| Microwave processing of advanced materials has been studied as an attractive alternative to conventional processing. In this dissertation, work was performed in two sections. The first section was a process study and focused on applying microwave theories to develop an integrated microwave adhesive bonding system in a single mode applicator with non-invasive, on-line monitoring and precise controlling features. The second section was a fundamentals study and focused on investigating microwave heating mechanism to provide explanations for rapid bonding using microwaves.; A single mode microwave heating method was first applied in adhesive bonding process. The microwave heating mode was chosen based on theoretical computation and experimental characterization so that the adhesive was placed at the strongest electric field. Rapid and selective heating of the adhesive was observed. However, only limited material size could be uniformly heated with single mode microwave method because of the non-uniform electric field distribution. To solve this problem, a variable frequency mode switching method (switch between modes with complementary heating patterns) was used to improve the bonding uniformity of large-size materials. Theoretical heating modes were applied in this process. A control system was developed to provide rapid, stable, and uniform bonding by adjusting microwave power and frequency. The control software was programmed with LabVIEW. To determine the bonding cycle on-line, a new method (with corresponding software) was invented based on microwave theories and experimental discoveries. Compared with thermal adhesive bonding, the microwave method reduced the bonding cycle vastly and obtained equal or even higher bonding strength for the materials used in this research.; The reduction in bonding cycle resulted from faster curing of the epoxy adhesive with microwaves. The phenomenon of microwave fast curing of epoxy was investigated with a new method by studying the effect of carbon additive on microwave curing of epoxy. It was hypothesized that during microwave curing, carbon absorbed most of the microwaves and weakened the localized superheating of the epoxy functional groups. If localized superheating were the main mechanism of rate enhancement in microwave curing of epoxy, then the curing rate should decrease at the presence of carbon. Microwave curing experiments were carried out at three different temperatures with various carbon concentrations. New correlations for reaction rate constants as functions of carbon concentration and temperature were proposed. Results showed microwave curing rate decreased with increasing carbon concentration. Thus, this study suggested reaction rate enhancement in microwave curing of epoxy result from localized superheating of the functional groups. However, the carbon particles used in this study were activated and might adsorb the amine from the epoxy resin because carbon particles might have higher temperature than the bulk resin in microwave curing. This will result in decrease in microwave curing rate. Further study is required to elucidate this problem. In addition, at higher carbon concentration, the required microwave power was lower to maintain the same bulk temperature. This might also result in decrease in microwave curing rate at higher carbon concentration. |
