全文获取类型
收费全文 | 2910篇 |
免费 | 169篇 |
国内免费 | 140篇 |
专业分类
化学 | 1007篇 |
晶体学 | 9篇 |
力学 | 107篇 |
综合类 | 9篇 |
数学 | 747篇 |
物理学 | 1340篇 |
出版年
2024年 | 3篇 |
2023年 | 45篇 |
2022年 | 25篇 |
2021年 | 43篇 |
2020年 | 65篇 |
2019年 | 51篇 |
2018年 | 39篇 |
2017年 | 57篇 |
2016年 | 53篇 |
2015年 | 67篇 |
2014年 | 101篇 |
2013年 | 155篇 |
2012年 | 94篇 |
2011年 | 121篇 |
2010年 | 93篇 |
2009年 | 177篇 |
2008年 | 189篇 |
2007年 | 270篇 |
2006年 | 230篇 |
2005年 | 138篇 |
2004年 | 172篇 |
2003年 | 148篇 |
2002年 | 132篇 |
2001年 | 123篇 |
2000年 | 96篇 |
1999年 | 90篇 |
1998年 | 80篇 |
1997年 | 98篇 |
1996年 | 70篇 |
1995年 | 37篇 |
1994年 | 21篇 |
1993年 | 15篇 |
1992年 | 19篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 15篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1959年 | 1篇 |
排序方式: 共有3219条查询结果,搜索用时 78 毫秒
81.
The photochemical reaction channels of cyclobutanone have been studied at the CASSCF level with a 6‐31G* basis set. Starting from the n‐π* excited‐state (S1) cyclobutanone, the three reactions can take place: decarbonylation (produce CO and cyclopropane or propylene), cycloelimination (produce ketene and ethylene), and ring expansion (produce oxacarbene). Our computation indicates that decarbonylation products CO and triplet trimethylene are formed on the triplet n‐π* excited state (T1) in a stepwise way via a biradical intermediate after intersystem crossing (ISC) to T1 from S1. And, then, the triplet trimethylene undergoes a second ISC to the ground state (S0) to produce the singlet trimethylene from which cyclopropane can be produced rapidly only overcoming a 1 to 2‐kcal/mol barrier while propylene can be formed as a secondary product. The cycloelimination products ketene and ethylene are formed on the S0 in a concerted mechanism after internal conversion (IC) to S0 from S1 via a biradical conical intersection. The reaction channels corresponding to ring expansion on the S0, T1, and S1 states have also been discussed, and the likeliest reaction path is that oxacarbene is formed on the ground state following S1/S0 internal conversion. The surface topology of cyclobutanone on the S1 surface is characterized by a transition state separating the minimum from the S1/S0 conical intersection, which is consistent with the previous computations and can explain the wavelength dependence of the fluorescence emission yield. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 相似文献
82.
Fractal dimension of a carious tooth surface was determined using an electrochemical method. The method was based on time-dependent diffusion towards electrode surfaces, which is one of the most useful and reliable methods for the determination of fractal dimension of electrode surfaces. For this purpose, the tooth was covered with a gold layer, which acted as an electrode in electrochemical experiments. It is suggested that the fractal dimension can be used as a quantitative measure of the state of dental surfaces. The method presented demonstrates the power of electrochemical techniques for the determination of fractal dimension of surface of non-conducting objects. 相似文献
83.
Abraham F. Jalbout 《International journal of quantum chemistry》2007,107(6):1532-1539
This work presents the results of solvation of electrons on several hypothetical cyclooctane and cyclohexane molecular surfaces, using the hydrogen fluoride (HF) dimer. These complexes were constructed with extensive OH groups on one side of a hydrocarbon surface (i.e., cyclohexane sheets), which creates hydrogen‐bonded networks that can form, increasing the dipole moment of the system. Concurrently, the hydrogen atoms on the opposite side of the surface form a pocket of positive charge that can attract excess electrons. Two possible orientations for HF dimer solvation on eight molecular surfaces that have been demonstrated to be stable toward electron detachment are examined. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
84.
The Studies of the Heterogeneous Properties of Solid Surfaces by Means of the Derivatograph Q-1500 D
A simple method has been adopted to obtain an important information on the heterogeneous properties of materials studied,
adsorbed liquid films and liquid-solid interactions. The method utilizes Q-TG mass loss and the first derivative Q-DTG mass
loss curves with respect to temperature and time obtained during programmed liquid thermodesorption in quasi-isothermal conditions.
The values of the adsorption capacity, total porosity of material, the value of active centers, desorption energy distribution
and mesopore-size distribution functions obtained by this method are in good agreement with those estimated on the basis of
independent methods. The theoretical and experimental results provided novel and unique data on the heterogeneity of solid
surfaces, properties of liquid adsorbed films and thermal stability of the liquid/solid interfaces.
This paper presents the more important results obtained so far in the studies the liquid/solid systems by means of the Paulik's
classical and quasi-isothermal techniques.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
85.
86.
In this study, well-defined, high density poly(2-(dimethylamino)ethyl methacrylate) [poly(DMAEMA)] brushes were fabricated by the combination of the self-assembly of a monolayer of RAFT agent and surface-mediated RAFT polymerization. The whole fabrication process of the poly(DMAEMA) was followed by water contact angles, grazing angle-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. Kinetic studies revealed a linear increase in poly(DMAEMA) film thickness with polymerization time, indicating that the chain growth from the surface was a controlled process. Characterization of the poly(DMAEMA) brushes, such as molecular weight and thickness determination, were measured by gel permeation chromatography, and ellipsometry, and the grafting density was estimated. The pH response of the poly(DMAEMA) brushes was further investigated and the results verified the “brush-like” to “mushroom-like” transition of the poly(DMAEMA) chains due to the reversible protonation/deprotonation upon changing the solution pH. 相似文献
87.
88.
Cangyi Chen Ping Tang Feng Qiu 《Journal of polymer science. Part A, Polymer chemistry》2014,52(24):1583-1599
Binary polymer brushes, including mixed homopolymer brushes and diblock copolymer brushes, are an attractive class of environmentally responsive nanostructured materials. Owing to microphase separation of the two chemically distinct components in the brush, multifaceted nanomaterials with functionalized and patterned surfaces can be obtained. This review summarizes recent progress on the theory and simulations related to binary polymer brushes grafted to flat, spherical, and cylindrical substrates, with a focus on patterned morphologies of multifaceted hairy nanoparticles, an intriguing class of hybrid nanostructured particles (e.g., nanospheres and nanorods). In particular, powerful field theory and particle-based simulations suitable for revealing novel structures on these patterned surfaces, including self-consistent field theory and dissipative particle dynamics simulations, are emphasized. The unsolved yet critical issues in this research field, such as dynamic response of binary polymer brushes to environmental stimuli and the hierarchical self-assembly of binary hairy nanoparticles, are briefly discussed. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1583–1599 相似文献
89.
Multiaxial mechanical behavior of aramid fibers and identification of skin/core structure from single fiber transverse compression testing 下载免费PDF全文
Judith Wollbrett‐Blitz Sébastien Joannès Rémi Bruant Christophe Le Clerc Marc Romero De La Osa Anthony Bunsell Alba Marcellan 《Journal of Polymer Science.Polymer Physics》2016,54(3):374-384
The transverse and longitudinal mechanical properties of aramid fibers like Kevlar? 29 (K29) fibers are strongly linked to their highly oriented structure. Mechanical characterization at the single fiber scale is challenging especially when the diameter is as small as 15 µm. Longitudinal tensile tests on single K29 fibers and single fiber transverse compression test (SFTCT) have been developed. Our approach consists of coupling morphological observations and mechanical experiments with SFTCT analysis by comparing analytical solutions and finite element modeling. New insights on the analysis of the transverse direction response are highlighted. Systematic loading/unloading compression tests enable to experimentally determine a transverse elastic limit. Taking account of the strong anisotropy of the fiber, the transverse mechanical response sheds light on a skin/core architecture. More importantly, results suggest that the skin of the fiber, typically representing a shell of one micrometer in thickness, has a transverse apparent modulus of 0.2 GPa. That is around more than fifteen times lower than the transverse modulus of 3.0 GPa in the core. By comparison, the measured longitudinal modulus is about 84 GPa. The stress distribution in the fiber is explored and the critical areas for damage initiation are discussed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 374–384 相似文献
90.