Impurity phase

Microstructurc. Crystal size, porosity, and impurity phases play a major role in fixing the fracture characteristics and toughness of an abrasive grain. As an example, rapidly cooled fused aluminum oxide has a microcrystalline stmcture promoting toughness for heavy-duty grinding appHcations, whereas the same composition cooled slowly has a macrocrystalline stmcture more suitable for medium-duty grinding. 

Fig. 4.5. Schematic of top left corner of the "silicon-impurity" phase diagram. To make things simple, we assume that the liquidus and solidus lines ore straight. The impurity concentration in the solid is then always less than that in the liquid by the factor k (called the distribution coefficient).

MB 20- solutions of M atoms in /3-rh. B MB. ioo hectoborides, various modifications of B and impurity phases... 

These impurity phases deteriorate the catalytic activity due to reduction of surface areas. 

Apart from these, there are volume defects that cannot conveniently be described in any other terms. The most important of these consist of regions of an impurity phase—precipitates—in the matrix of a material (Fig. 3.39). Precipitates form in a variety of circumstances. Phases that are stable at high temperatures may not be stable at low temperatures, and decreasing the temperature slowly will frequently lead to the formation of precipitates of a new crystal structure within the matrix of the old. Glasses, for example, are inherently unstable, and a glass may slowly recrystallize. In this case precipitates of crystalline material will appear in the noncrystalline matrix. 

The electron paramagnetic resonance experiments on the yttrium ceramics, on the other hand, are ambiguous. In the study by Murrieta et al. [55], the EPR signal of YBa2Cu307 sample was interpreted as a superposition of two different lines attributed to the Cul and Cu2 sites. In some other studies of yttrium ceramics, the EPR signal was not detected or was attributed to an impurity phase [56]. Thus, Ilirther more refined EPR experiments are needed to confirm the location of the unpaired spin in the cluster. 

Phase purity of the sample. Crystalline impurity phases present in a powder sample (e.g. residual amounts of starting materials from a synthetic procedure) contribute additional peaks to the experimental powder XRD pattern. As a result, the pattern may look substantially different from that of a pure sample of the main phase. Clearly, careful inspection should be carried out to assess the presence of impurity phases, particularly with regard to residual amounts of unreacted starting materials. 

The quality of a ceramic sample is a function of the degree to which it consists of the desired product. An essential tool for the characterization of a polycrystalline sample is powder X-ray diffraction. The powder pattern is a fingerprint of the sample. For a sample to be declared single phase, all low angle peaks (below 60 20 for CuKq radiation) which are above the noise must be accounted for. Powder X-ray diffraction is often unable to see impurity phases present below the 5% level. Visual inspection (using a microscope)... 

The original specific heat experiments on BaPb xB Og by Methfessel et al (60) immediately raised the prospect that an unusual mechanism was operative in this newly found system. Their finding of no heat capacity anomaly at Tc could actually have a number of possible interpretations, including an impurity phase giving rise to superconductivity, a non-phonon mechanism, or some new form of conductivity. 

Copper acetate was used in Ref. 38 it was noted that if chloride was used instead of acetate, no deposition occurred, and this was attributed to adsorption of chloride on the substrate (Pt). The berzelianite phase with a small amount of umangite impurity was obtained. The composition and phase of the film could be altered by electrochemical cathodic polarization (in an aqueous K2SO4 solution). Initially, there occurred an increase in lattice parameters and decrease in x (Cu2-A Se). With continued polarization, a phase change occurred until eventually only orthorhombic Cui xSe was present in the film. The umangite phase also disappeared, and it was believed that this impurity phase catalyzed the phase transformation. The change in composition during cathodic polarization was attributed to reduction of zerovalent Se to Se, which was dissolved in the solution. Based on the study of Fohner and JeUinek [41] discussed earlier, this explanation can be interpreted as reduction of Sei ( monovalent Se) to Se (divalent Se). 

F — ferromagnetic A — antiferromagnetic P — paramagnetic imp — impurity phase SR — spin reorientation Tq — Curie temperature Tn — Ndel temperature a and c — tetragonal lattice parameters, z — coordinate of B with c as its unit. 

Microstruvture. Cry stal size, porosity, and impurity phases play a major role in fixing the fracture characteristics and toughness of an abrasive grain. 

The carbothermal nitridation of oxides is a common method of preparation of nitrides. It is generally applicable to nitrides of high thermal stability like TiN, VN and CrN. Its main drawback is that it produces impure phases. 

It is important to point out that the observed two-component EPR spectra are an intrinsic property of the lightly doped LSCO and are not due to conventional chemical phase separation. We examined our samples using x-ray diffraction, and detected no impurity phases. Moreover, the temperature dependence of the relative intensities of the two EPR signals rules out macroscopic inhomogeneities and points towards a microscopic electronic phase separation. The qualitatively different behavior of the broad and narrow EPR signals indicates that they belong to distinct regions in the sample. First we notice that the broad line vanishes at low temperatures. This... 

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