Preparation, Microwave Absorption Properties and Oxidation Resistance of High-Entropy (Zr,Hf,Nb,Ta)C Microcuboids

Abstract: In response to the difficulty of the synthesis of high-entropy carbides (HECs), a pressureless low-temperature sintering strategy was utilized to prepare (Zr,Hf,Nb,Ta)C powders with three different transition metal proportion, which powder mixtures of ZrC, HfC, NbC, TaC and Ni were initially annealed at 1 500 ℃ under a flowing Ar atmosphere, and then etched by the nitric acid solution in order to dissolve the metal matrix. The results show that all samples exhibit cuboid morphology with an average size of 2 μm and the exposure of (100) face. Due to high dielectric constant, (Zr_1/4Hf_1/4Nb_1/4Ta_1/4)C microcuboids show good microwave absorption performance, and its minimum value of reflection loss attains -48.86 dB at a thickness of 3.5 mm and a frequency of 6.16 GHz. As-fabricated HECs samples also possess good oxidation resistance at 800~1 200 ℃. Moreover, oxidation products are irrelevant to the transition metal proportion and oxidation temperature, which are composed of orthorhombic Nb₂O₅-Ta₂O₅ solid solution (o-(Nb_xTa_1-x)₂O₅), monoclinic ZrO₂-HfO₂ solid solution (m-(Zr_xHf_1-x)O₂) and orthorhombic HfO₂(o-HfO₂). Notably, the synergy effect caused by transition metal elements significantly influences the phase transition process of oxides in comparison with monocarbide. In specific, the existence of Hf inhibits the transition of Nb₂O₅ from orthorhombic to monoclinic, in the meantime, assists the transition of ZrO₂ from tetragonal to monoclinic even at 800 ℃. Moreover, the addition of Nb and Ta accelerates the phase transition of HfO₂ from monoclinic to orthorhombic under atmospheric pressure.

Key words: high-entropy carbides ceramics, (Zr,Hf,Nb,Ta)C, microcuboid, microwave absorption property, oxidation resistance