全文获取类型
收费全文 | 421篇 |
免费 | 11篇 |
专业分类
化学 | 255篇 |
晶体学 | 10篇 |
力学 | 8篇 |
数学 | 23篇 |
物理学 | 136篇 |
出版年
2023年 | 4篇 |
2022年 | 2篇 |
2021年 | 8篇 |
2020年 | 7篇 |
2019年 | 8篇 |
2018年 | 3篇 |
2017年 | 6篇 |
2016年 | 7篇 |
2015年 | 7篇 |
2014年 | 9篇 |
2013年 | 17篇 |
2012年 | 23篇 |
2011年 | 22篇 |
2010年 | 14篇 |
2009年 | 7篇 |
2008年 | 25篇 |
2007年 | 13篇 |
2006年 | 18篇 |
2005年 | 26篇 |
2004年 | 21篇 |
2003年 | 11篇 |
2002年 | 13篇 |
2001年 | 11篇 |
2000年 | 14篇 |
1999年 | 8篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 10篇 |
1995年 | 10篇 |
1994年 | 7篇 |
1993年 | 3篇 |
1992年 | 10篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1987年 | 5篇 |
1986年 | 10篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1980年 | 2篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1973年 | 10篇 |
1972年 | 3篇 |
1971年 | 4篇 |
1967年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有432条查询结果,搜索用时 421 毫秒
91.
Cabeza M Heuze I Bratoeff E Murillo E Ramirez E Lira A 《Chemical & pharmaceutical bulletin》2001,49(9):1081-1084
The pharmacological activity of four new progesterone derivatives: 4-bromo-17alpha-(p-fluorobenzoyloxy)-4-pregnene-3,20-dione (7), 4-bromo-17alpha-(p-bromobenzoyloxy)-4-pregnene-3,20-dione (8), 4-bromo-17alpha-(p-chlorobenzoyloxy)-pregnene-3,20-dione (9) and 4-bromo-17alpha-(p-toluoyloxy)-4-pregnene-3,20-dione (10) was determined. These compounds were evaluated as 5alpha-reductase inhibitors on gonadectomized hamster seminal vesicles and flank organs. The pharmacological data of this study indicate that compounds 7 and 9 having at C-17 p-fluorobenzoyloxy and p-chlorobenzoyloxy ester functions respectively showed the highest antiandrogenic effect as measured by the reduction of the weight of the seminal vesicles. In the flank organ model, the same compounds 7 and 9 exhibited a smaller diameter, 1.8 and 1.0 mm, respectively, than the commercially available finasteride 3 (2.3 mm), thus indicating a higher inhibitory effect on 5alpha-reductase enzyme. Steroid 7 showed a higher inhibitory activity on the conversion of T to DHT (Fig. 3) than the presently used finasteride, thus indicating a higher antiandrogenic effect. The nonsubstituted benzoyloxy ester (compound 15) showed a lower antiandrogenic activity as measured in the seminal vesicles model than the p-substituted benzoyloxy compounds. 相似文献
92.
Milagrosa Ramirez del Solar Luis Esquivias 《Journal of Sol-Gel Science and Technology》1994,3(1):41-46
A first approach to the ultrastructural evolution during the sintering process of TiO2-SiO2 aerogels has been done on the basis of the gelation kinetics and dried gel previous analysis, in order to tailor the mixed glass sol-gel processing.Small Angle X-Ray Scattering (SAXS) technique was used to study the evolution of the ultrastructure of porous aerogels partially densified at several temperatures. The analysis of the scattering intensity profiles on the basis of the Porod's Two Phase Media Model gives valuable information about pore-solid matrix interface evolution on sintering. 相似文献
93.
94.
95.
Michael M. Yamano Rachel R. Knapp Aurapat Ngamnithiporn Melissa Ramirez Kendall N. Houk Brian M. Stoltz Neil K. Garg 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(17):5709-5713
The chemistry of strained cyclic alkynes has undergone a renaissance over the past two decades. However, a related species, strained cyclic allenes, especially heterocyclic derivatives, have only recently resurfaced and represent another class of valuable intermediates. We report a mild and facile means to generate the parent 3,4‐oxacyclic allene from a readily accessible silyl triflate precursor, and then trap it in (4+2), (3+2), and (2+2) reactions to provide a variety of cycloadducts. In addition, we describe a catalytic, decarboxylative asymmetric allylic alkylation performed on an α‐silylated substrate, to ultimately permit access to an enantioenriched allene. Generation and trapping of the enantioenriched cyclic allene occurs with complete transfer of stereochemical information in a Diels–Alder cycloaddition through a point‐chirality, axial‐chirality, point‐chirality transfer process. 相似文献
96.
Y. Kudriatsev A. Villegas S. Gallardo G. Ramirez R. Asomoza V. Mishurnuy 《Applied Surface Science》2008,254(16):4961-4964
We investigated the work function (WF) change of a silicon surface being under cesium ion bombardment and simultaneous oxygen flooding with various oxygen pressures at the sample surface. It was found that WF of Cs+ ion sputtered Si decreases under oxygen flooding. This decrease provides an essential grow of secondary ion yields of some negative ions, sputtered from Si. At the same time Si− ion yield decreases approximately in two times. In the paper we have discussed possible explanations of our experimental data: we considered a surface composition change, formation of surface dipoles and work function change caused by oxygen adsorption, and their relationships between each other. 相似文献
97.
Redouane Chebout Didier Tichit Géraldine Layrac Akila Barama Bernard Coq Iuliana Cota Eliana Ramirez Rangel Francisco Medina 《Solid State Sciences》2010,12(6):1013-1017
A new route for the preparation of basic mixed oxides using nanocomposite precursors obtained from layered double hydroxides (LDH) was investigated. These nanocomposites have been prepared by intercalation of negatively charged guest entities containing Ca2+, Sr2+, Ba2+ or La3+ cations in the interlayer space of host Mg/Al LDH by anionic exchange. The guest entities have been previously prepared by complexation of the required cations in the presence of edta or citrate chelating anions. Intercalation of [Mn+(edta)](4?n)? or [La(cit)OH]? complexes is proved by elemental analysis, XRD analysis and FT-IR spectroscopy. However, in order to satisfy the charge equilibrium in the materials, (edta)4? and (citrate)3? species are also co-intercalated in the nanocomposites. The basic properties of the mixed oxides obtained after thermal decomposition of the nanocomposites precursors were evaluated in the disproportionation of 2-methyl-3-butyn-2-ol (MBOH) and the transesterification of 1-phenylethanol with diethylcarbonate (DEC). The Sr- and Ba-containing mixed oxides exhibit basicity close to the one of MgO and Mg(La)O mixed oxides. A comparative study of the reactivity of La-containing mixed oxides obtained following different preparation routes, i.e. coprecipitation or anionic exchange, allows to conclude that the nanocomposite route led to the most active samples. 相似文献
98.
99.
We search for signatures of the extra neutral gauge boson Z′, predicted in some extensions of the Standard Model, from the analysis of some distributions for p+p→μ++μ−+X, where the only exotic particle involved is Z′. In addition to the invariant mass and charge asymmetry distributions, we propose in our search to use the transverse momentum distribution (pT) as an observable. We do our calculation for two values of the LHC center of mass energy (7 and 14 TeV), corresponding to 1 and 100 fb−1 of luminosity, in order to compare our findings from some models with the distributions following from the Standard Model. By applying convenient cuts in the invariant mass, we show that the final particles pT distributions can reveal the presence of an extra neutral gauge boson contribution. We also claim that it is possible to disentangle the models considered here and we emphasize that the minimal version of the model, based on SUC(3)×SUL(3)×UX(1) symmetry, presents the more clear signatures for Z′ existence. 相似文献
100.
Ma Y Ramirez A Singh KJ Keresztes I Collum DB 《Journal of the American Chemical Society》2006,128(48):15399-15404
Lithium diisopropylamide-mediated dehydrobrominations of exo-2-bromonorbornane, 1-bromocyclooctene, and cis-4-bromo-tert-butylcyclohexane were studied in THF solutions and THF solutions with added hexamethylphosphoramide (HMPA). Rate studies reveal a diverse array of mechanisms based on mono-, di-, and trisolvated monomers as well as triple ions. The results are contrasted with analogous eliminations in THF in the absence of HMPA. 相似文献