全文获取类型
收费全文 | 14174篇 |
免费 | 3044篇 |
国内免费 | 990篇 |
专业分类
化学 | 2159篇 |
晶体学 | 42篇 |
力学 | 819篇 |
综合类 | 86篇 |
数学 | 619篇 |
物理学 | 4779篇 |
无线电 | 9704篇 |
出版年
2024年 | 58篇 |
2023年 | 184篇 |
2022年 | 370篇 |
2021年 | 392篇 |
2020年 | 467篇 |
2019年 | 356篇 |
2018年 | 400篇 |
2017年 | 533篇 |
2016年 | 615篇 |
2015年 | 662篇 |
2014年 | 1106篇 |
2013年 | 945篇 |
2012年 | 1076篇 |
2011年 | 1149篇 |
2010年 | 861篇 |
2009年 | 869篇 |
2008年 | 961篇 |
2007年 | 997篇 |
2006年 | 888篇 |
2005年 | 783篇 |
2004年 | 608篇 |
2003年 | 634篇 |
2002年 | 472篇 |
2001年 | 476篇 |
2000年 | 382篇 |
1999年 | 314篇 |
1998年 | 267篇 |
1997年 | 239篇 |
1996年 | 223篇 |
1995年 | 180篇 |
1994年 | 143篇 |
1993年 | 113篇 |
1992年 | 105篇 |
1991年 | 78篇 |
1990年 | 53篇 |
1989年 | 49篇 |
1988年 | 27篇 |
1987年 | 34篇 |
1986年 | 24篇 |
1985年 | 23篇 |
1984年 | 21篇 |
1983年 | 15篇 |
1982年 | 19篇 |
1981年 | 10篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1974年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 750 毫秒
41.
本文介绍了近年来普通物理实验课程体系和教学内容改革的实践,并针对目前实验教学中仍存在的一些问题提出了开放式实验教学的改革思路。 相似文献
42.
Charles J. Colbourn Sosina S. Martirosyan Gary L. Mullen Dennis Shasha George B. Sherwood Joseph L. Yucas 《组合设计杂志》2006,14(2):124-138
A covering array CA(N;t,k, v is an N × k array such that every N × t subarray contains all t‐tuples from v symbols at least once, where t is the strength of the array. Covering arrays are used to generate software test suites to cover all t‐sets of component interactions. The particular case when t = 2 (pairwise coverage) has been extensively studied, both to develop combinatorial constructions and to provide effective algorithmic search techniques. In this paper, a simple “cut‐and‐paste” construction is extended to covering arrays in which different columns (factors) admit different numbers of symbols (values); in the process an improved recursive construction for covering arrays with t = 2 is derived. © 2005 Wiley Periodicals, Inc. J Combin Designs 14: 124–138, 2006 相似文献
43.
基于校园网的实验管理系统的设计 总被引:2,自引:1,他引:1
主要讨论了应用实验室管理系统的必要性,并从用户需求分析、系统总体设计、功能模块的设计等方面对B/S实验室管理系统的开发进行了详细的阐述。 相似文献
44.
45.
46.
采用现代高频功率变换技术的有源功率因数校正(Power Factor Corrector,PFC)技术是解决高频开关变换器谐波污染的有效手段。与传统的PFC电路相比,有源PFC电路的输入电流接近正弦波且与输与电压同相位,能有效抑制电流波形畸变和谐波,因此避免了对同一电网设施的干扰。在PFC电路中,Boost变换器是研究和应用得最多的一种变换器。本文着重分析了Boost电路在不连续导电模式状态下,PFC电路的临界条件,对实际电路结构的设计有很好的指导意义。 相似文献
47.
We derive an explicit autocorrelation function (ACF) formula of state of polarization for a fiber transmission system with polarization mode dispersion (PMD) and polarization dependent loss (PDL), which is found to agree well with Monte Carlo simulation. Then we use the new ACF to investigate the combined effect of PMD and PDL on the polarization multiplexed scheme. We find the performance of the polarization multiplexed scheme can be deteriorated more severely than the case without PDL. 相似文献
48.
CAO Wei-feng WANG Wen-boBeijing University of Posts Telecommunications Beijing P.R. China 《中国邮电高校学报(英文版)》2005,12(4)
1IntroductionCurrently,smart antenna technology can greatly en-hance system performance and i mprove the system ca-pacity so that it has been considered as a keytechnologyinthe third generation wireless communicationsystems .The researches on channel model , beamforming algo-rithms and receive algorithms have been widely carriedout and a great many of proposition have beensubmittedtothe 3th Generation Project Partner (3GPP) . Fromthe viewpoint of link-level and system-level research,many asp… 相似文献
49.
Yang Dai Faquan Li Xuewu Cheng Zhiling Jiang Shunsheng Gong 《Optics & Laser Technology》2007,39(7):1374-1379
In Shack–Hartmann wavefront sensor (SHWS), the behavior of the irradiance pattern produced by the micro-lens array is important for an accurate centroid estimation. In this paper, the behavior of a micro-lens array in SHWS is analyzed using Fourier optics, and reveals that in addition to the main, expected spots, secondary spots with smaller intensities also appeared as a result of diffraction by the small dimensions of the micro-lens and interference from the different micro-lenses. This result is confirmed by comparing with a irradiance pattern taken from an actual SHWS. The additional error in centroid estimation caused by these secondary spots is discussed and relationship to the parameters of the micro-lens of SHWS is analyzed. 相似文献
50.
We present a simplified relativistic configuration interaction method (SRCI), by which all the energy levels and oscillator strengths in each transition array can be calculated. There exist generalized quasi-sum relations for the calculated oscillator strengths in the relevant transition arrays by the SRCI and by an unresolved transition array model (UTA). Based on UTA and the detailed configuration accounting (DCA), with a fully relativistic treatment incorporated with the quantum defect theory, the X-ray absorption spectra for any middle- and high-Z plasmas or composite plasmas can be calculated with much less computational efforts. The gross features of calculated spectra by DCA-UTA are in agreement with the relevant experimental measurements, except some detailed structures in some narrow spectral ranges. Such detailed structures can be calculated by SRCI incorporated with DCA-UTA. As an illustrative example, the absorption spectra of Ge plasmas are calculated by DCA-UTA-SRCI and are in agreement with the experimental opacity data. Therefore, the theoretical method (DCA-UTA-SRCI) verified by experimental measurements will not only be a basic tool to provide “precision” opacity data for the inertial confinement fusion research (ICF) and studies in stellar physics but also can be used to analyze the relevant diagnostic measurements for ICF plasmas. 相似文献