首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   287篇
  免费   8篇
  国内免费   1篇
化学   168篇
晶体学   2篇
力学   5篇
数学   64篇
物理学   57篇
  2023年   8篇
  2022年   2篇
  2021年   6篇
  2020年   6篇
  2019年   11篇
  2018年   17篇
  2017年   6篇
  2016年   26篇
  2015年   8篇
  2014年   13篇
  2013年   21篇
  2012年   25篇
  2011年   23篇
  2010年   11篇
  2009年   11篇
  2008年   15篇
  2007年   15篇
  2006年   10篇
  2005年   11篇
  2004年   7篇
  2003年   6篇
  2002年   6篇
  2001年   4篇
  2000年   2篇
  1999年   4篇
  1998年   2篇
  1997年   1篇
  1996年   1篇
  1995年   1篇
  1993年   2篇
  1989年   2篇
  1987年   1篇
  1986年   3篇
  1985年   1篇
  1983年   1篇
  1979年   1篇
  1978年   1篇
  1975年   1篇
  1974年   1篇
  1967年   1篇
  1962年   1篇
  1958年   1篇
排序方式: 共有296条查询结果,搜索用时 15 毫秒
91.
Asymmetric induction is observed in the additioin of chlorine to alkenes as well as in the synthesis of 0-acetyl cyanohydrins under phase-transfer conditions using (?)-benzylcinchonidinium chloride and (?)-N-dodecyl-N-methylephedrinium bromide as chiral catalysts.  相似文献   
92.
In this study, we analyzed fibrillarin nucleolar protein expression in CBs of spermatogenic cells from testicular follicles of Triatoma sordida and Triatoma infestans. In the structural and ultrastructural analysis, it was used impregnation by silver ions, immunocytochemistry, immunofluorescence and transmission electron microscopy using antibodies against fibrillarin. Regarding the results, the fibrillarin nucleolar protein marked the nucleus and some cytoplasmic spots of germ cells during spermatogenesis in triatomines. These data suggest that fibrillarin could be a constituent of the CB that was most likely derived from nucleolar fragmentation. This is the first time that fibrillarin protein expression has been shown in the CB during spermatogenesis progression in triatomines. The knowledge regarding CB constituents may help to expand the understanding of the physiological role of this structure and the role that it plays in the reproductive biology of triatomines, which are vectors of Chagas Disease.  相似文献   
93.
We study an SIS epidemiological model for a sexually transmitted infection in a monogamous population where the formation and breaking of couples is governed by individual preferences. The mechanism of couple recombination is based on the so-called bar dynamics for the marriage problem. We compare the results with those of random recombination – where no individual preferences exist – for which we calculate analytically the infection incidence and the endemic threshold. We find that individual preferences give rise to a large dispersion in the average duration of different couples, causing substantial changes in the incidence of the infection and in the endemic threshold. Our analysis yields also new results on the bar dynamics, that may be of interest beyond the field of epidemiological models.  相似文献   
94.
In this work we present a proton NMR comparative study of the molecular order in the smectic C2 (), smectic C^* () and anticlinic smectic C phases ( and ) of two liquid crystalline compounds. The second moments of the experimental proton NMR spectra are well explained taking into account two different molecular orientations models. From the analysis of our experimental results it was possible to estimate the tilt angles for all the smectic C phases studied in this work. Received 15 September 1999 and Received in final form 18 January 2000  相似文献   
95.
Steady-state fluorescence of 4'-dimethylamino-3-hydroxyflavone (DMA3HF) was observed in supercritical carbon dioxide (scCO(2)). Excited-state intramolecular proton transfer (ESIPT) occurs resulting in two well-separated emission bands corresponding to the normal and tautomer forms. As the scCO(2) density exceeds 0.7 g/mL, the relative intensity of the two bands tends to a constant value, comparable to that observed for organic solvents with ET(30) = 33.0 +/- 0.5 kcal/mol, such as toluene and di-n-butyl ether. At lower densities, the substantial decrease of the total fluorescence intensity (a 600-fold decrease as the pressure decreases from 100 to 80 bar) is accompanied by an even more accentuated decrease of the tautomer fluorescence. This can be explained by a shift in the equilibrium between normal and tautomer forms, concomitant with a more efficient quenching of the less solvated fluorophore, that may change the thermodynamic control of the relative population of the two emissive species to a kinetic control.  相似文献   
96.
The reaction of cadmium(II) acetate dehydrate with the complexing agents tropolone (tropH), 2-hydroxynaphthoquinone (nqH), 1,2-dihydroxyanthraquinone (alizH2), and 1,8-dihydroxyanthraquinone (chryzH2), afforded the complexes [Cd(trop)2] 1, [Cd2(nq)4(H2O)4]·3H2O 2·3H2O, [Cd(alizH)2(H2O)3] 3, and [Cd(chryzH)2(H2O)2] 4, respectively. The X-ray molecular structure of the dinuclear complex 2·3H2O reveals that each cadmium(II) atom is seven-coordinated. The nature of water molecules (lattice or coordinated) was discerned by thermogravimetric studies. The 113Cd NMR spectra were discussed in light of the chemical exchange.  相似文献   
97.
Cyclic oligomer fractions of ethylene terephthalate c(ET)n and 1,4‐cyclohexylenedimethylene terephthalate c(CT)n were obtained by cyclodepolymerization of their respective polyesters, the former containing around 80 mol % of trimer and the latter with around 70 mol % of trimer to pentamer. Mixtures of these fractions at selected compositions were subjected to ring opening copolymerization to give a series of poly(ethylene‐co‐cyclohexylenedimethylene terephthalate) copolyesters with ET/CT comonomer ratios ranging from 90/10 to 10/90. The copolyesters were characterized by GPC and NMR, and their thermal properties were evaluated by DSC and TGA. They had essentially the same composition as the feed from which they were produced and had an average‐weight molecular weights between 30,000 and 40,000 g/mol with polydispersities between 2 and 2.7. The distribution of the monomeric units in these copolyesters was essentially at random although it evolved to be a blocky microstructure as the contents in the two comonomers became more dissimilar. Their thermal behavior was the expected one for these types of copolyesters with crystallinity and heating stability decreasing with the content in CT units. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5954–5966, 2009  相似文献   
98.
Naturally occurring tartaric acid was used as raw material for the synthesis of novel linear polyurethanes (PURs) bearing two carboxylate side‐groups in the repeating unit. Aliphatic and aromatic PURs were obtained by reaction in solution of alkyl and benzyl tartrates with hexamethylene diisocyanate and 4,4′‐methylene‐bis(phenyl isocyanate), respectively. All the novel PURs were thermally stable and optically active. The aliphatic carboxylate‐containing PURs had Mw in the 40–70 kDa range, with PD between 2.1 and 2.5; all were semicrystalline polymers with melting temperatures between 100 and 150 °C and Tg in the 50–80 °C range. The aromatic PURs were amorphous materials with molecular weights between 18 kDa and 25 kDa and Tg above 130 °C. Hydrogenolysis of the PUR made from hexamethylene diisocyanate and benzyl tartrate yielded PURs containing up to 40% of free carboxylic side‐groups. The tartrate‐derived PURs displayed enhanced sensitivity to hydrolysis compared with their unsubstituted 2,6‐PUR homologs. The PURs bearing free carboxylic groups were unique in being degraded by water upon incubation under physiological conditions. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2391–2407, 2009  相似文献   
99.
Consider a firm providing home-delivery of small parcels within a short period of time. The paper proposes a method to estimate the productivity of each driver on any fictitious route using linear regressions. The method separates the trip time in terms of the time spent: with customers, driving, and all other activities. Our tests show that productivity, measured in terms of number of customers visited per hour, vary across drivers. Even more, the same driver’s productivity vary for peak and off-peak demand periods considerably. This methodology should enhance the reliability of the driver scheduling programs, since it reduces the error and bias of the productivity offered by the assigned set of drivers. It shall also improve the reliability of the delivery time promised to the customer.  相似文献   
100.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号