Gibbs free energies of formation of molybdenum carbide and tungsten carbide from 1173 to 1573 K

The gas equilibrium method of CH₄/H₂ has been widely used for measuring carbon potential. However, it has been reported that this method is not applicable at high temperatures since the equilibrium between CH₄ and H₂ is disturbed by the reaction of CH₄ with moisture in the system. Nevertheless, this method should be applicable theoretically at high temperatures below which CH₄ decomposition can be neglected because the equilibrium between CH₄ and H₂ reaches constant ratio in spite of the reaction. Since the role of moisture is to oxidize the sample during the measurements under the oxygen potential determined byPh ₂ o/ph ₂ ratio, the Gibbs free energies of formation of Mo₂C and WC were successfully measured from 1173 to 1573 K by keeping the moisture level in the system low enough not to oxidize the sample. The experimental results are expressed by the following equations which were derived by least squares treatments of the data: Mo₂C:ΔG = -68270 + 8.23T J mol^-1 WC:ΔG = -52330 + 14.06T J mol^-1 These values were in good agreement with those measured by M. Gleiseret al. for narrow tempareture ranges using the CO/CO₂ gas equilibrium method.     

Magnesium-thermic synthesis of molybdenum carbide in ion melts

The physicochemical aspects of the synthesis of the powders of molybdenum carbide by the magnesium-thermic reduction of its oxide in melts of lithium, sodium, and potassium carbonates are considered. The thermodynamic evaluation of reactions based on the synthesis is given. The influence of the melt temperature on the granulometric characteristics of the carbides is revealed. It is shown that the powders with the largest specific surface are formed in the melt of lithium carbonate (Mo₂C of 7.96 m²/g) at 750°C.     

Determination of thermodynamic activity of carbon in alloyed molybdenum carbide

Conclusions It is demonstrated that the thermodynamic properties of carbon in the carbide {ie736-01}. and {ie736-02}, vary greatly under the action of alloying, and that the effect shows up at concentrations of alloying elements less than 1%.The complex nature of the concentration dependence of carbon bond energy in alloyed carbides is due to the simultaneous effect of a variety of factors. The contribution of a particular factor to the overall bond energy differs with different concentrations.Translated from Poroshkovaya Metallurgiya, No. 9, pp. 64–70, September, 1967.     

The composition of eta carbide phase in 2 1/4 Cr-1 Mo Steel

The metal-to-carbon ratio in the eta phase in 2 1/4 Cr-1 Mo steel exposed to sodium at 566°C for 26,500 h, and probably under a variety of service conditions, approaches four, rather than six, a fact which may be influenced by the presence of Si, O, and perhaps Cr. Silicon is present in the eta phase, probably on a metal sublattice, while oxygen is probably present on the metalloid sublattice. The observation is supported by both structural and compositional data from the literature.     

The composition of eta carbide phase in 2 1/4 Cr-1 Mo Steel

The metal-to-carbon ratio in the eta phase in 2 1/4 Cr-1 Mo steel exposed to sodium at 566°C for 26,500 h, and probably under a variety of service conditions, approaches four, rather than six, a fact which may be influenced by the presence of Si, O, and perhaps Cr. Silicon is present in the eta phase, probably on a metal sublattice, while oxygen is probably present on the metalloid sublattice. The observation is supported by both structural and compositional data from the literature.     

Carbon diffusion in Mo2C as determined from carburization of Mo

A diffusion coefficient of C in nonstoichiometric α-Mo2C has been determined from the growth kinetics of the carbide layer. The results conform to the relationship:Dc (in cm²/s) = 68.86 ± 1.51 exp [(-294.77 ± 4.98)/RT] for the temperature range of 1273 to 1673 K, with the activation energy in kJ/mole. The growth rate,Kp, of the carbide thickness can be expressed as:Kp (in cm²/s) = 32.63 ± 1.52 exp [(-319.06 ± 5.12)/RT].     

Free energy of formation of Mo2C and the thermodynamic properties of carbon in solid molybdenum

The activity of C in the two-phase region Mo+Mo₂C has been obtained from the C content of iron rods equilibrated with metal+carbide powder mixtures. From this activity data the free energy of formation of α-Mo₂C has been determined as ΔG _f ^o (α-Mo₂C) (1270 to 1573 K)=−47,530−9.46T±920 J/mol. This is in good agreement with the expression obtained from gas-equilibration studies by Gleiser and Chipman, ΔG _f ^o (α-Mo₂C) (1200 to 1340 K)=−48,770−7.57 J/mol, but both our and Gleiser and Chipman's values are about 10 pct lower than those of Pankratz, Weller and King calculated from ΔH _f,298 ^o andC _p vs T data. With the aid of available data for the solid solubility of C in Mo, the thermodynamic properties of C in the terminal solid solution have been calculated as J/mol, J/mol and , the excess entropy ofC in the solid solution assumingC is in the octahedral interstices =43.4±8.2 J/deg.-mol.     

An investigation of the synthesis of highly dispersed composite powders containing titanium carbide and molybdenum

Conclusions The possibility was shown of production of composite ultradispersed powder materials directly in the plasmachemical process, eliminating the stage of long mixing of the components of the composite. The process of combined reduction of titanium chloride to titanium carbide and of molybdenum chloride to metallic molybdenum was optimized. The maximum degree of transformation is obtained with a discharge energy of 1.5 kJ in the whole range of investigated consumptions.The molybdenum carbide composite powders obtained have a size of about 10 nm and are characterized by uniform distribution of Mo and TiC.Translated from Poroshkovaya Metallurgiya, No. 2(302), pp. 58–62, February, 1988.     

Kinetics of carbide formation from reduced iron in CO-H2-H2S mixtures

Gravimetric measurements were made for the kinetics of the carbide formation from reduced iron by preventing the decomposition of iron carbides and the deposition of carbon by utilizing the effect of sulfur. Particles of a hematite ore were completely reduced in the H₂-H₂S mixture and converted to iron carbide in a CO-H₂-H₂S-CO₂-H₂O-Ar or CO-CO₂-COS-N₂ mixture at 873 to 1073 K. The carbide formation kinetics was drastically accelerated by the addition of a few percent H₂ to CO and reached the maximum at about 20 pct H₂. From morphology observation, it was assumed that the carburization reaction on the surface of pores in the reduced iron determines the carbide formation kinetics, and the carburization rate, df _θ/dt, is expressed at the product of a function of carburization rate, g(p _i, T), and the relative reaction surface area at the time t, 1 − f _θ. Applying the integrated rate equation, -ln (1−f _θ)=g(p _i, T)t, to the experimental results, the value of g(p _i, T) was obtained. By analyzing the carbide formation rate, it was assumed that the elementary reactions determining the carbide formation rate are CO+H(ad) → [C]+OH(ad) and OH(ad)+H₂= H₂O+H(ad), except in the low ratio of , where the rate determining elementary reaction shifts to O(ad)+CO → CO₂. The overall rate Eq. [22] has been proposed for the carbide formation from the reduced iron in CO-H₂-H₂S-CO₂-H₂O mixtures. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.