全文获取类型
收费全文 | 1906篇 |
免费 | 85篇 |
国内免费 | 8篇 |
专业分类
化学 | 1385篇 |
晶体学 | 19篇 |
力学 | 74篇 |
数学 | 290篇 |
物理学 | 231篇 |
出版年
2023年 | 19篇 |
2022年 | 37篇 |
2021年 | 42篇 |
2020年 | 55篇 |
2019年 | 40篇 |
2018年 | 31篇 |
2017年 | 25篇 |
2016年 | 55篇 |
2015年 | 49篇 |
2014年 | 68篇 |
2013年 | 117篇 |
2012年 | 127篇 |
2011年 | 169篇 |
2010年 | 74篇 |
2009年 | 77篇 |
2008年 | 132篇 |
2007年 | 130篇 |
2006年 | 106篇 |
2005年 | 109篇 |
2004年 | 93篇 |
2003年 | 95篇 |
2002年 | 103篇 |
2001年 | 21篇 |
2000年 | 15篇 |
1999年 | 21篇 |
1998年 | 25篇 |
1997年 | 26篇 |
1996年 | 21篇 |
1995年 | 14篇 |
1994年 | 12篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1989年 | 9篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1895年 | 1篇 |
1887年 | 2篇 |
排序方式: 共有1999条查询结果,搜索用时 31 毫秒
181.
182.
The relative displacement rates of the halide substituent from 2‐fluoro‐ and 2‐chloropyridines by EtONa in EtOH at +25° were assessed by competition kinetics. The 2‐fluoropyridine reacts 320 times faster than the chloro analog. A CF3 group increases the reactivity more than single halogen atoms do, whatever the element, and the latter are superior to Me3Si groups. Substituents accommodated at the 4‐position operate through their inductive effect, whereas at the 3‐position, this action may be attenuated by steric hindrance. Almost all 5‐substituents enhance the rate of the nucleophilic substitution occurring at the 2‐position. The sole exception concerns the F‐atom at the 5‐position which retards the reaction, presumably by lone‐pair/lone‐pair repulsion with the negative charge building up at the central C‐atom of the intermediate Meisenheimer complex. The substituent effects are additive. Therefore, by using the increments derived from the present work, the rates of future reactions should be predictable with fair accuracy. 相似文献
183.
Mode coupling between the fundamental core mode of an as-drawn optical fiber operating far from cutoff and higher-order modes in the plastic jacketing has been observed. In one fiber, the transmission spectrum of the core exhibited five distinct minima between 1200 and 1900 nm. By approximating the fiber jacket as an asymmetric slab waveguide of infinite lateral extent, the wavelengths of these transmission features were successfully predicted using coupled-mode theory. Since conditions at the jacket surface influence the transmission spectrum, this coupling effect potentially enables new applications for environmental sensing. 相似文献
184.
We consider the problem of laminar mixed convection flow between parallel, vertical and uniformly heated plates where the governing dimensionless parameters are the Prandtl, Rayleigh and Reynolds numbers. Using the method based on the centre manifold theorem which was derived from the general theory of dynamical systems, we reduce a three-dimensional simplified model of ordinary differential amplitude equations emanating from the original Navier-Stokes system of the problem in the vicinity of a trivial stationary solution. We have found that when the forcing parameter, the Rayleigh number, increases beyond the critical value Ras, the stationary solution is a pitchfork bifurcation point of the system. 相似文献
185.
Fibrous thermal insulations are widely used to conserve energy in ambient to high temperature applications including buildings, solar collectors, heat exchangers, furnaces and thermal protection systems of reusable launch vehicles. It has long been recognised that zirconia has the lowest thermal conductivity of commercial refractories. The thermal conductivity of a zirconia fibrous medium is strongly dependent of its bulk density; high bulk densities of zirconia fibers provide the most effective insulation at high temperatures. Lee's theory for radiative transfer through fibrous media is used in this paper. The two-flux model is applied to determine the backward and forward parameters of a medium of zirconia fibers oriented in parallel planes. Theoretical calculations of the backscatter factor and absorption ratio of this medium are carried out in the visible spectrum for different size parameters of the fibers and for three different temperatures. Our results show that the backscatter factor of zirconia fibrous insulations is maximum, and therefore the heat transfer by the fibrous medium is the lowest, for a size parameter of 0.45 for all the temperatures studied. We also observed that the backscatter factor decreases with increasing temperature. 相似文献
186.
Synthesis of Carbon/Sulfur Nanolaminates by Electrochemical Extraction of Titanium from Ti2SC
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)
Dr. Meng‐Qiang Zhao Morgane Sedran Zheng Ling Maria R. Lukatskaya Olha Mashtalir Michael Ghidiu Boris Dyatkin Darin J. Tallman Prof. Thierry Djenizian Prof. Michel W. Barsoum Prof. Yury Gogotsi 《Angewandte Chemie (International ed. in English)》2015,54(16):4810-4814
Herein we electrochemically and selectively extract Ti from the MAX phase Ti2SC to form carbon/sulfur (C/S) nanolaminates at room temperature. The products are composed of multi‐layers of C/S flakes, with predominantly amorphous and some graphene‐like structures. Covalent bonding between C and S is observed in the nanolaminates, which render the latter promising candidates as electrode materials for Li‐S batteries. We also show that it is possible to extract Ti from other MAX phases, such as Ti3AlC2 , Ti3SnC2 , and Ti2GeC, suggesting that electrochemical etching can be a powerful method to selectively extract the “M” elements from the MAX phases, to produce “AX” layered structures, that cannot be made otherwise. The latter hold promise for a variety of applications, such as energy storage, catalysis, etc. 相似文献
187.
Franck Charmantray Nadia Touisni Laurence Hecquet Thierry Noguer Christine Mousty 《Electroanalysis》2015,27(6):1341-1344
This paper describes the development of an amperometric biosensor based on galactose oxidase (GAOx) immobilization within a laponite clay film deposited on Carbon Screen‐Printed Electrodes modified by electrodeposited Prussian Blue and coated with poly‐(O‐phenylenediamine) (PPD/PB/CSPEs). Amperometric performances of GAOx@laponite/PPD/PB/CSPEs bioelectrodes were determined using several GAOx substrates. Using these modified electrodes the reduction of enzymatically generated hydrogen peroxide was performed at ?0.2 V vs. Ag‐AgCl. In an initial attempt, E.Coli transketolase activity on its immobilized form was followed using a bienzymatic GAOx‐TK biosensor. 相似文献
188.
Avin Ramanjooloo Thierry Cresteil Cindy Lebrasse Girish Beedessee Preeti Oogarah Rob W.M. van Soest 《Natural product research》2015,29(4):383-387
This report describes the use of α-glucosidase to evaluate the anti-diabetic potential of extracts from marine sponges collected in the Mauritius waters. Initial screening at 1.0 mg/mL of 141 extracts obtained from 47 sponge species revealed 10 extracts with inhibitory activity greater than 85%. Seven of the 10 extracts were further tested at 0.1 and 0.01 mg/mL and only the methanol extract of two sponges namely Acanthostylotella sp. (ASSM) and Echinodictyum pykei (EPM) showed inhibition activity greater than 60% at 0.1 mg/mL with an IC50 value of 0.16 ± 0.02 and 0.04 ± 0.01 mg/mL, respectively, while being inactive at 0.01 mg/mL. 相似文献
189.
190.
Maria D. Manrique-Juarez Dr. Fabrice Mathieu Dr. Victoria Shalabaeva Dr. Jean Cacheux Sylvain Rat Dr. Liviu Nicu Dr. Thierry Leïchlé Dr. Lionel Salmon Dr. Gábor Molnár Dr. Azzedine Bousseksou 《Angewandte Chemie (International ed. in English)》2017,56(28):8074-8078
We report on a bistable MEMS device actuated by spin-crossover molecules. The device consists of a freestanding silicon microcantilever with an integrated piezoresistive detection system, which was coated with a 140 nm thick film of the [Fe(HB(tz)3)2] (tz=1,2,4-triazol-1-yl) molecular spin-crossover complex. Switching from the low-spin to the high-spin state of the ferrous ions at 338 K led to a reversible upward bending of the cantilever in agreement with the change in the lattice parameters of the complex. The strong mechanical coupling was also evidenced by the decrease of approximately 66 Hz in the resonance frequency in the high-spin state as well as by the drop in the quality factor around the spin transition. 相似文献