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

Abstract

The high temperature heat content of a high-purity sample of ZrB? has been measured in a copper-block drop calorimeter over the range 410" to 1125"K., and the results correlated with low temperature heat capdcity data on a similar sample. Derived thermodynamic properties over the range 0" to 2000°K. are presented based on experimental data up to 1200" K. and on extrapolations of computer-selected fits of the experimental points. FIVE SETS OF H I G H T E M P E R A T U R E thermody-namic data ( I , 2, 7, 8, IO) and a single low temperature study ( 1 I ) have previously been available for ZrB? 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. I n a cross-check of data, Pears and coworkers (9) made runs on the same ZrB? used by Barnes and coworkers ( I ) 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. T h e AVCO results are pulse-type heat capacity measurements (2) over the temperature range 2000" to 2500' K. and seem to differ drastically from conventional extrapolations of the lower temperature results, as do the results of Prophet (IO).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 ( I 1 ) for their low-temperature studies. EXPERIMENTALThe calorimeter used in this work has been described previously ( 4 ) and the samples were contained in platinum or platinum-rhodium capsules under an argon atmosphere.Temperature measurements were made with a Pt us. Pt-10% R h thermocouple calibrated against a reference which had been recently calibrated a t the National Bureau of Standards. T h e reliability of the calorimeter was checked by rerunning a standard sample of synthetic sapphire.The heat content for the empty Pt capsules was measured at several temperatures and found to check well with the equation of Kelley ( 6 ) while the heat content of the Pt-10% R h capsule was calculated from Kelley's data on the elements.

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.…”

Synthesis of ultra-refractory transition metal diboride compounds

“…Through the 1950s and 1960s, research on diborides, particularly those of Ti, Zr and Hf, typically utilized commercially available powders. [84][85][86] Some of the exceptions were the phase equilibria studies by Rudy 87 and the examination of borides of alkali, 88 alkaline earth, 89 rare-earth, 90 or lesscommon transition metals 91 as well as studies of ternary and higher borides. 92,93 This trend continues to hold true to the present time whereby the majority of studies of the more common diborides (e.g., TiB2, ZrB2, and HfB2) use commercial powders whereas self-synthesis is mainly used for other borides.…”

Thermodynamic Analysis of the Phase Equilibria in the Fe-Zr-B System

“…31) Concerning the thermodynamic properties of this binary system, both the enthalpy of formation [37][38][39] and the heat capacity 40) of ZrB 2 are available in the literature. Thermodynamic assessment of this binary system has been carried out by Rogl and Potter, 41) where the allotropic transformation of Zr and the formation of ZrB were not taken into account.…”