Re-exchange of Fe and Cu at the interface in sintered Nd–Fe–B magnets: A method to eliminate Fe precipitation at grain boundaries

According to the decoupling hypothesis for magnetic grains, the coercivity in sintered Nd–Fe–B magnets is increased after Cu doping, which is due to the formation of non-magnetic grain boundaries. However, this method partially fails, and ferromagnetic Fe-segregation occurs at the grain boundary. We discovered both experimentally and through calculation that the Fe content at the grain boundaries can be tuned across a wide range by introducing another element of Ag. Segregated Fe at high temperature at the grain boundary re-dissolves into Nd₂Fe₁₄B grains during annealing at low temperature. Both configurable and magnetic entropies contribute a large driving force for the formation of nonmagnetic grain boundaries. Almost zero Fe content could be achieved at the grain boundaries of sintered Nd–Fe–B magnet.     

Possible room-temperature ferromagnetism in SnO2 nanocrystalline powders with nonmagnetic K doping

Sn_1 − xK_xO₂ nanocrystalline powders were synthesized by the sol-gel method. The room-temperature ferromagnetism was observed in K-doped samples, and the saturation magnetization increases with the K concentration. It indicates the ferromagnetism induced by holes injecting is possible. First-principles calculations were employed to analyze the effect of defects on the structural and magnetic properties. Taking into account the structural and valence state characterization, the inhomogeneous distribution of dopants and the holes compensation effect of K and H interstitials in crystal were thought to play important roles in reducing the magnetic moments in samples.     

Orientation effect on the electronic transport properties of C24 fullerene molecule

The transport properties of the cage-like molecule depend on its orientation between the electrodes, but the investigation on the mechanism has not been found. Using first-principle density-functional theory (DFT) and non-equilibrium Green’s function (NEGF) formalism for quantum transport calculation, we study the electronic transport properties of C₂₄ fullerene molecule with different orientations in Au–C₂₄–Au two-probe system. The effects of k-point sampling on the Brillouin zone are explored. Our results show that the negative differential resistance of C₂₄ molecule is found in such a system and can be tuned by the molecule's orientation in the two-probe system. We also proposed a mechanism for it. The I–V characteristic under bias voltage is determined. The present findings could be helpful for the application of the C₂₄ molecule in the field of single molecular devices or nanometer electronics.     

A first-principles measure for the twinnability of FCC metals

Twinnability is the property describing the ease with which a metal plastically deforms by twinning relative to deforming by dislocation-mediated slip. In this paper a theoretical measure for twinnability in face-centered-cubic (fcc) metals is obtained through homogenization of a recently introduced criterion for deformation twinning (DT) at a crack tip in a single crystal. The DT criterion quantifies the competition between slip and twinning at the crack tip as a function of crack orientation and applied loading. The twinnability of bulk material is obtained by constructing a representative volume element of the material as a polycrystal containing a distribution of microcracks and integrating the DT criterion over all possible grain and microcrack orientations. The resulting integral expression depends weakly on Poisson's ratio and significantly on three interfacial energies: the stacking-fault energy, the unstable-stacking energy and the unstable-twinning energy. All these four quantities can be computed from first principles. The weak dependence on Poisson's ratio is exploited to derive a simple and accurate closed-form approximation for twinnability which clarifies its dependence on the remaining material parameters. To validate the new measure, the twinnability of eight pure fcc metals is computed using parameters obtained from quantum-mechanical tight-binding calculations. The ranking of these materials according to their theoretical twinnability agrees with the available experimental evidence, including the low incidence of DT in Al, and predicts that Pd should twin as easily as Cu.     

Synthesis and structure of a new halophosphate Sr3P3O10Cl with the flexible [P3O10]5− anions

A new halophosphate crystal Sr₃P₃O₁₀Cl was grown in molten chloride flux media. It crystallizes in the centrosymmetric orthorhombic space group Pnma (No. 62) with a = 10.617(2) Å, b = 10.736(2) Å, and c = 8.7354(17) Å. In the structure, the basic building unit is the [P₃O₁₀]⁵⁻ anion, which is consist of three PO₄ tetrahedra by sharing the corner oxygen atoms. The two Sr atoms and the Cl atom are linked to construct an infinite [Sr₃Cl]⁵⁺ chain. The [P₃O₁₀]⁵⁻ anions are interconnected with the [Sr₃Cl]⁵⁺ chains to form a three-dimensional frameworks. Additionally, the first-principle calculation was employed to obtain the band structures and densities of states.     

H在Mg(0001)表面吸附、解离和扩散的第一性原理研究

采用基于密度泛函的第一性原理方法, 同时结合Nudged Elastic Band方法, 系统研究了H2分子和H原子在Mg(0001)表面的吸附过程. 给出了H2分子的解离路径和势垒, 结果表明H2分子的吸附过程中仅存在物理吸附; 在给出H原子在Mg(0001)表面的吸附势能面的基础上, 进一步研究了H原子在Mg(0001)表面及体内的扩散过程. 计算发现, Mg(0001) slab存在表面效应, 且对H原子的表面扩散影响较明显. 在此基础上, 通过比较解离、扩散和放氢环节的激活能数据, 为H2分子的解离和氢化物的放氢过程是速控步骤这一结论提供了理论支持.     

N掺杂p-型ZnO的第一性原理计算

采用基于密度泛函理论(DFT)的第一性原理平面波超软赝势方法, 计算了纤锌矿ZnO和N掺杂p-型ZnO晶体的电子结构, 分析了N掺杂p-型ZnO晶体的能带结构、电子态密度、差分电荷分布以及H原子和N2分子对p-型掺杂ZnO的影响.     

Mn掺杂Fe2B晶体结构、力学性能及电子结构的第一性原理研究

基于密度泛函理论的第一性原理计算方法,本文系统地研究了Fe8-xMnxB4 (x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, 6, 7 ,8)的晶体结构、机械性能和电子结构。计算得到Fe2B的晶格常数与实验值相符,所有相都具有良好的热力学稳定性和机械稳定性。随着Mn掺杂浓度逐渐增大,Fe8-xMnxB4的各向异性先减弱后增强,Fe7.75Mn0.25B4的各向异性最弱。当Mn掺杂浓度较低时,Fe8-xMnxB4的硬度略微降低,韧性增强。除了Fe7Mn1B4、Fe6Mn2B4、Fe5Mn3B4、Fe4Mn4B4之外,其余的Fe8-xMnxB4相的韧性均比Fe2B好。由电子结构可以发现,Fe8-xMnxB4的力学性能主要由Fe-B键或Mn-B键决定。Mn掺杂到Fe2B中会使得B-B共价键增强,Fe2B的本征脆性得到改善,同时Fe2B的磁性不断减弱。     

Study on mechanism of photocatalytic performance of La-doped TiO2/Ti photoelectrodes by theoretical and experimental methods

TiO₂ photoelectrodes with various nanostructures have been successfully prepared by the anodization method. The morphology, microstructure and optical properties of as-prepared photoelectrodes were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet/visible light diffuse reflectance spectra (UV/vis/DRS), surface photovoltage spectroscopy (SPS) and photocurrent. The electronic structure and optical properties of La doped/undoped TiO₂ photoelectrodes with different crystal structures were calculated by the density function theory. The photocatalytic and photoelectrocatalytic activities of as-prepared photoelectrodes were evaluated. The results showed that the anodization potentials played a crucial role in the surface morphology and microstructure. Both results of theoretical calculations and experimental tests demonstrated that La-doped photoelectrodes were more sensitive to light than undoped one. The difference of photoelectrodes performance was ascribed to the crystal configuration, impurity energy levels and long-range orientation moving of photogenerated carriers.     

Interatomic exchange in Mn-doped III-V semiconductors

Density-functional calculations are used to determine the electronic structure and magnetic properties of dilute magnetic semiconductors with the composition X_1−xMn_xN (X=Al, Ga, In, x=6.25% and 12.5%). Emphasis is on the interatomic exchange as a function of the Mn-Mn distance. Our superlattice calculations show that the Mn dopants are spin-polarized with a half-metallic band gap and a magnetic moment of 4 μ_B per Mn atom at x=6.25 and 12.5%. The Mn (3d) bands lie in the band gap but partially hybridize with valence band or N 2p electrons, depending on the group-III element and on the spin direction. To calculate the exchange interaction parameters J_ij, we have used a Green-function approach. The interaction between Mn atoms extends over several interatomic interactions and is mediated by nitrogen (2p) electrons. The exchange is always ferromagnetic and largest for the first nearest neighbors, but substantial ferromagnetic interactions persist over Mn-Mn distances up to sixth nearest neighbors in the considered supercell.