全文获取类型
收费全文 | 107044篇 |
免费 | 7504篇 |
国内免费 | 9160篇 |
专业分类
化学 | 58378篇 |
晶体学 | 1751篇 |
力学 | 4294篇 |
综合类 | 799篇 |
数学 | 28892篇 |
物理学 | 29594篇 |
出版年
2023年 | 1093篇 |
2022年 | 1281篇 |
2021年 | 2310篇 |
2020年 | 2503篇 |
2019年 | 2652篇 |
2018年 | 2342篇 |
2017年 | 2353篇 |
2016年 | 2961篇 |
2015年 | 2824篇 |
2014年 | 4046篇 |
2013年 | 7493篇 |
2012年 | 4831篇 |
2011年 | 5956篇 |
2010年 | 5099篇 |
2009年 | 6622篇 |
2008年 | 7228篇 |
2007年 | 7467篇 |
2006年 | 6435篇 |
2005年 | 5171篇 |
2004年 | 4659篇 |
2003年 | 4553篇 |
2002年 | 4863篇 |
2001年 | 3488篇 |
2000年 | 3360篇 |
1999年 | 2926篇 |
1998年 | 2740篇 |
1997年 | 1807篇 |
1996年 | 1704篇 |
1995年 | 1691篇 |
1994年 | 1616篇 |
1993年 | 1264篇 |
1992年 | 1309篇 |
1991年 | 906篇 |
1990年 | 772篇 |
1989年 | 690篇 |
1988年 | 583篇 |
1987年 | 521篇 |
1986年 | 447篇 |
1985年 | 474篇 |
1984年 | 463篇 |
1983年 | 228篇 |
1982年 | 367篇 |
1981年 | 349篇 |
1980年 | 234篇 |
1979年 | 241篇 |
1978年 | 185篇 |
1977年 | 169篇 |
1976年 | 112篇 |
1974年 | 63篇 |
1973年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 645 毫秒
71.
Francesca Al Francesca Miraglia Alessandro Orticoni Elda Judica Maria Cotelli Paolo Maria Rossini Fabrizio Vecchio 《Entropy (Basel, Switzerland)》2020,22(11)
Human brain, a dynamic complex system, can be studied with different approaches, including linear and nonlinear ones. One of the nonlinear approaches widely used in electroencephalographic (EEG) analyses is the entropy, the measurement of disorder in a system. The present study investigates brain networks applying approximate entropy (ApEn) measure for assessing the hemispheric EEG differences; reproducibility and stability of ApEn data across separate recording sessions were evaluated. Twenty healthy adult volunteers were submitted to eyes-closed resting EEG recordings, for 80 recordings. Significant differences in the occipital region, with higher values of entropy in the left hemisphere than in the right one, show that the hemispheres become active with different intensities according to the performed function. Besides, the present methodology proved to be reproducible and stable, when carried out on relatively brief EEG epochs but also at a 1-week distance in a group of 36 subjects. Nonlinear approaches represent an interesting probe to study the dynamics of brain networks. ApEn technique might provide more insight into the pathophysiological processes underlying age-related brain disconnection as well as for monitoring the impact of pharmacological and rehabilitation treatments. 相似文献
72.
73.
Tatiana Monaretto Andre Souza Tiago Bueno Moraes Victor Bertucci-Neto Corinne Rondeau-Mouro Luiz Alberto Colnago 《Magnetic resonance in chemistry : MRC》2019,57(9):616-625
The traditional way to enhance signal-to-noise ratio (SNR) of nuclear magnetic resonance (NMR) signals is to increase the number of scans. However, this procedure increases the measuring time that can be prohibitive for some applications. Therefore, we have tested the use of several post-acquisition digital filters to enhance SNR up to one order of magnitude in time domain NMR (TD-NMR) relaxation measurements. The procedures were studied using continuous wave free precession (CWFP-T1) signals, acquired with very low flip angles that contain six times more noise than the Carr–Purcell–Meiboom–Gill (CPMG) signal of the same sample and experimental time. Linear (LI) and logarithmic (LO) data compression, low-pass infinity impulse response (LP), Savitzky–Golay (SG), and wavelet transform (WA) post-acquisition filters enhanced the SNR of the CWFP-T1 signals by at least six times. The best filters were LO, SG, and WA that have high enhancement in SNR without significant distortions in the ILT relaxation distribution data. Therefore, it was demonstrated that these post-acquisition digital filters could be a useful way to denoise CWFP-T1, as well as CPMG noisy signals, and consequently reducing the experimental time. It was also demonstrated that filtered CWFP-T1 method has the potential to be a rapid and nondestructive method to measure fat content in beef and certainly in other meat samples. 相似文献
74.
Gas-phase 21Ne nuclear magnetic resonance spectra were measured at the natural abundance of 21Ne isotope for samples consisting of pressurized neon up to 60 bar at room temperature and applying the magnetic field of the strength B0 = 11.7574 T. It showed that the nuclear magnetic resonance frequency is linearly dependent on the density of gaseous neon. The resonance frequency was extrapolated to the zero-density point, and it permitted the determination of the 21Ne nuclear magnetic moment, μ(21Ne) = 0.6617774(10) μN. The present value of μ(21Ne) is not influenced by the bulk magnetic susceptibility of neon and interactions between neon atoms; therefore, it is more precise and reliable than the previous result obtained for μ(21Ne). 相似文献
75.
Michiel Janssen Prof. Dirk E. De Vos 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(45):10724-10734
The practical application of Shilov-type Pt catalysis to the selective hydroxylation of terminal aliphatic C−H bonds remains a formidable challenge, due to difficulties in replacing PtIV with a more economically viable oxidant, particularly O2. We report the potential of employing FeCl2 as a suitable redox mediator to overcome the kinetic hurdles related to the direct use of O2 in the Pt reoxidation. For the selective conversion of butyric acid to γ-hydroxybutyric acid (GHB), a significantly enhanced catalyst activity and stability (turnover numbers (TON)>30) were achieved under 20 bar O2 in comparison to current state-of-the-art systems (TON<10). In this regard, essential reaction parameters affecting the overall activity were identified, along with specific additives to attain catalyst stability at longer reaction times. Notably, deactivation by reduction to Pt0 was prevented by the addition of monodentate pyridine derivatives, such as 2-fluoropyridine, but also by introducing varying partial pressures of N2 in the gaseous atmosphere. Finally, stability tests revealed the involvement of PtII and FeCl2 in catalyzing the non-selective overoxidation of GHB. Accordingly, in situ esterification with boric acid proved to be a suitable strategy to maintain enhanced selectivities at much higher conversions (TON>60). Altogether, a useful catalytic system for the selective hydroxylation of primary aliphatic C−H bonds with O2 is presented. 相似文献
76.
77.
78.
Ping Chen Yanhong Hao Xinxin Wang Prof. Dr. Dan Yuan Prof. Dr. Yingming Yao Prof. Dr. Lutz Ackermann 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(30):7292-7297
Highly position selective alkylations of N-alkylindoles at C7-positions have been enabled by cationic zirconium complexes. The strategy provides a straightforward access to install alkyl groups at C7-positions of indoles without a complex directing group. Mechanistic studies provided support for the importance of Brønsted acids in the catalytic manifold. 相似文献
79.
Activated carbon production from almond shells using phosphoric acid activation agent was achieved by applying both conventional heating and microwave heating in succession. The morphology and surface properties of activated carbon were studied using thermogravimetric and differential gravimetric analysis, Fourier-transform infrared spectroscopy, scanning electron microscopy, and Brunauer–Emmett–Teller analysis. A surface area of 1128 m2/g was achieved by optimizing the microwave power (500?W), microwave application time (15?minutes), conventional heating time (45?minutes), conventional heating temperature (500?°C), and the phosphoric acid:sample ratio (0.7:1). An adsorption capacity of methylene blue of 148?mg/g and an iodine value of 791?mg/g was obtained for the prepared activated carbon. 相似文献
80.
Pan Huang Dawen Xu Robert M. Reich Felix Kaiser Boping Liu Fritz E. Kühn 《Tetrahedron letters》2019,60(24):1574-1577
A first example of an Et2Zn mediated silylation of 1-aklynes is reported. A series of functional groups are tolerated in this reaction. Mechanistic studies support Zn alkynilides as intermediates in the reaction. This reaction protocol provides a practical method for the preparation of alkynylsilanes and expands the application of organometallic zinc in organic synthesis. 相似文献