全文获取类型
收费全文 | 203934篇 |
免费 | 2364篇 |
国内免费 | 834篇 |
专业分类
化学 | 91928篇 |
晶体学 | 2461篇 |
力学 | 7603篇 |
综合类 | 7篇 |
数学 | 17703篇 |
物理学 | 54033篇 |
无线电 | 33397篇 |
出版年
2021年 | 1119篇 |
2020年 | 1232篇 |
2019年 | 1220篇 |
2018年 | 1296篇 |
2017年 | 1288篇 |
2016年 | 2591篇 |
2015年 | 2047篇 |
2014年 | 2770篇 |
2013年 | 8263篇 |
2012年 | 6661篇 |
2011年 | 8402篇 |
2010年 | 5455篇 |
2009年 | 5979篇 |
2008年 | 8484篇 |
2007年 | 8891篇 |
2006年 | 8643篇 |
2005年 | 8099篇 |
2004年 | 7281篇 |
2003年 | 6457篇 |
2002年 | 6211篇 |
2001年 | 7390篇 |
2000年 | 5776篇 |
1999年 | 4626篇 |
1998年 | 3762篇 |
1997年 | 3694篇 |
1996年 | 3575篇 |
1995年 | 3250篇 |
1994年 | 3164篇 |
1993年 | 3073篇 |
1992年 | 3313篇 |
1991年 | 3269篇 |
1990年 | 2914篇 |
1989年 | 2829篇 |
1988年 | 2714篇 |
1987年 | 2281篇 |
1986年 | 2201篇 |
1985年 | 3104篇 |
1984年 | 3052篇 |
1983年 | 2487篇 |
1982年 | 2662篇 |
1981年 | 2529篇 |
1980年 | 2485篇 |
1979年 | 2370篇 |
1978年 | 2457篇 |
1977年 | 2376篇 |
1976年 | 2321篇 |
1975年 | 2296篇 |
1974年 | 2176篇 |
1973年 | 2287篇 |
1972年 | 1361篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
C. S. Kubrusly 《Applied Mathematics Letters》1989,2(4):349-352
It is given a simple and unified new proof for the following well-known stability condition: an infinite-dimensional time-invariant discrete linear system is uniformly asymptotically stable if and only if the associate Lyapunov equation has a unique strictly positive solution. The proof is partially based on an application of Rota's model construction technique. 相似文献
242.
This is a review of two electrical equivalent circuits for multi-element magnetostatic wave transducers. The two circuit models are identified as a transmission line and a two terminal model. Both models have been extended to all three principal MSW modes of propagation to the point where computer programs have been written to plot phase and amplitude response as measured by commercial network analyzers. A review is provided of the basic assumptions, similarities and differences, advantages and disadvantages, and limitations of the two models. The useful range of them covers many cases of practical interest in the one to twelve Ghz frequency range. 相似文献
243.
A method for determining the axial ratio of an antenna illuminated by an imperfectly circularly polarized source is described. This method requires measurements of the right- and left-handed amplitude components and their relative phases for both the receiving and transmitting (or source) antennas. From these measured data and, of course, the theory of power transfer between the source and receiving antennas, an algorithm is derived for the actual axial ratio of the receiving antenna. Illustrative results showing the effects of the phase angle are presented. 相似文献
244.
We report viscometric data collected in a Couette rheometry on dilute, single‐solvent polystyrene (PS)/dioctyl phthalate (DOP) solutions over a variety of polymer molecular weights (5.5 × 105 ≤ Mw ≤ 3.0 × 106 Da) and system temperatures (288 K ≤ T ≤ 318 K). In view of the essential viscometric features, the current data may be classified into three categories: The first concerns all the investigated solutions at low shear rates, where the solution properties are found to agree excellently with the Zimm model predictions. The second includes all sample solutions, except for high‐molecular‐weight PS samples (Mw ≥ 2.0 × 106 Da), where excellent time–temperature superposition is observed for the steady‐state polymer viscosity at constant polymer molecular weights. No similar superposition applies at a constant temperature but varied polymer molecular weights, however. The third appears to be characteristic of dilute high‐molecular‐weight polymer solutions, for which the effects of temperature on the viscosity curve are further complicated at high shear rates. The implications concerning the relative importance of hydrodynamic interactions, segmental interactions, and chain extensibility with increasing polymer molecular weight, system temperature, and shear rate are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 787–794, 2006 相似文献
245.
Binding energy spectra of the valence electrons of the open shell molecule NO have been obtained up to 55 eV at azimuthal angles of 0° and 7° using binary (e, 2e) spectroscopy at an impact energy of 1200 eV. The momentum distribution has been obtained for the least tightly bound (unpaired) electron, removal of which leads to formation of the X 1Σ+ ground state of NO+. Momentum distributions have also been measured at 21.0 and 40.5 eV. The measured momentum distributions are compared with several literature wavefunctions of varying complexity. They are found to be in excellent agreement with those calculated using the natural spin orbital wavefunctions of Kouba and Ohrn. 相似文献
246.
Coherent heterodyne detection in optical communication is degraded by phase instabilities present when a semiconductor laser is used as a local oscillator. Postdetection processing that accounts for these instabilities is suggested, based on a diffusion-process model for oscillator instabilities and a maximum-likelihood sequence-estimator of information symbols modulating the received light. 相似文献
247.
C.-A. Pillet 《Communications in Mathematical Physics》1991,140(2):241-274
We develop a scheme based on pseudo-differential operators to analyze the propagation of excitations in inhomogeneous extended systems. This method is used in a very specific situation, however we think that it has some generality and should apply to various other problems of current interest. We study the well known two-dimensionalsymmetric model of solidification introduced by Langer and Turski. Assuming the existence of Ivantsov-like steady-state solutions, we calculate their excitation spectrum. We show that there are no unstable propagating modes if the Gibbs-Thomson effect is taken into account. This proves that the growth of needle-crystals is stable with respect to side-branching. 相似文献
248.
Szczech J.B. Megaridis C.M. Gamota D.R. Jie Zhang 《Electronics Packaging Manufacturing, IEEE Transactions on》2002,25(1):26-33
An emerging selective metallization process to fabricate fine-line conductors based on drop-on-demand (DOD) ink jet printing and novel nano-particle fluid suspensions (NPFS) was studied. The suspensions consist of 1-10 nm silver or gold particulates that are homogeneously suspended in an organic carrier. A piezo-electric droplet generator driven by a bipolar voltage signal is used to dispense 50-70 μm diameter droplets traveling at 1-3 m/s before impacting a compliant polyimide substrate. The deposit/substrate composite is subsequently processed at 300°C for 15 min to allow for complete evaporation of the carrier and for sintering of the nano-particles, thereby yielding a finished circuit interconnect. Test vehicles created using this technique exhibit features as fine as 120 μm wide and 1 μm thick with resistivities on the order of 3.5×10-5 Ωcm. The circuitry performed well under environmental conditioning. As expected, repeatability of circuitry fabrication showed sensitivity to the generation of steady, satellite-free droplets. In an effort to generate droplets consistently, it is essential to develop a strong fundamental understanding of the correlation between device excitation parameters and dispensed fluid properties, and to resolve the microrheological behavior of the NPFS when flowing through the droplet generator 相似文献
249.
Lee C.K. Hui S.Y.R. Shu-HungChung H. Shrivastava Y. 《Power Electronics, IEEE Transactions on》2002,17(1):94-100
In this paper, a feasibility study of a randomized space vector switching scheme for three-level inverters is reported. Based on the traditional space vector concept for pulse width modulation (PWM) inverters, the sequence of the space vector is randomized in this scheme. The overall vector, resulting from the randomization, is mathematically identical to that of the traditional space vector. It has been found that the randomized scheme can effectively spread the switching noise over a wide frequency range and at the same time keep the steady-state link capacitor voltage balanced. The proposed scheme has been implemented in real time in a digital controller and its features have been experimentally confirmed 相似文献
250.
F. D. Saccone C. E. Rodrí guez Torres F. H. S nchez O. Gutfleisch 《Physica B: Condensed Matter》2002,320(1-4):312-315
In the present work, a quantitative analysis of the phase compositions by Mössbauer effect spectroscopy of solid and conventional hydrogen disproportionated Pr13.7Fe80.3B6.0 and Pr13.7Fe63.5Co16.7Zr0.1B6.0 alloys was carried out. Significant amounts of intermediate borides t-Fe3B and Pr(Fe, Co)12B6 were detected after solid hydrogen disproportionation treatment in Pr13.7Fe80.3B6.0 and Pr13.7Fe63.5Co16.7Zr0.1B6.0 alloys, respectively. After conventional hydrogenation–disproportionation–desorption–recombination treatment these phases were not detected and in no case residual Pr2Fe14B-phase was found. It was observed that the amount of intermediate borides after disproportionation can be correlated with the degree of texture after recombination at various temperatures. 相似文献