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   409篇
  免费   47篇
  国内免费   11篇
电工技术   12篇
综合类   21篇
化学工业   57篇
金属工艺   29篇
机械仪表   25篇
建筑科学   33篇
矿业工程   16篇
能源动力   12篇
轻工业   27篇
水利工程   4篇
石油天然气   6篇
武器工业   2篇
无线电   62篇
一般工业技术   48篇
冶金工业   6篇
原子能技术   10篇
自动化技术   97篇
  2025年   2篇
  2024年   12篇
  2023年   18篇
  2022年   19篇
  2021年   32篇
  2020年   43篇
  2019年   30篇
  2018年   21篇
  2017年   31篇
  2016年   16篇
  2015年   10篇
  2014年   15篇
  2013年   30篇
  2012年   63篇
  2011年   40篇
  2010年   32篇
  2009年   8篇
  2008年   6篇
  2007年   3篇
  2006年   1篇
  2005年   1篇
  2003年   1篇
  2002年   1篇
  2001年   5篇
  1999年   4篇
  1997年   1篇
  1996年   1篇
  1995年   1篇
  1993年   1篇
  1991年   2篇
  1990年   1篇
  1989年   1篇
  1987年   4篇
  1986年   4篇
  1984年   1篇
  1983年   1篇
  1978年   1篇
  1968年   1篇
  1967年   1篇
  1964年   2篇
排序方式: 共有467条查询结果,搜索用时 0 毫秒
1.
    
A novel process of calcification-leaching for bastnaesite concentrate(REFCO_3) was proposed. The prior calcification was carried out in the system of NaOH-CaO-H_2O and the lgC-pH pattern for Ce-F-Ca-C-H_2O system was drawn. The thermodynamics result indicates that decomposition for bastnaesite requires certain alkaline condition, but excessive alkalinity also causes decomposition of CaF_2. XRD and SEM-EDS analyses on the calcification-leaching process reveal that bastnaesite first decomposes into RE(OH)_3 and CaF_2. Then, by HCl leaching rare earths were extracted,while CaF_2 was left in the leaching residue. In addition, effects of temperature, time, NaOH and CaO on the calcification were investigated. The results show that the leaching rate of rare earths(REs)reaches 72.5 wt%, at the same time 99.2 wt% of F is left in leaching residue with 20 wt% NaOH and 38 wt% CaO at 493 K for 180 min.  相似文献   
2.
    
The crushed-rock layer is a highly porous medium that has been used to ensure the stability of embankment in permafrost regions. At present, depending on different boundary conditions (impermeable and permeable) of crushed-rock layer in embankment, the crushed-rock embankments are divided into two kinds of structures in the construction of Qinghai–Tibetan railway in China. One is a closed-boundary crushed-rock embankment; the other is an open-boundary crushed-rock embankment. In order to investigate the influence of boundary conditions (impermeable and permeable) on the cooling effect of a crushed-rock embankment, two numerical models of the unsteady two-dimensional hydrokinetic equations for incompressible fluid are presented to analyze the velocity and temperature characteristics of crushed-rock embankment with different embankment heights under impermeable and permeable boundary conditions for a period of 50 years. The results indicate: (1) the boundary conditions (impermeable and permeable) of crushed-rock embankment can have a very large impact on the heat transfer pattern within it in windy permafrost regions of Qinghai–Tibetan Plateau. The cooling effect of the closed crushed-rock embankment mainly relies on natural convection within crushed-rock layer, which is caused by the thermal boundary condition, but the cooling effect of the open crushed-rock embankment is due to the heat transfer enhancement because of internal forced convection induced by the external low temperature air flow (wind); (2) from the temperature distributions of crushed-rock embankments, it can be found that, under the assumption that the air temperature will be warmed up by 2.6 °C in a period of 50 years and in the areas where the mean annual air temperature is − 4.0 °C, when embankment is low, the cooling effects of crushed-rock embankment have no obvious difference under the two boundary conditions, and the cooling effect of closed crushed-rock embankment is only a little better than that of open one; however, when embankment is high, the boundary conditions cause a distinct influence on the temperature distribution of crushed-rock embankment, and the cooling effect under the permeable boundary condition is far better than that under the impermeable boundary condition. However, the asymmetric temperature distribution problem of the high crushed-rock embankment, caused by permeable boundary and external wind, must be considered when it is designed and constructed.  相似文献   
3.
基于总体最小二乘的红外图像去噪   总被引:2,自引:1,他引:2       下载免费PDF全文
杨鸿森 《激光与红外》2008,38(9):961-964
针对红外图像存在的加性、乘性及混合噪声,采用从图像中截取图像块,再用图像块的线性结合对原图像进行去噪,总体最小二乘算法用来求解其中的系数向量,充分考虑了噪声图像中存在的不确定性,通过这组系数得到去噪后的红外图像.在对像素点空间关系权重的求解上,采用模糊核聚类算法将红外图像粗略进行聚类,归为同一类的像素点之间存在较强空间约束关系,否则认为它们之间存在较弱空间约束.通过与维纳滤波算法比较,仿真结果证明了总体最小二乘去噪算法在红外图像的视觉质量和信噪比改善两个方面的有效性.最后通过比较无噪红外图像与去噪红外图像的直方图表明总体最小二乘去噪算法的优越性.  相似文献   
4.
复卷机伺服张力控制系统设计及优化   总被引:1,自引:0,他引:1       下载免费PDF全文
本研究通过对复卷机工艺流程进行分析,分别对纸幅张力、速度和纠偏的控制系统进行了设计及优化,采用罗克韦尔PLC作为主控制器,结合伺服驱动器和伺服电机等,实现对纸幅张力的稳定控制。该控制系统在实际生产中取得了良好的控制效果,张力调整时间由之前的10 s缩短到5 s内,并且无超调量。  相似文献   
5.
面源红外诱饵使用的过程中,合理有效的干扰策略是干扰红外制导导弹的关键,针对这一问题,通过理论分析与仿真模拟相结合得到面源红外诱饵对抗红外制导导弹的干扰策略。首先以红外成像制导导弹为例,介绍红外制导导弹的仿真模型,然后将攻击过程分为锁定前与锁定后,以命中率为评价指标,得到两个阶段下的最佳干扰策略,包括诱饵的最佳释放时机、释放间隔以及载机应采取的机动动作等。  相似文献   
6.
将亲水基团引入硝化纤维素分子中是消除硝基涂料VOC含量过高缺陷的重要手段。为获得最佳反应条件,分析了异氰酸酯与硝化纤维素分子反应动力学的特点;通过测定不含催化剂及不同催化剂浓度、不同反应温度下异氰酸酯浓度随时间的变化,配合数学处理,得到了两者反应时的活化能、最佳催化剂浓度、不同条件下的反应速率常数等参数,推导了反应机理。认为异氰酸酯与硝化纤维素反应为二级反应,催化剂的加入可极大提高反应速率,并降低活化能50%。  相似文献   
7.
介绍了氟含量的4种检测方法在牙膏分析领域的应用进展,目的是为在不同实验条件下选择准确、简便、快速测定牙膏中氟含量的检测方法提供借鉴。  相似文献   
8.
根据经验数据初步设计离心式驱动器叶轮,用CATIA软件建立其实体模型并利用FLUENT软件进行流场分析,得出流量-压力、流量-效率曲线图。分析表明:当叶轮转速为5 500 r/min、流量在12~30 L/min内时,流体压力可超过26.6 kPa(200 mmHg),工作效率可达80%以上。并利用流体质点速度分析影响效率的主要因素。  相似文献   
9.
    
《Ceramics International》2022,48(17):25049-25055
Recently, composite materials with outstanding absorption properties, like extraordinary absorbing capability, light weight, and thin in size, are required to solve the challenges of electromagnetic pollution. In addition, most of the work is based on the optimization of absorber material structure, and microstructure. In the current work, we improved the reflection loss feature of Bi0.5Nd0.5FeO3 nanopowders via decoration with polyindole polymer by tuning the filler loading of the nanocomposite in the matrix. XRD, UV–Vis, XPS, and FESEM were used to determine the physicochemical features of the as-prepared nanocomposite. The minimum RL was lowered further with the increasing filler loading at 25 wt%. The lower RL of ?22 dB was noticed for 2.2 mm thickness at 11.5 GHz. The maximum value of the SER for a 25 wt% sample was 5.5, whereas 19 dB and 24.5 dB values were recorded for SEA and SET, respectively. The resonance peak above 11.5 GHz demonstrated the better outcome of the absorber at high frequency. Good impedance matching characteristics, conductive features, dielectrics, and magnetic losses were all credited with the excellent reflection loss and electromagnetic interference shielding efficiency. The as-prepared nanocomposite materials that have been proven are interesting prospects for electromagnetic reflection loss and interference shielding that is lightweight, flexible, and extremely effective.  相似文献   
10.
针对一些电动机对供电品质要求较高的问题,提出了一种新的正弦波变换器实现方法。通过分析单极性倍频调制PWM原理,用高速TMS320F2812DSP作处理芯片,驱动电路采用IGBT智能模块M57959L,结果使得正弦变换器体积减小、可靠性增强。设有合理的过电压保护和LC带阻滤波以及RLC低通滤波双重滤波,保证工频升压变压器输出高质量的正弦电压交流电。最后进行DC-AC仿真实验,并对样机进行正弦波形测量,验证了方案的正确性。  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