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   53155篇
  免费   2489篇
  国内免费   156篇
电工技术   715篇
综合类   66篇
化学工业   10608篇
金属工艺   2137篇
机械仪表   3241篇
建筑科学   1102篇
矿业工程   25篇
能源动力   2111篇
轻工业   3891篇
水利工程   271篇
石油天然气   89篇
武器工业   2篇
无线电   7836篇
一般工业技术   10669篇
冶金工业   7344篇
原子能技术   658篇
自动化技术   5035篇
  2024年   51篇
  2023年   570篇
  2022年   870篇
  2021年   1484篇
  2020年   1079篇
  2019年   1176篇
  2018年   1429篇
  2017年   1414篇
  2016年   1753篇
  2015年   1294篇
  2014年   2089篇
  2013年   3008篇
  2012年   3267篇
  2011年   3889篇
  2010年   2815篇
  2009年   2921篇
  2008年   2812篇
  2007年   2185篇
  2006年   2031篇
  2005年   1726篇
  2004年   1572篇
  2003年   1520篇
  2002年   1329篇
  2001年   1134篇
  2000年   996篇
  1999年   1047篇
  1998年   2735篇
  1997年   1635篇
  1996年   1188篇
  1995年   793篇
  1994年   642篇
  1993年   640篇
  1992年   323篇
  1991年   321篇
  1990年   299篇
  1989年   271篇
  1988年   229篇
  1987年   209篇
  1986年   143篇
  1985年   138篇
  1984年   92篇
  1983年   70篇
  1982年   49篇
  1981年   55篇
  1980年   48篇
  1979年   32篇
  1978年   34篇
  1977年   103篇
  1976年   172篇
  1975年   22篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Exposure to pollutants in the air over a long period of time slowly degrades aluminum stranded conductors steel reinforced (ACSR) of overhead transmission lines. Presently, a large number of ACSR conductors have exceeded their forecast useful life. Therefore, it is required to assess the condition of most conductors to determine an economic maintenance schedule. This paper describes a method for evaluating the current condition of aged conductors based on dominant factors such as years in service, environment index, and conductor configuration. The paper presents a diagnostic tool with a fuzzy inference system (FIS) to predict the deterioration degree corresponding to the lifetime of aged conductors based on experts' knowledge. This system can be used as an effective guide to perform nondestructive diagnosis and maintenance of old ACSR conductors.  相似文献   
82.
Experimental studies were made of the influence of mixing and casting techniques and of curing time upon the compressive strength and hardness of methyl methacrylate based polymer concrete. The Rockwell K and F scales were found to be suitable for the hardness determinations. Over a wide range of hardness and strength values, the average compressive strength (fc) varied linearly with the average hardness (RK) according to the relation fc (psi) = 560 RK - 12000, irrespective of casting and curing variables. The weaker materials exhibited greater variations in hardness. Hardness testing is found to provide a meaningful and convenient method for evaluating the quality of polymer concrete.  相似文献   
83.
The feasibility of a new alloy design concept utilizing the principle of ‘tungsten bronze effect’ is critically evaluated for the development of metallic bipolar plates for proton exchange membrane fuel cell (PEMFC). An austenitic stainless steel (ASS) is modified with W and La to improve the stability of the passive film in an acidic environment as well as to reduce the contact resistance by the tungsten bronze effect. The experimental ASS containing W and La was evaluated in a simulated PEMFC environment of H3PO4 and H2SO4 solutions at 80 °C, and the electrical property was evaluated by performing a contact resistance test. The test results show that the ASS modified with W and La has good passive film stability for corrosion resistance and low contact resistance. The X-ray photoelectron spectroscopy (XPS) analysis clearly suggests the possibility of the tungsten bronze effect from the change in valency state of W6+ to W5+ in the passive film formed on the modified ASS. The feasibility of a new alloy design concept utilizing the ‘tungsten bronze effect’ is well demonstrated; however, more study is highly required for the development of metallic bipolar plates of PEMFC.  相似文献   
84.
85.
86.
Ancylostoma duodenale is still the dominant hookworm species in the Mediterranean area, India, China and Japan. In the present study, biopsied materials were taken from the small intestine of 30 patients infected only with A. duodenale and 12 cross matched controls. The results showed some pathological changes in severely infected cases. However, normal or insignificant changes were seen in the enzymatic activity of the intestinal mucosa.  相似文献   
87.
This study reports the establishment of alpha-amylase-producing human parotid pleomorphic adenoma cell lines (2HP and 2HP1) which have been maintained in culture for over 1 yr. The procedures required preparation of cellular clumps from tumor tissue and plating them on plasma clot or precoated dishes. During the initial phase of growth they required modified MCDB-153 medium without serum. When cells showed signs of degeneration they were changed to MCDB-153 medium containing first 2% and then 10% heat inactivated fetal bovine serum. Although cells grew well in MCDB-153 containing 10% serum, the epithelial cell morphology was not distinct. Therefore, the growth and morphology of cells grown in MCDB-10% serum were compared with those in RPMI growth medium containing 10% fetal bovine serum and F12 containing 10% agammaglobulin newborn bovine serum. Although the growth of cells was a little slower in F12 medium than those in MCDB and RPMI, the epithelial cell morphology was maintained better than in other growth media. The cells of 2HP and 2HP1 produce low levels of alpha-amylase and relatively high levels of alpha-amylase mRNAs of 1176 and 702 bp and contain neurofilament-160, a neuronal-specific marker. The cells of 2HP1 are tumorigenic when tested in athymic mice, but the cells of 2HP are not. The establishment of amylase-producing human parotid adenoma cell lines of different characteristics in culture provides a new opportunity to study the mechanisms of differentiation and transformation, and regulation of alpha-amylase in these cells.  相似文献   
88.
89.
In this study a series of 3-arylisoquinoline derivatives were synthesized and cytotoxicity against human melanoma tumor cell evaluated, and a three dimensional quantitative structure-activity relationship was investigated using the comparative molecular field analysis (CoMFA). The results suggested that the electrostatic, steric and hydrophobic factors of 3-arylisoquinolines were strongly correlated with the antitumor activity. Considerable predictive ability (cross-validated r2 as high as 0.721) was obtained through CoMFA.  相似文献   
90.
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