Fibers turbostratic

Uses. Hot-pressed hBN is useful for high temperature electric or thermal insulation, vessels, etc, especially in inert or reducing atmospheres, and for special materials such as IITV semiconductors (qv). Its low thermal expansion makes it resistant to thermal shock. The powder can be used as a mold release agent or as thermal insulation. Boron nitride is also available in fiber form (19). BN deposited pyrolyticaHy on refractory substrates at 1200—1800°C has a turbostratic stmcture and low porosity it has greater chemical resistance and is impervious to helium. 

PAN fibers develop a structure with little point-to-point relationship between atoms in neighboring basal planes. This structure is labeled the turbostratic configuration and is characterized by interplanar spacing values greater than 0.344 nm. The crystallite size in the direction normal to the basal planes, or stack height (L, ), in turbostratic graphite is typically less than 5 nm. 

BN fibers are used in aluminum matrices because they are one of the few materials easily wet by molten aluminum. hBN-AlN composites are described ". Gel methods resulted in an hAlN-turbostratic hBN composite. zBN-based composites, e.g., with Al, Si, Ti nitride matrices, were reported. A nanostructure composite was made by low temperature conversion of an aqueous solution of simple Al, B, and N-containing compounds to intermediate precursor (gel) materials. BN-AIN composite fabrication is also described . 

PC, pyrolytic carbon CF, carbon fibers GLF, glassUke fibers AC, activated carbon (including fibers or cloth) CB, carbon blacks. Carbons with spacing greater than 0.344 nm are turbostratic. 

The meridional width is used to calculate the apparent length parallel to the fiber axis, Lafiber axis, L,j., by the Ruland and Wamen formulas, respectively [1] [4] [32]. The interlayer spacing door, is derived from the 002 reflection along the equatorial direction. It characterizes the turbostratic feature of the carbon. A parameter characterizing the misorientation of the carbon layers with respect to the fiber axis, Z, is derived from the half maximum width of the azimuthal distribution intensity curve along the 002 arc (arc opening). 

The transverse radius of curvature of the graphene layers, n, increases from the core of a fiber to its surface, and graphene sheets near the fiber surface usually lie parallel to it. This structure is the cause of the notable skin/core effect and the low reactivity of the fiber surface. The reactivity of aromatic carbons is lower in-plane than at the layer edge. The turbostratic character of carbon in PAN based HM carbon fibers and the occurrence of elongated pores explain their low density (1.8-1.9 g/cm ). 

Carbon fibers consist mainly of polyaromatic carbons and effectively exist as structurally disordered (turbostratic) carbon. The crystal structure of a true graphite with a distance of... 

Figure 5.15 a. Crystal structure of graphite crystal, b. Structure of turbostratic carbon. Source Reprinted from Hoffman WP, Hurley WC, Liu PM, Owens TW, The surface topology of non-shear treated pitch and PAN carbon fibers as viewed by STM, J Mater Res, 6,1685, 1991. 

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