全文获取类型
收费全文 | 46513篇 |
免费 | 6138篇 |
国内免费 | 4823篇 |
专业分类
化学 | 24581篇 |
晶体学 | 465篇 |
力学 | 1755篇 |
综合类 | 298篇 |
数学 | 3991篇 |
物理学 | 11215篇 |
无线电 | 15169篇 |
出版年
2024年 | 109篇 |
2023年 | 846篇 |
2022年 | 1056篇 |
2021年 | 1492篇 |
2020年 | 1464篇 |
2019年 | 1501篇 |
2018年 | 1243篇 |
2017年 | 1310篇 |
2016年 | 1815篇 |
2015年 | 1924篇 |
2014年 | 2278篇 |
2013年 | 2978篇 |
2012年 | 3550篇 |
2011年 | 3745篇 |
2010年 | 2851篇 |
2009年 | 2931篇 |
2008年 | 3112篇 |
2007年 | 2997篇 |
2006年 | 2719篇 |
2005年 | 2339篇 |
2004年 | 1823篇 |
2003年 | 1505篇 |
2002年 | 1492篇 |
2001年 | 1256篇 |
2000年 | 1082篇 |
1999年 | 965篇 |
1998年 | 816篇 |
1997年 | 730篇 |
1996年 | 755篇 |
1995年 | 617篇 |
1994年 | 568篇 |
1993年 | 510篇 |
1992年 | 485篇 |
1991年 | 401篇 |
1990年 | 357篇 |
1989年 | 268篇 |
1988年 | 220篇 |
1987年 | 152篇 |
1986年 | 179篇 |
1985年 | 181篇 |
1984年 | 123篇 |
1983年 | 77篇 |
1982年 | 84篇 |
1981年 | 59篇 |
1980年 | 58篇 |
1979年 | 63篇 |
1978年 | 51篇 |
1976年 | 47篇 |
1974年 | 50篇 |
1973年 | 47篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
41.
42.
Supermedia-enhanced Internet-based telerobotics 总被引:4,自引:0,他引:4
Elhajj I. Ning Xi Wai Keung Fung Yun-Hui Liu Hasegawa Y. Fukuda T. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》2003,91(3):396-421
This paper introduces new planning and control methods for supermedia-enhanced real-time telerobotic operations via the Internet. Supermedia is the collection of video, audio, haptic information, temperature, and other sensory feedback. However, when the communication medium used, such as the Internet, introduces random communication time delay, several challenges and difficulties arise. Most importantly, random communication delay causes instability, loss of transparency, and desynchronization in real-time closed-loop telerobotic systems. Due to the complexity and diversity of such systems, the first challenge is to develop a general and efficient modeling and analysis tool. This paper proposes the use of Petri net modeling to capture the concurrency and complexity of Internet-based teleoperation. Combined with the event-based planning and control method, it also provides an efficient analysis and design tool to study the stability, transparency, and synchronization of such systems. In addition, the concepts of event transparency and event synchronization are introduced and analyzed. This modeling and control method has been applied to the design of several supermedia-enhanced Internet-based telerobotic systems, including the bilateral control of mobile robots and mobile manipulators. These systems have been experimentally implemented in three sites test bed consisting of robotic laboratories in the USA, Hong Kong, and Japan. The experimental results have verified the theoretical development and further demonstrated the stability, event transparency, and event synchronization of the systems. 相似文献
43.
The approach to risk management for RF and microwave radiation has been to establish guidelines for maximum permissible levels of exposure. These guidelines are based on evaluations of the relevant scientific literature and, with safety margins to account for uncertainties and gaps in scientific knowledge, offer a significant level of protection against established hazards of RF and microwave radiation for short-term exposures. One can argue, on this basis, that the precautionary principle - specifically, safety factors -have been used to guide the making and promulgation of existing permissible-exposure standards for RF and microwave radiation. Therefore, discussions on the appropriateness of the precautionary principle for RF radiation may be "much ado about nothing". Clearly, this approach to "pre caution" neither has stifled technological development, nor has it helped to enforce the acceptance of zero risk, but it provided a basis for action when science was unable to give a clear answer. 相似文献
44.
This article presents a graph-theoretic method for constructing low-density parity-check (LDPC) codes from connected graphs without the requirement of large girth. This method is based on finding a set of paths in a connected graph, which satisfies the constraint that any two paths in the set are either disjoint or cross each other at one and only one vertex. Two trellis-based algorithms for finding these paths are devised. Good LDPC codes of practical lengths are constructed and they perform well with iterative decoding. 相似文献
45.
Yen-Shin Lai Juo-Chiun Lin 《Power Electronics, IEEE Transactions on》2003,18(5):1211-1219
A new hybrid fuzzy controller for direct torque control (DTC) induction motor drives is presented in this paper. The newly developed hybrid fuzzy control law consists of proportional-integral (PI) control at steady state, PI-type fuzzy logic control at transient state, and a simple switching mechanism between steady and transient states, to achieve satisfied performance under steady and transient conditions. The features of the presented new hybrid fuzzy controller are highlighted by comparing the performance of various control approaches, including PI control, PI-type fuzzy logic control (FLC), proportional-derivative (PD) type FLC, and combination of PD-type FLC and I control, for DTC-based induction motor drives. The pros and cons of these controllers are demonstrated by intensive experimental results. It is shown that the presented induction motor drive is with fast tracking capability, less steady state error, and robust to load disturbance while not resorting to complicated control method or adaptive tuning mechanism. Experimental results derived from a test system are presented confirming the above-mentioned claims. 相似文献
46.
Chun-Tsen Lu Kun-Wei Lin Huey-Ing Chen Hung-Ming Chuang Chun-Yuan Chen Wen-Chau Liu 《Electron Device Letters, IEEE》2003,24(6):390-392
A new and interesting Pd-oxide-Al/sub 0.3/Ga/sub 0.7/As MOS hydrogen sensor has been fabricated and studied. The steady-state and transient responses with different hydrogen concentrations has been measured at various temperatures. Based on the large Schottky barrier height and presence of oxide layer, the studied device exhibits a high hydrogen detection sensitivity and wide temperature operating regime. The studied device exhibits the low-leakage current and obvious current changes when exposed to hydrogen-contained gas. Even at room temperature, a very high hydrogen detection sensitivity of 155.9 is obtained when a 9090 ppm H/sub 2//air gas is introduced. Furthermore, when exposed to hydrogen-contained gas at 95/spl deg/C, both the forward and reverse currents are substantially increased with increased hydrogen concentration. In other words, the studied device can be used as a hydrogen sensor under the applied bidirectional bias. Under the applied voltage of 0.35 V and 9090 ppm H/sub 2//air hydrogen ambient, a fast adsorption response time about 10 s is found. The transient and steady-state characteristics of hydrogen adsorption are also investigated. 相似文献
47.
应用Nd:YAG激光(波长1.06μm,功率5w,照射时间0.5-3 min)和Ar+激光(波长488nm,功率70mw,照射时间5-15min)对长期保存的不同保存形式的塔胞藻(Pyramidomonassp.)进行辐照处理。研究了不同剂量激光辐照对藻体生长和叶绿素含量的影响。实验结果表明:照射剂量为60s的Nd:YAG激光及剂量为15min的Ar+激光对液体保存形式的藻种有较明显的促长效果,接种后这两种激光处理组的比生长速率分别较对照提高达53.33%和28.89%,叶绿素含量分别增加73.33%和65.69%。两种激光对液体保存藻种的活化效果均好于固体保存种。 相似文献
48.
玻璃微珠的表面化学镀银及红外辐射性能研究 总被引:6,自引:1,他引:5
采用化学镀方法在玻璃微珠表面镀银,考察了预处理条件、反应温度和反应时间等因素对玻璃微珠表面镀银的影响,并通过扫描电镜和X-射线衍射分析仪,对镀银后玻璃微珠的表面形貌和结构进行了观察和表征。将镀银玻璃微珠用于涂料中,考察了玻璃微珠在涂料中的应用及其红外辐射率的变化,探讨了化学镀条件对涂料红外辐射率的影响,结果表明,在控制反应温度和浓度的条件下,可使镀银玻璃微珠的红外辐射率由原来的1.02降为0.70,将其应用于涂料后,涂层的红外辐射率为0.80。 相似文献
49.
50.
Hao Wang Ying Lin Biao Chen 《Signal Processing, IEEE Transactions on》2003,51(10):2613-2623
We investigate non data-aided channel estimation for cyclically prefixed orthogonal frequency division multiplexing (OFDM) systems. By exploiting channel diversity using only two receive antennas, a blind deterministic algorithm is proposed. Identifiability conditions are derived that guarantee the perfect channel retrieval in the absence of noise. In the presence of noise, the proposed method has the desired property of being data efficient-only a single OFDM block is needed to achieve good estimation performance for a wide range of SNR values. The algorithm is also robust to input symbols as it does not have any restriction on the input symbols with regard to their constellation or their statistical properties. In addition, this diversity-based algorithm is computationally efficient, and its performance compares favorably to most existing blind algorithms. 相似文献