全文获取类型
收费全文 | 331篇 |
免费 | 14篇 |
国内免费 | 2篇 |
专业分类
化学 | 218篇 |
力学 | 2篇 |
数学 | 78篇 |
物理学 | 49篇 |
出版年
2023年 | 2篇 |
2021年 | 3篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2017年 | 7篇 |
2016年 | 9篇 |
2015年 | 12篇 |
2014年 | 6篇 |
2013年 | 13篇 |
2012年 | 16篇 |
2011年 | 28篇 |
2010年 | 12篇 |
2009年 | 11篇 |
2008年 | 12篇 |
2007年 | 17篇 |
2006年 | 19篇 |
2005年 | 9篇 |
2004年 | 13篇 |
2003年 | 11篇 |
2002年 | 8篇 |
2001年 | 10篇 |
2000年 | 11篇 |
1999年 | 5篇 |
1998年 | 4篇 |
1997年 | 7篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 6篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1957年 | 1篇 |
1954年 | 1篇 |
1939年 | 1篇 |
1937年 | 1篇 |
1935年 | 1篇 |
排序方式: 共有347条查询结果,搜索用时 15 毫秒
61.
Martiskainen O Sztanke K Matosiuk D Pihlaja K 《Rapid communications in mass spectrometry : RCM》2006,20(17):2548-2552
The electron ionization mass spectra of the title compounds (1: a R = H, b 2-CH(3), c 4-CH(3), d 2,3-diMe, e 2-OCH(3), f 4-OCH(3), g 2-Cl, h 3-Cl, i 4-Cl, j 3,4-diCl) were recorded at 70 eV to determine the effects of substituents and the possible keto-enol tautomerism. The compounds showed several common fragment ions but also fragment ions which divided them into three classes, namely 1a-1d (parent compound and Me-substituted derivatives), 1e and 1f (MeO-substituted derivatives), and 1g-1j (Cl-substituted derivatives). The presence of the HOCN(+.) ion as well as the exponential dependence of its total ion current in the case of p- and also 3-Cl-substituted compounds (1a, c, f, h-j) on the Hammett sigma constants and the loss of CHO or one or two HOCN moieties can be somewhat easier explained by the presence of the enol form but as a whole the results support the predominance of the keto form, in parallel to the situation in solution. 相似文献
62.
Yaar Dürüst Cevher Altu Jari Sinkkonen Olli Martiskainen Kalevi Pihlaja 《Journal of heterocyclic chemistry》2006,43(5):1267-1274
63.
Avoiding Carbothermal Reduction: Distillation of Alkoxysilanes from Biogenic,Green, and Sustainable Sources
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Richard M. Laine Joseph C. Furgal Phi Doan David Pan Vera Popova Xingwen Zhang 《Angewandte Chemie (International ed. in English)》2016,55(3):1065-1069
The direct depolymerization of SiO2 to distillable alkoxysilanes has been explored repeatedly without success for 85 years as an alternative to carbothermal reduction (1900 °C) to Simet, followed by treatment with ROH. We report herein the base‐catalyzed depolymerization of SiO2 with diols to form distillable spirocyclic alkoxysilanes and Si(OEt)4. Thus, 2‐methyl‐2,4‐pentanediol, 2,2,4‐trimethyl‐1,3‐pentanediol, or ethylene glycol (EGH2) react with silica sources, such as rice hull ash, in the presence of NaOH (10 %) to form H2O and distillable spirocyclic alkoxysilanes [bis(2‐methyl‐2,4‐pentanediolato) silicate, bis(2,2,4‐trimethyl‐1,3‐pentanediolato) silicate or Si(eg)2 polymer with 5–98 % conversion, as governed by surface area/crystallinity. Si(eg)2 or bis(2‐methyl‐2,4‐pentanediolato) silicate reacted with EtOH and catalytic acid to give Si(OEt)4 in 60 % yield, thus providing inexpensive routes to high‐purity precipitated or fumed silica and compounds with single Si?C bonds. 相似文献
64.
High and low molecular weight (Mw) carboxymethyl celluloses (CMC) were adsorbed on a well-characterized fiber substrate (long fibers of a commercial bleached birch kraft pulp with the carboxylic acid groups in Na-form) to increase the charge of the fibers in a controlled fashion. The Mw played a role in the utilization of CMCs as a strength additive in paper sheets nearly doubling the tensile strength with the high Mw CMC. Swelling properties of the CMC treated fibers were measured with water retention value (WRV). The WRV increased more with the high Mw CMC. The swelling was further tuned by two highly cationic polyelectrolytes; high Mw poly(diallyldimethyl ammonium chloride) (PDADMAC) and low Mw polybrene (hexadimethrine bromide, [3,6]-ionene). They were chosen because of their known ability to neutralize the anionic charge either exclusively on the surface or in the whole fiber, respectively. Adsorption of PDADMAC could reduce WRV of the CMC pre-treated fibers to the level of the untreated reference, while polybrene adsorbed pulps with 3–10 times more cationic polyelectrolyte deswelled the fibers only slightly more than the surface neutralized fibers. These results indicated surface conformation differences with low and high Mw CMCs. While the conformation did play a role after physical alteration (drying and rewetting) of the fibers, the paper sheets produced from these fibers showed remarkable differences. In extreme cases, the strength of the paper could be retained after drying (low Mw CMC + PDADMAC) or paper, resistant to disintegration, could be achieved (CMC + polybrene). 相似文献
65.
Spatially resolved OSL (SR-OSL) measurements are made with a scanning or imaging system that records luminescence emissions from known sources within a sample. This work is a first step toward the development of an SR-OSL methodology based on the imaging of ultraviolet OSL emissions with an EMCCD camera. We present UV OSL images of natural quartz extracts, OSL/TL/IR images of sandstone, image-based OSL measurements from aluminum oxide grains, and dose recovery experiments on aluminum oxide and Risø calibration quartz. Camera sensitivity and quantitative reproducibility indicate that EMCCD-based OSL dating is feasible. Further experimentation is necessary to adequately define the error sources and measurement limitations of EMCCDs in the OSL regime. 相似文献
66.
In this paper we suggest, numerically simulate and discuss a tapered plasmonic nanostructure which allows efficient transmission of the wave beam energy into a weakly subwavelength (λ/4 … λ/2) spatial region within a broad frequency range. The frequency stable location of this region becomes possible due to unusual plasmonic modes we have theoretically revealed and used. Our structure is promising for prospective transducers between conventional optical waveguides and subwavelength nanoguides or nanocircuits. 相似文献
67.
Octa(aminophenyl)silsesquioxane as a nanoconstruction site. 总被引:10,自引:0,他引:10
R Tamaki Y Tanaka M Z Asuncion J Choi R M Laine 《Journal of the American Chemical Society》2001,123(49):12416-12417
68.
Journal of Statistical Physics - The study and analysis of real-world social, communication, information and citation networks for understanding their structure and identifying interesting patterns... 相似文献
69.
We consider in this paper the numerical solution of the Falkner-Skan differential equation, modelling under some similarity assumptions the boundary layer equation. We look for the extremal solution of this third order differential equation. The methods we use are basically the Newton method with a shooting process, which is coupled with a continuation method: they allow us to follow the solution arcs which contain regular and turning point solutions. 相似文献
70.
The room temperature oxidation of porous silicon 总被引:1,自引:0,他引:1
The room temperature oxidation of porous silicon was studied using isothermal methods. The oxidation was found to depend on the type of the porous silicon. The microcalorimetric signals from the oxidation of the p+- and n-type porous silicon in dry air were different. In humid air the signals from the oxidation could not be distinguished from the strong signal due to adsorption of water vapour, but when the samples were placed in water similar differences were observed. The reason for differences in reactions is discussed. The oxidation in different liquids was also studied. The signal from reactions in methanol and ethanol were found to be 100 times higher than in water. In FTIR studies the reaction gas produced by reactions between alcohols and the porous silicon, silane (SiH4) was found in the gas. Traces of SiOCH3 and SiOC2H5 groups were also found in FTIR spectra indicating Si---O---CxHy passivation of the surface. 相似文献