全文获取类型
收费全文 | 42347篇 |
免费 | 4265篇 |
国内免费 | 2801篇 |
专业分类
化学 | 3311篇 |
晶体学 | 100篇 |
力学 | 1460篇 |
综合类 | 416篇 |
数学 | 4156篇 |
物理学 | 8106篇 |
综合类 | 31864篇 |
出版年
2024年 | 208篇 |
2023年 | 534篇 |
2022年 | 1174篇 |
2021年 | 1178篇 |
2020年 | 967篇 |
2019年 | 880篇 |
2018年 | 791篇 |
2017年 | 1116篇 |
2016年 | 1190篇 |
2015年 | 1341篇 |
2014年 | 2302篇 |
2013年 | 2127篇 |
2012年 | 2417篇 |
2011年 | 2667篇 |
2010年 | 2095篇 |
2009年 | 2294篇 |
2008年 | 2537篇 |
2007年 | 3299篇 |
2006年 | 2940篇 |
2005年 | 2667篇 |
2004年 | 2161篇 |
2003年 | 1962篇 |
2002年 | 1740篇 |
2001年 | 1471篇 |
2000年 | 1219篇 |
1999年 | 1021篇 |
1998年 | 852篇 |
1997年 | 795篇 |
1996年 | 637篇 |
1995年 | 494篇 |
1994年 | 422篇 |
1993年 | 358篇 |
1992年 | 301篇 |
1991年 | 246篇 |
1990年 | 219篇 |
1989年 | 189篇 |
1988年 | 182篇 |
1987年 | 130篇 |
1986年 | 87篇 |
1985年 | 42篇 |
1984年 | 44篇 |
1983年 | 8篇 |
1982年 | 20篇 |
1981年 | 23篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 11篇 |
1959年 | 4篇 |
1955年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
L. Jackowska-Strumillo J. Sokolowski A. Żochowski A. Henrot 《Computational Optimization and Applications》2002,23(2):231-255
The new method is proposed for the numerical solution of a class of shape inverse problems. The size and the location of a small opening in the domain of integration of an elliptic equation is identified on the basis of an observation. The observation includes the finite number of shape functionals. The approximation of the shape functionals by using the so-called topological derivatives is used to perform the learning process of an artificial neural network. The results of computations for 2D examples show, that the method allows to determine an approximation of the global solution to the inverse problem, sufficiently closed to the exact solution. The proposed method can be extended to the problems with an opening of general shape and to the identification problems of small inclusions. However, the mathematical theory of the proposed approach still requires futher research. In particular, the proof of global convergence of the method is an open problem. 相似文献
92.
Qingfeng Zhang Shanjia Xu 《International Journal of Infrared and Millimeter Waves》2006,27(11):1539-1552
A novel uniplanar 2-D composite right/left-handed transmission line (CRLH-TL) structure is proposed based on a general transmission
line theory and the way to increase the relative operating bandwidth in the left-handed (LH) region and lower the LH operating
frequency is illustrated. In addition, a new method to extract parameters and calculate the Bloch impedance of the structure
is presented. Numerical results of the dispersion as well as the extracted parameters are given, which are calculated based
on full-wave simulation. The present 2-D uniplanar CRLH-TL structure is applicable to 2-D left-handed materials in lower and
wider LH frequency range.
Supported by the National Natural Science Foundation of China (No. 60371010) (No. 60471037) (No. 63531020) 相似文献
93.
M.Yu. Tretyakov V.V. Parshin M.A. Koshelev A.P. Shkaev A.F. Krupnov 《Journal of Molecular Spectroscopy》2006,238(1):91-97
Extension of the working frequency of modern resonator spectrometers into submillimeter wave range is described. Experimental record of atmospheric absorption spectrum covering 45-370 GHz range is demonstrated for the first time, and measured water vapor J′ka′, kc′-Jka, kc = 51,5-42,2 at 325 GHz line parameters are presented. For the first time pressure lineshift for the 325-GHz water vapor line is measured. Further extension of working range is discussed. New estimations of physical limits of time needed for measurements of absorption in the whole Backward Wave Oscillator (BWO) range are given for phase continuous synthesizer regime. Basic schemes of fast broadband continuous phase synthesized sources are discussed. Verification of the previous measurements of water vapor 31,3-22,0 at 183 GHz line parameters is presented. Comparisons with ringdown resonator spectrometers are given. 相似文献
94.
利用气体放电双探针法研究了等离子体的I-V曲线中的电流I相对于电压V轴交点的不对称性,并提出2种可能的解释:一认为是由于两探针表面积不同引起的;二认为是由于探针所在处等离子体电位不等引起的.本文利用仪器的工艺误差和调换放电管电压的方法,对提出的2种可能原因分别进行验证,并指出第二种解释的合理性,并对其进行了理论分析. 相似文献
95.
96.
论网络时代高校图书馆的思想道德教育 总被引:2,自引:1,他引:1
陈章华 《科技情报开发与经济》2005,15(23):11-13
网络对大学生的思想道德修养具有重要而深刻的影响。高校图书馆应充分认识到加强大学生思想道德教育的重要意义和自己在其中应负的责任,采取积极有效的措施,培养和提高大学生的思想道德素质。 相似文献
97.
Orthogonally linear polarized lasers(Ⅰ)--principle and devices 总被引:1,自引:0,他引:1
Two kinds of orthogonally polarized lasers, i.e. Zeeman dual-frequency lasers and four-frequency ring lasers (laser gyros) have been developed since the invention of lasers, in which circularly polarized lights oscillate. This paper summarizes recent progress of the study on orthogonally linear polarized lasers with the standing wave cavity. Firstly, the expression of producing orthogonally linear polarized lights in standing wave cavity, i.e. laser frequency splitting, is given. Almost all the birefringence effects made in laser cavity are used to produce orthogonally linear polarized lights. The effect includes quartz crystal birefringence effect, calcite birefringence effect,stress (photo-elastic) birefringence effect and electro-optical birefringence effect. Secondly, several physical phenomena of orthogonally linear polarized lasers are discovered such as aberrance of frequency splitting curves caused by optical activity of quartz crystal, order-passing of longitudinal modes with frequency splitting and strong modes competition. Finally, because the traditional Zeeman dual frequency laser cannot output frequency difference larger than 3 MHz, the approaches of obtaining larger frequency difference are studied. The sequential results, several kinds of orthogonally polarized lasers, are described, such as birefringence dual frequency lasers outputting a frequency difference from 40 MHz to hundreds of megahertz, birefringence-Zeeman dual frequency lasers outputting a frequency difference from 1 MHz to hundreds of megahertz, the LD pumped YAG birefringence dual frequency laser outputting frequency difference of several gigahertz, and the lasers whose longitudinal mode spacing is c/4L instead of c/2L. 相似文献
98.
利用VBA技术实现对离散高程点的三角构网,快速定位三角形。将其用于土方开挖边线处理,在此基础上重新构网。实际计算结果表明,将三角网格用于土方工程填挖量计算,可以提高计算精度并可实现填挖后的三维显示。 相似文献
99.
刘海龙 《科技情报开发与经济》2006,16(3):230-232
论证了电力系统建设时间同步网的必要性,提出网同步才是真正的同步,并结合山西电力已建的数字同步网及数据通信网,提出了利用山西电网已建的同步时钟系统实现全省各变电站时间同步的初步解决方案。 相似文献
100.
网络文化对高校学生负面影响的理性思考 总被引:4,自引:0,他引:4
张路 《重庆三峡学院学报》2006,22(4):102-104
随着网络信息技术的迅猛发展,互联网对大学生的影响越来越大,网络文化是一把双刃剑,它具有深远的意义和沉重的“负面效应”。为此,笔者对网络文化的特点及网络文化对大学生的负面影响进行了分析总结,并在此基础上探寻高校教育的新对策,以增强教育的实效性。 相似文献