全文获取类型
收费全文 | 3533篇 |
免费 | 246篇 |
国内免费 | 153篇 |
专业分类
电工技术 | 68篇 |
综合类 | 221篇 |
化学工业 | 127篇 |
金属工艺 | 65篇 |
机械仪表 | 89篇 |
建筑科学 | 329篇 |
矿业工程 | 20篇 |
能源动力 | 108篇 |
轻工业 | 10篇 |
水利工程 | 84篇 |
石油天然气 | 14篇 |
武器工业 | 8篇 |
无线电 | 1249篇 |
一般工业技术 | 304篇 |
冶金工业 | 5篇 |
原子能技术 | 10篇 |
自动化技术 | 1221篇 |
出版年
2024年 | 4篇 |
2023年 | 77篇 |
2022年 | 55篇 |
2021年 | 80篇 |
2020年 | 111篇 |
2019年 | 80篇 |
2018年 | 144篇 |
2017年 | 255篇 |
2016年 | 289篇 |
2015年 | 196篇 |
2014年 | 400篇 |
2013年 | 281篇 |
2012年 | 277篇 |
2011年 | 258篇 |
2010年 | 227篇 |
2009年 | 260篇 |
2008年 | 157篇 |
2007年 | 175篇 |
2006年 | 116篇 |
2005年 | 78篇 |
2004年 | 46篇 |
2003年 | 49篇 |
2002年 | 35篇 |
2001年 | 53篇 |
2000年 | 31篇 |
1999年 | 34篇 |
1998年 | 16篇 |
1997年 | 10篇 |
1996年 | 17篇 |
1995年 | 13篇 |
1994年 | 12篇 |
1993年 | 8篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 27篇 |
1986年 | 19篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1966年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有3932条查询结果,搜索用时 15 毫秒
1.
High dimensionality in real-world multi-reservoir systems greatly hinders the application and popularity of evolutionary algorithms, especially for systems with heterogeneous units. An efficient hierarchical optimization framework is presented for search space reduction, determining the best water distributions, not only between cascade reservoirs, but also among different types of hydropower units. The framework is applied to the Three Gorges Project (TGP) system and the results demonstrate that the difficulties of multi-reservoir optimization caused by high dimensionality can be effectively solved by the proposed hierarchical method. For the day studied, power output could be increased by 6.79 GWh using an optimal decision with the same amount of water actually used; while the same amount of power could be generated with 2.59 × 107 m3 less water compared to the historical policy. The methodology proposed is general in that it can be used for other reservoir systems and other types of heterogeneous unit generators. 相似文献
2.
We present an optimization-based unsupervised approach to automatic document summarization. In the proposed approach, text summarization is modeled as a Boolean programming problem. This model generally attempts to optimize three properties, namely, (1) relevance: summary should contain informative textual units that are relevant to the user; (2) redundancy: summaries should not contain multiple textual units that convey the same information; and (3) length: summary is bounded in length. The approach proposed in this paper is applicable to both tasks: single- and multi-document summarization. In both tasks, documents are split into sentences in preprocessing. We select some salient sentences from document(s) to generate a summary. Finally, the summary is generated by threading all the selected sentences in the order that they appear in the original document(s). We implemented our model on multi-document summarization task. When comparing our methods to several existing summarization methods on an open DUC2005 and DUC2007 data sets, we found that our method improves the summarization results significantly. This is because, first, when extracting summary sentences, this method not only focuses on the relevance scores of sentences to the whole sentence collection, but also the topic representative of sentences. Second, when generating a summary, this method also deals with the problem of repetition of information. The methods were evaluated using ROUGE-1, ROUGE-2 and ROUGE-SU4 metrics. In this paper, we also demonstrate that the summarization result depends on the similarity measure. Results of the experiment showed that combination of symmetric and asymmetric similarity measures yields better result than their use separately. 相似文献
3.
Traditional maximum power point tracking (MPPT) methods can hardly find global maximum power point (MPP) because output characteristics curve of photovoltaic (PV) array may have multi local maximum power points in irregular shadow, and thus easily fall into the local maximum power point. To address this drawback, Considering that sliding mode variable structure (SMVS) control strategy have such advantages as simple structure, fast response and strong robustness, and P&O method have the advantages of simple principle and convenient implementation, so a new algorithm combining SMVS control method and P&O method is proposed, besides, PI controller is applied to reduce system chattering caused by switching sliding surface. It is applied to MPPT control of PV array in irregular shadow to solve the problem of multi-peak optimization in partial shadow. In order to verity the rationality of the proposed algorithm, the experimental circuit is built, which achieves MPPT control by means of the proposed algorithm and P&O method. The experimental results show that compared with the traditional P&O algorithm, the proposed algorithm can fast track the global MPP, tracking speed increases by 60% and the relative error decreased by 20%. Moreover, the system becomes more stable near the MPP, the fluctuations of output power is greatly reduced, and thus make full use of solar energy. 相似文献
4.
5.
This paper studies the price uniformity in the Chinese gasoline market, using station-level data of Hohhot city, Inner Mongolia. We first document that the mode prices of the gasoline stations are consistent with the price ceilings set by the government, implying that the price ceiling regulation in the Chinese gasoline market may serve as a focal point for the gasoline stations to reach price uniformity. We corroborate the focal point hypothesis by providing evidence showing that some stations would “jump” to the ceilings as their prices approach the ceilings. Also, we find that local market structure, distance between stations, station capacity, market characteristics, and past pricing behavior could affect the probability of gas stations to price at the ceilings. Moreover, a higher price ceiling would reduce the probability that stations reach price uniformity. Our results provide another piece of evidence to the literature regarding the unintended effect of price ceiling regulation. 相似文献
6.
《Optical Fiber Technology》2006,12(3):265-267
In this paper, we proposed a high birefringence photonic bandgap fiber (PBGF) with elliptical air holes in cladding and circular air hole in core. The forbidden gaps of the honeycomb structure are calculated when the circular air holes are deformed to the elliptical shape. And the birefringence of the PBGFs is investigated by using a full-vector finite element method (FEM) and the numerical results are presented. 相似文献
7.
《Optical Fiber Technology》2006,12(3):238-242
We measure the system impacts due to the amplitude of group-delay (GD) ripple in single and cascaded chirped fiber Bragg gratings (CFBGs). Signals with smaller pulse width result in smaller performance variation at the same data rate. A 65-ps peak-to-peak GD ripple induces 0.9, 1.7, and 2.7 dB maximum penalties for 10, 20, and 40-Gb/s, respectively. We also find that cascading gratings with random ripple causes much less degradation than cascading gratings with the same ripple profile. 相似文献
8.
This paper discusses an analytical method to study the effects of the vane-parameters on the gain-frequency response of an azimuthally periodic vane-loaded cylindrical waveguide interaction structure for the gyro-traveling wave tube (gyro-TWT) amplifier in the small-orbit TE01 waveguide mode configuration. The vane-loaded gyro-TWT enables the higher gain than that of the smooth-wall device. An optimum choice of the vane dimensions causes a change in the frequency corresponding to the peak-gain. 相似文献
9.
This investigation proposes a fixed collision rate (FCR) back-off algorithm for wireless networks. The proposed scheme takes advantage of the central unit (CU) in a wireless network to broadcast a common back-off window size to all the users, significantly alleviating the unfairness of bandwidth utilization in conventional binary exponential back-off (BEB) algorithms. It is shown that, when maximum throughput is achieved, collision rate is almost a constant for any traffic load. In the operation of the FCR, the CU dynamically adjusts the back-off window size to keep the collision rate at a constant level for maximum throughput. Simulation results demonstrate that the unfairness of bandwidth utilization in the BEB is significantly lessened and the throughput can be maintained at e-1≈0.368 when the number of users approaches infinity. The capture effect even further improves system performance. 相似文献
10.
荀殿栋 《信息安全与通信保密》1996,(3)
Datacryptor 64E是数据包端一端保密机,用于保护经公用和专用X.25包交换网传输的敏感而有价值的信息。在传输之前先将数据加密,加密后的数据经网络传输,在其终点予以解密。 Datacryptor 64E仅对包的数据字段加密,地址和控制信息仍为明文,以保证数据包正确地通过网的路由传输。既然本设备用于端一端加密,这就意味着它仅安装在网络的终点处,而不是安装在每个网络的链路上。 相似文献