全文获取类型
收费全文 | 282361篇 |
免费 | 3168篇 |
国内免费 | 973篇 |
专业分类
化学 | 130756篇 |
晶体学 | 3689篇 |
力学 | 9906篇 |
综合类 | 2篇 |
数学 | 27133篇 |
物理学 | 75249篇 |
无线电 | 39767篇 |
出版年
2020年 | 2009篇 |
2019年 | 2253篇 |
2018年 | 2718篇 |
2017年 | 2613篇 |
2016年 | 4161篇 |
2015年 | 2757篇 |
2014年 | 4279篇 |
2013年 | 11892篇 |
2012年 | 8523篇 |
2011年 | 10507篇 |
2010年 | 7153篇 |
2009年 | 7434篇 |
2008年 | 10237篇 |
2007年 | 10665篇 |
2006年 | 9926篇 |
2005年 | 9427篇 |
2004年 | 8526篇 |
2003年 | 7664篇 |
2002年 | 7607篇 |
2001年 | 8641篇 |
2000年 | 6877篇 |
1999年 | 5575篇 |
1998年 | 4889篇 |
1997年 | 4939篇 |
1996年 | 4545篇 |
1995年 | 4337篇 |
1994年 | 4246篇 |
1993年 | 4287篇 |
1992年 | 4471篇 |
1991年 | 4528篇 |
1990年 | 4283篇 |
1989年 | 4106篇 |
1988年 | 4083篇 |
1987年 | 3540篇 |
1986年 | 3380篇 |
1985年 | 4463篇 |
1984年 | 4626篇 |
1983年 | 3873篇 |
1982年 | 4173篇 |
1981年 | 4015篇 |
1980年 | 3868篇 |
1979年 | 3874篇 |
1978年 | 4089篇 |
1977年 | 3897篇 |
1976年 | 4096篇 |
1975年 | 3658篇 |
1974年 | 3765篇 |
1973年 | 4105篇 |
1972年 | 2544篇 |
1971年 | 1998篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
91.
Markus Bödenler Ludovic de Rochefort P. James Ross Nicolas Chanet Geneviève Guillot Gareth R. Davies 《Molecular physics》2019,117(7-8):832-848
ABSTRACTFast field-cycling (FFC) nuclear magnetic resonance relaxometry is a well-established method to determine the relaxation rates as a function of magnetic field strength. This so-called nuclear magnetic relaxation dispersion gives insight into the underlying molecular dynamics of a wide range of complex systems and has gained interest especially in the characterisation of biological tissues and diseases. The combination of FFC techniques with magnetic resonance imaging (MRI) offers a high potential for new types of image contrast more specific to pathological molecular dynamics. This article reviews the progress in FFC-MRI over the last decade and gives an overview of the hardware systems currently in operation. We discuss limitations and error correction strategies specific to FFC-MRI such as field stability and homogeneity, signal-to-noise ratio, eddy currents and acquisition time. We also report potential applications with impact in biology and medicine. Finally, we discuss the challenges and future applications in transferring the underlying molecular dynamics into novel types of image contrast by exploiting the dispersive properties of biological tissue or MRI contrast agents. 相似文献
92.
Tereshchenko E. D. Cherniakov S. M. Yurik R. Yu. Rietveld M. T. Häggström I. 《Radiophysics and Quantum Electronics》2020,62(10):667-676
Radiophysics and Quantum Electronics - We present the results of comparing the total electron content measurements based on GLONASS satellite signals and the EISCAT UHF incoherent scatter radar... 相似文献
93.
Umie F.M. Haziz Rosenani A. Haque Shun-Ze Zhan Hassan H. Abdallah Mohd R. Razali 《应用有机金属化学》2020,34(10):e5818
The synthesis and characterizations for a series of dinuclear gold (I)-di-NHC complexes, 1–8 through the trans-metalation method of their respective silver (I)-di-NHC complexes, i–viii are reported (where NHC = N-heterocyclic carbene). The successful complexation of a series of unusual non-symmetrical and symmetrical di-NHC ligands, 3,3'-(ethane-1,2-diyl)-1-alkylbenzimidazolium-1'-butylbenzimidazolium (with alkyl = methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, benzyl) with the gold (I) ions are suggested by elemental analysis, Fourier transform-infrared, 1H- and 13C-NMR data. The 13C-NMR spectra of 1–8 show a singlet sharp peak in the range of 190.00–192.00 ppm, indicating the presence of a carbene carbon that bonded to the gold (I) ion. From single crystal X-ray diffraction data, the structure of complex 6 with the formula of [di-NHC-Au (I)]2·2PF6 is obtained [where NHC = 3,3'-(ethane-1,2-diyl)-1-hexylbenzimidazolium-1'-butylbenzimidazolium]. The photophysical study in solid state of 6 displays an intense photoluminescence with a strong emission maxima, λem = 480 nm, upon excitation at 340 nm at room temperature. Interestingly, the emission maximum at 77 K shows a structural character with a strong peak at 410 nm, a medium at 433 nm and a weak at 387 nm, accompanied by a tail band to about 500 nm. 相似文献
94.
95.
Radiophysics and Quantum Electronics - We consider whether it is possible in principle to retrieve the key parameters of the mixed layer in the upper ocean (its thickness, bulk eddy viscosity and... 相似文献
96.
97.
Vanessa R. Machado Louis P. Sandjo Giovanni L. Pinheiro Milene H. Moraes Mario Steindel Moacir G. Pizzolatti 《Natural product research》2018,32(3):275-281
The natural product lupeol 1 was isolated from aerial parts of Vernonia scorpioides with satisfactory yield, which made it viable to be used as starting material in semisynthetic approach. Ten lupeol derivatives 2–11 were prepared by classical procedures. Including, five new esters derivatives 7–11, which were obtained by structural modifications in the isopropylidene fragment. All semisynthetic compounds and lupeol 1–11 were confirmed by 1H NMR, 13C NMR and HRMS. Their antiprotozoal activity was evaluated in vitro against L. amazonensis and T. cruzi. Derivative 6 showed the best antitrypanosomal activity (IC50 = 12.48 μg/mL) and the lowest cytotoxic derivative (CC50 = 161.50 μg/mL). The mechanism of action of the most active derivatives (4, 6 and 11) is not dependent from the enzyme trypanothione reductase. 相似文献
98.
K.P. Pipe R.J. Ram 《Photonics Technology Letters, IEEE》2003,15(4):504-506
By measuring the total energy flow from an optical device, we can develop new design strategies for thermal stabilization. Here we present a comprehensive model for heat exchange between a semiconductor laser diode and its environment that includes the mechanisms of conduction, convection, and radiation. We perform quantitative measurements of these processes for several devices, deriving parameters such as a laser's heat transfer coefficient, and then demonstrate the feasibility of thermal probing for the nondestructive wafer-scale characterization of optical devices. 相似文献
99.
Surveys the field of super resolution (SR) processing for compressed video. The introduction of motion vectors, compression noise, and additional redundancies within the image sequence makes this problem fertile ground for novel processing methods. In conducting this survey, though, we develop and present all techniques within the Bayesian framework. This adds consistency to the presentation and facilitates comparison between the different methods. The article is organized as follows. We define the acquisition system utilized by the surveyed procedures. Then we formulate the HR problem within the Bayesian framework and survey models for the acquisition and compression systems. This requires consideration of both the motion vectors and transform coefficients within the compressed bit stream. We survey models for the original HR image intensities and displacement values. We discuss solutions for the SR problem and provide examples of several approaches. 相似文献
100.
High slew-rate CMOS operational amplifier 总被引:1,自引:0,他引:1
A 0.8 /spl mu/m CMOS operational amplifier configuration with a slew rate in excess of 2 V/ns and a unity gain bandwidth of 55 MHz with a load capacitance of 15 pF is proposed. This employs a dynamic technique that turns on a large current source when the rate of change of input is larger than a pre-decided value. 相似文献