Mechanosynthesis of nanocrystalline ZrB2-based powders by mechanically induced self-sustaining reaction method

1 Mechanosynthesis of nanocrystalline ZrB2-based powders by mechanically induced self-sustaining reaction method 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, Americo Vespucio 49, 41092 Sevilla, Spain *Corresponding Author, Email: [email protected] Tel: +989127387902, Fax: +982177240480 Abstract Preparation of nanocrystalline ZrB2–based powder by aluminothermic and magnesiothermic reductions in M/ZrO2/B2O3 (M = Al or Mg) systems was investigated. In this research, high energy ball milling was employed to persuade necessary conditions for the occurrence of a mechanically induced self-sustaining reaction (MSR). The happening of MSR reactions was recorded by a noticeable pressure rise in the system during milling. Ignition times for ZrB2 formation by aluminothermic and magnesiothermic reductions were found to be 13 and 6 minutes, respectively. Zirconium diboride formation mechanism in both systems was explained through the analysis of the relevant sub-reactions. Keywords: Zirconium diboride; Mechanosynthesis; Reaction mechanism; Ball milling 1. Introduction Ultra-high temperature materials (UHTMs) are a group of materials that are chemically and physically stable at very high temperatures in extreme reactive environments. Several borides, carbides, and nitrides of group IV and V elements are 2 anticipated to be potentially suitable for such applications. All borides, carbides, and nitrides in this category have similar properties, such as high melting point and hardness due to the strong covalent bonding. However, the borides tend to have higher thermal conductivity compared to the carbides and nitrides, which gives them better thermal shock resistance. Diboride zirconium has been found to be a suitable material for high-temperature applications because of its excellent properties such as high melting point (greater than 3000°C), high hardness and strength at room and elevated temperatures, good thermal and electrical conductivities, low thermal expansion coefficients, chemical inertness, good oxidation resistance, and high thermal stability [1]. The distinctive features of these materials make them good selections to be employed in various applications from industrial applications such as cutting tools, wear resistant coatings, molten metal crucibles and high temperature electrodes to military ones such as hypersonic flights, atmospheric re-entry vehicles, and rocket propulsion systems. Among the borides, ZrB2 and HfB2 have attracted the most attention because of their superior oxidation resistance, which is a consequence of the stability of ZrO2 and HfO2 oxide formed on these materials at high temperatures in oxidizing atmospheres [2]. It is of great importance to select the proper method of synthesis because of its significant effect on the properties of the products. Zirconium diboride can be synthesized by various techniques including solid state methods, liquid state methods and gas state methods [3]. Solid state synthesis of ZrB2 can be achieved through several different routes including the reaction of elemental Zr and B [1,4-6]; borothermic reduction of ZrO2 [7,8]; carbothermic reduction of ZrO2 and B2O3 [9,10]; reduction of ZrO2 by boron carbide [11-13]; and metallothermic reduction of ZrO2 and B2O3 [14-20]. 3 Literature review [3] shows that metallothermic reduction route has been more interesting due to its cheaper raw material as well as the high exothermic nature of the involving self-sustainable reactions. Research works concerning the metallothermic process of ZrB2 synthesis based on self-propagating high temperature synthesis (SHS) reactions have been performed by various methods, such as reactive hot pressing [6], arc resistive ignition [17], and high energy ball milling [10,16,20]. Highly exothermic chemical reactions can be induced by high energy ball milling leading to a selfsustaining reaction within the milling vial after the elapse of a critical milling period, called the ignition time. This kind of process, known as a mechanically induced selfsustaining reaction (MSR) [21], is similar to thermally ignited SHS systems. In contrast to the conventional SHS procedure, MSR process has the kinetically favorable side benefit of mixing of the reactants as well as the subsequent homogenization of the products together with their extremely particle size reduction in one single step [4]. Mechanically induced self-sustaining reaction processes are often accomplished at short times due to the instantaneous nature of involving SHS reactions. ZrB2 synthesis by use of high energy ball milling has been examined by a group of researchers. Setoudeh and Welham [16] studied ZrB2 formation by conducting the magnesiothermic reduction reaction by milling, using ZrO2 and B2O3 as Zr and B bearing source. In this study, no MSR process occurred and ZrB2 was synthesized after 15 hours of milling which is comparable to a normal mechanical alloying. Akgun et al. [20] also attempted to produce zirconium diboride by mechanochmical treatment of ZrO2-B2O3-Mg. Zirconium diboride has been obtained after 30 hours of milling in their effort. 4 Therefore, the intention of the present research work was to investigate the mechanosynthesis of ZrB2 powder by use of ZrO2 and B2O3 as starting material and application of mechanically induced self-sustaining reactions (MSR) of both aluminoand magnesiothermic reduction which has not been ever reported. The self-sustaining reactions are as follows: ZrO2 + B2O3 + (10/3)Al → ZrB2 + (5/3)Al2O3 (1) ΔG°298= –722 kJ, ΔH°298= –746 kJ, Tad ≈ 2330 K ZrO2 + B2O3 + 5Mg → ZrB2 + 5MgO (2) ΔG°298= –931 kJ, ΔH°298= –960 kJ, Tad ≈ 2900 K 2. Material and methods The stoichiometric amounts of monoclinic ZrO2 (99%, Aldrich, USA), B2O3 (98%, Fluka, USA), Al (99%, Aldrich, USA), and Mg (99%, Riedel-deHaen, Germany) powders were used as starting materials to synthesize ZrB2. High energy ball milling was conducted in a planetary ball mill (Pulverisette7, Fritsch, Germany) at 600 RPM using a ball-to-powder mass ratio of 30:1. The milling vial and balls (15 mm) were made of hardened chromium steel. Milling experiments were performed under highpurity argon gas of 5bar pressure. The vial was purged with argon gas several times, and the determined pressure was adjusted before the start of the milling. The vial was connected to the gas cylinder during the milling experiments by a rotating union and a flexible polyamide tube. The pressure changes vs. time were monitored by a SMC solenoid valve (model EVT307-5DO-01F-Q, SMC Co ., Tokyo, Japan) to record the ignition occasion. Once the MSR reaction was occurring, a sharp peak emerged in this diagram due to the pressure rise resulted from temperature increase. The position of this 5 peak shows the ignition time. The system used in this work has already been shown elsewhere [22]. X-ray powder diffraction (XRD) analysis was performed to determine the structural changes of powder during milling experiments. A Panalitycal X'Pert diffractometer (45 kV, 40 mA) with Cu Kα radiation (λ=0.15406 nm) was used for XRD analysis. The XRD patterns were recorded in the 2θ range of 10–80° (step size 0.05° and time per step 1s). The crystallite sizes of samples were estimated by broadening analysis of XRD peaks using Williamson–Hall formula: ε Dθsin λ45.0 θcot 2 β (3) where β is the peak breadth in mid height, θ is the Bragg diffraction angle, λ is the Xray wave length, D is the average crystallite size and ε is the mean value of internal strain. 3. Results and Discussion 3.1. Aluminothermic reduction Fig. 1 shows the variation of gas pressure inside the vial versus milling time in Al/B2O3/ZrO2 system (reaction 1). A great pressure rise can be clearly observed at about 13 min milling representing a great exothermic reaction. It implies that the MSR reaction occurs at this point and the ignition time is around 13 min. The XRD patterns of Al, ZrO2 and B2O3 powder mixture as-received and after different milling times are shown in Fig. 2. The XRD pattern of as-received powder mixture included only the sharp peaks of Al (ICCD PDF #03-0932), ZrO2 (ICCD PDF #13-0307) and B2O3 (ICCD PDF #06-0297). As can be seen the products of reaction (1), ZrB2 (ICCD PDF #34-0423) and Al2O3 (ICCD PDF #11-0661), were completely developed with sharp peaks after 13 min milling, although traces of Al and ZrO2 were 6 still observed. This is a typical behavior in mechanosynthesis reactions, especially in MSR systems [21], due to entrapment of some powders in dead zones of milling vial. With increasing the milling time, unprocessed powders gradually find the chance of being subjected to the ball incidences and gaining sufficient energy to be locally reacted. There was no peak in Fig.1 after 13 min indicating that the amount of unreacted agglomerates and/or entrapped particles is negligibly small. They can be locally ignited so that temperature and pressure do not sensibly rise. As seen in Fig. 2, a specific kind of ZrO2 (designated as T-ZrO2) has been detected among unreacted materials in the samples milled for 13 min (ignition point). Zirconium dioxide has normally a monoclinic crystal structure at room temperature. It transforms at high temperatures to tetragonal type. Hence, it seems that a portion of starting monoclinic zirconia has transformed to tetragonal one due to the great increase of temperature resulted from highly exothermic MSR reaction. As Fig. 2 shows, a very slight amount of elemental Zr can be recognized in the XRD pattern related to the sample after 13 min milling. The Fig. 2 shows the XRD patterns of samples milled for 1 and 2 hours. Although slight amounts of zirconium dioxide can be observed after 1 h milling, it is obvious that the peaks of remaining Al and ZrO2 completely disappeared after 2 h milling. The corresponding peaks of ZrB2 and Al2O3 were broadened and their intensity gradually reduced during long-term ball milling suggesting the introduction of lattice defects into the crystal structures and decreasing the grain size of the product into nanoscales. The crystallite size and internal strain of ZrB2 after 2 h ball milling were calculated to be around 83 nm and 0.65 %, respectively. The mechanism of ZrB2 formation in the present system can be explained as follows, where the main reaction seems to have the following three sub-reactions: 7 2Al + B2O3 → 2B + Al2O3 (4) ΔG°298= –389 kJ, ΔH°298= –404 kJ, Tad ≈ 2310 K (4/3) Al + ZrO2 → Zr + (2/3) Al2O3 (5) ΔG°298= –15 kJ, ΔH°298= –20 kJ, Tad ≈ 500 K Zr + 2B → ZrB2 (6) ΔG°298= –318 kJ, ΔH°298= –322 kJ, Tad ≈ 3200 K Aluminum reduces boron oxide and zirconium oxide to form elemental boron and zirconium. ZrB2 can be subsequently formed by the reaction between these two elements. With regard to the adiabatic temperatures (Tad) of above equations, reactions 4 and 6 possess the necessary conditions to satisfy Merzhanov criterion [20] to proceed in an SHS manner. But, the reduction of ZrO2 by Al (reaction 5) does not satisfy this condition, i.e. aluminum cannot thermodynamically reduce zirconia in an SHS manner. The reduction behavior of B2O3 and ZrO2 by Al and Mg can be compared in the Ellingham diagram as shown in Fig. 3. It is seen from this diagram that Al-Al2O3 line approximately coincides that of Zr-ZrO2, especially at low temperatures. Consequently, Al does not have the sufficient thermodynamic ability to dissociate zirconia. When Al, B2O3 and ZrO2 are simultaneously present in the system, Al reduces boron oxide in an SHS manner to yield elemental boron together with the release of a giant amount of heat which 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. Then, ZrB2 can be synthesized by the reaction between these two elements. This approach can be confirmed by the presence of a slight amount of remaining elemental zirconium in the XRD pattern exactly after ignition time. Great negative values of ΔG° and ΔH° of the following reaction can provide further support for the mentioned mechanism. 8 3ZrO2 + 10B = 3ZrB2 + 2B2O3 (7) ΔG°298= –221 kJ, ΔH°298= –219 kJ, Tad ≈ 770 K Thereafter, B2O3 produced in the above reaction reduces again with aluminum. All these reactions can occur instantaneously if a suitable condition is provided. In order to examine the proposed mechanism, sub-reactions were studied. The MSR manner of the reaction between Zr and B has been already confirmed [1, 3] and hence, this sub-reaction was not dealt with again in the present work. The stoichiometric amounts of Al/B2O3 and Al/ZrO2 according to the reactions 4 and 5 were mixed and milled in the same conditions. For reaction 4, the ignition time obtained from the corresponding pressure-time curve (Fig. 1) was found to be about 25 min, which is longer than the ignition time of reaction 1 due to the lower exothermicity of reaction 4. Fig. 4(a) shows the XRD pattern of un-milled and milled Al-B2O3 powder mixtures. It can be seen that powder mixture after ignition included Al2O3 and a little remaining initial materials. Boron element cannot be observed in the pattern most likely due to its amorphization as a result of the high temperature rise and high cooling rate in the MSR reaction. For reaction 5, no pressure rise was observed during the long time of milling as expected from the Ellingham diagram (Fig. 3). Fig. 4(b) shows the XRD pattern of un-milled and milled Al-ZrO2 powder mixtures until 4 h. It is evident that the only phenomenon is peak broadening of initial materials and no reaction occurred during the long time of milling, indicating that Al cannot individually reduce zirconia in this condition, as mentioned above, due to the very small enthalpy of reaction 5. Introduction of boron oxide to this system can motivate the decomposition of zirconia because of the addition of a great deal of heat to the system. 9 It should be pointed out that the product resulted from aluminothermic reduction is a composite of ZrB2–Al2O3. Although Al2O3 can be removed by an appropriate NaOH leaching process [23], it should be studied in detail elsewhere. The principal aim of the present work has been the examination of the feasibility of MSR synthesis of ZrB2 during high energy milling technique. Furthermore, there are some research works focused on the formation of diboride–alumina composites as attractive materials in some applications [18, 19, 24]. 3.2. Magnesiothermic reduction The change of internal pressure of vial versus milling time for the Mg/B2O3/ZrO2 system (reaction 2) was shown in Fig. 1. The emergence of an enormous peak at about 6 min demonstrates the occurrence of an intense exothermic reaction. The much higher pressure rise which also happens sooner in the case of magnesiothermic reaction (reaction 2) compared to that of aluminothermic reaction (reaction 1) is a reasonable consequence of the fact that the former reaction is noticeably more vigorous than the latter one due to its higher heat generation capacity (enthalpy). XRD patterns of initial mixture and milled samples are illustrated in Fig. 5. In the pattern related to initial powder mixture only the peaks of Mg (ICCD PDF #35-0821), ZrO2 and B2O3 compounds can be seen. After the elapse of 6 minutes of milling (ignition time), the products of reaction 2, i.e. ZrB2 and MgO (ICCD PDF #04-0829), have been completely formed, although some traces of Mg and ZrO2 can be still observed. Local reactions between remaining materials can occur at the extended milling time. Fig. 5 shows the XRD pattern of samples milled for 2 and 3 h. The disappearance of the remaining compounds after 3 h milling is obvious. The crystallite 16 Fig. 1. The change of the pressure inside the vial versus milling time due to the occurrence of self-sustaining ignition in different systems. Fig. 2. X-ray diffraction patterns of as-received and as-milled samples of Al/ZrO2/B2O3 system. 17 Fig. 3. Ellingham diagram of Mg-MgO, Zr-ZrO2, Al-Al2O3 and B-B2O3 systems. 18 Fig. 4. X-ray diffraction patterns of initial and as-milled samples in (a) Al/B2O3 and (b) Al/ZrO2 systems. The MSR reaction occurred in (a), but no ignition was observed in (b). 19 Fig. 5. X-ray diffraction patterns of as-received and as-milled samples of Mg/ZrO2/B2O3 system. 20 Fig. 6. X-ray diffraction patterns of as-received and as-milled samples in (a) Mg/B2O3 and (b) Mg/ZrO2 systems. The MSR reaction occurred in (a), but no ignition was observed in (b).