Pressure-temperature diagram, diamond

Figure 9. Pressure-temperature diagram for diamond synthesis showing typical conditions (P, Ti). The supersaturation Ac is a function of P Ty giving rise to different crystal morphology at different conditions.

Fig. 2. Phase diagram approximation of carbon, indicating pressure-temperature parameters favoring yield of graphite and diamond. See also phase diagram in the article on Carbon...

Figure 15.6 The (graphite + diamond) phase diagram, including the pressure-temperature region for diamond synthesis with ferrous metals and their alloys as solvent catalysts. Reproduced with permission from H. M. Strong, Early Diamond Making at General Electric , Am. J. Phys., 57, 794-802 (1989). Published by the American Association of Physics Teachers.

It is evident from the phase diagram that diamond may be obtained in a very wide pressure-temperature range, thus allowing several synthesizing methods to work in various regions. Those mainly applied are conversion of graphite to diamond by a flux method, direct conversion by shock wave compression, and direct conversion by static compression. Synthetic diamond is mostly produced by the flux method, which will be outlined below. 

Ammonium dinitramide and dinitro azetidinium dinitramide For both of these materials the pressure/temperature and reaction phase diagram have been determined using a high-temperature-high-pressure diamond anvil cell with FTIR spectroscopy, Raman spectroscopy and optical microscopy. For ammoninm dinitramide energy dispersive X-ray diffraction was also employed (Russell et al. 1996, 1997). 

In monotropy, one polymorph is always more stable than the other at all temperatures below their melting points (Fig. 7b). This definition is based on the assumption that the pressure remains eonstant. An alternative definition is that, if the pressure temperature phase diagram does not allow a polymorph to be in equilibrium with its vapor phase below the critical point, it is the unstable monotrope, otherwise it is an enantiotrope. This definition recognizes that some monotropes may be thermodynamically stable at elevated pressures and temperatures, e.g., diamond, which is the metastable polymorph of carbon under ambient conditions. 

To gain a balanced general view of the many ways in which man has crystallized diamond, the thermodynamically stable and metastable phases of elemental carbon and reaction dynamics between them, over obtainable pressures and temperatures, should be considered. The most up to date phase and transformation diagrams are to be found in a review by Bundy et al. [9]. Figure 7 is an adaptation of the pressure-temperature phase transition diagram taken from this review article, together with further information gleaned from the extensive diamond- and carbon-related scientific literature. 

Figure 4. Schematic phase diagram of metastable silicon in the pressure-temperature (P, T) plane discussed in [20,113]. The thick solid line represents the liquid-crystal (cubic diamond) transition line, extended into the j6-Tin phase. The dotted lines represent the liquid-/S-Tin and the Cubic diamond-yS-Tin transition lines. The thin line is the liquid-liquid phase transition line ending at a critical point represented by a filled circle. The dashed lines represent the spinodals associated with the liquid-liquid transition. The oval symbol represents the amorphous-liquid transition as predicted by some of the earlier experiments. [With permission from McMillan [20,113].]...

Figure 23. The ph ase diagram of supercooled silicon in pressure temperature (P, T) plane obtained from simulations using the SW potential. The phase diagram shows the location of (i) the liquid-crystal phase boundary [115]—thick solid line, (ii) the liquid-gas phase boundary and critical point—line and a star, (iii) the liquid-liquid phase boundaiy and critical point—filled diamond and a thick circle, (iv) the liquid splnodal—filled circle (v) the tensile limit—open circle (vi) the density maximum (TMD) and minimum (TMinD) lines— filled and open squares, and (vii) the compressibility maximum (TMC) and minimum (TMinC) line—filled and open circle. Lines joining TMD and TMinD (dot-dashed), TMC and TMinC (solid), Spinodal (black dotted line) are guides to the eye.

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. 

The phase diagram for carbon, shown here, indicates the extreme conditions that are needed to form diamonds front graphite, (a) At 2000 K, what is the minimum pressure needed before graphite changes into diamond (b) What is the minimum temperature at which liquid carhon can exist... 

Figure 5.4 The phase diagram of carbon showing the two solid-state extremes of diamond and graphite. Graphite is the thermodynamically stable form of carbon at room temperature and pressure, but the rate of the transition C iamond) — C aphite) is virtually infinitesimal...

An interesting application of this method is the preparation of diamond films which may be obtained from a precursor such as CH4, C2H2 and H2 activated by heating, microwaves, etc. typically at 600-1000°C at a reduced pressure. The direct deposition from the gas to the surface results in the formation of metastable diamond whereas, according to the phase diagram (see Fig. 5.37), the production of stable bulk diamond requires very high pressure and temperature. Kinetically, the... 

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