Preparation of composite powders containing TiB2 and ZrB2 particles through combustion synthesis of TiO2–ZrO2–B2O3–Mg system
Introduction
Transition metal borides like TiB2 and ZrB2 with ultrahigh melting point, superior thermal stability and high hardness are used as refractory components, hard phase and abrasives in cermets and reinforcements in metal and ceramic matrix composites [[1], [2], [3], [4], [5]]. These compounds are produced by a large variety of methods such as carbothermal reduction [[5], [6], [7]], sol-gel [8], ball milling [3,[9], [10], [11]], direct reaction of pure elements [12] and combustion synthesis [1,2,7,[13], [14], [15], [16], [17]]. Carbothermal method is the most popular synthesis route. However, the reaction is extremely endothermic and needs prolonged heating above 1800 °C that increases the production cost [6,7]. Alternatively, combustion synthesis (CS) is a fast and easy-echo process that usually consumes metallic oxides as raw materials [1,13,15,[18], [19], [20], [21]]. Oxides often are mixed with Mg, Al or Zn elemental powders and an exothermic reduction reaction occurs [20,22]. The MgO and ZnO unwanted phases then are removed by acid leaching [10,13,17,19,20]. Sometimes, the liberated heat causes the reaction to propagate along the unreacted mixture in a self-sustaining mode, that it is called self-propagating high-temperature synthesis (SHS) [23].
In recent years many studies have focused on the production of titanium borides and zirconium boride powders by CS process. Jinyun et al. [15] considered the production of TiB2 powder via combustion synthesis of the B2O3–TiO2–Mg system in air atmosphere. They showed TiB2 along with small concentrations of TiO2 and TiN were detected in the combustion product after acid leaching. Moreover, the addition of TiB2 as a diluent agent in the range of 0 and 20 wt% decreased adiabatic temperature from 3110 K to 2896 K. Pure TiB2 was obtained through microwave assisted self-propagating high-temperature synthesis of the TiO2–B2O3–Mg system in Ar environment and subsequent HCl leaching by Ghangari et al. [14]. They stated Mg loss during the reaction led to developing Mg3B2O6 by product. However, the addition of 40 wt% excess Mg eliminated unwanted phases and decreased the product grain size. Weimin et al. [16] studied the chemistry of the reaction in the TiO2–B2O3–Mg mixture and proposed a reaction sequence for this system. They also managed to produce nearly pure TiB2 by this method. All these studies confirmed TiB2 can successfully be synthesized by the SHS reaction of the TiO2–B2O3–Mg system. Meanwhile, the results of those studies considered synthesis of ZrB2 via SHS reaction show some contradictions. For example, Zheng et al. [17] successfully prepared ZrB2 powders with high purity by the combustion synthesis of Mg–ZrO2–B2O3 system. In contrary, Gadakary et al. [1] showed a large amount of ZrO2 remained in the combustion product of the ZrO2–H3BO3–Mg system after acid leaching. Consequently, they added extra Mg and boric acid to the reaction product and decreased the amount of ZrO2 after double synthesis process. Similar results were reported by Akgünet al. [9] as they could not remove the residual ZrO2 from the CS products of the ZrO2–H3BO3–Mg mixture even by 30 h pre-milling. Cordova and Shafirovich [13] showed the addition of 20 wt% Mg and 30 wt% NaCl as a diluent agent decreased the concentration of oxygen in the ZrB2 obtained via SHS reaction of the ZrO2–H3BO3–Mg system. Theses contradictions in the reporting outcomes may be due to using different reactant's particle size, green compact's density, compact size, reaction mechanism, mixing method, etc. Therefore, further studies will help to confirm the results of previous investigations. Accordingly, the first part of this research was conducted on the synthesis of TiB2 and ZrB2 by the SHS reaction in the TiO2–B2O3–Mg and ZrO2–B2O3–Mg systems, respectively. The primary results were consistent with those reported by Gadakary et al. [1] and confirmed that ZrO2 remained in the products after acid leaching. It was assume this may be due to the weak exothermic nature of the ZrO2–B2O3–Mg system. Providing that this hypothesis was true, coupling this system with a strong one like the TiO2–B2O3–Mg system could be helpful in increasing the exothermicity of the reaction and eliminating ZrO2 content in the final product. Therefore, in the next step, both systems were mixed to attain two purposes: (a) to prepare composite powders containing both ZrB2 and TiB2 to utilize the benefits of both borides and, (b) to decrease the amount of ZrO2 remained in the products most probably through altering the reaction mechanism. Thereupon, the present study deals with the results of combustion synthesis reaction preformed on the (2-x)TiO2‒xZrO2‒2B2O3‒(10 + y)Mg system. In order to have composites with various TiB2/ZrB2 ratios, the x-value altered between 0 and 2. Moreover, several concentrations of excess Mg between 0 and 25 wt% (0 ≤ y ≤ 2.5 mol) were added to obtain more pure products.
Section snippets
Materials and method
Titanium oxide (>99%, < 1 μm), zirconium oxide (>99%, 2–5 μm), boron oxide (>98%, < 1–2 μm) and magnesium ((>99%, 50–200 μm) powders were used as starting materials. Then, different mixtures of these powders were prepared according to the following reaction.
In this reaction, the x-value varied between 0 and 2 with 0.5 intervals to control the TiB2/ZrB2 ratio in the final composites. Moreover, different amounts of excess Mg (y = 0, 0.5, 1.5
Thermodynamic considerations and reaction front velocity
In the combustion synthesis, the heat liberated from an exothermic reaction leads to obtain the final product. The amount of heat released by a reaction can theoretically be estimated using thermodynamic functions. The mentioned heat increases the temperature of the products. In the adiabatic condition, the maximum temperature (Tad) can be calculated by the following equation [23,24]:where, ΔH(298) is the difference between the molar formation enthalpy of the
Conclusion
The aim of the present study was characterization of features and the quality of the products obtained through combustion synthesis of the (2-x)TiO2–xZrO2–2B2O3–(10 + y)Mg system. The x-value varied between 0 and 2 to attain composites with different TiB2/ZrB2 ratios. Moreover, various concentrations of excess Mg were added (0 ≤ y ≤ 2.5 mol) to the reaction as the diluent agent. The main results are summarized as follows:
- 1.
Adiabatic temperature decreased with increasing both x and y values from
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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