Liquid Phase Growth Of Diamond At Low Pressure

Gem diamond crystals have been grown by CVD methods at partial vacuum, but as graphite is the stable phase, the growth rate of diamond must be kept low lest graphitic carbons are formed. On the other hand, the growth of gem diamonds in molten alloy (e.g. invar composition) under ultrahigh pressure can be increased, but the volume of the apparatus is severely limited. It would be ideal if diamond can grow in liquid phase at atmospheric pressure. In this case, not only the growth rate can be high, but also there is no limitation in number of crystals growing simultaneously.

During the growth of CVD diamond, hydrogen atoms must be around to stabilize diamondoid bonds. If a carbon solvent is also the catalyst for generating hydrogen atoms, diamond can grow without the accumulation of graphite at low pressures. The liquid phase is also a diamond catalyst (e.g. Fe, Co, Ni, Mn, Pd or its alloys) that may be alloyed with rare earth elements (La, Ce) to suppress its melting point (e.g. to 400-800°C). Such alloys have

Figure 11.19. A literature example of possible gas species in an original mixture of CH4 (0.5%) and H2. The plot is based on the gas pressure at 20 torr.
Phast Diagram Calcium Titanate
Figure 11.20. The phase diagrams of cerium alloy with either Co or Ni indicated that the eutectic compositions can be melted below 600°C.

high solubilities of both carbon atoms and hydrogen atoms. Pd or Pt may be also be used as the liquid phase to enhance the catalytic absorption of hydrogen atoms. In the molten alloy, both carbon and hydrogen atoms hidden in voids of the metal atoms. The dissolved carbon atoms are surrounded by catalytic metal atoms, at the same time hydrogen atoms are nearby so the carbon atoms may maintain the diamondoid structure until they are deposited on the diamond seed (Fig. 11.20).

With the presence of a liquid catalyst that can dissolve both carbon atoms and hydrogen atoms, diamond seeds (e.g. nanodia-monds) may be mixed with a diamondoid nutrient (e.g. adamantine) in such a way that diamond seeds are growing at the expense of diamondoid nutrient. An example of the reactor is shown below (Fig. 11.21).

The above idea of diamond growth was tested by Prof. Chhiu-Tsu Lin and his Ph.D. candidate Chien-Chung Teng at Northern Illinois University. The following photographs represented the data taken from the initial experiments (Figs. 11.22 and 11.23).

Liquid ¡-(Ni. PJ)(U, Cc) +■ Nino Diamond (Gmphnc) electricity feedthrough

Figure 11.21. The growth of diamond seeds in a diamond cooker with a molten catalyst and diamond nutrient thoroughly mixed by convection current. The left shows the concept; and the right, the schematic of an experimental reactor.

Liquid ¡-(Ni. PJ)(U, Cc) +■ Nino Diamond (Gmphnc) electricity feedthrough

Figure 11.21. The growth of diamond seeds in a diamond cooker with a molten catalyst and diamond nutrient thoroughly mixed by convection current. The left shows the concept; and the right, the schematic of an experimental reactor.

Figure 11.22. The raw materials used for cooking diamond included nanodiamond (left diagram) as seeds, nano nickel as catalyst (middle diagram), and adamantine nano particles (right diagram) as nutrient.
Figure 11.23. The Raman spectra of the materials mixture before and after the cooking experiment (800°C for 10 hours). Note that the pronounced increase of diamond peak at 1332 cm-1.

The above illustration indicated that multiple diamond seeds (loose or fixed) could be grown in a liquid system at ambient pressure. Such a novel idea, if implemented on a production scale, may revolutionize the way diamond is synthesized by either CVD or ultrahigh pressure. The ample availability of large diamond crystals can pave the road for the rapid advancement of diamond devices, including diamond semiconductors. The widespread use of diamond devices will mark the arrival of diamond age.

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