In situ synthesis of ZrC/SiC nanocomposite via carbothermic reduction of binary xerogel
Introduction
Recently the researchers paid a great attention to find reasonable materials to be utilized under severe and extreme environment in order to be applied in highly technological applications like Hypersonic flight and rocket propulsion. Ultra High Temperature Ceramic (UHTCs), based on the carbides, nitrides, and borides of some transition metals had received much attention in this field. These materials were found to be potential candidates for thermal protection system in hypersonic vehicles due to their excellent and unique combination of high melting points, good thermal-shock resistance and superior ablation/oxidation resistance [1], [2], [3], [4], [5], [6]. Particularly, zirconium carbide (ZrC) has attracted much attention due to its exhibits various outstanding physical and chemical properties such as high melting point (>3000 °C), relatively low density, high modulus of elasticity and weak damage sensitivity under irradiation. Moreover, it has ability to form refractory zirconium oxide scales with a melting point of 2770 °C, which enables them to withstand temperatures up to 2500 °C [7], [8]. Also, the ZrC had been considered as one of the promising materials for the Inert Matrix Fuels (IMF) in the IV generation nuclear reactor systems, due to its excellent neutronic, high-temperature mechanical properties and resistance to corrosion by fission products [9], [10].
On the other hand, SiC is already a widely used ceramic particularly as abrasives and in composites used for high-temperature structural applications [11], [12]. It is utilized in a powder form or as thin layers in many industrial and research fields. It can be used in photoelectronics and as hard wear-resistant coatings due to some advantages such as blue emission properties, and mechanical properties, respectively [13]. Additionally, it is outstanding materials as a protective barrier for corrosion or thermal oxidation. Indeed, SiC conventional or nanostructured ceramics are receiving a great interest for applications in the nuclear industry and are, therefore, the subject of various studies [14]. Meanwhile, the oxidation of traditional silicon carbide and its matrix composites at high temperature is one of its disadvantageous limitations [15]. Besides, the poor oxidation resistance of ZrC makes it unfeasible to be used alone in an oxidation atmosphere [16]. Therefore, ZrC and SiC cannot be used separately in ultra high temperature environments due to SiC decomposition and ZrC oxidation [17], [18]. However, it can be seen that, if ZrC is introduced to SiC ceramics to form ZrC/SiC composite, the combination of the passivating character of SiC and higher melting temperature, hardness and thermal stability of ZrC should generate a kind of high-performance ceramics [19]. Moreover, this combination might produce a composite able to endure more aggressive environments [20], [21].
In addition, despite the advantages mentioned above for ZrC, the diffusion and application of this promising class of ceramics is significantly hindered according to the difficulties encountered in the fabrication of dense bodies [22]. Such low intrinsic sinterability can be overcome by adopting severe sintering temperature and pressure conditions. Generally, introducing a suitable sintering aid such as SiC [23], [24], [25], [26], Si3N4 [27], MoSi2 [28], and TaSi [29], is a useful way to improve the consolidation conditions as well as the oxidation resistance of the carbide and boride UHTCs. Hereby, synthesis of ZrC/SiC composite would enhance the sinterability of the final product. In addition, the synthesis of this composite in nanosize would enhance the surface energy of the final powder and therefore its sinterability.
The conventional methods for carbide synthesis such as solid state synthesis and ball milling are energy and time consuming. Also, the final product suffers from contamination of impurities and is inhomogeneous because the powders are mixed together on a relatively coarse scale (e.g., micrometer-scale) and attrition of milling media increase content of impurities [17], [30], [31], [32]. On the other hand, the sol–gel method is regarded as an effective way for the low temperature synthesis of ultra-fine ceramic powders according to the intimate contact of the reactants [33].
The present study will focus on the elaboration of a new way to synthesize a binary solid carbide mixture of ZrC and SiC using combination of sol–gel and conventional carbothermic reduction techniques. Such combination between these two techniques offers some advantages compared to conventional powder processing techniques, such as lower reaction temperatures and shorter reaction times (due to the intimate contact of the reactants). Furthermore, using the sol–gel process will reduce the kinetic barriers between the formed metal oxide and the carbon particles created in pyrolysis of metal alkoxide polymer due to the homogeneous dispersion of reactants in the precursor material. Increasing the contact area of the nano-grains allows carbothermic reduction between the metal oxide and carbon particles to take place at a lower temperature and a shorter time as compared to the conventional methods. Moreover, by using the molecular precursors and controlling the synthesis conditions, it will be possible to prepare homogeneous and pure multicomponent systems.
A binary xerogel containing Zr and Si elements will be firstly prepared from metal alkoxide and then subjected to a carbothermic reduction at 1500 °C for 3 h in an argon gas flow. The synthesized powder will be thoroughly investigated using the XRD, FT-IR, XPS, SEM, TEM and thermal analysis. Based on the obtained results the mechanism of reaction during carbothermic reduction of the binary xerogel will be postulated.
Section snippets
Preparation of binary gel containing Zr and Si
The starting materials are zirconium n-propoxide Zr(OPr)4 (70% in propanol, Fluka Chemie GmbH, Switzerland), Tetraethylorthosilicate TEOS (Fluka Chemie GmbH, Switzerland), acetic acid AcOH and lactose (Al Nasr Pharmaceutical Chemical Co., Egypt).
The sols containing both zirconium and silicon were separately prepared. The zirconium based sol was prepared via dissolving the lactose in acetic acid at 70 °C. Then the solution was cooled down to room temperature. A dropwise addition of zirconium n
Results
The influence of the carbothermic reduction of the binary xerogel containing Zr and Si elements (at 1500 °C for 3 h and at a constant argon flow rate of (1 l/min)) on the phase composition of the synthesized powder was investigated via XRD analysis. Fig. 1 show the XRD pattern of the synthesized powder. Thirteen peaks are clearly identified in the pattern. Ten of them at 2θ of 33.02, 38.3, 55.4, 66.1, 69.5, 82.3, 91.6, 94.8, 107.5 and 117.5 were identified to be correspondence for the ZrC phase
Discussion
A careful examination of the XRD pattern revealed the existence of sharp and intense peaks with the absence of any humps. This observation indicates that the reaction between the starting precursors might go to completion leading to the formation of distinct phases without the presence of an amorphous one. In addition, the absence of any extra peaks in the pattern reflecting that the synthesized powder almost consisted of a composite of zirconium and silicon carbides. However, all peaks,
Conclusions
A carbotheramic reduction reaction of binary xerogel containing Zr and Si was found to be a successful way of synthesizing a nano ZrC/SiC composite. The size of the produced nanocomposite was found to be around 50 nm for both phases with the absence of any residual carbon. Meanwhile, traces of zirconium oxycarbide phase were found to exist in the synthesized powder. The homogeneous distribution of both phases has been confirmed through the thermogravimetric analysis. However, the importance of
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