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1.
The vast majority of materials shrink in all directions when hydrostatically compressed; exceptions include certain metallic or polymer foam structures, which may exhibit negative linear compressibility (NLC) (that is, they expand in one or more directions under hydrostatic compression). Materials that exhibit this property at the molecular level--crystalline solids with intrinsic NLC--are extremely uncommon. With the use of neutron powder diffraction, we have discovered and characterized both NLC and extremely anisotropic thermal expansion, including negative thermal expansion (NTE) along the NLC axis, in a simple molecular crystal (the deuterated 1:1 compound of methanol and water). Apically linked rhombuses, which are formed by the bridging of hydroxyl-water chains with methyl groups, extend along the axis of NLC/NTE and lead to the observed behavior. 相似文献
2.
Hall LJ Coluci VR Galvão DS Kozlov ME Zhang M Dantas SO Baughman RH 《Science (New York, N.Y.)》2008,320(5875):504-507
Most materials shrink laterally like a rubber band when stretched, so their Poisson's ratios are positive. Likewise, most materials contract in all directions when hydrostatically compressed and decrease density when stretched, so they have positive linear compressibilities. We found that the in-plane Poisson's ratio of carbon nanotube sheets (buckypaper) can be tuned from positive to negative by mixing single-walled and multiwalled nanotubes. Density-normalized sheet toughness, strength, and modulus were substantially increased by this mixing. A simple model predicts the sign and magnitude of Poisson's ratio for buckypaper from the relative ease of nanofiber bending and stretch, and explains why the Poisson's ratios of ordinary writing paper are positive and much larger. Theory also explains why the negative in-plane Poisson's ratio is associated with a large positive Poisson's ratio for the sheet thickness, and predicts that hydrostatic compression can produce biaxial sheet expansion. This tunability of Poisson's ratio can be exploited in the design of sheet-derived composites, artificial muscles, gaskets, and chemical and mechanical sensors. 相似文献
3.
At pressures up to 125 atmospheres, helium failed to anesthetize mice; at slightly higher pressures (135 to 145 atmospheres) it proved lethal. With Italian newts (Triturus italicus), whose sensitivity to anesthesia by nitrogen is similar to that of mice, responsiveness was lost at pressures between 165 and 245 atmospheres, whether the pressure was achieved with helium or neon, or hydrostatically. It was concluded that the anesthetic pressures of helium and neon, for mice and newts, are higher than the tolerable mechanical pressures. 相似文献
4.
The processes involved in the dissolution and growth of crystals are closely related. Atomic force microscopy (AFM) of faceted pits (called negative crystals) formed during quartz dissolution reveals subtle details of these underlying physical mechanisms for silicates. In imaging these surfaces, the AFM detected ledges <1 nanometer (nm) high that were spaced 10 to 90 nm apart. A dislocation pit, invisible to optical and scanning electron microscopy measurements and serving as a ledge source, was also imaged. These observations confirm the applicability of ledge-motion models to dissolution and growth of silicates; coupled with measurements of dissolution rate on facets, these methods provide a powerful tool for probing mineral surface kinetics. 相似文献
5.
Laser Brillouin spectroscopy was used to determine the adiabatic single-crystal elastic stiffness coefficients of silicon dioxide (SiO(2)) in the alpha-cristobalite structure. This SiO(2) polymorph, unlike other silicas and silicates, exhibits a negative Poisson's ratio; alpha-cristobalite contracts laterally when compressed and expands laterally when stretched. Tensorial analysis of the elastic coefficients shows that Poisson's ratio reaches a maximum value of -0.5 in some directions, whereas averaged values for the single-phased aggregate yield a Poisson's ratio of -0.16. 相似文献
6.
Recently, artificially constructed metamaterials have become of considerable interest, because these materials can exhibit electromagnetic characteristics unlike those of any conventional materials. Artificial magnetism and negative refractive index are two specific types of behavior that have been demonstrated over the past few years, illustrating the new physics and new applications possible when we expand our view as to what constitutes a material. In this review, we describe recent advances in metamaterials research and discuss the potential that these materials may hold for realizing new and seemingly exotic electromagnetic phenomena. 相似文献
7.
Kalsin AM Fialkowski M Paszewski M Smoukov SK Bishop KJ Grzybowski BA 《Science (New York, N.Y.)》2006,312(5772):420-424
Self-assembly of charged, equally sized metal nanoparticles of two types (gold and silver) leads to the formation of large, sphalerite (diamond-like) crystals, in which each nanoparticle has four oppositely charged neighbors. Formation of these non-close-packed structures is a consequence of electrostatic effects specific to the nanoscale, where the thickness of the screening layer is commensurate with the dimensions of the assembling objects. Because of electrostatic stabilization of larger crystallizing particles by smaller ones, better-quality crystals can be obtained from more polydisperse nanoparticle solutions. 相似文献
8.
The single-crystal elastic moduli of the modified spinel structure (beta phase) of magnesium orthosilicate (Mg(2)SiO(4)) have been measured by Brillouin spectroscopy under ambient conditions. Single crystals with dimensions up to 500 micrometers were grown at 22 gigapascals and 2000 degrees C over a period of 1 hour. Growth of crystals larger than 100 micrometers was achieved only when the pressure was within 5 percent of the pressure of the phase boundary separating the beta- and gamma-phase stability fields. A comparison of the elastic properties of the modified spinel phase with those of the olivine phase suggests that the 400-kilometer seismic discontinuity in the earth's mantle can be described by a mantle with 40 percent olivine. These results confirm that the 400-kilometer discontinuity can be due to the transition from olivine to modified spinel. The amount of olivine that must be present is less than that in a pyrolite model, although the results do not exclude pyrolite as a possible mantle model. 相似文献
9.
丘陵区土壤颗粒的分形维数及其应用 总被引：26，自引：0，他引：26
通过对四川省宜宾地区 2 3个耕层土样的颗粒组成分析 ,分别应用AR法和杨培岭法计算了不同质地的颗粒的分形维数 ,其结果是砂壤土、轻壤土、中壤土、重壤土和粘土分形维数的平均值依次递增。回归相关分析表明土粒分形维数与粒径 0 0 5～ 1 0 0mm颗粒含量关系显著负相关 ,与 0 0 1～ 0 0 5mm和 0 0 0 5～ 0 0 1mm颗粒含量关系不显著 ,与 0 0 0 1～ 0 0 0 5mm和小于 0 0 0 1mm颗粒 (粘粒 )含量呈极显著正相关 ;它们与有机质呈显著正相关 ,与土壤容重呈极显著负相关。分形维数可作为土壤质地的综合性的定量化指标 相似文献
10.
11.
Systematic variation in DNA length yields highly ordered repressor-operator cocrystals 总被引：2，自引：0，他引：2
Crystals have been grown that contain the operator-binding domain of the lambda repressor and the lambda operator site OL1. Crystallization conditions were tested with a set of DNA fragments, ranging in length from 17 to 23 base pairs. The best crystals were grown with a 20-base pair DNA fragment. These crystals have space-group symmetry P2I, with unit cell dimensions a = 37.1 A, b = 68.8 A, c = 56.8 A, and a beta angle of 91.5 degrees. They diffracted to at least 2.5 A resolution. High resolution data from these crystals should allow the direct determination of how a repressor recognizes its operator site. 相似文献
12.
The agricultural communicator is a key link in transmitting information to farmers. If agricultural communicators' ethics are compromised, the resulting biases in news production could have serious detrimental effects on the quality of information conveyed to farmers. But, to date, agricultural communicators' perceptions of ethical problems they encounter at work has not been examined. This study looks at the dimensions of ethical concerns for topics area (agricultural) journalists as defined by practitioners. To determine these dimensions, we sent open ended questionnaires (50 percent response rate) to members of two professional agricultural journalist associations: the Newspaper Farm Editors of America and the American Agricultural Editors' Association. Agricultural communicators overwhelmingly focus on one specific threat to objectivity—advertising pressure. Both NFEA and AAEA respondents indicated that agricultural journalists' responses to advertising pressure adversely affected the entire profession. The responses indicated that agricultural writers were concerned with the different types of pressures and the effects of advertising pressure on the industry as a whole. NFEA and AAEA respondents mentioned both indirect pressure, “freebies,” conferences, trips and press releases from advertising or public relations sections of agri-business firms, and direct pressures from advertisers, salesmen and publishers. The respondents were clearly more comfortable when newspaper policy protected them from advertising pressure and when they had techniques to reduce this pressure. The editors' and reporters' perceptions of advertising pressure clearly indicates that advertising abuses are a clear and present danger and one worthy of far more attention than it has previously received. 相似文献
13.
为研究小麦主要库、流、源器官与产量和品质的关系,分析评价其对产量和品质的影响,为小麦优质高产育种提供参考,首先分析旗叶、穗下节和籽粒与产量性状和品质特性间的相关性,然后以旗叶、穗下节和籽粒为自变量,以产量性状和品质特性为因变量进行多元回归分析。结果表明:(1)较大的旗叶面积是保证小麦产量和品质的前提;旗叶的长、宽和面积对产量均有促进作用,且旗叶宽较旗叶长对产量的影响更明显;旗叶宽与沉降值、面团稳定时间、面团形成时间和拉伸面积呈显著或极显著负相关,且较旗叶长对品质的影响更明显。(2)高秆品种,穗下第一节长/穗下第二节长与产量呈负相关,与品质呈正相关;矮秆品种,穗下第一节长/穗下第二节长与产量呈正相关,与品质呈负相关。无论是高秆还是矮秆,穗下第一节直径均与产量呈正相关,且在矮秆中的作用更显著;穗下第一节直径与籽粒蛋白质含量,湿面筋含量呈正相关,与沉降值、面团稳定时间和面团形成时间呈负相关。(3)粒厚对产量的作用最大,粒宽与粒长次之;粒长、粒宽和粒厚与小穗数、穗粒质量和千粒质量均为正相关;粒宽、粒厚与穗长和穗粒数呈负相关,且粒厚的负作用更大;粒长与籽粒容重呈极显著负相关,较粒宽和粒厚对籽粒容重的影响更明显;粒宽与籽粒蛋白质含量、湿面筋含量、沉降值、面团稳定时间和面团形成时间呈显著或极显著负相关,较粒厚对品质的负作用更大;粒长与除容重外的其他品质特性均呈显著正相关。小麦主要库、流、源器官与产量和品质间存在较为复杂的关系,通过协调各因素间的关系,可以达到源足、库大和流畅的要求,从而实现优质高产的育种目标。 相似文献
14.
G-protein coupled receptors（GPCRs） have a relatively conservative seven transmembrane helix（7tm） regions, and their N and C termini are various. In order to strengthen the features of GPCR families, N and C termini were removed in this study, then frequency features in the form of single amino acid and dipeptide compositions for recognition of human GPCRs were analyzed and extracted based on the compressed amino acid alphabets. Based on these features, classifiers were developed using support vector machine（SVM）. The ability of different compressed methods was investigated. The testing results demonstrated that the suitable choice of compressed method combined with amino acid composition information could get good performance for the recognition of human GPCRs. 相似文献
15.
Microscopic inclusions of aqueous fluids trapped in interstices in quartz and other crystals provide novel systems for the deliberate study of liquids under tension. Liquids under tension should differ in interesting ways from those at ambient pressure or compressed liquids because attractive, rather than repulsive, forces should dominate their behavior. Static tensions in excess of 100 megapascals (~1000 atmospheres) have been obtained reproducibly. Video-recorded observations of the final liquid rupture process, coupled with extrapolations of data at positive pressure, suggest that the homogeneous vapor nucleation point was reached in two of the cases studied. Raman spectra of the fluids at -80 megapascals show that an isothermal volume stretch of -5 percent by volume has only a weak effect on the spectral features and is similar to the effect of isobaric heating. 相似文献
16.
Volume diffusion models predict that crystals with large diffusion dimensions can record a wide range of thermal conditions in the Earth's crust. Direct measurements of the zoning of radiogenic argon-40 in single muscovite porphyroblasts, from a complex terrain in the Vermont Appalachians, record multiple crustal events that span 150 million years. The crystal radius was the effective dimension for argon diffusion (approximately 1000 micrometers). Late deformation features inside the crystals locally decreased the diffusion dimension and promoted loss of argon-40. Zoning patterns of radiogenic isotopes, as observed in this study, are an increasingly important diagnostic tool for studying the thermal record of tectonic processes. 相似文献
17.
Transmission electron microscopy of experimentally deformed amphibolite suggests that submicroscopic intracrystalline tubes formed around linear defects may be a previously unrecognized kind of diffusion pathway. Deformed and compositionally altered plagioclase and amphibole crystals include moderate densities of linear defects that morphologically resemble unit dislocations but display unusual contrast. During prolonged electron irradiation, the core regions of the defects expand to well-defined tubes that are approximately 20 nanometers in diameter. Both observations suggest that the regions about the defect cores are glassy and were filled with silicate-water fluid during the experiments. Intracrystalline transport along these tubes would likely be several orders of magnitude faster than traditionally conceived processes of solid-state volume diffusion, grain-boundary solvent transfer, and ordinary pipe diffusion along dislocation cores. 相似文献
18.
Wood TA Lintuvuori JS Schofield AB Marenduzzo D Poon WC 《Science (New York, N.Y.)》2011,334(6052):79-83
Colloidal particles immersed in liquid crystals frustrate orientational order. This generates defect lines known as disclinations. At the core of these defects, the orientational order drops sharply. We have discovered a class of soft solids, with shear moduli up to 10(4) pascals, containing high concentrations of colloidal particles (volume fraction φ ? 20%) directly dispersed into a nematic liquid crystal. Confocal microscopy and computer simulations show that the mechanical strength derives from a percolated network of defect lines entangled with the particles in three dimensions. Such a "self-quenched glass" of defect lines and particles can be considered a self-organized analog of the "vortex glass" state in type II superconductors. 相似文献
19.
The self-organization of pyramidal PbSe islands that spontaneously form during strained-layer epitaxial growth of PbSe/Pb1-xEuxTe (x = 0.05 to 0.1) superlattices results in the formation of three-dimensional quantum-dot crystals. In these crystals, the dots are arranged in a trigonal lattice with a face-centered cubic (fcc)-like A-B-C-A-B-C vertical stacking sequence. The lattice constant of the dot crystal can be tuned continuously by changing the superlattice period. As shown by theoretical calculations, the elastic anisotropy in these artificial dot crystals acts in a manner similar to that of the directed chemical bonds of crystalline solids. The narrow size distribution and excellent control of the dot arrangement may be advantageous for optoelectronic device applications. 相似文献
20.
户外用压缩防腐木——ACQ——D木材的处理技术初探 总被引：1，自引：0，他引：1
为研究压缩处理在木材防腐方面的应用,该研究测定了经过不同水浴和喷水预处理后的径向压缩木材在胺溶铜季铵盐--D型防腐剂中浸泡后的吸液量和变形回复率,并测定了由此制备得到的压缩防腐木的表面硬度、厚度方向上的密度分布和CuO保持量分布情况。结果表明:①变形回复率和吸液量之间有很好的对应关系,一般变形回复率越大,吸液量也越大;②经过预处理的压缩试材和素材相比,吸液量可以提高1.5倍左右,表面硬度可提高3～4倍;未经预处理的压缩试材和素材相比,吸液量可以提高2倍以上,但是表面硬度的提高不大;③ 经过预处理的压缩防腐木,其厚度方向上的密度分布比较均匀,另外CuO的保持量比未经预处理的压缩防腐木高,并且呈现表层高、逐步向内层递减的趋势。由此看出,由该研究方法制备的压缩防腐木可同时实现表面密实化和提高防腐性能的目的,尤其适用于户外设施中。 相似文献