PbWO4:(Sb,Y)晶体的发光和闪烁性能研究

本文报道了用多坩埚下降法生长的大尺寸PbWO4:(Sb,Y)晶体的光谱和闪烁性能.基于透射光谱、X射线激发的发射谱、紫外激发及其发射谱、光产额和超短脉冲X射线激发荧光寿命等方面的测试,讨论了Sb,Y双掺杂对PbWO4晶体的光谱和闪烁性能的作用.结果表明:Sb,Y双掺杂能显著改善PbWO4晶体的光谱性能和闪烁性能,使PbWO4晶体在短波方向的透过率明显提高;对于尺寸为23×23×20mm3的掺杂晶体样品,光产额最大值大约为50 p.e./MeV,约为BGO光产额的6.0;;发光成份中有1.9ns和15.8ns两个衰减时间常数的快成份.     

掺杂TGS晶体的生长及热释电性能研究

选择重稀土离子Dy3 为掺杂阳离子,DL-丙氨酸与L-谷氨酸部分取代甘氨酸分子,生长了不同掺杂配比的TGS晶体。生长和测试实验表明,掺杂TGS晶体较纯TGS晶体易于生长。将掺杂晶体生长溶液的pH值控制在1~4,可改变掺杂晶体的结晶习性。用ICP发射光谱测试了掺杂晶体中稀土元素的含量,用X射线粉末衍射法测定了晶格参数,结果表明:元素已进入晶体,晶格参数稍有变化,但掺杂晶体的对称性仍为C2-2。通过测量掺杂晶体的电滞回线,得到了内偏压场,还测量了各样品的热释电系数、自发极化强度,作了温度曲线,并分析了各掺杂剂对提高热释电性能和锁定极化的作用。结果表明:是有应用前景的热释电材料。     

NaF、LiF掺杂钨酸铅晶体的性能研究

采用坩埚下降法生长了NaF、LiF掺杂钨酸铅(PbWO4)晶体,研究了掺杂晶体的透射光谱、光产额和X射线激发发射谱等发光性能.结果表明:与未掺杂PbWO4晶体相比,NaF掺杂可以显著提高PbWO4晶体在350nm附近的透过率,其X射线激发发射谱中出现新的发光峰;而LiF掺杂引起了晶体可见光范围内的强烈吸收,造成PbWO4晶体光产额的显著下降.基于影响PbWO4晶体闪烁性能的原因分析,提出了采用其它价态金属氟化物掺杂来提高PbWO4晶体光产额的新思路.     

掺杂CLBO晶体的生长及性能研究

本文采用顶部籽晶泡生法进行掺杂CLBO晶体的生长及性能研究.分别掺入稀土离子Nd3+、Ce3+、Eu3+,均生长出透明晶体.通过测量掺杂CLBO晶体的荧光谱和定量分析晶体中杂质含量,证实这些杂质确实已掺入CLBO晶体中. 对掺杂晶体进行了吸潮特性和倍频效应的研究,发现掺杂CLBO晶体的吸潮性能有很大的改善,激光倍频转换效率仍然很高.     

钨酸铅闪烁晶体垂直梯度凝固法生长及光学性能研究

本文报道了垂直梯度凝固法(VGF)生长PWO晶体的研究结果.通过优化工艺参数,成功获得直径25mm、长度140mm的PWO晶体.通过测试PWO晶体的XRD、透过光谱、荧光光谱等,研究了所得晶体的光学性能.结果表明:VGF法生长PWO晶体在350～420 nm处的光学透过率明显提高,荧光发光主峰位于435 nm,是快发光峰,但慢发光比例有所增加.     

钨酸铅闪烁晶体的双掺杂性能研究

本文采用坩埚下降法生长出了PbWO4:(F,Er)晶体和PbWO4:(F,Nd)晶体,并且对此两种晶体透过性能和闪烁发光性能进行了测试分析.透过光谱结果显示,PbWO4:(F,Er)晶体和PbWO4∶(F,Nd)晶体在350 nm至700nm范围内的透过率比纯的钨酸铅均有较大的提高.紫外激发发射光谱测试结果表明,通过双掺杂可以提高钨酸铅晶体闪烁发光的强度,并且PbWO4:(F,Er)双掺杂晶体在波长为527 nm处和546 nm处均形成了较强的发光峰.通过对掺杂晶体进行XRD分析可知,阴阳离子的同时掺入并未引起钨酸铅晶体结构的明显改变.     

Yb3+掺杂稀土钽铌酸盐的晶体场从头计算研究

Yb³⁺掺杂晶体材料是重要的激光材料,在超短脉冲激光、大功率激光等领域有重要应用前景,但长期以来很难通过实验拟合确定Yb3+晶体场参数,尤其是低对称体系,从头计算是解决此问题的重要途经.本文介绍了适合计算稀土离子掺杂晶体的从头计算DV-Xα方法和有效哈密顿量模型,用该方法计算了Yb³⁺掺杂M型和M'型钽铌酸盐的晶场参数和旋轨耦合参数,得到的能级结构和实验能很好地符合,并发现了Yb³⁺掺杂钽铌酸盐的晶场强度参数随稀土原子序数呈现规律变化.表明结合DV-Xα计算和有效哈密顿量方法是计算Yb³⁺掺杂低对称钽铌酸盐晶体场的有效途径,结果显示Yb3+掺杂钽铌酸盐晶体有望成为新型全固态激光工作物质.     

ZnO:Sb掺杂机理的第一性原理研究

为了研究ZnO:Sb的掺杂机理,本文运用第一性原理密度泛函理论计算了理想纤锌矿ZnO和SbO 、SbZn 、SbZn-2VZn三种Sb掺杂ZnO晶体模型的几何结构、能带结构和电子态密度.计算结果表明:sb的掺人使得晶格发生不同程度的膨胀,其中以SbZn-2VZn复合缺陷模型的膨胀最小,键长最短,说明此结构的化学稳定性最高.通过能带和态密度的分析可知,ShO和SbZn模型存在不合理性,而SbZn-2VZn复合缺陷中的VZn可以使价带产生非局域化空穴载流子.定量计算进一步确认了SbZn-2VZn构型的可填充电子数最多,合理解释了晶体导电性的提高.形成能计算表明,在富氧条件下SbZn-2VZn的形成能最低,说明在富氧条件下掺杂Sb更有利于实现ZnO的p型化.     

Er3+掺杂钨酸铅晶体的发光和光谱特征

本试验测试了Er3 掺杂钨酸铅晶体(PbWO4:Er3 )的吸收光谱,依据J-O理论,首次计算报道了光谱项特征:J-O强度参数、量子荧光效率、荧光分支比等,Ω2=3.75×10-20cm2,Ω4=0.67×10-20cm2,Ω6=0.41×10-20cm2。计算证实,PWO:Er3 中几乎有80%的激发能量非辐射跃迁转移致4I13/2能级,4I13/2的计算寿命是5200μs,J-O计算显示,在PWO中产生4I13/2→4I15/2和发射1.53μm.有高的几率。讨论了不同浓度Er3 掺杂对于吸收系数和PbWO4晶体光学吸收边的影响,讨论了Er3 掺杂PbWO4晶体的光致发光和X射线激发发光光谱,PbWO4:Er3 晶体中存在着从PbWO4基质到Er3 离子的能量传递,发光光谱的分析表明,这种能量传递是共振能量传递。     

Ce3+单掺和Ce3+,Eu3+共掺杂CaF2晶体生长与光学特性研究

采用Bridgman方法在氩气氛保护的单晶炉内生长出了Ce3+掺杂的以及Ce3+和Eu3+共掺的CaF2晶体,利用Ce2O3和E2O3分别与氢氟酸反应合成了CeF3和EuF3掺杂试剂,对掺杂晶体生长的起始原料-特定组成的CaF2、CeF3、EuF3混合物进行了高温氟化处理,确定了掺杂晶体的物相,讨论了掺杂浓度对晶体紫外波段透过率的影响.此外,还初步分析了Ce3+和Eu3+共掺的CaF2晶体作为紫外滤光材料的可行性.     

Relationship of Morphological Types of Dynamic Forms of Crystals

Crystallography Reports - The relationship of morphological spectra (sets of data on the morphological types of real polyhedral crystals and their probabilities under current physicochemical...     

A review of measurement of thermophysical properties of silicon melt

Measurements of thermophysical properties of Si melt and supplementary study of X-ray scattering/diffraction by the authors' group were reviewed. The values obtained differed variously from those of literature. Density was 2–3% larger, surface tension 20–30% smaller, viscosity up to 40% larger, electrical conductivity 8% smaller, spectral emissivity more or less in good agreement with literature values, and thermal diffusivity a few percent larger. An anomalous density jump was found near the melting point. Surface tension and viscosity also showed anomaly. A strange time-dependent change of density was observed over 3 h after melting. X-ray analyses suggested a slight change in local atom ordering, but showed no sign of cluster formation. An addition of 0.1 at% gallium caused the density jump to disappear, while that of boron caused no change. An EXAFS study of the former melt indicated a strong interaction between Ga and Si atoms as if molecules of GaSi₃ existed. The implications of the measured properties are a possibility of soft-turbulence in an Si melt in a relatively large crucible, a more complicated manner of intake of oxygen depleted molten Si from the free surface region to underneath the growing crystal, and a relaxation of the melt after melting arising from trapped gas species.     

Generation of crystal structures of heteromolecular compounds by the method of discrete modeling of packings

Within the method of discrete modeling of packings, an algorithm of generation of possible crystal structures of heteromolecular compounds containing two or three molecules in the primitive unit cell, one of which has an arbitrary shape and the other (two others) has a shape close to spherical, is proposed. On the basis of this algorithm, a software package for personal computers is developed. This package has been approved for a number of compounds, investigated previously by X-ray diffraction analysis. The results of generation of structures of five compounds—four organic salts (with one or two spherical anions) and one solvate—are represented.     

Development of Pauling's concepts of geometric characteristics of atoms

The evolution of the geometric characteristics introduced by Pauling and their dependence on the specific features of the structure and chemical bonds have been considered. The values of the covalent and van der Waals radii are given as well as their relationships and mutual transitions.     

Investigation of the Mechanism of Influence of Stress Corrosion on the Development of Macroplastic Instabilities of Aluminum–Magnesium Alloy

Crystallography Reports - Macroscopic jumps of plastic deformation (few percent in amplitude) on creep curves of aluminum–magnesium alloy, caused by a local effect of concentrated solution of...     

Velocities of Disappearance and Lifetime of Faces of Growing Crystals

The formulae for absolute R_disap and relative R velocities of disappearance and lifetime τ of faces of growing crystals have been derived for stationary growth. It was shown that the quantities are determined by the relative growth velocity R_A/R_critA of the vanishing face A with respect to the critical growth velocity R_critA and by the geometry of a crystal expressed by the trigonometric functions of interfacial angles β and γ formed between face A and the adjacent faces. R increases and τ decreases with the increase in R_A/R_critA to certain limiting values. The calculations have been verified and illustrated by the experimental results for triclinic potassium bichromate (KBC) crystals. Results enable ones to predict values of velocities of disappearance and lifetimes of undesirable, supplementary faces of any real crystal.     

Comparative analysis of two methods of calculation of the orientation of domain walls in ferroelastics

Two types of domain-wall equations are analyzed: the equations derived by the Sapriel method and the equations obtained by interface matching of the thermal-expansion tensor. It is shown that, for W-type domain walls, these methods yield the same equations. For W′-type domain walls, the equations obtained by different methods coincide for proper ferroelastics and differ for improper ferroelastics.     

Dependence of the parameters of equations of viscous flow on chemical composition of silicate melts

The temperature dependence of viscosity of silicate melts is discussed in the framework of the Avramov–Milchev (AM) equation. The composition is described by means of two parameters: the molar fraction, x, and the “lubricant fraction”, l. The molar fraction is the sum of the molar parts x_i of all oxides dissolved in SiO₂, the molar fraction of the latter being 1 ? x. It is shown that, with sufficient precision, two of the parameters of the AM equation can be presented as unique functions of the molar fraction. On the other hand, x is not sufficient to determine properly the reference temperature T_r , at which viscosity is η_r = 10¹³ [dPa.s]. Therefore, additional parameter, “lubricant fraction” l, is introduced. For each of the components, l_i is a product of molar part x_i and a specific dimensionless coefficient 0 ≤ k_i ≤ 1 accounting for the specific contribution of this component to the increased mobility of the system. It is demonstrated that, for l > 0, the reference temperature is related to the “lubricant fraction” l through the reference temperature T_r,SiO2 of pure SiO₂.     

Mechanism of thermal expansion of structural modifications of zinc pyrovanadate

Crystal chemical characteristics of the α and β modifications of Zn₂V₂O₇ are calculated based on in situ high-temperature X-ray measurements. The expansion of the structure is found to be strongly anisotropic up to the negative volumetric thermal expansion of the α-Zn₂V₂O₇ unit cell in the temperature range of 300–600°С, α _V =–17.94 × 10^–6 1/K. The transformations of the “hard” and “soft” sublattices with an increase temperature and at the phase transition are considered in detail. It is shown that the negative volumetric thermal expansion of α-Zn₂V₂O₇ is due to the degeneracy of the zigzag-like shape of zinc–oxygen columns at constant distances between their vertices.