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   6155篇
  免费   409篇
  国内免费   287篇
电工技术   375篇
综合类   314篇
化学工业   1838篇
金属工艺   837篇
机械仪表   269篇
建筑科学   290篇
矿业工程   96篇
能源动力   163篇
轻工业   491篇
水利工程   19篇
石油天然气   193篇
武器工业   45篇
无线电   378篇
一般工业技术   825篇
冶金工业   269篇
原子能技术   65篇
自动化技术   384篇
  2024年   9篇
  2023年   59篇
  2022年   94篇
  2021年   133篇
  2020年   142篇
  2019年   111篇
  2018年   86篇
  2017年   169篇
  2016年   183篇
  2015年   179篇
  2014年   301篇
  2013年   352篇
  2012年   432篇
  2011年   543篇
  2010年   381篇
  2009年   511篇
  2008年   374篇
  2007年   438篇
  2006年   387篇
  2005年   350篇
  2004年   296篇
  2003年   269篇
  2002年   215篇
  2001年   152篇
  2000年   120篇
  1999年   94篇
  1998年   79篇
  1997年   71篇
  1996年   56篇
  1995年   44篇
  1994年   46篇
  1993年   44篇
  1992年   32篇
  1991年   26篇
  1990年   10篇
  1989年   11篇
  1988年   9篇
  1987年   7篇
  1986年   2篇
  1985年   11篇
  1984年   2篇
  1983年   4篇
  1982年   4篇
  1981年   4篇
  1975年   1篇
  1974年   1篇
  1964年   1篇
  1959年   1篇
  1958年   2篇
  1951年   2篇
排序方式: 共有6851条查询结果,搜索用时 234 毫秒
81.
82.
In this paper, spatial shift‐share decomposition is analysed when applied to Italian data on regional business change at plant level, over the period 2004–2009. A new type of spatial decomposition, which looks more effectively at neighbourhood influence, is introduced here. Notable results emerge from the empirical investigation. First, it can be seen that the spatial level of aggregation greatly affects results. Second, evidence of neighbourhood advantage in the Southern NUTS 3 regions is found, together with opposite results for the Central‐Northern NUTS 3 regions. Finally, evidence of positive industrial mix effects is only found in Central‐Northern Italy.  相似文献   
83.
马悦 《工业催化》2017,25(9):60-62
等温反应器的运行空速条件远远高于绝热反应器,需要催化剂具有更高的活性,但这样会加速催化剂的结焦,影响选择性及催化剂寿命。中国石油大庆石化公司联合中国石油石油化工研究院兰州中心针对等温反应器的特点,制备了LY-C2-12D催化剂,通过应用改进,平均乙烯选择性为70%,连续运行时间超过6个月,运行稳定性和绿油生成量得到明显改善,经济效益显著。  相似文献   
84.
Three various 12CrNi2 alloy steels were successfully fabricated on 35CrMo alloy steel by laser melting deposition with the aim of investigating the effect of powder oxygen content on formability of 12CrNi2 alloy steel. The phase constituents, macroscopic morphology, microstructure, and microhardness of powders and samples were studied by some modern analysis methods. The experimental results showed that with the increase of the oxygen content from 220 to 600?ppm, the sphericity and density of the powders gradually decreased which led to the appearance of pores in the as-deposited samples. The phase constituent of three as-deposited samples with different oxygen contents was α-Fe(M) phase. The percentage porosity in the macroscopic morphology increased with the increase of the oxygen content. Meanwhile, the high oxygen content contributed to more pores and the crack formation in the microstructure. With the increase of oxygen content, the microhardness of the as-deposited samples would decrease.  相似文献   
85.
86.
我公司自主设计的发电锅炉启动疏水扩容器系统里,在节流管部件中常会应用12Cr1MoVG与15CrMoG组焊结构,由于这两种材料自身常用的PWHT工艺不同,为简化PWHT过程,本文就锅炉制造中此类管接头常用的PWHT工艺与国家标准NB/T 47015-2011推荐的PWHT工艺进行对比研究,综合实验结果和制造工艺流程情况,给出更加合适的PWHT工艺。  相似文献   
87.
Blend fabrics of cotton and polyester are widely used in apparel, but high flammability becomes a major obstacle for applications of those fabrics in fire protective clothing. The objective of this research was to investigate the flame retardant finishing of a 50/50 polyester/cotton blend fabric. It was discovered previously that N,N′‐dimethyloldihydroxyethyleneurea (DMDHEU) was able to bond a hydroxy‐functional organophosphorus oligomer (HFPO) onto 50/50 nylon/cotton blend fabrics. In this research, the HFPO/DMDHEU system was applied to a 50/50 polyester/cotton twill fabric. The polyester/cotton fabric treated with 36% HFPO and 10% DMDHEU achieved char length of 165 mm after 20 laundering cycles. The laundering durability of the treated fabric was attributed to the formation of polymeric cross‐linked networks. The HFPO/DMDHEU system significantly reduced peak heat release rate (PHRR) of cotton on the treated polyester/cotton blend fabric, but its effects on polyester were marginal. HFPO/DMDHEU reduced PHRR of both nylon and cotton on the treated nylon/cotton fabric. It was also discovered that the nitrogen of DMDHEU was synergistic to enhance the flame retardant performance of HFPO on the polyester/cotton fabric.  相似文献   
88.
89.
90.
Limited research has been conducted on the integration of Tablet-PCs in classroom instruction. This paper reports a qualitative study which investigates the acceptance of Tablet-PCs, seen as technological innovation, amongst teachers. The research approach intends to complement research on the acceptance of technology through a more detailed qualitative examination. Semi-structured interviews were conducted with 18 teachers during a pilot project introducing Tablet-PCs to classroom instruction at three different schools. The findings indicate diversity in the attitude of teachers towards the technology, but also with regards to the performance expectancy and the facilitating conditions.  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