Formation mechanism of ZrB2-Al2O3 nanocomposite powder by mechanically induced self-sustaining reaction

1 Formation mechanism of ZrB2–Al2O3 nanocomposite powder by mechanically induced self-sustaining reaction M. Jalaly 1,*, M.Sh. Bafghi 1, M. Tamizifar 1, F.J. Gotor 2 1. School of Metallurgy and Materials Engineering, Iran University of Science & Technology (IUST), Narmak, Tehran 16846-13114, Iran 2. Instituto de Ciencia de Materiales de Sevilla (CSIC-US), Americo Vespucio 49, 41092 Sevilla, Spain *Corresponding Author, Email: [email protected] Tel: +989127387902, Fax: +982177240480 Abstract ZrB2–Al2O3 nanocomposite powder was produced by aluminothermic reduction in Al/ZrO2/B2O3 system. In this research, high energy ball milling was used to produce the necessary conditions to induce a mechanically induced self-sustaining reaction (MSR). The ignition time of the composite formation was found to be about 13 minutes. The synthesis mechanism in this system was investigated by examining the corresponding sub-reactions as well as changing the stoichiometry of reactants. Thermal behavior of the system was also studied. Keywords: Zirconium diboride; Mechanosynthesis; Mechanism; Nanocomposite 1. Introduction ZrB2 is a well-known material belonging to the Ultra-High Temperature Ceramics class [1]. The distinctive features of this class of materials make them good selections for use in various applications such as hypersonic flights, atmospheric re-entry vehicles, and rocket propulsion systems. Zirconium diboride has attracted much attention because of 2 its superior oxidation resistance, which is a consequence of the stability of ZrO2 formed on these materials at high temperatures in oxidizing atmospheres [2]. Ceramic matrix composites consisting of high temperature borides and some other industrial ceramic materials have been of interest in recent years. Alumina is one of the most common used ceramics in various applications such as grinding media, cutting tools, crucibles, tube furnaces and liners, owing to its several intrinsic characteristics like high hardness, high melting point, good chemical inertness, high wear resistance, and low cost. Nevertheless, some mechanical properties of alumina are not good enough for several applications. The mechanical strength of these materials can significantly improve by the addition of strong compounds like zirconium diboride [3]. TiB2–Al2O3 composite that was prepared by mixing alumina and diboride powders, showed excellent mechanical properties [4]. The most important applications of ZrB2 are in the high-temperature fields where it is used as a refractory material; hence, the effect of alumina on the oxidation rate of ZrB2 ceramic should be essentially investigated. Few reports exist on the high temperature oxidation resistance of ZrB2– Al2O3 composites. Recently, Li et al. [5] examined the oxidation kinetics of Al2O3/ZrB2/ZrO2 composite prepared by mixing and hot-pressing. Self-propagating high temperature synthesis (SHS) has recently been used extensively for the preparation of refractory materials such as carbides, silicides, nitrides, and various composite materials. This kind of synthesis is characterized by its significant negative enthalpy and high adiabatic temperature (Tad) of above 1800 K [6]. Literature survey shows a great attention to the metallothermic reduction of ZrO2 and B2O3 in 3 preparation of ZrB2 because of its cheaper raw material as well as the high exothermic nature of the involving self-sustaining reactions compared to the other synthesis routes [7-12]. Magnesium has been frequently used to reduce zirconium and boron oxides [7- 11] due to good feasibility of MgO leaching, although aluminium has also been employed in a few cases [3, 12] for inducing the reduction reaction in this system. Using Al as reducing agent has also the advantage of in-situ making of ZrB2–Al2O3 which is a valuable composite. When a self-sustaining reaction is induced by the high-energy ball milling of reactants after a critical milling period, called the ignition time, such a mechanochemical process is referred to as mechanically induced self-sustaining reaction (MSR) [6]. MSR and SHS are different methods, although there are common features between both processes. In contrast to the conventional SHS procedure, MSR process has the favorable side benefit of mixing of the reactants as well as the subsequent homogenization of the products together with intensive particle size reduction of both reactants and products, all just in one single step. All research works that were performed on the ZrB2 synthesis by use of Al as reducing agent, were carried out by thermally combustive SHS methods, rather than MSR reactions [3, 11]. Furthermore, reaction mechanism has not also been clearly explained. The aim of the present work, therefore, was to investigate the mechanosynthesis of ZrB2–Al2O3 powder by aluminothermic reduction using ZrO2 and B2O3 as starting materials and by means of mechanically induced self-sustaining reaction (MSR), which has not been reported so far. The mechanistic explanation of ZrB2 formation in this 4 system was also another target, which has been accomplished through the investigation of sub-reactions, as well as the study of the influence of the reactants stoichiometry. 2. Experimental The raw materials, monoclinic ZrO2 (99%, Aldrich, USA), B2O3 (98%, Fluka, Germany), and Al (99%, Aldrich, USA) powders, were used to produce zirconium diboride–alumina composite powder. Elemental zirconium (99%, Alfa Aesar, Germany) and boron (97%, amorphous, Aldrich, USA) were also used for studying a sub-reaction. The starting materials were subjected to the high energy ball milling in a modified planetary ball mill (Pulverisette7, Fritsch, Germany). The rotational speed and ball-to- powder mass ratio were 600 rpm and 30:1, respectively. The milling vial and balls (15 mm) were made of hardened chromium steel. All milling experiments were conducted under 5 bar of high-purity argon gas. The vial was purged with argon gas several times, and the desired pressure was adjusted before the start of the milling. The connection of the vial to the gas cylinder during the milling experiments was maintained by a rotating union and a flexible polyamide tube. The pressure change vs. time was monitored by a SMC solenoid valve (model EVT307-5DO-01F-Q, SMC Co ., Tokyo, Japan) to record the ignition time. A sharp peak due to the pressure rise appears when the MSR reaction occurs. The position of this peak represents the ignition time. The system used in this work has already been shown elsewhere [13]. Thermal behavior of the as-blended mixture was studied by differential scanning calorimetry (DSC) in a TA Instrument Q600 analyzer using a constant heating rate of 20°C/min from room temperature to 1400°C. The DSC measurement was done under a 5 flowing helium atmosphere. Furthermore, isothermal annealing of mixtures was carried out at different temperatures for 30 min under a flowing argon atmosphere at a pressure of 1 bar in a horizontal tubular furnace (IGM1360 model no. RTH-180-50-1H, AGNI, Germany). Structural features of the samples were investigated using X-ray diffraction (XRD) analysis by means of a PANalitycal X'Pert diffractometer (45 kV, 40 mA) with Cu Kα radiation (λ=0.15406 nm) XRD analyser. The crystallite size of sample was estimated by broadening analysis of XRD peaks using Williamson–Hall formula [14]. Scanning electron microscopy (SEM) images were obtained by a Hitachi S-4800 SEM- Field Emission Gun microscope. Transmission electron microscopy (TEM) images were taken using a 200 kV Philips CM200 microscope equipped with a SuperTwin objective lens and a tungsten filament (point resolution Ø=0.25 nm). Powdered samples were dispersed in ethanol, and droplets of the suspension were deposited onto a holey carbon film. 3. Results and Discussion 3.1. Mechanosynthesis The initial purpose of the present work was to synthesize zirconium diboride-alumina composite. In order to achieve this goal, the following reaction was considered: ZrO2 + B2O3 + (10/3)Al → ZrB2 + (5/3)Al2O3 (1) ΔG°298= –722 kJ, ΔH°298= –746 kJ, Tad ≈ 2330 K 6 Stoichimetric amounts of starting materials (Table 1) were milled under aforementioned conditions. The change of internal pressure of the vial versus milling time is shown in Fig. 1. The large pressure rise observed at approximately 13 min milling demonstrates the occurrence of a highly exothermic MSR reaction. Fig. 2 shows the XRD patterns of Al, ZrO2 and B2O3 powder mixture as-received and after different milling times. The XRD pattern of as-blended mixture included only the sharp peaks of Al (ICCD PDF #03-0932), ZrO2 (ICCD PDF #13-0307) and B2O3 (ICCD PDF #06-0297) compounds, which were converted to ZrB2 (ICCD PDF #34- 0423) and Al2O3 (ICCD PDF #11-0661) after 13 min milling (just after ignition). Small amounts of Al and ZrO2 were still remained which is a typical behavior of mechanosynthesis reactions, especially in the MSR situations [12], due to the entrapment of some powders in the dead zones of milling vial. As Fig. 2 shows, a tetragonal ZrO2 phase (nominated as T-ZrO2) was detected among the remaining materials in the sample that was milled for 13 min (the ignition point). In fact, zirconium dioxide transformed from its monoclinic crystal structure at room temperature to a tetragonal form, due to the significant temperature rise resulted from the highly exothermic MSR reaction. Because the cooling rate after ignition is very high, there are traces of transformed zirconia remained in the tetragonal state at room temperature. As shown in Fig. 2, a very slight amount of elemental Zr was detected in the XRD pattern for the sample milled for 13 min, most likely due to the incomplete reaction of reduced zirconium with boron. Elemental boron, which may remain due to the incomplete reaction with zirconium, could not be detected, most likely due to its very little quantity and/or its amorphization because of rapid cooling after ignition. 7 By increasing the milling time, powders trapped in the dead zones can be gradually subjected to the ball impacts and locally reacted. Fig. 2 shows the XRD patterns of samples milled for 1 and 2 hours. A slight amount of zirconium dioxide was observed after 1 h milling, but it was entirely disappeared after 2 h milling. The crystallite size of ZrB2 after 2 h ball milling was calculated to be approximately 83 nm. The electron microgarphs of a sample milled for 2 h are shown in Fig. 3. As these micrographs shows, the product powder contains agglomerates which are composed of particles of sub-micrometric and nanometric sizes with a combination of semi-spherical and platelet morphology. The TEM micrograph in Fig. 3 (b) shows dark polyhedron ZrB2 particles surrounded by bright alumina nanoparticles. A bigger, single crystal of hexagonal ZrB2 along with alumina nanoparticles is indicated in a larger magnification in Fig. 3 (c). 3.2. Reaction mechanism 3.2.1. During milling The mechanistic study of the formation of ZrB2–Al2O3 composite during milling in the present work can be divided into two sections. The overall reaction (Rea.1) was initially investigated by the study of involved sub-reactions. At the second step, the effect of boron oxide amount on the nature of the reaction was studied. To form zirconium diboride, zirconium and boron elements must be obtained through the reduction of their corresponding oxides. The sub-reactions involved in this system can be written as follows: 8 2Al + B2O3 → 2B + Al2O3 (2) ΔG°298= –389 kJ, ΔH°298= –404 kJ, Tad ≈ 2310 K (4/3) Al + ZrO2 → Zr + (2/3) Al2O3 (3) ΔG°298= –15 kJ, ΔH°298= –20 kJ, Tad ≈ 500 K Zr + 2B → ZrB2 (4) ΔG°298= –318 kJ, ΔH°298= –322 kJ, Tad ≈ 3200 K In the above reactions, aluminum reduces boron oxide and zirconium oxide to yield elemental boron and zirconium. ZrB2 can be subsequently formed by the reaction between these two elements. According to the adiabatic temperatures (Tad) and enthalpy values of the above reactions, Reas. 2 and 4 meet the required conditions to satisfy Merzhanov’s criterion [12] to proceed in a self-sustaining manner. However, the reduction of ZrO2 by Al (Rea. 3) does not satisfy these conditions, and it is hence anticipated to gradually proceed as an ordinary reaction. When Al, B2O3 and ZrO2 are all present in one system, Al reduces boron oxide in a selfsustaining manner to yield elemental boron together with the release of a great deal of heat; this heat increases the system temperature inside the milling vial to such a level which can trigger the reaction between Al and zirconia to form elemental Zr. Consequently, ZrB2 can be synthesized by the reaction between these two elements. The highly exothermic reaction of the reduced B and Zr (Rea. 4) can exert an additional amount of heat to the system, thus causing the further ZrO2 reduction to proceed more easily. All these three reactions occur simultaneously; thus only one peak is observed in the pressure-time graph of overall reaction (Rea. 1). This postulation can be confirmed 9 by the existence of a trace amount of residual zirconium as the XRD pattern just after the ignition time (Fig. 2) shows. Although elemental boron was not detected in the XRD pattern, it may be present in consequence of incomplete reaction of ZrB2 formation and remaining elemental zirconium. Boron can be possibly interpreted to have been present in an amorphous form, if this possible mechanism is factual. In order to examine the proposed mechanism, these sub-reactions were separately studied. Stoichiometric amounts of Al/B2O3, Al/ZrO2 and Zr/B were mixed according to the Reas. 2-4 and milled under the same conditions. For Rea. 2, the ignition time was found to be about 25 min (Fig. 4), which is a little longer than the ignition time of Rea. 1, due to lower exothermicity of Rea. 2. Reaction 1 includes Al/B2O3 ignition along with Zr/B ignition which can cause the system to be more rigorous than reaction1 (only Al/B2O3 ignition). Therefore, the ignition time is shorter in the overall system. The XRD patterns of initial materials along with that of sample milled up to the ignition point (25 min) are shown in Fig. 5. It can be seen that the ignited sample contains aluminum oxide, which is an indication of the feasibility of the reduction of boron oxide by Al. Small amounts of the starting materials were also observed, perhaps due to their entrapment in the dead zones of the vial, as mentioned earlier. However, elemental boron was not detected in the XRD pattern, most likely due to its amorphization as a result of the high temperature rise during the MSR reaction and high cooling rate after ignition. The formation of amorphous boron as a result of metallothermic reduction of boron oxide has already been described in the literature [15,16]. 16 treatments, which can make the conditions suitable for Zr and Al to form zirconium aluminides. 4. Conclusion High energy ball milling technique was successfully applied for mechanosynthesis of ZrB2–Al2O3 nanocomposite by means of the aluminothermic reduction. Synthesis in Al/B2O3/ZrO2 system found to possess a self-sustaining nature having an ignition time of 13 min. Examination of the sub-reactions revealed that boron oxide is easily reduced by Al, while Al cannot reduce ZrO2 to Zr in a self-sustaining manner. It was concluded that the great deal of heat generated through the reduction of boron oxide by Al together with the large amount of heat released by the reaction between the reduced B and Zr, is capable of activating the reduction of ZrO2. This mechanism was found to be in good agreement with the general trend observed during thermal treatment of the system with an exception of formation of intermediate zirconium aluminides in the latter case. The amount of boron oxide was recognized to be a critical parameter which can alter the magnitude of reaction heat and consequently cause the system to undergo a transition from a gradual state to a self-sustaining one. Acknowledgements This work was financially supported by the Spanish government under grant No. MAT2011-22981, which was financed in part by the European Regional Development Fund of 2007-2013. The work has been based on an initiation proposed by the School of Metallurgy and Materials Engineering of Iran university of science and technology as the Ph.D thesis subject of Mr. M. Jalaly who was granted the permission to accomplish 17 his experiments with the facilities and co-supervision of Prof. F.J. Gotor in Instituto de Ciencia de Materiales de Sevilla, Sevilla, Spain. References [1] M.A. Aviles, J.M. Cordoba, M.J. Sayagues, F.J. Gotor, Mechanochemical synthesis of Ti1- xZrxB2 and Ti1-xHfxB2 solid solutions, Ceramics International 37 (2011) 1895–1904. [2] W.G. 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Takacs, Self-sustaining reactions induced by ball milling, Progress in Materials Science 47 (2002) 355–414. [13] F.J. Gotor, M. Achimovicova, C. Real, P. Balaz, Influence of the milling parameters on the mechanical work intensity in planetary mills, Powder Technology 233 (2013) 1–7. [14] G. K. Williamson, W. H. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Metallurgica 1 (1953) 22-31. [15] V. Sundaram, K. V. Logan, R. F. Speyer, Aluminothermic reaction path in the synthesis of a TiB2-Al2O3 composite, Journal of Materials Research 12 (1997) 1681-1684. [16] J. Wang, Y. Gu, Z. Li, W. Wang, Z. Fu, Synthesis of nano-sized amorphous boron powders through active dilution self-propagating high-temperature synthesis method, Materials Research Bulletin 48 (2013) 2018-2022. 19 Fig. 1. Pressure inside the vial versus milling time for Al/ZrO2/B2O3 system. Fig. 2. X-ray diffraction patterns of as-blended and milled samples of Al/ZrO2/B2O3 system. 20 Fig. 3. Electron micrographs of Al/ZrO2/B2O3 system after 2 h milling, (a) SEM and (b) TEM images. Fig. 4. Pressure inside the vial versus milling time for Al/B2O3 and Zr/B systems. 21 Fig. 5. X-ray diffraction patterns of initial and milled samples of Al/B2O3 system. Fig. 6. X-ray diffraction patterns of initial and milled samples of Al/ZrO2 system. 22 Fig. 7. X-ray diffraction patterns of initial and milled samples of Zr/B system. Boron cannot be observed among initial mixture due to its amorphous state. Fig. 8. Calculated adiabatic temperature and room temperature enthalpy of Rea. 5 series versus B2O3 molar amount. 23 Fig. 9. X-ray diffraction patterns of the different compositions (y = 0.7–0.9 in Rea. 5) after milling. Fig. 10. DSC curve of as-blended mixture in Al/ZrO2/B2O3 system. 24 Fig. 11. X-ray diffraction patterns of the blended powders in Al/ZrO2/B2O3 system after heating at 700, 950 and 1050°C (30 min dwelling at maximum temperature and then cooling to room temperature). Fig. 12. X-ray diffraction patterns of the blended powders in Al/ZrO2/B2O3 system after heating at 1150, 1300, 1400 and 1500°C (30 min dwelling at maximum temperature and then cooling to room temperature). 25 Table 1. The weight (g) of components involved in the Reaction 5 for the different y values. y 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 B2O3 - 0.127 0.238 0.335 0.420 0.495 0.563 0.624 0.679 0.729 0.774 ZrO2 2.435 2.260 2.108 1.975 1.858 1.754 1.661 1.578 1.502 1.434 1.371 Al 0.712 0.759 0.801 0.837 0.869 0.897 0.922 0.945 0.965 0.984 1.001