Structural characterization of phase transition of Al2O3 nanopowders obtained by polymeric precursor method

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Abstract

Nanocrystalline Al2O3powders have been synthesized by the polymeric precursor method. A study of the evolution of crystalline phases of obtained powders was accomplished through X-ray diffraction, micro-Raman spectroscopy and refinement of the structures through the Rietveld method. The results obtained allow the identification of three steps on the γ-Al2O3 to α-Al2O3 phase transition. The single-phase α-Al2O3 powder was obtained after heat-treatment at 1050 ° C for 2 h. A study of the morphology of the particles was accomplished through measures of crystallite size, specific surface area and transmission electronic microscopy. The particle size is closely related to γ-Al2O3 to α-Al2O3 phase transition.

Introduction

Ultrafine and nanosized single-metal oxide powders have been given a lot of attention as a possibility for functional materials of electrical parts and structural materials of mechanical parts [1]. Currently and for the foreseeable future, the most important types of nanoparticles are simple oxides, such as Al2O3, used in established applications [2], [3]. During sintering and shaping of oxidic materials for practical applications, use of nano sized particles as starting materials can be of great advantage because of the availability of large surface areas of the nanoparticles [4].

Conventional methods for synthesizing α-Al2O3powder involve solid state thermally driven transformations from the hydrates of aluminium oxide. The extent of conversion to the corundum structure depends on the temperature and the time of thermal treatment. Total conversion occurs on heating above 1230 ° C [5].

We have previously described the use of polymeric precursor method as a means of producing alumina powders and influence of cobalt in the formation of α-Al2O3at around 1050 ° C [6]. In addition, the influence of cobalt localization in spectroscopic transitions of alumina phases was evaluated. On the other hand, the influence of particle size was not investigated.

In this way, the object of this study is to evaluate the particle size of Al2O3nanopowders prepared by means of polymeric precursor method and to study the relationship between phase transition.

Section snippets

Experimental procedure

Preparations used alumina nanoparticles that had been synthesized by means of polymeric precursor method in experiments that will be described in more detail elsewhere [6]. Summarizing, ammonium alum – NH4Al(SO4)2was first dissolved in distilled water and ammonium hydroxide – NH4OH in order to form aluminum hydroxide – Al(OH)3. With addition of citric acid anhydrous – C6H8O7, the formation of aluminum citrate takes place. The polymerization occurred upon the addition of ethylene glycol – C2H6O2

Results and discussion

Fig. 1, Fig. 2, Fig. 3 display the diffraction patterns, micro-Raman spectroscopy and transmission electronic micrographs of the Al2O3 system in three steps of phase transition.

The first step is the presence of single phase γ-Al2O3 (Fig. 1).

The second step is the phase transition with the presence of both γ-Al2O3 and α-Al2O3 phase (Fig. 2).

The third step is the presence of single phase α-Al2O3 (Fig. 3).

As indicated by Fig. 1(a), at temperatures up to 700 °C, the patterns are ascribed to an

Conclusion

The steps of phase transition γα-Al2O3were in number of three when characterized by X-ray diffraction (XRD), micro-Raman spectroscopy and transmission electronic microscopy (TEM) techniques. In the first step, in the range 800–950 °C, the synthesized powder is constituted of γ-Al2O3 according to results presented in XRD patterns. In this step, the cubic symmetry of γ-Al2O3 structure (space group Fd3m) leads to a low signal of micro-Raman spectra with particle size about 10 nm, as observed in

Acknowledgments

The authors gratefully acknowledge the financial support of the Brazilian financing agencies FAPESP (proc. 00/12353-5)/CEPID, CNPq (proc. 150330/2003-0) and Fundação Araucária.

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