 Proceedings of Engineering and Technology Innovation , vol. 2, 2016, pp. XX - XX 17 Combustion Synthesis of Mullite/Metal Boride Composites Chun-Liang Yeh * , Chi-Chian Chou, Che-Han Kang Department of Aerospace and Systems Engineering, Feng Chia University, Taichung, Taiwan. Received 14 March 2016; received in revised form 27 April 2016; accept ed 12 May 2016 Abstract Formation of co mposite materia ls co mposed of mullite (3Al2O32SiO2) and transition meta l borides (NbB2 and TaB2) was studied by self-propagating high-temperature synthesis (SHS). Starting mate ria ls inc luded not only meta l o xides (Nb2O5 and Ta2O5) and boron ox- ide (B2O3) as the sources of meta llic e le ments and boron, but also Al and Si powders as the reducing agents. The evolution of mu llite fro m in situ formed SiO2 and Al2O3 and synthesis of NbB2 and Ta B2 were investigated. The effect of e xcess Si addit ion was studied on the co mbus- tion temperature, fla me -front propagation ve- locity, and phase composition of the fina l product. For formation of the Nb B2/mu llite composites, the combustion velocity about 2.5 mm/s and reaction temperature a round 1500 o C decreased slightly as the Si content increased . However, a considerable decrease in co mbustion front velocity fro m 2.74 to 1.43 mm/s and in reaction temperature fro m 1600 to 1250 o C was observed for the production of the TaB2/mullite composites . The XRD patterns of the fina l products confirmed the role of e xcess Si in the improve ment of silicothermic reduction of B2O3 and subsequent evolution of NbB2, Ta B2, and mu llite. The EDS analysis indicated an atomic proportion close to that of 3Al2O32SiO2 for the mullite grains synthesized in this study. Keywor ds : combustion synthesis , mullite, tran- sition metal borides , XRD, SEM 1. Introduction Mullite co mposites have attracted great at- tention due to the broadness of their industrial applications. Mullite with stoichiometries ranging fro m re latively silica-rich 3Al2O32SiO2 (3:2 mu llite) to alu mina-rich 2Al2O3SiO2 (2:1 mu llite) is the only stable crystalline phase in the alu minosilicate system [1]. Fo r further imp roved fracture toughness, fle xu ra l strength, and ther- ma l shock resistance, mullite-based composites containing ZrO2 and Al2O3 we re e xtensively studied [2-3]. Late ly, other cera mic addit ives such as TiC and TiB2 have gained increasing attention [4-5]. A variety of processing tech- niques with d iffe rent starting materials have been employed to prepare mu llite in e ither monolithic or co mposite forms, and they include the sol-gel method, reaction sintering, spark plasma sintering, solution combustion synthesis, self-propagating high-temperature synthesis (SHS), and therma l e xp losion. The SHS method takes advantage of highly exothermic reactions, and has the merits of low energy require ment, short processing time, simplic ity, high produ c- tivity, and structural and functional diversity of final products [6-7]. In addit ion to the y ield of Al2O3, high e xothermicity of the thermite reac - tion is a great benefit fo r the SHS process. Therefore, the thermite-based SHS technique has been utilized to prepare mullite co mbined with ceramic or intermetallic phases. This study aims to fabricate Nb B2/mu llite and TaB2/ mu llite co mposites by combustion synthesis in the SHS mode. The SHS process involves coreduction of metal o xides (Nb2O5 and Ta2O5) and B2O3 by Al and Si. Due to the evaporation loss of Si at elevated temperatures, the effect of e xcess Si addit ion was studied on the combustion temperature, fla me -front veloc- ity and phase composition. 2. Method The starting materials of this study include Nb2O5 (Stre m Che micals, < 45 m, 99.9%), Ta2O5 (Alfa Aesar, < 45 m, 99.85% ), B2O3 (Stre m Che micals, 99.9% ), Al (Showa Che mica l Co., < 45 m, 99.9% ), and Si (St re m Che micals, < 45 m, 99% ). The reactant mixtures we re formulated as 13/ 11Nb2O5 (or Ta2O5) + 26/ 11B2O3 + 6Al + (2x )Si with x = 1.0–2.0 for studying the effect of e xcess Si (x > 1.0) addi- tion. *Corresponding aut hor. Email: clyeh@fcu.edu.t w Proceedings of Engineering and Technology Innovation , vol. 2, 2016, pp. 17 - 19 18 Copyright © TAETI Sa mple co mpacts were prepared in a cylin- drical shape with 7 mm in dia meter, 12 mm in length, and 50% in re lative density by com- pressing the well-mixed reactant powders. Co mbustion experiments were performed under high-purity (99.99% ) a rgon of 0.15 MPa. The combustion propagation rate and reaction tem- perature were prec isely measured. Deta ils of the e xperimental setup and technique were reported elsewhere [8]. 3. Results and Discussion A typical co mbustion sequence illustrated in Fig. 1 is associated with formation of TaB2/mu llite fro m a powder compact with x = 1.0. It is evident that upon ignition the combus- tion front was readily formed and propagated progressively along the sample, confirming sufficiency of the reaction exothermicity. Fig. 1 A sequence of self-sustaining combustion wave For the format ion of the Nb B2/mu llite composite, the combustion velocity of about 2.5 mm/s and react ion te mperature of a round 1500 ℃ decreased slightly as the Si content e xceeded the stoichiometric a mount. This implies a re - duction in the reaction e xothermic ity, because the endothermic mu llit ization process is en- hanced. However, a considerable decrease in combustion front velocity fro m 2.74 to 1.43 mm/s and in reaction te mperature fro m 1600 to 1250 ℃ was observed for the production of the TaB2/ mullite co mposites . This was attributed to the yield of TaSi2 a lo wer e xothermic phase in comparison to TaB2. In contrast, no silic ide phases existed in the NbB2/mullite composite. Besides mullite, niobiu m borides of three phases, NbB, Nb3B4, and NbB2, we re identified in the final product. The presence of NbB and Nb3B4 as the minor phases means a defic iency of boron. This was probably caused by lack of sufficient Si to fully carry out meta llothermic reduction of B2O3. For the reactant co mpact with e xcess Si of x = 2.0, both NbB and Nb 3B4 are considerably reduced. The XRD pattern of the product synthesized fro m an e xactly stoichio metric sample is shown in Fig. 2(a). First of all, the evolution of mu llite fro m in situ formed SiO2 and Al2O3 was achieved and TaB2 was identified as the do mi- nant boride phase, confirming coreduction of Ta2O5 and B2O3 by both Al and Si. However, the presence of free Al2O3 means that the evapora- tion loss of Si brings about a shortage of SiO2. The lack of Si correspondingly caused incom- plete reduction of Ta2O5 and B2O3. For the Si-e xcess samples of x = 1.5 and 2.0, Fig. 2(b) and (c) show almost no free Al2O3 and a slight decrease in TaB for the resulting products . This implies that the degree of silicothermic reduc- tion of B2O3 was enhanced. However, e xcess Si considerably increased the amount of TaSi2, indicative of lack of boron for the formation of TaB2. Fig. 2 XRD patterns of TaB2/mullite synthesized fro m samp les with Si of (a ) x = 1.0, (b ) x = 1.5, and (c) x = 2.0 Fig. 3 SEM micrograph and EDS spectrum of NbB2/ mullite synthesized fro m a samp le of x = 1.5 Proceedings of Engineering and Technology Innovation , vol. 2, 2016, pp. 17 - 19 19 Copyright © TAETI The typical microstructure of fracture sur- face of the NbB2/mu llite co mposite synthesized fro m a sa mple of with x = 1.5 is d isplayed in Fig. 3. For those plate-like gra ins formed into a structure with rectangular opens, the EDS anal- ysis advises their atomic proportion of O:Al:Si = 63.09:28.05:8.86, wh ich matches closely with that of 3:2 mu llite. The other constituent NbB2 e xists as fine particles distributed over thin mullite plates. 4. Conclusions Preparation of NbB2/mullite and TaB2/mullite was conducted by combustion synthesis in the SHS mode involv ing metallothermic reduction of Nb2O5, Ta2O5, and B2O3. Co mbustion of the sample adopting two reducing agents (Al and Si) was more e xothermic and speedy than that with pre-added SiO2 and using Al as the only re- ductant. It was found that the Si content about 50 mo l% in e xcess of the stoichiometric quantity was required by the Si-containing sa mples to facilitate the silicothermic reduction of B2O3 to the utmost extent. As a result, the format ion of NbB2 or Ta B2 and mullite was improved. The mu llite gra ins synthesized in this study showed an atomic proportion close to that of 3Al2O32SiO2. In conclusion, the advantages of utilizing two reductants include more e xothe r- mic co mbustion, higher self-sustainability, shorter reaction time, and better phase trans- formation. Acknowledgement This research was sponsored by the Minis- try o f Sc ience and Technology of Ta iwan under the grant of MOST 104-2221-E-035-057. References [1] H. Schneider, J. Schreuer, and B. 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