Crystallites, turbostratic

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. 

The basic building block of carbon is a planar sheet of carbon atoms arranged in a honeycomb structure (called graphene or basal plane). These carbon sheets are stacked in an ordered or disordered manner to form crystallites. Each crystallite has two different edge sites (Fig. 2) the armchair and zig-zag sites. In graphite and other ordered carbons, these edge sites are actually the crystallite planes, while in disordered soft and hard carbons these sites, as a result of turbostratic disorder, may not... 

Graphite is commonly produced by CVD and is often referred to as pyrolytic graphite. It is an aggregate of graphite crystallites, which have dimensions (L ) that may reach several hundred nm. It has a turbostratic structure, usually with many warped basal planes, lattice defects, and crystallite imperfections. Within the aggregate, the crystallites have various degrees of orientation. When they are essentially parallel to each other, the nature and the properties of the deposit closely match that of the ideal graphite crystal. 

Isotropic carbon is obtained by the pyrolysis of a hydrocarbon, usually methane, at high temperature (1200-1500°C) in a fluidized bed on a graphite substrate.Under these conditions, a turbostratic structure is obtained which is characterized by very little ordering and an essentially random orientation of small crystallites. In contrast to graphite which is highly anisotropic, such a structure has isotropic properties (see Ch. 7). Isotropic carbon is completely inert biologically. Its properties are compared to alumina, another common implant material, in Table 17.8. Notable is its high strain to failure. 

As in many other anthracitic substances, diffuse bands also appear near 43° and 80° 26. These represent two-dimensional (hk) reflections only since the turbostratic disorder of graphitic layers which characterizes amorphous carbons does not permit (hkl) three-dimensional atomic planes other than (001). Hirsch (19) proposed that the position of the (11) band was a function of carbon content of the sample, related to the crystallite layer diameter (L ). With increasing rank, the (11) reflection shifts towards smaller 26 values, representing greater bond lengths and larger crystallite size. Using values... 

The asymmetry of peak shape is preserved in anthraxolite heated to 1200°C. showing that turbostratic disorder persists in spite of a general enhancement of ordering. The band is also sharper and narrower. This may be interpreted to mean either that fewer class intervals are represented in the crystallite size distribution or that increased ordering of aromatic lamellae has reached the point where graphite (hid) planes are more common. Diffraction peaks of both (100) and (101) fall with the 2-A. band. 

Pinnick (57), from a study of crystallographic changes in carbon blacks between 1000° and 3000° C., suggests that for basal planes having diameters below 150 A. there is little tendency for the turbostratic structure to be lost. Above 150 A. interplanar forces, which increase as the square of the diameter, become great enough to cause orientation of the basal planes. Pinnick also finds that the diameter of the carbon black particle serves as an upper limit to the diameter of the crystallite which can be made from it. 

On the basis of the results of investigations on a large number of commercial carbon blacks synthesized under various conditions it was established [60] that the microtexture of all the materials could be described in terms of the characteristics represented in Fig. 1. Depending on the production method of a material its parameters L, Li, and y may vary. A decrease in y and an increase in L give evidence for the ordering of the particle structure. Figure 2(a) displays a schematic view of a section of a carbon black particle, where individual crystallites are visible [60]. The surface of each of the crystallites visible in this figure has a turbostratic structure. 

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