全文获取类型
收费全文 | 75087篇 |
免费 | 7700篇 |
国内免费 | 14301篇 |
专业分类
化学 | 67740篇 |
晶体学 | 2585篇 |
力学 | 1322篇 |
综合类 | 597篇 |
数学 | 5053篇 |
物理学 | 19791篇 |
出版年
2024年 | 223篇 |
2023年 | 1274篇 |
2022年 | 2475篇 |
2021年 | 2428篇 |
2020年 | 2747篇 |
2019年 | 2479篇 |
2018年 | 2064篇 |
2017年 | 2719篇 |
2016年 | 2788篇 |
2015年 | 2439篇 |
2014年 | 3343篇 |
2013年 | 6881篇 |
2012年 | 4642篇 |
2011年 | 5069篇 |
2010年 | 4345篇 |
2009年 | 5111篇 |
2008年 | 5055篇 |
2007年 | 5141篇 |
2006年 | 4841篇 |
2005年 | 4333篇 |
2004年 | 4070篇 |
2003年 | 3306篇 |
2002年 | 2757篇 |
2001年 | 2270篇 |
2000年 | 2182篇 |
1999年 | 1759篇 |
1998年 | 1500篇 |
1997年 | 1273篇 |
1996年 | 1102篇 |
1995年 | 1058篇 |
1994年 | 1003篇 |
1993年 | 816篇 |
1992年 | 702篇 |
1991年 | 532篇 |
1990年 | 352篇 |
1989年 | 324篇 |
1988年 | 277篇 |
1987年 | 186篇 |
1986年 | 157篇 |
1985年 | 165篇 |
1984年 | 128篇 |
1983年 | 57篇 |
1982年 | 103篇 |
1981年 | 129篇 |
1980年 | 101篇 |
1979年 | 97篇 |
1978年 | 66篇 |
1977年 | 71篇 |
1976年 | 43篇 |
1973年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Eugenio?Garcia-Pa?eda Pilar?Guardado Alfredo?MaestreEmail author 《Journal of solution chemistry》2004,33(10):1277-1293
Partial molar volumes at infinite dilution, V02, of alkali–metal halides (LiCl, NaCl KCl RbCl CsCl, NaBr, KBr, KI), tetra-n-alkylammonium bromides, R4NBr (R=Me, Et, n-Pr, n-Bu, n-Pen), NaBPh4, and Ph4PCl have been determined in binary solvent mixtures of water with 2-methyl-2-butanol covering the water-rich region and the alcohol-rich region at 298.15 K. V02 for alkali–metal halides show relatively little dependence on the solvent composition. However, in the case of hydrophobic electrolytes the observed effects are more pronounced. A good linear dependence between V02(R4NBr) and the molecular weight of the tetra-n-alkylammonium cation is found. Limiting single-ion volumes have been obtained using the assumption that V0(Ph4P+)–V0(BPh–4)=2.0 cm3-mol–1. The trends in the single-ion volumes are discussed in both solvent regions. 相似文献
952.
I.K. Battisha 《Journal of Sol-Gel Science and Technology》2004,30(3):163-172
Visible up-conversion emissions at (435, 545, 580, 675 and 690 nm) and (437, 547 575 and 675 nm) have been observed from the sol-gel derived nano-crystalline Ho3+: BaTiO3 powders and thin films respectively, under 808 nm laser diode excitation emissions. Combined with the energy level structure of Ho3+ ions and the kinetics of the visible emissions, the up-conversion mechanism has been analyzed and explained. The blue, green and red emissions of both samples has been attributed to the ground state-directed transition from (5F1), (5S2) and (5F5), which are populated through excited state absorption (ESA) for 808 nm excitation. Nano-structure pure barium titanate and doped with different concentrations of Ho3+ ions in the from of powder and thin film have been prepared by sol-gel technique, using barium acetate (Ba(Ac)2), and titanium butoxide (Ti(C4H9O)4), as precursors. The thin films were prepared by sol-gel spin coating method. The as-grown thin films and powders were found to be amorphous, which crystallized to the tetragonal phase after heating at 750°C in air for 30 minutes. The crystallite sizes of the thin film and powder both doped with 4% Ho3+ ions was found to be equal to 11 and 16 nm, respectvely. 相似文献
953.
Peter G. Dormer Amude M. Kassim Johnnie L. Leazer Jr. Feng Xu Edward G. Corley Jimmy O. DaSilva David M. Tschaen 《Tetrahedron letters》2004,45(28):5429-5432
A general method for the preparation of syn-2,3-disubstituted-2,3-dihydro-1,4-benzoxathiin rings from 2-mercaptoethanols and quinone ketals is presented. This ring system is produced by Michael addition of a 2-mercaptoethanol to a quinone ketal, followed by cyclization of the initial Michael adduct, and subsequent aromatization to afford a syn-2,3-disubstituted-1,4-benzoxathiin in fair to good chemical yield. Several chiral syn-2,3-disubstituted-2,3-dihydro-1,4-benzoxathiin rings were prepared with this method from enantioenriched 2-mercaptoethanols. No loss of enantiopurity was observed. 相似文献
954.
In the past several decades, much attention has been paid to optically active, C2-asymmetric 1,1′-binaphthalene-2,2′-diol and its derivatives because of their application in inducing chirality in asymmetric synthesis[1,2]. Enantiomerically pure 1,1′-binaphthalene-2,2′-diol has been prepared by many methods ranging from the classical resolution via crystallization of the diastereoisomeric salts[3] to asymmetric oxidative coupling[4~6]. 相似文献
955.
There are many organic pollutants in the environment, such as polychlorinated biphenyl, polycyclic aromatic hydrocarbons, dichlorodiphenyl-trichloroethane (DDT), and polychlorinated naphthalene. These organic pollutants are persistent,liposoluble and easily cumulated in organism; consequently, the potential toxicity will be high. Risk assessment of industrial chemicals is currently carried out using scanty experimental data, because many of these chemicals have very little or no test data. S… 相似文献
956.
A study on the reactivity of 3,4-dihydro-2-pyridones 9, derived from (S)-phenylglycinol, toward different bases is presented. Using a catalytic amount of bases, the cis bicyclic lactams were obtained with excellent diastereoselectivities. 相似文献
957.
Rikuhei Tanikaga Yoshimasa MatsumotoMaki Sakaguchi Yohei KoyamaKentaro Ono 《Tetrahedron letters》2003,44(35):6781-6783
Lipase-mediated acetylations of trans- and cis-2-substituted cyclohexanols gave the corresponding (1R)-cyclohexyl acetates and (1S)-cyclohexanols in high yields and ee, but c-4-tert-butyl-c-2-ethenyl-r-1-cyclohexanol was unreactive owing to the steric interaction between the axial OH group and the axial H atoms at the 3- and 5-positions. In the cis-isomer the OH group occupies an equatorial position to bind to the lipase, and less bulky axial alkenyl and alkynyl groups might not so much prevent acetylations than an alkyl group. 相似文献
958.
D. Tirzite A. Krauze A. Zubareva G. Tirzitis G. Duburs 《Chemistry of Heterocyclic Compounds》2002,38(7):795-800
The alkylation of 3-cyano-1,4-dihydropyridine-2(3H)-thiones or the condensation of an aromatic aldehyde, cyanothioacetamide, acetylacetone, and methyl iodide in the presence of piperidine has given a series of novel 5-acetyl-2-alkylthio-4-aryl-6-methyl-1,4-dihydropyridine-3-carboxylic acid nitriles. A compound was obtained from 3,5-di(tert-butyl)-4-hydroxybenzaldehyde in the molecule of which were combined the active part of the antioxidant ionol and a 1,4-dihydropyridine ring. It was found that, among the compounds synthesized, the highest antiradical activity occurred in a compound having two hydroxyl groups in the 4-phenyl substituent. 相似文献
959.
LIN Zhou-Bin HU Zu-Shu ZHANG Li-Zhen WANG Guo-Fu 《结构化学》2002,21(6):637-639
<正>The crystal of Nd0.06Y0.94Sr6Sc(BO3)6 with the dimensions up to 35 mm × 28 mm × 13 mm was grown by a top-seeded solution growth method from Li6B4O9 flux. The grown crystal was characterized by X-ray powder diffraction. The optical absorption of the crystal shows that it has a strong absorption band at 8070 A, and the absorption coefficient is 2.17cm-1 with a FWHM of 41 A, which can match with the wavelength of the diode-laser (LD) and is suitable for the LD pumping. Based on the Judd-Ofelt theory, the three parameters of line oscillator strength Ω(λ) (λ = 2, 4 and 6) of the Nd3+ion in the crystal were calculated as follows: Ω2= 1.194 × 10-20, Ω4= 4.186 × 10-20 and Ω6 = 3.351 × 10-20cm2, which are relatively larger. The results indicate that the crystal Nd0.06Y0.94Sr6Sc(BO3)6 may be a kind of high-efficient laser material for diode-pumped. 相似文献
960.