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JOINING CERAMICS USING MICROWAVE ENERGY IFTIKHAR AHMAD and RICHARD SILBERGLITF Technology Assessment & Transfer, Inc., 133 Defense Highway, Suite 212, Annapolis, MD 21401. ABSTRACT In the past several years there has been an explosive growth in the use of microwave energy for the processing of a host of materials. Microwave energy provides rapid internal heating which results in an overall reduction in the processing time. The important features of microwave processing are described, as well as several applications. Microwave energy has been used by a few groups for the joining of alumina, mullite, silicon nitride and silicon carbide. The work performed by these groups will be reviewed. Typically, a single mode microwave applicator has been used to join ceramics at temperatures ranging between 1250'C - 1800'C. Microwave joining of ceramics was achieved in a matter of minutes, in contrast to hours reported by conventional methods. The strength of the joints was equal to or greater than the as-received materials. Joining of specimens of sintered silicon carbide (Hexoloy Tm ) using interlayers, and direct joining of reaction bonded silicon carbide (RBSC) to itself and HexoloyTM has been accomplished recently. Both single mode and multimode microwave applicators were used and larger specimens of RBSC having complex shapes were joined using hybrid heating. The paper describes microwave joining apparatus, techniques and results.

INTRODUCTION The use of microwave energy for industrial applications is not new. In the past microwave applications have been mainly confined to communications. Other uses of microwave power include medical and biological applications, as well as heating. Heating applications exist in the consumer, commercial, industrial and scientific areas. The most common application is heating food in consumer microwave ovens. A comprehensive historical review of microwave heating is given by Osepchuk [1]. There is little evidence of RF heating, and even less of microwave heating, before the Second World War. However, the patent literature includes some references to using microwave energy to affect materials properties for industrial purposes. In particular, Kassner (having patents on a spark-gap microwave generator) believed that it is possible to achieve useful changes in molecular state, and hence the chemistry of materials, without heating [1]. During the Second World War, there were efforts to measure dielectric properties of various materials. This work was done as a necessary task in the development of the radar, telephone and communication systems. The work begun under Von Hippel at the MIT Laboratory for Insulation Research and the relevant lectures given by experts in this area appeared as a book "Dielectric Materials and Applications" [2]. This work provides the foundation for all future efforts in radio frequency and microwave heating.

Industrial Microwave Heating Applications Microwave processing, which until recently dealt principally with food processing [3], is now expanding drastically to