全文获取类型
收费全文 | 69525篇 |
免费 | 7602篇 |
国内免费 | 5090篇 |
专业分类
电工技术 | 6708篇 |
技术理论 | 3篇 |
综合类 | 6159篇 |
化学工业 | 4553篇 |
金属工艺 | 2843篇 |
机械仪表 | 5282篇 |
建筑科学 | 12274篇 |
矿业工程 | 2475篇 |
能源动力 | 2003篇 |
轻工业 | 2969篇 |
水利工程 | 3542篇 |
石油天然气 | 3962篇 |
武器工业 | 1424篇 |
无线电 | 5675篇 |
一般工业技术 | 6398篇 |
冶金工业 | 2322篇 |
原子能技术 | 635篇 |
自动化技术 | 12990篇 |
出版年
2024年 | 308篇 |
2023年 | 915篇 |
2022年 | 1824篇 |
2021年 | 2137篇 |
2020年 | 2559篇 |
2019年 | 2014篇 |
2018年 | 1935篇 |
2017年 | 2559篇 |
2016年 | 2791篇 |
2015年 | 2995篇 |
2014年 | 4613篇 |
2013年 | 4735篇 |
2012年 | 5438篇 |
2011年 | 5544篇 |
2010年 | 4207篇 |
2009年 | 4306篇 |
2008年 | 3991篇 |
2007年 | 4635篇 |
2006年 | 4156篇 |
2005年 | 3626篇 |
2004年 | 2980篇 |
2003年 | 2421篇 |
2002年 | 2062篇 |
2001年 | 1740篇 |
2000年 | 1388篇 |
1999年 | 1154篇 |
1998年 | 931篇 |
1997年 | 720篇 |
1996年 | 694篇 |
1995年 | 577篇 |
1994年 | 440篇 |
1993年 | 311篇 |
1992年 | 282篇 |
1991年 | 176篇 |
1990年 | 166篇 |
1989年 | 128篇 |
1988年 | 77篇 |
1987年 | 49篇 |
1986年 | 53篇 |
1985年 | 34篇 |
1984年 | 34篇 |
1981年 | 24篇 |
1980年 | 67篇 |
1979年 | 49篇 |
1965年 | 26篇 |
1964年 | 34篇 |
1960年 | 23篇 |
1959年 | 32篇 |
1956年 | 29篇 |
1955年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
The even-flow harvest scheduling problem arises when the forestry agency has evolved into a rigid non-declining even-flow policy. In this paper, we investigate model formulation and solution strategies for the even-flow harvest scheduling problem. A multiple-objective linear programming problem is formulated for even-flow harvest scheduling problems with multiple-site classes and multiple periods. The aim of this problem is to simultaneously maximize a desired harvest-volume per hectare for each period of planning horizon and the total economic return. A block diagonal constraint structure, with many sets of network sub-problems and a set of coupling constraints, is identified in this linear programming problem. A longest path method for each of network sub-problems and a primal-dual steepest-edge algorithm for the entire problem are developed. The developed algorithm has been coded in Borland C++ and implemented on a personal computer. An illustrative example is used to display the detailed procedure for the developed algorithm and a real-world case study is used to show the trade-off between desired even-flow harvest volume policy and total economic return. Results show the potential benefits of this approach. 相似文献
32.
非自喷井解释方法一直是困扰试井界的难题之一。自1995年底应用DKS2.0软件中DST压恢分析法先后处理了大港油田200多层实测资料以来,发现一部分井与常规分析法的处理结果完全不同。这些井多出现在均质无限大油藏,且其压力及压力导数曲线开口小、导数曲线驼峰较小。通过对DST压恢分析法在大港油田应用情况的分析,认为此方法适用于均质非自喷井,尤其是处理低渗且地层受污染井的结果比较符合地层的实际地质情况,其成果的应用对地层改造有一定的指导作用。 相似文献
33.
介绍85W/90卡车手动变速箱油的组分和添加剂,提出符合85W/90粘度级别的基础油,讨论影响85W/90卡车手动变速箱油性能的各种因素,给出符合85W/90卡车手动变速箱油规格的油品配方。 相似文献
34.
The causes of lightning outage are subdivided into direct lightning strokes and induced lightning strokes, which are identified by the characteristics of the lightning overvoltage. In the past, lightning protection devices were directed mainly toward the latter, and attention has been focused on the installation of lightning protection devices, ground wires, and reinforcement of insulators. However, lightning outages continue to occur. Thus it is extremely important to clarify the fault characteristics of lightning surges and to study the effectiveness of various lightning protection devices by considering both direct lightning stroke and induced lightning stroke in order to prevent lightning outage in the future. In this research, the electromagnetic transients program (EMTP) has been applied to the direct lightning stroke, and the induced lightning outage analysis program for multiple conductor systems has been applied to the induced lightning stroke to study the effectiveness of lightning protection devices provided by combination of various lightning protection devices. The most effective lightning protection schemes are analyzed and evaluated based on verification tests on the full scale models as well as economic considerations. 相似文献
35.
随着GIS的工作电压不断提高和发展,一系列新的现象和问题也同时出现。从国外一些科研,设计,制造部门所研究发表的内容看,大致有隔离开关操作和高速瞬变过程,粒子影响和对干扰定位的技术,元件的监督,现场试验等课题,本文对以上问题加以综合介绍。 相似文献
36.
37.
Sidhartha R. Das Basheer M. Khumawala 《International Journal of Flexible Manufacturing Systems》1991,3(2):121-147
Flexible manufacturing systems (FMSs) are a class of automated systems that can be used to improve productivity in batch manufacturing. Four stages of decision making have been defined for an FMS—the design, planning, scheduling, and control stages. This research focuses on the planning stage, and specifically in the area of scheduling batches of parts through the system.The literature to date on the FMS planning stage has mostly focused on the machine grouping, tool loading, and parttype selection problems. Our research carries the literature a step further by addressing the problem of scheduling batches of parts. Due to the use of serial-access material-handling systems in many FMSs, the batch-scheduling problem is modeled for a flexible flow system (FFS). This model explicitly accounts for setup times between batches that are dependent on their processing sequence.A heuristic procedure is developed for this batch-scheduling problem—the Maximum Savings (MS) heuristic. The MS heuristic is based upon the savings in time associated with a particular sequence and selecting the one with the maximum savings. It uses a two-phase method, with the savings being calculated in phase I, while a branch-and-bound procedure is employed to seek the best heuristic solution in phase II. Extensive computational results are provided for a wide variety of problems. The results show that the MS heuristic provides good-quality solutions. 相似文献
38.
39.
郑文新 《水科学与工程技术》2007,(4):25-27
黑河水利枢纽右岸溢洪洞为缩短引渠左导墙的长度,减少施工开挖量,降低施工难度,减少工程投资,进行优化试验.通过对7种不同的方案对比分析,选择了合适的体型,满足了工程要求. 相似文献
40.
Different qualities of concrete have been fire tested using different geometries of the specimens as well as different load levels and load configurations. The main objective with the study was to examine a test methodology consisting of a full‐scale test and different small scale‐tests for determining the probability of spalling and the amount of spalling of fire exposed concrete structures. A reference specimen was defined as a one‐sided fire exposed slab with the dimensions 1800 × 1200 mm2 giving an exposed area of 1500 × 1200 mm2. A number of concrete qualities with different probabilities for spalling, were tested using the reference specimen. These tests showed that the reference specimens worked well giving the expected test results. Small specimens were manufactured in different shapes with the same concrete as the one used in the reference tests. These small specimens were tested either at the same time as the reference specimens in the large furnace or afterwards on a small‐scale furnace where the fire exposed surface was 450 × 360 mm2. The test results clearly show the increased probability and the increased amount of spalling by using external compressive loading. The results also show that by using pre‐stress through bars or wires the load can be lost due to heating of the bars/wires which results in a decreased amount of spalling. The boundary of the specimen also affects the amount of spalling. The spalling around the edges was in all tests less than the spalling on the central parts of the exposed area. It could also be noted that the spalling did not pass completely through any of the specimens. The reason for this is probably that the water/vapour could migrate out from the unexposed surface of the specimen. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献