全文获取类型
收费全文 | 1790篇 |
免费 | 78篇 |
国内免费 | 7篇 |
专业分类
化学 | 1408篇 |
晶体学 | 6篇 |
力学 | 12篇 |
数学 | 119篇 |
物理学 | 330篇 |
出版年
2023年 | 4篇 |
2022年 | 19篇 |
2021年 | 42篇 |
2020年 | 33篇 |
2019年 | 37篇 |
2018年 | 18篇 |
2017年 | 22篇 |
2016年 | 52篇 |
2015年 | 58篇 |
2014年 | 47篇 |
2013年 | 103篇 |
2012年 | 108篇 |
2011年 | 142篇 |
2010年 | 92篇 |
2009年 | 69篇 |
2008年 | 121篇 |
2007年 | 117篇 |
2006年 | 112篇 |
2005年 | 115篇 |
2004年 | 71篇 |
2003年 | 63篇 |
2002年 | 53篇 |
2001年 | 22篇 |
2000年 | 30篇 |
1999年 | 23篇 |
1998年 | 16篇 |
1997年 | 15篇 |
1996年 | 19篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 14篇 |
1992年 | 19篇 |
1991年 | 14篇 |
1990年 | 19篇 |
1989年 | 15篇 |
1988年 | 13篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 8篇 |
1976年 | 4篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 5篇 |
排序方式: 共有1875条查询结果,搜索用时 15 毫秒
981.
982.
983.
By exploiting a variational technique based upon projecting over the Poho?aev manifold, we prove existence of positive solutions for a class of nonlinear fractional Schrödinger equations having a nonhomogenous nonautonomous asymptotically linear nonlinearity. 相似文献
984.
Dr. J. Reyes Flores Dr. Griselda Castruita-De León Dr. Gleb Turlakov Dr. Eduardo Arias Dr. Ivana Moggio Dr. Sagrario M. Montemayor Dr. Román Torres Dr. Raquel Ledezma Dr. Ronald F. Ziolo Dr. Julio González-Torres 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(7):2493-2505
Two series of 2,5-di(butoxy)phenyleneethynylenes, one halogenated ( n PEC4-X ; n=2, 3, or 4) and the other boron-dipyrromethene (BODIPY) terminated ( n PEC4-By ; n=3, 4, or 5; By=BODIPY), were synthesized monodirectionally by the step-by-step approach and the molecular structure was corroborated by NMR spectroscopy (1H, 13C-DEPTQ-135, COSY, HSQC, HMBC, 11B, 19F) and MALDI-TOF mass spectrometry. The multiplicity and J-coupling constants of 1H, 11B, and 19F/11B NMR signals revealed, in the n PEC4-By series, that the phenyl in the meso position of BODIPY becomes electronically part of the conjugation of the phenyleneethynylene chain, whereas BODIPY is electronically isolated. The photophysical, electrochemical, and theoretical studies confirm this finding because the properties of n PEC4-By are comparable to those of the n PEC4-X oligomers and BODIPY, indicating negligible electron communication between BODIPY and the n PEC4 moieties. Nevertheless, energy transfer (ET) from n PEC4 to BODIPY was rationalized by spectroscopy and theoretical calculations. Its yield decreases with the n PEC4 conjugation length, according to the increase in distance between the two chromophores, resulting in dual emission for the longest oligomer in which ET is quenched. 相似文献
985.
Fernando Sanchez Lasheras Celestino Ordóñez Javier Roca-Pardiñas Francisco Javier de Cos Juez 《Mathematical Methods in the Applied Sciences》2020,43(14):8032-8041
Solar observation is the branch of astronomy devoted to the study of the Sun. When the light wavefront that comes from the Sun penetrates the atmosphere, it suffers some distortions caused by optically turbulent layers that change the wavefront's shape and morphology. Therefore, in order to obtain a good-quality image, it is necessary to correct the induced error. This is done by applying adaptive optics (AO) techniques. In the case of the present research, it is performed with the help of a Single Conjugate Adaptive Optics System (SCAO). The reconstruction technique proposed in this research is a SCAO based on convolutional neural networks (CNNs). This research develops and assesses a real-time tomographic reconstructor based on CNN, able to correct the error introduced by the atmosphere in the light wavefront received from the Sun. The CNN was trained and validated using data from the Durham AO Simulation Platform as input information. This platform incorporates certain solar functionalities that have been employed in the present research, allowing us to simulate a solar telescope. The normalized errors obtained for both ReLu and Leaky ReLu kernels were promising, without showing statistically significant differences among kernels in the value of RMSE volts of the deformable mirror commands. When different kernel dimensions are compared, statistically significant differences are found, showing that RMSE volts of the deformable mirror commands are lower for 3 × 3 kernels when compared with those of dimensions 5 × 5 and 7 × 7. As far as the authors know, this is the first time that an AO system based on CNN has been developed for solar telescopes. 相似文献
986.
Ani Ozcelik Dr. Daniel Aranda Sara Gil-Guerrero Xaquín A. Pola-Otero Dr. Maria Talavera Liangxuan Wang Dr. Santosh Kumar Behera Dr. Johannes Gierschner Prof. Ángeles Peña-Gallego Dr. Fabrizio Santoro Dr. Raquel Pereira-Cameselle Dr. J. Lorenzo Alonso-Gómez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(72):17342-17349
Several theoretical studies have proposed strategies to generate helical molecular orbitals (Hel-MOs) in [n]cumulenes and oligoynes. While chiral even-[n] cumulenes feature Hel-MOs, odd-[n] cumulenes may also present them if the terminal groups lie in different planes. However, the proposed systems have been either experimentally unfeasible or resulted in opposite pseudo-degenerated Hel-MOs. We hereby demonstrate the introduction of a remarkable energy difference between helical orbitals of opposite twist by fixing the torsion angle between the terminal groups in butadiyne fragments. To experimentally lock the conformation of the terminal groups, we designed and synthesized cyclic architectures by combining acetylenes with chiral spirobifluorenes. The high stability of these systems with distinct helical orbitals allowed their isolation and full characterization. In our view, these results constitute a step further in the development of real systems presenting helical molecular orbitals. 相似文献
987.
Dr. Enrique Sanchez Perez Dr. Ritu Toor Dr. Pierrick Bruyat Dr. Céline Cepeda Dr. Mélissa Degardin Dr. Jérôme Dejeu Dr. Didier Boturyn Dr. Liliane Coche-Guérente 《Chemphyschem》2021,22(21):2231-2239
Among non-covalent bonds, the host-guest interaction is an attractive way to attach biomolecules to solid surfaces since the binding strength can be tuned by the nature of host and guest partners or through the valency of the interaction. For that purpose, we synthesized cyclodecapeptide scaffolds exhibiting in a spatially controlled manner two independent domains enabling the multimeric presentation of guest molecules on one face and the other face enabling the potential grafting of a biomolecule of interest. In this work, we were interested in the β-cyclodextrin/ferrocene inclusion complex formed on β-CD monolayers functionalized surfaces. By using surface sensitive techniques such as quartz crystal microbalance and surface plasmon resonance, we quantified the influence of the guest valency on the stability of the inclusion complexes. The results show a drastic enhancement of the affinity with the gradual increase of guest valency. Considering that the sequential binding events are equal and independent, we applied the multivalent model developed by the Huskens group to extract intrinsic binding constants and an effective concentration of host. 相似文献
988.
Dr. Marianna Gambino Dr. Martin Veselý Dr. Matthias Filez Dr. Ramon Oord Dr. Dario Ferreira Sanchez Dr. Daniel Grolimund Dr. Nikolai Nesterenko Dr. Delphine Minoux Marianne Maquet Dr. Florian Meirer Prof. Dr. Bert M. Weckhuysen 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(10):3950-3955
Ni contamination from crude oil in the fluid catalytic cracking (FCC) process is one of the primary sources of catalyst deactivation, thereby promoting dehydrogenation–hydrogenation and speeding up coke growth. Herein, single-particle X-ray fluorescence, diffraction and absorption (μXRF-μXRD-μXAS) tomography is used in combination with confocal fluorescence microscopy (CFM) after thiophene staining to spatially resolve Ni interaction with catalyst components and study zeolite degradation, including the processes of dealumination and Brønsted acid sites distribution changes. The comparison between a Ni-lean particle, exposed to hydrotreated feedstock, and a Ni-rich one, exposed to non-hydrotreated feedstock, reveals a preferential interaction of Ni, found in co-localization with Fe, with the γ-Al2O3 matrix, leading to the formation of spinel-type hotspots. Although both particles show similar surface zeolite degradation, the Ni-rich particle displays higher dealumination and a clear Brønsted acidity drop. 相似文献
989.
Juan M. Sanchez 《Journal of separation science》2020,43(13):2708-2717
Ordinary least squares is widely applied as the standard regression method for analytical calibrations, and it is usually accepted that this regression method can be used for quantification starting at the limit of quantification. However, it requires calibration being homoscedastic and this is not common. Different calibrations have been evaluated to assess whether ordinary least squares is adequate to quantify estimates at low levels. All calibrations evaluated were linear and heteroscedastic. Despite acceptable values for precision at limit of quantification levels were obtained, ordinary least squares fitting resulted in significant and unacceptable bias at low levels. When weighted least squares regression was applied, bias at low levels was solved and accurate estimates were obtained. With heteroscedastic calibrations, limit values determined by conventional methods are only appropriate if weighted least squares are used. A “practical limit of quantification” can be determined with ordinary least squares in heteroscedastic calibrations, which should be fixed at a minimum of 20 times the value calculated with conventional methods. Biases obtained above this “practical limit” were acceptable applying ordinary least squares and no significant differences were obtained between the estimates measured using weighted and ordinary least squares when analyzing real‐world samples. 相似文献