Reflection powders

For transmission measurements, samples are typically pressed into thin wafers, whereas for measurements in diffuse reflection powders are used. To suppress regular reflection and particularly to work in a range in which F(p) is proportional to the concentration, samples often have to be diluted. The diluent should be a nonabsorbing standard that does not interact with the sample, and the mixing procedure should be nondestructive (some samples are sensitive to mechanical stress (Section 2.4)). 

Conversely, the use of parafocusing reflection powder diffractometers widely used in other areas of materials science can lead to minor complications. Essenhally, the sample does not absorb sufficiently for it to appear to be infinitely thick as in the case of metals and this will lead to modification to the intensity values which will need to be corrected before interpretation. 

It has recently been shown that the flaming disaster of the Hindenburg Zeppelin in 1937, in which 36 lives were lost, may have been caused by static electricity igniting the outer fabric. This was shown to contain an iron oxide pigment and reflecting powdered aluminum. Such a combination, known as a thermite mixture, results in the highly exothermic Gouldshmidt reaction (first reported in 1898) ... 

Useful as the base for reflecting powders Easy to evacuate in vacuum insulation... 

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

The use of DRIFTS for the characterization of surfaces has to date been limited, but has recently been used for applications in fields as diverse as sensors development [12], soils science [13], forensic chemistry [14], corrosion [15], wood science [16] and art [F7]. Given that there is in general no reason for preferring transmission over difilise reflectance in the study of high-area powder systems, DRIFTS is likely to become much more popular in the near fiiture. 

Figure C2.17.8. Powder x-ray diffraction (PXRD) from amoriDhous and nanocry stalline Ti02 nanocrystals. Powder x-ray diffraction is an important test for nanocrystal quality. In the top panel, nanoparticles of titania provide no crystalline reflections. These samples, while showing some evidence of crystallinity in TEM, have a major amoriDhous component. A similar reaction, perfonned with a crystallizing agent at high temperature, provides well defined reflections which allow the anatase phase to be clearly identified.

More often, however, microporosity is associated with an appreciable external surface, or with mesoporosity, or with both. The effect of microporosity on the isotherm will be seen from Fig. 4.11(a) and Fig. 4.12(a). In Fig. 4.11(a) curve (i) refers to a powder made up of nonporous particles and curve (ii) to a solid which is wholly microporous. However, if the particles of the powder are microporous (the total micropore volume being given by the plateau of curve (ii)), the isotherm will assume the form of curve (iii), obtained by summing curves (i) and (ii). Like isotherm (i), the composite isotherm is of Type II, but because of the contribution from the Type 1 isotherm, it has a steep initial portion the relative enhancement of adsorption in the low-pressure region will be reflected in a significantly increased value of the BET c-constant and a shortened linear branch of the BET plot. 

Fig. 3. Infrared spectra of polystyrene (a) transmission of a 33-p.m thick polystyrene film (b) diffuse reflection of polystyrene powder at a concentration of...

Opacifiers are fine inorganic powders, usually white, that are used to reduce the transparency of ceramic gla2es and porcelain enamels. The coating becomes opaque because the particles of the opacifter scatter and reflect the incident light. When inorganic pigments are combined with white opacifiers, pastel colors are obtained. 

Usually the rate, Q, is far in excess of the required rate, especially if the bulk material consists primarily of coarse particles. Slowing down the discharge rate requires a feeder. Fine powders, on the other hand, have considerably lower maximum discharge rates when exiting from a mass flow bin, because of the interaction between air (or gas) and solid particles as reflected in the permeabiUty of the material. 

The dramatic improvements in the physical and chemical properties of tantalum powder produced by the sodium reduction process are evident in the lessening of chemical impurities (see Table 5). The much-improved chemistry reflects the many modifications to the process put in place after 1990. 

Titanium pyrophosphate [13470-09-2] TiP20y, a possible uv reflecting pigment, is a white powder that crystallizes ia the cubic system and has a theoretical density of 3106 kg/m. It is iasoluble ia water and can be prepared by heating a stoichiometric mixture of hydrous titania and phosphoric acid at 900°C. 

Indexings and Lattice Parameter Determination. From a powder pattern of a single component it is possible to determine the indices of many reflections. From this information and the 20-values for the reflections, it is possible to determine the unit cell parameters. As with single crystals this information can then be used to identify the material by searching the NIST Crystal Data File (see "SmaU Molecule Single Stmcture Determination" above). 

Another indication of the growth of the powder coating market is reflected in the membership statistics of the Powder Coating Institute (PCI). Prom 1987 to 1991 the number of members associated with powder coating manufacturers increased from 5 to 22 equipment suppHers from 3 to 9 custom apphcators from 1 to 29 and suppHers to the powder coating industry from 5 to 38 (78). 

Upases. The idea of using Upases in the wash process dates back to 1913 when O. Rn hm suggested a dding pancreatin [8049-47-6] to detergent formulations. Many patents have demonstrated that Upases can improve the removal of fatty stains when used in powder and liquid detergents, special presoakers, or other cleaning agents. Intense research activity is also reflected in the literature (43—45). 

This technique is based on the Dewar flask, which is a donble-walled vessel with reflective surfaces on the evacuated side to reduce radiation losses. Figure 11-66 shows a typical laboratory-size Dewar. Figure 11-67 shows a semiportable type. Radiation losses can be further reduced by filling the cavity with powders such as perlite or silica prior to pulling the vacuum. 

FIG. 20-76 The yield loci of a powder, reflecting the increased shear stress required for flow as a function of apphed normal load. YLl through YL3 represent yield loci for increasing previous compaction stress. EYL and VYL are the effective and wall yield loci, respectively. 

Usually, particle size has relatively little effect on Raman line shapes unless the particles are extremely small, less than 100 nm. For this reason, high-quality Raman spectra can be obtained from powders and from polycrystalline bulk specimens like ceramics and rocks by simply reflecting the laser beam from the specimen surface. Solid samples can be measured in the 90° scattering geometry by mounting a slab of the solid sample, or a pressed pellet of a powder sample so that the beam reflects from the surface but not into the entrance slit (Figure 3). 

---