Oxide grains

Standard ceramic processing conditions are controlled so that the resulting ceramic microstmcture is composed of semiconducting 2inc oxide grains (pZnO < IHcm) and electrically insulating grain boundaries > 10 0cm)). The microstmcture is thus similar to that of the BaTiO thermistor and... 

Both single-layer protective and duplex-layer protective scales have a PBR of 2.1, whereas breakaway scales show a PBR of approximately 2.7, containing around 23% porosityand graphitic carbon grains (up to 6% by weight) between the oxide grains... 

Substantial abundance anomalies occur among the heavy oxygen isotopes 170 and 180, which are underabundant by up to about 4 per cent relative to 160 in oxide grains of certain of the CAIs, compared with the bulk composition in which the isotope ratios are closer to a terrestrial standard. The intriguing feature of these anomalous ratios is that, in common with some other meteorites, but in contrast to terrestrial and lunar samples, the relative deviations of the two heavy isotopes are equal most normal fractionation processes would cause 180 to have twice the anomaly of 170, as indeed is observed in terrestrial samples and more differentiated meteorites, where the anomalies are also usually much smaller. While there has been speculation that there might be a substantial admixture of pure 160 from a supernova, there are fractionation mechanisms that may be able to account for the effect, e.g. photo-dissociation of molecules affected by selfshielding (R. Clayton 2002). In this case, it is possible that the terrestrial standard is enriched in the heavy O-isotopes while the inclusions have more nearly the true solar ratio. 

Fig. 2. Fe-oxide grains occurring as small inclusions in an actinolite grain from a till sample collected near the Discovery outcrop. Scale bar is 100 pm in length.

Other meteorite classes like C2, CO and ordinary chondrites contain much smaller inclusions less than 1 mm (MacPherson et al. 1988) and require ion microprobe techniques to evaluate the isotopic compositions. On the least metamorphosed side. Cl have very few inclusions or oxide grains, but are the carrier of the greatest amounts of stellar nanodiamond and other carbides (Anders and Zirmer 1993). As will be shown for Cr anomalies in carbonaceous chondrites, the survival of the mineral carriers of the anomalies also depends on the metamorphic grade of the meteorites. Nevertheless, isotopic anomalies have also been formd in higher metamorphic grade from other classes, especially in the reduced enstatite chondrites. 

Hibonite rich inclusions of the CM2 and oxide grains in carbonaceous chondrites... 

Nittler LR, Alexander CMOD, Walker RM, Gao X, Zinner EK (1994) Meteoritic oxide grain from supernova found. Nature 370 443-446... 

Figure 2.3 shows a schematic view of the nano crystalline sensor material. It consists of single-crystalline tin-oxide grains with a typical size of 10 nm and a narrow size distribution [68]. The grains are in loose contact. The lower graph in Fig. 2.3 schematically represents the conduction band of the layer. 

The preparation of a ferrofluid emulsions is quite similar to that described for double emulsions. The starting material is a ferrofluid oil made of small iron oxide grains (Fe203) of typical size equal to 10 nm, dispersed in oil in the presence of an oil-soluble surfactant. The preparation of ferrofluid oils was initially described in a US patent [169]. Once fabricated, the ferrofluid oil is emulsifled in a water phase containing a hydrophilic surfactant. The viscosity ratio between the dispersed and continuous phases is adjusted to lie in the range in which monodisperse fragmentation occurs (0.01-2). The emulsification leads to direct emulsions with a typical diameter around 200 nm and a very narrow size distribution, as can be observed in Fig. 1.33. 

Polished thin sections or iron oxides grains polished in epoxy mounts were analyzed using Universite Laval CAMECA SX-100 5-VVDS electron microprobe under a beam of 15 kV at 100 nA, using a range of natural and synthetic standards. After counting over the peak for 20 to 30 sec, background is measured on both sides for 10 sec. These settings yield minimum detection limits (mdl) as low as 20 ppm for elements such as K, Ca, Al, Si, Ti and Mg, 50 ppm for Mn, Cr and V, 200 ppm for Cu,... 

Most presolar silicon carbide and oxide grains and a significant fraction of presolar silicate grains found in meteorites come from low- to intermediate-mass stars in the asymptotic giant branch (AGB) phase (see Chapter 3). Evidence for this conclusion derives from two sources (1) spectroscopic observations of the envelopes of these stars and (2) comparison... 

Oxygen isotopic compositions of presolar oxide grains compared with those of red giant stars. Stellar data are shown without error bars, which are large on this scale. Both data sets are characterized by higher 170/160 ratios and lower 180/160 ratios compared to solar oxygen. Stellar data from Smith and Lambert (1990). 

There is clear evidence that live 26A1 was incorporated into presolar silicon carbide and aluminum oxide grains (see Chapter 5). These grains acquired their 26 Al in the atmospheres of the dying stars in which they formed. It had almost certainly decayed away long before the grains reached the solar system. In presolar grains,26 Al serves as a probe of nucleosynthesis in the parent stars. 

The obtained CNT-Fe-Al203 powder is composed of clean isolated CNTs and small-diameter bundles of CNTs, surrounding all the oxide grains as a web. SEM and HRTEM studies have shown that most CNTs are SWCNTs or DWCNTs (80%), with only a small proportion of three- to six-walled CNTs, and have external diameters between 0.7 and 5 nm. The method has been extended to MgAl204 (Fig. 12.3) and MgO based powders, using Mg(i A.)MxAl204 or Mg,, M/) (M = Co, Fe or Ni) as starting materials.28,29 The products which were used to prepare dense composites were CNT-Fe-... 

Fig. 6. Electric space charge layer in small oxide grains on metal films...

Fig. 8 demonstrates the curving of the bands in the three oxides. Because the diameter of the oxide grains is about ten times smaller than the thickness of a space charge layer, the charge carrier concentration in the grains corresponds to the arrows marked II. At the bottom of Fig. 7 is shown how the various experi-... 

Before going on, a short remark is necessary regarding the material to which the measured activation energy may be attributed. The oxide layer on the metal support is not coherent. The approximately 1000 A thick oxide grains do not... 

The reaction may take place at the same time, but independently, on the oxide grains and on the metal surface not covered by oxide. 

The reaction may take place on the oxide grains only. The electron concentration of the grains is changed by the formation of a space charge layer. 

Nittler, L. R. (1997) Pre solar oxide grains in meteorites. In Astrophysical Implications of the Laboratory Study of Presolar Materials, T. J. Bematowicz E. K. Zinner, Eds., pp. 59-84. AIP Conf. Proc. 402. 

H. Li, F. Czerwinski, J.A. Szpunar. The role of oxide grain boundary character distribution in nickel oxidation kinetics // Defect Diffusion Forum.- 2001.- V. 194-199.-P.1683-1688. 

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