Volume 22, Issue 6, November 1993
Index of content:
Thermodynamic Properties of Synthetic Sapphire (α‐Al2O3), Standard Reference Material 720 and the Effect of Temperature‐Scale Differences on Thermodynamic Properties22(1993); http://dx.doi.org/10.1063/1.555931View Description Hide Description
Comparison of the National Institute of Standards and Technology’s Standard Reference Material 720 certificate values for heat capacity with those obtained from recent experimental determinations indicated the possibility of a systematic error in the certificate values. Selected experimental determinations of enthalpy increments and heat capacities were fitted in order to obtain a representation of the thermodynamic properties of α‐Al2O3, a sample of which is the standard reference material (SRM‐720) for calibration of some types of calorimeters. The fitted equation and calculated values of the heat capacity, the relative enthalpy, and the entropy are given. The new values are more accurate and result from a better representation of the experimental values than did the 1982 SRM‐720 certificate values. Additionally, the general problem of the effect of changes in practical temperature scales on thermodynamic properties is briefly discussed, using the results for α‐Al2O3. A recent report from the I.U.P.A.C. Commission on Thermodynamics gave a method for the conversion of thermodynamic properties for changes in practical temperature scale. The I.U.P.A.C. method is shown to be not generally correct. A better method for estimation of these changes is given.
Thermodynamic Properties of Gaseous Silicon Monotelluride and the Bond Dissociation Enthalpy m (SiTe) at T→022(1993); http://dx.doi.org/10.1063/1.555932View Description Hide Description
Statistical‐thermodynamic calculations have been combined with the results of high‐temperature Knudsen‐effusion studies of the vaporization of Si2Te3 to calculate the thermodynamic properties of SiTe(g) from T→0 to T=2000 K. The dissociationenthalpyD ○ m (SiTe, T→0) is (448±8) kJ⋅mol−1; its value is discussed vis‐a‐vis the other silicon monochalcogenides.
The Disilicides of Tungsten, Molybdenum, Tantalum, Titanium, Cobalt, and Nickel, and Platinum Monosilicide: A Survey of Their Thermodynamic Properties22(1993); http://dx.doi.org/10.1063/1.555922View Description Hide Description
A critical evaluation is presented of the thermodynamic properties of six disilicides and one monosilicide that are important in the manufacture of very large scale integrated circuits. Values of the standard molar enthalpies of formation Δf H at T=298.15 K and p°=0.1 MPa are recommended as follows: WSi2, −(79±5) kJ⋅mol−1; MoSi2, −(137±4) kJ⋅mol−1; TiSi2, −(171±11) kJ⋅mol−1; CoSi2, −(103±15) kJ⋅mol−1; NiSi2, −(88±12) kJ⋅mol−1; TaSi2, −(120±20) kJ⋅mol−1; and PtSi, −(119±7) kJ⋅mol−1. Equations are given for the molar enthalpy increments of some of the silicides along with a few evaluated standard Gibbs free energies of formation. Brief descriptions of methods of synthesis and limited structural information have also been included.
Evaluated Bimolecular Ion‐Molecule Gas Phase Kinetics of Positive Ions for Use in Modeling Planetary Atmospheres, Cometary Comae, and Interstellar Clouds22(1993); http://dx.doi.org/10.1063/1.555940View Description Hide Description
Recommendations of reaction rate coefficients and product distributions for bimolecular positive ion‐molecule reactions of importance in planetary atmospheres, cometary comae, and interstellar clouds are presented. Two publications Anicich and Huntress, 1986, Ap. J. Supplement Series 62, 553 and Anicich, 1993, Ap. J. Supplement Series 84, 215 served as the basis for this evaluation, which covers the literature from 1965 through 1991 with some additional citations missed in the original surveys.
22(1993); http://dx.doi.org/10.1063/1.555933View Description Hide Description
The biennial review of atomic weight A r (E), determinations, and other cognate data has resulted in changes for the standard atomic weight of indium from 114.82±0.01 to 114.818±0.003, for tungsten from 183.85±0.03 to 183.84±0.01, and for osmium from 190.2±0.1 to 190.23±0.03 due to new high precision measurements. Recent investigations on silicon and antimony confirmed the presently accepted A r values. The footnote ‘‘g’’ was added for carbon and potassium because it has come to the notice of the Commission that isotope abundance variations have been found in geological specimens in which these elements have an isotopic composition outside the limits for normal material. The value of 272 is recommended for the 14N/15N ratio of N2 in air for the calculation of atom percent 15N from measured δ 15N values. Because many elements have a different isotopic composition in nonterrestrial materials, recent data on nonterrestrial material are included in this report for the information of the interested scientific community.