全文获取类型
收费全文 | 2256篇 |
免费 | 41篇 |
国内免费 | 6篇 |
专业分类
化学 | 1770篇 |
晶体学 | 22篇 |
力学 | 33篇 |
综合类 | 1篇 |
数学 | 94篇 |
物理学 | 383篇 |
出版年
2021年 | 15篇 |
2020年 | 27篇 |
2019年 | 23篇 |
2018年 | 17篇 |
2016年 | 33篇 |
2015年 | 23篇 |
2014年 | 38篇 |
2013年 | 112篇 |
2012年 | 87篇 |
2011年 | 95篇 |
2010年 | 69篇 |
2009年 | 52篇 |
2008年 | 115篇 |
2007年 | 115篇 |
2006年 | 119篇 |
2005年 | 123篇 |
2004年 | 113篇 |
2003年 | 107篇 |
2002年 | 133篇 |
2001年 | 53篇 |
2000年 | 54篇 |
1999年 | 36篇 |
1998年 | 30篇 |
1997年 | 26篇 |
1996年 | 38篇 |
1995年 | 26篇 |
1994年 | 35篇 |
1993年 | 35篇 |
1992年 | 28篇 |
1991年 | 24篇 |
1990年 | 24篇 |
1989年 | 18篇 |
1988年 | 21篇 |
1987年 | 22篇 |
1986年 | 10篇 |
1985年 | 37篇 |
1984年 | 35篇 |
1983年 | 18篇 |
1982年 | 35篇 |
1981年 | 43篇 |
1980年 | 33篇 |
1979年 | 25篇 |
1978年 | 23篇 |
1977年 | 22篇 |
1976年 | 13篇 |
1975年 | 17篇 |
1974年 | 13篇 |
1973年 | 26篇 |
1972年 | 7篇 |
1971年 | 7篇 |
排序方式: 共有2303条查询结果,搜索用时 157 毫秒
1.
Dr. Manussada Ratanasak Takumi Murata Taishin Adachi Prof. Jun-ya Hasegawa Prof. Tadashi Ema 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(58):e202202210
BPh3 catalyzes the N-methylation of secondary amines and the C-methylenation (methylene-bridge formation between aromatic rings) of N,N-dimethylanilines or 1-methylindoles in the presence of CO2 and PhSiH3; these reactions proceed at 30–40 °C under solvent-free conditions. In contrast, B(C6F5)3 shows little or no activity. 11B NMR spectra suggested the generation of [HBPh3]−. The detailed mechanism of the BPh3-catalyzed N-methylation of N-methylaniline ( 1 ) with CO2 and PhSiH3 was studied by using DFT calculations. BPh3 promotes the conversion of two substrates (N-methylaniline and CO2) into a zwitterionic carbamate to give three-component species [Ph(Me)(H)N+CO2−⋅⋅⋅BPh3]. The carbamate and BPh3 act as the nucleophile and Lewis acid, respectively, for the activation of PhSiH3 to generate [HBPh3]−, which is used to produce key CO2-derived species, such as silyl formate and bis(silyl)acetal, essential for the N-methylation of 1 . DFT calculations also suggested other mechanisms involving water for the generation of [HBPh3]− species. 相似文献
2.
3.
4.
5.
Takeshi Murashige Hideo Fujikake Hiroto Sato Hiroshi Kikuchi Taiichiro Kurita Fumio Sato 《Optical Review》2004,11(6):349-352
We have confirmed light diffraction of aligned polymer fibers obtained by a phase separation of an anisotropic-phase solution of liquid crystal and polymer. He—Ne laser light passing through the polymer fibers was scattered in the axis vertical to the fibers, and had two peaks of light intensity symmetrical to the center of the transmitting laser spot. The two peaks were found to be caused by light diffraction due to the periodic polymer-fiber dispersion because the peaks corresponded to values calculated by intervals between the fibers. The periodical fiber networks are considered to be formed by anisotropic spinodal decomposition. This effect can be used to measure the dispersion order of the polymer fibers. © 2004 The Optical Society of Japan 相似文献
6.
The binding properties of metal ions to a pectic polysaccharide, rhamnogalacturonan II (RG-II), from plant cell walls were analyzed by size-exclusion HPLC/ICP-MS. The dimeric RG-II borate complex (dRG-II-B) from sugar beet and red wine contained 0.8 - 1.1 mol/mol of B, 0.8 - 1.1 mol/mol of Ca, 0.1 - 0.3 mol/mol of Sr, and 0.03 - 0.07 mol/mol of Ba. The treatment of dRG-II-B with Sr2+, Ba2+, Pb2+ or La3+ exchanged the originally present Ca, Sr and Ba. In contrast, monomeric RG-II (mRG-II), which contained approximately 0.1 mol/mol of Ca, formed complexes with La3+, Eu3+, and Lu3+ added to the solution, but did not do so with Ca2+, Sr2+, Ba2+ and Pb2+. The HPLC/ICP-MS and HPLC/RI (refractive index detector) analysis of the partially hydrolyzed mRG-II that was treated with La3+ indicated that the side chains and backbone of mRG-II together form a lanthanoid binding site. 相似文献
7.
A large-volume (100 microl) injection-ETAAS with W-treated PG furnace combined with a phosphate modifier was applied to the determination of unpolluted levels of Cd in tap, snow and river-water samples. The limit of detection of 1.1 ng l(-1) was observed for a 4 w/v% NH4H2PO4 modifer. Matrix interference studies were tested for major ion species well found in fresh water. The direct determination of Cd in certified river water (12 +/- 2 ng l(-1)) was carried out and the observed value was in agreement with the certified one. The good recoveries of Cd added to real environmental water samples were also observed. This method was applied to the determination of Cd in unpolluted environmental water samples. 相似文献
8.
Mineo Hashizume Hiroaki Inoue Kiyofumi Katagiri Atsushi Ikeda Jun-Ichi Kikuchi 《Journal of Sol-Gel Science and Technology》2004,31(1-3):99-102
Morphological characterization of the organic-inorganic vesicular nanohybrids, Cerasomes, was performed in aqueous media from two aspects. Firstly, a novel Cerasome-forming lipid having two triethoxysilyl groups in the head moiety was synthesized and the physical property of the Cerasome was investigated. While the morphological stability of the Cerasomes, as evaluated from the vesicular collapse behavior against a micelle-forming nonionic surfactant, Triton-X 100, was extremely higher than that of the conventional phospholipid liposome, the stabilities were comparable to each other for the Cerasomes derived from the dual- and single-head lipids. On the other hand, the surface property of the Cerasome formed with the dual-head lipid more closely resembled the colloidal silica particles rather than that derived from the single-head lipid, as suggested by zeta-potential measurements. Secondly, the effect of the media pH on the morphological stability of the Cerasome formed with the single-head lipid was evaluated and appeared as a time difference in obtaining the morphological stability of the Cerasome. These morphological characteristics of the Cerasomes could be mainly owing to the development of the siloxane network on the vesicular surface. 相似文献
9.
Sonoko Kikuchi Kazuteru Kawauchi Sadatsugu Ooki Masaru Kurosawa Hisashi Honjho Teruo Yagishita 《Analytical sciences》2004,20(8):1111-1112
Using Raman spectroscopy without any preparation, we analyzed the brominated flame retardant, polybromodiphenyl ethers (PBDEs), which has been prohibited by the European Union. The typical Raman bands of PBDEs, which are clearly different from those of other brominated flame retardants, are from 100 to 300 cm(-1). In our analysis, the detection limit was about 100 ppm, and the analysis took about 1 min, whereas the usual method of solvent extraction using gas chromatography/mass spectroscopy takes 50 h. 相似文献
10.
In this paper, we hall discuss the projective flatness of complex Finsler metrics by investigating the geometry of projective bundles associated with a holomorphic vector bundle. 相似文献