全文获取类型
收费全文 | 62122篇 |
免费 | 14591篇 |
国内免费 | 3704篇 |
专业分类
化学 | 51547篇 |
晶体学 | 495篇 |
力学 | 1689篇 |
综合类 | 136篇 |
数学 | 4043篇 |
物理学 | 8370篇 |
无线电 | 14137篇 |
出版年
2024年 | 93篇 |
2023年 | 572篇 |
2022年 | 705篇 |
2021年 | 984篇 |
2020年 | 2474篇 |
2019年 | 3817篇 |
2018年 | 2012篇 |
2017年 | 1667篇 |
2016年 | 4685篇 |
2015年 | 4867篇 |
2014年 | 5091篇 |
2013年 | 5875篇 |
2012年 | 5088篇 |
2011年 | 4295篇 |
2010年 | 4362篇 |
2009年 | 4352篇 |
2008年 | 3979篇 |
2007年 | 3269篇 |
2006年 | 2903篇 |
2005年 | 2847篇 |
2004年 | 2339篇 |
2003年 | 2111篇 |
2002年 | 2817篇 |
2001年 | 2049篇 |
2000年 | 1859篇 |
1999年 | 902篇 |
1998年 | 502篇 |
1997年 | 471篇 |
1996年 | 439篇 |
1995年 | 369篇 |
1994年 | 307篇 |
1993年 | 237篇 |
1992年 | 262篇 |
1991年 | 234篇 |
1990年 | 191篇 |
1989年 | 167篇 |
1988年 | 126篇 |
1987年 | 102篇 |
1986年 | 108篇 |
1985年 | 109篇 |
1984年 | 96篇 |
1983年 | 73篇 |
1982年 | 61篇 |
1981年 | 54篇 |
1980年 | 43篇 |
1978年 | 43篇 |
1977年 | 46篇 |
1976年 | 47篇 |
1974年 | 45篇 |
1973年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
ThestudyofphotoionizationandphotodissociationprocessesinducedbyintensefemtosecondlaserpuIses(>lo"W/cm')withpolyatomicmoleculesbecomesofinterestbecausesomenewphenomenahavebeenobserved'-3.Sofar,mostoftheinvestigationsfocusontheexperimentalexplorationofphotoionizationprocesses'-'.Butthephotoionizationmechanismofpolyatomicmoleculesinanintensefslaserfieldisstillambiguous'.,.Incontrasttothephotoionizationprocesses,theunderstandingoffragmentationofmolecularionisevenpoorer.Corkumetal,'.,'reportedthef… 相似文献
962.
We propose a numerical procedure for the calculation of the electrostatic repulsion force between two identical, parallel surfaces immersed in anab electrolyte solution. These surfaces are coated with an ion-penetrable membrane carrying fixed charges. The amount of fixed charges is governed by the dissociation of the functional groups in the membrane phase. The effect of pH on the degree of dissociation of these functional groups is taken into account. The difficulty of extensive use of Jacobi elliptic function in the numerical treatment of Poisson-Boltzmann equation can be circumvented by resorting to the present algorithm. 相似文献
963.
The determination of lanthanides by Inductively Coupled Plasma Mass Spectrometry (ICP‐MS) is complicated by several spectral overlaps from M+, MO+ or MOH+ ions formed in the ICP. Especially, it is essential to avoid the spectral interferences from lighter lanthanide and Ba polyatomic ions on middle or heavier lanthanides. To tackle this problem, we have developed a mathematical correction method, which reduces all the spectral overlaps from oxide species of Pr, Nd, Ce and Sm over Gd, Tb, Dy and Ho, and Gd, Tb over Yb and Lu. It can also successfully correct the oxide and hydroxide interference of Ba over Eu. The effectiveness of the proposed the mathematical correction scheme is demonstrated for the USGS Standard Rock samples AGV‐1 and G‐2. The results show that the experimental data obtained by applying the mathematical correction scheme for lanthanides is in good agreement with the reported values, using pneumatic and ultrasonic nebulisation methods, for their ICP‐MS analysis. 相似文献
964.
H. Poschenrieder H.‐D. Stachel B. Wiesend K. Polborn 《Journal of heterocyclic chemistry》2003,40(1):61-69
Ozonolysis of the pyrrolidinediones 4 afforded the pyrrolidinetriones 5 , which in the presence of Lewis acids were converted into maleimide 6 . Analogously, ozonolysis of the pyrrolidinones 7 gave the pyrrolidinediones 8 , which were converted into the pyridinetriones 11a, b via Lewis acid catalyzed isomerization to yield the trihydroxypyridones 10 and ensuing air oxidation. In solution two tautomeric forms of the pyridinetriones 11 may exist both of which represent hydroxy‐azabenzoquinones. In two steps compounds 11 were transformed into the azaquinone derivatives 19 . Representatives of another type of azaquinones are compounds 28a, b. These were generated in two steps from the pyridones 25 . The azaquinone 28a reacted easily with acidic compounds yielding the adducts 26, 27 and 29 or with 2‐butenal forming the cycloadduct 30 . 相似文献
965.
A novel micro-injector has been fabricated for capillary electrophoresis (CE). It was successfully used for the determination of some indole derivatives for example melatonin (MT), serotonin (5-HT), tryptophan (Trp), and 5-hydroxy-tryptophane (5-HTrp) in the rat pineal gland by capillary electrophoresis with electrochemical detection (CE-EC). CE was performed in 0.20 mol L(-1) phosphate buffer (pH 8.0). The compounds investigated can be well separated and detected within 15 min. The working electrode used was a 300-microm diameter carbon electrode positioned opposite the outlet of the capillary. The relationship between peak current and analyte concentration was highly linear in the range from 0.10 to 500 micromol L(-1); detection limits (S/N = 3) were 0.03-0.13 micromol L(-1). The proposed method has been successfully used to analyze real biological samples. 相似文献
966.
Three novel perylene polyimides (PPIs) containing p‐n diblock units were designed and synthesized for use in dye‐sensitized mesoporous TiO2 solar cells. They all dissolve in m‐cresol and N‐methyl‐2‐pyrrolidone (NMP). Their visible light absorption, electrochemical and photoelectrochemical properties were systematically studied. The polyimides have band gap energies of 2.16, 2.19 and 2.25 eV deduced from ultraviolet–visible absorption spectra, and electron affinity (Ea) and ionization potential (IP) of ?3.93 and 6.10 eV for PPI1, ?3.94 and 6.13 eV for PPI2, ?3.93 and 6.59 eV for PPI3, respectively, deduced from cyclic voltammogram. Experimental data show that introduction of 4,4′‐bisaminetriphenylamine cannot only greatly enhance optic‐electro conversion efficiency, but also enhance the dissolubility which in favorable for making the devices. The relationship of structure and properties of PPI is discussed and the mechanism of photocurrent generation is explained. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
967.
The conjecture of Tuinier (J. Colloid Interface Sci. 258 (2003) 45) for the electrical potentials near a cylindrical surface and near a spherical surface under the conditions of symmetric electrolyte and large scaled radius are derived by solving the corresponding Poisson-Boltzmann equation. The surface charge density-surface potential relations for these surfaces are also derived under the conditions of constant surface potential. We show that the level of surface charge density for planar, cylindrical, and spherical surfaces follows the order spherical surface > cylindrical surface > planar surface. 相似文献
968.
969.
1 Introduction and Main Results Consider the first Piola-Kirchhoff stress P defined as[1]P(F)=?W(F)/?F,(1.1)where F and W refer to the deformation tensor and strain energy density.Due to the principle of material frame-indifference and the material symmetry,two important constraints should be satisfied by these relations[1]. 相似文献
970.
ONBAHADURASYMPTOTICEFFICIENCYINASEMIPARAMETRICREGRESSIONMODELLIANGHUA(梁华);CHENGPING(成平)(InstituteofSystemsScience,theChineseA... 相似文献