全文获取类型
收费全文 | 336篇 |
免费 | 31篇 |
国内免费 | 18篇 |
专业分类
化学 | 62篇 |
力学 | 27篇 |
综合类 | 9篇 |
数学 | 126篇 |
物理学 | 72篇 |
无线电 | 89篇 |
出版年
2023年 | 6篇 |
2022年 | 4篇 |
2021年 | 7篇 |
2020年 | 10篇 |
2019年 | 8篇 |
2018年 | 3篇 |
2017年 | 8篇 |
2016年 | 12篇 |
2015年 | 7篇 |
2014年 | 22篇 |
2013年 | 27篇 |
2012年 | 23篇 |
2011年 | 15篇 |
2010年 | 18篇 |
2009年 | 30篇 |
2008年 | 17篇 |
2007年 | 20篇 |
2006年 | 23篇 |
2005年 | 15篇 |
2004年 | 13篇 |
2003年 | 11篇 |
2002年 | 7篇 |
2001年 | 3篇 |
2000年 | 9篇 |
1999年 | 10篇 |
1998年 | 5篇 |
1997年 | 13篇 |
1996年 | 1篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有385条查询结果,搜索用时 437 毫秒
21.
C.L. Canedy W.W. Bewley J.R. Lindle J.A. Nolde D.C. Larrabee C.S. Kim M. Kim I. Vurgaftman J.R. Meyer 《Journal of Electronic Materials》2008,37(12):1780-1785
We report an experimental investigation of four interband cascade lasers with wavelengths spanning the mid-infrared spectral
range, i.e., 2.9 μm to 5.2 μm, near room temperature in pulsed mode. One broad-area device had a pulsed threshold current density of only 3.8 A/cm2 at 78 K (λ = 3.6 μm) and 590 A/cm2 at 300 K (λ = 4.1 μm). The room-temperature threshold for the shortest-wavelength device (λ = 2.6 μm to 2.9 μm) was even lower, 450 A/cm2. A␣cavity-length study of the lasers emitting at 3.6 μm to 4.1 μm yielded an internal loss varying from 7.8 cm−1 at 78 K to 24 cm−1 at 300 K, accompanied by a decrease of the internal efficiency from 77% to 45%. 相似文献
22.
Based on the property analysis of interferential multispectral images, a novel compression algorithm of partial set partitioning in hierarchical trees (SPIHT) with classified weighted rate-distortion optimization is presented.After wavelet decomposition, partial SPIHT is applied to each zero tree independently by adaptively selecting one of three coding modes according to the probability of the significant coefficients in each bitplane.Meanwhile the interferential multispectral image is partitioned into two kinds of regions in terms of luminous intensity, and the rate-distortion slopes of zero trees are then lifted with classified weights according to their distortion contribution to the constructed spectrum.Finally a global ratedistortion optimization truncation is performed.Compared with the conventional methods, the proposed algorithm not only improves the performance in spatial domain but also reduces the distortion in spectral domain. 相似文献
23.
24.
In this paper, the pattern memory of the second-order neural networks with time-delay are investigated based on the stability theory. Several sufficient conditions are obtained such that the equilibrium point is locally exponential stable when the equilibrium point located in the saturation region. These conditions can be directly derived from the synaptic connection weights and the external input of the second-order neural networks, and be verified easily. In addition, four examples are given to show the effectiveness of the results. 相似文献
25.
Received May 28, 1996 / Revised version received May 1, 1998 Published online October 9, 1998 相似文献
26.
Proton nuclear magnetic resonance (NMR) spin-spin relaxation measurements were made on three commercial-grade polyethylenes in the melt state, free of solvent. All samples exhibit a three-component relaxation behavior, with components being assigned to amorphous low-molecular weight material (non-network fraction), amorphous entangled network fraction, and an ordered or high-segmental-density fraction, in order of decreasing relaxation times. Sample thermal history is shown to have a considerable effect on the overall relaxation behavior, and therefore on the relative amounts of each of the three components in the melt. An adequate thermal treatment of samples produces an equilibrium melt with invariant composition of the three fractions. The effects of thermal history on the relative amount of high-segment-density regions in the melt parallels its effect on the fraction of crystalline material in the solid polymer. These results are evidence for the persistence of ordered regions in polyethylene at temperatures well above the crystalline melting point of the polymer. We further comment on the nature of the two slower relaxing components and present examples of how the components manifest themselves in other polymer characterization techniques. © 1992 John Wiley & Sons, Inc. 相似文献
27.
Eva Lesanska 《Applications of Mathematics》2002,47(1):9-23
The problem is to determine the optimum size of nonsensitiveness regions for the level of statistical tests. This is closely connected with the problem of the distribution of quadratic forms. 相似文献
28.
电子碰撞激发X光激光的增益特性依赖于电子密度Ne、电子温度Te、增益区宽度D R和介质速度梯度dv/dz等四个表征等离子体内部状态的参数。以类氖-锗离子为例研究了反转和增益特性对Te、Ne的依赖关系,并在典型的增益区宽度(D R=100 μm)和介质速度梯度(dv/dz=1.3×109s-1)下讨论了共振线俘获对增益特性影响,给出波长为19.6nm, 23.2nm和23.6nm三条激光线的增益目标区域。还讨论了双电子复合过程对离子布居的重要影响。 相似文献
29.
V. G. Krotov 《Mathematical Notes》1997,62(4):439-448
In the critical case αp=n functions from the Hardy-Sobolev spacesH
α
p
(B
n
) have a limit almost everywhere on the boundary along certain regions of exponential contact with the boundary. It is proved
in the paper that the maximal operator associated with these regions is bounded as an operator fromH
α
p
(B
n
) toL
p
(ϖB
n
).
Translated fromMatematicheskie Zametki, Vol. 62, No. 4, pp. 527–539, October, 1997.
Translated by N. K. Kulman 相似文献
30.
Sandra S. Ferreira Dário Ferreira Célia Nunes João T. Mexia 《Mathematical Methods in the Applied Sciences》2023,46(1):356-367
Sub-models of mixed linear models are considered. The independence of these sub-models leads to sufficient statistics for the parameters relevant for their densities. Using pivot variables, confidence regions are obtained as well hypothesis testing for variance components, estimable functions, and estimable vectors. In addition, to compare the estimators and the models, we present the histograms with the empirical joint densities for positive and negative parts of the estimators. The figures, for the two-dimensional charts, contain the corresponding UMVUE and are all unimodal with the UMVUE near the mode. The nearness of the estimators and the modes validates the presented methodology and allows the safe use of induced densities. A numerical example applied to real data is presented. 相似文献