Transformation processes, diamond

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 mobiUty for the transformation, and the high temperature in turn demands a high pressure (above 12 GPa 120 kbar) for diamond stabihty. 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) stmcture was discovered (25). 

Like graphite, C60 can be transformed into diamond, but the process requires less stringent conditions. It has also been found that Cso becomes a superconductor at low temperature. Another interesting characteristic of Cso is that when it is prepared in the presence of certain metals, the Cso cage can enclose a metal atom. In some cases, other materials can be enclosed within the C60 cage in a "shrink wrapped" manner to form "complexes" that are described as endohedral. It has also been possible to prepare metal complexes of Cso that contain metal-carbon bonds. A compound of this type is (C6H5P)2PtC60. 

Problem 1.2. The transformation of diamond to graphite is an example of a thermodynamically favorable but kinetically frustrated process. A kinetically frustrated process is one that will not occur over a reasonable scientifically observable time period. Come up with another example of a process that is thermodynamically favorable but kinetically frusttated. 

There are no obstacles in searching for novel approaches to diamond synthesis by studying new growth processes electrolysis, hydrothermal (15) and laser assisted. Graphite has been transformed to diamond by laser process (16,17) and some laser assisted CVD processes were... 

At 25°C and 1 atm, graphite is the stable form of carbon. Diamond, in principle, should slowly transform to graphite under ordinary conditions. Fortunately for the owners of diamond rings, this transition occurs at zero rate unless the diamond is heated to about 1500°C, at which temperature the conversion occurs rapidly. For understandable reasons, no one has ever become very excited over the commercial possibilities of this process. The more difficult task of converting graphite to diamond has aroused much greater enthusiasm. 

AH = 2.9 kj mol-1 at 300 K and 1 atm, there is no low-energy pathway for the transformation, so the process is difficult to carry out. However, synthetic diamonds are produced on a large scale at high temperature and pressure (3000 K and 125kbar). The conversion of graphite to diamonds is catalyzed by several metals (i.e., chromium, iron, and platinum) that are in the liquid state. It is believed that... 

Diamond is transformed into graphite when heated by a powerful electric current between carbon poles, and both diamond and graphite can be indirectly converted into charcoal. The artificial production of the diamond, however, is a more difiticult process but the late Professor Moissan succeeded in effecting it, so far as very small diamonds are... 

Much of the quartz in the fly ash originates from the coal as silt- and sand-sized particles, and it remains in the ash because it survives thermal transformation during the combustion process (Helmuth 1987). Small amounts of volatilized Si may also oxidize to form very fine crystals of quartz within the fly ash glass (Diamond 1984 Hubbard et al. 1984). Although bituminous coal ash may contain more than 50 wt% analytical Si02, only 5-10 wt% of it is present in the form of quartz (McCarthy et al. 1990). Some Si is present in the mineral mullite, but the majority of it is in the amorphous glass phase. 

The multimedia urban model (MUM) is a fugacity-based mass balance model that treats the movement of POPs in an urban environment and links emissions to ambient chemical concentrations, and thus outdoor exposure (Diamond et al., 2001). MUM considers longterm, average conditions of chemical transport and transformation among six environmental compartments in urban areas (air, soil, surface water, sediment, vegetation and surface film see Figure 6.1) shows a concepmal version of the model). The model does not estimate event-specihc processes as do meteorological-based air or stormwater models. 

However, even though this transformation is thermodynamically favored, the diamond allotrope still exists at high pressures and over long time periods. That is, if a particular phase transformation is predicted as spontaneous, the acmal rate of that process will depend on the kinetics of the transformation. Since the sp carbon bonds in diamond are extremely strong, the kinetics governing the migration of carbon atoms between diamond-graphite is extremely slow at normal temperatures and... 

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