Diamond, synthetic

According to figure 39.2 diamond is stable only at very high pressure. All the valuable jewehy diamonds, and the industrial diamonds too, should spontaneously form graphite. That diamond really exists at atmospheric pressure and room temperature is of course due to the fact that transformation of diamond to graphite occurs with very low speed at low temperatures. 

In an inert atmosphere diamond is rapidly transformed to graphite at 800°C. Because of that diamond tools cannot be used for machining (lathing, drilling) of steel. 

Synthetic diamond accounts for more than 90% of industrial diamonds used in the world. The world production of synthetic diamond was in 2001 about 600 million carats. USA accounted for 50%, Russia for 13%, South Africa and Ireland for 10% each, Japan for 5%, Belarus, Sweden and China for 3-4% each. 

The commercial availability of synthetic diamond offered two advantages. First, there was potentially unlimited availability of industrial diamond compared to the limited volumes of suitable natural material and, second, it offered the opportunity of engineering material to have specific properties suited to particular industrial applications. 

Following the successful commercial synthesis of diamond in the 1950s, the second hardest material known, cubic boron nitride, cBN, was introduced to the market in the 1960s and is complementary to diamond. The iron, and its alloying elements, in ferrous materials has a tendency to react chemically with diamond under machining conditions and this can reduce the efficiency of the tool. cBN, however, although not as hard as diamond, does not react chemically with iron and is therefore particularly well suited to machining hard ferrous materials. 

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Very high temperature and pressure on graphite in the presence of a metal catalyst gives synthetic diamonds big enough for many industrial uses. 

Very small synthetic diamonds have been made industrially by subjecting graphite to pressures in the range 5.5-b.9 GN m , at temperatures between 1500 and 2700 K. The diamonds produced are very small but competitive with natural diamonds for use in industrial cutting and grinding wheels. 

Diamond. Diamond [7782 0-3] is the hardest substance known (see Carbon, diamond, natural). It has a Knoop hardness of 78—80 kN/m (8000—8200 kgf/m ). The next hardest substance is cubic boron nitride with a Knoop value of 46 kN/m, and its inventor, Wentorf, beheves that no manufactured material will ever exceed diamond s hardness (17). In 1987 the world production of natural industrial diamonds (4) was about 110 t (1 g = 5 carats). It should be noted that whereas the United States was the leading consumer of industrial diamonds in 1987 (140 t) only 260 kg of natural industrial diamonds were consumed this is the lowest figure in 48 years (4), illustrating the impact that synthetic diamonds have made on the natural diamond abrasive market. 

Synthetic Diamond. In 1955 the General Electric Company announced the successful production of diamonds (see Carbon, diamond, synthetic) from graphite under very high pressure and temperature ia the presence of a metal catalyst. It was later reported that a Swedish company, Allmana Svenska Electriska AB (ASEA), had succeeded ia ptoduciag diamond ia 1953 (35). 

A.brasives as a Whole Norton Company, U.S. Washington Mills, U.S. Treibacher, Austria Pechiney, France ESK, Germany and Exolon-ESK, U.S. Superabrasives (Synthetic Diamond and CBN) General Electric, U.S. De Beers, UK and Tomei, Japan ... 

Annual production of powdered BN is ca 180—200 metric tons per year and its cost is 50—250/kg, depending on purity and density. The price of cubic boron nitride is similar to that of synthetic diamond bort. Hot-pressed, dense BN parts are 3—10 times more expensive than reaction-sintered parts. 

Boron and carbon form one compound, boron carbide [12069-32-8] B C, although excess boron may dissolve ia boron carbide, and a small amount of boron may dissolve ia graphite (5). Usually excess carbon appears as graphite, except for the special case of boron diffused iato diamonds at high pressures and temperatures, eg, 5 GPa (50 kbar) and 1500°C, where boron may occupy both iaterstitial and substitutional positions ia the diamond lattice, a property utilized ia synthetic diamonds (see Carbon, diamond, synthetic). 

The high elastic modulus, compressive strength, and wear resistance of cemented carbides make them ideal candidates for use in boring bars, long shafts, and plungers, where reduction in deflection, chatter, and vibration are concerns. Metal, ceramic, and carbide powder-compacting dies and punches are generahy made of 6 wt % and 11 wt % Co ahoys, respectively. Another apphcation area for carbides is the synthetic diamond industry where carbides are used for dies and pistons (see Carbon). 

Soon after the first successful diamond synthesis by the solvent—catalyst process, a pilot plant for producing synthetic diamond was estabUshed, the efficiency of the operation was increased, production costs declined, and product performance was improved while the uses of diamond were extended. Today the price of synthesized diamond is competitive with that of natural diamonds. 

Fig. 8. (a) Synthetic diamond grit for resinoid or vitreous bond (free-cutting) abrasive wheels, and (b) synthetic diamond grit for metal bond abrasive... 

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... 

The Properties of Natural and Synthetic Diamond, cd. J.E. Eield, Academic Press, London, 1992. 

An engineer s ability to distinguish a natural from a synthetic diamond is tested independently on 10 different occasions. Wliat is the probability of 7 correct identifications if tlie engineer is only guessing (i.e., has a probability of making a correct identification of 0.5) ... 

Figure 8.4 Phase diagram of carbon showing regions of importance for the production of synthetic diamond. ...

Many ceramic applications are high value and small volume, so energy expenditure is high. Ferroelectric magnets, electronic substrates, electrooptics, abrasives such as silicon carbide and diamond, are examples. Diamond is found naturally, and made synthetically by the General Electric Company at high pressure and temperature. Synthetic diamonds for abrasives require less energy to make than the value in Table 4 nevertheless, the market is carefully divided between natural and synthetic diamonds. 

There are two types of diamonds, synthetic and natural. Synthetic diamonds are man made and are used in PDC STRATAPAX type bit designs. STRATAPAX PDC bits are best suited for extremely soft formations. The cutting edge of synthetic diamonds are round, half-moon shaped or pointed. 

Figure 4-150 shows the major components and design of the PDC bit. The polycrystalline diamond compacts, shown in Figure 4-151. The polycrystalline diamond compacts (of which General Electric s) consist of a thin layer of synthetic diamonds on a tungsten carbide disk. These compacts are produced as an integral blank by a high-pressure, high-temperature process. The diamond layer consists of many tiny crystals grown together at random orientations for maximum strength and wear resistance.

Ceramic-coated disposable inserts, including silicon nitride, boron nitride, titanium nitride (TIN), titanium carbide (TIC) and sintered synthetic diamond ... 

Charles Friedel (1832-1899) was horn in Strasbourg, France, and studied at the Sorbonne in Paris. Trained as both 3 mineralogist and a chemist, he was among the first to attempt to manufacture synthetic diamonds. He was professor of mineralogy at the School of Mines before becoming professor of chemistry at the Sorbonne (1884-1899). 

The pressure needed to make synthetic diamonds from graphite is 8 X I04 atm. Express this pressure in (a) Pa ... 

A polycrystalline synthetic diamond electrode was used, values of A o range between 6-10 and 2.4-10 cm s . 

A synthetic diamond electrode was used values of range between 8.5-10 and 1.2-10 cm s. ... 

Like synthetic diamond, C-BN is normally obtained by high-pressure processing. Efforts to synthesize it by CVD at low pressure are promising. It is deposited in an electron-cyclotron-resonance (ECR) plasma from a mixture of BF3 and either ammonia or nitrogen at 675°C on an experimental basis.F l Like CVD diamond, it is also deposited by the hot-filament method using diborane and ammonia diluted with hydrogen at 800°C.P 1... 

Eden, R. C., Application of Synthetic Diamond Substrate for Thermal Management of High Performance Electronic Multi-Chip Modules, in Applications of Diamond Films and Related Materials, (Y. Tzeng, etal., eds.), Elsevier Science Publishers, pp. 259-266(1991)... 

Dischler, B. S. Wild, C. (eds.) 1998 Low-pressure synthetic diamond. Berlin Springer. This book goes into more detail about the technical aspects of making CVD diamond. 

Spear, K. E., Dismukes, J. P. 1994 Synthetic diamond emerging CVD science and technology. New York Wiley. This book gives a useful description of the chemistry and physics behind diamond CVD, as well as various novel applications for CVD diamond. 

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... 

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