The possibility of the production of reaction bonded aluminium oxide using microwave energy

Abstract

Recently many investigations of the use of microwaves in the area of material processing have been conducted. One of them involves the use of microwave energy to make metal-ceramic composites [1, 2]. Metal is known to reflect microwaves, but when fine metal powders are used, metal-ceramic powder mixtures can be heated due to the surface ohmic current of metal powder. Conventional ceramic sintering usually results in large shrinkage (15-20%), which causes problems such as fracture, and leads to difficulty in predicting the size of final products and in forming complicated shapes. Many processes have been developed to reduce this large shrinkage, one of which is the production of reaction bonded aluminium oxide (RBAO) [3]. Using this process, it is possible to produce low-shrinkage alumina, because the volume expansion of A1 powders during oxidation in A1-A1203 mixture can reduce the shrinkage during sintering. Since the starting powder of the RBAO process is metal-ceramic powder mixture which is suitable for heating in the microwave cavity, we investigated the possibility of making RBAO using microwave energy and the effects of the powder characteristics on the oxidation and sintering behaviour. Microwave processing experiments were carried out in air using a domestic microwave oven (Daewoo KOR111, 2.45 GHz, 650 W). The alumina powder used was reagent grade (99.6 wt% purity, A-12, IKEI Co., Japan) agglomerate, with average particle size of 16/xm, consisting of small alumina grain ( -0 .9 /xm) . The A1 powder was reagent grade (99.9 wt % purity, Junsei Chemical Co. Ltd, Japan) with average particle size of 23/~m and flake-like shape. The AI:A1203 ratio was 35:65 (vol %). After weighing, the A1-AI203 mixture was stirred by magnetic bar, then mixed using a mortar and pestle for 1 h. In order to prevent hydration and oxidation, ethanol was used as the mixing solvent. After drying, the powders were pressed into 4 g cylindrical pellets under 1.5 tonnes in a 10 mm diameter cylindrical die. For the oxidation experiment, a low temperature insulating brick was placed on the bottom of the microwave cavity, with a high temperaute insulating brick placed above. A hole a little larger than the size of the specimen was drilled in the middle of the top surface of the insulating brick, and another insulating brick was placed as a cover. For the sintering experiment an SiC plate was placed above the high temperature insulating brick as a heater,