D. C. electrical properties of hot-pressed nitrogen ceramics

The d.c. electrical properties of some hot-pressed polycrystalline nitrogen ceramics have been measured between 18 and 500° C in applied electric fields up to 1.1×10⁴ Vcm^–1. The materials examined were Si₃N₄, 5wt%, MgO/Si₃N₄ and two sialons havingz=3.2 andz=4.0. The conduction in all the materials showed similar general features. The time dependent charging (I _c) and discharging currents (I _D) were observed which followed a I(t)t_–n law at room temperature withn=0.7 to 0.8. The exponentn forI _c decreased with increasing temperature. The current density-field (J _s-E) characteristics were ohmic in applied fields of less than 3×10³ Vcm^–1; conductivity increased with electric field above that range. Above about 280° C, a was independent ofE, its temperature dependence following log T ^–1. Below about 230° C conductivity fitted a exp (–B/T _1/4) law in both low and high fields. There is a good correlation between the temperature and field variations of time dependent current and the steady current. The conductivities were in the range of 10^–15 to 10^–16–1 cm^–1 at 18° C and rose to 4×10^–10 to 2×10^{–12 –1} cm^–1 at 500° C. The activation energies were in the range of 1.45 to 1.80 eV and 0.05 to 0.15 eV at above 300° C and near room temperature respectively. Various models to explain the data are considered.