SYNTHESIS AND CHARACTERIZATION OF NANODIAMONDS FROM DIFFERENT CARBON PRECURSORS"

DIAMOND AND DIAMOND-LIKE CARBONS

J.B.Donnet 1, C. Le Moigne 1, T.K. Wang 1, and M. Samirant 2 I Lab. Chimie Physique, ENSCMu, 3, rue A. Werner, 68093 Mulhouse cedex, France, 21nstitut de

Recherche Franco-Allemand de Saint-Louis, 5, Rue du Gdndral Cassagnou, 68300 Saint Louis, France

Introduction

Synthetic diamonds can be prepared by static, dynamic and chemical vapor deposition (CVD) methods. The static method has been used industrially since a long time and the CVD method has been considerably improved in recent years. However, the dynamic method advanced just smoothly and the mechanisms of diamond formation is still controversed.

In this study, diamonds have been synthesized with two dynamic methods: (1) The explosion method using directly the carbon rich explosives as precursors; (2) The shock wave method using the projectile impact and different carbon precursors. The diamonds obtained have a powder form with the particle size in the nanometer scale. They have been characterized by several methods such as the X-ray diffraction (XRD), electron diffraction (ED) and transmission electron microscopy (TEM).

Experimental

Diamond synthesis (1) Explosion method A mixture of explosives was

used as either high pressure (HP) and high temperature (HT) producer or a carbon-rich precursor. The mixture is composed of three explosives: trinitro-2,4,6 toluene (C7HsN306, TNT), cyclo- 1,3,5-trimethylene-2,4,6- trinitramine (C3H6N606, RDX) and pentrite (CsHsO12N4, PETN). The mixture was placed to the center of a closed metal vessel equipped with two thermocouples measuring the temperature till 1250°C, two optic fibers connected to two oscilloscopes to detect the illumination signals during 1 microsecond and a system for the analysis of the gases produced by the explosion. The experiments were realized in air, nitrogen and water respectively.

(2) Shock wave method. Different carbon precursors, such as graphite and carbon blacks mixed with copper powder (5/95 w/w), were compressed into a cylindrical slice of 12 mm in diameter and 1 mm in thickness and confined into a still capsule. The projection system was similar to that used by others [1 ]. The explosive used for the projection was a TNT/RDX mixture (65/35) and can give a detonation velocity of 8 km/s.

Results and Discussion

(1) Explosion method The HP and HT produced during the explosion in

our experiments are sufficient to transform the carbon contained in explosives into diamonds. But the yields depend strongly on the environmental media used. In the air, the explosive was totally converted into gases and no solid deposit was recovered. The light illumination and temperature detection showed a post-combustion process. In nitrogen gas, this combustion was inhibited and a diamond containing soot mixture was obtained. If the explosives (-250g) are immersed in water, the diamond yields were seemingly improved. In the carbonaceous solid mixture,, diamond content can reach 60% in weight after the elimination of graphitic carbon.

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Figure 1. X-ray diffraction (XCuKC1.5406A) pattem of purified diamond obtained by the explosion method

X-ray diffraction (Fig. 1) and electron diffraction results show that the diamonds obtained by explosion method has a cubic crystalline structure. The size of the diamond particles are quite homogeneous and are about 5-7 nm in diameter (Fig. 2). This agrees well with the specific surface area measurement by nitrogen adsorption (-~280m2/g). Interestingly, the surface of the particles obtained directly by the explosion in water medium contains many hydroxyl groups.

3 0 2

When the nanodiamond particles were heated at 1500°C in vacuum, onion-like carbon was obtained (Fig. 3). The of the onion-like particle has nearly the same size as the diamond precursor, suggesting a in-situ trans- formation.

heterogeneity of the diamond particle size and morphology favors the reconstructive diamond formation mechanism.

Intensi ty 6 0 . (111)d 55 i. (002)g | 50 1

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Figure 4. X-ray diffraction pattern (XCuKC1.5406/~) of a diamond-containing carbon obtained by projectile impact on carbon black N 110.

Figure 2. TEM image of the nanodiamond particles obtained with explosion method.

Figure 3. Onion-like carbon obtained by heating the nanodiamond at 1500°C in vacuum.

(2) Shock wave method Diamonds were obtained both with graphite and

carbon black (Fig. 4) precursors. Copper was proved to be necessary as quenching agent and catalyst. Typically, the diamond-containing carbon particles have a diamond core and graphitic shell structure (Fig. 5). The diamond core can be monocristalline (Fig. 5) or polycristalline. The graphitic shell structure contributes at least partly to the (002)g peak of the X-ray diffraction pattern (Fig. 4) and could be formed by the regraphitization process. The

Figure 5. TEM image of a carbon particle with a diamond core and graphitic shell, obtained by the shock wave method with the carbon black N 110 as precursor.

Conclusions

Nanodiamonds were synthesized by both explosion and shock wave methods. The results suggest a recons- tructive mechanism for diamond formation.

References:

1. F. R. Norwood, R. A. Graham, A. Sawaoka, in Shock Waves in Condensed Matter (ed. Y. M. Gupta), Plenum Press, New York, 1986, p.837

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