全文获取类型
收费全文 | 2931篇 |
免费 | 6篇 |
国内免费 | 231篇 |
专业分类
化学 | 2737篇 |
力学 | 7篇 |
数学 | 13篇 |
物理学 | 411篇 |
出版年
2022年 | 2篇 |
2021年 | 1篇 |
2018年 | 1篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2015年 | 1篇 |
2014年 | 5篇 |
2013年 | 1篇 |
2012年 | 258篇 |
2011年 | 324篇 |
2010年 | 40篇 |
2009年 | 125篇 |
2008年 | 427篇 |
2007年 | 300篇 |
2006年 | 350篇 |
2005年 | 348篇 |
2004年 | 244篇 |
2003年 | 185篇 |
2002年 | 139篇 |
2001年 | 87篇 |
2000年 | 107篇 |
1999年 | 35篇 |
1998年 | 13篇 |
1997年 | 8篇 |
1996年 | 37篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 46篇 |
1992年 | 24篇 |
1991年 | 4篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1986年 | 3篇 |
1979年 | 1篇 |
排序方式: 共有3168条查询结果,搜索用时 31 毫秒
991.
Glycogen synthase kinase-3 (GSK-3) targets encompass proteins implicated in AD and neurological disorders. The functions of
GSK-3 and its implication in various human diseases have triggered an active search for potent and selective GSK-3 inhibitors.
In this sense, QSAR could play an important role in studying these GSK-3 inhibitors. For this reason, we developed QSAR models
for GSK−3α, linear discriminant analysis (LDA), and artificial neural networks (ANNs) from nearly 50,000 cases with more than 700 different
GSK−3α inhibitors obtained from ChEMBL database server; in total we used more than 20,000 different molecules to develop the QSAR
models. The model correctly classified 237 out of 275 active compounds (86.2%) and 14,870 out of 15,970 non-active compounds
(93.2%) in the training series. The overall training performance was 93.0%. Validation of the model was carried out using
an external predicting series. In these series, the model classified correctly 458 out of 549 (83.4%) compounds and 29,637
out of 31,927 non-active compounds (83.4%). The overall predictability performance was 92.7%. In this study, we propose three
types of non-linear ANN as alternative to already existing models, such as LDA. Linear neural network: LNN: 236:236-1-1:1
which had an overall training performance of 96% proved to be the best model. In addition, we did a study of the different
fragments of the molecules of the database to see which fragments had more influence in the activity. This can help design
new inhibitors of GSK−3α. This study reports the attempts to calculate, within a unified framework probabilities of GSK−3α inhibitors against different molecules found in the literature. 相似文献
992.
Classically, optical systems are considered to have a fundamental resolution limit due to diffraction. Many strategies for
improving both axial and lateral resolutions are based on a priori information about the input signal. These strategies lead
to a numerical aperture improvement. However these are still limited by the wave nature of light. By using fluorescence technique
one theoretically can reach unlimited resolution. The key point is to use the nonlinear dependence of the fluorescence emission
rate on the intensity of the applied illumination. In this paper we present simulation as well as experimental results which
show the advantage and the problems of using the nonlinear fluorescence effect in super resolution systems as well as discussing
the nonlinear phenomena concerning the fluorescence process. The results show that the nonlinear fluorescence effect is accompanied
by severe quenching, bleaching and saturation phenomena. As consequence, super resolution using saturated structured illumination
method in living biological samples becomes severely restricted. 相似文献
993.
Pelegrín-García D 《The Journal of the Acoustical Society of America》2011,129(3):1161-1164
Recently, a paper written by Brunskog Gade, Paya?-Ballester and Reig-Calbo, "Increase in voice level and speaker comfort in lecture rooms" [J. Acoust. Soc. Am. 125, 2072-2082 (2009)] related teachers' variation in vocal intensity during lecturing to the room acoustic conditions, introducing an objective parameter called "room gain" to describe these variations. In a failed attempt to replicate the objective measurements by Brunskog et al., a simplified and improved method for the calculation of room gain is proposed, in addition with an alternative magnitude called "voice support." The measured parameters are consistent with those of other studies and are used here to build two empirical models relating the voice power levels measured by Brunskog et al., to the room gain and the voice support. 相似文献
994.
Mumm HP Chupp TE Cooper RL Coulter KP Freedman SJ Fujikawa BK García A Jones GL Nico JS Thompson AK Trull CA Wilkerson JF Wietfeldt FE 《Physical review letters》2011,107(10):102301
We report the results of an improved determination of the triple correlation DP·(p(e)×p(v)) that can be used to limit possible time-reversal invariance in the beta decay of polarized neutrons and constrain extensions to the standard model. Our result is D=[-0.96±1.89(stat)±1.01(sys)]×10(-4). The corresponding phase between gA and gV is ?AV=180.013°±0.028° (68% confidence level). This result represents the most sensitive measurement of D in nuclear β decay. 相似文献
995.
Onset and dynamics of vortex-antivortex pairs in polariton optical parametric oscillator superfluids
Tosi G Marchetti FM Sanvitto D Antón C Szymańska MH Berceanu A Tejedor C Marrucci L Lemaître A Bloch J Viña L 《Physical review letters》2011,107(3):036401
We study, both theoretically and experimentally, the occurrence of topological defects in polariton superfluids in the optical parametric oscillator (OPO) regime. We explain in terms of local supercurrents the deterministic behavior of both the onset and dynamics of vortex-antivortex pairs generated by perturbing the system with a pulsed probe. Using a generalized Gross-Pitaevskii equation, including photonic disorder, pumping and decay, we elucidate the reason why topological defects form in couples and can be detected by direct visualizations in multishot OPO experiments. 相似文献
996.
An effective negative refractive index (NRI) is demonstrated and experimentally verified for the first two propagation bands of a fishnet-like metamaterial at millimeter-wave frequencies. The dual-band NRI behavior is achieved by engineering the diffraction order (±1, ±1) associated with the internal mode supported between holey layers to correspond with the second propagation band. In addition to the experimental interferometric technique that accounts for the handedness of the propagation, numerical results are given to predict the dual-band effective NRI and to confirm dual-band negative refraction for a prism composed of the proposed metamaterial. 相似文献
997.
Primo A Marino T Corma A Molinari R García H 《Journal of the American Chemical Society》2011,133(18):6930-6933
When irradiated with visible light (λ > 400 nm) 1 wt % gold-supported ceria nanoparticles generate oxygen from water (10.5 μmol·h(-1)) more efficiently than the standard WO(3) (1.7 μmol·h(-1)) even under UV irradiation (9.5 μmol·h(-1)). This remarkable photocatalytic activity arises from a novel preparation method to reduce the particle size of ceria (5 nm) by means of electrostatic binding of Ce(4+) to alginate gel, subsequent supercritical CO(2) drying, and calcination. The low loading of Au is crucial for the observed high catalytic activity. 相似文献
998.
本文报道了以光学活性乳酸为手性元对目标化合物的所有可能的四个异构体所进行的合成。根据合成样品与天然产物的核磁及旋光数据比较,标题化合物的相对及绝对构型都得以完全确立,其完整结构定为(2S,3S)-4,6-庚二炔-2,3-二醇。 相似文献
999.
a. Lab for Shock Wave Detonation Physics Research Southwest Institute of Fluid Physics P.O.Box - Chengdu Sichuan P.R.China b. Institute of Applied Physics Sichuan Union University Chengdu Sichuan P.R.China c. Current 《原子与分子物理学报》1996,(4)
SHOCKCOMPRESSIONBEHAVIOROFBROMOFORMPengShangqianga,b,cJingFuqiana,bHuJinbiaoa,bGouQingquanba.LabforShockWaveandDetonationPhy... 相似文献