全文获取类型
收费全文 | 1154篇 |
免费 | 36篇 |
国内免费 | 232篇 |
专业分类
化学 | 967篇 |
力学 | 19篇 |
综合类 | 1篇 |
数学 | 33篇 |
物理学 | 402篇 |
出版年
2024年 | 9篇 |
2023年 | 102篇 |
2022年 | 36篇 |
2021年 | 29篇 |
2020年 | 26篇 |
2019年 | 27篇 |
2018年 | 17篇 |
2017年 | 27篇 |
2016年 | 36篇 |
2015年 | 36篇 |
2014年 | 30篇 |
2013年 | 88篇 |
2012年 | 62篇 |
2011年 | 86篇 |
2010年 | 61篇 |
2009年 | 91篇 |
2008年 | 117篇 |
2007年 | 66篇 |
2006年 | 74篇 |
2005年 | 57篇 |
2004年 | 51篇 |
2003年 | 38篇 |
2002年 | 32篇 |
2001年 | 33篇 |
2000年 | 27篇 |
1999年 | 26篇 |
1998年 | 23篇 |
1997年 | 18篇 |
1996年 | 17篇 |
1995年 | 16篇 |
1994年 | 3篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 8篇 |
1982年 | 4篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 4篇 |
1967年 | 1篇 |
排序方式: 共有1422条查询结果,搜索用时 15 毫秒
61.
以多孔Al2O3陶瓷为基体材料, 采用浸渍法担载NiO后用2B铅笔修饰NiO/Al2O3表面, 通过化学镀法沉积约5 μm厚的金属钯, 还原后成功制得Pd/Pencil/Ni/Al2O3膜. 为进行对比, 还制备了未担载镍的Pd/Pencil/Al2O3膜. 膜的表面和断面形貌分别采用扫描电镜和金相显微镜观测, 膜的透氢动力学通过H2/N2单气体法测试, 并以成分为H2 77.8%, CO 5.2%, CO2 13.5%和CH4 3.5%的原料氢测定了膜的氢分离效果. 结果表明, 未载镍的Pd/Pencil/Al2O3膜只具有氢分离作用, 而Pd/Pencil/Ni/Al2O3膜还可以有效地将钯膜泄漏的CO和CO2转化为甲烷, 因而成为双功能型钯膜. 这种双功能膜尤其适用于面向质子交换膜燃料电池(PEMFC)的氢气分离, 既有效解决了PEMFC对氢燃料中CO格外敏感的难题, 又提高了对钯膜缺陷的容忍度, 因而延长了钯膜的使用寿命. 相似文献
62.
采用两步还原法制得Co@Pt/C核壳结构催化剂, 其中Co与Pt 的总质量分数为20%. 通过改变金属前驱体的用量, 制备了不同Co:Pt 原子比的Co@Pt/C 催化剂, 以20% (w) Co@Pt(1:1)/C 与20% (w) Co@Pt(1:3)/C 表示. 采用透射电镜(TEM)、光电子射线能谱分析(XPS)、循环伏安(CV)、线性扫描伏安(LSV)等方法考察了其结构与性能, 并与实验室早先制备的40% (w) Co@Pt/C 催化剂进行了比较. 自制20% Co@Pt(1:1)/C 与20% Co@Pt(1:3)/C 催化剂的金属颗粒直径约为2.2-2.3 nm, 在碳载体上分散均匀, 粒径分布范围较窄, 电化学活性比表面积(ECSA)分别为56 和60 m2·g-1, 均超过商用催化剂20% Pt/C(E-tek) (ECSA=54 m2·g-1). 20%Co@Pt(1:1)/C 与20% Co@Pt(1:3)/C 的半波电位相较于40% Co@Pt(1:1)/C 和40% Co@Pt(1:3)/C 均向正向移动, 表现出更好的氧还原(ORR)催化活性, 并有望降低催化剂的成本, 在质子交换膜燃料电池领域表现出良好的应用前景. 相似文献
63.
利用3,3,4,4-四氟二苯砜、十氟联苯、6F-双酚A及6,13-双三蝶烯二酚,通过亲核取代共聚及后磺化方法制备了2个系列不同磺化度的磺化双三蝶烯型聚芳醚砜,并通过氢核磁共振波谱(1H NMR)对其化学结构进行了表征.研究发现,所得磺化聚芳醚砜均表现出了优异的热稳定性.此类膜材料具有优良的机械性能、尺寸稳定性、氧化稳定性及高温低湿度条件下高的质子传导率.透射电子显微镜的结果表明,聚合物主链中大量氟原子的引入显著改善了聚合物的相分离结构,并且随着聚合物主链中氟含量的增加,亲水区域明显增大.这也是含氟磺化双三蝶烯型聚芳醚砜质子交换膜材料在高温低湿度条件下具有高质子传导率的主要原因. 相似文献
64.
Dr. Carolin Müller Alexander Schwab Dr. Nicholas M. Randell Dr. Stephan Kupfer Prof. Benjamin Dietzek-Ivanšić Dr. Murielle Chavarot-Kerlidou 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(18):e202103882
The design of photoactive systems capable of storing and relaying multiple electrons is highly demanded in the field of artificial photosynthesis, where transformations of interest rely on multielectronic redox processes. The photophysical properties of the ruthenium photosensitizer [(bpy)2Ru( oxim-dppqp )]2+ ( Ru ), storing two electrons coupled to two protons on the π-extended oxim-dppqp ligand under light-driven conditions, are investigated by means of excitation wavelength-dependent resonance Raman and transient absorption spectroscopies, in combination with time-dependent density functional theory; the results are discussed in comparison to the parent [(bpy)2Ru(dppz)]2+ and [(bpy)2Ru( oxo-dppqp )]2+ complexes. In addition, this study provides in-depth insights on the impact of protonation or of accumulation of multiple reducing equivalents on the reactive excited states. 相似文献
65.
《印度化学会志》2022,99(12):100799
A charge transfer hydrogen bonded complex was prepared and experimentally explored in an acetonitrile (ACN) medium between the proton acceptor (electron donor) 2, 3-Diamino-5-bromopyridine and the proton donor (electron acceptor) chloranilic acid. The stoichiometry of the charge transfer complex is 1:1. The Benesi-Hildebrand equation is used to calculate the molar absorptivity (εCT), association constant (KCT) and other spectroscopic physical characteristics. The solid compound was synthesized and studied using several spectroscopic methods. The presence of charge and proton transfers in the resultant complex was supported by 1H NMR, FT-IR and SEM-EDX investigations. The complex DNA binding ability was investigated using electron absorption spectroscopy, and the CT complex binding mechanism is intercalative. The intrinsic binding constant (Kb) value is 5.2 × 106M?1. The good binding affinity of the CT complex makes it potentially suitable for usage as a pharmaceutical in the future. Molecular docking calculations have been performed between CT complex and DNA (ID = 1BNA) to study the CT-DNA interaction theoretically. To corroborate the experimental findings, calculations based on DFT were carried out in the gas and PCM analysis where the existence of charge and hydrogen transfers. Finally, good agreement between experimental and theoretical computations was observed confirming that the basis set used is appropriate for the system under examination. 相似文献
66.
质子交换膜燃料电池(PEMFC)商业化应用的瓶颈仍然是贵金属催化剂导致的成本问题。然而,目前对于催化层中纳米尺度全氟磺酸离聚物(以Nafion为代表)薄膜中质子传导的问题研究不足,无法完善三相界面的成型规律,进而指导催化层设计。在催化层浆料制备过程中,分散溶剂对Nafion的分散形态有直接影响,可能对催化层成型后附着在催化剂颗粒表面Nafion薄膜的微观结构有潜在影响,进而影响Nafion薄膜的质子传导能力。因此,在本文中利用分子自组装技术模拟催化层离聚物薄膜的聚集过程,于模型基底上制备厚度精确可控的纳米尺度Nafion薄膜,并通过微观测试表征技术探索并建立纳米尺度Nafion离聚物的微观结构模型,阐明分散溶剂对Nafion薄膜微观结构及质子传导的影响。研究发现Nafion薄膜在纳米尺度下的质子电导率比体相膜的质子电导率低一个数量级,使用介电常数较小的醇类溶剂可以使Nafion薄膜形成更有利于质子传导的微观结构,使Nafion薄膜的质子电导率得到提高。相关研究结果为优化PEMFC催化层结构,改善PEMFC催化层中质子传导问题提供给了依据。 相似文献
67.
Mixed Quantum Classical Reaction Rates based on the Phase Space Formulation of the Hierarchical Equations of Motion
下载免费PDF全文
![点击此处可从《化学物理学报(中文版)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In a previous work [J. Chem. Phys. 140 , 174105 (2014)], we have shown that a mixed quantum classical (MQC) rate theory can be derived to investigate the quantum tunneling effects in the proton transfer reactions. However, the method is based on the high temperature approximation of the hierarchical equation of motion (HEOM) with the Debye-Drude spectral density, and results in a multi-state Zusman type of equation. We now extend this theory to include quantum effects of the bath degrees of freedom. By writing the full HEOM into a multidimensional partial differential equation in phase space, we can define a new reaction coordinate, and the previous method can be generalized to the full quantum regime. The validity of the new method is demonstrated by using numerical examples, including the spin-Boson model, and the double well model for proton transfer reaction. The new method is found to resolve some key problems of the previous theory based on high temperature approximation, including possible numerical instability in long time simulation and wrong rate constant at low temperatures. 相似文献
68.
A Mechanistic Switch in C-H Bond Activation by Elusive Fe\begin{document}$^\text{V}$\end{document} (O)(TAML) Reaction Intermediate: A Theoretical Study
下载免费PDF全文
![点击此处可从《化学物理学报(中文版)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The divergent behavior of C-H bond oxidations of aliphatic substrates compared to those of aromatic substrates shown in Gupta's experiment was mechanistically studied herein by means of density functional theory calculations. Our calculations reveal that such difference is caused by different reaction mechanisms between two kinds of substrates (the aliphatic cyclohexane, 2, 3-dimethylbutane and the aromatic toluene, ethylbenzene and cumene). For the aliphatic substrates, C-H oxidation by the oxidant Fe\begin{document}$^{\rm{V}}$\end{document} (O)(TAML) is a hydrogen atom transfer process; whereas for the aromatic substrates, C-H oxidation is a proton-coupled electron transfer (PCET) process with a proton transfer character on the transition state, that is, a proton-coupled electron transfer process holding a proton transfer-like transition state (PCET(PT)). This difference is caused by the strong \begin{document}$\pi$\end{document} -\begin{document}$\pi$\end{document} interactions between the tetra-anionic TAML ring and the phenyl ring of the aromatic substrates, which has a "pull" effect to make the electron transfer from substrates to the Fe=O moiety inefficient. 相似文献
69.
Electrochemical hydrogen storage in porous carbon materials is emerging as a cost-effective hydrogen storage and transport technology with competitive power and energy densities. The merits of electrochemical hydrogen storage using porous conductive carbon-based electrodes are reviewed. The employment of acidic electrolytes in such storage systems is compared with alkaline electrolytes. The recent innovations of a proton battery for smaller-scale electricity storage, and a proton flow reactor system for larger (grid)-scale storage and bulk export of hydrogen produced from renewable energy, are briefly described. It is argued that such systems, along with variants proposed by others, all of which rely on electrochemical hydrogen storage in porous carbons, can contribute to the search for energy storage technologies essential for the transition to a zero-emission global economy. 相似文献
70.
Shangwei Zhou Paul R. Shearing Dan J.L. Brett Rhodri Jervis 《Current Opinion in Electrochemistry》2022
Proton exchange membrane fuel cells are considered a promising power supply system with high efficiency and zero emissions. They typically work within a relatively narrow range of temperature and humidity to achieve optimal performance; however, this makes the system difficult to control, leading to faults and accelerated degradation. Two main approaches can be used for diagnosis, limited data input which provides an unintrusive, rapid but limited analysis, or advanced characterisation that provides a more accurate diagnosis but often requires invasive or slow measurements. To provide an accurate diagnosis with rapid data acquisition, machine learning methods have shown great potential. However, there is a broad approach to the diagnostic algorithms and signals used in the field. This article provides a critical view of the current approaches and suggests recommendations for future methodologies of machine learning in fuel cell diagnostic applications. 相似文献