全文获取类型
收费全文 | 4779篇 |
免费 | 523篇 |
国内免费 | 387篇 |
专业分类
化学 | 2392篇 |
晶体学 | 14篇 |
力学 | 159篇 |
综合类 | 22篇 |
数学 | 507篇 |
物理学 | 1108篇 |
无线电 | 1487篇 |
出版年
2024年 | 14篇 |
2023年 | 97篇 |
2022年 | 76篇 |
2021年 | 121篇 |
2020年 | 117篇 |
2019年 | 117篇 |
2018年 | 100篇 |
2017年 | 72篇 |
2016年 | 175篇 |
2015年 | 142篇 |
2014年 | 201篇 |
2013年 | 294篇 |
2012年 | 365篇 |
2011年 | 372篇 |
2010年 | 286篇 |
2009年 | 274篇 |
2008年 | 363篇 |
2007年 | 277篇 |
2006年 | 289篇 |
2005年 | 237篇 |
2004年 | 195篇 |
2003年 | 222篇 |
2002年 | 168篇 |
2001年 | 137篇 |
2000年 | 104篇 |
1999年 | 93篇 |
1998年 | 79篇 |
1997年 | 88篇 |
1996年 | 85篇 |
1995年 | 72篇 |
1994年 | 59篇 |
1993年 | 43篇 |
1992年 | 45篇 |
1991年 | 28篇 |
1990年 | 31篇 |
1989年 | 23篇 |
1988年 | 17篇 |
1987年 | 18篇 |
1986年 | 19篇 |
1985年 | 22篇 |
1984年 | 24篇 |
1983年 | 14篇 |
1982年 | 14篇 |
1981年 | 10篇 |
1980年 | 17篇 |
1979年 | 16篇 |
1978年 | 7篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1974年 | 7篇 |
排序方式: 共有5689条查询结果,搜索用时 343 毫秒
21.
22.
Whilst the space volume of muffler in noise control system is often constrained for maintenance in practical engineering work, the maximization on muffler’s performance becomes important and essential. In this paper, a novel approach genetic algorithms (GAs) based on the principles of natural biological evolution will be used to tackle this optimization of muffler design [M. Mitchell, An Introduction to Genetic Algorithms, The MIT Press, Cambridge, MA, 1996]. Here, the shape optimization of multi-segments muffler coupled with the GA searching technique is presented. The techniques of binary genetic algorithms (BGA) together with the commercial MATLAB package [G. Lindfield, J. Penny, Numerical Method Using Matlab, second ed., Prentice Hall, Englewood Cliffs, NJ, 2000] are applied in GA searching. In addition, a numerical case of pure tone elimination with 2-5 segments on muffler is introduced and fully discussed. To achieve the best optimization in GA, several GA parameters are on trial in various values. Results show that the GA operators, including crossover mutation and elitism, are essential in accuracy. Consequently, results verify that the optimal sound transmission loss at the designed frequency of 500 Hz is exactly maximized. The GA optimization on multi-segments muffler proposed in this study surely provides a quick and correct approach. 相似文献
23.
Yingyu Hu Guoqiao Lai Yongjia Shen Yongfang Li 《Monatshefte für Chemie / Chemical Monthly》2004,33(2):1167-1172
Zinc(II) and magnesium(II) phthalocyanines bearing four substituted propylenedithiotetrathiafulvalene (PDT-TTF) units were synthesized and characterized by NMR, MS, EA, and UV/VIS spectra. The absorption peaks of two target molecules were hypsochromically shifted compared with the phthalocyanine parent compound. Studies using cyclic voltammetry indicated that introduction of Zn and Mg as central metals enhanced the electronwithdrawing between the phthalocyanine core and the PDT-TTF unit, thus losing one electron to form (PDT-TTF)+ becomes more difficult. 相似文献
24.
Chenn-Jung Huang Wei-Kuang Lai Sheng-Yu Hsiao Hao-Yu Liu Rui-Lin Luo 《International Journal of Wireless Information Networks》2004,11(3):131-146
In this paper, a routing protocol which utilizes the characteristics of Bluetooth technology is proposed for Bluetooth-based mobile ad hoc networks. The routing tables are maintained in the master devices and the routing zone radius for each table is adjusted dynamically by using evolving fuzzy neural networks. Observing there exists some useless routing packets which are helpless to build the routing path and increase the network loads in the existing ad hoc routing protocols, we selectively use multiple unicasts or one broadcast when the destination device is out of the routing zone radius coverage of the routing table. The simulation results show that the dynamic adjustment of the routing table size in each master device results in much less reply time of routing request, fewer request packets and useless packets compared with two representative protocols, Zone Routing Protocol and Dynamic Source Routing. 相似文献
25.
Yen-Jen Chang Shanq-Jang Ruan Feipei Lai 《Very Large Scale Integration (VLSI) Systems, IEEE Transactions on》2003,11(4):568-580
Power consumption is an increasingly pressing problem in modern processor design. Since the on-chip caches usually consume a significant amount of power, it is one of the most attractive targets for power reduction. This paper presents a two-level filter scheme, which consists of the L1 and L2 filters, to reduce the power consumption of the on-chip cache. The main idea of the proposed scheme is motivated by the substantial unnecessary activities in conventional cache architecture. We use a single block buffer as the L1 filter to eliminate the unnecessary cache accesses. In the L2 filter, we then propose a new sentry-tag architecture to further filter out the unnecessary way activities in case of the L1 filter miss. We use SimpleScalar to simulate the SPEC2000 benchmarks and perform the HSPICE simulations to evaluate the proposed architecture. Experimental results show that the two-level filter scheme can effectively reduce the cache power consumption by eliminating most unnecessary cache activities, while the compromise of system performance is negligible. Compared to a conventional instruction cache (32 kB, two-way) implemented with only the L1 filter, the use of a two-level filter can result in roughly 30% reduction in total cache power consumption. Similarly, compared to a conventional data cache (32 kB, four-way) implemented with only the L1 filter, the total cache power reduction is approximately 46%. 相似文献
26.
27.
Kuo-Kai Shyu Chiu-Keng Lai Yao-Wen Tsai Ding-I Yang 《Industrial Electronics, IEEE Transactions on》2002,49(3):558-565
A robust controller which is designed by employing variable-structure control and linear-quadratic method is presented for a permanent-magnet synchronous motor (PMSM) position control system. It is to achieve accurate control performance in the presence of plant parameter variation and load disturbance. In addition, it possesses the design flexibility of the conventional state feedback control. It is applied to the position control of a PMSM. Simulation and experimental results show that the proposed approach gives a better position response and is robust to parameter variations and load disturbance 相似文献
28.
Jih-Sheng Lai 《Industrial Electronics, IEEE Transactions on》1997,44(1):71-80
This paper summarizes recently developed soft-switching inverters and proposes two possible options for electric propulsion motor drive applications. The newly developed soft-switching inverter employs an auxiliary switch and a resonant inductor per phase to produce a zero voltage across the main switch, so that the main switch can turn-on at the zero-voltage condition. Both the auxiliary switch and the resonant inductor are operating at a fractional duty and, thus, are small in size as compared to the main inverter circuit components. Operation modes in a complete zero-voltage switching cycle for the single-phase soft-switching inverter are described in detail, with graphical explanations. The circuit operation was first verified by a computer simulation, and then further tested with a 1 kW single-phase and a 100 kW three-phase inverter. Both simulation and experimental results are presented to show the superior performance in efficiency improvement, EMI reduction and dv/dt reduction of the proposed soft-switching inverters 相似文献
29.
30.
Wengu Chen Yixin Lai 《分析论及其应用》2006,22(2):195-200
Let μ be a Borel measure on Rd which may be non doubling. The only condition that μ must satisfy is μ(Q) ≤ col(Q)n for any cube Q () Rd with sides parallel to the coordinate axes and for some fixed n with 0 < n ≤ d. The purpose of this paper is to obtain a boundedness property of fractional integrals in Hardy spaces H1 (μ). 相似文献