The formation of diamond from carbynoid material at ambient pressure

Diamond alrd Related Materials 6 ( 1997 ) 1743- 1746

he formation of diamond from carbynoi

material at a

Y.P. Kudryavtsev *, S.E. Evsyukov A. N. Nestmymv

htitute

ofOrgtmoehent Received

Cor~ppow~ds.Russiun Acndetny of Sciences. II 7813 Moscow, Russia 25 June 1996; accepted 6 May 1997

Abstract

Ultra-dispersed diamond (UDD) has been obtained by heating at the ambient pressure of carbynoid material, prepared by dehydrochlorination of poly(vinylidene chloride) in the presence of a small amount of UDD. Q 1997 Elsevier Science S.A. Kc)‘w~I&: Carbyne; Low pressure; Microparticle;

Synthetic diamond

1. Introduction

Mutual transformations of various carbon materials and allotropic forms of carbon constitute an area of particular interest within the science of elemental carbon. The transformation of graphitic materials to diamond (including shock-induced dynamic transformation) has been extensively studied for a long time and several different mechanisms have been proposed for this kind of transformation [ 1.21. An intriguing hypothesis recently proposed by Hcimann [2] invokes the role of linear carbon allotropes (carbynes) as precursors and mediating structures during dynamic transformation 01 graphite to diamond. Indeed, the splitting of the planar graphitic sheet into one-dimensional carbyne chains involves a low-energy mechanism characterized by shifting electrons and small displacements of carbon atoms [J-5], which appears to be energetically more favorable compared with high-energy buckling or puckering of graphite layers to form a three-dimensional diamond lattice [2]. Subsequent compression of the linear carbon chains up to some critical inter-chain distance’ will result in the collapse of the chain structure and, possibly. attainment of sp3 configuration. The fact that carbynes were found at the impurity levels in natural graphites [7] and diamonds [S] supports this hypothesis. In these * Corresponding author. * The calculation of the interaction potential of two polyyne chains on the basis of a molecular interaction model showed that with increasing inter-chain interaction the one-dimensional structure becomes unstable [6]. The equilibrium inter-chain distance was shown to depend on the chain length: the distance increases with the number of carbon atoms in the chains. 0925-9635/97/$17.00 Q 1997 Elsevier Science S.A. All rights reserved. PII §0925-9635(97)00134-9

cases carbyne can be regarded as “frozen” intermediate or residual structures, respectively. However, in 1973 Kasatochkin et al. [8,9] reported that carbyne synthesized by oxidative dehydropolycondensation of acetylene did not transform into diamond under conditions sufficient to achieve diamond synthesis from graphite (9 GPa. 1800 ‘C, 5 min). On the other haI Sobolev ct (11.[IO] reported on the transfo carbyne (pi:. chaoite obtained in dynamic loading experiments) into lonsdaleire and cubic diamond at 1330 C and a static pressure of 5.5 GPa within a short period of time (I I min). The formation of grapbitc was observed at higher temperatures (2 1530 C ). 0f no& is the disappearance of diffraction lines correspon the x-carbyne phase from the X-ray diffraction ( pattern and the appearance of lines correspondi carbyne reported to occur during the high-pressure/hightemperature treatment in both cases [&IO]. In this regard. it is interesting to speculate whether the /I-phase is really carbyne or whether it is a partially (but regularly) cross-linked carbynoid structure (similar to the “layer-chain carbon” reported by Melnitchenko c’t al. [I I]) being. in effect. a missing i termediate in the transformation of carbyne into diamond. It has also been found that carbynoid materials obtained by chemical dehydrohalogenation of poly(vinylidene halides) transform into diamond under a static pressure of 7.7 GPa at 1400-1700 ‘C without the use of the yield at 1700 “C was virtually quanlitawever, the shock compression of a chemically similar material was recently reported to produce mostly graphite with a small amount of diamond and traces of a new carbon phase possessing a hexagonal

cell parameter ii =0.338 nm [ 13,141. The latter phase may also turn out to be an intermediate structure related to (or derived from) carbyne. Presumably, diamond in this case could be produced from carbyne by a soft shear deformation “umklapp” mode [ 151. Ail the processes mentioned above employed high pressures and temperatures. The present paper reports on the formation of diamond from carbynoid material (“amorphous carbyne”) at ambient pressure and moderate temperature.

2. Experimental Starting carbynoid material was prepared by chemical dehydrochlorination of poly(vinylidene chloride) (PVDC) powder using a reported procedure 1161. The starting material was mixed mechanically with a catalytic amount (approximately ! wt.%) of ultra-dispersed diamond ( UDD) obtained by the detonatioli of high explosives [ 17-191. The black mixture was then heated in a glass tube at 360-370 “C for several hours under Ar flow at ambient pressure. The product of thermal treatment was studied by means of infra-red (IR) spectroelectron scopy. X-ray analysis and transmission microscopy (TEM ) techniques using, respectively, UR-20, DRON-3. and JEM-IOOC instruments.

exults ml discussion

During thermal trcatmcnt of the starting tuixturc at -370 C. the gradual lightening of the black powder was observed. The transformation process was h&J to bc complctod when the color of the pow&r became white (or light-gray) and uniform. Normally, it takes approximately IO h. Noteworthy is that the product revealed high stability with respect to oxidizing agents (a hot l:I HNOJ:H2S0, mixture and aqueous HF solution) customarily used for purification of diamond from graphitic and inorganic impurities. The IR absorption spectrum of the product, compared with that of the starting material, demonstrates clearly pronounced changes (Fig. I ). The absorption band at 2170 cm-‘, attributable to the carbyne fragments [20,21] in dehydrochlorinated PVDC [Fig. I a], disappears completely from the spectrum of the heated product [Fig. 1b], as does the strong band at 1600 cm-l atid a shoulder at 1710cm-‘, attributable to double C-C bonds (both conjugated and cumulated [ 16,20,21]) and carbonyl groups 1221, respectively. At the same time, the spectrum of the product is very similar to that of the UDD used in original mixture as a catalyst [Fig. 1(c)l, suggesting that the transformation of carbynoid material into UDD might occur during thermal treatment. 360

, I 600

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1000

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1200

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II

1

I

1600

1400

’

1800

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’

2000

1

’

2200

’

t

2400

Wave number, cm“ Fig. 1. IR spectra of (a) the starting mixture [carbynoid material+ LJDD ( I wt.%)]: (b) the product of its thermal treatment: and (c) original UDD used as a catalyst. The samples were pressed with KBr into pellets. It should be noted that owing to the low concentration of UDD in the starting mixture. its contribution to the IR spectrum is not noticeable (cf. spectrum a).

The X-ray analysis of the resulting white powder allowed one to confirm this conjecture. The X-ray diffraction pattern of the heated product (Fig. 2) exhibits three marked peaks at 44”, 75”20’ and 91”30’ corresponding to interplanar spacings of 2.05, 1.26, and I .07 A, respectively, which are characteristic of cubic diamond [I]. It was not possible to separate the transformation product from the original catalytic UDD bccausc their properties turned out to bc virtually idenlical. Bi\setl on ~hc amowlt of starting carbynoid material, the yield of ultimate UDD was found to be approximately 20%. The results of TEM studies of the diamond powder thus obtained are consistent with those of X-ray analysis (Fig. 3). The size of individual diamond crystallites was

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Fig. 2. X-ray diflraction

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70

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pattern of the heated product.

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by extensive cross-linking to form three-dimensional carbonaceous network. Thermal decomposition of oxygen-containing species may bring about the chain scissions and evolution of carbon oxides (CO and CO?). The decomposition of side groups dismisses steric hindrances that prevented carbon chains from crosslinking, and further heating gives rise to the cleavage of multiple bonds in the carbyne fragments to form the diamond skeleton. Preliminary results of differential thermal analysis (DTA) of dehydrohalogenated PVDC are consistent with above considerations. The DTA heating curve (Fig. 4) exhibits three marked thermal effects in the 150-330 ‘C range (very broad), at 365 “C (strong) and at 395 “C. Apparently, the peak in the 150-330 ‘C range can be assigned to the decomposition and elimination of side groups being accompanied by gradual crosslinking. The high-intensity peak at 365 ‘YZcan be attributed to the break of multiple bonds in the carbyne fragments to form the diamond framework, whereas the peak at 395 ‘C cannot be unambiguously interpreted and has yet to be dealt with.

4. Conclusion

Fig. 3. TEM micrograph (n) and electron dilrrnction pattorn ( b) diamond powder produced from carbynoid material.

01’ IIIC

estimated to be approximately 50 A, as inferred from the width of the diffraction maxima. According to the data of recent 13C solid-state NMR studies, the carbynoid material produced by chemical dehydrohalogenation of PVDC has a complex structure owing to competitive side reactions of nucleophilic substitution, cross-linking, and some secondary transformations of both intermediate and ultimate products [22]. The straightforward elimination of hydrogen chloride results in the formation of short carbyne fragments of both polyyne and cumulene structure, whereas the side reactions produce various defects (such as alkoxyand hydroxy-groups, carbonyl species and cross-links) randomly distributed in the polymer chains. Presumably, the gentle pyrolysis of such carbynoid material results in gradual elimination of side groups being accompanied

The high-temperature pyrolysis of an organic polymer precursor has been recently reported by Bianconi et rd. to produce diamond-like carbon [23]. The process in that case was performed at 1000 “C, whereas the “gentle pyrolysis” of carbynoid materials reported in the presem paper provides a promising approach to the syntbcsis of diamond under surprisingly mild conditions. The particles 01’original UDD that was used ;1s a ciilalyst the tritt~slbrtnitti~tl oI‘ tll~’ seem to serve as nuclei during carbynoid material into diamond. However, an exact

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Temperature,

300

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Fig. 4. DTA heating curve of the carbynoid material.

450

transformation mechanism is yet not clear. This lvill be a subject of further investigations.

References [I] A.V. Kurdyumov, A.N. Pilyankevich, Phase Transformations in Carbon and Boron Nitride, Naukova Dumka, Kiev, 1979 (in Russian). [2] R.B. Heimann, Diamond Related Mater. 3 (1994) 1151l1157. [3] A.G. Whittaker. Science 200 (1978) 763-764. [4] R.B. Heimann, J. Kleiman, N.M. Salansky, Nature 306 (1983) 164-167. [5] R.B. Heimann, J. Kleiman. N.M. Salansky, Carbon 22 (1984) 147-156. [6] Y.P. Kudryavtsev, SE. Evsyukov, V.G. Babaev, M.B. Guseva, V.V. Khvostov. L.M. Krechko. Carbon 30 (1992) 213 -221. [7] A.G. Whittaker, Carbon I7 (1979) 21-24. [8] V.I. Kasatochkin, L.E. Sterenberg, M.E. Kazakov, V.N. Slesarev. L.V. Belousova, Dokl. Akad. Nauk SSSR 209 (1973) 388-39l.(in Russian) [9] V.I. Kasatochkin, V.V. Korshak, Yu.P. Kudryavtsev, A.M. Sladkov. L.E. Sterenberg, Carbon I I (1973) 70-72. [IO] V.V. Sobolev, V.Y. Slobodskoi, LT. Kozlov. Mineralog. Sbornik (L’vov) 43 ( 1989) 6668.(in Russian) [II] V.M. Melnitchenko, Yu.N. Nikulin, A.M. Sladkov, Carbon 23 (1985) 3-7.

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