Ranking Of Catalysts For Diamond Synthesis

Based on the above model of "atomic matching" and "touch and go", the relative catalytic power of transition metals can be calculated for their ability to nucleate, grow, and form diamond. Moreover, if the catalyst has a low melting point, it can also reduce the pressure of diamond synthesis. A lower synthesis pressure can allow a larger reaction volume for a higher output, or a longer carbide life for a lower cost. Hence, a low melting point is another merit to be considered for selecting the proper catalyst for diamond synthesis (Fig. 4.7).

Figure 4.7. The relative catalytic power of transition metals for graphite to diamond transition under high pressure. N = Nucleation Index, G = Growth Index, T = Transformation Index, and A = All-merits Index. Note the dome-shaped curves that indicate transition metals with about half full d-orbitals; these metals are both efficient solvents of carbon atoms and effective catalysts for diamond formation.

Figure 4.7. The relative catalytic power of transition metals for graphite to diamond transition under high pressure. N = Nucleation Index, G = Growth Index, T = Transformation Index, and A = All-merits Index. Note the dome-shaped curves that indicate transition metals with about half full d-orbitals; these metals are both efficient solvents of carbon atoms and effective catalysts for diamond formation.

MA 1UA UA UIA UliA UiilA IB MB

Sc

Ti

U

Cr

Mn

Fe

Co

Ni

Cu

Zn

Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Ag

Cd

La

Hf

Ta

W

Re

Os

Ir

Pi

Au

Hg

22

l<3

13

5

3

2

1

4

25

28

24

20

17

11

7

8

9

14

26

29

23

21

18

16

12

18

6

15

27

30

Figure 4.8. The ranking (numbers shown in the lower diagram) of catalyst merits of transition metals for graphite to diamond transition. In the figure, heavily stippled elements are the 12 original catalysts identified by General Electric scientists (Bovenkerk et al., 1959). Lightly stippled elements are other likely catalysts (Sung and Tai, 1997).

Figure 4.8 shows the ranking of transition metals according to the above merit scale as catalysts for graphite to diamond conversion.

Based on the above ranking scale, the most powerful catalysts are listed in the decreasing order of: Fe, Co, Ni, Mn, and Cr. If low melting points are factored in, the order is changed to: Co, Fe, Mn, Ni, and Cr (Fig. 4.8). These five transition metals are indeed the most commonly used catalysts for diamond synthesis in the industry. The catalyst composition can be analyzed from the metal inclusions trapped inside diamond during its synthesis. For example, General Electric's saw diamonds were formed from a catalyst of FeNiCr. The saw diamonds formed by De Beers used the catalysts FeCo or FeNi. Russian scientists formed diamond using NiMn as catalysts. Chinese diamond makers used mainly NiMnCo. The agreement of theoretical predictions with empirical experience supports the validity of the above described catalytic mechanism for graphite to diamond transition.

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