常温常压下BiMnO_3纳米粉末的制备与物性分析

钙钛矿型BiMnO3是同时具有铁磁性和铁电性的多铁性材料,在传感器,信息存储等方面具有潜在的应用前景.但是它的制备条件非常苛刻,需要高温、高压、密封等,这极大地限制了它的应用与发展.本文采用共沉淀法,以MnCl2·4H2O,Bi2O3,NaOH,HNO3和聚乙二醇为材料,在100℃的水溶液中成功地制备出了BiMnO3材料,此方法大大降低了制备的温度和压强,从而节约了能源,降低了成本.同时利用X射线衍射仪(XRD),透射电子显微镜(TEM),振动样品磁强计(VSM)等对其结构、相貌及磁性等进行了检测,并讨论了制备条件及煅烧温度对BiMnO3物性的影响.     

Synthesis and photoluminescence properties of Nd2O3 nanoparticles modified by sodium bis 2-ethylhexyl sulfosuccinate

This paper reports that Nd₂O₃ nanoparticles modified by AOT(sodium bis(2-ethylhexyl) sulfosuccinate) were prepared using microemulsion method in the system of water and propanol/AOT/toluene. Transmission electron microscopy shows that the Nd₂O₃ nanoparticles take the shape of sphere with 18\,nm and 31nm with different preparation. The organic sol of Nd₂O₃ nanoparticles is very stable at room temperature. X-ray diffraction results show that the product has hexagonal phase structure. Two ultraviolet emission band at 344\,nm and 361\,nm corresponding to the transition of ⁴D_3/2→4I_9/2 and 2P_3/2→⁴I₁₁₂ or ⁴D_3/2 →⁴I_13/2 were observed.     

The structure of monolayer SiO2 on Mo(1 1 2): A 2-D [Si-O-Si] network or isolated [SiO4] units?

In this letter, atomically resolved scanning tunneling microscopic (STM) images obtained from monolayer SiO₂/Mo(1 1 2) are presented. The results are consistent with a previously proposed structural model of isolated [SiO₄] units based on vibrational features observed by high-resolution electron energy loss spectroscopy (HREELS) and infrared reflection-absorption spectroscopy (IRAS), and oxygen species identified by ultra-violet photoemission spectroscopy (UPS). These results are inconsistent with a structural model that assumes a two-dimensional (2-D) [Si-O-Si] network. These data illustrate that a metal substrate, although coated with an oxide thin layer, can be directly imaged at the atomic-scale with STM.     

The structure of a long-period stacking phase in an Mg–Al–Y alloy studied by electron microscopy

A new phase with a 10H-type long-period stacking (LPS) structure was found in an Mg₇₅Al₁₀Y₁₅ alloy annealed at 823?K. The LPS structure in the Mg₇₅Al₁₀Y₁₅ alloy annealed at 823?K for 2?h has an ordered arrangement of L1₂-type structural Al₆Y₈ clusters on the two-dimensional plane parallel to the c-plane of hexagonal Mg lattice and a disordered arrangement along the c-axis, whereas a perfectly ordered structure along the c-axis, which has a period with two times of that of the 10H-type LPS structure, was established by annealing at 823?K for 24?h. The structural model of the ordered LPS phase is proposed by high-resolution images taken with a Cs-corrected scanning transmission electron microscope and also electron diffraction patterns.     

Spin polarization effect for Mn2 molecule

The density functional theory method （DFT） （b3p86） of Gaussian 03 has been used to optimize the structure of the Mn2 molecule. The result shows that the ground state of the Mn2 molecule is an 11-multiple state, indicating a spin polarization effect in the Mn2 molecule, a transition metal element molecule. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state does not mingle with wavefunctions of higher-energy states. So the ground state for Mn2 molecule being of an 11-multiple state is the indicative of spin polarization effect of the Mn2 molecule among those in the transition metal elements： that is, there are 10 parallel spin electrons in a Mn2 molecule. The number of non-conjugated electrons is the greatest. These electrons occupy different spacious orbitals so that the energy of the Mn2 molecule is minimized. It can be concluded that the effect of parallel spin in the Mn2 molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell-Sorbie potential functions with the parameters for the ground state and other states of the Mn2 molecule are derived. The dissociation energy De for the ground state of the Mn2 molecule is 1.4477 eV, equilibrium bond length Re is 0.2506 nm, vibration frequency ωe is 211.51 cm^-1. Its force constants f2, f3, and f4 are 0.7240 aJ·nm-2, -3.35574 aJ·nm^-3, 11.4813 aJ·nm^-4 respectively. The other spectroscopic data for the ground state of the Mn2 molecule ωeχe, Be, αe are 1.5301 cm^-1, 0.0978 cm^-1, 7.7825×10^-4 cm^-1 respectively.     

High-pressure polymorphic transformation of rutile to alpha-PbO2-type TiO2 at {011}R twin boundaries

The presence of nano-scale lamellae of the α-PbO₂-type polymorph of TiO₂ sandwiched between twinned rutile inclusions in jadeite has been confirmed by electron diffraction and high-resolution transmission electron microscopy, backed up by image simulation techniques, from ultrahigh-pressure jadeite quartzite at Shuanghe in the Dabie Mountains, China. The crystal structure is orthorhombic with lattice parameters a = 4.58 Å, b = 5.42 Å, c = 5.02 Å and space group Pbcn. A three-dimensional structural model has been constructed for the rutile to α-PbO₂-type TiO₂ phase transformation based on high-resolution electron microscopic images. Computer image simulation and structural model analysis reveal that rutile {0 1 1}_R twin interface is a basic structural unit of α-PbO₂-type TiO₂. Nucleation of α-PbO₂-type TiO₂ lamellae 1–2 nm thick is caused by the displacement of one half of the titanium cations within the {0 1 1}_R twin slab. This displacement reduces the Ti–O–Ti distance and is favored by high pressure.     

Spontaneous magnetostriction of Y2Fe16Al compound

The structure and magnetic properties of Y₂Fe₁₆Al compound have been investigated by means of x-ray diffraction and magnetization measurements. The Y₂Fe Fe₁₆Al compound has a hexagonal Th$_{2}$Ni$_{17}$-type structure. Negative thermal expansion was found in Y₂Fe₁₆Al compound in the temperature range from 332 to 438K by x-ray dilatometry. The coefficient of the average thermal expansion is \alpha =-3.4\times 10^-5K^-1. The spontaneous magnetostrictive deformations from 293 to 427K have been calculated based on the differences between the experimental values of the lattice parameters and the corresponding values extrapolated from the paramagnetic range. The result shows that the spontaneous volume magnetostrictive deformation \textit{$\omega $}$_{\rm S}$ decreases from 5.4$\times $10^-3to near zero with temperature increasing from 293 to 427K, the spontaneous linear magnetostrictive deformation \textit{$\lambda $}$_{\rm c}$ along the $c$ axis is much larger than the spontaneous linear magnetostrictive deformation \textit{$\lambda $}$_{\rm a}$ in basal-plane in the same temperature range except near 427K.     

Spin polarization effect for Fe2 molecule

This paper uses the density functional theory （DFT）（B3p86） of Gaussian03 to optimize the structure of Fe2 molecule. The result shows that the ground state for Fe2 molecule is a 9-multiple state, which shows spin polarization effect of Fe2 molecule of transition metal elements for the first time. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state does not mingle with wavefunctions with higher energy states. So, that the ground state for Fe2 molecule is a 9-multiple state is indicative of the spin polarization effect of Fe2 molecule of transition metal elements. That is, there exist 8 parallel spin electrons. The non-conjugated electron is greatest in number. These electrons occupy different spacious tracks, so that the energy of the Fe2 molecule is minimized. It can be concluded that the effect of parallel spin of the Fe2 molecule is laFger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell Sorbie potential functions with the parameters for the ground state and other states of Fe2 molecule are derived. Dissociation energy De for the ground state of Fe2 molecule is 2.8586ev, equilibrium bond length Re is 0.2124nm, vibration frequency we is 336.38 cm^-1. Its force constants f2, f3, and f4 are 1.8615aJ.nm^-2, -8.6704aJ.nm^-3, 29.1676aj.nm^-4 respectively. The other spectroscopic data for the ground state of Fe2 molecule weXe, Be, αe are 1.5461 cm^-1, 0.1339cm^-1, 7.3428× 10^-4 cm^-1 respectively.     

Long-period shear structure in lanthanum cuprate

In non-superconducting, oxygen-deficient La₂CuO_4–x a long-period shear structure is formed to accommodate the local non-stoichiometry. Based on the results of electron diffraction and high-resolution electron microscopy a model is proposed for this new structure. The model is based on edge-sharing octahedra along the crystallographic shear planes. Observations suggest that the shear takes place above the tetragonal-orthorhombic phase transition. The space group of the new phase has been determined as Plcl.     

Synthesis and characterization of vanadium nanoparticles on activated carbon and their catalytic activity in thiophene hydrodesulphurization

Vanadium nanoparticles (∼7 nm) stabilized on activated carbon were synthesized by the reduction of VCl₃·3THF with K[BEt₃H]. This material was characterized by inductive coupled plasma-atomic emission spectroscopy (ICP-AES), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analyses. The catalytic performance of the carbon-supported vanadium was studied using thiophene hydrodesulfurization (HDS) as model reaction at 300 °C and P = 1 atm. The catalytic activity of the vanadium carbide phase on the activated carbon carrier was more significant than that of the reference catalysts, alumina supported NiMoS. The method proposed for the synthesis of such a catalyst led to an excellent performance of the HDS process.     

Magnetocaloric effect in Gd6Co1.67Si3 compound with a second-order phase transition

The magnetic properties and the magnetic entropy change AS have been investigated for Gd6Co1.67Si3 compounds with a second-order phase transition. The saturation moment at 5 K and the Curie temperature TC are 38.1μB and 298 K, respectively. The AS originates from a reversible second-order magnetic transition around TC and its value reaches 5.2 J/kg.K for a magnetic field change from 0 to 5T. The refrigerant capacity （RC） of Gd6Co1.67Si3 are calculated by using the methods given in Refs.[12] and [21], respectively, for a field change of 0 5T and its values are 310 and 440 J/kg, which is larger than those of some magnetocaloric materials with a first-order phase transition.     

Si3N4的晶体化和ZrN/Si3N4纳米多层膜的超硬效应

研究了Si₃N₄层在ZrN/Si₃N₄纳米多层膜中的晶化现象及其对多层膜微结构与力学性能的影响. 一系列不同Si₃N₄层厚度的ZrN/Si₃N₄纳米多层膜通过反应磁控溅射法制备. 利用X射线衍射仪、高分辨透射电子显微镜和微力学探针表征了多层膜的微结构和力学性能. 结果表明，由于受到ZrN调制层晶体结构的模板作用，溅射条件下以非晶态存在的Si₃N₄层在其厚度小于0.9 nm时被强制晶化为NaCl结构的赝晶体，ZrN/Si₃N₄纳米多层膜形成共格外延生长的柱状晶，并相应地产生硬度升高的超硬效应. Si₃N₄随层厚的进一步增加又转变为非晶态，多层膜的共格生长结构因而受到破坏，其硬度也随之降低.     

Synthesis, structure and antiferromagnetic behaviour of brannerite MnV2O6

Brannerite MnV 2 O 6 with plate-like shape is successfully synthesized by hydrothermal method.Its crystal structure and morphology are investigated by x-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscope (TEM),high resolution transmission electron microscopy (HRTEM) and select area electronic diffraction (SAED).The results show that the brannerite MnV 2 O 6 with monoclinic structure has a uniform plate-like shape with a diameter of about 5-8 μm and a thickness of about 500 nm.SAED patterns further confirm the structure of the brannerite MnV 2 O 6 and the single crystalline character of the plate crystal.Magnetic properties are measured by superconducting quantum interference device (SQUID) in a temperature range of 2-300 K under a magnetic field of 1 T.The magnetic measurement results indicate that the material undergoes an antiferromagnetic transition with a N’eel temperature of 17 K.Above 50 K,the inverse susceptibility is fitted well to the Curie-Weiss law with a calculated moment of 5.98 μ B.Finally,the origin of antiferromagnetic behaviour in the brannerite MnV 2 O 6 is explained by means of Anderson model.     

Twin Boundaries in Zinc Oxide with Additions of Gallium Oxide

Twin boundaries (TBs) in ZnO sintered with small additions of Ga₂O₃ have been characterized with advanced methods of transmission electron microscopy (TEM). The TBs and accompanying inversion domain boundaries are on {011¯3} planes of ZnO. The Ga content of the TB corresponds to an effectively half occupied {011¯3} plane determined from compositional maps calculated from electron spectroscopic images using electron filtering TEM. The structure of the TBs were investigated by high-resolution TEM, and images of focus series were used to reconstruct the complex electron wave. Simulated electron waves based on structure models of the TB were quantitatively compared with the reconstructed wave to identify and to refine atom positions. The twins can be considered to be created by a mirror operation on a {011¯3} plane of ZnO, and two alternating closed-packed polyhedral clusters of oxygen ions can be identified as building units of the TB structure. Unit 1 is occupied with Zn²⁺ by simply continuing ZnO₄ tetrahedra of the same type from both crystals to the TB. Using arguments of local charge balance unit 2 can only be occupied with the trivalent Ga³⁺ ion. The Ga³⁺ position was refined with high precision (±5 pm), and the resulting polyhedron is a GaO₅ square pyramid. The pyramids form densely occupied columns parallel to the twin axis [21¯1¯0]. The analysis of the TB structure yields a fractional occupancy of the boundary plane by Ga of 0.5, which is in good agreement with the result of the chemical composition measurement with energy filtered TEM.     

Judd--Ofelt analysis of spectra and experimental evaluation of laser performance of Tm3+ doped Lu2SiO5 crystal

This paper reports that the Tm^3＋：Lu2SiO5 （Tm：LSO） crystal is grown by Czochralski technique. The roomtemperature absorption spectra of Tm：LSO crystal are measured on a b-cut sample with 4 at.% thulium. According to the obtained Judd-Ofelt intensity parameters Ω2=9.3155×10^-20 cm^2, Ω4=8.4103×10^-20 cm^2, Ω6=1.5908×10^-20 cm^2, the fluorescence lifetime is calculated to be 2.03 ms for ^3F4 → ^3H6 transition, and the integrated emission cross section is 5.81×10^-18 cm^2. Room-temperature laser action near 2μm under diode pumping is experimentally evaluated in Tm：LSO. An optical-optical conversion efficiency of 9.1% and a slope efficiency of 16.2% are obtained with continuouswave maximum output power of 0.67 W. The emission wavelengths of Tm：LSO laser are centred around 2.06μm with spectral bandwidth of -13.6 nm.     

Synthesis and characterization of axially periodic Zn2SnO4 dendritic nanostructures

Zn2SnO4 （ZTO） nanowires with a unique dendritic nanostructure were synthesized via a simple one-step thermal evaporation and condensation process. The morphology and microstructure of the ZTO nanodendrite have been investigated by means of field emission scanning electron microscopy （SEM）, x-ray diffraction （XRD） and high-resolution transmission electron microscopy （HRTEM）. SEM observation revealed the formation of branched nanostructures and showed that each branch exhibited a unique periodic structure formed by a row of overlaid rhombohedra of ZTO nanocrystals along the axis of the nanobranch. HRTEM studies displayed that the branches grew homoepitaxially as single-crystalline nanowires from the ZTO nanowire backbone. A possible growth model of the branched ZTO nanowires is discussed. To successfully prepare branched structures would provide an opportunity for both fundamental research and practical applications, such as three-dimensional nanoelectronics, and opto-electronic nanodevices.     

Annealing effects on the structure and electrical characteristics of amorphous Er2O3 films

Amorphous Er 2 O 3 films are deposited on Si (001) substrates by using reactive evaporation.This paper reports the evolution of the structure,morphology and electrical characteristics with annealing temperatures in an oxygen ambience.X-ray diffraction and high resolution transimission electron microscopy measurement show that the films remain amorphous even after annealing at 700 C.The capacitance in the accumulation region of Er 2 O 3 films annealed at 450 C is higher than that of as-deposited films and films annealed at other temperatures.An Er 2 O 3 /ErO x /SiO x /Si structure model is proposed to explain the results.The annealed films also exhibit a low leakage current density (around 1.38 × 10 4 A/cm 2 at a bias of 1 V) due to the evolution of morphology and composition of the films after they are annealed.     

AlN/Si3N4纳米多层膜的外延生长与力学性能

采用射频磁控溅射方法制备单层AlN, Si₃N₄薄膜和不同调制周期的AlN/Si₃N₄纳米多层膜.采用X射线衍射仪、高分辨透射电子显微镜和纳米压痕仪对薄膜进行表征.结果发现,多层膜中Si₃N₄层的晶体结构和多层膜的硬度依赖于Si₃N₄层的厚度.当AlN层厚度为4.0nm、 Si₃N₄层厚度 关键词： 3N₄纳米多层膜')" href="#">AlN/Si₃N₄纳米多层膜 外延生长 应力场 超硬效应     

Crystal structure of a new superconducting high-pressure phase in the YBaCuO-System

A superconducting (T _c=40K) high-pressure phase recently discovered in the system of perovskite type YBaCuO structures is investigated by high-resolution transmission electron microscopy and electron diffraction in order to find its crystal structure. A structure model is proposed on the basis of a comparison between the observed images and image simulations based on crystal chemical considerations. The new phase has aB-centered orthorhombic cell with a monoclinic primitive cell. The primitive cell is composed of two subunits. The first of these is identical with the unit cell of orthorhombic YBa₂Cu₃O_7–x (1-2-3 structure) withT _c=92K. In contrast to the first subunit, the second one contains two adjacent Cu–O chains but is identical otherwise. This second subunit has been observed as one type of planar defect in the 1-2-3 structure. It is therefore concluded, that other stacking polytypes composed of these two different units could exist. The structure of the new phase is compared to the structures of the other known high-T _c superconductors. The chemical formula for the new phase can be written as Y₂Ba₄Cu₇O_14+x, with x0.5±0.2.     

Atomic structure and stability of elliptical carbon onion

Elliptical carbon onions were produced from amorphous carbon by electron beam irradiation. Atomic structure and structural stability of the onion were investigated by high-resolution electron microscopy, molecular mechanics calculation and molecular orbital calculation, and a structure model of C₂₄@ C₈₄ was proposed at the center of the elliptical onion. Received 30 November 2000