全文获取类型
收费全文 | 5826篇 |
免费 | 238篇 |
国内免费 | 28篇 |
专业分类
化学 | 3908篇 |
晶体学 | 71篇 |
力学 | 161篇 |
数学 | 437篇 |
物理学 | 1515篇 |
出版年
2024年 | 4篇 |
2023年 | 30篇 |
2022年 | 82篇 |
2021年 | 154篇 |
2020年 | 104篇 |
2019年 | 106篇 |
2018年 | 86篇 |
2017年 | 93篇 |
2016年 | 149篇 |
2015年 | 170篇 |
2014年 | 219篇 |
2013年 | 377篇 |
2012年 | 422篇 |
2011年 | 456篇 |
2010年 | 293篇 |
2009年 | 244篇 |
2008年 | 426篇 |
2007年 | 364篇 |
2006年 | 374篇 |
2005年 | 307篇 |
2004年 | 288篇 |
2003年 | 237篇 |
2002年 | 234篇 |
2001年 | 168篇 |
2000年 | 111篇 |
1999年 | 95篇 |
1998年 | 54篇 |
1997年 | 29篇 |
1996年 | 45篇 |
1995年 | 56篇 |
1994年 | 34篇 |
1993年 | 38篇 |
1992年 | 29篇 |
1991年 | 15篇 |
1990年 | 31篇 |
1989年 | 29篇 |
1988年 | 14篇 |
1987年 | 15篇 |
1985年 | 22篇 |
1984年 | 10篇 |
1983年 | 11篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 10篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1969年 | 3篇 |
排序方式: 共有6092条查询结果,搜索用时 31 毫秒
911.
Poly(ethylene glycol)(PEG)‐based interpenetrating polymeric network (IPN) hydrogels were prepared for the application of enzyme immobilization. Poly(acrylamide)(PAAm) was chosen as the other network of IPN hydrogel and different concentration of PAAm networks were incorporated inside the PEG hydrogel to improve the mechanical strength and provide functional groups that covalently bind the enzyme. Formation of IPN hydrogels was confirmed by observing the weight per cent gain of hydrogel after incorporation of PAAm network and by attenuated total reflectance/Fourier transform infrared (ATR/FTIR) analysis. Synthesis of IPN hydrogels with higher PAAm content produced more crosslinked hydrogels with lower water content (WC), smaller Mc and mesh size, which resulted in enhanced mechanical properties compared to the PEG hydrogel. The IPN hydrogels exhibited tensile strength between 0.2 and 1.2 MPa while retaining high levels of hydration (70–81% water). For enzyme immobilization, glucose oxidase (GOX) was immobilized to PEG and IPN hydrogel beads. Enzyme activity studies revealed that although all the hydrogels initially had similar enzymatic activity, enzyme‐immobilizing PEG hydrogels lost most of the enzymatic activity within 2 days due to enzyme leaching while IPN hydrogels maintained a maximum 80% of the initial enzymatic activity over a week due to the covalent linkage between the enzyme and amine groups of PAAm. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
912.
913.
We investigated the effect of several computational variables, including the choice of the basis set, application of symmetry constraints, and zero-point energy (ZPE) corrections, on the structural parameters and predicted ground electronic state of model 5-coordinate hemes (iron(II) porphines axially coordinated by a single imidazole or 2-methylimidazole). We studied the performance of B3LYP and B3PW91 with eight Pople-style basis sets (up to 6-311+G*) and B97-1, OLYP, and TPSS functionals with 6-31G and 6-31G* basis sets. Only hybrid functionals B3LYP, B3PW91, and B97-1 reproduced the quintet ground state of the model hemes. With a given functional, the choice of the basis set caused up to 2.7 kcal/mol variation of the quintet-triplet electronic energy gap (DeltaEel), in several cases, resulting in the inversion of the sign of DeltaEel. Single-point energy calculations with triple-zeta basis sets of the Pople (up to 6-311G++(2d,2p)), Ahlrichs (TZVP and TZVPP), and Dunning (cc-pVTZ) families showed the same trend. The zero-point energy of the quintet state was approximately 1 kcal/mol lower than that of the triplet, and accounting for ZPE corrections was crucial for establishing the ground state if the electronic energy of the triplet state was approximately 1 kcal/mol less than that of the quintet. Within a given model chemistry, effects of symmetry constraints and of a "tense" structure of the iron porphine fragment coordinated to 2-methylimidazole on DeltaEel were limited to 0.3 kcal/mol. For both model hemes the best agreement with crystallographic structural data was achieved with small 6-31G and 6-31G* basis sets. Deviation of the computed frequency of the Fe-Im stretching mode from the experimental value with the basis set decreased in the order: nonaugmented basis sets, basis sets with polarization functions, and basis sets with polarization and diffuse functions. Contraction of Pople-style basis sets (double-zeta or triple-zeta) affected the results insignificantly for iron(II) porphyrin coordinated with imidazole. Poor performance of a "locally dense" basis set with a large number of basis functions on the Fe center was observed in calculation of quintet-triplet gaps. Our results lead to a series of suggestions for density functional theory calculations of quintet-triplet energy gaps in ferrohemes with a single axial imidazole; these suggestions are potentially applicable for other transition-metal complexes. 相似文献
914.
The human organic anion transporter 4 (hOAT4) has been identified as the fourth isoform of OAT family. hOAT4 contributes to move several negatively charged organic compounds between cells and their extracellular milieu. The functional characteristics and regulatory mechanisms of hOAT4 remain to be elucidated. It is well known that caveolin plays a role in modulating proteins having some biological functions. To address this issue, we investigated the co-localization and interaction between hOAT4 and caveolin-1. hOAT4 and caveolin-1 (mRNA and protein expression) were observed in cultured human placental trophoblasts isolated from placenta. The confocal microscopy of immuno-cytochemistry using primary cultured human trophoblasts showed hOAT4 and caveolin-1 were co-localized at the plasma membrane of the cell. This finding was confirmed by Western blot analysis using isolated caveolae-enriched membrane fractions and immune-precipitates from the trophoblasts. When synthesized cRNA of hOAT4 along with scrambled- or antisense-oligodeoxynucleotide (ODN) of Xenopus caveolin-1 were co-injected to Xenopus oocytes, the [3H]estrone sulfate uptake was significantly decreased by the co-injection of antisense ODN but not by scrambled ODN. These findings suggest that hOAT4 and caveolin-1 share a cellular expression in the plasma membrane and caveolin-1 up-regulates the organic anionic compound uptake by hOAT4 under the normal physiological condition. 相似文献
915.
Choi BO Hwang JH Kim J Cho EM Cho SY Hwang SJ Lee HW Kim SJ Chung KW 《Experimental & molecular medicine》2008,40(3):354-360
Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome is a genetically heterogeneous mitochondrial disorder with variable clinical symptoms. Here, from the sequencing of the entire mitochondrial genome, we report a Korean MELAS family harboring two homoplasmic missense mutations, which were reported 9957T>C (Phe251Leu) transition mutation in the cytochrome c oxidase subunit 3 (COX3) gene and a novel 13849A>C (Asn505His) transversion mutation in the NADH dehydrogenase subunit 5 (ND5) gene. Neither of these mutations was found in 205 normal controls. Both mutations were identified from the proband and his mother, but not his father. The patients showed cataract symptom in addition to MELAS phenotype. We believe that the 9957T>C mutation is pathogenic, however, the 13849A>C mutation is of unclear significance. It is likely that the 13849A>C mutation might function as the secondary mutation which increase the expressivity of overlapping phenotypes of MELAS and cataract. This study also demonstrates the importance of full sequencing of mtDNA for the molecular genetic understanding of mitochondrial disorders. 相似文献
916.
Choi JS Choi HJ Jung DC Lee JH Cheon J 《Chemical communications (Cambridge, England)》2008,(19):2197-2199
Co@Pt-Au nanoparticles, which have enhanced magnetism and high stability in aqueous media, are utilized in conjunction with MRI to monitor the structural evolution of Abeta assemblies, especially Abeta protofibrils in the early reversible stages. 相似文献
917.
Let {ξ
j
; j ∈ ℤ+
d
be a centered stationary Gaussian random field, where ℤ+
d
is the d-dimensional lattice of all points in d-dimensional Euclidean space ℝd, having nonnegative integer coordinates. For each j = (j
1
, ..., jd) in ℤ+
d
, we denote |j| = j
1
... j
d
and for m, n ∈ ℤ+
d
, define S(m, n] = Σ
m<j≤n
ζ
j
, σ2(|n−m|) = ES
2
(m, n], S
n
= S(0, n] and S
0
= 0. Assume that σ(|n|) can be extended to a continuous function σ(t) of t > 0, which is nondecreasing and regularly varying with exponent α at b ≥ 0 for some 0 < α < 1. Under some additional conditions, we study limsup results for increments of partial sum processes and prove as well the law of the iterated logarithm for such partial sum processes.
Research supported by NSERC Canada grants at Carleton University, Ottawa 相似文献
918.
Ki-Young Choi Seung-Man Yang Kyu-Chul Lee Haiil Ryu Chan Hee Lee Jungho Seo Mancheol Suh 《Transition Metal Chemistry》2008,33(1):99-105
The tridentate ligand 2-Oximino-3-thiosemicarbazone-2,3-butanedione (Hotsb) reacts with MCl2 (M = Ni2+ or Cu2+) to give rise to the mononuclear complexes [Ni(Hotsb)2]Cl2 · H2O (1) and [Cu(Hotsb)Cl2] · H2O (2). These complexes have been characterized by X-ray crystallography, spectroscopy, and cyclic voltammetry. The nickel(II) ion
in (1) is in a six-coordinate octahedral environment being bonded to the two protonated tridentate ligands which occupy mer positions. The copper(II) ion in (2) is in a five-coordinate square-pyramidal geometry, in which the basal plane is made up the two nitrogens, sulfur, and chloride
atom, while the other chloride atom is coordinated at the axial position. The cyclic voltammogram of the complexes displays
two one-electron waves corresponding to MII/MIII and MII/MI processes. The electronic as well as infrared spectral properties of the title complexes are reported and discussed. 相似文献
919.
Shon YS Choi D Dare J Dinh T 《Langmuir : the ACS journal of surfaces and colloids》2008,24(13):6924-6931
This article presents a synthesis method for nanoparticle-cored dendrimers (NCDs), which have dendritic architectures around a monolayer-protected gold nanoparticle. The synthesis method is based on a strategy in which the synthesis of monolayer-protected nanoparticles is followed by adding dendrons on functionalized nanoparticles by a single coupling reaction. NMR spectroscopy, IR spectroscopy, and thermogravimetric analysis (TGA) characterizations confirmed the successful coupling reaction between dendrons with different generations ([G1], [G2], and [G3]) and COOH-functionalized nanoparticles ( approximately Au201L71). The dendrimer wedge density also could be controlled by reacting nanoparticles having different loading of COOH groups ( approximately 60 and approximately 10% COOH of the 71 ligands per gold nanoparticle) with functionalized dendrons. Transmission electron microscope results showed that this synthesis strategy maintains the average size of the nanoparticle core during dendron coupling reactions. This control over the composition and core size makes the systematic study of NCDs with different generations possible. The chemical stability of NCDs was found to be affected by dendron generation around the nanoparticle core. The current-potential response of NCD films on microelectrode arrays exhibited better electrical conductivity for NCDs with lower dendron generation. 相似文献
920.
Sang-Wook Park Byoung-Sik Choi Dae-Won Park Jae-Wook Lee 《Research on Chemical Intermediates》2008,34(8-9):881-889
The ionic liquid (IL) of 1,1,3,3-tetramethyl guanidinum trifluoroacetate, which was synthesized by reacting 1,1,3,3-tetramethylguanidine (TMG) and trifluoroacetic acid, was used as a catalyst of the reaction between carbon dioxide and glycidyl methacrylate (GMA)., The initial absorption rate of carbon dioxide into GMA solutions containing IL was measured in a semi-batch stirred tank with a plane gas-liquid interface at 101.3 kPa to obtain the reaction kinetics. The reaction rate constants of the reaction between carbon dioxide and GMA were evaluated from analysis of the mass transfer mechanism accompanied by the elementary reactions based on the film theory. Solvents such as toluene,N-methyl-2-pirrolidinone and dimethyl sulfoxide affected the reaction rate constants. 相似文献