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   2864篇
  免费   307篇
  国内免费   75篇
化学   542篇
晶体学   8篇
力学   187篇
综合类   76篇
数学   530篇
物理学   1903篇
  2025年   21篇
  2024年   37篇
  2023年   57篇
  2022年   100篇
  2021年   104篇
  2020年   70篇
  2019年   76篇
  2018年   78篇
  2017年   100篇
  2016年   98篇
  2015年   150篇
  2014年   187篇
  2013年   199篇
  2012年   160篇
  2011年   188篇
  2010年   131篇
  2009年   174篇
  2008年   201篇
  2007年   125篇
  2006年   140篇
  2005年   131篇
  2004年   102篇
  2003年   110篇
  2002年   74篇
  2001年   75篇
  2000年   57篇
  1999年   51篇
  1998年   43篇
  1997年   29篇
  1996年   31篇
  1995年   18篇
  1994年   15篇
  1993年   12篇
  1992年   15篇
  1991年   9篇
  1990年   9篇
  1989年   8篇
  1988年   10篇
  1987年   10篇
  1986年   3篇
  1985年   7篇
  1984年   9篇
  1983年   4篇
  1982年   7篇
  1981年   2篇
  1980年   2篇
  1973年   1篇
  1971年   1篇
  1970年   1篇
  1969年   1篇
排序方式: 共有3246条查询结果,搜索用时 0 毫秒
991.
    
A new 8 ‐pulse P hase M odulated binomial‐like selective inversion pulse sequence, dubbed ‘8PM’, was developed by optimizing the nutation and phase angles of the constituent radio‐frequency pulses so that the inversion profile resembled a target profile. Suppression profiles were obtained for both the 8PM and W5 based excitation sculpting sequences with equal inter‐pulse delays. Significant distortions were observed in both profiles because of the offset effect of the radio frequency pulses. These distortions were successfully reduced by adjusting the inter‐pulse delays. With adjusted inter‐pulse delays, the 8PM and W5 based excitation sculpting sequences were tested on an aqueous lysozyme solution. The 8 PM based sequence provided higher suppression selectivity than the W5 based sequence. Two‐dimensional nuclear Overhauser effect spectroscopy experiments were also performed on the lysozyme sample with 8PM and W5 based water signal suppression. The 8PM based suppression provided a spectrum with significantly increased (~ doubled) cross‐peak intensity around the suppressed water resonance compared to the W5 based suppression. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   
992.
    
A nondestructive and sensitive method is developed to determine mixed pesticides of acetamiprid, chlorpyrifos and carbendazim in apple samples by surface‐enhanced Raman spectroscopy (SERS). Self‐modeling mixture analysis (SMA) was used to identify and extract the Raman signals of each pesticide from the spectra of apples contaminated with mixed pesticides. Results indicate that the obtained SERS signal intensities of each pesticide in their mixture have no obvious difference to the signal intensities of the corresponding pure pesticide at a low concentration. The lowest detectable level of acetamiprid, chlorpyrifos and carbendazim in apple are 0.0054 mg/kg, 0.064 mg/kg and 0.014 mg/kg, respectively, which are sensitive enough for identifying apple contaminated with pesticides above the maximum residue limit. The predicted values of each pesticide in their mixture are obtained using the prediction model based on the Raman signal of the single pesticide. The correlation coefficients of predicted values and actual values are 0.893 for acetamiprid, 0.926 for chlorpyrifos and 0.938 for carbendazim, respectively. The method presents the ultrasensitive SERS performance for quantifying residual pesticides in apple samples without sample pre‐treatment, showing great potential to serve as a useful means for monitoring pesticide residues used in mixed state. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   
993.
    
The direct transmission of digital baseband signals has practical significance in the field of Ethernet terminal connection, high-speed digital communication, data transmission of various types of information peripherals. The signal amplitude gradually decays while the transmission distance increases. Also the attenuation is proportional to the signal frequency, resulting in signal distortion and receiving error. It is a common method for digital baseband signal transmission to use pre-emphasis chip and equalizer chip to improve the transmission quality with a wide range of mature applications. This paper describes a new type of digital signal transmission method, as the receiver using analog-to-digital converter, instead of equalizer chip, to achieve the multi-level quantization of receiving time-domain data waveform. The waveform of the transmitted digital high and low level signal is sampled into multi-bit values. Then, the paper realizes adaptive frequency domain equalization based on soft threshold and makes use of multi-level quantization soft information for error correction. Error correcting code is mainly used to correct the error caused by the channel bandwidth limit, external noise or interference in the process of data transmission, so as to improve the stability and reliability of the transmission. The paper uses the two-stage error correcting codec system based on both Turbo and BCH coding, to achieve the high performance of Turbo code, and good characters of respond time and complexity. The transmitter outputs 12.5 MHz pseudo-random sequence through a 199.93 meter unshielded balanced twisted pair transmission medium. And the receiver circuit using a 62.5MSPS analog-to-digital converter over-samples the waveform to 8-level quantity. The output error of a 65536 bit pseudo-random sequence is less than 8 bits, and the error correction can be further improved by 8b-10b codec. Compared with the traditional pre-emphasis and balanced interface ICs connection, the method described in this article has the advantages of longer transmission distance, better flexibility and wider scope of use.  相似文献   
994.
    
《化学:亚洲杂志》2017,12(21):2780-2789
As a newly developed technique, photoelectrochemical (PEC) immunoassays have attracted great attention in recent years because of their low cost and desirable sensitivity. Because the detection signal originates from the photoelectric conversion of photoelectric materials, the appearance and application of quantum dots (QDs), which possess unique photophysical properties and regulated optoelectronic characteristics, has taken the development of PEC immunoassays to new heights. This review concisely introduces the general mechanism of QDs‐based photoelectric conversion for immunoassays and summarizes the current advances in QD applications in immunoassays. Given that signal strategies and photoactive materials are the key elements in PEC biosensor systems, we comprehensively highlight the state‐of‐the‐art signaling strategies and various applications of QDs in PEC immunoassays to introduce advances in QDs‐based PEC immunoassays. Finally, challenges and future developmental trends are briefly discussed  相似文献   
995.
    
The present contribution focuses on the estimation of the geometric acceleration and of the geometric jolt (namely, the derivative of the acceleration) of a multidimensional, structured gyroscopic signal. A gyroscopic signal encodes the instantaneous orientation of a rigid body during a full three‐dimensional rotation that is regarded as a trajectory in the curved space SO(3) of the special orthogonal matrices. The geometric acceleration and jolt associated to a gyroscopic signal are evaluated through the rules of calculus prescribed by differential geometry. Such an endeavor is motivated by recent studies on the smoothness of human body movement in biomechanical engineering, sports science, and rehabilitation neuroengineering. Two indexes of smoothness are compared, namely, a normalized proper geometric acceleration index and a normalized proper geometric jolt index. Our investigation concludes that, in the considered experiments with measured signals, for relatively low values of the acceleration and of the jolt indexes, such indexes are strongly positively correlated. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   
996.
The chirp heterodyne technique allows a directional discrimination of laser Doppler velocity measurements without the use of additional frequency shift elements. This paper presents, for the first time, the use of a chirp frequency-modulated solid-state laser, emitting in the green spectral range. Experimental results of a fluid flow measurement verify the function of the realised chirp heterodyne laser Doppler velocimeter.  相似文献   
997.
两种高速CMOS图像传感器的应用与测试   总被引:2,自引:0,他引:2  
孙宏海  刘艳滢 《中国光学》2011,4(5):453-460
采用MI-MV13和LUPA-1300-2两种不同厂家型号的高速CMOS图像传感器,设计了分辨率为1 280×1 024的300~500 frame/s高速数字工业相机,并在实验室条件下对设计相机进行了关键性能指标对比测试,得到了光谱响应及量子效率、增益、动态范围、暗电流、读出噪声、光电响应非均匀性等测试结果。测试分析显示,LUPA-1300-2的峰值量子效率为50%,比MI-MV13的峰值量子效率高12%,与厂家的参考指标基本一致。测试结果证明:该测试方法正确,对两种高速CMOS图像传感器的关键性能指标的测试客观可信,所设计的高速CMOS摄像机的性能基本满足高帧频摄像的要求。  相似文献   
998.
在原子辐射跃迁原理的基础上,依据光磁共振实验的基本实验现象建立简化模型,给出光抽运过程中跃迁速率方程,理论计算了光抽运信号光强随外场变化关系以及粒子跃迁弛豫时间随外场的变化关系,理论计算结果与实验符合.  相似文献   
999.
张云峰  韩广良 《光学技术》2005,31(2):166-169
以两片由TI公司生产的数字信号处理器TMS320C6203B为核心,用可编程逻辑阵列CPLD进行逻辑控制,采用现场可编程门阵列FPGA作图像的预处理和进行双数字信号处理器(DSP)之间的通讯,实现了实时相关的图像处理。此系统实时性好,可直接利用数字图像的灰度特征,在低信噪比的情况下目标跟踪点漂移小,目标跟踪能够较好地适应不同灰度分布的背景。  相似文献   
1000.
激光测距中的几种虚警抑制措施   总被引:2,自引:0,他引:2  
激光测距的关键在于光电微弱信号的检测,如何调和可靠性和灵敏度之间的矛盾是弱检问题的关键。激光测距系统包括主控电路、发射电路、接收电路、计时电路以及其他一些功能电路,其中与虚警的产生和抑制密切相关的部分主要是接收电路和主控电路。从硬件设计和主控电路软件算法两方面,对测距机虚警抑制方法进行了分析,其虚警抑制方法对其他激光测距系统具有普遍意义。  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