全文获取类型
收费全文 | 92944篇 |
免费 | 791篇 |
国内免费 | 702篇 |
专业分类
化学 | 33596篇 |
晶体学 | 896篇 |
力学 | 7146篇 |
综合类 | 30篇 |
数学 | 33967篇 |
物理学 | 18802篇 |
出版年
2022年 | 121篇 |
2021年 | 191篇 |
2020年 | 179篇 |
2019年 | 191篇 |
2018年 | 10549篇 |
2017年 | 10334篇 |
2016年 | 6363篇 |
2015年 | 1139篇 |
2014年 | 658篇 |
2013年 | 1149篇 |
2012年 | 4379篇 |
2011年 | 11151篇 |
2010年 | 6122篇 |
2009年 | 6492篇 |
2008年 | 7229篇 |
2007年 | 9400篇 |
2006年 | 803篇 |
2005年 | 1919篇 |
2004年 | 2117篇 |
2003年 | 2559篇 |
2002年 | 1636篇 |
2001年 | 717篇 |
2000年 | 710篇 |
1999年 | 467篇 |
1998年 | 426篇 |
1997年 | 359篇 |
1996年 | 433篇 |
1995年 | 369篇 |
1994年 | 318篇 |
1993年 | 286篇 |
1992年 | 238篇 |
1991年 | 251篇 |
1990年 | 258篇 |
1989年 | 233篇 |
1988年 | 222篇 |
1987年 | 216篇 |
1986年 | 193篇 |
1985年 | 291篇 |
1984年 | 294篇 |
1983年 | 219篇 |
1982年 | 250篇 |
1981年 | 209篇 |
1980年 | 220篇 |
1979年 | 211篇 |
1978年 | 167篇 |
1977年 | 205篇 |
1976年 | 172篇 |
1975年 | 154篇 |
1974年 | 148篇 |
1973年 | 162篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A new Schiff base has been synthesized from 4-aminoantipyrine and 3-formylsalicylic acid. The ligand has a dianionic tetradentate compartmental OONO donor system. The cobalt(II), nickel(II), copper(II) and dioxouranium(VI) complexes exist in phenolato-bridged dinuclear species, while palladium(II) gives a mononuclear complex with free –COOH groups. The complexes have been characterized by elemental analyses, i.r., u.v.-vis, thermal and magnetic measurements. 相似文献
992.
Wanxing Wei Yuanjiang Pan Yaozu Chen Cuiwu Lin Tengyou Wei Shukai Zhao 《Chemistry of Natural Compounds》2005,41(1):17-21
Five compounds, terephthalic acid mono-[2-(4-carboxy-phenoxycarbonyl)-vinyl] ester (1), (E)-3-(5′-hydroperoxy-2,2′-dihydroxy[1,1′-biphenyl]-4-yl)-2-propenoic acid (2), 3,4,5-trihydroxybenzoic acid (3), succinic acid (or butanedioic acid) (4), and 2,3,4,5,6-pentahydroxybenzoic acid (5), were isolated from Phyllanthus urinaria. The structures of these compounds were elucidated by means of spectral techniques including IR, MS, and 1D/2D NMR. 1 and 2 are new compounds.__________Published in Khimiya Prirodnykh Soedinenii, No. 1, pp. 14–17, January–February, 2005. 相似文献
993.
Mariko Kusabe Hiromitsu Kozuka Satoru Abe Hiroshi Suzuki 《Journal of Sol-Gel Science and Technology》2007,44(2):111-118
Hydroxypropylcellulose (HPC)–titania hybrid thin films were prepared by sol–gel method where titanium tetraisopropoxide Ti(OC3H7
i
)4 was hydrolyzed under acidic conditions in the presence of HPC, followed by dip-coating and drying at 120 °C for 24 h. The
viscosity average molecular weight of HPC was 55,000–70,000 or 110,000–150,000, and the TiO2/(HPC + TiO2) mass ratio ranged from 0 to 1, which was calculated on the assumption that all Ti(OC3H7
i
)4 is converted into TiO2. The films were 0.35–1.0 μm thick, transparent in visible region and opaque in ultraviolet (UV) region, where the optical
absorption coefficient in UV region increased with increasing titania content. The refractive index increased with increasing
titania content, ranging from 1.6 to 1.8 for the hybrid thin films. The pencil hardness increased from 6B to 5H, the durability
in hot water significantly increased and the contact angle of water on films increased from 35° to 89° with increasing titania
content. Crack-free films could be deposited on organic polymer substrates irrespective of titania or HPC contents, where
cracking did not occur at higher HPC contents even when the substrate was bent. 相似文献
994.
Résumé Les auteurs ont étudié le comportement chromatographique sur plusieurs phases stationnaires polaires et non polaires de composés cyclopropaniques stéréoisomêres: où R=H ou CH3 et X=CH, COOCH3 ou CH2OH. Les résultats sont interprétés en fonction de la configuration de X par rapport à , de la nature électronique de X et de l'orientation du phényle par rapport au plan du cyclopropane.
Relationship between chemical structure and retention. VII. Cyclopropane compounds
Summary The chromatographic behaviour, on several polar and non polar phases, of stereoisomeric cyclopropanes has been studied R=H or CH3 and X=CN, COOCH3, CH2OH. The results are interpreted in terms of the X versus configuration, the electronic nature of X and the orientation of the phenyl ring in relation to plane of the cyclopropane ring.相似文献
995.
Marco-A. De Paoli R. J. Waltman A. F. Diaz J. Bargon 《Journal of polymer science. Part A, Polymer chemistry》1985,23(6):1687-1698
An electrically conductive plastic material was obtained by the electrochemical polymerization of pyrrole in a poly(vinyl chloride) matrix. The transmission electron microscopy shows that polypyrrole is uniformly distributed in the matrix. The conductivity of the composites fall in the range of 5 to 50 S/cm, and their mechanical properties, as measured in a stress—strain test, are very similar to those of pure poly(vinyl chloride). These can be further improved by addition of poly(chloroprene) rubber as a plasticizer. 相似文献
996.
The stability of spontaneous thin layers and thin layers formed upon cathodical polarization of Ti in KOH solutions have been
studied by potentiostatic and ellipsometric methods. At open circuit potential (OCP) the strongly adherent films, whose thickness
depends on the concentration of the KOH solution, were formed. During the cathodic polarization the transformation of these
films to weakly adsorbed precipitated layers on the electrode surface was observed. Comparing the theoretically computed curves
with the experimental Ψ vs Δ loci measured ellipsometrically, the complex indices of refraction and the thickness of the generated films, from 3.6
to 60 nm in 1 M KOH and from 36 to 105 nm in 5 M KOH (adherent to the electrode surface), were determined. At OCP the rate
of film growth increases with increasing the concentration of KOH solution. Cathodic polarizations change the chemical composition
and retard the rate of film growth. Based on the ellipsometric and electrochemical data the chemical compositions of the formed
films consisted of TiO2, Ti2O3, TiO2·H2O, Ti(OH)3 and TiOOH·nH2O. 相似文献
997.
Xiufang Xu Guichang Wang Xuezhuang Zhao Yinming Pan Yunxiao Liang Zhenfeng Shang 《Journal of mathematical chemistry》2007,41(2):143-160
In this paper, the fuzzy symmetry of some prototypical linear molecules has been analyzed. The results show that some molecular
orbitals (MOs) are less symmetrical but some others are more symmetrical than the molecular skeleton, which the MOs correspond
to. The membership functions of space inversion for MOs are closely related to the chemical characteristics of the MOs. Sometimes,
although the symmetry of a molecular skeleton is not obvious, however that of some MO is quite obvious. The membership functions
of the fuzzy inversion symmetry depend on the choice of the position of the center of inversion. As compared to those of diatomic
molecules and linear tri-atomic molecules, the linear polyatomic molecules in which a distinctive fuzzy symmetry of space
translation may exist, and thus a significant effect on their properties can be expected. 相似文献
998.
Microcapsules containing healing agents have been used to develop the self-healing polymeric composites. These microcapsules
must possess special properties such as appropriate strength and stability in surrounding medium. A new series of microcapsules
containing dicyclopentadiene (DCPD) with melamine–formaldehyde (MF) resin as shell material were synthesized by in situ polymerization
technology. These microcapsules may satisfy the requirements for self-healing polymeric composites. The chemical structure
of microcapsule was identified by using Fourier transform infrared (FTIR) spectrometer. The morphology of microcapsule was
observed by using optical microscope (OM) and scanning electron microscope. Size distribution and mean diameter of microcapsules
were determined with OM. The thermal properties of microcapsules were investigated by using thermogravimetric analysis and
differential scanning calorimetry. Additionally, the self-healing efficiency was evaluated. The results indicate that the
poly(melamine–formaldehyde) (PMF) microcapsules containing DCPD have been synthesized successfully, and their mean diameters
fall in the range of 65.2∼202.0 μm when the adjusting agitation rate varies from 150 to 500 rpm. Increasing the surfactant
concentration can decrease the diameters of microcapsules. The prepared microcapsules are thermally stable up to 69 °C. The
PMF microcapsules containing DCPD can be applied to polymeric composites to fabricate the self-healing composites. 相似文献
999.
Xu-Chun Wang Jian-Feng Li He-Ping Chen Jie-Ying Wu Hong-Ping Zhou Jia-Xiang Yang Bao-Kang Jin Yu-Peng Tian 《Transition Metal Chemistry》2007,32(5):551-557
The novel compound, 3-(E)-ferrocenyl-vinyl-N-hexylcarbazole (FVHC) was first synthesized and fully characterized by elemental analysis, IR, 1H-NMR, single-crystal X-ray diffraction, ultraviolet (UV) absorption, cyclic voltammograms (CV) and optical limiting (OL)
measurements. The result of single crystal X-ray diffraction for the compound reveals that the ferrocenyl and carbazole groups
are approximately coplanar, and bridged by double-bond with E configuration, showing that there is a well-delocalized π-electron
system in the molecule. The electrochemical investigation indicated that the electron in the FVHC may partially be delocalized
over the π-conjugated system and CT process in functionalized carbazole systems. Besides, the compound exhibited strong UV
absorption and large optical limiting effect, indicating promising potential applications as useful OL materials. 相似文献
1000.
F. De Meester J. M. Frre J. L. Piette P. Jacquemin L. Grooters G. Llabres S. Defays 《Journal of heterocyclic chemistry》1992,29(2):535-541
In an attempt to rationalize a synthesis of penicillin analogs modified at C(3), we have isolated the 3R(β)-carbinolamide derivative 4a. The trityl substituent on N(6′) seems to be responsible for the inversion of configuration which occurs at C(3) during the acid hydrolysis of the isocyanate intermediate. An hydrogen bond is formed on the β-face of the bell-shaped bicyclic skeleton between the N(6′)-nitrogen lone pair and the C(3) hydroxyl group. On standing, the carbinolamide analog slowly isomerizes to its expanded bicyclic isomer 4b , but the starting material may be easily recovered by treatment with acid. The postulated intermediate during isomerization, i.e., the open aldehyde form, does not accumulate. Substitutions of the hydroxyl group at C(3) lead to a variety of compounds with the biologically active 3S(α) configuration. These may be used to study the importance of the carboxyl group of penicillins in their interaction with β-lactamases at the molecular level. 相似文献