TIME 2 Synthetic Diamonds

Since that time, synthetic diamond films have developed into an important high-tech product employed for many purposes. In comparison to other forms of diamond, the most attractive difference is the facile generation of diamond coated workpieces in almost any desired shape. The preparation of thin layers, for example, for electronic applications, became possible as well only after the development of CVD methods. 

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. 

For years it was thought that diamonds were made of carbon atoms, just like graphite and coal, but no one could demonstrate this. In 1955 scientists were able to produce the tremendous pressure (over 100,000 times normal) and temperatures over 2,500°C to form a synthetic diamond from graphite that appears to be as real as a naturally formed diamond. However,... 

In the last 40 yr, the development of synthetic diamond in various forms has fueled a revolution in the use of diamond as an engineering material. The process of HPHT diamond synthesis was responsible for stunning growth in the abrasives market. During that time, the world s consumption of diamond abrasive materials increased from 5 to over lOOtons/yr. 

Figure 23. Comparison of ESR signals from a single crystal of synthetic diamond in the TEioi cavity and in the loop-gap resonator. Spectra were obtained at room temperature with 2G field modulation. Spectrometer gains are indicated. A, Absorption signal, 1 mW, 9.3 GHz, 1 second time constant. B, Absorption signal, 2 pW, 8.8 GHz, 0.25 second time constant. C, Dispersion signal, other conditions same as in B. D, Dispersion signal, 1 mW, 8.8 GHz, 0.25 second time constant. From [53], with permission.

There is only one known acceptor in diamond, responsible for the p-type conductivity of the lib diamonds. For some time, it was assumed that this acceptor was aluminium [49], but it has been suggested [43] and finally shown conclusively [38] that boron was indeed responsible for the p-type conductivity and the spectroscopic properties of type lib blue diamonds. Natural lib diamonds had been identified ca. 1954 (see Sect. 2.11), and synthetic lib diamonds were obtained at the beginning of the 1960s [80]. Boron is commonly introduced as a dopant in synthetic diamonds and its ionization energy ) is 370 meV [177]. The discrete acceptor spectrum of B extends approximately 70 meV below ) and is superimposed on the two- and three-phonon spectra of Cdiam- Boron acceptor absorption lines are observed at 305, 347 and 363 meV ( 2780, 2800, and 2930 cm 1) at RT, giving phonon-assisted transitions near 464 and 504meV (see [140], and references therein). 

Nanodiamond as such is not really a new material. Indeed, essential experiments on the production of nanoscale diamond crystallites have already been performed in the late 1950s and early 1960s. No later than 1959, DeCarh and Jamieson found that tiny diamond crystalHtes could be obtained from the action of a shock wave on graphitic material. The procedure was patented in the 1960s, and from that time on, the DuPont Corp. has been producing about 2 million karat of synthetic diamond per year in this manner (Section 5.3.2). 

When used for jewelry, synthetic single crystals can be more costly than naturally occurring ones. For example, the cost of a gem-grade faceted synthetic diamond may be 10 times that of an equivalent natural diamond. However, flux-grown emeralds and rubies are about one-tenth the cost of natural stones of comparable quality. (The synthetic stones are often much more perfect than the natural ones.) Large single crystals of cubic ZrOi, which are used... 

Frequently, when large, high-quality synthetic diamonds are discussed, the question of their potential use as gemstones is raised. At the time of writing, none of the major manufacturers of synthetic diamonds markets material for gem use. How-... 

Silicon carbide and alumina still dominate the abrasive industry at the present time. However their performance in the grinding of superalloys, ceramics, reinforced plastics, and other hard materials is generally unsatisfactory. This has led to the development of new abrasives such as synthetic diamond and cubic boron nitride. Cubic boron nitride was first synthesized in 1957 and has been available commercially since the 1970 s. Although not as hard as diamond, c-BN does not react with carbide formers such as Fe, Co. Ni, Al, Ta, and B at 1000 (while diamond does). However, it reacts with aluminum at 1050°C, with Fe and Ni alloys containing Al above 12S0"C, and with water and water-soluble oils.1 1... 

Changing the diamond grits strength measure from 32 H to 82 H at constant diamond concentration results in 20-fold increase in fatigue life of the composite. At the same time the decrease in synthetic diamond concentration from 37 5% down to 18.7%, the other parameters being constant, results in 8-fold increase in a number of fatigue cycles to failure of DCM specimens. 

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