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971.
A series of bifunctional phosphine-thiourea organic catalysts based on natural amino acid scaffolds were designed and prepared. L-threonine-derived bifunctional phosphine catalysts were found to be very efficient in promoting asymmetric Morita-Baylis-Hillman (MBH) reaction of acrylates with aromatic aldehydes, affording the desired MBH adducts with up to 90% ee. To gain mechanistic insights into the reaction, the effects of adding various additives on the MBH reaction were investigated. We propose that the hydrogen bonding interactions between the thiourea moiety of the catalyst and the enolate intermediate are crucial for the stereochemical outcome of the reaction. The method described in this report may provide a practical solution to the enantioselective MBH reaction of simple acrylates. 相似文献
972.
Shen H Wang H Zhou C Niu JZ Yuan H Ma L Li LS 《Dalton transactions (Cambridge, England : 2003)》2011,40(36):9180-9188
Here we report a new "green" method to synthesize Zn(1-x)Cd(x)Se (x = 0-1) and stable red-green-blue tricolor Zn(1-x)Cd(x)Se core/shell nanocrystals using only low cost, phosphine-free and environmentally friendly reagents. The first excitonic absorption peak and photoluminescence (PL) position of the Zn(1-x)Cd(x)Se nanocrystals (the value of x is in the range 0.005-0.2) can be fixed to any position in the range 456-540 nm. There is no red or blue shift in the entire reaction process. Three similar sizes of alloyed Zn(1-x)Cd(x)Se nanocrystals with blue, green, and yellow emissions were successfully selected as cores to synthesize high quality blue, green, and red core/shell nanocrystal emitters. For the synthesis of core/shell nanocrystals with a high quantum yield (QY) and stability, the selection of shell materials has been proven to be very important. Therefore, alternative protocols have been used to optimize thick shell growth. ZnSe/ZnSe(x)S(1-x) and CdS/Zn(1-x)Cd(x)S have been found as an excellent middle multishell to overcoat between the alloyed Zn(1-x)Cd(x)Se core and ZnS outshell. The QYs of the as-synthesized core/shell alloyed Zn(1-x)Cd(x)Se nanocrystals can reach 40-75%. The Cd content is reduced to less than 0.1% for Zn(1 -x)Cd(x)Se core/shell nanocrystals with emissions in the range 456-540 nm. More than 15 g of high quality Zn(1-x)Cd(x)Se core/shell nanocrystals were prepared successfully in a large scale, one-pot reaction. Importantly, the emissions of such thick multishell nanocrystals are not susceptible to ligand loss and stability in various physiological conditions. 相似文献
973.
Multidrug resistance (MDR) continues to be a major obstacle for successful anticancer therapy. In this work, fractions from 17 clinically used antitumour traditional Chinese medicinal herbs were tested for their potential to restore the sensitivity of MCF-7/ADR and A549/Taxol cells to a known antineoplastic agent. The effects of these fractions were evaluated by MTT method and an assay of the cellular accumulation of doxorubicin. Fractions from the PB group (herbs with the ability to promote blood circulation and remove blood stasis) showed more significant effects than fractions from the CH group (herbs with the ability to clear away heat and toxic materials). Fractions from CH?Cl? extracts were more effective than fractions from EtOAc extracts. Five herbs (Curcuma wenyujin, Chrysanthemum indicum, Salvia chinensis, Ligusticum chuanxiong Hort. and Cassia tora L.) could sensitise these resistant cancer cells at a non-toxic concentration (10?μg?mL?1), and markedly increased doxorubicin accumulation in MCF-7/ADR cells, which necessitates further investigations on the active ingredients of these herbs and their underlying mechanisms. 相似文献
974.
Zou X Liu D Zhong L Yang B Lou Y Hu B Yin Y 《Analytical and bioanalytical chemistry》2011,401(4):1251-1261
In this study, we describe a method for highly specific enrichment of phosphopeptides with multifunctional chitosan–glycidyl
methacrylate (GMA)–iminodiacetic acid (IDA)–Fe (III) nanospheres for direct analysis by matrix-assisted laser desorption–ionization
mass spectrometry (MALDI-MS). This is the first time that chitosan has been used to create nanospheres support material for
selective enrichment of phosphopeptides by modification with GMA, derivatization with IDA, and loading with Fe (III) ions.
Chitosan-GMA-IDA-Fe (III) nanospheres with a diameter of 20 to 100 nm have multifunctional chemical moieties which confer
unique properties, good dispersibility in highly acidic binding buffers, as well as good biocompatibility and chemical stability
which improves their specific interaction with phosphopeptides using various types of acid binding buffers. The process of
enrichment is very simple, quick, efficient, and specific. Its high specificity and efficiency for purification of phosphopeptides
is reflected in the very low and substoichiometric amounts of phosphopeptides which can be detected, in quantities as low
as 1:3,000 M ratios. Compared with other state-of the-art technologies such as the use of conventional Fe3+-IMAC and TiO2, these chitosan nanosphere techniques show superior specificity and sensitivity. Moreover, the resultant chitosan-GMA-IDA-Fe3+ nanosphere-absorbed phosphopeptides can be either directly analyzed by MALDI-TOF MS analysis or eluted and further analyzed
by nano-LC-MS/MS. 相似文献
975.
In this note, for the case of , we prove the existence of global-in-time finite energy weak solution of the equations of a two-dimensional magnetohydrodynamics with Coulomb force, where γ denotes the adiabatic exponent. The value is the optimal lower bound of γ to establish global-in-time finite energy weak solution under current frame. 相似文献
976.
In this paper, a key assumption similar to that of Li and Chen is introduced by virtue of a gap function for a class of parametric
set-valued weak vector variational inequalities in Banach spaces. By using this key assumption, sufficient and necessary conditions
of the continuity and Hausdorff continuity of the solution set mapping for such parametric set-valued weak vector variational
inequalities are given in Banach spaces when the image space is infinite dimensional. The results presented in this paper
generalize and improve some main results of Li and Chen. 相似文献
977.
Zhong ChenWei Jiang 《Applied mathematics and computation》2011,217(19):7790-7798
The purpose of this paper is to obtain the approximation solution of linear and strong nonlinear weakly singular Volterra integral equation of the second kind, especially for such a situation that the equation is of nonsmooth solution and the situation that the problem is a strong nonlinear problem. For this purpose, we firstly make a transform to the equation such that the solution of the new equation is as smooth as we like. Through modifying homotopy perturbation method, an algorithm is successfully established to solve the linear and nonlinear weakly singular Volterra integral equation of the second kind. And the convergence of the algorithm is proved strictly. Comparisons are made between our method and other methods, and the results reveal that the modified homotopy perturbation is effective. 相似文献
978.
In this paper, we consider the heat flow for the Hsystem with constant mean curvature in higher dimensions. We give sufficient conditions on the initial data such that the heat flow develops finite time singularity. We also provide a new set of initial data to guarantee the existence of global regular solution to the heat flow, that converges to zero in W 1,n with the decay rate t 2/(2-n) as time goes to infinity. 相似文献
979.
This paper proposes a descent method to solve a class of structured monotone variational inequalities. The descent directions are constructed from the iterates generated by a prediction-correction method [B.S. He, Y. Xu, X.M. Yuan, A logarithmic-quadratic proximal prediction-correction method for structured monotone variational inequalities, Comput. Optim. Appl. 35 (2006) 19-46], which is based on the logarithmic-quadratic proximal method. In addition, the optimal step-sizes along these descent directions are identified to accelerate the convergence of the new method. Finally, some numerical results for solving traffic equilibrium problems are reported. 相似文献
980.
A periodic mathematical model of cell populations affected by periodic radiation is presented and studied in this paper. We obtain some sufficient conditions on the permanence and extinction of the system. Furthermore, criteria on the existence and global asymptotic stability of unique positive periodic solutions are established. Some numerical examples are shown to verify our results. A discussion is presented for further study. 相似文献