| Emerging as a new class of materials,high-entropy borides have the advantages of strong resistances to oxidation,high hardness,high melting points and excellent electrical conductivities.Therefore,they have great potentials for applications in mineral mining bits,tram and train wheels,wearing-resistant parts of microelectronic devices,and protecting layers used in aerospace.However,their properties in extreme environments have been hardly studied.Thus,on the basis of the definition of multi-component high-entropy compounds,two high-entropy borides(Ta0.2Nb0.2Zr0.2Cr0.2Ti0.2)B2(denoted as TNZCT)and(Ta0.167Nb0.167Zr0.167Hf0.167Ti0.167Cr0.167)B2(denoted as TNZHTC)have been designed and then successfully prepared by us using spark plasma sintering method.A series of structural and morphological characterizations were performed on the two high-entropy borides:the structures were characterized by X-ray diffraction(XRD);the micro-morphologies were observed by scanning electron microscopy(SEM);the types and contents of the constituent elements were analyzed at the microscopic and macroscopic scales using energy dispersive X-ray spectrometer(EDS)and X-ray fluorescence spectrometer(XRF).Further,their electrical conductivity and magnetism at ambient environment,as well as the structural stability and electrical resistivity at high pressure were investigated.Research results show that the two prepared high-entropy borides are single-phase solid solutions both of which are in the hexagonal crystal structures.Microscopic analysis shows that the elements in the samples are uniformly distributed.Macroscopic analysis shows that the contents of the contained transition metal elements in each sample were approximately equal in contents.At ambient pressure,the electrical conductivities of both the samples are on the orders of 107 S/m,indicating their excellent electrical conductance.With the decrease in the temperature,TNZCT changes from paramagnetic to ferromagnetic.(Ta0.167Nb0.167Zr0.167Hf0.167Ti0.167Cr0.167)B2 is diamagnetic at room temperature.However,with the deceases in the temperature,it becomes paramagnetic,and continued decreases in thetemperature makes it ferromagnetic eventually.High pressure X-ray diffraction and Raman spectroscopy show that the structure of the two high-entropy borides have strong stabilities toward compression:even though the pressure was increased up to?55 GPa,there were no phase transitions occurred and hence they could maintain their original hexagonal crystal structures.The bulk modulus derived from the Rietveld fittings of the X-ray diffraction patterns of the two five-and six-component high-entropy borides are,respectively,353.6±7.0 and 303.5±5.0 GPa.Using these values,the estimated Vickers hardness of the two high-entropy borides based on an empirical formula are,respectively,34.1±9.4 and 29.2±8.0 GPa.These indicate that the hardness of the two high-entropy borides is very high.The mass densities of the five-and six-component high-entropy borides are 5.11 and 4.64 g/cm3,respectively.High-pressure resistivity measurements show that the resistivity of the high-entropy boride TNZCT drops sharply at?0-4 GPa,and then it stays almost the same until?12 GPa.The resistivity of the high-entropy boride TNZHTC drops sharply at?0-3 GPa and it changes only slightly when it is compressed to?6 GPa.In conclusion,our designed and synthesized five-and six-component high-entropy borides possess the characteristics of high structural stability toward compression,high bulk modulus and hardness,as well as high conductivity with low pressure-coefficient.These make them potentially superior materials for use in extreme environments.The results from this work provide fundamental scientific knowledge and useful guidance for the development of their new applications. |
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