Recent Developments in Microwave Joining
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MRS BULLETIN/NOVEMBER 1993
few years, work at QuesTech, Inc.10 and Toyota R&D Center11 using single-mode resonant cavities demonstrated that-Al2O3, mullite, and Si3N4 could all be joined to themselves in minutes using microwaves, as compared with hours using conventional techniques. The resulting joints were as strong as the as-received materials and were not detectable by microscopic observation. Homogeneity in the vicinity of the joints was retained with little difference in the microstructure before and after joining. These results clearly indicated that, in small specimens (0.32-0.95 cm diameter rods and tubes), engineering quality joints of both oxide and non-oxide ceramics could be made in very short times using modest microwave power (a few hundred watts), modest compression (a few MPa), and with minimal surface preparation (only that necessary for good contact and alignment). The balance of this article will discuss recent efforts to expand microwave
joining to encompass materials and configurations required for specific industrial applications.
Microwave Joining Techniques A microwave joining system consists of a microwave power source (most often a 2.45 GHz magnetron such as found in home microwave ovens), a waveguide to direct the microwave energy to the samples, and an applicator (typically a single-mode or multimode resonant cavity). Figures la-lc show some of the types of microwave applicators that can be used to join ceramics. Figure la depicts the rectangular singlemode cavity originally used by Badot12 to heat and sinter ceramic oxides. This type of cavity was adapted and used in both the QuesTech and Toyota microwave joining work described previously. The microwaves enter the applicator through an opening (or iris) in the front wall of the cavity. Irises of different sizes and shapes have been used in microwave joining experiments, including an adjustable compound iris that allows a wide variation of the coupling in order to minimize reflected power. The samples to be joined (rods and tubes) are placed in the cavity through holes in either wall, depending on the desired orientation to the electric field. (As illustrated in Figure la, the samples are parallel to the field.) The cavity length is chosen to allow a single mode of the electromagnetic field, with a maximum at the sample position. Since the presence of the samples perturbs the field, an adjustable backwall is provided to allow tuning of the cavity to maintain a reso-
Adjustable Backwall
Figure la. Single-mode cavity applicator for microwave heating of ceramics (After Badot, Reference 12). As shown, specimens are parallel to the electric field.
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Recent Developments in Microwave Joining
sure is lined with a microwave susceptor (typically a thin layer of SiC), which heats initially, then radiates energy to the sample in the joint region only. The result is
Figure, lb. Mitcred-bcnd single-mode cavity applicator provides an enhanced region of high electric field for long specimens passing through the applicator walls.
nant condit
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