Electrical properties of hot-pressed nitrogen ceramics

Abstract

Some electrical properties of hot-pressed Si(_3)N(_4), 5 W/o MgO/Si(_3)N(_4) and two sialons, Si(_6-z). Al(_z).O(_z).N(_8-z) having Z = 3.2 and Z = 4.0 have been measured, using a variety of techniques, between 18 c and l000 C. The electrical behaviour of all the materials showed similar general features. The d.c. conductivities were in the range of 10(^-15)-10(^-16)Ω(^-1)cm(^-1) at 18 C and rose to 10(^-6)-10(^-5)Ω(^-1)cm(^-1) at 1000 C. The current density-field (J3-E) characteristics were ohmic in applied fields of less than 3 x 10(^3) volt cm(^-1), conductivity increased with electric field above that range. Above about 280 C, o(_dc) was independent of E, its temperature dependence following log o(_dc) aT(^-1). Below about 230 c conductivity fitted a o α exp ((^-B/)T(^1/4)) law in both low and high fields. The activation energies were in the range of 1.45-1.80 eV and 0.05-0.15 eV at above 300 C and near room temperatures respectively. Time dependent charging (I(_C)) and discharging (I(_D) currents were observed which followed a I(t)α t(^-n) law with n = 0.7-0.8 at room temperature. The exponent n for I(_C) decreased with increasing temperature. Hall effect and thermoelectric power measurements enabled the Hall mobility to be estimated as less than 10(^-4) cm(^2)v(^-1)sec(^-1) at above 400 C and showed that the materials were all p-type between 400 and 900 C and n-type above 900 C. The drift mobility obtained from observations of transit time effects was 1 x 10(^-8)-5 x 10(^-9) cm(^2)v(^-1)sec(^-1). Various hopping models to explain the data are considered. At room temperature the variation of conductivity (o’ (w) α w(^n) with frequency over the range 200 Hz to 9.3 GHz followed the o’(w) α w(^n) law with n = 0.9. The dielectric constant (Σ') and loss (tano) both fell slightly over this frequency range, the average values at 10(^5) Hz being about 9.5 and 5 X 10(^-3) respectively. At temperatures up to 500 C the data fits well with the 'Universal dielectric law' Σ"(w) α w(^n-1) and approximately fits the Kramers-Kronig relation Σ"(w)/(_Σ’(w) – Σ). = cot ((^nπ)/(_2)) with 0.5 < n < 1. The exponent n decreases with increasing temperature. The effects may be caused by either non-Debye dipolar or hopping charge phenomena. Similar studies of electrical properties for 30 m/o Li-sialon and 14.3 m/o Y-sialon were also made in an attempt to relate the electrical properties and compositions of pure and doped sialons.