全文获取类型
收费全文 | 15805篇 |
免费 | 1335篇 |
国内免费 | 649篇 |
专业分类
化学 | 15489篇 |
晶体学 | 324篇 |
力学 | 24篇 |
综合类 | 15篇 |
数学 | 64篇 |
物理学 | 1873篇 |
出版年
2024年 | 16篇 |
2023年 | 165篇 |
2022年 | 547篇 |
2021年 | 479篇 |
2020年 | 478篇 |
2019年 | 358篇 |
2018年 | 299篇 |
2017年 | 273篇 |
2016年 | 551篇 |
2015年 | 551篇 |
2014年 | 591篇 |
2013年 | 1156篇 |
2012年 | 715篇 |
2011年 | 676篇 |
2010年 | 753篇 |
2009年 | 761篇 |
2008年 | 797篇 |
2007年 | 814篇 |
2006年 | 782篇 |
2005年 | 719篇 |
2004年 | 759篇 |
2003年 | 692篇 |
2002年 | 1421篇 |
2001年 | 393篇 |
2000年 | 325篇 |
1999年 | 323篇 |
1998年 | 298篇 |
1997年 | 214篇 |
1996年 | 419篇 |
1995年 | 396篇 |
1994年 | 143篇 |
1993年 | 117篇 |
1992年 | 132篇 |
1991年 | 82篇 |
1990年 | 56篇 |
1989年 | 72篇 |
1988年 | 69篇 |
1987年 | 41篇 |
1986年 | 41篇 |
1985年 | 56篇 |
1984年 | 43篇 |
1983年 | 25篇 |
1982年 | 21篇 |
1981年 | 19篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 13篇 |
1975年 | 7篇 |
1971年 | 48篇 |
1970年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
The parameters that affect the shape of the band profiles of acido‐basic compounds under moderately overloaded conditions (sample size less than 500 nmol for a conventional column) in RPLC are discussed. Only analytes that have a single pKa are considered. In the buffer mobile phase used for their elution, their dissociation may, under certain conditions, cause a significant pH perturbation during the passage of the band. Two consecutive injections (3.3 and 10 μL) of each one of three sample solutions (0.5, 5, and 50 mM) of ten compounds were injected on five C18‐bonded packing materials, including the 5 μm Xterra‐C18 (121 Å), 5 μm Gemini‐C18 (110 Å), 5 μm Luna‐C18(2) (93 Å), 3.5 μm Extend‐C18 (80 Å), and 2.7 μm Halo‐C18 (90 Å). The mobile phase was an aqueous solution of methanol buffered at a constant WWpH of 6, with a phosphate buffer. The total concentration of the phosphate groups was constant at 50 mM. The methanol concentration was adjusted to keep all the retention factors between 1 and 10. The compounds injected were phenol, caffeine, 3‐phenyl 1‐propanol, 2‐phenyl butyric acid, amphetamine, aniline, benzylamine, p‐toluidine, procainamidium chloride, and propranololium chloride. Depending on the relative values of the analyte pKa and the buffer solution pH, these analytes elute as the neutral, the cationic, or the anionic species. The influence of structural parameters such as the charge, the size, and the hydrophobicity of the analytes on the shape of its overloaded band profile is discussed. Simple but general rules predict these shapes. An original adsorption model is proposed that accounts for the unusual peak shapes observed when the analyte is partially dissociated in the buffer solution during its elution. 相似文献
42.
Biological transformation of volatile organic compounds is one of the key factors that influence contaminant-plume evolution
and thus natural attenuation. In this study we investigate the effect of biological transformation on the transport of contaminants
in the aqueous and gaseous phases. The analysis includes the study of the effect of density-driven advection of contaminants
in the gaseous phase on multiphase and multispecies flow, fate and transport modeling in the subsurface. Trichloroethylene
(TCE) and its two byproducts, dichloroethylene and vinyl chloride, are analyzed as the target contaminants. Our results indicate
that density-driven advection of the gaseous phase, which is initiated by evaporation of TCE as a nonaqueous phase liquid,
increases the downward and also the lateral migration of TCE within the unsaturated zone. This process also influences the
location of high-concentration zones of the byproducts of TCE in the unsaturated and the saturated zones. Biotransformation
of TCE contributes to the reduction of dissolved TCE plume development as expected. The daughter byproducts, which are introduced
into the subsurface system, show distinct transport patterns as they are affected by their independent degradation kinetics
and density-driven advection. These observations, which are based on our simulation results for biotransformation and transport
of TCE and its byproducts, are useful in evaluating the natural attenuation processes, its potential health hazards and also
the evaluation of potential plume development at contaminated sites. 相似文献
43.
Titanium tetrachloride promoted reaction of silyl ketene acetals with epoxides, followed by acidic work-up, affords butanolides in moderate/good yields. With epihalohydrins the reaction is regioselective and occurs at the less substituted end of the epoxide; the γ-haloalkyl-γ-butanolides thus obtained can be further transformed into various products. 相似文献
44.
45.
Chung-Cherng Lin 《Journal of Physics and Chemistry of Solids》2004,65(5):913-921
The phase transition of a synthetic clinoenstatite in a diamond-anvil cell has been studied by using Raman spectroscopy at various pressures and room temperature. The phenomena observed in clinoenstatite have been compared with that observed in orthoenstatite. It is found that the pressure-induced phase transitions in the two enstatites are reversible, but with different transition pressures and transition behavior. An analysis of Raman spectra has revealed that the two enstatites have different high-pressure polymorphs. This result suggests that the space group of the high-pressure polymorph of orthoenstatite is not of C2/c, and that orthoenstatite and orthoferrosilite have different transition routes at room temperature and high pressure. The compressional behavior of the high-PC2/c enstatite is also discussed according to the pressure dependences of Raman frequencies. 相似文献
46.
Ivan Horvath Viera Luptakova 《Journal of inclusion phenomena and macrocyclic chemistry》1991,11(3):257-265
Benzothiazolium compounds exhibit pronounced antimicrobial activities and stimulation effects on plant growth. When applied to fields they can pollute soil colloids. In contact with the soil they can potentially interact with a clay fraction giving rise to clay organocomplexes. Model intercalation complexes were prepared using monoionic montmorillonite and a variety of water-soluble benzothia zolium salts. It was found that the adsorption into the silicate interlayer space proceeds via a cation exchange process. Substantial differences were observed in the extent of the reaction between non-substituted andN-substituted species. The sulphur atoms present in the benzothiazolium molecules repel the surface oxygen atom: this is considered to be the factor responsible for prevention of insertion of the guest cations perpendicular to the layers. 相似文献
47.
48.
The influence of humic substances on sorption and methylation processes for inorganic- and organotin species is presented. Four sediment samples from different locations of the Rivers Elbe, Mulde and Spittelwasser, Germany, with different organotin and humic contents were selected to extract the humic and fulvic acids. The various fractions—the original sediment, the humic acid, the fulvic acid and the residual sediment—were analysed for their organotin content. The individual buyltin species show quite different distribution patterns. Monobutyltin is found mostly associated with humic acids. Dibutyltin shows a nonunique behaviour. At low total organotin content, dibutyltin is found bonded to humic and fulvic acids, whereas at high organotin content dibutyltin is distributed more with the residual sediment. Most of the tributyltin remains in the sediment unextracted; only small quantities of it are in the fulvic acid fraction. Tetrabutyltin is only in the humic acid fraction when it binds to humic matter; it mostly remains in the sediment. General observations indicate that ionic butyltin species bind to fulvic acids whereas the non-polar tetrabutyltin is not found in the fulvic acid fractions in any of the samples. The appearance of monomethyl- and dimethyl-tin species in the humic and fulvic acid fractions after the alkaline extraction was surprising. There is a correlation between the humic content of the sample and the formation of methyltin species. Evidence is provided by experiments that humic substances act as methylation agents. 相似文献
49.
Summary. The photocycloaddition of aldehydes and α-ketoesters to 2,5-dimethyl-4-isobutyloxazole leads to bicyclic oxetanes with high
to moderate (exo) diastereoselectivity that can be easily ring-opened to give α-amino-β-hydroxyketones. 相似文献
50.
M. G. Bettoli M. Ravanelli L. Tositti O. Tubertini L. Guzzi W. Martinotti G. Queirazza M. Tamba 《Radiation Physics and Chemistry》1998,52(1-6):327-331
Decomposition by ionizing radiations of p-chlorophenol and tetrachloroethylene in synthetic water samples at about 2 mg Cl L−1, has been studied on laboratory-scale experiments. Bicarbonate/carbonate and nitrate ions, at two concentration levels (20 and 200 mg HCO3−1 and 1 and 50 mg NO3−L−) were added to synthetic samples in order to evaluate their influence on decomposition yield. At 5 kGy γ dose level, a quantitative degradation of p-chlorophenol is obtained whereas only qualitative consideration can be drawn on tetracholoroethylene. Comparative study with respect to degradation of p-cholophenol solutions (about 2 mg Cl L−1) by γ-rays and electron beam irradiation treatment at 0.5 kGy dose level, are in progress; preliminary results indicate that irradiation with γ-rays seems to be more efficient in terms of removal efficiency respect to electron beams source. 相似文献