全文获取类型
收费全文 | 44540篇 |
免费 | 4130篇 |
国内免费 | 2748篇 |
专业分类
化学 | 25455篇 |
晶体学 | 186篇 |
力学 | 1328篇 |
综合类 | 954篇 |
数学 | 9402篇 |
物理学 | 14093篇 |
出版年
2023年 | 425篇 |
2022年 | 723篇 |
2021年 | 1903篇 |
2020年 | 1117篇 |
2019年 | 1067篇 |
2018年 | 818篇 |
2017年 | 1053篇 |
2016年 | 1348篇 |
2015年 | 1303篇 |
2014年 | 1925篇 |
2013年 | 2820篇 |
2012年 | 2018篇 |
2011年 | 2204篇 |
2010年 | 2063篇 |
2009年 | 2553篇 |
2008年 | 2643篇 |
2007年 | 2795篇 |
2006年 | 2193篇 |
2005年 | 1511篇 |
2004年 | 1357篇 |
2003年 | 1399篇 |
2002年 | 3793篇 |
2001年 | 1472篇 |
2000年 | 1022篇 |
1999年 | 828篇 |
1998年 | 681篇 |
1997年 | 539篇 |
1996年 | 581篇 |
1995年 | 490篇 |
1994年 | 504篇 |
1993年 | 537篇 |
1992年 | 479篇 |
1991年 | 328篇 |
1990年 | 287篇 |
1989年 | 225篇 |
1988年 | 258篇 |
1987年 | 206篇 |
1986年 | 228篇 |
1985年 | 333篇 |
1984年 | 243篇 |
1983年 | 141篇 |
1982年 | 292篇 |
1981年 | 473篇 |
1980年 | 425篇 |
1979年 | 469篇 |
1978年 | 370篇 |
1977年 | 283篇 |
1976年 | 239篇 |
1974年 | 77篇 |
1973年 | 150篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
采用基于第一性原理的密度泛函理论研究了四角晶相二氧化铪(t-HfO2)体相及 其(001)表面的原子几何与电子结构.理论计算结果表明,t-HfO2(001)表面不会 产生重构现象.与体相电子结构相比, t-HfO2(001)表面态密度明显高于体相态 密度.其次,表面原子的态密度更靠近费米能级(EF),价带往低能量处移动,并 有表面态产生.计算结果表明了t-HfO2表面禁带宽度明显低于体相的禁带宽度. t-HfO2(001)的表面态产生以及表面禁带宽度减小是由于Hf原子与O原子的配位 数减少,表面原子周围的环境发生变化而引起的.
关键词:
密度泛函理论
2(001)')" href="#">t-HfO2(001)
表面电子结构 相似文献
33.
We employ a first-principles plane wave method with the
relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter
(HGH) scheme in the frame of DFT to calculate
the equilibrium lattice parameters and the thermodynamic properties of
AlB2 compound with hcp structure. The obtained lattice parameters are in
good agreement with the available experimental data and those calculated by
others. Through the quasi-harmonic Debye model, obtained successfully are
the dependences of the
normalized lattice parameters a/a0 and c/c0 on pressure P, the normalized
primitive cell volume V/V0 on pressure P, the variation of the thermal
expansion α with pressure P and temperature T, as well as the Debye
temperature \ThetaD and the heat capacity CV on pressure P and
temperature T. 相似文献
34.
Second-order random wave solutions for interfacial internal waves in N-layer density-stratified fluid 下载免费PDF全文
This paper studies the random internal wave equations describing the density interface displacements and the velocity potentials of N-layer stratified fluid contained between two rigid walls at the top and bottom. The density interface displacements and the velocity potentials were solved to the second-order by an expansion approach used by Longuet-Higgins (1963) and Dean (1979) in the study of random surface waves and by Song (2004) in the study of second- order random wave solutions for internal waves in a two-layer fluid. The obtained results indicate that the first-order solutions are a linear superposition of many wave components with different amplitudes, wave numbers and frequencies, and that the amplitudes of first-order wave components with the same wave numbers and frequencies between the adjacent density interfaces are modulated by each other. They also show that the second-order solutions consist of two parts: the first one is the first-order solutions, and the second one is the solutions of the second-order asymptotic equations, which describe the second-order nonlinear modification and the second-order wave-wave interactions not only among the wave components on same density interfaces but also among the wave components between the adjacent density interfaces. Both the first-order and second-order solutions depend on the density and depth of each layer. It is also deduced that the results of the present work include those derived by Song (2004) for second-order random wave solutions for internal waves in a two-layer fluid as a particular case. 相似文献
35.
High index of refraction via quantum interference in a three-level system of Er^3+-doped yttrium aluminium garnet crystal 下载免费PDF全文
A simple three-level system is proposed to produce high index of refraction with zero absorption in an Er^3+-doped yttrium aluminium garnet (YAG) crystal, which is achieved for a probe field between the excited state 4I13/2 and ground state 4I15/2 by adjusting a strong coherent driving field between the upper excited state 4I11/2 and 4I15/2. It is found that the changes of the frequency of the coherent driving field and the concentration of Er^3+ ions in the YAG crystal can maximize the index of refraction accompanied by vanishing absorption. This result could be useful for the dispersion compensation in fibre communication, laser particle acceleration, high precision magnetometry and so on. 相似文献
36.
A profit-maximizing supply chain network design model with demand choice flexibility 总被引:1,自引:0,他引:1
Zuo-Jun Max Shen 《Operations Research Letters》2006,34(6):673-682
We present a profit-maximizing supply chain design model in which a company has flexibility in determining which customers to serve. The company may lose a customer to competition if the price it charges is too high. We show the problem formulation and solution algorithm, and discuss computational results. 相似文献
37.
Formalization for problems of multicriteria decision making under uncertainty is constructed in terms of guaranteed and weak
estimates. A relevant definition of the vector maximinimax value is given. Parameterization and approximation of maximum,
minimax, and maximinimax values based on the inverse logical convolution are suggested. An application for multicommodity
networks is considered.
Received: December 13, 2000 / Accepted: August 21, 2001?Published online May 8, 2002 相似文献
38.
Recursive algorithms for the computation of standard deviation and average deviation are derived and their applications in data acquisition are discussed. The relative speeds and accuracies of the two algorithms are compared for synthetic data. The performance of recursive estimation under shot and proportional noise limitations is also described. As an example of the utility of these algorithms, absorbance data with constant confidence intervals are collected regardless of incident and transmitted intensities. The desired precision is specified prior to data acquisition and used to control signal-averaging of the data in real time. 相似文献
39.
40.
基于小波变换和数学形态学的运动物体检测 总被引:1,自引:0,他引:1
李晓亮 《南昌大学学报(理科版)》2006,30(6):624-626,630
提出了一种基于帧差法和小波变换相结合的运动目标检测方法,充分利用帧差法计算简单和小波变换的多尺度特性。实验表明,这种方法可以有效地从复杂自然场景的图像序列中检测出完整的运动目标。而且能够有效的抑制噪声。同时减少计算时间,满足检测的实时性要求。 相似文献