全文获取类型
收费全文 | 42737篇 |
免费 | 4782篇 |
国内免费 | 3758篇 |
专业分类
化学 | 22413篇 |
晶体学 | 273篇 |
力学 | 1071篇 |
综合类 | 748篇 |
数学 | 8543篇 |
物理学 | 18229篇 |
出版年
2023年 | 366篇 |
2022年 | 557篇 |
2021年 | 1438篇 |
2020年 | 1050篇 |
2019年 | 1080篇 |
2018年 | 840篇 |
2017年 | 883篇 |
2016年 | 1164篇 |
2015年 | 1284篇 |
2014年 | 1631篇 |
2013年 | 2864篇 |
2012年 | 1933篇 |
2011年 | 2326篇 |
2010年 | 2154篇 |
2009年 | 2776篇 |
2008年 | 2963篇 |
2007年 | 3102篇 |
2006年 | 2475篇 |
2005年 | 1810篇 |
2004年 | 1529篇 |
2003年 | 1644篇 |
2002年 | 1518篇 |
2001年 | 1407篇 |
2000年 | 1123篇 |
1999年 | 920篇 |
1998年 | 841篇 |
1997年 | 638篇 |
1996年 | 681篇 |
1995年 | 600篇 |
1994年 | 608篇 |
1993年 | 603篇 |
1992年 | 593篇 |
1991年 | 408篇 |
1990年 | 322篇 |
1989年 | 301篇 |
1988年 | 292篇 |
1987年 | 255篇 |
1986年 | 254篇 |
1985年 | 383篇 |
1984年 | 285篇 |
1983年 | 160篇 |
1982年 | 333篇 |
1981年 | 507篇 |
1980年 | 454篇 |
1979年 | 492篇 |
1978年 | 386篇 |
1977年 | 298篇 |
1976年 | 263篇 |
1974年 | 86篇 |
1973年 | 168篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
11.
Motoko S. Asano Sho Hashimoto Takuya Shinozuka Yasutaka Fushimi 《Molecular physics》2019,117(19):2664-2672
ABSTRACTInteraction between a zinc porphyrin (ZnPor) as the end-group and poly(9,9-di-n-octylfluorene-2,7-vinylene) (PFV) as the main chain in a porphyrin end-modified fluorescent conjugated polymer, ZnPFV, was studied by time-resolved electron paramagnetic resonance (EPR) and fluorescence spectroscopy. While fluorescence from the PFV part of ZnPFV showed a spectral profile almost identical to that of a PFV oligomer without end-modification, the emission spectrum of the ZnPor part exhibited a much broader profile compared to that of the reference zinc porphyrin monomer. Based on the analysis of lifetimes and quantum yields, it was found that radiative rate constant of the ZnPor part was enhanced by nearly three times. The observed unusual enhancement in the radiative rate constant was rationalised in terms of a partial π-conjugation between the end group and the main chain, as a result of co-planarisation in fluid solution. On the other hand, the time-resolved EPR spectrum of ZnPFV at 100?K basically showed a similar spectral pattern to that of the reference zinc porphyrin, but with significant differences in zero-field spitting parameters and initial population ratios. The π-system of the excited triplet state is deduced to deviate from D4h symmetry in the end zinc porphyrin groups. The obtained results show that interaction of the porphyrin end group with the main chain of the polymer significantly influences the excited singlet state properties of the porphyrin, while its triplet state properties were affected to a lesser extent. 相似文献
12.
13.
A simple, sensitive and rapid ultra‐high‐performance liquid chromatography–electrospray ionization–tandem mass spectrometry method was developed and validated for the quantification of warfarin and 7‐hydroxy warfarin in Sprague Dawley (SD) rats. Animals were administered a single dose of warfarin sodium formulations (crystalline and amorphous) at 12 mg/kg via oral gavage and blood was drawn over a 96‐h time course. Sample process recoveries, matrix effect and analyte stability were determined. The linearity for warfarin and 7‐hydroxy warfarin was from 5 to 2000 ng/mL in blank SD rat plasma. Correlation coefficients (r2) for standard calibration curves were >.98 and analytes quantified within ±15% of target at all calibrator concentrations. The average percent accuracy and precision for intra‐ and inter‐day were 93.7%–113.8% and ≤12.1%, respectively, for warfarin and 7‐hydroxy warfarin, across the quality control standards (5, 10, 500, 1800 and 2000 ng/mL). Acceptable analytical recovery (>55%) was achieved with process efficiencies >41.5% and matrix effects <139.9% over the analytical range. Both analytes were stable in stock solution, autosampler, benchtop and three cycles of freeze–thaw with percent accuracy ≥90.2% and precision (percent relative standard deviation) ≤14%. The validated method was successfully applied to a pre‐clinical bioavailability study of crystalline and amorphous warfarin sodium formulations in SD rats. 相似文献
14.
Sachin B. Pandya Urmila H. Patel Kaushik P. Chaudhary Bhavesh N. Socha Nikita J. Patel Bhupesh S. Bhatt 《应用有机金属化学》2019,33(12)
Novel cobalt complex of 4‐amino‐N‐(6‐chloropyridazin‐3‐yl)benzene sulfonamide (sulfachloropyridazine) has been synthesized and characterized by elemental analysis, FT‐IR spectroscopy and magnetic susceptibility (VSM). Cobalt complex of Sulfachloropyridazine (Co‐SCP) crystallized in monoclinic space group P21/n with Z = 4. The structure is solved by direct method and refined to R = 0.099 for 4720 reflections with I ?4σ(I). The results of FT‐IR spectra suggest the binding of cobalt atom to the sulfonamide ligand which is in agreement with the crystal structure determination. In crystal structure, molecule is linked via, C‐H … π, C‐Cl … π and π … π intermolecular interactions. The computational studies like the optimization energy and root means square deviation compare with single crystal structure, frontier molecular orbital (Homo‐Lumo energy) and binding energy of the Co‐SCP has been carried out using DFT/B3LYP level of theory in gaseous phase. Hirshfeld surfaces and the 2D‐fingerprint analysis are performed to study the nature of interactions and their measurable contributions towards crystal packing. The interaction of the complex with DNA is investigated using viscosity measurement and absorption titration studies. The result shows the complex bind to DNA with intercalative mode with high DNA‐binding constant (Kb). Also, in vivo and in vitro cytotoxic studies are performed using S. pombe cells and brine shrimp lethality bioassay. DNA‐cleavage study shows better cleaving ability of the complex. 相似文献
15.
Yasuhito Tanaka 《Operations Research Letters》2019,47(5):406-409
We study a dynamic free-entry oligopoly with sluggish entry and exit of firms under general demand and cost functions. We show that the number of firms in a steady-state open-loop solution for a dynamic free-entry oligopoly is smaller than that at static equilibrium and that the number of firms in a steady-state memoryless closed-loop solution is larger than that in an open-loop solution. 相似文献
16.
This paper develops a framework to deal with the unconditional superclose analysis of
nonlinear parabolic equation. Taking the finite element pair $Q_{11}/Q_{01} × Q_{10}$ as an example,
a new mixed finite element method (FEM) is established and the $τ$ -independent superclose
results of the original variable $u$ in $H^1$-norm and the flux variable $\mathop{q} \limits ^{\rightarrow}= −a(u)∇u$ in $L^2$-norm are deduced ($τ$ is the temporal partition parameter). A key to our analysis is an
error splitting technique, with which the time-discrete and the spatial-discrete systems are
constructed, respectively. For the first system, the boundedness of the temporal errors is obtained. For the second system, the spatial superclose results are presented unconditionally, while the previous literature always only obtain the convergent estimates or require
certain time step conditions. Finally, some numerical results are provided to confirm the
theoretical analysis, and show the efficiency of the proposed method. 相似文献
17.
18.
19.
In this paper, we study the following Klein–Gordon–Maxwell system Using variational methods, we obtain the existence of ground state solutions under some appropriate assumptions on and . 相似文献
20.
The electrochemical behavior of austenitic stainless steel (Type 304) in 3 M sulfuric acid with 3.5% recrystallized sodium chloride at specific concentrations of butan-1-ol was investigated with the aid of potentiodynamic polarization, open circuit measurement and weight loss technique. Butan-1-ol effectively inhibited the steel corrosion with a maximum inhibition efficiency of 78.7% from weight-loss analysis and 80.9% from potentiodynamic polarization test at highest concentration studied. Adsorption of the compound obeyed the Freundlich isotherm. Thermodynamic calculations reveal physiochemical interactions and spontaneous adsorption mechanism. Surface characterizations showed the absence of corrosion products and topographic modifications of the steel. Statistical analysis depicts the overwhelming influence and statistical significance of inhibitor concentration on the inhibition performance. 相似文献