The Electrical Properties at Low Field of Aluminum - Hydroxyl-Terminated Polybutadiene Composites.

DC and AC electrical conduction properties of composites of hydroxyl-terminated polybutadiene (HTPB), a solid rocket propellant binder, filled with different contents of 13 μm aluminum particles have been investigated in the temperature range of -60^circC to +70 ^circC and relative humidity of 25%. An automated system was developed for dc and ac measurements in a controlled temperature and relative humidity environment. DC studies have centered on I-V-T characteristics and current relaxation for composites having aluminum weight fraction of 0.025, 0.05, 0.10 and 0.20. Resistivities measured in a range of electric fields below 2 times 10^4 V/cm showed conduction is essentially ohmic over the whole temperature range. Current -time measurements have been performed as a function of temperature in a field level of 8.8 times 10^3 V/cm. Using a fast fourier transform technique, a frequency dependent loss factor was calculated from the resorption current in a temperature range of -30^ circC to 0^circC. A thermally-activated relaxation peak, observed at very low frequencies, was interpreted as the interfacial polarization at the crystalline-amorphous interfaces. The degree of crystallinity of HTPB increased with decreasing temperature and content of aluminum filler. Moreover, the volume conductivity ratio of amorphous and crystalline phases, ranging from 4.0 to 75.0 times, increased with decreasing temperature and increasing content of aluminum filler. This crystallization phenomenon of HTPB at low temperatures was confirmed by the x-ray diffraction patterns. In ac conduction, one relaxation, attributed to the glass transition of trans -units of HTPB, was observed at a temperature of about -40^circC and frequency of 10^5 Hz. The dependence of dissipation factor, tan(delta), on temperature and frequency were similar in all samples, regardless of the content of the aluminum. However, the dielectric constant, epsilon^' , depended upon the aluminum content.