全文获取类型
收费全文 | 82623篇 |
免费 | 18040篇 |
国内免费 | 1862篇 |
专业分类
电工技术 | 3075篇 |
技术理论 | 4篇 |
综合类 | 3053篇 |
化学工业 | 24532篇 |
金属工艺 | 2653篇 |
机械仪表 | 3374篇 |
建筑科学 | 5732篇 |
矿业工程 | 1051篇 |
能源动力 | 2267篇 |
轻工业 | 9790篇 |
水利工程 | 1018篇 |
石油天然气 | 2541篇 |
武器工业 | 315篇 |
无线电 | 12390篇 |
一般工业技术 | 16939篇 |
冶金工业 | 2939篇 |
原子能技术 | 668篇 |
自动化技术 | 10184篇 |
出版年
2024年 | 157篇 |
2023年 | 643篇 |
2022年 | 1091篇 |
2021年 | 1805篇 |
2020年 | 2599篇 |
2019年 | 4106篇 |
2018年 | 4189篇 |
2017年 | 4661篇 |
2016年 | 4994篇 |
2015年 | 5489篇 |
2014年 | 5836篇 |
2013年 | 7426篇 |
2012年 | 5349篇 |
2011年 | 5194篇 |
2010年 | 5071篇 |
2009年 | 5008篇 |
2008年 | 4422篇 |
2007年 | 4212篇 |
2006年 | 4061篇 |
2005年 | 3380篇 |
2004年 | 2910篇 |
2003年 | 2836篇 |
2002年 | 3023篇 |
2001年 | 2623篇 |
2000年 | 2385篇 |
1999年 | 1780篇 |
1998年 | 1257篇 |
1997年 | 1058篇 |
1996年 | 1002篇 |
1995年 | 796篇 |
1994年 | 653篇 |
1993年 | 602篇 |
1992年 | 401篇 |
1991年 | 282篇 |
1990年 | 297篇 |
1989年 | 199篇 |
1988年 | 164篇 |
1987年 | 124篇 |
1986年 | 91篇 |
1985年 | 67篇 |
1984年 | 52篇 |
1983年 | 42篇 |
1982年 | 39篇 |
1981年 | 27篇 |
1980年 | 23篇 |
1979年 | 11篇 |
1978年 | 10篇 |
1977年 | 12篇 |
1976年 | 15篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
891.
Development of a system‐based water balance model to assess interrelated physical and anthropogenic constraints in lake and reservoir watersheds,applied to Coeur d'Alene Lake,Idaho, USA
下载免费PDF全文
![点击此处可从《Lakes & Reservoirs: Research and Management》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dylan R. Hedden‐Nicely Fritz Fiedler 《Lakes & Reservoirs: Research and Management》2017,22(3):215-228
This study presents a methodology for the development of a water balance model that uses publicly available data in a manner useful to water scientists and managers who manage complex lake and reservoir watersheds. The approach was applied to Coeur d'Alene Lake, a naturally occurring lake that is controlled by a dam located on its outlet (Spokane River) in North Idaho, USA. As in many other areas, the region surrounding Coeur d'Alene Lake has experienced high rates of population growth in recent years, and there is concern that, as consumptive water use increases, the lake will eventually not be able to be managed to simultaneously maintain the federally mandated minimum flow requirements in the Spokane River and also maintain the target summertime elevation of 2128 feet (648.6 m) for recreation and hydropower purposes. The complexity caused by the competing uses at Coeur d'Alene Lake makes it an excellent case study for similarly characterized watersheds. Both a natural flow model and regulated elevation model were developed, and sensitivity analysis was conducted on both models to evaluate which lake processes have the greatest effects on lake elevation, thereby requiring the most attention. A ‘low‐flow’ scenario was modelled to demonstrate the usefulness of the model and to inform Coeur d'Alene Lake regional stakeholders regarding the interrelationship between current water policy and the lake's physical behaviour under stressed conditions that could result from climate change. Model result indicates that, while lake elevation may be maintained at the summertime elevation of 2128 feet (648.6 m) under a low‐flow scenario, the outflows in the Spokane River start to approach the minimum flow requirements in the month of August. The developed approach is useful where publicly available data exist and allows for economic, yet rigorous, water resources systems evaluation without requiring significant field data collection. 相似文献
892.
Benchmarking Fluvial Dynamics for Process‐Based River Restoration: the Upper Rhine River (1816–2014)
下载免费PDF全文
![点击此处可从《河流研究与利用》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Multi‐temporal analysis of river‐floodplain processes is a key tool for the identification of reference conditions or benchmarks and for the evaluation of deviations or deficits as a basis for process‐based river restoration in large modified rivers. This study developed a methodology for benchmarking fluvial processes at river segment level, focusing on those interrelations between morphodynamics (aggradation, erosion, channel shift) and vegetation succession (initial, colonization, transition) that condition habitat structure. Habitat maps of the free‐flowing Upper Rhine River downstream from Iffezheim dam (France–Germany border) were intersected with a geographic information system‐based approach. Patches showing trajectories of anthropization, changeless, progression and regression allowed for the identification of natural and human‐induced processes over almost 200 years. Before channelization, the riverine system was characterized by a shifting habitat mosaic with natural heterogeneity, high degree of surface water connectivity and equilibrium between progression and regression processes. On the other hand, the following 175 years of human interventions led to severe biogeomorphologic deficits evidenced by loss of natural processes and habitat heterogeneity, hydrological disconnection between the river and its floodplain and imbalance of progression versus regression dynamics. The main driving forces of change are found in hydromorphological impacts (channelization, regulation and hydropower plant construction). Regression processes are now almost absent and have to be the objective of process‐based river restoration measures for the studied river‐floodplain system. A sustainable view on water management and river restoration should aim at a more resilient riverine system by balancing the recovery of natural processes with societal needs. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
893.
894.
895.
896.
Cristina López‐Dehesa José A. González‐Marcos Juan R. González‐Velasco 《应用聚合物科学杂志》2007,103(5):2843-2848
Poly(1‐trimethylsilyl‐1‐propyne) (PTMSP) membranes have been used to separate ethanol–water mixtures by pervaporation. This polyacetylene is known to present high affinity toward ethanol, showing high selectivity and ethanol permeation flux. The performance of this polymer in the separation of alcohol–water solutions has been evaluated over long periods (572 h) at a high temperature (75°C) to examine the deterioration of the transport properties in the separation of 50 wt % ethanol–water solutions. Although PTMSP membranes present good characteristics for the separation of gases and liquid mixtures, their organic selectivity decrease with the operating time because of the relaxation processes of the polymeric chains, which affect the free volume of the polymer, the deterioration being more evident for concentrated solutions. The effects of the operation temperature on the characteristic parameters of pervaporation have also been studied to establish how this variable affects the performance of PTMSP membranes. The selectivity increases slightly with the operation temperature, but the effect of the temperature on the separation factor decreases as membranes are degraded with the operation time. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2843–2848, 2007 相似文献
897.
Yun‐Jia Li Man Xu Jun‐Qiang Feng Xiao‐Long Cao Yan‐Fei Yu Zhi‐Min Dang 《应用聚合物科学杂志》2007,106(5):3359-3365
Through the use of polyethylenes with different crystallinities as matrices, the effects of the matrix crystallinity on the percolation threshold and dielectric behavior of percolative composites have been investigated. The results suggest that the percolation threshold is negatively related to the matrix crystallinity, whereas the enhancement of the dielectric constant is positively related to the matrix crystallinity. A two‐dimensional diagram is proposed to illustrate such relationships. In addition, it has been found that the insulator–conductor transition is much flatter in low‐crystallinity‐matrix‐based composites, and this may be favorable for preparing threshold composites with a high dielectric constant and a low loss tangent. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007 相似文献
898.
Biodegradability and mechanical properties of agro‐flour–filled polybutylene succinate biocomposites
The objective of this study was the production of rice husk flour (RHF) and wood flour (WF) filled polybutylene succinate (PBS) biocomposites as alternatives to cellulosic material filled conventional plastic (polyolefins) composites. PBS is one of the biodegradable polymers, made from the condensation reaction of 1,4‐butanediol and succinic acid that can be naturally degraded in the natural environment. We compared the mechanical properties between conventional plastics and agro‐flour–filled PBS biocomposites. We evaluated the biodegradability and mechanical properties of agro‐flour–filled PBS biocomposites according to the content and filler particle size of agro‐flour. As the agro‐flour loading was increased, the tensile and impact strength of the biocomposites decreased. As the filler particle size decreased, the tensile strength of the biocomposites increased but the impact strength decreased. The addition of agro‐flour to PBS produced a more rapid decrease in the tensile strength, notched Izod impact strength, and percentage weight loss of the biocomposites during the natural soil burial test. These results support the application of biocomposites as environmentally friendly materials. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1513–1521, 2005 相似文献
899.
对湿式PU革透湿机理进行了概述,通过对湿式PU革吸湿性和透湿率的测试,详细讨论了基布吸湿性,PU膜结构,密度,厚度及填充料纤维素,表面印刷层,干法贴膜层及表面纹工艺对湿式PU革透湿性能的影响,并相应提出了提高湿式PU革透湿性的工艺方法。 相似文献
900.
The polyamide-6/attapulgite nanocomposites were prepared via an in situ polymerization route with attapulgites pre-modified with cetyltrimethylammonium bromide (CTAB) and toluene-2,4-diisocyanate (TDI). Morphology observation showed that the exfoliated attapulgite fibers were well dispersed in the polyamide-6 matrix on a nanometer scale and formed a percolation network structure. The rheological behaviors of such polymer/fibrous clay nanocompostie samples were investigated by an ARES rheometer with parallel plate geometry. The storage moduli (G′), loss moduli (G″), and dynamic viscosities of these samples increased monotonically with attapulgite content at low frequencies. The presence of attapulgites caused these nanocomposite melts to have solid-like behaviors and slower relaxation. This behavior can be explained in terms of the development of a grafting-percolated fibrous-silicate network structure. Monte Carlo simulations were performed to determine the critical threshold for attapulgites fibers in 3D. The calculated critical threshold from simulations fitted the results of our rheological experiments very well. 相似文献