Abstract
Hot-pressing technique is very advantageous to form easily a body with high purity and high density without large grain growth. Thus hot-pressed alumina has outstanding mechanical strength and excellent electrical properties.
Dielectric properties of alumina depend on purity, density, and surface conditions as well as many other factors. Humidity also influences dielectric properties to a large extent.
An effort was made to examine the influence of porosity on dielectric properties of hot-pressed alumina.
Four kinds of alumina powder were used, as shown in Table 1. Hot-pressing of alumina was carried out at 1500°-1900°C and 200kg/cm2 for 10minutes in graphite molds heated by high-frequency induction. Disk specimens prepared were 32mm in diameter and about 3.5mm thick, and of total porosity between 0% and 14%. Dielectric property measurements were carried out using Q-meter, at the frequency range from 5×104 cps to 5×107 cps, in atmospheres with the relative humidity of 0% to 60% at a room temperature.
The relations between the kind of alumina and the dielectric property were not found clearly in hot-pressed alumina specimens with low porosity. The dielectric loss of hot-pressed alumina was mainly determined by its porosity as well as by relative humidity of atmosphere and was dependent on frequency. At lower frequencies the dielectric loss increased with increasing porosity. At higher frequencies the dielectric loss increased independently of porosity. As the porosity increased, the minimum value of the energy loss occured at higher frequencies. Water adsorbed on open pores may be mainly contributing to the dielectric loss. The effect of relative humidity on the dielectric loss depended on open pore fraction. The dielectric constant of hot-pressed alumina was in the range of values 10 to 13. The effect of relative humidity on the dielectric constant was only found in alumina specimens with high porosity. Although the dielectric loss was influenced considerably by humidity, the dielectric constant was hardly affected at all.
