全文获取类型
收费全文 | 1048篇 |
免费 | 14篇 |
国内免费 | 105篇 |
专业分类
化学 | 938篇 |
晶体学 | 2篇 |
力学 | 9篇 |
综合类 | 1篇 |
数学 | 140篇 |
物理学 | 77篇 |
出版年
2023年 | 199篇 |
2022年 | 13篇 |
2021年 | 10篇 |
2020年 | 18篇 |
2019年 | 26篇 |
2018年 | 29篇 |
2017年 | 26篇 |
2016年 | 13篇 |
2015年 | 17篇 |
2014年 | 48篇 |
2013年 | 73篇 |
2012年 | 35篇 |
2011年 | 57篇 |
2010年 | 42篇 |
2009年 | 66篇 |
2008年 | 60篇 |
2007年 | 72篇 |
2006年 | 55篇 |
2005年 | 49篇 |
2004年 | 55篇 |
2003年 | 17篇 |
2002年 | 25篇 |
2001年 | 11篇 |
2000年 | 9篇 |
1999年 | 13篇 |
1998年 | 19篇 |
1997年 | 11篇 |
1996年 | 21篇 |
1995年 | 7篇 |
1994年 | 11篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 8篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1978年 | 1篇 |
1970年 | 1篇 |
1968年 | 2篇 |
排序方式: 共有1167条查询结果,搜索用时 187 毫秒
101.
Ngoc Binh Vo Le Anh Nguyen Tung Lam Pham Duy Tien Doan Thanh Binh Nguyen Quoc Anh Ngo 《Tetrahedron letters》2017,58(25):2503-2506
A procedure to exclusively obtain 3′S-cyanoanhydrovinblastine 12 from two naturally occurring vinca-alkaloids (catharanthine and vindoline) in one step with good yield is described. Stereoselective reductions of 12, providing straightforward access to three new vinca-alkaloids, including two diastereomers 3′S-cyano-(4′R,5′-dihydro)-anhydrovinblastine and 3′S-cyano-(4′S,5′-dihydro)-anhydrovinblastine as well as (3′S-aminomethyl)-(4′S,5′-dihydro)-anhydrovinblastine in good yields is also reported. 相似文献
102.
Long Chen František Hartl Howard M. Colquhoun Barnaby W. Greenland 《Tetrahedron letters》2017,58(19):1859-1862
A library of 18 conjugated, rod-like compounds with either one or two viologen residues were synthesized and characterized electrochemically. Containing up to 8 aromatic/heterocyclic rings in conjugation, the members of the library differ in the substitution pattern of electron-withdrawing or -donating groups on the aromatic substituents of the viologen units. The first and second half-wave potentials of each member were found to be linearly correlated with the calculated pKb values of the aromatic end-groups. This relationship will enable the half-wave potentials of related, novel, substituted viologen species to be predicted using a simple, empirical formula. 相似文献
103.
Perovskites as Precursors for Ni/La2O3 Catalysts in the Dry Reforming of Methane: Synthesis by Constant pH Co‐Precipitation,Reduction Mechanism and Effect of Ru‐Doping 下载免费PDF全文
Stefanie Kühl Hendrik Düdder Frank Girgsdies Kevin Kähler Martin Muhler Malte Behrens 《无机化学与普通化学杂志》2017,643(16):1088-1095
LaNiO3 perovskite is an interesting precursor for Ni/La2O3 catalysts for the dry reforming of methane at high temperatures. Precursors have been synthesized by co‐precipitation without, with 2.5 at %, and with 5 at % Ru doping. The presence of Ru leads to a stabilization of the perovskite structure and hinders the decomposition into NiO and Ruddlesden‐Popper mixed oxides Lan+1NinO3n+1, which was observed for the Ru‐free sample upon calcination at 1000 °C (n = 3). Upon reduction in hydrogen, a mechanism involving at least two steps was observed and the first major step was identified as the partial reduction of the precursor leading to a LaNiO2.5‐like intermediate. The second major step is the reduction to Ni metal supported on La2O3 independent of the Ru content of the catalyst. In the presence of Ru, indications for Ni‐Ru alloy formation and for a higher dispersion of the metallic phase were found. The catalytic activity in DRM of the catalyst containing 2.5 % Ru was superior to the catalysts with more or without Ru. Furthermore, the propensity of coke formation was reduced by the presence of Ru. 相似文献
104.
Surabhi Gupta Popuri Sureshbabu Adesh Kumar Singh Shahulhameed Sabiah Jeyakumar Kandasamy 《Tetrahedron letters》2017,58(10):909-913
A simple and efficient method for the deoxygenation of amine N-oxides to corresponding amines is reported using the green and economical reagent phenylboronic acid. Deoxygenation of N,N-dialkylaniline N-oxides, trialkylamine N-oxides and pyridine N-oxides were achieved in good to excellent yields. The reduction susceptible functional groups such as ketone, amide, ester and nitro groups are well tolerated with phenylboronic acid during the deoxygenation process even at high temperature. In addition, an indirect method for identification and quantification of tert-amine N-oxide is demonstrated using UV–Vis spectrometry which may be useful for drug metabolism studies. 相似文献
105.
以氧化石墨烯(GO)为原料,N-溴代丁二酰亚胺(NBS)为溴代试剂,硫代硫酸钠为还原剂,通过羧基化、溴化和还原三步法,采用自由基反应的方式制备了溴功能化还原氧化石墨烯(rGOBr).通过X射线衍射、扫描电子显微镜、红外光谱、拉曼光谱以及X射线光电子能谱等手段对rGOBr的结构、微观形貌和元素组成进行了表征.结果表明,溴元素以共价键的形式分布在石墨烯表面.本方法原料来源广泛、操作简单且条件温和,为石墨烯的溴功能化提供了一条新途径. 相似文献
106.
The chemo- and regioselectivity of the reduction, oxidation and Wittig reaction of polyfunctional pyrroles, containing a variety of reactive centres was investigated. The reaction of 3,5-dichloropyrrole-2,4-dicarboxaldehydes with potassium permanganate leads to regioselective oxidation of the 2-formyl group, while the Wittig reaction with 1 equiv of a triphenylphosphorane produced the 2-alkenyl substituted pyrroles. 相似文献
107.
Umberto M. Battisti Marina M. Carrozzo Carlo Parenti Luigino Troisi 《Tetrahedron letters》2010,51(33):4433-4436
A simple and efficient synthetic path for N-1 or N-2 alkyl-substituted 2-aminobenzenesulfonamides was developed based on regioselective reduction with NaBH3CN in different solvents. 相似文献
108.
Nickel and iron substituted LaCoO3 with rhombohedrally distorted perovskite structure were obtained in the temperature range of 600-900 °C by thermal decomposition of freeze-dried citrates and by the Pechini method. The crystal structure, morphology and defective structure of LaCo1−xNixO3 and LaCo1−xFexO3 were characterized by X-ray diffraction and neutron powder diffraction, TEM and SEM analyses and electron paramagnetic resonance spectroscopy. The reducibility was tested by temperature programmed reduction with hydrogen. The products of the partial and complete reduction were determined by ex-situ XRD experiments. The replacement of Co by Ni and Fe led to lattice expansion of the perovskite structure. For perovskites annealed at 900 °C, there was a random Ni, Fe and Co distribution. The morphology of the perovskites does not depend on the Ni and Fe content, nor does it depend on the type of the precursor used. LaCo1−xNixO3 perovskites (x>0.1) annealed at 900 °C are reduced to Co/Ni transition metal and La2O3 via the formation of oxygen deficient Brownmillerite-type compositions. For LaCo1−xNixO3 annealed at 600 °C, Co/Ni metal, in addition to oxygen-deficient perovskites, was formed as an intermediate product at the initial stage of the reduction. The interaction of LaCo1−xFexO3 with H2 occurs by reduction of Co3+ to Co2+ prior to the Fe3+ ions. The reducibility of Fe-substituted perovskites is less sensitive towards the synthesis procedure in comparison with that of Ni substituted perovskites. 相似文献
109.
In this paper, we focus on the reducibility of graphene oxide by hydrazine hydrate. The main emphasis is placed on the problem of the dosage of hydrazine hydrate and the reaction time on the reduction of chemical groups. This paper describes a system for the analysis of FT-IR. A model is developed for the FT-IR analysis using exponential decay function (Y=Y0e−RX). The height of the peaks in FT-IR spectra of the chemical groups was fitted with exponential decay function. Decay constant R is given the name of Reducibility Factor R. Emphasis is placed on the decay constant R in the fitting functions by which the reducibility of the chemical groups on graphene oxide is evaluated. It is found that R is different for different chemical groups. The sequence is ROH>RC=O>RC−O−C. This means that C−O−C is the most difficult one to be reduced, OH is the most easy, and C=O is between C−O−C and OH. The experimental results reveal a basis for the research of the application of graphene oxide. 相似文献
110.
The chemomechanical properties and microstructural stability of nanocrystalline PrxCe1 − xO2 − δ solid solutions are studied as a function of temperature by in situ X-ray diffraction measurements under oxidizing conditions at P(O2) ~ 200 mbar. The chemical expansion coefficient of nanocrystalline powder specimens, operative at intermediate temperatures during which Pr4+ is reduced to Pr3+, is found to be similar to that obtained for coarse-grained PrxCe1 − xO2 − δ. This is contrary to reports regarding variation of physical and chemical properties with crystallite size. The thermal expansion coefficient, measured under conditions for which PrxCe1 − xO2 − δ is highly oxygen deficient, was found to be greater than that measured for fully oxidized PrxCe1 − xO2 − δ, with potential sources of these changes discussed. Moreover, the microstructure of nanocrystalline PrxCe1 − xO2 − δ is observed to have excellent stability at working temperatures below 800 °C, enabled by the inherent microstrain in the structure, highlighting the potential application of this material for solid state electrochemical devices. 相似文献