全文获取类型
收费全文 | 106184篇 |
免费 | 11364篇 |
国内免费 | 6641篇 |
专业分类
电工技术 | 8881篇 |
技术理论 | 3篇 |
综合类 | 8814篇 |
化学工业 | 15320篇 |
金属工艺 | 6283篇 |
机械仪表 | 7372篇 |
建筑科学 | 9223篇 |
矿业工程 | 3262篇 |
能源动力 | 3145篇 |
轻工业 | 8717篇 |
水利工程 | 2560篇 |
石油天然气 | 4888篇 |
武器工业 | 1200篇 |
无线电 | 12700篇 |
一般工业技术 | 10924篇 |
冶金工业 | 4125篇 |
原子能技术 | 1356篇 |
自动化技术 | 15416篇 |
出版年
2024年 | 536篇 |
2023年 | 1749篇 |
2022年 | 3539篇 |
2021年 | 4468篇 |
2020年 | 3501篇 |
2019年 | 2729篇 |
2018年 | 3129篇 |
2017年 | 3511篇 |
2016年 | 3087篇 |
2015年 | 4439篇 |
2014年 | 5781篇 |
2013年 | 6352篇 |
2012年 | 7446篇 |
2011年 | 7707篇 |
2010年 | 7094篇 |
2009年 | 7005篇 |
2008年 | 7092篇 |
2007年 | 6387篇 |
2006年 | 6344篇 |
2005年 | 5110篇 |
2004年 | 3931篇 |
2003年 | 3791篇 |
2002年 | 4430篇 |
2001年 | 3752篇 |
2000年 | 2547篇 |
1999年 | 1976篇 |
1998年 | 1279篇 |
1997年 | 1064篇 |
1996年 | 918篇 |
1995年 | 797篇 |
1994年 | 633篇 |
1993年 | 444篇 |
1992年 | 386篇 |
1991年 | 277篇 |
1990年 | 214篇 |
1989年 | 195篇 |
1988年 | 127篇 |
1987年 | 92篇 |
1986年 | 63篇 |
1985年 | 43篇 |
1984年 | 30篇 |
1983年 | 26篇 |
1982年 | 36篇 |
1981年 | 24篇 |
1980年 | 22篇 |
1979年 | 19篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1959年 | 17篇 |
1951年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
分析了新乡观测到的VLF相位周期滑移现象,研究了产生相位周期滑移的原因,揭示了相位周期滑移的实质:VLF传播相位周期滑移是发生在日出或日落过渡期间一种特殊的多模干涉现象,它是由传播路径昼夜分界处的低电离层不连续性所引起的模转换干涉所造成的. 相似文献
82.
Jianhua Liu Jian Li 《Mobile Computing, IEEE Transactions on》2006,5(9):1133-1142
We consider improving the performance of orthogonal frequency-division multiplexing (OFDM)-based mobile broadband wireless communication (MBWC) systems. We show that a recently approved packet-based MBWC standard can cause the resulting systems to suffer from severe performance degradations for time-varying channels due to the lack of a mechanism for tracking the time-varying channels needed for coherent detection. We consider both packet design and signal processing to deal with time-varying channels. For the packet design, we segment an entire packet into multiple subpackets—with each subpacket having zero tail bits to reset the convolutional channel encoder—so that the detection/decoding result of each subpacket can be used to update the channel response for this subpacket. For signal processing, we use weighted polynomial fitting and prediction at the receiver to improve the channel tracking accuracy. Simulation results show that the coherent detection based on our new scheme can significantly outperform the commonly suggested differential modulation/detection methods. 相似文献
83.
Multimodality image registration via pixel migration is a powerful approach. However, it suffers from a serious problem--the global maximum on the sum of squared gradient magnitude (SSG) surface does not correspond to the correct solution of registration. To solve the problem, we partition the search space into feasible and infeasible regions. The genetic algorithm (global optimizer) is used to obtain a good initial estimate of registration parameters and followed by a fast refining with Powell's approach (local optimizer). The experimental results demonstrate that the use of this modified pixel migration algorithm on multisensor image registration is very effective. 相似文献
84.
Lei Zhou Yu-Dong Yao Heffes H. Ruifeng Zhang 《Wireless Communications, IEEE Transactions on》2006,5(4):779-788
A reduced-rate retransmission (RRR) scheme is proposed for improving the throughput performance of spread-spectrum packet radio networks. The scheme takes advantages of the available multi-rate scalable source coding techniques. It assumes that several versions of a data packet with different sizes (number of information bits) are available. The transmission of a packet starts from its full-size version. If the full-size version is not correctly received, its half-size version is used in the retransmission. If further retransmissions are needed, the quarter-size version and so on are used. The shrunk packets are transmitted either in a minislot if the processing gain is kept the same, or occupying a slot duration by increasing the processing gain proportionally. In both cases, the effective signal to interference ratio for a packet is increased. As a result, the system throughput is improved. Theoretical and numerical results are provided in this paper which illustrate the throughput improvement. Another advantage of the proposed RRR scheme is that the packet-size reduction provides finer granules for link adaptation. Therefore, it is especially suitable for multimedia applications for which codes of variable rate for the source data are available and which can tolerate gracefully degraded quality of service. The performance of the proposed scheme in fading channels is also addressed. 相似文献
85.
86.
Environmental factors such as oxygen, temperature, and microbial species may have significant effects on decomposition of biodegradable polymers. A representative biodegradable, thermoplastic polymer, poly(3‐hydroxybutyrate‐co‐hydroxyvalerate) (PHBV), was decomposed in an aqueous medium under controlled laboratory conditions by soil microbes for the intrinsic degradation kinetics and the effects of the environmental factors on polymer biodegradation. The amount of proteins, including the PHBV depolymerases, that attached to the polymer surfaces was quite constant during the period of significant mass loss of the polymer specimens. The microbial polymer degradation followed a zero‐order rate model, so the residual mass fraction of PHBV films declined linearly with time. The mixed aerobic microbial organisms from fertile soil showed a higher activity of polymer degradation than an aerobic PHBV‐producing bacterium and the mixed anaerobes in the same soil. The mixed anaerobic microorganisms from barren soil decomposed the polymer at a slower rate than the anaerobes from fertile soil, and this was attributed to fewer microbial cells in the barren soil instead of the difference in the microbial species. The temperature effect on PHBV degradation can be described with an Arrhenius equation, and the activation energy is around 16 kcal/mol. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 205–213, 2003 相似文献
87.
88.
Stewart C. Kai Shen Dwarkadas S. Scott M.L. Jian Yin 《Distributed Systems Online, IEEE》2004,5(10):1-1
The growth of the Internet and of various intranets has spawned a wealth of online services, most of which are implemented on local-area clusters using remote invocation (for example, remote procedure call/remote method invocation) among manually placed application components. Component placement can be a significant challenge for large-scale services, particularly when application resource needs are workload dependent. Automatic component placement has the potential to maximize overall system throughput. The key idea is to construct (offline) a mapping between input workload and individual-component resource consumption. Such mappings, called component profiles, then support high-performance placement. Preliminary results on an online auction benchmark based on J2EE (Java 2 Platform, Enterprise Edition) suggest that profile-driven tools can identify placements that achieve near-optimal overall throughput. 相似文献
89.
90.
Hogari K. Tetsutani S. Jian Zhou Yamamoto F. Sato K. 《Lightwave Technology, Journal of》2003,21(2):540-545
Many cables containing 1.3-/spl mu/m zero-dispersion single-mode (SM) optical fibers are installed in trunk and access networks. Recently, there have been a number of studies on wavelength-division-multiplexing (WDM) systems designed to increase transmission capacity and flexibility. If we can construct WDM systems using SM optical-fiber cable networks designed to transmit using wavelengths in the 1.3-/spl mu/m window (O-band), this will prove very effective in reducing construction costs. It is therefore important to examine the wavelength dependence of the transmission characteristics of SM optical-fiber cables and networks that have already been installed and in which several optical fibers are joined. In this paper, we describe the measured optical characteristics of SM optical-fiber cables and installed optical-fiber cable networks at various wavelengths. The optical characteristics were stable in the 1.46 to 1.625-/spl mu/m wavelength range and we confirmed that the installed SM optical-fiber cable networks could be used for WDM system applications. 相似文献