Microstructure of the microwave fast-sintered MgAl2O4 ceramics
A.A. Sorokin, S.V. Egorov, Yu.V. Bykov, A.G. Eremeev, V.V. Kholoptsev, K.I. Rybakov, S.S. Balabanov, A.V. Belyaev, A.G. Litvak
EPJ Web of Conferences
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
... 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  . In these experiments, many (mostly oxide) ceramics have been sintered nearly to full density in several seconds. In particular, in  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  . 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  . 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.