全文获取类型
收费全文 | 3783篇 |
免费 | 652篇 |
国内免费 | 551篇 |
专业分类
化学 | 2955篇 |
晶体学 | 38篇 |
力学 | 185篇 |
综合类 | 42篇 |
数学 | 450篇 |
物理学 | 1316篇 |
出版年
2024年 | 12篇 |
2023年 | 67篇 |
2022年 | 135篇 |
2021年 | 133篇 |
2020年 | 150篇 |
2019年 | 150篇 |
2018年 | 142篇 |
2017年 | 116篇 |
2016年 | 154篇 |
2015年 | 172篇 |
2014年 | 187篇 |
2013年 | 251篇 |
2012年 | 306篇 |
2011年 | 340篇 |
2010年 | 227篇 |
2009年 | 204篇 |
2008年 | 283篇 |
2007年 | 191篇 |
2006年 | 191篇 |
2005年 | 186篇 |
2004年 | 165篇 |
2003年 | 154篇 |
2002年 | 188篇 |
2001年 | 154篇 |
2000年 | 109篇 |
1999年 | 89篇 |
1998年 | 73篇 |
1997年 | 44篇 |
1996年 | 91篇 |
1995年 | 66篇 |
1994年 | 46篇 |
1993年 | 29篇 |
1992年 | 30篇 |
1991年 | 27篇 |
1990年 | 28篇 |
1989年 | 16篇 |
1988年 | 14篇 |
1987年 | 15篇 |
1986年 | 9篇 |
1985年 | 15篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1974年 | 1篇 |
1957年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有4986条查询结果,搜索用时 0 毫秒
91.
The formation of cyclic sulfates (4) from syn,syn-3-[N-(benzyloxycarbonyl)amino] 1,2-diols provides a common intermediate to access other diastereomers via two inversion procedures. Thermolysis of the cyclic sulfates in acetonitrile normally leads to inversion of the distal hydroxyl group to form a 1,3-oxazin-2-one (6). Catalytic hydrogenation of the cyclic sulfates under basic conditions (NEt(3)) results in inversion at the proximal hydroxyl group to form a 1,3-oxazolidin-2-one (5). 相似文献
92.
FuShengWANG BingKuiSONG XiaoLiHAN BaoGuiZHANG 《中国化学快报》2004,15(12):1487-1490
Black lead, Ti-Ru and Ti-PbO2 were used as anode and stainless steel was used as cathode The electrolytic process of producing hypophosphorous acid with four-compartment electrodialytic cell was studied. The comparison of some factors, such as anodic voltage, product concentration and current efficiency, of black lead, Ti-Ru, and Ti-PbO2 electrodes was conducted. As a result, theTi-PbO2 electrode is the optimal anode material used, it can be in electrolytic process for producing hypophosphorous acid. 相似文献
93.
有害杂草的微量元素分析 总被引:18,自引:0,他引:18
紫茎泽兰和水葫芦均属有害杂草,大规模防治的有效方法还未找到,本文分析研究了采自昆明郊区的紫茎泽兰和滇池水域的水葫芦中微量元素的含量,为进一步开发利用这两种杂草提供科学依据。 相似文献
94.
95.
TEOS-MTES基SiO2溶胶微结构的SAXS研究 总被引:3,自引:0,他引:3
正硅酸乙酯(TEOS)为前驱体,在碱性条件下制备含有无定形SiO2颗粒的溶胶,以甲基三乙氧基硅烷(MTES)在酸性条件下获得聚甲基硅氧链,二者混合后应用同步辐射X射线进行混合溶胶的SAXS散射强度测定,计算了溶胶的平均回转半径、平均粒径、两相界面层厚度、散射体体积分数、两相间比表面积等参数,辅以光子相关光谱法(PCS)和透射电子显微镜(TEM)观测溶胶粒度,证实SiO2颗粒被MTES混合物连接成族团.实验发现所测混合溶胶样品均表现出对Porod定理的负偏离,说明溶胶中颗粒与溶剂之间存在很明显的两相间界面层. 相似文献
96.
97.
98.
It was believed that electroosmotic mobility μeo is inversely proportional to the square root of the ionic strength L But the linear relationship for regression analysis was expressed differently in different papers. The paper studied the linear expression of the mathematical relationship between μeo and c (background buffer concentration) by mathematical transform and real experimental data.μeo values of fused silica capillary were determined in four buffer systems. Their experimental conditions were controlled carefully for decreasing temperature difference AT and pH difference ApH in 50 μm ID capillary, in which no double layer overlap existed. The linear relationship between the reciprocal of electroosmotic mobility and the square root of concentration (or ionic strength) was derived by mathematical method. The regression analysis of experimental data was shown to well correspond to the relationship. The constants in regression equation could be well defined and the calculated results were acceptable. 相似文献
99.
Eight ionic organotin compounds [R2SnCl2(2-quin)]−(HNEt3)+ have been synthesized by reactions of 2-quinH with R2SnCl2 (R = PhCH21, 2-Cl-C6H4CH22, 4-Cl-C6H4CH23, 2-F-C6H4CH24, 4-F-C6H4CH25, 4-CN-C6H4CH26, Ph 7, 2,4-Cl2-C6H3CH28) in the presence of organic base NEt3, and their structures have been characterized by elemental analysis, IR and multinuclear NMR (1H, 13C, 119Sn) spectroscopies. The structure of [(2,4-Cl2-C6H3CH2)2SnCl2(2-quin)]−(NEt3)+ (8) has been determined by X-ray diffraction study. Studies show that compound 8 has a monomeric structure with the central tin atom six-coordinate in a distorted octahedral configuration and the nitrogen atoms of the 2-quin ligands are coordinating to the tin atom in all the eight compounds. 相似文献
100.
Applications of poly(dimethylsiloxane) (PDMS)-based microfluidic systems are more popular nowadays. Previous fabrication methods of the masters for PDMS microchannels require complicated steps and/or special device. In this paper, we demonstrated that the toner printed on the transparency film with the office laser printer (1200 dpi) can be used as the positive relief of the masters. The transparency film was printed in two steps in order to obtain the same printing quality for the crossed lines. With the laser-printed master, the depth of the fabricated PDMS microchannels was ca. 10 microm and the smallest width was ca. 60 microm. Surface characteristics of the PDMS/PDMS microchannels were performed with SEM. Their electrokinetic properties were investigated by the aids of the measurement of electroosmotic flow (EOF) and the Ohm's curve. Using the PDMS/PDMS microchip CE systems, electroactive biological molecules and non-electroactive inorganic ions were well separated, respectively. This simple approach could make it easy to carry out the studies of PDMS microfluidic systems in more general labs without special devices. 相似文献