Microwave heating of lossy films on metal surface

Finite element calculations have been performed on a T-septum waveguide to investigate the possibility of heating a lossy dielectric film coated on a metal surface using microwaves. The results indicate that additional lossless dielectric loading is required above the septum to provide the necessary electric field to heat the lossy film. A parametric study reveals that significant power deposition can occur in the film provided the permittivity of the dielectric loading is high. Measurements on an applicator have shown that the temperature of a lossy film painted on a metal surface can be increased by more than 40°C. The temperature is related to thickness of the film, septum height, and the position and permittivity of the low loss dielectric loading. It has been also been shown that the necessary electric field in the coated film can further be increased by using a nonlinear distribution of permittivity of the dielectric loading. The power loss in the film is again related to various distributions of permittivity of the loading as well as the height of the T-septum. Finite element method was used to compute the field structure inside the waveguide applicator. Two symmetrical portions are assumed for the applicator and analysis are done by considering one of the half sections formed by introducing a magnetic wall. A novel compact Y-septum waveguide applicator is proposed for the single mode microwave heating of loads of small dimensions (e.g. a printed circuit board or a film) and is compared to a T-septum waveguide applicator of the same dimensions by varying the septum height, septum width, septum thickness and septum angle. Power Frequency Characterisation of different types of paints such as Solar Acrylic, Polyester Resin, Wattyl Instant Estapol and Metal Gloss Polypropylene with three different thicknesses (1.5 mm, 1.0 mm and 0.05 mm) was conducted using two different types of Variable Frequency Microwave (VFM) facilities. Temperature characterisation was also done in two different power outputs of 30W, 60W and 90W and all the above studies were done in both the VW1500 with frequency range of 6.5GHz to 18GHz and Microcure 2100 facilities with frequency range of 2.5GHz to 8GHz VFM facilities. After analysing the diagrams and data collected it was found that all different materials behaved in a different way to parameters like thickness, power input, bandwidth frequency and sweep time. Results also showed that special care should be taken when designing a microwave applicator with variable frequency for heating various materials and VFM facility is a very useful tool to identify these parameters and design an applicator for most favourable conditions so that hot spots and thermal run away can be avoided for specific materials. A simple fibre optic probe has been developed to measure temperature in microwave environment. The basic principle of operation of the probe is discussed together with its structure and construction. Temperature characteristics of the probe are analysed as well as the prediction of its temperature range and characteristics. The construction of a fibre optic thermal switch is presented. A new constant called the 'temperature bandwidth - sensitivity product' of the probe is defined which determines the temperature range and sensitivity of the fibre optic probe. Also stability studies were conducted by thermally cycling the temperature probe as well as the construction of a temperature probe is explained to carry out measurements near liquid nitrogen temperature.