Unusually tight aggregation in detonation nanodiamond: Identification and disintegration
Introduction
In 1963 a group of Soviet scientists discovered single crystals of cubic diamond particles in soot produced by detonating an oxygen-deficient TNT/hexogen composition in inert media without using any extra carbon source (Fig. 1) [1], [2]. The average size of crystals has been determined to be 4–5 nm from the half-widths of X-ray diffraction peaks [2], [3]. The discovery offered a remarkably advantageous method of producing novel ‘dispersed ultrananocrystalline diamond’ (DUNCD according to Gruen’s nomenclature [4]).
Although the historic discovery was kept secret for unduly long time for security reasons [5], research activities in this novel diamond increased sharply within the ex-Soviet countries after the first report was published in 1988 [6] and even industrial production was soon started in Siberia [2, see also Section 2.1 below]. However, the primary particles remained un-identified, this class of man-made diamond never attracted much attention in the western countries [7], [8], [9] and the industrial production was shortly suspended. All these problems were caused by the continued failure in disintegrating nanodiamond aggregates (NDA) into primary particles. Researchers have often remarked on the peculiar modes of aggregation in the NDA, and even briefly discussed about the presence of extremely tight core [10], [11], [12], [13], [14], which seemed to be of different nature from the conventional aggregates that occur due to the active surface of small particles [15], [16], [17]. Nevertheless, no systematic study on the persistent aggregation in the NDA has been reported in the past. Stimulated by the recent discovery of higher diamondoid molecules from petroleum gas [18], we began seeking ways of isolating the smallest nanodiamond crystals from the detonation soot.
In this paper we briefly describe the nature of differential aggregation modes in NDA as studied by the combination of dynamic light scattering (DLS), HRTEM and SEM, present a model for the core aggregate and disclose the only practicable method that we find to break up the latter to release the long-wanted primary particles.
Section snippets
Materials
NDA samples N1–N4 (Table 1) were prepared in our laboratory in St. Petersburg following an industrial procedure [19] by detonating Composition B, a 65:35 mixture of 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (hexogen) in an atmosphere of carbon dioxide (dry synthesis) or in water (wet synthesis). In dry synthesis, fresh soot was collected from the wall of the detonation chamber and ploughed with magnets to remove magnetic constituents that had been cleaved off from
Assessment of purity
Quantitative analysis of diamond contents of NDAs was performed only on commercial products because of the large sample-size required (ca 0.5 g). Diamond contents of A, B and B′ obtained in this way increased from 62% to 77% with the year of production (see Section 2). Elemental analyses of the samples used (Table 1) reveal significant amounts of hetero-atoms (H, N, O) and ashes in addition to carbon. Newer products give higher carbon, and lower oxygen and ash contents. These results attest to
What is special about core aggregates?
Why are core aggregates so hard to disintegrate whereas larger ones are not? Before presenting our interpretation to this question, let us define the three major events that occur during the deposition of carbon atoms from detonation mixture:
(1) Growth of diamond crystals (section B in Fig. 7). After the explosion, the excess or unburned carbon atoms from explosive molecules are engulfed into the rapidly advancing shock wave and exposed to high-temperature/high-pressure conditions corresponding
Acknowledgement
Work described here is partially supported by JSPS, NEDO and Futaba Corporation. A.K. gratefully acknowledges postdoctoral fellowships from the Humboldt Foundation and the JSPS, 2000–2002, and a Liebig scholarship of the Fonds der Chemischen Industrie. We are indebted to the Netsuren Co. for carrying out stirred milling experiments for us, Dr. H. Oka for some of the TEM pictures of the primary particles and to T. Ogawa, F. Villers and M. Takahashi for technical assistance. We gratefully
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