全文获取类型
收费全文 | 3513篇 |
免费 | 122篇 |
国内免费 | 22篇 |
专业分类
化学 | 2075篇 |
晶体学 | 11篇 |
力学 | 145篇 |
数学 | 497篇 |
物理学 | 697篇 |
无线电 | 232篇 |
出版年
2023年 | 42篇 |
2022年 | 59篇 |
2021年 | 121篇 |
2020年 | 107篇 |
2019年 | 96篇 |
2018年 | 65篇 |
2017年 | 54篇 |
2016年 | 142篇 |
2015年 | 98篇 |
2014年 | 143篇 |
2013年 | 186篇 |
2012年 | 241篇 |
2011年 | 272篇 |
2010年 | 153篇 |
2009年 | 135篇 |
2008年 | 218篇 |
2007年 | 210篇 |
2006年 | 216篇 |
2005年 | 197篇 |
2004年 | 153篇 |
2003年 | 133篇 |
2002年 | 132篇 |
2001年 | 57篇 |
2000年 | 28篇 |
1999年 | 29篇 |
1998年 | 33篇 |
1997年 | 41篇 |
1996年 | 37篇 |
1995年 | 22篇 |
1994年 | 25篇 |
1993年 | 19篇 |
1992年 | 9篇 |
1991年 | 18篇 |
1990年 | 9篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 10篇 |
1984年 | 10篇 |
1983年 | 10篇 |
1982年 | 12篇 |
1981年 | 15篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1975年 | 9篇 |
1974年 | 3篇 |
1973年 | 6篇 |
排序方式: 共有3657条查询结果,搜索用时 218 毫秒
11.
Clelia Marchionna Stefano Panizzi 《Mathematical Methods in the Applied Sciences》2007,30(11):1295-1309
We introduce a set of conserved quantities of energy‐type for a strictly hyperbolic system of two coupled wave equations in one space dimension. The system is subject to mechanical boundary conditions. Some of these invariants are asymmetric in the sense that their defining quadratic form contains second order derivatives in only one of the unknowns. We study their independence with respect to the usual energies and characterize their sign. In many cases, our results provide sharp well‐posedness and stability results. Finally, we apply some of our conservation laws to the study of a singular perturbation problem previously considered by J. Lagnese and J. L. Lions. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
12.
13.
Fabrizo Granelli Honggang Zhang Xiaofei Zhou Stefano Maranò 《Mobile Networks and Applications》2006,11(4):487-499
The paper illustrates the novel concept of Cognitive UWB Radio, a wireless system based on UWB transmission able to self-adapt to the characteristics of the surrounding environment. First, salient features of UWB transmission are reviewed, and then, application of UWB characteristics to the problem of cognitive radio systems is discussed. Current ongoing studies and achieved results on a possible physical layer based on Prolate Spheroidal Wave Functions and consistent with the Cognitive UWB Radio paradigm are presented and analyzed, and their application to the sensor network scenario is discussed. 相似文献
14.
Giovanni Tartarini Alessandra Lena Davide Passaro Lorenzo Rosa Stefano Selleri Pier Faccin Enrico Maria Fabbri 《Optical and Quantum Electronics》2006,38(9-11):869-876
A comprehensive numerical tool has been developed for the evaluation of the performances of Radio over Fiber (RoF) links intended for wireless signal distribution.At the transmitter end an appropriate set of rate equations allows to model the optical source as a solitary laser or as an appropriately injection locked laser. The optical channel is modeled putting into account the combined effect of fiber dispersion, laser source non ideal performances (e.g. non-linear effects, frequency chirp), and quadratic detection of the receiving photodiode. The simulation model developed can be a useful tool at the design stage allowing a preliminary evaluation of the characteristics of real RoF links. 相似文献
15.
Two different problems are proposed as approximations of the usual system modelling natural convection under the Oberbeck-Boussinesq assumptions. The error is evaluated by means of the norm of its gradient in the Hilbert space. The average Nusselt number is also estimated. 相似文献
16.
DRIFTS study of surface reactivity to NO2 by zinc nanoparticle aggregates and zinc hollow nanofibers
Zinc nanostructures synthesized with different morphologies from the same evaporation/condensation technique are studied with concern to surface reactivity to NO2 by Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS). Synthesis of nanopowders is obtained, according to previous work, by gas flow thermal evaporation at 540 °C of bulk Zn grains. Two types of Zn powders are obtained and studied in experiments. The first one is collected on the cold walls of the reactor as a deposit produced by thermophoretic effect. It is constituted by grains (∼10 μm) originated by the stratification of smaller aggregates (∼200 nm) and isolated primary particles (∼50 nm) born in the gas flow. The second type of powder is grown from the condensation of Zn chemical vapors within the expansion orifice of the quartz reactor after relatively long time (∼1 h) deposition process. It is constituted mainly by hollow Zn nanofibers with external and internal diameter about 100 and 60 nm. Preliminary characterization of the two types of powders is made by SEM, TEM, XRD. Thereafter, the two types of samples are studied by DRIFTS at variable temperature (VT). Comparison is made between the home-synthesized nanopowders with respect to commercial Zn standard dust. The Zn hollow nanofibers when exposed to NO2 are found to exhibit dramatic reactivity, which is not observed at all either in the case of clustered aggregate zinc or of commercial Zn dust powders. Results indicate that, at increasing temperature from RT to 300 °C, the hollow nanofibers surface reacts distinctively with adsorbant gas NO2, with contemporary formation of a progressively growing narrow absorption band at 2500 cm−1 and contemporary depression of a doublet (∼1600-1628 cm−1) band. In order to justify this striking spectral feature, we propose the occurring of a possible polymerization process at nanofibers surface where most probably as a consequence of pre-treatment and exposure to gas NO2 a very thin film of ZnO is formed. The possible role of huge specific surface of hollow nanofibers as inferred by preliminary SEM, TEM, XRD studies is discussed. 相似文献
17.
Online weighted flow time and deadline scheduling 总被引:1,自引:0,他引:1
Luca Becchetti Stefano Leonardi Alberto Marchetti-Spaccamela Kirk Pruhs 《Journal of Discrete Algorithms》2006,4(3):339
In this paper we study some aspects of weighted flow time. We first show that the online algorithm Highest Density First is an O(1)-speed O(1)-approximation algorithm for P|ri,pmtn|∑wiFi. We then consider a related Deadline Scheduling Problem that involves minimizing the weight of the jobs unfinished by some unknown deadline D on a uniprocessor. We show that any c-competitive online algorithm for weighted flow time must also be c-competitive for deadline scheduling. We then give an O(1)-competitive algorithm for deadline scheduling. 相似文献
18.
A computer search in the finite projective spaces PG(n, q) for the spectrum of possible sizes
k
of complete k-caps is done. Randomized greedy algorithms are applied. New upper bounds on the smallest size of a complete cap are given for many values of n
and q. Many new sizes of complete caps are obtained. 相似文献
19.
Giancarlo Marconi Sandra Monti Francesco Manoli Stefano Ottani 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):279-282
The Induced Circular Dichroism of the complex formed by inclusion of Licochalcone-A (a powerful antimalarian of natural origin)
and β-cyclodextrin was measured and calculated. The association constant was determined by titration experiments of ICD and
fluorescence. The lowest energy conformations of the complex were obtained through docking procedures and their Circular Dichroism
was evaluated within the Tinoco framework. The geometry of the complex was found to be consistent with the main experimental
features. 相似文献
20.