Al-Cu-Fe-Zn系准晶的形成规律研究

本文通过快速冷凝方法制得了Al-Cu-Fe-Zn系合金粉末。这种快冷合金粉末在室温下为准晶相和晶态相共存结构,当把粉末加热到773K左右时,晶态相开始消失并且转变为准晶相,加热到1000K晶态相基本上完全转变为准晶相,继续升温准晶相又逐渐减少,晶态相重新产生和增加。本文对Al-Cu-Fe-Zn系快冷合金粉末的加热相变、准晶形成规律、准晶的成分范围进行了详细的研究。     

Quasicrystalline valence bands in decagonal AlNiCo

Quasicrystals are metallic alloys that possess perfect long-range structural order, in spite of the fact that their rotational symmetries are incompatible with long-range periodicity. The exotic structural properties of this class of materials are accompanied by physical properties that are unexpected for metallic alloys. Considerable progress in resolving the geometric structures of quasicrystals has been made using X-ray and neutron diffraction, and concepts such as the quasi-unit-cell model have provided theoretical insights. But the basic properties of the valence electronic states--whether they are extended as in periodic crystals or localized as in amorphous materials--are still largely unresolved. Here we investigate the electronic bandstructure of quasicrystals through angle-resolved photoemission experiments on decagonal Al71.8Ni14.8Co13.4. We find that the s-p and d states exhibit band-like behaviour with the symmetry of the quasiperiodic lattice, and that the Fermi level is crossed by dispersing d-bands. The observation of free-electron-like bands, distributed in momentum space according to the surface diffraction pattern, suggests that the electronic states are not dominated by localization.     

Al-Cu-Fe-Mn,Al-Cu-Fe-Cr和Al-Cu-Fe-Cr-Mn多元系准晶的制备

著者提出了一项配制新型准晶系列的成分加和原则,根据这个原则配制合金,并且采用快速冷凝制得了Al-Cu-Fe-Mn,Al-Cu-Fe-Cr和Al-Cu-Fe-Cr-Mn系合金粉末。通过对所得合金粉末进行XRD,DTA,HXRD和TEM等仪器的测试结果表明,Al-Cu-Fe-Mn,Al-Cu-Fe-Cr和Al-Cu-Fe-Cr-Mn均为新的多元素准晶。文章还对三种准晶系的热稳定性及准晶相加热时相结构转变进行了研究。     

固体中的原子相关性和Bragg衍射

提出一个描述固体结构的新方法——原子相关性方法。给出原子相关性、相关性基矢、相关性格子及其倒格子的定义,分析对比原子相关性在晶体、准晶和非晶中的不同表现。从原子相关性出发讨论了固体的衍射问题,说明准晶也能产生明锐的Bragg衍射并导出普遍形式的Laue方程。     

Selective assembly on a surface of supramolecular aggregates with controlled size and shape.

The realization of molecule-based miniature devices with advanced functions requires the development of new and efficient approaches for combining molecular building blocks into desired functional structures, ideally with these structures supported on suitable substrates 1-4.Supramolecular aggregation occurs spontaneously and can lead to controlled structures if selective and directional non-covalent interactions are exploited. But such selective supramolecular assembly has yielded almost exclusively crystals or dissolved structures 5; the self-assembly of absorbed molecules into larger structures 6-8, in contrast, has not yet been directed by controlling selective intermolecular interactions. Here we report the formation of surface-supported supramolecular structures whose size and aggregation pattern are rationally controlled by tuning the non-covalent interactions between individual absorbed molecules.Using low-temperature scanning tunnelling microscopy, we show that substituted porphyrin molecules adsorbed on a gold surface form monomers, trimers, tetramers or extended wire-like structures. We find that each structure corresponds in a predictable fashion to the geometric and chemical nature of the porphyrin substituents that mediate the interactions between individual adsorbed molecules.Our findings suggest that careful placement of functional groups that are able to participate in directed non-covalent interactions will allow the rational design and construction of a wide range of supramolecular architectures absorbed to surfaces.     

A study of quasicrystal structure factors by using quantitative CBED

The procedure of simulating convergent beam electron diffraction (CBED) pattern of quasicrystals by dynamical theory is described. The simulated patterns are generally coincide with the experimental patterns. The variations of intensity distribution in CBED pattern with the amplitude and phase of the structure factor of quasicrystal are calculated with dynamical theory. The sensitivity of intensity distribution to the structure factor is investigated. Foundation item: Supported by the National Natural Science Foundation of China (59871034) Biography: Zhao Chun (1974-), female, Master candidate. Research direction: diffraction physics.     

Complete photonic bandgaps in 12-fold symmetric quasicrystals

Photonic crystals are attracting current interest for a variety of reasons, such as their ability to inhibit the spontaneous emission of light. This and related properties arise from the formation of photonic bandgaps, whereby multiple scattering of photons by lattices of periodically varying refractive indices acts to prevent the propagation of electromagnetic waves having certain wavelengths. One route to forming photonic crystals is to etch two-dimensional periodic lattices of vertical air holes into dielectric slab waveguides. Such structures can show complete photonic bandgaps, but only for large-diameter air holes in materials of high refractive index (such as gallium arsenide, n = 3.69), which unfortunately leads to significantly reduced optical transmission when combined with optical fibres of low refractive index. It has been suggested that quasicrystalline (rather than periodic) lattices can also possess photonic bandgaps. Here we demonstrate this concept experimentally and show that it enables complete photonic bandgaps--non-directional and for any polarization--to be realized with small air holes in silicon nitride (n = 2.02), and even glass (n = 1.45). These properties make photonic quasicrystals promising for application in a range of optical devices.     

Designing intermediate-range order in amorphous materials

Amorphous materials are commonly understood to consist of random organizations of molecular-type structural units. However, it has long been known that structural organizations intermediate between discrete chemical bonds and periodic crystalline lattices are present even in liquids. Numerous models--including random networks and crystalline-type structures with networks composed of clusters and voids--have been proposed to account for this intermediate-range order. Nevertheless, understanding and controlling structural features that determine intermediate-range order in amorphous materials remain fundamental, yet presently unresolved, issues. The most characteristic signature of such order is the first peak in the total structure factor, referred to as the first sharp diffraction peak or 'low Q' structure. These features correspond to large real-space distances in the materials, and understanding their origin is key to unravelling details of intermediate-range order. Here we employ principles of crystal engineering to design specific patterns of intermediate-range order within amorphous zinc-chloride networks. Using crystalline models, we demonstrate the impact of various structural features on diffraction at low values of Q. Such amorphous network engineering is anticipated to provide the structure/property relationships necessary to tailor specific optical, electronic and mechanical properties.     

甲壳质及其衍生物溶致液晶的研究进展

介绍了甲壳质及其衍生物形成液晶态的基本结构条件及制备液晶性甲壳质衍生物的一些主要的化学改性途径。讨论了结构因素对甲壳质衍生物液晶临界浓度的影响，甲壳质衍生物形成的主要液晶态织构，以及温度对甲壳质衍生物／二氯乙酸溶液的相变的影响。简述了甲壳质的成纤性和液晶纺丝。     

Dodecagonal tiling in mesoporous silica

Recent advances in the fabrication of quasicrystals in soft matter systems have increased the length scales for quasicrystals into the mesoscale range (20 to 500 ?ngstr?ms). Thus far, dendritic liquid crystals, ABC-star polymers, colloids and inorganic nanoparticles have been reported to yield quasicrystals. These quasicrystals offer larger length scales than intermetallic quasicrystals (a few ?ngstr?ms), thus potentially leading to optical applications through the realization of a complete photonic bandgap induced via multiple scattering of light waves in virtually all directions. However, the materials remain far from structurally ideal, in contrast to their intermetallic counterparts, and fine control over the structure through a self-organization process has yet to be attained. Here we use the well-established self-assembly of surfactant micelles to produce a new class of mesoporous silicas, which exhibit 12-fold (dodecagonal) symmetry in both electron diffraction and morphology. Each particle reveals, in the 12-fold cross-section, an analogue of dodecagonal quasicrystals in the centre surrounded by 12 fans of crystalline domains in the peripheral part. The quasicrystallinity has been verified by selected-area electron diffraction and quantitative phason strain analyses on transmission electron microscope images obtained from the central region. We argue that the structure forms through a non-equilibrium growth process, wherein the competition between different micellar configurations has a central role in tuning the structure. A simple theoretical model successfully reproduces the observed features and thus establishes a link between the formation process and the resulting structure.     

鳜鱼背鳍骨针的微结构分析研究

以鳜鱼背鳍骨针作为模型矿物,使用场发射扫描电镜(FESEM)、X-射线衍射(XRD)、傅里叶变换红外光谱(FT-IR)、热重分析(TGA)、透射电镜(TEM)和选区花样衍射(SAED)等手段进行了详尽的分析.结果显示,背鳍骨针存在分级结构,即纳米尺寸的矿物颗粒在有机质的作用下定向排列形成了束状,且彼此平行排列形成了片层结构,进而有序叠加形成了表面光滑的柱状骨针.骨针中的矿物相均为非晶磷酸钙(Amorphous calcium phosphate,ACP),因此本文认为可能是ACP直接构成了具有特定形状的背鳍骨针.     

大块准晶材料的制备

给出了一种大块准晶材料的制取方法,并且对所得材料的性能进行了研究。采用快速凝固制取几乎完全为二十面体相的准晶粉末,通过爆炸成形得到大块准晶材料。研究结果表明,在Al-Mn,Al-Cr和Al-Mn-Cr合金系列中,调整合金成分和冷却速度能够制得几乎完全为二十面体相的Al_(77.5)Mn_(22.5),Al_(82)Cr_(18)和Al_(78)Mn_(18)Cr_4准晶粉末,经爆炸成形后,所得的大块准晶材料仍然保持其二十面体相结构。     

Experimental measurement of the photonic properties of icosahedral quasicrystals

Quasicrystalline structures may have optical bandgap properties-frequency ranges in which the propagation of light is forbidden-that make them well-suited to the scientific and technological applications for which photonic crystals are normally considered. Such quasicrystals can be constructed from two or more types of dielectric material arranged in a quasiperiodic pattern whose rotational symmetry is forbidden for periodic crystals (such as five-fold symmetry in the plane and icosahedral symmetry in three dimensions). Because quasicrystals have higher point group symmetry than ordinary crystals, their gap centre frequencies are closer and the gaps widths are more uniform-optimal conditions for forming a complete bandgap that is more closely spherically symmetric. Although previous studies have focused on one-dimensional and two-dimensional quasicrystals, where exact (one-dimensional) or approximate (two-dimensional) band structures can be calculated numerically, analogous calculations for the three-dimensional case are computationally challenging and have not yet been performed. Here we circumvent the computational problem by doing an experiment. Using stereolithography, we construct a photonic quasicrystal with centimetre-scale cells and perform microwave transmission measurements. We show that three-dimensional icosahedral quasicrystals exhibit sizeable stop gaps and, despite their quasiperiodicity, yield uncomplicated spectra that allow us to experimentally determine the faces of their effective Brillouin zones. Our studies confirm that they are excellent candidates for photonic bandgap materials.     

Al-Mn准晶Ⅰ相的热力学特性函数与结合能

通过ＤＳＣ测量和Ｘ光衍射定量相分析，研究了Ａｌ－Ｍｎ准晶二十面体相（Ⅰ相）晶化转变 的热力学；在一定的近似条件下，定量地得到了准晶Ⅰ相的结合能以及Ｇｉｂｂｓ自由能．     

酰胺基西夫碱化合物的合成及其相态行为

设计合成了一系列通式为CH₃CONH—C₆H₄N=CH—C₆H₄—OC_nH_2n+1(n=1,2,4,6,12)的含酰胺基的西夫碱化合物。通过熔点测定、¹H-NMR和IR等方法，对化合物的结构进行了表征。并通过DSC检测，考察了柔性链上碳原子数对介晶相态的影响。结果表明，当碳原子数n≥ 4时，随着碳原子数的增加，介晶相清亮点温度 (T_cl)逐渐降低，介晶相温度区间 (ΔT)逐渐变窄。     

新稀土金属间化合物晶体结构及高温晶格热膨胀性能研究

[目的]研究稀土-过渡族金属合金体系中的新稀土金属间化合物,挖掘稀土合金与化合物的新应用。[方法]利用扫描电子显微镜和能谱仪分析合金样品及其组成物相的成分,采用X射线粉末衍射技术研究和测定新金属间化合物的晶体结构及高温晶格热膨胀性能,并对所研究的新金属间化合物的高温热膨胀性能进行总结分析。[结果]不同的合金元素可以形成结构类型相同的晶体结构,得到了部分新金属间化合物的晶格热膨胀系数,这些新稀土金属间化合物的平均晶格热膨胀系数在10~(-6)到10~(-5)数量级之间,它们的平均单胞体积热膨胀系数也在同一数量级。[结论]化学性质相近的合金元素可以相互替代形成同构型金属间化合物。利用X射线粉末衍射同构法可测定新化合物的晶体结构,该法适用于解析二元、三元乃至多元新化合物的晶体结构。所研究的新稀土金属间化合物的热膨胀系数分别满足其各自所属晶系的热膨胀关系。     

A first-order liquid-liquid phase transition in phosphorus

First-order structural phase transitions are common in crystalline solids, whereas first-order liquid-liquid phase transitions (that is, transitions between two distinct liquid forms with different density and entropy) are exceedingly rare in pure substances. But recent theoretical and experimental studies have shown evidence for such a transition in several materials, including supercooled water and liquid carbon. Here we report an in situ X-ray diffraction observation of a liquid-liquid transition in phosphorus, involving an abrupt, pressure-induced structural change between two distinct liquid forms. In addition to a known form of liquid phosphorus--a molecular liquid comprising tetrahedral P4 molecules--we have found a polymeric form at pressures above 1 GPa. Changing the pressure results in a reversible transformation from the low-pressure molecular form into the high-pressure polymeric form. The transformation is sharp and rapid, occurring within a few minutes over a pressure range of less than 0.02 GPa. During the transformation, the two forms of liquid coexist. These features are strongly suggestive of a first-order liquid-liquid phase transition.     

树枝状CdS纳米结构的合成及光学性质

分别以丙烯基硫脲和五水氯化镉为硫源及镉源,利用水热法合成了树枝状CdS复杂纳米结构,利用X射线衍射谱、场发射扫描电子显微镜、透射电子显微镜、高分辨电子显微镜和电子衍射等对产物的形貌和结构进行表征.利用室温荧光光谱和紫外-可见光吸收谱对产物的光学性质进行研究.结果表明,所得产物是六方晶系的CdS,并且树枝状CdS是单晶.研究了不同硫源和镉源对产物形貌的影响,同时对树枝状CdS复杂纳米结构的形成机制进行了探讨.     

机械合金化AlCuFe经退火形成准晶体的结构变化

为了研究AlCuFe准晶体的形成机理,采用了X射线衍射(XRD)和扩展X射线吸收精细结构(EXAFS)2种分析技术,有效地克服了高度无序体系结构分析的困难.研究结果表明,在球磨过程中Cu原子进入Al的晶格点阵,经过不同球磨时间分别形成金属间化合物Al2 Cu或者Cu9 Al4,而Fe原子保持原来的体心立方(bcc) α...