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0 1000 2000 3000 4000 5000 6000

Temperature (K)

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

0 1000 2000 3000 4000 5000 6000

Temperature (K)

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

Graphite is thermodynamically more stable than diamond at low pressures and the reverse is true at high pressures; the standard free energy for the solid state transition from graphite to diamond is +2.90 kJ-mol"1 at 25°C. The equilibrium phase boundary between graphite and diamond increases linearly with temperature and pressure from -1.7 GPa, 0 K to the diamond-graphite-liquid (d-g-1) triple point at -12 GPa, 5000 K. The melting line for diamond increases steeply with temperature above the d-g-1 triple point. The melting line for graphite lies between the d-g-1 triple point and the graphite-liquid-vapour triple point at -0.011 GPa, 5000 K and it passes through a temperature maximum at

Transitions between the different forms of carbon are characterised by high activation energies so that metastable forms of carbon can persist for long periods under conditions where another form of carbon is thermodynamically stable. The stability of diamonds under ambient conditions is the most obvious example. The region marked 'A' on the diagram defines the pressure, temperature, (P,T) region for the catalysed transformation of graphite to diamond. This is not a solid-solid transformation since the graphite is dissolved in the catalyst and the solute carbon atoms re-precipitate as diamond [6]. The point 'B' defines the much higher threshold P,T values for rapid uncatalysed solid-solid transformation of graphite to diamond, while in the P,T region denoted as 'C', diamond is rapidly converted to graphite by a solid-solid transformation. Various other solid-solid transformations can be produced by appropriate P,T cycles, e.g., rapid shock compression/quench and flash heating. These are described by Bundy et al, [62] but are omitted from Fig. 6 for the sake of clarity. As an example, 'D' denotes the P,T, region where single crystal graphite is slowly converted to hexagonal diamond by application of a multiaxial stress of at least 12 GPa with the principal stress parallel to the

Liquid carbon is formed at high temperatures and pressures. The balance of evidence, reviewed by Bundy et al [62], indicates that liquid carbon is a semi-metal and recent work does not support the earlier suggestion that there is an insulating liquid carbon phase. Carbynes have been proposed [63,64] as a thermodynamically stable phase at low pressures and high temperatures (-0.2 GPa, 5000 K) but this remains controversial (see Section 2.3 ). Some theoretical studies [65] indicate that a metallic form of carbon, BC8, is stable at very high pressures (above -1 TPa). However, other theoretical [66-68] studies suggest that cubic diamond is more stable than possible metallic forms up to 1.3-2.3 TPa. The high pressure-high temperature (up to -2 TPa, 14000 K) phase and transformation diagram for carbon has been reviewed by Sekine [69].

Solar Panel Basics

Solar Panel Basics

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