On the formation of Al2O3 nanofibers during self-propagating high-temperature synthesis of TiO2–Al–C system in various environments

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Abstract

The present study deals with the formation of Al2O3 fibers during self-propagating high-temperature synthesis of the TiO2–Al–C system triggered in argon, oxygen, nitrogen and air environments. Results showed TiC and Al2O3 were the primary products obtained in the argon and oxygen atmospheres. However, the dissolution of nitrogen in the structure of TiC resulted in the formation of Ti(C, N) after synthesis in nitrogen and air environments. Al2O3 fibers just were extensively attained in an air atmosphere with a low partial pressure of oxygen. Long nanofibers were primarily formed at the surface of the compact, while short fibers were developed at the inner part; especially adjacent to the internal pores. The formation mechanism of Al2O3 fibers was proposed as (a) the formation of Al2O gas through reaction of Al with O2 in the environment having low oxygen partial pressure like air; (b) further reaction of Al2O vapor with O2 and the development of Al2O3; and (c) precipitation of Al2O3 in the form of fibers by vapor-solid mechanism.

Introduction

In recent years, many efforts have been conducted to synthesize advanced materials by self-propagating high-temperature synthesis (SHS) route. Various kinds of materials including borides [[1], [2], [3]], nitrides [[4], [5], [6]], carbides [[7], [8], [9]], intermetallics [10], and composites [11,12] were successfully produced by this method. In some cases, the non-equilibrium nature of the process has resulted in the formation of some compounds with particular morphologies. For example, the development of Al2O3 fibers were reported during combustion synthesis of TiO2–Al–C system and various mechanisms were proposed for their formation [[13], [14], [15], [16], [17], [18], [19]]. These fibers have extraordinary mechanical, thermal, and corrosion resistance properties. So, they can be used as reinforcement in composites, catalysis, absorption, and aerospace materials [20]. Mostaan et al. [16] reported the development of Al2O3 nanofibers during mechanically induced SHS of Al–TiO2–C–Cu system in an argon environment. They stated the difference between the growth rates of Al2O3 in various crystallographic plans was responsible for the formation of nanofibers. Xia et al. [19] indicated that Al2O3 whiskers formed in the outer surface of the TiO2–Al–C compacted mixture triggered in an argon environment. They claimed the high cooling rate experienced by the surface of the compact in comparison to its inner part was responsible for the development of Al2O3 whiskers. In another research, Amel-Farzad et al. [13,14] synthesized Al2O3 nanofibers through the SHS of the TiO2–Al–C system in the air atmosphere. They expressed nanofibers were formed by direct reaction of Al vapor with oxygen and its simultaneous condensation. In other words, the metal/gas reaction was proposed as the dominant mechanism for the formation of alumina nanofibers. Furthermore, they confirmed the proposed mechanism by adding extra Al2O3 to the primary mixture to increase the Al2O3 nanofibers yield. Both Xia et al. [19] and Amel-Farzad et al. [13,14] used the same primary mixture containing Al, TiO2 and C powders with almost similar particle size to prepare TiC/Al2O3 composites by the SHS reaction. The only difference was the synthesis atmosphere. Xia et al. [19] showed that the Al2O3 nano-fiber yield was limited to a certain parts of the compact surface when the reaction triggered in the Ar atmosphere. In contrary, a large amount of Al2O3 fibers was obtained both at the surface and inner part of the compact synthesized in the air atmosphere [13,14]. Therefore, it can be concluded that variation in the synthesis environment might possess a significant influence on combustion products of the TiO2–Al–C system and Al2O3 fiber yield. Although, many studies have investigated different aspects of the SHS reaction in the TiO2–Al–C system, to the best knowledge of the author, no published work has been considered the effect of synthesis atmosphere on the combustion products and Al2O3 fiber yield in the mentioned system. Therefore, the present study aims to deepen the understanding about the effect of synthesis atmosphere including Ar, O2, N2 and air on the combustion products of the TiO2–Al–C system, as well as the formation mechanism of Al2O3 nano-fibers by the SHS method.

Section snippets

Materials and method

The starting materials including TiO2 (Anatase, Guangxi Jinmao Titanium Co. Ltd, <45 μm); Al (Rankem, A2265, <100 μm) and C (Merck, 102184) powders were provided. The primary mixture for synthesis was prepared after weighing the powders according to the following reaction:3TiO2+4Al+3C=3TiC+2Al2O3

Mixing was performed in a planetary mill with the ball to powder ratio of five for 10 min. Then, compacts with 13 mm in diameter and 18–20 mm in height were produced by compressing the mixture in a

Product appearance

Fig. 1 presents the appearance of SHS products. In the case of the sample synthesized in the argon environment, the surface retains a black color as observed in Fig. 1(a). Fig. 1(b) shows that a mixture of black and brownish colors was obtained on the surface after synthesis in nitrogen. In contrast, the surface of the samples synthesized in the oxygen and air environments was covered with a white color product as shown in Fig. 1(c) and (d), respectively. However, Fig. 1(d) illustrates that for

Conclusion

The present study aimed to explore the SHS reaction products of the TiO2–Al–C system combusted in various environments including argon, oxygen, nitrogen, and air. Moreover, the formation mechanism of Al2O3 fibers was considered. The outstanding results can be summarized as follows.

  • 1.

    Synthesis in argon and oxygen led to the formation of TiC and Al2O3 as the primary phases in the products. However, unreacted TiO2 and C were detected as minor constituents after synthesis in the oxygen environment.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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