全文获取类型
收费全文 | 2267篇 |
免费 | 48篇 |
国内免费 | 4篇 |
专业分类
化学 | 1516篇 |
晶体学 | 21篇 |
力学 | 18篇 |
数学 | 109篇 |
物理学 | 655篇 |
出版年
2021年 | 15篇 |
2020年 | 20篇 |
2019年 | 23篇 |
2018年 | 24篇 |
2017年 | 15篇 |
2016年 | 32篇 |
2015年 | 30篇 |
2014年 | 25篇 |
2013年 | 88篇 |
2012年 | 84篇 |
2011年 | 97篇 |
2010年 | 61篇 |
2009年 | 61篇 |
2008年 | 138篇 |
2007年 | 137篇 |
2006年 | 124篇 |
2005年 | 135篇 |
2004年 | 125篇 |
2003年 | 105篇 |
2002年 | 107篇 |
2001年 | 63篇 |
2000年 | 51篇 |
1999年 | 27篇 |
1998年 | 24篇 |
1997年 | 27篇 |
1996年 | 38篇 |
1995年 | 21篇 |
1994年 | 29篇 |
1993年 | 21篇 |
1992年 | 27篇 |
1991年 | 25篇 |
1990年 | 25篇 |
1989年 | 22篇 |
1988年 | 21篇 |
1987年 | 11篇 |
1986年 | 21篇 |
1985年 | 36篇 |
1984年 | 50篇 |
1983年 | 24篇 |
1982年 | 29篇 |
1981年 | 32篇 |
1980年 | 26篇 |
1979年 | 37篇 |
1978年 | 25篇 |
1977年 | 27篇 |
1976年 | 22篇 |
1975年 | 16篇 |
1973年 | 15篇 |
1972年 | 11篇 |
1969年 | 11篇 |
排序方式: 共有2319条查询结果,搜索用时 15 毫秒
51.
Ishikawa A Takata T Kondo JN Hara M Kobayashi H Domen K 《Journal of the American Chemical Society》2002,124(45):13547-13553
A Ti-based oxysulfide, Sm(2)Ti(2)S(2)O(5), was studied as a visible light-driven photocatalyst. Under visible light (440 nm < or = lambda < or = 650 nm) irradiation, Sm(2)Ti(2)S(2)O(5) with a band gap of approximately 2 eV evolved H(2) or O(2) from aqueous solutions containing a sacrificial electron donor (Na(2)S-Na(2)SO(3) or methanol) or acceptor (Ag(+)) without any noticeable degradation. This oxysulfide is, therefore, a stable photocatalyst with strong reduction and oxidation abilities under visible-light irradiation. The electronic band structure of Sm(2)Ti(2)S(2)O(5) was calculated using the plane-wave-based density functional theory (DFT) program. It was elucidated that the S3p orbitals constitute the upper part of the valence band and these orbitals make an essential contribution to the small band gap energy. The conduction and valence bands' positions of Sm(2)Ti(2)S(2)O(5) were also determined by electrochemical measurements. It indicated that conduction and valence bands were found to have satisfactory potentials for the reduction of H(+) to H(2) and the oxidation of H(2)O to O(2) at pH = 8. This is consistent with the results of the photocatalytic reactions. 相似文献
52.
Patnaik A Okudaira KK Kera S Setoyama H Mase K Ueno N 《The Journal of chemical physics》2005,122(15):154703
Near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy was adopted to probe the unoccupied electronic states of C60 anchored onto an organized assembly of 11-amino-1-undecane thiol on Au(111). The polarization dependence of the intensity of pi* resonance associated with C60 pi network revealed the self-assembled monolayer (SAM) system to be oriented with an average molecular tilt angle of 57 degrees with respect to the surface normal. Invoking the absence of solid-state band dispersion effects and in comparison to solid C60 and /or 1-ML C60/Au(111), the electronic structure of the resulting assembly was found dominated by spectral position shift and linewidth and intensity changes of the lowest unoccupied molecular orbital (LUMO), LUMO+1, and LUMO+2 orbitals. The latter implied hybridization between N Pz of -NH2 group of thiolate SAM and pi levels of C60, resulting in a nucleophilic addition with a change in the symmetry of C60 from Ih to C1 in the SAM. Occurrence of a new feature at 285.3 eV in the NEXAFS spectrum, assigned previously to pi* graphitic LUMO, signified the formation of aggregated clusters, (C60)n of C60 monomer. Low tunneling current scanning tunneling microscopy confirmed them to be spherical and stable aggregates with n approximately 5. 相似文献
53.
54.
Ethyl pentafluoropropanethioate, prepared by treating ethanethiol with hexafluoro-1, 2-epoxypropane in the presence of triethylamine, was found to be a convenient pentafluoropropionylating agent for amines and alcohols. 相似文献
55.
56.
57.
A carrier transport model to explain the high-frequency response in high-speed MQW lasers is described. The ambipolar approximation, which is unsuitable for dealing with the high-speed carrier dynamics in MQW structures, was not adopted for small-signal analysis. The carrier transport effect can be characterized by four time constants: the electron transport time, bmn; the hole transport time, bmp; the electron escape time, wbn; and the hole escape time, wbp. The frequency response was interpreted as the sum of the constant response term due to the fast electron current and the roll-off term due to the slow hole transport time. The ratio of the electron contribution to the total response was proportional to the ratio of electron contribution to the total differential gain, , and reciprocally proportional to n0 = 1 + bmn/wbn. The value of was calculated to be about 0.5 for typical MQW lasers. The roll-off frequency is mainly determined by . The ratio p0 = 1 + bmp/wbp affects the resonant frequency and the damping rate in the high-bias condition. 相似文献
58.
59.
We investigate the Ward identities of theW
symmetry in the super-Liouville theory coupled to the super-conformal matter of central charge
. The theory is classified into two chiralities. For the positive chirality, all gravitationally dressed scaling operators are generated from theq–1 gravitational primaries by one of the ring generators in the R-sector acting on them repeatedly. After fixing the normalizations of the dressed scaling operators, we find that the Ward identities are expressed in the form of theusual
W
q
algebra constraints as in the bosonic case:W
n
(k+1)
,=0, (k=1,...,q–1;nZ1–k), where the equations for even and oddn come from the currents in the NS- and the R-sector respectively. The non-linear terms come from the anomalous contributions at the boundaries of moduli space. The negative chirality is defined by interchanging the roles ofp andq. Then we get theW
p
algebra constraints. 相似文献
60.