Investigation of pitch–sulphur mixtures used as binder in the preparation of black ceramics

Since tens of years, sulphur is a very classical additive in the composition of black ceramics because, if added in small amounts (i.e., less than 10 wt.% of the binder), it improves their mechanical properties without strongly degrading their graphitability and their conductivity. Macroscopic properties of black ceramics strongly depend on the quality of the interfaces between grains of solid filler (graphite, carbon black) and coke originating from the pyrolysis of the binder; thus, the binder should correctly wet the filler prior to the carbonisation process. Coal-tar pitch usually exhibits a high affinity for carbon grains. In the present work, the effect of the amount of sulphur on the viscosity of the molten binder and on the way it wets the solid filler was investigated. For that purpose, the penetration of mixtures of coal-tar pitch and sulphur inside a bed of carbon particles was studied as a function of temperature. The influence of the sulphur content on composition, optical texture, and transport properties of the derived black ceramics, as well as the way it was mixed with the other constituents, were also examined. It was found that the preparation of black ceramics may be achieved easily and in safe conditions, without evolution of H2S, using pitch–sulphur mixtures first molten in a bladed mixer, and having sulphur contents within the range 10–20 wt.%. The presence of sulphur increases the coke yield but lowers the ability to graphitise, thus limiting the transport properties of the resultant black ceramic. The optimal concentration of sulphur should thus be a compromise between mechanical strength and other physical properties.