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   289837篇
  免费   30775篇
  国内免费   20032篇
电工技术   17957篇
技术理论   10篇
综合类   28159篇
化学工业   40065篇
金属工艺   14888篇
机械仪表   15250篇
建筑科学   27772篇
矿业工程   7103篇
能源动力   7272篇
轻工业   17919篇
水利工程   6239篇
石油天然气   10569篇
武器工业   2947篇
无线电   28708篇
一般工业技术   33401篇
冶金工业   9261篇
原子能技术   3467篇
自动化技术   69657篇
  2024年   1262篇
  2023年   3885篇
  2022年   8673篇
  2021年   9418篇
  2020年   8122篇
  2019年   7079篇
  2018年   7194篇
  2017年   8655篇
  2016年   10717篇
  2015年   12544篇
  2014年   17887篇
  2013年   17567篇
  2012年   18377篇
  2011年   19370篇
  2010年   15630篇
  2009年   17531篇
  2008年   17255篇
  2007年   20032篇
  2006年   18045篇
  2005年   15129篇
  2004年   11927篇
  2003年   10833篇
  2002年   9365篇
  2001年   7196篇
  2000年   6975篇
  1999年   5840篇
  1998年   4729篇
  1997年   4233篇
  1996年   4016篇
  1995年   3808篇
  1994年   3469篇
  1993年   2198篇
  1992年   2147篇
  1991年   1518篇
  1990年   1251篇
  1989年   1019篇
  1988年   882篇
  1987年   513篇
  1986年   368篇
  1985年   527篇
  1984年   547篇
  1983年   540篇
  1982年   456篇
  1981年   475篇
  1980年   383篇
  1979年   150篇
  1978年   147篇
  1977年   98篇
  1975年   80篇
  1962年   69篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
大泵站超驼峰水位运行压缩空气断流系统的设计方法   总被引:5,自引:0,他引:5  
阐述了压缩空气断流的原理,提出了压缩空气断流装置的一般构成和设计方法,首次应用气体喷射器作为压缩空气减压增量变换器,不但及时地增加了补气量,防止断流后流道压力的下降,而且大大减小了储气罐容积,使该项技术得以满足工程实际的要求,为同类泵站的技术改造  相似文献   
992.
We have investigated the electrochemical noise behavior of carbon steel in fully deaerated aqueous bicarbonate solutions, and discussed the optimum conditions of the noise analysis for estimating corrosion rate of the steel. Noise of the potential difference and of the short-circuit current between two identical steel coupons were successfully measured. The time-series noise patterns were transformed into frequency domain by fast Fourier transformation, and then their power spectrum densities (PSDs) at a frequency were determined to be compared with the corrosion rate. The PSDs of the potential and of the current varied with changing environmental factors of bicarbonate concentration, pH, and immersion time. The factors also controlled the corrosion rate of the steel. The PSDs were associated with the corrosion rate, and then it was found that the PSDs of the potential and of the current showed linear correlation with the corrosion rate in log-log scale. There was also linear relationship between the corrosion rate and a spectral noise resistance obtained from the PSDs of the potential and the current. The linearities of the three correlations were better at a lower analyzed frequency. Furthermore, the PSDs of the current and the noise resistance indicated more linear correlation with the corrosion rate than that of the potential. As the simplicity of the measurement system is additionally considered, it is concluded that the PSD of the current noise at an analyzed frequency of 3 mHz is the optimum conditions for estimating the corrosion rate from 10−2 to 100 A m−2 in this study.  相似文献   
993.
高砂比压裂技术研究   总被引:5,自引:1,他引:4  
蒋廷学  李斐 《钻采工艺》1998,21(2):13-16
应用由美国NSI公司引进的“stimplan”拟三维压裂设计软件,对影响裂缝内支撑剂铺置浓度的诸因素进行了灰色关联度分析,从中找出了主要的影响因素。并以高砂比压裂的技术指标为依据,研究了在目前压裂工艺水平下,进行高砂比压裂所必须遵循的原则。文中还对国内的高砂比压裂进行了实例分析,进一步论证了文中的观点。  相似文献   
994.
由于密码分析研究的进展及DES自身的弱点,原64比特的DES将不能作为数据加密的标准算法而长期存在。在原来工作的基础上,本文提出了256比特的分组密码方案。密码算法由基于混沌映射的数字滤波器构造。  相似文献   
995.
The enantiomers of catechin and epicatechin were separated by chiral capillary electrophoresis using modified cyclodextrins as chiral selectors. Various conditions for the separation system were optimized, including the pH value and the concentration of the running buffer. A baseline separation of the catechin and epicatechin enantiomers could be achieved by using 0.1 mol l−1 borate buffer (pH 8.5) with 12 mmol l−1 (2-hydroxypropyl)-γ-cyclodextrin as chiral selector, a fused-silica capillary with 40 cm effective length (75 μm I.D.), +18 kV applied voltage, a temperature of 20 °C and direct UV detection at 280 nm. The method was applied to different plant food samples. (+)-Catechin and (−)-epicatechin could be verified as the most common flavan-3-ols. In the case of guaraná, however, we were able to identify all four enantiomers, both (+)- and (−)-catechin and (+)- and (−)-epicatechin, as naturally occurring compounds. This finding was verified by further isolation and purification of the flavan-3-ols and subsequent LC–MS analysis. This method allows for the identification of the authenticity of guaraná through the analysis of the catechin and epicatechin enantiomers, additionally to the conventional methods like HPLC.  相似文献   
996.
刘可辛  罗升旭 《微电子学》1989,19(3):16-18,7
本文报导了MOS结构在准静态测试中的异常电容-电压曲线。讨论了它们产生的原因。  相似文献   
997.
全国农田水利建设先进县的经验与启示   总被引:3,自引:2,他引:1  
针对农田水利建设下滑的被动局面,各地积极探索,从加强领导、拓宽投入渠道、创新体制和机制等方面积极采取措施,取得了很大成效。对国务院2006年表彰的100个全国农田水利建设先进县(市、区)中的35个县(市、区)进行了调研,总结了它们的主要做法、经验,提出农田水利建设先进县的三点启示。  相似文献   
998.
999.
Heat-shock induction of heat-shock protein genes is due to a specific promoter element (the heat-shock element, HSE). This study used lacZ under HSE control (HSE-lacZ) to characterize HSE activity in Saccharomyces cerevisiae cells of different physiological states and differing genetic backgrounds. In batch fermentations HSE-lacZ induction by heat shock was maximal in exponential growth, and showed marked decline with the approach to stationary phase. Expression in the absence of heat shock was unaffected by growth phase, indicating that the growth-dependent expression of many yeast heat-shock genes uses promoter elements in addition to the HSE. Heat-induced expression was strongly influenced by the temperature at which cultures were grown. While basal, uninduced expression was constant during growth at different temperatures to 30 degrees C, induction by transfer to 39 degrees C was reduced by increases in growth temperature as low as 18-24 degrees C. Maximal HSE-lacZ induction (30- to 50-fold) was in cultures grown at low temperatures (18-24 degrees C), then heat shocked at 39 degrees C. Ethanol was a poor inducer. Mutations having little effect on HSE-lacZ expression included a respiratory petite; ubi4 (which inactivates the poly-ubiquitin gene); also ubc4 and ubc5 (which each inactivate one of the ubiquitin ligases involved in degradation of aberrant protein). pep4-3 increased both basal and induced beta-galactosidase about two-fold, probably because of slower turnover of this enzyme in pep4-3 strains.  相似文献   
1000.
PDC钻头在胜利油田深井钻井中的应用   总被引:5,自引:2,他引:3  
胜利油田为提高深井钻井速度,试验应用了PDC钻头,结果表明,深井钻井应用PDC钻头可大幅度提高机械钻速,缩短建井周期,降低钻井成本,经济效益明显。通过大量的现场试验,扩大了PDC钻头在胜利油田的使用范围,并取得很好的效果,初步形成了PDC钻头推广应用区域模式。详细介绍了胜利油田不适用PDC钻头钻进的地层岩性,给出了钻头选型及钻井参数选择原则,并对应用效果进行了分析。  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