全文获取类型
收费全文 | 322234篇 |
免费 | 4814篇 |
国内免费 | 849篇 |
专业分类
电工技术 | 5272篇 |
综合类 | 183篇 |
化学工业 | 51797篇 |
金属工艺 | 12502篇 |
机械仪表 | 9256篇 |
建筑科学 | 7989篇 |
矿业工程 | 1903篇 |
能源动力 | 7441篇 |
轻工业 | 30434篇 |
水利工程 | 3505篇 |
石油天然气 | 7214篇 |
武器工业 | 20篇 |
无线电 | 33426篇 |
一般工业技术 | 62970篇 |
冶金工业 | 59816篇 |
原子能技术 | 7686篇 |
自动化技术 | 26483篇 |
出版年
2021年 | 2879篇 |
2019年 | 2561篇 |
2018年 | 4985篇 |
2017年 | 4967篇 |
2016年 | 5293篇 |
2015年 | 3452篇 |
2014年 | 5478篇 |
2013年 | 14406篇 |
2012年 | 8904篇 |
2011年 | 11171篇 |
2010年 | 9084篇 |
2009年 | 9971篇 |
2008年 | 10506篇 |
2007年 | 10154篇 |
2006年 | 8839篇 |
2005年 | 8163篇 |
2004年 | 7585篇 |
2003年 | 7269篇 |
2002年 | 7242篇 |
2001年 | 6870篇 |
2000年 | 6547篇 |
1999年 | 6719篇 |
1998年 | 17413篇 |
1997年 | 12628篇 |
1996年 | 9575篇 |
1995年 | 7136篇 |
1994年 | 6461篇 |
1993年 | 6308篇 |
1992年 | 4651篇 |
1991年 | 4620篇 |
1990年 | 4466篇 |
1989年 | 4466篇 |
1988年 | 4401篇 |
1987年 | 3730篇 |
1986年 | 3736篇 |
1985年 | 4321篇 |
1984年 | 4110篇 |
1983年 | 3717篇 |
1982年 | 3550篇 |
1981年 | 3689篇 |
1980年 | 3535篇 |
1979年 | 3460篇 |
1978年 | 3539篇 |
1977年 | 4079篇 |
1976年 | 5370篇 |
1975年 | 3221篇 |
1974年 | 3052篇 |
1973年 | 3091篇 |
1972年 | 2683篇 |
1971年 | 2514篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
991.
Valla M. Montagna G. Castello R. Tonietto R. Bietti I. 《Solid-State Circuits, IEEE Journal of》2005,40(4):970-977
A direct conversion 802.11a receiver front-end including a synthesizer with quadrature VCO has been integrated in a 0.13-/spl mu/m CMOS process. The chip has an active area of 1.8 mm/sup 2/ with the entire RF portion operated from 1.2 V and the low frequency portion operated from 2.5 V. Its key features are a current driven passive mixer with a low impedance load that achieves a low 1/f noise corner and an high I-Q accuracy quadrature VCO. Measured noise figure is 3.5 dB with an 1/f noise corner of 200 kHz, and an IIP3 of -2 dBm. The synthesizer DSB phase noise integrated over a 10 MHz band is less than -36 dBc while its I-Q phase unbalance is below 1 degree. 相似文献
992.
G. Lucovsky J.G. Hong C.C. Fulton N.A. Stoute Y. Zou R.J. Nemanich D.E. Aspnes H. Ade D.G. Schlom 《Microelectronics Reliability》2005,45(5-6):827
This paper uses X-ray absorption spectroscopy to study the electronic structure of the high-k gate dielectrics including TM and RE oxides. The results are applicable to TM and rare earth (RE) silicate and aluminate alloys, as well as complex oxides comprised of mixed TM/TM and TM/RE oxides. These studies identify the nature of the lowest conduction band d* states, which define the optical band gap, Eg, and the conduction band offset energy with respect to crystalline Si, EB. Eg and EB scale with the atomic properties of the TM and RE atoms providing important insights for identification high-k dielectrics that meet performance targets for advanced CMOS devices. 相似文献
993.
O. G. Devoino M. A. Kardapolova S. Laber E. A. Feldstein O. V. Diyachenko 《Journal of Engineering Physics and Thermophysics》2004,77(2):372-376
The wear resistance of Ni–Cr–B–Si–C coatings after laser treatment with a doping covering under dry friction conditions has been investigated. The microstructure of coatings after fusion by a gas burner and a laser beam and the roughness and waviness parameters of the friction surfaces before and after wear have been investigated. 相似文献
994.
da Costa D.B. Yacoub M.D. Filho J.C.S.S. Fraidenraich G. Mendes J.R. 《Communications Letters, IEEE》2006,10(1):13-15
This paper provides simple, exact, new closed-form expressions for the generalized phase crossing rate of Nakagami-m fading channels. Sample numerical results obtained by simulation are presented that validate the formulations developed here. A special case of this formulation is the Rayleigh case, whose result agrees with that obtained elsewhere in the literature. In passing, several new closed-form results concerning the statistics of the envelope, its in-phase and quadrature components, phase, and their time derivatives are obtained. 相似文献
995.
Perello J. Escalona E. Spadaro S. Comellas J. Junyent G. 《Communications Letters, IEEE》2006,10(12):852-854
In this letter, we propose two novel mechanisms which enable GMPLS LMP to cope with the automatic discovery of all-optical transport planes. The feasibility of our contributions and their performances are assessed by simulations as well as experimental results over the ASON/GMPLS CARISMA field-trial 相似文献
996.
New modulation formats are presented that increase the transmission rate over that of conventional systems, without degrading the bit error rate (BER) and with minimal bandwidth variation. A time-varying function called an "extra phase variation function" (EPVF) is added to the discrete phase of conventional modulation formats such as M-PSK and QAM to transmit additional data bits. A receiver configuration is presented that allows the decoding of information represented by the discrete and extra phases. It is shown that in an additive white Gaussian noise (AWGN) channel, the BER performance of the bits carried by the discrete phase and the extra phase in the modified format improves over the BER of conventional modulation formats. 相似文献
997.
Water retention times less than 3 h from inlet to outlet were necessary to meet new effluent requirements without extending the footprint of the existing plant. Nitrogen removal was required at the existing, high loaded, direct precipitation plant and space was not readily available. The staff at the VEAS WWTP took on the task to design a new process at their own risk. Selected solutions and ups and downs in the water treatment during the past 10 years of operations and further developments are presented in this paper. Always asking for well-proven solutions might make life easy. The intent of this paper is to encourage some of our colleagues to be more daring in their approach to new challenges. However, expect sleepless nights in order to solve the unexpected problems along the way. 相似文献
998.
Parity space approach and H2 approach are two important fault detection approaches. This paper studies the relationship between these two approaches, which reveals frequency domain characteristics of the optimal solution of the parity space approach on the one side and provides a numerical solution of the H2-optimal design of residual generators on the other side. 相似文献
999.
Microstructural development and mechanical properties of interrupted aged Al-Mg-Si-Cu alloy 总被引:2,自引:0,他引:2
The effects of a recently developed interrupted aging procedure on the microstructural development and mechanical properties
of the commercial Al-Mg-Si-Cu alloy 6061 have been studied using transmission electron microscopy (TEM), differential scanning
calorimetry (DSC), and mechanical testing. This so-called T6I6 temper involves partially aging the alloy at a typical T6 temperature
(the underaging stage), quenching, then holding at a reduced temperature (in this case 65 °C) to facilitate further hardening
(the secondary aging stage), prior to final aging to peak properties at, or close to, the initial aging (T6) temperature (the
reaging stage). The T6I6 aging treatment produces simultaneous increases in tensile properties, hardness, and toughness, as
compared with conventional T6. The overall improvement in the mechanical properties of 6061 T6I6 is associated with the formation
of a greater number of finer, and more densely dispersed, β″ precipitates in the final microstructure. Secondary precipitation
took place during the interrupted aging stage of the T6I6 temper, resulting in the formation of a large number of Guinier-Preston
(GP) zones that served as precursors to the needlelike β″ precipitates when elevated temperature aging was resumed. 相似文献
1000.
Chen Q. Hong Y. Chen G. Hill D. J. 《IEEE transactions on circuits and systems. I, Regular papers》2006,53(12):2692-2704
In this paper, one kind of intermittency generated by a discontinuous system is studied. Although this system, which is composed of two switched subsystems coupled with a high strength, is nonsmooth, the mechanism of this kind of intermittency can be analyzed with several explicit relations between the intermittency characteristics and the system control parameters. In particular, estimates of "steady-state" values of the system (in the laminar phases) and a critical value for this intermittency can be derived, which are helpful in relevant control systems design. Moreover, some power laws for the observed intermittency are obtained and discussed 相似文献