Synthesis Of High-entropy Transition Metal Diboride Powders

As a new type of recently emergent ceramic materials,high-entropy diborides,were referred to as a sort of vital materials applied in aerospace and energy fields owing to its unique combination of prominent mechanical and chemical properties.Synthesis of high-quality powders of high-entropy diborides is crucial to preparing high-quality ceramics and advances its development,for highly pure and fine powders could not only significantly reduce sintering temperatures,but also help to acquire ceramics with small grain size,and increase the relative density of prepared ceramics as well.Nevertheless,there is no any reported method as yet to synthesize high-entropy diborides powders.Aiming on preparing high-entropy diborides powders,we successfully prepared high-entropy diborides powders via three different ways dubbed borothermal reduction,carbo/borothermal reduction and molten-salts method,respectively.In this study,we systematically investigated the effects of ratios of raw materials,synthesis temperatures on the morphology and phase compositions of products.Afterward,we analyzed the morphology,crystal structures,particle sizes and chemical uniformity in detail,determining the chemical reactions through synthesis processes by thermodynamical analyses combined with first principle calculation,revealing the mechanism of the formation of the high-entropy diborides nanostructures.Detailed studies and results are shown as follows.As a prototype of synthesis process,(Hf_0.2Zr_0.2Ti_0.2Nb_0.2Ti_0.2)B₂high-entropy diborides powders were successfully synthesized by using relatively cheaper metal oxides and boron as raw materials via borothermal reduction route at 1973 K in inert gas atmosphere.The results show that the products consist of hexagonal structural single crystalline particles with a average particle size of 310 nm and high chimical componential uniformity from nanocale to microscale.The mix enthalpy has been calculated to be-0.232 k J/mol by first principle calculation and thermaldynamical analysis demonstrate that the products were generated via one step reaction between metal oxides and boron.Meanwhile,the as-synthesized products contain 2.06%oxygen in weight.To the problem of high oxygen content in products prepared via borothermal reduction,a new route named carbo/borothermal reduction was designed to synthesize highly pure high-entropy diboride powders with low oxygen content.Specifically,the(Hf_0.25Nb_0.25Ta_0.25Ti_0.25)B₂high-entropy diboride powders with 0.49%oxygen content in weight were readily prepared by using raw materials of four corresponding transition metal oxides,carbon powders and boron carbide at the temperature of 2073K for 60 min.The as-synthesized powders were comprised of well crystallized hexagonal high-entropy diboride with an average particle size of 260 nm.Meanwhile,five componential elements are evenly distributed in products from micro scale to nanoscale.To problems of larger particle size of products and relatively high temperatures required in synthesis process via two aforementioned method,on the basis of borothermal reduction approach,a molten salts were proposed to introduce into the synthesis system to reduce synthesis temperature and particle size of products.KCl was adapted as molten salts in our experiments and(Ta_0.2Nb_0.2Ti_0.2W_0.2Mo_0.2)B₂high-entropy powders were successfully prepared via this route at the temperature of1423 K for 30 min.The characterizations of as-synthesized powders shows that as-synthesized products consist of flower-like nanostructures each of which was assembled by dozens of nanorods with length of 100?200 nm and diameter of 10?20 nm.Meanwhile,the products were tested to be hexagonal structural single-crystalline particles with highly uniform chemical compositions form micro scale to nano scale.A layer-by-layer growth model based on interface-control mechanism in Ostwald Ripening process were proposed to interpret the growth and formation of as-synthesized high-entropy-biborides nanoflowers.Apart from that,the oxygen content of as-synthesized products is up to 2.57%in weight.