is considered as one of the major candidates for various applications that require a combination of properties including high optical / IR transparency and excellent mechanical performance, particularly in harsh environments. Among many different techniques to produce translucent or transparent spinel, the two-stage method based on the pressureless or hot press sintering with subsequent hot isostatic pressing (HIP) proved itself the most successful. But high pressure (≥150 MPa) and high

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... ure (≥1600 °C) commonly used in HIP greatly limit the potential of its use. Recently, a significant attention has been attracted to the investigation of very rapid ("flash") sintering of ceramics achieved by applying a dc or ac voltage to the sample [1] . In these experiments, many (mostly oxide) ceramics have been sintered nearly to full density in several seconds. In particular, in [2] the MgAl 2 O 4 spinel was flash sintered to a relative density of 97.9 % at the applied DC field of 1000 V/cm. This paper reports on the fast microwave sintering of MgAl 2 O 4 ceramics to nearly full density under high-rate heating and zero isothermal hold time. Magnesium aluminate spinel powder was produced from isopropoxide MgAl 2 (OPr i ) 8 by method described in detail elsewhere [3] . Specific surface area of powder was about 100 m 2 /g after calcining in air at T = 900 °C. Samples with 15 mm dia and 2.5 mm thickness were uniaxially pressed at 300 MPa to a relative density of 39 % of the theoretical value. Some samples were impregnated with yttrium oxide which was introduced by dipping them into 0.5 mol/l water solution of Y(NO 3 ) 3 with consequent heating in air at 70 °C. The content of Y 2 O 3 in these samples was equal to 0.8-1.0 % wt. Samples were heated in the applicator of a gyrotron system with a microwave power up to 6 kW at a frequency of 24 GHz with a feedback computer control of the power. The details of the experimental scheme can be found in [4] . Samples were heated at 2-4 MPa pressure of the residual air at a rate of 50 °C/min up to 1250 °C and then at rates of 15...200 °C/min to the preset maximum temperature of sintering, T max , in the range of 1400-1780 °C. When the preset T max was reached, the microwave power was switched off automatically and a sample cooled down along with the thermal insulation. Shown in Fig. 1 are the relative densities of the pristine and Y-doped samples heated at a rate of 100 °C/min as the function of the temperature T max . The relative density of the Y-doped samples exceeded 99 % at all rates of heating to T max ≥ 1500 °C, whereas it was about 98 % for the pristine samples at T max ≥ 1600 °C. The relative densities of doped and pristine samples sintered conventionally in the similar regimes at T max = 1500 °C were only 92.2 and 89.1%, respectively.