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Dynamic synthesis of diamonds
Diamond and Related Materials 9 (2000) 887–892 www.elsevier.com/locate/diamond
Dynamic synthesis of diamonds J.B. Donnet a, *, E. Fousson a,b, T.K. Wang a, M. Samirant b, C. Baras b, M. Pontier Johnson c a Laboratoire de Chimie Physique, ENSCMu, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France b Institut franco-allemand de recherches de Saint-Louis (ISL), 5 rue du Ge´ne´ral Cassagnou, BP 34, 68301 Saint-Louis Cedex, France c Continental Carbon Company, 10655 Richmond Avenue, Suite #100, Houston, Texas 77042, USA
Abstract Nanometer-size diamonds were produced by firing of high explosive mixtures in water confinement. This simple method avoids the use of inert gas and is efficient enough to prevent the oxidation and graphitization of recovered diamonds. Studies of thermal and luminous phenomena were performed to examine eventually post-combustion phase. Condensed carbon yields of 30–55% were achieved for different explosive compositions, some of them containing metallic, carbonaceous or organic additives. For all the experiments, the presence of a diamond phase was revealed by X-ray diffraction, SEM, and TEM. To remove materials still unconverted to diamond, various selective oxidation treatments (with KNO /KOH, H O /HNO mixtures) were carried out, 3 2 2 3 leading to light gray ultradispersed diamond aggregates with a yield up to 60%. Otherwise, shock wave compression synthesis of diamond has been realized by using a planar impact system at 2 km/s. Compressed carbon products were obtained from several carbon precursors (graphite, carbon black, fullerenes, organic substances, …) mixed with a diverse metal matrix which acts as a cooling agent and stops the process of retro-graphitization. XRD analysis shows that diamond can be produced from many carbon materials with quenching as well as appropriate pressures and temperatures. © 2000 Elsevier Science S.A. All rights reserved. Keywords: High explosive detonation; Nanodiamonds; Shock compression; Transmission electron microscopy
1. Introduction For ten years nanodiamonds have been obtained by detonation of pure and composite CHNO explosives with a negative oxygen balance [1,2]. One feature of successful conditions is to preserve the diamonds from oxidation and graphitation by using an inert gas in the explosion tank, in addition to decreasing the effective temperature of detonation products. Environmental conditions, such as the nature of the inert gas, initial pressure and temperature, have been studied by many authors [3–6 ]. The carbon yield from the explosives depends on both their composition and density. Another way to produce diamond is the dynamic compression of carbon precursors by shock loading [7,8]. To produce shock waves, the main techniques involve explosives, guns or light-gas guns. Further, it was found that addition of a metallic component increases the diamond yield [9]. It acts as heat sink and thus restricts * Corresponding author. Tel.: +33-3-89-33-6845; fax: +33-3-89-33-6815. E-mail address: [email protected] (J.B. Donnet)
the phenomena of regraphitization. Moreover it most probably has a role of catalyst and chemically and physically favors the phase transition of carbon.
2. Experimental details 2.1. High explosive detonation Condensed carbon products were produced by firing TNT/RDX composite explosives with or without additives and HMX/TNT compositions ( Table 1). The pressure and temperature reached in the detonation wave zone for RDX/TNT compositions were evaluated at 20–30 GPa, and 3–4.103 K respectively [10], which correspond to the region of diamond stable phase in the P,T diagram for carbon [11]. For HMX composition, detonation rate and pressure were slightly higher. Synthesis time was close to the microsecond range. TNT is highly under-oxygenated (−73.9%) and therefore it is a source of solid carbon. RDX and HMX are less under-
0925-9635/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 9 25 - 9 63 5 ( 9 9 ) 00 2 1 5- 0
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J.B. Donnet et al. / Diamond and Related Materials 9 (2000) 887–892
Table 1 High explosive compositions Firing
Explosive composition
Treated soot (g)
Condensed carbon yield (%)
1 2 3 4 5 6 7
RDX/TNT: 65/35 RDX/TNT: 65/35+5% Al RDX/TNT: 65/35+2.07% B RDX/TNT: 30/70 RDX/TNT: 40/60+5% N110 RDX/TNT: 65/35+5% adamantane HMX/TNT: 30/70
21.6 25.6 25.2 34.6 46.0 22.9 35.8
36.8 43.6 42.9 44.9 54.1 32.6 46.6
Table 2 Some of the compressed compositions with pressures values and phases obtained Shot no.
Composition and density
Pressures P , P (GPa) 1 2
Phases indentified
1 2 3 4 5 6 7 8
Graphite Ceylan/Co 5 g/cm3 Carbon black N110/Co 5 g/cm3 N110/Cu 5 g/cm3 Gr. Ceylan/Cu 5 g/cm3 Acid treated saccharose/Cu 5 g/cm3 N110/Co 6.5 g/cm3 Fullerenes/Cu 5 g/cm3 Adamantane/Co 5 g/cm3
50.5–27.4 50.5–27.3 50.6–26.8 50.5–26.8 50.7–26.9 51.2–38.1 51.2–27.2 51.4–n.c.
G+D G+D G+D G+D D G+D G+D G+D
oxygenated (−21.6%) and produce the energy necessary to achieve the diamond state [12]. The explosive charges were cast into cylinders with a diameter of 35 mm and the mass was fixed at 250 g of explosive matter. For each charge a Bickford No.8 initiator and a 38 g PETN booster (which does not contribute to carbon yield ) were used. The detonation experiments were performed in water confinement. This method avoids the use of inert gas. The water surrounding the charge provides efficient cooling of detonation products and thus prevents the conversion of obtained diamonds into other carbon forms. Fig. 1 shows the experimental setup. The detonation experiments were done in a steel tank of 8.5 m3. The explosive charge was placed in a polyethylene bag containing 2 l of water which was suspended at the tank center. The thermal and luminous phenomena were recorded via photomultiplier cell, photocell and thermocouple.
Fig. 2. Planar impact assembly and sample capsule.
Fig. 1. Explosion tank before TNT/RDX firing.
The detonation products contained some impurities such as fragments from the tank walls (Fe O , TiO ), 2 3 2 copper from the detonator, and PE from the water container. Large size impurities were eliminated by
J.B. Donnet et al. / Diamond and Related Materials 9 (2000) 887–892
simple filtration of the suspension. After water evaporation the solid residue was treated with boiling concentrated sulfuric acid for 2 h and then washed thoroughly and dried. All the final products were analyzed by XRD ˚ ) in the range 10–100° (2h)]. [l =1.5406 A CuKa To remove materials still unconverted to diamond, a selective oxidation treatment with KNO /KOH or 3 H O /HNO mixture was performed [13,14]. This oper2 2 3 ation produced a light gray powder containing mainly ultradispersed diamond. 2.2. Shock wave compression synthesis It was been realized using a planar impact system at 2 km/s (Fig. 2). The original technique was described by Duvall [15]. Capsules containing the specimen are shock-treated by the impact of a supersonic stainless steel plate. The same device has been used by other authors [6,16,17]. The impact steel plate is accelerated downwards by the linear detonation of a high speed explosive charge and impacts the top face of the capsules lodged in the steel container. The symmetrical plate allows, through a confining effect, an increase of the throwing speed. The angle of inclination, a, determined by the Taylor relation, depends on both the detonation rate and the respective masses of the explosive and the steel plates; it is adjusted precisely to ensure planar impact.
Fig. 3. XRD spectra of (a) acid purified soot, and (b) H 0 /HNO 2 2 3 oxidized soot.
889
A numerical simulation realized with D2D code shows that the wave crossing the sample is not ideally plane, but has an inhomogeneous distribution of pressure and temperature. This is due to many lateral and rear relaxations. To reduce these differences, the diameter/thickness ratio of the sample should be made as large as possible. So for all the experiments, the dimensions were fixed to 12 mm diameter and 1 mm thickness. Other conditions were: flyer velocity of 2.1 km/s and sample densities varying from 60% to 85% of the theoretical densities. Pressures were determined using the principle of mass, momentum impulse and energy conservation [18]. We use also Hugoniot relations for the different materials involved. For porous samples, or mixtures, Hugoniot data were calculated considering the average volumetric mass [19]. Recovered capsules were mechanically cut open to take the sample out. After elimination of the metal matrix by HNO , XRD analysis of the compressed 3 products was performed (Debye–Scherrer mode with CuK radiation in the range 10–95° (2h)). a 3. Results and discussion 3.1. High explosive detonation For all the experiments involving explosive detonation, a single luminous flux was observed showing that no post-combustion phase occurs and that water confinement is efficient enough. XRD of acid-treated soot ( Fig. 3, curves labeled a) systematically show a broad ˚ corresponding to the d distance of peak for d~3.4 A 002 a disturbed graphitic carbon. The other peaks for d= ˚ , d=1.26 A ˚ and d=1.075 A ˚ correspond to the 2.076 A three first reticular distances d , d , and d of cubic 111 220 311 diamond. As shown in Table 1, adjunction of aluminum and boron powders in RDX/TNT compositions causes an increase in the condensed carbon yield. They seem to act partially as reducing agent towards the oxygen in the explosive substance. The best soot yields are obtained of course for compositions rich in TNT, or with carbonaceous additives. After oxidation treatment by H O /HNO , X-ray 2 2 3 examination of the product (Fig. 3, curves labeled b) indicates the elimination of the graphitic phase contribution. For hexolite 65/35 residues, the diamond yield after purification is close to 60%. SEM of the raw powder revealed porous aggregates which are composed of nanometer-size grains. More distinctly TEM ( Fig. 4) shows particles with different shapes and phases. We distinguish: spherical grains (a); more or less curved domains (b) composed of concentric sp2 layers with interlayer distances about 0.37–0.38 nm; dense particles of diamond (c); and some turbostratic
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J.B. Donnet et al. / Diamond and Related Materials 9 (2000) 887–892
Fig. 4. TEM of purified nanodiamonds powder.
ribbons (d). These features are in good agreement with Kuznetsov’s observations [20]. For diamond phase, contrary to spherical sp2 particles, the probability of having a favorable orientation towards the incident beam is very small so that it is difficult to distinguish sp3 planes. Diamond presence has also been revealed by dark-field image where it appears as luminous spots with 5–10 nm dimension. Examination of the eventual fullerenes present in the soot was undertaken after toluene extraction by Soxhlet. But no trace of fullerenes was found through HPLX and GC-MS techniques. 3.2. Shock wave compression synthesis Mixtures of carbon products such as graphite, carbon black, fullerenes and adamantane with a diverse metal
matrix (Co, Cu, Ni) compressed by the mouse-trap device (Fig. 2) led, almost always, to cubic diamond formation (some of the experiences are reported in Table 2). Maximum applied pressures were around 50 GPa with duration of shock compression of 0.37; 0.88 and 1.15 ms. P corresponds to the pressure in the 2 sample before the shock wave reaches the reflector, and P is the maximum pressure in the steel target, a pressure 1 which is attained by the sample after some reflections. As can be seen, all the systems lead to diamond and most of them with sp2 carbon. The XRD patterns of some selected compositions are presented in Fig. 5. The diamonds obtained were all in cubic crystalline form ˚ correwith, systematically, a clear peak for d=2.065 A sponding to the reticular distance d . The d and 111 220 d peaks are relatively weak. 311 With graphite as precursor, diamond formation is
J.B. Donnet et al. / Diamond and Related Materials 9 (2000) 887–892
891
Fig. 5. XRD spectra of shock compressed carbon.
accompanied by decrease of one order in the sp2 phase. However the inverse phenomenon is observed for N110 carbon black, with the distance between sp2 layers ˚ to ~3.45 A ˚ and approaching changing from ~3.6 A the d value of graphite. 002 Apparently adamantane does not have a positive effect on diamond yield in the explosive detonation method, but diamond is clearly observed in the shockcompressed product. The content of diamond and graphite phases can be estimated by the intensity of (111) peak/intensity of (002) peak ratio. We found that raising the shock wave duration slightly favors diamond formation. Finally, it is interesting to note that the carbon precursor prepared by acid dehydration of saccharose only gives diamond crystalline phase, without sp2 carbon trace. This product is less ordered than graphite or carbon black and shows a much better sp /sp ratio. 2 3 This series of experiments show that parameters such as the structure and size of precursors, and the nature, size and shape of the metal powders are essential for diamond transition. Better results are observed for a smaller size of precursor particles and when carbon has a disorganized structure. This indicates, therefore, that the mechanism of diamond formation is reconstructive. The shock compression product of carbon black with cobalt contains several carbon forms (Fig. 6): spherical empty particles, diamonds and an apparently monocrystalline form shown by arrows. This last form is quite similar to carbyne reported in Ref. [21]: platelets with well-defined edges and angles — but this is not confirmed by electron diffraction. Furthermore, dense diamond phases seem to grow on the base of the platelets. The
empty particles could be the result of the sample treatment by acid and this must be confirmed.
Acknowledgements We wish to thank Mrs Kessler, Patarin and Baron (LMM-CNRS No.428) for X-ray diffraction access and facilities. This study was made financially possible with the support of the CNRS, the ISL and Conseil Re´gional d’Alsace.
References [1] V.M. Titov, V.F. Anisichkin, I.Yu. Mal’kov, in: 9th Symp. on Detonation, Portland (1989) 407. [2] N.R. Greiner, D.S. Phillips, J.D. Johnson, F. Volk, Nature 333 (1998) 440. [3] F. Volk, F. Schedlbauer, in: 9th Symp. on Detonation, Portland (1989) 962. [4] V.L. Kuznetsov, A.L. Chuvilin, E.M. Moroz, V.N. Kolomiichuk, Sh.K. Shaikhutdinov, Yu.V. Butenko, I.Yu. Mal’kov, Carbon 32 (1994) 873. [5] E.A. Petrov, G.V. Sakovitch, P.M. Brylyakov, in: M.A. Meyers, L.E. Murr, K.P. Staudhammer ( Eds.), Shock-wave and Highstrain-rate Phenomena in Materials, Marcel Dekker, New York, 1992, p. 48. [6 ] J.B. Donnet, C. Lemoigne, T.K. Wang, C.M. Peng, M. Samirant, A. Eckhardt, Bull. Soc. Chem. Fr. 134 (1997) 875. [7] P.S. DeCarli, J.C. Jamieson, Science 133 (1961) 1821. [8] E.J. Wheeler, D. Lewis, Mat. Res. Bull. 10 (1975) 687. [9] D.G. Morris, J. Appl. Phys. 51 (4) (1980) 2059. [10] J. Baute, R. Chirat, in: Proc. 8th Symp. on Detonation, Alburquerque (1985) 521. [11] F.P. Bundy, Mat. Res. Soc. Symp. Proc. 383 (1995) 3.
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Fig. 6. TEM of compressed N110/Co mixture. [12] J. Ko¨hler, R. Meyer, Explosives, 4th EditionVCH, Weinheim, 1993. [13] G. Charlot, in: Les me´thodes de la chimie analytique, Analyse quantitative mine´rale, 5th Edition, Masson et Cie, Paris, 1966, p. 567. [14] T.M. Gubarevich, V.F. Pyaterikov, I.S. Larionova, V.Yu. Dolmatov, R.R. Sataev, A.V. Tyshetskaya, L.I. Poleva, J. Appl. Chem. USSR 65 (11) (1992) 2075 translated from Russian. [15] G.E. Duvall, Response of Metals to High Velocity Deformation Vol. 9 Wiley-Interscience, New York, 1961. [16 ] F.R. Norwood, R.A. Graham, A. Sawaoka, in: Y.M. Gupta ( Ed.), Shock Waves in Condensed Matter, Plenum Press, New York, 1986, p. 837.
[17] A.B. Sawaoka, Shock waves, Material Science, Springer, Tokyo, 1993. Chapter 1. [18] M. De´fourneaux, The´orie et mesure des ondes de choc dans les solides, ENSTA, 1975. [19] P.J. Torvik, A simple theory for shock propagation in homogeneous mixtures, AFIT TR 70-3, School of Engineering, Air Force Institute of Technology, Air University, 1970. [20] V.L. Kuznetsov, M.N. Aleksandrov, I.V. Zagoruiko, A.L. Chuvilin, E.M. Moroz, V.N. Kolomiichuk, V.A. Likholobov, P.M. Brylyakov, G.V. Sakovitch, Carbon 29 (1991) 665. [21] K. Yamada, G. Burkhard, T. Tanabe, A.B. Sawaoka, Carbon 34 (12) (1996) 1601.
Dynamic synthesis of diamonds J.B. Donnet a, *, E. Fousson a,b, T.K. Wang a, M. Samirant b, C. Baras b, M. Pontier Johnson c a Laboratoire de Chimie Physique, ENSCMu, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France b Institut franco-allemand de recherches de Saint-Louis (ISL), 5 rue du Ge´ne´ral Cassagnou, BP 34, 68301 Saint-Louis Cedex, France c Continental Carbon Company, 10655 Richmond Avenue, Suite #100, Houston, Texas 77042, USA
Abstract Nanometer-size diamonds were produced by firing of high explosive mixtures in water confinement. This simple method avoids the use of inert gas and is efficient enough to prevent the oxidation and graphitization of recovered diamonds. Studies of thermal and luminous phenomena were performed to examine eventually post-combustion phase. Condensed carbon yields of 30–55% were achieved for different explosive compositions, some of them containing metallic, carbonaceous or organic additives. For all the experiments, the presence of a diamond phase was revealed by X-ray diffraction, SEM, and TEM. To remove materials still unconverted to diamond, various selective oxidation treatments (with KNO /KOH, H O /HNO mixtures) were carried out, 3 2 2 3 leading to light gray ultradispersed diamond aggregates with a yield up to 60%. Otherwise, shock wave compression synthesis of diamond has been realized by using a planar impact system at 2 km/s. Compressed carbon products were obtained from several carbon precursors (graphite, carbon black, fullerenes, organic substances, …) mixed with a diverse metal matrix which acts as a cooling agent and stops the process of retro-graphitization. XRD analysis shows that diamond can be produced from many carbon materials with quenching as well as appropriate pressures and temperatures. © 2000 Elsevier Science S.A. All rights reserved. Keywords: High explosive detonation; Nanodiamonds; Shock compression; Transmission electron microscopy
1. Introduction For ten years nanodiamonds have been obtained by detonation of pure and composite CHNO explosives with a negative oxygen balance [1,2]. One feature of successful conditions is to preserve the diamonds from oxidation and graphitation by using an inert gas in the explosion tank, in addition to decreasing the effective temperature of detonation products. Environmental conditions, such as the nature of the inert gas, initial pressure and temperature, have been studied by many authors [3–6 ]. The carbon yield from the explosives depends on both their composition and density. Another way to produce diamond is the dynamic compression of carbon precursors by shock loading [7,8]. To produce shock waves, the main techniques involve explosives, guns or light-gas guns. Further, it was found that addition of a metallic component increases the diamond yield [9]. It acts as heat sink and thus restricts * Corresponding author. Tel.: +33-3-89-33-6845; fax: +33-3-89-33-6815. E-mail address: [email protected] (J.B. Donnet)
the phenomena of regraphitization. Moreover it most probably has a role of catalyst and chemically and physically favors the phase transition of carbon.
2. Experimental details 2.1. High explosive detonation Condensed carbon products were produced by firing TNT/RDX composite explosives with or without additives and HMX/TNT compositions ( Table 1). The pressure and temperature reached in the detonation wave zone for RDX/TNT compositions were evaluated at 20–30 GPa, and 3–4.103 K respectively [10], which correspond to the region of diamond stable phase in the P,T diagram for carbon [11]. For HMX composition, detonation rate and pressure were slightly higher. Synthesis time was close to the microsecond range. TNT is highly under-oxygenated (−73.9%) and therefore it is a source of solid carbon. RDX and HMX are less under-
0925-9635/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 9 25 - 9 63 5 ( 9 9 ) 00 2 1 5- 0
888
J.B. Donnet et al. / Diamond and Related Materials 9 (2000) 887–892
Table 1 High explosive compositions Firing
Explosive composition
Treated soot (g)
Condensed carbon yield (%)
1 2 3 4 5 6 7
RDX/TNT: 65/35 RDX/TNT: 65/35+5% Al RDX/TNT: 65/35+2.07% B RDX/TNT: 30/70 RDX/TNT: 40/60+5% N110 RDX/TNT: 65/35+5% adamantane HMX/TNT: 30/70
21.6 25.6 25.2 34.6 46.0 22.9 35.8
36.8 43.6 42.9 44.9 54.1 32.6 46.6
Table 2 Some of the compressed compositions with pressures values and phases obtained Shot no.
Composition and density
Pressures P , P (GPa) 1 2
Phases indentified
1 2 3 4 5 6 7 8
Graphite Ceylan/Co 5 g/cm3 Carbon black N110/Co 5 g/cm3 N110/Cu 5 g/cm3 Gr. Ceylan/Cu 5 g/cm3 Acid treated saccharose/Cu 5 g/cm3 N110/Co 6.5 g/cm3 Fullerenes/Cu 5 g/cm3 Adamantane/Co 5 g/cm3
50.5–27.4 50.5–27.3 50.6–26.8 50.5–26.8 50.7–26.9 51.2–38.1 51.2–27.2 51.4–n.c.
G+D G+D G+D G+D D G+D G+D G+D
oxygenated (−21.6%) and produce the energy necessary to achieve the diamond state [12]. The explosive charges were cast into cylinders with a diameter of 35 mm and the mass was fixed at 250 g of explosive matter. For each charge a Bickford No.8 initiator and a 38 g PETN booster (which does not contribute to carbon yield ) were used. The detonation experiments were performed in water confinement. This method avoids the use of inert gas. The water surrounding the charge provides efficient cooling of detonation products and thus prevents the conversion of obtained diamonds into other carbon forms. Fig. 1 shows the experimental setup. The detonation experiments were done in a steel tank of 8.5 m3. The explosive charge was placed in a polyethylene bag containing 2 l of water which was suspended at the tank center. The thermal and luminous phenomena were recorded via photomultiplier cell, photocell and thermocouple.
Fig. 2. Planar impact assembly and sample capsule.
Fig. 1. Explosion tank before TNT/RDX firing.
The detonation products contained some impurities such as fragments from the tank walls (Fe O , TiO ), 2 3 2 copper from the detonator, and PE from the water container. Large size impurities were eliminated by
J.B. Donnet et al. / Diamond and Related Materials 9 (2000) 887–892
simple filtration of the suspension. After water evaporation the solid residue was treated with boiling concentrated sulfuric acid for 2 h and then washed thoroughly and dried. All the final products were analyzed by XRD ˚ ) in the range 10–100° (2h)]. [l =1.5406 A CuKa To remove materials still unconverted to diamond, a selective oxidation treatment with KNO /KOH or 3 H O /HNO mixture was performed [13,14]. This oper2 2 3 ation produced a light gray powder containing mainly ultradispersed diamond. 2.2. Shock wave compression synthesis It was been realized using a planar impact system at 2 km/s (Fig. 2). The original technique was described by Duvall [15]. Capsules containing the specimen are shock-treated by the impact of a supersonic stainless steel plate. The same device has been used by other authors [6,16,17]. The impact steel plate is accelerated downwards by the linear detonation of a high speed explosive charge and impacts the top face of the capsules lodged in the steel container. The symmetrical plate allows, through a confining effect, an increase of the throwing speed. The angle of inclination, a, determined by the Taylor relation, depends on both the detonation rate and the respective masses of the explosive and the steel plates; it is adjusted precisely to ensure planar impact.
Fig. 3. XRD spectra of (a) acid purified soot, and (b) H 0 /HNO 2 2 3 oxidized soot.
889
A numerical simulation realized with D2D code shows that the wave crossing the sample is not ideally plane, but has an inhomogeneous distribution of pressure and temperature. This is due to many lateral and rear relaxations. To reduce these differences, the diameter/thickness ratio of the sample should be made as large as possible. So for all the experiments, the dimensions were fixed to 12 mm diameter and 1 mm thickness. Other conditions were: flyer velocity of 2.1 km/s and sample densities varying from 60% to 85% of the theoretical densities. Pressures were determined using the principle of mass, momentum impulse and energy conservation [18]. We use also Hugoniot relations for the different materials involved. For porous samples, or mixtures, Hugoniot data were calculated considering the average volumetric mass [19]. Recovered capsules were mechanically cut open to take the sample out. After elimination of the metal matrix by HNO , XRD analysis of the compressed 3 products was performed (Debye–Scherrer mode with CuK radiation in the range 10–95° (2h)). a 3. Results and discussion 3.1. High explosive detonation For all the experiments involving explosive detonation, a single luminous flux was observed showing that no post-combustion phase occurs and that water confinement is efficient enough. XRD of acid-treated soot ( Fig. 3, curves labeled a) systematically show a broad ˚ corresponding to the d distance of peak for d~3.4 A 002 a disturbed graphitic carbon. The other peaks for d= ˚ , d=1.26 A ˚ and d=1.075 A ˚ correspond to the 2.076 A three first reticular distances d , d , and d of cubic 111 220 311 diamond. As shown in Table 1, adjunction of aluminum and boron powders in RDX/TNT compositions causes an increase in the condensed carbon yield. They seem to act partially as reducing agent towards the oxygen in the explosive substance. The best soot yields are obtained of course for compositions rich in TNT, or with carbonaceous additives. After oxidation treatment by H O /HNO , X-ray 2 2 3 examination of the product (Fig. 3, curves labeled b) indicates the elimination of the graphitic phase contribution. For hexolite 65/35 residues, the diamond yield after purification is close to 60%. SEM of the raw powder revealed porous aggregates which are composed of nanometer-size grains. More distinctly TEM ( Fig. 4) shows particles with different shapes and phases. We distinguish: spherical grains (a); more or less curved domains (b) composed of concentric sp2 layers with interlayer distances about 0.37–0.38 nm; dense particles of diamond (c); and some turbostratic
890
J.B. Donnet et al. / Diamond and Related Materials 9 (2000) 887–892
Fig. 4. TEM of purified nanodiamonds powder.
ribbons (d). These features are in good agreement with Kuznetsov’s observations [20]. For diamond phase, contrary to spherical sp2 particles, the probability of having a favorable orientation towards the incident beam is very small so that it is difficult to distinguish sp3 planes. Diamond presence has also been revealed by dark-field image where it appears as luminous spots with 5–10 nm dimension. Examination of the eventual fullerenes present in the soot was undertaken after toluene extraction by Soxhlet. But no trace of fullerenes was found through HPLX and GC-MS techniques. 3.2. Shock wave compression synthesis Mixtures of carbon products such as graphite, carbon black, fullerenes and adamantane with a diverse metal
matrix (Co, Cu, Ni) compressed by the mouse-trap device (Fig. 2) led, almost always, to cubic diamond formation (some of the experiences are reported in Table 2). Maximum applied pressures were around 50 GPa with duration of shock compression of 0.37; 0.88 and 1.15 ms. P corresponds to the pressure in the 2 sample before the shock wave reaches the reflector, and P is the maximum pressure in the steel target, a pressure 1 which is attained by the sample after some reflections. As can be seen, all the systems lead to diamond and most of them with sp2 carbon. The XRD patterns of some selected compositions are presented in Fig. 5. The diamonds obtained were all in cubic crystalline form ˚ correwith, systematically, a clear peak for d=2.065 A sponding to the reticular distance d . The d and 111 220 d peaks are relatively weak. 311 With graphite as precursor, diamond formation is
J.B. Donnet et al. / Diamond and Related Materials 9 (2000) 887–892
891
Fig. 5. XRD spectra of shock compressed carbon.
accompanied by decrease of one order in the sp2 phase. However the inverse phenomenon is observed for N110 carbon black, with the distance between sp2 layers ˚ to ~3.45 A ˚ and approaching changing from ~3.6 A the d value of graphite. 002 Apparently adamantane does not have a positive effect on diamond yield in the explosive detonation method, but diamond is clearly observed in the shockcompressed product. The content of diamond and graphite phases can be estimated by the intensity of (111) peak/intensity of (002) peak ratio. We found that raising the shock wave duration slightly favors diamond formation. Finally, it is interesting to note that the carbon precursor prepared by acid dehydration of saccharose only gives diamond crystalline phase, without sp2 carbon trace. This product is less ordered than graphite or carbon black and shows a much better sp /sp ratio. 2 3 This series of experiments show that parameters such as the structure and size of precursors, and the nature, size and shape of the metal powders are essential for diamond transition. Better results are observed for a smaller size of precursor particles and when carbon has a disorganized structure. This indicates, therefore, that the mechanism of diamond formation is reconstructive. The shock compression product of carbon black with cobalt contains several carbon forms (Fig. 6): spherical empty particles, diamonds and an apparently monocrystalline form shown by arrows. This last form is quite similar to carbyne reported in Ref. [21]: platelets with well-defined edges and angles — but this is not confirmed by electron diffraction. Furthermore, dense diamond phases seem to grow on the base of the platelets. The
empty particles could be the result of the sample treatment by acid and this must be confirmed.
Acknowledgements We wish to thank Mrs Kessler, Patarin and Baron (LMM-CNRS No.428) for X-ray diffraction access and facilities. This study was made financially possible with the support of the CNRS, the ISL and Conseil Re´gional d’Alsace.
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