Historical High Pressure Synthesis Of Diamond

Numerous methods using high pressure technology have been invented to synthesize diamond in the past. In fact, high pressure technology was developed largely because of the need to synthesize diamond.

Hannay developed sealed steel tubes that could contain organic volatiles at high temperatures to about 0.2 GPa (1880). Parsons built a piston-cylinder apparatus that could bring pressures up to 1 GPa (1888). The Nobel laureate Moissan quenched molten steel to attain a pressure of about 0.5 GPa (1894). However, it was the other Nobel laureate, P. W. Bridgman, who invented truncated anvils that could penetrate the barrier to reach the ultrahigh pressure of 5 GPa (1946). An extensive review of high pressure technologies, in particular those for diamond synthesis, is presented elsewhere (Sung, 1997).

Although Hannay and Moissan claimed success at synthesizing diamond, no one has since been able to repeat their process. Most scientists believe that their diamonds were either carbides or other compounds, as in the case of Hannay; or seeded natural diamonds, as in the case of Moissan. Although Bridgman routinely squeezed graphite to 20 GPa, a pressure well within the stability field of diamond, and heated the sample to over 1000°C, this brutal force approach still did not yield any diamond. This lead to Bridgman's (1955) lament, "graphite is nature's best spring."

Diamond synthesis is one of few areas where chemists succeeded while physicists have failed. Although there were discussions of using catalysts to accelerate the formation of diamond (e.g. Russians already mentioned using iron as the carbon solvent for diamond synthesis), Bridgman was a physicist, so he did not pay much attention to this chemical aspect. It would take some chemists a decade later to apply catalysts and collapse that "graphite spring" to form diamond at a much lower pressure than what Bridgman attempted.

The first artificial diamond was created by ASEA (General Electric Company of Sweden) engineers on February 15,1953 (Fig. 4.3). The diamond was formed in a high pressure apparatus designed by Baltzar von Platen (Liander, 1955). This apparatus was composed of a large cubic press that contained six anvils arranged in a shape of a split-sphere. The sample volume was over 40 cm3, an enormous size at that time. The first diamond was produced from a mixture of cementite (Fe3C) and graphite. The charge was compressed to a pressure of about 75 Kb and heated to a temperature of over 1500°C by a thermistic reaction. After more than 3 min of heating, several dark diamond crystals hundreds of microns in size were formed.

Figure 4.3. ASEA's split sphere high pressure assembly designed by von Platen (top left). The actual apparatus revealed the inside structure (top right). The assembly drawing of the apparatus (bottom left). The center cell assembly (top right), and the diamond ASEAclaimed that was synthesized in 1953 (bottom right).

Figure 4.3. ASEA's split sphere high pressure assembly designed by von Platen (top left). The actual apparatus revealed the inside structure (top right). The assembly drawing of the apparatus (bottom left). The center cell assembly (top right), and the diamond ASEAclaimed that was synthesized in 1953 (bottom right).

On December 16,1954, Tracy Hall of General Electric Company in the United States also was successful in synthesizing diamond. He used a much simpler belt apparatus of his own design (Hall, 1970). The sample generated had a volume of less than 0.1 cm3. It contained a mixture of troilite (FeS) and graphite. The sample was first compressed to a pressure of about 70 Kb and then heated by passing through an electric current to a temperature of about 1600°C. After heating for two minutes, several minute diamond crystals were formed. The diamond did not grow in the sample mixture as originally intended. Instead, it was embedded in a solid end cap made of tantalum. The end cap was used to lead the electric current to the sample.

Subsequent research from General Electric scientists led to the discovery that diamonds could be formed at high pressure by the catalytic action of molten metal (Fig. 4.4). According to these researches, this metal must contain one or more elements selected

Figure 4.4. GE's belt apparatus as invented by Hall (top left), and the first diamonds that was synthesized in 1954 using this apparatus (top right). The early GE press used at "Diamond Mine" of corporate R&D (bottom).

from a list that contained eight Group VIIIB elements (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt) and three other transitional metals (Mn, Cr, and Ta) (Bundy et al., 1955).

The success in synthesizing diamond in 1950s represented a major feat of science and technology that, although in a more minuscule scale, may be compared to that of the landing of men on the moon in 1969. This success came from contributions of more than one group (including the Swedish scientists) and it is difficult to attribute the contribution to just a few individuals. Perhaps it was for this reason that the Nobel Prize was not awarded for such an epoch making event.

In 1961, diamond was converted directly from graphite without the use of any catalyst by the scientists DeCarIi and Jamieson from DuPont. The conversion was triggered by shock compression from an explosion that created a momentary (a few microseconds in length) pressure of ~350 kb and a temperature of ~770°C.

In 1963, direct graphite to diamond conversion was also achieved by Francis Bundy of the General Electric Company. This time the transition took place under a static pressure of about 120 kb and a transient (a few milliseconds in length) temperature of about 3000°C. The temperature of the sample was raised by releasing an electric flash from a capacitor at high voltage (Bundy, 1963). The possible mechanisms of such direct graphite to diamond transitions are discussed elsewhere (Sung and Tai, 1997b).

In 1972, another milestone was reached when the General Electric scientists Strong and Wentorf announced their success in making gem diamonds (Fig. 4.5). These diamonds were grown under controlled pressures that deviated within 0.1 GPa, and temperatures that fluctuated by less than 10°C. The time of synthesis lasted up to 1 week. In order to avoid the pressure decay caused by the volume reduction (about one-third) associated with the graphite to diamond transition, the carbon nutrient used was in the form of minute crystals of diamond itself. These crystals were dissolved in the hot zone of a molten metal. The dissolved carbon atoms then diffused toward a cold region (about 50°C) where they precipitated onto a diamond seed. This temperature gradient method can grow diamonds several milligrams an hour. Today, the largest single crystal diamond synthesized in this fashion is over 25 carats (over 30 carats for imperfect crystals) as demonstrated by De Beers' scientists.

Figure 4.5. The cell design of the temperature gradient method for growing gem diamond (upper diagram). The polished carat-sized diamond synthesized in the 1970s by GE (bottom left) and the 25 carat diamond crystal grown by De Beers in the 1990s (bottom right).

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