Crystal blocks

Fig. 6. Volume of crystals destroyed, VJj, as compared with the total volume of material destroyed (including non-crystalline regions) Vj beneath the in-denter against the reciprocal value of crystal block volume. Applied load 15 N 17)...

Since the increase of the elastic modulus of oriented polymers with draw ratio is, to a large extent, the consequence of the tie molecules or intercrystalline bridges interconnecting crystal blocks within the fibrils 72) one may anticipate a correlation to exist between indentation anisotropy and modulus. Recent data 23) illustrating the... 

For this measurement, the sample was a Te02 optical quality crystal of 2 x 2 x 3 cm3, corresponding to a mass of 75.493 g. It was sustained by four pure tin cylinders, which kept the crystal blocked inside a copper frame, as shown in Fig. 12.5. The length of the cylinders was chosen to compensate the thermal contraction of the crystal, down to the lowest reached temperatures. 

By inclining the flask A, applying suction at 3 and connecting 1 to a source of inert gas, the mother liquor may be drawn into the sintered glass funnel C without the crystals blocking the end of the tube D. 

The gray-green precipitate is filtered on a glass filter and washed twice with a little 2 M nitric acid. The precipitate is then stirred mechanically at 20°C. with 2000 ml. of water containing 5 ml. of 15 M nitric acid until all the crystal blocks are dispersed, and the suspension is filtered. To the filtrate... 

FIG. 2.15 Tie molecules in the amorphous layer between subsequent crystal blocks of the micro-fibril. At low strain (a) a single tie molecule (A), at medium strain (b) two tie molecules (B), and at the highest strain (e) three molecules (C) are stretched up to the rupture point (after Peterlin, 1971). 

As mentioned before, the conventional drawing process leads to the well-known fibrillar structure, which still contains a majority of folded chains (Fig. 19.14). In drawing above the "natural" draw ratio further unfolding takes place. Such a process at very high draw ratios must be conducted with the utmost care since critical concentrations of stress on the folded chain surface of the crystal blocks must be avoided they lead to fracture. The super-drawing can be carried out in one and two stages. 

An atom in the half-crystal position (also called kink position) (Fig. 1), is bonded (i) with a semi-infinite crystal block, (ii) with a semi-infinite crystal lattice plane, and (iii) with a semi-infinite crystal row. Attaching or detaching one atom to and from the half-crystal position a new half-crystal position is created and this is what makes this position a repeatable step in the successive building or disintegration of the bulk crystals. 

Extinction (see Section 2.2.2) is the phenomenon that reduces the observed intensity of the incident or diffracted beams by internal scattering and backscattering parallel to the incident beam direction. Primary extinction is assumed to take place when the different crystal blocks (domains) are sufficiently mutually misoriented that the reduction in intensity takes place oifly within each block. In secondary extinction, the size of the blocks is assumed to be so small that loss of intensity within each block is negligible. Instead, the reduction takes place by adjacent blocks scattering and rescattering the... 

The effects of impregnation of activated carbons with potassium hydroxide on the efficiency of NO removal was studied by Lee and coworkers [106, 107]. They found that KOH creates the selective adsorption sites (increases basicity of carbons by the presence of OH ) for NO, adsorption. As a result of the surface reaction, KNO2 and KNO3 are formed. Formation of salt crystals blocks the porosity of the materials and diminishes the NO, removal capacity. It was proposed that the surface basic OH ions delay oxidation of KNO2 to KNO3 and thus result in an increase in the surface adsorptivity. 

The simple model implies an infinite perfect crystal. The crystal specimen studied is necessarily finite (rarely more than 0.5 mm in size) and, if perfect by conventional criteria, would be utterly unsuitable for collection of intensity data of the kind required for ordinary structure determination and refinement. What is needed is an ideally imperfect crystal shot through with dislocations and intergrain boundaries so that it behaves, so far as diffraction is concerned, like a mosaic of perfect crystal blocks of the order of micrometers or tenths of micrometers in size, tilted with respect to one another by angles of the order of a few seconds of arc and scattering independently (incoherently) with respect to one another. The assumption of an ideally imperfect ... 

Sen T, Tiddy GJT, Casci JL, Anderson MW (2004) Synthesis and characterization of hierarchically ordered porous silica materials. Chem Mater 16 2044 Deng Y, Liu C, Yu T, Liu F, Zhang F, Wan Y, Zhang L, Wang C, Tu B, Webley PA, Wang H, Zhao D (2007) Facile synthesis of hierarchically porous carbons from dual colloidal crystal/block copolymer template approach. Chem Mater 19 3271 Luo Q, Li L, Yang B, Zhao D (2000) Three-dimensional ordered macroporous structures with mesoporous silica walls. Chem Lett 29 378... 

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