Preferred Orientation Texture

By introducing preferred orientation in polycrystalline ceramics, the advantage of the resulting anisotropies may be used for specihc purposes of interest. Anisotropy is a directionally-dependent property of a material, physical or mechanical. 

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Many compounds, including clay minerals, form needle- or plateshaped crystals. With finely dispersed minerals, the electron diffraction method can give a special kind of diffraction pattern, the texture pattern, which contains a two dimensional distribution of a regularly arranged set of 3D reflections [2], Specimens of fine-grained lamellar or fiber minerals, prepared by sedimentation from suspensions onto supporting surfaces or films, form textures in which the component microcrystals have a preferred orientation. Texture patterns of lamellar crystals tilted with respect to the electron beam are called oblique texture electron diffraction patterns [1]. 

Figure 7.15. Schematic representation of polycrystalline randomly oriented substrate (a) and substrate with preferred orientation (texture) (h).

A component or object in which grain orientations are completely random is said to have no texture. If the material exhibits some preferred orientation, it is said to have a texture. The texture can be weak, moderate, or strong, depending on the percentage of grains that have the preferred orientation. Texture is produced in most materials, either as an unintended by-product of the processing method or with the intention to exploit favorable properties in one orientation. In some cases, processing methods or routes are chosen explicitly to develop a preferred texture. 

V.K. Pecharsky, L.G. Akselrud, and P.Y. Zavalij, Method for taking into account the influence of preferred orientation (texture) in a powdered sample by investigating the atomic structure of a substance, Kristallografiya 32, 874 (1987). Engl, transl. Sov. Phys. Crystallogr. 32,514(1987). 

POLYCRYSTALLINE GRAPHITE is a GRAPHITE MATERIAL with coherent crystallographic domains of limited size regardless of the perfection and preferred orientation (texture) of their crystalline structure. 

For a physical mixture, the powder diffraction pattern is the sum of the patterns of the individual materials. The diffraction pattern can therefore be used to identify the crystalline phases in a mixture. The concentrations of the crystalline phases can be determined by methods based on comparing the intensities of the diffraction peaks with standards (6-8). If the crystal structures of the phases are known, the concentration of each phase can be detamined by Rietveld analysis (20,21). In the Rietveld method, a theoretical diffraction pattern is computed and the difference between the theoretical and observed patterns is minimized. For quantitative analysis, some care should be taken with specimen preparation if accurate and reliable results are to be obtained. The effects of factors such as preferred orientation, texturing, and particle size broadening must be minimized. 

Texture. Texture refers to the degree of crystallographic alignment in a polycrystalline material. A sample has no texture if the crystallographic alignment of the particles is completely random. The amount of texture of a material depends on the percentage of particles that possess a preferred orientation. Texture has a profound influence on the properties of many materials. 

Lane Camera. This flat-film camera can record both transmission and back-reflection patterns. The principle is illustrated in Figure 14 Figure 15 shows a flat-film pattern. Transmission patterns with polychromatic radiation yield information about the symmetry of a cry.stal (Laue symmetry) or with monochromatic radiation the orientation of the crystallites (preferred orientation, texture) such patterns are recorded, for exitmple. from fibers. Back-reflection patterns permit the nondestructive examination of workpiece.s. 

Coming back to the detailed analysis of diffraction patterns, we note that such efforts can be in practice more complicated for real samples for different reasons. First of all, the crystallites (grains) inside a polycrystalline sample might have a preferred orientation (texture), and accordingly, the Bragg reflexes of all other orientations are extremely suppressed in their intensity compared to those expected from calculated structure factors. Such a behavior can be expected e.g. in the case of epitaxially grown thin films that adopt the structure or at least the orientation of the substrate. This is observed e.g. for the passive films on iron discussed above [4], and in part also for those on Ni [19] but also for electrodeposited metal films. 

Chemical identification of unknowns by XRD relies in the accurate determination of a set of ii-spacings for the various crystallographic orientations. The data are screened against database of reference materials which are typically powder data with no preferred orientation. Accuracy of XRD identification of unknown species depends on the careful preparation of the samples, if powder form is required. On the other hand, if samples are not in powder form, care must be taken to account for missing lines in the XRD pattern and for abnormal intensity ratio in the observed peaks due to preferred orientation (texture). Detection limits in this case are within 0.1-1 wt%, which is worse than the ppm or ppb levels provided by surface analysis methods such as XPS or SIMS (discussed in this book). Chemical determination by XRD is limited to crystalline phases rally, but compounds can be identified down to their polymorphic phases. 

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. 

Crystallography is a very broad science, stretching from crystal-structure determination to crystal physics (especially the systematic study and mathematical analysis of anisotropy), crystal chemistry and the geometrical study of phase transitions in the solid state, and stretching to the prediction of crystal structures from first principles this last is very active nowadays and is entirely dependent on recent advances in the electron theory of solids. There is also a flourishing field of applied crystallography, encompassing such skills as the determination of preferred orientations, alias textures, in polycrystalline assemblies. It would be fair to say that... 

Metallurgists originally, and now materials scientists (as well as solid-state chemists) have used erystallographic methods, certainly, for the determination of the structures of intermetallic compounds, but also for such subsidiary parepistemes as the study of the orientation relationships involved in phase transformations, and the study of preferred orientations, alias texture (statistically preferential alignment of the crystal axes of the individual grains in a polycrystalline assembly) however, those who pursue such concerns are not members of the aristocracy The study of texture both by X-ray diffraction and by computer simulation has become a huge sub-subsidiary field, very recently marked by the publication of a major book (Kocks el al. 1998). 

Kocks, U.F., Tome, C.N. and Wenk, H.-R (1998) Texture and Anisotropy Preferred Orientations in Polycrystals and their Effects on Materials Properties (Cambridge University Press, Cambridge). 

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