全文获取类型
收费全文 | 400篇 |
免费 | 13篇 |
国内免费 | 1篇 |
专业分类
化学 | 284篇 |
晶体学 | 15篇 |
力学 | 3篇 |
数学 | 36篇 |
物理学 | 76篇 |
出版年
2023年 | 2篇 |
2022年 | 17篇 |
2021年 | 23篇 |
2020年 | 5篇 |
2019年 | 16篇 |
2018年 | 7篇 |
2017年 | 8篇 |
2016年 | 13篇 |
2015年 | 8篇 |
2014年 | 12篇 |
2013年 | 30篇 |
2012年 | 30篇 |
2011年 | 27篇 |
2010年 | 27篇 |
2009年 | 21篇 |
2008年 | 15篇 |
2007年 | 27篇 |
2006年 | 21篇 |
2005年 | 19篇 |
2004年 | 14篇 |
2003年 | 12篇 |
2002年 | 13篇 |
2001年 | 5篇 |
1999年 | 1篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 1篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1973年 | 4篇 |
1972年 | 1篇 |
1967年 | 2篇 |
排序方式: 共有414条查询结果,搜索用时 0 毫秒
11.
M.E. Mora-Ramos C.A. Duque E. Kasapoglu H. Sari I. Sökmen 《Journal of luminescence》2012,132(4):901-913
In this work we are studying the intense laser effects on the electron-related linear and nonlinear optical properties in GaAs–Ga1?xAlxAs quantum wells under applied electric and magnetic fields. The calculated quantities include linear optical absorption coefficient and relative change of the refractive index, as well as their corresponding third-order nonlinear corrections. The nonlinear optical rectification and the second and third harmonic generation coefficients are also reported. The DC applied electric field is oriented along the hererostructure growth direction whereas the magnetic field is taken in-plane. The calculations make use of the density matrix formalism to express the different orders of the dielectric susceptibility. Additionally, the model includes the effective mass and parabolic band approximations. The intense laser effects upon the system enter through the Floquet method that modifies the confinement potential associated to the heterostructure. The results correspond to several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation. They suggest that the nonlinear optical absorption and optical rectification are nonmonotone functions of the dimensions of the heterostructure and of the external perturbations considered in this work. 相似文献
12.
Munetaka Takeuchi Yasuo Kameda Yasuhiro Umebayashi Sari Ogawa Takaaki Sonoda Shin-ichi Ishiguro Miho Fujita Mitsuru Sano 《Journal of Molecular Liquids》2009,148(2-3):99-108
Self-diffusion coefficients of Li+ DLi+, PF6− DPF6− and solvent propylene carbonate (PC) DPC in LiPF6−PC solutions were determined at 298 K by the pulse gradient spin echo (PGSE) NMR technique over the salt concentration range of 0.1–3.0 M (M = mol dm– 3). The order of the diffusion coefficients was found to be DLi+ < DPF6− < DPC over the concentration range examined, and they were monotonically decreased with increasing the salt concentration. Haven ratio Λ/ΛNMR, where Λ and ΛNMR represent the ionic conductivity measured electrochemically and that estimated via the Nernst-Einstein equation using the diffusion coefficient, respectively, was evaluated as the measure of the ion–ion interaction in the LiPF6–PC solutions. Though Λ/ΛNMR values for LiPF6-solutions decrease with increasing the salt concentration, they were greater than those for LiBF4–PC solutions over the whole concentration range examined, which indicates that the ion pair formation ability of PF6– ion is weaker than that of the BF4– ion. The smaller value of the ionic conductivity for the highly concentrated LiPF6–PC solution (above 2.0 M) than that of the LiBF4-solutions can be attributed to the more rapidly increased viscosity relative to the LiBF4-solution. Classic molecular dynamics (MD) simulations for the respective LiPF6 and LiBF4-solution of 0.5 and 1.0 M were also carried out based on the effective pair potentials. Diffusion coefficients, ionic conductivity and Haven ratio for these solutions were calculated from MD trajectories, and they qualitatively agree with those evaluated by experiments. Pair correlation functions gLiO(r) (for Li+–O (PC) pair) and gLiPF6(r) (for Li+–PF6– pair) or gLiBF4(r) (for Li+–BF4– pair) revealed that the lithium ion weakly forms the contact ion pairs with PF6–, whilst strongly with BF4–, which supports the present experimental results. Moreover, the simulation results show that both anions in the contact ion pairs predominantly take the monodentate form, which is in contrast to the multidentate coordination predicted by ab initio calculation in gas phase. 相似文献
13.
The dissociation constants of carminic acid (7-D-glucopyronosyl-3,5,6,8-tetra- hydroxy-1-methyl-9,10-dioxo-anthracene-2-carboxylic acid) (CA), together with the stability constants of its Cu(II), Zn(II), Ni(II), Co(II) and Hg(II) complexes, were studied potentiometrically in aqueous medium at 25.0?(1)?°C, and at the ionic background of 0.1?mol?dm?3 of NaCl, and determined with the SUPERQUAD computer program. It has been observed that carminic acid has five dissociation constants, and for H5L their values are 3.39?(7), 5.78?(7), 8.35?(7), 10.27?(7), and 11.51?(7). This ligand behaves as a bi-dentate ligand, and the carboxyl and the ortho hydroxy groups of the ligand coordinate to the metal ions. Various metal complexes were produced in solution under the experimental conditions, for each metal ion used, including hydrolyzed species. The species distribution curves of the complexes formed in the solution were calculated and reviewed. The stability of the complexes was found to follow the order: Cu(II) > Zn(II) > Ni(II) > Co(II) > Hg(II). 相似文献
14.
Sari Lehto Lauri Niinistö Inkeri Yliruokanen 《Fresenius' Journal of Analytical Chemistry》1993,346(6-9):608-611
Summary For the analysis of electroluminescent thin films, X-ray fluorescence (XRF) provides a convenient method as both the concentration of the dopant and the film thickness can be determined rapidly and non-destructively. An XRF method for the determination of thickness and lead concentration in lead doped calcium sulfide thin films was developed. Calibration standards made of polyvinyl alcohol and gelatin as well as filter paper standards were used. In addition, the applicability of a fundamental parameter program UniQuant was investigated. For comparison, the concentrations of lead and calcium were determined after dissolution by atomic absorption spectrometry. Generally, the correlation between the different methods excluding the use of filter paper standards was satisfactory. When the dopant concentration was very low or very high, however, the fundamental parameter program yielded best results. Determination of thicknesses by XRF was made by comparing the sulfur K
intensities of the sample and those of a zinc sulfide standard. A correction factor for molar masses and densities was applied. The thicknesses obtained were compared to those measured with a profilometer after etching and the deviations were found to be less than 10%.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday 相似文献
15.
Journal of Mathematical Chemistry - A new implicit-explicit local differential transform method (IELDTM) is derived here for time integration of the nonlinear (2?+?1)-dimensional... 相似文献
16.
Enny Ratnaningsih Dewi Handayani Fatiha Khairunnisa Ihsanawati Sari Dewi Kurniasih Bill Mangindaan Sinta Rismayani Cica Kasipah Zeily Nurachman 《Applied biochemistry and biotechnology》2013,170(1):32-43
Staphylococcus sp. WL1 lipase (LipFWS) was investigated for methanolysis of crude palm oil (CPO) at moderate temperatures. Experiments were conducted in the following order: searching for the suitable bacterium for producing lipase from activated sludge, sequencing lipase gene, identifying lipase activity, then synthesising CPO biodiesel using the enzyme. From bacterial screening, one isolated specimen which consistently showed the highest extracellular lipase activity was identified as Staphylococcus sp. WL1 possessing lipFWS (lipase gene of 2,244 bp). The LipFWS deduced was a protein of 747 amino acid residues containing an α/β hydrolase core domain with predicted triad catalytic residues to be Ser474, His704 and Asp665. Optimal conditions for the LipFWS activity were found to be at 55 °C and pH 7.0 (in phosphate buffer but not in Tris buffer). The lipase had a K M of 0.75 mM and a V max of 0.33 mM?min?1 on p-nitrophenyl palmitate substrate. The lyophilised crude LipFWS performed as good as the commonly used catalyst potassium hydroxide for methanolysis of CPO. ESI-IT-MS spectra indicated that the CPO was converted into biodiesel, suggesting that free LipFWS is a worthy alternative for CPO biodiesel synthesis. 相似文献
17.
Katell Fiselier Florian Grundböck Karsten Schön Oliver Kappenstein Karla Pfaff Christoph Hutzler Andreas Luch Koni Grob 《Journal of chromatography. A》2013,1271(1):192-200
So far the majority of the measurements of mineral oil saturated hydrocarbons (MOSH) and mineral oil aromatic hydrocarbons (MOAH) were obtained from on-line high performance liquid chromatography–gas chromatography–flame ionization detection (on-line HPLC–GC–FID). Since this technique is not available in many laboratories, an alternative method with more easily available tools has been developed. Preseparation on a small conventional liquid chromatographic column was optimized to achieve robust separation between the MOSH and the MOAH, but also to keep out the wax esters from the MOAH fraction. This was achieved by mixing a small portion of silica gel with silver nitrate into highly activated silica gel and by adding toluene into the eluent for the MOAH. Toluene was also added to the MOSH fraction to facilitate reconcentration and to serve as a keeper preventing loss of volatiles during solvent evaporation. A 50 μl volume was injected on-column into GC–FID to achieve a detection limit for MOSH and MOAH below 1 mg/kg in most foods. 相似文献
18.
A new nickel(II) σ‐aryl complex, trans‐chloro(9‐phenanthrenyl)bis(triphenylphosphine)nickel(II), was used as a precatalyst for the Suzuki–Miyaura coupling reactions of aryl chlorides. The catalytic conditions were optimized by investigating the cross‐coupling of p‐chloroanisole with phenylboronic acid. The results show that this complex is efficient for both electron‐rich and electron‐deficient aryl chlorides, though it gives better yields for activated arylboronic acids than deactivated ones. All isolated cross‐coupled biaryl products have been characterized by 1H and 13C NMR, and their spectral data are consistent with those reported. Side products from the coupling of arylboronic acid with the precatalyst complex have also been isolated and characterized, which is helpful for understanding the coupling mechanism. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
19.
A more consistent, straightforward, and economical protocol for generation of stannylene species and their reaction with BnBr leading to products of O-monobenzylation of diols has been set. It has shown to be specially indicated for substrates bearing vicinal trans 1,2-diol moieties on cyclohexane backbones, which are more resistant to these transformations. Such protocol has been successfully applied to myo-inositol derivatives and acyclic diols. 相似文献
20.
Simulation of aerated lagoon using artificial neural networks and multivariate regression techniques
Karla Patricia Oliveira-Esquerre Aline C. da Costa Roy Edward Bruns Milton Mori 《Applied biochemistry and biotechnology》2003,106(1-3):437-449
The aim of this study was to develop an empirical model that provides accurate predictions of the biochemical oxygen demand
of the output stream from the aerated lagoon at International Paper of Brazil, one of the major pulp and paper plants in Brazil.
Predictive models were calculated from functional link neural networks (FLNNs), multiple linear regression, principal components
regression, and partial least-squares regression (PLSR). Improvement in FLNN modeling capability was observed when the data
were preprocessed using the PLSR technique. PLSR also proved to be a powerful linear regression technique for this problem,
which presents operational data limitations. 相似文献