全文获取类型
收费全文 | 4220篇 |
免费 | 510篇 |
国内免费 | 132篇 |
专业分类
电工技术 | 154篇 |
综合类 | 183篇 |
化学工业 | 352篇 |
金属工艺 | 21篇 |
机械仪表 | 120篇 |
建筑科学 | 143篇 |
矿业工程 | 26篇 |
能源动力 | 46篇 |
轻工业 | 2429篇 |
水利工程 | 580篇 |
石油天然气 | 89篇 |
武器工业 | 77篇 |
无线电 | 88篇 |
一般工业技术 | 163篇 |
冶金工业 | 38篇 |
原子能技术 | 15篇 |
自动化技术 | 338篇 |
出版年
2025年 | 2篇 |
2024年 | 91篇 |
2023年 | 127篇 |
2022年 | 160篇 |
2021年 | 181篇 |
2020年 | 224篇 |
2019年 | 241篇 |
2018年 | 178篇 |
2017年 | 219篇 |
2016年 | 224篇 |
2015年 | 206篇 |
2014年 | 209篇 |
2013年 | 329篇 |
2012年 | 302篇 |
2011年 | 262篇 |
2010年 | 199篇 |
2009年 | 185篇 |
2008年 | 168篇 |
2007年 | 200篇 |
2006年 | 158篇 |
2005年 | 98篇 |
2004年 | 94篇 |
2003年 | 94篇 |
2002年 | 106篇 |
2001年 | 72篇 |
2000年 | 57篇 |
1999年 | 38篇 |
1998年 | 45篇 |
1997年 | 46篇 |
1996年 | 50篇 |
1995年 | 31篇 |
1994年 | 33篇 |
1993年 | 35篇 |
1992年 | 53篇 |
1991年 | 32篇 |
1990年 | 13篇 |
1989年 | 13篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 13篇 |
1985年 | 12篇 |
1984年 | 18篇 |
1983年 | 11篇 |
1982年 | 10篇 |
1980年 | 2篇 |
1977年 | 2篇 |
1959年 | 1篇 |
排序方式: 共有4862条查询结果,搜索用时 0 毫秒
1.
Waters in the Great Lakes basin contain more than 400 contaminant chemicals that potentially affect fishery resources, commerce, and human inhabitants. We determined in the laboratory the effects of selected contaminants on the toxicity of the widely used lampricides TFM (3-trifluoromethyl-4-nitrophenol) and Bayer 73 (2′,5-dichloro-4′-nitrosalicylanilide) to three species of fish—rainbow trout (Salmo gairdneri), white sucker (Catostomus commersoni), and fathead minnow (Pimephales promelas). The fish were exposed to paired mixtures of lampricides and selected contaminants in standardized, acute static toxicity tests to determine the resulting type of response—less than additive, additive, or greater than additive (synergistic). As expected, the toxicities of combinations of lampricides with organic pesticides, metal, industrial or municipal pollutants, and tannic acid were mostly additive. However, the toxicity of a combination of TFM, Delnav, and malathion was synergistic, and extremely small quantities of each chemical became lethal when mixed. The concentration that produced 50% mortality was 1.64 mg/L for TFM alone but only 0.041 mg/L for TFM with the pesticides. Toxicities of the pesticides in the combination also increased commensurately. The triple combination of chemicals produced extraordinary synergism and effectively demonstrated the hazards that may result if certain chemical combinations occur in the aquatic environment. However, synergism is not the only kind of toxic action that produces hazards to aquatic organisms. All three types of toxic action are of concern because toxic units produced by contaminant chemicals add to the toxic units of applied management chemicals. Since the toxicity of the majority of chemical combinations is simply additive, this cumulative toxic action contributes more total units to aquatic environments than the extreme actions of less than additive and synergism. The toxicity of the lampricide TFM, as well as other management chemicals, is reinforced by the presence of any contaminant that contributes additional units of toxicity. Therefore, all types of cumulative toxic action should be of concern to people and agencies involved with protecting the environment. 相似文献
2.
One hypothesis for the transcontinental and intra-Great Lakes basin transfer of round gobies (Neogobius melanostomus) has been that round gobies were pumped into the ballast water of ships. During June 2005 in Lake Erie, we obtained evidence of a vertical migration of round goby larvae, when we collected 167 round goby larvae in surface ichthyoplankton net tows at night and zero during day. These results complemented similar findings from the Muskegon River estuary of Lake Michigan during 2003 and 2004, documenting diel vertical migration for the first time in larval round gobies. We suggest vertical migration behavior may have allowed larval round gobies to be transported to and within the Great Lakes via ballast water and dispersed in the Great Lakes via advection of 6.5–8.5-mm long larvae at the surface. Based on our results, if ballast water was only taken on near the surface during daylight hours from May through September when larval round gobies were present, it would have mitigated the spread of round gobies throughout the Great Lakes. 相似文献
3.
ABSTRACTThe United States and Canada have begun renegotiating the Columbia River Treaty, which is an international model for transboundary water governance. This paper identifies six institutional factors that will affect negotiations pertaining to fish passage during the renegotiation of the treaty: geographical advantage, issue linkage, a basin commission, the duration of agreements, negotiating autonomy and side payments. These factors and the methods used to determine them can be applied to other transboundary river basins where basin states have a history of transboundary resource governance. This analysis also serves as a policy-relevant resource for Columbia River Treaty negotiators and stakeholders. 相似文献
4.
李泽民 《华中科技大学学报(城市科学版)》1997,(3)
在借鉴国内几个著名大城市快速路系统规划成果的基础上,着重对快速路系统规划中的互通式立交、出入匝道的最小间距以及辅道设计等进行了研讨.此外,对尚待深化研究的快速路系统的合理规模问题,进行了初次探讨,提出了校核、判断其规模合理性的方法及算式,这对准备进行快速路网规划的其他大城市,避免规模不当,将不无裨益. 相似文献
5.
6.
短期电力负荷预测在电力系统安全调度、经济运行方面起到关键作用。在用最小二乘支持向量机进行负荷预测时,参数选择将直接影响预测精度。为了提高LSSVM负荷预测精度,文中提出一种基于Levy变异自适应视野人工鱼群-蛙跳算法对LSSVM进行参数优化的方法。以某县负荷、天气等历史数据对LSSVM进行训练,建立LAVAFSA-SFLA-LSSVM、AFSA-LSSVM、LAFSA-SFLA-LSSVM共3种预测模型,对该地区某日24 h的电力负荷进行预测。算例结果表明,LAVAFSA-SFLA-LSSVM预测精度比AFSA-LSSVM和LAFSA-SFLA-LSSVM更高,预测误差更小。 相似文献
7.
本文介绍了一种鱼类耳石样品的扫描电镜制样方法,通过对草鱼耳石进行包埋,抛磨,并用合适的腐蚀剂处理,用SEM成功地观察了耳石的日轮及超微结构,该方法简便易行,保证所观察耳石样品的微观结构完整与清晰。 相似文献
8.
杨军严 《电网与水力发电进展》2006,22(4):80-82
本文对水利水电工程对水生动物栖息的水环境造成阻隔产生的负面影响作了具体分析,并简单介绍了国内外所采用的过鱼方法。认为应在工程中修建、补建鱼道;在江河流域规划不建工程的支流,以加强水生野生动物种质资源保护。 相似文献
9.
10.
基于自适应人工鱼群算法的多用户检测器 总被引:22,自引:0,他引:22
将智能优化算法应用到多用户检测器(MUD)问题中,是近年来改善MUD性能的一个研究方向。人工鱼群算法(AFSA)是一种新的智能优化算法,该算法具有一些遗传算法和粒子群算法不具备的特点。但是用其解决离散优化问题时,该算法保持探索与开发平衡的能力较差,且在算法运行后期搜索的盲目性较大,从而影响了该算法搜索的质量和效率。为了克服这些缺点,本文对该算法进行了改进,得到两种自适应人工鱼群算法(AAFSA_FP和AAFSA_SP),并首次用其构建了新的多用户检测器。仿真结果表明,该方法与基于遗传算法的多用户检测器和基于粒子群算法的多用户检测器相比,在误码率、抗远近效应的能力和收敛速度等方面都有明显的改善。 相似文献