全文获取类型
收费全文 | 717篇 |
免费 | 8篇 |
国内免费 | 2篇 |
专业分类
化学 | 266篇 |
晶体学 | 4篇 |
力学 | 9篇 |
数学 | 154篇 |
物理学 | 294篇 |
出版年
2020年 | 14篇 |
2019年 | 10篇 |
2018年 | 5篇 |
2017年 | 7篇 |
2016年 | 7篇 |
2015年 | 11篇 |
2014年 | 6篇 |
2013年 | 31篇 |
2012年 | 21篇 |
2011年 | 17篇 |
2010年 | 8篇 |
2009年 | 11篇 |
2008年 | 23篇 |
2007年 | 29篇 |
2006年 | 28篇 |
2005年 | 14篇 |
2004年 | 25篇 |
2003年 | 18篇 |
2002年 | 17篇 |
2001年 | 9篇 |
2000年 | 23篇 |
1999年 | 10篇 |
1997年 | 11篇 |
1996年 | 6篇 |
1995年 | 12篇 |
1994年 | 13篇 |
1993年 | 15篇 |
1992年 | 14篇 |
1991年 | 17篇 |
1990年 | 23篇 |
1989年 | 9篇 |
1988年 | 13篇 |
1987年 | 10篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 16篇 |
1983年 | 12篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 16篇 |
1979年 | 8篇 |
1978年 | 15篇 |
1977年 | 14篇 |
1976年 | 12篇 |
1975年 | 15篇 |
1974年 | 16篇 |
1973年 | 12篇 |
1971年 | 6篇 |
1970年 | 6篇 |
1964年 | 4篇 |
排序方式: 共有727条查询结果,搜索用时 0 毫秒
21.
A stochastic (Monte Carlo) simulation is used to investigate thermodynamic and kinetic contributions from the reversible A <--> B reaction in capillary electrophoresis (CE). The effects of equilibrium constant, rate constant, and electrophoretic mobility on the molecular zone profiles and the corresponding statistical moments are evaluated. As the reaction approaches steady state, the velocity of the zone is governed by the equilibrium constant and the electrophoretic mobilities of the reacting molecules. When the equilibrium constant is less than unity, the mean zone velocity is more similar to that of the reactant A. Conversely, when the equilibrium constant is greater than unity, the velocity is more similar to that of the product B. The extent of zone-broadening and asymmetry at steady state is dependent upon the equilibrium constant, the characteristic reaction lifetime, and the electrophoretic mobility difference between reacting molecules. If all other parameters are held constant, the plate height is greatest and skew is least when the equilibrium constant is unity. The plate height increases linearly with the characteristic reaction lifetime and electrophoretic mobility difference, whereas the skew is independent of these parameters. These conclusions have important implications for the elucidation of thermodynamic and kinetic information from experimental data. 相似文献
22.
C.G. Newman B.N. Diel D.P. Paquin M.A. Ring 《Journal of organometallic chemistry》1977,137(3):281-286
Methylation of SiH4, MeSiH3, Si2H6, GeH4 and B2H6, but not of PH3 or AsH3, was observed during reaction (230–324°C) with GaMe3. The products from the SiH4 and Si2H6 reactions were MeSiH3, Me2SiH2 and Me3SiH. The GeH4-derived products were similar, with Me4Ge also being formed. The only methylated products from B2H6 was BMe3. The silane reactions were surface-catalyzed (presumably by surface hydroxyl groups), while those of GeH4 and B2H6 may have occurred via gas-phase free radical processes. 相似文献
23.
Roger H. Newman 《Cellulose (London, England)》2004,11(1):45-52
Solid-state 13C NMR spectroscopy was used to characterize a bleached softwood kraft pulp in the never-dried state and after cycles of drying and remoistening. Changes in NMR signal strengths indicated that growth of crystalline domains involved cocrystallization rather than accretion of cellulose from noncrystalline domains. A cluster of C-4 signals at 89.4 ppm, assigned to the interiors of crystalline domains, grew at the expense of C-4 signals at 84.0 and 84.9 ppm, assigned to the well-ordered surfaces of crystalline domains. Irreversible changes were not detected until the moisture content dropped below 18%. They were enhanced by a second drying/remoistening cycle, but showed little further change on subsequent cycles. The necessary conditions resembled those reported for hornification, suggesting that cocrystallization might provide a mechanism for hornification. 相似文献
24.
Tomato polyphenol oxidase B is spatially and temporally regulated during development and in response to ethylene 总被引:1,自引:0,他引:1
Plant polyphenol oxidases (PPOs) are ubiquitous plastid-localized enzymes. A precise analysis of PPO function in plants has been complicated by the presence of several family members with immunological cross reactivity. Previously we reported the isolation of genomic clones coding for the seven members of the tomato (Solanum lycopersicum) PPO family (A, A', B, C, D, E, and F). Here we report the complex spatial and temporal expression of one of the members, PPO B. The PPO B promoter was sequenced and subjected to homology analysis. Sequence similarities were found to nucleotide sequences of genes encoding enzymes/proteins active in the following systems: phenylpropanoid biosynthesis, signal transduction and responsiveness to hormones and stresses, fruit and seed proteins/enzymes, and photosynthesis. Chimeric gene fusions were constructed linking PPO B 5' flanking regions to the reporter gene, b-glucuronidase (GUS). The resultant transgenic plants were histochemically analyzed for GUS activity in various vegetative and reproductive tissues, and evaluated for PPO B responsiveness to ethylene induction. It was shown that PPO B expression was tissue specific, developmentally regulated, ethylene induced, and localized predominantly to mitotic or apoptotic tissues. 相似文献
25.
This article describes a portable microfluidic technology for determining the minimum inhibitory concentration (MIC) of antibiotics against bacteria. The microfluidic platform consists of a set of chambers molded in poly(dimethylsiloxane) (PDMS) that are preloaded with antibiotic, dried, and reversibly sealed to a second layer of PDMS containing channels that connect the chambers. The assembled device is degassed via vacuum prior to its use, and the absorption of gas by PDMS provides the mechanism for actuating and metering the flow of fluid in the microfluidic channels and chambers. During the operation of the device, degas driven flow introduces a suspension of bacterial cells, dissolves the antibiotic, and isolates cells in individual chambers without cross contamination. The growth of bacteria in the chambers in the presence of a pH indicator produces a colorimetric change that can be detected visually using ambient light. Using this device we measured the MIC of vancomycin, tetracycline, and kanamycin against Enterococcus faecalis 1131, Proteus mirabilis HI4320, Klebsiella pneumoniae, and Escherichia coli MG1655 and report values that are comparable to standard liquid broth dilution measurements. The device provides a simple method for MIC determination of individual antibiotics against human pathogens that will have applications for clinical and point-of-care medicine. Importantly, this device is designed around simplicity: it requires a single pipetting step to introduce the sample, no additional components or external equipment for its operation, and provides a straightforward visual measurement of cell growth. As the device introduces a novel approach for filling and isolating dead-end microfluidic chambers that does not require valves and actuators, this technology should find applications in other portable assays and devices. 相似文献
26.
Andreas Tengå Sven Lidin Prof. Jean‐Philippe Belieres Prof. Nathan Newman Prof. Yang Wu Ulrich Häussermann Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(27):6704-6710
Although free from structural disorder , the new intermetallic compound Cd13?xInySb10 (see figure) displays similarly low thermal conductivity values as disordered thermoelectric β‐Zn4Sb3 with an isostructural framework.
27.
By incorporating ferrocene into the hydrophobic membrane of PEG-b-PCL polymersome nanoparticles it is possible to selectively visualize their core using Transmission Electron Microscopy (TEM). Two different sizes of ferrocene-loaded polymersomes with mean hydrodynamic diameters of approximately 40 and 90 nm were prepared. Image analysis of TEM pictures of these polymersomes found that the mean diameter of the core was 4–5 times smaller than the mean hydrodynamic diameter. The values obtained also allow the surface diameter and internal volume of the core to be calculated. 相似文献
28.
Robert M. Przeslawski Suzanne Newman Edward R. Thornton Madeleine M. Joullié 《合成通讯》2013,43(19):2975-2980
The commercially available α-ketoxime of camphorquinone (1) was reduced to the corresponding exo,exo-amino alcohol (2) in a two-step, one-pot procedure using sodium borohydride and nickel(II) chloride hexahydrate. 相似文献
29.
Treatment of 5,5-dialkyl-N-nitrosooxazolidones (1) with Lithium ethoxide (and other bases) under certain conditions yields reaction products the formation of which seems best explained by assuming that unsaturated carbenes [A] are involved.3–5 相似文献
30.
Oleg B. Shchekin Peter J. Schmidt Fahong Jin Nate Lawrence Kenneth J. Vampola Helmut Bechtel Danielle R. Chamberlin Gerd O. Mueller 《固体物理学:研究快报》2016,10(4):310-314
Droop, the decrease of efficiency with increased power density, became a major topic with InGaN LEDs, after its introduction in 2007. This paper provides insight into droop in localized center luminescence phosphors, exemplified here by Eu2+ doped materials. This topic is of increasing importance, as high brightness blue LEDs have reached outputs >1 W/mm2. The nonlinearities in phosphor quantum efficiency result in drive‐dependent color point shift and reduction of overall efficiency of phosphor converted white LEDs which utilize Eu2+ activated phosphors. The efficiency quenching can be traced back to two processes, well‐known in laser physics, excited state absorption or/and cross relaxation by Foerster/Dexter transfer. Both processes lead to reduction in phosphor efficiency, but they can be differentiated. Understanding the root cause of efficiency quenching opens ways to minimize the practical consequences. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) 相似文献