Papers by Keyword: First-Principle Calculations

Abstract: The metal-air batteries, especially the Zinc-air batteries, are great solutions to the growing energy crisis with excellent rechargeable capacity. ORR is the key electro-chemical reaction in Zinc-air batteries, and the development of the ORR efficiency is being studied extensively. The doping of transition metal in Co₃O₄, with the basement of N-doped graphene have been confirmed to have catalytic activity which can be comparable to Pt/C. Herein, the Fe-doped Co₃O₄ supported by N-doped graphene is constructed as the catalyst of ORR, and that without Fe doping is also constructed as comparison. Through first-principle calculation, it shows that the adsorption energies to O₂ on the same site of each surface and on different sites on Fe-doped one. The partial density of state of the O₂ adsorption system shows the effects of electron transfer and orbital hybridization on catalysis, which provide evidence to the catalytic mechanism with Fe doping. The energy changes of each step in ORR on catalyst with Fe doping and without Fe doping show the shortcomings of the simulation, including the spin of Fe atoms. Thus the study confirms that the adding of Fe contributes to the catalystic capability compared to the pure Co₃O₄.

Abstract: Either at higher temperatures or when a certain alloying element content is exceeded, γ-TiAl alloys contain the β phase (bcc) or its ordered derivate β_o (B2). The relatively soft β phase can facilitate hot deformation, but β_o is detrimental for creep strength and ductility. Thus, knowledge about β_o→β phase transformation is desirable. Surprisingly, even for the binary Ti-Al system it is under discussion whether the ordered β_o phase exists. Also, the effect of alloying elements on the β phase ordering is still unclear. In the present work the ordering of the β phase in binary Ti-(39,42,45)Al and ternary Ti-42Al-2X alloys (X=Fe, Cr, Nb, Ta, Mo) which was experimentally investigated by neutron and high energy X-ray diffraction is compared with the results of first principles calculations using density functional theory. Except for Cr the experimentally determined and the predicted behavior correspond.

Abstract: A density functional theory (DFT) is used to investigate the energetics of an epoxy resin adhere on a tin oxide and a hydroxylated-alumina surface within a supercell approach. Self—consistent geometry optimization is performed for models of adhesion interface, which is comprised of a fragment of epoxy resin and hydroxylated-Al₂O₃ (001), and SnO2(001) and (110) surface. The epoxy resin studied was simplified fragment based on diglycidyl ether of bisphenol A (DGEBA). It is found that the distance between the resin and the surface where the adhesion force is maximized is substantially the same for all models. Analysis of the energy-distance plot reveals that the fragment of DGEBA molecule adhere most strongly to the SnO₂(001) surface, suggesting that the adhesion force is induced by van der Waals (vdW) interaction.

Abstract: ABO₃ perovskite ceramics due to their chemical nature and size difference of the cations A (where A is a divalent metal) and B (where B is a tetravalent metal) have non-centro-symmetric polymorphs and display significant piezoelectric properties. One path to improve piezoelectric properties is through alloying these materials. In order to assess the feasibility of this, we have investigated the structure, elastic and piezoelectric properties of prototypical cubic and tetragonal phases of ABO₃ bulk ceramic oxides and their alloys: {A_xA`<sub>(1-x)</sub>}BO<sub>3</sub> and A{B<sub>x</sub>B`_(1-x)}O₃ by density functional theory based first-principle calculations. Using 2x2x2 super cells as models in our calculations, we have covered the full alloying range by varying concentration, x, in steps of 12.5%. We have created models using Ba, Sr, Pb, for A and A`, and Ti, Zr for B and B` both in cubic and tetragonal super cells. Here, we will report the structural and piezoelectric properties of tetragonal phases of ABO₃ bulk ceramic oxides and their alloys.

Abstract: The Te-Te van der Waals-type secondary bond in stable mGeTe·nSb₂Te₃ (GST) pseudo-binary chalcogenides was investigated by means of ab initio calculations. The generalized gradient approximation of Perdew-Burke-Ernzerhof pseudopotential describes the Te-Te secondary bond very well. Whereas the local density approximation pseudopotential over-estimates the bond energy by underestimating the repulsive forces between the adjacent Te atoms. The electron localization function illustrates the chemical bonding nature of the Te-Te secondary bond. Our present results will shed insights on the secondary bonds in GST phase change materials.

Abstract: A theoretical study on Ru_1-xPd_xZr solid solutions has been carried out by means of periodic density functional theory (DFT) at generalized gradient approximation (GGA) level. Stability and martensite transformation analysis were performed based on the formation energy and electronic structures. The results show that the ternary phase Ru₂Pd₆Zr₈ is stable because the de-mixing energy of Ru₂Pd₆Zr₈ is-0.05eV. The calculated cohesive energy, bond length and density of states showed that the orthorhombic-Ru₂Pd₆Zr₈ is more stable than bcc-Ru₂Pd₆Zr₈, for the martensite transformation occurs when about 70% of the Ru atoms are replaced by Pd atoms. The studies provide a theoretical basis to the development and application of Ru_1-xPd_xZr solid solution.

Abstract: We investigate the electronic structures and optical properties of ZnO with antisite defects O_Zn using the density function pseudopotential method. Our results show that the Fermi level shifts into the conduction band after introducing one or two O_Zn defects into ZnO supercell, indicating that the system displays a metallic-like characteristic. Moreover, the antisite defects lead to a redshift of the optical absorption edge and obvious optical absorption in the visible light region. Especially, the optical properties are influenced by the configurations of two O_Zn defects in our considered ZnO supercell. The strongest optical absorption occurs when the two defects are connected by-Zn-O-Zn-bond in the ab plane. These findings are possibly applicable for designing new optoelectronic and photoelectrochemical devices with improved low energy light absorption.

Abstract: We demonstrate that the formation of carbonate-like moiety in SiO₂ could be the origin of negative fixed charge in SiC thermal wet oxidation, based on first-principles calculations. We find that negative CO₃ ion appears in SiO₂ due to not only the existence of residual C atoms that are expected to be emitted from the SiC/SiO₂ interface, but also a large enough number of incorporated H atoms during wet oxidation.

Abstract: Rare earth silicates (RE2_SiO₅ or RE₂Si₂O₇; RE=Er, Lu, Y, Yb etc.) possess a low silica activity and good thermal stability, making them good candidates for EBC top coating materials. We have performed first-principles total energy calculations for the Er₂O₃-SiO₂ pseudo binary system. The crystal lattice parameters and internal atomic coordinates of Er₂SiO₅ and Er₂Si₂O₇ are optimized, and the electronic structure and formation enthalpies are calculated. The present study provides a theoretical basis for exploring practical applications of the materials.

Abstract: The structural and electronic properties of Ti-doped SnO₂ with 6.25 at.% are investigated with the first principle calculations based on the density functional theory within the generalized gradient approximation. The calculation results indicate that the crystal structure of Sn_0.9375Ti_0.0625O₂ possesses a smaller volume; the bond length of Ti-O is shorter than that of Sn-O; the relative angle θ change value of Sn-O-Sn→Ti-O-Ti is about 1.07%. Ti-O bond possesses more covalent ingredient and stronger bond energy than Sn-O bond. After the replacement of one Ti atom, O atom bonded with Ti atom possessed fewer electrons, the ratio of charges possessed by Ti atom and O atom dose not agree with the stoichiometry of compound, create more holes at the top of VB of Sn_0.9375Ti_0.0625O₂, and lead to the increase of the conductivity.