全文获取类型
收费全文 | 256940篇 |
免费 | 20928篇 |
国内免费 | 10694篇 |
专业分类
电工技术 | 14889篇 |
技术理论 | 16篇 |
综合类 | 15401篇 |
化学工业 | 42572篇 |
金属工艺 | 13747篇 |
机械仪表 | 15925篇 |
建筑科学 | 20952篇 |
矿业工程 | 6591篇 |
能源动力 | 7308篇 |
轻工业 | 16324篇 |
水利工程 | 4482篇 |
石油天然气 | 13671篇 |
武器工业 | 1927篇 |
无线电 | 31802篇 |
一般工业技术 | 32114篇 |
冶金工业 | 13108篇 |
原子能技术 | 2745篇 |
自动化技术 | 34988篇 |
出版年
2024年 | 519篇 |
2023年 | 3942篇 |
2022年 | 6466篇 |
2021年 | 10037篇 |
2020年 | 7542篇 |
2019年 | 6358篇 |
2018年 | 6903篇 |
2017年 | 7921篇 |
2016年 | 7297篇 |
2015年 | 9758篇 |
2014年 | 12625篇 |
2013年 | 15945篇 |
2012年 | 16239篇 |
2011年 | 18337篇 |
2010年 | 15538篇 |
2009年 | 15302篇 |
2008年 | 14648篇 |
2007年 | 13801篇 |
2006年 | 13918篇 |
2005年 | 12088篇 |
2004年 | 8127篇 |
2003年 | 7021篇 |
2002年 | 6434篇 |
2001年 | 5793篇 |
2000年 | 5771篇 |
1999年 | 6509篇 |
1998年 | 6005篇 |
1997年 | 4916篇 |
1996年 | 4415篇 |
1995年 | 3793篇 |
1994年 | 3145篇 |
1993年 | 2490篇 |
1992年 | 1893篇 |
1991年 | 1453篇 |
1990年 | 1129篇 |
1989年 | 973篇 |
1988年 | 751篇 |
1987年 | 531篇 |
1986年 | 418篇 |
1985年 | 346篇 |
1984年 | 209篇 |
1983年 | 206篇 |
1982年 | 175篇 |
1981年 | 152篇 |
1980年 | 148篇 |
1979年 | 107篇 |
1978年 | 65篇 |
1977年 | 66篇 |
1976年 | 81篇 |
1975年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
以TD—SCDMA既有进展及面临的挑战与NGBWM的发展走向为基础,重点论述自主创新导向下的TD—SCDMA与NGBWM务实发展战略思考。 相似文献
992.
993.
994.
Chen J.J. Gao G.-B. Chyi J.-I. Morkoc H. 《Electron Devices, IEEE Transactions on》1989,36(10):2165-2172
Avalanche breakdown behavior at the collector junction of the GaAs/AlGaAs HBT (heterojunction bipolar transistor) has been studied. Junction breakdown characteristics displaying hard breakdown, soft breakdown, and negative resistance breakdown behavior were observed and are interpreted by analysis of localized microplasma effects, uniform microplasma-free behavior, and associated current gain measurements. Light emission from the collector-base junction of the GaAs/AlGaAs HBT was observed and used to investigate breakdown uniformity. Using a simple punchthrough breakdown model, the theoretical breakdown curves at different collector doping concentrations and thicknesses were computed and found to be in agreement with maximum breakdown voltages measured from devices displaying the most uniform junction breakdown. The serious current gain degradation of GaAs/AlGaAs HBTs at low current densities was analyzed in connection with the measurement of a large collector-emitter breakdown voltage. The unexpected functional relationship between the collector-emitter breakdown voltage and collector-base breakdown voltage is explained by the absence of a hole-feedback effect for devices not exhibiting transistor action 相似文献
995.
Chen Guodong 《International Journal of Pressure Vessels and Piping》1985,19(1):19-30
In this paper, according to the theory of thin shells, a basic equation in complex form is derived for conical thin shells with linearly varying wall thickness under symmetric loads, and the uniformly valid asymptotic solutions are given. 相似文献
996.
997.
强化措施抓好落实全面推进农村饮水安全工作 总被引:6,自引:0,他引:6
民以食为天,食以水为先。获得安全饮用水是人类生存的基本需求。
党中央、国务院领导高度重视饮水安全工作,要求把“切实保护好饮用水源,让群众喝上放心水”作为首要任务,把“让人民群众喝上干净的水、呼吸清新的空气,有更好的工作和生活环境”作为政府工作的目标。 相似文献
998.
999.
Robustness analysis of responses of vibration control structures with uncertain parameters using interval algorithm 总被引:1,自引:0,他引:1
Variations in system parameters due to uncertainties may result in system performance deterioration. Uncertainties in modeling of structures are often considered to ensure that control system is robust with respect to response errors. Hence, the uncertain concept plays an important role in vibration control of the engineering structures. The paper discusses the robustness of responses of vibration control systems with the uncertain parameters. The vibration control problem of an uncertain system is approximated by a deterministic one. The uncertain parameters are described by interval variables. The uncertain state matrix is constructed directly using system physical parameters and avoided to use bounds in Euclidean norm. The feedback gain matrix is determined based on the deterministic systems, and then it is applied to the actual uncertain systems. A method to calculate the upper and lower bounds of responses of the closed-loop system with uncertain parameters is presented. The upper bounds of responses can be used to estimate the robustness of responses of the controlled system with uncertain parameters. Two numerical examples are given to illustrate the applications of the present approach. 相似文献
1000.
Surveying instructor and learner attitudes toward e-learning 总被引:2,自引:2,他引:0
The trend of using e-learning as a learning and/or teaching tool is now rapidly expanding into education. Although e-learning environments are popular, there is minimal research on instructors’ and learners’ attitudes toward these kinds of learning environments. The purpose of this study is to explore instructors’ and learners’ attitudes toward e-learning usage. Accordingly, 30 instructors and 168 college students are asked to answer two different questionnaires for investigating their perceptions. After statistical analysis, the results demonstrate that instructors have very positive perceptions toward using e-learning as a teaching assisted tool. Furthermore, behavioral intention to use e-learning is influenced by perceived usefulness and self-efficacy. Regarding to learners’ attitudes, self-paced, teacher-led, and multimedia instruction are major factors to affect learners’ attitudes toward e-learning as an effective learning tool. Based on the findings, this research proposes guidelines for developing e-learning environments. 相似文献