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   5644篇
  免费   604篇
  国内免费   555篇
化学   3043篇
晶体学   71篇
力学   343篇
综合类   53篇
数学   1428篇
物理学   1865篇
  2025年   23篇
  2024年   135篇
  2023年   180篇
  2022年   306篇
  2021年   268篇
  2020年   272篇
  2019年   175篇
  2018年   169篇
  2017年   208篇
  2016年   210篇
  2015年   235篇
  2014年   321篇
  2013年   410篇
  2012年   444篇
  2011年   444篇
  2010年   373篇
  2009年   350篇
  2008年   232篇
  2007年   253篇
  2006年   190篇
  2005年   144篇
  2004年   94篇
  2003年   80篇
  2002年   83篇
  2001年   95篇
  2000年   61篇
  1999年   70篇
  1998年   67篇
  1997年   36篇
  1996年   25篇
  1995年   25篇
  1994年   22篇
  1992年   22篇
  1991年   19篇
  1989年   18篇
  1988年   66篇
  1987年   106篇
  1986年   95篇
  1985年   18篇
  1973年   17篇
  1972年   36篇
  1971年   35篇
  1970年   23篇
  1969年   17篇
  1968年   25篇
  1967年   47篇
  1966年   27篇
  1965年   24篇
  1964年   21篇
  1963年   17篇
排序方式: 共有6803条查询结果,搜索用时 15 毫秒
1.
《Nuclear Physics A》2007,781(1-2):81-87
It has been recently suggested that one might drastically shorten the alpha lifetime of nuclear waste products, if these are embedded in metals at low temperatures. Using quantum mechanical tunneling arguments, we show that such an effect is likely to be very small, if present at all.  相似文献   
2.
The influence of swirl (flow rotation) on the stability of a rod in annular leakage flow is investigated. Under the assumption of laminar flow and plane vibrations (no whirling), it is shown that the swirl acts, in effect, as an elastic foundation with negative foundation stiffness, the magnitude being proportional to the mean circumferential flow rate squared. Consequently, swirl always lowers the critical axial flow speed in case of divergence instability of a rod of finite length. Numerical analysis is needed to predict the effect of swirl in case of flutter instability of a finite rod; this is not performed here. However, for the flutter-like instability of travelling waves in an infinite rod-channel system, it is shown analytically that swirl again always lowers the critical axial flow speed. Finally, it is found that by circumferential flow alone, the travelling waves are extinguished at a certain flow rate, followed by a divergence-like instability.  相似文献   
3.
The article presents an analytical model to simulate experimental imperfections in the realization of an X-ray absorption spectroscopy experiment, performed in transmission or fluorescence mode. Distinction is made between sources of systematic errors on a time-scale basis, to select the more appropriate model for their handling. For short time-scale, statistical models are the most suited. For large time-scale, the model is developed for sample and beam imperfections: mainly sample inhomogeneity, sample self-absorption, beam achromaticity. The ability of this model to reproduce the effects of these imperfections is exemplified, and the model is validated on real samples. Various potential application fields of the model are then presented.  相似文献   
4.
He XC 《Talanta》1991,38(3):319-323
The ion flotation of rhodium(III) and palladium(II) with some anionic surfactants has been investigated. Two flotation procedures are proposed for the separation of some platinum metals, based on differences in the kinetic properties of the chloro-complexes of rhodium(III), palladium(II) and platinum(IV). The first involves the selective flotation of Rh(H(2)O)(3+)(6) from PdCl(2-)(4) and PtCl(2-)(6) in dilute hydrochloric acid with sodium dodecylbenzenesulfonate (SDBS). After precipitation of the hydroxide and redissolution in dilute acid, the Rh(III) is converted into Rh(H(2)O)(3+)(6), Pd(II) and Pt(IV) remaining as PdCl(2-)(4) and PtCl(2-)(6) respectively, and separation is achieved by floating the Rh(H(2)O)(3+)(6) with SDBS. The second is for separation of Pd(II). Prior to flotation, the solution of PdCl(2-)(4) and PtCl(2-)(6) is heated with ammonium acetate to convert PdCl(2-)(4) into Pd(NH(3))(2+)(4). The chloro-complex of Pt(IV) is unaffected. The complex cation, Pd(NH(3))(2+)(4), is then selectively floated with SDBS. The procedures are fast, simple and do not require expensive reagents and apparatus.  相似文献   
5.
This paper investigates the influences of wind speed and of heat-convection coefficient on the temperature prediction of a slab. Numerical calculation of a slab temperature found that wind speed varies the slab temperature in a degree of 2–10 °C. More varying degrees occur at midday and in sunny day but less, at midnight and in a cloud day. These degrees also depend on the used heat-convection coefficients, which have different values in different models. Special emphases are paid to unearth the correlation between different heat convection coefficients and find the best alternative in the slab-temperature prediction.  相似文献   
6.
对12个通用的三效循环系统以及这12个通用循环系统组成的76种可能的密闭循环系统进行了评测。根据通用三效循环系统的压力和溶解度要求,对通用三效循环系统进行了分析,并充分考虑各部件设计中的问题和COPs数值,以及处于高腐蚀环境中部件的数目。通过分析,确定了四个性价比较优的系统:Alkitrate Topping循环系统,Pressure Staged Envelope循环系统,High Overlap循环系统和Dual Loop循环系统。  相似文献   
7.
The Al-Cu-TM (TM = transition metal) alloy system has attracted great attention for both excellent glass-forming ability and its interesting physical properties. In this work, an investigation into the crystal, electrical and elastic properties of the AlCu2TM (TM = Ti, Zr, and Hf) compounds has been conducted by first-principles calculations based on density-functional theory. The fully relaxed structure parameters of the AlCu2TM compounds are in good agreement with previous experimental and other theoretical results. Besides, the cohesive energies of all the AlCu2TM compounds have been evaluated. The energy band and densities of state of these compounds are also obtained. According to the calculated single crystal elastic constants, all the compounds are mechanically stable. The polycrystalline bulk moduli, shear moduli, Young’s moduli and Poisson’s ratio have been deduced by using Voigt-Reuss-Hill (VRH) approximations. The calculated negative Cauchy pressure and ratio of bulk modulus to shear modulus indicated that the AlCu2TM compounds are ductile materials. The Debye temperatures of the AlCu2TM compounds decrease with increasing the TM (Ti, Zr, and Hf) atomic number.  相似文献   
8.
建立了ICP-AES测定高浓度基体中微量杂质元素的偏最小二乘方法(PLS)。研究表明,PLS能有效校正高浓度基体干扰引起的测量误差,比多元光谱拟合法(MSF)能承受的基体浓度更高。当基体与杂质的含量比为1 000∶1~20 000∶1时,该方法的加标回收率在95%~105%之间。对于干扰效应与基体浓度呈非线性相关的体系,普通PLS的预测准确度不高,但使用基于样品浓度矩阵变换的偏最小二乘法(LIN-PPLS),则明显改善了预测的准确度。分别用MSF、普通PLS和LIN-PPLS对水系沉积物国家标准物质GBW07312中的Co,Pb和Ga进行测定,结果表明,LIN-PPLS的预测准确度优于普通PLS,而普通PLS的预测准确度优于MSF。  相似文献   
9.
结合物理实验的教学功能,介绍几款巧用二极管的创新性物理实验的设计与制作。  相似文献   
10.
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