Study On Structures And Properties Of C-/B-/Si-based Materials By High-pressure Experiments And First-principles

With the development of instruments and computer science,DAC（diamond anvil cell）high-pressure experiment and first-principles calculation have been indispensable experimental and theoretical methods of material investigations,respectively.The materials based on C,B and Si,respectively,occupy significant position in the field of science research and practical application.Carbon allotropes with excellent physical and chemical properties are essentially fundamental materials of many industries;Si-B compounds possess great potential for thermoelectric component under extreme conditions;Al-Mg-Si alloys are extensively used in many industrial applications,due to their excellent formability,mechanical strength,corrosion resistance.To explore the bonding behaviors,microstructures,mechanical properties and obtain a deep understanding of the relations between their structures and properties,in this thesis,structural details,stabilities and elastic properties,electronic structures of C-/B-/Si-based materials were investigated and compared employing DAC high-pressure experiments and first-principles calculations.The main results are as follows:Graphite and glassy carbon were compressed in diamond anvil cell（DAC）,at the same time,in situ first-and second-order Raman spectra of the two carbon materials were collected.It can be found that G,2D and D″+D band performed dispersion and width broadening with the increasing pressure.When glassy carbon was compressed to about 44 GPa,G band（characteristic Raman peak of sp² C-C bond）was barely observed.Combined with Lorentz fit and other means of data analysis,it can be predicted that the distance of sp² C neighbor layers were shrunk and new bonds were formed between C atoms in closed interlayers accompanying with sp²-sp³ atomic hybridization changing.Based on first-principles calculations,the diamond-Si（atmospheric phase）and seven high-pressure phases of silicon were studied systematically.It was found that Si-VI phase showed mechanical instability while it is in accordance with previous experimental observation,and all the considered phases were thermodynamical stable.The mechanical properties of Si-VII phase are beyond others,since it has the highest bulk moduli B（272.7 GPa）and shear moduli G（94.61 GPa）,and the good ductility of Si-VII was predicted by its highest Pugh ratio（2.88）.From the electronic structure,the semiconducting potentials of Si-IV and SC16 were confirmed with band gap of0.537 and 0.026 eV,respectively.The first-principles study was carried on to Si-B system including elementary substance,compounds and high pressure phases.All the considered phases were satisfied with the criterion of mechanical and thermodynamical stabilities.The empirical equation about B and G of different Si-B phases with related Si atomic fractions are linear fitted as:B=185.4-68.425x_Si,and G=155.29-108.64x_Si.The highest bulk moduli B（216.0 GPa）and shear moduli G（234.0 GPa）belongs to HPγ-B₂₈ phase,and SiB₄（A^U=4.00）is the most anisotropic structure.In addition to diamond-Si,there were indirect band gap with 1.53、1.58 and 0.41 eV found in electronic structures ofα-rh B,γ-B₂₈ and SiB₆,respectively.Three new alloy structures,P-1-Mg₄Si₇,C2/m-MgAl₂Si₂ and Pccn-Mg₄AlSi₃,were proposed in this first-principles research of Al-Mg-Si alloys.Compared with previously suggested structures,those phases are more energetically and mechanically favorable.The empirical equation about B of different Al-Mg-Si alloys with related Mg atomic fractions is linear fitted as:B=69.2-20.7x_Mg.Among the precipitates,MgSi₂ and MgAlSi exhibits higher bulk modulus,73.1 and 67.7 GPa due to the tightly bound Si-Si/Si-Al covalent networks.