Zirconium Diboride: Heat Capacity and Thermodynamic Properties from 5° to 350° K.

Abstract

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-

Fluorine bomb calorimetry. Enthalpies of formation of the diborides of ziconium and hafnium

“…Specimens of the diborides were supplied by Arthur D. Little, Inc., in the form of zone-refined bars, designation numbers ZrB2-85 and HfB2-773. Similar zonerefined bars were also supplied to a number of other investigators (2,19,22,44,45,47,49).…”

“…Recent interest in refractory borides for possible high temperature applications has prompted a number of concurrent studies of the thermodynamic properties of zirconium and hafnium diborides. Some of these studies deal with vaporization processes (2,19,21,23,44,51), oxygen bomb calorimetry (7,14), low- (47,49) and high- (45) temperature heat capacities, and relative stabilities (4,33,38). In this work accurate values of the enthalpies of forma-tion were sought by the method of fluorine bomb calorimetry.…”

Heats of Formation of Metallic Borides by Fluorine Bomb Calorimetry

“…Westrum and Feick [81] meas-ued the heat capacity and tabulated the heat capacity$ entropy, enthalpy, and free energy functions foi" zirconium diboride in the temperature range 50 to 350 OK.…”

Thermodynamic Properties of Inorganic Substances VII. The High Temperature Heat Content of Zirconium Diboride.

“…Five SETS OF HIGH TEMPERATURE thermodynamic data (1, 2, 7, 8, 10) and a single low temperature study (11) have previously been available for ZrB2 samples of 95-99% purity but large positive and negative discrepancies (10-20% or more) have been observed even though all the calorimeters have been calibrated in standard ways. In a cross-check of data, Pears and coworkers (9) made runs on the same ZrB2 used by Barnes and coworkers (1) and reproduced the results of Barnes within a few per cent, which indicates that the reported large differences between samples are real and not measurement errors.…”

“…This report presents high temperature measurements on a sample of ZrB2 prepared by Feick (3) which is from the same batch as the sample used by Westrum and Feick (11) for their low-temperature studies.…”