全文获取类型
收费全文 | 3956篇 |
免费 | 1023篇 |
国内免费 | 391篇 |
专业分类
化学 | 1432篇 |
晶体学 | 73篇 |
力学 | 463篇 |
综合类 | 117篇 |
数学 | 590篇 |
物理学 | 2695篇 |
出版年
2024年 | 9篇 |
2023年 | 32篇 |
2022年 | 89篇 |
2021年 | 90篇 |
2020年 | 130篇 |
2019年 | 94篇 |
2018年 | 103篇 |
2017年 | 166篇 |
2016年 | 184篇 |
2015年 | 142篇 |
2014年 | 230篇 |
2013年 | 294篇 |
2012年 | 299篇 |
2011年 | 295篇 |
2010年 | 271篇 |
2009年 | 279篇 |
2008年 | 273篇 |
2007年 | 279篇 |
2006年 | 266篇 |
2005年 | 245篇 |
2004年 | 232篇 |
2003年 | 177篇 |
2002年 | 160篇 |
2001年 | 170篇 |
2000年 | 145篇 |
1999年 | 77篇 |
1998年 | 103篇 |
1997年 | 76篇 |
1996年 | 69篇 |
1995年 | 70篇 |
1994年 | 41篇 |
1993年 | 45篇 |
1992年 | 48篇 |
1991年 | 31篇 |
1990年 | 24篇 |
1989年 | 16篇 |
1988年 | 30篇 |
1987年 | 12篇 |
1986年 | 14篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1974年 | 5篇 |
1973年 | 2篇 |
排序方式: 共有5370条查询结果,搜索用时 437 毫秒
101.
Dimethyl 1′,1′-ureylenedi(1-ferrocenecarboxylate) (1) formed during the synthesis of 1-amino, 1′-ferrocenecarboxylic acid shows virtual molecular centrosymmetry. Electronic coupling between the two Fc groups through the ureylene bridge results in both Fc groups being individually oxidizable (ΔE1/2?0.14 V). The possible existence of intermolecular electronic communication has discussed. The oxidation was followed by spectroelectrochemistry. The separation between the two halfwave potentials ΔE1/2=137±5 mV and the comproportionation constant Kc=207. 相似文献
102.
103.
Xinbo Zhang Danzi Sun Wenya Yin Yujun Chai Minshou Zhao 《Journal of Solid State Electrochemistry》2006,10(4):236-242
This paper presents results concerning structure and electrochemical characteristics of the La0.67Mg0.33 (Ni0.8Co0.1Mn0.1)
x
(x=2.5–5.0) alloy. It can be found from the result of the Rietveld analyses that the structures of the alloys change obviously
with increasing x from 2.5 to 5.0. The main phase of the alloys with x=2.5–3.5 is LaMg2Ni9 phase with a PuNi3-type rhombohedral structure, but the main phase of the alloys with x=4.0–5.0 is LaNi5phase with a CaCu5-type hexagonal structure. Furthermore, the phase ratio, lattice parameter and cell volume of the LaMg2Ni9phase and the LaNi5 phase change with increasing x. The electrochemical studies show that the maximum discharge capacity increases from 214.7 mAh/g
(x=2.5) to 391.1 mAh/g (x=3.5) and then decreases to 238.5 mAh/g (x=5.0). As the discharge current density is 1,200 mA/g, the high rate dischargeability (HRD) increases from 51.1% (x=2.5) to 83.7% (x=3.5) and then decreases to 71.6% (x=5.0). Moreover, the exchange current density (I
0) of the alloy electrodes first increases and then decrease with increasing x from 2.5 to 5.0, which is consistent with the variation of the HRD. The cell volume reduces with increasing x in the alloys, which is detrimental to hydrogen diffusion and accordingly decreases the low-temperature dischargeability
of the alloy electrodes. 相似文献
104.
In the system of SDS/n-C5H11OH/n-C7H16/H2O with the weight ratio of SDS/n-C5H11OH/H2O system at5.0/47.5/47.5, the upper phase of the system was W/O microemulsion, and the lower phase was the bicontinuous microemulsion. When the n-heptane content was less than 1%, with the increase of the n-heptane content, the capacitance (Co, Cod) in the upper phase (W/O) dropped, the capacitance (CB1, CBld) in the lower phase (BI) raised. At the same time, the W/O-BI inteffacial potential (ΔE), capacitance (Ci), and charge-transfer current (ict) decreased.After the n-heptane content reached 1%, with the increase of the n-heptane content, ΔE, Ci and ict demonstrated no significant change. 相似文献
105.
M. E. Rogers T. E. Glass S. J. Mecham D. Rodrigues G. L. Wilkes J. E. McGrath 《Journal of polymer science. Part A, Polymer chemistry》1994,32(14):2663-2675
New strategies for the synthesis of perfectly alternating segmented polyimide-polydimethyl siloxane copolymers were developed by utilizing a transimidization method. Imide oligomers endcapped with 2-aminopyrimidine were reacted with aminopropyl terminated (dimethyl siloxane) oligomers to afford perfectly alternating segmented imide siloxane copolymers. The polymerization was conducted in solvents such as chlorobenzene and chlorofrom. High molecular weight, fully imidized perfectly alternating segmented imide siloxane copolymers were obtained within 2 h at temperatures of 60-110°C. The mechanism of the reaction was further elucidated via model compounds and NMR characterization. The block copolymers exhibited two Tgs due to the microphase separation of the polyimide and polysiloxane phases. The Tg of the polyimide phase was a function of the length of the polyimide block. However, partial phase mixing was also evident from the DSC results on the imide siloxane copolymers prepared with low molecular weight polyimide segments. Thermooxidative stability and tensile properties of the perfectly alternating segmented imide siloxane copolymers were found to be principally dependent on the amount of poly (dimethyl siloxane) incorporated in the copolymer and did not correlate with the poly (dimethyl siloxane) or polyimide block lengths. The stress-strain behavior of both solvent cast films or molded films is also reported. © 1994 John Wiley & Sons, Inc. 相似文献
106.
Bruno Ameduri Bernard Boutevin Fouad Malek 《Journal of polymer science. Part A, Polymer chemistry》1994,32(16):3161-3169
The synthesis of maleimides that have pyrazolic or bipyrazolic pendant groups is described. Their homopolymerization and their copolymerization with 2-chloroethyl vinyl ether (CEVE) is reported. The homopolymerizations of such maleimides were performed under various conditions and led to low molecular-weight polymers. However, alternating copolymers were obtained from CEVE as comonomers whatever the monomers feed compositions. A similar behavior was also observed for maleimides that do not exhibit any spacer, whereas for bulky vinyl ethers, random copolymers were produced. A comparison of the thermal behavior between these copolymers (glass transition temperatures, Tg, and decomposition temperatures) and other copolymers having different spacers between the nitrogenated cycles and the chain are related. Thus, an important decrease of Tg, was observed when C3H6CO2CH2 groups were used as the spacer instead of methylene groups. Moreover, the thermal weakness of these copolymers may come from the substituents of the vinyl ether and is discussed. © 1994 John Wiley & Sons, Inc. 相似文献
107.
Lei Wang Guang-ming Zhu Yue-zhong Meng Chun-mei Gao 《高分子科学》2007,(4):419-426
A novel sulfonated poly(arylene ether) containing triphenylmethane moieties was synthesized by the sulfonation of a designed parent polymer using chlorosulfonic acid as sulfonation agent. The sulfonation took place at the para position of the pendant phenyl rings because of the specially designed parent polymer. The position and degree of sulfonation were characterized by ^1H-NMR and elemental analysis. The sulfonated polymers are highly soluble in common organic solvents, such as dimethylsulfoxide, N,N'-dimethylacetamide, dimethylformamide, ethylene glycol monomethyl ether, and can be readily cast into tough and smooth films from solutions. The films showed good thermal and hydrolysis stabilities. Moreover, Fenton's reagent test revealed that the films exhibited superior stability to oxidation. The proton conductivities of the films were comparable with Nation 117 under same conditions. The membrane electrode assembly (MEA) prepared with the asmade film (706 EW, 100 μm dry thickness) shows better cell performance than Nation 115-MEA in the whole current density range. 相似文献
108.
Hans-Jörg Jacobasch Günther Bauböck Josef Schurz 《Monatshefte für Chemie / Chemical Monthly》1986,117(10):1133-1144
An apparatus for the determination of zetapotential is described, which allows measurement of both streaming potential and streaming current as well as electroosmosis with one and the same fiber diaphragm with various electrodes (calomel, Ag/AgCl, palladium). Measurements with glass fibers, fibers of polyacrylonitrile and of polyester, and with cellulose pulp show that identical values for the electrokinetic parameters are obtained independent of voltage applied during electroosmosis resp. pressure difference during streaming measurements. This allows the conclusion that the zetapotential in dilute electrolyte solution is here an unequivocally determinable figure.
Herrn Prof. Dr.R. C. Schulz mit den herzlichsten Glückwünschen zum 65. Geburtstag gewidmet. 相似文献
109.
The factors influencing the electrochemical behaviour of a supercapacitor have been partly examined in this work. The effects
of so-called intrinsic parameters, i.e. exchange current density, unit cell length and double layer (DL) capacitance; as well
as the so-called application parameters, i.e. cell current, on the cell potential discharge time have been considered. The
contributions of each type of capacitors, DL capacitor and faradaic supercapacitor under various states of operation and material
have been analyzed, and the competing (compensating) effects of the two types of capacitors as regards to the discharge and
power characteristics manifested by current–potential and energy–power (Ragone plots) are elucidated. 相似文献
110.
Soncini A Domene C Engelberts JJ Fowler PW Rassat A van Lenthe JH Havenith RW Jenneskens LW 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(4):1257-1266
Why are some (4n+2)π systems aromatic, and some not? The ipsocentric approach to the calculation of the current density induced in a molecule by an external magnetic field predicts a four‐electron diatropic (aromatic) ring current for (4n+2)π carbocycles and a two‐electron paratropic (antiaromatic) current for (4n)π carbocycles. With the inclusion of an electronegativity parameter, an ipsocentric frontier‐orbital model also predicts the transition from delocalised currents in carbocycles to nitrogen‐localised currents in alternating azabora‐heterocycles, which rationalises the differences in (magnetic) aromaticity between these isoelectronic π‐conjugated systems. Ab initio valence‐bond calculations confirm the localisation predicted by the naïve model, and coupled‐Hartree–Fock calculations give current‐density maps that exhibit the predicted delocalised‐to‐localised/carbocycle–heterocycle transition. 相似文献