全文获取类型
收费全文 | 324465篇 |
免费 | 2932篇 |
国内免费 | 854篇 |
专业分类
化学 | 157672篇 |
晶体学 | 4360篇 |
力学 | 12530篇 |
综合类 | 10篇 |
数学 | 33729篇 |
物理学 | 86528篇 |
无线电 | 33422篇 |
出版年
2020年 | 2455篇 |
2019年 | 2813篇 |
2018年 | 3631篇 |
2017年 | 3589篇 |
2016年 | 5254篇 |
2015年 | 3157篇 |
2014年 | 5107篇 |
2013年 | 12576篇 |
2012年 | 9653篇 |
2011年 | 11824篇 |
2010年 | 8378篇 |
2009年 | 8441篇 |
2008年 | 11809篇 |
2007年 | 12361篇 |
2006年 | 11577篇 |
2005年 | 10749篇 |
2004年 | 9914篇 |
2003年 | 8921篇 |
2002年 | 8821篇 |
2001年 | 8856篇 |
2000年 | 7144篇 |
1999年 | 5646篇 |
1998年 | 5225篇 |
1997年 | 5213篇 |
1996年 | 4896篇 |
1995年 | 4542篇 |
1994年 | 4620篇 |
1993年 | 4310篇 |
1992年 | 4723篇 |
1991年 | 4741篇 |
1990年 | 4606篇 |
1989年 | 4391篇 |
1988年 | 4434篇 |
1987年 | 4063篇 |
1986年 | 3856篇 |
1985年 | 5072篇 |
1984年 | 5395篇 |
1983年 | 4496篇 |
1982年 | 4688篇 |
1981年 | 4588篇 |
1980年 | 4468篇 |
1979年 | 4409篇 |
1978年 | 4728篇 |
1977年 | 4637篇 |
1976年 | 4636篇 |
1975年 | 4285篇 |
1974年 | 4339篇 |
1973年 | 4480篇 |
1972年 | 3160篇 |
1971年 | 2629篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Existence conditions are given for maximum likelihood (ML) parameter estimates for several families of 2-parameter software-reliability Poisson-process models. For each such model, the ML equations can be expressed in terms of one equation in one unknown. Bounds are given on solutions to these one equation problems to serve as initial intervals for search algorithms like bisection. Uniqueness of the solutions is established in some cases. Solutions are also tabulated for certain simple cases. Results are given for ungrouped failure data (exact times are available for all failures). ML estimation problems for such a situation are treated as limiting cases of problems based on failure times grouped into intervals of decreasing mesh 相似文献
62.
A new practical design approach for minimum-phase FIR or IIR filters, setting out from a high dimensionality FIR linear-phase prototype is described. The novelty of this technique lies in overcoming the inherent problem of finding the roots of a high order polynomial with repeated and/or very closely clustered roots 相似文献
63.
Many current implementations of protocols such as the Transmission Control Protocol/Internet Protocol (TCP/IP) are inefficient because data are often accessed more frequently than necessary. Three techniques that reduce the need for memory bandwidth are proposed. The techniques are copy-on-write, page remapping, and single-copy. Afterburner, a network-independent card that provides the services that are necessary for a single-copy protocol stack, is described. The card has 1 MByte of local buffers and provides a simple interface to a variety of network link adapters, including HIPPI and asynchronous transfer mode (ATM). Afterburner can support transfers to and from the link adapter card at rates up to 1 Gbit/s. An implementation of TCP/IP that uses the features provided by Afterburner to reduce the movement of data to a single copy is discussed. Measurements of the end-to-end performance of Afterburner and the single-copy implementation of TCP/IP are presented 相似文献
64.
Corazza G.E. Ferrari G. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(11):3003-3008
New bounds are proposed for the Marcum Q-function, which is defined by an integral expression where the 0th-order modified Bessel function appears. The proposed bounds are derived by suitable approximations of the 0th-order modified Bessel function in the integration region of the Marcum Q-function. They prove to be very tight and outperform bounds previously proposed in the literature. In particular, the proposed bounds are noticeably good for large values of the parameters of the Marcum Q-function, where previously introduced bounds fail and where exact computation of the function becomes critical due to numerical problems 相似文献
65.
The radiation emitted by charged, scalar particles in a Schwarzschild field with maximal acceleration corrections is calculated classically and in the tree approximation of quantum field theory. In both instances the particles emit radiation that has characteristics similar to those of gamma-ray bursters. 相似文献
66.
D. V. Vyalykh A. M. Shikin G. V. Prudnikova A. Yu. Grigor’ev A. G. Starodubov V. K. Adamchuk 《Physics of the Solid State》2002,44(1):164-170
This paper reports on the first experimental observation of quantum-well states and sp-type resonances in thin single-crystal gold, silver, and copper layers formed on single-crystal W(110) surfaces, which result from spatial localization of Bloch-type electronic wave functions in a quantum well with potential barriers at the vacuum/metal and metal/W(110) interfaces. The quantization of the valence-band electronic structure in Au/W(110), Ag/W(110), and Cu/W(110) systems was studied experimentally using angle-resolved photoelectron spectroscopy. 相似文献
67.
Compensated SC integrator stages are analysed and compared with respect to their sensitivities to finite amplifier gain effects. Simple single-stage amplifiers may be used in such circuits even in highly selective filtering applications. This may allow an extension of the present frequency range of SC circuits. 相似文献
68.
A narrow linewidth has been obtained in a new 1.5 ?m distributed feedback laser integrated monolithically with a tunable external cavity. The linewidth of 18 MHz has been achieved by tuning the current in the external cavity, and the FM response was flat from 100 kHz to 500 MHz. 相似文献
69.
Rajiv Manohar G. Tripathi A.K. Srivastava A.K. Prajapati 《Journal of Physics and Chemistry of Solids》2006,67(11):2300-2304
Dielectric properties of polymer-liquid crystal mixture, having constituent polymer, poly-butyl methacrylate (PBMA) and liquid crystal, cholesteryl nonanoate, are reported as a function of frequency and temperature. The measurement has been done in a temperature range of 300-375 K and frequency range of 100 Hz-10 MHz. The dielectric permittivity and dielectric loss shows significant changes with the addition of polymer molecules in liquid crystal. The significant feature of composite formation is that the pure liquid crystal and polymer do not show dielectric relaxation in the frequency range covered, while the composite shows relaxation peak at a particular frequency. The optical transmittance of pure liquid crystal and composite has also been measured and compared. 相似文献
70.
J. Maul I. Strachnov S. Karpuk P. Bernhard A. Oelsner G. Schönhense G. Huber 《Applied Physics A: Materials Science & Processing》2006,82(1):43-47
We report morphologic changes of metallic surfaces at the onset of ablation, starting from gentle ablation to the emergence of ablation craters. The evolution of both observed melting zones and of ablation craters therein are investigated in dependence of the ablation laser fluence for nanosecond ultraviolet laser pulses. Further, consequences of crater formation for cluster synthesis within the released atomic vapor are pointed out. PACS 52.38.Mf; 79.20.Ds; 65.40.De 相似文献