A close look at valences in hexahedral element meshes

This paper discusses using valences in objective functions for topological modification of 3D hexahedral meshes. For topological optimization of 2D quadrilateral meshes, node valence (i.e. number of element edges attached to each node) is used to maximize the number of regular nodes (i.e. nodes with four attached edges). Difficulties in developing 3D hexahedral local topology modifications have limited the success of hexahedral topology optimization, although published literature suggests using an object function based on node valence. However, in this paper, we show that node valence is not a consistent measure of good hexahedral element topology, and objective functions based on node valence can lead to element topology, which will only admit concave element shapes. Instead, we propose that objective functions based on edge valence (i.e. number of quadrilateral faces attached to each element edge) will provide a consistent measure of element topology. Copyright © 2010 John Wiley & Sons, Ltd.     

About copper valence and superconductivity

We describe the copper valence in superconductors based on our arguments on La₂O₃ crystal structure. In order to explain the two oxygen sites in La₂O₃, it has been supposed that O _II ¹⁻ ions occupy the tetrahedral site while O _II ²⁻ occupy the octahedral site. Oxygen ions in tetrahedral site form a covalent bond with lanthanum, which can be written [LaO]¹⁺. Considering the chemical and crystallographic properties of Tl and Bi compounds, [TlO]¹⁺ and [BiO]¹⁺ groups appear as defined by strong covalent bonds between Bi or Tl and O. This leads to the supposition that the four oxygen ions which coordinate to copper in Tl and Bi copper oxide-based superconductors are O _II ¹⁻ ions. The bivalent character of copper is then obtained through covalent bonds. For La_2−x Sr _x O₄ compounds, copper is supposed to have valence three, but spectroscopic studies point out bivalent copper. We show that krypton shell of Sr is responsible for the lack of one unit of valence as expected from krypton compounds for example KrF₂.     

Charge Transfer in YBCO Under Pressure with Bond Valence Sum Approach

The bond valence sum method has been developed to high pressures considering B₀(p) for calculating the valences of atoms. This approach has the advantages of finding a universal B₀(p) for all the atoms -at least in YBCO, ability to evaluate the valency of each atom, and charge neutrality requirement of the unit cell. As a result, a new structural refinement has led to a more exact structural detail. The position of apical oxygen is found to be specially crucial for hole distribution between the superconducting CuO₂ planes and Cu–O charge reservoir chains. Under pressure, the hole concentration in CuO₂ planes increases, but in the Cu–O chains decreases, consistent with the pressure induced charge transfer model in HTSC. Our results resolve the anomalous behavior of dT_c/dp in YBCO.     

An accurate method of iodometric titration to measure copper valence of high-T c superconductors

The double iodometric titration is the method widely used to determine copper valence of high-T _c superconductors. There are some problems that remain to be solved, and certain procedures are used: adding KSCN to release adsorbed iodine, adding HN₄HF₂ to eliminate the interference of some dopants, and using a new calculating equation to accurately calculate experiment results. Al- and Fe-doped samples were prepared and their copper valence, 2 +p, was determined by the improved double iodometric titration method. With these samples, how to use the above procedures to improve experiment results is discussed. We find that the iodometric titration method described here can be used to determine copper valence and oxygen content for doped and undoped samples with high accuracy.     

Evidence that attitude accessibility augments the relationship between speeding attitudes and speeding behavior: A test of the MODE model in the context of driving

According to the MODE model of attitude-to-behavior processes, attitude accessibility augments attitude–behavior correspondence, reflecting an automatic influence of attitudes on behavior. We therefore tested whether attitude accessibility moderates the attitude–behavior relationship in a context that is governed by characteristically automatic behavior, namely driving. In study 1 (correlational design), participants (N = 130) completed online questionnaire measures of the valences and accessibilities of their attitudes towards speeding. Two weeks later, online questionnaire measures of subsequent speeding behavior were obtained. Attitude valence was a significantly better predictor of behavior at high (mean + 1SD) versus low (mean – 1SD) levels of attitude accessibility. In study 2 (experimental design), attitude accessibility was manipulated with a repeated attitude expression task. Immediately after the manipulation, participants (N = 122) completed online questionnaire measures of attitude valence and accessibility, and two weeks later, subsequent speeding behavior. Increased attitude accessibility in the experimental (versus control) condition generated an increase in attitude–behavior correspondence. The findings are consistent with the MODE model’s proposition that attitudes can exert an automatic influence on behavior. Interventions to reduce speeding could usefully increase the accessibility of anti-speeding attitudes and reduce the accessibility of pro-speeding attitudes.     

Determination of oxygen contents and cation valences in the superconducting bismuth and thallium cuprates

The oxygen contents and valences of cations in various superconducting cuprates such as Bi₂Sr₂(Ca, Y)Cu₂O _y , Tl₂Ba₂Ca _n –1Cu _n O _y , etc. which contain two kinds of multi-valence metals have been determined by novel wet chemical methods. The wet chemical methods are based on the differences in oxidation power of each cation and at the last step an electrochemical method is employed for measuring the amount of reducing agent, i.e., CuCl. For superconducting Bi-based cuprates, a modified constant-current coulometry has been developed. For the superconducting Tl-based cuprates a constant-potential electrolysis method which makes it easy to control the electrolysis potential has been applied. The results obtained for Bi₂Sr₂(Ca, Y)Cu₂O _y have shown that both Bi- and Cu-valences decrease with increasing Y, which is consistent with the results of earlier works. The results obtained for the double Tl-O layer superconductors show that the Tl valence is about +3 in all the thallium cuprates and different from the value obtained from XPS measurements of earlier works. The reason for this discrepancy is discussed.     

Mössbauer spectroscopy and X-ray diffraction studies of Mg–Al–Fe–O mixed spinel sulphur transfer catalysts

Mossbauer spectroscopy with X-ray diffraction technique was used to characterize the chemical valences and coordination environments of iron ions in Mg–Fe–Al–O mixed spinel catalysts under different treatments. It was found that the values of quadrupole splitting gradually altered with the calcining temperature, which was connected with the crystalline size of the catalysts. During the thermal treatment, a probable electron density transfer from the oxygen ion to the iron ion in the Fe–O bond resulted in the values of isomer shift decreasing linearly with calcining temperature. The absorption lines were fitted when the atomic fraction of magnesium increased from 1 to 2 in the Mg–Al–Fe–O sample, indicating that a multiphase coexisted in this sample and iron ions had different chemical valences and different coordinated environments. When a sulphated MgFe0.2Al1.8O4 sample was reduced by hydrogen at 500 °C, various iron species with different chemical valences were identified. Fe3O4 phase with an anion defect, created in when the sulphated sample was reduced, was proposed as an active species during O2 and SO2 simultaneous adsorption on the reduced sulphated sample.     

钼酸钡薄膜的原电池及电解型电化学制备技术比较研究

分别采用恒电流电化学技术和原电池电化学技术在钼片上制备出了具有单一白钨矿结构的钼酸钡薄膜,并对薄膜的相结构、表面形貌、元素价态及钼材料的利用率进行了深入的比较研究.研究发现,两条电化学路线制备的钼酸钡薄膜都是结晶良好的,具有单一的白钨矿结构;晶粒呈四方锥形;薄膜中的钡、钼和氧元素分别表现为 2、 6和-2价.但采用原电池路线时,钼酸钡薄膜更加致密,晶粒更加均匀,表面较为平坦;而且,电化学溶解的钼能够更好地凝聚成钼酸钡附着在阳极表面;与之相对应,在恒电流条件下,有相当大一部分钼酸根离子扩散进入了电解液中.     

Phase transitions in the Ba1?xKxBiO3 system and other oxide superconductors

The mechanisms of dielectric-to-metal and metal-to-superconductor phase transitions are proposed. The antiphase-ordered array of heavy holes localized on the apical oxygen-bismuth bonds creates a dielectric gap on the Fermi surfce. With doping by potassium, light carriers are introduced, which reduce the Coulomb potential of ordered heavy holes and dielectric gap. Heavy holes tunneling between the neighboring apical O^–1 and O^–2 ions produce local charge fluctuations. Their interactions with short-wavelength optical phonons yield the coupled tunneling-phonon modes. Those local boson modes lead to the pairing of light-conducting carriers and to superconductivity with a short coherence length. The local pairing may explain the observed upward curvature ofH _c2(T) in the range fromT _c =30 K down to 4 K and up to 16 T. After obvious rearrangements, the mechanism can be applied for all bismuth oxide-based and copper oxide-based superconductors.     

The effect of surface relaxation and atomic vibration on the equilibrium shape of gold and copper crystallites

Summary The free energy of surfaces along the <011> pole in gold and copper is determined to assess the effect of surface relaxation and atomic vibration on the equilibrium crystal shape of gold and copper. The Wulff construction is performed on the -plots to determine the equilibrium shape of gold and copper crystallites at different temperatures. It is shown that surface relaxation and atomic vibration do not have any discernible effect on the equilibrium shape of EAM gold or copper crystallites. The equilibrium shape of EAM gold crystallites is formed entirely from {111} and {100} facets, while that of EAM copper shows small {110} facets in addition to the {111} and {100} facets.