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.
Rare‐Earth Polymorphism and Phase Equilibria in Rare‐Earth Oxide‐Water Systems
A high-water-pressure high-temperature study was conducted of the systems Y/sub 2/O/sub 3/-H/sub 2/O, Sm/sub 2/O/sub 3/-H/sub 2/O, N d/sub 2/O/sub 3/-H/sub 2/O, and La/sub 2/O/sub 3/-- H/sub 2/O. The trihydroxides La(OH)/sub 3/ , Nd(OH)a, an d this group. On the other hand, the oxyhydroxides YOOH, SmOOH, and NdOOH are isomorphous; LaOOH, if it exists, cannot be stable in the presence of water at room temperature. Pressure-temperature curves for the equilibrium between the trinydroxide, oxyhydroxide, and a new hydroxy compoud were determined for each of the three smaller ions. The reversibility of the transition between the A-, B-, and C-type rare earths was established, and new x-ray data on the B structure are listed. Ths relations between the polymorphs as a function of ionic size are changed from those given by Goldschmidt. The very marked difference in stability in air between the A- and C-type structures suggests the obvious desirability of converting rareearth oxide cermnics into the C structure with proper compositional variation and thermal treatment. (auth)
Neurology, psychology, and extraordinary religious experiences
Temporal lobe epilepsy and certain personality disorders often result in experiences described as “religious.” TLE research suggests a possible neurological basis for such experiences. Immediately the question arises about the authenticity of these experiences as religious. An experience is authentic if it furthers the authentic growth of the subject, regardless of what triggered it. So pathology may occasion authentic religious experiences, even as history exemplifies. For practical purposes, the further question about God in religious experience is secondary. The exception, miraculous occurrences, should not be granted without sufficient reason. This approach dissolves all conflict between science and faith.