The electromagnetic properties of ferrite composites and the dielectric and dynamic mechanical relaxation of liquid crystal polymer composites were examined.
A approach to calculate the effective complex permittivity and permeability, and the reflection loss in a resonant absorber of ferrite composites was proposed using the Bruggeman effective medium approach based on invariant Debye-Hertz potential coefficients. The validity of this approach was tested for MnZn ferrite-silicone rubber, NiZn ferrite-silicone rubber, MnZn ferrite-carbon-silicone rubber and MnZn ferrite-mortar composites with ferrite and carbon powder volume fraction varying in the frequency range between 45 MHz - 10 GHz. For two-phase ferrite composites, the agreement between the calculated results by the effective medium theory and experimental results was excellent, which indicates that the frequency dependence of electromagnetic properties and the reflection loss of the ferrite composites can be predicted. However, for three-phase composites containing conductive particles, the effective medium theory could not be applied quantitatively because of the anomalous behaviour of conductive composites such as percolation. The contour map of the reflection loss in a resonant absorber was suggested with respect to the thicknesss of absorber and frequency, which shows clearly the characteristics of resonant absorber.
Detailed investigation by dielectric and dynamic mechanical spectroscopies over a temperature range from -100℃ to 250℃ reavealed the multiple relaxations in liquid crystalline polyester(LCP) and short glass fibre reinforced polyester composites, which were denoted as $\alpha_e$, $\beta_e$, $\beta_e^\prime$, for dielectric analysis and $\alpha_m$, $beta_m$ for dynamic mechanical analysis in conformity with reducing temperature. The spectroscopic data obtained showed that $\alpha_e$ and $\alpha_m$ relaxation were the glass transitions of samples, $\beta_e$ and $\beta_m$ relaxations were likely to be related to the flexibility of main chain and dipole motion, $\beta_m^\prime$ was expected to be not the ordinary relaxation of glass state but phase transition. The LCP and LCP composites exhibited strong dynamic mechanical anisotropy due to molding flow direction. As the glass fiber content increased, the reinforcing effect was improved due to the increase in the effective stroage modulus, while the modulus anisotropy was reduced.
