High temperature phases, laboratory

In general, high-temperature phases are prepared in the laboratory by... 

For the following discussions, the results are conveniently presented according to the interests or emphases of a particular laboratory effort. Much recent work has come from various authors while at the Ames Laboratory. A wealth of information is available for the simplest rare earths (La and Ce) that have no high-temperature phase changes. The reader is directed to an excellent summary of work on the CeH system up to 1985, by Schlapbach et al. (1986a). In early NMR studies by Barnes et al. (1980), a clear transition at about 210 K is attributed to a metal-to-semiconductor transition. Also, a narrow band gap ( 0.01 eV) appears below that temperature, as discussed at some length in sections 6.1 and 6.2. The authors also identify the off-center displacement of o-site atoms. 

The intermetallic Ni-Ti system has the imusual property of after being distorted, returning to its original shape when heated. This was the first of the shape memory alloys (SMAs) and was discovered by accident at the Naval Ordnance Laboratory, hence its name Nitinol. Other SMAs include Cu-Al-Ni, Cu-Zn-Al, and Fe-Mn-Si alloys. The shape memory mechanism depends on a martensitic solid-state phase transition that takes place at a modest temperature (50°C—150°C), depending on the alloy. The high temperature phase is referred to as austenite and the low temperature phase is called martensite (following the terminology of the Fe-FeCa system). 

R. Dolloff. Research Study to Determine the Phase Equilibrium Relations of Selected Metal Carbides at High Temperatures, Research Laboratory National Carbon Company, Wadd Technical Report, Parme, Ohio, 1960, p. 60. 

It should be emphasized that very little is quantitatively known about how the total pressures of plutonium-bearing species vary with oxygen potential, stoichiometry, and temperature in the bivariant region of oxygen-deficient plutonia between the phase limits at very high temperatures. New (though limited) oxygen potential data have been obtained in our laboratory above the fluorite, diphasic, and sesquioxide phases in the Pu-0 system at 1750, 2050, and 2250 K. 

Thermochemical data are also available from the Internet. Some examples are the NIST Chemical Kinetics Model Database (http //kinetics.nist. gov/CKMech/), the Third Millennium Ideal Gas and Condensed Phase Thermochemical Database for Combustion (A. Burcat and B. Ruscic, ftp //ftp. technion.ac.il/pub/supported/aetdd/thermodynamics/), and the Sandia National Laboratory high-temperature thermodynamic database (http //www.ca.sandia. gov/HiTempThermo/). 

Acenaphthene occurs naturally in coal tar. Based on laboratory analysis of 7 coal tar samples, acenaphthene concentrations ranged from 350 to 12,000 ppm (EPRl, 1990). Detected in Dyr aged coal tar film and bulk coal tar at concentrations of 5,800 and 5,900 mg/kg, respectively (Nelson et al, 1996). A high-temperature coal tar contained acenaphthene at an average concentration of 1.05 wt % (McNeil, 1983). Lee et al. (1992a) equilibrated 8 coal tars with distilled water at 25 °C. The maximum concentration of acenaphthene observed in the aqueous phase was 0.3 mg/L. 

Reactions in gas phase are not commonly applied in laboratory seale because of the need of high temperature, other extreme conditions and difficult instruments in most eases, but three examples where gases were introduced into flow reaetor will be presented here. 

Waste-Form Stability. If they occur at all, solid state transformations in dry glass and UO2 matrices will be too slow under the temperature conditions of service to be observable in the laboratory at the same temperature. Here we need to extrapolate from high temperature laboratory conditions to low temperature service conditions. It will be desirable to develop an intimate knowledge of the processes of phase separation and devitrification of sodium borosilicate glasses at temperatures below the softening point by meticulous application of electron microscopic. X-ray crystallographic and other techniques. The glasses will contain inactive elements representative of the fission product... 

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