Gibbs energies of formation of chromium carbides
- PDF / 532,234 Bytes
- 6 Pages / 598 x 778 pts Page_size
- 1 Downloads / 214 Views
I.
INTRODUCTION
HYPERSTOICHIOMETRIC mixed carbides of plutonium and uranium have been under development as fuel for fast breeder reactors. The fuel is normally in the form of pellets enclosed in AISI 316 stainless steel cladding tubes. Depending upon the difference in carbon potential, transport of carbon between the fuel and the clad can occur at points of contact. The flux of carbon may affect the stability and distribution of chromium carbide precipitates, which control the chemical and physical properties of stainless steel. Accurate data on Gibbs energies of formation of chromium carbides are useful for analyzing the morphological stability of the carbide precipitates in a carbon chemical potential gradient. According to a recent phase diagram,ill there are three compounds in the Cr-C system: Cr23C6, Cr7C3, and Cr3C2. Several investigators[2 131have determined the thermodynamic properties of chromium carbides. The Gibbs energies of formation of these carbides were measured by using solid-gas equilibration,t2~.lO] solid-state electromotive force (EMF), r5 8] effusion,t2] and isopiestic[9] techniques. However, the results are in serious disagreement. Small and Rybat~q have discussed the possible errors associated with the various techniques. They have suggested that the differences in the stoichiometry of the carbides, nonequilibrium conditions prevailing during the investigation, and oxygen dissolution in carbide phases are possible sources of uncertainty in gas-solid equilibrium studies. They have indicated that the data obtained from solid-state EMF measurements with CaF2 electrolyte are more reliable. However, Kulkarni and Worrell E21 consider the results obtained using polycrystalline CaF2 electrolyte to be less reliable because of possible electronic conduction along the grain boundaries in the electrolyte. Subsequently, Coltters and Beltontrl have made measurements on chromium carbides using a new solid electrolyte BaF2-BaC 2.
However, their results are not in agreement with those recommended by Small and Ryba. t113 Hence, in the present work, the Gibbs energies of formation of Cr3C2, Cr7C 3, and Cr23C 6 were redetermined in the temperature range 973 to 1173 K by using the methane-hydrogen gas equilibration technique. II.
Heterogeneous equilibrium between a biphasic solid carbide sample and the gas phase was employed for the determination of the thermodynamic activity of carbon in the condensed phase. In a binary system containing a metal and carbon, the chemical potential of carbon is unambiguously established by a mixture of two coexisting phases at constant temperature and pressure. At equilibrium, the carbon activity in the gas phase is equal to the carbon activity in the solid mixture. If the gas phase consists of a mixture of hydrogen and methane, the Pcm/P~2ratio in the gas phase is a measure of the carbon potential, by virtue of the reaction [C]carbide -~- 2 (H2) ---> (CH4)
METALLURGICALAND MATERIALSTRANSACTIONSA
[1]
The carbon potential, relative to graphite as the standard state, is given
Data Loading...
