Abstract
In the present paper the effect of moisture on the dielectric behaviour of hot-pressed AlN ceramic having different volume percentages of porosity is reported. The annealed samples do not show any dielectric dispersion at room temperature in the audiofrequency range. However, a strong dispersion is observed after the samples have been exposed to moisture (relative humidity, 60%–95%); the effect is greater in the sample having the higher porosity. A quite different behaviour of moisture absorption is observed at low porosities (4 vol.% or less) from that at high porosities (9 and 15 vol.%); this is explained in terms of closed and open porosity in AlN samples. The recovery studies show that the low porosity samples return to the original state simply by evacuating the sample holder at room temperature. However, in the case of the higher porosity samples, water may be retained inside the sample even up to 600 K. The highly dense sample (porosity, 0.2 vol.%) did not show any appreciable effect of moisture; this indicates that highly dense samples of AlN can be used for insulation without moisture having any appreciable effect.
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