全文获取类型
收费全文 | 1674篇 |
免费 | 79篇 |
专业分类
化学 | 1067篇 |
晶体学 | 6篇 |
力学 | 95篇 |
数学 | 319篇 |
物理学 | 266篇 |
出版年
2024年 | 3篇 |
2023年 | 10篇 |
2022年 | 9篇 |
2021年 | 26篇 |
2020年 | 29篇 |
2019年 | 25篇 |
2018年 | 40篇 |
2017年 | 32篇 |
2016年 | 44篇 |
2015年 | 56篇 |
2014年 | 78篇 |
2013年 | 145篇 |
2012年 | 123篇 |
2011年 | 142篇 |
2010年 | 84篇 |
2009年 | 57篇 |
2008年 | 127篇 |
2007年 | 119篇 |
2006年 | 96篇 |
2005年 | 94篇 |
2004年 | 66篇 |
2003年 | 56篇 |
2002年 | 60篇 |
2001年 | 19篇 |
2000年 | 19篇 |
1999年 | 15篇 |
1998年 | 11篇 |
1996年 | 15篇 |
1995年 | 11篇 |
1994年 | 12篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 13篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 9篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1974年 | 5篇 |
1973年 | 4篇 |
1887年 | 1篇 |
排序方式: 共有1753条查询结果,搜索用时 15 毫秒
981.
The electrical and rheological properties of an ethylene vinyl acetate (EVA) copolymer filled with bamboo charcoal were investigated. The composites were prepared by melt process in an internal batch mixer. Size distribution analysis showed that d(50) and d(90) values of the bamboo charcoal particles are 12.7 and 40 μm, respectively, with a mean diameter of 22 μm. Scanning electron microscopy proved that the particles of bamboo charcoal present a rectangular shape. The electrical percolation threshold was observed at 0.18 volume fraction (35 wt%) of bamboo. Beyond the percolation threshold, a considerable increase in electrical properties was observed up to a limit value of 10-2 S/m. The rheological percolation was studied from different rheological models. As a result, the rheological percolation threshold was observed at 0.3 volume fraction (50 wt%) of bamboo charcoal contents. So, the electrical percolation occurs before the rheological percolation. This is principally due to the filler’s characteristics such as the specific surface area, the aspect ratio, and the surface properties. Finally, the bamboo charcoal confers high electrical properties to the EVA composite without inducing strong changes in its viscoelastic properties. 相似文献
982.
Soheila Sameni Catherine Jeunesse Dr. Dominique Matt Dr. Loïc Toupet Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(40):10446-10456
The first diphosphines based on a double calixarene, namely 1,4 (or 1,3)‐bis‐(5‐diphenylphosphino‐25,26,27,28‐tetrapropoxycalix[4]aren‐17‐yl)benzene ( L2 , L3 ) were each prepared in four steps starting from 5,17‐dibromo‐25,26,27,28‐tetrapropoxycalix[4]arene. Upon reaction of L2 with [Au(tht)(thf)]BF4, (tht=C4H8S) a rigid metallo‐capsule was quantitatively formed, which adopts an oblique form owing to the distinct nature of the spacers linking the two calixarene half‐spheres. In the solid state, the 1,4‐substituted phenylene linker is turned towards the gold ion, suggesting the existence of weak bonding interactions between two aromatic CH protons of this ring and the metal centre (Au???H=2.67 Å). In contrast to this gold complex, the related silver complex shows dynamic behaviour in solution, the exchange between two enantiomeric oblique forms being facilitated by the greater stereochemical flexibility of AgI vs. AuI. A heteronuclear 109Ag{1H} HMQC experiment established strong correlations between the CH protons of the phenylene linker and the 109Ag ion. Dynamic behaviour similar to that observed for the silver complex was further observed in trans‐[PtCl2? L2 ], a chelate complex that could be obtained quantitatively from L2 and [PtCl2(PhCN)2]. The intended formation of a chelate complex leading to a capsule with an endo‐oriented metal centre was achieved by reacting L3 with [Pd(allyl)(thf)2]BF4. The complex thus formed constitutes the first organometallic transition metal complex embedded in a cavity with large portals. Binding of [RuCl2(p‐cymene)] to L2 and L3 resulted in self‐compacting bimetallic complexes in which each calixarene basket entraps a Ru(p‐cymene) unit, thereby forming molecules occupying a minimal volume. 相似文献
983.
Mathematical Programming - The maximum number of edge-disjoint spanning trees in a network has been used as a measure of the strength of a network. It gives the number of disjoint ways that the... 相似文献
984.
Ricerche di Matematica - In this paper we propose a non linear mathematical model describing the process of biodegradation of organic pollutants by means of fungi, and using glucose to sustain its... 相似文献
985.
We introduce a q-analog of the true-polyanalytic Bargmann transform on . 相似文献
986.
A random coefficient autoregressive process in which the coefficients are correlated is investigated. First we look at the existence of a strictly stationary causal solution, we give the second-order stationarity conditions and the autocorrelation function of the process. Then we study some asymptotic properties of the empirical mean and the usual estimators of the process, such as convergence, asymptotic normality and rates of convergence, supplied with appropriate assumptions on the driving perturbations. Our objective is to get an overview of the influence of correlated coefficients in the estimation step through a simple model. In particular, the lack of consistency is shown for the estimation of the autoregressive parameter when the independence hypothesis in the random coefficients is violated. Finally, a consistent estimation is given together with a testing procedure for the existence of correlation in the coefficients. While convergence properties rely on ergodicity, we use a martingale approach to reach most of the results. 相似文献
987.
988.
In this paper we propose a dynamic programming algorithm to compare two quotiented ordered trees using a constrained edit distance. An ordered tree is a tree in which the left-to-right order among siblings is significant. A quotiented ordered tree is an ordered tree T with an equivalence relation on vertices and such that, when the equivalence classes are collapsed to super-nodes, the graph so obtained is an ordered tree as well. Based on an algorithm proposed by Zhang and Shasha [K. Zhang, D. Shasha, Simple fast algorithms for the editing distance between trees and related problems, SIAM Journal on Computing 18 (6) (1989) 1245–1262] and introducing new notations, we describe a tree edit distance between quotiented ordered trees preserving equivalence relations on vertices during computation which works in polynomial time. Its application to RNA secondary structures comparison is finally presented. 相似文献
989.
Laure Monnereau Dr. David Sémeril Dr. Dominique Matt Dr. Loïc Toupet Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(30):9237-9247
Five conical calix[4]arenes that have a PPh2 group as the sole functional group anchored at their upper rim were assessed in palladium‐catalysed cross‐coupling reactions of phenylboronic acid with aryl halides (dioxane, 100 °C, NaH). With arylbromides, remarkably high activities were obtained with the catalytic systems remaining stable for several days. The performance of the ligands is comparable to a Buchwald‐type triarylphosphane, namely, (2′‐methyl[1,1′‐biphenyl]‐2‐yl)diphenylphosphane, which in contrast to the calixarenyl phosphanes tested may display chelating behaviour in solution. With the fastest ligand, 5‐diphenylphosphanyl‐25,26,27,28‐tetra(p‐methoxy)benzyloxy‐calix[4]arene ( 8 ), the reaction turnover frequency for the arylation of 4‐bromotoluene was 321 000 versus 214 000 mol(ArBr).mol(Pd)?1. h?1 for the reference ligand. The calixarene ligands were also efficient in Suzuki cross‐coupling reactions with aryl chlorides. Thus, by using 1 mol % of [Pd(OAc)2] associated with one of the phosphanes, full conversion of the deactivated arenes 4‐chloroanisole and 4‐chlorotoluene was observed after 16 h. The high performance of the calixarenyl–phosphanes in Suzuki–Miyaura coupling of aryl bromides possibly relies on their ability to stabilise a monoligand [Pd0L(ArBr)] species through supramolecular binding of the Pd‐bound arene inside the calixarene cavity. 相似文献
990.
D.P.R. Queiroz F. Fraïsse B. Fayolle M. Kuntz J. Verdu 《Radiation Physics and Chemistry》2010,79(3):362-364
The degradation of an epoxy-amine network exposed to gamma irradiation in oxygen atmosphere has been studied by using a variety of analytical methods, including infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and sol–gel analysis. Results show that the oxidation of epoxy systems grows with the irradiation dose. Hydroperoxides, which are species resulting from oxidation, were identified and quantified by DSC. As indicated by the sol–gel analysis, the mechanism of degradation of chain scission seems to be predominant over crosslinking. The modifications induced by irradiation reflect in a greater capacity of water absorption. 相似文献