High Temperature And High Pressure Synthesis And Physical Properties Of New Boron-rich Superhard Functional Materials

Social and economic development and industrial progress are closely related to the design and synthesis of materials,and superhard materials are playing an indispensable role in industrial processing,national defense construction,military aviation and other fields.There are many kinds of hardness test methods,we usually Vickers hardness is greater than 40 GPa material is called superhard material,Vickers hardness is greater than 80 GPa material is called extremely hard material.Diamond is a typical representative of superhard materials,whose hardness is often around 80-120 GPa,which is the hardest material known at present.Therefore,diamond has been widely concerned and applied.However,there are still several problems in the application of diamond,such as low thermal stability,poor toughness and easy to react with transition metal elements,so its application scope is greatly limited.Therefore,the design of a new type of multifunctional material with high hardness,high toughness and high thermal stability has been one of the hot spots in the field of superhard materials.Superhard materials also contain an important system--strong covalent bond system based on B-C-N-O.Because these elements are very similar to the ion radius of carbon,it is easy to form a variety of hybrid orbitals,which is very likely to form a structure similar to diamond,and boron is one of the important elements in this system.Therefore,boron-rich materials have been considered by researchers as potential superhard multifunctional materials with many important properties and are playing an increasingly important role in the field of superhard materials science.In recent years,a large number of theories and experiments have proved that the metal borides in boron-rich materials will have the potential of high hardness and superconductivity,and it is a new type of multi-functional material with high hardness.Therefore,the research upsurge on boron-rich materials has been raised.In this paper,two studies have been carried out on the use of boron-rich materials to improve the toughness of diamond and the development of new boron-rich multifunctional materials and the exploration of physical properties by using high temperature and high pressure synthesis technology.Firstly,high purity nano-diamond powder B₄C was mixed evenly by high temperature and high pressure method,and prepressed for two hours in a domestic six-side top large press at a pressure and temperature of 6.5 GPa and 2023-2223?.Then,the prepressed samples were crushed and synthesized at higher pressure and high temperature(18 GPa,2000?)using two top presses to obtain a more compact sintered sample,with B₄C evenly distributed at the boundary of nano-diamond sintered.The hardness test of the sample shows that the toughness of the polycrystalline diamond sample with 5%B₄C mass ratio is increased to18.37 MPa M^0.5 under the condition of absolute high Vickers hardness(90 GPa),which is mainly due to the fact that the B₄C at the boundary of the nano-diamond can effectively prevent the crack from extending.Secondly,a new Na-B compound,NaB₄,was synthesized by a domestic six-sided top press under high temperature and pressure for the first time in combination with the theoretical prediction.The sample was characterized and its physical properties were tested.By means of X-ray diffraction spectrum and Raman spectrum,we analyzed the rule that the sample phase changes with the temperature under the same pressure point(1.5-5GPa),found the temperature point of sample phase transition under different pressures,and made the phase diagram.Secondly,we carried out electrical,thermal and hardness tests on the sample,and found that the sample presented metallization and showed a superconducting trend at 4 K.The mechanical properties of NaB₄ were tested and found to have a hardness of about 26 GPa.This new Na-B compound is of great significance for the exploration of multifunctional hard materials.Due to the complex structure of the material,the structure and proportion of the sample cannot be directly determined only by XRD results.We analyzed the structure of the synthesized samples,and found an interesting new honeycombed boron structure under spherical aberration electron microscopy.On this basis,the structure model of this material was established.At the same time,this material has good metal conductivity,which indicates that it will be a new kind of hard multifunctional material.