Gibbs energy of formation of nickel chromite
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I.
INTRODUCTION
II.
THERMODYNAMIC data of the Ni-Cr-O system are of great importance for the applications of many high-temperature alloys. The standard Gibbs energy of formation of nickel chromite, NiCr204, has been determined over a wide range of temperatures by different techniques, tl-5] However, considerable disagreement exists among the reported results. For example, Davies and SmeltzerI6J studied the thermodynamic properties of NiCr204 coexisting with Cr203 and Ni at 1173 to 1373 K using electrochemical measurements. They reported that the equilibrium partial pressures of oxygen in the NiCr204-Cr203-Ni system w e r e - 8 • 10 -16, 1 • 10 -13, and 6 • 10-12 atm at 1173, 1273, and 1373 K, respectively. The standard Gibbs energies of formation of NiCr204, i.e., AG~(NiCr204), at 1173, 1273, and 1373 K were, therefore, calculated to be -982.8, -946.0, and -910.9 kJ/mole, respectively, with an estimated error of ---5.0 kJ/mole. However, Schaefer]7~ recently measured the Gibbs energy of formation of NiCr204 at 1173 to 1260 K using the same technique and arrived at the following expression: AGf (NiCr204) = -1356.3 + 0.333T -+ 1.7 (kJ/mole)
[1]
Equation [1] indicates that the Gibbs energies of formation are -965.7, -932.4, and -899.1 kJ/mole at 1173, 1273, and 1373 K, respectively. Differences between these two studies E6"71are evident. Figure 1 summarizes the equilibrium partial pressures of oxygen reported by various authors [1-4,6,7J for different temperature ranges (in 1/T). The numbers in Figure 1 refer to the reference citations. Apparently, the results are not in good agreement but are scattered along the P(O2) axis over approximately five orders of magnitude. Thermodynamically, such a significant discrepancy in the equilibrium oxygen pressure is not acceptable and requires further experimental investigation. Therefore, this study was undertaken to re-evaluate the thermodynamics of the NiCr204-Cr203-Ni system.
SI-CHENG KUNG, Senior Research Engineer, is with the Babcock and Wilcox Company, Alliance, OH 44601. Manuscript submitted July 2, 1990. METALLURGICALTRANSACTIONSB
(kJ/mole)
EXPERIMENTAL PROCEDURES
High-purity NiO (99.99 pet) and Cr203 (99.999 pct) powders obtained from Aldrich Chemical Co., Milwaukee, WI, were used in this investigation. Nickel powder with a particle size of -100 mesh and a purity of 99.9+ pet was purchased from Alfa Products, Ward Hill, MA. To prepare the NiCr204-Cr203-Ni working electrode, powders of NiO and Cr203 in about 2: 3 molar ratio were first mixed and ground. The mixed oxide powders thus obtained were further mixed and ground with the nickel in about 1 : 1 volume ratio. The powder mixture comprised of NiO, Cr203, and Ni was pressed into pellets (--1/16-in. thickness • 3/4-in. diameter) using a steel press. The pellets were slowly heated under vacuum to 1000 ~ in a high-temperature furnace for 24 hours, followed by heating in ultrahigh purity (UHP)-grade argon at 1000 ~ for 3 days. The high-purity argon was further deoxidized by passing through an oxygen getter containi
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