全文获取类型
收费全文 | 99157篇 |
免费 | 12566篇 |
国内免费 | 7490篇 |
专业分类
化学 | 59546篇 |
晶体学 | 904篇 |
力学 | 5994篇 |
综合类 | 322篇 |
数学 | 17009篇 |
物理学 | 35438篇 |
出版年
2024年 | 170篇 |
2023年 | 1539篇 |
2022年 | 1737篇 |
2021年 | 2533篇 |
2020年 | 3079篇 |
2019年 | 2850篇 |
2018年 | 3593篇 |
2017年 | 3644篇 |
2016年 | 4117篇 |
2015年 | 3834篇 |
2014年 | 4391篇 |
2013年 | 5755篇 |
2012年 | 9634篇 |
2011年 | 8953篇 |
2010年 | 6235篇 |
2009年 | 5756篇 |
2008年 | 5107篇 |
2007年 | 4675篇 |
2006年 | 4317篇 |
2005年 | 7448篇 |
2004年 | 6248篇 |
2003年 | 4141篇 |
2002年 | 2204篇 |
2001年 | 1744篇 |
2000年 | 1440篇 |
1999年 | 1699篇 |
1998年 | 1408篇 |
1997年 | 1356篇 |
1996年 | 1330篇 |
1995年 | 1089篇 |
1994年 | 961篇 |
1993年 | 782篇 |
1992年 | 842篇 |
1991年 | 731篇 |
1990年 | 635篇 |
1989年 | 487篇 |
1988年 | 430篇 |
1987年 | 325篇 |
1986年 | 290篇 |
1985年 | 240篇 |
1984年 | 171篇 |
1983年 | 127篇 |
1982年 | 97篇 |
1981年 | 85篇 |
1980年 | 77篇 |
1979年 | 61篇 |
1977年 | 58篇 |
1976年 | 109篇 |
1975年 | 60篇 |
1973年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
991.
The ethanol effect on the Trichoderma reesei cellulases was studied to quantify and clarify this inhibition type. To determine inhibition parameters of crude cellulase
and purified exoglucanase Cel7A, integrated Michaelis-Menten equations were used assuming the presence of two inhibitors:
cellobiose as the reaction product and ethanol as a possible bioproduct of cellulose fermentation.
It was found that hydrolysis of cellulose by crude enzyme follows a model that considers noncompetitive inhibition by ethanol,
whereas Cel7A is very slightly competitively inhibited. Crude cellulase is much more inhibited (K
iul=K
icl=151.9 mM) than exoglucanase Cel7A (K
icl=1.6 × 1015 mM). Also, calculated inhibition constants showed that cellobiose inhibition is more potent than ethanol inhibition both for
the crude enzyme as well as exoglucanase Cel7A. 相似文献
992.
Sébastien Fiorucci Jer?me Golebiowski Daniel Cabrol-Bass Serge Antonczak 《Chemphyschem》2004,5(11):1726-1733
Flavonoids are naturally occurring phenol derivatives present in substantial amounts in a large variety of plants, fruits and vegetables daily eaten by humans. Most of these compounds exhibit several interesting biological activities, such as antiradical and antioxidant actions. Indeed, by complexation with specific enzymes, flavonoids are notably liable to metabolize molecular dioxygen. On the basis of experimental results describing oxygenolysis of the flavonoid quercetin, activated by the enzyme quercetin 2,3-dioxygenase (2,3-QD),ur attention has focused on the role of metal center in the activation of the substrate quercetin. Thus, in the present study, by means of DFT calculations at the B3LYP/ 6-31(+)G* level on model molecular systems, we describe different mechanisms for dioxygen metabolization by quercetin. Stationary points are described, and energetic and structural analyses along the reaction paths are reported. Our calculations show that the copper cation must act as an oxidant towards the substrate and that the reaction proceeds through a 1,3-cycloaddition. 相似文献
993.
The hydrosilylation reaction of octa(hydrido)silsesquioxane with ω‐halo‐1‐alkenes and other unsaturated substrates allows attachment of 8 long‐chain functionalized alkyls on the cubic Si8O12 skeleton. Pt/C and H2PtCl6 have been adopted as the catalysts, the yields being 74–98% for compounds 2–9 . For terminal alkenes, the hydrosilylation follows the anti‐Markonikov's rule. The morphological state of 2–9 ranges from viscous liquid to crystalline materials. The pyrolysis results indicate that 7 , a material of hard spherical core and soft flexible shell, may likely act as nanometer‐size ball bearings up to more than 400 °C. The X‐ray structure of 9 reveals that the molecule is required to possess a center of symmetry crystallographically. The linear arms on 9 , except for two of them, are virtually all‐trans in conformation, not counting the ω‐C‐Cl bond. 相似文献
994.
995.
Trace analysis of rapamycin in human blood by micellar electrokinetic chromatography 总被引:1,自引:0,他引:1
A capillary electrophoretic method with UV detection at 278 nm has been developed for analysis of the immunosuppressant rapamycin (sirolimus) in human blood at low microgram per liter levels. Separation has been achieved in an acidic carrier electrolyte containing sodium dodecylsulfate and 30% (v/v) acetonitrile. For sample clean-up and preconcentration, an off-line solid-phase extraction step using a silica-based reversed-phase material and an on-capillary focussing technique were employed. The latter allows the injection of increased sample volumes without excessive band broadening. Although this new method is less sensitive than existing liquid chromatographic procedures combined with mass spectrometry, it is fully suited to routine analysis of rapamycin in blood from patients treated with this drug. Last but not least the low costs make it an attractive alternative to established methods. 相似文献
996.
固相萃取-气相色谱-质谱分析肉样中盐酸克伦特罗的残留量 总被引:25,自引:12,他引:25
建立了固相萃取 气相色谱 质谱联机分析肉样中盐酸克伦特罗残留量的方法。对盐酸克伦特罗在C1 8和离子交换固相萃取柱上的保留行为进行了研究 ,发现不同浓度盐酸克伦特罗的固相萃取回收率在 75 %~ 95 %之间 ;肉样中的加标回收率在 70 %~ 83 % ;相对标准偏差在 4 .95 %~ 1 3 .4 % ;最低检出限为 1 μg kg。盐酸克伦特罗的硅烷化产物 ,采用选择离子的模式进行检测 ( 86、2 4 3、2 62、2 77) ,衍生物的峰面积与样品浓度在0 .0 0 2 5 2~ 2 .0 2mg L范围内呈良好的线性关系 ,线性回归系数大于 0 .9999。 相似文献
997.
998.
999.
Hae‐Jeong Lee Christopher L. Soles Da‐Wei Liu Barry J. Bauer Wen‐Li Wu 《Journal of Polymer Science.Polymer Physics》2002,40(19):2170-2177
X‐ray reflectivity has been used to determine the mass uptake of probe molecules in porous thin films supported on thick silicon wafers. The adsorption occurs by capillary condensation when the films are exposed to probe vapor at controlled partial vapor pressures. The probe solvent partial pressure was varied by mixing saturated air and dry air at constant temperature or by changing sample temperature at a constant vapor concentration. Pore size distribution in the films can be calculated from the probe uptake with typical porosimetric approaches such as the application of the Kelvin equation to convert partial pressure into pore size. For illustration, the pore size distribution of three different nanoporous thin films, the primary candidate of ultra‐low‐k interlevel dielectrics in the next generation of integrated circuit chips, was determined with this technique. These samples represent different generations of low‐k dielectrics developed by industry. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2170–2177, 2002 相似文献
1000.