The effects of NH4Cl addition and particle size of Al powder in AlN whiskers synthesis by direct nitridation

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Highlights

  • The diameters of AlN whiskers were from 140 nm to 340 nm by using different amounts of NH4Cl and 3 μm Al powder.

  • In the case of using NH4Cl more than 40wt%, pure AlN without any unreacted Al was formed as final products.

  • Using of NH4Cl and Al powder with particle size of 45 μm, leads to obtain AlN whiskers with 630 nm to 870 nm in diameter.

  • By adding 50wt% NH4Cl, pure AlN was formed. The diameter of the whiskers was increased by increasing in NH4Cl content in starting materials.

  • Increase in diameter of AlN whisker was resulted by using coarse Al powder. By adding NH4Cl to Al, thermodynamically spontaneous cholororination-nitridation reactions in vapor phase were increased and whiskers and pure AlN powder were produced.

Abstract

In this study, AlN whiskers were prepared in a tube furnace at 1000 °C for 1 h under 500 cc/min nitrogen gas flow. Al powders with particle sizes of 3 μm and 45 μm and NH4Cl were used as raw materials. SEM, TEM and XRD analyses were applied to characterize AlN whiskers. The diameters of AlN whiskers were from 140 nm to 340 nm by using different amounts of NH4Cl and 3 μm Al powder. In the case of using NH4Cl more than 40 wt.%, pure AlN without any unreacted Al was formed as final product. Using NH4Cl and Al powder with a particle size of 45 μm leads to obtain AlN whiskers with 630 nm to 870 nm in diameter. By adding 50 wt.% NH4Cl, pure AlN was formed. The diameter of the whiskers was increased by increasing the NH4Cl content in starting materials (about 200 nm). Also increase in diameter of AlN whisker was resulted by using coarse Al powder. By adding NH4Cl to Al, thermodynamically spontaneous chlorination–nitridation reactions in vapor phase were increased and whiskers and pure AlN powder were produced. It seems that the formation of whiskers in this work was related to solid–gas (VS) mechanisms.

Introduction

Aluminum nitride (AlN) has a hexagonal wurtzite crystal structure with the space group P63mc. AlN lattice constants are a = 3.11°A, c = 4.98°A and c/a = 1.601 which are comparable to ideal wurtzite structure. Aluminum atoms are arranged in a hexagonal close-packed (HCP) structure and nitrogen atoms fill half of tetrahedral sites [1].

Whiskers commonly refer to structures in which one of the dimensions is much longer than other dimensions. The defining characteristic of whiskers is their single-crystal structure. AlN whiskers can be described as fine, high purity single-crystal fibers. The length to diameter ratio range is from 5 to 1000. Due to their single-crystal structure, the whiskers exhibit unique physical and chemical properties. In addition, their strength tends to approach theoretical values. The earliest reports about the growth of AlN whiskers were released in early 1960. Aluminum nitride is studied as an ideal material for semiconductor substrate applications [2]. It possesses properties such as high thermal conductivity, low thermal expansion coefficient, high density, high electrical resistivity, and good dielectric properties, which make it an effective heat-dissipating medium [3], [4], [5]. AlN whiskers have good compatibility with SiC, Al2O3, ZrO2, Si3N4 and TiB2 [6]. The whiskers have also led to an increase in critical thermal shock [7]. Several methods to synthesize AlN whiskers have been investigated, including carbothermal reduction and nitridation [8], [9], [10], direct nitridation [11], [12], self-propagating high-temperature synthesis (SHS) [13], [14], [15], chemical vapor deposition (CVD), sublimation recrystallization [16], and catalytic growth using aluminum and carbon complex [17], [18]. For commercial applications, AlN whiskers need to be produced using cost effective techniques. Direct nitridation has been used as a low cost and simple method for AlN whisker production.

Portnoi and Gribkov's [19] found that the optimum temperature for AlN whisker growth could be between 1600 and 1700 °C in direct nitridation. And also it is found that raising the temperature over 1750 °C leads to vaporization of aluminum which, in turn, results in a rise in the degree of super-saturation, thereby leading to the further gas-phase deposition of aluminum nitride. The obtained whiskers had short length and large diameter and contained the defects. It was established that preferential whisker growth takes place at lower concentration of nitrogen. The direct nitridation method requires a high temperature which prevents realization of complete nitridation. Therefore, in this method ammonium chloride is added to aluminum powder at 1000 °C under nitrogen gas flow through solid-gas reactions [11], [12]. Radwan and Bahgat [12] reported that the evolution of gases due to the sublimation or decomposition of ammonium chloride produces several pores, which allows more reaction at 1000 °C. They further found that increasing the amount of ammonium chloride up to 50 wt.% promoted nitridation up to 95.6%. The resultant powder was white and contained nanowhiskers by adding 40 wt.% ammonium chloride. Also, Radwan, Bahgat and El-Geassy [11] found that the addition of ammonium chloride to 50 wt.% exhibited a different reaction pathway, compared with liquid–gas nitridation. In their experiment, no droplets were observed on the tip of the whiskers, suggesting that the whiskers were grown through a vapor-phase mechanism. They contended that AlN whiskers were grown through spontaneous intermediate chlorination–nitridation sequences in the vapor phase.

In the present study, AlN whiskers were synthesized through direct nitridation. This is a simple low-cost technique for the production of AlN whiskers. The effect of ammonium chloride on the diameter of the whiskers and the purity of AlN was initially examined. Then, the effect of aluminum powder size in the mixture of raw materials on the diameter of the synthesized whiskers was studied.

Section snippets

Experimental procedure

Fine and semispherical aluminum powder (3 μm) with 99% purity according to Table 1 (supplied by Merck, Germany), coarse and semispherical aluminum powder (45 μm) with 99% purity (supplied by Aluminium Bahrain), nitrogen gas (N2) with 99.999% purity (supplied by Farafan Gas Co.), and ammonium chloride powder of 99% purity (supplied by Merck, Germany) were weighted and mixed (Table 2).

The samples were put in graphite crucibles and then were placed in a horizontal controlled atmosphere tube furnace.

Results and discussion

The resultant synthesized samples were grayish-white. It appears that the porosity in the samples increased with increasing ammonium chloride. Central part of the samples was examined by SEM. Fig. 1 shows SEM micrographs of the samples A1, A2, A3, A4, A5 and A6 and Fig. 2 shows XRD patterns of these samples.

As shown in Fig. 1-a, sample A1 was in the form of molten aluminum balls. As for sample A2, by the addition of 10 wt.% ammonium chloride, a greater formation of AlN occurred, which can be

Conclusion

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    Addition of ammonium chloride to Al powder promoted the formation of whiskers and enhanced the nitridation conversion through spontaneous intermediate chlorination–nitridation in the gas phase. Also, with an increase in ammonium chloride concentration the diameter of the whiskers increases up to 200 nm in the presence of Al powder with a particle size of 3 μm and up to 240 nm in the presence of Al powder with a particle size 45 μm.

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    The diameter of the whiskers increases as a function of increasing

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