全文获取类型
收费全文 | 4174篇 |
免费 | 210篇 |
国内免费 | 41篇 |
专业分类
化学 | 2741篇 |
晶体学 | 41篇 |
力学 | 132篇 |
数学 | 441篇 |
物理学 | 1070篇 |
出版年
2024年 | 15篇 |
2023年 | 38篇 |
2022年 | 162篇 |
2021年 | 221篇 |
2020年 | 170篇 |
2019年 | 166篇 |
2018年 | 168篇 |
2017年 | 135篇 |
2016年 | 203篇 |
2015年 | 144篇 |
2014年 | 211篇 |
2013年 | 374篇 |
2012年 | 304篇 |
2011年 | 302篇 |
2010年 | 239篇 |
2009年 | 192篇 |
2008年 | 210篇 |
2007年 | 179篇 |
2006年 | 137篇 |
2005年 | 74篇 |
2004年 | 99篇 |
2003年 | 55篇 |
2002年 | 56篇 |
2001年 | 32篇 |
2000年 | 28篇 |
1999年 | 31篇 |
1998年 | 20篇 |
1997年 | 29篇 |
1996年 | 26篇 |
1995年 | 12篇 |
1994年 | 26篇 |
1993年 | 20篇 |
1992年 | 19篇 |
1991年 | 11篇 |
1990年 | 19篇 |
1989年 | 13篇 |
1988年 | 16篇 |
1987年 | 16篇 |
1985年 | 23篇 |
1984年 | 23篇 |
1983年 | 15篇 |
1982年 | 20篇 |
1981年 | 13篇 |
1980年 | 38篇 |
1979年 | 11篇 |
1977年 | 15篇 |
1976年 | 11篇 |
1973年 | 11篇 |
1969年 | 9篇 |
1968年 | 8篇 |
排序方式: 共有4425条查询结果,搜索用时 15 毫秒
901.
In this paper, a fractional photon model is proposed by considering the photon-exchange interaction in the reverse process of the non-linear three-photon mixing interaction. The phase matching condition for various types of uniaxial crystals can be simplified and classified by this model with the consideration of the unequal fractional photon-exchange of the two generated waves in the reverse manner. This also leads to the explanation of observing the phase mismatch and output bandwidth of the resonant and non-resonant waves in a cavity. Finally, a quantitative approach to this model, with experimental results, is also demonstrated. 相似文献
902.
Ahmad S. Shawali Nagwa M. S. Harb Khadiga O. Badahdah 《Journal of heterocyclic chemistry》1985,22(5):1397-1403
The spectra (1H nmr, ir and uv) of a series of 3-arylazo-4-hydroxycoumarins 1a-m indicate that such compounds exist predominantly in the keto hydrazone form 1A both in solid state and in solution. The acid dissociation constants of the series studied were determined spectrophotometrically in 80 vol% ethanol-water mixture at 27° and ionic strength of 0.1. The results of the correlations of these constants by Hammett and Yukawa-Tsuno equations exclude the presence of the hydroxyazo form 1B in equilibrium with 1A in agreement with the spectral data. Also, the HMO method has been used to study tautomerism in such compounds. The results are also in full agreement with both the spectral and linear free energy correlations, the hydrazone form 1A being the most stable. It is further shown that both the intermolecular and intramolecular hydrogen bonding favor the hydrazone tautomer. 相似文献
903.
Al-Eideh Basel M.; Al-Refai Ahmad S. A.; Sbeiti Wafaa M. 《IMA Journal of Management Mathematics》2004,15(1):39-51
The Maximum Likelihood estimator is used within a lognormaldiffusion process and closed form analytical solutions are obtained.The monthly CPI forecasts are estimated for the period between1970 and 2002. The quarterly estimates of inflation rates areobtained from monthly forecasts rather than from quarterly data.This has significantly improved the estimates of inflation rates.The model also produced a superior fit as compared to randomwalk and GARCH(p,q)-M models. The adopted approach is foundto be simple, economical and generally suitable for modellingstochastic processes that reflect aggregation over time stemmingfrom many factors, and in which the transition path betweenconsecutive states is relatively smooth. 相似文献
904.
A simple multi-scroll hyperchaotic system 总被引:2,自引:0,他引:2
We propose a simple autonomous hyperchaotic system that can generate multi-scroll attractors. The proposed system has a canonical structure, one control parameter, and a switching-type nonlinearity. If multiple breakpoints are added to the system nonlinearity, multi-scroll behavior can be obtained. We numerically demonstrate hyperchaotic behavior of the proposed system, under different nonlinearities, as its control parameter is changed. Furthermore, we study hyperchaos in the proposed system when it assumes a fractional order, and demonstrate that hyperchaotic behavior can be obtained in systems less than fourth order. Throughout the study, hyperchaos is verified by examining the Lyapunov spectrum, where the presence of multiple positive Lyapunov exponents in the spectrum is indicative of hyperchaos. 相似文献
905.
Arif Rafiq Nazir Ahmad Mir Farooq Ahmad .Mathematics Department COMSATS Institute of Information Technology Islamabad Pakistan .Centre for Advanced Studies in Pure Applied Mathematics Bahauddin Zakariya University Multan Pakistan 《应用数学和力学(英文版)》2007,(7)
In account of the famous ebyev inequality,a rich theory has appeared in the literature.We establish some new weighted ebyev type integral inequalitíes.Our proofs are of independent interest and provide new estimates on these types of inequalities. 相似文献
906.
In this paper, we introduce a new class of generalized implicit vector variational-like inequalities in Hausdorff topological vector spaces and Banach spaces which contain implicit vector equilibrium problems, implicit vector variational inequalities and implicit vector complementarity problems as special cases. We derive some new results by using the KKM–Fan theorem, under compact and noncompact assumptions on underlying convex sets. 相似文献
907.
E.F. Moore M.P. Carpenter R.V.F. Janssens T.L. Khoo T. Lauritsen D. Ackermann I. Ahmad H. Amro D. Blumenthal S. Fischer G. Hackman D. Nisius F. Hannachi A. Lopez-Martens S. J. Asztalos R.M. Clark M.A. Deleplanque R.M. Diamond P. Fallon R. Krueken I-Y. Lee A.O. Macchiavelli F.S. Stephens J.A. Becker L. Bernstein L.P. Farris E.A. Henry A. Korichi 《Zeitschrift für Physik A Hadrons and Nuclei》1997,358(2):219-223
The lifetimes of states in Superdeformed (SD) bands in 192,194Hg have been measured using the Doppler shift attenuation method. Intrinsic quadrupole moments have been extracted for band 1 of 192Hg and for bands 1, 2, and 3 of 194Hg. It was found that the quadrupole moments for the “identical” SD bands in these nuclei are the same. In the same experiment, one-step transitions linking SD states to yrast and near yrast states for bands 1 and 3 of 194Hg have been identified. From this observation, excitation energies, spins and probable parities have been assigned to SD states in these bands. 相似文献
908.
Natural radioactivity has been measured in marble stone samples collected from Dabaa'h, Ajloun and Azrak areas using a gamma spectroscopy technique. The measured 226Ra activities in the stones collected from Dabaa'h quarries varied from 264.9 Bq kg−1 to 369.5 Bq kg−1. In samples from Ajloun and Azrak the maximum values of 226Ra were 28.3 Bq kg−1 and 23.41 Bq kg−1, respectively. The measured values of 232Th were less than 17.89 Bq kg−1. The average values of the radium equivalent activities as well as external and internal hazard indices were also calculated. It was observed that for marble stones from Dabaa'h area, estimated average value of the external hazard index is 0.873 and that of internal hazard index is 1.71. 相似文献
909.
K. Wendt S. A. Ahmad C. Ekström W. Klempt R. Neugart E. W. Otten 《Zeitschrift für Physik A Hadrons and Nuclei》1988,329(4):407-411
The hyperfme structure and isotope shift in the 6s 2 S 1/2?6p 2P3/2 line of Ba II (455.4 nm) have been measured by collinear fast-beam laser spectroscopy for the neutron-rich isotopes139–146Ba and148Ba. Nuclear moments and mean square charge radii of these isotopes have been recalculated. The isotope shift of the isotope148Ba (T1/2=0.64 s) could be studied for the first time, yieldingδ〈r2〉138,148=1.245(3) fm2. 相似文献
910.
Summary Complexes of pentachlorotantalum with the Schiff bases: bis(vanillin)benzidine, bis(vanillin)-o-dianisidine, bis(acetylacetone)benzidine, bis(p-dimethylaminobenzaldehyde)-o-dianisidine, bis(anisaldehyde)-1, 3-propanediamine and bis(p-dimethylaminobenzaldehyde)-o-phenylenediamine have been prepared and characterized by molar conductance, decomposition temperature, elemental and t.g. analyses and i.r. spectral measurements. The conductances reveal that pentachlorotantalum (1 mole) interacts with all the ligands (1 mole), all five chloride ions thus forming simple adducts. A comparative study of the i.r. spectra of the parent ligands and their complexes allows the coordination sites to be ascertained. The studies show that tantalum(V) chloride prefers to form complexes of high coordination number. 相似文献