全文获取类型
收费全文 | 940篇 |
免费 | 76篇 |
国内免费 | 179篇 |
专业分类
化学 | 1146篇 |
晶体学 | 8篇 |
力学 | 4篇 |
综合类 | 3篇 |
物理学 | 34篇 |
出版年
2023年 | 10篇 |
2022年 | 28篇 |
2021年 | 24篇 |
2020年 | 23篇 |
2019年 | 26篇 |
2018年 | 35篇 |
2017年 | 27篇 |
2016年 | 41篇 |
2015年 | 43篇 |
2014年 | 38篇 |
2013年 | 107篇 |
2012年 | 39篇 |
2011年 | 48篇 |
2010年 | 28篇 |
2009年 | 46篇 |
2008年 | 46篇 |
2007年 | 55篇 |
2006年 | 52篇 |
2005年 | 40篇 |
2004年 | 48篇 |
2003年 | 56篇 |
2002年 | 37篇 |
2001年 | 24篇 |
2000年 | 35篇 |
1999年 | 34篇 |
1998年 | 31篇 |
1997年 | 27篇 |
1996年 | 23篇 |
1995年 | 22篇 |
1994年 | 22篇 |
1993年 | 18篇 |
1992年 | 14篇 |
1991年 | 7篇 |
1990年 | 7篇 |
1989年 | 1篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 4篇 |
排序方式: 共有1195条查询结果,搜索用时 0 毫秒
91.
Linear alkylbenzene sulfonic acid, the largest-volume synthetic surfactant, in addition to its excellent performance, is important
due to its biodegradable environmental friendliness, as it has a straight chain and is prepared by the sulphonation of linear
alkylbenzenes (LAB). To ensure environmental protection, the commercial benzene alkylation catalysts HF or AlCl3 are replaced and we have developed a clean LAB production process using a pillared clay catalyst capable of not only replacing
the conventional homogeneous catalysts, but also having high selectivity for the best biodegradable 2-phenyl LAB isomer. Pillared
clay catalysts having high Br?nsted acidity show efficient conversion in gas phase alkylation of benzene with 1-octene with
a good 2-phenyl octane selectivity. 相似文献
92.
V. Renugopalakrishnan G. Madrid G. Cuevas A. T. Hagler 《Journal of Chemical Sciences》2000,112(1):35-42
Density functional theory was applied to the calculation of molecular structures of N-methyl formamide (NMF), N,N-dimethyl
formamide (DMF), and N,N-dimethyl acetamide (DMA). DFT calculations on NMF, DMF, and DMA were performed using a combination
of the local functional of Vosko, Wilk, and Nusair (VWN) with the nonlocal exchange functional of Becke and the nonlocal correlational
functional of Lee, Yang, and Parr (BLYP). The adiabatic connection method (ACM) of Becke has also been used, for the first
time, for the calculation of molecular structures of NMF, DMF, and DMA. The calculated molecular structures are in excellent
agreement with the experimental geometries of NMF and DMA derived from gas-phase electron-diffraction studies. Sparse experimental
data on the gas-phase geometry of DMF reported in the literature compares well with the DFT results on DMF. DFT emerges as
a powerful method to calculate molecular structures. 相似文献
93.
Joakim Balogh 《Advances in colloid and interface science》2010,159(1):22-31
Microemulsions based on nonionic surfactants of the ethylene oxide alkyl ether type CmEn, have been studied thoroughly for around 30 years. Thanks to the considerable amount of published data available on these systems, it is possible to observe trends to make predictions of phase diagrams not yet determined. Strey and Kahlweit, and subsequently Sottmann and Strey, with coworkers have studied and published phase diagrams for systems with a fixed ratio of oil to water, varying the surfactant, the so-called Kahlweit fish-cut diagrams. Some properties of the phase diagrams can be scaled to become general and not system dependent. Here are shown two examples of scaling data from phase diagrams and the use of trends to determine phase diagrams, both inside and outside a dataset. The trends of microemulsions with fixed ratio of surfactant to oil, the so-called Lund-cut diagrams, are also investigated. The trends are used to determine a new phase diagram and this is compared with previously unpublished experimental data on C12E5-Octadecane-Water system. The scalings and trends make it possible to get good estimations of many of the important properties of the phase diagrams, both temperatures and surfactant concentrations of interest, by investigating one sample in the 3-phase region of the balanced fish-cut diagram. 相似文献
94.
Summary Retention indices and their energy equivalents were determined for ten alkylthiazoles analysed on the stationary phases Apiezon
L, Triton X-305 and Carbowax 1000. It was found that the energy contributions of identical alkyl groups to the partial molar
free energy of sorption of a molecule are not equivalent. Smaller energy contribututions are due to the α-position of alkyl
groups with respect to the heteroatoms in the thiazole ring (α-effect). This effect was most pronounced for 2-alkylthiazoles.
Higher sorption energies were found for dimethylthiazoles with two methylgroups in a vicinal position. The retention indices
were predicted for ten other di- and tri-substituted methyl, ethyl and propyl thiazoles on the basis of the so-called α- and
ortho-effects. The predicted values are in good agreement with the experimentally determined values.
Dedicated to Prof. Dr. E. sz. Kováts (Ecole Polytechnique Fédérale de Lausanne) on the occasion of his sixtieth birthday. 相似文献
95.
Akikazu Matsumoto Ryuji Kotaki Takayuki Otsu 《Journal of polymer science. Part A, Polymer chemistry》1991,29(12):1707-1715
Bulk polymerization of alkyl N,N-dialkylfumaramates (FAE) and maleamates (MAE) was performed in the presence of a radical initiator. It has been found that FAE is more reactive than MAE when the reactivity of the two geometrical isomers was compared for their homo- and copolymerizations. From investigation on the effect of ester and N-substituents of these monomers, it has been found that the isopropyl ester shows a higher reactivity than the methyl ester and that N-ethyl and n-butyl substitution gives polymers with high molecular weight of more than several thousands. The resulting substituted polymethylenes from FAE and MAE were characterized and compared with each other. The isomerization of MAE to FAE with morpholine as an isomerization catalyst and monomer-isomerization radical polymerization were also investigated. 相似文献
96.
研究了芳环上取代基诱导效应能和共轭效应能的分离,Ph(X)n的(X)n基属性从能量角度提出量化解释及应用,发现各类芳环取代基都不同程度存在两种σ_c、σ_I效应,且存有群轨道作用。 相似文献
97.
K. S. Birdi 《Colloid and polymer science》1982,260(6):628-631
In microemulsions consisting of four components, i.e. detergent — water — oil — cosurfactant, the free energy of transfer from the continuous oil phase to the interfacial region for the cosurfactant is reported. From the effect of temperature on the free energy, the entropy and the enthalpy values are also reported. The effect of chain length of the alcohol (cosurfactant) is also described. It is further shown, that if the oil phase consists of hexadecane, then the free energy changes as a linear function of the number of carbon atoms in the cosurfactant. On the other hand, if the oil phase is benzene, the cosurfactant chain length has very little effect. These data are analyzed with respect to the microemulsion structure and stability. 相似文献
98.
烷基在针状焦形成中的作用 总被引:2,自引:0,他引:2
采用煤焦油沥青的甲苯可溶物和废聚苯乙烯共碳化制备针状焦,考察了中间相沥青中烷基含量对针状焦性能的影响。结果表明,通过添加废聚苯乙烯共碳化,中间相沥青中烷基的质量分数从12.0%增加到33.3%,可以制备出热膨胀系数更低、光学各向异性排列更好的针状焦。代表光学各向异性指数的两个量,在光学结构单元中,平行于热膨胀系数方向的轴向分矢量平均长度和光学结构单元的矢量平均长度,分别从20.8μm 和23.4μm增加到28.4μm和28.8μm,热膨胀系数从0.8×10-6 /K降低到0.1×10-6 /K。针状焦形成过程中中间相沥青的烷基含量增加使体系黏度降低,有利于光学各向异性相的融并和定向排列。烷基的增多在固化阶段产生足量的气体,在向外逸出过程中使融并中间相的芳香平面大分子沿轴向排列的更为规整。 相似文献
99.
100.
Dr. Shihui Li Dr. Meiyan Wang Bo Liu Lei Li Prof. Jianhua Cheng Dr. Chunji Wu Dr. Dongtao Liu Prof. Dr. Jingyao Liu Prof. Dr. Dongmei Cui 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(47):15493-15498
The first four‐coordinate methanediide/alkyl lutetium complex (BODDI)Lu2(CH2SiMe3)2(μ2‐CHSiMe3)(THF)2 (BODDI=ArNC(Me)CHCOCHC(Me)NAr, Ar=2,6‐iPr2C6H3) ( 1 ) was synthesized by a thermolysis methodology through α‐H abstraction from a Lu–CH2SiMe3 group. Complex 1 reacted with equimolar 2,6‐iPrC6H3NH2 and Ph2C?O to give the corresponding lutetium bridging imido and oxo complexes (BODDI)Lu2(CH2SiMe3)2(μ2‐N‐2,6‐iPr2C6H3)(THF)2 ( 2 ) and (BODDI)Lu2(CH2SiMe3)2(μ2‐O)(THF)2 ( 3 ). Treatment of 3 with Ph2C?O (4 equiv) caused a rare insertion of Lu–μ2‐O bond into the C?O group to afford a diphenylmethyl diolate complex 4 . Reaction of 1 with PhN=C?O (2 equiv) led to the migration of SiMe3 to the amido nitrogen atom to give complex (BODDI)Lu2(CH2SiMe3)2‐μ‐{PhNC(O)CHC(O)NPh(SiMe3)‐κ3N,O,O}(THF) ( 5 ). Reaction of 1 with tBuN?C formed an unprecedented product (BODDI)Lu2(CH2SiMe3){μ2‐[η2:η2‐tBuNC(=CH2)SiMe2CHC?NtBu‐κ1N]}(tBuN?C)2 ( 6 ) through a cascade reaction of N?C bond insertion, sequential cyclometalative γ‐(sp3)‐H activation, C?C bond formation, and rearrangement of the newly formed carbene intermediate. The possible mechanistic pathways between 1 , PhN?C?O, and tBuN?C were elucidated by DFT calculations. 相似文献