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   7324篇
  免费   1115篇
  国内免费   797篇
化学   5104篇
晶体学   75篇
力学   411篇
综合类   42篇
数学   820篇
物理学   2784篇
  2024年   29篇
  2023年   145篇
  2022年   278篇
  2021年   268篇
  2020年   274篇
  2019年   308篇
  2018年   234篇
  2017年   235篇
  2016年   334篇
  2015年   370篇
  2014年   401篇
  2013年   480篇
  2012年   646篇
  2011年   670篇
  2010年   424篇
  2009年   369篇
  2008年   428篇
  2007年   401篇
  2006年   401篇
  2005年   313篇
  2004年   238篇
  2003年   218篇
  2002年   161篇
  2001年   159篇
  2000年   160篇
  1999年   184篇
  1998年   148篇
  1997年   136篇
  1996年   124篇
  1995年   118篇
  1994年   111篇
  1993年   79篇
  1992年   76篇
  1991年   85篇
  1990年   55篇
  1989年   34篇
  1988年   33篇
  1987年   30篇
  1986年   19篇
  1985年   19篇
  1984年   9篇
  1983年   20篇
  1982年   5篇
  1981年   2篇
  1980年   2篇
  1979年   1篇
  1977年   1篇
  1971年   1篇
排序方式: 共有9236条查询结果,搜索用时 0 毫秒
121.
利用脉冲火花预电离方式,设计了一种脉冲火花预电离触发的触发管气体开关。开关工作电压等级为100 kV,工作介质采用干燥空气,开关主间隙10 mm,电极材料采用304不锈钢,触发结构设计成盘环嵌套结构。实验结果表明:预电离能够显著减小低工作系数下触发管气体开关的触发时延和抖动。对于ns级快脉冲触发,预电离时刻越早,开关击穿时延和抖动越低。在30 kV/8 ns触发脉冲作用下,脉冲预电离触发的触发管开关在80%工作系数时,平均时延约为40 ns,抖动小于1 ns。  相似文献   
122.
基于MCNP程序对300#研究堆首炉堆芯进行精细建模,通过并行计算方式得到了实验临界棒位下堆芯的有效增殖因数为1.002 29,与临界值之间的相对误差为0.229%,验证了物理模型的正确性。探讨并解决了并行计算的中断与接续问题,提出了体通量计数与点探测器计数应用中的合理化建议,即对大体积空间计数时尽量使用体通量计数。计算值与实验值对比结果表明:两者在3 MW功率水平下热中子通量密度相差4.6%,符合得较好。  相似文献   
123.
We report on 980-nm InGaAs/GaAs lasers with dual-lobed far field based on a single-sided Bragg reflection waveguide (BRW). The high slope efficiency ~0.92 W/A and a continuous wave (CW) output power >1.5 W (3.2 W pulsed) have been obtained. The threshold current density is as low as 253 A/cm2 for a 1.5-mm-long device and the transparency current density is only 140 A/cm2. The further analysis shows the intrinsic reason for the single-lobed or the dual-lobed far-field distribution is determined by the mode shape in the cavity, not the single-sided or dual-sided BRW structure. The condition to achieve a narrow single-lobed far-field distribution is discussed.  相似文献   
124.
介绍了为开展漏孔检测、氘-氦分辨性能及真空检漏技术等相关实验研究而进行的四极质谱计分析与测量系统的研制,描述了真空室结构、抽气设备的选型、测量仪器的选择及控制保护子系统工作方式。对系统进行了包括极限真空、残余气体质谱分析、漏放气率、质谱计性能等测试,结果表明本系统达到了设计要求。  相似文献   
125.
Sun X  Gao H  Zeng B  Xu S  Liu W  Cheng Y  Xu Z  Mu G 《Optics letters》2012,37(5):857-859
Multiple filamentation has been observed when focusing a femtosecond laser pulse into a methanol solution with an axicon. It is found that multiple long filaments are located on the central spot and ring structures of the quasi-Bessel beam created by the axicon. Since the quasi-Bessel profile is determined by the axicon properties, the axicon has been suggested as a simple optics to control multiple filaments.  相似文献   
126.
In this paper, a new beamformer which combines the eigenspace-based minimum variance (ESBMV) beamformer with the Wiener postfilter is proposed for medical ultrasound imaging. The primary goal of this work is to further improve the medical ultrasound imaging quality on the basis of the ESBMV beamformer. In this method, we optimize the ESBMV weights with a Wiener postfilter. With the optimization of the Wiener postfilter, the output power of the new beamformer becomes closer to the actual signal power at the imaging point than the ESBMV beamformer. Different from the ordinary Wiener postfilter, the output signal and noise power needed in calculating the Wiener postfilter are estimated respectively by the orthogonal signal subspace and noise subspace constructed from the eigenstructure of the sample covariance matrix.We demonstrate the performance of the new beamformer when resolving point scatterers and cyst phantom using both simulated data and experimental data and compare it with the delay-and-sum (DAS), the minimum variance (MV) and the ESBMV beamformer. We use the full width at half maximum (FWHM) and the peak-side-lobe level (PSL) to quantify the performance of imaging resolution and the contrast ratio (CR) to quantify the performance of imaging contrast. The FWHM of the new beamformer is only 15%, 50% and 50% of those of the DAS, MV and ESBMV beamformer, while the PSL is 127.2 dB, 115 dB and 60 dB lower. What is more, an improvement of 239.8%, 232.5% and 32.9% in CR using simulated data and an improvement of 814%, 1410.7% and 86.7% in CR using experimental data are achieved compared to the DAS, MV and ESBMV beamformer respectively. In addition, the effect of the sound speed error is investigated by artificially overestimating the speed used in calculating the propagation delay and the results show that the new beamformer provides better robustness against the sound speed errors. Therefore, the proposed beamformer offers a better performance than the DAS, MV and ESBMV beamformer, showing its potential in medical ultrasound imaging.  相似文献   
127.
A new ternary intermetallic compound, Nd2Cu0.8Ge3, was synthesized and its crystal structure was determined by Rietveld refinement of X-ray powder diffraction data. The Nd2Cu0.8Ge3 compound crystallizes in space group I41/amd (No. 141), with a tetragonal a-ThSi2 structure type, and a=0.41783(2) nm, c=1.43689(9) nm, Z=2 and Dcalc=7.466 g/cm3. Using the high temperature powder X-ray diffraction (HTXRD) technique, the lattice thermal expansion behavior of the compound was investigated in the temperature range of 298–648 K, and the result shows that its unit-cell parameters increased anisotropically when temperature increased. The magnetic susceptibility measured in the temperature range of 5–300 K indicated antiferromagnetic order of Nd2Cu0.8Ge3 at low temperatures, and the magnetic susceptibility can be well described over the range of 50–300 K using Curie–Weiss law. The calculated effective magnetic moment (μeff) is 3.53 μB and dominated by the contribution of the Nd3+ ions.  相似文献   
128.
A simultaneous measurement method for the retardation and the fast axis angle of the eighth-wave plate in real time is proposed. The beam emitted from the laser passes through the circular polarizer and the eighth-wave plate to be measured successively, and then is split by the Dammann grating to form three sub-beams. They are analyzed by an analyzer array and detected by a detector array. Three detection signals are obtained simultaneously to calculate the retardation and the fast axis angle in real time. In experiments, a crystal quartz sample is measured at different fast axis angles. The average and standard deviation of its retardation respectively are 40.9°and 0.5°. The maximum measurement deviation of the fast axis angle is 2.1°. The usefulness of the method is verified.  相似文献   
129.
Structural Health Monitoring (SHM) based on fiber Bragg grating (FBG) sensor network has attracted considerable attention in recent years. However, FBG sensor network is embedded or glued in the structure simply with series or parallel. In this case, if optic fiber sensors or fiber nodes fail, the fiber sensors cannot be sensed behind the failure point. Therefore, for improving the survivability of the FBG-based sensor system in the SHM, it is necessary to build high reliability FBG sensor network for the SHM engineering application. In this study, a model reconstruction soft computing recognition algorithm based on genetic algorithm-support vector regression (GA-SVR) is proposed to achieve the reliability of the FBG-based sensor system. Furthermore, an 8-point FBG sensor system is experimented in an aircraft wing box. The external loading damage position prediction is an important subject for SHM system; as an example, different failure modes are selected to demonstrate the SHM system's survivability of the FBG-based sensor network. Simultaneously, the results are compared with the non-reconstruct model based on GA-SVR in each failure mode. Results show that the proposed model reconstruction algorithm based on GA-SVR can still keep the predicting precision when partial sensors failure in the SHM system; thus a highly reliable sensor network for the SHM system is facilitated without introducing extra component and noise.  相似文献   
130.
Digital image correlation (DIC) has received a widespread research and application in experimental mechanics. In DIC, the performance of subpixel registration algorithm (e.g., Newton-Raphson method, quasi-Newton method) relies heavily on the initial guess of deformation. In the case of small inter-frame deformation, the initial guess could be found by simple search scheme, the coarse-fine search for instance. While for large inter-frame deformation, it is difficult for simple search scheme to robustly estimate displacement parameters and deformation parameters simultaneously with low computational cost. In this paper, we proposed three improving strategies, i.e. Q-stage evolutionary strategy (T), parameter control strategy (C) and space expanding strategy (E), and then combined them into three population-based intelligent algorithms (PIAs), i.e. genetic algorithm (GA), differential evolution (DE) and particle swarm optimization (PSO), and finally derived eighteen different algorithms to calculate the initial guess for qN. The eighteen algorithms were compared in three sets of experiments including large rigid body translation, finite uniaxial strain and large rigid body rotation, and the results showed the effectiveness of proposed improving strategies. Among all compared algorithms, DE-TCE is the best which is robust, convenient and efficient for large inter-frame deformation measurement.  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