The researcher will develop a process control system for the microwave joining of advanced ceramics. Ceramics have the chemical and physical properties necessary for advanced engines and industrial and chemical process systems, but brittleness and the lack of reliable in-process methods to detect critical flaws make manufacturing of ceramic components very difficult. Joining is thus a critical technology impeding the widespread commercial use of ceramics. It has been demonstrated that direct heating of the joint interface with microwave energy produces strong bonds in a fraction of the time necessary with conventional methods. Precise control of the heating process is necessary to exploit this result, because the dielectric loss factor of ceramics increases with temperature, leading to positive feedback and thermal runaway. Control over the joining process will be achieved through development of the following automated feedback loops: (1) control of the power to the (magnetron) microwave source based upon measurements of the surface temperature of the samples being joined, and subsequent calculation of the interior temperature via solution of the thermal diffusion equation; and (2) control of the resonant frequency of the microwave cavity applicator and the coupling of the microwave power to the loaded cavity based upon measurement of the amplitude and the rate of change of the reflected power. The controllers will be based upon linearized models derived from numerical solutions to the coupled electromagnetic and thermal diffusion equations. If inexpensive, reliable methods for joining ceramic can be developed, it may be possible to manufacture ceramic components will tailored properties and then join them together, as is currently done with metals. Such a method may greatly expand the application of ceramics to machine components.
