纳米二氧化钛材料的相变和声子限制效应(英文)

拉曼光谱用于研究二氧化钛材料的相变和声子限制效应。化学溶液方法制备了TiO2纳米晶材料。其平均粒度为 6 8- 2 7 9nm。最低频率 1 5 2cm- 1 Eg 模随粒度减小出现蓝移和加宽。在声子限制模型下 ,理论上对不同粒度的TiO2 纳米晶 (6 8,1 0 3和 2 7 9nm)的频移和线宽进行计算 ,结果与实验吻合得很好。研究了TiO2 纳米晶锐钛矿 -金红石相变 ,其相变温度为 6 5 0 - 6 90℃ ,比体块TiO2 的相变温度 1 0 0 0℃低 ,表明了相变的尺寸效应     

嵌埋Ni颗粒的BaTiO_3/SrTiO_3薄膜的相变特性

采用高温X射线原位衍射和变温介电谱对SrTiO3基底上外延生长的BaTiO3(嵌埋Ni颗粒)薄膜进行了相变特性分析。从X射线衍射和介电谱的分析结果得出,BaTiO3的相变温度点转变为弥散的温度区间。在这个弥散的相变温度区间内,由于基底和薄膜之间的失配,以及嵌埋Ni颗粒的应力作用,薄膜的介电响应弥散剧烈,并偏离德拜弛豫。分析Cole-Cole图获知,BaTiO3薄膜在四方相转变为立方相的相变过程中同时存在几种极化机制,在高温状态下介电损耗随温度增大而增大。降温过程中,薄膜没有立即恢复四方相,可能是基底和Ni颗粒的共同作用影响了相变弛豫。     

The Synthesis and Characterisation of Nanocrystalline Lithium Niobate

Nanocrystalline LiNbO₃ powder with grain sizes below 10 nm have been successfully prepared using wet chemical techniques, namely the polymer precursor method (and variations of it) and a double alkoxide sol-gel method. These have grain sizes of 4.5 nm and 1.6 nm respectively which, as far as we are aware, are the smallest nanocrystals of LiNbO₃, produced to date. The materials have been characterised using Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) and X-ray Powder Diffraction (XRPD) and compared to samples prepared by High-Energy Ball Milling, which have also been characterised by these techniques.     

低温拉曼光谱研究钛酸铋尺寸驱动的相变

本文对粒径为25~75 nm的钛酸铋纳米晶在-190℃温度的低温拉曼光谱进行了研究,结果表明,在钛酸铋纳米晶中,除了存在已知的尺寸驱动的铁电正交-顺电四方的结构相变外,还存在尺寸驱动的细微的单斜-正交的结构相变。     

Grain Growth Kinetics of BaTiO3 Nanocrystals During Calcining Process

以醋酸钡和钛酸四丁酯为原料,采用溶胶-凝胶法合成了纳米BaTiO3粉体;运用差示/热重、X射线衍射及透射电镜对前驱体凝胶和产物进行了表征,并根据XRD结果,研究了纳米BaTiO3的晶格常数、晶格畸变度和晶粒尺寸随焙烧温度及时间的变化。结果表明,焙烧温度与时间对纳米BaTiO3晶格常数的影响不明显;随焙烧温度或时间的延长,纳米BaTiO3的晶格畸变度减小,晶粒尺寸增大,但晶格畸变度和晶粒尺寸更敏感于焙烧温度. 基于扩散控制机理的传统模型探讨了焙烧过程纳米BaTiO3晶粒生长动力学,得出其晶粒生长指数为7,晶粒生长活化能为75.49 kJ/mol. 将基于扩散与反应共同控制机理的新型等温模型应用于本研究中,结果表明,新型等温模型更能真实地反映纳米BaTiO3焙烧过程中的晶粒生长行为,说明纳米BaTiO3晶粒生长过程同时受溶质扩散和表面反应控制,其藕合晶粒生长活化能为27.23 kJ/mol.     

X-ray study of structural phase transitions in nanocrystalline LaMnO3+δ perovskite

The nanocrystalline LaMnO_3+δ perovskite was synthesized by the microwave-assisted glycothermal method and calcined at several temperatures up to 1500°C. The prepared samples were examined by the X-ray powder diffraction with the aim to show that LaMnO_3+δ exhibits the size-induced structural phase transitions. The as-received nanocrystals of LaMnO_3+δ are of tetragonal, pseudo-cubic symmetry not known for bulk material. The samples calcined at temperatures 750–1100°C have trigonal symmetry known from the high-temperature phase of LaMnO₃ single crystal. The samples calcined from 1200°C to 1500°C have two phases: trigonal and orthorhombic. Thus, the observed phase sequence is inverted with respect to that of the bulk material, which is the characteristic of the size-induced mechanism of phase transitions in the nanocrystals. The critical crystallite sizes for both structural transitions were evaluated as 20 and 100?nm.     

SnO2 ∶ Eu3+ 纳米晶的水热法制备及发光性能

采用水热法制备了Eu³⁺掺杂SnO₂纳米发光粉,样品在不同温度下热处理得到不同粒径尺寸的纳米颗粒。利用X射线衍射(XRD)与光致发光(PL)谱对样品进行表征。XRD分析表明:SnO₂ ∶ Eu³⁺样品均为纯相 金红石结构。PL测量表明:水热法直接制备的样品的激发谱由Eu³⁺的f-f本征激发峰组成,而经过热处理后样品的激发谱由O^2--Eu³⁺电荷迁移带和Eu³⁺的f-f本征激发组成;样品的发光强度与颗粒大小有密切关系。     

Investigation of size-driven phase transition in bismuth titanate nanocrystals by Raman spectroscopy

Raman spectroscopy was used to investigate the lattice dynamics and structural transformations in bismuth titanate (Bi₄Ti₃O₁₂) nanocrystals prepared by a chemical coprecipitation technique. The crystal structure of the samples of different grain sizes was determined by X-ray-diffraction analysis. The evolution of the Raman spectrum with grain size was characterized by an intensity decrease, a broadening of the line width, a frequency shift, and the disappearance of the Raman mode. The results revealed the appearance of a size-driven phase transition from orthorhombic to tetragonal phases at a critical size of 44 nm. This result is quite consistent with the X-ray-diffraction measurement and differential thermal analysis. The origin was attributed to the grain-size effect and explained by the surface-energy mechanism. Received: 26 June 2002 / Accepted: 18 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-25/3595535, E-mail: msz@nju.edu.cn     

纳米晶Si在高压下的电学性质与金属化相变

 在金刚石压砧装置上,采用电阻和电容测量方法,研究了粒径为15~18 nm和80 nm的纳米晶Si在室温下、24 GPa内的电阻、电容与压力的关系。实验结果表明,它们分别在19~17 GPa和14 GPa左右发生了金属化相变。     

Optical properties of Si-rich SiO2 films in relation with embedded Si mesoscopic particles

Optical properties of Si-rich SiO₂ films prepared by an RF cosputtering method are discussed. From the infrared and Raman spectroscopy together with the electron microscopy, it is shown that Si mesoscopic particles embedded in solid matrices with the sizes ranging from ˜ 10 nm (nanocrystals) to less than ˜1 nm (clusters) can be obtained by the cosputtering and post-annealing. The absorption and photoluminescence spectra are presented. For our samples, a red luminescence peak analogous to that of porous Si is observed for films containing Si clusters rather than nanocrystals. Raman spectra which evidence the success in the heavy doping of B atoms into Si nanocrystals are also discussed.     

Nanocrystalline gold with small size: inverse Hall–Petch between mixed regime and super-soft regime

ABSTRACT

Molecular dynamics simulations were used to study the atomic mechanisms of deformation of nanocrystalline gold with 2.65–18?nm in grain size to explore the inverse Hall–Petch effect. Based on the mechanical responses, particularly the flow stress and the elastic-to-plastic transition, one can delineate three regimes: mixed (10–18?nm, dislocation activities and grain boundary sliding), inverse Hall-Petch (5–10?nm, grain boundary sliding), and super-soft (below 5?nm). As the grain size decreases, more grain boundaries present in the nanocrystalline solids, which block dislocation activities and facilitate grain boundary sliding. The transition from dislocation activities to grain boundary sliding leads to strengthening-then-softening due to grain size reduction, shown by the flow stress. It was further found that, samples with large grain exhibit pronounced yield, with the stress overshoot decrease as the grain size decreases. Samples with grain sizes smaller than 5?nm exhibit elastic-perfect plastic deformation without any stress overshoot, leading to the super-soft regime. Our simulations show that, during deformation, smaller grains rotate more and grow in size, while larger grains rotate less and shrink in size.     

Pb_(0.85)Ca_(0.15)TiO_3纳米晶铁电—顺电相变的拉曼光谱研究

采用变温ＦＴ Ｒａｍａｎ技术测定了三种粒径Ｐｂ０．８５Ｃａ０．１５ＴｉＯ３纳米晶的居里温度。１７８９ｎｍ、２５．６９ｎｍ、和３６２５ｎｍ粒子的Ｔｃ分别为２７４℃,２６５℃,２９４℃。认为尺寸效应和掺杂引起的缺陷均对Ｔｃ产生影响。掺杂及焙烧不充分引起的晶体缺陷使居里点弥散。Ｅ（１ＴＯ）软模具有弱阻尼特征     

Comparing specific features of the structural formation of aluminum alloys during severe and intense plastic deformation

We compare the deformation behavior and specific features of the structural formation of two aluminum alloys, AMTs and V95, upon megaplastic (MPD) and severe plastic (SPD) deformation. It is established that upon SPD by dynamic channel angular pressing, a submicron crystalline structure is formed with grain sizes of 200 to 600 nm; and that upon MPD by high quasi-hydrostatic pressure shearing, a nanostructure is formed with grain sizes of 55 to 100 nm. The sequence of the phase and structure transition is established for an increase in the rate of deformation and velocity of the materials. Mechanisms of elastic energy relaxation are determined as a function of the extent of dislocation mobility.     

Size quantization effect in GaAs nanocrystals

GaAs nanocrystals have been synthesized by an electrochemical route from the acidic solutions of metallic gallium and arsenic oxide. Structural analysis by transmission electron microscopy (TEM), reveales the presence of orthorhombic phase, in contrast to the usual cubic phase of bulk GaAs. Well separated GaAs nanocrystals having sizes in the range of 7 to 15 nm are identified in the TEM micrograph. Optical absorption shows a strong excitonic absorption band on account of quantum size effect associated with GaAs nanocrystals. Photoluminescence, recorded at 300 K under ultraviolet excitation (369 nm), shows the HOMO-LUMO transition band as well as the red shifted bands. Micro-Raman analysis shows a phonon mode centered at 250 cm⁻¹ associated with a point defect.     

Co:BaTiO3纳米复合薄膜的制备及其结构

用脉冲激光沉积技术(PLD)在MgO(100)基底上生长了嵌埋Co纳米晶的BaTiO3复合薄膜. 分别利用x射线衍射(XRD)、原子力显微镜(AFM)以及拉曼光谱(Raman)对薄膜的微观结构、表面 形貌进行了表征. 结果表明该薄膜为c轴取向的四方晶体结构，薄膜表面均匀、致密、 具有原子尺度的光滑性，其均方根表面粗糙度(RMS)达到015nmCo以纳米晶形式嵌埋BaTi O3基体中，呈单分散性均匀分布，其粒径随激光脉冲数的增加而增大. Co：BaTiO3纳米 复合薄膜拉曼峰的强度随钴纳米晶粒径的增加明显减弱，但是峰的宽度逐渐增加. 关键词： Co：BaTiO3 纳米复合薄膜 脉冲激光沉积     

纳米多晶体的热力学函数及其在相变热力学中的应用

以往关于纳米材料热力学的研究，绝大多数以界面的热力学函数表征整体纳米材料的热力学性质，这种近似处理，对于尺寸超过几十纳米的较粗纳米材料，在相变热力学中对特征转变温度和临界尺寸等重要参量的预测，将导致很大误差. 应用“界面膨胀模型”和普适状态方程，研究了纳米晶界的热力学特性，进一步发展了纳米晶整体材料热力学函数的计算模型，给出了单相纳米多晶体的焓、熵和吉布斯自由能随界面过剩体积、温度，以及晶粒尺寸发生变化的明确表达式. 以Co纳米晶为例，分析了界面与整体纳米多晶体热力学函数的差异，确定了相变温度与晶粒尺寸的依赖关系，以及一定温度下可能发生相变的临界尺寸. 关键词： 纳米多晶体 热力学函数 相变热力学     

Photoluminescence and Raman spectroscopy studies of Eu3+-doped rutile TiO2 nanocrystals at high pressures

Nanocrystal samples (particle size about 90 nm) of Eu³⁺-doped rutile titanium dioxide (TiO₂) nanocrystals (rutile Eu³⁺/TiO₂ nanocrystals) were synthesized by the sol–gel method with hydrothermal treatment. The pressure effect on photoluminescence (PL) and Raman spectra of the rutile Eu³⁺/TiO₂ nanocrystals was investigated with a diamond anvil cell under hydrostatic pressure condition. Raman spectra of the samples at high pressures indicated that the critical pressure for the transition from the rutile phase to a new baddeleyite-type phase was between 10 and 14.2 GPa. The position of Raman bands shifted to high wavenumbers and the PL intensity of ⁵D ₀→⁷F ₂ transition of Eu³⁺ decreased down to zero with the increase of pressure before the phase transition occurred. After releasing the pressure, the rutile phase was not recovered and a α-PbO₂-type phase was observed at ambient pressure.     

锐钛矿二氧化钛超细纳米晶的三声子互作用

本文通过对粒径为2.2-25.5nm的锐钛矿二氧化钛超细纳米晶在83-293 K温度范围内的变温拉曼光谱的研究,得到了三声子互作用对拉曼频率和线宽的贡献随粒径的变化关系。结果表明锐钛矿二氧化钛超细纳米晶的三声子相互作用随粒径减小而加强。     

Measuring grain rotation at the nanoscale

In this paper, we introduced a method to measure grain rotation of nanomaterials under external stress using a high pressure diamond anvil cell and the Laue microdiffraction technique at a synchrotron facility. We used tungsten carbide marker crystals to investigate grain rotation activities of 3 and 500?nm nickel media. Our results show that the grain rotation of 3 and 500?nm nickel nanocrystals increase with pressure and finally rotation of 500?nm nickel tends to stop at a lower pressure/stress level than 3?nm nickel. 3?nm nickel nanocrystals show a higher rotation magnitude than 500?nm nickel nanocrystals. Our measurements show an effective method to study the grain rotation of nanomaterials especially in ultrafine nanocrystals.     

Doped ions (Co2+, Fe3+) tuning morphologies and magnetic properties of indium oxide nanoparticles

Transition-metal ion-doped indium oxide was potentially used in spintronic devices. In this article, 10 at.% Co²⁺- and Fe³⁺-doped indium oxide nanocrystals were, respectively, prepared by direct solvothermal method. The influences of doped ions on their morphologies and magnetic properties of indium oxide nanocrystals were investigated. It was found that the doped ions could tune the microscopic morphologies and crystalline structures of indium oxide nanoparticles. Co²⁺-doped samples were polycrystalline nanoflowers aggregated by about 5 nm nanocrystals while Fe³⁺-doped specimens were single-crystalline nanocubes with crystalline sizes of about 16 nm. Meanwhile, the magnetic transition from diamagnetism to paramagnetism was also realized by doping Co²⁺ or Fe³⁺ ions into the indium oxide host matrices. In addition, their magnetic properties were further demonstrated to be intrinsic and not caused by impurities.