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   129862篇
  免费   10447篇
  国内免费   5537篇
电工技术   7323篇
技术理论   12篇
综合类   8154篇
化学工业   21456篇
金属工艺   6851篇
机械仪表   7695篇
建筑科学   10179篇
矿业工程   3511篇
能源动力   3601篇
轻工业   8250篇
水利工程   2169篇
石油天然气   7025篇
武器工业   923篇
无线电   16324篇
一般工业技术   16645篇
冶金工业   6515篇
原子能技术   1466篇
自动化技术   17747篇
  2024年   593篇
  2023年   2182篇
  2022年   3668篇
  2021年   4893篇
  2020年   3770篇
  2019年   3183篇
  2018年   3622篇
  2017年   4058篇
  2016年   3583篇
  2015年   4689篇
  2014年   5873篇
  2013年   7441篇
  2012年   7842篇
  2011年   8521篇
  2010年   7466篇
  2009年   7276篇
  2008年   7198篇
  2007年   6761篇
  2006年   7007篇
  2005年   6134篇
  2004年   4237篇
  2003年   3763篇
  2002年   3581篇
  2001年   3399篇
  2000年   3256篇
  1999年   3738篇
  1998年   3123篇
  1997年   2740篇
  1996年   2456篇
  1995年   2060篇
  1994年   1654篇
  1993年   1325篇
  1992年   1058篇
  1991年   787篇
  1990年   640篇
  1989年   531篇
  1988年   406篇
  1987年   312篇
  1986年   218篇
  1985年   186篇
  1984年   117篇
  1983年   91篇
  1982年   92篇
  1981年   58篇
  1980年   49篇
  1979年   33篇
  1978年   25篇
  1977年   25篇
  1976年   40篇
  1973年   11篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
介绍了一个能为工厂设置零件编码系统和开发零件分类系统的工具系统。该系统能适应用户的具体环境,能对零件的特征信息做较详细的描述和建立相应目标的零件分类系统,从而能在GT(成组技术)基础上为机械加工零件开发CAD/CAPP/CAM集成系统提供支持,以满足CIMS的需要。  相似文献   
72.
Male strain A/J mice were exposed for 6 h a day, 5 days a week to environmental tobacco smoke (ETS) generated from Kentucky 1R4F reference cigarettes. Chamber concentrations were 87 mg/m3 of total suspended particulate matter (TSP), 246 p.p.m. of CO and 16 mg/m3 of nicotine. After 5 months, 33% of the ETS exposed and 11% of the control animals had one or several lung tumors; the difference was statistically not significant. A second group of animals exposed for 5 months to ETS was allowed to recover for another 4 months in filtered air. When they were killed, 85% of the ETS animals had lung tumors (average number per lung: 1.4 +/- 0.2), whereas in the control group 38% had lung tumors (average number of lung tumors in all animals 0.5 +/- 0.2). The differences in tumor incidence and multiplicity were statistically significant. More than 80% of all tumors were adenomas, the rest adenocarcinomas. When animals were pretreated with a carcinogen, lung tumor multiplicity was lower in the ETS exposed animals after 5 months compared with controls injected with a carcinogen and kept in air. However, after an additional 4 month recovery period in air, lung tumor multiplicities were the same in ETS plus carcinogen exposed mice as in carcinogen-treated air-exposed controls. Histopathologic and morphometric analysis of the lung tissue failed to reveal any differences between ETS exposed and control animals. However, immediately after ETS exposure, immunohistochemistry revealed increased staining for CYP1A1 in airway epithelia and lung parenchyma; following recovery in air, the staining disappeared again. Analysis of cell kinetics showed an initial burst of increased DNA synthesis in the epithelial cells of the airways and a smaller early positive response in the parenchyma. Feeding of butylated hydroxytoluene during ETS exposure did not modulate lung tumor development. It was concluded that ETS is a pulmonary carcinogen in strain A/J mice.  相似文献   
73.
阐述了国内外羰基镍测定的各种方法以及在工业与环境保护等领域的应用状。  相似文献   
74.
The present study determined tumorigenicity, tumor classification and DNA damage induced in infant mice by benzo[a]pyrene (B[a]P) or Manufactured Gas Plant (MGP) residues after a single exposure. Male and female B6C3F1 mice were exposed to B[a]P or MGP residue from a single environmental site (MGP-4) and males were also exposed to MGP residue composite from seven different sites (MGP-M7). At 26, 39 and 52 weeks after exposure tumorigenesis was assessed in lung, forestomach and liver. Formation and persistence of DNA adducts were quantified by 32P-postlabeling. Exposure of males to B[a]P induced liver tumors in a dose and time dependent manner. MGP induced more advanced tumors than B[a]P. Only a single liver tumor was found in MGP-4 treated females. No forestomach and few pulmonary adenomas were induced in males or females. MGP-4, MGP-M7 or B[a]P induced DNA adducts in males and females. Adducts in liver, lung and forestomach peaked on different days and decreased at different rates. At 24 h post-exposure, no significant differences in initial DNA adduct levels occurred in males and females exposed to MGP-4 or B[a]P. Lack of DNA damage (adducted DNA) did not account for non-responsiveness of lung and forestomach in B6C3F1 genders as well as in liver in females. MGP tumorigenicity could not be accounted for solely by B[a]P content nor did it reflect additivity of B[a]P and other carcinogenic polycyclic aromatic hydrocarbons (PAHs) in MGP. Synergy among MGP-PAHs, presence of unidentified carcinogens and/or promoters in MGP may account for MGP potency. The B6C3F1 infant male model is a convenient and rapid assay for assessing MGP liver tumorigenicity and potency.  相似文献   
75.
气流式加速度传感器的敏感机理   总被引:2,自引:0,他引:2  
介绍了在研制气流式加速度传感器过程中,为对气流式加速度传感器进行建模,作者对气流式加速度传感器的工作机理进行了理论性探讨。在分析过程中以流体力学、传热学和惯性理论为基础并结合实验结果,建立了适合于气流式加速度传感器的气体运动方程和能量方程。同时还讨论了热线测量加速度的方法及根据实验分析了输入加速度对腔体内温度及传感器输出电压的影响,从而为进一步研究一系列气流式传感器的工作机理提供了方法。  相似文献   
76.
PURPOSE: The purpose of this study was to assess the value of 3-dimensional sonography in the diagnosis of congenital müllerian duct anomalies, which cause infertility, preterm labor, and first trimester abortion. METHODS: A prospective study was undertaken in which 40 patients with histories of repeated spontaneous abortions or infertility were first examined using conventional 2-dimensional sonography or hysterosalpingography. Three-dimensional transvaginal sonography was then performed. RESULTS: Twenty-eight women had müllerian duct abnormalities, and 12 women had normal uterine anatomy. Müllerian duct defects detected in this study were unicornuate uterus (3), bicornuate uterus (3), complete or partial septate uterus (12), arcuate uterus (9), and didelphic uterus (1). The diagnosis of müllerian duct anomalies in these patients was confirmed by laparoscopic and/or hysteroscopic examinations. Three-dimensional sonography demonstrated all congenital uterine abnormalities with a sensitivity and specificity of 100%. Separate uterus and bicornuate uterus could be correctly diagnosed using 3-dimensional sonography in 11 (92%) of 12 cases and 3 (100%) of 3 cases, respectively. These 2 abnormalities were commonly confused with each other using hysterosalpingography and conventional sonography. CONCLUSIONS: Three-dimensional sonography with image reconstruction is less expensive and less invasive than hysterosalpingography for the assessment of uterine anatomy and diagnosis of müllerian duct abnormalities. The ability to visualize both the uterine cavity and the myometrium on a 3-dimensional scan facilitates the diagnosis of uterine anomalies and enables the differentiation of septate from bicornuate uteri for preoperative surgical planning.  相似文献   
77.
78.
讨论了指数自回归模型的辨识问题,证明了该模型最小二乘估计的目标函数的非凸性,并给出了使该函数为凸的条件,最后给出了辨识该模型的算法及该算法的收敛性,并以数值例子加以说明。  相似文献   
79.
80.
本文介绍了广播发射台利用计算机技术、网络技术、遥控遥测技术、现代测试手段,对设备进行自动化控制与管理;实现设备技术指标自动测试,发射台无人值班、有人留守,建立机器的计算机档案制度,确保安全优质播出的方法和经验。  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