L. Bencze

Thermochemistry of the Ni-Hf system—Intermetallic phases

The vaporization of alloys of the Ni-Hf system was investigated in the temperature range between 1200 and 1650 K by Knudsen effusion mass spectrometry. The different compositions of the 16 alloy samples investigated covered the complete homogeneity range of the Ni-Hf system. The partial pressure of Ni was determined over all samples. The thermodynamic activities of Ni and Hf in the alloys were evaluated from these pressures and by Gibbs-Duhem integration. In addition, Gibbs energies of formation, enthalpies of formation, and entropies of formation resulted for the nine intermetallic phases of the Ni-Hf system. Beside similar thermodynamic data for the evaporation reactions were studied. The data obtained are discussed and a method for distinguishing the congruent melting compounds from the peritectic ones by defining stability factors calculated from the Gibbs energies of formation is suggested.

Vapour pressure and mixing thermodynamic properties of the KNbO3–NaNbO3 system

Equilibrium vapour pressures of sodium and potassium over a KxNa1−xNbO3 solid solution within its whole compositional range at temperatures between 1173 K and 1303 K were determined by Knudsen Effusion Mass Spectrometry. It should be noted that the thermodynamic equilibrium between the condensed and the vapour phase could be established only after prolonged annealing (more than 10 h at 1263 K). The equilibrium vapour pressure of potassium over K0.5Na0.5NbO3 (KNN) is a few times larger than that of sodium, i.e., 8 × 10−3 Pa as compared to 3 × 10−3 Pa at 1263 K. From the obtained results, the excess thermodynamic functions for the pseudo-binary KNbO3–NaNbO3 system were evaluated. The excess Gibbs energy was found to be positive, the excess enthalpy is close to zero, while the negative excess entropy indicates a partial ordering of alkaline ions in the solid solution. The comparison of the obtained results to the well-established lead-based piezoelectric systems revealed, that the vapour pressure of alkalis over the respective niobates at 1200 K is almost three orders of magnitude lower as compared to the values reported for lead oxide over Pb(Zr,Ti)O3.

MASS SPECTROMETRIC INVESTIGATION OF THE EVAPORATION PROPERTIES OF LEAD OXIDE

Partial pressures of molecular species over solid PbO were measured in the temperature interval between 890 K and 1100 K using Knudsen-cell mass spectrometry. The assignment of ions is discussed in detail. Structural and vibrational parameters of the gas phase for (PbnOn) (n = 1,2,3,4) were calculated by the ab initio molecular orbital method at different approximation levels. Third law enthalpies of dissociation of Pb2O2 and Pb4O4 to monomeric PbO were obtained as (250.6 ± 6) kJ/mol and (835.0 ± 15) kJ/mol, respectively. The possibility of direct measurement of the dissociation pressure of PbO by a mass spectrometer is demonstrated.

Knudsen cell mass spectrometric investigation of the PbO-ZrO2-TiO2 system

Knudsen effusion mass spectrometry (KEMS) was used for direct determination of lead oxide activity as a function of temperature in various regions of the PbO-ZrO(2)-TiO(2) system. From the results, the enthalpy, Gibbs free energy and entropy of formation of PbTiO(3) (PT), PbZrO(3) (PZ) and Pb(Zr,Ti)O(3) (PZT) were evaluated. In addition, the single phase widths of Pb(Zr(0.5)Ti(0.5))O(3) and PbTiO(3) perovskite structures were determined at 1100 K. The reaction rate of PZT synthesis in vacuo was followed by direct measurement of the change of PbO activity with time. Lead oxide activity in stoichiometric Pb(Zr(0.5)Ti(0.5))O(3), PbTiO(3) and Pb(0.968)(Zr(0.5)Ti(0.5))O(2.968) (3% lead deficient) at 850 degrees C was found to be 0.40, 0.45 and 0.1, respectively. PZT, PT and PZ powder samples prepared by a solid state procedure were also measured, all revealing lead deficiency. Copyright 1999 John Wiley & Sons, Ltd.

THE EVAPORATION THERMODYNAMICS OF LITHIUM IODIDE. MASS SPECTROMETRIC AND AB INITIO STUDIES

The vaporization of LiI was investigated in the range between 583 and 726 K by Knudsen effusion mass spectrometry (KEMS). The ionization or the appearance energies (IE or AE) of all kinds of ions formed from the equilibrium vapour over solid lithium iodide were determined using Vogt and Pascuals deconvolution method. For the determination of vapour pressure two methods, viz. mass-loss Knudsen effusion and Knudsen effusion mass spectrometry were applied. The mean natural logarithms of the equilibrium vapour pressures (in Pa) of monomer and all kinds of oligomers, that can be detected using KEMS, as a function of temperature, can be expressed as follows: The molecular structure and the harmonic vibrational frequencies of (LiI)n species (n = 1,2,3,4) were predicted using ab initio molecular orbital methods. Both the bond dissociation energies and the enthalpy changes of dissociation of (LiI)n oligomers were evaluated and compared with the measured enthalpy change data. Using the calculated geometry and the vibration frequencies, the thermodynamic functions of (LiI)n could be calculated, making it possible to compare the second and third law thermodynamic data.

Thermodynamic properties of B2-AlFeNi Alloys. Part I: Investigation by knudsen effusion mass spectrometry

The vaporization of Al-Fe-Ni alloys has been investigated in the temperature range 1180 to 1508 K by Knudsen effusion mass spectrometry (KEMS). Fourteen different compositions were examined in the B2 region: 10 compositions at two fixed Al concentrations,xAl=0.45 andxAl=0.50 plus four extra compositions at constantxNi/xFe=1. For the first time, reliable partial pressures and thermodynamic activities of Al, Fe, and Ni have been evaluated from the measured ion intensities for both the alloy and the pure element. Gibbs energies, partial molar enthalpies, and entropies of formation for all the components have also been obtained. The relative partial molar enthalpies and entropies were found to be nearly temperature independent over the wide temperature ranges investigated. At 1400 K, the Gibbs energy of formation of Al0.50Fe0.25Ni0.25 and Al0.45Fe0.275Ni0.275, with Al(liq), Fe(fcc), and completely paramagnetic Ni(fcc,cpm) as reference states, are −37.9±0.42 kJ/mol and −38.1±0.42 kJ/mol, respectively. At the same temperature, the enthalpies of formation of Al0.50Fe0.25Ni0.25 and Al0.45Fe0.275Ni0.275, with the same reference states, are −51.5±1.7 kJ/mol and −49.2±1.7 kJ/mol, respectively.

Determination of thermodynamic activities by mass spectrometry in {xNaI + (1 − x)CsI}

Abstract A (Knudsen cell + mass spectrometer) has been used to study thermodynamic activities in { x NaI + (1 − x )CsI}. The mass spectrum of the vapours over the mixture has been measured over a temperature interval of 930 to 1050 K. From the ion intensities, thermodynamic activities were calculated by applying the Gibbs-Duhem and monomer-to-dimer ratio technique. The NaI and CsI activities over the full composition range, the excess partial molar and excess molar enthalpies, the corresponding entropy changes, and the excess chemicals potentials and excess molar Gibbs energies are reported. This study yields negative excess molar enthalpies over the whole composition range which, together with the other thermodynamic quantities, indicate deviation of the mixture from ideality; that is, considerable interaction forces exist between NaI and CsI molecules in the melts.

Mass spectrometric determination of appearance energies for ions formed from CoF4 and CoF3 molecules

Knudsen cell mass spectrometry was applied to the evaluation of the ionization efficiency curves for the ions originating from CoF4 molecules. Cobalt tetrafluoride was obtained in the gas phase over the CoF 3(s)‐ TbF4(s) system in the temperature range from 640 to 690 K. From the ionization efficiency curves the appearance energies of the ions formed from the CoF4 molecules were determined by means of Vogt’s deconvolution method. Clausius-Clapeyron plots for the ions from CoF4 molecules were measured. Evaporation of pure CoF3(s) was carried out, and the appearance energies of the ions formed from CoF3 molecules were determined. The ionization energies for CoF4 and CoF3 molecules were found to be (14.3 0.2) and (13.3 0.1) eV, respectively. Copyright # 2000 John Wiley & Sons, Ltd.