全文获取类型
收费全文 | 281003篇 |
免费 | 25276篇 |
国内免费 | 17686篇 |
专业分类
电工技术 | 19571篇 |
综合类 | 24581篇 |
化学工业 | 34751篇 |
金属工艺 | 18405篇 |
机械仪表 | 17367篇 |
建筑科学 | 17104篇 |
矿业工程 | 8977篇 |
能源动力 | 6046篇 |
轻工业 | 25830篇 |
水利工程 | 8676篇 |
石油天然气 | 7195篇 |
武器工业 | 3021篇 |
无线电 | 30355篇 |
一般工业技术 | 32579篇 |
冶金工业 | 10413篇 |
原子能技术 | 3946篇 |
自动化技术 | 55148篇 |
出版年
2024年 | 1159篇 |
2023年 | 3215篇 |
2022年 | 8160篇 |
2021年 | 10271篇 |
2020年 | 7442篇 |
2019年 | 5095篇 |
2018年 | 19580篇 |
2017年 | 19246篇 |
2016年 | 15198篇 |
2015年 | 9512篇 |
2014年 | 11394篇 |
2013年 | 13516篇 |
2012年 | 19755篇 |
2011年 | 26992篇 |
2010年 | 25096篇 |
2009年 | 21287篇 |
2008年 | 23408篇 |
2007年 | 23849篇 |
2006年 | 14033篇 |
2005年 | 12408篇 |
2004年 | 9113篇 |
2003年 | 6530篇 |
2002年 | 5436篇 |
2001年 | 4528篇 |
2000年 | 3730篇 |
1999年 | 1293篇 |
1998年 | 360篇 |
1997年 | 238篇 |
1996年 | 224篇 |
1995年 | 176篇 |
1994年 | 119篇 |
1993年 | 126篇 |
1992年 | 134篇 |
1991年 | 92篇 |
1990年 | 73篇 |
1989年 | 89篇 |
1988年 | 65篇 |
1987年 | 43篇 |
1986年 | 38篇 |
1980年 | 58篇 |
1979年 | 40篇 |
1968年 | 43篇 |
1966年 | 43篇 |
1965年 | 48篇 |
1959年 | 86篇 |
1958年 | 39篇 |
1957年 | 36篇 |
1955年 | 63篇 |
1954年 | 68篇 |
1951年 | 52篇 |
排序方式: 共有10000条查询结果,搜索用时 120 毫秒
71.
An agriglass composition containing different oxides acts as a slow release for macro and micro nutrients and was chosen to improve maize yield under most important abiotic stresses which affecting agriculture development; salinity and drought. A field experiment was performed in salt affected soil (EC =?7.5 dSm??1) by using different water deficit rates (I1 = 100, I2 = 85 and I3 = 70% of maize water requirements). Irrigation levels were located in main plots. Every main-plot divided into six sub-plots contained glassy fertilizer treatments [F1 = 55 kg fed?1 with 1/2 mm diameter of agriglass (fed. =?4200 m2), F2 = 55 kg fed?1 with 1 mm diameter, F3 = 80 kg fed?1 with 1/2 mm diameter, F4 = 80 kg fed?1 with 1 mm diameter, F5 = Recommendations of Ministry of Agriculture and F6 = control]. The experimental results demonstrated that, ears, straw, grains and biological yields increased with increasing both water and agriglass rates. Application of agriglass as a slow release fertilizer improved yield more than mineral fertilizer. Some growth parameters, water use efficiency (IWUE), macronutrients concentration and their relations were included. Other studies on residual effect of agriglass and the annual application rates to withstand salinity and drought stress by strategic crops are required. 相似文献
72.
73.
为智能化地识别警戒作业人员出现的低觉醒、注意力下降的生理状态,本文介绍了一种基于FPGA和脑电信号处理的低觉醒状态检测与唤醒系统,系统通过传感器从大脑头皮采集脑电信号,转换为数字信号,经傅里叶变换获取了脑电信号的θ相对能量、α相对能量、重心频率、谱熵等4个特征量,由4个特征量表征低觉醒状态并运用支持向量机对低警戒状态进行识别,当识别出低觉醒状态时采用声音报警模块发出声音,唤醒警戒作业人员。设计系统能够较好地识别出低觉醒状态,识别率达90.8%,可为提高警戒作业工作绩效提供一种可穿戴的智能装备。 相似文献
74.
Ahmed Obaid M. Alzahrani M. Sh. Abdel-wahab Meshari Alayash M. S. Aida 《Journal of Materials Science: Materials in Electronics》2018,29(19):16317-16324
In this work, p-NiO/n-ZnO heterostructures were successfully prepared at room temperature using RF sputtering technique. The influence of ZnO layer thickness on the performance of the heterojunction was investigated. The deposited ZnO layers have a hexagonal Wurtzite structure with preferable growth orientations along (002) and (103) for thinner films. Increasing the thickness results in more crystallographic orientation randomness. The current–voltage measurements of the realized heterojunctions showed a clear rectifying behavior. The measured ideality factor varies from 2.5 to 1.6 according to the thickness of ZnO layer. The series resistance of the device is enlarged with increasing ZnO thickness. The deduced parameters from the I–V characteristics suggest that 200 nm is the optimal thickness of the ZnO layer according to our experimental conditions. We attribute the relatively better performance of this thickness to achieving reasonable compensation between serial resistance and ideality factor. The best heterojunction was tested and successfully used as a UV detector. 相似文献
75.
Video transmission over IEEE 802.11e wireless networks still shows poor performance for large bandwidth demand and frequently changed environments. Thus, several enhancements of IEEE 802.11e were proposed. On the other hand, big frames and simultaneous sending of adjacent frames always cause packet dropping for buffer overflow. In the past, we proposed an IEEE 802.11e enhancement named DFAA and a content aware mechanism to solve the above problems. The motivation of this paper is to find a proper way to integrate these two mechanisms. A DFAA enhancement (DFAA-E) is proposed to make up the insufficiency of content aware mechanism. Experiments results show that the combination of DFAA-E and content aware mechanism improves the video decoded quality greatly. And its performance can be further enhanced by selecting the suitable settings of certain parameters. 相似文献
76.
按照管道加工工艺的不同,管道可分为内径控制管和外径控制管。通过热轧工艺生产的外径控制无缝钢管,可满足火力发电厂汽水系统中绝大部分管道的使用要求。对于超(超)临界机组的主蒸汽和高温再热蒸汽管道采用的P91/P92材质的大口径厚壁无缝钢管,由于对材料性能和加工工艺有特殊要求,因此宜采用内径控制管。 相似文献
77.
Yachao Dong Christos T. Maravelias Norman F. Jerome 《Optimization and Engineering》2018,19(4):937-976
We study a maritime inventory routing problem, in which shipments between production and consumption nodes are carried out by a fleet of vessels. The vessels have specific capacities and can be chartered under different agreements. The inventory levels of all consumption nodes and some production nodes should be maintained within specified bounds; for the remaining production nodes, orders should be picked up within pre-defined time windows. We propose a discrete-time mixed-integer programming model. In the face of new information and uncertainty, this optimization model has to be re-solved, as the horizon is rolled forward. We discuss how to account for different sources of uncertainty. We present a rolling-horizon reoptimization framework that allows us to study different policies that impact the quality of the implemented solution, so we can identify the optimal set of policies. 相似文献
78.
79.
对5种活性焦水洗再生前后的脱硫性能进行实验研究,通过比表面积(BET)、X射线光电子能谱(XPS)方法表征水洗再生工艺对活性焦孔隙结构和表面化学性质的影响,揭示影响活性焦水洗再生性能的关键因素。结果表明:活性焦脱硫硫容的大小和水洗再生率的高低没有直接关系;微孔比表面积较大的活性焦脱硫性能较好,中大孔孔容较大的活性焦水洗再生率较高,同时具备较大的微孔比表面积和中大孔孔容结构的活性焦适宜水洗再生工艺;活性焦表面极性含氧官能团含量越高水洗再生效果越好;活性焦AC5经过6次水洗再生循环,微孔中的硫酸经过多次累积后可以被水洗出,稳定再生率保持在45%~50%,最适用于活性焦水洗再生工艺。 相似文献
80.
如何在海量不确定数据集中提高频繁模式挖掘性能是目前研究的热点.传统算法大多是以期望、概率或者权重等单一指标为数据项集支持度,在大数据背景下,同时考虑概率和权重支持度的算法难以兼顾其执行效率.为此,本文提出一种基于Spark的不确定数据集频繁模式挖掘算法(UWEFP),首先,为了同时兼顾数据项的概率和权重,计算一项集的最大概率权重值并进行剪枝;然后,为了减少对数据集的多次扫描,结合Spark框架的优点,设计了一种具有FP-tree特征的新颖的UWEFP-tree结构进行模式树的构建及挖掘;最后在Spark环境下,以UCI数据集进行实验验证.实验结果表明本文的方法在保证挖掘结果的同时,提高了效率. 相似文献