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.
ZIRCONIA AND CERIA BASED CERAMICS AND U AND MECHANICAL PROPERTIES
Zirconia (ZrO 2 ) and ceria (CeO 2 ) based ceramics is widely applied in electrochemical devices due to its high ionic conductivity coupled with high mechanical properties. Nowadays, the task of the decrease in the working temperature of such devices is of current interest, there are a lot of papers dealing with this problem. The present paper considers the conductivity vs composition dependencies for such ceramics, it is shown that triple systems are very perspective compositions, especially with hafnium oxide additions. The effect of ceramics microstructure on the conductivity is discussed, in particular the dependence of the grain size on the electrochemical properties, a special attention is paid to nanosized ceramics. Experimental results obtained in this field are compared with those computed using BLM model for microsized case and n-GCM model for nanosized one. The data on the grain size effect on the ceramics mechanical properties are summarized, it is shown that the decrease in the grain size usually results in the ceramics hardening.
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