Catalytic growth of aluminum nitride whiskers by a modified carbothermal reduction and nitridation method

doi:10.1016/j.jcrysgro.2005.09.004

Journal of Crystal Growth

Volume 285, Issue 4, 15 December 2005, Pages 566-571

https://doi.org/10.1016/j.jcrysgro.2005.09.004 Get rights and content

Abstract

Aluminum nitride (AlN) whiskers were synthesized by calcining an [Al(Hedta)(acetone)] (H₄edta=ethylenediaminetetraacetic acid) complex, which contained a very small amount of iron as a catalyst, under flowing nitrogen in the temperature range 1200–1500 °C, and then burning out the residual carbon. At relatively low reaction temperatures (around 1200 °C), most AlN whiskers were bead-shaped and chain-like, whereas at higher reaction temperatures the whiskers were acicular and six-sided prismatic. An iron droplet was observed on the tip of AlN whiskers, strongly supporting that the whiskers grow through the vapor–liquid–solid (VLS) mechanism. We suggest that whether or not the diameter of the whisker is uniform along its length depends on relative rates between diffusion of AlN vapor into an iron droplet and growth of the whisker.

Introduction

Aluminum nitride (AlN) has good thermomechanical and electronic properties, such as high thermal conductivity, high electrical resistivity, high mechanical strength, a thermal expansion coefficient close to that of silicon, and a low dielectric constant [1]. Thus, it has attracted extensive interest for applications as electrical packaging materials and as components in structural composites. In recent years, one-dimensional structured AlN whiskers have been suggested for use as a filler in polymers to increase the thermal conductivity. High thermal conductivity polymers have a wide variety of applications, from sealants and potting compounds for electronic applications to heat-dissipating structural, adhesive, or insulating materials [1], [2]. AlN whiskers were fabricated by various methods, such as the direct nitridation of metallic aluminum [3], [4], the combustion synthesis using a mixture of aluminum and AlN [5], and the conventional carbothermal reduction and nitridation (CRN) method calcining a mixture of alumina and carbon at 1800 °C [6], [7], [8]. Recently, we reported the synthesis of AlN particles and whiskers by employing the modified CRN method, in which a series of basic dicarboxylate Al(III) complexes Al(OH)(C_n₊₂H₂_nO₄)·xH₂O were used as precursors [9], [10], [11].

In the present work, we synthesize AlN whiskers by using an [Al(Hedta)(acetone)] (H₄edta=ethylenediaminetetraacetic acid) complex (hereafter, it will be called as AHE) as a new precursor, and investigate the change in morphologies of AlN whiskers with the reaction temperature. The growth of AlN whiskers has been mainly explained by the vapor–solid (VS) [4], [8] or vapor–liquid–solid (VLS) mechanism [6], [7], [8]. In general the VLS mechanism is evidenced by the presence of catalyst liquid droplets attached to whiskers tips. But one characteristic of AlN whiskers produced by the conventional CRN method is the lack of droplets on the whisker tips [6], [7], [8].

Section snippets

Experimental procedure

A slurry of freshly prepared Al(OH)₃ (obtained by the addition of 1 M aqueous ammonia to a solution of Al₂(SO₄)₃·18H₂O (98%+, Aldrich, 33.32 g, 0.05 mol) in water (500 mL)) was treated with H₄edta (29.23 g, 0.1 mol) in water (600 mL) and refluxed for 12 h. A clear solution obtained by filtering any unreacted H₄edta was concentrated to 30 mL. Acetone was then added with rigorous stirring at room temperature, and the resulting crude white crystals were filtered off and washed with acetone. The molecular

Results and discussion

Fig. 1 shows X-ray diffraction (XRD) patterns of samples calcined at different reaction temperatures (1100–1500 °C). In the sample calcined at 1100 °C for 15 h only diffraction peaks assigned to γ-alumina were detected, but in the sample calcined at 1200 °C for 5 h diffraction peaks assigned to AlN were detected together with those assigned to γ-alumina. The intensities of the former peaks increased with the reaction temperature. In the case of the samples calcined above 1250 °C, there were no

Conclusions

The AHE complex, which contains a very small amount of Fe as an impurity, was a good precursor for preparation of AlN whiskers. An iron droplet on the whiskers tips was observed, strongly supporting that the AlN whiskers grow out of the droplet at the root of the whisker through the VLS mechanism. The difference in the morphology with reaction temperatures is believed to be due to the relative rates between diffusion of AlN vapor into an iron droplet and growth of the whisker. We suggest that

Acknowledgments

This work was supported by Yeungnam University Research Grant (106117). The XRD patterns and SEM were recorded at the Yeungnam University Instrumental Analysis Center.

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