Graphite phase change

Shown is the sketch of the phase diagram of carbon. For the phase change from graphite to liquid carbon along the line PQ, choose the correct... 

Most of the well-documented investigations of 2-D phase changes have featured the adsorption of Ar, Kr and Xe on the basal, (000 1), face of graphite (see Dash, 1975 Suzanne and Gay, 1996), but detailed work has also been undertaken on a number of other systems such as Ar, Kr and CH4 on layered halides (Larher, 1992) and cubic crystals of MgO (Coulomb et al., 1984). In addition, phase diagrams have been constructed for the adsorption of certain polar molecules on graphite (Terlain and Larher, 1983). 

It is significant that the second calorimetric peak and the associated isotherm sub-step were detectable only if the graphitized thermal black had been heated at temperatures above 1700°C. These results suggest that the 2-D phase transformation is very sensitive to the perfection of the surface basal planes and this is a further indication that the phase change leads to the development of a commensurate structure. 

The highly distinctive form of a Type VI isotherm is due to a stepwise layer-by-layer adsorption process. Such isotherms are given by the adsorption of simple non-polar molecules (e.g. argon, krypton and xenon) on uniform surfaces (e.g. the basal plane of graphite). The steps become less sharp as the temperature is increased. The vertical risers can be regarded as the adsorbed layer boundaries and the centres of the treads (inflection points) as the layer capacities. When present, sub-steps are associated with two-dimensional phase changes in the monolayer. Useful information concerning the surface uniformity and adsorbate structure can be obtained from the relative layer capacities and the presence of sub-steps. 

These results demonstrate that nanostrucmred carbon materials can be evaporated or undergo phase changes/transformations upon laser excitation of significantly lower intensity compared to bulk materials. Even the power of laser pointers commonly used during conference presentations or in the class room (up to 100 mW) is enough to heat carbon samples up to 3,500°C, evaporate and redeposit graphitic nanocarbons or convert nanodiamond into carbon onions, when focused to a spot size of several micrometers. 

In the two-dimensional physisorbed environment the nearest-neighbor distance increases from 3.99 A in the bulk a-phase to 4.26 A in the commensurate monolayer on graphite. This change reduces the quadrupole-quadmpole and dipole-dipole interaction energies to about 72% and 82% of the bulk values, respectively. Additional features complicating the extrapolation from bulk to expected monolayer behavior are the different number of nearest neighbors and the geometry of the coordination shells, as well as the surface interactions with the substrate. How the similarities and differences between bulk N2 and CO carry over to the two-dimensional physisorbed situation is the focus of Sections III and IV and Sections V and VI, respectively. 

The phase change between graphite and diamond is difficult to observe directly. Both substances can be burned, however. From these equations, calculate AH° for the conversion of diamond into graphite. 

Phase change material pellets consist of a tnixmre of paraffin (n-octadecane), a polymer, and a thermal conductivity improver (expandable graphite, graphite microfiber pieces, or graphite powder), a nucleating agent (sodium and calcium chloride and 1-octa-decanol). ... 

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