全文获取类型
收费全文 | 1502篇 |
免费 | 45篇 |
国内免费 | 5篇 |
专业分类
化学 | 926篇 |
晶体学 | 1篇 |
力学 | 48篇 |
数学 | 361篇 |
物理学 | 216篇 |
出版年
2022年 | 12篇 |
2021年 | 14篇 |
2020年 | 9篇 |
2019年 | 22篇 |
2018年 | 17篇 |
2017年 | 15篇 |
2016年 | 36篇 |
2015年 | 42篇 |
2014年 | 42篇 |
2013年 | 101篇 |
2012年 | 92篇 |
2011年 | 123篇 |
2010年 | 71篇 |
2009年 | 68篇 |
2008年 | 118篇 |
2007年 | 90篇 |
2006年 | 81篇 |
2005年 | 75篇 |
2004年 | 88篇 |
2003年 | 46篇 |
2002年 | 56篇 |
2001年 | 15篇 |
2000年 | 23篇 |
1999年 | 19篇 |
1998年 | 19篇 |
1997年 | 11篇 |
1996年 | 15篇 |
1995年 | 11篇 |
1994年 | 15篇 |
1993年 | 10篇 |
1992年 | 7篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 14篇 |
1983年 | 6篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 9篇 |
1978年 | 18篇 |
1976年 | 6篇 |
1975年 | 7篇 |
1974年 | 6篇 |
1973年 | 6篇 |
1969年 | 4篇 |
排序方式: 共有1552条查询结果,搜索用时 15 毫秒
31.
Ben Salah M Vilminot S André G Richard-Plouet M Bourée-Vigneron F Mhiri T Kurmoo M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(8):2048-2057
[Mn(3)(OH)(2)(SO(4))(2)(H(2)O)(2)] and its deuterated analogue were synthesized by a hydrothermal technique and characterized by differential thermal analysis, thermogravimetric analysis, and IR spectroscopy. Its nuclear structure, determined by single-crystal X-ray analysis and Rietveld analysis of neutron powder-diffraction data, consists of a 3D network of chains of edge-sharing Mn(1)O(6), running along the c axis, connected by the apices of Mn(2)O(6) and SO(4) units. It is isostructural to the nickel analogue. Determination of the magnetic structure and measurements of magnetization and heat capacity indicate the coexistence of both magnetic long-range ordering (LRO) and short-range ordering (SRO) below a Néel temperature of 26 K, while the SRO is retained at higher temperatures. The moments of the two independent Mn atoms lie in the bc plane, and that of Mn(1) rotates continuously by 54 degrees towards the c axis on decreasing the temperature from 25 to 1.4 K. While the SRO may be associated with frustration of the moments within a Mn(3) trimer, the LRO is achieved by antiparallel alignment of the four symmetry-related trimers within the magnetic unit cell. A spin-flop field, measured by dc and ac magnetization on a SQUID, is observed at 15 kOe. 相似文献
32.
Katritzky AR Xu YJ Vakulenko AV Wilcox AL Bley KR 《The Journal of organic chemistry》2003,68(23):9100-9104
Molecules were prepared with substituted nitrobenzyl groups covalently bonded to N-(4-hydroxy-3-methoxybenzyl)acetamide (2) by ether or carbonate linkages. These compounds decomposed under irradiation at 363 nm. Those with carbonate linkages decomposed at slower rates than those with ether linkages. Molecules with dimethoxy-substituted benzyl groups decomposed more slowly than monomethoxy-substituted benzyl groups due to the electronic characteristics of the benzylic carbon. 相似文献
33.
Galtchouk Leonid I. Pergamenshchikov Serge M. 《Statistical Inference for Stochastic Processes》2022,25(1):127-158
Statistical Inference for Stochastic Processes - We consider drift estimation problems for high dimension ergodic diffusion processes in nonparametric setting based on observations at discrete... 相似文献
34.
Crystal structure of di (1-n-dodecylpyridine) decahydro-closo-decaborate(2-) (C5H5N-C12H25)2[B10H10]
A. V. Virovets N. N. Vakulenko V. V. Volkov N. V. Podberezskaya 《Journal of Structural Chemistry》1994,35(3):339-344
Using X-ray analysis, we have determined the crystal structure of di(1-n-dodecylpyridine) decahydrocloso-decaborate(2-), (py-C12H25)2[B10H10] (I), a yellow salt-like substance obtained by reaction of (py-C12H25)Br and K2[B10H10] in an aqueous solution. Compound I melts at 1250 without decomposition and luminesces under ultraviolet radiation (λmax=555 nm at 298 K). In I, quaternary pyridine bases, containing a hydrocarbon radical with the C12 chain, are combined with the closo-cluster hydroborate anion [B10H10]2-. This results in a charge transfer structure having shortened nonvalent (equatorial B) H.…H(C of pyridine) contacts of 2.41(8) Å. Crystals I are orthorhombic with a=8.584 (1), b=7.739(1), c=31.183(5) Å, Vcell=2071.4(5) Å3, space group Pnmm, Z=2, dcalc=0.986 g/cm3 (a Syntex P21 automatic diffractometer, λCUKα, Nmsd/used=1654/711, Raniso=0.076, Rw=0.080, w=1/(σ(Fobs)2+0.0007·F obs 2 ). Substantial changes are observed in the vibration range of the valent B?H bonds in the IR spectrum of I as compared to that of pure ionic K2[B10H10], confirming the interaction. 相似文献
35.
Rajabian Mahmoud Gravelle Denis V. Vacquié Serge 《Plasma Chemistry and Plasma Processing》2004,24(2):261-284
An investigation of the plasma jet generated by a dc argon–nitrogen plasma torch, operated in association with a controlled-pressure chamber, is presented. The purpose of this article is to describe a study of the properties of a subsonic plasma jet under such operating conditions, when its transition to supersonic flow regime is nearly complete. The goal is that of performing plasma diagnostics not only in the initial region of the jet but also in the downstream region where the plasma emission is weak. For this purpose two different diagnostic methods are used. The first approach is based on non-intrusive optical emission spectroscopy, which yields both excitation and rotational temperatures as well as electron number density fields. The zone investigated by this method extends from the torch exit to about 10 nozzle diameters downstream. The second approach consisted of the use of the intrusive enthalpy probe technique for the measurement of the plasma gas temperature, mainly in the tail region of the plasma jet. In the present work, the effects of axial and radial distances across the jet, on the temperature and electron density profiles are discussed for subsonic flow conditions. Interesting features revealed are the data shown for the various diagnostic methods, which either disagree or overlap with each other. Finally, our results show the need for involving non-equilibrium models for the argon–nitrogen plasma due to the presence of significant differences between the temperatures of light and heavy particles. 相似文献
36.
In order to identify possible signatures of the most typical structures adopted by guanine-rich oligonucleotides, we submitted them to the crossed fire of circular dichroism (CD) and electrophoresis. These signatures show up in the circular dichroism spectra even when simultaneously present within the same molecule. Guanine-rich oligonucleotides, when structured, manifest themselves by CD contributions around 260 or 295 nm. For instance, positive bands at 264 nm and 295 nm, respectively, signal the parallel and antiparallel guanine quartets, while a positive band around 261 nm may indicate the presence of a (parallel?) Hoogsteen duplex. A positive band at 264 nm may also reflect the presence of rigidly and unusually oriented GpT and TpG steps within loops. The signatures are additive with those of other structural features of the same molecule, such as hairpins or Watson-Crick duplexes, whose bands are observed at 280 nm. 相似文献
37.
The esterfication of ethylene glycol (EG) with benzoic acid (BA) is an acid-catalyzed reaction which leads, as a first intermediate, to ethylene glycol monobenzoate (EGMB). This compound has an ? OH group which is more active towards carboxylic groups than the first ? OH group of EG. Moreover, it is a key intermediate in the formation of sideproducts containing diethylene glycol (DEG) moieties. The condensation reaction of EGMB was particularly studied in the presence of carboxylic acids or metal derivatives as catalysts. Titanium is the most active. Sb, which is widely used in the industrial synthesis of PET, needs to be present at a high level of concentration to show an activity. The presence of carboxylic groups in the reaction medium does not affect its activity, but it is necessary to remove the ethylene glycol as soon as it is formed in order to keep it active during the condensation reaction. 相似文献
38.
[structures: see text] A versatile (S)-3-(hydroxymethyl)butane-1,2,4-triol building block has been synthesized starting from D-isoascorbic acid, a common food preservative. The key transformation in this approach was the introduction of branching through a high yield and fully regioselective epoxide opening. This flexible synthon has been elaborated to a new class of (dihydro-)N-homo(phyto)ceramides. 相似文献
39.
[reaction: see text] The phosphoramidite 11 was prepared in three steps from methyl 2-mercaptoacetate and demonstrated efficiency in the synthesis of conventional 5'-/3'-phosphate/thiophosphate monoester derivatives of 2'-deoxyribonucleosides and DNA oligonucleotides. Moreover, the use of 11 has enabled the preparation of the dinucleoside phosphorothioate analogue 26 in high yields (>95%) with minimal cleavage (<2%) of the thermolytic thiophosphate protecting group. 相似文献
40.
Boudjahem AG Monteverdi S Mercy M Bettahar MM 《Langmuir : the ACS journal of surfaces and colloids》2004,20(1):208-213
We have studied the effect of silica of quartz-type on the reducibility of nickel acetate in aqueous hydrazine (80 degrees C, pH = 10-12) and metal particle formation. The obtained materials were characterized by X-ray diffraction, transmission electron microscopy, and thermodesorption experiments. With nickel acetate alone, the reduction was partial (45%) and a metal film at the liquid-gas interface or a powdered metal precipitate with an average particle size of 120 nm was obtained. In the presence of silica as the surfactant, the reduction of nickel acetate was total and the nickel phase deposited as a film on the support with an average particle size of 25 nm. Supported nickel acetate was also totally reduced. Crystallites of a mean particle size of about 3 nm were obtained. Decreasing the nickel content or increasing the hydrazine/nickel ratio decreased the metal particle size. Whiskers were formed for low nickel loadings. Hydrogen thermal treatment of the reduced phase showed that the organic acetate fragment, belonging to the precursor salt, still remained strongly attached to the nickel phase. The amount of the retained organic matrix depended on the metal particle size. Surface defects are suggested as active sites, which enhanced nickel ion reduction in the presence of silica as the surfactant or support. Metal-support interactions and the nucleation/ growth rate were the main factors determining the size and morphology of the supported metal particles formed. The organic matrix covered the reduced nickel phase. 相似文献