全文获取类型
收费全文 | 278192篇 |
免费 | 2298篇 |
国内免费 | 707篇 |
专业分类
化学 | 149349篇 |
晶体学 | 4265篇 |
力学 | 12133篇 |
综合类 | 9篇 |
数学 | 30964篇 |
物理学 | 84477篇 |
出版年
2020年 | 2200篇 |
2019年 | 2420篇 |
2018年 | 3229篇 |
2017年 | 3215篇 |
2016年 | 4685篇 |
2015年 | 2860篇 |
2014年 | 4543篇 |
2013年 | 11144篇 |
2012年 | 8671篇 |
2011年 | 10653篇 |
2010年 | 7460篇 |
2009年 | 7198篇 |
2008年 | 10006篇 |
2007年 | 10207篇 |
2006年 | 9609篇 |
2005年 | 8812篇 |
2004年 | 8063篇 |
2003年 | 7251篇 |
2002年 | 7148篇 |
2001年 | 7458篇 |
2000年 | 5787篇 |
1999年 | 4470篇 |
1998年 | 3990篇 |
1997年 | 3992篇 |
1996年 | 3750篇 |
1995年 | 3435篇 |
1994年 | 3593篇 |
1993年 | 3296篇 |
1992年 | 3746篇 |
1991年 | 3741篇 |
1990年 | 3649篇 |
1989年 | 3513篇 |
1988年 | 3622篇 |
1987年 | 3519篇 |
1986年 | 3331篇 |
1985年 | 4446篇 |
1984年 | 4714篇 |
1983年 | 3933篇 |
1982年 | 4071篇 |
1981年 | 4006篇 |
1980年 | 3979篇 |
1979年 | 3934篇 |
1978年 | 4232篇 |
1977年 | 4147篇 |
1976年 | 4198篇 |
1975年 | 3881篇 |
1974年 | 3920篇 |
1973年 | 4076篇 |
1972年 | 2868篇 |
1971年 | 2385篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
M.S. Anagnostou I. Panagiotopoulos A. Kostikas D. Niarchos G. Zouganelis 《Journal of magnetism and magnetic materials》1994,130(1-3):57-62
From the analysis of Mössbauer data for Y2Fe17 and Y2Fe17N3−δ at various temperatures the hyperfine fields for 4f, 6g, 12j, 12k iron sites were estimated as a function of temperature. The reduced magnetizations calculated from the values of the hyperfine fields are fitted with a mean field model for four interacting sublattices using a computer program. The estimated exchange interaction from the fitting procedure between the 4f sites is found strongly negative (antiferromagnetic) in Y2Fe17 whereas in Y2Fe17N3−δ it increases and becomes weak negative following a modified Slater-Néel curve. The rest of the exchange interactions are found positive or weak negative depending on the distances between the Fe atoms. 相似文献
112.
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
113.
Tatiana B. Mikenas Vladimir A. Zakharov Lyudmila G. Echevskaya Mikhail A. Matsko 《Journal of polymer science. Part A, Polymer chemistry》2007,45(22):5057-5066
The effects of polymerization temperature, polymerization time, ethylene and hydrogen concentration, and effect of comonomers (hexene‐1, propylene) on the activity of supported catalyst of composition LFeCl2/MgCl2‐Al(i‐Bu)3 (L = 2,6‐bis[1‐(2,6‐dimethylphenylimino)ethyl] pyridyl) and polymer characteristics (molecular weight (MW), molecular‐weight distribution (MWD), molecular structure) have been studied. Effective activation energy of ethylene polymerization over LFeCl2/MgCl2‐Al(i‐Bu)3 has a value typical of supported Ziegler–Natta catalysts (11.9 kcal/mol). The polymerization reaction is of the first order with respect to monomer at the ethylene concentration >0.2 mol/L. Addition of small amounts of hydrogen (9–17%) significantly increases the activity; however, further increase in hydrogen concentration decreases the activity. The IRS and DSC analysis of PE indicates that catalyst LFeCl2/MgCl2‐Al(i‐Bu)3 has a very low copolymerizing ability toward propylene and hexene‐1. MW and MWD of PE produced over these catalysts depend on the polymerization time, ethylene and hexene‐1 concentration. The activation effect of hydrogen and other kinetic features of ethylene polymerization over supported catalysts based on the Fe (II) complexes are discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5057–5066, 2007 相似文献
114.
115.
116.
117.
118.
W.G. Schmidt 《Applied Physics A: Materials Science & Processing》2002,75(1):89-99
There has been renewed interest in the structure of III-V compound semiconductor (001) surfaces caused by recent experimental
and theoretical findings, which indicate that geometries different from the seemingly well-established dimer models describe
the surface ground state for specific preparation conditions. I review briefly the structure information available on the
(001) surfaces of GaP, InP, GaAs and InAs. These data are complemented with first-principles total-energy calculations. The
calculated surface phase diagrams are used to explain the experimental data and reveal that the stability of specific surface
structures depends largely on the relative size of the surface constituents. Several structural models for the Ga-rich GaAs (001)(4×6)
surface are discussed, but dismissed on energetic grounds. I discuss in some detail the electronic properties of the recently
proposed cation-rich GaAs (001)ζ(4×2) geometry.
Received: 18 May 2001 / Revised version: 23 July 2001 / Published online: 3 April 2002 相似文献
119.
The adsorption of nonionic surfactant Triton X-100 on quartz sand and methylated quartz sand from water and toluene was investigated by means of spectrophotometry, the radiotracer technique, and wetting angle measurements. 相似文献
120.