Elsevier

Ceramics International

Volume 44, Issue 12, 15 August 2018, Pages 14355-14362
Ceramics International

On the self-propagating high-temperature synthesis of tungsten boride containing composite powders from WO3–B2O3–Mg system

https://doi.org/10.1016/j.ceramint.2018.05.044Get rights and content

Abstract

In this research, the self-propagating high-temperature synthesis characteristics of the 2WO3‒(1 + x)B2O3‒(9 + y)Mg system were studied using H3BO3 as the boron oxide source. The addition of extra B2O3 and Mg was considered as the main process parameter to obtain tungsten borides phases. XRD results showed that tungsten element was the only phase remained after leaching of the products obtained with the stoichiometric composition. The extra B2O3 and Mg acted as the diluent agents and decreased the adiabatic temperature and the reaction velocity as well as the products’ particle size. Moreover, tungsten boride containing composite powders were obtained. However, the type of the tungsten boride in the composites was dependent on the diluent agent. Fourier transformed infrared spectroscopy analysis showed that nearly-pure composite powders were obtained after acid leaching. Finally, based on the XRD phase analysis and differential thermal analysis results, a mechanism was suggested for the formation of the SHS products in this system.

Introduction

Nowadays, there are many attempts to synthesize different materials, including metals, ceramics, intermetallics, and composites from inexpensive raw materials via simple methods. Among these methods, self-propagating high-temperature synthesis (SHS) is a quick, easy-echo and energy-saving process, which has been used to obtain various materials for many years [1], [2], [3], [4], [5], [6], [7], [8], [9]. This process is based on the utilization of the heat released from exothermic reactions. To obtain the desired material by the SHS route, the reaction between the initial mixtures, which usually includes a metal oxide (MO) and a reducing metallic agent (i.e., Al, Mg or Zn), is initiated by a local heat source [10], [11], [12], [13], [14]. The reaction liberates a lot of heat, which in turn can assist the formation of metal carbides (MC), borides (MB), nitrides (MN) as well as intermetallics, etc. via the subsequent reactions between the reduced metal (M) with carbon, boron, nitrogen and their compounds [4], [5], [10], [15]. Up to now, many studies have been focused on the synthesis of various ceramics by the SHS process [1], [8], [10], [12]. Transition metal borides like Zr–B, Ti–B, Ta–B, V–B and especially W–B are interesting candidates, which can be obtained by this process [5], [16], [17], [18], [19]. These compounds have shown unique properties including good corrosion resistance, excellent wear properties, high melting point, chemical inertness in corrosive media and high thermal and electrical conductivity [18], [20], [21]. According to W–B phase diagram, there are four types of tungsten boride phases including W2B, WB, W2B5 or WB2 and WB4 [22]. However, in the present study, the synthesis of WB intermediate phase with the highest melting point (2800 °C) was considered as the promising target.

There are few studies on the synthesis of tungsten borides by the SHS route and its associated processes such as mechano-chemical reactions. Nasiri-Tabrizi et al. [23] investigated the effects of milling time and the excess boron oxide addition, as well as the application of carbon and magnesium as the reducing agents on the formation of WB through mechano-chemically induced SHS reactions of the WO3–B2O3–(Mg, C) system. Their results indicated that no new phase was detected after 120 h milling of the initial reactants in the presence of carbon reducing agent. However, WB was obtained after 1 h milling of WO3–B2O3–Mg system containing 30 wt% extra boron oxide. In addition, with increasing the extra B2O3 up to 150 wt%, WB and W2B5were formed as the predominant phases. Finally, MgO by-product was leached out by HCl solution and a WB‒W2B5 pure nano-composite powder was obtained.

In similar research, Bahrami-Karkevandi et al. [24] considered the stability of tungsten borides obtained via mechano-chemical reaction of the WO3–B2O3–Mg system. They indicated that W, W2B, WB and MgO were formed at the early stage of the milling process. Then, WB disappeared after 60 min milling. After that, no phase transformation was detected with increasing the milling time to 1800 min. They also removed the MgO by acid leaching to obtain W‒W2B composite powder.

Yeh and Wang [18] have prepared the WB and W2B5 powders through self-propagating high-temperature synthesis of the WO3–B–W system. They showed that the combustion temperature and the reaction velocity declined with increasing the B and W concentrations in the initial reactants. Moreover, WB and W2B5 were synthesized by altering the B/W elemental ratio in the starting mixture. In another experiment, Yazici and Derin [20], [21] obtained a pure tungsten boride phase by the magnesiothermic reactions of calcium tungstate (CaWO4) and tungsten oxide (WO3) in the presence of B2O3. They reported the optimum leaching parameters into the HCl solution including immersion time and temperature, the solid/liquid ratio (S/L) and acid concentration values.

As mentioned before, there are some studies on the production of tungsten borides by the SHS method in the open literature. Nevertheless, the combustion synthesis of the WO3–B2O3–Mg system having acid boric (H3BO3) as the boron oxide source has not been considered, yet. It is worth noting that acid boric is more easy-echo than the B2O3, while it transforms to boron oxide during heating below 400 °C. Therefore, the present study was intended (a): To investigate the SHS characteristics of the WO3–B2O3–Mg system using H3BO3 as the B2O3 source, (b): To study the effects of extra B2O3 and Mg additions on the reaction velocity, as well as the structure and microstructure of the products, and finally (c): To propose a reaction mechanism by applying the phase analysis and the simultaneous thermal analysis (STA) methods.

Section snippets

Thermodynamic calculations

It was previously mentioned that the SHS reactions propagate by utilizing the heat released from an exothermic reaction. However, each exothermic reaction may not be self-sustaining. In other words, the self-propagating mode can occur when the amount of the heat released in the reaction front is higher than that of losing to the surroundings as well as to the products and the remaining reactants. Due to the high heating and cooling rates of the small samples in the SHS process, it can be

Experimental procedure

WO3 (20–100 µm), H3BO3 (< 2 µm) and Mg (100–300 µm) powders, all from Merck company, were used as the starting materials. Tungsten boride (WB) can be synthesized from these raw materials via the following reaction:2WO3+2H3BO3+9Mg2WB+9MgO+3H2O

However, it is well-established that acid boric decomposes to boron oxide and water vapor below 400 °C through the subsequent reaction [27]:2H3BO32B2O3+3H2O

Therefore, to simplify the synthesis reaction, the concentrations of acid boric in the initial

Results and discussion

Fig. 1 shows the XRD patterns of the combustion products in the 2WO3‒B2O3‒9Mg system before and after acid leaching. Theoretically, it is expected that acid boric transforms to B2O3 during heating at low temperatures. Then, WO3 and B2O3 react with Mg to form W, B, and MgO. Finally, the reaction of W and B results to the formation of WB. These sequential reactions are as follows:WO3+3MgW+3MgOG1000=840kJB2O3+3Mg2B+3MgOG1000=460kJ2W+2B2WBG1000=94kJ

However, unlike the suggested reactions,

Conclusions

In the present research, self-propagating high-temperature synthesis of the 2WO3‒(1 + x)B2O3‒(9 + y)Mg system with various extra boron oxide and magnesium molar concentrations was investigated. The obtained results can be summarized as follows:

  • 1.

    Unlike the theoretical expectations, W was the only phase remained after acid leaching of the combustion products of the 2WO3‒B2O3‒9Mg system. It showed that the combustion reaction started with the reduction of WO3 by Mg.

  • 2.

    Both the adiabatic temperature

Acknowledgment

The authors thank Dr. Davod Mohebbi-Kalhori for proof-reading of the article.

References (33)

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