全文获取类型
收费全文 | 32099篇 |
免费 | 5579篇 |
国内免费 | 5876篇 |
专业分类
化学 | 24398篇 |
晶体学 | 682篇 |
力学 | 1685篇 |
综合类 | 590篇 |
数学 | 3757篇 |
物理学 | 12442篇 |
出版年
2024年 | 106篇 |
2023年 | 535篇 |
2022年 | 1120篇 |
2021年 | 1187篇 |
2020年 | 1279篇 |
2019年 | 1312篇 |
2018年 | 1126篇 |
2017年 | 1252篇 |
2016年 | 1463篇 |
2015年 | 1605篇 |
2014年 | 1983篇 |
2013年 | 2485篇 |
2012年 | 2932篇 |
2011年 | 3009篇 |
2010年 | 2398篇 |
2009年 | 2277篇 |
2008年 | 2488篇 |
2007年 | 2304篇 |
2006年 | 2163篇 |
2005年 | 1765篇 |
2004年 | 1370篇 |
2003年 | 1001篇 |
2002年 | 1004篇 |
2001年 | 896篇 |
2000年 | 873篇 |
1999年 | 627篇 |
1998年 | 416篇 |
1997年 | 364篇 |
1996年 | 301篇 |
1995年 | 304篇 |
1994年 | 289篇 |
1993年 | 223篇 |
1992年 | 189篇 |
1991年 | 158篇 |
1990年 | 164篇 |
1989年 | 110篇 |
1988年 | 99篇 |
1987年 | 66篇 |
1986年 | 83篇 |
1985年 | 57篇 |
1984年 | 27篇 |
1983年 | 31篇 |
1982年 | 26篇 |
1981年 | 15篇 |
1980年 | 17篇 |
1979年 | 14篇 |
1978年 | 9篇 |
1971年 | 6篇 |
1957年 | 4篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Binhang Yan Pengcheng Xu Xuan Li Cliff Yi Guo Yong Jin Yi Cheng 《Plasma Chemistry and Plasma Processing》2012,32(6):1203-1214
Liquid hydrocarbons including n-hexane, cyclohexane and toluene are pyrolyzed in H2/Ar plasma to investigate the effects of feedstock properties and key operating conditions (e.g., the feedstock specific input power and residence time) on the reaction performance. The experiments verify that the non-aromatic hydrocarbons show better chemical reactivity than partially aromatic substances. Meanwhile, the straight-chain alkanes and cycloalkanes have better yields of ethylene during the pyrolysis. The results also demonstrate that the pyrolysis reactions are almost completed within the first 0.8?ms in Ar/H2 plasma independent of the feed substances (i.e., liquid hydrocarbons), where the increased feedstock specific input power enhances the reactant conversions and correspondingly raises the yields of acetylene. At a feedstock specific input power of 4.7?×?104?kJ/kg, the n-hexane conversion is over 90?% and the yield of acetylene reaches 70?%. In addition, when using n-hexane as the feedstock, very little coke is formed during the course of reaction. Comprehensive comparisons of the current experiments with the data reported in the literature are made to point out the key influencing factors, i.e., the effective mass ratio of C/H (R C/H) in the gaseous phase and the quench temperature. Both two factors would need to be enhanced in order to get a better performance. Finally, the improvements on the specific energy requirement of this process are discussed. 相似文献
992.
The novel title compound 4-chlorobenzaldehyde (2-trifluoromethyl-5,6,7,8- tetrahydrobenzo[4,5]thieno[2,3-d]pyrimidin-4-yl)hydrazone monohydrate (C18H14ClF3N4SH2O, Mr = 428.86) has been synthesized by a condensation reaction of 4-chlorobenzaldehyde with (2-trifluoromethyl-5,6,7,8-tetrahydrobenzo[4,5]thieno[2,3-d]pyrimidin-4-yl)hydrazine, and its crystal structure was determined by single-crystal X-ray diffraction. The crystal belongs to monoclinic, space group P21/c with a = 7.4252(7), b = 26.344(2), c = 10.3095(9) , = 109.407(2)°, V = 1902.0(3) 3 , Z = 4, Dc = 1.498 g/cm3 , μ = 0.356 mm-1 , F(000) = 880, the final R = 0.0564 and wR = 0.1681 for 2343 observed reflections with I > 2 (I). X-ray diffraction analysis reveals that the title hydrazone molecule is nearly planar except for the cyclohexene and trifluoromethyl moieties. In the crystal packing, the molecules form stacks by a three-dimensional framework, which results from intermolecular N(3)-H(3)···O(1), O(1)-H(1B)···N(2), O(1)- H(1B)···N(4) and O(1)-H(1A)···F(1) hydrogen bonds via water molecules together with π-π stacking interactions. Molecular geometry of the title compound in the ground state optimized by B3LYP functional with 6-311G** basis sets indicates that the calculations are in agreement with the experimental data. The preliminary bioassay suggested that the title compound exhibits relatively good fungicidal activity against Fusarium oxysporium f.sp.vasinfectum and Dothiorella gregaria. 相似文献
993.
994.
Chia-Wen Lu Yan Cao Hui Li Cathleen Webb Wei-Ping Pan 《Journal of Thermal Analysis and Calorimetry》2014,116(3):1241-1248
Titanium dioxide (TiO2), especially in its anatase form, is an effective photocatalyst under ultraviolet (UV) light. The particle size of TiO2 is a critical factor to determine its photoactivity based on its quantum effectiveness under light irradiations. Thus, nanocrystalline TiO2 has been widely accepted to significantly enhance this effect. The sol–gel method is generally used to synthesize the anatase form of nanocrystalline TiO2. In this study, we expanded the synthesis method of TiO2 to high pressures under direct heating (hydrothermal method) and indirect heating (microwave-assisted method). It was found that pH value is one of the major factors to control nano-sizes of TiO2 particles, and the neutral condition in all methods is preferable for controlling the sizes of the prepared TiO2 particles. The microwave-assisted method further improves quality of synthesized nano-size TiO2 below 10 nm. These results have been confirmed by both the direct size measurement using TEM images and indirect determination using XRD peaks. The collected samples are further analyzed using UV–Vis spectroscopy to identify the particle size-dependent photoreactivity and to confirm the effectiveness of microwave-assisting under neutral conditions. DSC is also a powerful tool to identify the crystalline transition of TiO2. 相似文献
995.
Hang Ma Yang He Ruo-Feng Huang Xiao-Hui Zhang Jing Pan Jia-Qiang Li Chao He Xue-Ge Ling Xuan-Lun Wang Yan Xiong 《中国化学快报》2014,25(10):1327-1330
Under neat conditions,an efficient method for synthesis of imidoesters has been developed using cyanatobenzenes and dicarbonyl compounds.Nucleophilic addition spontaneously occurred between the two kinds of materials at room temperature with yields of up to 90%.A mechanism directed towards to the imidoester formation has been proposed. 相似文献
996.
997.
998.
999.
1000.