全文获取类型
收费全文 | 155301篇 |
免费 | 2672篇 |
国内免费 | 460篇 |
专业分类
化学 | 85869篇 |
晶体学 | 1669篇 |
力学 | 5640篇 |
综合类 | 5篇 |
数学 | 17892篇 |
物理学 | 47358篇 |
出版年
2020年 | 1065篇 |
2016年 | 2155篇 |
2015年 | 1737篇 |
2014年 | 2177篇 |
2013年 | 6594篇 |
2012年 | 4743篇 |
2011年 | 6069篇 |
2010年 | 3609篇 |
2009年 | 3237篇 |
2008年 | 5523篇 |
2007年 | 5560篇 |
2006年 | 5409篇 |
2005年 | 5171篇 |
2004年 | 4420篇 |
2003年 | 3957篇 |
2002年 | 3819篇 |
2001年 | 4148篇 |
2000年 | 3229篇 |
1999年 | 2575篇 |
1998年 | 2221篇 |
1997年 | 2188篇 |
1996年 | 2243篇 |
1995年 | 2144篇 |
1994年 | 1953篇 |
1993年 | 2003篇 |
1992年 | 2079篇 |
1991年 | 1932篇 |
1990年 | 1902篇 |
1989年 | 1868篇 |
1988年 | 1884篇 |
1987年 | 1877篇 |
1986年 | 1759篇 |
1985年 | 2470篇 |
1984年 | 2579篇 |
1983年 | 2167篇 |
1982年 | 2484篇 |
1981年 | 2326篇 |
1980年 | 2338篇 |
1979年 | 2312篇 |
1978年 | 2480篇 |
1977年 | 2354篇 |
1976年 | 2359篇 |
1975年 | 2271篇 |
1974年 | 2103篇 |
1973年 | 2339篇 |
1972年 | 1367篇 |
1971年 | 1075篇 |
1969年 | 1037篇 |
1968年 | 1177篇 |
1967年 | 1221篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
A fast-frequency modulated (FM) diode laser has been optically narrowed using the technique of resonant optical feedback, to provide linewidths in each FM mode of ≈ 200 kHz peak-to-peak. With a drive frequency of 50 MHz and modulation index of 0.2, the FM laser has been used for the first time to obtain Doppler-free FM spectra of the Rb D1 line at 795 nm. The potential use of this system for laser frequency stabilisation is discussed. 相似文献
102.
W. Richard Strark 《Mathematical Logic Quarterly》1989,35(4):311-320
103.
104.
When solving large complex optimization problems, the user is faced with three major problems. These are (i) the cost in human time in obtaining accurate expressions for the derivatives involved; (ii) the need to store second derivative information; and (iii), of lessening importance, the time taken to solve the problem on the computer. For many problems, a significant part of the latter can be attributed to solving Newton-like equations. In the algorithm described, the equations are solved using a conjugate direction method that only needs the Hessian at the current point when it is multiplied by a trial vector. In this paper, we present a method that finds this product using automatic differentiation while only requiring vector storage. The method takes advantage of any sparsity in the Hessian matrix and computes exact derivatives. It avoids the complexity of symbolic differentiation, the inaccuracy of numerical differentiation, the labor of finding analytic derivatives, and the need for matrix store. When far from a minimum, an accurate solution to the Newton equations is not justified, so an approximate solution is obtained by using a version of Dembo and Steihaug's truncated Newton algorithm (Ref. 1).This paper was presented at the SIAM National Meeting, Boston, Massachusetts, 1986. 相似文献
105.
106.
Ludmila C. Fidale Sarah Köhler Martin H.G. Prechtl Thomas Heinze Omar A. El Seoud 《Cellulose (London, England)》2006,13(5):581-592
The concentrations of water, W, and electrolytes present in solutions of LiCl in N,N-dimethylacetamide, LiCl/DMAc, and of tetrabutylammonium fluoride. x-hydrate in DMSO, TBAF.xW/DMSO can be accurately and expediently determined by three independent methods, UV–vis, FTIR and EMF measurement. The first relies on the use of solvatochromic probes whose spectra are sensitive to solution composition. It is applicable to W/LiCl/DMAc solutions but not to TBAF.xW/DMSO, because the charge-transfer complex bands of the probes are suppressed by strong interactions with the latter electrolyte. Integration of νOH band of water may be employed in order to determine [W], hence [electrolyte] by weight difference. EMF measurement uses ion-selective electrodes in order to determine [electrolyte], hence [W] by weight difference. Results of the latter method were in excellent agreement with those of FTIR. The reason for the failure of Karl Fischer titration is addressed, and the relevance of the results obtained to functionalization of cellulose under homogenous solution conditions is briefly commented on. 相似文献
107.
The occurrence and formation of black spots areas in PolyLED devices has been studied by time-of-flight SIMS (TOFSIMS). The composition, shape and position of the black spots is visualised by three-dimensional (3D)-TOFSIMS depth-profiling. It has been established that the formation of non-emissive spots is due to the growth of aluminium oxide clusters at the AlBa/polymer interface. Electron injection in the black spots is lost by the resulting local increase of the resistivity of the cathode. 相似文献
108.
W. Kappus 《Zeitschrift für Physik B Condensed Matter》1981,45(2):113-121
A model concerning adatom structures is proposed. Attractive nearest neighbour interactions, which may be of electronic nature lead to 2-dimensional condensation. Every pair bond causes and elastic dipole. The elastic dipoles interact via substrate strains with an anisotropics
–3 power law. Different types of adatoms or sites are permitted and many-body effects result, from the assumptions. Electric dipole interactions of adatoms are included for comparison. The model is applied to the W(110) surface and compared with superstructures experimentally found in the W(110)-0 system. It is found that there is still lack for an additional next-nearest neighbour interaction. 相似文献
109.
Gábor Dörnyei Marietta Bárczai-Beke Gábor Blaskó Péter Péchy Csaba Szántay 《Tetrahedron letters》1982,23(28):2913-2916
Regioselective demethylation of 3,4-dihydropapaveraldine (a) at 7 and 3' positions affords a properly substituted diphenolic key intermidiate (d) for the synthesis of reticuline and N-norreticuline. 相似文献
110.
The timing relationship between a synchronously pumped mode-locked dye laser and its pump laser is explored experimentally and theoretically. The experimental data is in quantitative agreement with numerical solutions of a semiclassical model that includes no free parameters. The data also agrees qualitatively with an approximate, analytic distillation of this model which is presented here for the first time. Our study has a direct bearing on the jitter and the long term stability of these important ultrashort pulse lasers. 相似文献