Inherent anisotropy in transition metal diborides and microstructure/property tailoring in ultra-high temperature ceramics—A review

Two‐step synthesis process for high‐entropy diboride powders

“…Transition metal diborides, which are one type of ultra‐high temperature ceramic (UHTC) material, are being widely studied for potential applications such as thermal protection systems in hypersonic or atmospheric re‐entry vehicles, components in rocket motors, and cutting tools . ZrB 2 and HfB 2 are the most studied boride UHTCs due to their melting temperatures in excess of 3000°C, thermal conductivities that can be over 100 W/m K at 25°C, superior mechanical properties, and good chemical stability …”

“…1-3 ZrB 2 and HfB 2 are the most studied boride UHTCs due to their melting temperatures in excess of 3000°C, thermal conductivities that can be over 100 W/m K at 25°C, superior mechanical properties, and good chemical stability. [4][5][6] Recently, high-entropy ultra-high temperature ceramics (HE-UHTCs), including high-entropy boride (HEB), [7][8][9][10][11][12][13] high-entropy carbide (HEC), [14][15][16][17][18] and high-entropy nitride (HEN) 19 ceramics have been reported. HEB materials are solid solutions of individual transition metal diborides that have attracted increasing interest due to the high thermal and electrical conductivities, and chemical stability.…”

“…Several process for the synthesis of HEB powders have been reported. [8][9][10][11][12] 4 C, and C. Oxide phases remained in the synthesized powders due to the loss of B during BCTR and the oxides were persistent through the densification of the HEB ceramics. Based on the previous studies, the presence of residual oxide impurities or excess B in HEB powders or precursors has been common among reported synthesis methods.…”

Growth behavior of TiB2 hexagonal plates prepared via a molten‐salt‐mediated carbothermal reduction

“…Titanium diboride (TiB 2 ), one of the transition metal borides, is widely studied due to its high hardness, high melting point, low density, and excellent chemical stability and has potential applications in wears parts, high temperature structural components, and ceramic cutting tools . Unlike other materials with similar properties, such as TiC, ZrB 2 , and ZrC, the research on TiB 2 is non‐satisfying, especially in terms of morphology control, which has a significant impact on physical and chemical properties as reported .…”

Molten salt synthesis, characterization, and formation mechanism of superfine (HfxZr1‐x)B2 solid‐solution powders

“…The development of new generation of space vehicles has recently been reviving an strong interest into a special class of refractory materials popularly known as the ultrahigh‐temperature ceramics (UHTCs) in that they possess potential applications in thermal protection structures and engine parts in next‐generation reentry space vehicles . Among the existing UHTCs, HfB 2 and ZrB 2 ceramics are the optimal candidates for thermal protection structures and engine parts in next‐generation reentry space vehicles owning to their unique combination of chemical stability, high electrical and thermal conductivities, and resistance to erosion/corrosion . Specially, ZrB 2 and HfB 2 can form the continuous single‐phase (Hf, Zr)B 2 solid‐solution ceramics due to their similar crystal structures (hexagonal, space group of P6/mmm) .…”