Pressure and temperature dependences of the elastic constants of compression-annealed pyrolytic graphite

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Abstract
Changes of ultrasonic transit times in compression‐annealed pyrolytic graphite (CAPG) were measured as a function of temperature (4–300°K at atmospheric pressure) and of pressure (0–20 kbar at 295°K). From the low‐temperature results, obtained for five independent acoustic modes, the temperature variations of all five elastic constants were calculated using the thermal expansion data of Bailey and Yates. From the high‐pressure data, taken for four independent modes, the changes of all five elastic constants and of the unit cell dimensions with pressure were calculated under the assumption that the small a‐axis compressibility is independent of pressure. The initial pressure derivatives of C33 and C44 are, respectively, about 25 and 35% lower than those obtained by Green et al. for CAPG. The pressure dependences of the volume and c‐axis compressibilities are in good agreement with measurements on natural graphite crystals by Lynch and Drickamer and by Bridgman, and in less satisfactory agreement with data of Kabalkina and Vereshchagin. The results support the contention that CAPG has bulk elastic properties very similar to those of single‐crystalline graphite.