全文获取类型
收费全文 | 234946篇 |
免费 | 45040篇 |
国内免费 | 32720篇 |
专业分类
化学 | 157766篇 |
晶体学 | 2697篇 |
力学 | 15227篇 |
综合类 | 1135篇 |
数学 | 30258篇 |
物理学 | 105623篇 |
出版年
2024年 | 670篇 |
2023年 | 3714篇 |
2022年 | 5338篇 |
2021年 | 6105篇 |
2020年 | 7362篇 |
2019年 | 7295篇 |
2018年 | 6762篇 |
2017年 | 6723篇 |
2016年 | 9606篇 |
2015年 | 9492篇 |
2014年 | 11184篇 |
2013年 | 15748篇 |
2012年 | 18158篇 |
2011年 | 20031篇 |
2010年 | 18807篇 |
2009年 | 18571篇 |
2008年 | 13464篇 |
2007年 | 12057篇 |
2006年 | 11136篇 |
2005年 | 9889篇 |
2004年 | 8963篇 |
2003年 | 7232篇 |
2002年 | 6622篇 |
2001年 | 6438篇 |
2000年 | 5286篇 |
1999年 | 5778篇 |
1998年 | 4859篇 |
1997年 | 4536篇 |
1996年 | 4869篇 |
1995年 | 4841篇 |
1994年 | 4712篇 |
1993年 | 4388篇 |
1992年 | 3899篇 |
1991年 | 3345篇 |
1990年 | 2819篇 |
1989年 | 2659篇 |
1988年 | 2406篇 |
1987年 | 1670篇 |
1986年 | 1733篇 |
1985年 | 1307篇 |
1984年 | 1269篇 |
1983年 | 586篇 |
1982年 | 1085篇 |
1981年 | 855篇 |
1980年 | 848篇 |
1979年 | 557篇 |
1978年 | 553篇 |
1977年 | 650篇 |
1976年 | 1064篇 |
1972年 | 537篇 |
排序方式: 共有10000条查询结果,搜索用时 562 毫秒
971.
972.
Kj?rand Iversen Per Nilsson 《International Journal of Computers for Mathematical Learning》2007,12(2):113-133
This paper focuses on the different ways in which students in lower secondary school (14–16 year olds) experience compound
random events, presented to them in the form of combined junctions. A carefully designed ICT environment was developed enabling
the students to interact with different representations of such structures. Data for the analysis was gathered from two interview
sessions. The analysis of the interaction is based on constructivist principles on learning; i.e. we adopted a student-oriented
perspective, taking into consideration the different ways students try to make sense of chance encounters.
Our results show how some students give priority to geometrical and physical concerns, and we discuss how seeking causal explanations
of random phenomena may have encouraged this. With respect to numerically oriented models a division strategy appears to stand
out as the preferred one. 相似文献
973.
974.
本文用AES研究了P-InP/TiPdAu热处理前后的界面特性,结果表明:TiPd层对Au的内扩散和In的扩散有阻挡作用。以TiPdAu作InGaAsP/InP双异质结发光管的p面电极、镀Au作热沉,采用In焊料,研究了器件的可靠性问题,在室温大气气氛中;70℃存储,70~80℃带电老化,三种条件下长时间考核结果表明:器件的I-V特性正常,末见正向压降明显变化。还比较了Au-Zn材料作p面电极用TiPdAu作肖脱基势垒限制层制成的器件和用TiPdAu作电极材料制成的深Zn扩散型器件在老化过程中的特性变化,后二种结构的器件,在长期老化过程中,有源区中有大面积DSD生长和增殖。 相似文献
975.
976.
A two-dimensional model for the simulation of a binary dendritic growth with convection has been developed in order to investigate the effects of convection on dendritic morphologies. The model is based on a cellular automaton (CA) technique for the calculation of the evolution of solid/liquid (s/l) interface. The dynamics of the interface controlled by temperature, solute diffusion and Gibbs–Thomson effects, is coupled with the continuum model for energy, solute and momentum transfer with liquid convection. The solid fraction is calculated by a governing equation, instead of some approximate methods such as lever rule method [A. Jacot, M. Rappaz, Acta Mater. 50 (2002) 1909–1926.] or interface velocity method [L. Nastac, Acta Mater. 47 (1999) 4253; L. Beltran-Sanchez, D.M. Stefanescu, Mat. and Mat. Trans. A 26 (2003) 367.]. For the dendritic growth without convection, mesh independency of simulation results is achieved. The simulated steady-state tip velocity are compared with the predicted values of LGK theory [Lipton, M.E. Glicksmanm, W. Kurz, Metall. Trans. 18(A) (1987) 341.] as a function of melt undercooling, which shows good agreement. The growth of dendrite arms in a forced convection has been investigated. It was found that the dendritic growth in the upstream direction was amplified, due to larger solute gradient in the liquid ahead of the s/l interface caused by melt convection. In the isothermal environment, the calculated results under very fine mesh are in good agreement with the Oseen–Ivanstov solution for the concentration-driven growth in a forced flow. 相似文献
977.
D. Fong J. H. Hamilton A. V. Ramayya J. K. Hwang C. Goodin K. Li J. Kormicki J. O. Rasmussen Y. X. Luo S. C. Wu I. Y. Lee A. V. Daniel G. M. Ter-Akopian G. S. Popeko A. S. Fomichev A. M. Rodin Yu. Ts. Oganessian M. Jandel J. Kliman L. Krupa J. D. Cole M. A. Stoyer R. Donangelo W. C. Ma 《Physics of Atomic Nuclei》2006,69(7):1161-1167
The hot bimodal fission of 252Cf is reexamined with new high-statistics data. We constructed a γ-γ-γ coincidence cube for binary fission and LCP-gated γ-γ matrix for ternary fission. By identifying the secondary fission fragments from their γ-ray transitions, we measured the yields for various fission splits. The normal neutron yield distribution is found to be
Gaussian for Xe-Ru. However, the binary fission split of Ba-Mo is found to exhibit a bimodal neutron distribution with the
“hot mode” corresponding to ≈3.1% of the total yield. In α ternary fission, the first measurements of yields for specific fission splits are presented. The Te-α-Ru and Xe-α-Mo neutron yields fit well with a single mode, but the Ba-α-Zr split shows evidence for an enhanced hot mode with an intensity of ≈13.8% of the normal mode.
The text was submitted by the authors in English. 相似文献
978.
Jiaqiang Wang Hua Fang Ying Li Junjie Li Zhiying Yan 《Journal of molecular catalysis. A, Chemical》2006,250(1-2):75-79
Cobalt-doped mesoporous titania with a crystalline framework synthesized by surfactant templating method presented highly selective (99%) and reasonable conversion rate (49%) of catalytic oxidation of para-chlorotoluene to para-chlorobenzaldehyde in acetic acid using aqueous hydrogen peroxide as oxidant for the first time. Recycling of the catalyst indicates that the catalyst can be used a number of times without losing its activity to a greater extent. By contrast, cobalt-doped mesoporous titania without a crystalline structure and cobalt doped the commercial titania, Degussa P25 prepared by impregnation method with the similar concentration of cobalt were found inactive. The effects of catalyst concentration, reaction time, reaction temperature, and solvents on the performance of the catalyst were also investigated. 相似文献
979.
A functional equation of nonlinear iterates is discussed on the circle S1 for its continuous solutions and differentiable solutions. By lifting to R, the existence, uniqueness and stability of those solutions are obtained. Techniques of continuation are used to guarantee the preservation of continuity and differentiability in lifting. 相似文献
980.
魏利 《数学的实践与认识》2006,36(4):232-239
利用不动点定理,分别证明了在自反Banach空间中极大单调算子值域扰动的抽象结论和在H ilbert空间中m增生映射值域的扰动结果,这些结论是对以往一些工作的推广;然后,利用文中的新结论讨论了一类微分方程解的存在性. 相似文献