全文获取类型
收费全文 | 4078篇 |
免费 | 150篇 |
国内免费 | 35篇 |
专业分类
化学 | 2863篇 |
晶体学 | 22篇 |
力学 | 110篇 |
综合类 | 2篇 |
数学 | 721篇 |
物理学 | 545篇 |
出版年
2023年 | 21篇 |
2022年 | 51篇 |
2021年 | 125篇 |
2020年 | 90篇 |
2019年 | 87篇 |
2018年 | 78篇 |
2017年 | 75篇 |
2016年 | 163篇 |
2015年 | 115篇 |
2014年 | 147篇 |
2013年 | 224篇 |
2012年 | 284篇 |
2011年 | 344篇 |
2010年 | 202篇 |
2009年 | 174篇 |
2008年 | 262篇 |
2007年 | 242篇 |
2006年 | 222篇 |
2005年 | 219篇 |
2004年 | 204篇 |
2003年 | 135篇 |
2002年 | 124篇 |
2001年 | 59篇 |
2000年 | 43篇 |
1999年 | 43篇 |
1998年 | 34篇 |
1997年 | 39篇 |
1996年 | 48篇 |
1995年 | 44篇 |
1994年 | 35篇 |
1993年 | 34篇 |
1992年 | 21篇 |
1991年 | 36篇 |
1990年 | 20篇 |
1989年 | 24篇 |
1988年 | 16篇 |
1987年 | 14篇 |
1986年 | 22篇 |
1985年 | 26篇 |
1984年 | 25篇 |
1983年 | 13篇 |
1982年 | 26篇 |
1981年 | 10篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1974年 | 3篇 |
1973年 | 3篇 |
排序方式: 共有4263条查询结果,搜索用时 93 毫秒
81.
82.
83.
Rafael Bilbao María Benita Murillo Angela Millera Jos Francisco Mastral 《Thermochimica Acta》1991,190(2):163-173
An experimental system that allows the use of large particle sizes and the simulation of different operating conditions was built to study the thermal decomposition of lignocellulosic materials. The values of solid conversion and of temperature obtained at different points using spherical particles of pine wood 2 cm in diameter are shown. The conversion values are compared with those calculated from the equations obtained in a thermobalance for small particle sizes. 相似文献
84.
Inspired by the determination of the structure of the nitrogenase enzyme cofactor by Rees et al., the binding of an N2 molecule to some model iron-sulfur compounds was investigated usingab initio calculations. Side-on and end-on coordination to one two and four iron centers were investigated. In most cases, the N2 is loosely bound and retains its internal triple bond, but a few examples are found where the N2 is “activated” and has a longer N-N bond length. 相似文献
85.
Carmen R. Alonso-Cruz Francisco J. Ortiz-López Ernesto Suárez 《Tetrahedron letters》2005,46(32):5265-5268
The reaction of derivatives of 3-acetyl-d-glucal, 3-acetyl-l-rhamnal, 3-acetyl-d-galactal, and 3-acetyl-d-lactal with sodium benzenesulfinate in acid medium catalyzed by HgSO4 afforded diastereoisomeric mixtures of the corresponding 2,3-dideoxy-3-(phenylsulfonyl)-hexopyranoses through a Ferrier rearrangement. The anomeric alkoxyl radical fragmentation of these γ-hydroxy sulfones using the system (diacetoxyiodo)benzene and iodine gave vinyl sulfones with structures of 1,2-dideoxy-4-O-formyl-2-(phenylsulfonyl)-pent-1-enitol and configurations d-erythro, l-erythro, and d-threo at the two stereogenic centers. 相似文献
86.
Functionalized keto-enamines 6 were obtained by nucleophilic addition of enol ethers to the imine moiety of 2-azadienes derived from dehydroaspartic esters 4. Reactions of 2-azadiene 4c containing three electron-withdrawing substituents (CO(2)R) with enol ethers 5 in the presence of lithium perchlorate led to the formation of tetrahydropyridine derivatives 7 in a regio- and stereoselective fashion. 2H-[1,3]-oxazines 10 and pyridine derivatives 12 and 13 were obtained by heterocycloaddition reactions of electron-poor azadienes 4d-g containing two electron-withdrawing substituents (4-O(2)N-C(6)H(4), CO(2)R) in positions 1 and 4 with carbonyl derivatives (ethyl glyoxalate 9a and diethyl ketomalonate 9b) and the electron-deficient olefin tetracyanoethylene 11. 相似文献
87.
Coupled-column liquid chromatography with fluorescence detection was applied to the determination of o-phenylphenol and bitertanol residues in orange and banana fruits. After extraction with a mixture of acetone, dichloromethane-petroleum ether, and ethyl acetate, an extract aliquot of 100 microL was injected directly without any additional clean-up into the chromatographic system using two reversed phase C18 coupled columns. The LC-LC approach allowed automated sample clean up of the vegetal extracts, leading to a simple and rapid analytical procedure, with limits of quantification between 0.01 and 0.05 mg kg(-1). Recovery experiments performed on orange and banana samples fortified at different concentrations (0.01 - 4 mg kg(-1)) gave average recoveries between 70 and 113% with relative standard deviations lower than 15%. The procedure developed was finally applied to orange and banana samples from different geographical locations and the results were confirmed by GC-MS. 相似文献
88.
A bulky tripodal tris(carbene)borate ligand, prepared from 1-tert-butylimidazole, is cleanly transferred to iron(II) by a magnesium reagent. 相似文献
89.
A capillary zone electrophoresis (CZE) method with ultraviolet-visible detection has been established and validated for the determination of five phenothiazines: thiazinamium methylsulfate, promazine hydrochloride, chlorpromazine hydrochloride, thioridazine hydrochloride, and promethazine hydrochloride in human urine. Optimum separation was obtained on a 64.5 cm x 75 microm bubble cell capillary using a buffer containing 150 mM tris(hydroxymethyl)aminomethane and 25% acetonitrile at pH 8.2, with temperature and voltage of 25 degrees C and 20 kV, respectively. Naphazoline hydrochloride was used as an internal standard. Field-amplified sample injection (FASI) has been applied to improve the sensitivity of the detection. Considering the influence of parameters affecting the on-line preconcentration (nature of preinjection plug, sample solvent composition, injection times, and injection voltage) and due to the significant interactions among them, in this paper we propose for the first time the application of a multivariate approach to carry out the study. The optimized conditions were as follows: preinjection plug of water for 7 s at 50 mbar, electrokinetic injection for 40 s at 6.2 kV, and 32 microm of H3PO4 in the sample solvent. Also, a solid-phase extraction (SPE) procedure is developed to obtain low detection limits and an adequate selectivity for urine samples. The combination of SPE and FASI-CZE-UV allows adequate linearities and recoveries, low detection limits (from 2 to 5 ng/mL), and satisfactory precisions (3.0-7.2% for an intermediate RSD %). 相似文献
90.
Fátima Nunes Serralha José Madeira Lopes Francisco Lemos Duarte Prazeres Maria Aires-Barros Joo Rocha Joaquim Cabral Fernando Ramôa Ribeiro 《Reaction Kinetics and Catalysis Letters》2000,69(2):217-222
Novel [CrIII(amp)(bipy)(Cl)] (1) (H2amp = N-(hydroxyphenyl)salicyldimine; bipy = 2,2-bipyridyl) and [CrIII(app)(bipy)(Cl)]+ (2) (H2app = N-(hydroxyphenyl)pyridine-2-carboxaldimine; bipy = 2,2-bipyridyl) complexes have been synthesized and characterized by physico-chemical methods. Complexes 1 and 2 have been employed as catalysts in the oxidation of both saturated and unsaturated hydrocarbons using tert-butylhydroperoxide (t-BuOOH). The significance of the results with respect to oxo-functionalization of C-H bonds both in unsaturated and saturated hydrocarbons is noted. 相似文献