Gibbs free energies of formation of molybdenum carbide and tungsten carbide from 1173 to 1573 K
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J mo1-1 J mol -~.
These values were in good agreement with those measured by M. Gleiser et al. for narrow temperature ranges using the CO/CO2 gas equilibrium method.
I.
INTRODUCTION
T.E following three methods have been used for the measurements of carbon potentials,* AGc (=RT In a~ where R *For solid samples, AGe is equal to the partial molar Gibbs energy per 1 g-atom carbon. So, carbon potentials of MOECand WC are equal to Gibbs free energies of formation, AG of them, respectively.
and T are gas constant and absolute temperature, respectively, and ac is the carbon activity of sample) of solid samples: (a) Knudsen cell mass spectrometry, (b) Electromotive force measurements, (c) Gas equilibrium methods (CH4/H2, CO/CO2). Using the method (a), E. K. Storms et al. l measured the carbon potential, AGe, of UC2, but this method can be applicable only at high temperatures above approximately 1800 K because the vapor pressure of the sample is large enough for the measurement above that temperature. A. Onishi et al. 2 estimated AGe of UC from the value of au (uranium activity) measured by the method (b). Thus, this method is generally an indirect one. Method (c) is applicable to various carbides and AGe of the sample could be measured directly. In case of adopting a gas equilibrium method, either of the following two reactions has been widely used: 3'4'5 C + COz = 2CO,
K1 = p2o/ac'Pco2
[1]
C + 2H2 = CH4,
K2 = PCH,/ac" p22
[2]
where p and ac are the partial pressure and the carbon activity, respectively.
T. IWAI, Research Scientist, I. TAKAHASHI, Research Associate, and M. HANDA, Head of Plutonium Fuel Laboratory, are with Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken 319-11, Japan. Manuscript submitted August 15, 1985. METALLURGICALTRANSACTIONSA
The application of reaction [1] to the samples which are easily oxidized is limited as reaction [1] is a competitive one with reaction [3]: 2CO + O2 = 2CO2,
K3 = pZoJPo2" p2o
[3]
Moreover, the equilibrium of reaction [1] is disturbed by the following reaction, the decomposition of CO: 6'7 2CO = CO2 + C
[4]
M. Gleiser et al. 8 measured the carbon potential of Mo2C by the following sophisticated method to avoid the sample oxidation during the experiment: 2MOO2 + 6CO = Mo2C + 5CO2
[5]
The value of AGe of Mo2C is not so precise since it is estimated from oxygen potential, AGo2 ( = R T In Po2 where Po2 is the partial pressure of oxygen) of MOO2. In the application of reaction [21, it has been reported that this method is not applicable at high temperatures because the equilibrium between C H 4 and H2 is disturbed by the following reaction of CH4 with the residual moisture in the system: CH4 + H 2 0 = C O + 3H2, K6 = Pco"
p3H2/PcH4 " PH20
[6]
In the measurement of AGe in Fe-C alloys using labeled 14CH4, T. Ellis et al. 9 recognized that Pco varied with PH20, but the ratio of PCH4/PH2 was kept constant and AGe was determinable. Taking into consideration the results described above, we considered that the role of the moisture in the system
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