全文获取类型
收费全文 | 1938篇 |
免费 | 58篇 |
国内免费 | 26篇 |
专业分类
化学 | 1220篇 |
晶体学 | 31篇 |
力学 | 47篇 |
数学 | 397篇 |
物理学 | 327篇 |
出版年
2022年 | 18篇 |
2021年 | 27篇 |
2020年 | 27篇 |
2019年 | 26篇 |
2018年 | 18篇 |
2017年 | 19篇 |
2016年 | 40篇 |
2015年 | 37篇 |
2014年 | 35篇 |
2013年 | 101篇 |
2012年 | 108篇 |
2011年 | 94篇 |
2010年 | 61篇 |
2009年 | 58篇 |
2008年 | 98篇 |
2007年 | 114篇 |
2006年 | 109篇 |
2005年 | 102篇 |
2004年 | 89篇 |
2003年 | 68篇 |
2002年 | 92篇 |
2001年 | 23篇 |
2000年 | 44篇 |
1999年 | 26篇 |
1998年 | 30篇 |
1997年 | 22篇 |
1996年 | 27篇 |
1995年 | 19篇 |
1994年 | 33篇 |
1993年 | 31篇 |
1992年 | 38篇 |
1991年 | 24篇 |
1990年 | 22篇 |
1989年 | 17篇 |
1988年 | 15篇 |
1986年 | 16篇 |
1985年 | 14篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 15篇 |
1978年 | 16篇 |
1977年 | 11篇 |
1976年 | 14篇 |
1975年 | 15篇 |
1974年 | 14篇 |
1973年 | 15篇 |
1972年 | 13篇 |
1971年 | 14篇 |
排序方式: 共有2022条查询结果,搜索用时 187 毫秒
101.
正In the paper"A numerical investigation of the acoustic mode waves in a deviated borehole penetrating a transversely isotropic formation"by Liu et al.[1]numerical experiments are reported for a dipole sonic logging scenario with a deviated borehole penetrating a VTI anisotropic medium.In such a situation it is important to clearly define the meaning of group and phase velocities as this has led to much confusion in the literature as discussed by Miller,Horne and Walsh[2]. 相似文献
102.
Two terthiophenes incorporating the synthon 3,4-(1,2-phenylenedioxy)thiophene (PheDOT) have been developed. Specifically, 2,5-bisthienyl-3,4-(1,2-phenylenedioxy)thiophene (BTh-PheDOT 1) and 2,5-bisethylenedioxythienyl-3,4-(1,2-phenylenedioxy)thiophene (BEDOT-PheDOT 2) were electropolymerized to form electroactive polymer films (P1 and P2) that switched between two highly colored states with the more electron-rich EDOT derivative P2 observed to switch at a lower potential. Additionally, both P1 and P2 displayed moderate to low optical bandgaps of 1.8 and 1.6 eV, respectively. Crystal structures of BTh-PheDOT showed the monomer to be nearly planar with π -stacking observable between monomers. These findings demonstrate the potential of PheDOT as an electroactive synthon for the formation of well-ordered systems. 相似文献
103.
104.
The interaction of the moieties of benzene, cyclobutadiene, cyclopentadinyl anion, and the cyclopentadianide cation upon each other and upon a CC bond connecting pairs of these rings is investigated computationally. The resulting non-fused bicycles include biphenyl, phenylcyclobutadiene, phenylcyclopentadienylium, phenylcyclopentadienide, pentafulvalene, cyclobutadienyl–cyclopentadienylium, cyclobutadienyl–cyclopentadienide, and bicyclobutadiene. The relative stability and aromaticity are assessed from hydrogenation energies, aromatic stabilization energies, ring separation energies, nucleus-independent chemical-shift, harmonic oscillator model of aromaticity, and natural bond orbital analysis. Calculations are performed with density functional theory (B3LYP) and Møller–Plesset perturbation theory of second order (MP2). Enthalpy quantities are also determined by G3. When both rings are aromatic in character, the bridging bond is mostly σ in character. When one or both of the rings is antiaromatic, the bridging bond has significant π character. Systems with contrasting aromaticities have CC bridging bonds of lengths between CC single bond lengths and CC double bond lengths and where the systems were charged, the charge is evenly distributed between the rings. 相似文献
105.
A short and efficient synthesis of the potent 5-lipoxygenase inhibitor CMI-977 is described using as the key step a stereoselective anomeric oxygen to carbon rearrangement of an alkynyl stannane tetrahydrofuranyl ether derivative mediated by boron trifluoride etherate. 相似文献
106.
An investigation into the transmission spectrum of a ferroelectric liquid crystal device is undertaken. This is done both for an initial static state and during a switching process. Comparisons are made between experimental data and theoretical predictions. The dynamic internal director configurations in the device is shown to be consistent with a simple model during both monopolar and bipolar addressing pulses. 相似文献
107.
Dr. Freddi Philippart Marcus Arlt Steve Gotzen Dr. Stefanie‐Joana Tenne Dr. Marco Bocola Dr. Hsui‐Hui Chen Dr. Leilei Zhu Prof. Ulrich Schwaneberg Prof. Jun Okuda 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(41):13865-13871
A β‐barrel protein hybrid catalyst was prepared by covalently anchoring a Grubbs–Hoveyda type olefin metathesis catalyst at a single accessible cysteine amino acid in the barrel interior of a variant of β‐barrel transmembrane protein ferric hydroxamate uptake protein component A (FhuA). Activity of this hybrid catalyst type was demonstrated by ring‐opening metathesis polymerization of a 7‐oxanorbornene derivative in aqueous solution. 相似文献
108.
109.
The ability of B atoms on two different molecules to engage with one another in a noncovalent diboron bond is studied by ab initio calculations. Due to electron donation from its substituents, the trivalent B atom of BYZ2 (Z=CO, N2, and CNH; Y=H and F) has the ability to in turn donate charge to the B of a BX3 molecule (X=H, F, and CH3), thus forming a B⋅⋅⋅B diboron bond. These bonds are of two different strengths and character. BH(CO)2 and BH(CNH)2, and their fluorosubstituted analogues BF(CO)2 and BF(CNH)2, engage in a typical noncovalent bond with B(CH3)3 and BF3, with interaction energies in the 3–8 kcal/mol range. Certain other combinations result in a much stronger diboron bond, in the 26–44 kcal/mol range, and with a high degree of covalent character. Bonds of this type occur when BH3 is added to BH(CO)2, BH(CNH)2, BH(N2)2, and BF(CO)2, or in the complexes of BH(N2)2 with B(CH3)3 and BF3. The weaker noncovalent bonds are held together by roughly equal electrostatic and dispersion components, complemented by smaller polarization energy, while polarization is primarily responsible for the stronger ones. 相似文献
110.
Jeongyun Heo Thomas A. Sobiech Hilliard L. Kutscher Lee Chaves Dinesh K. Sukumaran Shanta Karki Admire Dube Paras N. Prasad Jessica L. Reynolds 《Macromolecular bioscience》2021,21(1)
A nanoformulation composed of curdlan, a linear polysaccharide of 1,3‐β‐linked d ‐glucose units, hydrogen bonded to poly(γ ‐glutamic acid) (PGA), was developed to stimulate macrophage. Curdlan/PGA nanoparticles (C‐NP) are formulated by physically blending curdlan (0.2 mg mL?1 in 0.4 m NaOH) with PGA (0.8 mg mL?1). Forster resonance energy transfer (FRET) analysis demonstrates a heterospecies interpolymer complex formed between curdlan and PGA. The 1H‐NMR spectra display significant peak broadening as well as downfield chemical shifts of the hydroxyl proton resonances of curdlan, indicating potential intermolecular hydrogen bonding interactions. In addition, the cross peaks in 1H‐1H 2D‐NOESY suggest intermolecular associations between the OH‐2/OH‐4 hydroxyl groups of curdlan and the carboxylic‐/amide‐groups of PGA via hydrogen bonding. Intracellular uptake of C‐NP occurs over time in human monocyte‐derived macrophage (MDM). Furthermore, C‐NP nanoparticles dose‐dependently increase gene expression for TNF‐α, IL‐6, and IL‐8 at 24 h in MDM. C‐NP nanoparticles also stimulate the release of IL‐lβ, MCP‐1, TNF‐α, IL‐8, IL‐12p70, IL‐17, IL‐18, and IL‐23 from MDM. Overall, this is the first demonstration of a simplistic nanoformulation formed by hydrogen bonding between curdlan and PGA that modulates cytokine gene expression and release of cytokines from MDM. 相似文献