High Temperature Composite Materials and Dielectromagnetic Composites for Microwave Applications

Abstract

In the part I, we investigated the microstructures, mechanical properties, and oxidation behavior of hot pressed BN in the presence of sintering additives Al2O3, Y2O3 and SiO2. BN platelets size in the sintered samples grew from ~5 to ~30 times for the use of all three oxides, and the use of Al2O3 and Y2O3, correspondingly. The excessive growth of BN platelets in samples containing Al2O3 and Y2O¬3 caused them to misalign which, in turn, resulted in its low relative density (92.0%). The use of SiO2 mitigated this grain growth so that BN platelets aligned better to gain a higher relative density (99.5%). Flexural strength and elastic modulus of BN were proportional to their densities.

Oxidation experiments conducted at 1200oC in flowing dry air showed borate glass droplets were formed on all of oxidized BN samples. The addition of SiO2 resulted in the formation of a glass layer before the appearance of these glass droplets. The presence of glass droplets was a result of the poor wetting of liquid B2O3 on BN and the dominance of the formation of B2O3 to its evaporation. Their size evolution described the “breadth figure” theory, similar to the formation of water droplets on a flat surface from the saturated water vapor air.

Substructures observed inside the glass droplets contained high and consistent Al:Y atomic ratio (5:7) in all samples. The evaporation of B2O3 isolated Al2O3, Y2O3 in the form of immiscible liquid phase to borate.

In the part II, we investigated the formulation of equivalent permittivity and permeability with isotropic and anisotropic Co2Z-polymer composition. These two properties of isotropic Co2Z-LDPE/Co2Z-Silicone composites increased with Co2Z composition. However, their permittivity was always higher than that of their permeability.

Permittivity and permeability of anisotropic Co2Z-Silicone composites were split into high and low values along the parallel and perpendicular directions to the alignment direction of Co2Z particles. The separation at 20 vol% Co2Z was strongest, attained 46% anisotropy so that its parallel permeability approached closer to that of its perpendicular permittivity. However, the low permittivity of Co2Z required a higher electric field to increase its particles’ alignment.