March 6-11, 2016
We report the results of flash microwave sintering of various oxide ceramics to full density in a time less than several minutes. Ultra-rapid microwave sintering has been observed on Al2O3, Y2O3, MgAl2O4, and (Yb0.05La0.1Y0.85)2O3 samples compacted from nanosized and submicron powders.
The samples were heated in the applicator of a gyrotron system for microwave processing of materials with a maximum microwave power of up to 6 kW at a frequency of 24 GHz with a feedback computer control over the power . Two different processing modes were used for the heating in the temperature range from T0 = 800 °C to Tmax. One was a computer control over the microwave power ensuring the heating at a fixed rate (50, 100, 150, or 200 °C/min). The other one was heating under a fixed microwave power input to the applicator of about 5 kW, while the heating rate was as high as 5000 °C/min. In both cases, when the preset Tmax was reached, microwave power was switched off and the sample cooled down along with the thermal insulation.
The sintered (Yb0.05La0.1Y0.85)2O3 samples were translucent with density over 0.99 of the theoretical value. The SEM results suggest that the rate of heating strongly affects the microstructure of samples. At the 50 °C/min rate, isolated rounded particles 0.1-0.4 μm in diameter were formed along the boundaries of grains. With an increase in the heating rate, the particles merged together forming layers up to 0.3 μm in thickness which surrounded grains. Due to enhanced mass transport along the quasi-melted boundaries of grains the samples of almost full density were obtained in less than 20 s from the start of fast heating at T0 . The average grain size decreased from approximately 10 to 1 μm with an increase in the heating rate from 50 to 1350 °C/min.
Also, near full density (ρ > 0.98 ρth) samples of Al2O3-ceramics (Tmax = 1550 °С, heating rate 250 °C/min), Y2O3-ceramics (Tmax = 1600–1700 °С, 100–150 °C/min)and MgAl2O4-ceramics (Tmax ≥ 1800 °С, 100 °C/min) were obtained by microwave heating in the regimes with zero hold time.
Similar to the studies of flash sintering in dc / low-frequency ac fields, the effective conductivity of the samples exhibited a sharp increase at the onset of the flash sintering stage (Fig. 1). The microwave power deposited per unit volume of sample, estimated from the energy balance equation, was in the range 10 - 250 W/cm3 as the heating rate varied from 50 to 5000 °C/min. It should be noted that an electric power deposition of this order of magnitude was observed in the experiments on flash sintering of various oxide ceramics under a dc or low-frequency ac electrical current . Possible mechanisms responsible for the flash sintering effect in dc/ac and microwave fields are discussed.
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