全文获取类型
收费全文 | 14480篇 |
免费 | 452篇 |
国内免费 | 322篇 |
专业分类
化学 | 9041篇 |
晶体学 | 100篇 |
力学 | 404篇 |
综合类 | 30篇 |
数学 | 1886篇 |
物理学 | 3793篇 |
出版年
2022年 | 117篇 |
2021年 | 183篇 |
2020年 | 175篇 |
2019年 | 175篇 |
2018年 | 170篇 |
2017年 | 138篇 |
2016年 | 327篇 |
2015年 | 298篇 |
2014年 | 360篇 |
2013年 | 777篇 |
2012年 | 619篇 |
2011年 | 718篇 |
2010年 | 515篇 |
2009年 | 493篇 |
2008年 | 660篇 |
2007年 | 699篇 |
2006年 | 562篇 |
2005年 | 579篇 |
2004年 | 576篇 |
2003年 | 576篇 |
2002年 | 611篇 |
2001年 | 440篇 |
2000年 | 398篇 |
1999年 | 307篇 |
1998年 | 223篇 |
1997年 | 203篇 |
1996年 | 221篇 |
1995年 | 236篇 |
1994年 | 227篇 |
1993年 | 176篇 |
1992年 | 168篇 |
1991年 | 175篇 |
1990年 | 191篇 |
1989年 | 154篇 |
1988年 | 137篇 |
1987年 | 135篇 |
1986年 | 123篇 |
1985年 | 216篇 |
1984年 | 231篇 |
1983年 | 159篇 |
1982年 | 178篇 |
1981年 | 146篇 |
1980年 | 151篇 |
1979年 | 138篇 |
1978年 | 115篇 |
1977年 | 162篇 |
1976年 | 142篇 |
1975年 | 118篇 |
1974年 | 110篇 |
1973年 | 114篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
281.
The potentiometric determination of zinc in chloride solutions with a coated wire electrode is described. The electroactive membrane contains the chlorozincate(II) salt of Aliquat 336S in poly (vinyl chloride). The electrode shows a useful response over the range 10-5 M–10-1 M zinc(II) in 3 M total chloride solutions in the pH range 1.5–6. The electrode is more responsive to the divalent tetrachlorozincate(II) than to the monovalent trichlorozincate(II) species. Applications in compleximetric titrations, and to the analysis of zinc oxide and zinc concentrates are reported. 相似文献
282.
Summary The chelating agent, 2-thenoyltrifluoroacetone has been employed for rapid extraction and colorimetric determination of milligram amounts of copper(II) in one operation. At pH 2.4–6.0 copper(II) is extracted quantitatively from an aqueous solution by TTA-benzene in a single extraction. The green-coloured copper(II)-TTA chelate solution in benzene obeysBeer's law at 430 m over the range of 16–180g copper per millilitre. The coloured system is stable for 143 hours. It can tolerate silver, mercury(II), bismuth (<5 mg) and small amounts (<100 mg) of citrate and tartrate, whereas cobalt(II), nickel(II), iron(III), aluminium(III), cerium(IV), thorium and zirconium seriously interfere. The proposed method is reproducible to within ±1.4%. 相似文献
283.
Noppe H De Wasch K Poelmans S Van Hoof N Verslycke T Janssen CR De Brabander HF 《Analytical and bioanalytical chemistry》2005,382(1):91-98
An analytical procedure enabling routine analysis of four environmental estrogens at concentrations below 1 ng L–1 in estuarine water samples has been developed and validated. The method includes extraction of water samples using solid-phase extraction discs and detection by gas chromatography (GC) with tandem mass spectrometry (MS–MS) in electron-impact (EI) mode. The targeted estrogens included 17- and 17-estradiol (aE2, bE2), estrone (E1), and 17-ethinylestradiol (EE2), all known environmental endocrine disruptors. Method performance characteristics, for example trueness, recovery, calibration, precision, accuracy, limit of quantification (LOQ), and the stability of the compounds are presented for each of the selected estrogens. Application of the procedure to water samples from the Scheldt estuary (Belgium – The Netherlands), a polluted estuary with reported incidences of environmental endocrine disruption, revealed that E1 was detected most frequently at concentrations up to 7 ng L–1. aE2 was detected once only and concentrations of bE2 and EE2 were below the LOQ.Presented at the 9th FECS Conference on Chemistry and the Environment, Bordeaux, France, 29 August–1 September 2004 相似文献
284.
The formation and stability of protonated diamines-carboxylic ligand complexes was studied potentiometrically (H(+)-glass electrode). Species formed are ALH(r) (A=cadaverine, putrescine, L=acetate, malate, tartrate, malonate, citrate, 1,2,3-propanetricarboxylate, 1,2,3,4-butanetetracarboxylate and glutamate; r=1...m+1, where m is the maximum degree of protonation of the carboxylic ligand), and their stability is a function of charges involved in the formation reaction. For the equilibrium H(i)A(i+)+H(j)L((j-z))=ALH((i+j-z))(i+j) the following linear relationships can be written: logK(1j)=-0.25+0.75 |j-z|, logK(2j)=0.50+0.90 |j-z| (by also considering some ethylenediamine and 1,2-diaminopropane complexes). Medium effects were considered. Comparison was made with analogous inorganic polyanion complexes. The simplest relationships -DeltaG(0)=6.5+/-0.3 and -DeltaG(0)=7.9+/-0.6 kJ mol(-1)n(-1) (n=number of possible salt bridges) were found for carboxylic and inorganic anions, respectively. 相似文献
285.
Curve crossing in two excited states of IBr was studied by means of photodissociation with a pulse dye laser. The ratios of the formed ground state Br(2P and excited Br(2P) were measured at various wavelengths. They are compared with ratios calculated with the Landau-Zener formula, using parameters given by Child. The agreement between the measurements and the calculations is very good. 相似文献
286.
Cross sections and threshold energies are compared for radiation from fragments produced by electron impact on methane, ethylene, ethane and acetylene. Some previous measurements have been repeated. The emission cross sections for corresponding Balmer radiation are within 10% equal for these hydrocarbons. Also the thresholds for Balmer radiation lie close together. These results can be explained in a model where H fragments arise from Rydberg states excited by promotion of an inner valence electron to a non-bonding orbital. In this model a comparison between dissociative ionization yielding H+ and dissociative excitation yielding H+ is made. For radiation from molecular fragments it is shown that the CH(A2 Δ-X2Π) emission cross sections are particularly high in the case of acetylene. The electron impact data appear to be consistent with photoabsorption data. 相似文献
287.
De Riccardis F Di Filippo M Garrisi D Izzo I Mancin F Pasquato L Scrimin P Tecilla P 《Chemical communications (Cambridge, England)》2002,(24):3066-3067
The Na+ transporting properties of the first member of a new class of artificial ionophores, based on a C2-symmetric polyhydroxylated steroid dimer, are described. 相似文献
288.
Norbert Klempier Anna De Raadt Herfried Griengl Gottfried Heinisch 《Journal of heterocyclic chemistry》1992,29(1):93-95
Chemoselective hydrolysis of heterocyclic nitriles can be achieved by an easy to use immobilized biocatalyst prepared from Rhodococcus sp. Pyrimidine-2-carbonitrile ( 2a ) and 3-chloropyridazine-4-carbonitrile ( 4a ) were converted into the corresponding amides while from 2-ethoxycarbonyl-4-pyridinecarbonitrile ( 1a ), 6-methylpyridazine-3-carbonitrile ( 3a ), 3-chloropyridazine-4-carbonitrile ( 4a ), 3-ethoxycarbonyl-4,5-dihydroisoxazole-5-carbonitrile ( 8a ), indole-3-carbonitrile ( 9a ) and indole-3-ylacetonitrile ( 10a ) the acids were formed. 相似文献
289.
290.
Kang De Yao Tao Peng Han Bao Feng Yu Ying He 《Journal of polymer science. Part A, Polymer chemistry》1994,32(7):1213-1223
The aqueous swelling kinetics of a series of crosslinked chitosan (cr-CS) with glutaraldehyde (GA) interpenetrating polyether hydrogels have been studied as functions of pH, the N-deacetylation degree of chitosan, the amount of crosslinking agent, the electrolyte composition in solution, temperature, and gel composition. Based on these results, the swelling mechanism of the hydrogels was discussed. The release profiles of chlorhexidini acetas from the semi-IPN were also investigated. © 1994 John Wiley & Sons, Inc. 相似文献