全文获取类型
收费全文 | 3917篇 |
免费 | 159篇 |
国内免费 | 30篇 |
专业分类
化学 | 2457篇 |
晶体学 | 17篇 |
力学 | 117篇 |
数学 | 867篇 |
物理学 | 648篇 |
出版年
2023年 | 37篇 |
2022年 | 46篇 |
2021年 | 93篇 |
2020年 | 73篇 |
2019年 | 86篇 |
2018年 | 67篇 |
2017年 | 81篇 |
2016年 | 154篇 |
2015年 | 115篇 |
2014年 | 134篇 |
2013年 | 229篇 |
2012年 | 280篇 |
2011年 | 317篇 |
2010年 | 189篇 |
2009年 | 198篇 |
2008年 | 269篇 |
2007年 | 245篇 |
2006年 | 225篇 |
2005年 | 185篇 |
2004年 | 185篇 |
2003年 | 111篇 |
2002年 | 120篇 |
2001年 | 56篇 |
2000年 | 62篇 |
1999年 | 38篇 |
1998年 | 39篇 |
1997年 | 32篇 |
1996年 | 44篇 |
1995年 | 22篇 |
1994年 | 25篇 |
1993年 | 22篇 |
1992年 | 32篇 |
1991年 | 21篇 |
1990年 | 30篇 |
1989年 | 13篇 |
1988年 | 22篇 |
1987年 | 13篇 |
1986年 | 13篇 |
1985年 | 23篇 |
1984年 | 26篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 15篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1974年 | 7篇 |
1971年 | 6篇 |
1963年 | 5篇 |
排序方式: 共有4106条查询结果,搜索用时 171 毫秒
991.
Firms face a continuous process of technological and environmental changes that requires them to make managerial decisions in a dynamic context. However, costs and constraints prevent firms from making instant adjustments towards optimal conditions and may cause inefficiency to persist in time. We propose a dynamic inefficiency specification that captures differences in the adjustment costs among firms and non-persistent effects of inefficiency heterogeneity. The model is fitted to a ten year sample of Colombian banks. The new specification improves model fit and have effects on efficiency estimations. Overall, Colombian banks present high inefficiency persistence but important differences between institutions are found. In particular, merged banks present low adjustment costs that allow them to recover rapidly efficiency losses derived from merging processes. 相似文献
992.
The Crank–Nicolson–Galerkin finite element method for a nonlocal parabolic equation with moving boundaries 下载免费PDF全文
Rui M. P. Almeida José C. M. Duque Jorge Ferreira Rui J. Robalo 《Numerical Methods for Partial Differential Equations》2015,31(5):1515-1533
The aim of this article is to establish the convergence and error bounds for the fully discrete solutions of a class of nonlinear equations of reaction–diffusion nonlocal type with moving boundaries, using a linearized Crank–Nicolson–Galerkin finite element method with polynomial approximations of any degree. A coordinate transformation which fixes the boundaries is used. Some numerical tests to compare our Matlab code with some existing moving finite element methods are investigated. © 2014 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 31: 1515–1533, 2015 相似文献
993.
994.
Dul MC Ferrando-Soria J Pardo E Lescouëzec R Journaux Y Ruiz-García R Cano J Julve M Lloret F Fabelo O Pasán J Ruiz-Pérez C 《Inorganic chemistry》2010,49(24):11264-11266
Two new heterometallic Ni(II)(n)Cu(II)((9-n)) complexes [n = 1 (2) and 2 (3)] have been synthesized following a multicomponent self-assembly process from a n:(3 - n):2:6 stoichiometric mixture of Ni(2+), Cu(2+), L(6-), and [CuL'](2+), where L and L' are the bridging and blocking ligands 1,3,5-benzenetris(oxamate) and N,N,N',N',N'-pentamethyldiethylenetriamine, respectively. Complexes 2 and 3 possess a unique cyclindrical architecture formed by three oxamato-bridged trinuclear linear units connected through two 1,3,5-substituted benzenetris(amidate) bridges, giving a triangular metallacyclophane core. They behave as a ferromagnetically coupled trimer of two (2)/one (3) S = (1)/(2) Cu(II)(3) plus one (2)/two (3) S = 0 Ni(II)Cu(II)(2) linear units with overall S = 1 Ni(II)Cu(II)(8) (2) and S = (1)/(2) Ni(II)(2)Cu(II)(7) (3) ground states. 相似文献
995.
Jorge Acevedo-Martínez Igor G. Zenkevich Ramón Carrasco-Velar 《Chromatographia》2010,71(9-10):881-889
A simple algorithm is proposed for prediction of linear retention indices, RI, of organic compounds with different structures. The algorithm is based on the hypothesis that any structural moiety of a molecule contributes to gas chromatographic retention to a different extent, depending on its molecular environment. For a given moiety the mean structural increment (MSI) is calculated from the difference between the retention indices of two molecules, one containing it and one not, in different compound families. The mean of these values is the MSI for the corresponding moiety. The correlation between predicted and experimental values affords r 2 = 0.992 and the mean relative error is 1.65% for n = 92 compounds. 相似文献
996.
Roser Sabater i Serra Apostolos Kyritsis Jorge L. Escobar Ivirico Andreu Andrio Balado Jose Luis Gómez Ribelles Polykarpos Pissis Manuel Salmerón-Sánchez 《Colloid and polymer science》2010,288(5):555-565
Poly(L-lactide) networks (PmLA) hydrophilized with different amounts of 2-hydroxyethyl acrylate (HEA) were investigated by
dielectric relaxation spectroscopy, thermally stimulated depolarization currents, and differential scanning calorimetry. The
incorporation of HEA units in the PmLA network, with the aim of modulating the water sorption capacity of the system, results
in a material with a complex behavior. The system consists of phase-separated microdomains richer in one or the other comonomers
that constitute the network. Initially, the addition of smalls amount of HEA units in the network gives rise to a one-phase,
two-component system; however, when the amount of HEA in the system increases, a new phase (HEA-rich one) is formed containing
some mLA chains that modify the main relaxation mode of these domains and the local dynamics of the system. The structure
of the system has been analyzed by comparing the relaxational modes in the PmLA and PHEA homonetworks with those in the copolymer
networks. 相似文献
997.
Dayene M. Carvalho Jorge L. B. MacielJr. Leandro P. Ravaro Rogério E. Garcia Valdemir G. Ferreira Luis V. A. Scalvi 《Journal of Sol-Gel Science and Technology》2010,55(3):385-393
The fluid flow of the liquid phase in the sol-gel-dip-coating process for SnO2 thin film deposition is numerically simulated. This calculation yields useful information on the velocity distribution close
to the substrate, where the film is deposited. The fluid modeling is done by assuming Newtonian behavior, since the linear
relation between shear stress and velocity gradient is observed. Besides, very low viscosities are used. The fluid governing
equations are the Navier–Stokes in the two dimensional form, discretized by the finite difference technique. Results of optical
transmittance and X-ray diffraction on films obtained from colloidal suspensions with regular viscosity, confirm the substrate
base as the thickest part of the film, as inferred from the numerical simulation. In addition, as the viscosity increases,
the fluid acquires more uniform velocity distribution close to the substrate, leading to more homogenous and uniform films. 相似文献
998.
Jorge P. López‐Alonso Dr. Miguel A. Pardo‐Cea Irene Gómez‐Pinto Dr. Irene Fernández Dr. Avijit Chakrabartty Prof. Enrique Pedroso Prof. Carlos González Prof. Douglas V. Laurents Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(18):5314-5323
KIA7, a peptide with a highly restricted set of amino acids (Lys, Ile, Ala, Gly and Tyr), adopts a specifically folded structure. Some amino acids, including Lys, Ile, Ala, Gly and His, form under the same putative prebiotic conditions, whereas different conditions are needed for producing Tyr, Phe and Trp. Herein, we report the 3D structure and conformational stability of the peptide KIA7H, which is composed of only Lys, Ile, Ala, Gly and His. When the imidazole group is neutral, this 20‐mer peptide adopts a four‐helix bundle with a specifically packed hydrophobic core. Therefore, one‐pot prebiotic proteins with well‐defined structures might have arisen early in chemical evolution. The Trp variant, KIA7W, was also studied. It adopts a 3D structure similar to that of KIA7H and its previously studied Tyr and Phe variants, but is remarkably more stable. When tested for ribonucleolytic activity, KIA7H, KIA7W and even short, unstructured peptides rich in His and Lys, in combination with Mg++, Mn++ or Ni++ (but not Cu++, Zn++ or EDTA) specifically cleave the single‐stranded region in an RNA stem–loop. This suggests that prebiotic peptide–divalent cation complexes with ribonucleolytic activity might have co‐inhabited the RNA world. 相似文献
999.
Bañuelos-Prieto J Agarrabeitia AR Garcia-Moreno I Lopez-Arbeloa I Costela A Infantes L Perez-Ojeda ME Palacios-Cuesta M Ortiz MJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(47):14094-14105
Asymmetrically substituted BODIPY analogues of the dye PM567 have been synthesised from 2-acylpyrroles and pyrroles that bear indene, fluorene or difluorene units. The type of linkage between the fluorene and the BODIPY core plays an important role in the photophysics of the BODIPY chromophore. Indeed, an aliphatic bridge gives rise to an energy-transfer process between the chromophores, whereas a vinyl spacer allows an electronic interaction between them, leading to a large red shift of the spectral bands. The laser action of the new dyes has been analysed under transversal pumping at 10 Hz repetition rate, in both liquid phase and incorporated into solid polymeric matrices. Lasing efficiencies of up to 40% were reached with high photostabilities with the laser output remaining at the initial level after 1×10(5) pump pulses in the same position of the sample. The laser action of the new dyes outperforms the laser behaviour of commercial dyes that emit in the same spectral region. The replacement of fluorene by indene quenches the fluorescence and laser emission, but allows the development of an iron cation fluorescent sensor. 相似文献
1000.