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51.
Comparison of phantom materials for use in quality assurance of microbeam radiation therapy 下载免费PDF全文
Matthew Cameron Iwan Cornelius Dean Cutajar Jeremy Davis Anatoly Rosenfeld Michael Lerch Susanna Guatelli 《Journal of synchrotron radiation》2017,24(4):866-876
Microbeam radiation therapy (MRT) is a promising radiotherapy modality that uses arrays of spatially fractionated micrometre‐sized beams of synchrotron radiation to irradiate tumours. Routine dosimetry quality assurance (QA) prior to treatment is necessary to identify any changes in beam condition from the treatment plan, and is undertaken using solid homogeneous phantoms. Solid phantoms are designed for, and routinely used in, megavoltage X‐ray beam radiation therapy. These solid phantoms are not necessarily designed to be water‐equivalent at low X‐ray energies, and therefore may not be suitable for MRT QA. This work quantitatively determines the most appropriate solid phantom to use in dosimetric MRT QA. Simulated dose profiles of various phantom materials were compared with those calculated in water under the same conditions. The phantoms under consideration were RMI457 Solid Water (Gammex‐RMI, Middleton, WI, USA), Plastic Water (CIRS, Norfolk, VA, USA), Plastic Water DT (CIRS, Norfolk, VA, USA), PAGAT (CIRS, Norfolk, VA, USA), RW3 Solid Phantom (PTW Freiburg, Freiburg, Germany), PMMA, Virtual Water (Med‐Cal, Verona, WI, USA) and Perspex. RMI457 Solid Water and Virtual Water were found to be the best approximations for water in MRT dosimetry (within ±3% deviation in peak and 6% in valley). RW3 and Plastic Water DT approximate the relative dose distribution in water (within ±3% deviation in the peak and 5% in the valley). PAGAT, PMMA, Perspex and Plastic Water are not recommended to be used as phantoms for MRT QA, due to dosimetric discrepancies greater than 5%. 相似文献
52.
The motion of molecules across channels and pores is critically important for understanding mechanisms of many cellular processes.
Here we investigate the mechanism of interactions in the molecular transport through nanopores by analyzing exactly solvable
discrete stochastic models. According to this approach the channel transport is viewed as a set of chemical transitions between
discrete states. It is shown that the strength and spatial distribution of molecule/channel interactions can strongly modify
the particle current. Our analysis indicates that the most optimal transport is achieved when the binding sites are near the
entrance or exit of the pore depending on the sign of interaction potential. In addition, the role of intermolecular interactions
during the channel transport is studied, and it is argued that an increase in the flux can be observed for some optimal interaction
strength. The mechanisms of these phenomena are discussed. 相似文献
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A series of organotin(IV) compounds R3Sn(A) where R = Me or Ph and A is a chromogenic nitrophenolate ligand were prepared and studied as possible colorimetric sensors for anions (F−, Cl−, Br−, AcO−, H2PO4−). Equilibrium constants for a complete set of reactions between R3Sn(A) with A = 2‐amino‐4‐nitrophenolate (ANP) or 4‐nitrophenolate and anions (X−) involving formation of complexes R3Sn(A)(X)− and substitution products R3Sn(X) and R3Sn(X)2− were determined by UV‐vis and 1H NMR titrations in MeCN and DMSO. The binding selectivity was AcO− > F− > H2PO4− > Cl− ≫ Br− in both solvents and both for R = Me and Ph with higher affinity for R = Ph. Compounds with A = ANP were found to have the optimum properties as anion sensors allowing optical detection of F−, AcO− and H2PO4− anions in the 5–100 µM range by appearance of an intense absorption band of free ANP resulting from its substitution with the analyte. Selectivity and affinity of anion interactions with R3Sn(ANP) are similar to those for thiourea receptors, but the organotin receptor produces a much larger naked eye detected optical signal, operates equally well in nonpolar and polar solvents and tolerates the presence of up to 20% vol. of water in DMSO. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
55.
Viktor Gerasimenko Vyacheslav Shtyk Anatoly Zagorodny 《Central European Journal of Physics》2011,9(1):71-77
Evolution equations for marginal generalized microscopic phase densities are introduced. The evolution equations for average
values of microscopic phase densities are derived and a solution of the initial-value problem for the hydrodynamic type hierarchy
obtained is constructed. 相似文献
56.
A method of ring artefact suppression in X-ray computerised tomography (CT) reconstructions is proposed. The method is based on the assumption that a sinogram is a smooth function along the horizontal spatial coordinate. Methods based on the theory of ill-posed problems are applied to find a regularised solution. An analytical formula for the solution is proposed allowing fast ring artefact suppression. Its performance is demonstrated for parallel beam synchrotron X-ray tomography. 相似文献
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The hitherto unsolved problem of the origin of the unusually high nitrogen inversion-rotation (NIR) barriers in 7-azabicyclo[2.2.1]heptanes (the bicyclic effect) was examined using the natural bond orbital (NBO) approach. Reinvestigating the NIR barrier for tropane by DNMR, we found that NIR barriers increase smoothly on going from nitrogen-bridged bicyclic systems of a larger ring size to the smaller ring homologous systems. The experimental NIR barriers are reproduced with good accuracy using the MP2/6-31G level of theory. The NBO analysis for these and other azabicycles led to the conclusion that the height of these barriers is mostly determined by the energy of the sigma-orbitals of the C(alpha)(-)C(beta) bonds as well as the nitrogen lone pair. Thus, the bicyclic effect is actually an extreme case of a common C(alpha-)N-C(alpha) tripyramid geometry-NIR barrier dependence for N-bridged bicyclic amines. By establishing the rate-determining role of the C(alpha-)N-C(alpha) tripyramid fragment for NIR, we have derived the first sufficiently accurate quantitative correlations amine geometry-NIR barrier for monocyclic as well as bicyclic N-H and N-Me amines (i.e., for an amine set which also includes the bicyclic effect systems). 相似文献
60.
The shot noise in the current through a quantum dot is calculated as a function of voltage from the high-voltage Coulomb-blockaded regime to the low-voltage Kondo regime. Using several complementary approaches, it is shown that the zero-frequency shot noise (scaled by the voltage) exhibits a nonmonotonic dependence on voltage, with a peak around the Kondo temperature. Beyond giving a good estimate of the Kondo temperature, it is shown that the shot noise yields additional information on the effects of electronic correlations on the local density of states in the Kondo regime, unaccessible in traditional transport measurements. 相似文献