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					Journal of Crystal Growth 234 (2002) 9–11

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Formation of a-Si3N4 whiskers with addition of NaN3 as catalyst
Y.G. Caoa,c,*, H. Chena, J.T. Lib, C.C. Geb, S.Y. Tanga, J.X. Tanga, X. Chena
Key Lab of New Packaging Materials and Technology of China, National Packaging Corporation, Zhuzhou Engineering College, Zhuzhou 412008, Hu Nan Province, People’s Republic of China b Laboratory Special Ceramic & Metallurgy, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China c Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603-23, Beijing 100080, People’s Republic of China Received 20 March 2001; accepted 16 August 2001 Communicated by L.F. Schneemeyer
a

Abstract a-Si3N4 whiskers with diameters of about 0.1 mm and length of 3–5 mm were fabricated through combustion synthesis route under very low nitrogen pressure of 0.5–1 MPa. The measured maximum combustion temperature is about 1863 K which is rather lower than the adiabatic temperature (about 4400 K) and the decomposition temperature (about 2170 K) of Si3N4 under atmospheric pressure. The addition of NaN3 catalyst plays a key role in the formation of a-Si3N4 whiskers under such low nitrogen pressure. r 2002 Published by Elsevier Science B.V.
PACS: 81.05.Ea Keywords: A2. Combustion synthesis; B1. Nitrides

1. Introduction Si3N4 fiber/whiskers can be used as reinforcement in ceramic-matrix composites having high fracture toughness. Rodriguez [1] reported the b-Si3N4 fibers through combustion synthesis (CS) under 100 atm nitrogen pressure ðPN2 Þ: However, it is not a cost-effective route due to the high N2 pressure. In this paper, we report the formation of

Si3N4 whiskers through CS route under 5–30 atm of PN2 with the addition of NaN3.

2. Experimental procedures The additives of NaN3 has a purity of above 99.5%, in which free alkali is 0.2%, heavy metal 0.002%, weight loss on drying 0.2%, infusible substance 0.02%. The reactants with mixing ratios of Si : Si3N4 : NaN3=1 : 1 : 1 (wt%) (sample SSN1), Si : Si3N4 : NaN3=2 : 1 : 2 (sample SSN3), and Si : Si3N4=2 : 1 (sample SS3) were ball milled by sintered Si3N4

*Corresponding author. Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603-23, Beijing 100080, People’s Republic of China. E-mail address: yonggecao@hotmail.com (Y.G. Cao).

0022-0248/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 6 9 0 - 6

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Y.G. Cao et al. / Journal of Crystal Growth 234 (2002) 9–11

balls for 30 h in ethanol medium. After being sieved and dried, the mixtures were placed in a porous graphite crucible of size +20 Â 30 mm2, and thereafter the crucible was placed in a selfdesigned autoclave. The system was evacuated and back-filled with 0.5–3 MPa of nitrogen pressure. Details of the CS process are given elsewhere [2]. After being ignited by a tungsten coil, the combustion reaction was completed within 5 min. During the propagation of the front wave, the combustion temperature was recorded by two WRe3/W-Re25 thermocouples with +0.2 mm which were inserted into the top and the bottom of the sample, respectively. X-ray powder diffraction and scanning electron microscopy (SEM) were used to characterize the phase and the morphology of the combusted products.

1500
Temperature (˚C)

1000

500

0 0 30 60 90 Time (s) 120 150

Fig. 2. Combustion temperature profile dependence on time of sample SSN1 combusted under 1 MPa of nitrogen pressure.

3. Results and discussion Fig. 1 shows the X-ray powder diffraction pattern of sample SSN1 which is the combusted product under 1 MPa of PN2 : Only Si3N4 phases are observed in Fig. 1 which demonstrates that the Si powders have been nitrided completely. About 76.2% of a-Si3N4 and 23.8% of b-Si3N4 are involved in the final products calculated using the method reported in Ref. [3]. Fig. 2 shows the curve of combustion temperature vs. time for the sample SSN1 under 1 MPa of PN2 : Two typical peaks 14351C and 15901C are observed which correspond to the nitridation of
α
4000

Fig. 3. SEM observation of the combusted product of sample SSN1 under 1 MPa of nitrogen pressure.

α α α α

α=α- S i3N4 β=β - S i3N4

α
2000

α β

α
0 20

β

α α β α αα α β α α α βα α α α β α α
40 60 T WO - THETA 80

Fig. 1. X-ray powder diffraction pattern of sample SSN1 which is the combusted product under 1 MPa of PN2 :

the surface of silicon particles and the second nitridation of the inner part of silicon particles, respectively [4]. The propagation velocity of the combustion front is 0.035 cm sÀ1. Fig. 3 shows the whiskers morphology of the inner part of the combusted products of sample SSN1 under 1 MPa of PN2 observed by SEM. These Si3N4 whiskers have length of about 3–5 mm and diameter of about 0.1–1 mm. No droplets were found on the tip of whiskers. In our experiment, it was impossible to ignite the silicon particles under 1 MPa of PN2 if NaN3 was not added. Holt [5] has reported the nitrida-

CPS

Y.G. Cao et al. / Journal of Crystal Growth 234 (2002) 9–11

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tion of transient metals involving the following reactions: 3Me+NaN3-3MeN+Na(m). Since NaN3 can be decomposed at temperature of about 200–3001C it can both lower the maximum combustion temperature and supply sufficient nitrogen. In our experiment, it corresponds to the reaction 9Si+4NaN3-3Si3N4+4Na(m), in which Na is evaporated under high temperature. The measured maximum combustion temperature (about 1863 K) shown in Fig. 2 is rather lower than the adiabatic temperature (about 4400 K) and the decomposition temperature (about 2170 K) of Si3N4 under atmospheric pressure [2], which can ensure the propagation of combustion wave and get the desirable solid products. The decomposed nitrogen, however, can easily escape under low nitrogen pressure which will lead to the inhomogeneous distribution of nitrogen in the openings and hence result in the incomplete nitridation of silicon. The addition of Si3N4 can be used as diluents and crystal seeds so as to complete the sufficient nitridation. In order to observe the effects of NaN3 on the nitridation of silicon particles, the reactants of two samples, Si : Si3N4 : NaN3=2 : 1 : 2 (sample SSN3) and Si : Si3N4=2 : 1 (sample SS3), were combusted under 3 MPa of PN2 : The final products of sample SS3 contain three phases: free silicon (1.25 wt%), a-Si3N4 (54.7 wt%), b-Si3N4 (44.05 wt%); comparing to sample SS3, the final products of sample SSN3 have only 65 wt% of a-Si3N4 and 35 wt% of b-Si3N4 without free silicon. This result demonstrates that NaN3 can increase the nitridation ratio and proportion a-Si3N4 phase even under 3 MPa of PN2 : The SEM observation of samples SS3 and SSN3 shows the agglommerated and whiskers morphologies, respectively, which confirms that NaN3 can also act as a catalyst to promote the crystal growth of Si3N4 whiskers.

The combustion reaction of mixtures of silicon, NaN3 and Si3N4 cannot be ignited under very low PN2 such as less than 0.5 MPa.

4. Conclusions The silicon nitride whiskers were fabricated through combustion synthesis routes with the addition of NaN3 under 0.5–1 MPa of nitrogen pressure. NaN3 can lower the maximum combustion temperature and the required nitrogen pressure, as well as increase the nitridation ratio and a-Si3N4 proportion. It also acts as a catalyst for the crystal growth of Si3N4 whiskers.

Acknowledgements The authors greatfully acknowledge the supports from the Chinese Hu Nan Natural Science Foundation under Grant SSY2066 and Key Lab of New Materials & Technology of China National Packaging Corporation.

References
[1] M.A. Rodriguez, N.S. Makhonin, J.A. Escrina, I.P. Borovinskaya, M.F. Barba, J.E. Iglesias, J.S. Moya, Adv. Mater. 7 (1995) 745. [2] Osamu Yamada, Kiyoshi Hirao, Mitsue Kozumi, Yoshinari Miyamoto, J. Am. Ceram. Soc. 72 (1989) 1735. [3] P.G. Pigeon, A. Varma, J. Mater. Sci. Lett. 11 (1992) 1370– C. [4] A.G. Merzhahov, in: Z.A. Munir, J.B. Holt (Eds.), Combustion and Plasma Synthesis of High Temperature Materials, VCH, New York, 1990, p. 1. [5] J.B. Holt, Synthesis of fine-grained a-Si3N4 by a combustion process, US patent number 4,944,930, July 31, 1990.


				
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