A low-temperature technique for measuring enthalpies of formation
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A low-temperature technique for measuring enthalpies of formation T. P. Weihs,a) T. W. Barbee, Jr., and M. A. Wall Lawrence Livermore National Laboratory, Livermore, California 94550 (Received 28 September 1995; accepted 30 January 1996)
A technique to accurately measure the formation enthalpies of transition metal compounds at relatively low temperatures using thick multilayer foils and differential scanning calorimetry is demonstrated. The enthalpy of formation of Cu51 Zr14 was measured using 25 mm thick, free-standing Cu–Zr multilayer foils. The multilayers were deposited onto Si substrates using a planetary, magnetron source sputtering system. They were removed from their substrates, cut into 6 mm diameter specimens, and scanned in temperature from 50 ±C to 725 ±C in a differential scanning calorimeter. Three distinct exothermic reactions were systematically observed. The heats from the first two reactions were summed and then analyzed using a simple model that accounts for interfacial reactions and heat losses during deposition. The enthalpy of formation for Cu51 Zr14 was measured to be 14.3 6 0.3 kJymol. This quantity agrees with the single value of DHf 14.07 6 1.07 kJymol reported in the literature for this Cu–Zr compound. The advantages of measuring formation enthalpies using thick multilayer foils and low temperature calorimetry are discussed.
I. INTRODUCTION
Enthalpies of formation (DHf ) are commonly used to predict the stability and the formation of alloys and compounds,1–3 and they are a primary source of data for alloy theory. DHf is typically measured using high temperature techniques such as drop calorimetry or reaction calorimetry.2–6 In the case of transition elements, temperatures in excess of 1000 ±C are often needed to complete formation reactions. Accurate determination of DHf at these temperatures is experimentally demanding, and great care must be taken to avoid environmental contamination. Many of the reactions characteristic of environmental contamination are highly exothermic so that small amounts of contamination may result in large errors in measured heats. Such difficulties have limited the availability of enthalpies of formation for transition metal alloys, and may be a primary cause of the scatter in the reported data.4–6 In this paper we report the results of a Differential Scanning Calorimetry (DSC) study of the exothermic heats of solid-state reactions in macroscopic, nanostructured, free-standing, multilayer foils. These results demonstrate that enthalpies of formation for intermetallic compounds can be accurately measured at low temperatures under conditions where environmental contamination is minimized or eliminated. Metals and alloys can be deposited into a multilayer structure in which the alternate layers of metal A and a)
Currently at Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218.
metal B measure only nanometers in thickness.7 The nanoscale dimensions of the layers de
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