全文获取类型
收费全文 | 55598篇 |
免费 | 1096篇 |
国内免费 | 427篇 |
专业分类
化学 | 28532篇 |
晶体学 | 511篇 |
力学 | 2290篇 |
综合类 | 13篇 |
数学 | 11273篇 |
物理学 | 14502篇 |
出版年
2023年 | 148篇 |
2022年 | 298篇 |
2021年 | 513篇 |
2020年 | 467篇 |
2019年 | 477篇 |
2018年 | 1517篇 |
2017年 | 1758篇 |
2016年 | 1426篇 |
2015年 | 1187篇 |
2014年 | 1255篇 |
2013年 | 2141篇 |
2012年 | 5046篇 |
2011年 | 4444篇 |
2010年 | 3084篇 |
2009年 | 2667篇 |
2008年 | 2300篇 |
2007年 | 2235篇 |
2006年 | 2119篇 |
2005年 | 5845篇 |
2004年 | 5150篇 |
2003年 | 3274篇 |
2002年 | 1461篇 |
2001年 | 957篇 |
2000年 | 661篇 |
1999年 | 512篇 |
1998年 | 359篇 |
1997年 | 364篇 |
1996年 | 381篇 |
1995年 | 297篇 |
1994年 | 321篇 |
1993年 | 298篇 |
1992年 | 434篇 |
1991年 | 356篇 |
1990年 | 286篇 |
1989年 | 245篇 |
1988年 | 235篇 |
1987年 | 177篇 |
1986年 | 136篇 |
1985年 | 206篇 |
1984年 | 140篇 |
1983年 | 123篇 |
1982年 | 140篇 |
1981年 | 106篇 |
1979年 | 92篇 |
1978年 | 104篇 |
1977年 | 103篇 |
1976年 | 173篇 |
1975年 | 138篇 |
1974年 | 119篇 |
1973年 | 150篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
31.
We consider the one-dimensional steady-state semiconductor deviceequations modelling a pnpn device. There are two relevant scalingsof the equations corresponding to small and large applied voltages.In both scalings, the semiconductor equations can be consideredas singularly perturbed. It turns out that the small-voltagescaling breaks down for current values between two saturationcurrents. In that interval, the large-voltage scaling has tobe employed. For both scalings, we derive the first-order termsof an asymptotic expansion and show that the reduced problemhas a solution. An example verifies that the current-voltagecurves obtained have the expected qualitative structure. 相似文献
32.
Vernon Barger Cheng-Wei Chiang Paul Langacker Hye-Sung Lee 《Physics letters. [Part B]》2004,580(3-4):186-196
We study the effects of an extra U(1)′ gauge boson with flavor changing couplings with fermion mass eigenstates on certain B meson decays that are sensitive to such new physics contributions. In particular, we examine to what extent the current data on Bd→φK and Bd→η′K decays may be explained in such models, concentrating on the example in which the flavor changing couplings are left-chiral. We find that within reasonable ranges of parameters, the Z′ contribution can readily account for the anomaly in SφKS but is not sufficient to explain large branching ratio of Bd→η′K with the same parameter value. SφKS and Sη′KS are seen to be the dominant observables that constrain the extra weak phase in the model. 相似文献
33.
Sungyong Kim Lingxin Chen Sangyeop Lee Gi Hun Seong Jaebum Choo Eun Kyu Lee Chil-Hwan Oh Sanghoon Lee 《Analytical sciences》2007,23(4):401-405
A rapid DNA analysis has been developed based on a fluorescence intensity change of a molecular beacon in a PDMS microfluidic channel. Recently, we reported a new analytical method of DNA hybridization involving a PDMS microfluidic sensor using fluorescence energy transfer (FRET). However, there are some limitations in its application to real DNA samples because the target DNA must be labelled with a suitable fluorescent dye. To resolve this problem, we have developed a new DNA microfluidic sensor using a molecular beacon. By monitoring the change in the restored fluorescence intensity along the channel length, it is possible to rapidly detect any hybridization of the molecular beacon to the target DNA. In this case, the target DNA does not need to be labelled. Our experimental results demonstrate that this microfluidic sensor using a molecular beacon is a promising diagnostic tool for rapid DNA hybridization analysis. 相似文献
34.
Could simple intraannular-arm macrocyclic systems exist in enantiopure stable forms? The effective synthesis of two representative compounds of such a class, their resolution into enantiomers, and experiments justifying their stability toward racemization are presented. 相似文献
35.
N. Wright und W. C. Lee 《Fresenius' Journal of Analytical Chemistry》1940,120(1-2):39-40
Ohne Zusammenfassung 相似文献
36.
The transverse momentum spectra for pions observed by WA80 and NA35 collaborations are analysed within a fireball model with collective isentropic expansion and a realistic freeze-out criterion. By varing the initial state of the fireball, an excellent fit to the data is achieved for the whole measured range ofP T . Slight differences in the data for the spectral slopes from central and pheripheral collisions originate in our model from the difference in the size of the fireball and in the number of participating nucleons in central and peripheral collisions. Using additional information from two-pion correlations, we can extrapolate our model back from the freeze-out point (determined from the spectra) to the initial state; we find that an initial energy density of 1.5–2GGeV/fm3 is sufficient to explain the data from central O+Au collisions at 200A GeV. 相似文献
37.
This paper proposes and analyzes a GaAs traveling-wave optical modulator which uses a modulated coplanar strip electrode with periodic cross-tie overlay. This slow-wave structure can be designed to satisfy phase velocity and impedance matching conditions simultaneously. The dominant conductor loss in the slow-wave structure is reduced using the modulated coplanar strip electrode. The calculated 3-dB modulation bandwidth (100 GHz) is much wider than the bandwidth limit (30 GHz) of conventional electrode structures that are limited by phase velocity mismatch.This work was supported by the U.S. Army Research Office under contract DAAL-03-88-K-0005 and the Texas Advanced Technology Program. 相似文献
38.
Micha? Chmielewski 《Tetrahedron letters》2004,45(31):6007-6010
Comparison of the anion binding properties of a series of uncharged macrocyclic tetraamides reveal significant effects of the receptor's size on the strength of its anion complexes. This study allowed for estimation of the optimal size of a macroring for complexation of common anions. 相似文献
39.
Stuart P. RomerilVictor Lee Jack E. Baldwin Timothy D.W. ClaridgeBarbara Odell 《Tetrahedron letters》2003,44(42):7757-7761
The absolute configuration of the marine sponge alkaloid pyrinodemin A is established by organic synthesis. 相似文献
40.
Aiqing Zhang Xiangdan Li Changwoon Nah Keejun Hwang Myong‐Hoon Lee 《Journal of polymer science. Part A, Polymer chemistry》2003,41(1):22-29
A soluble aromatic polyimide was chloromethylated via a reaction with chloromethyl methyl ether in the presence of tin(IV) chloride to produce a new starting material for the modification of aromatic polyimides. The chemical structure of the resulting polymer was confirmed by 1H NMR and Fourier transform infrared spectroscopy. The maximum number of chloromethyl groups per repeat unit was 1.81. The chloromethylated polyimide was stable up to 250 °C and soluble in both chloroform and tetrahydrofuran. So that its utilization for further modification could be demonstrated, cinnamic acid was reacted with the formed polyimide, and it produced a new photosensitive polyimide with a cinnamoyl side chain. The photosensitivity of the resulting polyimide was investigated with ultraviolet spectroscopic methods. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 22–29, 2003 相似文献