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   731篇
  免费   36篇
  国内免费   17篇
化学   241篇
力学   25篇
综合类   6篇
数学   71篇
物理学   441篇
  2025年   2篇
  2024年   6篇
  2023年   16篇
  2022年   21篇
  2021年   17篇
  2020年   15篇
  2019年   14篇
  2018年   18篇
  2017年   26篇
  2016年   51篇
  2015年   41篇
  2014年   54篇
  2013年   57篇
  2012年   26篇
  2011年   48篇
  2010年   42篇
  2009年   43篇
  2008年   44篇
  2007年   29篇
  2006年   29篇
  2005年   27篇
  2004年   14篇
  2003年   24篇
  2002年   16篇
  2001年   11篇
  2000年   9篇
  1999年   17篇
  1998年   13篇
  1997年   9篇
  1996年   10篇
  1995年   4篇
  1994年   3篇
  1993年   4篇
  1992年   5篇
  1991年   1篇
  1990年   4篇
  1989年   1篇
  1988年   2篇
  1987年   2篇
  1986年   2篇
  1982年   3篇
  1979年   1篇
  1978年   1篇
  1972年   1篇
  1970年   1篇
排序方式: 共有784条查询结果,搜索用时 15 毫秒
1.
The band artifact in steady-state free precession can be reduced by synthesizing the multiple images obtained through different phase increments of successive radiofrequency pulses. Even though the complex summation method was reported to be effective in reducing the band artifact, it has the pitfalls of intensity abnormality and sensitivity to the phase abnormality. Two new methods have been developed for more reliable reduction of the band artifact than the complex summation method. One method is to sum the complex images partially and to take the maximum intensity of the partially summed images. The other method is to sum the free induction decay (FID) and primary echo components of the Fourier series that are obtained through Fourier analysis of the complex base images. Both proposed methods were compared with other magnitude (maximum intensity projection, spectrally decomposed synthesis, sum-of-squares, nonlinear averaging) and complex-based (complex summation, magnitude-weighted complex summation) methods experimentally at 3 T for the phantom and volunteer's head imaging. Both proposed methods were confirmed to maintain the advantage of the complex summation in reducing both the dark and bright band artifacts while reducing the intensity abnormality and sensitivity to the phase abnormality from that of the complex summation method over a wide range of flip angles and relaxation times.  相似文献   
2.
唐云 《低温与超导》2005,33(3):61-64
主要研究8mm波段,雷达FMCW信号的时域及频域特性,通过对理论值与实测值的比较、分析,验证了8mm收发模块的可行性,并为下级信号处理作了准备。  相似文献   
3.
Zhang G  Zhang S  Wang Y 《Ultrasonics》2000,38(10):961-964
In the paper, adaptive time-frequency decomposition by basis pursuit (BP) is utilized to improve ultrasonic flaw detection in highly-scattering materials as an alternative to the Wavelet Transform technique. The detection of ultrasonic pulses using the BP is described. Computer simulation was performed to verify the signal detection improvements for an ultrasonic wave embodied in white noise, and numerical results show good detection even for signal-noise ratio (SNR) of -18 dB. The improvement in detection is experimentally verified using cast steel samples with artificial flaws.  相似文献   
4.
A Digital Equipment Corporation PDP 8/S Computer is interfaced with an EPR Spectrameter to achieve signal enhancement.

A computer-based system for spectrum accumulation and noise averaging of EPR or NMR signals has been designed and implemented. The computer-based system was chosen over other perhaps more conventional systems because of its greater flexibility in the processing of spectra, especially after the accumulation is complete.(1)  相似文献   
5.
Effects of the reflection mirrors mounted to the sample holder of a fluorimeter have been investigated. Analytical expressions are given for the emission intensity measured as a function of the concentration or optical density of the sample, in case one or two reflection mirrors are attached beside the sample holder to intensify the emission signals. The emission intensity calculated as a function of the sample concentration agreed well with the experimental data. By mounting two reflection mirrors, the emission signal was intensified by a factor of near 3.5. However, the degree of intensification depended strongly on the sample concentration and the linearity between the sample concentration and the emission intensity deviated significantly with increasing the sample concentration.  相似文献   
6.
In this work we have performed a detailed analysis, using signal processing tools, to study time series of data (temperature proxy) extracted from the GRIP ice-core records and we relate it with the evolution of atmospheric CO2 within the last glacial period. Our method is based in considering the warm periods known as Dansgaard–Oeschger (D/O) events and posterior returns to the cold stage as a climatic cycle. After the warming phase, D/O events relax to the initial cold state in three different ways, what gives rise to three classes of cycles. Also, the Younger/Dryas–Bolling/Allerod (Y/D–B/A) cycle corresponds to one of the classes obtained. We have found that all cycles start with identical warming phases which seem completely unrelated to variations in CO2 concentration. We discuss on the consequences for global climatology of such steady pattern of cycles.  相似文献   
7.
8.
Hanle technique is used for the lifetime measurement of the atomic excited states. Field dependent Hanle signal is Lorentzian under ideal conditions. Any departure from ideal situation is reflected in the shape of the Hanle signal resulting in erroreous measurements. The effect of one such factor, a finite interaction cross section, is discussed here.  相似文献   
9.
傅里叶变换技术在紫外可见光谱区的应用   总被引:13,自引:3,他引:13  
何锡文  陈鼎 《分析化学》1994,22(1):94-100
本文评述了傅里叶变换在紫外可见光谱区的应用,探讨了傅里叶变换在紫外可见光谱学以及信号处理两方面的内容。详细介绍了傅里叶变换在紫外可见区遇到的问题、主要优点和发展前景。  相似文献   
10.
端粒酶是真核细胞维持端粒长度的关键逆转录酶,其生物活性的高低可以为多种癌症的临床诊断和预后治疗提供有价值的信息.本研究以人宫颈癌细胞(HeLa细胞)裂解液中的端粒酶为研究对象,通过借助杂交链式反应辅助多重信号放大策略,提出了一种新颖、灵敏的检测端粒酶电化学方法.首先将端粒酶的延伸引物自组装在金电极表面,当端粒酶存在时,端粒酶能够催化引物的延伸,产生与发卡环探针H1部分互补的序列,进而引发杂交链式反应,形成由两个发卡环探针(H1和H2)交替杂交而形成的DNA长链.由于H1和H2末端均修饰有生物素,加入链霉亲和素修饰辣根过氧化物酶后,辣根过氧化物酶被被连接到电极表面,催化邻苯二胺氧化生成2,3-二氨基吩嗪,产生显著的电化学信号.实验结果表明,本研究建立的端粒酶电化学检测方法高效、可行,线性范围宽,灵敏度高,可以检测每毫升10个HeLa细胞裂解液中的端粒酶.本方法具有较好的选择性,能有效区分端粒酶和对照蛋白.  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