Rare earth oxide vaporization

For the synthesis of materials, the reactants are placed in the copper crucible. An arc is struck by allowing the cathode to touch the anode. The current is raised slowly while the cathode is simultaneously withdrawn so as to maintain the arc. The arc is then positioned so that it bathes the sample in the crucible. The current is increased until the reactants melt When the arc is turned off, the product solidifies in the form of a button. Because of the enormous temperature gradient between the melt and the water-cooled crucible, a thin solid layer of the sample usually separates the melt from the copper hearth in this sense, the sample forms its own crucible and hence contamination with copper does not take place. Contamination of the sample by tungsten vaporizing from the cathode can be avoided by using water-cooled cathodes. The arc method has been successfully used for the synthesis of various oxides of Ti, V and Nb. A number of lower-valence rare-earth oxides, LnO, 5 have been prepared by arc fusion of LnjOj... 

Materials which have extremely low volatility and the potential to serve therefore as non-reactive substrates and supports include zirconia, titania, and alumina. The rare earth oxides, represented by lanthana and ceria, also show very low volatility and can be expected to resist vapor-transport-assisted sintering and corrosion. Of the alkaline earth oxides, MgO is superior to all others, although the alkaline earth aluminate complex oxides can be much more stable (e. g., by a factor of 1000 for BaO) than the alkaline earth oxides. 

The direct oxidation of propylene by molecular oxygen is a low-selective reaction. The propylene oxide yield can be raised by limiting the conversion rate to a low value, about 10 to 15 per cent, by using more selective catalysts, or by achieving co-oxidation with a more oxidizable compound than propylene (acetaldehyde, isobutyraldehyde etc.). Many patents have been Hied concerning this process, but without any industrial implementation. Among them is the liquid phase oxidation of propylene on a rare earth oxide catalyst deposited on silica gel (USSR), or in the presence of molybdenum complexes in chlorobenzene or benzene (JFP Instiiut Francois du Petrole. Jefferson ChemicalX vapor phase oxidation on modified silver catalysts (BP British Petroleum IFP, or on ... 

Several rare earth element oxides are components of high 7 superconducting materials. Their preparation is discussed elsewhere in this book (see Chapter 2). In general, rare earth metal oxides can be obtained by the chemical vapor deposition of the appropriate metal /S-diketonates or carboxylates [96]. Volatile metal alkoxides also should be potentially useful precursors for the preparation of rare earth oxides by CVD. Although several volatile rare earth element alkoxides have been reported in recent years [97-101], detailed information concerning their decomposition behavior is not available at this time. 

Trevisan, G., Depaus, R., Non-congruent vaporization of some rare earth oxides, Z. Naturforsch., 28a, (1973), 37-47. Cited on page 568. 

The enthalpies of formation of the gaseous species of the rare earth oxides are generally derived from mass spectrometric studies. The enthalpy of formation of a gaseous rare earth monoxide can be derived from the vapor pressure of the MO species and its rate of change as a function of temperature above the condensed phase ... 

Another method often employed is to minimize vaporization differences by using a spark-type of excitation which introduces the sample into the excitation zone by a sputtering process. At the Ames Laboratory, this second approach has been emphasized. Uniarc excitation, which is a combination of a spark and a unidirectional ac arc, has provided reliable analyses with adequate sensitivity for many impurities in most of the rare earth elements, and with a precision of better than 10%. A briquetted sample of one part rare earth oxide mixed with nine parts of pelleting graphite is used. The complex rare earth matrix spectra often causes spectral interference with the more sensitive spectral lines of impurity elements. For Pr and Nd, which have the most complex spectra, this problem is particularly serious, and alternate analytical approaches must be used if determinations at lower concentration levels are required. 

Some elements, such as the rare earths and the refractory metals, have a high affinity for oxygen, so vaporization of these elements in a normal vacuum of about 10 4 Pa, would lead to the formation of at least a surface layer of oxide on a deposited film. The evaporation of these elements therefore requires the use of ultra-high vacuum techniques, which can produce a pressure of 10-9 Pa. 

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