首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   7825篇
  免费   707篇
  国内免费   457篇
电工技术   1449篇
技术理论   1篇
综合类   1299篇
化学工业   473篇
金属工艺   130篇
机械仪表   215篇
建筑科学   1305篇
矿业工程   348篇
能源动力   495篇
轻工业   194篇
水利工程   421篇
石油天然气   249篇
武器工业   40篇
无线电   446篇
一般工业技术   318篇
冶金工业   384篇
原子能技术   24篇
自动化技术   1198篇
  2024年   14篇
  2023年   79篇
  2022年   164篇
  2021年   195篇
  2020年   245篇
  2019年   177篇
  2018年   161篇
  2017年   184篇
  2016年   214篇
  2015年   292篇
  2014年   557篇
  2013年   441篇
  2012年   517篇
  2011年   605篇
  2010年   518篇
  2009年   563篇
  2008年   520篇
  2007年   648篇
  2006年   584篇
  2005年   482篇
  2004年   389篇
  2003年   369篇
  2002年   254篇
  2001年   225篇
  2000年   148篇
  1999年   116篇
  1998年   64篇
  1997年   62篇
  1996年   39篇
  1995年   36篇
  1994年   25篇
  1993年   17篇
  1992年   15篇
  1991年   13篇
  1990年   14篇
  1989年   10篇
  1988年   4篇
  1987年   2篇
  1985年   2篇
  1984年   6篇
  1983年   2篇
  1981年   3篇
  1964年   3篇
  1963年   1篇
  1960年   2篇
  1958年   1篇
  1957年   1篇
  1956年   1篇
  1955年   1篇
  1954年   1篇
排序方式: 共有8989条查询结果,搜索用时 15 毫秒
1.
The performance of gallium promoted cobalt-ceria catalysts for ethanol steam reforming (ESR) was studied using H2O/C2H5OH = 6/1 mol/mol at 500 °C. The catalysts were synthetized via cerium-gallium co-precipitation and wetness impregnation of cobalt. A detailed characterization by N2-physisorption, XRD, H2-TPR and TEM allowed the normalization of contact time and rationalization of the role of each catalysts component for ESR. The gallium promoted catalyst, Co/Ce90Ga10Ox, was more efficient for the ethanol conversion to H2 and CO2, and the production of oxygenated by-products (such as, acetaldehyde and acetone) than Co/CeO2. The catalytic performance is explained assuming that: (i) bare ceria is able to dehydrogenate ethanol to ethylene; (ii) Ce–O–Ga interface catalyzes ethanol reforming; (iii) both Ce–O–Co and Ce–O–Ga interfaces takes part in acetone production; and (iv) cobalt sites further allow C–C scission. It is suggested that a cooperative role between Co and Ce–O–Ga sites enhance the H2 and CO2 yields under ESR conditions.  相似文献   
2.
2018年非洲猪瘟疫情出现以来,全国及黑龙江省生猪生产、猪肉市场受到了较大的影响,生猪、能繁母猪存栏减少,生猪出场价格也经历了明显波动。2019年春节以后,随着省外调出量增加和猪源减少,黑龙江省生猪出场价格进入上升通道,超过了全国平均水平,白条猪肉批发价格和猪肉市场零售价格随之上行。通过对2019年黑龙江省生猪生产环节、屠宰加工环节、猪肉市场零售环节进行调研,对生猪出场价格、白条猪肉批发价格、猪肉零售价格、生猪和能繁母猪产能等指标进行分析,阐述了以生猪和猪肉市场为重点的肉类产销形势,为促进肉类产业高质量发展、提高市场保供能力提供参考。  相似文献   
3.
从制冷空调企业的现状出发,利用校企协同、产教融合的创新组织模式,整合暖通行业、企业和学校的资源优势,构建一套“协同创新”的制冷空调课程体系,从根本上提高建环专业学生工程素养和创新能力,以满足暖通行业、企事业单位对具有实践及创新能力的高素质应用型技术人才需求。  相似文献   
4.
ABSTRACT

Electric heat pumps feature prominently in projected energy transitions in the UK and elsewhere. Owing to their high electricity consumption, heat pumps are viewed as important targets for demand-side response (DSR). Findings are presented from a field trial of a new control system that aims to optimize heat pump performance, including under time-varying tariff conditions. The trial involved monitoring 76 properties with heat pumps, but without dedicated heat storage; 31 of these received the control system. Interviews were conducted with a subsample of 12 participants. The controller successfully evened out electricity demand over the day (reducing the evening peak), but this was associated with increased late night and daytime temperatures. Interview participants reported some disturbance owing to overnight heating and noise, as well as usability issues with the controller interface and hardware. These issues present risks to the future acceptability of such systems. While the system delivered short-term demand reductions successfully, longer-term demand shifting risked causing unacceptable disturbance to occupants. Future control systems could overcome some of the issues identified in this pioneering trial through more effective zoning, using temperature caps or installing dedicated heat storage, but these may either limit the available flexibility or be challenging to achieve.  相似文献   
5.
马飞燕  刘文 《建筑施工》2022,44(1):86-88
基于某超高层建筑群项目的主体施工阶段,对超高层劲性结构建筑群项目施工过程中可能遇到的问题,诸如大型机械立体协同、钢结构与钢筋连接节点优化、钢结构桁架层施工等重难点进行总结分析,并提出相应的解决方案以供探讨,可为同类型工程提供参考经验。  相似文献   
6.
《云南化工》2019,(9):174-175
基础化学实验课程是学习化学的基础,对于提高学生对化学理论知识的理解、培养学生科研素质起到非常重要的作用。公共基础化学实验室的开放,使不同专业的学生有更多的机会可以接触化学实验,增强对化学基础知识的了解和掌握,培养学生实践能力和创新能力。介绍了如何建立开放化学实验室准入制度、实验室分区和安全管理制度、实验室跟进和激励制度,并给出了一些中外合作的探索措施,为化学实验室的开放管理和运行提供参考。  相似文献   
7.
In this paper, we empirically examine the application of a host of techniques employed to measure price efficiency through long-range dependence using prices of monthly oil contracts. Using a series of methods, we analyse the volatility (daily absolute returns) of WTI and Brent oil prices for nine different contracts with maturity, ranging from 1 to 9 months, during the sample period of 1990-2017. We use bootstrapping to compute the confidence interval of the parameter of long-range dependence. Our results indicate that on an average, there is no long-range dependence in the volatility of oil price contracts at least at the 10-percent level of significance. Moreover, our results of rolling estimates suggest that the normality assumption does not affect the results considerably, and the results are robust to different rolling window sizes. While the results of the efficiency index suggest that the efficiency of oil returns vary with time, the futures contracts for Brent oil are found to be less efficient compared to WTI oil. The long-term futures contracts are more efficient than short-term contracts.  相似文献   
8.
This study aims to examine the relationship between financial development, energy price, real income, and energy consumption in 17 emerging economies. In doing so, the financial development has been handled with three different dimensions (banking sector, stock market, and bond market) and the effects of each financial development dimension on energy consumption is investigated. For this purpose, the annual data of 1991–2015 are analyzed using the common correlated effect (CCE) estimator to take into account the cross-sectional dependence. The results show that bond market development is the most efficient dimension to reduce the energy consumption.  相似文献   
9.
Studying the distribution of the patent cooperation networks from the perspective of assignees provides a very important reference to improve the analysis of the market situation, master the layout of industrial technology and seek partners or mergers and acquisitions. This study uses the Derwent patent database and the patent metric approach to investigate the cooperative network structure of the assignees. The overall patent output in the artificial intelligence field on a global scale exhibited a rapid growth, and the proportion of cooperative patents significantly increased; the cooperation structure between the assignees was loose, and the innovation efficiency was low.  相似文献   
10.
Territorial Impact Assessment (TIA) is proposed as a critical tool in order to investigate the way policies affect spatial development and hence territorial cohesion. The paper introduces a TIA methodological model in relation to transport sector. The design of the methodological model is based on a quantitative evaluation framework that comprises a set of selected indicators applied in a cross-border area of southeast Europe, influenced by the operation of the Greek motorway system of Egnatia and its vertical axes. The indicators are grouped into three main fields of TIA, i.e. transport intermodality, polycentric spatial organization and cross-border territorial cooperation. The TIA’s indicators are approximated by quantitative analysis. The procedure concludes to an overall composite indicator measuring the ‘level of territorial cohesion’ along with some core results and the expected added value. It also highlights some key technical aspects about the routines used and proceeds to suggestions for further refinement.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号