热处理过程对纳米TiO2结构和性能的影响

以钛酸丁酯为钛源,采用溶胶-凝胶法制备纳米TiO2,结合TG-DTA,XRD,TEM等分析了不同热处理温度对纳米TiO2的晶型结构、晶粒粒径及微观形貌的影响.以甲基橙溶液为目标降解物,探讨了热处理温度对纳米TiO2光催化活性影响.利用Eastman的粒子生长理论对晶粒生长的动力学过程进行初步分析.研究表明:随着热处理温度升高,TiO2粒径逐渐从11.2 nm增大到78.6 nm;热处理温度为450～ 550℃时,纳米TiO2晶粒以锐钛矿为主,温度升至650℃时,出现了锐钛矿和金红石的混合相(质量比A∶R =9∶1),此时晶粒对甲基橙的降解率达到97.75;.而煅烧温度高于850℃后,TiO2几乎完全为金红石相,光催化活性显著下降.50;锐钛矿型TiO2转变金红石型TiO2的温度约为730℃,锐钛矿和金红石相晶粒表观活化能分别18.15 kJ/mol和42.56 kJ/mol;晶粒生长最快温度分别为546℃和1280℃.     

纳米TiO2薄膜的制备与光催化性能研究

以四氯化钛为前驱体,采用水解沉淀法,在不同的水解温度下,在硅藻土基多孔陶瓷上负载纳米TiO2薄膜,结合XRD和TEM对负载的纳米TiO2粒径进行了表征。对比分析了测试方法及水解温度对纳米TiO2粒径的影响。以罗丹明B为目标降解物,考察了水解温度对光催化剂活性的影响。结果表明:样品经550℃煅烧后,TiO2薄膜为锐钛矿型;水解温度为50℃时TiO2粒径小于水解温度为75℃时所负载纳米TiO2粒径;两种方法所测TiO2粒径有一定差异:dXRD>dTEM;水解温度为50℃所负载纳米TiO2薄膜,紫外光照300 min,对罗丹明B的去除率为89.9%,而75℃时,样品在紫外光照330 min,对罗丹明B的去除率为78.6%。     

单溶剂体系中CdSe纳米晶体的形貌调控研究

采用非水溶液方法,分别在十六胺(HDA)或邻苯二甲酸辛酯(DOP)的单溶剂体系中,通过调节前体加入速度和反应温度制备出六角形、多臂、短棒、长棒状以及四脚架形CdSe纳米晶体,并用透射电镜(TEM)对所得产品进行了表征.结果表明,前体的过饱和度强烈地影响CdSe纳米晶体的形貌,过饱和度的提高有导致六方纤锌矿结构(011-)方向上择向生长的趋势;两种单溶剂体系中在CdSe纳米晶体的形貌调控上有不同的影响;200 ℃不利于得到高质量的CdSe纳米晶体.     

水热合成硅酸锌纳米粉体及其形成机理的研究

以醋酸锌为锌源、正硅酸乙酯为硅源水热合成硅酸锌(Zn2 SiO4)晶体.研究了反应时间、温度、pH以及不同的反应溶剂对Zn2 SiO4晶体生长的影响.采用X射线衍射仪(XRD)分析样品物相,扫描电镜(SEM)和透射电镜(TEM)对样品晶粒聚集成球的过程在结构和形貌上进行表征.利用Johnson-Mehl-Avrami (JMA)方程对Zn2 SiO4晶体进行生长动力学分析.结果表明:随着温度升高与反应时间的延长,球状Zn2SiO4不断长大,结晶性能逐渐增强.水热合成的Zn2 SiO4晶体Avrami指数n145℃=0.55、n165℃=0.60、n205℃=0.71、n185℃=0.85,表明晶体的形成有从扩散机制向成核机制转变的趋势.     

电解液温度对二氧化钛纳米管阵列结构及光电性能的影响

以氟化铵(NH4F)、乙二醇溶液为电解液,在不同的电解液温度(0～50℃)条件下,采用电化学阳极氧化法制备二氧化钛(TiO2)纳米管阵列.采用扫描电子显微镜(SEM)、X射线衍射仪(XRD)、紫外-可见分光光度计(UV-Vis)对TiO2纳米管阵列的形貌、结构和光学性能进行表征.结果表明,电解液温度高于20℃均能制备出纳米管阵列.光照下电解液温度40℃时制备的TiO2纳米管阵列具有最高光电流密度(Iph)0.2408 mA/cm2.以活性艳红X-3B为目标降解物,在室温、紫外光照射条件下考察了电解液温度对光催化活性的影响,结果显示电解液温度40℃时制备的TiO2纳米管阵列具有最高的光催化活性.     

NiTiO3纳米晶的溶胶-凝胶制备工艺研究

以无机镍盐和钛酸四丁酯为起始原料,采用溶胶-凝胶法制备了NiTiO3纳米晶.采用X射线衍射(XRD)、综合热分析仪(DSC/TG)以及透射电镜(TEM)对制备的纳米晶进行了测试和表征.研究了不同镍源、后处理温度对钛酸镍纳米晶相组成以及显微结构的影响.结果表明:以硝酸镍为起始原料更易获得单一物相的NiTiO3纳米晶;以硝酸镍为原料,乙醇为溶剂,n(柠檬酸):n(Ni2++Ti4+)=1:1,后处理温度为700 ℃为制备NiTiO3纳米晶的最佳工艺条件.由谢乐公式推算得出NiTiO3晶粒生长符合Brook关系式,计算得到晶粒生长活化能为Ea=43.07 kJ/mol.     

助溶剂NH4I条件下CuI晶体的降温法生长及性能研究

以碘化铵(NH4I)作为助溶剂,采用溶液降温法生长CuI晶体.在50 ～60℃温区、6 mol/L助溶剂浓度条件下生长出厘米级尺寸的透明块状晶体.采用XRD、综合热分析(TG/DTA)对晶体进行结构表征,晶体属于γ相,相变温度分别为370℃(γ相→β相)和409℃(β相→α相).利用透射光谱、光致发光谱分析晶体的光学性能,晶体在可见区波段透过率达到70;,在426 nm附近有一个明显的带边特征峰,并伴有一个412 nm的肩峰.霍尔测试表明晶体为p型半导体,相应迁移率为11.88cm2 ·V-1 ·s-1.     

乙醇对3-D多脚状ZnO微晶的形貌调控及作用机制

以氯化锌(ZnCl2)和氢氧化钠(NaOH)为原料,通过低温(85℃)陈化的一步合成法制备出了三维(3-D)多脚状ZnO微晶,并对其生长机制进行了初步研究.结果表明,由于该反应体系晶体成核和生长的速度太快,初级粒子(晶核)及次级粒子之间频繁的相互碰撞,聚集生长导致了特殊的3-D结构.乙醇通过破坏晶粒之间的"架桥效应",阻止晶粒之间的硬团聚,从而抑制了晶粒的聚集生长.以纯水作溶剂时,得到平均粒径约2μm的3-D多脚状ZnO微晶;以乙醇-水混合溶剂作反应介质时,随乙醇含量增加,出现大量未发生聚集生长的次级粒子;而以纯乙醇作溶剂时,得到平均粒径约30nm的单分散椭球状ZnO纳米颗粒.     

PAN/TiO2复合微粒的制备与表征

采用沉淀法合成聚丙烯腈(PAN),并通过化学吸附法制备得到PAN/TiO2纳米复合微粒.采用透射电镜(TEM)、X射线衍射(XRD)、红外光谱(IR)、X射线光电子能谱(XPS)、紫外可见漫反射吸收光谱(DRS)等方法对复合微粒的结构和性能进行了分析表征.结果表明,复合微粒的形貌、晶型和粒径和纯二氧化钛比较基本没有发生改变;PAN/TiO2纳米复合微粒在可见光区的光吸收能力增强.对甲基橙降解实验研究结果表明,所制备的PAN/TiO2纳米复合微粒具有良好的光催化降解性能,其14 h对甲基橙的降解去除率可以达到86.5;.     

Al2TiO5/Al2O3陶瓷复合材料的晶粒异向生长

通过机械激活法制备了Al2TiO5/Al2O3(nAl2TiO5: nAl2O3=1: 4)的陶瓷复合材料.利用X射线衍射(XRD)、扫描电镜(SEM)以及能谱(EDS)分析了不同的温度制度下Al2TiO5/Al2O3陶瓷复合材料的晶粒生长特性.X射线衍射谱图(XRD)显示烧结样品的主晶相为α-Al2O3和Al2TiO5.SEM和EDS分析显示,较低温度下烧结(1350 ℃)时Al2TiO5晶粒为等轴状.当温度升高到1450 ℃时,Al2TiO5开始异向长大,形成棒状晶粒.随着温度的升高(1500 ℃),棒状Al2TiO5的长径比继续增加.而Al2O3则始终为片状和等轴晶粒,且异向生长的Al2TiO5晶粒填充于Al2O3三角晶界处.研究表明,烧结温度的升高会促进晶体异向生长并增加其体积分数.Al2TiO5会阻碍Al2O3基体颗粒进一步生长,所以当Al2TiO5颗粒平均尺寸随着烧结温度增加而增大,Al2O3基体晶粒生长几乎停滞.     

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...     

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.     

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.     

A SAXS study of kinetics of aggregation of silica sols

SAXS in situ experiments on the evolution of TMOS solutions during hydrolysis and polycondensation lead to power laws with scaling exponents ≈ 2. It is suggested that this could be the result of the polydispersity of the samples and that only an apparent fractal dimension can be obtained in this way. Kinetic studies tend to indicate that agglomeration in the sol is the result of a diffusion-controlled process.     

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.     

Study of the possibility of interactive control of the surface profile of X-ray optical elements

We investigate the possibility of controlling the curvature parameters of parabolic mirrors that are modular elements of two types consisting of a base and thin inserts placed at the opposite side of the work surface. In the first type of modular elements, bending is controlled by the difference in the coefficients of the thermal expansion of the base and inserts. In the second type of elements, the profile is changed by the piezoelectric straining of the inserts under an electric field. A correlation is established between the parameters of modular elements and their surface curvature profile.     

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...     

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.     

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.     

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.