全文获取类型
收费全文 | 27760篇 |
免费 | 4457篇 |
国内免费 | 3196篇 |
专业分类
化学 | 20004篇 |
晶体学 | 328篇 |
力学 | 1669篇 |
综合类 | 297篇 |
数学 | 3241篇 |
物理学 | 9874篇 |
出版年
2024年 | 42篇 |
2023年 | 544篇 |
2022年 | 654篇 |
2021年 | 820篇 |
2020年 | 1105篇 |
2019年 | 1028篇 |
2018年 | 858篇 |
2017年 | 814篇 |
2016年 | 1284篇 |
2015年 | 1173篇 |
2014年 | 1491篇 |
2013年 | 1950篇 |
2012年 | 2411篇 |
2011年 | 2387篇 |
2010年 | 1727篇 |
2009年 | 1622篇 |
2008年 | 1782篇 |
2007年 | 1635篇 |
2006年 | 1573篇 |
2005年 | 1271篇 |
2004年 | 1050篇 |
2003年 | 900篇 |
2002年 | 906篇 |
2001年 | 767篇 |
2000年 | 643篇 |
1999年 | 704篇 |
1998年 | 570篇 |
1997年 | 477篇 |
1996年 | 482篇 |
1995年 | 425篇 |
1994年 | 401篇 |
1993年 | 335篇 |
1992年 | 286篇 |
1991年 | 256篇 |
1990年 | 260篇 |
1989年 | 176篇 |
1988年 | 134篇 |
1987年 | 100篇 |
1986年 | 108篇 |
1985年 | 76篇 |
1984年 | 46篇 |
1983年 | 40篇 |
1982年 | 42篇 |
1981年 | 25篇 |
1980年 | 13篇 |
1979年 | 7篇 |
1977年 | 3篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 468 毫秒
71.
A new reagent, potassium permanganate adsorbed on aluminum silicate, suitable for the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds is described. 相似文献
72.
An experimental method is developed to examine the near tip deformation at the mesoscopic scale level. The differential interference contrast (DIC) method is used by application of the Nomarski prism in polarized microscope for measuring the out of surface deformation. The method is very sensitive to small height changes detected by different interference color. Discussed are results for the crack tip deformation field. 相似文献
73.
Xin‐Gui Li Run‐Feng Chen Mei‐Rong Huang Mei‐Fang Zhu Qun Chen 《Journal of polymer science. Part A, Polymer chemistry》2004,42(9):2073-2092
A new series of copolymers was synthesized through the oxidative polymerization of pyrrole (PY) and o‐phenetidine (PHT) with inorganic oxidants in acidic media. The polymerization parameters including the mixing method of the oxidant with the comonomer, the comonomer ratio, the time, the temperature, the oxidant, the organic medium, and the acid were systematically optimized for the synthesis of copolymers with high yields, intrinsic viscosities, and solubility. The resultant copolymers were characterized by elemental analysis, infrared, ultraviolet–visible, solution high‐resolution 1H NMR and solid‐state high‐resolution 13C NMR, circular dichroism spectroscopy, and cyclic voltammetry. The results showed that the PY observed content in the copolymers was much higher than the PY feed content. The regular variation of the polymerization yield, intrinsic viscosity, solubility, macromolecular structure, and electroactivity of the resulting polymers with the comonomer ratio, together with the complete solubility of a PY/PHT (10/90) polymer in highly polar solvents, indicated the formation of real random copolymers containing both PY and PHT units rather than a mixture of two homopolymers. However, the polymers containing more than 59 mol % PY were not homogeneous copolymers consisting of soluble and insoluble parts. A semiquantitative relationship between the polymerization yield or solubility of the copolymers and the polarity index of the organic solvents was examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2073–2092, 2004 相似文献
74.
Shu‐Ling Huang Wei‐Kuo Chin W. P. Yang 《Journal of Polymer Science.Polymer Physics》2004,42(18):3476-3486
The tetramethoxysilane (TMOS)/2‐hydroxylethyl methacrylate (HEMA) hybrid gels were synthesized with acid and base catalysts, via the in situ polymerization of HEMA, with and without the cosolvent methanol. With methanol in the TMOS/HEMA sol, the enhanced esterification and depolymerization reactions of the silanols resulted in a slower growth of silica particles. The silica particles that were synthesized with an acid catalyst were less than 40 nm. The thermal resistance of the poly(2‐hydroxyethyl methacrylate) (PHEMA) chains was enhanced by the addition of colloidal silica. The Fourier transform infrared characterizations and the exothermal peaks on the differential scanning calorimetry traces of these hybrid gels indicated chemical hybridization occurring as a result of condensation of the colloid silica and PHEMA at higher temperatures. Hence, the residual weight content of the hybrid gel after its synthesis with the base catalyst was even higher than the content of TMOS in the hybrid sol. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3476–3486, 2004 相似文献
75.
Akihide Itoh Taisuke Nagasawa Yanbei Zhu Kyue-Hyung Lee Eiji Fujimori Hiroki Haraguchi 《Analytical sciences》2004,20(1):29-36
In order to elucidate the distributions of the elements among the particulate and dissolved fractions in pond water, major-to-ultratrace elements in different sizes of particles as well as in the filtrate passed through the 0.05 microm filter were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). The different sizes of particle samples (ca. 100-300 microg each) were collected on the membrane filters with pore sizes of 10, 3.0, 1.2, 0.4, 0.2 and 0.05 microm, respectively, by sequential fractionation. As a result, about 40 elements in different sizes of particles could be determined by ICP-AES and ICP-MS, after acid digestion using HNO3/HF/HClO4. Then, the fractional distribution factors of major-to-ultratrace elements among the particulate and dissolved fractions were estimated from the analytical results. The total contents of Al, Fe, Ti, REEs (rare earth elements), Bi, Pb and Ag in the particulate fractions (larger than 0.05 microm) were more than 80-90%, while those of Ca, Sr, Cs, W, Ba, Mn and Co in the dissolved fraction, which corresponded to the filtrate passed through the 0.05 microm membrane filter, were more than 80%. It was further found that the fractional distributions of Cu and Zn in the dissolved fraction were ca. 50%. In addition, the enrichment factors (EFs) of the elements in the particulate fractions with particle sizes of 3.0-10 microm and 0.05-0.2 microm were estimated to elucidate their geochemical characteristics in natural water. 相似文献
76.
We developed two DNA-conjugated polymers, one based on polyallylamine and the other on polyacrylic acid, for use in DNA chips. A 30-mer single-stranded DNA probe and thioctic acid were covalently attached to polyallylamine as sidechains. The same single-stranded DNA and 3-(pyridyldithio)propionyl hydrazide were covalently attached to polyacrylic acid as sidechains. Both DNA-conjugated polymers could be specifically immobilized onto a gold sensor substrate by a self-assembly technique. The interactions between fully matched DNA and each DNA-conjugated polymer were investigated by surface plasmon resonance. A gold surface modified with either DNA-conjugated polymer recognized fully matched DNA much better than unmatched DNA. The hybridization selectivity and efficiency of DNA-conjugated polyallylamine was optimized by adjusting the pH so as to reduce the effects of cationic polymer sidechains. The hybridization selectivity and efficiency of DNA-conjugated polymers were higher than those of a conventional immobilized thiol-based DNA. The coating of DNA-conjugated polymers reduced nonspecific adsorption of DNA by the gold substrate. DNA-conjugated polyacrylic acid was more selective toward fully matched DNA than was DNA-conjugated polyallylamine. Therefore, DNA-conjugated polymers show promise for application in novel DNA chips. 相似文献
77.
78.
在异质纳米结构表面发生的新现象是当前研究的热点.最近发现,尽管甘氨酸在纯Ag表面只 能作物理吸附,蒸镀在Cu表面的单层Ag岛却能在Cu的帮助下,出现对甘氨酸作化学吸附的能力,这种现象是溢流效应的一种反映.蒸镀在Ag表面的Cu岛也能帮助附近裸露的Ag表面获得 化学吸附甘氨酸的能力,虽然这里已不是单原子层的银了.结果说明这种溢流现象来源于CuA g在表面的纳米结构共存,而不只是这种共存的某个结构所特有的.但是,由于Cu的表面能大 于Ag,所以即使是在室温下,Cu岛也会逐渐地被一单层Ag原子完全覆盖,从而失去溢
关键词:
溢流
甘氨酸
Cu
Ag(111) 相似文献
79.
建立傅里叶变换步进扫描时间分辨光电导光谱,并研究太阳电池中与转换效率密切相关的少数载流子寿命.实验选取三种典型的硅太阳电池(单晶硅样品1、多晶硅样品2和多晶硅样品3 ),发现其瞬态光电导的上升和衰退曲线可以分别用两个简单的指数函数描述.由于有复合中心的参与,复合过程中少数载流子的寿命比产生过程中的寿命短.为验证实验结果的可靠性,采用了提取样品少数载流子的体寿命和计算其有效扩散长度两种方法.通过与太阳电池暗伏安特性和负载特性研究相结合,进一步分析和讨论了少数载流子寿命与短路电流、开路电压和转换效率的关系.同时探讨了步进扫描时间分辨光谱实验的其他用途.
关键词:
步进扫描
时间分辨
硅太阳电池
瞬态光电导 相似文献
80.