全文获取类型
收费全文 | 517691篇 |
免费 | 6821篇 |
国内免费 | 1416篇 |
专业分类
电工技术 | 10091篇 |
综合类 | 734篇 |
化学工业 | 76645篇 |
金属工艺 | 18290篇 |
机械仪表 | 15002篇 |
建筑科学 | 12915篇 |
矿业工程 | 1657篇 |
能源动力 | 14467篇 |
轻工业 | 47110篇 |
水利工程 | 4375篇 |
石油天然气 | 6433篇 |
武器工业 | 21篇 |
无线电 | 67392篇 |
一般工业技术 | 98897篇 |
冶金工业 | 98754篇 |
原子能技术 | 9607篇 |
自动化技术 | 43538篇 |
出版年
2021年 | 4056篇 |
2019年 | 3730篇 |
2018年 | 6271篇 |
2017年 | 6213篇 |
2016年 | 6399篇 |
2015年 | 4358篇 |
2014年 | 7628篇 |
2013年 | 23727篇 |
2012年 | 12417篇 |
2011年 | 17153篇 |
2010年 | 13610篇 |
2009年 | 15659篇 |
2008年 | 16414篇 |
2007年 | 16254篇 |
2006年 | 14511篇 |
2005年 | 13180篇 |
2004年 | 12916篇 |
2003年 | 12862篇 |
2002年 | 12287篇 |
2001年 | 12781篇 |
2000年 | 12026篇 |
1999年 | 12791篇 |
1998年 | 31703篇 |
1997年 | 22493篇 |
1996年 | 17437篇 |
1995年 | 13355篇 |
1994年 | 12093篇 |
1993年 | 11779篇 |
1992年 | 8613篇 |
1991年 | 8270篇 |
1990年 | 8068篇 |
1989年 | 7761篇 |
1988年 | 7538篇 |
1987年 | 6347篇 |
1986年 | 6437篇 |
1985年 | 7343篇 |
1984年 | 6629篇 |
1983年 | 6306篇 |
1982年 | 5781篇 |
1981年 | 5862篇 |
1980年 | 5500篇 |
1979年 | 5319篇 |
1978年 | 4951篇 |
1977年 | 6020篇 |
1976年 | 7941篇 |
1975年 | 4281篇 |
1974年 | 4136篇 |
1973年 | 4065篇 |
1972年 | 3480篇 |
1971年 | 2979篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Faulkner D.W. Payne D.B. Stern J.R. Ballance J.W. 《Lightwave Technology, Journal of》1989,7(11):1741-1751
Some of the options for optical technology within the local loop environment are examined. In particular, passive shared access networks have been considered in some detail. These networks show great promise for delivering existing telephone services to small to medium business customers (4-30 lines) economically by the early 1990s. Extending fiber to the home will also be possible by virtue of a similar passive network infrastructure for customers requiring new broadband services beyond the single telephone line. For one-line plain old telephone service (POTS) customers, an intermediate approach of terminating the fiber network at the final network distribution point, with copper retained for the final leg, may be used prior to the provision of broadband services. A key feature of the passive optical network architecture is the use of wavelength-division multiplexing (WDM) as an upgrade strategy, allowing graceful upgrading from telephone services to multichannel high-definition television (HDTV) on gigabit/second bearers and full two-way switched broadband services employing wavelength routing across the network 相似文献
992.
993.
Conversion of acrylonitrile-based precursors to carbon fibres 总被引:1,自引:0,他引:1
Mukesh K. Jain M. Balasubramanian P. Desai A. S. Abhiraman 《Journal of Materials Science》1987,22(1):301-312
The progress of stabilization of two compositions of acrylic fibres with various orientations has been followed by a variety of techniques. The thermooxidative treatments for stabilization have been carried out in a continuous process and also in a batch process under free shrinkage, constant length and constant tension conditions. The morphological model of acrylic fibres consists of an alternating sequence of laterally ordered and laterally disordered regions along the fibre direction. This structure is consistent with the observations based on small-angle X-ray scattering of copper- impregnated precursor fibres and thermomechanical response, thermal stress development, calorimetry, wide- and small-angle X-ray scattering and sonic modu-lus measured at different extents of stabilization. Lateral as well as orientational order in these fibres can be increased markedly through a high-temperature deformation process prior to stabilization. An increase in perfection and extent of order is observed in the early stages of stabilization. There is also a simultaneous decrease in the orientation of the disordered phase at this stage and the extent of this decrease depends on the axial constraints imposed on the fibre. Little difference in the rate of stabilization is observed as measured by density or oxygen uptake for fibres with different extents of orientation, lateral order or restraint. Fibres containing itaconic add, a stabilization catalyst did show an increased rate of stabilization. Inferences have been drawn regarding additional research pertaining to achieving high order in precursor fibres, minimizing orientational relaxation during oxidative stabilization, and the techniques for monitoring the extents of the stabilization treatment and the changes in relevant morphological parameters. 相似文献
994.
995.
A. J. Hugo D. M. Jakelski G. Stanitsas G. R. Sullivan R. R. Hudgins P. L. Silveston 《加拿大化工杂志》1986,64(2):349-351
In the approach to a cycle-invariant state in the forced feed concentration cycling of the CO oxidation over a NiO catalyst, a very long transient has been observed lasting 80 ks or more. This transient appears to be associated only with CO pretreatment of the catalyst and is not a general phenomenon. The slow convergence to the cycle-invariant state reported in certain modelling calculations in the literature appears to have a different cause. 相似文献
996.
997.
998.
Zhibin Xiong Haitao Liu Chunxiang Zhu Sin J.K.O. 《Electron Devices, IEEE Transactions on》2005,52(12):2629-2633
In this paper, a new polysilicon CMOS self-aligned double-gate thin-film transistor (SA-DG TFT) technology is proposed and experimentally demonstrated. The self-alignment between the top- and bottom-gate is realized by a backlight exposure technique. The structure has an ultrathin channel region (300 /spl Aring/) and a thick source/drain region. Experimental results show that this technology provides excellent current saturation due to a combination of the effective reduction in the drain field and the full depletion of the ultrathin channel. Moreover, for n-channel devices, the SA-DG TFT has a 4.2 times higher on-current (V/sub gs/=20V) as compared to the conventional single-gate TFT. Whereas for the p-channel devices, the SADG TFT has a 3.6 times higher on-current (V/sub gs/=-20V) compared to the conventional single-gate device. 相似文献
999.
O Baeder-Bederski M Dürr M Borneff-Lipp P Kuschk R Netter G Daeschlein P Mosig R A Müller 《Water science and technology》2005,51(9):205-212
Different types of constructed wetlands in a pilot-plant system were fed with identical municipal waste water to compare the sanitisation process in two-stage systems. With combinations of a vertical and a horizontal flow filter an E. coli reduction of 5 log10 with an E. coli influent concentration of 10(7) MPN/100 ml was achieved. Using different filter materials in each stage the total performance of the two-stage system was independent from the sequence of these materials. However, using coarser filter material in the first stage makes the filter less prone to clogging and is thus the preferential option with regard to operational reliability. 相似文献
1000.
On the macroscopic optimization of multicell wireless systems with multiuser detection and multiple antennas - uplink analysis 总被引:1,自引:0,他引:1
In this paper, we consider a system with K single-antenna client users, n/sub B/ base stations (each base station has n/sub R/ antennas), as well as a centralized controller. A client user could be associated with a single base station at any time. All the base stations operate at the same frequency and have optimal multiuser detection per base station which cancels intracell interference only. We consider a general problem of uplink macroscopic resource management where the centralized controller dynamically determines an appropriate association mapping of the K users with respect to the n/sub B/ base stations over a macroscopic time scale. We propose a novel analytical framework for the above macroscopic scheduling problems. A simple rule is to associate a user with the strongest base station (camp-on-the-strongest-cell), and this has been widely employed in conventional cellular systems. However, based on the optimization framework, we found that this conventional approach is in fact not optimal when multiuser detection is employed at the base station. We show that the optimal macroscopic scheduling algorithm is of exponential complexity, and we propose a simple greedy algorithm as a feasible solution. 相似文献