Equilibration temperature

Advantages and disadvantages of HS-GC over regular GC are summarised in Table. 4.26. HS-GC fingerprinting chromatograms obviously include only the volatile components present and do not provide a complete picture of sample composition on the other hand, when solvent extraction is used, all the soluble sample constituents are removed, including also those having no appreciable vapour pressure at the equilibration temperature. Headspace analysis enhances the peaks of volatile trace components. 

The peak temperatures that asteroids experienced can be estimated from chemical exchange reactions between minerals - so-called geothermometers. For example, the exchange of calcium between coexisting orthopyroxene and clinopyroxene in highly metamorphosed chondrites has been used to estimate their equilibration temperatures (Slater-Reynolds and McSween, 2005). For ordinary chondrites, these temperatures range up to -1175 K. The experimental conditions at which achondrites melt provide minimum temperatures for their parent bodies. Melting of achondrites typically requires temperatures of>1200 K. 

In practice, equilibration protocols can be rather involved. Large portions of the system may be held frozen initially while subregions are relaxed. Ultimately, the entire system is relaxed (i.e., all the degrees of freedom diat are being allowed to vary) and, once the equilibration temperature has reached the desired average value, one can begin to collect statistics. 

Water equilibration Temperature, °C E /Ed % Vbwfobd % Water uptake, %... 

Maintain a layer of solid salts in the slurries during the whole period of equilibration to confirm that the solutions always remain saturated. Generally, the solutions should be slushes with 4 mm of solution over the crystal layer. At each equilibration temperature used, the aw (ERH) of the atmosphere above the saturated solution will be different (Table A2.3.1 and Table A2.3.2). 

Metals distribution, which in some degree should relate to asphaltene distribution, are shown for the actual equilibration temperatures in Table IX. Nickel showed the expected enrichment at all temperatures. Vanadium responded similarly except at 403 and 603°F. Progressive sulfiding (0.6, 0.7, 1.4% S) and coke lay-down (0.8, 1.5, 7.8% C) were observed for the used catalysts and hence represent an experiment complication. 

Stuart et al. [127] studied the analysis of volatile organic compounds using an automated static headspace method. Recoveries decreased in the following order water, pure sand, sandy soil, clay and topsoil. A full evaporation technique that uses little or no aqueous phase and higher equilibration temperature gave the most reproducible analyte recoveries. 

Annealing experiments on meteorites and terrestrial feldspars, and studies of separated components from meteorites, suggest that the high and low temperature forms of feldspar have distinctive TL peak temperatures and widths and thus provide a new means of exploring equilibration temperatures and cooling rates. 

Air-purge = 75mL/min, cell pressure (P) = ambient, equilibration temperature = 25 °C. 

Oxygen thermometry for minerals in igneous and metamorphic rocks was surveyed by Epstein and Taylor (1967). These authors reviewed older analytical and experimental work and showed how useful many equilibrium silicate and oxide pairs (quartz-muscovite, quartz-magnetite, quartz-gamet among many others) can be for recalculation of equilibration temperatures. 

The comparable equilibrium behavior of p-type a-Si H films is shown in Fig. 6.7. The density of band tail holes, has a smaller activation energy than the n-type material and is constant at the highest doping levels. The increase in conductivity activation energy at the equilibration temperature is less obvious than in the n-type material (see Fig. 5.2), but the frozen state is identified by the dependence on thermal history. The equilibration temperature is lower than in n-type material by about 50 °C and the relaxation times in Fig. 6.5 are also correspondingly shorter. 

Thermal equilibration effects are also present in undoped a-Si H (Smith et al. 1986). The dangling bond density varies reversibly with temperature, as is shown in Fig. 6.8 for samples of different deposition conditions. The experiment is performed by rapidly cooling the material from different temperatures to freeze in the equilibrium configuration and measuring the ESR spin density of the g = 2.0055 resonance. The defect density increases four-fold between 200 C and 400 °C, with an activation energy of 0.15-0.2 eV. The relaxation times are slower than in the doped material at the same temperature, and the equilibration temperature of about 200 °C, for a cooling rate of... 

Table 6.2. Comparison of the measured and calculated equilibration temperatures for different doping types...

Conductivity data for n-type and p-type samples above the equilibration temperature are shown in Fig. 7.4 and similar data are in Fig. 5.2. The conductivity is activated with a prefactor of 100-200 cm for both doping types and the activation energy is 0.3-0.4 eV in n-type material and 0.4-0.6 eV in p-type. In the thermal equilibrium regime, the Fermi energy is pinned by the defect and dopant states and consequently the statistical shift is small, as is discussed in Section 6.2.2. Yp may be calculated from a numerical... 

The first term on the right hand side of Eq. (7.18) is the usual statistical shift determined at a constant electron density. However, the electron density is itself temperature dependent above the equilibration temperature because of the structural changes. The second term on the... 

Fig. 7.4. Temperature dependence of the conductivity above the equilibration temperature. The top left comer shows the high temperature...

The electronic structure is frozen below the equilibration temperature and the density of electrons is constant. The second term in Eq. (7.18) is therefore zero and the statistical shift of the Fermi... 

The origin of these effects is not at all clear. The bias effects and the room temperature persistent photoconductivity have similar annealing properties. There is also an obvious similarity between the annealing curves and those for the frozen-in excess conductivity of bulk doped a-Si H (e.g. Fig. 6.3). It is therefore probable that carrier-induced defect creation is the origin of the changes in conductivity and that annealing to the equilibration temperature restores the initial state. However there is as yet no complete explanation for the non-ohmic behavior and why it depends on the applied bias. 

Many minerals contain minor elements whose distribution among possible sites can be ordered or disordered. Olivine, for example, in addition to Mg, Si, and O usually contains Fe and smaller amounts of Mn and Ni, which substitute for Mg atoms in the two nonequivalent Ml and M2 sites. The distribution of Mg, Fe, Mn, and Ni in Ml and M2 sites in a specimen of San Carlos olivine was determined by Tafto and Spence (1982) using ALCHEMI, and more recently, McCormick, Smyth, and Lofgren (1987) have used the same technique to determine the site-occupancies of these minor elements in synthetic olivines as a function of equilibration temperature. Recall from Section 7.4 that ALCHEMI is not fully quantitative for the determination of site-occupancy in some structures because of the so-called localization effect. However, because these studies of olivine indicate that planar ALCHEMI can be used with confidence in this important mineral, it is therefore appropriate to consider this particular application of the technique as an example. 

The results of McCormick et al. (1987) indicate that the degrees of ordering at different equilibration temperatures differ by less than the analytical errors, although there may be significant differences in ordering between olivines that have been quenched from temperatures of 300° C (or higher)... 

Figure 8.54. Proportions of Fe, Mn, and Ni in Ml sites of San Carlos olivine (solid symbols, Tafto and Spence 1982) and in synthetic olivine as a function of equilibration temperature (open symbols, McCormick et al. 1987). Error bars are 1 e.s.d.

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