不同龄期柠条茎秆的拉剪强度试验与分析

[目的]为优化柠条联合收获机切割过程中的平茬参数,本文研究了不同平茬位置与不同生长年限柠条茎秆的生物力学性质。[方法]采用完全随机区组设计方法,分别对柠条茎秆样本进行拉伸与剪切试验。[结果]试验测得柠条茎秆的抗拉强度均值为138N·mm~(-2),柠条茎秆的抗剪强度均值为35N·mm~(-2);柠条茎秆的拉伸断裂强度与平茬位置和生长年龄都密切相关,且生长年龄对其试验指标的影响比平茬位置对其的影响要大,随生长年龄的增加,柠条茎秆抗拉强度升高;柠条茎秆的抗剪强度与生长年龄相关,而与柠条的平茬位置关系不大。[结论]在柠条联合收获机平茬作业时,应当首先考虑柠条的生长年龄,其次考虑平茬位置。本研究为柠条联合收获机的设计与作业条件的选择提供理论依据。     

Demineralization of porous solids

When a porous ionic solid is placed in acid, the acid will dissolve surface material. When this dissolved material and the acid diffuse into the solid's pores, they can precipitate more solid. If the acid is buffered, the diffusing species can bring about precipitation in some regions and dissolution in others. When the porous solid contains several chemical species, the diffusion can precipitate one species and dissolve another. The results have implications for the demineralization of teeth.     

Melting of two-dimensional solids

Recent theoretical predictions indicate that melting of a two-dimensional solid may be caused by spontaneous creation of dislocations. The theory predicts that melting occurs by a two-step process involving an intermediate phase, called the hexatic phase, in which there is order in the local crystalline axes but not in the positions of atoms. These ideas are being tested by numerical simulations and by experiments on electrons on liquid helium, liquid crystal films, and rare gas layers adsorbed on graphite. Experiments on liquid crystal films indicate that the three-dimensional analog of the hexatic phase exists, and xenon on graphite exhibits a melting transition close to the form predicted.     

Two-dimensional rare gas solids

Monolayers of rare gas atoms adsorbed onto the basal planes of graphite play the same prototype role in two dimensions that rare gas liquids and solids do in three dimensions. In recent experiments such novel phenomena as continuous melting, the lack of true crystallinity in two dimensions, orientationally ordered fluid phases, and melting from a solid to a reentrant fluid with decreasing temperature have been observed. Because the forces in these rare gas monolayers are simple and well understood, by studying them the investigator can examine a direct interface between experiment and first principles. In order to understand the phases and phase transitions that occur in such materials, it is necessary to consider the geometrical matching of the rare gas overlayer to the graphite substrate. It turns out that in two dimensions both the local and the long-distance behavior are important. These two-dimensional rare gas solids may be effectively probed with synchrotron x-ray techniques, and the results of a series of synchrotron x-ray scattering studies of these solids are presented.     

Prediction of new low compressibility solids

An empirical model and an ab initio calculation of the bulk moduli for covalent solids are used to suggest possible new hard materials. The empirical model indicates that hypothetical covalent solids formed between carbon and nitrogen are good candidates for extreme hardness. A prototype system is chosen and a first principles pseudopotential total energy calculation on the system is performed. The results are consistent with the empirical model and show that materials like the prototype can have bulk moduli comparable to or greater than diamond. It may be possible to synthesize such materials in the laboratory.     

High-resolution nuclear magnetic resonance of solids

The development of line-narrowing techniques, such as magic-angle spinning (MAS) and high-power decoupling, has led to powerful high-resolution nuclear magnetic resonance approaches for solid samples. In favorable cases (for instance, where high abundances of protons are present) cross polarization (CP) provides a means of circumventing the time bottleneck caused by inefficient spinlattice relaxation in many solids. The combined CP-MAS approach for carbon-13 with proton decoupling has become a popular and routine experiment for organic solids. For many nuclides with spin quantum number /> (1/2) the central nuclear magnetic resonance transition can be employed in high-resolution experiments that involve rapid sample spinning. A continuing stream of advances holds great promise for the use of high-resolution techniques for the characterization of solids by a wide range of nuclides.     

Predicting new solids and superconductors

It is now possible to start with a simple model of a solid composed of atomic cores and itinerant valence electrons and compute the total energy for a given structural arrangement of atoms with enough precision to predict the existence of new solids and their properties. The application of the model based on the pseudopotential method is described with silicon chosen as a prototype material. With only information about the constituent atoms, the electronic, structural, vibrational, and even superconducting properties of solids can be calculated from first principles. The successful predictions of superconductivity in highly condensed hexagonal silicon and the existence of new high-pressure semiconductor phases are highlighted. A discussion is presented of the use of the method to discover new stable or metastable solids at high pressures.     

n-Type conducting CdSe nanocrystal solids

A bottleneck limiting the widespread application of semiconductor nanocrystal solids is their poor conductivity. We report that the conductivity of thin films of n-type CdSe nanocrystals increases by many orders of magnitude as the occupation of the first two electronic shells, 1Se and 1Pe, increases, either by potassium or electrochemical doping. Around half-filling of the 1Se shell, a peak in the conductivity is observed, indicating shell-to-shell transport. Introducing conjugated ligands between nanocrystals increases the conductivities of these states to approximately 10(-2) siemens per centimeter.     

Fast ionic transport in solids

The discovery of inorganic solids with ionic conductivities comparable to those of aqueous electrolytes has revolutionized solid-state electrochemistry. Sodium beta alumina, a Na(+) conductor, and LixTiS(2), an intercalation compound with simultaneous Li(+) and electronic conductivity, are two of the best and most versatile fast ionic conductors. A wide variety of cations can replace Na(+) in beta alumina and Li(+) in LixTiS(2) and change the properties of the materials. Sodium beta alumina and LixTiS(2) are currently used in the development of high-energy density batteries for electric vehicles and electrical utility load leveling. Current research in solid ionic conductors is exploring new intercalation compounds, solid polymer electrolytes, and alkali ion and proton transport in crystalline solids.     

Ion beam spectroscopy of solids and surfaces

Ion beams are important new probes for characterizing the chemistry and structure of a wide variety of materials. When beams of particles with energies of approximately 1000 electron volts are used, as in secondary ion mass spectrometry, it is possible to detect ions ejected from the top layer of the material with sensitivities we below the picogram level. A number of theoretical developments now permit analysis of the geometry of adsorbed atoms and molecules on surfaces from the angular distributions of the ejected particles. Much surface chemical information can also be deduced from ejected molecular cluster ions. In addition, the observation of clusters with weights up to nearly 20,000 atomic mass units promises to expand applications of mass spectrometry to the analysis of biomolecules and the sequencing of proteins.     

Thermochemistry and aqueous solubilities of hydrotalcite-like solids

Hydrotalcites are used in technology as catalysts and anion exchangers and are important sinks for environmental contaminants. Their compositional variability makes it important, but difficult, to estimate their aqueous solubility. We report calorimetric measurements of the heats of formation of cobalt-aluminum hydrotalcite phases. The heat and free energy of formation from the elements are equal to those of mechanical mixtures of binary compounds, namely hydroxides and carbonates. The interlayer anion is much more important than the cation in determining the solubility of the hydrotalcite phase and its ability to contain or release heavy metals to the environment. Because hydrotalcites do not have an unreactive polymer as a structural core, their aqueous stability will change dramatically with composition, particularly with anion content. This simple mechanical mixture model allows prediction of aqueous solubilities and trace metal retention in a variety of geochemical settings.     

High-resolution nuclear magnetic resonance of inorganic solids

Recent improvements in instrumentation and technique now permit the observation of high-resolution nuclear magnetic resonance spectra of many nuclei in inorganic solids. The application of nuclear magnetic resonance to the study of the structures of materials of interest in chemistry, earth science, and materials science are discussed together with a prognosis for future work.     

平行边壁剪切流的雷诺数与摩阻流速特性初探

应用涡动力学观点,分析平行边壁剪切流的流动特性和从层流到湍流转捩的特点;对雷诺数和摩阻流速的概念及他们之间的关系进行分析和评述;给出了雷诺数的不同表达形式,把它与脉动过程和失稳机制联系起来,弥补以往的动力学参数缺乏流动物理机制的不足;最后,指出摩阻流速实际上是反映流场与边壁互相作用的一个系统整体参数.     

框-剪结构考虑墙肢剪切变形时剪力墙合理数量

从框架-剪力墙结构协同工作的连续化分析原理出发，建立了在水平荷载作用下考虑剪力墙剪切变形影响时的平衡微分方程．按照我国现行高层建筑混凝土结构技术规程（JGJ3—2002）和建筑抗震设计规范（GB50011—2001），推导了考虑剪力墙剪切变形影响时满足层间侧移角限值的抗震剪力墙计算公式，并将其与未考虑剪力墙剪切变形影响时的计算公式进行比较，分析了它们在确定剪力墙合理数量上的相似点与不同点．然后引入了各种修正系数，通过图表探讨了剪切变形系数对剪力墙合理数量的影响．最后给出算例，反映出系数对计算结果的影响程度，并分析了引起差异的原因，该方法简单实用，可用于高层建筑结构的初步设计阶段，