Carbons hexagonal graphite structure

The second stage involves heating the fibers for a further period at 1770 K to eliminate all elements other than carbon. This carbonization is believed to involve cross-linking of the chains to form the hexagonal graphite structure, and this final heat treatment can affect the mechanical properties to a marked extent, as shown in Figure 15.17. The major application so far is in composite structures where they act as extremely effective reinforcing fibers. These reinforced plastic composites find uses in the aircraft industry, in the small-boat trade, and as ablative composites. 

Its structure resembles that of graphite, but the latter s flat planes of carbon hexagons are replaced in boron nitride by planes of hexagons of alternating B and N atoms (Fig. 14.27). Unlike graphite, boron nitride is white and does not conduct... 

The AlB2-type structure can be considered a filled-up WC structure type. The B atoms form a hexagonal net and centre all the A1 trigonal prisms. The arrangement of the boron atoms in their layers is the same as that of carbon in graphite (63 layers). (See also the nThSi2, tI12, description for a comparison between the planar... 

White powder, hexagonal graphite-like form or cubic crystal cubic form similar to diamond in its crystal structure, and reverts to graphite form when heated above 1,700°C density 2.18 g/cm melts at 2,975°C (under nitrogen pressure) sublimes at 2,500°C at atmospheric pressure insoluble in water and acid attacked by hot alkalies and fused alkali carbonates not wetted by most molten metals or glasses. 

In this process, diamond forms from graphite without a catalyst. The refractory nature of carbon demands a fairly high temperature (2500—3000 K) for sufficient atomic mobility for the transformation, and the high temperature in turn demands a high pressure (above 12 GPa 120 kbar) for diamond stability. The combination of high temperature and pressure may be achieved statically or dynamically. During the course of experimentation on this process a new form of diamond with a hexagonal (wurtzitic) structure was discovered (25). 

Crystal Structure. Diamonds prepared by the direct conversion of well-crystallized graphite, at pressures of about 13 GPa (130 kbar), show certain unusual reflections in the x-ray diffraction patterns (25). They could be explained by assuming a hexagonal diamond structure (related to wurtzite) with a = 0.252 and c = 0.412 nm, space group P63 frame — D3h with four atoms per unit cell. The calculated density would be 3.51 g/cm3, the same as for ordinary cubic diamond, and the distances between nearest neighbor carbon atoms would be the same in both hexagonal and cubic diamond, 0.154 nm. 

Its structure resembles that of graphite, but the latter s flat planes of carbon hexagons are replaced in boron nitride by planes of hexagons of alternating B and N atoms (Fig. 14.31). Unlike graphite, it is white and does not conduct electricity. Under high pressure, boron nitride is converted to a very hard, diamondlike crystalline form called Borazon. In recent years, boron nitride nanotubes similar to those formed by carbon have been synthesized (Section 14.18), and they have been found to be semiconducting (see Box 14.2). 

PI5.9 Boron nitride (BN) is isoelectronic with carbon and the B, C, and N atoms are about the same size. The result is that BN forms crystal structures similar to those of carbon, in that it crystallizes in a hexagonal (graphite-like hBN) and a cubic (diamond like cBN) structure. The data summarized at the end of the problem are available for the two forms of BN.17... 

Elastic neutron diffraction was first performed (analyzer in Fig. 1 set to zero energy transfer) to establish the structure of the monolayer at low temperature. Three Bragg reflections were observed which could be indexed by a triangular lattice having a nearest-neighbor distance about 10% smaller than required for a 3 X /3 R30° commensurate structure (every third carbon hexagon in the graphite basal plane occupied). 

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