yttrium silicates under CMAS attack. Investigation of the ytterbium silicate mixtures exposes their superior resistance to CMAS, which is even higher than the resistance of the pure monosilicate. Also their decreased CTE suggests they will display better performance than the pure monosilicate.
Al-contaminated Ta-substituted Li7La3Zr2O12 (LLZ:Ta), synthesized via solid-state reaction, and Al-free Ta-substituted Li7La3Zr2O12, fabricated by hot-press sintering (HP-LLZ:Ta), have relative ...densities of 92.7% and 99.0%, respectively. Impedance spectra show the total conductivity of LLZ:Ta to be 0.71 mS cm–1 at 30 °C and that of HP-LLZ:Ta to be 1.18 mS cm–1. The lower total conductivity for LLZ:Ta than HP-LLZ:Ta was attributed to the higher grain boundary resistance and lower relative density of LLZ:Ta, as confirmed by their microstructures. Constant direct current measurements of HP-LLZ:Ta with a current density of 0.5 mA cm–2 suggest that the short circuit formation was neither due to the low relative density of the samples nor the reduction of Li–Al glassy phase at grain boundaries. TEM, EELS, and MAS NMR were used to prove that the short circuit was from Li dendrite formation inside HP-LLZ:Ta, which took place along the grain boundaries. The Li dendrite formation was found to be mostly due to the inhomogeneous contact between LLZ solid electrolyte and Li electrodes. By flatting the surface of the LLZ:Ta pellets and using thin layers of Au buffer to improve the contact between LLZ:Ta and Li electrodes, the interface resistance could be dramatically reduced, which results in short-circuit-free cells when running a current density of 0.5 mA cm–2 through the pellets. Temperature-dependent stepped current density galvanostatic cyclings were also carried out to determine the critical current densities for the short circuit formation. The short circuit that still occurred at higher current density is due to the inhomogeneous dissolution and deposition of metallic Li at the interfaces of Li electrodes and LLZ solid electrolyte when cycling the cell at large current densities.
The ionic conductivity of solid electrolytes is dependent on synthesis and processing conditions, ie, powder properties, shaping parameters, sintering time (ts), and sintering temperature (Ts). In ...this study, Na3Zr2(SiO4)2(PO4) was sintered at 1200 and 1250°C for 0‐10 hours and its microstructure and electrical performance were investigated by means of scanning electron microscopy and impedance spectroscopy. After sintering under all conditions, the sodium super‐ionic conductor‐type structure was formed along with ZrO2 as a secondary phase. The microstructure investigation revealed a bimodal particle size distribution and grain growth at both Ts. The density of samples increased from 60% at 1200°C for 0 hours to 93% at 1250°C for 10 hours. The ionic conductivity of the samples increased with ts due to densification and grain growth, ranging from 0.13 to 0.71 mS/cm, respectively. The corresponding equivalent circuit fitting for the impedance spectra revealed that grain boundary resistance is the prime factor contributing to the changing conductivity after sintering. The activation energy of the bulk conductivity (Ea,bulk) remained almost constant (0.26 eV) whereas the activation energy of the total conductivity (Ea) exhibited a decreasing trend from 0.37 to 0.30 eV for the samples with ts = 0 and 10 hours, respectively—both sintered at 1250°C. In this study, the control of the grain boundaries improved the electrical conductivity by a factor of 6.
In current-rate flash-sintering experiments the current is injected into the specimen from the very start and then increased at a constant rate, while the furnace is held at a constant temperature. ...The power supply remains under current control. The flash is induced at low current densities which reduces local heating at the electrodes. It leads to a uniform grain size across the entire gage length of the dog-bone specimen. This work pertains to 10 mol.% gadolinium-doped ceria flash sintered at current-rates ranging from 50 mA min−1 to 1000 mA min−1 at a furnace temperature of 680 °C. Full densities are obtained at a current density limit of 200 mA mm–2. Densification is shown to depend only on the instantaneous value of the current density, and not on the current-rate. The grain size, however, is shown to become finer at higher current-rates. A preliminary analysis of the “energy deficit”, that is, the estimated power input corresponding to the temperature as measured with a pyrometer, and the actual power consumption, estimates that huge concentrations of Frenkel defects may be introduced in the flash process.
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Abstract
The microstructural evolution during sintering involves the formation and pinch‐off of pore channels. The sintering of 3 particles in 3 dimensions is a simple model for studying the ...pinch‐off of a pore channel. We simulate the solid‐state sintering of 3 particles by using Brakke's Surface Evolver program, which incorporates coupled grain‐boundary diffusion and surface diffusion. The pinch‐off of pore channel divides the sintering process into 2 stages; the initial stage and the later stage. The contact area has a noncircular shape bounded by both surface triple junction and triple junction in the later stage. A general method is presented to determine the sintering force acting on the noncircular contact. The mechanical approach of densification, where the relative motion of particles is driven by both sintering force and applied force, is applicable not only for the initial stage, but also for the later stage.
The development of high-capacity, high-performance all-solid-state batteries requires the specific design and optimization of its components, especially on the positive electrode side. For the first ...time, we were able to produce a completely inorganic mixed positive electrode consisting only of LiCoO2 and Ta-substituted Li7La3Zr2O12 (LLZ:Ta) without the use of additional sintering aids or conducting additives, which has a high theoretical capacity density of 1 mAh/cm2. A true all-solid-state cell composed of a Li metal negative electrode, a LLZ:Ta garnet electrolyte, and a 25 μm thick LLZ:Ta + LiCoO2 mixed positive electrode was manufactured and characterized. The cell shows 81% utilization of theoretical capacity upon discharging at elevated temperatures and rather high discharge rates of 0.1 mA (0.1 C). However, even though the room temperature performance is also among the highest reported so far for similar cells, it still falls far short of the theoretical values. Therefore, a 3D reconstruction of the manufactured mixed positive electrode was used for the first time as input for microstructure-resolved continuum simulations. The simulations are able to reproduce the electrochemical behavior at elevated temperature favorably, however fail completely to predict the performance loss at room temperature. Extensive parameter studies were performed to identify the limiting processes, and as a result, interface phenomena occurring at the cathode active material/solid–electrolyte interface were found to be the most probable cause for the low performance at room temperature. Furthermore, the simulations are used for a sound estimation of the optimization potential that can be realized with this type of cell, which provides important guidelines for future oxide based all-solid-state battery research and fabrication.
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