Pergamon PII: SOOO&6223(98)00019-0 Carbon Vol. 36, No. 5-6, pp. 795-800, 1998 0 1998 Elsevier ScienceLtd Printed in Great Britain. All rights reserved 0008-6223/98 $19.00+ 0.00 LASER PYROLYSIS OF CARBON-NITROGEN GAS-PHASE COMPOUNDS: AN ATTEMPTED APPROACH TO CARBON NITRIDE FORMATION R. ALEXANDRESCU,‘** S. COJOCARU,~ A. CRUNTEANU,’ S. PETCU,’ R. CIREASA,~ I. VOICU,~ I. MORJAN,’ D. FATU~ and F. HUISKENC aNational Institute for Lasers, Plasma and Radiation Physics, Lasers Department, P.O. Box MC-36, R-76900 Bucharest, Romania bDepartment of Physical Chemistry, University of Bucharest, Bd. Republicii 13, Bucharest, Romania ‘Max-Planck-Institut fur Stromungsforschung, Bunsenstrasse 10, D-37073 Gottingen, Germany zyxwvutsrqponmlkjih (Received 30 October 1997; accepted in revised form 4 November 1997) Abstract-We describe the possibility to synthesize carbon nitride fine powder by laser pyrolysis of acetylene/nitrous oxide/ammonia mixtures. The powders were produced in a flow reactor by varying the relative concentrations of the sensitizers (SF&H.,). The kinetics of the reaction was monitored by IR spectrophotometry of the exhaust gases. The powder crystallinity and bond structure was studied by X-ray diffraction (XRD) and IR transmission spectrometry, respectively. The majority of XRD data suggests the presence of the predicted a- and g-C,N, structures, with a concentration depending on starting conditions. The presence of C-N bonded phases seems relevant. Under the present conditions, the slight contamination of powders by SF, decomposition products could not be avoided. 0 1998 Elsevier Science Ltd. All rights reserved. Key Words-A. Carbon composites, C. spectrophotometry. 1. INTRODUCTION The widespread interest in the preparation of carbon nitride materials is generated by their ultrahard prop- erties, theoretically predicted years ago by Liu and Cohen [ 11. Since then, a large number of studies, both experimental and theoretical, have been devoted to the synthesis of carbon nitrides [2-41, the vast majority of them reporting results on the nucleation and growth of CN, thin solid films [ 5-101. Although structural evidence for the formation of B-&N, phases have been reported [5,9,10] recent theoretical calculations predict a- and cubic C3N4 as new, low- compressibility, super-hard carbon nitrides [ 31. Attempts to preparing solid carbon nitrides (in the bulk or powdery form) were merely chemical [ 111, using as starting materials high-molecular-weight nitrogen-containing organic precursors. It was shown that thermodynamic effects of the simultaneous appli- cation of elevated temperatures and ultrahigh pres- sures allow for the creation of new carbon nitride phases [ 121. We have reported recently on the possibility of C,N, fine powder synthesis by the laser pyrolysis of sensitized acetylene/nitrous oxide/ammonia mixtures [ 131. The CO2 laser synthesis from gaseous precursors in a flow reactor is an attractive process since the very sharp temperature gradients developed at high pressures (hundreds of Torr) on a microsecond time scale allow for the fast cooling of the nucleated *Corresponding author. Fax: +401423 1791; e-mail: ralexa@roifa.ifa.ro particles and for the formation of solid compounds with variable and controlled composition [ 141. Such a system could quench the sample and retain it metastably at ambient conditions. In this paper we have studied the powder crystall- inity and bond structure at the variation of the relative concentration of SF&H4 sensitizers. The majority of X-ray diffraction (XRD) data is consis- tent with a mixture of the predicted a- and B-C3N, structure with a concentration depending on starting conditions. By IR transmission measurements, the existence of the C-N bonded phases and the absence of the triple bonded C=N was observed. The slight contamination of powders by SF, decomposition could not presently be avoided. 2. EXPERIMENTAL In the CO2 laser synthesis method the reactant gases are heated by laser absorption in the well- defined irradiated region. In the present experiment the process is based on the resonance between the laser radiation and a sensitizer (energy transfer agent) added to the ternary C2H2/N20/NH3 chemical system. The synthesis was induced by a CW CO2 laser (maximum power 130 W). Because the funda- mental vibrations of the reactants are out of the emission range of the CO2 laser (l/1 = 944 cm-‘), mixtures of sulfur hexafluoride and ethylene (SF&H, = 1:l and SFs:C2H4= 1:2) were used as energy transfer agents. The detailed features of the experimental apparatus 795