全文获取类型
收费全文 | 3036篇 |
免费 | 82篇 |
国内免费 | 10篇 |
专业分类
化学 | 2286篇 |
晶体学 | 34篇 |
力学 | 34篇 |
数学 | 143篇 |
物理学 | 631篇 |
出版年
2022年 | 13篇 |
2021年 | 17篇 |
2020年 | 37篇 |
2019年 | 48篇 |
2018年 | 37篇 |
2017年 | 18篇 |
2016年 | 57篇 |
2015年 | 46篇 |
2014年 | 66篇 |
2013年 | 146篇 |
2012年 | 144篇 |
2011年 | 171篇 |
2010年 | 102篇 |
2009年 | 87篇 |
2008年 | 151篇 |
2007年 | 147篇 |
2006年 | 143篇 |
2005年 | 144篇 |
2004年 | 121篇 |
2003年 | 132篇 |
2002年 | 121篇 |
2001年 | 84篇 |
2000年 | 85篇 |
1999年 | 52篇 |
1998年 | 33篇 |
1997年 | 36篇 |
1996年 | 39篇 |
1995年 | 27篇 |
1994年 | 32篇 |
1993年 | 41篇 |
1992年 | 37篇 |
1991年 | 46篇 |
1990年 | 48篇 |
1989年 | 32篇 |
1988年 | 43篇 |
1987年 | 43篇 |
1986年 | 35篇 |
1985年 | 60篇 |
1984年 | 43篇 |
1983年 | 22篇 |
1982年 | 34篇 |
1981年 | 35篇 |
1980年 | 22篇 |
1979年 | 34篇 |
1978年 | 33篇 |
1977年 | 41篇 |
1976年 | 31篇 |
1975年 | 22篇 |
1974年 | 17篇 |
1973年 | 21篇 |
排序方式: 共有3128条查询结果,搜索用时 467 毫秒
101.
The synthesis of 1 alpha- and/or 1 beta-methylcarbapenems, 2-unsubstituted and 2-(5-methyl-1,3,4-thiadiazol-2-yl)thiomethyl derivatives having a 1-(hydroxy)methylethylidene or cyclic carbonate side chain at the C-6 position, is described. The in vitro antibacterial activities of these compounds and their corresponding 1-unsubstituted carbapenems are compared. 相似文献
102.
Kenneth E. Laintz Shuichi Iso Yoshiro Meguro Zenko Yoshida 《Journal of separation science》1994,17(8):603-606
The chromatographic behaviors of lanthanide chelates of acetylacetone (ACAC), trifluoroacetylacetone (TFA), thenoyltrifluoroacetone (TTA), dipivaloylmethane (THD), 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione (FOD), and the thenoyltrifluoroacetonepyridine (TTA·Py) adduct were investigated using packed column supercritical fluid chromatography. Mobile phases consisting of neat and alcohol modified CO2 were used with a phenyl packed stationary phase. Lanthanide complexes of ACAC, THD, and FOD were shown to have better chromatographic performance compared to the corresponding chelates with TFA, TTA, and TTA·Py. In particular, TTA complexes such as Eu(TTA)3 showed characteristic thermal decomposition in the mobile phase at elevated temperature. In addition, retention behavior was found to be a temperature dependent function of volatility and solubility for all chelates studied. 相似文献
103.
Crystal structures of the silver complexes derived from tetrabenzo[16]annulene with AgOTf and AgClO(4) are different, although these two complexes show similar (1)H NMR spectra reflecting a similar clathrate structure in solution. The silver complex of pentabenzo[20]annulene with AgClO(4) adopts a clathrate structure both in the solid state and in solution. 相似文献
104.
[reaction: see text] (Z)-2-Fluoro-1-alkenyl(phenyl)iodonium salts were stereoselectively prepared by the reaction of alkynyl(phenyl)iodonium salts with aqueous HF in good yields. The method is applicable to the synthesis of fluoroalkenyliodonium salts having functional groups such as ketone, ester, and chloride. (Z)-2-Fluoro-1-alkene, (Z)-2-fluoro-2-alkenoate, and (Z)-beta-fluoroenyne could be stereoselectively prepared from the fluoroalkenyliodonium salt. 相似文献
105.
Matsui J Yoshida S Mikayama T Aoki A Miyashita T 《Langmuir : the ACS journal of surfaces and colloids》2005,21(12):5343-5348
Semiconducting thin films consisting of regioregular poly(3-hexylthiophene) (RR-PHT) and poly(N-dodecylacrylamide) (pDDA) were constructed by the Langmuir-Blodgett (LB) technique. A mixture of RR-PHT and pDDA spread from a chloroform solution on a water surface forms a stable monolayer, which can be transferred onto solid substrates by the LB method, yielding a well-defined polymer LB film. Surface morphology studies of the LB film indicate that the RR-PHT is dispersed uniformly throughout the surface. The polymer thin film was chemically doped by contacting with FeCl3 acetonitrile solution, and a conductivity of 5.6 S/cm was achieved. Further, the LB film was utilized as the semiconducting film in the field-effect transistor (FET), and mobilities of 2.2 x 10(-4) and 4.4 x 10(-4) cm2 V(-1) s(-1) were obtained by analyzing the saturated and linear regions of the current-voltage characteristic, respectively. 相似文献
106.
Biodegradable polyurethanes are an interesting alternative to many applications that involve plastics since they can minimize environmental problems caused by the low rates of natural degradation of synthetic polymers. In addition, since waterborne polyurethanes are based on aqueous dispersions, they restrict the use of organic solvents during processing and application of the polymer, thus contributing furthermore to reduce environmental damage. In this work, aqueous anionic polyurethane dispersions (PUD) with tailorable susceptibility for hydrolysis were synthesized by progressively replacing polypropylene glycol (PPG) with a biodegradable polycaprolactone diol (PCL) as soft segments. The hard segments were formed by extending isophorone diisocyanate (IPDI) with hydrazine (HZ). Dimethylol propionic acid (DMPA) was used as ionic center and triethyl amine (TEA) as neutralizer. The degree of phase separation was evaluated mainly by infrared spectroscopy (FTIR) and small angle X-ray scattering (SAXS). The results indicated that phase separation between hard and soft segments of poly(ester-urethane) is more significant than that of poly(ether-urethane). Data obtained from SAXS experiments indicated that phase separation within soft domains can also be present in samples containing both polyester and polyether soft segments. Hydrolytic degradation of the polymers in buffer solution of pH 7.4 and alkaline solution was performed as an initial test. The results showed that the fraction of polyester soft segments in the polyurethanes can be used to tailor the susceptibility of the materials to hydrolytic attack. Polyurethanes having higher contents of polyester were more promptly hydrolytically degraded than polyurethanes containing only polyether segments. 相似文献
107.
Peroxotungstate immobilized on ionic liquid-modified silica as a heterogeneous epoxidation catalyst with hydrogen peroxide 总被引:6,自引:0,他引:6
Yamaguchi K Yoshida C Uchida S Mizuno N 《Journal of the American Chemical Society》2005,127(2):530-531
Peroxotungstate immobilized on ionic liquid-modified SiO2 is capable of heterogeneously epoxidizing a wide range of olefins with the maintenance of the catalytic activity of homogeneous analogue. The epoxidation was immediately stopped by the removal of the catalyst, and no tungsten species could be found in the filtrate after the removal of the catalyst. These results can rule out any contribution to the observed catalysis from the tungsten species that leached into the reaction solution, and the observed catalysis is truly heterogeneous in nature. Furthermore, the catalyst was reusable without the loss of the catalytic performance. 相似文献
108.
The reaction of an N-acyliminium ion with an alkyl iodide and hexabutyldistannane took place to give the alkylation product. A mechanism involving the addition of an alkyl radical to an N-acyliminium ion to produce the corresponding radical cation has been suggested. 相似文献
109.
Nitrate forms an insoluble ternary complex, Ag(phen)(2)NO(3), with silver in the presence of 1,10-phenanthroline (phen). A simple and direct conductometric titration of nitrate, based on the formation of this ternary complex, has been investigated. The sample solution is adjusted to pH 4.3 with acetate buffer and titrated with 0.1M Ag(phen)(+)(2) complex solution in 40% ethanol-water mixture. The relative error is less than 0.5% and relative standard deviation less than 0.4%. The effect of ammonium ion and urea has also been studied. The method has been successfully applied to determination of nitrate-nitrogen in fertilizer. 相似文献
110.
Synthesis of 12-deoxyroyleanone, cryptoquinone, 11,14-dihydroxy-8,11,13-abietatrien-7-one, and related derivatives from dehydroabietic acid 总被引:1,自引:0,他引:1
Yoh-ichi Matsushita Yoshihisa Iwakiri Satoru Yoshida Kazuhiro Sugamoto Takanao Matsui 《Tetrahedron letters》2005,46(21):3629-3632
Naturally occurring abietane quinones and hydroquinone, namely, 12-deoxyroyleanone (1a), cryptoquinone (4a), and 11,14-dihydroxyabieta-8,11,13-trien-7-one (5a), together with the epimers of tryptoquinones D (2) and F (3), were first synthesized from dehydroabietic acid (6). 相似文献