全文获取类型
收费全文 | 4525篇 |
免费 | 743篇 |
国内免费 | 443篇 |
专业分类
化学 | 3300篇 |
晶体学 | 42篇 |
力学 | 270篇 |
综合类 | 47篇 |
数学 | 435篇 |
物理学 | 1617篇 |
出版年
2024年 | 11篇 |
2023年 | 128篇 |
2022年 | 134篇 |
2021年 | 179篇 |
2020年 | 230篇 |
2019年 | 170篇 |
2018年 | 146篇 |
2017年 | 127篇 |
2016年 | 202篇 |
2015年 | 179篇 |
2014年 | 255篇 |
2013年 | 272篇 |
2012年 | 394篇 |
2011年 | 374篇 |
2010年 | 277篇 |
2009年 | 271篇 |
2008年 | 298篇 |
2007年 | 281篇 |
2006年 | 279篇 |
2005年 | 190篇 |
2004年 | 134篇 |
2003年 | 117篇 |
2002年 | 107篇 |
2001年 | 86篇 |
2000年 | 86篇 |
1999年 | 103篇 |
1998年 | 90篇 |
1997年 | 90篇 |
1996年 | 65篇 |
1995年 | 90篇 |
1994年 | 74篇 |
1993年 | 47篇 |
1992年 | 58篇 |
1991年 | 45篇 |
1990年 | 40篇 |
1989年 | 26篇 |
1988年 | 17篇 |
1987年 | 14篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有5711条查询结果,搜索用时 31 毫秒
141.
Although heme protein-based, amperometric nitric oxide (NO) biosensors have been well documented in previous studies, most have been conducted in anaerobic conditions. Herein we report a novel hemoglobin-based NO biosensor that is not only very sensitive but also usable in air. The heme protein was entrapped in a sodium montmorillonite film, which was immobilized at a pyrolytic graphite electrode surface. Film-entrapped hemoglobin can directly exchange electrons with the electrode, and this process has proven to favor the catalytic reduction of oxygen. In addition, NO induced a cathodic potential shift of the catalytic reduction peak of oxygen. This potential shift was proportional to the logarithm of NO concentration ranging from 4.0 × 10−11 to 5.0 × 10−6 mol/L. The detection limit has been estimated to be 20 pM, approximately four orders lower than previously reported amperometric detectors. 相似文献
142.
The solution structure of Cobeta-5'-deoxyadenosylimidazolylcobamide, Ado(Im)Cbl, the coenzyme B(12) analogue in which the axial 5,6-dimethylbenzimidazole (Bzm) ligand is replaced by imidazole, has been determined by NMR-restrained molecular modeling. A two-state model, in which a conformation with the adenosyl moiety over the southern quadrant of the corrin and a conformation with the adenosyl ligand over the eastern quadrant of the corrin are both populated at room temperature, was required by the nOe data. A rotation profile and molecular dynamics simulations suggest that the eastern conformation is the more stable, in contrast to AdoCbl itself in which the southern conformation is preferred. Consensus structures of the two conformers show that the axial Co-N bond is slightly shorter and the corrin ring is less folded in Ado(Im)Cbl than in AdoCbl. A study of the thermolysis of Ado(Im)Cbl in aqueous solution (50-125 degrees C) revealed competing homolytic and heterolytic pathways as for AdoCbl but with heterolysis being 9-fold faster and homolysis being 3-fold slower at 100 degrees C than for AdoCbl. Determination of the pK(a)'s for the Ado(Im)Cbl base-on/base-off reaction and for the detached imidazole ribonucleoside as a function of temperature permitted correction of the homolysis and heterolysis rate constants for the temperature-dependent presence of the base-off species of Ado(Im)Cbl. Activation analysis of the resulting rate constants for the base-on species show that the entropy of activation for Ado(Im)Cbl homolysis (13.7 +/- 0.9 cal mol(-1) K(-1)) is identical with that of AdoCbl (13.5 +/- 0.7 cal mol(-1) K(-1)) but that the enthalpy of activation (34.8 kcal mol(-1)) is 1.0 +/- 0.4 kcal mol(-1) larger. The opposite effect is seen for heterolysis, where the enthalpies of activation are identical but the entropy of activation is 5 +/- 1 cal mol(-1) K(-1) less negative for Ado(Im)Cbl. Extrapolation to 37 degrees C provides a rate constant for Ado(Im)Cbl homolysis of 2.1 x 10(-9) s(-1), 4.3-fold smaller than for AdoCbl. Combined with earlier results for the enzyme-induced homolysis of Ado(Im)Cbl by the ribonucleoside triphosphate reductase from Lactobacillus leichmannii, the catalytic efficiency of the enzyme for homolysis of Ado(Im)Cbl at 37 degrees C can be calculated to be 4.0 x 10(8), 3.8-fold, or 0.8 kcal mol(-1), smaller than for AdoCbl. Thus, the bulky Bzm ligand makes at best a <1 kcal mol(-1) contribution to the enzymatic activation of coenzyme B(12). 相似文献
143.
Shao-Chen Yang Qihuang Gong Zongju Xia Y. H. Zou Y. Q. Wu D. Qiang Y. L. Sun Z. N. Gu 《Applied physics. B, Lasers and optics》1992,55(1):51-53
The nonlinear third-order optical susceptibility of C70 in a toluene solution is measured for the first time by the method of degenerate four-wave mixing using 10 ns laser pulses at 1.06 m. The third-order susceptibility X
in
(3)
is 5.6×10–12 esu for a C70 toluene solution at a concentration of 0.476 g/l. The correspondent magnitude of the hyperpolarizability 1111 of the C70 molecule is 1.2×10–30 esu which is in a good agreement with the prediction given by the model of a free electron in a spherical box. 相似文献
144.
145.
146.
147.
Xujiao Ma Yajie Yang Rongchen Ma Yunfeng Zhang Xiaoqin Zou Shoujun Zhu Xin Ge Ye Yuan Wei Zhang Guangshan Zhu 《Chemical science》2020,11(44):12187
Despite their remarkable mechanical, optical, and electrical properties, inorganic particles and dynamic polymer assemblies encounter difficulties in their compatibility with regards to structural order and complexity. Here, covalent organic frameworks (COFs) constructed through reversible coupling reactions were exploited as dynamic porous polymers to prepare inorganic nanocrystal-polymer assemblies. Under an in situ growth process, carbon quantum dots (CDs) were gradually prepared in the COF cavity, with a narrow size distribution (2 ± 0.5 nm). The well-established assemblies achieve effective energy transfer from the inorganic to the organic part (efficiency > 80%), thus rendering a ∼130% increase in quantum yield compared with the pristine COF network. Notably, the hybrid material realizes a simple, selective, and sensitive diagnostic tool for urine copper, surpassing the detection limit of COF solid by 150 times. Beyond the scientific and fundamental interests, such hybrid assemblies are attractive from technological perspectives as well, for example, in energy storage, electronics, catalysis, and optics.Despite their remarkable mechanical, optical, and electrical properties, inorganic particles and dynamic polymer assemblies encounter difficulties in their compatibility with regards to structural order and complexity. 相似文献
148.
Furostanol saponins with inhibitory action against COX-2 production from Tupistra chinensis rhizomes
Kun Zou Jun Zhi Wang Jun Wu Yuan Zhou Chuang Liu Fei Jun Dan Ya Xiong Zhang Jin Yang 《中国化学快报》2007,18(10):1239-1242
Two furostanol saponins were obtained from the n-butanol fraction of methanol extract from Tupistra chinensis rhizomes,a folk medicine of Shennongjia Forest District of Hubei Province.Their structures were determined as (25S)-26-O-(β-D-glucopyranosyl)- furost-1β,3β,22α,26-tetrol-3-O-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→2)-β-D-glucopyranoside (1) and (25R)- 26-O-(β-D-glucopyranosyl)-furost-1β,3β22α,26-tetrol 3-O-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→2)-β-D-glu- copyranoside (2),on basis of chemical and spectroscopic evidences.1 and 2 displayed marked inhibitory action towards COX-2 production in macrophages of the rat abdomen induced by LPS at 20μg/mL. 相似文献
149.
Zhang B Li JF Zhong QL Ren B Tian ZQ Zou SZ 《Langmuir : the ACS journal of surfaces and colloids》2005,21(16):7449-7455
Core-shell Au-Pt nanoparticles were synthesized by using a seed growth method and characterized by transmission electron microscopy, X-ray diffraction, and UV-vis spectroscopy. Au(core)-Pt(shell)/GC electrodes were prepared by drop-coating the nanoparticles on clean glassy carbon (GC) surfaces, and their electrochemical behavior in 0.5 M H2SO4 revealed that coating of the Au core by the Pt shell is complete. The electrooxidation of carbon monoxide and methanol on the Au(core)-Pt(shell)/GC was also examined, and the results are similar to those obtained on a bulk Pt electrode. High quality surface-enhanced Raman scattering (SERS) spectra of both adsorbed CO and thiocyanate were observed on the Au(core)-Pt(shell)/GC electrodes. The potential-dependent SERS features resemble those obtained on electrochemically roughened bulk Pt or Pt thin films deposited on roughened Au electrodes. For thiocyanate, the C-N stretching frequency increases with the applied potential, yielding two distinctly different dnu(CN)/dE. From -0.8 to -0.2 V, the dnu(CN)/dE is ca. 50 cm(-1)/V, whereas it is 90 cm(-1)/V above 0 V. The bandwidth along with the band intensity increases sharply above 0 V. At the low-frequency region, Pt-NCS stretching mode at 350 cm(-1) was observed at the potentials from -0.8 to 0 V, whereas the Pt-SCN mode at 280 cm(-1) was largely absent until around 0 V and became dominant at more positive potentials. These potential-dependent spectral transitions were attributed to the adsorption orientation switch from N-bound dominant at the negative potential region to S-bound at more positive potentials. The origin of the SERS activity of the particles is briefly discussed. The study demonstrates a new method of obtaining high quality SERS on Pt-group transition metals, with the possibility of tuning SERS activity by varying the core size and the shell thickness. 相似文献
150.
Xu X Zhao Z Song P Zhou G Xu J Deng P Bourdet G Chanteloup JC Zou JP Fulop A 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(11-12):2444-2447
Infrared (1.2-1.6 microm) luminescence in a ytterbium aluminium garnet (YbAG) crystal, doped with Cr (0.05at.%) ions, was investigated under CW laser diode pumping (lambda=940 nm). The Cr4+ emission band was observed with its peak at 1.34 microm and measured to be about 1.3 times with respect to Yb3+ IR luminescence (lambda=1.03 microm). We demonstrate that for the excitation wavelength of 940 nm Yb3+ ions act as sensitizers of the 3B2(3T2)-3B1(3A2) emission of Cr4+ ions. This crystal is promising as a high-efficient system for tunable laser (1.2-1.6 microm) output. 相似文献