Some properties of reaction-bonded silicon nitride
Electron Spin Resonance, Pulse Echo Ultrasonics and electrical conductivity measurements have been used to study the effect of the unreacted silicon which is present in Reaction Bonded Silicon Nitride (RBSN) as a result of the manufacturing process. One of the techniques (ESR) has been found to be very sensitive to the unreacted silicon, and the PEG Ultrasonic measurements have suggested that weight gain should not be the sole criterion by which to judge RBSN for mechanical applications. ESR studies of unreacted silicon powder gave a signal, similar to that reported for amorphous silicon with g = 2.0055; the line is attributed to dangling bonds. ESR spectra have been found for both RBSN and Hot Pressed Silicon Nitride (HPSN) with g values closer to the free electron value. Measurements on partially reacted materials showed a complex signal whose shape changed considerably over the temperature range 4 to 300 K. The behaviour of this line, presumed to be the sum of the silicon and RBSN signals is probably attributably to differences in the relaxation rates of the two species. Determination of the elastic constants of the RLSN materials has shown that partially nitrided ceramics have lower strength than fully nitrided materials with similar densities, except in the region were the reaction is nearly complete (weight gain of 59% or more) when the effect of unreacted silicon is negligible, and the major factor governing strength is density. A.C.electrical measurements on high weight gain materials have shown dielectric constant (ɛ’) behaviour analagous to the mechanical strength in that the higher ɛ' has been found for the denser (less porous), but lower weight gain material. In contrast to this, however, the high weight gain material was found to have a lower tanδ this is consistent with the lower levels of silicon in the fully reacted ceramic. D.C. 'step response' measurements at room temperature gave results which fitted Jonscher's two stage relaxation theory. I(_D) (t) α (w(_p)t)(^n) + (w(_p)t)(^k) with n in the region 0.7 to 0.8 and k in the region 1.45 to 1.6. D.C. and A.C. results over the temperature range 100ºC to 900ºC suggested that the predominant conductivity up to 750ºC was hopping either in defect bands or localized states in the band tails.