On the Mechanism of Microwave Flash Sintering of Ceramics

Abstract

The results of a study of ultra-rapid (flash) sintering of oxide ceramic materials under microwave heating with high absorbed power per unit volume of material (10–500 W/cm3) are presented. Ceramic samples of various compositions—Al2O3; Y2O3; MgAl2O4; and Yb(LaO)2O3—were sintered using a 24 GHz gyrotron system to a density above 0.98–0.99 of the theoretical value in 0.5–5 min without isothermal hold. An analysis of the experimental data (microwave power; heating and cooling rates) along with microstructure characterization provided an insight into the mechanism of flash sintering. Flash sintering occurs when the processing conditions—including the temperature of the sample; the properties of thermal insulation; and the intensity of microwave radiation—facilitate the development of thermal runaway due to an Arrhenius-type dependency of the material’s effective conductivity on temperature. The proper control over the thermal runaway effect is provided by fast regulation of the microwave power. The elevated concentration of defects and impurities in the boundary regions of the grains leads to localized preferential absorption of microwave radiation and results in grain boundary softening/pre-melting. The rapid densification of the granular medium with a reduced viscosity of the grain boundary phase occurs via rotation and sliding of the grains which accommodate their shape due to fast diffusion mass transport through the (quasi-)liquid phase. The same mechanism based on a thermal runaway under volumetric heating can be relevant for the effect of flash sintering of various oxide ceramics under a dc/ac voltage applied to the sample.

Effect of Shear Stress on Isothermal Crystallization Behavior of CaO-Al2O3-SiO2-Na2O-CaF2 Slags

“…The larger the activation energy of crystallization, the more difficult it is for the crystal to precipitate. The Arrhenius formula [33,34] is used to calculate and analyze the crystallization activation energy of the mold fluxes at different agitation strengths (Equation (6)):$\mathrm{normalk}=A\exp false(-Ea/RTfalse)$ where Ea is the activation energy of crystallization, A is the pre-exponential factor, R is the gas constant, and T is the absolute temperature (notice the unit is K).…”

Fabrication of Scandia-Stabilized Zirconia Thin Films by Instant Flash Light Irradiation

“…Recently, doped ceria-based electrolyte materials, such as gadolinia-, yttria-, and scandia-doped ceria, have been investigated as oxide ion-conducting electrolytes because of their high ionic conductivity 3 superheated plasma gas. However, the plasma gas may be heated to a much higher temperature than required, depending on furnace geometry, plasma input power, and load density [20][21][22][23][24].…”

“…An arc plasma sintering process can sinter material in a few minutes using a superheated plasma gas. However, the plasma gas may be heated to a much higher temperature than required, depending on furnace geometry, plasma input power, and load density [20][21][22][23][24].In this study, we adopted a novel sintering technique with high-power flash light irradiation of a visible wavelength range from 380 to 980 nm. This innovative method considerably reduced the post-heat treatment process time from hours to seconds.…”

Microstructure of the microwave fast-sintered MgAl2O4 ceramics

“…in the effective high-frequency conductivity of the material of the sample, which is associated with the development of the thermal instability. The onset temperature of the instability decreases with an increase in the microwave power deposited per unit volume of sample.The mechanism of fast densification caused by the development of thermal instability has been suggested in [4,5]. In brief, if the temperature and the density of the deposited power are high enough, the particle surface premelts at a temperature well below the melting point of the bulk of grains due to the abundance of impurities and defects in the near-surface layer.…”

“…The mechanism of fast densification caused by the development of thermal instability has been suggested in [4,5]. In brief, if the temperature and the density of the deposited power are high enough, the particle surface premelts at a temperature well below the melting point of the bulk of grains due to the abundance of impurities and defects in the near-surface layer.…”