Turbostratic structure

Carbon blacks are synthetic materials which essentially contain carbon as the main element. The structure of carbon black is similar to graphite (hexagonal rings of carbon forming large sheets), but its structure is tridimensional and less ordered. The layers of carbon blacks are parallel to each other but not arranged in order, usually forming concentric inner layers (turbostratic structure). Some typical properties are density 1.7-1.9 g/cm pH of water suspension 2-8 primary particle size 14-250 nm oil absorption 50-300 g/100 g specific surface area 7-560 m /g. 

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. 

Other types of carbon (amorphous or transitional forms with turbostratic structure) consist of fragments of graphitelike regions cross-linked to a three-dimensional polymer by carbon chains. Unlike graphite, the transitional forms are organic semiconductors with electrical properties determined by delocalized rr-electrons. 

Whatever the precursor, the formation of an intermediate solid phase was always observed. It can be inferred from X-ray diffraction (Fig. 9.2.7) and infrared spectroscopy that this intermediate phase shows a lamellar, incompletely ordered structure (turbostratic structure) built up with parallel and equidistant sheets like those involved in the lamellar structure of the well-crystallized hydroxides Ni(OH)2 or Co(OH)2, these sheets are disoriented with intercalation of polyol molecules and partial substitution of hydroxide ions by alkoxy ions (29). The dissolution of this solid phase, which acts as a reservoir for the M(I1) solvated species, controls the concentration of these species and then plays a significant role in the control of the nucleation of the metal particles and therefore of their final morphological characteristics. For instance, starting from cobalt or nickel hydroxide as precursor in ethylene glycol, the reaction proceeds according to the following scheme (8) ... 

Metallic hydroxy-alkoxide with a turbostratic structure)... 

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 III-V 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 pyrolytically on refractory substrates at 1200—1800°C has a turbostratic structure and low porosity it has greater chemical resistance and is impervious to helium. 

The carbon atoms within each layer are arranged in almost the same manner as in graphite. The layers are nearly parallel to each other however, the relative position of these layers is random, so that there is no order as in the c direction of graphite ( turbostratic structure ) [4.3]. X-ray diffraction permits the determination of crystalline regions within the carbon black primary particle. These regions are... 

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. 

Turbostratic structure is characterized by a layer structure similar to h-BN the layers are mostly parallel but not aligned to the c-axis [10] (Fig. 4). 

The second important way to produce h-BN is the reaction of boric acid or alkali-borates with organic nitrogen compounds (melamin, urea, dicyanamide, guanidine) in nitrogen atmosphere [76-80]. These reactions are carried out at temperatures between 1000 °C and 2100 °C in N2 atmosphere. Before final thermal treatment, the product can be washed with methanol or diluted acids in order to remove all non-reacted products. For removing oxygen impurities a thermal treatment at 1500 °C in inert N2 or Ar atmosphere is used. BN with turbostratic structure (t-BN) is obtained which is characterized by partial or complete absence of three-dimensional order in the stacking of its atomic planes [81]. 

Hexagonal graphite Rhombohedral graphite Turbostratic structure... 

FIGURE 2.7 X-ray powder diffraction pattern of a petroleum coke heat-treated at 1000°C having a typical turbostratic structure. 

After 60h of grinding, the 100 line becomes broad and unsymmetrical, indicating that introducing too many stacking faults leads to a turbostratic structure [7],... 

In the case of the turbostratic structure the total reduced radial distribution function can be calculated as a sum of two contributions arising fi om correlations within a single layer and fi om averaged inter-layer correlations. The analytical formula has been derived by Mildner and Carpenter [14] for the averaged contribution fi om the randomly stacked graphitic layers. Now, the theoretical d(r) can be written as ... 

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. 

Charlier et al. [48] used the tight-binding model to study distorted stacking of graphene layers, termed pregraphitic or turbostratic carbon. The turbostratic structure was obtained by generating an amorphous cluster of graphene plates that... 

Structural and Surface Characterization of Carbon Products. Carbon products of the process were analyzed by a number of material characterization techniques, including x-ray diffraction, scanning electron microscopy. Auger electron spectroscopy, x-ray photoelectron spectroscopy, and others. X-ray diffraction studies revealed an ordered graphite-like (or turbostratic) structure of carbon products (Figure 4). 

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