The preparation and properties of pyrolytic graphite

Abstract

The use of the Ettingshausen effect as an alternative to thermoelectric effects for the production of useful cooling at low temperatures is discussed, and the reasons leading to the investigation of pyrolytic graphite as a possible material for Ettingshausen cooling are given. The Brown and Watt method for the deposition of pyrolytic graphite was successfully developed to enable good quality deposits with consistent properties to be obtained using surface deposition temperatures of 2200 C and propane as the hydrocarbon gas. The work showed the importance of the temperature gradient developed across the deposit in this method of growth. The temperature profile was estimated and the properties of the material showed good agreement with the values obtained in other workers' annealing studies. One particular bar of pyrolytic graphite was cleaved into a number of sections and its properties studied as a function of the effective formation temperature. Measurements were made of the a-direction thermal conductivity in the temperature range 70 to 300 K and of the crystallite preferred orientation. The thermal conductivity results were used to estimate the crystallite sizes and the values obtained agreed well with those obtained from the saturation of the mobility. The crystallite size was also measured directly by x-ray line broadening. A variance method was used to separate the strain component of the broadening. X-ray determinations of the crystallite size have always in the past yielded low values, but the. use of the proper correction for stacking faults gave good agreement with the in direct estimates. A thermal activation energy of 5-9 eV was obtained for crystallite growth. The graphitisation process is discussed and it is suggested that better quality material than any produced so far could be obtained by annealing highly oriented soot-free deposits at high temperatures. An upper limit of 100 μm is set for the crystallite size of such material produced at 3500 C. It is concluded that in the absence of phonon drag effects graphite is unlikely to be a suitable material for Ettingshausen cooling.