Mechanical and electronic properties of C60 under structure distortion studied with density functional theory

B3PW91/6-31G density functional method was employed to investigate the elastic, strength and electronic properties of C₆₀(I_h) in its ground electronic state (X ¹A_g). Most of the properties were examined for larger structure distortions. The over-all elastic constant were derived from the near-equilibrium potential energy curves (PECs) in five independent directions [according to symmetries by 1. D_5d, 2. D_3d, 3. D_2h, 4. C_2h(1), 5. C_2h(2)]. By extension of the distortions as large as the structure of C₆₀ was destroyed, the necessary energies were obtained, which quantitatively illuminated the stability of C₆₀ theoretically. Analytical polynomial fit of the full PECs reproduced the energy data accurately. Time-dependent B3PW91/6-31G analysis showed significant electronic spectra changes associated with the structure distortions. Elongation in the direction of D_5d and compression in that of D_2h encountered potential energy surface cross-linkages, which might be considered as a single electron pump for further application in designs of single electron devices.     

Effect of the shear and coupling elements of the Aij and Dij matrices on the impact behavior of glass, aramid and hybrid fabric composites

The impact behavior of glass, aramid and glass–aramid hybrid fabric epoxy matrix composites was evaluated. Delamination was identified as the main macroscopic failure mode of the composites. A simple relationship based on the macromechanical behavior of laminated composites was established, relating coupling—A₂₆, D₂₆—and shear—A₁₆, D₁₆—elements of the extensional and bending stiffness matrices to the total energy absorbed at impact. The results obtained show that the A₆₆/D₆₆ ratio is a relevant parameter of concern, correlated with the behavior of the composites under impact. Additionally, the coupling matrix B_ij was shown to restrain the macroscopic delamination of the composites.     

Stress invariants in an orthotropic damage space

In the classical theory of plasticity, it is often convenient to express failure criteria and/or flow rules in terms of stress invariants (I₁,I₂,I₃) and deviatoric stress invariants (J_2D, J_3D). Within the framework of continuum damage mechanics, the microscopic degradation of materials is reflected through the damage variables that conceive the notion of net-stress (the term net-stress has been used here in place of the term effective-stress since the latter already has an established meaning in Geotechnical Engineering). Any operative yield or failure criterion in the continuum damage mechanics will then need to be expressed in terms of the net-stress. This motivates an attempt to find the equivalent stress invariants in terms of the net-stress. In this paper, the stress invariants in an orthotropic damage space have been successfully derived and presented.     

Homogenization kinetics of a cast Ti48A12W0.5Si alloy

The homogenization kinetics of a cast Ti48A12W0.5Si alloy with a duplex microstructure was studied in terms of γ-phase dissolution and -grain growth. It was found that the measured volume fraction of remnant γ grains can be well simulated by a model of interface-controlled dissolution in a dislocation mechanism, instead of a diffusion-controlled one. The activation energy for the /γ interface reaction was found to be Q_int = 476 kJ mol⁻¹, which is much higher than the interdiffusion activation energy in TiAl alloy. The grain growth of phase during homogenization can be categorized into three stages. During the first stage, where the volume fraction of remnant γ grains is higher than about 10%, the growth of grains follows the parabolic law D = k₁t^0.2, and the activation energy for grain growth was calculated to be Q₁ = 442 kJ mol⁻¹, very close to the Q_int for /γ interface reaction. In the second stage, where few fine γ grains (1–10 vol.%) remained, a dramatic grain growth occurs. During the final stage, as the single phase is obtained, the coarsening of grains again satisfies the grain growth law D = k₃t^0.4, with the grain growth activation energy Q₃ of 147 kJ mol⁻¹, lower than the reported interdiffusion energy of γ phase.     

Triplet State Characteristics of Smaller Fullerenes

We studied the energetics and the geometry of the distorted triplet structures of C50, C₅₆ and C₅₈ theoretically within a simple one-electron-per-site model. Excited state wave functions were constrained into lower symmetries by appropriate group theoretical projection technics. Jahn-Teller distortions were monitored for degenerate vertical excitations. The distortion energies are typically a few tens of meV, the largest value (-0.1 eV) was found for the C_2v ³A₁ and ³B₁ states of C₅₀ Zero-neld-splittings of various triplets were evaluated; the signs and values of the resulting D and E parameters axe quite sensitive to the system and to the symmetry of the particular excited state.     

DP600双相钢在两种盐雾实验条件下的腐蚀行为对比研究

采用SEM、EDS、XRD以及EIS等技术,对比研究了DP600双相钢在中性盐雾(NSS)实验和循环盐雾腐蚀实验(CCT)两种不同加速实验条件下的腐蚀行为,并获得了腐蚀动力学规律。结果表明：在两种加速实验条件下,试样腐蚀失重均逐步增加,且CCT中的大于NSS中的;同时,初期腐蚀速率相差不大,在480 h时腐蚀速率均达到最大值,分别为1.89 g·m^-2·h^-1(NSS)和2.72 g·m^-2·h^-1(CCT)。两种实验条件下腐蚀产物主要是Fe₃O₄、α-FeOOH、γ-FeOOH、δ-FeOOH和α-Fe₂O₃,而CCT条件下同时也形成了较多的β-FeOOH。CCT条件下的锈层厚度大于NSS条件下的,且厚度增加趋势也更快。EIS结果表明：两种加速实验条件下试样溶液电阻和腐蚀产物膜电阻均逐步增大,电荷转移电阻均为先减小后增加。实验前期(≤480 h),NSS和CCT条件下的腐蚀速率可分别表达为：ΔD_1-1=0.7349t^{\mathrm{0.1522}},ΔD_2-1=0.3511t^{\mathrm{0.3313}};而在实验后期(>480 h),则分别为：ΔD_1-2=14.6239t^{-\mathrm{0.3236}},ΔD_2-2=6.8542t^{-\mathrm{0.1570}}。     

Ultraviolet Photoelectron Spectroscopy of Two Titanium Metal Atoms Encapsulated Metallofullerenes, Ti2@C80 and Ti2@C84

Ultraviolet photoelectron spectra (UPS) of Ti₂@C₈₀ (mixture of two isomers) and two Ti₂@C₈₄ isomers excited by synchrotron radiation light source are presented. The spectra of Ti₂@C₈₀ are complicated compared with those of other metallofullerenes, which is considered with an aid of molecular orbital (MO) calculation. The photoelectron spectra of two Ti₂@C₈₄ isomers are fairly analogous with each other and show less structure than that of Sc₂@C₈₄. This suggests that the symmetry of two isomers differs from D_2d symmetry of Sc₂@C₈₄.     

On The Differences Between Neutral and Negatively Charged C60 Dimers

The optimized ground state geometries and total energies have been determined for Ceo dimers with various structures. Neutral as well as negatively charged dimers were considered. Semiempirical calculations were performwith three different parametrizations: MNAand PM3. For the neutral and the charged dimers different structures have been found at the global energy minima. In the neutral cathe well known 66/66 cycloaddition product with D_2hst symmetry was found to be the most stable configuration. In contrathe charged dimer had a different structure: it is bound through a single carbon atom on each C₆₀ty) moiethe two balls being in trans position (C_2h symmetry). This is in accord with X-ray results on the quenched phase of A₁C₆₀ (A = K,Rb). Some results of the vibrational analysis on these dimers (neutral as well as charged) are also presented in this paper.     

Computations of the Energetics of C60F36 Isomers

Three isomers of C₆₀F₃₆ are known from experiment (C₃, T, and C₁ symmetry). However, computations do not agree on their relative stabilities. In order to clarify the situation, DFT computations are carried out with different basis sets and several functionals. While lower level computations yield the C₃ isomer as the most stable, higher computational treatments consistently point out the T isomer as the lowest potential-energy species.     

A Computational Treatment of 35 IPR Isomers of C88

The complete sets of 35 isolated-pentagon-rule (IPR) isomers of C₈₈ is described by the SAM1 (Semi-Ab-initio Model 1) quantum-chemical method. The separation energetics is also computed at the HF/STO-3G, HF/3-21G, and HF/4-31G levels. The SAM1 and HF/4-31G data mostly agree within a few kJ/mol. As the SAM1 energetics does not reproduce the recent NMR observations, entropy contributions are included, too, being based on the harmonic-oscillator and rigid-rotator model. Considerable temperature effects on the relative stabilities in the system are found. The ground-state structure of C₈₈ is a C₃ isomer, however, with an increase of temperature a C₂ structure becomes important. At still higher temperatures a near C₂ species is dominant. The results can be viewed as a good agreement with the available observations, and they further expand the family of the IPR sets where the thermodynamic equilibrium treatment allows for a satisfactory support of observations.     

Relative Stabilities of C74 Isomers

Relative stabilities of six C₇₄ fullerene cages are evaluated: one species obeying the isolated pentagon rule (IPR), three isomers with a pentagon-pentagon junction, two structures with one pentagon-pentagon pair and one heptagon. The computations are carried out using the Gibbs energy in a broad temperature interval. It is shown that the IPR cage (D_3h symmetry) prevails throughout. As low-lying electronic excited states are possible for the cages, their electronic partition functions are included into consideration. It is argued that for the special conditions of the fullerene synthesis/isolation, the electronic partition function based on the singlet excited states only should better reproduce the experimental population findings. The computations indicate that isolation of other C₇₄ cage, in addition to the IPR isomer, is less likely though not impossible.     

A one-dimensional model for non-linear behaviour of piezoelectric composite materials

In this paper, a one-dimensional analytical model is proposed to investigate the non-linear behaviour for piezoelectric and piezoelectric fibre reinforced composite (PFRC) materials in the fibre direction. The required linear and non-linear constants for purely piezoelectric materials are obtained using the curve-fitting method and the measured S₃–E₃ non-linear loops of the corresponding piezoelectric materials, whereas those for PFRC materials are determined by employing the quadratic non-linear constitutive equations for a purely piezoelectric material, the iso-field assumptions and linear and non-linear constants of the composite constituents. A numerical study is conducted. The numerical results reveal a significant effect of stress T₃ on S₃–E₃ non-linear behaviour for both soft PZT–5H ceramics and PZN–4.5%PT crystals. It is also found that the piezoelectric fibre volume fraction V_f and strain S₃ can significantly affect the T₃–E₃ non-linear behaviour for both PZT–5H/piezo-polymer polyvinylidene fluoride (PVDF) and PZN–4.5%PT/PVDF PFRC materials. A good correlation is noted between the piezoelectric constants d₃₃ and e₃₃ predicted using the present method and those recommended by manufacturer for PZT–5H ceramics and PZN–4.5%PT crystals.     

On secondary dendrite arm coarsening in peritectic solidification

A model for isothermal coarsening of secondary dendrite arms in peritectic reaction and transformation (liquid + primary-phase → peritectic-phase) is proposed to evaluate the secondary dendrite arm spacing (λ₂) of the primary phase in directional solidification of peritectic alloys. The model defines three stages for thin-arm dissolution (or thick-arm coarsening), i.e. the initial, intermediate and final stages: the initial thin-arm dissolution through the primary phase is sustained solely by the Gibbs–Thomson effect; the intermediate thin-arm dissolution through the peritectic phase is driven by Gibbs–Thomson effect but retarded by the peritectic reaction and transformation; the final dissolution through the primary and peritectic phases is enhanced by the Gibbs–Thomson effect and the phase transformation. The kinetics of peritectic reaction and transformation were found to be crucial to determine the thin-arm dissolution, which were characterized by the reaction constant (f) and the diffusion coefficient of solute in solid peritectic-phase (D_S), respectively. The present model shows that λ₂V^m is constant for a given Pb–Bi peritectic alloy, where V is growth velocity, and the factor, m, ranges from 1/3 to 1/2, rather than that normally observed (e.g. 1/3) for single-phase solidification. It is also notable that the calculated λ₂ for a Zn–7.37 wt.% Cu peritectic alloy was reasonably consistent with our earlier experiments for various growth velocities.     

Non-collinear structure of Cr trimer on the surface of non-magnetic metals

Magnetic structure of Cr trimer clusters on non-magnetic metal substrate has been calculated by recursion method within Anderson model. The values of the moments are enhanced relatively to bulk bcc Cr due to the reduction of coordination number. Angles between the moments depend on the relations between hopping parameters V_ij, determined by the interatomic distance in the cluster. We performed systematic calculations of the ground state for different values of parameters V₁₂=V₁₃ and V₂₃. For V₁₂V₂₃ ground state corresponds to moments on the atoms 2 and 3 with opposite direction, whereas the moment on atom 1 is perpendicular to them. For V₁₂=V₂₃ the angle between each pair of moments equals 2π/3 and the total moment of the trimer is zero. A decrease of V₂₃ versus V₁₂ leads to collinear state with moment on the atom 1 antiparallel to the moments on second and third. Calculation in external magnetic field reveals the co-existence of two self-consistent states with essentially different total magnetic moments. In weak field these solutions are very close in energy whereas with the increase of the field the state with larger moment is stabilized.     

Femtosecond Z-scan measurements of nonlinear refraction in nonlinear optical crystals

We report an investigation of third-order optical nonlinearities in several nonlinear optical crystals using the Z-scan technique with femtosecond laser pulses at 780 nm wavelength. The crystals studied include LiNbO₃:MgO, KTiOAsO₄, KTiOPO₄, β-BaB₂O₄ and LiB₃O₅, which are extensively used for ultrashort-pulse second-harmonic generation and optical parametric oscillation. The nonlinear refractive index n₂ in these crystals has been determined to be in the range from 10⁻¹⁶ to 10⁻¹⁵ cm²/W. No two-photon absorption has been observed. The experimental results are compared with the two-band model for the bound electronic Kerr nonlinearity. It is shown that the measured n₂ values in β-BaB₂O₄ and LiB₃O₅ are one order of magnitude smaller than those of LiNbO₃:MgO, KTiOAsO₄, KTiOPO₄, which is in agreement with the theoretical prediction.     

Effect of CH4 and H2 on CVD of SiC and TiC for possible fabrication of SiC/TiC/C FGM

SiC and TiC coatings were deposited by CVD on graphite substrates and the effect of the variation of the methane (CH₄) and hydrogen (H₂) ratio on deposition was investigated. SiCl₄, TiCl₄ and CH₄ were used as sources of Si, Ti and C. In case of the SiC coatings, stoichiometric SiC was obtained when the ratios of CH₄/(SiCl₄ + CH₄)andH₂/(SiCl₄ + CH₄) are 0.4 and 10, respectively. Stoichiometric TiC was also obtained under similar conditions. In order to obtain non-stoichiometric materials for possible fabrication of functionally gradient materials (FGM), a change of microstructure and composition was observed with changes of the CH₄ and H₂ ratio. As a result, SiC, TiC and C contents were more easily controlled by a change of the H₂ ratio compared to the CH₄ ratio for SiC and TiC deposition. It has been verified that the change of the H₂ ratio is more desirable for possible manufacturing of SiC/TiC/C FGM.     

热处理对真空压铸NZ30K镁合金微观组织及力学性能的影响

使用光学显微镜(OM)、扫描电镜(SEM)、能谱分析(EDS)、硬度测试和拉伸性能测试等方法,研究了热处理对真空压铸NZ30K镁合金微观组织及力学性能的影响。结果表明：铸态合金的宏观组织分为表层区和心部区,表层区组织由细小α-Mg等轴晶和分布在晶界的Mg₁₂Nd组成,心部区组织则由细小α-Mg等轴晶、粗大预结晶组织(ESCs)和分布在晶界的离异共晶Mg₁₂Nd组成。在固溶处理过程中心部区晶粒的长大比表层区更为显著,晶界迁移速率与晶粒尺寸不均匀呈正相关性,满足晶粒长大模型v=M₀ exp (-Q/RT) A (1/D₁-1/D₂)。合金的优化热处理工艺为540℃×6 h+200℃×8 h。与铸态合金(UTS=186.0±1.5 MPa,YS=131±2.5 MPa,EL=6.6±0.4%)相比,峰值时效态合金的抗拉强度和屈服强度分别提高到了223.6±4.1 MPa和172.8±2.9 MPa,但延伸率降低到了4.2±0.3%。其强度的提高主要得益于时效析出的片状纳米β"相能够有效地阻碍位错在基面上的滑移。铸态和热处理态合金的表层区断裂模式均为韧性断裂,而心部区的断裂模式在铸态下为准解理断裂、在固溶态下为解理断裂、在峰值时效态下为准解理断裂。     

Computer simulation of martensitic textures

We consider a Ginzburg-Landau model free energy F(ε, e₁, e₂) for a (2D) martensitic transition, that provides a unified understanding of varied twin/tweed textures. Here F is a triple well potential in the rectangular strain (ε) order parameter and quadratic e₁², e₂² in the compressional and shear strains, respectively. Random compositional fluctuations η(r) (e.g. in an alloy) are gradient-coupled to ε, ˜ − ∑_rε(r)[(Δ_x² − Δ_y²)η(r)] in a “local-stress” model. We find that the compatibility condition (linking tensor components ε(r) and e₁(r), e₂(r)), together with local variations such as interfaces or η(r) fluctuations, can drive the formation of global elastic textures, through long-range and anisotropic effective ε-ε interactions. We have carried out extensive relaxational computer simulations using the time-dependent Ginzburg-Landau (TDGL) equation that supports our analytic work and shows the spontaneous formation of parallel twins, and chequer-board tweed. The observed microstructure in NiAl and Fe_xPd_{1 − x} alloys can be explained on the basis of our analysis and simulations.     

Resistance changes in thin film integrated circuits caused by interdiffusion

The resistance changes in the thin film Au/Pd and Al/Au systems are investigated at a temperature of 350°C (Au/Pd) and in the range 100–200°C (Al/Au). Except for the original Au/Pd and Al/Au data, the experimental Au/Pd values published in the paper by Hall et al. are evaluated. The correct mathematical solutions for lateral diffusion from grain boundaries into grains and for interdiffusion in two-film samples are used in interpreting the experimental data. The evaluated values of the diffusion coefficient D satisfy the Arrhenius relationships and yield reliable information on the activation enthalpies H of the interdiffusion processes. However, the values of the frequency factors D₀ are less reliable, owing to their direct dependence on the grain size (Au/Pd) or on the film thicknesses (Al/Au).     

B3LYP/6-31G*//SAM1 Calculations of C36 Fullerene and quasi-Fullerene Cages

The recently isolated fullerene C₃₆ is computed by the SAM1 method with energetics refined at the B3LYP/6-31G* level. Twelve low-energy cages are considered in detail, exhibiting both fullerene (pentagons and hexagons) and quasi-fullerene (also squares and heptagons) pattern. In contrast to other fullerene systems, the SAM1 and B3LYP/6-31G* methods do produce somewhat different energetics. Consequently, the equilibrium isomeric composition at high temperatures is different, too. In the best affordable B3LYP/6-31G* approach the fullerene cage with the topological D_6d symmetry represents the most populated species at high temperatures. Hence, an agreement with the solid-state NMR experiment is achieved at the B3LYP/6-31G* theoretical level.