The Phase and Transition Diagram for Carbon

Elucidation of the phase relationships between the different forms of carbon is a difficult field of study because of the very high temperatures and pressures that must be applied. However, the subject is one of great technical importance because of the need to understand methods for transforming graphite and disordered forms of carbon into diamond. The diagram has been revised and reviewed at regular intervals [59-61] and a simplified form of the most recent diagram for carbon [62] is in Fig. 5. 

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Fig. 5. The phase and transition diagram for carbon (simplified from [62]).

Use the phase diagram for carbon in Exercise 8.14 (a) to describe the phase transitions that carbon would undergo if compressed at a constant temperature of 2000 K from 100 atm to 1 X 106 atm (b) to rank the diamond, graphite, and liquid phases of carbon in order of increasing density. 

Figures 9 and 10 show phase-behavior diagrams for David Lloydminster crude oil and the surfactant Neodol 25-3S in the presence of 1 wt% sodium carbonate. Phase-behavior measurements were carried out according to the method of Nelson et al. (52). The David Lloydminster oil field is near the Alberta-Saskatchewan border directly east of Edmonton. The oil has a density of 0.922 g/mL and a viscosity of 144 MPa s at 23 °C. The region of optimal phase behavior is shown at a surfactant concentration of 0.1 wt% in Figure 9. The region of optimal phase behavior is shaded. Above this region, type II +) behavior occurs, and type II(-) behavior occurs below the region of optimal phase behavior. Volume percent oil refers to the amount of oil present in the phase-behavior tube used. For a given oil-to-water ratio, a transition from type II(-) to type III to type II(+) occurs as salinity increases. As the amount of oil increases relative to the amount of aqueous phase, the same trend in phase behavior is seen.

Carbon Monoxide. There are close similarities between carbon monoxide and nitrogen. The molecules are isoelectronic, and the bond lengths and dissociation energies are quite comparable. The phase diagrams of the two compounds show the same trends in the moderate pressure range with a variety of phase transitions between essentially alike crystal structures [333], when allowance is made for the lack of the inversion center and the presence of a weak electric dipole moment in carbon monoxide. However, the behavior and stability at higher... 

Revised proposals for the phase diagrams of the Group 5 transition metal-carbon systems are given in Figure 4.3(a-c). In our opinion they are preferable with respect to the composition and decomposition temperature of the t, phases (within the limits given in Table 4.1) as well as the homogeneity ranges of the fi and 6 phases. 

In the systems Co-C and Ni-C and in the other transition metal-carbon systems not mentioned so far, no stable carbide phases are observed. The carbon solnbilities in the metals are of importance for the fabrication and properties of hardmetals (see Section 9.1.1). The phase diagrams are of the entectic type. Metastable carbide phases have been reported in rapidly qnenched Co-C and Ni-C alloys. 

PHASE EQUILIBRIA. A useful solvent for supercritical extraction, especially in food processing, is carbon dioxide, which has a critical point of 31.06 C and 73.8 bars (1070 Iby/in. ). The phase diagram for pure COj (Fig. 20.16) shows the equilibrium regions of solid, liquid, and gas and the conditions under which a supercritical fluid exists. In the supercritical region there is no distinction between liquid and gas and no phase transition from one to the other the supercritical fluid acts like a very dense gas or a light, mobile liquid. 

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