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Graphical abstract: - Highlights: • Tungsten-doped porous ZrB{sub 2}–SiC specimens were fabricated by reactive sintering. • Both sintering and mechanical properties were improved by doping with tungsten. • Optimum concentration to obtain high mechanical properties was W/Zr (mol) = 0.05. • Sintering might be promoted by self-accelerated diffusion. • Mechanical properties might be improved by solid solution strengthening. - Abstract: Tungsten-doped porous ZrB{sub 2}-20 vol.% SiC specimens with a high strength skeleton were fabricated by reactive sintering using metastable reactants, i.e., as-synthesized amorphous hydrous ZrO{sub 2}–WO{sub 3}/ZrO{sub 2}, amorphous boron, nanocarbon, and β-SiC. Doping with tungsten clearly promoted sintering and improved mechanical properties; the optimum tungsten concentration to obtain high mechanical properties was investigated. A single-phase solid solution (Zr,W)B{sub 2} was formed in the specimens with tungsten. We suggest that sintering was promoted by self-accelerated diffusion owing to the formation of point defects caused by doping with tungsten. The improvement of mechanical properties could be attributed to solid solution strengthening.

Firstly, an amorphous precursor of hydrous nano-ZrO2–WO3 powder was precipitated by a co-precipitation method in alcohol-water mixed solvent using ammonia as a precipitator. Then, a mixture of tungsten doped ZrB2 and W2B5 powder was successfully synthesized via a solid-state reaction by borothermal and carbothermal reduction using nanocarbon, amorphous boron powders and as-synthesized amorphous nano-ZrO2–WO3. Furthermore, thermodynamic calculations clarify the above-mentioned reactions may occur. The mass and heat flow of the samples were monitored by thermal analysis. The crystallographic structure was identified by X-ray diffractometry. The size and morphology of the particles were characterized by TEM microscopy. The combined effects of uniformly distributed tungsten and the complex physicochemical changes effectively improved the solid state reactions.

Zirconium diboride (ZrB2) nanoparticles were synthesized by sol–gel method using zirconium n-propoxide (Zr(OPr)4), boric acid (H3BO3), sucrose (C12H22O11), and acetic acid (AcOH). Clearly, it was a non-aqueous solution system at the very beginning of the reactions. Here, AcOH was used as both chemical modifier and solvent to control Zr(OPr)4 hydrolysis. Actually, AcOH could dominate the hydrolysis by self-produced water of the chemical propulsion, rather than the help of outer water. C12H22O11 was selected, since it can be completely decomposed to carbon. Thus, carbon might be accounted precisely for the carbothermal reduction reaction. Furthermore, we investigated the influence of the gelation temperature on the morphology of ZrB2 particles. Increasing the gelation temperature, the particle shapes changed from sphere-like particles at 65 °C to a particle chain at 75 °C, and then form rod-like particles at 85 °C. An in-depth HRTEM observation revealed that the nanoparticles of ZrB2 were gradually fused together to evolve into a particle chain, finally into a rod-like shape. These crystalline nature of ZrB2 related to the gelation temperature obeyed the “oriented attachment mechanism” of crystallography.

To achieve a ZrO2-B-C ternary solid-state reaction, a small amount of carbon was introduced into a binary system of ZrO2 and boron. Amorphous hydrous ZrO2 precursor was firstly prepared through precipitation method using zirconyl nitrate and ammonia at room temperature. Then, ZrB2 powder was successfully synthesized using the as-prepared amorphous hydrous nano-ZrO2 via borothermal and carbothermal reduction (BCTR) by amorphous boron and nanocarbon powders under flowing argon. Plate-like ZrB2 particles with a uniform distribution and ca. 1.0 μm in two-dimensions were obtained with a B/Zr molar ratio of 4, and 5 wt.% (total weight of dry hydrous ZrO2 and boron) nanocarbon at 1550 °C for 2 h.

Thermodynamically, ZrO 2 may react with boron to form B 2 O 3 /B 2 O 2 and ZrB 2 at room temperature. However, this reaction is incomplete at temperatures lower than 1550 °C, even with the use of metastable reactants, i.e. , as-synthesized amorphous hydrous nano-ZrO 2 and amorphous boron powders. In this study, a complete disintegration of ZrO 2 was achieved by introducing nanocarbon to the binary system of ZrO 2 and boron at 1550 °C. The metastable reactants affected the temperature required for the solid-state reactions and also strongly affected the kinetics of the transformation. Single crystal and plate-like ZrB 2 particles with a uniform distribution and a size of ca. 1.0 μm in two-dimensions were obtained using 5 wt.% nanocarbon and a B/Zr molar ratio of 4.