共查询到20条相似文献，搜索用时 15 毫秒
1.
Warren SC Messina LC Slaughter LS Kamperman M Zhou Q Gruner SM DiSalvo FJ Wiesner U 《Science (New York, N.Y.)》2008,320(5884):1748-1752
The synthesis of ordered mesoporous metal composites and ordered mesoporous metals is a challenge because metals have high surface energies that favor low surface areas. We present results from the self-assembly of block copolymers with ligand-stabilized platinum nanoparticles, leading to lamellar CCM-Pt-4 and inverse hexagonal (CCM-Pt-6) hybrid mesostructures with high nanoparticle loadings. Pyrolysis of the CCM-Pt-6 hybrid produces an ordered mesoporous platinum-carbon nanocomposite with open and large pores (>/=10 nanometers). Removal of the carbon leads to ordered porous platinum mesostructures. The platinum-carbon nanocomposite has very high electrical conductivity (400 siemens per centimeter) for an ordered mesoporous material fabricated from block copolymer self-assembly. 相似文献
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We have studied the self-assembly of amphiphilic dendrons extended with linear polyethylene oxide (PEO) chains and their ion complexes. Keeping the dendron core and linear PEO chain compatible allows for the combination of dendritic core-shell and conventional blockcopolymer characteristics for complex mesophase behavior. An unexpected sequence of crystalline lamellar, cubic micellar (Pm3n), hexagonal columnar, continuous cubic (Ia3d), and lamellar mesophases is observed. Multiple phase behavior within single compounds allows for the study of charge transport and mechanical property correlations as a function of structure. The results suggest an advanced molecular design concept for the next generation of nanostructured materials in applications involving charge transport. 相似文献
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Templating of mesoporous molecular sieves by nonionic polyethylene oxide surfactants 总被引：3，自引：0，他引：3
Mesoporous silica molecular sieves have been prepared by the hydrolysis of tetraethylorthosilicate in the presence of low-cost, nontoxic, and biodegradable polyethylene oxide (PEO) surfactants, which act as the structure-directing (templating) agents. This nonionic, surfactant-neutral, inorganic-precursor templating pathway to mesostructures uses hydrogen bonding interactions between the hydrophilic surfaces of flexible rod- or worm-like micelles and Si(OC(2)H(5))(4-x)(OH)(x) hydrolysis products to assemble an inorganic oxide framework. Disordered channel structures with uniform diameters ranging from 2.0 to 5.8 nanometers have been obtained by varying the size and structure of the surfactant molecules. Metal-substituted silica and pure alumina mesostructures have also been prepared by the hydrolysis of the corresponding alkoxides in the presence of PEO surfactants. These results suggest that nonionic templating may provide a general pathway for the preparation of mesoporous oxides. 相似文献
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The transition from body-centered cubic to hexagonal close-packed phase in iron has been studied in a diamond anvil cell with synchrotron radiation. The hexagonal close-packed phase, when it first appears, has a ratio of lattice parameters that is significantly larger than normal. This is attributed to a displacive mechanism that causes a distortion of the hexagonal close-packed structure in a body-centered cubic matrix. The hexagonal close-packed phase adjacent to a boundary with the body-centered cubic phase is stretched in the c direction and compressed in the a direction when it first forms. 相似文献
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Vaughan GB Heiey PA Luzzi DE Ricketts-Foot DA McGhie AR Fischer JE Hui YW Smith AL Cox DE Romanow WJ Allen BH Coustel N McCauley JP Smith AB 《Science (New York, N.Y.)》1991,254(5036):1350-1353
The high-temperature structure of solvent-free C(70) has been determined with high-resolution x-ray powder difraction and electron microscopy. Samples crystallized from solution form hexagonal close-packed crystals that retain an appreciable amount of residual toluene, even after prolonged heating. Samples prepared by sublimation, which contain no detectable solvent, are primarily face-centered cubic with some admixture of a hexagonal phase. The relative volume of the hexagonal phase can be further reduced by annealing. The structures of both phases are described by a model of complete orientational disorder. The cubic phase contains an appreciable density of stacking faults along the [111] direction. 相似文献
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A first principles theoretical approach shows that, at the density of the inner core, both hexagonal [hexagonal close-packed (hcp)] and cubic [face-centered-cubic (fcc)] phases of iron are substantially elastically anisotropic. A forward model of the inner core based on the predicted elastic constants and the assumption that the inner core consists of a nearly perfectly aligned aggregate of hcp crystals shows good agreement with seismic travel time anomalies that have been attributed to inner core anisotropy. A cylindrically averaged aggregate of fcc crystals disagrees with the seismic observations. 相似文献
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Triton's polar caps are modeled as permanent nitrogen deposits hundreds of meters thick. Complex temperature variations on Triton's surface induce reversible transitions between the cubic and hexagonal phases of solid nitrogen, often with two coexisting propagating transition fronts. Subsurface temperature distributions are calculated using a two-dimensional thermal model with phase changes. The phase changes fracture the upper nitrogen layer, increasing its reflectivity and thus offering an explanation for the surprisingly high southern polar cap albedo (approximately 0.8) seen during the Voyager 2 flyby. The model has other implications for the phase transition phenomena on Triton, such as a plausible mechanism for the origin of geyser-like plume vent areas and a mechanism of energy transport toward them. 相似文献
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Belonoshko AB Skorodumova NV Rosengren A Johansson B 《Science (New York, N.Y.)》2008,319(5864):797-800
Earth's solid-iron inner core is elastically anisotropic. Sound waves propagate faster along Earth's spin axis than in the equatorial plane. This anisotropy has previously been explained by a preferred orientation of the iron alloy hexagonal crystals. However, hexagonal iron becomes increasingly isotropic on increasing temperature at pressures of the inner core and is therefore unlikely to cause the anisotropy. An alternative explanation, supported by diamond anvil cell experiments, is that iron adopts a body-centered cubic form in the inner core. We show, by molecular dynamics simulations, that the body-centered cubic iron phase is extremely anisotropic to sound waves despite its high symmetry. Direct simulations of seismic wave propagation reveal an anisotropy of 12%, a value adequate to explain the anisotropy of the inner core. 相似文献
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An x-ray diffraction study of lead under pressure has shown that face-centered cubic structure transforms to the hexagonal close-packed structure at room temperature and a pressure of 130+/- 10 kilobars. The volume change for the transformation is -0.18+/- 0.06 cubic centimeter per mole. 相似文献
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Stierle A Renner F Streitel R Dosch H Drube W Cowie BC 《Science (New York, N.Y.)》2004,303(5664):1652-1656
Ultrathin Al2O3 layers on alloys are used as templates for model catalysts, tunneling barriers in electronic devices, or corrosion-resistant layers. The complex atomic structure of well-ordered alumina overlayers on NiAl110 was solved by surface x-ray diffraction. The oxide layer is composed of a double layer of strongly distorted hexagonal oxygen ions that hosts aluminum ions on both octahedral and tetrahedral sites with equal probability. The alumina overlayer exhibits a domain structure that can be related to characteristic growth defects and is generated during the growth of a hexagonally ordered overlayer (Al2O3) on a body-centered cubic (110) substrate (NiAl). 相似文献
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Safinya CR Sirota EB Bruinsma RF Jeppesen C Plano RJ Wenzel LJ 《Science (New York, N.Y.)》1993,261(5121):588-591
Synchrotron x-ray scattering studies were performed to probe the nonequilibrium structures of two layered systems at high shear rates: the smectic-A phase of the thermotropic liquid crystal 4-cyano-4'-octylbiphenyl (8CB) and the lamellar L(alpha) phases of surfactant membranes composed of sodium dodecyl sulfate and pentanol. Whereas the lamellar surfactant phases oriented primarily with their layers parallel to the shearing plates, as expected intuitively, in the corresponding high shear regime, the smectic-A liquid crystalline material oriented with the layers perpendicular to the shearing plates. A careful numerical study revealed that this surprising layer orientation results from nonlinear dynamics of the liquid crystal director and is caused by the flow distortion of thermal fluctuations. 相似文献
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"Tubules-Within-a-Tubule" Hierarchical Order of Mesoporous Molecular Sieves in MCM-41 总被引：1，自引：0，他引：1
The recently discovered mesoporous aluminosilicate MCM-41 consists of hexagonal arrays of nanometer-sized cylindrical pores. It is shown that this material can be synthesized by cooperative condensation of silicate and cylindrical cationic micelles. Careful control of the surfactant-water content and the rate of condensation of silica at high alkalinity resulted in hollow tubules 0.3 to 3 micrometers in diameter. The wall of the tubules consisted of coaxial cylindrical pores, nanometers in size, that are characteristic of those of MCM-41. The formation of this higher order structure may take place through a liquid crystal phase transformation mechanism involving an anisotropic membrane-to-tubule phase change. The hierarchical organization of this "tubules-within-a-tubule" particle texture is similar to that of the frustules of marine diatoms. 相似文献
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Thyroid hormone influence upon lung surfactant metabolism 总被引：6，自引：0，他引：6
Thyroid administration and thyroidectomy in the rat profoundly affect the morphological characteristics of the type II pneumonocyte and the quantitative harvest of lung surfactant obtained from alveolar washings. The correlation of the ultrastructural changes with quantitative alterations in lung surfactant is evidence that the lamellar bodies within this cell are the source of the surface-active phospholipids utilized at the air-liquid interphase of lung alveoli. Our findings suggest that in the rat, L-thyroxine may be a potent regulator of lung surfactant metabolism. 相似文献
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The Density of Hydrous Magmatic Liquids 总被引：2，自引：0，他引：2
Density measurements on several hydrous (相似文献
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对于由冷压陶瓷技术所开发的纯六方Ba（Ti0.95Fe0.05）O3-δ陶瓷，采用变温XRD技术测量该陶瓷在不同温度下的XRD谱，鉴定晶体结构．澄清了六方钛酸钡（BaTiO3）介电陶瓷材料的晶体结构是否在BaTiO3的居里温度（n=130℃）发生结构相变，并验证开发的变温X射线衍射（XRD）技术在无机粉体材料结构监控的可行性．结果表明：BaTiO3陶瓷在Tc发生四方-立方的结构相变，以45°附近两个分立的（002）／（200）衍射峰演变为单一对称的（200）衍射峰为标志；而六方Ba（Ti0.95Fe0.05）O3-δ陶瓷直到300℃都不发生结构相变．对于六方Ba（Ti0.95Fe0.05）O3-δ陶瓷，升高温度并不能导致吸附环境氧来填充氧空位，Fe3＋Vo-Fe3＋缺陷复合体所维持的晶格电中性仍占据统治地位，六方-立方结构相变不发生．该项研究为变温XRD技术在工业生产上对无机粉体材料的监控应用提供了技术依据． 相似文献
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Discs of finite size are a very rare form of stable surfactant self-assembly. It is shown that mixing of two oppositely charged single-chain surfactants can produce rigid nanodiscs as well as swollen lamellar liquid crystals with frozen bilayers. The crucial requirement for obtaining nanodisc self-assembly is the use of H+ and OH- as counterions. These counterions then form water and lower the conductivity to 10 microsiemens per centimeter. In the case of cationic component excess, a dilute solution of nanodiscs is in thermodynamic equilibrium with a lamellar phase. The diameter of the cationic nanodiscs is continuously adjustable from a few micrometers to 30 nanometers, with the positive charge located mainly around the edges. 相似文献
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Optical patterns in polarized light and x-ray reflections in the low-angle region were used to detect a shift from one liquid crystalline structure to another during polymerization. The polymerization took place in a Iotropic liquid crystal of water and sodium undecenoate, with a structure consisting of cylinders in a two-dimensional hexagonal close packing. After polymerization, a lamellar liquid crystalline structure was obtained. 相似文献
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Recent studies are leading to a better understanding of the formation of the earth's metal core. This new information includes: better knowledge of the physics of metal segregation, improved geochemical data on the abundance of siderophile and chalcophile elements in the silicate part of the earth, and experimental data on the partitioning behavior of siderophile and chalcophile elements. Extensive melting of the earth as a result of giant impacts, accretion, or the presence of a dense blanketing atmosphere is thought to have led to the formation of the core. Collision between a planet-sized body and the earth may have also produced the moon. Near the end of accretion, core formation evidently ceased as upper mantle conditions became oxidizing. The accumulation of the oceans is a consequence of the change to oxidizing conditions. 相似文献
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[目的]研究稻壳灰制备纳米SiO2的方法,为稻壳灰综合利用提供参考。[方法]在微波辐射条件下,采用稻壳灰为原料制备出硅酸钠。而后,又以这种硅酸钠为原料,用化学沉淀法制备出纳米SiO2。[结果]研究了焙烧温度、表面活性剂的用量及溶液pH值对纳米SiO2粒径的影响。硅酸的聚合速度与溶液的pH值有很大关系,而聚合速度直接影响到硅酸的分子量,进而影响SiO2的粒径,溶液pH值在5~6最好;焙烧温度对SiO2的粒径及形貌有较大影响,温度高了,团聚多,形貌不好;表面活性剂的用量需达到一定值才可防止粒子团聚。结果表明：在PEG添加量为≥0.15 g时,经过600℃焙烧后,可制备近似球形的纳米SiO2。[结论]利用微波辐射提取稻壳灰中的SiO2,通过化学沉淀法可制备近似球形的纳米SiO2。该方法操作简单,易于工业化,有利于稻谷壳灰的综合利用。 相似文献