Some electrical properties of lithium sialons

Abstract

Sialons are a new range of high strength, refractory materials which can be prepared by hot-pressing mixtures of silicon nitride (Si(_3)N(_4)) and alumina (Al(_2)O(_3)) at temperatures of about 17000C. The resulting ceramics have a general formula Si(_6-z),Al(_z)0(_z)N(_8-z) where z may vary from 3 to 4. Their crystal structure is similar to that of Si(_3)N(_4) but N(^3-) is replaced by O(^2-) and, to effect charge compensation, Si(^4+) is replaced by Al(^3+). Charge compensation can also be achieved by introducing other metal ions such as Li(^+), Na(^+), Mg(^2+)and Y(^3+). This leads to an even greater possible range of ceramics. The present work is concerned with one group of these, the lithium sialons. The small amount of previous work on the electrical properties of lithium sialons was confined to preliminary measurements of d.c. conductivity. Here the electrical behaviour of a range of lithium sialons has been studied and then compared with that of pure sialons and an yttrium sialon glass in order to determine the conductivity, mechanism operating ivithin the lithium sialon samples. The orders of magnitude of conductivity encountered lay.between 10(_7)(ohm, cm)(^-1) and 10(^-3)(ohm cm)(^-1). Techniques have been developed for the measurement of d.c. conductivity (σ(_dc)) from room temperature to 800 C and of a.c. conductivity (σ(_ac)) from 20 Hz to 25 kHz over the same temperature range. The measurements also enabled dielectric constant and losses to be determined at the some temperatures and frequencies. The results showed the presence of two conductivity regimes, one above and one below about 400 . It has also been observed that (σ(_dc)) decreases as a result of passing current through the specimen and a preliminary examination of this apparent solid state electrolysis has been made. All the results are discussed in comparison with the available data on pure sialons and some other nitrogen ceramics.