首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   539609篇
  免费   22415篇
  国内免费   16479篇
化学   247220篇
晶体学   6385篇
力学   24963篇
综合类   676篇
数学   74839篇
物理学   143466篇
无线电   80954篇
  2022年   5653篇
  2021年   7413篇
  2020年   7621篇
  2019年   7411篇
  2018年   18608篇
  2017年   17995篇
  2016年   17307篇
  2015年   10099篇
  2014年   13104篇
  2013年   22505篇
  2012年   24454篇
  2011年   33410篇
  2010年   23171篇
  2009年   23217篇
  2008年   26706篇
  2007年   28688篇
  2006年   18887篇
  2005年   18227篇
  2004年   15928篇
  2003年   14649篇
  2002年   13146篇
  2001年   12723篇
  2000年   10631篇
  1999年   9095篇
  1998年   8089篇
  1997年   7652篇
  1996年   7259篇
  1995年   6649篇
  1994年   6403篇
  1993年   5839篇
  1992年   6086篇
  1991年   5976篇
  1990年   5652篇
  1989年   5138篇
  1988年   5053篇
  1987年   4574篇
  1986年   4359篇
  1985年   5376篇
  1984年   5603篇
  1983年   4656篇
  1982年   4736篇
  1981年   4618篇
  1980年   4497篇
  1979年   4464篇
  1978年   4737篇
  1977年   4666篇
  1976年   4712篇
  1975年   4315篇
  1974年   4375篇
  1973年   4548篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
This brief presents a necessary and sufficient condition for testing positive, real, imaginary, and negative rational functions. A related term, the positive, imaginary, and negative polynomial, is defined and two necessary and sufficient conditions for testing it are given.  相似文献   
42.
Processing methods used in photonics and nanotechnology possess many limitations restricting their application areas such as high cost, inability to produce fine details, problems with scalability, and long processing time. Proximity field nanopatterning is a lithography method which surpasses these limitations. By using interference patterns produced by a two-dimensional phase mask, the technique is able to generate a submicron detailed exposure on a millimeter-size slab of light sensitive photopolymer, which is then developed like a photographic plate to reveal three-dimensional interference patterns from the phase mask. While it is possible to use simulations to obtain the interference patterns produced by a phase mask, realizing the mask dimensions necessary for producing a desired interference pattern is analytically challenging due to the intricacies of light interactions involved in producing the final interference pattern. An alternative method is to iteratively optimize the phase mask until the interference patterns obtained converge to the desired pattern. However, depending on the optimization technique used, one either risks a significant probability of failure or requires a prohibitive number of iterations. We argue that an optimization technique that is to take advantage of the physics of the problem using machine learning methods (here fuzzy learning) can lead to competent mask design. This technique is described in this letter.  相似文献   
43.
A 0.9 V 92 dB Double-Sampled Switched-RC Delta-Sigma Audio ADC   总被引:1,自引:0,他引:1  
A 0.9 V third-order double-sampled delta-sigma audio ADC is presented. A new method using a combination of a switched-RC technique and a floating switched-capacitor double-sampling configuration enabled low-voltage operation without clock boosting or bootstrapping. A three-level quantizer with simple dynamic element matching was used to improve linearity. The prototype IC implemented in a 0.13 CMOS process achieves 92 dB DR, 91 dB SNR and 89 dB SNDR in a 24 kHz audio signal bandwidth, while consuming 1.5 mW from a 0.9 V supply. The prototype operates from 0.65 V to 1.5 V supply with minimal performance degradation.  相似文献   
44.
To further evaluate the causality between endogenous pyrogen (EP)-induced fever and cyclic adenosine-3',5'-monophosphate (cyclic AMP) level, the effects of sodium salicylate (SS) on the febrile response and increased levels of cyclic AMP in both cerebrospinal fluid (c.s.f.) and plasma during EP-induced fever in rabbits were observed. The results suggest that cyclic AMP is probably involved in the central mediation of EP-induced fever and that increased concentration of cyclic AMP in c.s.f. associated with EP-induced fever is not the result of temperature elevation but appears to be caused by the increased synthesis in the central nervous system. In addition it is confirmed that blood is impossibly a contributory source of increased cyclic AMP in c.s.f. during EP fever, and that SS may act subsequent to the increase in cyclic AMP.  相似文献   
45.
46.
Crystalline materials that are transparent in the vacuum UV spectral region and currently used have been reviewed. Transmission of crystals of solid solutions with the fluorite structure Ca1?x R xF2+x (R = Sc, Y, La, Yb, Lu) in the UV and vacuum UV spectral regions has been investigated. It is shown that application of different methods of purification of fluorides from some impurities can significantly improve the optical quality of fluoride multicomponent crystals in the short-wavelength spectral region.  相似文献   
47.
48.
在本文中,给定一组有序空间数据点列及每个数据点的切矢向量,利用加权二次有理Bézier曲线对数据点作插值曲线,使该曲线具有C1连续性,并且权因子只是对相应顶点曲线附近产生影响,同调整两个相邻的权因子可以调整这两个相邻顶点之间的曲线和它的控制多边形.  相似文献   
49.
A three-dimensional examination of blood vessels is provided using MR data from seven cases. The vascular surfaces are constructed with an algorithm that automatically follows the selected artery or vein and generates a projected three-dimensional gradient shaded image. Fast 3DFT pulse sequences were optimized to enhance the time-of-flight contrast of the intravascular region. By increasing the surface threshold value in a three-dimensional head study, the flesh of a patient's face was peeled away to demonstrate the superfacial temporal artery. Gated cardiac images show the great vessels and cardiac chambers. A three-dimensional view of the aorta shows an irregular surface in the vicinity of an adrenal tumor. 3D MR exams provide a non-invasive technique for assessing vascular morphology in a clinical setting.  相似文献   
50.
Malaviya  N. Jauhari  G. 《Electronics letters》1987,23(20):1072-1073
An expression is derived for the power spectral density of a coded mark inversion (CMI) TV signal transmitted through a fibre-optic heterodyne system. An illustrative example is given, and its power spectral density has been computed numerically.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号