Investigation of the local structure of Cu2+ ions doped in alkali lead tetraborate glasses by their electron paramagnetic resonance and optical spectra

The local structure of the Cu²⁺ centers in alkali lead tetraborate glasses was theoretically studied based on the optical spectra data and high-order perturbation formulas of the spin Hamiltonian parameters (electron paramagnetic resonance g factors g_∥, g_⊥ and hyperfine structure constants A_∥, A_⊥) for a 3d⁹ ion in a tetragonally elongated octahedron. In these formulas, the relative axial elongation of the ligand O^2? octahedron around the Cu²⁺ due to the Jahn–Teller effect is taken into account by considering the contributions to the g factors from the tetragonal distortion which is characterized by the tetragonal crystal-field parameters Ds and Dt. From the calculations, the ligand O^2? octahedral around Cu²⁺ is determined to suffer about 19.2% relative elongation along the C₄ axis of the alkali lead tetraborate glass system, and a negative sign for A_∥ and a positive sign for A_⊥ for these Cu²⁺ centers are suggested in the discussion.     

Lanthanum Oxide Effects on the Structure of Calcium Phosphate Glasses

Calcium phosphate glasses, in which part of calcium oxide was replaced by lanthanum oxide, were prepared by using the conventional melt quench method. The structures of xLa₂O₃ · (50-x)CaO · 50P₂O₅ (x = 0, 1, 3, 6, 12 mol%) samples were investigated by X-ray diffraction (XRD), Raman spectrum, Fourier transform infrared spectrum (FTIR), and differential scanning calorimetry (DSC). The results show that lanthanum oxide acts as network modifier in the network space of glass structure. The glass formation occurs at an O/P ratio of about 3.0–3.12. At larger values, the crystalline phases calcium pyrophosphate (Ca₂P₂O₇) and calcium lanthanum phosphate [Ca₉La(PO₄)₇] are detected in the samples. Raman and FTIR spectra indicate that the structure of lanthanum-free sample is chain P–O–P bond metaphosphate–based Q² units. Glass structure will change to Q² and Q¹ units when the lanthanum oxide content is less then 6 mol%. When lanthanum oxide content increases to 9 and 12 mol% more nonbridging oxygen in the glass, resulting in the depolymerization of the phosphate network, the network of glass transforms to Q², Q¹, and Q⁰ units mixture. Based upon DSC results, T_g slightly decreases because of the depolymerization of microstructure. Endothermal peak of DSC curves indicate that crystal phases separate out from vitreous body with the addition of lanthanum oxide content.     

Inelasticity corrections for time-of-flight and fixed wavelength neutron diffraction experiments

Time-of-flight neutron diffraction methods are widely used to study the structure of liquids and glasses. The scattering nuclei in these experiments suffer nuclear recoil in the course of the neutron scattering process, which gives rise to distortions to both the self and distinct structure factors extracted from the data. These distortions are in general difficult to evaluate quantitatively, especially when the mass of the nucleus is similar to that of the neutron, such as when hydrogen is present in the material being studied. Traditional treatments of this effect generally assume the neutron energy is lower than the excitation energy of an atom, but for time-of-flight diffraction this is never the case, and the experiments typically sample a wide range of energy transfers from sub-meV to tens of eV. In addition, by attempting to produce an analytical correction, such methods invariably make a long list of approximations which can be difficult to justify in particular cases. Here, a model for the scattering kernal is developed based on the well known harmonic oscillator model [A.C. Zemach and R.J. Glauber, Phys. Rev. 101, 118 (1956)]. This is shown to have the correct first and second moments of the scattering kernal for both the self and distinct scattering, and is used to estimate the self and distinct scattering from a diatomic ‘dumbell’ molecule. The model gives a realistic account of the single atom scattering from light and heavy water over a wide range of incident neutron energies, but is not yet accurate enough to perform quantitative corrections. In lieu of a quantitative approach, a ‘top hat’ convolution method is developed to perform the subtraction of self scattering from real data, and to allow data from multiple detector banks to be merged into a single structure factor. The harmonic oscillator model is also used to address the question of inelasticity effects on the interference scattering. For the intramolecular correlations at least at low scattering angles up to ～40° it is found that the effect of inelasticity is rather small – around 0.6% on the estimated OH bond length for H₂O. Although the emphasis here is on time-of-flight diffraction, the model is quite general and can just as easily be applied to the case of fixed wavelength neutron diffraction where it also gives accurate results.     

High pressure electrical resistivity studies on Ga-Te glasses

Abstract

Bulk, semiconducting Ga_xTe_100-x (17 ≤ x ≤ 25) glasses are prepared by the meltingh quenching method and electrical resistivity measurements are carried out at high pressures up to 8 GPa and low temperatures down to 77 K in a Bridgman anvil system. It is found that all the Ga_xTe_100-x, glasses exhibit metallization under pressure, with a continuous decrease in electrical resistivity and activation energy for conduction. Further, the high pressure metallic phases of Ga_xTe_100-x samples are found to be crystalline.     

亚波长金属光栅结构的制备与矢量衍射理论分析

利用纳米压印结合溅射和反应离子刻蚀工艺制备了周期为1μm、占空比为0.2的亚波长金属光栅,利用紫外-可见-近红外光谱仪测量了光栅的0级反射光谱。在严格耦合波分析的基础上,把光栅区域电磁场的空间谐波通过勒让德多项式展开,使用多项式展开的谱分析法求解常微分方程,计算了该亚波长金属光栅的反射光谱及磁场分布。实验测量结果同矢量衍射理论计算结果都显示,该光栅在近红外、中红外波段具有表面等离子体共振现象。数值计算结果还表明,对于此类亚波长金属光栅,当光栅的深宽比增加时,其反射光谱中会出现更多的反射谷。     

结构自还原效应对铋掺碱土金属硅磷铝硼玻璃超宽带近红外发光的影响

本文报道了结构自还原对铋掺杂碱土金属硅磷铝硼玻璃超宽带近红外发光性质的影响. 以Eu作为对比, 在空气气氛中采用高温熔融法分别制备了Eu₂O₃和Bi₂O₃掺杂的35SiO₂-25AlPO₄-12.5Al₂O₃-12.5B₂O₃-15RO(R=Ca,Sr,Ba) 玻璃. 结果证实该玻璃中可发生Eu³⁺→Eu²⁺的高温自还原现象, 且随着碱土金属离子半径增大Eu²⁺ 的自还原性减弱; 同样条件下Bi位于1300 nm波段的近红外发光却随之增强, 而位于1100 nm波段近红外发光和源于Bi²⁺的红光则减弱. 根据结构自还原机理及碱土离子半径变化对玻璃近红外超宽带发光性质的影响, 讨论了Bi离子的近红外发光中心的归属. 上述研究表明玻璃结构自还原特性可以为Bi近红外发光机理研究以及高效Bi掺杂超宽带近红外发光玻璃的设计提供一种有效的思路和方法. 关键词： 玻璃 铋掺杂 近红外发光 自还原效应     

Al-Ni-RE非晶合金的晶化行为和热稳定性

本文制备了一系列Al-Ni-RE (RE=La, Ce, Y) 非晶合金薄带, 利用差示量热扫描仪和X射线衍射仪考察了非晶合金的晶化行为和初生相, 并分析了其与合金成分和原子特性间的关系. 结果表明: 在拓扑不稳定参数λ以有效原子半径修正为λ'后, 每一Al-Ni-RE非晶合金体系可由其两个临界值划分为纳米晶、纳米玻璃和玻璃三类; Al-Ni-RE非晶合金的晶化开始温度和混合焓与λ'成良好的线性关系, 即λ'能很好的表征Al基非晶合金的热稳定性. 关键词： Al基非晶 玻璃转变 初生相 热稳定性     

Physical,optical and electrical properties of calcium bismuth borate glasses

The variation in physical, optical and electrical properties has been investigated as a function of Bi₂O₃ content in 20CaO?·?xBi₂O₃?·?(80???x)B₂O₃ (0?≤?x?≤?60, in mol%) glasses. The samples were prepared by normal melt-quenching process, and the optical absorption and reflection spectra were recorded in the wavelength range of 400–950 nm. The fundamental absorption edge has been identified from the optical absorption spectra. The optical band gap, E _g, for indirect allowed and indirect forbidden transitions has been determined from the available theories and its value lies between 1.80–2.37 eV and 1.08–2.19 eV, respectively. The theoretical fitting of the optical absorption indicates that the present glass system behaves as an indirect gap semiconductor. The origin of the Urbach energy, ΔE, has been associated with the phonon-assisted indirect transitions. The refractive index and optical dielectric constant have been evaluated from the reflection spectra. The density and molar volume are found to depend on the molar concentration of Bi₂O₃. The values of DC electrical conductivity have been measured from 373 to 623 K and the activation energy has been calculated. Theoretical optical basicity has been reported as a function of the Bi₂O₃ content. The variations have been discussed in terms of structural changes.     

A SIMPLISTIC ANALYTICAL MODEL OF PERMEABILITY FOR OPEN-CELL METALLIC FOAMS

针对通孔金属泡沫中的渗透率预测及现有理论模型的局限性,发展了一种新的全解析渗透率模型. 该模型以立方体结构作为代表单元,采用基于追踪流体微团轨迹的分支算法解析求解代表单元内的流动迂曲度. 渗透率的表达形式简单且不含任何拟合或经验参数,仅是孔隙率与平均孔径的函数. 采用实验测量和文献数据对模型预测进行了验证. 结果表明:提出的模型能够在较为宽广的孔隙率(0.55~0.98) 和孔密度(5~100 PPI) 范围内预测孔通孔金属泡沫的渗透率;采用分支算法得到的流动迂曲度能够较好地描述流体在通孔金属泡沫中的流动特征;采用开孔率修正的解析模型亦能对半开孔泡沫材料的渗透率提供良好预测.