Electron diffraction and HREM image characteristics from quasicrystalline phases in alloys of Al-Mn and Al-Mn-Si

An investigation of the quasicrystalline phases in rapidly cooled alloys of Al₈₆Mn₁₄ and Al₇₄Mn₂₀Si₆ is carried out. The Al-Mn-Si alloy shows three different kinds of phase, the icosahedral phase, the crystalline α-type phase and a third phase which resembles the decagonal T phase found in alloys of Al-Mn. Both kind of compounds (Al-Mn and Al-Mn-Si) have icosahedral diffraction patterns along similar zone axes whose lowest order diffracted vectors are different in magnitude. In the past, these extra rings of reflections were associated with the presence of superstructures. The HREM images obtained along the five-fold axis display image contrast features of similar characteristics when the specimen thickness is increased. These kind of images also show high atomic density features which are curved and modulated in intensity. These features can be seen along the five-fold, three-fold and two-fold axis. Diffraction patterns along the main icosahedral axis give rise to spots with characteristic morphology. A qualitative insight on the nature of these effects can be obtained from simulated images of density waves in a field of phasons and phonons. The Fourier transform of these images can also give a qualitative understanding of some features in the experimental diffraction patterns.     

A Near-bulk Quasicrystal Alloy Formed in Zn-Mg-Y System

Zn-Mg-Y and Mg-Zn-Y-Zr ingots with different chemical compositions were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The icosahedral phases were found in all alloys studied in present work under the as-cast conditions. There are nearly all quasicrystal phases with some MgZn2 Laves phase in a Zn-28.31Mg-13.80Y alloy (wt pct).After annealing at 400℃ for 8 h, the quasicrystal phases transformed to an fcc "W" phase in an Mg-5.57Zn-1.08Y-0.60Zr alloy (wt pct).     

A possible crystallographic explanation for the five-fold diffraction symmetry in icosahedral phases

X-ray and electron diffraction data from the Al-Cu-Fe icosahedral phase are compared and analysed on the basis of the microcrystalline and multi-domain model developed by the author. It is shown that a crystallographic explanation is now possible for both the enigmatic five-fold symmetry and non-periodicity of reflections observed in electron diffraction patterns of icosahedral phases.     

Thermal stability of decagonal quasicrystal prepared from undercooled Al72Ni12Co16 alloy melt

The thermal stability of the single-phase decagonal quasicrystal produced directly from the undercooled Al₇₂Ni₁₂Co₁₆ alloy melt was investigated by different heat treatments. The optical microscopy, TEM, X-ray powder diffractometer (XRD) and differential thermal analyzer (DTA) techniques were adopted to observe the microstructure and to characterize the phase composition, respectively. Based on the analyses of the X-ray diffraction patterns and the DTA curves, two modifications of the decagonal quasicrystal existed in the annealed samples, which have a close structural relationship to the decagonal quasicrystal.     

Structure and tribological property of B2-based approximants

The present paper is concerned with a special group of approximants with B2 superstructures. In the first part, recent work on structural features of the B2 superstructure approximants is summarized. Experimental results obtained in Al-Cu-Mn and Al-Cu systems are presented, where a series of B2-based approximants are observed. These phases all have similar valence electron concentrations, in full support of thee/a-constant definition of approximants. Special emphasis is laid on the chemical twinning modes of the B2 basic structure in relation to the Al-Cu approximants. It is revealed that the B2 twinning mode responsible for the formation of local pentagonal atomic arrangements is of 180°/[111] type. This is also the origin of 5-fold twinning of the B2 phase on quasicrystal surfaces. Crystallographic features of phases B2, τ2, τ3,γ, and other newly discovered phases are also discussed. In all these phases, local pentagonal configurations are revealed. In the second part, dry tribological properties of some AlCuFe samples containing the B2-type phases are presented. The results indicated that the B2 phase having their valence ratio near that of the quasicrystal possesses low friction coefficient under various loads, comparable with the annealed quasicrystalline ingot. Such a result indicates that the B2-type phase withe/a near that of quasicrystal is indeed an approximant, which is in full support of the valence electron criterion for approximants.     

Étude par microscopie et diffraction électroniques de particules pentagonales de nickel obtenu par réduction d'un sel en phase liquide

Nickel particles prepared by reduction of a nickel salt in organic liquid media were investigated by electron diffraction and electron microscopy. Various shapes were observed for these particles, the mean size of which is about 300 Å. In this paper we report on the structure and morphology of particles with a hexagonal elongated shape and a pentagonal section. From dark field and bright field images and from diffraction patterns it is shown that the particles have a decahedral shape with anisotropic growth in the Λ₅ direction.     

Hydrothermal fabrication of copper sulfide nanocones and nanobelts

Novel copper sulfide nanocones and nanobelts have been hydrothermally fabricated at 140 °C for 24 h with acrylamide and sodium dodecyl benzene sulfonate (SDBS) as surfactants, respectively. X-ray diffraction (XRD) pattern indicates that the as-prepared samples are the pure hexagonal phase CuS. XPS spectra show that the ratio of Cu/S is about 1:1.084. The morphology was characterized by scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) techniques. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images reveal that nanocones and nanobelts grow along the [110] axis. The surfactant is found to be critical to the morphologies of the products. The possible formation mechanism is also discussed.     

Occurrence of faults/APBs in periodic to quasiperiodic transformations

The strip projection method has been used here to establish the structural relationships between various two dimensional quasiperiodic structures and certain well known periodic structures. These periodic structures, designated as unmodulated approximants, can transform into their quasiperiodic counterparts in a continuous fashion. The early stages of such transformations are characterized by the occurrence of faults or APBs. There is considerable high resolution electron microscopy (HREM) and electron diffraction evidence in favour of this in the case of the β-Mn to octagonal quasicrystal, the σ-phase to dodecagonal quasicrystal and the Al₅Ti₃ to quasiperiodic superlattice structure transformations.     

不同压力下纳米 SnO_2材料的特性

用水热法制得了在室温大气环境下稳定的半导体 SnO_2纳米材料。用 X 射线粉晶衍射、透射电镜、高分辨电镜及 M(?)ssbauer 谱等研究了不同压力下的纳米 SnO_2的特性。随着压力增加,所有谱线都有明显展宽,其特性明显区别于一般的多晶 SnO_2。用残缺态的概念解释了实验结果。     

Face centred cubic cobalt ultrathin-layers in Au/Co(111) multilayers: a study by electron diffraction and by HREM

The structure of various Co/Au(111) multilayers has been investigated, using electron diffraction and high resolution electron microscopy (HREM). The multilayers were grown by electron beam evaporation on a float glass substrate. The cobalt layers exhibit a face centred cubic (f.c.c.) structure with the cobalt [111] axis parallel to the gold [111] axis. For cobalt layers, this f.c.c. structure hadn't been observed in Co/Au multilayers before. Measurements were done on digitized electron diffraction patterns and high resolution images. An important volume expansion of the cobalt lattice and good epitaxial relations between cobalt and gold were observed.     

Synthesis and characterization of srilankite nanowires

We describe the synthesis and characterization of srilankite (Ti2ZrO6) nanowires. The nanowires are produced via hydrothermal synthesis with a TiO2/ZrO2 mixture under alkaline conditions. The zirconium titanate nanowires have median diameters of 60 nm and median lengths of 800 nm with the (022) axis along the length of the nanowire. Electron microscopy, energy dispersive X-ray spectroscopy, powder X-ray diffraction, and electron diffraction are used to characterize the phases and compare nanowires produced with varying molar ratios of Ti and Zr. Electron diffraction patterns produced from single nanowires show highly crystalline nanowires displaying a compositional-ordering superlattice structure with Zr concentrated in bands within the crystal structure. This is in contrast to naturally occurring bulk srilankite where Zr and Ti are randomly substituted within the crystal lattice. Streaking is observed in the electron diffraction patterns suggesting short-range ordering within the superlattice structure.     

Effect of cooling rate on the formation of metastable icosahedral quasicrystal phase in rapidly solidified Al-8.2 at % Mn alloy

The microstructure of rapidly solidified Al-8.2 at % Mn alloy was analysed by X-ray diffraction, transmission electron microscopy and energy-dispersive analysis of X-rays and the effect of cooling rate on the formation of the metastable icosahedral quasicrystal phase (IQP) was investigated. The formation of IQP was found to be sensitive to the cooling rate in a rapidly solidified alloy of a certain composition. A lower critical cooling rate at which metastable IQP starts to appear and an upper critical cooling rate at which IQP suppresses completely the stable crystalline phase exist. The fact that the amount and the manganese concentration of IQP change non-linearly with the cooling rate suggests that there is an optimum cooling rate at which both the amount of IQP and its solute concentration reach maximum values in an alloy of a certain composition.     

Microwave-hydrothermal synthesis and structural characterization of PX-phase single-crystalline PbTiO3 nanowires by electron microscopy

PX-phase PbTiO₃ (PT) nanowires were synthesized by microwave-hydrothermal process, and their microstructures were characterized by electron microscopy. The PX-phase PT nanowires exhibit acicular morphology with diameter sizes of 20-80 nm and length over 1 µm. They tend to grow into a regular structure with parallel arrangement along their long axis in the [001] direction. Selected area electron diffraction patterns demonstrate the PX-phase PT nanowires with a 3-fold modulated periodicity along the [110] direction and a 4-fold modulated periodicity in the [001] direction. These results were also confirmed by the high-resolution transmission electron microscopy images.     

NiFe2O4-BiFeO3纳米管的制备与磁电特性研究

采用溶胶-凝胶法在多孔氧化铝模板的纳米孔洞中成功地合成了xNiFe₂O₄-(1-x) BiFeO₃ (NFO-BFO)复合纳米管阵列。扫描电镜照片显示纳米管的外径约为70 nm, 内径约为50 nm, 长度约为80 μm。X射线衍射图谱显示, 通过自组装形成了钙钛矿结构的铁电相BFO和尖晶石结构的铁磁相NFO。磁性和铁电性的测试结果表明, 复合纳米管具有明显的室温铁磁性和电极化性能, 且易磁化方向沿着纳米管的长轴方向。随着NFO复合比例的增加, 纳米管阵列的磁性和电极化性能逐渐增强。NFO对复合纳米管阵列铁磁贡献的计算结果表明, NFO-BFO纳米管阵列中存在着磁电耦合效应。     

Electron microscopy study of striation contrast in Al-Cu-Co-Si decagonal quasicrystals

Detailed transmission electron microscopy study was carried out in single crystals of a decagonal phase in the Al-Cu-Co-Si quaternary system. X-ray diffraction and convergent beam electron diffraction patterns of the powder samples confirmed the structures to be decagonal quasicrystals. No microcrystalline nor crystalline phases could be identified. Thin slices normal to the 10-fold directions were prepared for transmission electron microscopy. Diffuse streaks along symmetric directions around the fundamental spots were observed in the diffraction patterns. Bright field images and dark field images showed discontinuous lines or striations lying perpendicular to the direction of diffuse streaking. The striation contrast appears to be originating from anti-phase boundary (APB) in the decagonal superstructures. The diffuse streaks seem to be a characteristic feature of a partially ordered decagonal superlattice structure. The atomic rearrangement or phasonic movement in certain symmetric directions along the pentagrids or Ammann lines in the structure has obviously caused the type of contrast observed in the images. The evolution of rhombic domains consisting of APBs in localized regions can be understood as one of the signature of an intermediate structural state formed prior to a superstructure formation.     

Structural stability and kinetics of hydrogen in TiZrNi quasicrystals

When hydrogen atoms slowly diffuse into a metastable structure, a coherence length of thin TiZrNi quasicrystal metallic ribbons is increased. Improved atomic order with addition of hydrogen is an uncommon phenomenon in metals, and may reflect the unordinary structural property of quasicrystals. Metastable TiZrNi quasicrystals prepared by rapid quenching of molten ingots were hydrogenated in a low pressure of hydrogen (lower than 1000 Torr) at high temperature. After completion of the pressure-composition-temperature measurements between 200 to 300 degrees C, X-ray diffraction data revealed that the full width at half maximum values of the diffraction peaks were decreased suggesting that the quasicrystal phase may stabilized by hydrogen inclusion. Annealing of the same sample at 200 degrees C without hydrogen yields no change of the relative intensity of diffraction peaks.     

中间相沥青基石墨纤维的微观结构

本文用透射电镜、X 射线衍射分析等方法观察和分析了辐射状中间相沥青基石墨纤维的显微结构。结果表明,中间相沥青基石墨纤维中同时存在着有序石墨和乱层石墨两种相。在有序石墨相中,石墨片层堆叠的有序程度较高,且在纵向沿纤维轴高度择优取向,在横向则基本上沿径向排列。有序石墨微晶一般呈略有弯曲的片状,并具有分枝结构,微晶在纵向沿纤维轴交错排列,而在横向则沿径向从芯部向表层不断延伸、分叉和交织。在乱层石墨相中,石墨片层堆叠的有序程度较差,且沿纤维轴的择优取向程度较低。乱层石墨相一般镶嵌在有序石墨微晶之间。此外,在某些地方还存在着严重的晶格畸变。     

Quasicrystalline Phase in As-cast Mg-Zn-Y-Zr Alloy

An icosahedral phase in as-cast Mg 5. 78Zn-0.89Y-0.48Zr (wt-%) alloy has been identified with transmission electron microscope (TPM) The selected area diffraction patterns (SADP) from the phase show a five-fold symmetry of a quasicrystalline structure. The energy dispersive X-ray spectra (EDXS) indicate that the quaternary phase has a chemical composition of 67.6Zn-1 9.4Mg-8,1 Y-4 9Zr in at -%     

Growth of Black Phosphorus Nanobelts and Microbelts

Black phosphorus nanobelts are fabricated with a one‐step solid–liquid–solid reaction method under ambient pressure, where red phosphorus is used as the precursor instead of white phosphorus. The thickness of the as‐fabricated nanobelts ranges from micrometers to tens of nanometers as studied by scanning electron microscopy. Energy dispersive X‐ray spectroscopy and X‐ray diffraction indicate that the nanobelts have the composition and the structure of black phosphorus, transmission electron microscopy reveals a typical layered structure stacked along the b‐axis, and scanning transmission electron microscopy with energy dispersive X‐ray spectroscopy analysis demonstrates the doping of bismuth into the black phosphorus structure. The nanobelt can be directly measured in scanning tunneling microscopy in ambient conditions.     

钴、镍纳米线及其异质结的制备与磁性研究

采用脉冲电沉积技术在氧化铝模板中制备了单晶钴、镍纳米线阵列和镍/钴纳米线异质结阵列.分别用场发射扫描电镜、透射电镜、X射线衍射仪、物理性能测试系统对纳米线阵列的微观形貌、结构和性能进行了表征与研究.结果表明,所制备的磁性纳米线有很大的长径比,易磁化方向均为纳米线长轴方向.纳米线异质结阵列在易磁化方向具有较大的矫顽力和矩形比,可用作高密度垂直磁记录材料.