全文获取类型
收费全文 | 341058篇 |
免费 | 12269篇 |
国内免费 | 6659篇 |
专业分类
化学 | 197849篇 |
晶体学 | 5408篇 |
力学 | 14856篇 |
综合类 | 337篇 |
数学 | 34903篇 |
物理学 | 106633篇 |
出版年
2022年 | 3228篇 |
2021年 | 3461篇 |
2020年 | 3831篇 |
2019年 | 3727篇 |
2018年 | 3582篇 |
2017年 | 3411篇 |
2016年 | 5879篇 |
2015年 | 5105篇 |
2014年 | 6538篇 |
2013年 | 15620篇 |
2012年 | 12579篇 |
2011年 | 14594篇 |
2010年 | 9683篇 |
2009年 | 9277篇 |
2008年 | 12311篇 |
2007年 | 12048篇 |
2006年 | 11517篇 |
2005年 | 10600篇 |
2004年 | 9255篇 |
2003年 | 8174篇 |
2002年 | 7931篇 |
2001年 | 9206篇 |
2000年 | 7190篇 |
1999年 | 6135篇 |
1998年 | 4988篇 |
1997年 | 4975篇 |
1996年 | 4872篇 |
1995年 | 4623篇 |
1994年 | 4204篇 |
1993年 | 3963篇 |
1992年 | 4572篇 |
1991年 | 4361篇 |
1990年 | 4169篇 |
1989年 | 4101篇 |
1988年 | 4035篇 |
1987年 | 4026篇 |
1986年 | 3765篇 |
1985年 | 5039篇 |
1984年 | 5099篇 |
1983年 | 4256篇 |
1982年 | 4639篇 |
1981年 | 4430篇 |
1980年 | 4380篇 |
1979年 | 4499篇 |
1978年 | 4814篇 |
1977年 | 4578篇 |
1976年 | 4573篇 |
1975年 | 4335篇 |
1974年 | 4195篇 |
1973年 | 4399篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
Sajeesh Kumar Madhurakkat Perikamana Sang Min Lee Jinkyu Lee Taufiq Ahmad Min Suk Lee Hee Seok Yang Heungsoo Shin 《Macromolecular bioscience》2019,19(4)
Plant derived flavonoids have not been well explored in tissue engineering applications due to difficulties in efficient formulations with biomaterials for controlled presentation. Here, the authors report that surface coating of epigallocatechin gallate (EGCG) on polymeric substrates including poly (L‐lactic acid) (PLLA) nanofibers can be performed via oxidative polymerization of EGCG in the presence of cations, enabling regulation of biological functions of multiple cell types implicated in bone regeneration. EGCG coating on the PLLA nanofiber promotes osteogenic differentiation of adipose‐derived stem cells (ADSCs) and is potent to suppress adipogenesis of ADSCs while significantly reduces osteoclastic maturation of murine macrophages. Moreover, EGCG coating serves as a protective layer for ADSCs against oxidative stress caused by hydrogen peroxide. Finally, the in vivo implantation of EGCG‐coated nanofibers into a mouse calvarial defect model significantly promotes the bone regeneration (61.52 ± 28.10%) as compared to defect (17.48 ± 11.07%). Collectively, the results suggest that EGCG coating is a simple bioinspired surface modification of polymeric biomaterials and importantly can thus serve as a promising interface for tuning activities of multiple cell types associated with bone fracture healing. 相似文献
123.
ABSTRACTQM(UB3LYP)/MM(AMBER) calculations were performed for the locations of the transition structure (TS) of the oxygen–oxygen (O–O) bond formation in the S4 state of the oxygen-evolving complex (OEC) of photosystem II (PSII). The natural orbital (NO) analysis of the broken-symmetry (BS) solutions was also performed to elucidate the nature of the chemical bonds at TS on the basis of several chemical indices defined by the occupation numbers of NO. The computational results revealed a concerted bond switching (CBS) mechanism for the oxygen–oxygen bond formation coupled with the one-electron transfer (OET) for water oxidation in OEC of PSII. The orbital interaction between the σ-HOMO of the Mn(IV)4–O(5) bond and the π*-LUMO of the Mn(V)1=O(6) bond plays an important role for the concerted O–O bond formation for water oxidation in the CaMn4O6 cluster of OEC of PSII. One electron transfer (OET) from the π-HOMO of the Mn(V)1=O(6) bond to the σ*-LUMO of the Mn(IV)4–O(5) bond occurs for the formation of electron transfer diradical, where the generated anion radical [Mn(IV)4–O(5)]-? part is relaxed to the ?Mn(III)4?…?O(5)- structure and the cation radical [O(6)=Mn(V)1]+ ? part is relaxed to the +O(6)–Mn(IV)1? structure because of the charge-spin separation for the electron-and hole-doped Mn–oxo bonds. Therefore, the local spins are responsible for the one-electron reductions of Mn(IV)4->Mn(III)4 and Mn(V)1->Mn(IV)1. On the other hand, the O(5)- and O(6)+ sites generated undergo the O–O bond formation in the CaMn4O6 cluster. The Ca(II) ion in the cubane- skeleton of the CaMn4O6 cluster assists the above orbital interactions by the lowering of the orbital energy levels of π*-LUMO of Mn(V)1=O(6) and σ*-LUMO of Mn(IV)4–O(5), indicating an important role of its Lewis acidity. Present CBS mechanism for the O–O bond formation coupled with one electron reductions of the high-valent Mn ions is different from the conventional radical coupling (RC) and acid-base (AB) mechanisms for water oxidation in artificial and native photosynthesis systems. The proton-coupled electron transfer (PC-OET) mechanism for the O–O bond formation is also touched in relation to the CBS-OET mechanism. 相似文献
124.
Jiatian Chen Le Yang Ying Li Qinghua Hou Lanlan Li Peng Jin 《International journal of quantum chemistry》2019,119(16):e25961
Inorganic planar ring-shape molecules with 4n + 2 π electrons are always the focus of experimental synthesis and theoretical research due to their potential aromaticity and stability. In this work, the whole series of five-membered heterocycle monoanions X nY5-n− (X, Y = group 15 elements; n = 1-4) were thoroughly investigated by means of density functional theory calculations. They all have large formation energies and HOMO-LUMO gap energies, suggesting the potential thermodynamic and kinetic stability. Their aromaticities are comparable to that of typical aromatic hydrocarbons. Their thermal stabilities were firmly established by the ab initio molecular dynamics simulations. As most of them are predicted for the first time, their various spectra were simulated for experimental characterization. Furthermore, we demonstrate that these five-membered cyclic anions can be employed as η5-ligand to construct novel all-inorganic metallocenes, which may serve as the building blocks of low-dimensional nanomaterials. 相似文献
125.
Nonlinear Dynamics - Global dynamics of a flexible asymmetrical rotor resting on vibrating supports is investigated. Hamilton’s principle is used to derive the partial differential governing... 相似文献
126.
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
127.
128.
Two new homosecoiridoids, named loniaceticiridoside (1) and lonimalondialiridoside (2), were isolated from an aqueous extract of the flower buds of Lonicera japonica. Their structures including the absolute configuration were determined by extensive spectroscopic studies, especially by 2D NMR and CD data analysis. A proposed biosynthetic pathway and preliminary investigations of the biological activity of compounds 1 and 2 are also discussed. 相似文献
129.
130.