[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

A b fig 337 hopg block structure a in the plane of cc

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(a) (b) Fig. 3.37. HOPG block structure: ( a ) in the plane of CC net, size 1 mm × 1 mm, 125 µ m under the surface; acoustic microscopy; ( b ) in perpendicular direction; elec- tron microscopy after laser etching according to a certain distribution, which determines the HOPG mosaic spread. These blocks contain a second, small-scale mosaic structure with a smaller mosaic spread. Mosaic spread of crystallites within individual blocks can be as low as 0.1 . The form of the HOPG blocks is very anisotropic: their length along the CC plane is about two orders larger than in the perpendicu- lar direction. Block structure can be visualized by acoustical microscopy [115] and by laser etching (Fig. 3.37a and b). It causes irregularities of the inten- sity, shape and symmetry of the diffraction patterns when a HOPG plate is scanned with a collimated X-ray beam. Bent HOPG Crystals The traditional technology of manufacturing bent HOPG crystals includes the process of annealing under pressure on concave or convex pistons [111]. However, the anisotropy of thermal expansion and the laminar structure are impediments to obtaining the required profiles with a proper accuracy, espe- cially when the shape is significantly more complicated than a simple cylinder, and the curvature radii are smaller than 100 mm. Here, process difficulties can lead to a drastic increase of the production costs. As a result, the first exper- iments with bent crystals used HOPG cylinders [116] or toroids assembled from a set of flat or singly curved HOPG crystals [117, 118]. To a certain extent, this approach involved both deviations of the shape obtained from the shape required and decreased reflectivities at the edges of separate crystals due to some damage during cutting. Some of these principle difficulties that arose during assembly restricted the range of applications of bent HOPG in analytical instrumentation.
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X-Ray Optics 147 The difficulties of this traditional approach to produce bent crystal initi- ated dedicated investigations aimed at a way to modify flat HOPG crystals into a bent form under chemical and thermomechanical treatment. The main difficulty was that such a treatment (e.g., intercalation by strong oxidizing agents [119]) led to a destruction of the material structure and a dramatic deterioration of the crystal mosaicity. Dedicated investigations of the HOPG structure [115] resulted in the mod- ification procedure [120] offering HOPG with both an excellent flexibility and an improved mosaic spread. The flexible HOPG can be easily stacked and mounted on a substrate of any form at room temperature [120]. In many cases, the crystals can be fixed on a substrate due to adhesion even without glue, if the substrate surface is smooth enough (e.g., polished). The method allows the creation of a wide range of crystal geometries ranging from conical and cylindrical [119], toroidal [121], paraboloidal [122] to spherical ones [123].
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