ZrB2 is of particular interest among ultra-high temperature ceramics because it exhibits excellent thermal resistance at high temperature, as well as chemical stability, high hardness, low cost, and good electrical and thermal conductivity, which meet the requirements of high-temperature components of hyper-sonic aircraft in extreme environments. In recent years, the development of ZrB2 powders’ synthesis method has broken through the classification of traditional solid-phase method, liquid-phase method, and gas-phase method, and there is a trend of integration of them. Solid-state synthesis, including the borothermic reduction method, carbothermic reduction method, and metallothermic reduction method, is mostly used because the process is simple and the raw materials are cheap and easily available.
Solid-state synthesis mainly includes direct reaction between elemental boron and zirconium powders, carbothermal reduction reaction, and magnesiothermic reduction reaction. In a direct reaction method, ZrB2 powders are obtained by completing the following reaction [45,46,47][36][37][38]:
Different additives were introduced into the SHS system in order to adjust the combustion temperature and isolate ZrB2 particles from each other to get well-dispersed nanoparticles. Zhang W. studied the Mg-ZrO2-B2O3-MgCl2 and Mg-ZrO2-B2O3-NaB4O7 system and Zhang T. studied the Mg-ZrO2-B2O3-MgO system for the preparation of ZrB2 powders by SHS [59,60,61][47][48][49]. Figure 1 shows FESEM images of the ZrB2 powders synthesized without any diluents and the EDS image of point A. Figure 2 shows XRD patterns of ZrB2 powders prepared with different content of NaB4O7.
NaCl (wt.%) | Crystal Size (nm) |
---|
0 | 25 |
5 | 20 |
10 | 18 |
15 | 16 |
20 | 13 |
The other approach for ZrB2 powder production is to use a non-pressed green mixture to avoid sintering and agglomeration at high temperature. Heat and mass transfer become difficult due to the porosity of non-compacted samples, and the reaction has difficulty in automatically propagating without any energy supply from outside after initiation. Therefore, the entire body of the sample needs to be placed in a high temperature environment and heated uniformly. Since the melting point of B2O3 is only 450 °C, the green mixture will become inhomogeneous due to B2O3 loss during the pre-heating process.
It can be seen from Figure 3 that well-dispersed ZrB2 powders were obtained when the green mixture was naturally packed in the crucible, while aggregates appeared in a large area when the packing density increased from 0.66 g/cm3 (a, naturally packed) to 1.32 g/cm3 (b, twice that of the naturally packed).
Synthesis in vacuum is a commonly used improved process. The change of Gibbs’s free energy can be used to determine whether the reaction can happen from thermodynamics. When ∆G < 0, the reaction is spontaneous; when ∆G > 0, the reaction is not spontaneous. The value of Gibbs free energy change in a non-standard state can be calculated based on that of the standard state according to Equation (86):
Thermal plasma synthesis is the most efficient technique for producing metal and ceramic nanopowders in continuous and scalable way [81,82,83,84,85,86,87][61][62][63][64][65][66][67].