全文获取类型
收费全文 | 2708篇 |
免费 | 459篇 |
国内免费 | 366篇 |
专业分类
化学 | 2610篇 |
晶体学 | 10篇 |
力学 | 3篇 |
综合类 | 10篇 |
数学 | 15篇 |
物理学 | 885篇 |
出版年
2024年 | 5篇 |
2023年 | 38篇 |
2022年 | 100篇 |
2021年 | 106篇 |
2020年 | 112篇 |
2019年 | 109篇 |
2018年 | 105篇 |
2017年 | 105篇 |
2016年 | 161篇 |
2015年 | 139篇 |
2014年 | 130篇 |
2013年 | 208篇 |
2012年 | 154篇 |
2011年 | 179篇 |
2010年 | 159篇 |
2009年 | 185篇 |
2008年 | 165篇 |
2007年 | 192篇 |
2006年 | 161篇 |
2005年 | 151篇 |
2004年 | 129篇 |
2003年 | 113篇 |
2002年 | 99篇 |
2001年 | 84篇 |
2000年 | 79篇 |
1999年 | 49篇 |
1998年 | 59篇 |
1997年 | 42篇 |
1996年 | 38篇 |
1995年 | 17篇 |
1994年 | 28篇 |
1993年 | 24篇 |
1992年 | 16篇 |
1991年 | 9篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 8篇 |
1983年 | 2篇 |
1982年 | 9篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1977年 | 4篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1972年 | 1篇 |
排序方式: 共有3533条查询结果,搜索用时 0 毫秒
101.
102.
Christoph Kaiser Dr. Thomas Halbritter Dr. Alexander Heckel Dr. Josef Wachtveitl 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(35):9160-9173
Photoacids attract increasing scientific attention, as they are valuable tools to spatiotemporally control proton-release reactions and pH values of solutions. We present the first time-resolved spectroscopic study of the excited state and proton-release dynamics of prominent merocyanine representatives. Femtosecond transient absorption measurements of a pyridine merocyanine with two distinct protonation sites revealed dissimilar proton-release mechanisms: one site acts as a photoacid generator as its pKa value is modulated in the ground state after photoisomerization, while the other functions as an excited state photoacid which releases its proton within 1.1 ps. With a pKa drop of 8.7 units to −5.5 upon excitation, the latter phenolic site is regarded a super-photoacid. The 6-nitro derivative exhibits only a phenolic site with similar, yet slightly less photoacidic characteristics and both compounds transfer their proton to methanol and ethanol. In contrast, for the related 6,8-dinitro compound an intramolecular proton transfer to the ortho-nitro group is suggested that is involved in a rapid relaxation into the ground state. 相似文献
103.
为实现单能质子对惯性约束聚变(ICF)内爆过程的诊断,使用FLUKA蒙特卡罗程序模拟分析了影响质子对内爆过程诊断的因素。通过设置不同初始质子数目,来分析质子产额对小球图像质量的影响,结果发现,要得到清晰图像,需要109的质子数。参照实验中常用的靶参数,模拟了直接驱动和间接驱动条件下的照相情况,分析了间接驱动情况下,金属腔对质子束的散射会造成内爆小球图像模糊,说明质子照相不适合间接驱动内爆。给出了一种时间分辨的质子照相方法,通过调整质子产生和内爆产生的时间差,实现了对内爆过程前后不同时刻的照相等。模拟表明,D3He反应产生的14.7 MeV的准单能质子具有的脉宽短、尺寸小的优点,可用于ICF内爆过程的动态照相。 相似文献
104.
The energy transfer from the upper triplet states of diphenylamine, naphthalene and triphenylene to toluene and hexene-1 is studed. Two mechanisms of dissipation transferred energy in solvents are discussed. 相似文献
105.
106.
Chikkannagari Nagamani Jing Guo S. Thayumanavan 《Journal of polymer science. Part A, Polymer chemistry》2012,50(6):1187-1196
Biaryl hydroxy polymers with orthogonal disposition of proton transporting ? OH moieties have been synthesized via conventional free radical polymerization. The polymers are characterized for their thermal stability and proton conductivity, and the results are compared with the corresponding styrenic hydroxy polymers. The orthogonal disposition of ? OH moieties in biaryl polymers does result in lower Ea for proton transport. However, the lower Ea values in biaryl polymers did not translate into a net increase in proton conductivity. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 相似文献
107.
Pure water has been characterized for nearly a century, by its dissociation into hydronium (H3O)1+ and hydroxide (HO)1- ions. As a chemical equilibrium reaction, the equilibrium constant, known as the ion product or the product of the equilibrium concentration of the two ion species, has been extensively measured by chemists over the liquid water temper-ature and pressure range. The experimental data have been nonlinear least-squares fitted to chemical thermodynamic-based equilibrium equations, which have been accepted as the industrial standard for 35 years. In this study, a new and statistical-physics-based water ion product equation is presented, in which, the ions are the positively charged protons and the negatively charged proton-holes or prohols. Nonlinear least squares fits of our equation to the experimental data in the 0-100 ℃ pure liquid water range, give a factor of two better precision than the 35-year industrial standard. 相似文献
108.
109.
Hsu‐Feng Lee Po‐Hsun Wang Yi‐Chiang Huang Wen‐Hung Su Ram Gopal Chun Che Lee Steven Holdcroft Wen‐Yao Huang 《Journal of polymer science. Part A, Polymer chemistry》2014,52(18):2579-2587
A series of sterically‐encumbered, sulfonated, poly(arylene ether) copolymers were synthesized and their proton conductivity examined. The series was prepared by copolymerizing a novel monomer, 2″,3″,5″,6″‐tetraphenyl‐[1,1′:4',1″:4″,1″':4″',1″″‐quinquephenyl]‐4,4″″‐diol, with 4,4'‐difluorobenzophenone and bisphenol A. Subsequent sulfonation and solution casting provided membranes possessing ion exchange capacities of 1.9 to 2.7 mmol/g and excellent mechanical properties (Young's modulus, 0.2–1.2 GPa; tensile strength, 35–70 MPa; elongation at break, 62–231%). Water uptake ranged from 34 to 98 wt% at 80 °C/100% RH. Proton conductivities ranged between 0.24 to 16 mS/cm at 80 °C/60% RH, and 3 to 167 mS/cm at 80 °C/95% RH. TEM analysis of the polymers, in the dehydrated state, revealed isolated spherical aggregates of ions, which presumably coalesce when hydrated to provide highly conductive pathways. The strategy of using highly‐encumbered polymer frameworks for the design of mechanically‐robust and dimensionally‐stable proton conducting membranes is demonstrated. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2579‐2587 相似文献
110.
ABSTRACTThe reaction of formic acid (HCOOH) with chlorine atom and amidogen radical (NH2) have been investigated using high level theoretical methods such BH&HLYP, MP2, QCISD, and CCSD(T) with the 6–311?+?G(2df,2p), aug-cc-pVTZ, aug-cc-pVQZ and extrapolation to CBS basis sets. The abstraction of the acidic and formyl hydrogen atoms of the acid by the two radicals has been considered, and the different reactions proceed either by a proton coupled electron transfer (pcet) and hydrogen atom transfer (hat) mechanisms. Our calculated rate constant at 298?K for the reaction with Cl is 1.14?×?10?13?cm3?molecule?1?s?1 in good agreement with the experimental value 1.8?±?0.12/2.0?×?10?13?cm3?molecule?1?s?1 and the reaction proceeds exclusively by abstraction of the formyl hydrogen atom, via hat mechanism, producing HOCO+ClH. The calculated rate constant, at 298?K, for the reaction with NH2 is 1.71?×?10?15?cm3?molecule?1?s?1, and the reaction goes through the abstraction of the acidic hydrogen atom, via a pcet mechanism, leading to the formation of HCOO+NH3. 相似文献