全文获取类型
收费全文 | 126724篇 |
免费 | 7510篇 |
国内免费 | 3673篇 |
专业分类
电工技术 | 6328篇 |
技术理论 | 7篇 |
综合类 | 11092篇 |
化学工业 | 9281篇 |
金属工艺 | 5281篇 |
机械仪表 | 14172篇 |
建筑科学 | 28546篇 |
矿业工程 | 3910篇 |
能源动力 | 3004篇 |
轻工业 | 5131篇 |
水利工程 | 3626篇 |
石油天然气 | 4117篇 |
武器工业 | 1444篇 |
无线电 | 7834篇 |
一般工业技术 | 13264篇 |
冶金工业 | 3006篇 |
原子能技术 | 672篇 |
自动化技术 | 17192篇 |
出版年
2024年 | 461篇 |
2023年 | 1253篇 |
2022年 | 2058篇 |
2021年 | 2660篇 |
2020年 | 2966篇 |
2019年 | 2270篇 |
2018年 | 2222篇 |
2017年 | 2805篇 |
2016年 | 3262篇 |
2015年 | 3514篇 |
2014年 | 9122篇 |
2013年 | 7191篇 |
2012年 | 8933篇 |
2011年 | 9591篇 |
2010年 | 8152篇 |
2009年 | 8436篇 |
2008年 | 7554篇 |
2007年 | 9075篇 |
2006年 | 7578篇 |
2005年 | 6691篇 |
2004年 | 5388篇 |
2003年 | 5076篇 |
2002年 | 4042篇 |
2001年 | 3406篇 |
2000年 | 2786篇 |
1999年 | 2181篇 |
1998年 | 1798篇 |
1997年 | 1515篇 |
1996年 | 1270篇 |
1995年 | 994篇 |
1994年 | 817篇 |
1993年 | 565篇 |
1992年 | 443篇 |
1991年 | 377篇 |
1990年 | 236篇 |
1989年 | 239篇 |
1988年 | 184篇 |
1987年 | 138篇 |
1986年 | 83篇 |
1985年 | 93篇 |
1984年 | 122篇 |
1983年 | 81篇 |
1982年 | 92篇 |
1981年 | 31篇 |
1980年 | 35篇 |
1979年 | 27篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1959年 | 11篇 |
1951年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
M. J. O’Sullivan C. G. Walker M. L. O’Sullivan T. D. Thompson A. B. Philpott 《Telecommunication Systems》2006,33(4):353-376
The problem of designing fibre-optic networks for local-access telecommunications generates (at least) three non-trivial subproblems.
In the first of these subproblems one must determine how many fibre-optic cables (fibres) are required at either end of a
street. In the next subproblem a minimum-cost network must be designed to support the fibres. The network must also provide
distinct paths from either end of the street to the central exchange(s). Finally, the fibre-optic cables must be placed in
protective covers. These covers are available in a number of different sizes, allowing some flexibility when covering each
section of the network. In this paper we describe a dynamic programming (DP) formulation for finding a minimum-cost (protective)
covering for the network (the third of the subproblems). This problem is a generalised set covering problem with side constraints
and is further complicated by the introduction of fixed and variable welding costs. The DP formulation selects covers along
each arc (in the network), but cannot exactly model the fixed costs and so does not guarantee optimality. We also describe
an integer programming (IP) formulation for assessing the quality of the DP solutions. The cost of the networks constructed
by the IP model is less than those designed using the DP model, but the saving is not significant for the problems examined
(less than 0.1%). This indicates that the DP model will generally give very good solutions. Furthermore, as the problem dimensions
grow, DP gives significantly better solution times than IP. 相似文献
62.
63.
64.
织金电视台通过对有线电视网络进行双向改造,为Internet宽带接入、数字电视、VOD等多媒体应用提供了运行平台,介绍织金县城区HFC双向网络设计方案。 相似文献
65.
The multihop optical network is the most appropriate solution to satisfy the increasing applications of Internet services. This paper extends the regular Kautz graph to one with multiple layers in order to produce more architectural variations. The connectivity between adjacent layers utilizes the systematic connection patterns of a regular Kautz graph. A routing algorithm based on its property is presented. Optical passive star (OPS) couplers are adopted to implement our new topologies. Three scheduling criteria that can solve the contention problem in the intermediate nodes are evaluated and compared in terms of their capability to improve the accessibility. 相似文献
66.
介绍了柳钢新建120万t/a链篦机-回转窑氧化球团工程的主要设计特点。该工程是我国第一条自行设计、设备国产化和采用清洁气体能源的链篦机-回转窑生产线。它的顺利投产,标志着我国炼铁炉料生产技术有了一个新的进步。 相似文献
67.
68.
大型往复式压缩机功率大、占地面积大,管道振动问题也更为突出。笔者结合实践经验,对大型往复式压缩机的配管设计进行探讨,说明了平面布置与管道防振设计的密切关系,并介绍了管道防振设计的方法和步骤,以及相对简单的复杂管系气柱固有频率的转移矩阵计算方法。 相似文献
69.
An approach is presented to satisfy the demand for simple criteria, guidelines and models for the preliminary sizing of horizontal subsurface flow (SF) constructed wetland systems. This approach eliminates time-consuming calculations and iterations by providing graphical solutions for wetland system sizing. Therefore, it can be used for the preliminary assessment of new or performance evaluation of existing subsurface flow constructed wetland systems. The validity of this methodology is checked with data from existing systems and is found to be quite satisfactory. This methodology is combined with simple equations predicting the maximum wetland capacity in summer, so as to assist designers in sizing installations in tourist areas with increased summer populations. Furthermore, based on this methodology, a sensitivity analysis is performed of the area requirements for wastewaters of various strengths and various design conditions and performance criteria. The results provide a useful overview to engineers and further simplify the design methodology of new subsurface flow constructed wetland systems. 相似文献
70.
石油化工可燃气体或蒸气排放系统的消防安全设计 总被引:1,自引:1,他引:0
石油化工生产装置大多是通过火炬燃烧排放易燃易爆气体或通过放空管将易爆气体直接排入大气中。文章分析了火炬系统和放空管的火灾危险因素,并从火炬的布置、高度、排放能力、自动监控装置以及放空管的气体排放浓度、安装位置、安全装置等方面,重点论述了火炬系统和放空管的消防安全设计要点。 相似文献