全文获取类型
收费全文 | 424982篇 |
免费 | 4282篇 |
国内免费 | 1020篇 |
专业分类
电工技术 | 7348篇 |
综合类 | 456篇 |
化学工业 | 65640篇 |
金属工艺 | 21974篇 |
机械仪表 | 15406篇 |
建筑科学 | 8817篇 |
矿业工程 | 4772篇 |
能源动力 | 8672篇 |
轻工业 | 25972篇 |
水利工程 | 6048篇 |
石油天然气 | 15421篇 |
武器工业 | 46篇 |
无线电 | 41250篇 |
一般工业技术 | 96116篇 |
冶金工业 | 65972篇 |
原子能技术 | 14195篇 |
自动化技术 | 32179篇 |
出版年
2021年 | 4270篇 |
2019年 | 4196篇 |
2018年 | 7738篇 |
2017年 | 8067篇 |
2016年 | 8610篇 |
2015年 | 4917篇 |
2014年 | 8308篇 |
2013年 | 18530篇 |
2012年 | 12080篇 |
2011年 | 15398篇 |
2010年 | 12473篇 |
2009年 | 14031篇 |
2008年 | 14033篇 |
2007年 | 13813篇 |
2006年 | 11396篇 |
2005年 | 10331篇 |
2004年 | 9923篇 |
2003年 | 9653篇 |
2002年 | 9225篇 |
2001年 | 9042篇 |
2000年 | 9009篇 |
1999年 | 8604篇 |
1998年 | 17833篇 |
1997年 | 13353篇 |
1996年 | 10327篇 |
1995年 | 7903篇 |
1994年 | 7279篇 |
1993年 | 7277篇 |
1992年 | 5986篇 |
1991年 | 5976篇 |
1990年 | 5871篇 |
1989年 | 5836篇 |
1988年 | 5600篇 |
1987年 | 5237篇 |
1986年 | 4984篇 |
1985年 | 5551篇 |
1984年 | 5300篇 |
1983年 | 5024篇 |
1982年 | 4640篇 |
1981年 | 4807篇 |
1980年 | 4677篇 |
1979年 | 4907篇 |
1978年 | 5089篇 |
1977年 | 5347篇 |
1976年 | 6569篇 |
1975年 | 4586篇 |
1974年 | 4564篇 |
1973年 | 4648篇 |
1972年 | 4019篇 |
1971年 | 3723篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
Piedade M. Sousa L. A. de Almeida T. M. Germano J. da Costa B. A. Lemos J. M. Freitas P. P. Ferreira H. A. Cardoso F. A. 《IEEE transactions on circuits and systems. I, Regular papers》2006,53(11):2384-2395
This paper presents a hand-held microsystem based on new fully integrated magnetoresistive biochips for biomolecular recognition (DNA hybridization, antibody antigen interaction, etc.). Magnetoresistive chip surfaces are chemically treated, enabling the immobilization of probe biomolecules such as DNA or antibodies. Fluid handling is also integrated in the biochip. The proposed microsystem not only integrates the biochip, which is an array of 16times16 magnetoresistive sensors, but it also provides all the electronic circuitry for addressing and reading out each transducer. The proposed architecture and circuits were specifically designed for achieving a compact, programmable and portable microsystem. The microsystem also integrates a hand-held analyzer connected through a wireless channel. A prototype of the system was already developed and detection of magnetic nanoparticles was obtained. This indicates that the system may be used for magnetic label based bioassays 相似文献
22.
The phenomenon of persistent tunneling photoconductivity was studied using the tunneling spectroscopy technique at liquid-helium temperature: the separation between the unoccupied levels in a δ-doped layer at the GaAs surface decreased after illumination. This decrease was due to an increase in the width of the quantum well of the δ-doped layer. For photon energies hv exceeding the GaAs band gap E g , this increase in the width of the quantum well was related to the accumulation of positive charge in the depth of GaAs induced by the generation of the electron-hole pairs and photoionization of deep centers. For hv < E g (including the case of CO2 laser), only photoionization is important. The experimental data agree with the self-consistent calculations. The critical temperature for the effect has been determined (T c = 45 K); at higher temperatures, the effect disappears. 相似文献
23.
The problem of synthesis of an isotropic reradiator providing for the transparency of a circular perfectly conducting cylinder illuminated by a TE-polarized plane wave is investigated. A general solution to the problem of diffraction from the cylinder with a reradiator is obtained, and a system of nonlinear equations for the synthesis problem is derived. Numerical solution of the synthesis problem is illustrated by a number of examples. 相似文献
24.
This letter presents a new polarizer which has a simple comb structure inside a circular waveguide. The electrical performance of the proposed comb polarizer is optimized by a circular waveguide radius and by the physical parameters of the comb plates. This polarizer is suitable for providing good performance in millimeter‐band application because of its simple structure and low fabrication cost. In our experiments the dual‐band comb polarizer designed in band 1(K) and band 2(Ka) showed good electrical performance without any tuning elements. 相似文献
25.
V. O. Yablonskii 《Chemical and Petroleum Engineering》2006,42(11-12):611-617
A mathematical model for separation of suspensions with a non-Newtonian disperse medium by dual-stage pressurized flotation in a cylindrical-conical hydrocyclone is developed. A system of differential equations of the convective diffusion and movement of a complex of particles-bubbles is solved by a numerical method. The concentration field is modeled, and integral separation indicators are determined. Values of structural parameters of the hydrocyclone for which the separation indicators depend heavily on the taper angle of the conical section of the housing and the rheological properties of the disperse medium are established. It is demonstrated that dual-stage pressurized flotation makes it possible to reduce considerably the residual concentration of solid-phase particles as compared with single-stage flotation. 相似文献
26.
27.
Quasi-static stress fields for a crack inclined to the property gradation in functionally graded materials 总被引:3,自引:0,他引:3
Summary. Quasi-static stress fields for a crack inclined to the direction of property gradation in functionally graded materials (FGMs)
are obtained through an asymptotic analysis coupled with Westergaard's stress function approach. The elastic modulus of the
FGM is assumed to vary exponentially along the gradation direction. The mode mixity due to the inclination of the property
gradient is accommodated in the analysis through superposition of opening and shear modes. The first four terms in the expansion
of the stress field are derived to explicitly bring out the influence of nonhomogeneity on the structure of the stress field.
Using these stress field contours of constant maximum shear stress, constant maximum principal stress, constant first stress
invariant and constant out of plane displacement are generated, and the effect of inclination of the property gradation direction
on these contours is discussed.
Received September 22, 2002
Published online: May 20, 2003
The financial support of National Science Foundation (NSF) under grant no. CMS 99000138 is gratefully acknowledged. 相似文献
28.
V. N. Tsygankov V. V. Safonov A. I. Kozlov V. P. Gavrilov 《Inorganic Materials》2003,39(10):1076-1078
The GeO2–NiO system is studied in the stability range of Ni2GeO4 by temperature-dependent resistivity measurements, in combination with x-ray diffraction, differential thermal analysis, and thermogravimetry. The resistivity of the starting mixtures and heat-treated samples is measured in the composition range 55–90 mol % NiO. The effect of air humidity on the resistivity of the samples is examined. 相似文献
29.
It is often required to detect a long weak signal in Gaussian noise, and frequently, the exact form of that signal is parameterized but not known. A bank of matched filters provides an appropriate detector. However, in some practical applications, there are very many matched filters, and most are quite long. The consequent computational needs may render the classical bank-of-filters approach infeasibly expensive. One example, and our original motivation, is the detection of chirp gravitational waves by an Earth-based interferometer. In this paper, we provide a computational approach to this problem via sequential testing. Since the sequential tests to be used are not for constant signals, we develop the theory in terms of average sample number (ASN) for this case. Specifically, we propose two easily calculable expressions for the ASN: one a bound and the other an approximation. The sequential approach does yield moderate computational savings, but we find that by preprocessing the data using short/medium fast Fourier transforms (FFTs) and an appropriate sorting of these FFT outputs such that the most informative samples are entered to a sequential test first, quite high numerical efficiency can be realized. The idea is simple but appears to be quite successful: Examples are presented in which the computational load is reduced by several orders of magnitude. The FFT is an example of an energy-agglomerating transform, but of course, there are many others. The point here is that the transform need not match the sought signal exactly in the sense that all energy becomes confined to a single sample; it is enough that the energy becomes concentrated, and the more concentrated the better. 相似文献
30.
Thomy V. Dubois L. Vanoverschelde C. Sozanski J.P. Pribetich J. 《IEEE sensors journal》2004,4(6):772-778
This paper describes a novel planar antenna sensor created for the purpose of noninvasive temperature measurements using microwave radiometry. In order to improve radiometric measurements in industrial applications, a new generation of sensors is introduced, composed of a metallic sheet. Simulations based upon the method of moments is used both to design and to determine their electromagnetic performances. This paper also describes a radiometric device using these sensors to measure and control the temperature of food products during deep freezing processes. The results and discussions are presented. 相似文献