全文获取类型
收费全文 | 1559篇 |
免费 | 67篇 |
国内免费 | 11篇 |
专业分类
化学 | 939篇 |
晶体学 | 4篇 |
力学 | 78篇 |
综合类 | 1篇 |
数学 | 377篇 |
物理学 | 238篇 |
出版年
2023年 | 8篇 |
2022年 | 13篇 |
2021年 | 21篇 |
2020年 | 23篇 |
2019年 | 23篇 |
2018年 | 27篇 |
2017年 | 21篇 |
2016年 | 53篇 |
2015年 | 53篇 |
2014年 | 49篇 |
2013年 | 95篇 |
2012年 | 94篇 |
2011年 | 108篇 |
2010年 | 74篇 |
2009年 | 70篇 |
2008年 | 89篇 |
2007年 | 100篇 |
2006年 | 89篇 |
2005年 | 83篇 |
2004年 | 81篇 |
2003年 | 76篇 |
2002年 | 62篇 |
2001年 | 22篇 |
2000年 | 19篇 |
1999年 | 24篇 |
1998年 | 13篇 |
1997年 | 19篇 |
1996年 | 21篇 |
1995年 | 13篇 |
1994年 | 15篇 |
1993年 | 13篇 |
1992年 | 11篇 |
1991年 | 13篇 |
1990年 | 15篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1985年 | 13篇 |
1984年 | 12篇 |
1983年 | 10篇 |
1982年 | 12篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1979年 | 5篇 |
1977年 | 11篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1970年 | 3篇 |
排序方式: 共有1637条查询结果,搜索用时 15 毫秒
1.
Jocelyn Sabatier Serge Poullain Patrick Latteux Jean Luc Thomas Alain Oustaloup 《Nonlinear dynamics》2004,38(1-2):383-400
Robust speed control of a low damped electromechanical system with backlash is studied, controlled load angular speed being not measured. The proposed control strategy combines a Luenberger observer (load angular speed and load torque disturbance estimations) and a robust CRONE controller. The observer provides estimation of the load angular speed and of the disturbance torque applied on the load. Through the computation of only three independent parameters (as many as a PID controller), the CRONE controller permits to ensure the robust speed control of the load in spite of plant parametric variations and speed observation errors. The proposed control strategy is applied to a four mass experimental test bench. 相似文献
2.
Ccile Nouvel Philippe Dubois Edith Dellacherie Jean‐Luc Six 《Journal of polymer science. Part A, Polymer chemistry》2004,42(11):2577-2588
The whole controlled synthesis of novel amphiphilic polylactide (PLA)‐grafted dextran copolymers was achieved. The control of the architecture of such biodegradable and potentially biocompatible copolymers has required a three‐step synthesis based on the “grafting from” concept. The first step consisted of the partial silylation of the dextran hydroxyl groups. This protection step was followed by the ring‐opening polymerization of D ,L ‐lactide initiated from the remaining OH functions of the partially silylated polysaccharide. The third step involved the silylether group deprotection under very mild conditions. Based on previous studies, in which the control of the first step was achieved, this study is focused on the last two steps. Experimental conditions were investigated to ensure a controlled polymerization of D ,L ‐lactide, in terms of grafting efficiency, graft length, and transesterification limitation. After polymerization, the final step was studied in order to avoid degradation of both polysaccharide backbone and polyester grafts. The chemical stability of dextran backbone was checked throughout each step of the synthesis. PLA‐grafted dextrans and PLA‐grafted (silylated dextrans) were proved to adopt a core‐shell conformation in various solvents. Furthermore, preliminary experiments on the potential use of these amphiphilic grafted copolymers as liquid/liquid interface stabilizers were performed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2577–2588, 2004 相似文献
3.
Luc Hillairet 《Annales Henri Poincare》2006,7(4):689-710
We consider Riemannian surfaces in which there are many embedded Euclidean cylinders. We show that quantitative assumptions
on the number of such cylinders provide estimates for the counting function of eigenvalues. These results in particular apply
to translation surfaces.
Communicated by Jens Marklof
Submitted: 3/03/ 2005 Accepted: 18/11/2005 相似文献
4.
L.J. Vanmaele P.J. De Clercq M. Vandewalle S.J. Halkes W.R.M. Overbeek 《Tetrahedron》1984,40(7):1179-1182
An efficient method for the preparation of crystalline α-hydroxy previtamin D3 is described. Also the stereoselective reduction of 1-keto previtamin is discussed; it was observed that with aluminum hydride is the most abundant product. This stands in contrast with previously reported LiAlH4 and NaBH4 mediated reductions. 相似文献
5.
Luc Maurette 《Tetrahedron》2004,60(44):10077-10098
Three different kinds of ring carbo-mers of [5]cyclitol ethers were targeted as challenging examples of functional [5]pericyclynes. Three tertiary pentaaryl-carbo-[5]cyclitol methyl ethers were synthesized through a [11+4] ring-closing double addition of triphenyl- and tri-p-anisyl-undecatetrayn-diides to dibenzoylacetylene. These compounds, obtained as oily mixtures of stereoisomers, are stable and can behave as acetylenic ligands of one or two Co2(CO)6 units. NMR analysis reveals that the broad diasteroisomeric dispersity of a triether, is consistently reduced in the symmetrized pentaether. Three bis-secondary triaryl-carbo-[5]cyclitol methyl ethers with adjacent CH(OR) vertices were synthesized through a similar [11+4] ring-closing process, where the same tetrayn-diides add to both the carbaldehyde ends of the (η2-OCH-CC-CHO)Co2(CO)6 complex. Despite the possibility of tautomeric isomerization, the occurrence of two adjacent bis-propargylic carbinol vertices does not diminish the stability of the [5]pericyclyne framework. Finally, two bis-secondary carbo-[5]cyclitol methyl ethers with non-adjacent CH(OH) vertices were synthesized through an alternative [10+5] ring-closing process. The bis-secondary carbo-[5]cyclitols are regarded as isohypsic equivalents of the challenging [C,C]5carbo-cyclopentadienyl cation. A diphenyl-hexaoxy-[5]pericyclyne with two non-adjacent secondary carbinol vertices was also prepared through a [10+5] ring-closing strategy: this molecule is an isohypsic equivalent of the previously calculated zwitterionic carbo-cyclopentadienone, which could be observed as a DCI/NH3-MS fragment after treatment with SnCl2/HCl. Analytical HPLC showed that the C11 triphenyl-undecatetrayne precursor of the [11+4] strategy was obtained as a statistical 1:2:1 mixture of the three possible diastereoisomers. Semi-preparative HPLC allowed for the resolution of this mixture. The pure major diastereoisomer was employed to prepare a partly resolved sample of pentamethoxy-pentaphenyl-[5]pericyclyne. Analytical HPLC showed that the latter corresponds to the statistical distribution of the expected three residual diastereoisomers. Semi-preparative HPLC finally afforded samples of diastereoisomerically pure pentamethoxy-[5]pericyclyne as crystalline solids. 相似文献
6.
7.
P. Luc R. Vetter C. Bauche-Arnoult J. Bauche 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1994,31(3):145-148
High accuracy measurements of hyperfine structure due to47Ti and49Ti in the 3d 2 4s 2 a 3 F 2?3d 2 4s4p z 5 D 1 absorption line at σ=18482.772 cm?1 have been performed by use of a Doppler-free experiment, where a beam of titanium atoms is crossed by a CW single mode tunable dye laser. They have allowed for the determination of isotope shifts between46Ti,47Ti,48Ti,49Ti and50Ti. By use of accurate values of mean square nuclear charge radii for the even isotopes, it has been possible to separate mass shifts from field shifts and to determine accurate values for the mean square nuclear charge radii of47Ti and49Ti. The field shift presents a marked odd-even staggering. 相似文献
8.
The results of D(-) ion desorption induced by 3-20 eV electrons incident on condensed CD(4), C(2)D(6), C(3)D(8), C(2)D(4), and C(2)D(2) are presented. These compounds were deposited in submonolayer amounts on the surfaces of multilayer solid films of Kr and nonporous and porous amorphous ice. While desorption of the D(-) anions proceeds via well-known processes, i.e., dissociative electron attachment (DEA) and dipolar dissociation, significant perturbations of these processes due to presence of the different film substrates are observed. We have shown that it is possible to distinguish between the character and nature of these perturbations. The presence of the nonporous ice perturbs the D(-) desorption intensity by affecting the intrinsic properties of the intermediate anion states through which dissociation proceeds. On the other hand, the presence of the porous ice introduces extrinsic effects, which can affect electron energy losses prior to their interaction with the hydrocarbon molecule and/or the energies and intensities of the fragment species after dissociation. Simple mechanisms responsible for the observed variations in the intensities of desorbed anionic signals are proposed and discussed. Electron transfer from transient anion states to electron states of the substrate film or nearby hydrocarbon molecules appear as the most efficient mechanism to reduce the magnitude of the DEA process. 相似文献
9.
The influence of using normal-phase and reversed-phase versions of four commercial polysaccharide stationary phases on chiral separations was investigated with capillary electrochromatography (CEC). Both versions of the stationary phases, Chiralcel OD, OJ, and Chiralpak AD, AS were tested for the separation of two basic, two acidic, a bifunctional, and a neutral compound. Different background electrolytes were used, two at low pH for the acid, bifunctional and neutral substances, and three at high pH for the basic, bifunctional and neutral ones. This setup allowed evaluating differences between both stationary-phase versions and between mobile-phase compositions on a chiral separation. Duplicate CEC columns of each stationary phase were in-house prepared and tested, giving information about the intercolumn reproducibility. In general, reversed-phase versions of the current commercial polysaccharide stationary phases are found to be best for reversed-phase CEC, even though at high pH no significant differences were seen between both versions. Most differences were observed at low pH. For acidic compounds, it was seen that an ammonium formate electrolyte performed best, which is also an excellent electrolyte if coupling with mass spectrometry is desired. For basic, bifunctional and neutral compounds, no significant differences between the three tested electrolytes were observed at high pH. Here, a phosphate buffer is preferred as electrolyte because of its buffering capacities. However, if coupling to mass spectrometry is wanted, the more volatile ammonium bicarbonate electrolyte can be used as an alternative. 相似文献
10.
John W. Tromp Robert J. Le Roy Simon Gerstenkorn Paul Luc 《Journal of Molecular Spectroscopy》1983,100(1):82-94
The disagreement of Danyluk and King's (Chem. Phys.25, 343 (1977)) rotational constants for levels lying near the dissociation limit of B-state I2 with the mechanical behavior predicted by near-dissociation theory is investigated. The discrepancies are shown to be much too large to be explained by either the neglect of centrifugal distortion effects in the original analysis or by rotational or spin-rotation coupling to a nearby repulsive 1u state. These differences are therefore attributed to experimental error, a conclusion which is confirmed by more recent experimental results. A reanalysis of the best available data for levels near the dissociation limit of B-state I2 then yields improved values for the B-state dissociation limit = 20 043.16 (±0.02) cm?1 of the vibrational index at dissociation v = 87.32 (±0.04) and of the long-range potential constant . This in turn implies a slightly improved ground-state dissociation energy of 0 = 12 440.18 (±0.02) cm?1. 相似文献