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.
Thermochemical Study of Salts of Oxygen-containing Acids in the Gas Phase: VI. Barium Metaborates
Vapor formation in the BaO-B2O3 system was studied by high-temperature mass spectrometry. The equilibrium reactions with participation of gaseous barium metaborates were determined. The standard enthalpies of formation of gaseous BaB2O4(g) and BaBO2(g) of -1424 ± 28 and 598 ± 15 kJ mol-1, respectively, were evaluated.
Vaporization thermodynamics of the lanthanum carbon system - Mass spectrometric determination of the dissociation energy of LaC2, LaC3, and LaC4
The vaporization of the lanthanum dicarbide–carbon system has been studied by Knudsen‐effusion mass spectrometry over the temperature range from 2267 to 2600°K. Experimentally determined reaction enthalpies were combined with published thermodynamic data to yield the atomization energies D0.atom°(LaC2) = 1257 ± 8 kJ mole−1, D0.atom°(LaC3) = 1759 ± 31 kJ mole−1, and D0.atom°(LaC4) = 2500 ± 16 kJ mole−1. Ion intensities of the parent molecular ions La+, LaC2+, LaC3+, and LaC4+ were combined with absolute effusion measurements from the literature to obtain vapor pressure equations (1900–2600°K): logPLa = (− 2.606 × 104 / T) + 10.162, logPLaC2 = (− 3.327 × 104 / T) + 12.813, logPLaC3 = (− 4.414 × 104 / T) + 14.557, and log PLaC4 = (− 4.301 × 104 / T) + 15.067, where the units of pressure are newtons per square meter.