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- Entropy terms in statistical thermodynamic analysis formula for non-stoichiometric interstitial compoundsPublication . Shohoji, NobumitsuABSTRACT: A series of statistical thermodynamic analyses were made since 1974 for different types of non-stoichiometric interstitial compounds MXx under simplifying a priori assumption of constant interaction energy E(X-X) between nearest neighbour interstitial atoms X within a homogeneity composition range of MXx at arbitrary temperature T [K]. Mode of distribution of X atoms in interstitial sites in MXx lattice is represented by number θ of available interstitial sites for occupation by X atoms per M atom and the value of θ is determined to fulfil the a priori assumption. Mode of atomic configuration would yield major contribution to entropy term ∆S that appears in conventional thermodynamic expression of Gibbs free energy of formation, ∆G, in form of T∆S. In the statistical thermodynamic formulation, contribution of tightly bound electron appearing in form of RT ln fX where fX refers to atomic partition function of X atom in the MXx lattice and R the universal gas constant. Judging from this mathematical form of the term, R ln fX is considered to represent entropic contribution from tightly bound electron to X atom in MXx lattice. In the published series of works on statistical thermodynamic analysis for non-stoichiometric interstitial compounds, calculated values for R ln fX were reported but they were not reviewed with serious attention because R ln fX was considered merely as a secondary factor compared to principal factor E(X-M) referring to interaction energy between X and M in MXx lattice that represents enthalpy ∆H in conventional thermodynamic term. In this review article, consideration is given exclusively to the factor R ln fX evaluated in statistical thermodynamic approach to non-stoichiometric interstitial compounds.
- Hydrogen absorption in epitaxial bcc V (001) thin films analysed by statistical thermodynamicsPublication . Shohoji, NobumitsuAndersson, Aits and Hjörvarsson of Uppsala Universitymeasured hydrogen uptake in epitaxial bcc (body centred cubic) vanadium (V) (001) thin films of thickness, 50 nm and 100 nm, over temperature range between 443 K and 513 K. The reported equilibriumpressure–temperature–composition (P–T–C) relationships for the epitaxial bcc V (001) thin films showed appreciable extent of enhancement ofH solubility comparedwith that for bulk bcc V. In this work, the reported equilibriumP–T–C relationships for the epitaxial bcc V(001) thin films by Andersson et al.were analysed in terms of statistical thermodynamics for H2 gas partial pressure p(H2) upto100 Pa andH/V mole atom ratio x in VHx up to 1. The present analysis results showed that, up to x=0.75, the state of H in the V latticewas comparable to that in bulk VHx specimen but that, in the range of x higher than 0.75, state of H in the thin film with the constrained basal plane condition was evidently distinguishable from that in non-constrained bulk VHx. This was concluded to be the consequence of the tetragonal distortion of the bcc lattice with biaxially constrained condition at the bottom surface of the VHx (001) thin film in the range of x exceeding 0.75
- Carbide phases synthesised from C/Mo powder compacts at specified sub-stoichiometric ratios by solar radiation heating to temperatures between 1600 C and 2500 CPublication . Granier, Bernard; Shohoji, Nobumitsu; Almeida Costa Oliveira, Fernando; Magalhães, Teresa; Fernandes, Jorge Cruz; Rosa, Luís GuerraThere are a number of distinguishable carbide phases in the binary Mo-C system depending on C/Mo ratio as well as on temperature. In a preceding work published in this journal, carbide formation performance for graphite/molybdenum powder mixtures at specified levels of substoichiometric C/Mo atom ratio (C/Mo = 1/1, 3/4, 2/3 and 1/2) by exposure to concentrated solar radiation in a solar furnace at PROMESCNRS in Odeillo (France) was reported at a target temperature 1900 C. In the present work, the similar carbide synthesis experiments were carried out at 1600 C as well as at temperature exceeding 2500 C. The target temperature setting was adjusted by controlling the downward deviation of the test piece top surface position from the exact focal spot of the parabolic mirror concentrator located above. In this solar furnace at PROMES-CNRS, temperature of the test piece was raised from ambient temperature to the target temperature within fractions of a second. Reaction products detected were hexagonal -MoC1 x and -Mo2C (high temperature sub-carbide phase) depending on the C/Mo ratio in the starting material as well as on the processing temperature. No evidence of formation of cubic -MoC1 x was detected by X-ray diffraction analysis for any test piece examined.