Orientation of crystallites

As we have seen, the orientation of crystallites in a thin film can vary from epitaxial (or single crystalline), to complete fiber texture, to preferred orientation (incomplete fiber texture), to randomly distributed (or powder). The degree of orientation not only influences the thin-film properties but also has important consequences on the method of measurement and on the difficulty of identifying the phases present in films having multiple phases. 

The development of the internal orientation in formation in the fiber of a specific directional system, arranged relative to the fiber axis, of structural elements takes place as a result of fiber stretching in the production process. The orientation system of structural elements being formed is characterized by a rotational symmetry of the spatial location of structural elements in relation to the fiber axis. Depending on the type of structural elements being taken into account, we can speak of crystalline, amorphous, or overall orientation. The first case has to do with the orientation of crystallites, the second—with the orientation of segments of molecules occurring in the noncrystalline material, and the third—with all kinds of structural constitutive elements. 

The quantitative assessment of the degree of crystallite orientation by x-ray examination is not free of ambiguity. From a comparative analysis [23] in which results obtained from the consideration of (105) and from three different variations of equatorial reflection were compared, the conclusion was that the first procedure can lead to underrated results, i.e., to the underestimation of the orientation. However, it can be assumed that this does not result from an incorrect procedure, but from ignoring the fact that the adjacent (105) reflex can overlap. The absence of the plate effect of the orientation is characteristic of the orientation of crystallites in PET fibers. The evidence of this absence is the nearly identical azimuthal intensity distributions of the diffracted radiation in the reflexes originating from different families of lattice planes. The lack of the plate effect of orientation in the case of PET fiber stretching has to do with the rod mechanism of the crystallite orientation. 

The orientation of crystallites in PET fibers can also be assessed quantitatively by means of IR spectro-graphic examination. In this case, the basis for the assessment are the values of dichroic ratio (R) of the crystalline absorption bands in the fiber spectrogram. The determination of the values of fc is made using Fraser s dependence [24,25] modified by Chranowski [26] ... 

In practice, commercially fabricated polymer items are generally oriented to some degree. The scattering patterns from such materials comprise arcs, which are parts of the full circles obtained from unoriented samples. The lengths and positions of these arcs reveal much about the orientation of crystallites within a sample. The shorter the arcs, the more oriented the sample. In cases of extreme orientation, as found in highly oriented fibers such as Kevlar , the scattering pattern can approach that of a single crystal. 

N3)2Ga N(CH2CH2NEt2)2 ] low volatility Horizontal hot-wall LP-CVD Growth temperature 750-950 °C, preferred orientation of crystallites perpendicular to c-plane of sapphire substrate, no additional N source 287... 

In order to observe a structure inside a bulk sample with TEM, one should prepare a replica of the fracture surface of the sample, or must make an ultra-thin section whose thickness is small enough for electron beams to pass through the section. Unfortunately, however, orientation of crystallites might be changed by microtoming. 

In catalysis, one does not expect the activity of a catalyst to be proportional to its surface area, since there is good evidence that in many instances catalytic action is limited to certain active regions which may constitute only a small fraction of the total surface area (129). As would be expected, the same reasoning holds true for gas-carbon reactions. Carbon is a multicrystalline material, which can present varying degrees of surface heterogeneity depending upon the size and orientation of the crystallites. In the broadest sense, two main orientations of crystallites in the carbon surface need be considered—(1) crystallites with their basal planes parallel to the surface and (2) crystallites with their basal planes perpendicular to the surface. 

This model is incorrect because the linear thermal expansivity for both components in the isotropic and oriented state is assumed to be the same. The concequences of this assumption are quite different for Px and For px, it does not play any essential role, because in the isotropic and oriented state perpendicular to the draw axis the thermal expansivity is determined solely by intermolecular interactions. For P, this suggestion may lead to a principal inconsistency. This conclusion is evident from comparison of the calculated and experimental -dependences of P and P for PE and PP according to Eqs. (107) and (108). For Px, the agreement between the model calculation and the experiment is quite satisfactory for all draw ratios. On the other hand, Eq. (108) does not describe the X-dependence of P( at all. This equation does not yield negative values of P even in case of a limited orientation of crystallites (fc = 1) because it is based on the suggestion that Pam is always positive and pam > Pcr. ... 

It must be remembered that all measurements are made on solids at low temperatures solutions are therefore frozen, and this may give rise to other problems. For example, different phases may be obtained with different cooling rates. It is also possible that orientation of crystallites may occur during freezing, which will affect the relative intensities of the eight lines of a subspectrum if this is not taken into account, there will be errors in the derived parameters. This is especially likely when there are overlapping subspectra. 

Note that angular brackets <... > are used for the average over the orientations of crystallites in reflection and a bar on the averaged quantity for the average over all orientations in the Euler space. 

Figure 10.16 gives data on orientation of crystallites in polypropylene containing various amounts of CaCO ). Maximum orientation of crystallites is obtained when the concentration of calcium carbonate is in the range of 15-20%. REFERENCES... 

Figure 10 shows the dry diffraction pattern of the CTAHF. The sample was air dried at ambient temperature with no applied stress. Discrete reflections appear along the equatorial line underneath the general ring pattern. The appearance of discrete reflections Is expected, as the removal of water from the matrix would allow the polymer chains to collapse Into more ordered regions. Most notably, there appears to be better orientation of crystallites with the fiber axis In comparison to the original sample. 

Investigation of texture has shown that every time after changing the axis of loading there occurs formation of qualitatively similar axial texture, in which the prevailing orientation of crystallites was stable. This means that... 

The pole distribution t ) or t ( , O) and the orientation parameter/ are defined and measured for individual poles. The pole orientation parameter by itself does not specify the state of orientation of crystallites in the sample. The orientation of an individual crystallite in space is uniquely, defined when the orientations of at least two nonparallel directions associated with the crystallite are given. Thus, in an effort to specify the average orientation of crystallite in a sample, one may evaluate the orientation parameters fa and / of two nonparallel poles, a and b, and represent... 

Figure 3.23 A point plotted on this diagram, giving the Hermans orientation parameters fa and fy for two orthogonal poles a and b in the crystal, represents the state of average orientation of crystallites in the sample.

Figure 8.42 Schematic of diffraction from a powdered crystalline sample. The powdered sample generates the concentric cones of diffracted X-rays because of the random orientation of crystallites in the sample. The X-ray tube exciting the sample is not shown in this diagram. The cones of diffracted light intersect X-ray film curved to Ht the diameter of the Rowland circle. The result is a series of curved lines on the X-ray film.

Table 5.3 shows that based on their origin, the pores can be categorized into two classes, intraparticle and interparticle pores. The intraparticle pores are further classified into two, intrinsic and extrinsic intraparticle pores. The former class owes its origin to the crystal structure, that in most activated carbons, large amounts of pores of various sizes in the nanometer range are formed because of the random orientation of crystallites these are rigid interparticle pores. 

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