| Single layer composites comprised of ceramic powders--{dollar}rm Pb(Mgsb{lcub}1/3{rcub}Nbsb{lcub}2/3{rcub})Osb3 (PMN), 0.67Pb(Mgsb{lcub}1/3{rcub}Nbsb{lcub}2/3{rcub})Osb3{dollar}-0.33PbTiO{dollar}sb3{dollar} (PMN-PT), {dollar}rm Fesb3Osb4,{dollar} silver coated glass spheres, nickel coated graphites--in a polymer-epoxy, Polyethylene (PE)-matrix were evaluated for their dielectric properties {dollar}(varepsilonspprime, varepsilonsp{lcub}primeprime{rcub}){dollar} in the X-Band (8-12 GHz) frequency regime by use of rectangular waveguides and an HP 8510 A two-port network analyzer. From the data obtained for normalized single layer composites it was observed that absorbance and reflectance increased and that transmittance decreased with volume loading (0-50 v/o). In order to minimize the unwanted reflectances and obtain a wider band absorbance in the microwave frequencies, multi-layer composites were proposed and evaluated.; A model was developed which described the interactions between transverse electromagnetic (TEM) waves and isotropic single and multi-layer composites. Good agreements were found between the experimental data and numerical results.; A comparison between single and multi-layer composites showed that the multi-layer yielded a low reflectance and transmittance over a broad band of frequencies; however, a single layer could be designed to have a far superior absorbance with minimal reflectance, and transmittance only for a narrow band of frequencies. Difficulty existed in matching layers in a multi-layer structure while attempting to keep the thickness to a minimum. The equivalent single layer of a multi-layer composite mimics the behavior predicted and measured for the multi-layer, but could not easily exist in a composite body with the properties measured and evaluated in this thesis.; A separate examination was also conducted on metal backed composite systems to obtain a non-reflecting condition, and a figure of merit. The figure of merit was determined to be a function of dielectric constant and loss, frequency, and thickness desired. It was found that the higher the dielectric constant and loss the thinner the thickness had to be to create a nonreflectance condition. The nonreflectance bandwidth was increased by introducing a series of low dielectric layers in front of the ideal nonreflectant system. Finally, an examination of the reduction in the size of the thickness of the ideal system showed that slight thickness increases or decreases shifted the resonance of nonreflectance condition down the frequency band. (Abstract shortened by UMI.)... |
