Crystalhte

This rule of thumb does not apply to all polymers. For certain polymers, such as poly (propylene), the relationship is complicated because the value of Tg itself is raised when some of the crystalline phase is present. This is because the morphology of poly(propylene) is such that the amorphous regions are relatively small and frequently interrupted by crystallites. In such a structure there are significant constraints on the freedom of rotation in an individual molecule which becomes effectively tied down in places by the crystalhtes. The reduction in total chain mobility as crystallisation develops has the effect of raising the of the amorphous regions. By contrast, in polymers that do not show this shift in T, the degree of freedom in the amorphous sections remains unaffected by the presence of crystallites, because they are more widely spaced. In these polymers the crystallites behave more like inert fillers in an otherwise unaffected matrix. 

Thermal decomposition of [Cu0Si(0 Bu)3]4 in the solid phase begins at ca. 100 °C under argon (by TGA) and results in formation of an amorphous material until roughly 600 °C, at which temperature Cu metal was detected (by PXRD) [105]. Conversely, decomposition under oxygen led initially to a material with Cu crystalhtes and small amounts of CU2O and CuO, and subsequent heating beyond 800 °C resulted in oxidation of all the copper to CuO. 

Bregoli LJ. 1978. Influence of platinum crystalhte size on electrochemical reduction of oxygen in phosphoric-acid. Electrochim Acta 23 489-492. 

Markovic N, GasteigerH, Ross PN. 1997a. Kinetics of oxygen reduction on ViQikl) electrodes Implications for the crystalhte size effect with supported Pt electrocatalysts. J Electrochem Soc 144 1591-1597. 

Y. G. Choi, G. Sakai, K. Shimanoe, Y. Teraoka, N. Miura, and N. Yamazoe, Preparation of size and habit-controlled nano crystalhtes of trmgsten oxide. Sensors Actuators B 93(1—3), 486-494... 

LDHs with Ni/Al molar ratio of 2.5 have been synthesized by a sol-gel method using nickel acetylacetonate and aluminium isopropylate as precursors. The sol-gel synthesized samples exhibited lower crystalhte dimensions and greater BET surface areas compared with coprecipitated samples prepared from an aqueous solution of nickel and aluminium nitrates [19,172]. 

Unfortunately, the X-ray diffraction patterns for different UZM-5 preparations showed broad peaks (due to the very small crystalhte size and anisotropic morphology) and were not of sufficient quality for ah initio structure solution. However, indexing the patterns did provide the crystal system (tetragonal) and approximate... 

Grains are generally understood here to be individual crystalhtes in a polycrys-taUine body of material. This definition is, however, not common to all authors. Thus, clumps of crystallites are referred to by some as grains, whereas others refer to such clumps as islands. In this discussion, ductihty is related to the maximum amount of strain to which a body may be subjected without breaking. 

OPTICAL PROPERTIES. The most commonly reported optical properties are optical transmission, with some smdies also on photoluminescence. The importance of the optical transmission for CdS in particular lies in its use in photovoltaic cells, where it acts as a window layer. The CdS should be as transparent as possible to the incoming radiation. The transmission is a function of thickness, bandgap, and fihn structure (is the fihn transparent or scattering ). The bandgap in most studies is constant (ca. 2.45 eV at room temperature), although somewhat larger values have been obtained due to size quantization in very small crystalhtes. 

Another polymer-mediated synthesis used a lyotropic liquid-crystal (LLC) template to obtain 4- to 7-nm crystalhtes of Pd (Ding and Gin, 2000). In this study, Pd2+ was ion exchanged into the channels of a cross-linked LLC and reduced with H2. The resultant catalysts were found to be active for efficient hydrogenation of benzaldehyde. 

It should be noted that for a polycrystal composed of cubic crystalhtes, the Voigt and Reuss approximations for the bulk modulus are equal to each other, as they should be since the bulk modulus represents a volume change but not shape change. Therefore, in a cube the deformation along the principal strain directions are the same. Hence, Eqs. 10.39 and 10.40 are equal and these equations also hold for an isotropic body. The... 

Figure 1.7 (a) In the hquid phase, molecules of both compounds are dispersed and all molecules are potentially reactive (b) when two sohds react, reaction can only occur at the point where the two crystalhtes are in direct contact (c) reaction between solids may form a thin hquid layer which increases the rate of reaction... 

An interesting observation reported in Table XLIX is the increase in the hydroquinone/catechol ratio from 1.44 to 1.99 when the dielectric constant of the medium is decreased from 58.9 to 39.2 by addition of methanol to water. A similar increase in the hydroquinone/catechol ratios was also observed in phenol hydroxylation catalyzed by TS-1 (266) in dioxane-water and tert-butyl alcohol-water mixtures. The para/ortho ratio increased nearly 10-fold when 10% dioxane was added to water. Similarly, the para/ortho ratio more than doubled (1.3-3.0) when 10% tert-butyl alcohol was added to water. An opposite trend, namely, a decrease in the para/ortho ratio from 1.4 to 0.6, was observed when 10% formamide (s = 108) was added to water. Because of geometric constraints in the MFI pores, catechol is expected to be formed more easily on the external surface of TS-1 crystalhtes than in the pores (97). Hydroquinone, less spatially demanding, can form in the TS-1 channels. A greater coverage of the hydrophobic... 

An explanation for this extrinsic phase separation is thus to imagine each crystalhte having its unit-cell parameters and stoichiometry controlled by its Mn + Mn + ratio, which ultimately determines whether the HTO phase transforms into to orbitally ordered LTO phase or the charge-ordered LTM one, or not at all (see Figure 5). 

Since we are considering a 2D system, we note that this is a monomolecular system and all of the centers of mass of the molecules are in the same plane (even if individual nuclei in the molecule are not). S Q), introduced earlier in this chapter, therefore only depends on the component of Q projected onto the scattering plane, Qparaiiei Just as Warren did, the case of lamellar systems like graphite and mica can now be considered, assuming that the scattering system has random orientations of the crystalhtes about an axis normal to the basal (or 2D) plane. Because the magnitude of the parallel component of the structure factor 5 (Q) is the relevant quantity... 

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