全文获取类型
收费全文 | 24069篇 |
免费 | 2058篇 |
国内免费 | 5309篇 |
专业分类
化学 | 27571篇 |
晶体学 | 178篇 |
力学 | 309篇 |
综合类 | 116篇 |
数学 | 432篇 |
物理学 | 2830篇 |
出版年
2024年 | 64篇 |
2023年 | 368篇 |
2022年 | 714篇 |
2021年 | 819篇 |
2020年 | 1211篇 |
2019年 | 1018篇 |
2018年 | 1063篇 |
2017年 | 978篇 |
2016年 | 1074篇 |
2015年 | 1023篇 |
2014年 | 1506篇 |
2013年 | 2308篇 |
2012年 | 1466篇 |
2011年 | 1685篇 |
2010年 | 1287篇 |
2009年 | 1431篇 |
2008年 | 1476篇 |
2007年 | 1629篇 |
2006年 | 1475篇 |
2005年 | 1380篇 |
2004年 | 1379篇 |
2003年 | 1034篇 |
2002年 | 612篇 |
2001年 | 504篇 |
2000年 | 512篇 |
1999年 | 435篇 |
1998年 | 391篇 |
1997年 | 339篇 |
1996年 | 316篇 |
1995年 | 303篇 |
1994年 | 251篇 |
1993年 | 196篇 |
1992年 | 195篇 |
1991年 | 144篇 |
1990年 | 111篇 |
1989年 | 103篇 |
1988年 | 85篇 |
1987年 | 53篇 |
1986年 | 49篇 |
1985年 | 56篇 |
1984年 | 49篇 |
1983年 | 26篇 |
1982年 | 36篇 |
1981年 | 36篇 |
1980年 | 27篇 |
1979年 | 25篇 |
1978年 | 27篇 |
1977年 | 29篇 |
1976年 | 36篇 |
1974年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
Gabor Schmera Adi Bulsara David Pierson Frank Moss Enrico Di Cera 《Journal of statistical physics》1993,71(5-6):1179-1190
We consider the class of experiments which can be characterized by a Fokker-PIanck dynamics corresponding to the overdamped motion of a state point in a suitable stochastic potential. We assume that the general form of the potential is known (or can be guessed with reasonable accuracy), but that its parameters are to be determined experimentally by measurements made with a noisy instrument. This possible method for determining the potential parameters, which exploits the system's own internal stochastic motion in order to explore rapidly its available parameter space, is substantially more efficient than traditional methods involving time averages of single point measurements, and yet does not appear to have been previously considered. The method could be important when, for example, the experiment must be completed in a limited time owing either to the expense of the experimental materials or to the temporary stationarity of the preparation, situations which are commonly encountered in experimental biochemistry and biology. 相似文献
32.
33.
The blends composed of polyamide 6 (PA6) and polyamide 66 (PA66) were obtained using two different preparation methods, one of which was the melt‐mixing through a twin‐screw extruder and the subsequent injection molding; and the other, the in situ blending through anionic polymerization of ε‐caprolactam in the presence of PA66. For the former, there existed a remarkable improvement in toughness but a drastic drop in strength and modulus; however, for the latter, a reverse but less significant trend of mechanical properties change appeared. Various characterizations were conducted, including the analyses of crystalline morphology, crystallographic form, and crystallization and melting behaviors using polarized optical microscopy (POM), wide‐angle X‐ray diffraction (WAXD), and differential scanning calorimetry (DSC), respectively; observation of morphology of fractured surface with scanning electron microscope (SEM); measurement of glass transition through dynamic mechanical analysis (DMA); and the intermolecular interaction as well as the interchange reaction between the two components by Fourier transform infrared spectrometry (FT‐IR) and 13C solution NMR. The presence and absence of interchange reaction was verified for the in situ and melt‐mixed blends, respectively. It is believed that the transreaction resulted in a drop in glass transition temperature (Tg) for the in situ blends, contrary to an increase of Tg with increasing PA66 content for the melt‐mixed ones. And the two kinds of fabrication methods led to significant differences in the crystallographic form, spherulite size and crystalline content and perfection as well. Accordingly, it is attempted to explain the reasons for the opposite trends of changes in the mechanical properties for these two blends. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1176–1186, 2007 相似文献
34.
We consider a spatially distributed hybrid system consisting of a convection/reaction system in which the reaction switches discontinuously in time between modes, independently at each spatial point on reaching “switching thresholds”. The model involves a novel formulation for evolution of the free boundary between the modal regions. 相似文献
35.
G. N. Merrill 《Journal of Physical Organic Chemistry》2007,20(1):19-29
Ab initio molecular orbital calculations were carried out on epibromohydrin (EBH) and epichlorohydrin (ECH) in an attempt to elucidate their reactivity with respect to a hard nucleophile, hydroxide. These systems were modeled in both the gas phase and a polar solvent under basic conditions. In the gas phase, it was determined that a direct displacement mechanism (nucleophilic attack at the C1 position) was operative for EBH, while an indirect pathway (nucleophilic attack at the C3 position and subsequent intramolecular displacement) was followed for ECH. In an acetone solution, only the indirect displacement mechanism was found to occur. An electrostatic argument is advanced to account for this behavior in polar solution. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
36.
Vicenç Branchadell Roser Pleixats Claude Commandeur Max Malacria 《Journal of organometallic chemistry》2003,687(2):337-345
Regioselectivity of the Tsuji-Trost reaction on allyl acetates and carbonates substituted with silyl groups at the olefinic moiety has been analyzed. Silicon atom in the central carbon atom increases the stability of the intermediate π-allylpalladium cation with respect to the isomeric π-allylpalladium cation featuring the silicon in a terminal carbon atom. 相似文献
37.
Arata Yajima Ayumi Yamaguchi Fumihiro Saitou Tomoo Nukada Goro Yabuta 《Tetrahedron》2007,63(5):1080-1084
A convergent synthetic strategy for abietane diterpenoids via B-alkyl Suzuki-Miyaura coupling and Lewis acid-mediated cyclization reactions is established. Asymmetric total synthesis of 12-deoxyroyleanone, an antileishmanial diterpene, is described. 相似文献
38.
Nanocrystalline FeS2 cathode material of lithium cell was synthesized from cheap materials of FeSO4, Na2S2O3, and sulfur by a hydrothermal process. The scanning electron microscopy analysis showed the obtained material was nano-sized,
about 500 nm. The X-ray powder diffraction analysis showed that the synthetic FeS2 material had two phases of the crystalline structure, pyrite and marcasite. The phase of marcasite seems to have no negative
effect on the electrochemical performance of the material. The synthetic FeS2 showed a significant improvement of electrochemical performance for Li/FeS2 cells. 相似文献
39.
J. Mass M. Avella J. Jimnez M. Callahan E. Grant K. Rakes D. Bliss B. Wang 《Superlattices and Microstructures》2007,42(1-6):306
Understanding the luminescence of ZnO is very important for some applications. In spite of the many studies carried out, there are still some points concerning the origin of some of the luminescence emissions in ZnO crystals that require additional study; in particular, the role of extended defects remains to be a matter of controversy. We present here a cathodoluminescence analysis of the defects generated by Vickers indentation in hydrothermal HTT crystals. Special emphasis was paid to the luminescence band peaking around 3.3 eV. The origin of this band is a matter of controversy, since it has been related to different causes, extended defects being one of the candidates for this emission. The CL images were acquired around crystal defects. It is observed that the 3.3 eV emission is enhanced around the crystal defects; though it is also observed, but weaker, out of the defect regions, which suggests that there exist two luminescence emissions peaking very close to 3.3 eV. The two emissions, one related to structural defects and the other to the LO phonon replica of the free excitonic band, appear very close each other and their relative intensity should determine the shape of the spectrum. 相似文献
40.
The hydrothermal synthesis and magnetic entropy change for the perovskite manganite La0.5Ca0.3Sr0.2MnO3 have been studied. The La0.5Ca0.3Sr0.2MnO3 can be produced as phase-pure, crystalline powders in one step from solutions of metal salts in aqueous potassium hydroxide solution at a temperature of 513 K in 72 h. Scanning electron microscopy shows that the materials are made up of cuboid-shaped particles in typical dimension of 4.0×2.5×1.6 μm. Heat treatment can improve the magnetocaloric effect for the hydrothermal sample. The maximum magnetic entropy change ΔSM for the as-prepared sample is 0.88 J kg−1 K−1 at 315 K for a magnetic field change of 2.0 T. It increases to 1.52 J kg−1 K−1, near its Curie temperature (317 K) by annealing the sample at 1473 K for 6 h. The hydrothermal synthesis method is a feasible route to prepare high-quality perovskite material for magnetic refrigeration application. 相似文献