Polycrystalline pellets

The membranes of the other ion-selective electrodes can be either homogeneous (a single crystal, a pressed polycrystalline pellet) or heterogeneous, where the crystalline substance is incorporated in the matrix of a suitable polymer (e.g. silicon rubber or Teflon). The equation controlling the potential is analogous to Eq. (6.3.9). 

The single crystals thus far prepared are too small for many physical property measurements. Also, the two techniques described for preparation of powders result in fine grained (and not sintered) materials which are also not appropriate for many measurements. A different technique, however, was recently developed which produces near theoretical density polycrystalline pellets (56). Stoichiometric mixtures of BaO, K02 and Bi2Os are mixed and melted in N2 gas and quickly quenched onto a copper block under the N2 atmosphere. The process must be performed... 

Considerable work has been devoted to the development of solid membranes that are selective primarily to anions. The solid-state membrane can be made of single crystals, polycrystalline pellets, or mixed crystals. The resulting solid-state membrane electrodes have found use in a great number of analytical applications. 

P, As) in which the aromatic molecules occur in stacks this situation results in metal-like electrical conductivity (a) (a = 0.12 0.046 ft-1 cm-1 for a polycrystalline pellet). This finding suggested the possibility of analogous behavior in the fluoroaromatic series. Materials containing dimeric cations, however, have not been isolated from reaction mixtures containing excess amounts of the neutral monomers or from controlled reduction of monocation salts. In each case, the cations are monomeric and magnetically independent of one another. 

Polycrystalline Pellet. Approximately 10 g. of V2O6 powder was placed... 

Inorganic Ions in Glasses and Polycrystalline Pellets as Huorescence Standard Reference Materials, Chapter in Accuracy in Spectrophotometry and Spectrofluorimetry, Ed. by N. B. S. (1972). 

Figure 13. (A) Variable temperature electrical conductivity data of (V) as pressed polycrystalline pellet and in "single crystal form. (B) Variable temperature thermolectric power data (Seebeck coefficient) of (V) as pressed polycrystalline pellet and in "single" crystal form.

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). 

For comparison, figure 3B shows results not only for the relatively most stable face of a single crystal (2) and a polycrystalline thin layer prepared by pasting 5), but also for a pressed pellet of CdSe (2). This last electrode shows stability behaviour intermediate between that of the two other electrodes. This behaviour is evident also in the decrease in output stability, which is less steep than that of the single crystal electrode. 

The particle size of many synthetic zeolites is too small for most applications and therefore they must be formed into polycrystalline aggregates (e.g. by pelletization). Binders are often added to improve the aggregate strength and durability. It must be kept in mind that these or other changes in the particle or aggregate morphology may significantly affect the equilibria or dynamics of adsorption. 

Conductivity of (BN) iSO F and comparison with Cg SO F. In our early studies (12), a four-probe technique was employed, in which four platinum wires were used for electrical contact, and the samples were prepared by pressing powdered polycrystalline material into pellets. Because the platinum wires and the pellet surface are not ideally flat, a uniform intimate contact could not be assured between the wires and the pellet. The boundary effects due to the polycrystalline nature of the pellet sample also render such conductivity measurements unreliable. Attempts to use a contactless radio frequency inductive technique described by Zeller et al. (22) failed because this technique is not sensitive to low conductivities. A four-point probe measurement (21) on an intercalated highly oriented boron nitride sample was used in the present set of conductivity measurements. The <7295k 1.5Scm . The specific conductivity increased with decreasing temperature (see Fig. 1), it having nearly twice the room temperature value at 77 K. This indicates metallic behavior. 

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