全文获取类型
收费全文 | 33067篇 |
免费 | 835篇 |
国内免费 | 744篇 |
专业分类
化学 | 6758篇 |
晶体学 | 958篇 |
力学 | 490篇 |
综合类 | 48篇 |
数学 | 21383篇 |
物理学 | 5009篇 |
出版年
2023年 | 75篇 |
2022年 | 205篇 |
2021年 | 193篇 |
2020年 | 229篇 |
2019年 | 635篇 |
2018年 | 680篇 |
2017年 | 445篇 |
2016年 | 483篇 |
2015年 | 490篇 |
2014年 | 910篇 |
2013年 | 2392篇 |
2012年 | 901篇 |
2011年 | 2304篇 |
2010年 | 1997篇 |
2009年 | 2119篇 |
2008年 | 2357篇 |
2007年 | 2324篇 |
2006年 | 1729篇 |
2005年 | 1401篇 |
2004年 | 1249篇 |
2003年 | 1015篇 |
2002年 | 955篇 |
2001年 | 640篇 |
2000年 | 646篇 |
1999年 | 679篇 |
1998年 | 658篇 |
1997年 | 536篇 |
1996年 | 629篇 |
1995年 | 636篇 |
1994年 | 590篇 |
1993年 | 478篇 |
1992年 | 497篇 |
1991年 | 347篇 |
1990年 | 312篇 |
1989年 | 330篇 |
1988年 | 217篇 |
1987年 | 231篇 |
1986年 | 187篇 |
1985年 | 276篇 |
1984年 | 244篇 |
1983年 | 132篇 |
1982年 | 208篇 |
1981年 | 233篇 |
1980年 | 164篇 |
1979年 | 113篇 |
1978年 | 165篇 |
1977年 | 128篇 |
1976年 | 121篇 |
1975年 | 53篇 |
1974年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
41.
42.
A. P. Vink E. van der Kolk P. Dorenbos C. W. E. van Eijk 《Optics Communications》2002,210(3-6):277-284
The optical properties of Ce3+ in CaSO4, SrSO4 and BaSO4 are reported. The Ce3+ ion shows 4f05d1→2F5/2,2F7/2 luminescence in all three sulphates. Co-doping with Na+ does not change the local surrounding of the Ce3+ ion, but enhances the amount of Ce3+ ions built in. Under optical excitation, besides the typical Ce3+ doublet emission in the ultraviolet spectral region, band emission around 445 nm was observed. This band emission was not assigned to emission from a Ce3+ centre, but to emission from an impurity-trapped exciton. Under X-ray excitation, both Ce3+ emission and an emission band around 380 nm was observed. This band was assigned to emission from a self-trapped exciton. 相似文献
43.
Summary In this paper a Gauss-Jordan algorithm with column interchanges is presented and analysed. We show that, in contrast with Gaussian elimination, the Gauss-Jordan algorithm has essentially differing properties when using column interchanges instead of row interchanges for improving the numerical stability. For solutions obtained by Gauss-Jordan with column interchanges, a more satisfactory bound for the residual norm can be given. The analysis gives theoretical evidence that the algorithm yields numerical solutions as good as those obtained by Gaussian elimination and that, in most practical situations, the residuals are equally small. This is confirmed by numerical experiments. Moreover, timing experiments on a Cyber 205 vector computer show that the algorithm presented has good vectorisation properties. 相似文献
44.
Wen Zhang Hui Huang Yu Ding Margherita Gavagnin Ernesto Mollo Guido Cimino Yue‐Wei Guo 《Helvetica chimica acta》2006,89(4):813-820
Three new polyoxygenated steroids, muricesteroid ( 1 ), and menellsteroids A ( 2 ) and B ( 3 ), were isolated from two species of the South China Sea gorgonian Muricella flexuosa and Menella verrucosa Brundin , respectively. The structures of these new compounds were elucidated on the basis of extensive spectroscopic analysis, chemical methods and comparison with known related compounds. 相似文献
45.
Homo-apioneplanocin A was efficiently synthesized via stereoselective hydroxymethylation, regio- and chemoselective hydroboration, and chemoselective oxidation as key steps from d-ribose. 相似文献
46.
Using Brown's construction (J. Algebra 15 (1970) 103) of an exact 6-term sequence for a fibration of groupoids we show how an exact 9-term sequence can be associated to a fibration of bigroupoids. Applications to topology and algebra are given. 相似文献
47.
The dielectric responses (i.e. the refractive indices and the second order nonlinear susceptibilities) of all orthorhombic rare earth molybdates have been studied on the basis of the relationship between dielectric responses and the average atomic number of constituent atoms of crystals. Both the linear and second order nonlinear optical responses at 1.064 μm decrease with increasing atomic number from La to Lu. 相似文献
48.
Bond covalency and valence of elements in HgBa2Can−1CunO2n+2+δ (n=1, 2, 3, 4) were calculated and their relationship with Tc was discussed. For both oxygen and argon annealed samples, the results indicated that with the increase of n, the trend of bond covalency of Hg-O and Cu-O was the same or opposite compared with that of superconducting temperature. This may suggest that the magnitudes of Cu-O and Hg-O bond covalency are important in governing the superconducting temperature. For the highest Tc sample, Hg had the lowest valence, implying that lower valence of Hg was preferred in order to produce higher Tc. For fixed n, the valence of Cu in oxygen annealed samples was larger than that in argon annealed samples, indicating that oxygen annealed samples produced more carriers than argon annealed samples. 相似文献
49.
Electrical impedance measurements of Na3H(SO4)2 were performed as a function of both temperature and frequency. The electrical conductivity and dielectric relaxation have been evaluated. The temperature dependence of electrical conductivity reveals that the sample crystals transformed to the fast ionic state in the high temperature phase. The dynamical disordering of hydrogen and sodium atoms and the orientation of SO4 tetrahedra results in fast ionic conductivity. In addition to the proton conduction, the possibility of a Na+ contribution to the conductivity in the high temperature phase is proposed. The frequency dependence of AC conductivity is proportional to ωs. The value of the exponent, s, lies between 0.85 and 0.46 in the room temperature phase, whereas it remains almost constant, 0.6, in the high-temperature phase. The dielectric dispersion is examined using the modulus formalism. An Arrhenius-type behavior is observed when the crystal undergoes the structural phase transition. 相似文献
50.
R.S. Gonnelli A. CalzolariD. Daghero G.A. UmmarinoV.A. Stepanov P. FinoG. Giunchi S. CeresaraG. Ripamonti 《Journal of Physics and Chemistry of Solids》2002,63(12):2319-2323
We studied the voltage and temperature dependency of the dynamic conductance of normal metal-MgB2 junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of MgB2 samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2Δ/kBTc=3.3. In out-of-plane, high-pressure point-contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Δ=2.6±0.2 meV (clean limit). 相似文献