Thermodynamic properties of the La2 O3 -ZrO2 system by Knudsen effusion mass spectrometry at high temperature.
RATIONALE
Zirconia doped with a lanthanum oxide system is of high interest due to its exceptional thermal stability for the development of high-performance ceramics. It possesses the beneficial properties of pure zirconia, such as heat resistance, mechanical strength and inertness, but also eliminates its main disadvantage, i.e. brittleness. At high temperatures, components of such systems may vaporize selectively, leading to significant change in composition, and hence, the thermal resistance, phase stability and performance of the ceramic materials. Therefore, information on the vaporization processes and thermodynamic properties of the La2 O3 -ZrO2 system is of great importance.


METHODS
Knudsen effusion mass spectrometry was used to study the vaporization processes and thermodynamic properties of solid solutions in the La2 O3 -ZrO2 system at 2110 K. Pure La2 O3 was used as a reference substance. Comprehensive characterization of the precursors and ceramics was performed via SEM, STA, XRD and PSD analysis.


RESULTS
Zirconia-doped lanthania precursor powders and ceramics with La2 O3 of 33.3, 50, 70 and 90 mol.% content were manufactured by solid-state synthesis and the original cryochemical technique. La, LaO, ZrO and ZrO2 were found to be the main vapor species over the samples studied. The activities and thermodynamic properties of La2 O3 were calculated. Via XRD analysis it was shown that the phase composition of xLa2 O3 -(100-x)ZrO2 powders (x = 0.33, 0.5, 0.7 and 0.9 mol. fraction) significantly depends on the synthesis technique chosen.


CONCLUSIONS
According to the XRD results combined with Rietveld refinement of the patterns, 33.3 La2 O3 -66.7 ZrO2 ceramics after solid-state synthesis are composed of well-formed cubic pyrochlore-type La2 Zr2 O7 with 5 wt.% admixture of monoclinic and cubic ZrO2 whereas 33.3 La2 O3 -66.7 ZrO2 precursor powders after cryochemical synthesis correspond to low-crystalline La(OH)3 . The components of the La2 O3 -ZrO2 system evaporate separately: there is no temperature range where lanthanum and zirconium gaseous species are present together. It was found that the activities of lanthania have low negative deviation from the ideal case.
Vaporization and thermodynamic properties of lanthanum hafnate
Abstract The superfine lanthanum hafnate powder was synthesized by the glycol-citrate method. The phase transitions and the changes of microstructure of the La2Hf2O7 sample were studied under different conditions of thermal treatment (973–1673 K, 2–4 h). The vaporization processes of La2Hf2O7 were established at temperatures 2297–2500 K by the high temperature mass spectrometric method. The thermodynamic properties in the La2O3-HfO2 system were obtained at temperatures 2297 and 2445 K.
Gaseous Phosphorus Compounds. IV. Thermodynamic Study of Gallium Monophosphide with a Mass Spectrometer and Dissociation Energy of Aluminum Diphosphide
Gaseous equilibria involving the GaP molecule have been studied by means of the Knudsen‐effusion technique in combination with mass spectrometric analysis of the vapor phase. In order to obtain the desired phosphorus and gallium partial pressures of comparable magnitude, mixtures of gallium and either WP or Th3P4 which were evaporated from a graphite Knudsen cell were employed. From the experimentally determined partial pressures the equilibrium constants and the enthalpies of the reactions Ga(g)+0.5 P2(g)=GaP(g), Ga(g)+0.5 P4(g)=GaP(g)+0.5 P2(g), 0.5 Ga2(g)+0.5 P2(g)=GaP(g), and GaAg(g)+0.5 P2(g)=GaP(g)+0.5 Ag2(g) have been determined. These reaction enthalpies have been combined with appropriate literature data to yield the dissociation energy, D0° = 54.0 ± 3.0 kcal mol−1, the heat of sublimation, ΔHs,298° = 111.2 ± 5.0 kcal mol−1, and the heat of formation ΔHf,298° = 90.3 ± 4.1 kcal mol−1, of gaseous gallium monophosphide. The molecule AlP2 has been observed over the congruently vaporizing AlP(s). The ...