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Ideal Strengths And Hydrogen Storage Of Light Elements Covalent Solids From First Principles Studies

Posted on:2008-06-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y ZhangFull Text:PDF
GTID:1101360242495157Subject:Condensed matter physics
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This dissertation is mainly focused on the mechanical and hydrogen storage prop-erties from the point of view of microscopic process and electron structure of lightelement (B, C, N) covalent solids. The first principles ab initio method has been em-ployed in our study.Since the successful synthesis of diamond and cubic BN (c-BN), two paths arefollowed in the search for new superhard materials. The first is to find materials with theextreme hardness of diamond and good thermal stability of c-BN. One of the exampleis the study of cubic BC2N, which is based on the simple idea of mixing diamond and c-BN. The second is to find materials with higher hardness than that of diamond. The C-Nbond is discovered as the shortest covalent bond. Those materials containing C-N bondsshould have extreme hardness according to the empirical model. It has been predictedby ab initio calculations that the bulk moduli of C3N4 compounds are comparable oreven higher than that of diamond. This result suggests the C3N4 compounds may havelarge hardness. One of the goals in this dissertation is to investigate whether these twopaths can eventually approach the success of finding new superhard materials.Moreover, the synthesized BC2N and C3N4 samples are in nano-crystalline form.So the data of accurately measured hardness is still lacking due to the size limitation.The most reliable way at present is to give theoretical predictions by means of firstprinciples calculation. Although there are some conventionally used parameters suchas bulk and shear moduli, it is better to use ideal strength as the criteria of materialhardness. The ideal strength is the lowest stress at which a perfect crystal becomesunstable and it sets an upper bound for material strength. Different from the elasticparameters, the ideal strength reflexes the mechanical properties at large strain wherethe bonding characteristics may change significantly. The empirical model based onthe equilibrium parameters could fail for this reason. As a result, the ideal strength ismuch reliable to evaluate the hardness measurement associated with deformation andstructural failure.We perform a detailed study for diamond and c-BN on the chemical bonds, total energy, and stress-strain relation at large strain. It is shown diamond and c-BN havesome interesting disparities on the break of chemical bonds, mechanical response, andbond relax modes, Diamond remains strong at large strain whereas c-BN is weakenednear the break point. In shear deformation, diamond deforms into graphitic structureperpendicular to the <111> direction. However, c-BN deforms into graphitic h-BNalong the <111> direction. We attribute this result to the different ionicity of C-Cbond and B-N bond.In the study of cubic BC2N, all of the 8-atom-zinc-blended structured cubic BC2Nare studied. Our results show that two of the most stable cubic BC2N have higher elasticmoduli than that of c-BN, but the calculated ideal tensile and shear strength are lower.Detailed atomistic analysis show the C-N is the weak bond of BC2N which weakensrapidly at large strain, thus lead to the sequential break of chemical bonds. This resultalso indicates the bonding environment and bonding characteristic at large strain iscrucial to material strength. The cubic BC6N has been studied too. It is interestingthat although the elastic moduli of BCxN material increase with the carbon content,the calculated ideal tensile strength is not enhanced. Moreover, the ideal shear strengthis even lowered due to the presence of C-N bonds and a sequential break of chemicalbonds. The result indicate that it is hard to obtain a superhard material by mixingdiamond and c-BN. The atomic hybridization will inevitably introduce the weak bondsand thus significantly lowers the ideal strength.We study the ideal strength of two crystalline C3N4: pc-C3N4 andβ-C3N4. pc-C3N4 has the overall highest bulk and shear moduli in C3N4 family, but its ideal strengthis lower than that of c-BN. During the cubic-graphite transformation, the sp3 lone pairelectrons in pc-C3N4 become sp2 lone pair electrons. The disparity between cubic andgraphitic phase in energy promotes the structural failure at early stage.β-C3N4 hassp2 lone pair electrons and the cage-like geometry leads to low anisotropy to externalloading. The calculated ideal strength is higher than that of pc-C3N4, but still lowerthan that of c-BN. As a result, carbon nitride should not be harder than diamond andc-BN.In addition, we propose a new adsorption media for metals, the graphenic C3N4(h-C3N4). This natural porous structure can tightly bind with transition metal atoms.The adsorption energy for metals is much higher than that of C60. This result suggests the way to the settlement of spontaneous clustering problem in metal doping nano-materials. Meanwhile, we find that the Ti decorated h-C3N4 is good for hydrogen ad-sorption. The H2 adsorption simulation announces up to 6.7wt% storage efficiency. Thebinding energy per H2 is within 0.1-0.4eV and suitable for use at ambient conditions.The total density of states (DOS), orbital projected density of states (PDOS), and differ-ential charger density are plotted and discussed. The metal-C3N4 interaction originatesfrom strong electro-negativity of N atoms and the hybridization between nonbondingp-like states of h-C3N4 and d electrons of transition metals. The adsorption process ofH2 is in accordance with the'Kubas'effect.
Keywords/Search Tags:first principles, light element covalent, superhard materials, hydrogenstorage
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