SOLGAS, an early computer program for calculating equilibrium in a chemical system, has been made more user-friendly, and several "bells and whistles" have been added. The necessity to include elemental species has been eliminated. The input of large numbers of starting conditions has been automated. A revised spreadsheet-based format for entering data, including non-ideal binary and ternary mixtures, simplifies and reduces chances for error. Calculational errors by SOLGAS are flagged, and several programming errors are corrected. Auxiliary programs are available to assemble and partially automate plotting of large amounts of data. Thermodynamic input data can be changed "on line." The program can be operated with or without a co-processor. Copies of the program, suitable for the IBM-PC or compatibles with at least 384 bytes of low RAM, are available from the authors. ix FOREWORD This is our second issuance of SOLGAS Refined. SOLGAS still does all the calculations described in the earlier report. In addition, SOLGAS calculates equilibrium in systems with both binary and ternary non-ideal solutions and can be used with aqueous solutions, including ionic species.
Vapor pressures of zirconium over zirconium diboride have been measured by the Knudsen technique over the temperature range 2150° to 2475°K. A new type of apparatus was constructed and used successfully in the study.
Zirconium diboride was determined to evaporate congruently at a composition of ZrB1.906(+0.025 or —0.010) by heating solid pressed plugs of both zirconium-rich and boron-rich material to constant composition at 2400°C to 2500°C. The over-all reaction is ZrB1.906(s)=Zr(g)+1.906 B(g)Three series of measurements were made using tungsten crucibles and different orifice sizes. Second-law and third-law treatments of the data did not agree. Thermodynamic calculations were made which indicated that water vapor at low background pressures would produce volatile oxides of both zirconium and boron. This reaction was investigated by adding water vapor to the system and the increased transport of zirconium was clearly demonstrated. Accordingly, each pressure measurement was corrected by a factor β, constant for each series, related to the background pressure.
From the corrected pressures, values of ΔH0o for the reaction were computed by the third-law treatment. From the three series a vaporization coefficient of 0.025±0.010 was computed, leading to an equilibrium ΔH0o of 458.3±6.5 kcal/mole for reaction 1, or a partial pressure of Zr over ZrB1.906 of 2.38×10—10 atm±20% at 2000°K. The variance between this and an expected value of 477.4 kcal/mole is presumed to be related, at least in part, to discrepancies in the heat of vaporization of zirconium.
The results verify the prediction that the Zr–B system exhibits a congruently evaporating phase and suggest that the congruently evaporating composition in many high-temperature systems will occur at nonstoichiometric compositions.
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