Electrical conductivity in hot-pressed nitrogen ceramics

Abstract

The a.c. and d.c. electrical conductivities of some hot-pressed polycrystalline nitrogen ceramics have been measured between 400 and 1000° C. The materials examined were Si₃N₄, 5.0% MgO/Si₃N₄ and two sialons, Si_(6−z) · Al _z · O _z · N_(8−z) having z ≈ 3.2 and z ≈ 4.0 respectively. The electrical behaviour of all the materials showed similar general features. The d.c. conductivities were about 10⁻¹⁰ Ω⁻¹ cm⁻¹ at 400° C and rose to between 10⁻⁶ and 10⁻⁵ Ω⁻¹ cm⁻¹ at 1000° C. The a.c. Data, taken over the frequency range 15 Hz to 5 kHz showed that below about 500° C the a.c. conductivity (σ _a.c.) varied with frequency as σ _a.c. ∝ ω ^s where 0.7 <s <1 indicative of a hopping process; in this temperature range the d.c. conductivity (σ _d.c.) agreed well with the relation σ _d.c. = A exp(−B/T ^1/4). Above 700° C both the a.c. and d.c. conductivities followed log σ ∝ T ⁻¹. Hall effect and thermoelectric power measurements enabled the Hall mobility to be estimated as less than 10⁻⁴ cm² V⁻¹ sec⁻¹ at 400° C and showed that the materials were all p-type below 900° C and n-type above 900° C. The electrical properties of all four materials are consistent with the presence of a glassy phase.