Физика / 2.Физика твердого тела

Titova Yu.V., Majdan D.A., Sholomova А.V.,
Aleksandrov D.Yu., Khisamutdinova А.V.

Samara State Technical University

Self-Propagating High-Temperature Synthesis
of Nanostructured AlN Powder with the Use of AlF₃ and NaN₃

One fundamental base of technological advance is the development of new materials meeting demands of modern technology. In 1967, when studying gas-free combustion of mixtures of powders of metals and nonmetals, Russian scientists (Academician A.G. Merzhanov, Professor I.P. Borovinskaya, and Professor V.M. Shkiro) in the academic borough of Chernogolovka near Moscow developed the new synthesis of compounds, including nitrides, which was called self-propagating high-temperature synthesis (SHS) [1].

In 1970, Professor V.S. Kosolapov of the Kuibushev Polytechnic Institute proposed using powders of solid inorganic azides, the application of which increases concentrations of reacting substances in the synthesis zone and eliminates filtration difficulties, instead of gaseous nitrogen as the nitrifying reagent in SHS [2]. This is the beginning of azide technology of self-propagating high-temperature synthesis (SHS-Az).

The investigation into the synthesis conditions and properties of aluminum nitride AlN is considered in numerous publications [3-9], the results of which were the origin of its application in modern engineering. Aluminum nitride has a forbidden band of 6.2 eV with the direct transitions, which is close to dielectric. Its resistivity is very high (greater than 1011 Ω), whereas permittivity is very low. Because of these properties, AlN is ideally appropriate for the use as a material of electronic substrates or the package of integrated circuits. It also has a low thermal expansion coefficient of 4.3 × 10^–6 K^–1 and a high thermal conductivity of 320 W m^–1 K^–1, which is three times higher than for aluminum oxide, while it is the most valuable known ceramic material [10].

The aim of the given work is to investigate the process for obtaining the nanostructured powder of aluminum nitride in the SHS mode at the excess pressure of nitrogen using sodium azide and aluminum fluoride.

The stoichiometric equation for the chemical reaction of obtaining aluminum nitride in the SHS-Az mode is the following:

AlF₃ + 3NaN₃ = AlN + 3NaF + 4N₂. (1)

In this study we investigated the combustion temperature and rate of the starting component mixture under conditions of a laboratory reactor with a constant pressure [2] and the chemical and phase compositions of combustion products as a function of the pressure of gaseous nitrogen in the reactor, the relative density of the starting mixture, and the sample diameter.

Based on the obtained data and technological considerations related to the use of pressing equipment, it is appropriate to use the apparent density of the charge for the further investigations. It is evident from data that the combustion temperature and rate of the AlF₃ + 3NaN₃ system increase with increasing sample diameter D attaining the maximum at D = 3 cm. Exceeding this value causes filtering difficulties for the nitrogen supply into the central part of the sample. External nitrogen participates at the after burning stage, which is why the desired product with a high N content forms [11]. Based on these data, we chose a sample diameter of 3 cm for the further investigation of the AlF₃ + 3NaN₃ system.

The surface topography and particle sizes of the AlN powder synthesized from the AlF₃ + 3NaN₃ system are shown in Fig. 1. It is evident from the presented photographs that aluminum nitride is synthesized in the form of particles containing whiskers ~100 nm in diameter, which can be classified as nanofibers.

Data of X-ray analysis (Fig. 2) testify that reaction products contain three phases: aluminum nitride, sodium hexafluoroaluminate, and sodium fluoride. The last two components are present only in unwashed combustion products and are removed by washing in distilled water.

a	b

Figure 1 – Surface topography of AlN powders synthesized in the “AlF₃ + 3NaN₃” system:

(a) ×1000 and (b) ×20000

Figure 2 – X-ray diffraction pattern of the washed product synthesized
in the “AlF₃ + 3NaN₃” system

Let us note that the binary system that we investigated makes it possible for the first time to synthesize the aluminum nitride powder in the form of nanofibers.

References

1. Amosov, A.P., Borovinskaya, I.P., and Merzhanov, A.G., Powder Technology of Self-Propagating High-Temperature Synthesis of Materials: Moscow: Mashinostroenie-1, 2007.

2. Amosov, A.P. and Bichurov, G.V. Azide Technology of Self-Propagating High-Temperature Synthesis of Micropowders and Nanopowders of Nitrides, Moscow: Mashinostroenie-1, 2007.

3. Kosolapov, V.T., Shmel’kov, V.V., Levashev, A.F., and Markov, Yu.M., 2nd All-Russia Conf. on Technological Combustion, Chernogolovka, 1978, pp. 129-130.

4. Prokudina, V.K., Shestakova, T.V., Borovinskaya, I.P., et al., Problems of Technological Combustion: Proc. 3rd All-Russia Conf. on Technological Combustion, Chernogolovka, 1981, vol. 2, pp. 5-8.

5. Zakorzhevskii, B.B., Borovinskaya, I.P., and Sachkova, H.B., Inorg. Mater., 2002, vol. 38, no. 11, p. 1131.

6. D’yachkov, L.G., Zhilyakov, L.A., and Kostanovskii, A.B., Tech. Phys., 2000, vol. 45, no. 7, p. 928.

7. Borets-Pervak, I.Yu., Kvantovaya Elektron., 1997, vol. 24, no. 3, pp. 265-268.

8. Grabis, Ya.P., Ubele, I.P., and Kuzyukevich, A.A., Latv. PSR Zinat. Akad. Vestis., Ser. Khimiya., 1982, no. 3, pp. 279–282.

9. Titova, Yu.V. and Shiganova, L.A., Proc. Int. Theor. and Pract. Conf. “Modern Innovations in Science and Technology”, Kursk, 2011, pp. 113-115.

10. Samsonov, G.V., Nitrides, Kiev: Naukova Dumka, 1969.

11. Bichurov, G.V., Self-Propagating High-Temperature Synthesis of Nitrides with Application of Inorganic Azides and Halogen Salts, Extended Abstract of Doctoral Dissertation, Samara: Samara State Tech. Univ., 2003.