全文获取类型
收费全文 | 579922篇 |
免费 | 28139篇 |
国内免费 | 13282篇 |
专业分类
电工技术 | 22601篇 |
技术理论 | 56篇 |
综合类 | 19875篇 |
化学工业 | 92110篇 |
金属工艺 | 27692篇 |
机械仪表 | 28584篇 |
建筑科学 | 31697篇 |
矿业工程 | 11835篇 |
能源动力 | 16834篇 |
轻工业 | 39553篇 |
水利工程 | 7432篇 |
石油天然气 | 26177篇 |
武器工业 | 2613篇 |
无线电 | 69958篇 |
一般工业技术 | 88522篇 |
冶金工业 | 66449篇 |
原子能技术 | 8241篇 |
自动化技术 | 61114篇 |
出版年
2023年 | 5757篇 |
2022年 | 10281篇 |
2021年 | 14566篇 |
2020年 | 11337篇 |
2019年 | 9847篇 |
2018年 | 12373篇 |
2017年 | 13598篇 |
2016年 | 12584篇 |
2015年 | 14366篇 |
2014年 | 19695篇 |
2013年 | 30422篇 |
2012年 | 26212篇 |
2011年 | 30755篇 |
2010年 | 25902篇 |
2009年 | 26018篇 |
2008年 | 26123篇 |
2007年 | 25429篇 |
2006年 | 25424篇 |
2005年 | 22464篇 |
2004年 | 16487篇 |
2003年 | 15532篇 |
2002年 | 14035篇 |
2001年 | 13854篇 |
2000年 | 13849篇 |
1999年 | 15936篇 |
1998年 | 24410篇 |
1997年 | 18041篇 |
1996年 | 15010篇 |
1995年 | 11829篇 |
1994年 | 10211篇 |
1993年 | 8856篇 |
1992年 | 6562篇 |
1991年 | 5922篇 |
1990年 | 5306篇 |
1989年 | 4826篇 |
1988年 | 4546篇 |
1987年 | 3677篇 |
1986年 | 3461篇 |
1985年 | 3532篇 |
1984年 | 3163篇 |
1983年 | 3018篇 |
1982年 | 2822篇 |
1981年 | 2667篇 |
1980年 | 2584篇 |
1979年 | 2366篇 |
1978年 | 2139篇 |
1977年 | 2638篇 |
1976年 | 3613篇 |
1975年 | 1779篇 |
1974年 | 1742篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
介绍了铁路电力试验车软件的设计思想、框架结构及其总体控制策略 ,并就其中的测试程序、数据库管理系统、串口通讯程序、复杂式样报表生成和系统安装程序的设计策略作了叙述。 相似文献
993.
The ray-tracing (RT) algorithm has been used for accurately predicting the site-specific radio propagation characteristics, in spite of its computational intensity. Statistical models, on the other hand, offers computational simplicity but low accuracy. In this paper, a new model is proposed for predicting the indoor radio propagation to achieve computational simplicity over the RT method and better accuracy than the statistical models. The new model is based on the statistical derivation of the ray-tracing operation, whose results are a number of paths between the transmitter and receiver, each path comprises a number of rays. The pattern and length of the rays in these paths are related to statistical parameters of the site-specific features of indoor environment, such as the floor plan geometry. A key equation is derived to relate the average path power to the site-specific parameters, which are: 1) mean free distance; 2) transmission coefficient; and 3) reflection coefficient. The equation of the average path power is then used to predict the received power in a typical indoor environment. To evaluate the accuracy of the new model in predicting the received power in a typical indoor environment, a comparison with RT results and with measurement data shows an error bound of less than 5 dB 相似文献
994.
E.A. Soliman A.K. Abdelmageed M.A. El-Gamal 《AEUE-International Journal of Electronics and Communications》2002,56(3)
In this paper a new artificial neural network (ANN) based model for the calculation of the method of moments (MoM) matrix elements is presented. Training sets that characterize the matrix elements are first constructed. These sets are then utilized to effectively train two radial basis function (RBF) neural networks to accurately estimate all the elements of the MoM matrix for any mesh used. The potential of the proposed approach is demonstrated in the case of a narrow microstrip line. The current distribution on the microstrip line produced by the trained RBF networks agrees very well with the exact distribution. In addition, the proposed ANN model is much faster than the conventional MoM procedure. 相似文献
995.
996.
Oracle9i数据库安全管理机制剖析 总被引:4,自引:0,他引:4
数据库安全和数据安全机制是数据库开发设计者目前必须考虑的问题之一。Oracle9i数据库管理系统在数据库安全方面为数据库管理员和应用开发者提供了很好的安全机制。文章从不同侧面和角度讨论Oracle9i数据库系统的安全管理机制,为实施更安全更灵活的数据库安全策略提出了全方位的技术思路。 相似文献
997.
钢坯在炉内的横向跑偏和纵向跑偏位移量,是步进式加热炉设备的重要性能之一.重点介绍了鞍钢1780热连轧生产线步进式加热炉钢坯跑偏测试的应用技术. 相似文献
998.
999.
1000.
We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system with multiple transmitter and receiver antennas over correlated frequency- and time-selective fading channels. It is shown that the product of the time-selectivity order and the frequency-selectivity order is a key parameter to characterize the outage capacity of the correlated fading channel. It is also observed that STCs with large effective lengths and ideal built-in interleavers are more effective in exploiting the natural diversity in multiple-antenna correlated fading channels. We then propose a low-density parity-check (LDPC)-code-based STC-OFDM system. Compared with the conventional space-time trellis code (STTC), the LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels. Compared with the previously proposed turbo-code-based STC scheme, LDPC-based STC exhibits lower receiver complexity and more flexible scalability. We also consider receiver design for LDPC-based STC-OFDM systems in unknown fast fading channels and propose a novel turbo receiver employing a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder, which can significantly reduce the error floor in fast fading channels with a modest computational complexity. With such a turbo receiver, the proposed LDPC-based STC-OFDM system is a promising solution to highly efficient data transmission over selective-fading mobile wireless channels 相似文献