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Zinc oxide densification mechanisms occurring during the cold sintering process (CSP) are examined by investigating specifically the effects of ion concentration in solution, temperature, pressure, and die sealing. The experiments suggest that mass transport through solution is a primary densification mechanism and that either a pre‐loaded solution or grain dissolution can supply migrating ions. Additionally, results indicate cold sintering zinc oxide requires a critical pressure value, above which densification is relatively pressure independent under the majority of process conditions. This critical pressure is related to thermal expansion of the liquid and determines the uniaxial pressure threshold for densification. The data supports a three‐stage interpretation of cold sintering, which includes quick compaction, grain rearrangement, and dissolution‐reprecipitation events. Further, it is observed that under the lowest temperature conditions a net decrease in particle size can occur during the cold sintering process. [ABSTRACT FROM AUTHOR]

Cold sintering of ZnO and MnO with dimethyl sulfoxide (DMSO)-based solutions is demonstrated. For ZnO ceramics, density values approach 99% theoretical when cold-sintered at 180 °C with DMSO-HOAc and DMSO-Zn(OAc)2 solutions. MnO densified with aqueous HOAc solutions produces ceramics of 84% theoretical density that contain significant amounts of Mn(OH)2 secondary phases. In comparison, using DMSO-HOAc solutions produces density values of 94% theoretical at 250 °C with trace quantities of Mn3O4, verified via X-ray diffraction. Scanning electron microscope analysis of sample fracture surfaces containing Mn3O4 reveals numerous crystallites smaller than 100 nm that nucleate on or between the considerably larger starting MnO grains. With increasing temperature, these precipitates appear to coalesce and fill the porosity that remains after initial compaction. These results identify an avenue to cold sinter metal oxides that, in the presence of aqueous media, favor hydroxide formation which inhibits further densification. [ABSTRACT FROM AUTHOR]

The refractory nature of BaTiO3 leads to limited densification and grain growth for films processed at low temperatures and a modest nonlinear dielectric response due to a marked sensitivity to physical scale and material quality. Adding liquid-forming sintering aids, common in bulk ceramics, to thin films enhances mass transport, leading to enhanced grain growth at lower temperatures. This work explores the effectiveness of a sputtered CuO buffer layer with BaO-B2O3 (BBO) fluxes to engineer the microstructure of BaTiO3 films. Grain size and homogeneity increase in the presence of even a ~1 nm CuO layer. In general, grain size increases from 75 to 370 nm with an addition of 2.2% BBO and 8 nm CuO. Room temperature capacitance in fluxed films increases by a factor of 5 over pure films, and ferroelectric phase transitions are clearly observable in dielectric measurements. CuO-BBO proves effective on (0001) Al2O3 and (100) MgO substrates, although all microstructures are notably finer for the latter. [ABSTRACT FROM AUTHOR]

This paper examines the influence of SiO2 doping on densification and microstructure evolution in Nd3xY3-3xAl5O12 (Nd:YAG) ceramics. Nd:YAG powders were doped with 0.035-0.28 wt% SiO2 and vacuum sintered between 1484° and 1750°C. 29Si magic-angle spinning nuclear magnetic resonance showed that Si41 substitutes onto tetrahedrally coordinated Al3+ sites. High-resolution transmission electron microscopy showed no grain boundary second phases for all silica levels in samples sintered at 1600°-1750°C. Coarsening was limited by a solute drag mechanism as suggested by cubic grain growth kinetics and transmission electron microscopy energy-dispersive X-ray spectroscopy observations of increased Nd31 concentration near grain boundaries. Increasing SiO2 content increased both densification and grain growth rate and led to increasingly coarsening-dominated sintering trajectories. Fine-grained (< 3 μm), highly transparent (> 82% real in-line transmission) ceramics were produced by sintering 0.035 wt% SiO2-doped ceramics at 1750°C for 8 h. Coarse-grained (18 μm), transparent samples were obtained with 0.28 wt%SiO2-doped Nd:YAG when sintered at 1600°C for 8 h. [ABSTRACT FROM AUTHOR]

Microstructural control in thin-layer multilayer ceramic capacitors (MLCCs) is one of the present-day challenges for maintaining an increase in capacitive volumetric efficiency. The present paper continues a series of investigations aimed at understanding and controlling the microstructural stability of ultrathin Ni electrodes in MLCCs. Here, a kinetic approach based on the control of sintering profiles is used. Ni–BaTiO3 MLCC chips (0805-type with 300 active layers) are nonisothermally sintered up to 900°–1300°C with different heating rates in the range from 200° to 3000°C/h. In general, the continuity of the Ni electrodes increases with heating rate. However, a strong nonlinear dependence of Ni electrode continuity on sintering temperature is observed. It is concluded that a low-melting interfacial liquid (Ni,Ba,Ti) alloy layer initiates at temperatures between 1000° and 1100°C when the Ni electrodes are under tension. This interfacial liquid phase accelerates a stress-induced diffusion and is the key cause of the severe electrode discontinuities during heating. At higher temperatures (above 1100°C), where compressive stresses are active, the interfacial liquid alloy layer facilitates some recovery of the Ni electrode microstructure. The formation of the interfacial liquid alloy layer can be kinetically controlled using fast-heating rates, which improves the Ni electrode continuity. [ABSTRACT FROM AUTHOR]

A novel approach to pressureless sintering based on the combination of rapid-rate sintering, rate-controlled sintering, and two-step sintering under a controlled atmosphere is proposed. This combined sintering method facilitates control of grain and pore morphology. The application of this sintering approach for pure nanocrystalline barium titanate powder enables the suppression of grain growth during the intermediate and final stages of sintering and the production of fully dense ceramics with 108 nm grain size. The grain growth factor is 3.5, which is three and 17 times smaller than rate-controlled and conventional sintering, respectively. [ABSTRACT FROM AUTHOR]

Color center formation was studied in vacuum sintered Nd3xY3-3xAl5O12 transparent ceramics. The primary color centers were F- and F+-centers as evidenced by optical absorption in the 250-400 nm wavelength range and the presence of an electron spin resonance (ESR) line at g=1.9977. Annealing in air at 1600 °C for 10 h reduced the number of color centers to below the detection limit of ESR. Color center formation is controlled by oxidation and reduction of Fe2+/3+ impurities. [ABSTRACT FROM AUTHOR]

• Novel controls system bridges gap between conventional and flash sintering. • FEA and in-situ flash sintering IR measurements correlated with microstructure. • Densification and grain growth trends follow classic dependency on thermal profile. • Bulk conductivity depends on flash sintering thermal profile, including quench. • Flash sintered samples consistently exhibit higher grain boundary conductivity. In flash sintering experiments, the thermal history of the sample is key to understanding the mechanisms underlying densification rate and final properties. By combining robust temperature measurements with current-ramp-rate control, this study examined the effects of the thermal profile on the flash sintering of yttria-stabilized zirconia, with experiments ranging from a few seconds to several hours. The final density was maximized at slower heating rates, although processes slower than a certain threshold led to grain growth. The amount of grain growth observed was comparable to a similar conventional thermal process. The bulk electrical conductivity correlated with the maximum temperature and cooling rate. The only property that exhibited behavior that could not be attributed to solely the thermal profile was the grain boundary conductivity, which was consistently higher than conventional in flash sintered samples. These results suggest that, during flash sintering, athermal electric field effects are relegated to the grain boundary. [ABSTRACT FROM AUTHOR]