The study of grain growth behavior and properties of high-entropy pseudobrookite (Mg,Co,Ni,Zn)Ti₂O₅ ceramics

It is well known that the grain size of high-entropy ceramics is quite small owing to the so-called sluggish diffusion effect. However, the abnormal grain growth behavior often occurs in high-entropy pseudobrookite ceramics, finally resulting in the formation of many abnormally grown grains with the grain size as large as 50 μm. To study this phenomenon, the grain growth behavior of high-entropy pseudobrookite ceramics is systematically investigated in this paper. The results demonstrate that the starting material powders firstly react with each other to form a high-entropy intermediate phase and calcined TiO₂ powders (TiO₂-1100 ℃), and then as the sintering temperature increases, the formed high-entropy intermediate phase further react with TiO₂-1100 ℃ to form high-entropy pseudobrookite ceramics. Thus, in this system, besides the sluggish diffusion effect, the grain sizes of the high-entropy intermediate phase and TiO₂-1100 ℃ also affect the morphology of high-entropy pseudobrookite. Compared to nano-sized TiO₂, micron-sized TiO₂ has lower sintering activity. Therefore, the high-entropy intermediate phase (Mg,Co,Ni,Zn)TiO₃ and TiO₂-1100 ℃ prepared with micron-sized starting materials exhibits lower grain sizes, finally resulting in the formation of high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ with small grain size. What’s more, nano-indentation and thermal conductivity tests are carried out on high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ with different morphologies. The results show that the hardness of high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ increases from 6.05 GPa to9.95 GPa as the grain size refinement, whereas thermal conductivity decreases from 2.091±0.006 Wm^-1K^-1 to 1.583±0.006 Wm^-1K^-1. All these results indicate that high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ with small grain size is a potential material for thermal protection.