全文获取类型
收费全文 | 72004篇 |
免费 | 8037篇 |
国内免费 | 4816篇 |
专业分类
电工技术 | 6290篇 |
技术理论 | 2篇 |
综合类 | 5761篇 |
化学工业 | 9854篇 |
金属工艺 | 4232篇 |
机械仪表 | 4927篇 |
建筑科学 | 5825篇 |
矿业工程 | 2550篇 |
能源动力 | 2075篇 |
轻工业 | 5776篇 |
水利工程 | 1819篇 |
石油天然气 | 3775篇 |
武器工业 | 930篇 |
无线电 | 9074篇 |
一般工业技术 | 7234篇 |
冶金工业 | 2866篇 |
原子能技术 | 963篇 |
自动化技术 | 10904篇 |
出版年
2024年 | 432篇 |
2023年 | 1294篇 |
2022年 | 2565篇 |
2021年 | 3612篇 |
2020年 | 2618篇 |
2019年 | 2066篇 |
2018年 | 2240篇 |
2017年 | 2599篇 |
2016年 | 2312篇 |
2015年 | 3411篇 |
2014年 | 4264篇 |
2013年 | 4950篇 |
2012年 | 5802篇 |
2011年 | 5874篇 |
2010年 | 5379篇 |
2009年 | 5092篇 |
2008年 | 4817篇 |
2007年 | 4627篇 |
2006年 | 4366篇 |
2005年 | 3551篇 |
2004年 | 2440篇 |
2003年 | 1845篇 |
2002年 | 1790篇 |
2001年 | 1398篇 |
2000年 | 1144篇 |
1999年 | 919篇 |
1998年 | 667篇 |
1997年 | 554篇 |
1996年 | 494篇 |
1995年 | 433篇 |
1994年 | 313篇 |
1993年 | 185篇 |
1992年 | 170篇 |
1991年 | 119篇 |
1990年 | 104篇 |
1989年 | 87篇 |
1988年 | 63篇 |
1987年 | 34篇 |
1986年 | 31篇 |
1985年 | 20篇 |
1984年 | 22篇 |
1983年 | 14篇 |
1982年 | 17篇 |
1981年 | 15篇 |
1980年 | 18篇 |
1979年 | 12篇 |
1977年 | 9篇 |
1976年 | 18篇 |
1959年 | 8篇 |
1951年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Spectral slope (S), describing the exponential decrease of the absorption spectrum over a given wavelength range, is an important parameter in the study of of chromophoric dissolved organic matter (CDOM) dynamics, and also an essential input parameter in remote sensing models. Furthermore, S is often used as a proxy for CDOM composition, including the ratio of fulvic to humic acids and molecular weight. The relative broad range in S values reported in the literature can be explained by the different spectral ranges and fitting methods used. A single exponential model is used to fit the S values for 17 investigations involving 458 samples in Lake Taihu from January to October in 2004. The average S value was 15.18 ± 1.39 μm−1 for the range of 280–500 nm, which fell within the range reported in the literature. The frequency distribution of S value basically obeyed a normal distribution. Significant differences in S values between summer and other seasons showed that phytoplankton degradation was one of the important sources of CDOM in summer, whereas CDOM mainly came from the river input in other seasons. Furthermore, the estimated S value decreased with increasing wavelength range used in regression. The maximum and minimum values derived from the regression were 17.89 ± 1.25 μm−1 and 13.62 ± 2.11 μm−1 for the wavelength ranges of 280–380 nm and 400–500 nm, respectively, a decrease of 23.9%. S values significantly decreased with the increase of CDOM absorption coefficients. CDOM absorption coefficients could be more appropriately estimated from exponential model introducing the variation of S with absorption coefficients, making them useful for a remote sensing bio-optical model of Lake Taihu. DOC-specific absorption coefficient a*(λ) and the parameter M describing molecular size of the humic molecules could also be used as a proxy for the sources and types of CDOM. A general relationship was found between S and a*(λ), and M values. S increased with the decrease of DOC-specific absorption coefficient and the increase of M corresponding to the decrease of molecular weight. 相似文献
42.
43.
多媒体业务对无线通信的带宽与品质提出了更高的要求,OFDM和MIMO由于满足了这一需求被越来越多的无线通信标准采用所为底层的调制和传输技术。频率同步问题是OFDM—MIMO物理层设计中正确解调的前提和基础,本文介绍了OFDM—MIMO的原理、特点和频率同步问题的研究现状。 相似文献
44.
Direct sequence code-division multiple access has been shown as the predominant access technology to provide high-speed multimedia services in next-generation wireless cellular networks. The DS-CDMA cellular network significantly differentiates itself from the traditional narrowband TDMA/FDMA system, in which great capacity gain can be explored by taking advantage of its unique characteristics. This article addresses resource management in DS-CDMA cellular networks. We first provide an overview of the DS-CDMA network and discuss the main challenges in the design of resource management schemes. We then examine several critical issues within the scope of resource management, including congestion control, rate and power control, and cell planning, with the emphasis on call admission control. Finally, we offer some insights and discussions on the performance aspects, strengths, and weaknesses of existing proposals. 相似文献
45.
Polyester‐based polyurethane/nano‐silica composites were obtained via in situ polymerization and investigated by Fourier‐transform infrared spectroscopy (FTIR), or FTIR coupled with attenuated total reflectance (FTIR‐ATR), Transmission electron microscopy (TEM), atomic force microscopy (AFM), an Instron testing machine, dynamic mechanical analysis (DMA) and ultraviolet‐visible spectrophotometry (UV‐vis). FTIR analysis showed that in situ polymerization provoked some chemical reactions between polyester molecules and nano‐silica particles. FTIR‐ATR, TEM and AFM analyses showed that both surface and interface contained nano‐silica particles. Instron testing and DMA data showed that introducing nano‐silica particles into polyurethane enhanced the hardness, glass temperature and adhesion strength of polyurethane to the substrate, but also increased the resin viscosity. UV‐vis spectrophotometry showed that nano‐silica obtained by the fumed method did not shield UV radiation in polyurethane films. Copyright © 2003 Society of Chemical Industry 相似文献
46.
Novel algorithms for shared segment protection 总被引:8,自引:0,他引:8
Dahai Xu Yizhi Xiong Chunming Qiao 《Selected Areas in Communications, IEEE Journal on》2003,21(8):1320-1331
The major challenges in designing survivable schemes are how to allocate a minimal amount of spare resources (e.g., bandwidth) using fast (e.g., polynomial-time) algorithms, and, in case a failure occurs, to be able to recover quickly from it. All existing approaches invariably make tradeoffs. We propose novel shared segment protection algorithms which make little or no compromise . We develop an elegant integer linear programming (ILP) model to determine an optimal set of segments to protect a given active path. Although the ILP approach is useful for a medium-size network, it is too time consuming for large networks. Accordingly, we also design a fast heuristic algorithm based on dynamic programming to obtain a near-optimal set of segments. Although the heuristic algorithm has a polynomial time complexity, it can achieve a bandwidth efficiency as high as some best-performing shared path protection schemes and, at the same time, much faster recovery than these shared path protection schemes. The proposed scheme is also applicable to a wide range of networking technologies, including Internet Protocol and wavelength-division multiplexing networks under the generalized multiprotocol label switched framework. 相似文献
47.
Achieving fast and bandwidth-efficient shared-path protection 总被引:4,自引:0,他引:4
Yizhi Xiong Dahai Xu Chunming Qiao 《Lightwave Technology, Journal of》2003,21(2):365-371
Dynamic provisioning of restorable bandwidth guaranteed paths is a challenge in the design of broad-band transport networks, especially next-generation optical networks. A common approach is called (failure-independent) path protection, whereby for every mission-critical active path to be established, a link (or node) disjoint backup path (BP) is also established. To optimize network resource utilization, shared path protection should be adopted, which often allows a new BP to share the bandwidth allocated to some existing BPs. However, it usually leads the backup paths to use too many links, with zero cost in term of additional backup bandwidth, along its route. It will violate the restoration time guarantee. In this paper, we propose novel integer linear programming (ILP) formulations by introducing two parameters (/spl epsi/ and /spl mu/) in both the sharing with complete information (SCI) scheme and the distributed partial information management (DPIM) scheme. Our results show that the proposed ILP formulations can not only improve the network resource utilization effectively, but also keep the BPs as short as possible. 相似文献
48.
本文提出了一种基于拓扑法的多关节机械手无碰路径规划方法,文中重点讨论了三关节机械手路径规划时如何划分状态空间、判别各连通块之间的连通性以及路径搜索等问题,最后我们给出了一个实验结果以示该方法的有效性。 相似文献
49.
本文论述了一种普及型电子式电能表的设计原理和实现方法,具体讨论了时分割乘法器和电流互感器的组成和误差分析,最后给出测试结果。 相似文献
50.