全文获取类型
收费全文 | 16966篇 |
免费 | 2467篇 |
国内免费 | 3609篇 |
专业分类
化学 | 15423篇 |
晶体学 | 593篇 |
力学 | 2157篇 |
综合类 | 113篇 |
数学 | 464篇 |
物理学 | 4292篇 |
出版年
2024年 | 49篇 |
2023年 | 272篇 |
2022年 | 504篇 |
2021年 | 786篇 |
2020年 | 951篇 |
2019年 | 693篇 |
2018年 | 600篇 |
2017年 | 686篇 |
2016年 | 960篇 |
2015年 | 917篇 |
2014年 | 1022篇 |
2013年 | 1502篇 |
2012年 | 1105篇 |
2011年 | 1023篇 |
2010年 | 953篇 |
2009年 | 1097篇 |
2008年 | 1103篇 |
2007年 | 1218篇 |
2006年 | 1065篇 |
2005年 | 1013篇 |
2004年 | 953篇 |
2003年 | 803篇 |
2002年 | 588篇 |
2001年 | 447篇 |
2000年 | 424篇 |
1999年 | 338篇 |
1998年 | 315篇 |
1997年 | 282篇 |
1996年 | 268篇 |
1995年 | 228篇 |
1994年 | 156篇 |
1993年 | 116篇 |
1992年 | 121篇 |
1991年 | 73篇 |
1990年 | 65篇 |
1989年 | 44篇 |
1988年 | 53篇 |
1987年 | 51篇 |
1986年 | 47篇 |
1985年 | 35篇 |
1984年 | 21篇 |
1983年 | 8篇 |
1982年 | 18篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1973年 | 7篇 |
1971年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
71.
72.
An FT-IR study of pyrrole self-association in CCl4 solutions was carried out. According to the IR measurements, pyrrole forms self-associated dimeric species via N-H?π hydrogen bonding. This was also confirmed by quantum chemical calculations for pyrrole monomer and dimer at B3LYP/6-31++G(d,p) level of theory. A T-shaped minimum was located on B3LYP/6-31++G(d,p) PES of pyrrole dimer characterized with a hydrogen bond of an N-H?π type, with centers-of-mass separation of monomeric units of 4.520 Å, H?π distance of 2.475 Å, the interplanar angle between the two monomeric units being 72.9°. The anharmonic vibrational frequency shift upon dimer formation calculated on the basis of 1D DFT vibrational potentials is in excellent agreement with the experimental data (84 vs. 87 cm−1). Harmonic vibrational analysis predicts somewhat smaller shift (68 cm−1). On the basis of NIR spectroscopic data, anharmonicity constants for the 2ν(N-H) and 2ν(N-H?π) vibrational transitions were calculated. The orientational dynamics of monomeric and self-associated pyrrole species was studied within the framework of the transition dipole moment time correlation function formalism. The period of essentially free rotation in the condensed phase reduces from 0.05 ps for the monomeric pyrrole to 0.02 ps for the proton-donor molecule within the dimer. 相似文献
73.
Nongelatin dichromated holographic film(NGD) is a new holographic recordingmaterial.Holograms recorded on this material have better environmental stability, higherdiffraction efficiency and stronger real time effect, etc..An experimental study to ascertainthe influence of two electron donors, namely,N,N-dimethylformamide(DMF) and dimethyl-sulfoxide(DMSO),on the real time diffraction efficiency(RTDE) of NGD holograms is car-ried out.The pressence of electron donors not only improves the sensitivity,but also enhancesthe real time effect greatly.RTDE of above 60% at±1 orders of NGD plane transimissiongrating is achieved by introducing electron donors. 相似文献
74.
75.
MDF materials are chemically bonded ceramic materials free of the macrodefects typical of hydraulic cement-based materials. MDF materials arising through reactions of sulfo-aluminate-ferrite belitic (SAFB) clinkers and/or Portland cements (PC) with two types of water-soluble polymer (hydroxy-propylmethyl cellulose {HPMC}, polyphosphate glass {poly-P}) are discussed. Mixes of low energy SAFB clinkers with Portland cement, HPMC and, especially poly-P comprise promising cross-linked compositions additional to the better known MDF materials formed from high alumina cement with polyvinylalcohol/acetate. The principles of co-ordination of P and C atoms (of the polymer) with Al and Fe atoms (originating from the cement) are highlighted from spectroscopic information on next-nearest-neighbour interactions, along with the effects of second co-ordination spheres. Polymers modify the interface through functional bonding/grafting of polymer chains onto the surfaces of cement grains. Both the cross-linked atomic structure and the interface coincide well with the model of functional polymers and represent a new type of atomic-level structure in polymer-modified cements. Interpretation is based on previous magnetic resonance and thermal analysis studies. The compactness of Al(Fe)-O-P cross-links reduces transport through the interfaces, increasing the interfacial interactions and resisting the unfavourable uptake of moisture and carbonation. 相似文献
76.
析氢反应动力学的交流阻抗法研究 总被引:1,自引:0,他引:1
利用交流阻抗法测定析氢电化学动力学参数,得到与极化曲线法一致的结果,为确定复杂析氢过程速率的决定步骤提供了判据。 相似文献
77.
To improve aircraft crash safety, conditions critical to occupants survival during a crash must be known. In view of the importance of this problem, studies of post-crash dynamic behavior of victims are necessary in order to reduce severe injuries. In this study, crash dynamics program SOM-LA/TA, incorporating a dynamic model of the human body with a finite element model of the seat structure was used. Modifications were performed in the program for reconstruction of an occupant's head impact with the interior walls or bulkhead. A viscoelastic-type contact force model was used to represent the compliance characteristics of the bulkhead. Correlated studies of analytical simulations with impact sled test results were accomplished. A parametric study of the coefficients in the contact force model was then performed in order to obtain the correlations between the coefficients and the Head Injury Criteria. A measure of optimal values for the bulkhead compliance and displacement requirements was thus achieved in order to keep the possibility of a head injury as little as possible. This information could in turn be used in the selection of suitable materials for the bulkhead, instrument panel, or interior walls of an aircraft. 相似文献
78.
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
79.
80.
仓库容量有限条件下的一类存贮管理模型 总被引:10,自引:1,他引:9
建立了一类仓库容量有限条件下存贮管理决策模型 ,给出最优存贮策略 . 相似文献