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1.
Diffusion couples, consisting of sapphire and fused silica, which were annealed in the temperature range from 1678 degrees to 2003 degrees C and analyzed by electron beam microprobe, have provided data on the stable phase equilibrium of the silica-alumina system. Under stable equilibrium conditions, the intermediate compound of this system, mullite (3Al(2)O(3) . 2SiO(2)), melts incongruently at 1828 degrees +/- 10 degrees C and its solid solution field extends from 70.5 to 74.0 percent (by weight) alumina. The stable phase diagram is a composite of the two binary eutectic diagrams: silica-mullite in the absence of alumina and silica-alumina in the absence of mullite. Under metastable conditions, mullite melts congruently at approximately 1890 degrees +/- 10 degrees C and its solid solution field extends to approximately 83.2 percent (by weight) alumina. 相似文献
2.
Monte Carlo and molecular dynamics methods have been used to study the shearing behavior of an atomic fluid between two plane-parallel solid surfaces having the face-centered cubic (100) structure. A distorted, face-centered cubic solid can form epitaxially between surfaces that are separated by distances of one to five atomic diameters. Under these conditions a critical stress must be overcome to initiate sliding of the surfaces over one another at fixed separation, temperature, and chemical potential. As sliding begins, a layer of solid exits the space between the surfaces and the remaining layers become fluid. 相似文献
3.
Phase changes in stratospheric aerosols were studied by cooling a droplet of sulfuric acid (H(2)SO(4)) in the presence of nitric acid (HNO(3)) and water vapor. A sequence of solid phases was observed to form that followed Ostwald's rule for phase nucleation. For stratospheric partial pressures at temperatures between 193 and 195 kelvin, a metastable ternary H(2)SO(4)-HNO(3) hydrate, H(2)SO(4) . HNO(3) . 5H(2)O, formed in coexistence with binary H(2)SO(4) . kH(2)O hydrates (k = 2, 3, and 4) and then transformed to nitric acid dihydrate, HNO(3) . 2H(2)O, within a few hours. Metastable HNO(3) . 2H(2)O always formed before stable nitric acid trihydrate, HNO(3).3H(2)O, under stratospheric conditions and persisted for long periods. The formation of metastable phases provides a mechanism for differential particle growth and sedimentation of HNO(3) from the polar winter stratosphere. 相似文献
4.
Precision isothermal solution calorimetry was used to determine the standard-state enthalpy of formation of a number of phases in the Y-Ba-Cu-O system. An analysis of the data indicates that YBa(2)Cu(4)O(8) is thermodynamically metastable under ambient conditions. Taken together with the results from previous investigations, these data show that all of the superconducting compounds in the Y-Ba-Cu-O system are thermodynamically metastable at low temperatures. 相似文献
5.
A recently designed single-crystal surface calorimeter has been deployed to measure the energy difference between two solid surface structures. The clean Pt{100} surface is reconstructed to a stable phase in which the surface layer of platinum atoms has a quasi-hexagonal structure. By comparison of the heats of adsorption of CO and of C(2)H(4) on this stable Pt{100}-hex phase with those on a metastable Pt{100}-(1x1) surface, the energy difference between the two clean phases was measured as 20 +/- 3 and 25 +/- 3 kilojoules per mole of surface platinum atoms. 相似文献
6.
San Carlos olivine crystals under laboratory conditions of 26 gigapascals and 973 to 1473 kelvin (conditions typical of subducted slabs at a depth of 720 kilometers) for periods of a few minutes to 19 hours transformed to the phase assemblage of perovskite and magnesiowustite in two stages: (i) the oxygen sublattice transformed into a cubic close-packed lattice, forming a metastable spinelloid, and (ii) at higher temperatures or longer run durations, this spinelloid broke down to perovskite and magnesiowustite by redistributing silicon and magnesium while maintaining the general oxygen framework. The breakdown was characterized by a blocking temperature of 1000 kelvin, below which olivine remained metastable, and by rapid kinetics once the reaction was activated. 相似文献
7.
Simulations and perturbation theory are used to study the molecular origins of friction in an ideal model system, a layer of adsorbed molecules sliding over a substrate. These calculations reproduce several surprising features of experimental results. In most cases, the frictional force on a solid monolayer has a different form from that observed between macroscopic solids. No threshold force or static friction is needed to initiate sliding; instead, the velocity is proportional to the force. As in experiments, incommensurate solid layers actually slide more readily than fluid layers. A comparison of experiment, simulation, and analytic results shows that dissipation arises from anharmonic coupling between phonon modes and substrate-induced deformations in the adsorbate. 相似文献
8.
采用微波法提取杏香兔耳风总黄酮,对微波法提取植物有效成分的新工艺进行了系统探索。采用乙醇浓度(40%～45%)、固液比(1∶10～1∶40)、微波炉功率(200～800 W)、微波作用时间(5～20 min)进行L1(6 45)正交试验研究,结果表明,试验的最佳提取条件为:用80%乙醇为溶剂,料液比1∶20,微波功率600 W下微波提取20 min。在此工艺条件下总黄酮的提取率为5.12%。 相似文献
9.
A new type of natural pyrrhotite, orthorhombic 11C type (a = 6.892, b = 11.952, c = 5.744 x 11 angstroms), and the hexagonal 6C type (a = 6.89, c = 5.76 x 6 angstroms) are described. Their compositions are Fe(10)S(11) and Fe(11)S(12), respectively. Pyrrhotites stable in nature have essentially stoichiometric composition, Fe(n)-(l)S(n) (n>/=8), with the structures of n/2C type for n even and of nC type for n odd. The solid solutions between Fe(11)S(12) and Fe(10)S(11), and between Fe(10)S(11) and Fe(9)S(10) are considered metastable in nature. 相似文献
10.
Newton RC 《Science (New York, N.Y.)》1966,153(3732):170-172
The metastable kyaniteandalusite equilibrium in the Al(2)SiO(5) system has been reversed at 700 degrees , 750 degrees , and 800 degrees C at elevated water pressures, with a variety of natural and synthetic kyanites and andalusites as starting materials. Sillimanite, the stable form of Al(2)SiO(5) under these conditions, did not appear. The value of the transition pressure at 750 degrees C is 6.6 +/- 0.4 kilobars, several kilobars below pressures given by several convergent previous determinations. The Al(2)SiO(5) pressure-temperature triple point now indicated lies far from the points found by others. The revised aluminum silicate phase diagram indicates that many rocks crystallized at lower pressures than formerly thought possible. 相似文献
11.
Cohen ML 《Science (New York, N.Y.)》1986,234(4776):549-553
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. 相似文献
12.
[目的]探讨生姜渗透脱水的最佳工艺条件,建立生姜渗透脱水的传质数学模型。[方法]以生姜为研究对象,以NaCl溶液为渗透液,在不同的渗透脱水条件下考察物料失水率、固形物增加率的变化。[结果]确定了不同条件下生姜失水率、固形物得率常数,建立了生姜渗透脱水传质模型。在一定实验范围内,生姜脱水率、固形物得率均随渗透时间、渗透液浓度及温度的增加而增加;最佳条件为：渗透液浓度20%,渗透时间1.00h,渗透温度50℃。[结论]该研究为生姜脱水工艺的改进提供了一定理论依据。 相似文献
13.
The rheology of suspensions (solid particles dispersed in a fluid) is controlled primarily through the volume fraction of solids. We show that the addition of small amounts of a secondary fluid, immiscible with the continuous phase of the suspension, causes agglomeration due to capillary forces and creates particle networks, dramatically altering the bulk rheological behavior from predominantly viscous or weakly elastic to highly elastic or gel-like. This universal phenomenon is observed for a rich variety of particle/liquid systems, independent of whether the second liquid wets the particles better or worse than the primary liquid. These admixtures form stable suspensions where settling would otherwise occur and may serve as a precursor for microporous polymer foams, or lightweight ceramics. 相似文献
14.
Crown-gall tumor initiation by Agrobacterium tumefaciens is inhibited by cell walls from normal dicotyledonous plants but not by cell walls from crown-gall tumors apparently because of bacterial adherence or nonadherence, respectively, to the different cell walls. Cell walls from normal and tumor tissues in culture also show this difference, indicating that the two types of tissue stably maintain this difference under these conditions. Habituated tissue cultures, which resemble crown-gall tumor cultures, however, form cell walls that are inhibitory like those of the normal cultures from which they are derived. Monocotyledonous plants do not act as hosts for Agrobacterium and bacteria-specific inhibition is not shown by cell walls from several species of grass, a monocot family. Cell wallsfrom "embryonic" tissues (dicot seedlings less than 2 centimeters long), unlike those from older seedlings, are non-inhibitory. Crown-gall tumors thus resemble embryonic tissues in this respect. 相似文献
15.
对于设备保温而言，两层保温材料夹缝处的温度即为外层保温的最大可能使用温度，而该蔡同时受到内，外层保温材料厚度变化的影响。为得到双层保温材料的经济厚度，就要合理调整内，外层保温材料厚度，根据传热学原理，建立了具有温度约束时的双层保温最优化目标函数和约束条件，采和内部惩罚函数法对约束最优化问题进行了求解。 相似文献
16.
The tunneling resistance between two ferromagnetic metal layers that are separated by a thin insulator depends on the relative orientation of the magnetization M of each layer. In a memory device, independent switching of the magnetically soft layer is achieved by making the other layer either exchange-biased or magnetically hard. The repeated reversal of M of the soft layer by field cycling can demagnetize the other magnetically hard layer and thus erase the tunnel junction's memory. The M of exchange-biased structures was stable at least to 10(7) cycles, whereas in hard structures, M generally decayed logarithmically with the number of field cycles. The decay was very sensitive to the thickness of the hard layer and the composition of the soft layer. However, no decay was observed when the moment reversal was accomplished by coherent rotation, establishing that demagnetization results from the formation and motion of domain walls in the soft layer during field cycling. 相似文献
17.
The filamentous roots of mustard (Raphanus sativus), radish (Brassica nigra), squash (Cucurbita pepo), and wheat (Triticum aestivum) are covered throughout their length with living nucleated root hairs which may measure 1600 micro or more. The outer walls of piliferous and nonpiliferous cells consist of successive layers of mucilage, cutin, and the cellulose-pectic framework of the cell. Plasmodesmata and pits occur on all cell walls. Under the electron microscope individual pores and pits in the microfibrillar wall are evident throughout the length of the root hair. The "semipermeable membrane" of the root hair zone is thus structurally complex. 相似文献
18.
为初步分析利福昔明(Rifaximin)在人工胃/肠液和肠道菌群及动物组织样品中的稳定性,将利福昔明分别与空白人工胃液(pH=1.3,不含酶)、空白人工肠液(pH=6.8,不含酶)、人工胃液(pH=1.3,含胃蛋白酶)、人工肠液(pH=6.8,含胰蛋白酶)、SD大鼠(雄性)肠道菌群及不同组织样品的匀浆液等共孵育不同时间后,采用高效液相色谱法(HPLC)检测剩余利福昔明百分比的变化。结果表明:1)在空白人工胃液中,随孵育时间的增加,剩余利福昔明百分比虽有下降,但仍在90%以上;在人工胃液中孵育6 h,其剩余百分比为84.03%;而在空白人工肠液和人工肠液中孵育6 h后,其剩余百分比则分别降至81.14%和78.12%。2)利福昔明在SD大鼠肠道菌群中孵育12 h后,虽发生部分降解,但剩余百分比仍在90%以上,总体呈稳定的趋势。3)利福昔明在空白SD大鼠的胃、肝、小肠和大肠及内容物的匀浆液中分别孵育6 h后,其剩余百分比分别为92.09%、75.95%、78.24%和83.90%。综上,利福昔明在酸性条件下较稳定,而在碱性条件下,胰蛋白酶、胃蛋白酶和肠壁、肝脏代谢酶等对其稳定性的影响亦较为有限。本研究将为兽用利福昔明口服制剂的开发提供数据支持。 相似文献
19.
Transcription of class III genes: formation of preinitiation complexes 总被引：125，自引：0，他引：125
20.
Long Term Effects of Farming System on Soil Water Content and Dry Soil Layer in Deep Loess Profile of Loess Tableland in China 
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下载免费PDF全文 Soil water is strongly affected by land use/cover in the Loess Plateau in China. Water stored in thick loessal soils is one of the most important resources regulating vegetation growth. However, soil water in the deep loess profile, which is critical for maintaining the function of the “soil water pool” is rarely studied because deep profile soil samples are difficult to collect. In this study, four experimental plots were established in 2005 to represent different farming systems on the Changwu Tableland: fallow land, fertilized cropland, unfertilized cropland, and continuous alfalfa. The soil water content in the 15-m-deep loess profiles was monitored continuously from 2007 to 2012 with the neutron probe technique. The results showed that temporal variations in soil water profiles differed among the four farming systems. Under fallow land, the soil water content increased gradually over time, first in the surface layers and later in the deep soil layers. In contrast, the soil water content decreased gradually under continuous alfalfa. The distributions of soil water in deep soil layers under both fertilized and unfertilized cropland were relatively stable over time. Thus farming system significantly affected soil water content. Seven years after the start of the experiment, the soil water contents in the 15-m-deep profiles averaged 23.4% under fallow land, 20.3% under fertilized cropland, 21.6% under unfertilized cropland, and 16.0% under continuous alfalfa. Compared to measurements at the start of the experiment, both fallow land and unfertilized cropland increased soil water storage in the 15-m loess profiles. In contrast, continuous alfalfa reduced soil water storage. Fertilized cropland has no significant effect on soil water storage. These results suggest that deep soil water can be replenished under the fallow and unfertilized farming systems. Dry soil layers (i.e., those which have soil water content less than the stable field water capacity) in the subsoil of the Changwu Tableland region can be classified as either temporary dry soil layers or persistent dry soil layers. Temporary dry soil layers, which typically form under annual crops, often disappear during wet years. Persistent dry soil layers generally develop under perennial vegetation. Even after removing the vegetation, persistent dry soil layers remain for several decades. This study provides information useful for the conservation and utilization of soil water resources in the Loess Tableland. 相似文献