全文获取类型
收费全文 | 29212篇 |
免费 | 3621篇 |
国内免费 | 1685篇 |
专业分类
化学 | 4893篇 |
晶体学 | 68篇 |
力学 | 4784篇 |
综合类 | 231篇 |
数学 | 14711篇 |
物理学 | 9831篇 |
出版年
2024年 | 62篇 |
2023年 | 429篇 |
2022年 | 383篇 |
2021年 | 541篇 |
2020年 | 840篇 |
2019年 | 746篇 |
2018年 | 759篇 |
2017年 | 809篇 |
2016年 | 819篇 |
2015年 | 752篇 |
2014年 | 1339篇 |
2013年 | 2555篇 |
2012年 | 1379篇 |
2011年 | 1887篇 |
2010年 | 1537篇 |
2009年 | 1940篇 |
2008年 | 1888篇 |
2007年 | 1876篇 |
2006年 | 1672篇 |
2005年 | 1475篇 |
2004年 | 1346篇 |
2003年 | 1232篇 |
2002年 | 1065篇 |
2001年 | 918篇 |
2000年 | 864篇 |
1999年 | 745篇 |
1998年 | 681篇 |
1997年 | 631篇 |
1996年 | 483篇 |
1995年 | 377篇 |
1994年 | 318篇 |
1993年 | 293篇 |
1992年 | 233篇 |
1991年 | 216篇 |
1990年 | 170篇 |
1989年 | 149篇 |
1988年 | 119篇 |
1987年 | 108篇 |
1986年 | 70篇 |
1985年 | 100篇 |
1984年 | 128篇 |
1983年 | 67篇 |
1982年 | 100篇 |
1981年 | 84篇 |
1980年 | 69篇 |
1979年 | 68篇 |
1978年 | 60篇 |
1977年 | 39篇 |
1976年 | 30篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
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
92.
Quan Sen JIU Department of Mathematics Capital Normal University Beijing P.R.China Cheng HE Institute of Applied Mathematics Academy of Mathematics System Sciences Chinese Academy of Sciences Beijing P.R.China 《应用数学学报(英文版)》2004,(4)
In this paper we are interested in the sufficient conditions which guarantee the regularityof solutions of 3-D ideal magnetohydrodynamic equations in the arbitrary time interval [0,T].Fivesufficient conditions are given.Our results are motivated by two main ideas:one is to control theaccumulation of vorticity alone;the other is to generalize the corresponding geometric conditions of3-D Euler equations to 3-D ideal magnetohydrodynamic equations. 相似文献
93.
M. A. Efendiev J. Fuhrmann S. V. Zelik 《Mathematical Methods in the Applied Sciences》2004,27(8):907-930
For the Boussinesq approximation of the equations of coupled heat and fluid flow in a porous medium we show that the corresponding system of partial differential equations possesses a global attractor. We give lower and upper bounds of the Hausdorff dimension of the attractor depending on a physical parameter of the system, namely the Rayleigh number of the flow. Numerical experiments confirm the theoretical findings and raise new questions on the structure of the solutions of the system. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
94.
Continuous dependence on a modelling parameter is established for solutions of a problem for a complex Ginzburg–Landau equation. A homogenizing boundary condition is also used to discuss the continuous dependence results. We derive a priori estimates that indicate that solutions depend continuously on a parameter in the governing differential equation. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
95.
Parabolic Capacity and Soft Measures for Nonlinear Equations 总被引:2,自引:0,他引:2
We first introduce, using a functional approach, the notion of capacity related to the parabolic p-Laplace operator. Then we prove a decomposition theorem for measures (in space and time) that do not charge the sets of null capacity. We apply this result to prove existence and uniqueness of renormalized solutions for nonlinear parabolic initial boundary-value problems with such measures as right-hand side. 相似文献
96.
通过对非饱和土非线性本构方程和场方程的线性化,推导出了非饱和土的线性本构方程和场方程,把线性方程表示为与Biot饱和多孔介质方程相似的形式;证明了Darcy定律对非饱和土的适用性;说明了Biot饱和多孔介质方程是这些线性方程的特征。所有这些都表明用混合理论处理非饱和土本构问题的正确性。 相似文献
97.
Sufficient conditions are established for the asymptotic behavior of solutionsof nonlinear delay differential equations x′(t)+sum from i=1 to m(pi(t)x(t-т_i))=F(t,x_t),t≥0where 0<т_1<т_2<…<т_m≤r,pi∈C([0,∞)),i=1,2,…,m,F∈C([0,∞)×C_0,R).C_0=C([-r,0],R)equipped with the sup norm ||·|| forsome r>0. A new result is established, some known results are improved. 相似文献
98.
99.
Vincent Maurice 《Surface science》2005,581(1):88-104
STM, STS, LEED and XPS data for crystalline θ-Al2O3 and non-crystalline Al2O3 ultra-thin films grown on NiAl(0 0 1) at 1025 K and exposed to water vapour at low pressure (1 × 10−7-1 × 10−5 mbar) and room temperature are reported. Water dissociation is observed at low pressure. This reactivity is assigned to the presence of a high density of coordinatively unsaturated cationic sites at the surface of the oxide film. The hydroxyl/hydroxide groups cannot be directly identify by their XPS binding energy, which is interpreted as resulting from the high BE positions of the oxide anions (O1s signal at 532.5-532.8 eV). However the XPS intensities give evidence of an uptake of oxygen accompanied by an increase of the surface coverage by Al3+ cations, and a decrease of the concentration in metallic Al at the alloy interface. A value of ∼2 for the oxygen to aluminium ions surface concentration ratio indicates the formation of an oxy-hydroxide (AlOxOHy with x + y ∼ 2) hydroxylation product. STM and LEED show the amorphisation and roughening of the oxide film. At P(H2O) = 1 × 10−7 mbar, only the surface of the oxide film is modified, with formation of nodules of ∼2 nm lateral size covering homogeneously the surface. STS shows that essentially the valence band is modified with an increase of the density of states at the band edge. With increasing pressure, hydroxylation is amplified, leading to an increased coverage of the alloy by oxy-hydroxide products and to the formation of larger nodules (∼7 nm) of amorphous oxy-hydroxide. Roughening and loss of the nanostructure indicate a propagation of the reaction that modifies the bulk structure of the oxide film. Amorphisation can be reverted to crystallization by annealing under UHV at 1025 K when the surface of the oxide film has been modified, but not when the bulk structure has been modified. 相似文献
100.
Zhu Yongliang; Pagilla Prabhakar R. 《IMA Journal of Mathematical Control and Information》2005,22(2):181-186
We derive some useful and easily computable necessary conditionsfor the existence of a positive semi-definite solution to thealgebraic Ricatti equation (ARE). A motivating example is givento highlight the usefulness of the conditions for controllerand observer designs for nonlinear systems. Further, an upperbound on the trace of the solution to the ARE is also derived. 相似文献