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   215207篇
  免费   27474篇
  国内免费   6981篇
电工技术   10375篇
技术理论   9篇
综合类   9893篇
化学工业   45389篇
金属工艺   11026篇
机械仪表   11525篇
建筑科学   12612篇
矿业工程   4255篇
能源动力   4762篇
轻工业   18124篇
水利工程   3777篇
石油天然气   9982篇
武器工业   1187篇
无线电   25506篇
一般工业技术   38530篇
冶金工业   12015篇
原子能技术   4992篇
自动化技术   25703篇
  2024年   704篇
  2023年   2586篇
  2022年   5261篇
  2021年   7308篇
  2020年   6572篇
  2019年   7340篇
  2018年   8443篇
  2017年   9360篇
  2016年   9257篇
  2015年   10152篇
  2014年   11904篇
  2013年   14727篇
  2012年   13212篇
  2011年   13695篇
  2010年   12216篇
  2009年   11727篇
  2008年   11199篇
  2007年   10478篇
  2006年   9568篇
  2005年   8057篇
  2004年   6343篇
  2003年   6220篇
  2002年   6199篇
  2001年   5617篇
  2000年   4930篇
  1999年   3866篇
  1998年   2734篇
  1997年   2316篇
  1996年   2043篇
  1995年   1794篇
  1994年   1519篇
  1993年   1294篇
  1992年   1238篇
  1991年   1110篇
  1990年   1118篇
  1989年   1039篇
  1988年   915篇
  1987年   861篇
  1986年   778篇
  1985年   728篇
  1984年   707篇
  1981年   677篇
  1979年   748篇
  1978年   782篇
  1977年   750篇
  1976年   763篇
  1975年   717篇
  1974年   719篇
  1973年   725篇
  1972年   708篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
The influence of minor amounts of pro‐ and anti‐oxidants on the kinetics of the autoxidation of fat has been evaluated. The reaction rates of oxygen with the substrates were found to follow the same basic equation, hitherto established for pure substrates. There is evidence that the surface of the reaction vessel also acts as a reaction catalyst and its effect is proportional to the area of glass in contact with lipids. Oxidation is enhanced by trace metal ions as well as by surface‐active compounds (e.g. hydroperoxides and sterols). Antioxidants such as α‐tocopherol and butylated hydroxyanisole inhibit the oxidation by delaying the start of oxygen consumption (the induction period) while retarders like amino acids only decrease the rate of oxidation. Thus pro‐ and anti‐ oxidants affect either the start or the rate of oxygen consumption. The empirical formula dx/dt = k [O2] (1‐x/n) f′(t) was found applicable to the different stages of oxidation.  相似文献   
982.
本文对多目标二次规划问题非劣解集的理论生成进行了探讨,在介绍二次规划单目标问题优化解求法的基础上,提出了在决策平面上互切点,共交点轨迹的概念.由此导出了非劣解集的一般解析式(对切点解问题)和另一类交点解问题时的非劣解集求法,然后介绍了在目标平面非劣解集的解析表达式.使多目标二次规划问题整个非劣解集的推求严格、明确和方便,有理论和实际意义。文中以一个综合利用水库参数的多目标优选作为例子阐述了应用,并求出了最终的最佳协调解.  相似文献   
983.
The process of mechanoluminescence transformation of a pulse pressure sensor is considered. The process consists in excitation of emission under the action of mechanical loading. An algorithm for use in processing the output optical signal of the sensor that makes it possible to determine an input shock pulse is presented. __________ Translated from Izmeritel’naya Tekhnika, No. 10, pp. 28–31, October, 2007.  相似文献   
984.
于奎伟  胡寿 《物探装备》2007,17(4):272-275
随着Macha遥爆系统在地震勘探中使用年限的增加,该系统在工作中时常出现各种各样的故障,本文根据其日常检修记录,分析总结了Macha系统常见的十一例故障,并从故障的现象、分析、排除三个方面对各个故障进行了详细的介绍。  相似文献   
985.
对ASON在当前应用中可能碰到的几个问题进行了分析,并提出了看法。  相似文献   
986.
A semi-Markov model is constructed that describes an arbitrarily structured multicomponent technological system and takes into account its calendar maintenance. Using an aggregation algorithm, approximate values of stationary reliability characteristics and an optimal term of maintenance work are determined for the system. __________ Translated from Kibernetika i Sistemnyi Analiz, No. 2, pp. 69–86, March–April 2006.  相似文献   
987.
Floods cause environmental hazards and influence on socio‐economic activities. In this study, we evaluated the historic flood frequency at a confluence in the middle Yellow River, China. A non‐parametric, multivariate, empirical, orthogonal function matrix model, which consists of time correlation coefficients of flood discharge at different gauge stations and flood events was used for the analysis of flood frequency. The model addresses the characteristics of confluent floods such as frequency and the probability in multiple tributary rivers. Flood frequency analysis is often coupled with studies of hydrological routing processes that reduce the flood capacity of the rivers. Flood routing to the confluence were simulated using kinematic wave theory. Results of this flood frequency analysis showed that flooding frequency has intensified in the past 500 years, especially during the 19th century. Flooding in streams above the confluence was more frequent than in streams below the confluence. Over the last 2000 years, concurrent flooding in multiple tributary rivers accounted for 67.5% of the total flooding in the middle Yellow River. Simulation of flood routing processes shows that the decreased flooding capacity and elevated river bed of the shrunken main channel leads to an increased flood wave propagation time (24–52.3 h) in the study area after 1995. The model indicates that human activities, such as constructions of the Sanmenxia Dam, have changed flood routing boundary conditions and have contributed to the increased flood frequency at the confluence. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
988.
TiO_2纤维的制备、表征及其光催化活性   总被引:3,自引:2,他引:1  
黄垒  彭峰  余皓  王红娟 《石油化工》2007,36(1):78-82
采用常压乙二醇法制备出在可见光下具有较高催化活性的TiO2纤维。扫描电子显微镜表征结果表明,TiO2纤维直径为0.8~2.0μm、长度为5~25μm;BET比表面积和X射线衍射表征结果表明,随焙烧温度的升高,比表面积减小,TiO2纤维晶型逐渐由锐钛矿型向金红石型转变;紫外-可见漫反射光谱分析结果表明,TiO2纤维在可见光区有明显的吸收。在可见光下对甲基橙和苯酚的降解实验表明,焙烧温度为400℃时TiO2纤维的催化活性最好,对甲基橙和苯酚的降解率分别达到90.3%和84.0%;所制备的TiO2纤维比溶胶-凝胶法制备的锐钛矿型TiO2在可见光下具有更好的催化活性,原因是其能带间隙减小和吸收波长扩展到可见光区所致。  相似文献   
989.
Large woody debris (LWD) can increase stream habitat heterogeneity by providing structure, altering flow patterns, enhancing sediment deposition, forming pools and retaining organic matter. In North America, the role of LWD has been studied extensively in streams of mature forests (e.g. Pacific Northwest), but few studies have assessed LWD in streams of younger forests (e.g. Midwestern USA). Our objectives were to: (1) quantify the volume and abundance of LWD in a set of Midwestern streams; (2) evaluate possible factors influencing LWD quantity; (3) identify the functional roles of LWD; and (4) compare LWD levels in the upper Midwest to those elsewhere in North America. In 2002 and 2003, we measured LWD and geomorphological variables in 15 low‐gradient streams draining previously logged watersheds in the Upper Peninsula of Michigan. Mean (±SE) LWD volume (0.77 ± 0.12 m3 100 m−2) and abundance (33 ± 3 pieces 100 m−1) were 71% and 10% lesser, respectively, than in streams of similar gradient elsewhere in North America. Channel shape (width:depth ratio) explained 30% of the variation in LWD volume (multiple stepwise regression, P = 0.015) while LWD length and length:channel width combined, explained 72% of the variation in LWD density (multiple stepwise regression, P < 0.0001). About 50% of the LWD either stored sediment or stabilized banks and 14% of the LWD formed pools, although pool density was not significantly related to LWD volume or density. LWD levels, overall, were low in upper Midwestern streams, but the relative importance of that LWD to ecosystem function may be magnified in these wood‐poor systems. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
990.
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