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   7892篇
  免费   854篇
  国内免费   471篇
电工技术   642篇
综合类   540篇
化学工业   1251篇
金属工艺   422篇
机械仪表   473篇
建筑科学   605篇
矿业工程   232篇
能源动力   244篇
轻工业   562篇
水利工程   220篇
石油天然气   287篇
武器工业   97篇
无线电   1014篇
一般工业技术   1083篇
冶金工业   371篇
原子能技术   106篇
自动化技术   1068篇
  2024年   32篇
  2023年   144篇
  2022年   274篇
  2021年   383篇
  2020年   318篇
  2019年   271篇
  2018年   262篇
  2017年   299篇
  2016年   307篇
  2015年   378篇
  2014年   485篇
  2013年   622篇
  2012年   623篇
  2011年   601篇
  2010年   501篇
  2009年   483篇
  2008年   459篇
  2007年   432篇
  2006年   363篇
  2005年   318篇
  2004年   210篇
  2003年   194篇
  2002年   236篇
  2001年   202篇
  2000年   120篇
  1999年   113篇
  1998年   114篇
  1997年   75篇
  1996年   69篇
  1995年   47篇
  1994年   40篇
  1993年   34篇
  1992年   26篇
  1991年   28篇
  1990年   16篇
  1989年   17篇
  1988年   8篇
  1987年   7篇
  1986年   12篇
  1985年   9篇
  1984年   12篇
  1983年   9篇
  1982年   11篇
  1981年   8篇
  1980年   6篇
  1979年   9篇
  1978年   4篇
  1976年   7篇
  1970年   3篇
  1969年   4篇
排序方式: 共有9217条查询结果,搜索用时 31 毫秒
71.
1-苯基-3-甲基-5-吡唑啉酮的合成工艺研究   总被引:1,自引:0,他引:1  
王树清  高崇 《染料与染色》2004,41(2):114-115
在无水乙醇的存在下,乙酰乙酸乙酯和苯肼为原料合成标题化合物、采用单因素实验法,得到最佳配比为乙酰乙酸乙酯:苯肼=1.05(摩尔比),无水乙醇50m,反应温度70℃,反应2小时,再回流7小时,收率90.9%,产品纯度99.0%以上。  相似文献   
72.
In previous works on current measured by along-track interferometric synthetic aperture radar (ATISAR), the decorrelation function of an ocean-surface backscattering signal was usually assumed to be a real Gaussian function, i.e. the phase term was omitted. In this study, it is proved that the omission of the phase term included in the decorrelation function results in a significant estimation bias that can be modelled by a higher-order function of time lag and that the coefficients of this higher-order function can be expressed as a series of higher-order Doppler spectral moments. This model is validated by the scatterometer data obtained from an experimental wind-wave tank. The estimation bias especially needs to be considered for the ATISAR system with a long time lag. Simulation results show that if the time lag is equal to the coherence time, the estimation bias of the current can reach about 0.2 m s?1, which is not insignificant in high-precision current-retrieving applications. However, because most real-life ATISAR systems, including TerraSAR-X, operate with time lags significantly shorter than the expected coherence time, the estimation biases in these systems are relatively small or even negligible. Finally, four possible compensation methods for the estimation bias are proposed and discussed.  相似文献   
73.
The alpine ecosystem is one of the most fragile ecosystems threatened by global climate change. The impact of climate variability on the vegetation dynamics of alpine ecosystems has become important in global change studies. In this study, spatially explicit gridded data, including the Moderate Resolution Imaging Spectroradiometer (MODIS) land-surface temperature (LST) product (MOD11A1/A2), the Tropical Rainfall Measuring Mission (TRMM) rainfall product (3B43), and MODIS net primary productivity (NPP) product (MOD17A3), together with meteorological observation data, were used to explore the spatio-temporal pattern of climate variability and its impact on vegetation dynamics from 2000 to 2012 in the Lancang River headwater area. We found that the variation patterns of LST, precipitation, and NPP in the study area showed remarkable spatial differences. From the northwest to the southeast the spatial variation of average annual LST exhibited a decreasing–increasing–decreasing–increasing pattern. At the same time, most of the study area exhibited an increasing LST during the growing season. The annual precipitation increased in the semi-arid northern part, whereas it decreased in the semi-humid southern part. The precipitation variability during the growing season has a pattern similar to the annual precipitation variability. Although the majority of the regions have seen an NPP increase from 2000 to 2012, the responses of the vegetation to the varied climate factors were spatially heterogeneous. The alpine–subalpine meadows in the high-altitude areas were more sensitive to climate variability in the growing season. It is argued that satellite remote-sensing products have great potential in investigating the impact of climate variability on vegetation dynamics at the finer scale, especially for the Lancang River headwater area with complex surface heterogeneity.  相似文献   
74.
Poly(ethylene terephthalate) films were treated with aqueous sodium hydroxide solutions of different concentrations for various times. The rate of weight loss increased with the addition of a swelling agent (methylene chloride) or a cationic surfactant. The surface roughness of the treated films was determined from atomic force microscopy (AFM) and pore diameter was obtained from scanning electron microscopy (SEM). In general, surface roughness was found to increase with increasing weight loss for the treated films. A maximum roughness was obtained for samples with a weight loss of approximately 15-20%, beyond which the roughness of the samples decreased. The addition of methylene chloride and surfactant resulted in an almost two-fold increase in the roughness for all treatment times investigated. The adhesion of electrolessly plated platinum film was dependent on the contact area produced by chemical treatment. Treatments producing smaller diameter pores of greater depth gave better adhesion.  相似文献   
75.
The aggregation pheromone ofGnathotrichus retusus was isolated and identified as (S)-(+)-sulcatol (6-methyl-5-hepten-2-ol). In laboratory and field experiments,G. retusus responded to (S)-(+)-sulcatol, but not to (±)-sulcatol, which was attractive to the sympatric species,G. sulcatus. G. sulcatus did not respond to optically pure (S)-(+)-sulcatol, but began to respond when 1% (R)-(–)-sulcatol was present in an enantiomeric mixture.Research supported by the National Science Foundation, U.S.A. (Grant BMS-74-13643), and the Natural Sciences and Engineering Research Council of Canada (Co-op grant A0243 and Operating Grant A3881 and A3785).  相似文献   
76.
重金属在宽叶香蒲人工湿地系统中的分布与积累   总被引:46,自引:0,他引:46  
本文通过对广东韶关凡口宽叶香蒲(Typha latifolia)人工湿地系统的进出水口,土壤剖面及四种优势植物体各部分器官中Pb,Zn,Cu和Cd含量进行分析。结果表明;重金属主要库存于土壤中。从水平上看,全量的重金属含量从进水口到出水口呈上升趋势,而有效态则相反;从土壤剖面的垂直分布上看,全量和有效态重金属含量都随土壤深度的增加而递减,四种植物都具有较强的吸收和富集重金属的能力,且主要富集在植物的地下部分。  相似文献   
77.
刘贤梅  闫冲 《计算机与数字工程》2014,(11):2022-2025,2098
为了提高Excel软件的数据处理效率,需建立一个能满足高精确度、高效率、易使用的计算环境。论文建立圆柱螺线优化插值模型,在Excel中通过五组测斜数据计算井眼曲率,再根据云计算可弹性增加计算节点的特点,将Sympho‐ny DE并行软件搭建在本地私有云环境下,通过增加计算节点数目对比运算效率与计算精度。结果表明:随着计算节点数的增加,运算效率大大提高,计算结果更加精确。  相似文献   
78.
实时通信技术在移动互联网的应用已得到广泛的认可,关系较为紧密的用户需要进行实时性更强的富媒体通信,这要求在网络连接并不稳定的移动互联网环境,提供相对可靠的实时通信机制.本文将使用ELGG社交网络平台,采用MQTT作为消息传输协议,利用手机广播用户已有的社交关系,设计实现了基于消息通知的实时通信机制体系结构,并且通过手机终端信息的交互验证了这种通信方式的可行性.  相似文献   
79.
Polyvinylpyrrolidone (PVP)/sodium montmorillonite (MMT) nanocomposites prepared via the solution intercalation method were investigated by UV/vis, SEM, X-ray diffraction, TEM, FT-IR and PLM (polarized light microscopy). PVP/MMT nanocomposites show exfoliation below 20 wt% MMT and intercalation above this concentration. Nanocomposites retain good optical clarity and increased thermal resistance with MMT content. The compatibility between PVP and MMT and their enhanced properties may be explained by hydrogen bonding interactions. In addition, the nanocomposites prepared under more rigorous mixing conditions show better transparency because the smaller particle sizes are induced. In addition, the study on optically clear PVP/MMT suspensions helps one to understand how optical anisotropy of MMT is affected by the existence of polymer in aqueous solution.  相似文献   
80.
低雾化低挥发性汽车用新型聚氨酯泡沫塑料助剂   总被引:1,自引:1,他引:1  
康普顿公司开发了一系列适用于消除或减少汽车内饰件中胺扩散或雾化的添加剂,其中包括快速、有效的生产用零扩散催化剂和低雾化表面活性剂。该系列添加剂能大大降低泡沫制品中的胺扩散,与异氰酸酯和多元醇原料能组成最佳组合,以保持良好的泡沫物理性能。  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