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   72474篇
  免费   8252篇
  国内免费   3649篇
化学   24789篇
晶体学   359篇
力学   5295篇
综合类   750篇
数学   12516篇
物理学   16631篇
无线电   24035篇
  2024年   190篇
  2023年   731篇
  2022年   1681篇
  2021年   1915篇
  2020年   2020篇
  2019年   1809篇
  2018年   1679篇
  2017年   2322篇
  2016年   2941篇
  2015年   2591篇
  2014年   3982篇
  2013年   5236篇
  2012年   4769篇
  2011年   4630篇
  2010年   3766篇
  2009年   4076篇
  2008年   4395篇
  2007年   4738篇
  2006年   4147篇
  2005年   3555篇
  2004年   3253篇
  2003年   2830篇
  2002年   2255篇
  2001年   1925篇
  2000年   1740篇
  1999年   1506篇
  1998年   1364篇
  1997年   1216篇
  1996年   1066篇
  1995年   987篇
  1994年   836篇
  1993年   639篇
  1992年   603篇
  1991年   540篇
  1990年   432篇
  1989年   306篇
  1988年   286篇
  1987年   192篇
  1986年   161篇
  1985年   208篇
  1984年   168篇
  1983年   88篇
  1982年   134篇
  1981年   99篇
  1980年   72篇
  1979年   97篇
  1978年   49篇
  1977年   33篇
  1976年   36篇
  1975年   15篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
环保意识调查问卷的Logistic模型   总被引:1,自引:0,他引:1  
本文应用多元统计分析中的方差分析 ,logistic模型等方法对环保意识调查问卷反馈的信息进行统计分析 ,建立被调查企业对环境保护意识的logistic模型 ,进而可对企业家的环境保护意识的好坏进行判断。  相似文献   
52.
This paper deals with positive solutions of degenerate and quasilinear parabolic systems not in divergence form: ut=up(Δu+av), vt=vq(Δv+bu), with null Dirichlet boundary conditions and positive initial conditions, where p, q, a and b are all positive constants. The local existence and uniqueness of classical solution are proved. Moreover, it will be proved that all solutions exist globally if and only if ab?λ12, where λ1 is the first eigenvalue of −Δ in Ω with homogeneous Dirichlet boundary condition.  相似文献   
53.
结合Liapunov泛函,研究了具连续时滞Lienard方程概周期解的存在唯一性和安全一致渐进稳定性。  相似文献   
54.
The conformational properties of polymers derived from isocyanodipeptides have been investigated with a combination of model calculations, X‐ray diffraction, and circular dichroism spectroscopy. Depending on the configuration of the side chains, defined arrays of hydrogen bonds along the polymeric backbone are formed. This leads to a well‐defined conformation as, for example, expressed in the formation of lyotropic liquid‐crystalline phases and increased helical stability. Upon the disruption of the hydrogen bonds by a strong acid, a less well‐defined macromolecular conformation is observed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1725–1736, 2003  相似文献   
55.
56.
Proton activation analysis was used for the determination of cadmium and lead in three sediment reference materials. The method is based on the111,112Cd(p, xn)111 In and the206,207,208Pb(p, xn)206Bi reactions.111In and206Bi were chemically separated by anion exchange. The results obtained were taken into account for the certification of the materials and are in the excellent agreement with the certified values.  相似文献   
57.
Twenty new cw FIR laser lines in CD3OH, optically pumped by a CO2 laser, are reported. The frequencies of 39 of the stronger laser lines were measured relative to stabilized CO2 lasers with a fractional uncertainty, as determined by the reproducibility of the FIR frequency itself, of 2 parts in 107.Contribution of the U.S. Government, not subject to copyright.  相似文献   
58.
In the framework of stochastic volatility models we examine estimators for the integrated volatility based on the pth power variation (i.e. the sum of pth absolute powers of the log‐returns). We derive consistency and distributional results for the estimators given high‐frequency data, especially taking into account what kind of process we may add to our model without affecting the estimate of the integrated volatility. This may on the one hand be interpreted as a possible flexibility in modelling, for example adding jumps or even leaving the framework of semimartingales by adding a fractional Brownian motion, or on the other hand as robustness against model misspecification. We will discuss possible choices of p under different model assumptions and irregularly spaced data. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
59.
The stress-strain state in the zone of load transfer in a uniaxially stretched specimen made of a unidirectional epoxy carbon-fiber-reinforced plastic (CFRP) is investigated. A parametric analysis of the influence of geometric and mechanical characteristics of the specimen on its stress-strain state is performed by means of finite-element modeling. The parameters allowing us to significantly reduce the dangerous concentration of transverse and tangential stresses are revealed. The mechanical tensile characteristics of a high-strength pultruded unidirectional CFRP are determined experimentally, and the size effect of its strength is estimated.  相似文献   
60.
We derived the WKB wave function for the general time-dependent quadratic Hamiltonian system using a unitary transformation method. We applied our research to sinusodially drived Caldirola–Kanai oscillator and confirmed that the time evolution of our approximated WKB wave function is similar to that of the exact one. This wave function can be used to analyze the interference between the probability amplitudes contributed by the area of overlap in phase space of quantum states.  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