全文获取类型
收费全文 | 1253篇 |
免费 | 43篇 |
国内免费 | 2篇 |
专业分类
化学 | 761篇 |
晶体学 | 3篇 |
力学 | 54篇 |
数学 | 215篇 |
物理学 | 265篇 |
出版年
2023年 | 28篇 |
2022年 | 48篇 |
2021年 | 52篇 |
2020年 | 34篇 |
2019年 | 40篇 |
2018年 | 30篇 |
2017年 | 23篇 |
2016年 | 55篇 |
2015年 | 62篇 |
2014年 | 46篇 |
2013年 | 66篇 |
2012年 | 101篇 |
2011年 | 98篇 |
2010年 | 64篇 |
2009年 | 39篇 |
2008年 | 61篇 |
2007年 | 60篇 |
2006年 | 71篇 |
2005年 | 54篇 |
2004年 | 39篇 |
2003年 | 49篇 |
2002年 | 30篇 |
2001年 | 8篇 |
2000年 | 14篇 |
1999年 | 7篇 |
1998年 | 7篇 |
1997年 | 4篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 7篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1972年 | 2篇 |
1955年 | 2篇 |
1891年 | 2篇 |
排序方式: 共有1298条查询结果,搜索用时 15 毫秒
111.
Leonelli F Blesi F Dirito P Trombetta A Ceccacci F La Bella A Migneco LM Marini Bettolo R 《The Journal of organic chemistry》2011,76(16):6871-6876
The problem of constructing diastereoselectively the C/D ring system of stemarane diterpenes from a bicyclo[2.2.2]octane intermediate was solved resulting in very simple synthesis of (+)-13-stemarene 1. The obtaining of the latter represents also a formal synthesis of (+)-18-deoxystemarin 2. In the key step, the epimeric mixture 10, dissolved in toluene, was converted by the action of TsOH into (+)-stemar-13-en-15-one 28. 相似文献
112.
Giancarlo Valente Federico De Martino Giuseppe Filosa Marco Balsi Elia Formisano 《Magnetic resonance imaging》2009
Spatial independent component analysis (ICA) is a well-established technique for multivariate analysis of functional magnetic resonance imaging (fMRI) data. It blindly extracts spatiotemporal patterns of neural activity from functional measurements by seeking for sources that are maximally independent. Additional information on one or more sources (e.g., spatial regularity) is often available; however, it is not considered while looking for independent components. In the present work, we propose a new ICA algorithm based on the optimization of an objective function that accounts for both independence and other information on the sources or on the mixing model in a very general fashion. In particular, we apply this approach to fMRI data analysis and illustrate, by means of simulations, how inclusion of a spatial regularity term helps to recover the sources more effectively than with conventional ICA. The improvement is especially evident in high noise situations. Furthermore we employ the same approach on data sets from a complex mental imagery experiment, showing that consistency and physiological plausibility of relatively weak components are improved. 相似文献
113.
Vittorio di Federico 《Meccanica》1991,26(2-3):129-133
After a brief review of the validity of Darcy's law, a nonlinear flow law is adopted for the analytical solution of a groundwater flow problem. A one-dimensional unsteady flow in plane geometry, with prescribed head at the boundaries, is studies. The solution of the analogous linear case is reviewed through the use of Boltzmann's transformation. A solution for nonlinear flow is obtained through a generalization of this transformation. Detailed expressions for specific discharge and drawdown are derived for two significant values of the exponent of the flow law. All results are presented in dimensionless form for a comparative analysis. Some significant cases are plotted. Finally, some implications of the adoption of a nonlinear flow law are discussed.
Sommario Dopo un breve riepilogo sui limiti di validità della legge di Darcy, si adotta una formulazione non lineare della legge del moto per la risoluzione analitica di un problema di filtrazione. Si studia un moto non stazionario monodimensionale in geometria piana, con carico assegnato al contorno. Viene richiamata la soluzione dell'analogo caso lineare, tramite l'applicazione della trasformata di Boltzmann. Una soluzione per il moto non lineare è ottenuta mediante la generalizzazione di tale trasformazione. Si ricavano le espressioni della velocità apparente di filtrazione e del drawdown per due valori significativi dell'esponente della legge del moto. Tutti i risultati sono presentati in forma adimensionale per una analisi comparativa. Alcuni casi significativi sono diagrammati. In conclusione, sono discusse alcune implicazioni dell'adozione di una legge del moto non lineare.相似文献
114.
Stefano Casarin Federico Aletti Giuseppe Baselli Marc Garbey 《Acta Mechanica Sinica》2017,33(2):284-294
The high demand for lung transplants cannot be matched by an adequate number of lungs from donors. Since fully ex-novo lungs are far from being feasible, tissue engi-neering is actively considering implantation of engineered lungs where the devitalized structure of a donor is used as scaffold to be repopulated by stem cells of the receiv-ing patient. A decellularized donated lung is treated inside a bioreactor where transport through the tracheobronchial tree (TBT) will allow for both deposition of stem cells and nour-ishment for their subsequent growth, thus developing new lung tissue. The key concern is to set optimally the boundary conditions to utilize in the bioreactor. We propose a pre-dictive model of slow liquid ventilation, which combines a one-dimensional (1-D) mathematical model of the TBT and a solute deposition model strongly dependent on fluid velocity across the tree. With it, we were able to track and drive the concentration of a generic solute across the airways, look-ing for its optimal distribution. This was given by properly adjusting the pumps' regime serving the bioreactor. A feed-back system, created by coupling the two models, allowed us to derive the optimal pattern. The TBT model can be easily invertible, thus yielding a straightforward flow/pressure law at the inlet to optimize the efficiency of the bioreactor. 相似文献
115.
Headphone rendering of nearby virtual sound sources represents to date an open issue in 3-D audio, due to a number of technical challenges and temporal requirements involved in the measurement of individual Head-Related Transfer Functions (HRTFs). In order to tackle this problem, we propose a filter model of near-field effects based on the Distance Variation Function (Kan et al., 2009). Thanks to its simple structure and low order, the model can be applied to any far-field virtual auditory display to yield a realistic and computationally efficient near-field compensation of spectral and binaural effects. The model is subjectively evaluated in two psychophysical experiments where the relative distance of pairs of virtually rendered sound sources is judged. Results show that even though sound intensity overshadows subtler near-field effects when it is available as a cue for distance, the model is capable of offering relative distance information of near lateral virtual sources when intensity cues are removed. Furthermore, performances of the model in relative distance rendering are compared to those of alternative near-field rendering methods available in the literature. 相似文献
116.
Federico Colombo Luigi Lentini Terenziano Raparelli Vladimir Viktorov 《Meccanica》2017,52(15):3645-3660
Active compensation is an effective method for increasing air bearing static and dynamic performance. This paper describes the design, modelling and experimental validation of an actively compensated externally pressurized gas bearing. The active compensation is obtained through the support compensation strategy. With this strategy, the system’s initial working position is restored by compensating for air gap variations through adjustments to the bearing vertical dimension. The described bearing consists in a conventional thrust bearing which is integrated with a multilayer piezoelectric actuator, a compliant mechanism and a digital controller. Nevertheless the non-linear nature of the air system, a simple linear model results to be an effective choice for neighbour of equilibrium conditions. Results demonstrate the good accuracy of the model and the system’s good capacity of rejecting external force disturbances. 相似文献
117.
Federico Ferreres David M. Pereira Patrícia Valentão Paula B. Andrade 《Rapid communications in mass spectrometry : RCM》2010,24(6):801-806
Apicultural products have been widely used in diet complements as well as in phytotherapy. Bee pollen from Echium plantagineum was analysed by high‐performance liquid chromatography/photodiode‐array detection coupled to ion trap mass spectrometry (HPLC‐PAD‐MSn) with an electrospray ionisation interface. The structures have been determined by the study of the ion mass fragmentation, which characterises the interglycosidic linkage in glycosylated flavonoids and differentiates positional isomers. Twelve non‐coloured flavonoids were characterised, being kaempferol‐3‐O‐neohesperidoside the major compound, besides others in trace amounts. These include quercetin, kaempferol and isorhamnetin glycosides, with several of them being isomers. Acetylated derivatives are also described. This is the first time that non‐coloured flavonoids are reported from this pollen, with MS fragmentation proving to be most useful in the elucidation of isomeric structures. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
118.
Federico Scaravelli Sergio Bacchi Ornella Curcuruto William Maton 《Tetrahedron letters》2010,51(39):5154-5156
An efficient and versatile synthetic method is described to synthesize diethylphosphonacetamides in a single step. 相似文献
119.
120.