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991.
A seven-step total synthesis of α-cyclopiazonic acid is reported from a commercially available 4-bromoindole.Salient feature of the work is the rapid formation of tetracyclic skeleton via a bioinspired [3+2] annulation to form the C/D rings. 相似文献
992.
Optical phase grating diffraction in a quasistatic electric field biased nematic liquid crystal film
Abstract It is known that an optical phase grating can be obtained when two mutually coherent laser beams overlap in a nematic liquid crystal. This is mainly due to director reorientation which contributes to a large optical non-linearity. It has been suggested by Herman and Serinko that a phase grating could be obtained in nematic liquid crystals if a D.C. field is used to bias it near the critical orientational Freedericksz transition. A homeotropic MBBA film biased by an electric field at 1 kHz has been studied. Two weak Ar+ laser beams were incident normally to the film with a small intersection angle (?0·4?). Using the picture of a director reorientation mechanism and a degenerate four wave mixing scheme, we have obtained the dependence of the diffraction beam intensity on that of the incident beam and the strength of the biased electric field. The theoretical prediction and experimental results both show that the phase grating diffraction can be dramatically enhanced by the coupling of the electric field to the optical field in the Freedericksz transition region. This is due to the critical behaviour of the sample at that transition. The prominently improved signal-to-noise ratio is discussed. 相似文献
993.
Ging-Ho Hsiue Yi-An Sha Shih-Jung Hsieh Ru-Jong Jeng Wen-Jang Kuo 《Liquid crystals》2013,40(3):365-374
A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved. 相似文献
994.
In this Letter, an efficient and chemoselective deprotection of aryl- and styrenyldithioketals (acetals) is described. After being carefully examined, 10% Pd/C and Amberlite 120 in refluxing methanol was found to be an excellent condition for the chemoselective deprotection of aryl- and styrenyldithioketals (acetals) in good yields. Under this condition, no deprotection and no reduction of alkyldithioketals (acetals) was observed. 相似文献
995.
B.-T. Liou S.-H. Yen Y.-K. Kuo 《Applied Physics A: Materials Science & Processing》2005,81(3):651-655
Numerical simulation based on first-principle calculations is applied to study the wurtzite AlxIn1-xN. Simulation results suggest that the Vegard’s law deviation parameter is 0.063 ± 0.014 Å for the a lattice constant, and -0.160 ± 0.015 Å for the c lattice constant. The band gap bowing parameter is 3.668 ± 0.147 eV with the lattice constants by means of the minimized equilibrium energy, and 3.457 ± 0.152 eV with the lattice constants derived from Vegard’s law. PACS 71.15.-m; 71.20.Nr; 81.05.Ea 相似文献
996.
Given a graph G, a proper labeling f of G is a one-to-one function . The bandwidth sum of a graph G, denoted by Bs(G), is defined by Bs(G)=min∑uvE(G)|f(u)-f(v)|, where the minimum is taken for all proper labelings f of G. In this paper, we give some results for the bandwidth sum problem for the join of k graphs G1,G2,…,Gk, where each Gi is a path, cycle, complete graph, or union of isolated vertices. We also discuss the bandwidth sum for the composition of two graphs G and H, where G and H are path, cycle, or union of isolated vertices. 相似文献
997.
Jeng‐Yue Wu Tzong‐Ming Wu Wei‐Yan Chen Shih‐Jung Tsai Wen‐Faa Kuo Gwo‐Yang Chang 《Journal of Polymer Science.Polymer Physics》2005,43(22):3242-3254
X‐ray diffraction and differential scanning calorimeter (DSC) methods have been used to investigate the crystallization behavior and crystalline structure of hexamethylenediamine (HMDA)‐modified maleic‐anhydride‐grafted polypropylene/clay (PP‐g‐MA/clay) nanocomposites. These nanocomposites have been prepared by using HMDA to graft the PP‐g‐MA (designated as PP‐g‐HMA) and then mixing the PP‐g‐HMA polymer in hot xylene solution, with the organically modified montmorillonite. Both X‐ray diffraction data and transmission electron microscopy images of PP‐g‐HMA/clay nanocomposites indicate that most of the swellable silicate layers are exfoliated and randomly dispersed into PP‐g‐HMA matrix. DSC isothermal results revealed that introducing 5 wt % of clay into the PP‐g‐HMA structure causes strongly heterogeneous nucleation, which induced a change of the crystal growth process from a three‐dimensional crystal growth to a two‐dimensional spherulitic growth. Mechanical properties of PP‐g‐HMA/clay nanocomposites performed by dynamic mechanical analysis show significant improvements in the storage modulus when compared to neat PP‐g‐HMA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3242–3254, 2005 相似文献
998.
M.-E. Lee Y.-C. Yeh Y.-H. Chung C.-L. Wu C.-S. Yang W.-C. Chou C.-T. Kuo D.-J. Jang 《Physica E: Low-dimensional Systems and Nanostructures》2005,26(1-4):422
The carrier capture and relaxation of type II ZnTe/ZnSe quantum dots have been investigated with ultrafast time-resolved photoluminescence upconversion. The carrier capture times were 7 and 38 ps for the Volmer–Weber mode and Stranski–Krastanow mode, respectively. We found that the carrier relaxation of QDs exhibits faster decay under the Volmer–Weber growth mode than under the Stranski–Krastanow growth mode. We attribute the difference of carrier relaxation to the wetting layer formed in the Stranski–Krastanow growth mode. 相似文献
999.
Matthew J McGrath J Ilja Siepmann I-Feng W Kuo Christopher J Mundy Joost VandeVondele Jürg Hutter Fawzi Mohamed Matthias Krack 《Chemphyschem》2005,6(9):1894-1901
A series of first-principles Monte Carlo simulations in the isobaric-isothermal ensemble were carried out for liquid water at ambient conditions (T=298 K and p=1 atm). The Becke-Lee-Yang-Parr (BLYP) exchange and correlation energy functionals and norm-conserving Goedecker-Teter-Hutter (GTH) pseudopotentials were employed with the CP2 K simulation package to examine systems consisting of 64 water molecules. The fluctuations in the system volume encountered in simulations in the isobaric-isothermal ensemble require a reconsideration of the suitability of the typical charge-density cutoff and the regular grid-generation method previously used for the computation of the electrostatic energy in first-principles simulations in the microcanonical or canonical ensembles. In particular, it is noted that a much higher cutoff is needed and that the most computationally efficient method of creating grids can result in poor simulations. Analysis of the simulation trajectories using a very large charge-density cutoff at 1200 Ry and four different grid-generation methods point to a significantly underestimated liquid density of about 0.8 g cm-3 resulting in a somewhat understructured liquid (with a value of about 2.7 for the height of the first peak in the oxygen-oxygen radial distribution function) for BLYP-GTH water at ambient conditions. In addition, a simulation using a charge-density cutoff at 280 Ry yields a higher density of 0.9 g cm-3, showing the sensitivity of the simulation outcome to this parameter. 相似文献
1000.
Shih‐Chi Chan Shiao‐Wei Kuo Hwo‐Shuenn She Ho‐May Lin Hsin‐Fang Lee Feng‐Chih Chang 《Journal of polymer science. Part A, Polymer chemistry》2007,45(1):125-135
Mono‐polyhedral oligomeric sillsesquioxane‐end capped poly(ε‐caprolactone) (mPPCL) can form inclusion complexes (ICs) with α‐ and γ‐cyclodextrins (CDs) but not with β‐CD. These CD ICs have been characterized with X‐ray diffraction, solid‐state 13C cross‐polarization/magic‐angle‐spinning NMR spectroscopy, 1H NMR spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and scanning electron microscopy. The poly(ε‐caprolactone) (PCL) chain of mPPCL is included within the channel provided by the CDs to form a columnar, crystalline structure. The PCL/CD ratios determined by 1H NMR spectroscopy for the ICs with α‐ or γ‐CDs are higher than the stoichiometries because of the steric hindrance of the bulky polyhedral oligomeric silsesquioxane chain end and result in a fraction of the ε‐caprolactone units free from complexation with the CDs. On the basis of these analyses, we propose some possible structures for these CD/mPPCL ICs. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 125–135, 2007 相似文献