The heat capacity of a characterized, zone-melted sample of zirconium diboride has been measured over the low temperature range by adiabatic calorimetry and found to be of a normal sigmoid shape without transitions or thermal anomalies. The values of the heat capacity at constant pressure (Cp), the entropy (S°), and the Gibbs function (-[G°-H §]/T) are 11.53, 8.59, and 3.25 cal. (g.f.m. °K.) "l, respectively, at 298.15°K.ALTHOUGH in increasing demand as a consequence of technological developments in nuclear reactors, missiles, and other high temperature applications of refractory materials, accurate thermodynamic data on carbides, borides, and related compositions on the group IV elements are relatively rare. The major difficulty is preparing these substances in high-purity, well-characterized states suitable for thermodynamic reference use. This study endeavors to procure reliable thermodynamic data on zirconium diboride (ZrB2) by adiabatic calorimetry. EXPERIMENTAL Cryostat and Technique. Measurements were made in the Mark II adiabatic, vacuum cryostat schematically similar to one previously described ( 20), but modified and improved in several respects to give an instrument of greater compactness, mechanical rigidity, and operating convenience. The cryostat is provided with an economizer, with perforated silver foil discs. This economizer serves as a heat exchanger between the bundle of electrical leads Constantan thermocouples monitor the difference in temperature between calorimeter and shield and between shield and ring. Three separate channels of recording electronic circuitry with proportional, rate, and reset control actions are used for shield control above 50°K. This instrumentation controls temperature to within about a millidegree over the temperature range and makes the uncertainty in the (enameled around it) and the heat capacity of the cold ef-
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