沉淀剂对ZnO压敏陶瓷缺陷结构和电气性能的影响

研究了不同沉淀剂(NH₄HCO₃和NaOH)对共沉淀法制备的ZnO压敏陶瓷性能的影响. 结果表明: 不同的沉淀剂对ZnO压敏陶瓷的微观结构及电气性能有明显的影响. 其中陶瓷微观结构的变化主要由沉淀剂本身的性质引起; 而电气性能的改变除了与微观结构相关外, 主要受不同沉淀剂对陶瓷晶界势垒参数的影响; 此外, 沉淀剂NaOH引入的Na⁺作为施主杂质离子掺杂ZnO压敏陶瓷, 增加晶粒中的自由电子浓度, 因此本征缺陷(锌填隙和氧空位)浓度受到抑制, 而锌填隙浓度相对于氧空位而言对施主离子掺杂更为敏感. 由此, 采用共沉淀法制备ZnO压敏陶瓷粉体时, 沉淀剂种类的选择很重要, 即使微量的杂质也会引起压敏陶瓷性能的较大改变, 应尽可能避免杂质离子的引入. 关键词： ZnO压敏陶瓷 缺陷结构 沉淀剂 介电性能     

达罗他胺中间体合成中杂质的发现和结构表征

达罗他胺是治疗前列腺癌的重要药物,在进行其合成工艺研究时,在第一步Suzuki偶联和第二步水解脱保护反应中,发现并纯化得到3个杂质A、B和C,其中杂质A和B来自第一步反应,杂质C来自第二步反应.通过高分辨质谱(HRMS)、核磁共振氢谱(¹H NMR)和核磁共振碳谱(¹³C NMR)表征方法,确定了杂质A和B的结构,分别为初始反应原料化合物2的脱硼酸频哪醇酯产物和初始反应原料化合物1的双偶联产物;借助HRMS、¹H NMR、¹³C NMR、¹H-¹H COSY、¹H-¹³C HSQC、¹H-¹³C HMBC和¹H-¹H NOESY方法确定了中间体化合物3和杂质C的准确结构,对其形成机理和规避方法也进行了讨论分析.     

用正电子湮没研究钙钛矿结构压电陶瓷中的点缺陷

用正电子湮没寿命谱技术对有意掺杂的或被杂质替代的钙钛矿结构压电陶瓷中的点缺陷进行了研究.掺杂了La³⁺的PbTiO₃，掺杂了Sr²⁺，Cd²⁺和La³⁺的Pb(Mg_1/3Nb_2/3)_0.375Ti_0.375Zr_0.25O₃，掺杂了Ca²⁺的BaT 关键词：     

ZnS掺Ag与Zn空位缺陷的电子结构和光学性质

本文基于第一性原理平面波赝势(PWP)和广义梯度近似(GGA)方法,对ZnS闪锌矿结构本体、掺入p型杂质Ag和Zn空位超晶胞进行结构优化处理. 计算了三种体系下ZnS材料的电子结构和光学性质,并从理论上给出了p型ZnS难以形成的原因. 详细分析了其平衡晶格常数、能带结构、电子态密度分布和光学性质.结果表明:在Ag掺杂与Zn空位ZnS体系中,由于缺陷能级的引入,禁带宽度有所减小,在可见光区电子跃迁明显增强. 关键词： 硫化锌 缺陷 电子结构 光学性质     

Ni4PrB的电子结构和磁性能研究

考虑Pr-4f及Ni-3d电子间的库仑作用U和交换作用J,采用局域自旋密度近似LSDA(Local spin-density approximation)及LSDA+U(在位库仑势)近似,对Ni₄PrB化合物进行结构优化,并计算体系电子结构,能带结构和磁性能. 结果显示,Ni₄PrB具备金属半导体性质,存在Pr-Ni铁磁耦合. U的引入对体系磁特性和结构稳定性有关键作用,加U前体系磁性来源为Ni原子磁矩,加U后体系磁性来源为Pr原子,且体系稳定性提高,U值的作用对于修正体系强关联有重要影响,可以合理描述由强关联和自旋排斥引发的排斥效应. 关键词： 密度泛函理论 电子键结构 磁性能 稀土过渡金属间化合物     

非晶态超导合金Zr87.7Si12.3中结构弛豫和结晶化对超导

对非易态超导合金Zr_87.7Si_12.3,在适当的温度下进行了lh等时退火处理.测量了其结构、超导临界参量T_c,H_(C2)及其转变宽度ΔT_c,ΔH_c2等的变化.发现些与金属-金属型非晶态合金不同的变化规律,并结合在结构弛豫过程中所发生的拓扑短程有序化和化学短程有序化,以及相应的结晶化机制,对实验结果作了讨论. 关键词：     

单层正三角锯齿型石墨烯量子点的电子结构和磁性

采用基于密度泛函理论的广义梯度近似(GGA),对不同尺寸(N=2—11)的单层正三角锯齿型石墨烯量子点(Z_N -GNDs)的结构进行优化,得到与实验数据较好符合的晶格常数,进一步计算得到不同尺寸下体系的自旋多重度、磁矩、电子态密度以及自旋电子密度.结果表明:所有体系都呈现金属性,在尺寸较小的体系中量子尺寸效应对电子结构的影响比较明显;与单层石墨烯片一样,sp²杂化作用和非键态电子在量子点中仍起到非常重要的作用;费米能级上有自旋向上的电子分布,体系的 关键词： 石墨烯 量子点 电子结构 磁性     

过渡金属W、Mn、V、Ti掺杂二维材料MoSi2N4的第一性原理计算

本文基于密度泛函理论(DFT)的第一性原理计算了W、Mn、V、Ti替位掺杂二维MoSi₂N₄后的几何结构、电子结构以及光学性质的变化.电子结构分析表明W、Mn、W、Ti替位掺杂二维MoSi₂N₄后的禁带宽度分别为1.806 e V、1.003 e V、1.218 e V和1.373 e V;四种过渡金属掺杂后MoSi₂N₄的带隙类型没有发生改变,均为间接带隙半导体;W掺杂后的杂质能级靠近价带顶,费米能级靠近价带顶,为p型半导体,杂质能级为受主能级;Mn掺杂后的杂质能级靠近导带底,费米能级靠近导带底,为n型半导体;V和Ti掺杂后杂质能级位于费米能级附近,为复合中心;光学性质分析表明,在2 e V～4 e V的能量区间内,W掺杂结构的吸收波长为336 nm,体系发生红移;Mn、V和Ti替位掺杂后的吸收波长分别为320 nm、358 nm和338 nm,且掺杂体系均发生蓝移.     

稀磁合金的杂质互作用效应

本文用格林函数方法讨论Tk(Kondo温度)时杂质互作用对Kondo效应的影响。对s-d互作用哈密顿量作自洽场近似时,同时计入导致Kondo效应和产生杂质互作用的切断项,求得了杂质系统的Kondo温度和低温电阻。结果表明:在稀磁合金中,杂质间的互作用效应使Kondo温度下降,并且使T《T_k时的电阻率温度变化曲线由(1-AT²)型变为(1+BT²)型(A,B>0),从而可能在Tk温区产生电阻极大。 关键词：     

固体电解质蒙脱石结构与电导性能研究

对一组提纯蒙脱石样品研究表明,它们具有不同的次微结构。红外(IR)谱中吸收带915和840cm^-1的变化及电子自旋共振(ESR)谱中g≈4.3等的变化反映了各样品次微结构的差异。研究了它们的离子导电性,讨论了结构对其电导性能的影响。 关键词：     

Magnetism without magnetic ions in non-magnetic perovskites SrTiO3, SrZrO3 and SrSnO3

Using the full-potential linearized augmented plane-wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange-correlation potential, we studied spin polarization induced by replacement of oxygen atoms by non-magnetic 2p impurities (B, C and N) in non-magnetic cubic SrMO₃ perovskites, where M=Ti, Zr and Sn. The results show that the magnetization may appear because of the spin–split impurity bands inside the energy gap of the insulating SrMO₃ matrix. Large magnetic moments are found for the impurity centers. Smaller magnetic moments are induced on the oxygen atoms around impurities. It is shown that SrTiO₃:C and SrSnO₃:C should be magnetic semiconductors while other compounds in this series (SrTiO₃:B, SrTiO₃:N and SrZrO₃:C) are expected to exhibit magnetic half-metallic or pseudo-half-metallic properties.     

Percolation mechanism of the diffusion of impurity atoms in dense surface layers

The molecular dynamics method is used to study the migration of an impurity atom on an unfilled square lattice. The calculations are performed on a lattice containing 2¹² × 2¹⁴ sites at various values of the ratio p of the frequencies of jumps impurity and lattice atoms and various relative concentrations of vacancies ? _V . In the limit of vanishingly small concentrations of vacancies, ? _V ? 1, the present simulation results are in agreement with our previous analytical results. With increasing ? _V , the diffusion coefficient of impurity atoms predicted by the simulations exceeds the result of the analytical theory, a behavior that can be explained by the growing influence of vacancy clusters, voids on the surface, in which the impurity atom can travel long distances. This is most clearly seen in the case of highly mobile impurity atoms (p ? 1), where within the characteristic time of displacement of impurity atoms, lattice atoms remain practically immobile, and the problem appears to be closely related to the percolation problem. In this case, up to ? _V ≈ 0.3, the diffusion coefficient is independent of p; then, such a dependence appears, and the diffusion coefficient increases sharply with ? _V .     

Off-centering of Pb and Sn impurities in GeTe induced by strong local stress

Noncentrality of large impurity atoms—Pb and Sn atoms—substituting for Ge atoms in a GeTe lattice has been discovered by means of EXAFS investigations in Ge_0.9Pb_0.1Te and Ge_0.85Sn_0.15Te samples. The transition of impurity atoms into a noncentral position under conditions of a strong local stress is explained by the participation of an unshared electron pair from the impurity atoms in the formation of the chemical bond. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 600–603 (25 April 1996)     

Thermoelectric properties of homogeneously and non-homogeneously doped CdTe15/16M1/16 (M=N,P, As,Sb) and Cd15/16TeM1/16 (M=Na,K, Rb,Cs)

The electrical transport properties of p-doped semiconductors CdTe_15/16M_1/16 (M=N, P, As, Sb) and Cd_15/16TeM_1/16 (M=Na, K, Rb, Cs) with two configurations are investigated through first-principles calculations combined with Boltzmann transport theory under the relaxation time approximation. It is found that N and Cs atoms in the homogeneous structure induce much sharper electron densities of states (DOSs) and flatter energy bands at the valence band edges than the rest of doped elements, resulting in much larger Seebeck coefficients. The calculations reveal that most of the Seebeck coefficients and electrical conductivities are impacted unfavorably by the conglomeration of impurity atoms considered. Though the power factors for homogeneous doping of N and Cs are comparatively smaller, the electronic figures of merit are much larger at 800–1000 K than the rest ones due to much smaller electronic thermal conductivities, therefore probably enhancing the thermoelectric figures of merit. The results show that doping the elements with electronegativities distinct from the host atoms can enhance the Seebeck coefficients and the thermoelectric performances of bulk semiconductors efficiently if the energy levels of doped atoms resonate with those of host atoms and the arrangement of doped atoms is modulated appropriately to avoid deteriorating the sharpness of the DOS (or transport distribution).     

Hierarchy of percolation thresholds and the mechanism for reduction of magnetic moments of transition metals intercalated into TiSe<Subscript>2</Subscript>

The concentration dependences of the effective magnetic moment of transition metal atoms intercalated into TiSe₂ are analyzed in the framework of the percolation theory. It is shown that, depending on the degree of localization of impurity states, the effective magnetic moment is determined by the overlap of 3d orbitals of transition metals or orbitals of titanium atoms coordinated by impurity atoms.     

SC-molecular/cluster calculations of the electric field gradient on 5sp-impurities (Ag-Xe) in cadmium — Volume and surface effects

The electric field gradient (EFG) on impurity atoms (Ag-Xe) in hcp Cd are calculated within the SC local density molecular/cluster approach using 27 atoms. The systematic trend and change of sign with increasing impurity charge, is well reproduced and explained by a successive population of hybridized p_z and p_x, p_y orbitals. Results on the volume and c/a dependence of the EFG will also be presented, as well as on the surface EFG.     

Annealing and oxidation effects after CF4/H2 dry etching for damage recovery

When a Si substrate, just after SiO₂ etching, is exposed to an additional etch using CF₄ + H₂ reactive sputter etching, carbon and fluorine atoms penetrate into the Si substrate to a depth of ≈ 100–150 Å. The effects of an annealing and oxidation atmosphere for this damage recovery and impurity redistribution in the Si substrate are investigated using IMA and Secco etching. Annealing in N₂ gas is only effective for removing fluorine atoms. After dry O₂ oxidation, though the carbon atoms are removed, the fluorine atoms are still confined in the oxidized layer. Under wet oxidation, carbon and fluorine atoms are removed as soon as the damaged layer is oxidized. Moreover, after 300 Å wet oxidation, no impurity atoms or defects are observed. Wet oxidation may serve as a pretreatment for Si selective epitaxial growth on a dry etched Si substrate.     

First-principle study of magnetism in Co-doped SnO2

The effects of Co dopants and oxygen vacancies on the electronic structure and magnetic properties of the Co-doped SnO₂ are studied by the first-principle calculations in full-potential linearized augmented plane wave formalism within generalized gradient approximations. The Co atoms favorably substitute on neighboring sites of the metal sublattice. Without oxygen vacancies, the Co atoms are at low spin state independent of concentration and distribution of Co atoms, and only the magnetic coupling between nearest-neighbor Co atoms is ferromagnetic through direct exchange and super-exchange interaction. Oxygen vacancies tend to locate near the Co atoms. Their presence strongly increases the local magnetic moments of Co atoms, which depend sensitively on the concentration and distribution of Co atoms. Moreover, oxygen vacancies can induce the long-range ferromagnetic coupling between well-separated Co atoms through the spin-split impurity band exchange mechanism. Thus the room temperature ferromagnetism observed experimentally in the Co-doped SnO₂ may originate from the combination of short-range direct exchange and super-exchange interaction and the long-range spin-split impurity band exchange model.     

Kinetics of the diffusion of an impurity atom on a square lattice

An analytical study of the migration of an embedded impurity atom over a solid surface under the influence of the diffusion of vacancies is presented. The case of small surface coverages of both vacancies ϑ _v and impurity atoms ϑ _i , with ϑ _i ≪ ϑ _v ≪ 1, is considered. It is shown that the realization of multiple collisions of a single impurity atom with vacancies imparts a Brownian character to its motion. At long times, the dependence of the mean square displacement on the time differs little from the linear, whereas the spatial density distribution is close to the Gaussian, features that makes it possible to introduce a diffusion coefficient. For the latter, an analytical expression is derived, which differs from the product of the diffusion coefficient of vacancies and their relative concentration only by a numerical factor η. The dependence of the diffusion coefficient of an impurity atom on the ratio of the frequency of its jumps to the frequency of jumps of vacancies is analyzed. In the kinetic mode, at ω ≪ 1, the diffusion coefficient of impurity atoms depends linearly on ω, whereas at ω ≫ 1, a saturation is observed; i.e., the dependence on the frequency of jumps of the impurity atom disappears. Nevertheless, the value of η remains less than unity, and no total entrainment of impurity atoms with vacancies occurs.     

Effects of substitutional impurity Au and Si atoms on antiphase boundary energies in Ti3Al: A first principles study

Effects of substitutional impurity atoms Au and Si on the energies of antiphase boundaries (APBs) on {1100} and (0001) planes in a Ti₃Al intermetallic compound were examined using first principles calculations. Au additions reduce the energies of APBs on both {1100} and (0001) planes by up to more than 40%. The reduction tends to be more remarkable especially when the added Au atom has larger number of Al atoms on its second-nearest neighbor sites rather than on first-nearest neighbor ones. In addition, in the case of Si addition, a significant energy reduction was found only for APBs on (0001) planes, and no remarkable dependence of APB energies on the coordinating atoms was found even for APBs on (0001) planes. These results are crucial to both understanding of the effect of APBs on the impurity diffusivity and predicting the ability of impurity atoms to stabilize antiphase domain structure that increases the strength of Ti₃Al dramatically.