全文获取类型
收费全文 | 3693篇 |
免费 | 74篇 |
国内免费 | 30篇 |
专业分类
化学 | 2936篇 |
晶体学 | 49篇 |
力学 | 58篇 |
数学 | 176篇 |
物理学 | 578篇 |
出版年
2022年 | 24篇 |
2021年 | 30篇 |
2020年 | 31篇 |
2019年 | 38篇 |
2018年 | 26篇 |
2017年 | 31篇 |
2016年 | 43篇 |
2015年 | 68篇 |
2014年 | 78篇 |
2013年 | 263篇 |
2012年 | 164篇 |
2011年 | 229篇 |
2010年 | 125篇 |
2009年 | 104篇 |
2008年 | 191篇 |
2007年 | 198篇 |
2006年 | 220篇 |
2005年 | 206篇 |
2004年 | 178篇 |
2003年 | 191篇 |
2002年 | 194篇 |
2001年 | 43篇 |
2000年 | 51篇 |
1999年 | 54篇 |
1998年 | 40篇 |
1997年 | 37篇 |
1996年 | 51篇 |
1995年 | 50篇 |
1994年 | 43篇 |
1993年 | 28篇 |
1992年 | 29篇 |
1991年 | 27篇 |
1990年 | 29篇 |
1989年 | 35篇 |
1988年 | 34篇 |
1987年 | 26篇 |
1986年 | 25篇 |
1985年 | 46篇 |
1984年 | 47篇 |
1983年 | 28篇 |
1982年 | 57篇 |
1981年 | 39篇 |
1980年 | 55篇 |
1979年 | 58篇 |
1978年 | 41篇 |
1977年 | 30篇 |
1976年 | 24篇 |
1975年 | 23篇 |
1974年 | 25篇 |
1973年 | 18篇 |
排序方式: 共有3797条查询结果,搜索用时 15 毫秒
101.
The conjugate addition reactions of BHA alkenoates with organolithiums in toluene or toluene-hexane at −78 °C were mediated by the chiral ligands 1 and 2 to give the corresponding 3-substituted alkanoates in high ees and high yields. The two ligands are complementary each other, 1 is effective for phenyl- and vinyllithiums to give the adducts in 64–93% ee, while 2 is effective for butyl- and ethyllithiums to give the adducts in 91–99% ee. 相似文献
102.
Jyunichi Koyanagi Katsumi Yamamoto Kouji Nakayama Akira Tanaka 《Journal of heterocyclic chemistry》1997,34(2):407-412
4,9-Dimethoxynaphtho[2,3-b]furan 9 was obtained in 91% yield via the reductive methylation of naphtho[2,3-b]furan-4,9-dione 2 . After treatment of 9 with butyllithium, the mixture was allowed to react with N,N-dimethylacetamide, followed by oxidization with cerium(IV) diammonium nitrate to give 2-acetylnaphtho[2,3-b]furan-4,9-dione 1 . 2-Formylnaphtho[2,3-b]furan-4,9-dione 13 and 2-trimethylsilyl-naphtho[2,3-b]furan-4,9-dione 14 were also obtained from 9 by a similar method. The halodesilylations of 14 easily gave 2-iodonaphtho[2,3-b]furan-4,9-dione 16 , 2-bromonaphtho[2,3-b]furan-4,9-dione 17 , and 2-chloronaphtho[2,3-b]furan-4,9-dione 18 in 82%, and 93% and 83% yield, respectively. Furthermore, the nitrodesilylation of 14 gave 2-nitronaphtho[2,3-b]furan-4,9-dione 3 in 77% yield. 相似文献
103.
104.
105.
Yu Chen Zu-En Huang Rui-Fang Cai Bo-Cheng Yu Osamu Ito Jun Zhang Wei-Wen Ma Chao-Fan Zhong Li Zhao Yu-Fen Li Lei Zhu Mamoru Fujitsuka Akira Watanabe 《Journal of Polymer Science.Polymer Physics》1997,35(8):1185-1190
The phenomena of aggregation and coalescence of fullerenes in the UV-laser ablation time-of-flight mass spectrometric investigation of C60-modified poly(N-vinylcarbazole) both in the positive and in the negative ion channels have been observed. The results indicate that in C60 chemically modified PVK (C60–PVK) copolymer the nascent fullerene fragments ruptured from main chain can easily coalesce into large fullerenes through collisions, whereas in the C60-doped PVK the aggregation and coalescence of C60 were relative weak due to nonbounding action and incomplete charge transfer behavior between C60 and PVK. Furthermore, the photoinduced electron transfer behavior between C60 and carbazole units in the C60 chemically modified poly(N-vinylcarbazole) in benzonitrile by laser flash photolysis at 355 nm has also been investigated. Efficiency of the anion radical of C60 in copolymer at 1080 nm is higher than that of the C60-doped poly(N-vinylcarbazole) polymers. The formation of a C60 radical anion may be ascribed to photoinduced electron transfer between C60 pendanted on the main chain backbone and the inter-, and intrachain carbazole units in the copolymer. © 1997 John Wiley & Sons, Inc. 35 : 1185–1190, 1997 相似文献
106.
Fumiko Kimura Tsunehisa Kimura Akira Sugisaki Mamoru Komatsu Hiroaki Sata Eiko Ito 《Journal of Polymer Science.Polymer Physics》1997,35(16):2741-2747
In situ Fourier transform infrared (FTIR) measurements were carried out to elucidate conformation changes occurring during the isothermal melt crystallization of poly(ethylene-2,6-naphthalate) (PEN). Based on the band assignments for the components of the amorphous, α-crystal form, and β-crystal form of PEN in film samples, the in situ data was analyzed in terms of the amorphous- and crystal-trans conformations. It was observed at a higher isothermal crystallization temperature that the formation of amorphous-trans conformations precedes the growth of crystals. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2741–2747, 1997 相似文献
107.
Yoshimori Takamori Minoru Morimoto Hitoshi Sashiwa Yoshihiro Shigemasa Yoshiharu Okamoto Saburo Minami Akira Matsuhashi 《Macromolecular Symposia》1997,120(1):11-18
Chitin from squid pen was effectively degraded by the chitinase from Bacillus sp. PI-7S, while the degradation of chitin with lysozyme proceeded very slowly. Among the various vital components studied, the degradation of chitin from squid pen was performed not by canine serum, canine tela subcutanea, canine tela liver, and equine serum but by bovine serum and caprine serum. Noteworthy is the fact that chitin sponge subcutaneously implanted in dog reported not to have chitinase was degraded in ca. 14 days. 相似文献
108.
Akira Nishimura Ryouga Shimada Yoshito Sakakibara Akira Koshio Eric Hu 《Molecules (Basel, Switzerland)》2021,26(10)
The aim of this study is to clarify the effect of doped metal type on CO2 reduction characteristics of TiO2 with NH3 and H2O. Cu and Pd have been selected as dopants for TiO2. In addition, the impact of molar ratio of CO2 to reductants NH3 and H2O has been investigated. A TiO2 photocatalyst was prepared by a sol-gel and dip-coating process, and then doped with Cu or Pd fine particles by using the pulse arc plasma gun method. The prepared Cu/TiO2 film and Pd/TiO2 film were characterized by SEM, EPMA, TEM, STEM, EDX, EDS and EELS. This study also has investigated the performance of CO2 reduction under the illumination condition of Xe lamp with or without ultraviolet (UV) light. As a result, it is revealed that the CO2 reduction performance with Cu/TiO2 under the illumination condition of Xe lamp with UV light is the highest when the molar ratio of CO2/NH3/H2O = 1:1:1 while that without UV light is the highest when the molar ratio of CO2/NH3/H2O = 1:0.5:0.5. It is revealed that the CO2 reduction performance of Pd/TiO2 is the highest for the molar ratio of CO2/NH3/H2O = 1:1:1 no matter the used Xe lamp was with or without UV light. The molar quantity of CO per unit weight of photocatalyst for Cu/TiO2 produced under the illumination condition of Xe lamp with UV light was 10.2 μmol/g, while that for Pd/TiO2 was 5.5 μmol/g. Meanwhile, the molar quantity of CO per unit weight of photocatalyst for Cu/TiO2 produced under the illumination condition of Xe lamp without UV light was 2.5 μmol/g, while that for Pd/TiO2 was 3.5 μmol/g. This study has concluded that Cu/TiO2 is superior to Pd/TiO2 from the viewpoint of the molar quantity of CO per unit weight of photocatalyst as well as the quantum efficiency. 相似文献
109.
Homoleptic cyclometalated iridium complexes with highly efficient red phosphorescence and application to organic light-emitting diode 总被引:7,自引:0,他引:7
Tsuboyama A Iwawaki H Furugori M Mukaide T Kamatani J Igawa S Moriyama T Miura S Takiguchi T Okada S Hoshino M Ueno K 《Journal of the American Chemical Society》2003,125(42):12971-12979
Phosphorescence studies of a series of facial homoleptic cyclometalated iridium(III) complexes have been carried out. The complexes studied have the general structure Ir(III)(C-N)(3), where (C-N) is a monoanionic cyclometalating ligand: 2-(5-methylthiophen-2-yl)pyridinato, 2-(thiophen-2-yl)-5-trifluoromethylpyridinato, 2,5-di(thiophen-2-yl)pyridinato, 2,5-di(5-methylthiophen-2-yl)pyridinato, 2-(benzo[b]thiophen-2-yl)pyridinato, 2-(9,9-dimethyl-9H-fluoren-2-yl)pyridinato, 1-phenylisoquinolinato, 1-(thiophen-2-yl)isoquinolinato, or 1-(9,9-dimethyl-9H-fluoren-2-yl)isoquinolinato. Luminescence properties of all the complexes at 298 K in toluene are as follows: quantum yields of phosphorescence Phi(p) = 0.08-0.29, emission peaks lambda(max) = 558-652 nm, and emission lifetimes tau = 0.74-4.7 micros. Bathochromic shifts of the Ir(thpy)(3) family [the complexes with 2-(thiophen-2-yl)pyridine derivatives] are observed by introducing appropriate substituents, e.g., methyl, trifluoromethyl, or thiophen-2-yl. However, Phi(p) of the red emissive complexes (lambda(max) > 600 nm) becomes small, caused by a significant decrease of the radiative rate constant, k(r). In contrast, the complexes with the 1-arylisoquinoline ligands are found to have marked red shifts of lambda(max) and very high Phi(p) (0.19-0.26). These complexes are found to possess dominantly (3)MLCT (metal-to-ligand charge transfer) excited states and have k(r) values approximately 1 order of magnitude larger than those of the Ir(thpy)(3) family. An organic light-emitting diode (OLED) device that uses Ir(1-phenylisoquinolinato)(3) as a phosphorescent dopant produces very high efficiency (external quantum efficiency eta(ex) = 10.3% and power efficiency 8.0 lm/W at 100 cd/m(2)) and pure-red emission with 1931 CIE (Commission Internationale de L'Eclairage) chromaticity coordinates (x = 0.68, y = 0.32). 相似文献
110.
Density functional theory (DFT) studies were performed to investigate the effect of substituents on the properties of benzdiyne derivatives. Twelve substituted benzdiynes-C(6)X(2), where X = F, Cl, Br, Me, CF(3), CN, OH, NO(2), NH(2), OMe, NMe(2), and Ph-were considered along with the unsubstituted 1,4-benzdiyne. The structures, vibrational frequencies, and IR intensities of these benzdiynes were studied with a popular three-parameter hybrid density functional (B3LYP) combined with the split-valence 6-31G(d) basis set and Dunning's correlation-consistent polarized triple-zeta (cc-pVTZ) basis set. The relative stabilities of the substituted benzdiynes were studied with the help of reaction energies of isodesmic reactions, which showed that the electron-withdrawing groups destabilized the benzdiynes more than they did the corresponding benzenes, whereas the electron-donating groups stabilized the benzdiynes more than they did their benzene counterparts. Correlation analyses revealed that field/inductive effects played a more important role than did resonance effects. The changes in atomic charges and spin populations due to the substituents were also studied. The asymmetric nu(Ctbd1;C) stretching modes obtained were close to the 1500-cm(-)(1) mark. Reinvestigation of the experimental results supported these results; a weak IR band at 1486 cm(-)(1) was assigned to this asymmetric stretching mode in C(6)(CF(3))(2) F. Some other benzdiynes also had large IR intensity values for their asymmetric nu(Ctbd1;C) vibrational modes due to the coupling with other vibrational modes. Heats of formation for the substituted benzdiynes were obtained from the reaction energies calculated at the B3LYP/cc-pVTZ level of theory. 相似文献