Graphitization polycrystalline diamond

Natural diamonds used for jewellery and for industrial purposes have been mined for centuries. The principal diamond mining centres are in Zaire, Russia, The Republic of South Africa, and Botswana. Synthetic diamonds are made by dissolving graphite in metals and crystallising diamonds at high pressure (12-15 GPa) and temperatures in the range 1500-2000 K [6] see section 3. More recently, polycrystalline diamond films have been made at low pressures by... 

S. Sumiya, T. Irifune, A. Kurio, S. Sakamoto, and T. Inoue, Microstructure Features of Polycrystalline Diamond Synthesized Directly from Graphite under Static High Pressure , Jour. Mater. Sci., 39,445 (2004). 

It has been reported that when Ceo is rapidly and nonhydrostatically compressed above 20 GPa at room temperature, it transforms into polycrystalline diamond [524]. Although Ceo can be considered as a folded graphite sheet, we must take into account that in the pentagons there is an important tetrahedral distortion making the transformation of Ceo into diamond likely easier than the HP-HT conversion from graphite, and it is possible to use this reaction for industrial production of diamonds. 

The non-diamond carbon phase in polycrystalline diamond films (often referred to as graphite, although this conclusion is far from accurate [23]) is first and foremost the disordered carbon in the intercrystallite boundaries. Their exposure to the film surface can be visualized by using a high-resolution SEM techniques [24] the intercrystallite boundaries thickness comes to a few nanometers. In addition to the intercrystallite boundaries, various defects in the diamond crystal lattice contribute to the non-diamond carbon phase, not to mention a thin (a few nanometers in thickness) amorphous carbon layer on top of diamond. This layer would form during the latest, poorly controlled stage of the diamond deposition process, when the gas phase activation has ceased. The non-diamond layer affects the diamond surface conduc-... 

Fig. 7. Cyclic voltammograms of background current for (a) and (b) polycrystalline diamond, (c) platinum, and (d) highly oriented pyrolytic graphite (basal plane) in 0.5 M H2SO4 (a) high-quality film (b) low-quality film [38], Reproduced by permission of The Electrochemical Society, Inc.

In Section 2 we showed that the properties of amorphous carbon vary over a wide range. Graphite-like thin films are similar to thoroughly studied carbonaceous materials (glassy carbon and alike) in their electrode behavior. Redox reactions proceed in a quasi-reversible mode on these films [75], On the contrary, no oxidation or reduction current peaks were observed on diamondlike carbon electrodes in Ce3+/ 41, Fe(CN)63 4. and quinone/hydroquinone redox systems the measured current did not exceed the background current (see below, Section 6.5). We conventionally took the rather wide-gap DLC as a model material of the intercrystallite boundaries in the polycrystalline diamond. Note that the intercrystallite boundaries cannot consist of the conducting graphite-like carbon because undoped polycrystalline diamond films possess excellent dielectric characteristics. 

Diamond-like Phases and Carbon-based Films. Raman spectra were used to characterise ion-irradiated diamond samples.296 Raman data could be used to identify features related to point defects in diamond.297 There is Raman evidence for the formation of polycrystalline diamond from graphite at high pressures and temperatures.298 Surface C-H groups on diamond nanocrystals were characterised by IR (vC-H) and ab initio calculations, e.g. the vC-H band on a C(111)1 x 1 site is at 2834 cm-1.299... 

In principle, there are two procedures that differ in the kind of starting material used. In the first process, the explosive is detonated mixed with a graphitic substance. Two things happen simultaneously then Firstly, a direct conversion of the already existing elemental carbon, and secondly, a condensation of carbon from the explosive. Together they result in the formation of a polycrystalline diamond product with particle dimensions almost idenhcal to those of the starting material. Hence, an at least partially martensitic process can be assumed for the mechanism of formation. The yield is about 17% relative to the carbon employed or, relative... 

Chemical Vapor Deposition Polycrystalline diamond Pyrolytic graphite... 

In addition, electron diffraction patterns of polycrystalline diamond are similar to those of basal-plane oriented polycrystalline graphite and, when analyzing mixtures of the two, it may be difficuitto separate one pattern from the other. Unfortunately, mixed graphite-carbon-diamond aggregates eire common in natural and synthetic materials. 

Fullerene-Diamond Transformation. The rapid compression of Cqo powder, to more than 150 atm in less than a second, caused a collapse of the fullerenes and the formation of a shining and transparent material which was identified as a polycrystalline diamond in an amorphous carbon matrix.O Thus the fullerenes are the first known phase of carbon that transforms into diamond at room temperature. Graphite also transforms into diamond but only at high temperatures and pressures (see Ch. 12, Sec. 3.0). 

Diamond and cBN powders produced by milling are essentially monocrystalline and dominate the market. However, polycrystalline diamond powder can also be produced by shock synthesis. Under suitable conditions, shock waves produced by explosively driven projectiles can produce HPHT conditions in confined volumes for a sufficient duration to achieve partial conversion of graphite into nanometer-sized diamond grains which can also sinter into micrometer-sized, polycrystalline partieles." This process was commercialized by DuPont to produce a polycrystalline DMP (trade name Mypolex ) that is more friable than monocrystalline DMP and is well suited to fine polishing applications. Hexagonal (graphite-hke) BN will also react under shock-synthesis conditions, but the dense, nanometersized particles that are produced are of the wurtzite phase (wBN) rather than the cubic phase. So far, nano-wBN has not achieved much commercial importance. 

Static Pressure Synthesis. Diamond can form direcdy from graphite at pressures of about 13 GPa (130 kbar) and higher at temperatures of about 3300—4300 K (7). No catalyst is needed. The transformation is carried out in a static high pressure apparatus in which the sample is heated by the discharge current from a capacitor. Diamond forms in a few milliseconds and is recovered in the form of polycrystalline lumps. From this work, and studies of graphite vaporization/melting, the triple point of diamond, graphite, and molten carbon is estimated to He at 13 GPa and 5000 K (Fig. 1)... 

Limited supply, increasing demand, and high cost have led to an intense search for an alternative, dependable source of diamond. This search led to the high pressure (ca 5 GPa (0.5 x 106 psi)), high temperature (ca 1500°C) (HP—HT) synthesis of diamond from graphite in the mid-1950s (153—155) in the presence of a catalyst—solvent material, eg, Ni or Fe, and the subsequent development of polycrystalline sintered diamond tools in the late 1960s (156). 

---