Electrical conductivity measurements as a microprobe for structure transitions in polysiloxane derived Si-O-C ceramics

Polysiloxanes [RSiO1.5](n) with R=CH3 (PMS) and C6H5 (PPS), respectively, were transformed to Si-O-C ceramics of variable composition and structure upon pyrolysis in inert atmosphere at 800-1500 degrees C. The electrical conductivities of the Si-O-C ceramics in air were measured at room temperature by using a shielded two point configuration. In situ measurements of the dc-conductivity during the pyrolytic conversion from the polymer to the ceramic phase were carried out up to 1500 degrees C with four point contacted carbon electrodes in inert atmosphere. During polymer-ceramic conversion excess carbon precipitates above 400 degrees C (PPS)-700 degrees C (PMS). At temperatures above 800 degrees C (PPS) and 1400 degrees C (PMS) coagulation and growth of the carbon clusters results in a percolation network formation. While below the percolation threshold electrical conductivity can be described according to Motts mechanism by variable-range-hopping of localized charge carriers, regular electron band conduction due to the instrinsic conductivity of turbostratic carbon (8x10(-4) (Omega cm)(-1)) predominates above. Thus, the in situ measurement of non-linear electrical property changes can be used as a microprobe of high sensivity to detect microstructural transformations during the pyrolysis of preceramic polymers. (C) 2000 Elsevier Science Ltd. All rights reserved.