全文获取类型
收费全文 | 5754篇 |
免费 | 475篇 |
国内免费 | 1211篇 |
专业分类
化学 | 3582篇 |
晶体学 | 33篇 |
力学 | 1047篇 |
综合类 | 80篇 |
数学 | 1585篇 |
物理学 | 1113篇 |
出版年
2024年 | 9篇 |
2023年 | 60篇 |
2022年 | 92篇 |
2021年 | 147篇 |
2020年 | 200篇 |
2019年 | 187篇 |
2018年 | 165篇 |
2017年 | 179篇 |
2016年 | 253篇 |
2015年 | 171篇 |
2014年 | 239篇 |
2013年 | 533篇 |
2012年 | 319篇 |
2011年 | 362篇 |
2010年 | 301篇 |
2009年 | 350篇 |
2008年 | 336篇 |
2007年 | 377篇 |
2006年 | 333篇 |
2005年 | 367篇 |
2004年 | 361篇 |
2003年 | 267篇 |
2002年 | 271篇 |
2001年 | 224篇 |
2000年 | 195篇 |
1999年 | 180篇 |
1998年 | 141篇 |
1997年 | 109篇 |
1996年 | 89篇 |
1995年 | 101篇 |
1994年 | 84篇 |
1993年 | 76篇 |
1992年 | 62篇 |
1991年 | 41篇 |
1990年 | 34篇 |
1989年 | 24篇 |
1988年 | 30篇 |
1987年 | 27篇 |
1986年 | 18篇 |
1985年 | 19篇 |
1984年 | 20篇 |
1983年 | 15篇 |
1982年 | 13篇 |
1981年 | 12篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1973年 | 4篇 |
排序方式: 共有7440条查询结果,搜索用时 78 毫秒
101.
The chromatographic separation properties of long, thin adsorption tubes enable substance-specific quantitative enrichment and reduction to be achieved when sampling and thermal desorption are carried out in the same flow direction. The specific retention volumes, and also the breakthrough and peak end volumes, of 69 compounds in the boiling range between-164 and 126°C and of a relative molecular mass between 16 and 119 were determined at temperatures between 30 and 130°C: normal alkanes, isoalkanes, cycloalkanes, alkenes, polyenes, alkynes, aromatics, ethers, alcohols, aldehydes, ketones, carboxylic acids and their esters, nitroalkanes, O-heterocycles, S-heterocycles, chloroalkanes, water, nitrogen monoxide, nitrogen dioxide, carbon dioxide, and sulfur dioxide. The correlation between adsorbent temperature and specific retention volume of these components, presented in the form of diagrams, permits the required quantity of adsorbent to be determined for a given sample volume. Contrary to literature sources, even extremely volatile compounds such as propane, propene, methanol, formaldehyde, formic acid, and chloromethane can be quantitatively retained on Tenax provided the operating conditions are appropriately selected. 相似文献
102.
103.
A kind of inorganic‐organic hybrid 18‐molybdodiphosphate nanoparticles ([(C4H9)4N]6P2Mo18Q62·4H2O) was firstly used as a bulk‐modifier to fabricate a three‐dimensional chemically modified carbon paste electrode (CPE) by direct mixing. The electrochemical behavior of the solid nanoparticles dispersed in the CPE in acidic aqueous solution was characterized by cyclic and square‐wave voltammetry. The hybrid 18‐molybdodiphosphate nanoparticles bulk‐modified CPE (MNP‐CPE) displayed a high electrocatalytic activity towards the reduction of nitrite, bromate and hydrogen peroxide. The remarkable advantages of the MNP‐CPE over the traditional polyoxometalates‐modified electrodes are their excellent reproducibility of surface‐renewal and high stability owing to the insolubility of the hybrid 18‐molybdodiphosphate nanoparticles. 相似文献
104.
Shigeru Matsuo Tomoyoshi Murakami Ryuichi Takasawa 《Journal of polymer science. Part A, Polymer chemistry》1993,31(13):3439-3446
Crystalline poly(arylene ether nitrile) could be prepared by the polycondensation of 2,6-dihalobenzonitrile with resorcinol at 200°C in N-methylpyrrolidone in the presence of sodium carbonate. A reaction temperature of at least 200°C was necessary to attain high molecular weight polymer. Spectral data indicated that the polymer had the structure of a poly(meta-phenylene ether) with pendent nitrile groups on every other phenylene unit. Despite this structure, the crystallinity and the crystallization rate of the polymer were greater than those of the corresponding polymer with a para-linked structure. The glass transition temperature and the melting temperature of the polymer were almost the same as those of poly(etheretherketone) (PEEKTM). A series of other new poly(arylene ether nitriles) were also examined. The polymers derived from 4,4′-biphenol, dihydroxytetra-phenylmethane, dihydroxydiphenylsulfone, and 1,5-isoquinolinediol had high glass transition temperatures. The poly(arylene ether nitriles) exhibited excellent tensile strength compared with the corresponding ketone- or sulfone-containing polymers. Comparing the three different kinds of polymers containing the same bisphenol units, the order of glass transition temperature was found to be sulfone- > nitrile- > ketone-containing polymers, while the order of tensile strength was nitrile- > ketone- > sulfone-containing polymers. The excellent mechanical properties are attributable to dipole-dipole interactions of nitrile groups. © 1993 John Wiley & Sons, Inc. 相似文献
105.
Crespi M. S. Silva A. R. Ribeiro C. A. Oliveira S. C. Santiago-Silva M. R. 《Journal of Thermal Analysis and Calorimetry》2003,72(3):1049-1056
The thermal behavior and non-isothermal kinetics of thermal decomposition of three different kinds of composting of the USR
like: stack with drilled PVC tubes (ST), revolved stack (SR) and stack with material of structure (SM), from the usine of
composing of Araraquara city, Săo Paulo state, Brazil, within a period of 132 days of composting were studied. Results from
TG, DTG and DSC curves obtained on inert atmosphere indicated that the cellulosic fraction present, despite the slow degradation
during the composting process, is thermally less stable than other substances originated from that process. Due to that behavior,
the cellulosic fraction decomposition could be kinetically evaluated through non-isothermal methods of analysis. The values
obtained were: average activation energy, Ea=248, 257 and 259 kJ mol-1 and pre- exponential factor, logA=21.4, 22.5, 22.7 min-1, to the ST, SR and SM, respectively. From Ea and logA values and DSC curves, Málek procedure could be applied, suggesting that the SB (Šesták-Berggren) kinetic model is the appropriated
one to the first thermal decomposition step.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
106.
It is well known that porphyrin derivatives play a key role in the primary process of photo-synthesis[1], in which porphyrins directly absorb the sunlight or indirectly acquire excitation en-ergy from light-harvesting antenna system to reach their excited state, and then donate electrons to quinone acceptors to yield a series of charge-separated species. In general, only first singlet ex-cited state of porphyrins is involved in energy transfer process[2]. However, highly excited state (S2 stat… 相似文献
107.
108.
Organoboranes, readily available via the hydroboration of unsaturated organic compounds, exhibit a remarkable versatility
in their reactions. The boron atom in these organoboranes can be readily converted into a wide variety of organic groups under
very mild conditions, providing simple versatile syntheses of organic compounds. Exploration of these substitution reactions
reveal that, with rare exceptions, the organoboranes transfer the alkyl group to other elements of synthetic interest with
complete retention of stereochemistry. Recently we have discovered a method of synthesizing essentially optically pure organoborane
intermediates. These optically active alkyl groups attached to boron can also be transferred with complete retention of optical
activity. Consequently, it is now possible to achieve by a rational synthesis the preparation of almost any optically active
compound with a chiral center, either R- or S-, in essentially 100% enantiomeric excess. 相似文献
109.
A. N. Ageev A. A. Aratskova L. D. Belyakova T. N. Gvozdovich A. V. Kiselev Ya. I. Yashin J. Kàlal F. Švec 《Chromatographia》1983,17(10):545-548
Summary The retention of benzene derivatives with nonpolar and polar substituents on a porous methacrylate copolymer containing epoxy groups using both nonpolar and polar eluents was investigated. When n-hexane is used as the eluent, the retention of n-alkylbenzenes and polymethylbenzenes is weaker than that of benzene. In the case of benzene derivatives containing polar functional groups their capacity ratios (k) on this porous polymer increases approximately linearly with the increase of the adsorbate molecules dipole moment. The retention characteristics of the methacrylate copolymer were compared with that of a styrene-divinylbenzene copolymer and silica gels with a hydroxylated surface and with a surface modified by chemically bonded alkylsilyl groups. 相似文献
110.
SynthesesandStudiesofPEG┐b┐PNIPABlockPolymersCAOWei-xiao**andZHANGTao(ColegeofChemistryandMolecularEnginering,PekingUniversit... 相似文献