全文获取类型
收费全文 | 3984篇 |
免费 | 674篇 |
国内免费 | 139篇 |
专业分类
化学 | 2193篇 |
晶体学 | 3篇 |
力学 | 450篇 |
综合类 | 28篇 |
数学 | 504篇 |
物理学 | 1619篇 |
出版年
2024年 | 20篇 |
2023年 | 73篇 |
2022年 | 116篇 |
2021年 | 94篇 |
2020年 | 141篇 |
2019年 | 136篇 |
2018年 | 130篇 |
2017年 | 178篇 |
2016年 | 151篇 |
2015年 | 177篇 |
2014年 | 176篇 |
2013年 | 289篇 |
2012年 | 302篇 |
2011年 | 279篇 |
2010年 | 189篇 |
2009年 | 219篇 |
2008年 | 236篇 |
2007年 | 258篇 |
2006年 | 211篇 |
2005年 | 216篇 |
2004年 | 161篇 |
2003年 | 166篇 |
2002年 | 136篇 |
2001年 | 89篇 |
2000年 | 108篇 |
1999年 | 80篇 |
1998年 | 58篇 |
1997年 | 52篇 |
1996年 | 57篇 |
1995年 | 43篇 |
1994年 | 35篇 |
1993年 | 33篇 |
1992年 | 42篇 |
1991年 | 40篇 |
1990年 | 21篇 |
1989年 | 20篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1979年 | 2篇 |
1971年 | 2篇 |
1969年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有4797条查询结果,搜索用时 0 毫秒
11.
Yirang Yuan 《Numerical Methods for Partial Differential Equations》2003,19(5):665-681
For the coupled system of multilayer fluid dynamics in porous media, the modified characteristic finite difference fractional steps method applicable to parallel arithmetic is put forward and two‐dimensional and three‐dimensional schemes are used to form a complete set. Some techniques, such as calculus of variations, energy method, piecewise biquadratic interpolation, multiplicative commutation rule of difference operators, decomposition of high order difference operators and prior estimates are adopted. Optimal order estimates in L2 norm are derived to determine the error in the approximate solution. This method has already been applied to the numerical simulation of multilayer fluid dynamics in porous media. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 665–681, 2003. 相似文献
12.
A Tandem Queue with Coupled Processors: Computational Issues 总被引:2,自引:0,他引:2
In Resing and Örmeci [16] it is shown that the two-stage tandem queue with coupled processors can be solved using the theory of boundary value problems. In this paper we consider the issues that arise when calculating performance measures like the mean queue length and the fraction of time a station is empty. It is assumed that jobs arrive at the first station according to a Poisson process and require service at both stations before leaving the system. The amount of work that a job requires at each of the stations is an independent, exponentially distributed random variable. When both stations are nonempty, the total service capacity is shared among the stations according to fixed proportions. When one of the stations becomes empty, the total service capacity is given to the nonempty station. We study the two-dimensional Markov process representing the numbers of jobs at the two stations. The problem of finding the generating function of the stationary distribution can be reduced to two different Riemann-Hilbert boundary value problems, where both problems yield a complete analytical solution. We discuss the similarities and differences between the two problems, and relate them to the computational aspects of obtaining performance measures. 相似文献
13.
We consider a composite material composed of fibres included in a resin which becomes solid when it is heated up (reaction of reticulation). The mathematical modelling of the cure process is given by a kinetic equation describing the evolution of the reaction of reticulation coupled with the heat equation. In this paper, we are interested in the computation of approximate solutions. We propose a family of discretized problems depending on two parameters (β1, β2) ε [0, 1]2 which split the linear and non‐ linear terms in implicit and explicit parts. We prove the stability and convergence of the discretization for any (β1, β2) ε [½, 1 ] × [0, 1]. We present also some numerical results. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
14.
A quick sample preparation method was used for the determination of sulphur and trace metals in oil and oily products by the ICP with a minitorch. Stable aqueous emulsions of oils and oil products were made by using two kinds of emulsifiers (one with low sulphur content, the other sulfonated). Aqueous standard solutions can be used for the preparation of calibration solutions. The contents of S, Al, Cr, Cu, Fe, Mg, Ni and Pb in aqueous emulsions were determined and the results were in good agreement with the composition of oil standard solutions used for control analysis procedure. 相似文献
15.
Sun Shufen 《Acta Mechanica Sinica》1987,3(1):44-51
The coupled model has been developed in this study for moisture transport and beat exchange in soil. The model is driven by
atmospheric conditions measured in a field. The concept that soil has a surface resistance to evaporation is introduced in
order to relate the evaporation from surface with the vapour flux in air. With the use of this concept, the simulation results
agree quite well with the measured data. 相似文献
16.
A recursive method is developed for the solution of coupled algebraic Riccati equations and corresponding linear Nash strategies of weakly interconnected systems. It is shown that the given algorithm converges to the exact solution with the rate of convergence ofO(2), where is a small coupling parameter. In addition, only low-order systems are involved in algebrdic computations; the amount of computations required does not grow per iteration and no analyticity assumption is imposed on the system coefficients.This work was supported by Rutgers University Research Council under Grant No. 2-02188. 相似文献
17.
选择不同电离电位的基体元素K、Na、La、Y和Mg,比较研究了它们对分析元素谱线强度的影响,其影响程度与谱线激发电位及基体元素电离电位有定量的相关关系。碱金属基体(K、Na)和非碱金属基体(La、Y和Mg)对分析元素检出限、背景强度及其相对标准偏差的影响显著不同,后者的记忆效应也大于前者。La的电离电位与Na相差不大。但其基体效应与Y相似,表明墓体效应与基体元素的化学性质有关。 相似文献
18.
The precision of isotopic measurements of Pb by thermal ionization mass spectrometry (TIMS) is limited by the fact that this element does not possess an invariant isotope ratio that can be used for the correction of mass fractionation by internal normalization. Multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) can overcome this limitation, because with plasma ionization, elements with overlapping mass ranges are thought to display identical mass discrimination. With respect to Pb, this can be exploited by the addition of Tl to the sample solutions; the mass discrimination factor obtained for Tl can then be used for the correction of the measured Pb isotope ratios. In this article we present the results of a detailed study that investigates the accuracy and precision of such an external correction technique for mass discrimination based upon the results of multiple analyses of a mixed standard solution of NIST SRM-981 Pb and SRM-997 Tl. Our data indicate that normalization of the Pb isotope ratios to the certified isotopic composition of SRM-997 Tl produces Pb isotopic results that are significantly lower than recently published reference values by TIMS. This systematic offset can be eliminated by renormalization of the Pb data to a different Tl isotopic composition to obtain an empirically determined mass discrimination factor for Pb that generates accurate results. It is furthermore shown that a linear law is least suited for the correction of mass discrimination, whereas a power or exponential law function provide significantly more accurate and precise results. In detail, it appears that a power law may provide the most appropriate correction procedure, because the corrected Pb isotope ratios display less residual correlations with mass discrimination compared to the exponentially corrected data. Using an exponential or power law correction our results, obtained over a period of over seven months, display a precision (2σ) of better than 60 parts per million (ppm) for 208Pb/206Pb and 207Pb/206Pb and of better than 350 ppm for 206Pb/204Pb, 207Pb/204Pb/204Pb, and 208Pb/204Pb. This represents a significant improvement compared to conventional TIMS techniques and demonstrates the potential of MC-ICPMS for routine, high-precision measurements of Pb isotopic compositions. 相似文献
19.
20.