Effect of the production temperature on the structure of Si3N4 nanofibers

Abstract

The effect of the production temperature on the morphology and structure of Si 3 N 4 nanofibers was investigated. It was shown that nanofibers produced in the temperature range of 1340-1360°C are amorphous while those produced at 1380°C and above are monocrystalline. Apart from the principal phase (á-Si 3 N 4 ) the imide Si 2 N 2 NH was also found in the reaction products.Silicon nitride Si 3 N 4 has a set of useful working parameters (high values for the hardness, strength, Young modulus, resistance to creep and thermal shock, and high chemical stability), which make it possible to use it for high-temperature work.Fibrous Si 3 N 4 was first produced by the reduction of SiO 2 in the presence of nitrogen at temperatures in the range of 1300-1500°C [1]. Microfibers of Si 3 N 4 are single crystals with diameters between one and several micrometers. Such microfibers are promising for the reinforcement of composite materials both with metallic and with ceramic matrices. Thus, for example, with a content of 30 vol.% of â-Si 3 N 4 whiskers in the Si 3 N 4 matrix it was possible to achieve a rupture toughness of 7.6-8.6 MPa·m 1/2 , which is twice as high as for the non-reinforced matrix of polycrystalline Si 3 N 4 [2].Even greater prospects are presented by the use of a reinforcing silicon nitride phase in the form of nanofibers, the synthesis of which was first reported by the authors of [3]. Nanofibers of Si 3 N 4 and Si 2 N 2 O with diameters of 4-40 nm and lengths of several micrometers were obtained [3] by the reaction of carbon nanotubes with a mixture of Si and SiO 2 powders in an atmosphere of nitrogen at 1400°C for 1 h.Interest in aspects of the production and investigation of Si 3 N 4 nanofibers has not diminished to this day, as witnessed by the fairly large number of publications on the subject (e.g., see [4][5][6][7][8]).Another promising method for the production of Si 3 N 4 nanofibers is the chemical gas-phase deposition method, in which silicon tetrachloride (SiCl 4 ) and also ammonia and/or nitrogen are used as reagents [4,9].Worthy of great attention is a simple method of producing Si 3 N 4 nanofibers such as the nitriding of silicon powder by nitrogen or ammonia. The first results on the production of cotton wool-like Si 3 N 4 by this method were published in 1982 [10], and the interest in this simple method has been revived in the last few years (see [11,12]).In the present work the results from an investigation into the effect of temperature on the morphology and structure of Si 3 N 4 nanofibers produced by nitriding silicon powder with ammonia are reported. 0040-5760/11/4701-0009