全文获取类型
收费全文 | 108572篇 |
免费 | 9843篇 |
国内免费 | 5245篇 |
专业分类
电工技术 | 7741篇 |
技术理论 | 17篇 |
综合类 | 12266篇 |
化学工业 | 9308篇 |
金属工艺 | 2900篇 |
机械仪表 | 5320篇 |
建筑科学 | 21975篇 |
矿业工程 | 5094篇 |
能源动力 | 3211篇 |
轻工业 | 8062篇 |
水利工程 | 5610篇 |
石油天然气 | 6662篇 |
武器工业 | 1319篇 |
无线电 | 5886篇 |
一般工业技术 | 7710篇 |
冶金工业 | 4653篇 |
原子能技术 | 782篇 |
自动化技术 | 15144篇 |
出版年
2024年 | 525篇 |
2023年 | 1580篇 |
2022年 | 2827篇 |
2021年 | 3425篇 |
2020年 | 3472篇 |
2019年 | 2818篇 |
2018年 | 2650篇 |
2017年 | 3043篇 |
2016年 | 3534篇 |
2015年 | 3802篇 |
2014年 | 7165篇 |
2013年 | 6892篇 |
2012年 | 7546篇 |
2011年 | 7985篇 |
2010年 | 6176篇 |
2009年 | 6580篇 |
2008年 | 5944篇 |
2007年 | 7373篇 |
2006年 | 6611篇 |
2005年 | 5768篇 |
2004年 | 4762篇 |
2003年 | 4127篇 |
2002年 | 3425篇 |
2001年 | 2917篇 |
2000年 | 2377篇 |
1999年 | 1925篇 |
1998年 | 1374篇 |
1997年 | 1241篇 |
1996年 | 1001篇 |
1995年 | 910篇 |
1994年 | 716篇 |
1993年 | 566篇 |
1992年 | 429篇 |
1991年 | 330篇 |
1990年 | 296篇 |
1989年 | 273篇 |
1988年 | 194篇 |
1987年 | 157篇 |
1986年 | 106篇 |
1985年 | 125篇 |
1984年 | 105篇 |
1983年 | 85篇 |
1982年 | 73篇 |
1981年 | 50篇 |
1980年 | 39篇 |
1979年 | 54篇 |
1978年 | 26篇 |
1977年 | 27篇 |
1976年 | 23篇 |
1975年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Fatigue crack propagation of multiple coplanar cracks with the coupled extended finite element/fast marching method 总被引:1,自引:0,他引:1
A numerical technique for modeling fatigue crack propagation of multiple coplanar cracks is presented. The proposed method couples the extended finite element method (X-FEM) [Int. J. Numer. Meth. Engng. 48 (11) (2000) 1549] to the fast marching method (FMM) [Level Set Methods & Fast Marching Methods: Evolving Interfaces in Computational Geometry, Fluid Mechanics, Computer Vision, and Materials Science, Cambridge University Press, Cambridge, UK, 1999]. The entire crack geometry, including one or more cracks, is represented by a single signed distance (level set) function. Merging of distinct cracks is handled naturally by the FMM with no collision detection or mesh reconstruction required. The FMM in conjunction with the Paris crack growth law is used to advance the crack front. In the X-FEM, a discontinuous function and the two-dimensional asymptotic crack-tip displacement fields are added to the finite element approximation to account for the crack using the notion of partition of unity [Comput. Meth. Appl. Mech. Engng. 139 (1996) 289]. This enables the domain to be modeled by a single fixed finite element mesh with no explicit meshing of the crack surfaces. In an earlier study [Engng. Fract. Mech. 70 (1) (2003) 29], the methodology, algorithm, and implementation for three-dimensional crack propagation of single cracks was introduced. In this paper, simulations for multiple planar cracks are presented, with crack merging and fatigue growth carried out without any user-intervention or remeshing. 相似文献
82.
83.
84.
85.
This review article is devoted to the materials containing niobium, which have been discovered or developed in the past few years and exhibit the potential application in heterogeneous catalysis. Niobium oxides and mixed oxides as well as sulfides, nitrides (oxynitrides), carbides (oxycarbides), and phosphates are considered. Among the catalytic processes in which Nb-containing materials were tested, liquid and gas phase oxidation is described in details, and the role of niobium in the prevention of the catalyst from SO2 poisoning is mentioned. 相似文献
86.
提出了用于水利项目方案评价的方法——基于相对最优方案的指标赋权模型,介绍了该方法的基本原理,建立了综合考虑保证出力、年发电量、单位千瓦投资等因素的基于相对最佳方案的指标赋权模型,并用实例加以说明,与模糊优选神经网络模型的评价结果相对比,研究表明该模型适合于水利项目方案的评价和优选。 相似文献
87.
88.
应用遗传算法设计和开发了针对爱立信GSM900系统的频率规划软件。在设计中,采用了交叉点存储、变异回退以及分割分步降维的方法,从而提高遗传算法的执行速度。 相似文献
89.
The problem of operating freeze drying of pharmaceutical products in vials placed in trays of a freeze dryer to remove free water (in frozen state) at a minimum time was formulated as an optimal control problem. Two different types of freeze dryer designs were considered. In type I freeze dryer design, upper and lower plate temperatures were controlled together, while in type II freeze dryer design, upper and lower plate temperatures were controlled independently. The heat input to the material being dried and the drying chamber pressure were considered as control variables. Constraints were placed on the system state variables by the melting and scorch temperatures during primary drying stage. Necessary conditions of optimality for the primary drying stage of freeze drying process in vials are derived and presented. Furthermore, an approach for constructing the optimal control policies that would minimize the drying time for the primary drying stage was given. In order to analyze optimal control policy for the primary drying stage of the freeze-drying process in vials, a rigorous multi-dimensional unsteady state mathematical model was used. The theoretical approach presented in this work was applied in the freeze drying of skim milk. Significant reductions in the drying times of primary drying stage of freeze drying process in vials were obtained, as compared to the drying times obtained from conventional operational policies. 相似文献
90.
随着印制电路板产业飞速发展,其引发的环境问题日益受到广泛的关注。然而长期以来如何有效地进行环境冲击评估是业内研究的重点和难点。而本文采用了一种智能的简化分析方法来进行全面的环境冲击评估,以提高评估的经济效益和准确度,并希望能为决策者提供一个可供参考的环境冲击评估的理念和方法。 相似文献