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   2556篇
  免费   732篇
  国内免费   166篇
化学   524篇
晶体学   10篇
力学   524篇
综合类   115篇
数学   186篇
物理学   2095篇
  2024年   4篇
  2023年   14篇
  2022年   51篇
  2021年   56篇
  2020年   65篇
  2019年   51篇
  2018年   69篇
  2017年   79篇
  2016年   112篇
  2015年   93篇
  2014年   179篇
  2013年   180篇
  2012年   183篇
  2011年   208篇
  2010年   156篇
  2009年   182篇
  2008年   154篇
  2007年   186篇
  2006年   170篇
  2005年   163篇
  2004年   129篇
  2003年   136篇
  2002年   122篇
  2001年   110篇
  2000年   90篇
  1999年   74篇
  1998年   62篇
  1997年   49篇
  1996年   41篇
  1995年   41篇
  1994年   37篇
  1993年   33篇
  1992年   30篇
  1991年   30篇
  1990年   26篇
  1989年   18篇
  1988年   19篇
  1987年   7篇
  1986年   13篇
  1985年   9篇
  1984年   8篇
  1983年   4篇
  1982年   6篇
  1981年   2篇
  1980年   2篇
  1979年   1篇
排序方式: 共有3454条查询结果,搜索用时 15 毫秒
991.
Mesoscopic damped double resonance mutual capacitance coupled RLC circuit is quantized by the method of damped harmonic oscillator quantization. The Hamiltonian is diagonalized by unitary transformation. The eigenenergy spectra of this circuit are given. The quantum fluctuations of the charges and current of each loop are researched in excitation state of the squeezed vacuum state, the squeezed vacuum state and in vacuum state. It is show that, the quantum fluctuations of the charges and current are related to not only circuit inherent parameter and coupled magnitude, but also quantum number of excitation, squeezed coefficients, squeezed angle and damped resistance. And, because of damped resistance, the quantum fluctuation decay along with time. PACS numbers: 03.65.-w,42.50.Lc.  相似文献   
992.
The paper reports the results of an experimental study on pressure drop during horizontal flow boiling of refrigerants R22, R507, R404A, R134a, R407C and R410A. The test section is a smooth, horizontal, stainless steel tube (6 mm I.D., 6 m length) uniformly heated by Joule effect. The experimental tests are carried out at an almost constant evaporating pressure of 7.0 bar varying the mass flow rate in the range 280–1,080 kg/m2 s. The experimental comparison clearly shown that the pressure drop of R22 is significantly higher as compared to all the other fluids. The results are compared against well-known pressure drop prediction methods. The available correlations can be used for both pure fluids and mixtures with no corrective factors, provided the mixture properties are evaluated at local compositions. The Chawla friction correlation is the best-fitting of our experimental data in combination with the heterogeneous momentum pressure drop model on the basis of the Rouhani-Axelsson void fraction correlation.  相似文献   
993.
The quantum theory for mesoscopic electric circuit with charge discreteness is briefly described. The effect of scattering in mesoscopic ‘pure’ inductance design circuit, just like in the mesoscopic metallic rings has been address. The quantum characteristics of charge diffusion has also been obtained explicitly. The case in finite temperature has been discussed as well.  相似文献   
994.
介绍利用耦合腔链等效电路模型计算加速管场分布的原理和方法. 把加速腔链等效为谐振电路, 通过求解回路方程组, 并引入分流阻抗等物理参数, 可计算加速腔链整管的电磁场幅值和相位分布. 这一计算方法对加速管动力学设计和加速管几何参数设计有很好的指导意义, 并在16MeV返波加速管设计调试过程中得到了应用和验证.  相似文献   
995.
We propose a new model for the elementary act of electron transfer between two species in solution. The central idea is that the solution in the immediate vicinity of each species may be represented by an equivalent circuit consisting of a Debye circuit shunted by a resistor. Based on this insight, we derive a new formula for the one-dimensional potential energy profile of a coupled donor–acceptor pair at finite (but large) separation d, along a charge-fluctuation reaction co-ordinate, at fixed radii of the transition states. The corresponding reorganisation energy of the reaction is also derived, and it is found to differ from that in the Marcus theory. In particular, the new model predicts that the reorganisation energy is independent of the static dielectric constant of the solution, whereas the old model predicts a strong dependence. The difference is traced to the fact that the Marcus theory omits consideration of the work required to form the charge fluctuations and focuses instead on the work required to localise the charge fluctuations. In general, the equivalent circuit approach permits many of the difficult-to-derive equations of non-equilibrium polarisation theory to be written down by inspection.  相似文献   
996.
Poly‐p‐phenylenebenzobisoxazole (PBO) contains a fully conjugated rod‐like backbone entailing excellent optoelectronic properties and superior stabilities. Poly(2,3‐dihydrothieno‐1,4‐dioxin):polystyrenesulfonate (PEDOT:PSS) is a hole transferring medium, which was spun into a thin‐film between PBO and indium‐tin‐oxide to facilitate photovoltaic (PV) effect by forming a donor‐acceptor interlayer to separate and to transport photoinduced charges. Optimum PBO thickness for PV heterojunctions was about 71 nm at which the hole transferring PEDOT:PSS generated the maximum short circuit current (Isc) at a thickness of 115 nm. By using a layer of lithium fluoride (LiF) as an electron transferring layer adhering to Al cathode, the most open circuit voltage (Voc) and Isc were achieved with a LiF thickness of 1–2 nm because of possible electric dipole effect leading to an increase of Voc from 0.7 to 0.92 V and of Isc from about 0.1 to 0.2 μA. No PV response was observed for all PBO homojunctions because of insufficient exciton separation into electrons and holes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 988–993, 2007  相似文献   
997.
本文提出了一个旨在改善电容器放电平顶的简单电路。从基本电路的分析出发,推出了补偿后的输出电压、平顶峰值电压、峰值电压出现时间以及平顶时间同电压补偿系数k和放电时间常数比λ间关系式和曲线族。结果表明,如参数选择得当,在过补偿工作状态下,可以获得满意的平顶和较长的平顶持续时间。  相似文献   
998.
X射线皮秒变象管分幅相机研究   总被引:1,自引:1,他引:0  
本文介绍了我们提出的一种新的实现皮秒分幅摄影的技术方案,其优点是对控制线路要求不高,给出了用计算机对此方案进行数值模拟的结果和新设计的用于这种分幅方案的变象管特性,用直流X射线源测出该变象管的静态空间分辨率为20lp/mm。用激光打靶产生的脉冲X射线测量了相机动态特性,已得到了四幅动态像,每幅曝光时间150ps,空间分辨率3lp/mm。  相似文献   
999.
本文详细介绍了西安光机所正在研制中的一种变象管瞬时光谱测量系统,它由WDG30型光栅光谱头与JTG 305型变象管分幅/扫描摄影机组成。文中给出了系统的总体设计考虑、变象管及其控制和主要性能参数。  相似文献   
1000.
Knowledge of X-ray tube spectral distribution is necessary in theoretical methods of matrix correction, i.e. in both fundamental parameter (FP) methods and theoretical influence coefficient algorithms. Thus, the influence of X-ray tube distribution on the accuracy of the analysis of thin films and bulk samples is presented. The calculations are performed using experimental X-ray tube spectra taken from the literature and theoretical X-ray tube spectra evaluated by three different algorithms proposed by Pella et al. (X-Ray Spectrom. 14 (1985) 125–135), Ebel (X-Ray Spectrom. 28 (1999) 255–266), and Finkelshtein and Pavlova (X-Ray Spectrom. 28 (1999) 27–32). In this study, Fe–Cr–Ni system is selected as an example and the calculations are performed for X-ray tubes commonly applied in X-ray fluorescence analysis (XRF), i.e., Cr, Mo, Rh and W. The influence of X-ray tube spectra on FP analysis is evaluated when quantification is performed using various types of calibration samples. FP analysis of bulk samples is performed using pure-element bulk standards and multielement bulk standards similar to the analyzed material, whereas for FP analysis of thin films, the bulk and thin pure-element standards are used. For the evaluation of the influence of X-ray tube spectra on XRF analysis performed by theoretical influence coefficient methods, two algorithms for bulk samples are selected, i.e. Claisse–Quintin (Can. Spectrosc. 12 (1967) 129–134) and COLA algorithms (G.R. Lachance, Paper Presented at the International Conference on Industrial Inorganic Elemental Analysis, Metz, France, June 3, 1981) and two algorithms (constant and linear coefficients) for thin films recently proposed by Sitko (X-Ray Spectrom. 37 (2008) 265–272).  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