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491.
492.
The activation energies for four Smiles reactions vary with the distance squared between the nucleophilic and electrophilic atoms (consistent with theoretical considerations and pertinent to organic and enzymatic catalyses). 相似文献
493.
494.
Jones LH Harwig CW Wentworth P Simeonov A Wentworth AD Py S Ashley JA Lerner RA Janda KD 《Journal of the American Chemical Society》2001,123(15):3607-3608
495.
The gem-dimethyl effect is the acceleration of cyclization by substituents in the chain and is often used in organic synthesis as a ring-closing effect. Calculations on cyclobutane, methylcyclobutane, and 1,1-dimethylcyclobutane are performed. 1,1-Dimethylcyclobutane is a four-membered carbon ring with gem-dimethyl substituents. Optimum equilibrium geometries, harmonic vibrational frequencies, and corresponding electronic energies are computed for all pertinent molecular systems using SCF theory, density functional theory (DFT), and second-order perturbation theory (MP2) with two triple-zeta quality basis sets, 6-311G(d,p) and 6-311G+(2df,2pd). Additional single-point calculations are performed using the optimized MP2/6-311G+(2df,2pd) geometries and coupled-cluster theory including single and double excitations and noniterative, linear triple excitations (CCSD(T)). Calculations indicate that 1,1-dimethylcyclobutane is more than 8 kcal mol-1 less strained than cyclobutane, that is, there is at least some thermodynamic component to the gem-dimethyl effect. 相似文献
496.
Waveguiding structures are one of the fundamental components of integrated photonic circuitry. Devices with low loss and a linear response across a wide wavelength range are especially desirable. In the present Letter, we have successfully developed and characterized low-loss silica waveguides integrated on a silicon substrate with a novel suspended cylinder geometry. The unique design creates a device that is effectively air clad, resulting in a large refractive index contrast for improved optical field confinement. The measured loss is constant from 658 to 1550 nm, and it is independent of the polarization of the input light and the input power. 相似文献
497.
A more flexible nucleotide building block for the synthesis of new DNA based porphyrin-zipper arrays is described. Changing the rigid acetylene linker between the porphyrin substituent and the 2'-deoxyuridine to a more flexible propargyl amide containing linkage leads in part to an increased duplex stability. The CD spectra reveal different electronic interactions between the porphyrins depending on the type of linker used. Molecular modelling suggests large variation of the relative orientation of the porphyrins within the major groove of the DNA. The porphyrins can be metallated post-synthetically with different metals as shown with zinc, cobalt and copper. The spectroscopic features do not alter drastically upon metallation apart from the CD spectra, and the stability of the metal complex is highly dependent on the nature of the metal. As shown by CD spectroscopy, the zinc porphyrin is rapidly demetallated at high temperatures. Globular structure determination using SAXS indicates that a molecular assembly comprised of a two to four helical bundle dominates in solution at higher concentrations (≥50 μM) which is not observed by spectroscopy at lower concentrations (≤1 μM). 相似文献
498.
Munro NH Green DW Dangerfield A McGrath KM 《Dalton transactions (Cambridge, England : 2003)》2011,40(36):9259-9268
Chitosan hydrogels are of considerable interest in synthetic biomimetic mineralisation strategies due to their favourable characteristics such as the presentation of a large surface area for crystal nucleation within a structured yet responsive scaffold. Chitosan hydrogels were prepared and subsequently calcium carbonate mineralisation was initiated using a method which combines alternate soaking of the films with precursor solutions followed by treatment with Kitano solution. This combined approach allows for increased extent of mineralisation, inducement of mineralisation uniformly throughout the hydrogel rather than only at the peripheral surface and ready scalability and shape manipulation. The base synthetic system is readily modified through the introduction of additives that manipulate the nucleation and growth of the calcium carbonate. Addition of poly(acrylic acid) inhibits nucleation and induces tangential crystal growth along the internal and external interfaces of the hydrogel. The resulting composite is comprised of stacked overlapping plates of calcium carbonate intercalated with carbohydrate. The method is applicable in combination with a variety of hydrogels including macroporous chitosan, chitosan-alginate bilayers and pure alginate hydrogels. The composite materials were analysed by SEM, XRD, microRaman spectroscopy and mechanical strength testing. 相似文献
499.
Zhilin Li Li Wang Eric Aspinwall Racheal Cooper Paul Kuberry Ashley Sanders Ke Zeng 《Mathematical Methods in the Applied Sciences》2015,38(18):4530-4539
Interface problems modeled by differential equations have many applications in mathematical biology, fluid mechanics, material sciences, and many other areas. Typically, interface problems are characterized by discontinuities in the coefficients and/or the Dirac delta function singularities in the source term. Because of these irregularities, solutions to the differential equations are not smooth or discontinuous. In this paper, some new results on the jump conditions of the solution across the interface are derived using the distribution theory and the theory of weak solutions. Some theoretical results on the boundary singularity in which the singular delta function is at the boundary are obtained. Finally, the proof of the convergency of the immersed boundary (IB) method is presented. The IB method is shown to be first‐order convergent in L ∞ norm. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
500.
Kimberly C. Clarke Alison M. Douglas Ashley C. Brown Thomas H. Barker L. Andrew Lyon 《Current Opinion in Colloid & Interface Science》2013,18(5):393-405
The development of tissue engineering scaffolds has focused on mimicking the natural biochemical and biophysical environment of the extracellular matrix (ECM). In this review, we describe a variety of strategies aimed at reproducing and also simplifying the ECM. Despite the progress that has been made, the degree of complexity that needs to be incorporated into these scaffolds is still not known. We begin by describing the ECM and its biological functions followed by outlining current efforts to engineer ECMs with both natural and synthetic polymers. We then focus on colloidal particles as potential artificial ECM components that could increase the complexity as modular building blocks. Drawing from examples from the literature we present the broad utility of colloids and describe how these applications could be useful in the development of ECM mimetic systems. 相似文献