全文获取类型
收费全文 | 31323篇 |
免费 | 3575篇 |
国内免费 | 2069篇 |
专业分类
化学 | 3400篇 |
晶体学 | 31篇 |
力学 | 4271篇 |
综合类 | 328篇 |
数学 | 19303篇 |
物理学 | 9634篇 |
出版年
2024年 | 70篇 |
2023年 | 367篇 |
2022年 | 408篇 |
2021年 | 519篇 |
2020年 | 939篇 |
2019年 | 921篇 |
2018年 | 843篇 |
2017年 | 811篇 |
2016年 | 882篇 |
2015年 | 748篇 |
2014年 | 1376篇 |
2013年 | 2652篇 |
2012年 | 1469篇 |
2011年 | 1914篇 |
2010年 | 1795篇 |
2009年 | 2035篇 |
2008年 | 2122篇 |
2007年 | 2095篇 |
2006年 | 1746篇 |
2005年 | 1719篇 |
2004年 | 1437篇 |
2003年 | 1391篇 |
2002年 | 1207篇 |
2001年 | 932篇 |
2000年 | 875篇 |
1999年 | 821篇 |
1998年 | 800篇 |
1997年 | 653篇 |
1996年 | 522篇 |
1995年 | 422篇 |
1994年 | 367篇 |
1993年 | 247篇 |
1992年 | 241篇 |
1991年 | 234篇 |
1990年 | 201篇 |
1989年 | 112篇 |
1988年 | 133篇 |
1987年 | 125篇 |
1986年 | 109篇 |
1985年 | 107篇 |
1984年 | 98篇 |
1983年 | 46篇 |
1982年 | 97篇 |
1981年 | 79篇 |
1980年 | 54篇 |
1979年 | 57篇 |
1978年 | 39篇 |
1977年 | 31篇 |
1976年 | 24篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
111.
Heterogeneity is typically the result of space variability of soil parameters at different scales. Soil anisotropy may be defined as the spatial persistence in some direction only, across coarse-grid elements, of heterogeneous structures with different characteristic lengths in different directions. One can account for the effect of these structures by upscaling soil properties. Analyzing flow in a strongly anisotropic structured soil at different scales evidences how transverse dispersion reduces to a subscale process, leading to mixing within the conductive structures. 相似文献
112.
113.
L. A. Utracki 《Journal of Polymer Science.Polymer Physics》2004,42(15):2909-2915
The Simha and Somcynsky (S–S) statistical thermodynamics theory was used to compute the solubility parameters as a function of temperature and pressure [δ = δ(T, P)], for a series of polymer melts. The characteristic scaling parameters required for this task, P*, T*, and V*, were extracted from the pressure–temperature–volume (PVT) data. To determine the potential polymer–polymer miscibility, the dependence of δ versus T (at ambient pressure) was computed for 17 polymers. Close proximity of the δ versus T curves for four miscible polymer pairs: PPE/PS, PS/PVME, and PC/PMMA signaled the usefulness of this approach. It is noteworthy, that the tabulated solubility parameters (derived from the solution data under ambient conditions) propounded the immiscibility of the PVC/PVAc pair. The computed values of δ also suggested miscibility for polymer pairs of unknown miscibility, namely PPE/PVC, PPE/PVAc, and PET/PSF. In recognizing the limitations of the solubility parameter approach (the omission of several thermodynamic contributions), these preliminary results are auspicious because they indicate a new route for estimating the miscibility of any polymeric material at a given temperature and pressure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2909–2915, 2004 相似文献
114.
115.
裂缝检测是目前国内外石油勘探界研究的一个热点问题,如何确定裂缝方位等参数是石油公司面临的难题,而解决该难题就要确定裂缝方位等参数与地震波场传播之间的定量关系.但是目前所采用的裂缝性地层介质模型不能完全定量地反映裂缝的方位特征和衰减特征.针对该问题,建立了具有任意裂缝方位的裂缝性地层介质模型;并构造了时间增量的方法,将非线性的卷积积分采用近似的方法实现,建立了以位移场表示的具有任意方位角的黏弹性方位各向异性介质的波动方程.该波动方程定量地给出了黏弹性波场特征与裂缝走向的关系,描述了黏弹性地震波在这种介质中的
关键词:
裂缝
各向异性
黏弹性
波动方程 相似文献
116.
对于超细长弹性杆静力学的Kirchhoff方程,用动力学的概念和方法研究其常值特解 和稳定性问题.计算了Kirchhoff方程相对固定坐标系、截面主轴坐标系以及中心线Frenet 坐标系的常值特解,进行了Kirchhoff动力学比拟,用一次近似理论分别讨论了它们的Lyapu nov稳定性,导出了若干稳定性判据,并在参数平面上绘出了稳定域.
关键词:
超细长弹性杆
Kirchhoff方程
常值特解
Lyapunov稳定性 相似文献
117.
118.
Wolfgang Knirsch 《Mathematische Nachrichten》2004,265(1):15-23
On weighted spaces with strictly plurisubharmonic weightfunctions the canonical solution operator of and the ‐Neumann operator are bounded. In this paper we find a class of strictly plurisubharmonic weightfunctions with certain growth conditions, so that they are Hilbert‐Schmidt operators between weighted spaces with different weightfunctions, if they are restricted to forms with holomorphic coefficients. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
119.
The control of piecewise-deterministic processes is studied where only local boundedness of the data is assumed. Moreover the discount rate may be zero. The value function is shown to be solution to the Bellman equation in a weak sense; however the solution concept is strong enough to generate optimal policies. Continuity and compactness conditions are given for the existence of nonrelaxed optimal feedback controls. 相似文献
120.
In 1983, a preconditioner was proposed [J. Comput. Phys. 49 (1983) 443] based on the Laplace operator for solving the discrete Helmholtz equation efficiently with CGNR. The preconditioner is especially effective for low wavenumber cases where the linear system is slightly indefinite. Laird [Preconditioned iterative solution of the 2D Helmholtz equation, First Year's Report, St. Hugh's College, Oxford, 2001] proposed a preconditioner where an extra term is added to the Laplace operator. This term is similar to the zeroth order term in the Helmholtz equation but with reversed sign. In this paper, both approaches are further generalized to a new class of preconditioners, the so-called “shifted Laplace” preconditioners of the form Δφ−k2φ with
. Numerical experiments for various wavenumbers indicate the effectiveness of the preconditioner. The preconditioner is evaluated in combination with GMRES, Bi-CGSTAB, and CGNR. 相似文献