Study On Physical Properties Of Binary Iron Nitrides And Zirconium Borides

Multi-functional hard materials with high hardness,corrosion resistance,excellent conductivity,magnetism have a wide range of application prospects,especially under extreme conditions,and play an irreplaceable role.However,it is very difficult to prepare multi-functional hard materials.For example,hardness and conductivity are two mutually restrictive properties in materials.Conventional superhard materials are mostly insulators or semiconductors,and the hardness of materials with high conductivity is generally low.This is because the localized electrons that cause high hardness and the free electrons that produce conductivity are two mutually resisting states of electrons in the structure,which are difficult to coexist.How to have both high-density localized electrons and free electrons in the same structure is a major challenge to explore conductive-high hardness/superhard materials.Transition metal light element compounds(TMLEs)are a treasure trove of multi-functional hard materials.The transition metal can provide high valence electrons,and the covalent bond grid constructed of light elements has a large number of localized electrons.In TMLEs,the compounds with high light element ratio have higher hardness;The compounds with low light element ratio have low hardness and good conductivity.However,the hardness and conductivity do not change monotonously with the ratio of light elements,and there are many disputes.In addition,the source of high hardness and excellent conductivity in TMLEs is also another focus of debate,especially in compounds with very high and very low light element ratios.Therefore,studying the physical mechanism of the origin of high hardness and conductivity in TMLEs not only has important scientific significance,but also has strong application value and considerable economic benefits.In this paper,two kinds of TMLEs with very low and very high ratio of light element compound are studied to explore the physical mechanism of the origin of high hardness and conductivity.The first one takes the iron metal with excellent conductivity as the skeleton and infiltrates into isolated N atoms to form?-Fe₂N.The conductivity is realized by iron-based skeleton,and the hardness is enhanced by bonding nitrogen atoms with metal atoms.The second one is based on the three-dimensional skeleton structure of light element B and introduces Zr atoms to form ZrB₁₂ with high boron content.The internal relationship between the electron localization and the conductive channel of ZrB₁₂ was explored.The innovative research results are as follows:The bulk?-Fe₂N was prepared under high pressure and high temperature,its crystal structure was confirmed,and its magnetic and other physical parameters were obtained.The single phase of?-Fe₂N was successfully prepared at a pressure of 5 GPa and a temperature of 1073 K,using the results of powder X-ray diffraction and structural refinement,it is confirmed that the space group is P312.N occupies three sites 1a,1d and 1e in the structure.Six iron atoms form an octahedron.N atoms are located in the center of the octahedron to form an N-Fe₆ octahedron structure.The results of thermogravimetry and differential thermal analysis show that the decomposition temperature of?-Fe₂N is about 670 K and completely decomposes at920 K.the decomposition process is a transformation process from high nitrogen phase to low nitrogen phase.The measurement of magnetic susceptibility and magnetization shows that the Curie temperature(Tc)of?-Fe₂N is about 250 K and the effective magnetic moment(?_eff)is about 5.16?B.The saturation magnetization(Ms)of?-Fe₂N at 2 K is about 1.2?B.Vickers hardness test and low-temperature electric transport test were used to confirm it experimentally?-Fe₂N is a high hardness material with metallic behavior.Based on the first-principle calculation,the physical mechanism of nitrogen increasing the hardness and decreasing the conductivity of iron-based structure has been theoretically given.Vickers hardness test results shown the Vickers hardness value of?-Fe₂N is about 7 GPa,which is 75%higher than that of iron element(4 GPa).The introduction of nitrogen can greatly improve the hardness of iron-based structure.The calculation of the electron localization function shows the addition of N atom does not form N-N covalent bond,there is ionic chemical bond between N atom and Fe atom.At the same time,the addition of N atom improves the compression resistance of the whole structure,which improves the hardness of?-Fe₂N.The electrical transport test shown?-Fe₂N exhibits metallic conductive behavior.The room-temperature resistivity of?-Fe₂N is about 172??·cm,which is two orders of magnitude higher than that of elemental iron(9.78??·cm).The calculations of electron band structure shown that all the conductive states are contributed by the 3d electrons of Fe,which is similar to that of iron.However,the bonding between Fe-N reduces the free electron content contribute by Fe-3d orbital,resulting in the reduction of?-Fe₂N conductivity.By means of low temperature electrical transport measurements and hardness characterization,it is confirmed that ZrB₁₂ with high hardness shows excellent metallic behavior,and the causes of high hardness are theoretically analyzed.The electrical transport and Seebeck coefficient tests shown the resistivity of ZrB₁₂ decreases with the decrease of temperature,exhibiting the metallic behavior.Its room temperature resistivity is small(only 18??·cm)and very close to that of Pt.Furthermore,the Seebeck coefficient of ZrB₁₂ is only 2.0?V·K^-1 at RT,which is also comparable to that of Cu and Pt,and could identify its excellent electrical transport behaviors as a metallic conductor.Vickers hardness test results shown the hardness of ZrB₁₂ is about 26 GPa,which is a few high hardness TMLEs with a hardness of more than 25 GPa.Its high hardness mainly due to the highly symmetrical B-B three-dimensional covalent bond network.The calculated stress-strain relationships shown the plastic deformation of boron cage structure in ZrB₁₂ crystal structure is realized through the reconstruction of B-B covalent bond network around it,which makes ZrB₁₂ have strong ability to resist shear plasticity and show high hardness.By first-principle calculation,it was found that the free electron conducting channel is formed in the three-dimensional covalent network structure of ZrB₁₂,which explains the physical mechanism of excellent conductivity in high boron compound ZrB₁₂.The calculations of electronic structure shown the conduction bands at the Fermi level are hybrid states mainly composed by B-2p and Zr-4d orbitals.The spatial distributions of band decomposed charge density in the real space of ZrB₁₂ crystal structure show that a large number of B-B covalent networks in ZrB₁₂ form localized?-bond orbitals,and some B-B covalent networks can form delocalized?-bond.The Zr-4d form an delocalized conductive channel with d-?-d bridge structure in the B-B three-dimensional covalent network by conducting with the B-B delocalized?-bond.The mess move of valence electrons is realized through the B-B delocalized?-bond,which makes ZrB₁₂ show abnormally excellent conductivity.Therefore,the coexistence of localized?-bond and delocalized?-bond is formed in ZrB₁₂,which realizes the compatibility of high hardness and excellent conductivity.In summary,the bonding between of a small amount of light elements and metal can improve the hardness,but lead to the decrease of the content of free electron and reduce the conductivity of the materials;In some TMLEs with high light element content,through charge transfer and orbital hybridization between metal and light elements,a three-dimensional covalent structure containing electron channels can be formed while preserving localized?-bond,and finally the coexistence of localized electrons and free electrons can be realized,showing high hardness and excellent conductivity in the material at the same time.This work provides new ideas for functional high hardness materials in the future.