Химия и химические технологи / 5

Химия и химические технологи / 5. Фундаментальные проблемы создания новых материалов и технологий

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

Samara State Technical University

Preparation of aluminum nitride nanopowder using
SHS azide technology

A distinctive feature of nitride compounds are broad areas of homogeneity where a great amount of vacant sites is formed while their crystal structure is maintained. In addition, like many other phases of implementation, nitrides are capable of forming numerous solid solutions, many of which are distinguished by improved operating characteristics.

The nitrides of elements of III group (B, Al, Ga, In) and, particularly, aluminum nitride occupy a special place among the nitride compounds. The application perspectiveness of AlN is specified by the width of its band gap (6.28 eV), high value of the critical electric breakdown, great heat resistance and mechanical strength [1]. Aluminum nitride is the only ceramic material that has an extremely interesting combination of high thermal conductivity and excellent insulation properties. Due to this it is widely used in energetics and microelectronics. For example, AlN is utilized in the manufacturing of printed circuit boards (substrates) in semiconductors, it serves as a heat sink for the led lighting technology and in high-power electronics. Moreover, aluminum nitride is viable for hardening aluminum alloys operating at high temperatures.

Nowadays there is a huge number of technologies for producing of aluminum nitride: direct nitriding, plasma-chemical synthesis, carbothermic synthesis, chemical deposition from the gas phase, the explosion of aluminum wire, etc.

This article [2] presents the results of plasma-chemical synthesis of aluminum nitride nanopowder using gaseous nitrogen and ammonia. A nanocomposite of Al/AlN with spherical particles sized 50-100 nm is generated when pure nitrogen is used as a reaction gas. Nitrogen content in the samples increases and the average particle size decreases by adding ammonia.

Aluminum nitride was obtained by the method of chemical deposition from the gas phase, using the system AlCl₃-NH₃-N₂ [3]. Gaseous aluminum chloride, ammonia and nitrogen were reacted at a temperature 1044 °C. The resulting powders’ particles have a spherical shape and an average size of less than 0.1 micrometers.

In the following test [4] the aluminum nitride was synthesized by the reaction of aluminum chloride and sodium azide. The mixture was placed in a reactor with a nitrogen atmosphere, and then the reactor was placed into a furnace for heating. The reaction temperature of 450 °C was maintained for 24 hours. The synthesized product was a micro-ribbon of AlN and bit long, straight fibers with a diameter of 40 to 60 nm and length of several micrometers.

The process of formation of aluminum nitride in terms of electrical explosion of aluminum wire was investigated by the authors of the present article [5]. As a result of the search for better conditions of electrical explosion, a powder with a content of AlN equal to 93% and a specific surface area of 14 m2/g.

However, these methods are associated with large power inputs and high-cost and complex equipment is required for them, the prepared powders have a strongly defect structure, because of the shock cooling of the end-product. The self-propagating high-temperature synthesis (SHS) of refractory compounds is free of these disadvantages. It is characterized by low power inputs; short process duration; high purity of products; the possibility to prepare new compounds, especially multiphase composites, which are difficult to synthesize using other technologies; and wide possibilities to control the dispersed structure of powders from single-crystal grains to nanodimensional particles [6, 7]

The method is based on the exothermic reaction of two or more chemical elements occurring in the directional mode of combustion. The process is carried out in a thin layer of the mixture of the initial agents after local initiation of the reaction (ignition via electric impulse with a duration of 3-5 seconds) and spontaneously propagates throughout the system due to heat transfer from the hot products to original unheated materials that do not require energy input.

The authors [8] applied the SHS method for the synthesis of highly dispersed AlN powder by combusting the initial mixture of aluminum and gasifying additive (NH₄Cl, NH₄F) in the nitrogen medium. It is stated that the degree of conversion of the aluminum to the nitride increases with an increase in the proportion of aluminum in the charge and reaches a maximum value (99.5 %) when the content of aluminum in the mixture in the amount of 50 wt %. The particle diameter of the aluminum nitride decreases from 8-12 μm to about 1-2 μm, and the specific surface area increases up to 1.5 m²/g with increasing of ammonium chloride proportion in the mixture up to 10 mass %.

Starting from 1970, the azide technology of self-propagating high-temperature synthesis (SHS-Az) has been developed at the Samara State Technical University. This technology makes it possible to fabricate micropowders and nanopowders of nitrides and composites based on them using the sodium-azide (NaN₃) powder as the nitriding reagent and haloid salts [9].

The goal of this study was to investigate the possibilities of applying the SHS-Az technology to fabricate the nanostructured aluminum nitride powder.

The stoichiometric equation of the preparation reaction of aluminum nitride in the SHS-Az mode is as follows:

AlF₃ + 3NaN₃ = AlN + 3NaF + 4N₂, (1)

Na₃AlF₆ + 3NaN₃ = AlN + 6NaF + 4N₂. (2)

The usage of halide salts containing aluminum (AlF₃, Na₃AlF₆) instead of metallic aluminum as a starting component reduces the combustion temperature and also enables the performing of the reaction on the atomic level. In this case, it is possible to synthesize the nanosized powders of aluminum nitride.

As the initial feedstock, we used the sodium azide powder of the “Pure” grade (assay percentage 98,71 wt.%), aluminum fluoride powder of the “Pure” grade (assay percentag 99,9 wt.%), sodium hexa-fluoroaluminate powder of the “Pure” grade (assay percentag99,0wt.%), nitrogen gas quality “1” (assay percentag 99,99 wt.%).

The procedure of experimental investigations in an SHS constant-pressure reactor with a volume of 4.5 L is described in monographs [9]. The cylindrical samples (diameter – 30 mm, height – 45 mm) of the initial powder mixture of the apparent density under an external nitrogen pressure in a reactor of 4 MPa were combusted.

We investigated the phase and chemical compositions, morphology, and particle size of combustion products. The phase composition was determined using an ARL X’TRA automated X-ray diffractometer. X’ray spectra were recorded using Cu radiation with continuous scanning in angle range 2θ = 20°–80° with a rate of 2 deg/min. The surface topography and morphology of powder particles were investigated using a JSM-6390A scanning electron microscope.

To preliminarily analyze the combustion temperature of the mixture of initial components and composition of synthesis products, thermodynamic calculations were performed using the Thermo computed program developed at the Institute of Structural Macrokinetics and Problems of the Materials Science of the Russian Academy of Sciences (Chernogolovka, Moscow region).

Table 1 presents the results of thermodynamic calculations, which show the values of the adiabatic temperature, reaction thermal effect, and composition of combustion products for the two main compositions of the initial mixture, which differ in haloid salts.

Table 1 – Results of thermodynamic calculations of combustion parameters

Composition no.	Charge composition	Adiabatic temperature [K]	Reaction thermal effect [kJ]	Synthesis products [mole]
Composition no.	Charge composition	Adiabatic temperature [K]	Reaction thermal effect [kJ]	AlN	NaF	N₂
1	AlF₃+3NaN₃	1682	–135	1	3	4
2	Na₃AlF₆+3NaN₃	1269	–107	1	6	4

From the analysis of the presented data for the compositions 1 and 2 follows, that the adiabatic temperature and the thermal effect are higher while using aluminum fluoride as a starting component than sodium hexa-fluoroaluminate. However, the yield of the desired product - aluminum nitride is the same in both speciations.

The results of the experimental determination of the combustion temperature and its rate, as well as the phase compositions under consideration, are presented in Table 2.

Table 2 – Combustion parameters and composition of synthesis products

Composition no.	Charge composition	Combustion temperature[°C]	Combustion rate[mm/c]	Phase composition
1	AlF₃+3NaN₃	1250	11	AlN, Na₃AlF₆, NaF
2	Na₃AlF₆+3NaN₃	950	6	AlN, Na₃AlF₆, NaF

It appears from Table 2 that, while using sodium fluoride the combustion temperature and its rate are higher, than by using sodium hexa-fluoroaluminate by 300 °С и 5 mm/s correspondingly. The combustion products of both speciations consist of three phases: aluminum nitride (AlN), sodium hexa-fluoroaluminate (Na₃AlF₆) and sodium fluoride (NaF). However, their ratio differs.

Figure 1 displays X’ray powder diffraction patterns of synthesis flushed and unflushed products synthesized of the speciation 1. the flushing was performed by diluting the powders with distilled water in ratio 1:10, roiling the suspension and than filtrating the desired products in the vacuum funnel. The filtered powder was blown dry to the constant weight in the vacuum oven.

а)

Figure 1 – The results of X-ray phase analysis of the products, synthesized

of the mixture “AlF₃+ NaN₃”:

a) before flushing; b) after flushing

The X-ray patterns demonstrate that the combustion produtcts of the temper “AlF₃ + 3NaN₃” consist of three phases: aluminum nitride (AlN), sodium hexa-fluoroaluminate (Na₃AlF₆) and sodium fluoride (NaF). The peak heights allow us to assume that NaF > Na₃AlF₆ > AlN. The amount of the desired product - aluminum nitride is low, so that the X-ray pattern of the unflushed products there are no peaks of AlN. A good solubility in water sodium fluoride is absolutely removed from the combustion products after flushing in the distilled water. On the contrary, due to bad solubility in water sodium hexa-fluoroaluminate exists in the combustion roducts. The quantative phase composition of the flushed combustion products of the temper “AlF₃ + 3NaN₃” displayed the fraction of aluminum nitride – 64% and sodium hexa-fluoroaluminate – 36%.

See figure 2. The X-ray patterns of the unflushed and flushed produts synthesized form speciation 2.

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а)

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Figure 2 – The results of X-ray phase analysis of the products, synthesized

of the mixture “Na₃AlF₆+ NaN₃”:

a) before flushing; b) after flushing

The X-ray patterns demonstrate that the combustion produtcts of the mixture “Na₃AlF₆+ NaN₃” consist of three phases: sodium fluoride (NaF), sodium hexa-fluoroaluminate (Na₃AlF₆) and aluminum nitride (AlN). The peak heights allow us to assume that NaF> Na₃AlF₆ >AlN. The quantative phase composition of the flushed combustion products of the temper “Na₃AlF₆ + NaN₃” displayed the fraction of aluminum nitride – 61% and sodium hexa-fluoroaluminate – 39%.

The process of flushing of sodium hexa-fluoroaluminate consisted of decomposing it into the fluorides of sodium and aluminum which are soluble in water by heating them in Ar at a temperature of 400 °С for 30 min and flushing it with water. This action allows extracting the high purity aluminum nitride powder (figure 3).

Figure 3 – The results of X-ray phase analysis of the products, synthesized of the mixture “AlF₃+3NaN₃” after been blown dry in the vacuum oven

See figure 3 we may see after been dry in the vacuum oven the product consists of one phase - aluminum nitride.

See figure 4. The photographs of microstructure of the aluminum nitride powders synthesized of speciation 1 and 2

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а)	b)

Figure 4. The microstructure of synthesized aluminum nitride powders:

a) “AlF₃+3NaN₃”; b) “Na₃AlF₆ + NaN₃”

The picture displays a crystal whiskers structure of aluminum nitride, synthesized of the mixture “AlF₃+3NaN₃”. The whiskers have a diameter from 100 to 200 nm and in length they are about 5 µm. Aluminum nitride, generated in the combustion of the temper “Na₃AlF₆ + NaN₃” has a structure of the spherical particles with a diameter from 50 to 140 nm.

The applying of energy-saving SHS azide technology allowed extracting high purity aluminum nitride micro and nanopowders of the speciations “AlF₃+3NaN₃” and “Na₃AlF₆ + NaN₃”. It is of tremendous value for using them in electronics and electric engineering. AlN is synthesized as the crystallized whiskers with the diameter 100-200 nm and spherical particles with the diameter 50-140 nm depending on the haloid salt used in the reaction.

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