全文获取类型
收费全文 | 434356篇 |
免费 | 35018篇 |
国内免费 | 19235篇 |
专业分类
电工技术 | 26056篇 |
技术理论 | 47篇 |
综合类 | 30773篇 |
化学工业 | 71014篇 |
金属工艺 | 25070篇 |
机械仪表 | 27198篇 |
建筑科学 | 33610篇 |
矿业工程 | 13236篇 |
能源动力 | 12077篇 |
轻工业 | 29842篇 |
水利工程 | 8435篇 |
石油天然气 | 26404篇 |
武器工业 | 3423篇 |
无线电 | 49178篇 |
一般工业技术 | 48832篇 |
冶金工业 | 22216篇 |
原子能技术 | 4535篇 |
自动化技术 | 56663篇 |
出版年
2024年 | 1400篇 |
2023年 | 6206篇 |
2022年 | 11178篇 |
2021年 | 16516篇 |
2020年 | 12232篇 |
2019年 | 9962篇 |
2018年 | 11557篇 |
2017年 | 13091篇 |
2016年 | 11926篇 |
2015年 | 16595篇 |
2014年 | 21245篇 |
2013年 | 25930篇 |
2012年 | 28151篇 |
2011年 | 31259篇 |
2010年 | 27488篇 |
2009年 | 26187篇 |
2008年 | 25702篇 |
2007年 | 24561篇 |
2006年 | 24715篇 |
2005年 | 21466篇 |
2004年 | 14631篇 |
2003年 | 12996篇 |
2002年 | 12316篇 |
2001年 | 10887篇 |
2000年 | 10552篇 |
1999年 | 11129篇 |
1998年 | 8862篇 |
1997年 | 7528篇 |
1996年 | 7052篇 |
1995年 | 5834篇 |
1994年 | 4688篇 |
1993年 | 3308篇 |
1992年 | 2611篇 |
1991年 | 1985篇 |
1990年 | 1569篇 |
1989年 | 1257篇 |
1988年 | 1037篇 |
1987年 | 652篇 |
1986年 | 533篇 |
1985年 | 362篇 |
1984年 | 276篇 |
1983年 | 209篇 |
1982年 | 185篇 |
1981年 | 130篇 |
1980年 | 149篇 |
1979年 | 84篇 |
1978年 | 54篇 |
1977年 | 58篇 |
1976年 | 87篇 |
1973年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 318 毫秒
971.
Central to cost-based competition is the capability to accurately predict the cost of delivering a project. Most literature on cost estimation focuses on specific estimation methods as generic techniques and little attention has been paid to the unique requirements at each project stage. This note attempts to identify the critical factors for effective estimation at various stages of typical construction projects. Drawing from organization control theory and cost estimating literature, this note develops a theoretical framework that identifies the critical factors for effective cost estimation during each project phase of a conventional construction project. The underlying logic is that as a cost estimating effort progresses, both task programmability and output measurability improve. As a result, control effort will shift from input-oriented control to a combination of output and behavior control. 相似文献
972.
Andreja ?gajnar Gotvajn Jana Zagorc-Kon?an Tatjana Ti?ler 《Canadian Metallurgical Quarterly》2007,133(1):89-94
The preoxidation of a highly polluted waste pharmaceutical fermentation broth using wet air oxidation (WAO) has been studied as a possible method for the effective removal of organics. The applied (pre)treatment method should enhance the biotreatability of the pharmaceutical fermentation broth in terms of reduced initial toxicity and higher biodegradability. Preliminary experiments in the pilot biological treatment plant were successful only at low organic loads, whereas the system collapses at higher ones. The characterization of the fermentation broth was started by common physicochemical analysis, whereas several bioassays were used to determine its impact on biological treatment plants and the environment. Toxicity prior to and after WAO was determined using the acute Vibrio fischeri test, measurement of inhibition of O2 consumption, and the Daphnia magna acute test. Ready biodegradability of the treated and untreated broth has also been assessed. WAO experiments were accomplished in the 2?L batch reactor at different temperatures (240/280°C) and operating pressures. WAO experiments confirmed reduction of the toxicity toward microorganisms, whereas oxidized wastewater was more toxic to daphnids. Biodegradability of the oxidized broth has also been enhanced. Further work has been focused on designing appropriate combination of WAO and biological processes. 相似文献
973.
We developed a stream classification system that is based on stream’s susceptibility to algal growth using a two-step approach. The model portrays algal biomass as a result of stream’s response to nutrient concentrations and the response is governed by various stream factors. In the first step, a nutrient-chlorophyll a relationship was developed to characterize nutrient’s effects on algal biomass. Residuals of the relationship were attributed to stream’s susceptibility to algal growth in response to nutrients and referred to as “observed” susceptibility. In the second step, conditions of other contributing factors were used to explain the variation in the residuals and the developed relationship was used to generate “predicted” susceptibility. Existing data compiled from various monitoring projects of Illinois streams and rivers were used to illustrate the approach. Streams were classified into three (high, medium, and low) categories based on their observed and predicted susceptibility values, respectively. With the available data, the model showed a 40-50% success rate for classifying the streams based on three observed and predicted susceptibility categories. Model entropy also was calculated for selecting the best model. The results show the important role of both nutrients and other contributing factors in explaining the variation of algal biomass. The study also suggests ways to fine tune the model and improve its accuracy, which would make the presented model a more viable tool for stream classification for establishing nutrient criteria to prevent surface streams from eutrophication. 相似文献
974.
975.
In 2004, the American National Standards Institute approved the Role-Based Access Control standard to fulfill "a need among government and industry purchasers of information technology products for a consistent and uniform definition of role based access control (RBAC) features". Such uniform definitions give IT product vendors and customers a common and unambiguous terminology for RBAC features, which can lead to wider adoption of RBAC and increased productivity. However, the current ANSI RBAC Standard has several limitations, design flaws, and technical errors that, it unaddressed, could lead to confusions among IT product vendors and customers and to RBAC implementations with different semantics, thus defeating the standard's purpose. 相似文献
978.
混凝土早期收缩开裂是加剧混凝土劣化并导致最终失效的一个重要因素.分析混凝土早期的自由收缩量值是进行收缩开裂研究的基础。为此,基于湿度扩散理论建立了混凝士早期的自生收缩与干燥收缩计算分析模型,经实例验证,计算结果与试验结果有较好的一致性。 相似文献
979.
The continuous reduction of chip size driven by the market demand has a significant impact on circuit design and assembly process of IC packages. Shrinking chip size and increasing I/O counts require finer bond pad pitch and bond pad size for circuitry layout. As a result, serious wire deflection during transfer molding process could make adjacent wires short, and this issue becomes more critical as a smaller wire diameter has to be applied for the finer pitch wire bonded IC devices.This paper presents a new encapsulation process development for 50 μm fine pitch plastic ball grid array package. Since reduced wire diameter decreases the bending strength of bonded wires significantly, wire deflection during molding process becomes quite serious and critical. Experiments on conventional transfer molding were conducted to evaluate wire span threshold with 23.0 μm diameter gold wire. The results show that the wire span threshold is about 4.1 mm, which is much shorter than the wire span threshold of over 5.0 mm for wire with 25.4 μm diameter. Finite element analysis shows there is a significant difference in the wire deflection between 23.0 μm gold wire and 25.4 μm gold wire diameter under the same action of mold flow. A novel encapsulation method is introduced using non-sweep solution. The wire span could be extended to over 5.0 mm with wire sweep less than 1%. Reliability tests conducted showed that all the units passed 1000 temperature cycles (−55 to 125 °C) with JEDEC moisture sensitivity level 2a (60 °C/60% relative humidity for 120 h) and 3 times reflow (peak temperature at 220–225 °C). It is believed that this solution could efficiently overcome the risk of wire short issues and improve the yield of ultra fine pitch wire bonds in high-volume production. 相似文献
980.