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1.
Summary The electrocapillary properties of polyacrylic acid have been studied by two methods. Exploratory measurements have been made of the effect of the polymer on the differential capacity of a mercury drop in 0.1 m sodium perchlorate. They showed that the polymer was strongly adsorbed over a wide range of potentials but that it did not appear to form a monolayer. The surface excess of polymer obtained from drop weight data showed a maximum at very low concentrations and then a decline at higher concentrations. The bulk of the work was carried out by making surface tension measurements, using a sessile mercury drop, in solutions of a fraction of polyacrylic acid (mol. wt. 7.02×104) in potassium chloride at 0.01, 0.1, 0.2, and 0.5 m at 25°C.The data have been used to evaluate the surface excesses of the polymer and of the inorganic ions. The distribution of K+ and Cl– in the electrical double layer and the contact adsorption of Cl– on the mercury were very little affected by the presence of the polymer. The surface excess of polymer was always found to be greatest at low concentrations, to decrease steeply at first as the concentration was increased and then to decrease more slowly at higher concentrations.Possible explanations of this behaviour are discussed and it is concluded that the rapid decrease is a consequence of molecular weight dispersion and the stronger adsorption of high molecular weight polymer. The slow decrease in surface excess at higher concentrations may be a result of configurational changes of the polymer molecules.Surface pressure data show that, despite this decrease in the surface excess, the surface coverage reaches a high level at very low polymer concentrations and then continues to increase slowly as the concentration of polymer is increased. This apparent contradiction is due to changes in configuration of the adsorbed polymer molecules. At higher bulk concentrations the chain configurations are more compact and each adsorbed molecule makes more contacts with and so occupies a greater area of the mercury surface than at low concentrations.The conclusion is reached that the surface excess of polymer is mostly contained in a layer probably more than 1000 Å thick. It consists of a concentrated and entangled mass of polymer chains. Relatively few of these chains are in contact with the mercury at any istant. The concentration in this surface layer decreases steadily with increasing distance from the mercury surface and it merges without discontinuity into the bulk solution.With 10 figures in 22 details 相似文献
2.
The chain-scission reaction which occurs in copolymers of styrene and acrylonitrile has been studied at temperatures of 262, 252, and 240°C. Under these conditions volatilization is negligible, and chain scission can be studied in virtual isolation. At 262°C three kinds of chain scission are discernible, namely, at weak links which are associated with styrene units, “normal” scission in styrene segments of the chain and scission associated with the acrylonitrile units. The rate constants for normal scission and scission associated with acrylonitrile units are in the ratio of approximately 1 to 30. The molecular weight of the copolymer has no effect on the rates of scission. At 252°C the same general behavior is observed for the copolymers containing up to 24.9% acrylonitrile. The 33.4% acrylonitrile copolymer is anomalous, however. At 240°C the trends observed at 262°C appear to break down completely although individual experiments are quite reproducible. This behavior at the lower temperatures is believed to be associated with the fact that the melting points of the various copolymers are in this temperature range. Thus the viscosity of the medium, which should be expected to have a strong influence on the chain scission reaction, will be changing rapidly with temperature, copolymer composition, and molecular weight in this temperature range. 相似文献
3.
Yao Chang Jiayue Yang Zhichao Chen Zhiguo Zhang Yong Yu Qingming Li Zhigang He Weiqing Zhang Guorong Wu Rebecca A. Ingle Matthew Bain Michael N. R. Ashfold Kaijun Yuan Xueming Yang Christopher S. Hansen 《Chemical science》2020,11(19):5089
Chemical processing in the stratospheres of the gas giants is driven by incident vacuum ultraviolet (VUV) light. Ethane is an important constituent in the atmospheres of the gas giants in our solar system. The present work describes translational spectroscopy studies of the VUV photochemistry of ethane using tuneable radiation in the wavelength range 112 ≤ λ ≤ 126 nm from a free electron laser and event-triggered, fast-framing, multi-mass imaging detection methods. Contributions from at least five primary photofragmentation pathways yielding CH2, CH3 and/or H atom products are demonstrated and interpreted in terms of unimolecular decay following rapid non-adiabatic coupling to the ground state potential energy surface. These data serve to highlight parallels with methane photochemistry and limitations in contemporary models of the photoinduced stratospheric chemistry of the gas giants. The work identifies additional photochemical reactions that require incorporation into next generation extraterrestrial atmospheric chemistry models which should help rationalise hitherto unexplained aspects of the atmospheric ethane/acetylene ratios revealed by the Cassini–Huygens fly-by of Jupiter.The vacuum ultraviolet photodissociation dynamics of ethane provide clues for modelling the atmospheric chemistry of the gas giants. 相似文献
4.
Colin D. Bain George M. Whitesides 《Angewandte Chemie (International ed. in English)》1989,28(4):506-512
The interfacial properties of organic materials are of critical importance in many applications, especially the control of wettability, adhesion, tribology, and corrosion. The relationships between the microscopic structure of an organic surface and its macroscopic physical properties are, however, only poorly understood. This short review presents a model system that has the ease of preparation and the structural definition required to provide a firm understanding of interfacial phenomena. Long-chain thiols, HS(CH2)nX, adsorb from solution onto gold and form densely packed, oriented monolayers. By varying the terminal functional group, X, of the thiol, organic surfaces can be created having a wide range of structures and properties. More complex systems can be constructed by coadsorbing two or more thiols with different terminal functional groups or with different chain lengths onto a common gold substrate. By these techniques, controlled degrees of disorder can be introduced into model surfaces. We have used these systems to explore the relationships between the microscopic structure of the monolayers on a molecular and supramolecutar scale and their macroscopic properties. Wettability is a macroscopic interfacial property that has proven of particular interest. 相似文献
5.
Ellipsometry, surface tensiometry, and contact-angle measurement have been used to study the transition between partial wetting and pseudo-partial wetting of surfactant solutions by alkanes. In the partial wetting regime, the air-water surface tension is the same with and without alkane. In the pseudo-partial wetting regime, the air-water surface tension is lowered by the presence of alkane, showing that oil is solubilised into the surfactant monolayer. A discontinuous change in the coefficient of ellipticity with increasing surfactant concentration provides unequivocal evidence for the first-order nature of the wetting transitions. Ellipsometry has been used to explore the generality of wetting transitions of alkanes (dodecane, hexadecane, and squalane) on surfactant solutions [dodecyltrimethylammonium bromide, tetredecyltrimethylammonium bromide, dibucaine hydrochloride, and Aerosol OT (AOT)]. Of the systems studied, only hexadecane on AOT solutions did not show a wetting transition. Excess alkane remains as a lens on the surface of the surfactant solutions at all concentrations, but the contact angle is a minimum at the wetting transition. A semiquantitative model for the variation of the contact angle with surfactant concentration is provided. 相似文献
6.
7.
Li Zhang James A. Bain Jian-Gang Zhu Leon Abelmann Takahiro Onoue 《Journal of magnetism and magnetic materials》2006
A method of heat-assisted magnetic recording (HAMR) potentially suitable for probe-based storage systems is characterized. In this work, field emission current from a scanning tunneling microscope (STM) tip is used as the heating source. Pulse voltages of 2–7 V were applied to a CoNi/Pt multilayered film fabricated on either bare silicon or oxidized silicon substrates. Different types of Ir/Pt and W STM tips were used in the experiment. The results show that thermally recorded magnetic marks are formed with a nearly uniform mark size of 170 nm on the film fabricated on bare silicon substrate when the pulse voltage is above a threshold voltage. The mark size becomes 260 nm when they are written on the identical film fabricated on an oxidized silicon substrate. The threshold voltage depends on the material work function of the tip, with W having a threshold voltage about 1 V lower than Pt. A synthesized model, which contains the calculation of the emission current, the simulation of heat transfer during heating, and the study of magnetic domain formation, was introduced to explain experimental results. The simulation agrees well with the experiments. 相似文献
8.
9.
Sloutskin E Sapir Z Bain CD Lei Q Wilkinson KM Tamam L Deutsch M Ocko BM 《Physical review letters》2007,99(13):136102
Millimolar bulk concentrations of the surfactant cetyltrimethylammonium bromide (CTAB) induce spreading of alkanes, H(CH(2))(n)H (denoted C(n)) 12< or =n< or =21, on the water surface, which is not otherwise wet by these alkanes. The novel Langmuir-Gibbs film (LGF) formed is a liquidlike monolayer comprising both alkanes and CTAB tails. Upon cooling, an ordering transition occurs, yielding a hexagonally packed, quasi-2D crystal. For 11< or =n< or =17 this surface-frozen LGF is a crystalline monolayer. For 18< or =n< or =21 the LGF is a bilayer with a crystalline, pure-alkane, upper monolayer, and a liquidlike lower monolayer. The phase diagram and film structure were determined by x-ray, ellipsometry, and surface tension measurements. A thermodynamic theory accounts quantitatively for the observations. 相似文献
10.
Bain AD Baron M Burger SK Kowalewski VJ Rodríguez MB 《The journal of physical chemistry. A》2011,115(33):9207-9216
Cyclohexane is an extremely flexible molecule that oscillates, at room temperature, between two clearly distinct and extreme conformations that cannot be distinguished at room temperature; so much so that the NMR spectrum is a single line that includes all 12 protons be they axial or equatorial. This raises the interesting question as to what happens when there are equal substituents at the 1 and 4 carbon atoms of the ring. Therefore substitution in the 1,4-positions in the cyclohexane ring has been the subject of considerable interest because some form of interconversion between extreme conformations could lead to the existence of a rather unusual behavior. To study this problem, the interconversion in (di- or tetra-1,4)-substituted six-membered cyclohexane-type rings, trans-1,4-dibromo-1,4-dicyanocyclohexane, was found to be a particularly suitable candidate. Although X-ray diffraction studies on the crystalline solid found the molecule to be centrosymmetric, it still shows a significant dipole moment μ in solution, as determined with a procedure that leads to the vapor phase values of μ. Furthermore, the low magnetic field proton NMR spectrum at ambient temperature appears as a single line, a situation that changes with increasing field intensity and different solvents. Both these effects are attributed to dynamics, because small distortions can easily disrupt the exact cancellation of the individual dipoles (which are quite strong) associated with each end of the molecule. The molecule can exist in two forms, with both the bromines in an axial geometry or both in an equatorial position. Interconversion between these forms is observed, as in the parent cyclohexane. The single NMR line observed at low magnetic fields is due to fast exchange and requires that the two forms have roughly equal populations. Spectra obtained at low temperature confirm this, and variable-temperature studies allow measurement of the rates, leading to an enthalpy of activation of 62 kJ mol(-1). More details of the interconversion are provided by some new calculation methods. Even for a relatively small molecule like this, calculation of a full potential energy surface is prohibitive. However, methods are now available to follow the molecule along the reaction coordinate in quite an efficient way. The results of these calculations lead to an extremely detailed picture of chair-chair interconversion in a di- and tetrasubstituted six-membered ring of the cyclohexane family. 相似文献