Shock synthesis of nanodiamonds from carbon precursors: identification of carbynes

C. R. Acad. Sci. Paris, Se´rie IIc, Chimie / Chemistry 3 (2000) 359 – 364 © 2000 Acade´mie des sciences / E´ditions scientifiques et me´dicales Elsevier SAS. Tous droits re´serve´s S1387 1609(00) 01149-X/SCO

Shock synthesis of nanodiamonds from carbon precursors: identification of carbynes ´ ric Foussona,b, Michel Samirantb, Tong Kuan Wanga, Jean-Baptiste Donneta,*, E Marie Pontier-Johnsonc, Andre´ Eckhardtd a

Laboratoire de chimie physique ENSCMu, 3 rue Alfred-Werner, 68093 Mulhouse cedex, France Institut franco-allemand de recherches de Saint-Louis, 5, rue du Ge´ne´ral-Cassagnou, BP 34, 68301 Saint-Louis cedex, France c Continental Carbon Company, 10655 Richmond Avenue, Suite c100, Houston, TX, 77042, USA d Institut de chimie des surfaces et interfaces, 15, rue Jean-Starcky, 68093 Mulhouse cedex, France b

Received 9 February 2000, accepted 5 June 2000 Communicated by Franc¸ois Mathey

Abstract – Carbon blacks or fullerenes mixed with a cobalt matrix have been shock compressed using a mouse-trap system. X-ray analysis has revealed the formation of cubic diamond and a turbostratic phase, while transmission electron microscopy and electron diffraction have demonstrated moreover the presence of a significant monocrystalline phase corresponding to the carbyne form. © 2000 Acade´mie des sciences / E´ditions scientifiques et me´dicales Elsevier SAS shock compression / transmission electron / microscopy / carbynes / nanodiamond Version franc¸aise abre´ge´e — Synthe`se par choc de nanodiamants a` partir de pre´curseurs carbone´s : identification de carbynes. Introduction. La synthe`se du diamant via compression par choc de pre´curseurs carbone´s a e´te´ e´voque´e dans la litte´rature [1, 2]. Nous avons employe´ un syste`me de rele`vement de plaques a` 2 km·s – 1 pour comprimer du graphite, des noirs de carbone, des fullere`nes ou des substances organiques. Diverses poudres me´talliques ont e´te´ ajoute´es, qui permettent d’arreˆter le processus de re´trographitation par refroidissement [3]. Les analyses par rayons X montrent que du diamant cubique, ainsi qu’une phase turbostratique, sont obtenus. Les observations par microscopie e´lectronique par transmission (MET), ainsi que par diffraction e´lectronique, des produits comprime´s indiquent que plusieurs varie´te´s allotropiques du carbone sont pre´sentes, en particulier des carbynes. Partie expe´rimentale. Les ondes de choc ge´ne´rant les hautes pressions sont cre´e´es graˆce au lancement d’un impacteur sur une cible contenant l’e´chantillon carbone´. La technique originale a e´te´ de´crite par Duvall [4]. Le meˆme montage expe´rimental a e´te´ utilise´ par d’autres auteurs [5–7]. L’onde traversant l’e´chantillon n’est pas re´ellement plane. Il y a des inhomoge´ne´ite´s dans la distribution des pressions et des tempe´ratures, dues aux nombreuses relaxations late´rales et arrie`res. Les pressions ont e´te´ de´termine´es graˆce au principe de conservation de la masse, de la quantite´ de mouvement et de l’e´nergie [8]. Pour les e´chantillons poreux, les donne´es d’Hugoniot ont e´te´ calcule´es en conside´rant leur masse volumique moyenne [9]. Des me´langes noir de carbone N110/Co ou fullere`nes C60/Co (5:95) ont e´te´ introduits dans des capsules en acier inoxydable ferme´es sous presse, de manie`re a` former des pastilles (1 mm d’e´paisseur pour 12 mm de diame`tre), avec une densite´ comprise entre 60 et 85 % de la densite´ the´orique. L’impacteur, e´pais de 3 mm, a e´te´ lance´ a` 2,1 km·s – 1, produisant des pressions de 55 GPa (dure´e de 0,88 ms). Les capsules re´cupe´re´es apre`s choc sont ouvertes me´caniquement et la matrice me´tallique est e´limine´e par traitement dans HNO3. Re´sultats. Pour un me´lange de noir de carbone N110 avec du cobalt, le diffractogramme RX du produit comprime´ (figure 1 ) re´ve`le la pre´sence d’une phase contenant des carbones polyaromatiques, avec des distances intercouches passant de 0,36 nm initialement a` 0,345 nm. Ceci de´montre une meilleure organisation des plans graphe`nes par rapport au mate´riau de de´part, bien que la structure demeure de type turbostratique. Trois autres pics sont pre´sents pour : d = 0,206 5 nm, 0,126 2 nm et 0,107 5 nm, correspondant aux distances re´ticulaires d111, d220 et d311 du diamant cubique. Outre les particules * Correspondence and reprints : E-mail address: [email protected] (J.-B. Donnet).

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NOTE / PRELIMINARY COMMUNICATION

Physical and theoretical chemistry / Chimie physique et the´orique

J.-B. Donnet et al. / C. R. Acad. Sci. Paris, Se´rie IIc, Chimie / Chemistry 3 (2000) 359 – 364 turbostratiques classiques, les cliche´s de MET mettent en e´vidence la pre´sence de deux micro-textures particulie`res. La premie`re (figure 2b) est forme´e de particules sphe´riques (30 – 80 mm) apparemment vides, et compose´es de quelques couches graphitiques concentriques distantes de 0,36–0,37 nm. Ces e´le´ments sont diffe´rents du noir de carbone de de´part, de par leur taille plus importante, leur partie centrale e´vide´e, leur meilleure organisation, et ressemblent aux coquilles observe´es dans le cas des carbones catalytiques [10]. La seconde phase (figure 2c) comporte des domaines monocristallins (30 – 60 nm), avec des plaquettes transparentes, parfois hexagonales, montrant des contours nets et rectilignes. Les formes observe´es sont similaires a` celles reporte´es dans la litte´rature [11]. Les diffe´rents cliche´s re´ve`lent des franges de re´seau espace´es de 0,46–0,48 nm, cette valeur e´tant le´ge`rement supe´rieure aux 0,446 nm de´termine´s par diffraction e´lectronique pour les carbynes e´tudie´s dans d’autres travaux [12]. Les plans carbynes sont extreˆmement fins et ne repre´sentent qu’une faible fraction du mate´riau. Ceci explique le fait qu’aucune re´flexion caracte´ristique ne soit observe´e pour les analyses RX. De plus, cette technique analytique semble, selon certains auteurs, mal adapte´e a` l’e´tude des carbynes [13]. Enfin, des structures de type ruban, constitue´es de franges paralle`les distantes d’environ 0,5 nm, ont e´te´ distingue´es (figure 2d). La diffraction e´lectronique (figure 3 ) a permis de de´terminer plusieurs distances re´ticulaires, dont les valeurs mesure´es a` partir des anneaux de diffraction (tableau, 1re colonne) sont en bon accord avec celles des formes chaoite et a-carbynes. Si la pre´sence de nanodiamants semble acquise au vu de l’interpre´tation des RX, elle n’a pu eˆtre confirme´e formellement par MET. En effet, les entite´s diamants n’ont pas e´te´ isole´es pre´cise´ment, a` cause de leur structure tre`s fortement agglome´re´e. Pour les produits issus de la compression des me´langes fullere`nes C60/Co, nous avons releve´ du diamant cubique, des particules graphitiques et des carbynes. La MET (figure 4 ) confirme la pre´sence de nombreux domaines carbynes superpose´s, oriente´s dans toutes les directions. Certains d’entre eux ont une forme de paralle´logramme, avec un espacement entre franges de 0,46 – 0,48 nm. Discussion. Une nouvelle forme allotropique du carbone, trouve´e dans le graphite du crate`re de Ries, a e´te´ re´ve´le´e la premie`re fois par El Goresy et Donnay [16], puis par Sladkov et Koudrayatsev [17], qui l’ont de´nomme´e « carbynes ». L’e´tude des diffe´rents types de carbynes (modifications a et b) a e´te´ mene´e ensuite par Kasatochkin et al. [18]. Depuis lors, cette forme de carbone a e´te´ observe´e dans de nombreuses expe´riences : de la chaoite et du carbone VI ont e´te´ retrouve´s apre`s chauffage de graphite hexagonal a` 2 550 K [19], puis par chauffage de carbone vitreux graˆce a` un laser CO2 [20]. Un me´lange contenant du diamant, une structure graphitique et de la chaoite a e´te´ re´cupe´re´ apre`s compression par choc de carbones vitreux et amorphe, me´lange´s a` du cuivre [21, 22]. Des carbynes ont e´te´ aussi forme´s par sublimation de noir d’ace´tyle`ne [23], par carbonisation de poly(fluorure de vinilyde`ne) [12], par compression de diamant [24], ainsi que par ablation laser de graphite pyrolytique [15]. Diffe´rents me´canismes pour la formation des carbynes ont e´te´ sugge´re´s. Le premier, propose´ par Whittaker [25], consiste en une transformation directe du graphite par rupture des liaisons simples et reports des e´lectrons sur les liaisons simples ou doubles adjacentes. Pour la compression dynamique de me´langes graphite/cuivre, les carbynes forme´s ont tout d’abord e´te´ perc¸us comme un produit interme´diaire de la regraphitation du diamant synthe´tise´ selon un proce´de´ de type SVLS [13]. La longueur des chaıˆnes carbynes de´pend de la tempe´rature et diminue pour des tempe´ratures plus e´leve´es [26]. Pour d’autres auteurs [11, 23, 24], les carbynes sont produits a` haute tempe´rature par condensation de vapeur plutoˆt que par re´trographitation du diamant. Dans le cas de la compression par choc de substances poreuses, le syste`me atteint des tempe´ratures et des pressions conside´rables. Si le flux d’e´nergie au niveau de la surface des particules est supe´rieur au processus de diffusion thermique dans la particule, le point de fusion du carbone peut eˆtre atteint, entraıˆnant ensuite la vaporisation du carbone lors de l’expansion adiabatique pendant le processus de rare´faction. Les carbynes peuvent eˆtre forme´s alors directement par condensation de la vapeur. Des re´sultats re´cents [15] ont permis de clarifier le me´canisme de constitution des carbynes. Il semble de´pendre essentiellement du parame`tre pression. Ainsi, pour des pressions faibles, les carbynes sont obtenus par condensation de vapeur, tandis que, pour des pressions e´leve´es, ils sont issus directement de la transformation du graphite par rupture des liaisons simples. La pre´sence dans notre cas de plusieurs espe`ces carbone´es, diamant, carbone turbostratique, carbone graphitique et carbynes s’explique par une he´te´roge´ne´ite´ des tempe´ratures, des pressions, ainsi que des distributions d’e´nergies externes et internes des particules. Conclusion. L’identification des carbynes dans les expe´riences de synthe`se de diamant par compression par choc est manifeste. © 2000 Acade´mie des sciences / E´ditions scientifiques et me´dicales Elsevier SAS Compression par choc / microscopie e´lectronique a` transmission / carbynes / nanodiamant

1. Introduction Diamond synthesis via shock compression of carbonaceous precursors has already been described [1, 2]. We employed a planar impact system at 2 km·s – 1. Starting materials such as graphite, carbon blacks, fullerenes or organic substances are mixed with di-

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verse metallic powders that act as cooling agent and stop the process of retro-graphitation [3]. X-ray diffraction (XRD) analysis shows that in most cases cubic diamond and turbostratic phase are obtained under appropriate conditions (temperature and pressure). Observations by transmission electron microscopy (TEM) and electron diffraction of the

J.-B. Donnet et al. / C. R. Acad. Sci. Paris, Se´rie IIc, Chimie / Chemistry 3 (2000) 359 – 364

compressed products indicate that complex mixtures of species are present, including several allotropic forms of carbon and in particular carbynes.

2. Experimental details The shock waves generating high pressures are created via throwing a supersonic impactor on a target that contains the carbon sample. The original technique was described by Duvall [4]. The same device has been used by other authors [5 – 7]. A numerical simulation realized with Dyna2D code shows that the wave crossing the sample is in reality not ideally plane with consequently inhomogeneous distribution of pressures and temperatures. This phenomenon is due to many lateral and rear relaxations. Pressures occurring in the sample are determined using the conservation of mass, momentum impulse and energy [8]. We used Hugoniot relations for the different materials involved. For the porous samples, Hugoniot data were calculated considering the average volumetric mass [9]. Thus the calculated speed of the flyer plate allows us to determine the pressures values in the carbonaceous product. Carbon blacks type N110 (with about 18 nm size grains) or C60 fullerene are mixed with Co powder (spherical particles of 20 – 100 mm diameter) in a ratio of 5:95 by weight. These substances are then introduced in the stainless steel capsules which are closed under press so as to form 1 mm thick and 12 mm large pellets with initial density varying from 60 –85 % of the theoretical maximal densities. The conditions for the firing are: flyer steel plate thickness 3 mm and throwing speed 2.1 km·s – 1. The generated pressure in the sample is close to 55 GPa according to the Hugoniots of the involved materials with shock duration of 0.88 ms. Recovered capsules after shock are mechanically cut open and the metallic matrix is removed by immersion of the sample in HNO3 68 % (d = 1.41). After drying, the carbonaceous product is analysed by X-ray diffraction — in Debye – Scherrer mode with lCuKa = 0.154 06 nm in the range 10 –95° (2 u) — and by transmission electron microscopy (TEM). The samples for TEM were prepared by dispersion of the product in chloroform and then deposition of suspension drops on a carbon membrane. Despite ultrasonic agitation, it stays difficult to separate elementary particles which still remain as aggregates. Consequently characterization has been rather performed on the borders of the clusters.

3. Results For a mixture of carbon black N110 with cobalt, the XRD pattern of the compressed products (figure 1 ) reveals the presence of a polyaromatic carbon phase, with inter-layer distance decreasing from original 0.36 nm to 0.345 nm, which indicates a better organization of graphenes relative to the starting material, though the structure is still turbostratic. In addition to this contribution, three other peaks are present: d = 0.206 5 nm, 0.126 2 nm and 0.107 5 nm, corresponding to the reticular distances of cubic diamond, d111, d220 and d311, respectively. Besides typical turbostratic particles, TEM images of the products (general view of an aggregate, figure 2a) exhibit two particular microtextures. The first one (figure 2b) is constituted of spherical particles that seem to be empty, measuring 30–80 nm and composed of a few concentric graphitic layers. These compounds are different from the starting carbon blacks because of their larger sizes of their empty core, and are better organized. They are very similarto catalytic carbon shells [10]. The interlayer distances in the graphitic shells are about 0.36–0.37 nm. The second phase (figure 2c) is composed of apparently mono-crystalline domains. These fields consist of crystallites as well as relatively transparent platelets, sometimes hexagonal, or showing outlines and well defined rectilinear edges. The observed forms are similar to that of carbynes reported in the literature [11]. The different pictures reveal well-ordered areas from 30–60 nm wide. The measurement of the distances between the lattice fringes is about 0.46– 0.48 nm. This is higher than the 0.446 nm value found in other works for chaoite or a-modification of carbynes by electron diffraction techniques [12]. It must be noticed that we used the same carbon microgrids for observation of several nanodiamond samples generated through detonation waves of high explosives or shock compression [5]. Carbynes were detected only on the samples obtained by the second technique, showing that it does not belong to the carbon sample support. Furthermore, layers of carbynes are very thin plates representing probably a small fraction of the total material. This could explain the fact that no reflections occur for the X-ray analysis. Actually it is hard to analytically characterize carbynes with bulk methods based on large grain populations specially for XRD as reported by others [13]. At last some ribbon-like structure, constituted of a few parallel fringes that are separated by :0.5 nm are also observed (figure 2d).

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pattern is too bright to clearly see the ring corresponding to the (110) contribution. However, the 0.46–0.48 nm d-spacing has been observed on lattice fringes of the TEM images. If the presence of nanodiamonds seems to be established by X-ray interpretation, it has not been absolutely confirmed by TEM. In fact diamond units have not been isolated on TEM pictures because of the strongly agglomerated structure.

Figure 1. XRD spectrum of compressed N110 / Co mixture.

Owing to the high agglomeration degree of particles, it was not easy to isolate a carbyne mono-crystal. The crystalline structure can be estimated from the diffraction rings of the electron diffraction pattern (figure 3 ). Values are shown in the first column of the table. The determined spacings are in good agreement with those of the different carbyne forms (a-carbyne and chaoite). The centre of the electron diffraction

In the shock-compressed product of a C60 fullerene / Co mixture, we also observed different carbon forms: cubic diamond, graphitic particles, and carbynes. TEM (figure 4 ) proves the presence of many overlapping carbyne fields oriented in all directions. Some of them appear as parallelogram shapes with 0.46–0.48 nm range spacing between carbon planes.

4. Discussion A new allotropic form of carbon, discovered inside graphite in the Ries crater, was revealed for the first

Figure 2. a. TEM image of an aggregate formed by shock compression of N110 / Co mixture. b. TEM image showing spherical particles composed of a few concentric graphitic layers. c. TEM image of a carbyne platelet. d. TEM image showing ribbon-like structure.

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found after heating hexagonal graphite at 2 550 K under reduced pressure [19] and vitreous carbon by CO2 laser [20]. A mixture containing diamond, a graphitic structure and chaoite was recovered after shock compression of vitreous and amorphous carbon mixed with copper [21, 22]. Furthermore, carbynes were obtained by acetylene black sublimation [23], by carbonization of poly(vinylidene fluoride) [12], via compression of diamond [24], and by laser ablation of pyrolytic graphite [15]. Different mechanisms for the carbyne formation have been suggested. The first mechanism proposed by Whittaker [25] consists of a direct transformation at high temperature of graphite to carbyne with splitting up of single bonds and report of the electrons on adjacent single or double bonds. For dynamic compression of graphite/copper mixtures, formed carbynes were first considered as intermediate products of re-graphitation of diamond synthesized via a solid–vapour–liquid–solid process [13]. Their chain lengths depend on the temperature, and decrease for higher temperatures [26].

Figure 3. Electron diffraction pattern of a carbyne mono-crystal.

time by El Goresy and Donnay [16], and then by Sladkov and Kudryavtsev [17], who called it ‘carbyne’. Investigation of different carbyne types (a and b-carbynes) was carried out by Kasatochkin et al. [18]. Then this form of carbon has been observed in many experiments: chaoite and carbon VI were

For other authors [11, 23, 24], carbynes are produced at high temperature by vapour condensation rather than through re-graphitation of diamond. In the case of shock compression of porous materials, the system reaches high temperature and high pressure. If the shock wave energy flow on the surface is greater than the thermal diffusion in the particle, the melting temperature may be reached on carbon grain surfaces. The carbon is then vaporized due to the high residual temperature and the influence of adiabatic expansion during the rarefaction process; carbynes are then produced directly through vapour condensation.

Table. Measured d-spacing in comparison with reference data dobs (nm)

0.306 0.259 0.215 0.177 weak 0.167 0.154 0.131 0.126 0.113

a-carbyne [11]a

chaoite [13]b

chaoite [14]c

carbyne [15]d

0.447

0.445

0.445 7

0.447

0.257 0.223 5

0.259 7 0.222 5

0.257 1 0.223 4

0.256 0.224

0.168 7 0.148 8 0.129 0.124

0.168 8 0.148 8

0.168 1 0.149 6 0.128 3 0.123 8 0.111 8

0.167 0.148 0.130 0.126

0.128 9

a

Carbyne obtained in shock compression experiments of annealed pyrolytic graphite. Found in compression of graphite/copper mixtures. c Produced by sublimation of pyrolytic graphite. d Obtained from pyrolytic graphite irradiated by neodymium laser. b

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the Rice University for a part of the TEM works. This study was partly supported by the Conseil re´gional d’Alsace.

References [1] [2] [3] [4] [5] [6]

[7] [8] Figure 4. TEM image of a compressed fullerene C60/Co mixture.

Recently, results [15] have clarified the mechanism of carbyne constitution. It seems to depend on pressure parameters. For low values, carbynes are achieved by vapour condensation, while at highpressure level they are obtained directly from the graphite through breaking of simple bonds.

[9]

[10] [11] [12] [13]

In our case, the presence of several carbon species, diamond, turbostratic phase and carbynes, is due to the heterogeneity of temperature, pressure, and external, internal energy distribution in particles. However, the pressure lasts a very short time (about a microsecond) while the temperature remains very high.

[14] [15] [16] [17] [18] [19] [20]

5. Conclusion

[21]

The identification of carbynes after applying a shock wave compression technique for diamond synthesis is beyond doubt.

[22] [23] [24] [25] [26]

Acknowledgements The authors wish to thank Mr Bruce E. Brinson and

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