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The Microscopic Mechanism And Mechanical Properties Of Transition-metal Compounds

Posted on:2015-01-23Degree:MasterType:Thesis
Country:ChinaCandidate:Z FuFull Text:PDF
GTID:2181330422975824Subject:Mechanical design and theory
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Superhard materials possess a combination of outstanding properties, such as highhardness, high melting point, resistance to corrosion and chemical stability, whichmakes them be used as wear-resistance-coating, drilling tools and cutting tools inmarine equipment, oil and gas exploration, machining and many other fields. Due to theinherent defects (such as diamond can react with oxygen and iron easily), traditionalsuperhard materials can not meet the actual needs and hunt for a more stable newsuperhard material has become an international focus. The combination oftransition-metal with very high densities of valence electrons and light element,including boron, carbon, nitrogen, oxygen to form short and strong covalent bonds, maybe a new approach to synthesize superhard materials. In this letter, we identify bydensity functional theory calculations that the mechanical properties and relatedmicroscopic mechanism of transition compounds including TiO2, WBxand MoBx(1) We have investigated structural phase transitions (phase transition order andtransition pressures) for various TiO2polymorphs under high pressure. The calculationsshow that the Pca21-type TiO2should be a viable phase under the proper condition oftemperature and pressure. However, the viability of two cubic phase (fluorite and pyrite)is eliminated in the whole pressure range of0~200GPa. These results support that theexperimentally assumed cubic TiO2should be the Pca21-type TiO2. For most TiO2phases, the result of calculations are agreement with previously experimentalobservations, with the decrease of equilibrium volume, the corresponding bulk modulussteadily increases. The only exception is the baddeleyite phase for which we find anunexpectedly lower bulk (149GPa) modulus as compared to previous experiments(290-304GPa). Finally, we further study high-pressure mechanical properties of theFe2P-type TiO2, the results point to the potential for the Fe2P-type TiO2to be anultrahard material under ultrahigh pressure. (2) We have systematically studied the stability and mechanical properties WBxbythe first-principles and we found a drastic reduction of hardness of about61%fromWB3with relatively low boron content to WB4with relatively high boron content. Thethree-dimensional covalent network consisting of boron honeycomb planesinterconnected with strong zigzag W-B bonds underlies the high hardness of WB3.Despite the strong intralayer and interstitial B-B bonds, the interlayer B-B nonbondingand the considerably weak zigzag W-B bonding allow the layers of WB4to cleavereadily by shear stress, which results in the anomalous softening of WB4.(3) We have performed first-principles calculations to investigate the systematicstability, mechanical properties, crystal structures, and electronic structures for MoBx. Itis found that an unexpected reduction of stability and hardness from the void-containedhP16and hR18structures to closepacked hP20(hP3) and hR21one, respectively.Furthermore, we further clarify that the anomalous variation of stability and hardnessoriginates from the filling of extra boron atoms resulting in the strong antibondinginteractions. These findings strongly challenge the general opinion to design superhardmaterials, only pursuing the dense TMBs with high boron content, and highlight theimportance of thermodynamic stability in the design of intrinsically hard materials.
Keywords/Search Tags:First-principles, Mechanical properties, Microscopic mechanism, Transition-metal compounds, superhard materials, High-pressure
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