Microcrystalline properties

The microcrystalline properties of metals are particularly influ ced by irradiation. Although low-alloy steel in modem reactor tanks are rather radiation resistant (provided they are free of Cu, P and S in urities), stainless steel (e.g. of the 18% Cr, 8% Ni type) has been found to become brittle upon irradiation due to the formation of microscopic helium bubbles, probably due to n,a reactions in Fe and inq)urities of light elem ts (N, B, etc.). This behavior is accentuated for metallic uranium in reactors because of the formation of fission products, some of which are gases. As a result of this radiation effect it is not possible to use uranium metal in modem power reactors, where high radiation doses are accumulated in a very short time. The fuel elemrats for power reactors are therefore made of nonmetallic uranium compounds. 

Microcrystalline properties of the plasticized PVC depend on the PVC type, plasticizer type and conditions of processing (heating and cooling temperatures and rates). These compositions and conditions may be varied to obtain required properties but there is a limit of how far creep and high temperature properties can be adjusted by formulation and processing characteristics. If lesser creep or stability of shape at elevated temperatures are required, then the polymer must be modified by the use of reactive plasticization. Figure 11.56 shows that the creep rate decreases with increased crosslink density caused by reactive plasticizer (triallyl cyanurate). The crosslinking increases the gel content and decreases the... 

Cane sugar is generally available ia one of two forms crystalline solid or aqueous solution, and occasionally ia an amorphous or microcrystalline glassy form. Microcrystalline is here defined as crystals too small to show stmcture on x-ray diffraction. The melting poiat of sucrose (anhydrous) is usually stated as 186°C, although, because this property depends on the purity of the sucrose crystal, values up to 192°C have been reported. Sucrose crystallines as an anhydrous, monoclinic crystal, belonging to space group P2 (2). 

Microcrystalline ceUuloses ate marketed under the trade name Avicel. The physical characteristics of microcrystalline ceUuloses differ markedly from those of the original ceUulose. The ftee-flowiag powders have particle sizes as smaU as 0.2—10 p.m. Avicel ceUuloses coated with xanthan gum, guar gum, or carboxy-methylceUulose to modify and stabilize their properties are also available. The Avicel products are promoted for use ia low calorie whipped toppiags andiciags andia fat-reduced salad dressiags and frozen desserts (see Fat substitutes). 

Bacterial Cellulose. Development of a new strain of Acetobacter may lead to economical production of another novel ceUulose. CeUulon fiber has a very fine fiber diameter and therefore a much larger surface area, which makes it physicaUy distinct from wood ceUulose. Its physical properties mote closely resemble those of the microcrystalline ceUuloses thus it feels smooth ia the mouth, has a high water-binding capacity, and provides viscous aqueous dispersions at low concentration. It iateracts synergisticaUy with xanthan and CMC for enhanced viscosity and stabUity. 

It may occasion surprise that an amorphous material has well-defined energy bands when it has no lattice planes, but as Street s book points out, the silicon atoms have the same tetrahedral local order as crystalline silicon, with a bond angle variation of (only) about 10% and a much smaller bond length disorder . Recent research indicates that if enough hydrogen is incorporated in a-silicon, it transforms from amorphous to microcrystalline, and that the best properties are achieved just as the material teeters on the edge of this transition. It quite often happens in MSE that materials are at their best when they are close to a state of instability. 

Polynuclear transition metal cyanides such as the well-known Prussian blue and its analogues with osmium and ruthenium have been intensely studied Prussian blue films on electrodes are formed as microcrystalline materials by the electrochemical reduction of FeFe(CN)g in aqueous solutionThey show two reversible redox reactions, and due to the intense color of the single oxidation states, they appear to be candidates for electrochromic displays Ion exchange properties in the reduced state are limited to certain ions having similar ionic radii. Thus, the reversible... 

The presence in these copolymers of hetero-substituted monomeric units randomly dispersed along the phosphazene skeleton brings about the extreme difficulty of the polymeric chains to be packed in regular structures. They lose, therefore, the original stereo-regularity of the parent phosphazene homopolymers (microcrystalline materials), and show only amorphous structures, with sharp decrease in the values of the Tg (collapsed up to about -90 °C) and with the onset of remarkable elastomeric properties [399,409,457]. 

Experimental observations exist according to which a foreign substrate may influence the catalytic properties of the microcrystalline metal supported by it, and the supported metal conversely may influence the catalytic properties of the substrate. 

These aspects of solvent property similarly apply to precoated impregnated silica gel plates, e.g., by ammonium sulfate, silver nitrate, or magnesium acetate, as well as to microcrystalline cellulose precoated plates. On preparative RP phases, water has the lowest elution power. Therefore, more polar or aqueous solvents should be preferred. In contrast to HPTLC RP-18 layers, on which such aqueous solutions remain as a drop on the surface and are not able to penetrate through the lipophilic layer, on preparative RP phases, pnre aqneons application solutions can be apphed owing to the minor degree of C18 modification. 

Burton JJ. 1974. Structure and properties of microcrystalline catalysts. Catal Rev 9 209-222. 

The electrical properties (dark conductivity and photoconductivity) are reported to first decrease and then increase upon increasing power [361]. The optical bandgap increases with increasing power, due to the increase of the hydrogen content [63, 82, 362, 363]. However, at very high power levels, microcrystalline silicon is formed [364], which causes the hydrogen content (and, consequently, the bandgap) to decrease. 

For suspensions primarily stabilized by a polymeric material, it is important to carefully consider the optimal pH value of the product since certain polymer properties, especially the rheological behavior, can strongly depend on the pH of the system. For example, the viscosity of hydrophilic colloids, such as xanthan gums and colloidal microcrystalline cellulose, is known to be somewhat pH- dependent. Most disperse systems are stable over a pH range of 4-10 but may flocculate under extreme pH conditions. Therefore, each dispersion should be examined for pH stability over an adequate storage period. Any... 

It may sometimes by necessary to supplement the properties of the drug so that it compresses more easily, and these needs have been realized by several manufacturers of excipients. Materials described as compression aids are now commercially available. Ideally, such adjuvants should develop mechanical strength while improving, or at least not adversely affecting, release characteristics. Among the most successful at meeting both these needs have been the microcrystalline celluloses (partially acid-hydrolyzed forms of cellulose). A number of grades are available based upon particle size and distribution. 

Although the true waxes, such as beeswax, are esters of fatty acids with alcohols, the term is often applied to certain solid substances which melt at fairly low temperatures, e.g., paraffin wax. Paraffin waxes (crystalline and microcrystalline) are obtained from the refining of petroleum, and are used in rubber compounding as protective agents. They have the property of blooming to the surface of the product, forming a thin film which replenishes itself if removed in service. 

Measurements of particle porosity are a valuable supplement to studies of specific surface area, and such data are particularly useful in the evaluation of materials used in direct compression processes. For example, both micromeritic properties were measured for several different types of cellulosic-type excipients [53]. Surface areas by the B.E.T. method were used to evaluate all types of pore structures, while the method of mercury intrusion porosimetry used could not detect pores smaller than 10 nm. The data permitted a ready differentiation between the intraparticle pore structure of microcrystalline and agglomerated cellulose powders. 

The sorption of water by excipients derived from cellulose and starch has been considered by numerous workers, with at least three thermodynamic states having been identified [82]. Water may be directly and tightly bound at a 1 1 stoichiometry per anhydroglucose unit, unrestricted water having properties almost equivalent to bulk water, or water having properties intermediate between these two extremes. The water sorption characteristics of potato starch and microcrystalline cellulose have been determined, and comparison of these is found in Fig. 11. While starch freely adsorbs water at essentially all relative humidity values, microcrystalline cellulose only does so at elevated humidity values. These trends have been interpreted in terms of the degree of available cellulosic hydroxy groups on the surfaces, and as a function of the amount of amorphous material present [83]. 

Property measurements of fullerenes are made either on powder samples, films or single crystals. Microcrystalline C6o powder containing small amounts of residual solvent is obtained by vacuum evaporation of the solvent from the solution used in the extraction and separation steps. Pristine Cgo films used for property measurements are typically deposited onto a variety of substrates (< . , a clean silicon (100) surface to achieve lattice matching between the crystalline C60 and the substrate) by sublimation of the Cr,o powder in an inert atmosphere (e.g., Ar) or in vacuum. Single crystals can be grown either from solution using solvents such as CS and toluene, or by vacuum sublimation [16, 17, 18], The sublimation method yields solvent-free crystals, and is the method of choice. 

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