全文获取类型
收费全文 | 3726篇 |
免费 | 177篇 |
国内免费 | 36篇 |
专业分类
化学 | 2462篇 |
晶体学 | 27篇 |
力学 | 111篇 |
数学 | 689篇 |
物理学 | 650篇 |
出版年
2023年 | 22篇 |
2022年 | 38篇 |
2021年 | 70篇 |
2020年 | 70篇 |
2019年 | 91篇 |
2018年 | 83篇 |
2017年 | 56篇 |
2016年 | 132篇 |
2015年 | 126篇 |
2014年 | 167篇 |
2013年 | 233篇 |
2012年 | 288篇 |
2011年 | 336篇 |
2010年 | 183篇 |
2009年 | 168篇 |
2008年 | 250篇 |
2007年 | 230篇 |
2006年 | 211篇 |
2005年 | 195篇 |
2004年 | 167篇 |
2003年 | 155篇 |
2002年 | 120篇 |
2001年 | 71篇 |
2000年 | 56篇 |
1999年 | 34篇 |
1998年 | 36篇 |
1997年 | 30篇 |
1996年 | 36篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 33篇 |
1992年 | 26篇 |
1991年 | 22篇 |
1990年 | 17篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 17篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1978年 | 7篇 |
1976年 | 6篇 |
1974年 | 4篇 |
1961年 | 3篇 |
1942年 | 6篇 |
1940年 | 3篇 |
排序方式: 共有3939条查询结果,搜索用时 15 毫秒
961.
Juan J Torres-Vega Alejandro Vásquez-Espinal Dr. Lina Ruiz Prof. María A Fernández-Herrera Prof. Luis Alvarez-Thon Prof. Gabriel Merino Prof. William Tiznado 《ChemistryOpen》2015,4(3):302-307
The electron delocalization of benzene (C6H6) and hexafluorobenzene (C6F6) was analyzed in terms of the induced magnetic field, nucleus-independent chemical shift (NICS), and ring current strength (RCS). The computed out-of-plane component of the induced magnetic field at a distance (r) greater than or equal to 1.0 Å above the ring center correlates well (R2>0.99) with the RCS value. According to these criteria, fluorination has two effects on the C6 skeleton; concomitantly, the resonant effects diminish the π electron delocalization and the inductive effects decrease the charge density at the ring center and therefore reduce the magnitude of the paratropic current generated in this region. The equilibrium between both effects decreases aromaticity in the fluorinated benzene derivatives. These results can be extrapolated to determine the aromaticity of any derivative within the series of fluorinated benzene derivatives (C6H(6−n)Fn, where n=1–5). 相似文献
962.
Haibin Gu Roberto Ciganda Ricardo Hernandez Patricia Castel Pengxiang Zhao Jaime Ruiz Didier Astruc 《Macromolecular rapid communications》2016,37(7):630-636
Diblock metallopolymer polyelectrolytes containing the two redox‐robust cationic sandwich units [CoCp′Cp]+ and [FeCp′(η6‐C6Me6)]+ (Cp = η5‐C5H5; Cp′ = η5‐C5H4‐) as hexafluorophosphate ([PF6]−) salts are synthesized by ring‐opening metathesis polymerization using Grubbs' third generation catalyst. Their electrochemical properties show full chemical and electrochemical reversibilities allowing fine determination of the copolymer molecular weight using Bard–Anson's electrochemical method by cyclic voltammetry.
963.
Convergence analysis of a DDFV scheme for a system describing miscible fluid flows in porous media 下载免费PDF全文
Claire Chainais‐Hillairet Stella Krell Alexandre Mouton 《Numerical Methods for Partial Differential Equations》2015,31(3):723-760
In this article, we prove the convergence of a discrete duality finite volume scheme for a system of partial differential equations describing miscible displacement in porous media. This system is made of two coupled equations: an anisotropic diffusion equation on the pressure and a convection‐diffusion‐dispersion equation on the concentration. We first establish some a priori estimates satisfied by the sequences of approximate solutions. Then, it yields the compactness of these sequences. Passing to the limit in the numerical scheme, we finally obtain that the limit of the sequence of approximate solutions is a weak solution to the problem under study. © 2014 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 31: 723–760, 2015 相似文献
964.
Numerical methods and software packages for solving dynamic optimization or optimal control problems require a suitable initial
estimation of the solution. This paper focuses on problems that arise in chemical processes described by complex dynamics.
We present a very simple method, based on Pontryagin’s Minimum Principle, to obtain an initial guess for the solution. Our
method presents numerous advantages: it is very easy to programme, it allows a wide range of problems to be addressed, the
computation time is very short, the initial guess is very close to the solution and is attracted to a global minimum. 相似文献
965.
The asymmetric transfer hydrogenation of aromatic ketones can be efficiently accomplished using catalysts that are based on platinum group metals which are more toxic and less abundant than iron. For that reason the discovery of iron based catalysts for the use in this transformation is important. To address this issue, we synthesized a new series of iron(II)-based precatalysts trans-[Fe(Br)(CO)(PPh(2)CH(2)CH═NCHRCHRN═CHCH(2)PPh(2))]BPh(4) (5a-5d) containing P-N-N-P ligands with the diamines (R,R)-1,2-diaminocyclohexane (a), (R,R)-1,2-diphenyl-1,2-diaminoethane (b), (R,R)-1,2-di(4-methoxyphenyl)-1,2-diaminoethane (c), and ethylenediamine (d) incorporated in the backbone using a convenient one-pot synthesis using readily available starting materials. All of the complexes, when activated with a base, show a very high activity in the transfer hydrogenation catalysis of acetophenone, using 2-propanol as a reducing agent under mild conditions. A comparison of the TOF of complexes 5a-5d show that the catalytic activity of complexes increase as the size of the substituents in the backbone of ligands increases (d < a < b = c). 相似文献
966.
Near-infrared (NIR) and mid-infrared (MIR) spectroscopies were used to characterize the hydroxyl groups present in a sepiolite and its calcination products at 250, 500 and 800 °C. Calcining the sepiolite at 250 °C was found to result in its dehydration through the loss of coordination water. Raising the calcination temperature to 500 °C caused the sepiolite structure to fold and hydroxyl groups in octahedral layers to be removed. Finally, calcination at 800 °C resulted in a phase change leading to enstatite, as confirmed by XRD spectroscopy. 相似文献
967.
Statistical uncertainty in the quantitative analysis of solid samples in motion by laser-induced breakdown spectroscopy (LIBS) has been assessed. For this purpose, a LIBS demonstrator was designed and constructed in our laboratory. The LIBS system consisted of a laboratory-scale conveyor belt, a compact optical module and a Nd:YAG laser operating at 532 nm. The speed of the conveyor belt was variable and could be adjusted up to a maximum speed of 2 m s− 1. Statistical uncertainty in the analytical measurements was estimated in terms of precision (reproducibility and repeatability) and accuracy. The results obtained by LIBS on shredded scrap samples under real conditions have demonstrated that the analytical precision and accuracy of LIBS is dependent on the sample geometry, position on the conveyor belt and surface cleanliness. Flat, relatively clean scrap samples exhibited acceptable reproducibility and repeatability; by contrast, samples with an irregular shape or a dirty surface exhibited a poor relative standard deviation. 相似文献
968.
Dario A. Donatti Dimas R. Vollet A. Ibañez Ruiz 《Journal of Sol-Gel Science and Technology》2010,54(2):243-248
Small-angle X-ray scattering (SAXS) was employed to study the nanostructural properties of poly(ethylene glycol)(PEG)/silica
hybrid wet gels prepared from hydrolysis of tetraethoxysilane (TEOS) in mixtures with PEG/(PEG + TEOS) molar ratio in the
nominal range between 0 and 0.8. The SAXS pattern was found to be well fitted by the scattering from a polymeric particle
of radius of gyration R
g with an internal structure of primary silica particles described by a polymeric constraint of a mass-fractal of dimension
D. R
g increases with the PEG quantity while D first increases from 2.24 for the pure TEOS-derived gel to reach values between 2.46 and 2.40 with the additions of PEG.
The correlation volume V
c as determined by SAXS for such a polymeric structure behaves as a volume-fractal and accordingly scales with R
g as V
c ~ R
g
D
. An overall mean value of about 1 nm could be estimated as an upper limit for the radius of the primary silica particle building
up the structure of all the wet gels, independent of the PEG addition. 相似文献
969.
970.
Elodie Boisselier Dr. Liyuan Liang Maria Dalko‐Csiba Dr. Jaime Ruiz Dr. Didier Astruc Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(20):6056-6068
Titrations of commercial diaminobutane (DAB) and polyamidoamine (PAMAM) dendrimers by vitamins C (ascorbic acid, AA), B3 (nicotinic acid), and B6 (pyridoxine) were monitored by 1H NMR spectroscopy using the chemical shifts of both dendrimer and vitamin protons and analyzed by comparison with the titration of propylamine. Quaternarizations of the terminal primary amino groups and intradendritic tertiary amino groups, which are nearly quantitative with vitamin C, were characterized by more or less sharp variations (Δδ) of the 1H chemical shift (δ) at the equivalence points. The peripheral primary amino groups of the DAB dendrimers were quaternarized first, but not selectively, whereas a sharp chemical‐shift variation was recorded for the inner methylene protons near the tertiary amines, thereby indicating encapsulation, when all the dendritic amines were quaternarized. With DAB‐G5‐64‐NH2, some excess acid is required to protonate the inner amino groups, presumably because of basicity decrease due to excess charge repulsion. On the other hand, this selectivity was not observed with PAMAM dendrimers. The special case of the titration of the dendrimers by vitamin B6 indicates only dominant supramolecular hydrogen‐bonding interactions and no quaternarization, with core amino groups being privileged, which indicates the strong tendency to encapsulate vitamins. With vitamin B3, a carboxylic acid, titration of DAB‐G3‐16‐NH2 shows that only six peripheral amino groups are protonated on average, even with excess vitamin B3, because protonation is all the more difficult due to increased charge repulsion, as positive charges accumulate around the dendrimer. Inner amino groups interact with this vitamin, however, thus indicating encapsulation presumably with supramolecular hydrogen bonding without much charge transfer. 相似文献