全文获取类型
收费全文 | 68575篇 |
免费 | 11801篇 |
国内免费 | 9113篇 |
专业分类
化学 | 49290篇 |
晶体学 | 898篇 |
力学 | 4338篇 |
综合类 | 631篇 |
数学 | 8155篇 |
物理学 | 26177篇 |
出版年
2024年 | 129篇 |
2023年 | 1267篇 |
2022年 | 1554篇 |
2021年 | 2309篇 |
2020年 | 2675篇 |
2019年 | 2618篇 |
2018年 | 2269篇 |
2017年 | 2231篇 |
2016年 | 3153篇 |
2015年 | 3279篇 |
2014年 | 4079篇 |
2013年 | 5197篇 |
2012年 | 6366篇 |
2011年 | 6493篇 |
2010年 | 4610篇 |
2009年 | 4571篇 |
2008年 | 4923篇 |
2007年 | 4225篇 |
2006年 | 3954篇 |
2005年 | 3228篇 |
2004年 | 2569篇 |
2003年 | 2062篇 |
2002年 | 1987篇 |
2001年 | 1629篇 |
2000年 | 1414篇 |
1999年 | 1410篇 |
1998年 | 1108篇 |
1997年 | 1019篇 |
1996年 | 1043篇 |
1995年 | 871篇 |
1994年 | 811篇 |
1993年 | 668篇 |
1992年 | 608篇 |
1991年 | 492篇 |
1990年 | 457篇 |
1989年 | 320篇 |
1988年 | 292篇 |
1987年 | 258篇 |
1986年 | 208篇 |
1985年 | 200篇 |
1984年 | 158篇 |
1983年 | 144篇 |
1982年 | 91篇 |
1981年 | 71篇 |
1980年 | 58篇 |
1979年 | 51篇 |
1978年 | 33篇 |
1977年 | 31篇 |
1976年 | 32篇 |
1975年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
A new dihydroberberine alkaloid, 7,8‐dihydro‐8‐methoxyberberine ( 1 ), along with six known compounds including two dihydroberberine alkaloids, 7,8‐dihydro‐8‐hydroxyberberine ( 2 ) and oxyberberine ( 3 ) and four protoberberine alkaloids, berberine ( 4 ), palmatine ( 5 ), jatrorrhizine ( 6 ) and columbamine ( 7 ), were isolated from the stems of Mahonia japonica. These compounds were characterized and identified by physical and spectral evidence. 相似文献
993.
An end-channel amperometric detector with a guide tube for working electrode was designed and integrated on a home-made glass microchip. The guide tube was directly patterned and fabricated at the end of the detection reservoir, which made the fixation and alignment of working electrode relatively easy. The fabrication was carried out in a two-step etching process. A 30 μm carbon fiber microdisk electrode and Pt cathode were also integrated onto the amperometric detector. The baseline separation of dopamine (DA), catechol (CA) and epinephrine (EP) was achieved within 80 s. Relative standard deviations of not more than 5.2% were obtained for both peak currents and migration times of DA and CA (n=5). Using standard adding method, DA in tLrine and plasma samples was detected. The recoveries were in the range of 83%—103%. 相似文献
994.
Two new dicyanamide coordination polymers, {Mn(dmpz)[N(CN)2]2}2 (1) and {Cu(dmpz)[N(CN)2]2}2 (2)(dmpz=3,5-dimethylpyrazole), were synthesized and characterized by single crystal X-ray diffraction analysis and IR spectroscopy. In 1 and 2 the metal ions have two different coordination modes, where one is coordinated to four dicyanamide anions and two monodentate dmpz molecules to form a slightly distorted octahedral geometry, while the other adopts octahedral geometry, surrounded by four nitrile N atoms and two amide N atoms of the dicyanamide anions. Both complexes contain two alternating chains that are parallel to each other. 相似文献
995.
Alkyl ketene dimer (AKD) sizing of paper involves the redistribution of the wax over the fibre surface upon heating. The two
major mechanisms widely studied so far are the spreading of an autophobic precursor of molten AKD and AKD vapour transport
and re-deposition on the fibre surface. All previous work assumed that the transport of AKD vapour could be expressed by the
change of water contact angle with substrates that were exposed to the vapour. Information regarding the chemical composition
of the vapour phase above the AKD wax has not been found in the literature. In this work, a simple method for analysing the
chemical composition of the vapour is established. Our preliminary results indicated that the chemical composition of AKD
vapour in the temperature range of 75–80 °C is dominantly fatty acids. This suggests that the sizing effect by actual AKD
molecules via the vapour deposition mechanism is likely to be insignificant in this temperature range. This also implies that
fatty acids play a positive role in AKD sizing. The chemical stability of AKD in this temperature range is also studied. 相似文献
996.
997.
Established high-throughput proteomics methods provide limited information on the stereostructures of proteins. Traditional technologies for protein structure determination typically require laborious steps and cannot be performed in a high-throughput fashion. Here, we report a new medium throughput method by combining mobility capillary electrophoresis (MCE) and native mass spectrometry (MS) for the 3-dimensional (3D) shape determination of globular proteins in the liquid phase, which provides both the geometric structure and molecular mass information of proteins. A theory was established to correlate the ion hydrodynamic radius and charge state distribution in the native mass spectrum with protein geometrical parameters, through which a low-resolution structure (shape) of the protein could be determined. Our test data of 11 different globular proteins showed that this approach allows us to determine the shapes of individual proteins, protein complexes and proteins in a mixture, and to monitor protein conformational changes. Besides providing complementary protein structure information and having mixture analysis capability, this MCE and native MS based method is fast in speed and low in sample consumption, making it potentially applicable in top–down proteomics and structural biology for intact globular protein or protein complex analysis.Using native mass spectrometry and mobility capillary electrophoresis, the ellipsoid dimensions of globular proteins or protein complexes could be measured efficiently. 相似文献
998.
在肽库中进行小肽筛选的初步研究 总被引:1,自引:0,他引:1
以合成六肽DGGSAA为模型对在噬菌体肽库中进行小肽筛选做了初步研究。结果表明,含有可形成氢键和离子键残基的小肽能够在噬菌体肽库中进行筛选。并用明显提高了筛选的专一性。 相似文献
999.
The substituent effects on O-H and O-CH(3) bond dissociation energies for a series of 18 para-substituted phenols (p-XC(6)H(4)OH) and 11 para-substituted anisoles have been studied using the density functional method in order to understand the origin of these effects. The calculated substituent effects agree well with experimental measurements for phenols but are substantially larger than the reported values for anisoles. Both ground-state effect and radical effect contribute significantly to the overall substituent effect. An electron-donating group causes a destabilization in phenols or anisoles (ground-state effect) but a stabilization in the phenoxy radicals (radical effect), resulting in reduced O-R bond dissociation energy. An electron-withdrawing group has the opposite effect. In most cases, the radical effect is more important than the ground-state effect. There is a good correlation between the calculated radical effects and calculated variations in charge and spin density on the phenoxy oxygen. This supports the concept that both polar and spin delocalization effects influence the stability of the phenoxy radical. While almost every para-substituent causes a stabilization of the phenoxy radical by spin delocalization, electron-donating groups stabilize and electron-withdrawing groups destabilize the phenoxy radical by the polar effect. 相似文献
1000.
The synthesis of mixed alcohols (C1-C5) from syngas has been studied at 6. 0 MPa over modified Co/CuLaZr catalysts. The molybdenum addition enhanced greatly the activity and the selectivity to alcohols. The improvement of hydrogenation capacity of the system via a reversible spillover effect of hydrogen could explain this effect, together with a great capacity of CO insertion, illustrated by the effect of C2H4 addition as a probe to syngas. Under our experimental conditions, a mixed alcohol production of 147.1g/kg. cat/h containing a proportion of 33. 1% of higher alcohols (C2 OH) was obtained with a selectivity to ROH of 52. 8%. 相似文献