全文获取类型
收费全文 | 430528篇 |
免费 | 19568篇 |
国内免费 | 12754篇 |
专业分类
化学 | 235755篇 |
晶体学 | 6445篇 |
力学 | 21287篇 |
综合类 | 673篇 |
数学 | 50871篇 |
物理学 | 147819篇 |
出版年
2022年 | 5957篇 |
2021年 | 7038篇 |
2020年 | 7452篇 |
2019年 | 7664篇 |
2018年 | 8867篇 |
2017年 | 8740篇 |
2016年 | 12307篇 |
2015年 | 9010篇 |
2014年 | 12976篇 |
2013年 | 23508篇 |
2012年 | 21393篇 |
2011年 | 24007篇 |
2010年 | 17435篇 |
2009年 | 17091篇 |
2008年 | 19688篇 |
2007年 | 18939篇 |
2006年 | 17523篇 |
2005年 | 15010篇 |
2004年 | 13349篇 |
2003年 | 11491篇 |
2002年 | 11214篇 |
2001年 | 12463篇 |
2000年 | 9498篇 |
1999年 | 7950篇 |
1998年 | 6650篇 |
1997年 | 6319篇 |
1996年 | 5997篇 |
1995年 | 5204篇 |
1994年 | 5032篇 |
1993年 | 4683篇 |
1992年 | 4978篇 |
1991年 | 5061篇 |
1990年 | 4733篇 |
1989年 | 4500篇 |
1988年 | 4148篇 |
1987年 | 4245篇 |
1986年 | 4010篇 |
1985年 | 4891篇 |
1984年 | 4857篇 |
1983年 | 4033篇 |
1982年 | 4090篇 |
1981年 | 3773篇 |
1980年 | 3701篇 |
1979年 | 3936篇 |
1978年 | 3921篇 |
1977年 | 3921篇 |
1976年 | 3895篇 |
1975年 | 3694篇 |
1974年 | 3621篇 |
1973年 | 3660篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
1.
2.
3.
提出用溶胶粒子表面修饰方法,结合溶胶凝胶技术制备无机催化膜.该方法的基本原理是利用合适的金属配合物在胶粒表面的吸附作用,经溶胶凝胶过程,将活性组分结合到无机膜中.实验测定结果表明:(NiEDTA)2-,VO-3,MoO2-4,(Pd(NH3)4)2+,PdCl2-4,PtCl2-6和RhCl3-6可用来修饰AlOOH溶胶.以Pd/γAl2O3催化膜的制备为例,经三次溶胶凝胶过程,可制得无裂缺的厚度为9μm的Pd/γAl2O3催化膜,膜材料的平均孔直径为6nm,Pd被均匀地分布在膜的顶层,其平均粒径为23nm. 相似文献
4.
本文是文[1-7]的继续,研究变权综合问题,从确定变权的经验公式入手引出了变权原理,给出了变权的公理化定义,讨论了与之有关的均衡函数及其梯度向量。 相似文献
5.
We consider the growth of a spherical crystal in a supersaturatedsolution. In the first part, existence and uniqueness resultsfor radially symmetric growth are obtained, provided that thesupersaturation is not too large; conversely, when the far-fieldsupersaturation exceeds a critical value, it is shown that theradially symmetric solution ceases to exist in finite time.In the second part, we examine the linear stability of a radiallysymmetric similarity solution (in which the radius grows ast?) to shape perturbations. The results are compared with previousquasi-static analyses, and, in particular, the critical radiusat which the crystal becomes unstable is found to be largerfor small supersaturations, but smaller for large supersaturations,than those predicted by the quasi-static analysis 相似文献
6.
7.
Zi-Cai Li 《Numerical Methods for Partial Differential Equations》1989,5(4):279-295
The nonconforming combination of Ritz-Galerkin and finite difference methods is presented for solving elliptic boundary value problems with singularities. The Ritz-Galerkin method is used in the subdomains including singularities, the finite difference method is used in the rest of the solution domain. Moreover, on the common boundary of two regions where two different methods are used, the continuity conditions are constrained only on the nodes of difference grids. Theoretical analysis and numerical experiments have shown that average errors of numerical solutions and their generalized derivatives can reach the convergence rate O(h2-δ), where h is the mesh spacing of uniform difference grids, and δ is an arbitrarily small, positive number. This convergence rate is better than O(h), obtained by the nonconforming combination of the Ritz-Galerkin and finite element methods. 相似文献
8.
9.
10.
In developing travel demand models it is generally assumed that the base-year data used in developing the parameters, as well as the forecasted data to be used as independent variables for the design year, are of acceptable quality. The purpose of this paper is to present the application of error propagation theory in assesing the predictive quality of one type of travel demand forecasting model (multinomial logit models) and to demonstrate how error considerations can be used as a tool for identifying the optimal model. The general conclusions of this study are that: (1) it is indeed possible to quantify errors in dependent variables in logit models as a consequence of errors in independent variables; and (2) error consideration can be used as a tool for identifying the optimal model from a set of candidate models. Further research is recommended to develop better insights into the phenomenon of error propagation so that the consideration of errors can be a factor in decisions on model selection. 相似文献