High-temperature NMR

NMR experiments were performed on a Bruker Avance 400WB NMR spectrometer, operating at 9.4 T. The crucible is placed inside a specific HT NMR probe, and is heated by a symmetrical irradiation using two CO2 lasers (Coherent, 250 W) [14]. Thanks to the high thermal conductivity of BN, the crucible acts as a small furnace. The power of the lasers is increased slowly up to the liquidus temperature in order to insure a stabilized and homogeneous temperature in the sample. 

For self-diffusion measurements, the HT PFG NMR probe is a 10 mm liquid probe (Bruker design) equipped with a gradient coil providing 5.5G/cm/A and with a gradient amplifier of 10 A. The thermal shielding of the radiofrequency and gradient coils is insured by several ceramic barriers. The NMR pulse sequence used here combines stimulated echoes and bipolar gradient pulses [17]. 

The eutectic composition of LiF-CaF2 (80.5 to 19.5 mol%) were simulated by MD. The interaction potential used in those simulations was calculated with the PIM which consists of a Born-Mayer pair potential forming together with an ionic polarisation [11]. The simulations were performed on a box containing 239 F , 39 Ca and 161 Li+ ions, and for temperatures ranging between 770 and OtWC. 

The local structure of the melt was determined with the calculation of the radial distribution function g(r), and the dynamic information was directly obtained from an estimation of the self-diffusion coefficient. 

A point defect model has been proposed for the brownmillerite structure, based on a study of electrical conductivity and em/measurements of Ba2ln205 [236]. The experimental results matched well with modelling, while evidence was found for protonic conduction at low temperatures. In modelling, the unoccupied oxygen sites relative to the perovskite structure, below Tj, are regarded as structural units and therefore are potential interstitial sites for 

Structure, corresponding to a doubling of the pseudocubic perovskite unit cell, gives rise to a texture consisting of microdomains with a mean size estimated 

This section discusses selected observations from oxygen permeation experiments. General trends have been summarized already in Section 10.6.1. 

In a number of studies, the oxygen fluxes through, e.g., SrCoo.sFeo.203 g have been reported to be significantly lower than claimed by Teraoka et al. [37]. Such 

Using a constant value of Pq at the oxygen-lean side of 2 mm thick disc membranes of Laj. rxCoOs (x = 0.2, 0.3, 0.4, 0.5 and 0.6), Van Doom et al. [148,149] showed that the activation energy for oxygen permeation in the 

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Characterization Techniques for Pitch Materials. Among a number of characterization techniques developed in Japan, the technique due to the members of the Society of Heavy Oil, led by Kunugi, stands out as particularly useful. The analytical data are treated by computer methods to construct average molecular structures for the carbonaceous materials. Sanada s group in Hokkaido University used high-temperature NMR and ESR data obtained by in situ measurements of pitch materials in molten salt (21). Much information on mesophase behavior during the heat-treatment process was obtained in this way. 

Isomerization of cryptand-containing spirobenzopyran 63 (Scheme 3) to the open-colored merocyanine 63 in the presence of Srl2 has been studied by H NMR spectroscopy at 500 MHz. In several deuterated solvents (e.g. CDC13, CD3CN, DMSO-d6) the H NMR signals of 63 were considerably broadened at room temperature. High-temperature NMR (85 °C) in DMSO-d6 succeeded in resolv-... 

By the same method, a mixture of tetrasulfones was pyrolyzed by Vogtle et al. to yield the extremely strained biphenylenophane 174 [62]. The isomer derived from 172 does not form. A four-fold intraannularly phenyl-substituted tetrasulfone was transformed by Vogtle et al. by sulfone pyrolysis into the sterically extremely crowded phane 176 [57]. The high-temperature NMR spectrum of 176 leads to the conclusion that at temperatures of more than 120 C the phenyl rings are able to pass the inner region of the macrocycle. 

The general problem of solution homogeneity represents a major area of concern in high-resolution solution NMR studies of complex hydrocarbon mixtures encountered in coal products. Although alternate NMR approaches could be used or developed (e.g., solid-state NMR, high-temperature NMR, ideal NMR solvents, etc.), the quantitative NMR approach used in the present study can distinguish this problem when complementary elemental combustion data is available. 

Recently, the most reliable high-temperature NMR experiments in fluoroaluminate melts have been performed using the laser-heated system developed by Lacassagne et al. (1997). The experimental device is schematically presented in Figure 10.5. 

Table 10.1. Typical acquisition conditions used for high-temperature NMR experiments...

In these liquids, for all the observed nuclei, the high-temperature NMR spectrum consists of a single, narrow line, characterized by its position (isotropic chemical shift) and its line width. This single sharp line reflects rapid exchange between the different available environments (rapid as compared to NMR time scales ranging from 10 to 10 Hz). Consequently, the observed peak position is the average of the chemical shifts of individual species, weighted by their respective populations. 

In a pioneering high-temperature NMR study, Stebbins et al. (1992) reported Al chemical shifts for four compositions of the NaF-AlF3-Al203 system that were systematically much higher in liquid than in the solid phases. They emphasized the dramatic effect of composition and temperature on the liquid structure and proposed an evolution of aluminum environment toward a higher fraction of a 4-fold coordinated environment of aluminum. 

Investigation of the system NasAlFe-Fe Oj, (Fe Oj, = Fe203, FeO) using high temperature NMR spectroscopy method was performed quite recently by Simko (2004). High-temperature NMR spectra of Al and Na chemical shifts were obtained at 1020°C. 

System Na3AlFs-Fe203. Al and Na high-temperature NMR spectra of molten NasAlFe with Fe203 content ranging from 0.5 up to 1 mole % are characterized by one detached Lorentzian peak. The values of the chemical shift increase linearly from 18.3 ppm for... 

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