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   199篇
  免费   6篇
化学   154篇
晶体学   3篇
力学   12篇
数学   14篇
物理学   22篇
  2023年   2篇
  2022年   10篇
  2021年   9篇
  2020年   7篇
  2019年   9篇
  2018年   8篇
  2017年   6篇
  2016年   11篇
  2015年   8篇
  2014年   5篇
  2013年   21篇
  2012年   13篇
  2011年   18篇
  2010年   11篇
  2009年   12篇
  2008年   10篇
  2007年   4篇
  2006年   4篇
  2005年   4篇
  2004年   6篇
  2003年   3篇
  2002年   2篇
  2000年   2篇
  1997年   2篇
  1996年   1篇
  1994年   2篇
  1991年   1篇
  1989年   2篇
  1987年   2篇
  1986年   1篇
  1985年   2篇
  1984年   1篇
  1983年   1篇
  1982年   1篇
  1976年   2篇
  1973年   1篇
  1965年   1篇
排序方式: 共有205条查询结果,搜索用时 15 毫秒
91.
A new aromatic diamine monomer, N-(4-(9H-carbazol-9-yl)phenyl)-3,5-diaminobenzamide, was successfully prepared in four steps using carbazole as starting material and polymerized with three aromatic tetracaboxylic acid dianhydrides via the conventional two-stage synthesis including the polyaddition and chemical cyclodehydration to produce a series of the aromatic polyimides. The polyimides were characterized by FT-IR, 1H NMR, and 13C NMR spectroscopy, differential scanning calorimetric (DSC) and thermo gravimetric analysis (TGA) analysis. The polyimides with inherent viscosities in the range of 0.38–0.46 dL/g showed excellent solubility in various solvents such as N-methyl-2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), pyridine and dioxane. DSC showed the glass transition temperatures (Tg) in the range of 277–288 °C. TGA showed that all polymers were stable, with 10% weights loss recorded above 524 °C in air atmosphere. Preliminary tests on films of the polyimides indicate that the materials are brittle.  相似文献   
92.
93.
In this paper, a new analytical method for vibration analysis of a cracked simply supported beam is investigated. By considering a nonlinear model for the fatigue crack, the governing equation of motion of the cracked beam is solved using perturbation method. The solution of the governing equation reveals the superhaxmonics of the fundamental frequency due to the nonlinear effects in the dynamic response of the cracked beam. Furthermore, considering such a solution, an explicit expression is also derived for the system damping changes due to the changes in the crack parameters, geometric dimensions and mechanical properties of the cracked beam. The results show that an increase in the crack severity and approaching the crack location to the middle of the beam increase the system damping. In order to validate the results, changes in the fundamental frequency ratios against the fatigue crack severities are compared with those of experimental results available in the literature. Also, a comparison is made between the free response of the cracked beam with a given crack depth and location obtained by the proposed analytical solution and that of the numerical method. The results of the proposed method agree with the experimental and numerical results.  相似文献   
94.
95.
96.
Summary Let X ∼ Np(μ,σ2Ip) and let s/σ2 ∼ χ n 2 , independent ofX, where μ and σ2 are unknown. This paper considers the estimation of μ (by δ) relative to a convex loss function given by (δ−μ)′[(1−α)Ip2+αQ](δ−μ)/[(1−α)p/σ2+α tr (Q)], whereQ is a knownp×p diagonal matrix and 0≦α≦1. Two classes of minimax estimators are obtained for μ whenp≦3; the first is a new result and the second is a generalization of a result of Strawderman (1973,Ann. Statist.,1, 1189–1194). A proper Bayes estimator is also obtained which is shown to satisfy the conditions of the second class of minimax estimators. The paper concludes by discussing the estimation of μ relative to another convex loss function. This work was supported by the Army, Navy and Air Force under Office of Naval Research Contract No. N00014-80-C-0093. Reproduction in whole or in part is permitted for any purpose of the United States Government.  相似文献   
97.
The critical properties of hexafluorobenzene were measured, and compared with the most reliable values reported in literature. The vapour pressure was determined from 226°C to the critical point and correlated by an equation of state.  相似文献   
98.
99.
100.
Transition Metal Chemistry - Reactions of (dppe)NiCl2 (dppe?=?1,2-bis(diphenylphosphino)ethane) with chlorido-substituted phenyl thiolates (RClS?) produced the corresponding...  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