全文获取类型
收费全文 | 73360篇 |
免费 | 6797篇 |
国内免费 | 5774篇 |
专业分类
化学 | 49479篇 |
晶体学 | 474篇 |
力学 | 1732篇 |
综合类 | 819篇 |
数学 | 7662篇 |
物理学 | 25765篇 |
出版年
2024年 | 133篇 |
2023年 | 702篇 |
2022年 | 2152篇 |
2021年 | 2524篇 |
2020年 | 1953篇 |
2019年 | 1989篇 |
2018年 | 1553篇 |
2017年 | 1802篇 |
2016年 | 2506篇 |
2015年 | 2528篇 |
2014年 | 2983篇 |
2013年 | 5010篇 |
2012年 | 3652篇 |
2011年 | 3944篇 |
2010年 | 3480篇 |
2009年 | 4501篇 |
2008年 | 4608篇 |
2007年 | 4762篇 |
2006年 | 3965篇 |
2005年 | 2994篇 |
2004年 | 2595篇 |
2003年 | 2413篇 |
2002年 | 4624篇 |
2001年 | 2187篇 |
2000年 | 1655篇 |
1999年 | 1397篇 |
1998年 | 1339篇 |
1997年 | 1084篇 |
1996年 | 1072篇 |
1995年 | 927篇 |
1994年 | 844篇 |
1993年 | 847篇 |
1992年 | 765篇 |
1991年 | 544篇 |
1990年 | 420篇 |
1989年 | 381篇 |
1988年 | 437篇 |
1987年 | 291篇 |
1986年 | 261篇 |
1985年 | 368篇 |
1984年 | 278篇 |
1983年 | 171篇 |
1982年 | 333篇 |
1981年 | 514篇 |
1980年 | 464篇 |
1979年 | 485篇 |
1978年 | 390篇 |
1977年 | 284篇 |
1976年 | 249篇 |
1973年 | 202篇 |
排序方式: 共有10000条查询结果,搜索用时 578 毫秒
851.
聚合物光老化、光稳定机理与光稳定剂(上) 总被引:6,自引:0,他引:6
大多数聚合物材料,包括塑料、橡胶、纤维、涂料等,在户外使用时会发生光老化而使使用寿命缩短。为有效防止聚合物材料的光劣化,人们已对聚合物的光老化和光稳定机理进行了广泛的研究并先后研发出了几个类型的光稳定剂。但由于户外应用范围不断扩大和环境不断恶化正使聚合物的光老化变得越来越复杂而光防护难度也日益加大,因此国内外有关聚合物光老化、光稳定机理的研究和光稳定剂的研发一直十分活跃。本文上篇主要阐述聚合物的光老化机理。 相似文献
852.
Fluorometric investigation of the interaction of bovine serum albumin with surfactants and 6-mercaptopurine 总被引:2,自引:0,他引:2
Hu YJ Liu Y Jiang W Zhao RM Qu SS 《Journal of photochemistry and photobiology. B, Biology》2005,80(3):235-242
Fluorescence quenching in solutions of bovine serum albumin has been investigated in the presence of 6-mercaptopurine and ionic surfactants. Spectroscopic analysis of the emission quenching at different temperatures revealed that the quenching mechanism of bovine serum albumin by 6-mercaptopurine was dynamic quenching mechanism. The Stern–Volmer quenching model has been successfully applied, and the activation energy of the interaction between 6-mercaptopurine and bovine serum albumin as much as 4.26 kJ mol−1 was calculated. The distance r between donor (bovine serum albumin) and acceptor (6-mercaptopurine) was obtained according to fluorescence resonance energy transfer (FRET). The result of synchronous fluorescence spectra shows that the conformation of bovine serum albumin has been changed at the present of 6-mercaptopurine. 相似文献
853.
L.A. Isupova I.S. Yakovleva I.I. Gainutdinov Yu.T. Pavlyukhin V.A. Sadykov 《Reaction Kinetics and Catalysis Letters》2004,81(2):373-382
The phase composition and microstructure of samples of the La1-xCaxFeO3-y system prepared via a ceramic route were characterized by M?ssbauer spectroscopy. In all cases, iron was found in the 3+ state. The ordering
of anion vacancies in the samples with the composition in the range of 0.8 > x ≥ 0.4, which corresponds to a microheterogeneous
solid solution, generates new distorted octahedral and fivefold/tetrahedral sites revealed by two typical sextets. The disordering
of this solid solution and small (10-100 ?) sizes of domains with a perovskite, braunmillerite or Grenier phase structure
caused the appearance of a superparamagnetic doublet, which grows with the Ca content up to x = 0.8 but disappears in the
sample of pure braunmillerite. The appearance of Fe cations in a distorted coordination correlates with the increased activity
of the samples with a microheterogeneous structure in the CO catalytic oxidation and with their reducibility by H2.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
854.
K. Dimroth A. Berndt F. Br A. Schweig R. Volland 《Angewandte Chemie (International ed. in English)》1967,6(1):34-40
The distribution of the unpaired electron over the oxygen and the 24 carbon atoms in the free 2,4,6-triphenylphenoxy radical was determined by electron spin resonance spectroscopy and quantum-mechanical approximation methods. The hyperfine splitting was evaluated with the aid of the spectra of triphenylphenoxyls deuterated in some or all of the substituent phenyl groups. The results of the quantum-mechanical approximations were checked by recording the ESR spectra of triphenylphenoxyls labeled with 13C in positions 1,2,3, or 4 of the central ring. The spin density distribution permits a first discussion of the 17O-coupling constants of correspondingly labeled triphenylphenoxyl and other organic free radicals. 相似文献
855.
Recent developments in quantitative surface analysis by Auger (AES) and x-ray photoelectron (XPS) spectroscopies are reviewed and problems relating to a more accurate quantitative interpretation of AES/XPS experimental data are discussed. Special attention is paid to consideration of elementary physical processes involved and influence of multiple scattering effects on signal line intensities. In particular, the major features of core-shell ionization by electron impact, Auger transitions and photoionization are considered qualitatively and rigorous approaches used to calculate the respective transition probabilities are analysed. It is shown that, in amorphous and polycrystalline targets, incoherent scattering of primary and signal Auger and photoelectrons can be described by solving analytically a kinetic equation with appropriate boundary conditions. The analytical results for the angular and energy distribution, the mean escape depth, and the escape probability as a function of depth of origin of signal electrons as well as that for the backscattering factor in AES are in good agreement with the corresponding Mote Carlo simulation data. Methods for inelastic background subtraction, surface composition determination and depth-profile reconstructions by angle-resolved AES/XPS are discussed. Examples of novel techniques based on x-ray induced photoemission are considered. 相似文献
856.
The combination of ethyl diazoacetate with aryl aldehydes in the presence of copper(I) or rhodium(II) catalysts results in
the formation of 1,3-dioxolane products in moderate to good yields. These reactions occur through a pathway that involves
ylide intermediates. Catalyst-dependent diastereocontrol is observed and suggests that metal-associated ylides are involved
in the product-determining step. The influence of aryl aldehyde substituents has been determined.
Current address.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 961–965, May, 1998. 相似文献
857.
V. D. Sheludyakov Z. V. Belyakova V. M. Shevchenko E. A. Chernyshev 《Russian Chemical Bulletin》1997,46(5):997-1002
Novel dialkylaminocarbenium salts with metallocomplex counter ions were prepared by the reaction of phosgene with either DMF
or tetramethylurea in the presence of metal chlorides. Reactions of organosilicon amides with phosgene gave corresponding
carbenium salts, while organosilicon ureas yielded aminoiminocarbenium salts. Dialkylaminochlorocarbenium salts were reduced
with hydrosilanes to give dialkylaminocarbenium, salts and can be easily hydrolyzed to afford either amides or ureas. Pathways
of the reaction with water and alcohols depend on the nature of reagent and the reaction conditions.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1035–1040, May 1997. 相似文献
858.
采用SCF-X_α-MS方法, 对于C(2×2)S/Fe(001)吸附体系, 选择Fe_5S和Fe_9S两种原子簇模型, 研究了该吸附体系的电子结构、吸附成键特征及其相互作用图象。结果表明, S吸附于Fe(001)单晶表面具有较强的定域性质, S原子与底物Fe原子之间的吸附相互作用主要表现为S(3p)-Fe(4s,3d)之间的轨道相互作用。通过对表面吸附键长的优化, 其结果与X.S.Zhang等的ARPEFS的实验结果一致。 相似文献
859.
Bonnie R. Hames Steven R. Thomas Amie D. Sluiter Christine J. Roth David W. Templeton 《Applied biochemistry and biotechnology》2003,105(1-3):5-16
New, rapid, and inexpensive methods that monitor the chemical composition of corn stover and corn stover-derived samples are
a key element to enabling the commercialization of processes that convert stover to fuels and chemicals. These new techniques
combine near infrared (NIR) spectroscopy and projection to latent structures (PLS) multivariate analysis to allow the compositional
analysis of hundreds of samples in 1 d at a cost of about $10 each. The new NIR/PLS rapid analysis methods can also be used
to support a variety of research projects that would have been too costly to pursue by traditional methods. 相似文献
860.
Most known perylene diimides are lipophilic, with few exceptions of hydrophilic derivatives. Even in the latter case, the compounds have limited water solubility and show a strong tendency to self-aggregation. In this paper we present the synthesis of four new perylene derivatives with three and four basic side chains, obtained by functionalizing the bay-area of perylene. These molecules show great solubility in aqueous media as hydrochlorides and their tendency to self-aggregate is remarkably reduced with respect to the previously synthesized two-chained perylene diimides. Their different spectroscopic properties in various solvents and conditions are reported and discussed. 相似文献