全文获取类型
收费全文 | 457096篇 |
免费 | 38276篇 |
国内免费 | 21603篇 |
专业分类
电工技术 | 29195篇 |
技术理论 | 74篇 |
综合类 | 54368篇 |
化学工业 | 53424篇 |
金属工艺 | 18401篇 |
机械仪表 | 24422篇 |
建筑科学 | 63741篇 |
矿业工程 | 25485篇 |
能源动力 | 16293篇 |
轻工业 | 30038篇 |
水利工程 | 23055篇 |
石油天然气 | 21312篇 |
武器工业 | 5207篇 |
无线电 | 29945篇 |
一般工业技术 | 33687篇 |
冶金工业 | 25855篇 |
原子能技术 | 4053篇 |
自动化技术 | 58420篇 |
出版年
2024年 | 1783篇 |
2023年 | 5283篇 |
2022年 | 10030篇 |
2021年 | 12620篇 |
2020年 | 12781篇 |
2019年 | 10134篇 |
2018年 | 9578篇 |
2017年 | 11851篇 |
2016年 | 14154篇 |
2015年 | 15457篇 |
2014年 | 28095篇 |
2013年 | 25275篇 |
2012年 | 31306篇 |
2011年 | 34003篇 |
2010年 | 26023篇 |
2009年 | 26774篇 |
2008年 | 24887篇 |
2007年 | 31432篇 |
2006年 | 29225篇 |
2005年 | 25592篇 |
2004年 | 21586篇 |
2003年 | 19190篇 |
2002年 | 15748篇 |
2001年 | 13203篇 |
2000年 | 11153篇 |
1999年 | 8913篇 |
1998年 | 6635篇 |
1997年 | 5799篇 |
1996年 | 5119篇 |
1995年 | 4302篇 |
1994年 | 3788篇 |
1993年 | 2799篇 |
1992年 | 2422篇 |
1991年 | 1765篇 |
1990年 | 1527篇 |
1989年 | 1375篇 |
1988年 | 1007篇 |
1987年 | 709篇 |
1986年 | 554篇 |
1985年 | 502篇 |
1984年 | 437篇 |
1983年 | 334篇 |
1982年 | 270篇 |
1981年 | 224篇 |
1980年 | 192篇 |
1979年 | 138篇 |
1978年 | 81篇 |
1977年 | 89篇 |
1975年 | 72篇 |
1959年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
151.
针对目前企业固定资产投资中存在的投资超概算问题 ,论述了实行工程预决算审计的必要性 ,结合江西铜业股份有限公司的实践阐明了如何实行工程预决算审计 ,以达到提高投资效益的目的。 相似文献
152.
本文主要从土壤 pH ,重碳酸盐 ,和氮素形态等几方面综述了各种土壤因子对土壤缺铁失绿的影响 ,并提出了进一步加强研究的方向 相似文献
153.
Collaborative work is an important part of tertiary education but it is very difficult to arrange and supervise for extremely large classes of students in their first year. The possibility that computer-mediated communication can be used to facilitate this type of learning is appealing from a pragmatic organisational point of view. This paper explains in detail what a ‘virtual team’ is in the educational context. It reports on an interpretive field study where students taking an introductory course were allowed to choose where and when they did the required collaborative work. The paper discusses the factors that should be taken into account when offering students the option of working as a virtual team. These include factors that influence the students’ choice. The project cannot be considered to have been successful but indicates reasons for the lack of success and suggests contexts in which it would be valuable to repeat the project. The importance of extensive preparation in terms of teaching the students necessary social and technological skills cannot be over emphasised. 相似文献
154.
Using the Internet, “public” computing grids can be assembled using “volunteered” PCs. To achieve this, volunteers download and install a software application capable of sensing periods of low local processor activity. During such times, this program on the local PC downloads and processes a subset of the project's data. At the completion of processing, the results are uploaded to the project and the cycle repeats. 相似文献
155.
Transport problems typically involve at least two types of constraints, on income and on time. Therefore, the indirect utility
function depends either on the income available after having subtracted the cost of the discrete alternative and on the free
time left after having worked and travelled by each competing option. In the typical linear-in-the-attributes and in-the-parameters
specification, that represents the first grade approximation of the indirect utility function, the effect of income and time
constraints cancel out and only the cost and time of the alternatives matter in the comparison between them. From a microeconomic
point of view this is equivalent to assume that income and time effects could be disregarded; which is not always the case.
To account for these effects the utility function should include second order attributes; however, in non-linear utility functions
it may not be easy to distinguish among several effects that could be relevant: direct preferences for good and leisure, and
simple interactions between attributes other than income and time effects. This paper analyses these effects from a theoretical
point of view focusing on the possible confounding problem in detecting income and time effects. We use a dataset collected
for a modal choice context and containing both revealed and stated preference data, and estimate several NL models examining
the effect of the different second-order terms on detecting income and time effects. We compared specifications including
square cost and time attributes, interactions between time and cost, cost divided by the income available to be spent on free
time, and time multiplied by free time. Our results confirm the strong effect of direct preferences for goods and leisure
time on choice, and the potential confounding effect between quadratic attributes and other non-linear omitted terms. Finally,
we also found that care should be taken in highlighting income and time effects using mixed data sources, since confounding
effects can occur when non-linearities are accounted for in both data sets. 相似文献
156.
针对100吨转炉设备安装过程中设备重量超载,超高,无法直接采用行车安装的情况,采用了设备分体移位吊装,现场整体组合焊接的施工工艺方案,设计专用安装工装,确保了设备安装精度并一次安装到位。 相似文献
157.
Many real-world problems are multi-objective optimization problems and evolutionary algorithms are quite successful on such problems. Since the task is to compute or approximate the Pareto front, multi-objective optimization problems are considered as more difficult than single-objective problems. One should not forget that the fitness vector with respect to more than one objective contains more information that in principle can direct the search of evolutionary algorithms. Therefore, it is possible that a single-objective problem can be solved more efficiently via a generalized multi-objective model of the problem. That this is indeed the case is proved by investigating the computation of minimum spanning trees.This work was supported by the Deutsche Forschungsgemeinschaft (DFG) as part of the Collaborative Research Center Computational Intelligence (SFB 531) and by the German-Israeli Foundation (GIF) in the project Robustness Aspects of Algorithms. 相似文献
158.
Aboubakar Sako Kenneth G. MacLeod Catherine M. O’Reilly 《Journal of Great Lakes research》2007,33(4):806-815
In this study, we measured growth trends in oxygen and carbon isotopic ratios in whole sagittal otoliths from three adult centropomid fish (Lates stappersii) from each of three sub-basins of Lake Tanganyika, East Africa. Sampling density was 20 to 50 samples per otolith. Both δ18O and δ13C values increase with age. The δ18O data suggest that otoliths were precipitated near the expected equilibrium with the ambient environment (ca. +3.5‰) and support a migration pattern from surface waters during larval stages to deeper waters (40 to 80 m) for mature fish. Relatively high δ18O values in the southern sub-basin are consistent with cooler temperatures in the region during seasonal upwelling. The δ13C increase from otolith core to edge is large (up to 4‰) and is interpreted as due to ontogenetic changes in diet and contributions from a decrease in the proportion of respired CO2 incorporated into otolith carbonate as metabolic rates of the fish dropped with maturity. The data seem to successfully reveal life strategy and migration patterns of L. stappersii, document regional differences in lake conditions, and provide a record of temperature within the water column during which the fish lived. Higher resolution studies and analyses of historical samples could be used to constrain modern and past growth patterns, and to reconstruct past temperature gradients and productivity patterns in the lake. 相似文献
159.
160.
U. Reimer
H. -J. Schek
《Data & Knowledge Engineering》1989,4(4):321-352The first part of this paper introduces the basic constructs of a frame representation model and gives a formal definition for them. Subsequently the nested relational model (NF2) is described as an extension of the classical relational model to support relation-valued attributes. In the second part of this paper both models are compared with each other and a mapping of frame representation structures to NF2 structures is specified. The structural similarities between frames and NF2 relations are made clear and it is shown that their main difference is due to the type polymorphism introduced by the concept hierarchy of the frame model. This causes type collisions to occur in the strictly typed NF2 model when frames are directly mapped to NF2 structures. Two solutions to this problem are suggested and compared. The paper concludes with a reformulation of query operations of the frame model in terms of NF2 algebra operations. 相似文献