全文获取类型
收费全文 | 166539篇 |
免费 | 37240篇 |
国内免费 | 31750篇 |
专业分类
化学 | 88503篇 |
晶体学 | 1770篇 |
力学 | 7854篇 |
综合类 | 586篇 |
数学 | 16865篇 |
物理学 | 68807篇 |
无线电 | 51144篇 |
出版年
2024年 | 843篇 |
2023年 | 2847篇 |
2022年 | 3232篇 |
2021年 | 3826篇 |
2020年 | 4215篇 |
2019年 | 5281篇 |
2018年 | 4780篇 |
2017年 | 5541篇 |
2016年 | 6977篇 |
2015年 | 7884篇 |
2014年 | 8433篇 |
2013年 | 11173篇 |
2012年 | 12418篇 |
2011年 | 13392篇 |
2010年 | 14759篇 |
2009年 | 14557篇 |
2008年 | 9043篇 |
2007年 | 8085篇 |
2006年 | 7571篇 |
2005年 | 6991篇 |
2004年 | 7141篇 |
2003年 | 5910篇 |
2002年 | 5748篇 |
2001年 | 5459篇 |
2000年 | 4292篇 |
1999年 | 4224篇 |
1998年 | 3626篇 |
1997年 | 3251篇 |
1996年 | 3641篇 |
1995年 | 3805篇 |
1994年 | 3763篇 |
1993年 | 3731篇 |
1992年 | 3277篇 |
1991年 | 2865篇 |
1990年 | 2350篇 |
1989年 | 2338篇 |
1988年 | 2168篇 |
1987年 | 1359篇 |
1986年 | 1412篇 |
1985年 | 1096篇 |
1984年 | 1153篇 |
1982年 | 999篇 |
1981年 | 818篇 |
1980年 | 850篇 |
1979年 | 598篇 |
1978年 | 588篇 |
1977年 | 676篇 |
1976年 | 1098篇 |
1973年 | 486篇 |
1972年 | 561篇 |
排序方式: 共有10000条查询结果,搜索用时 772 毫秒
231.
Maria Kordaki Athanasia Balomenou 《International Journal of Computers for Mathematical Learning》2006,11(1):99-135
This study focuses on the constructions in terms of area and perimeter in equivalent triangles developed by students aged 12–15 years-old, using the tools provided by Cabri-Geometry II [Labore (1990). Cabri-Geometry (software), Université de Grenoble]. Twenty-five students participated in a learning experiment where they were asked to construct: (a) pairs of equivalent triangles “in as many ways as possible” and to study their area and their perimeter using any of the tools provided and (b) “any possible sequence of modifications of an original triangle into other equivalent ones”. As regards the concept of area and in contrast to a paper and pencil environment, Cabri provided students with different and potential opportunities in terms of: (a) means of construction, (b) control, (c) variety of representations and (d) linking representations, by exploiting its capability for continuous modifications. By exploiting these opportunities in the context of the given open tasks, students were helped by the tools provided to develop a broader view of the concept of area than the typical view they would construct in a typical paper and pencil environment. 相似文献
232.
233.
Jin-Wei Shi Chi-Kuang Sun 《Lightwave Technology, Journal of》2002,20(11):1942-1950
We present the theory and design of a tapered line distributed photodetector (TLDP). In the previously demonstrated velocity-matched distributed photodetector (VMDP), high electrical bandwidth is achieved by proper termination in the input end to absorb reverse traveling waves, sacrificing one-half of the quantum efficiency. By utilizing the tapered line structure and phase matching between optical waves and microwaves in our analyzed structure, a traveling-wave photodetector is more realizable and ultrahigh bandwidth can be attained due to removal of the extra input dummy load that sacrifices one-half of the total quantum efficiency. To investigate the advantages of TLDP over VMDP, we calculate their electrical bandwidth performances by using an analytic photodistributed current model. We adopted low-temperature-grown (LTG) GaAs-based metal-semiconductor-metal (MSM) traveling-wave photodetectors as example unit active devices in the analytic bandwidth calculation for their high-speed and high-power performances. Both VMDP and TLDP in our simulation are assumed to be transferred onto glass substrates, which would achieve high microwave velocity/impedance and make radiation loss negligible. The simulated bandwidth of a properly designed LTG GaAs MSM TLDP is /spl sim/325 GHz, which is higher than the simulated bandwidth of the LTG GaAs MSM VMDP with an open-circuit input end (/spl sim/240 GHz) and is almost comparable to the simulated bandwidth of an input-terminated LTG GaAs MSM VMDP (/spl sim/330 GHz). This proposed method can be applied to the design of high-bandwidth distributed photodetectors for radio-frequency photonic systems and optoelectronic generation of high-power microwaves and millimeter waves. 相似文献
234.
235.
Multicast routing and bandwidth dimensioning in overlay networks 总被引:20,自引:0,他引:20
Multicast services can be provided either as a basic network service or as an application-layer service. Higher level multicast implementations often provide more sophisticated features and can provide multicast services at places where no network layer support is available. Overlay multicast networks offer an intermediate option, potentially combining the flexibility and advanced features of application layer multicast with the greater efficiency of network layer multicast. In this paper, we introduce the multicast routing problem specific to the overlay network environment and the related capacity assignment problem for overlay network planning. Our main contributions are the design of several routing algorithms that optimize the end-to-end delay and the interface bandwidth usage at the multicast service nodes within the overlay network. The interface bandwidth is typically a key resource for an overlay network provider, and needs to be carefully managed in order to maximize the number of users that can be served. Through simulations, we evaluate the performance of these algorithms under various traffic conditions and on various network topologies. The results show that our approach is cost-effective and robust under traffic variations. 相似文献
236.
Chih-Wea Wang Chi-Feng Wu Jin-Fu Li Cheng-Wen Wu Tony Teng Kevin Chiu Hsiao-Ping Lin 《Journal of Electronic Testing》2002,18(6):637-647
In this paper we propose a novel built-in self-test (BIST) design for embedded SRAM cores. Our contribution includes a compact and efficient BIST circuit with diagnosis support and an automatic diagnostic system. The diagnosis module of our BIST circuit can capture the error syndromes as well as fault locations for the purposes of repair and fault/failure analysis. In addition, our design provides programmability for custom March algorithms with lower hardware cost. The combination of the on-line programming mode and diagnostic system dramatically reduces the effort in design debugging and yield enhancement. We have designed and implemented test chips with our BIST design. Experimental results show that the area overhead of the proposed BIST design is only 2.4% for a 128 KB SRAM, and 0.65% for a 2 MB one. 相似文献
237.
238.
239.
Wen-Yan Yin Le-Wei Li Tat-Soon Yeo Mook-Seng Leong Pang-Shyan Kooi 《Electromagnetic Compatibility, IEEE Transactions on》2002,44(2):329-337
The direct integral equation is formulated for describing the current on the multiple perfectly conducting strips in cylindrical geometries for an E-polarization plane wave of normal incidence. By using the Galerkin's method, the surface currents on the conducting strips are expanded in the form of a series of Chebyshev polynomials of the first kind, while the unknown expanding coefficients are solved by a set of matrix equations of finite order with a fast convergence rate and a high accuracy. Furthermore, numerical results are presented to demonstrate the variation of the penetrated near-zone field in the presence of one, two, three, four and six cylindrical apertures, and the hybrid effects of both aperture number and aperture angular widths on the penetrated fields are investigated in detail 相似文献
240.