全文获取类型
收费全文 | 722篇 |
免费 | 36篇 |
国内免费 | 17篇 |
专业分类
化学 | 231篇 |
力学 | 4篇 |
数学 | 5篇 |
物理学 | 90篇 |
无线电 | 445篇 |
出版年
2023年 | 9篇 |
2022年 | 22篇 |
2021年 | 29篇 |
2020年 | 32篇 |
2019年 | 17篇 |
2018年 | 15篇 |
2017年 | 40篇 |
2016年 | 37篇 |
2015年 | 59篇 |
2014年 | 95篇 |
2013年 | 84篇 |
2012年 | 73篇 |
2011年 | 54篇 |
2010年 | 20篇 |
2009年 | 34篇 |
2008年 | 22篇 |
2007年 | 35篇 |
2006年 | 23篇 |
2005年 | 23篇 |
2004年 | 9篇 |
2003年 | 8篇 |
2002年 | 8篇 |
2001年 | 5篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1991年 | 3篇 |
1988年 | 1篇 |
1986年 | 2篇 |
排序方式: 共有775条查询结果,搜索用时 15 毫秒
71.
浊点萃取-流动注射电感耦合等离子体原子发射光谱法同时测定水中镉钴铜镍锌 总被引:3,自引:1,他引:2
本文提出了浊点萃取-流动注射电感耦合等离子体原子发射光谱(FI—ICP—AES)法同时测定水中镉、钴、铜、镍、锌的新方法。利用5-Br—PADAP将待测金属离子转化为水不溶性的螯合物,并萃取到表面活性剂Triton X-114的浓缩相,以乙醇-硝酸溶液稀释含富集离子的浓缩相,并以FI—ICP-AES法测定。考察了流动注射进样体积、积分时间、萃取体系介质酸度、螯合剂和表面活性剂用量等实验条件的影响。在折衷条件下,镉、钴、铜、镍和锌的浓缩倍率可达18、10、16、10和8,检出限分别为0.7μg/L、1.6μg/L、1.3μg/L、5.7μg/L、3.2μg/L。方法成功应用于自来水、河水和海水中痕量镉、钴、铜、镍和锌的分析。在0.02mg/L和0.10mg/L二个水平进行加入回收试验,回收率在80%与118%之间。 相似文献
72.
Jiming Li Patrick Minnis Hongru Yan Wencai Wang 《Journal of Quantitative Spectroscopy & Radiative Transfer》2011,112(2):361-375
Clouds alter general circulation through modification of the radiative heating profile within the atmosphere. Their effects are complex and depend on height, vertical structure, and phase. The instantaneous cloud radiative effect (CRE) induced by multi-layered (ML) and single-layer (SL) clouds is estimated by analyzing data collected by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), CloudSat, and Clouds and Earth’s Radiation Energy Budget System (CERES) missions from March 2007 through February 2008. The CRE differences between ML and SL clouds at the top of the atmosphere (TOA) and at the surface were also examined. The zonal mean shortwave (SW) CRE differences between the ML and SL clouds at the TOA and surface were positive at most latitudes, peaking at 120 W m−2 in the tropics and dropping to −30 W m−2 at higher latitudes. This indicated that the ML clouds usually reflected less sunlight at the TOA and transmitted more to the surface than the SL clouds, due to their higher cloud top heights. The zonal mean longwave (LW) CRE differences between ML and SL clouds at the TOA and surface were relatively small, ranging from −30 to 30 W m−2. This showed that the ML clouds only increased the amount of thermal radiation at the TOA relative to the SL clouds in the tropics, decreasing it elsewhere. In other words, ML clouds tended to cool the atmosphere in the tropics and warm it elsewhere when compared to SL clouds. The zonal mean net CRE differences were positive at most latitudes and dominated by the SW CRE differences. 相似文献
73.
74.
Abdulrahman Alharthi Madini O. Alassafi Robert J. Walters Gary B. Wills 《Telematics and Informatics》2017,34(2):664-678
Saudi universities have at their disposal a huge number of low cost IT resources to aid in teaching, research and learning. By migrating to cloud services, Saudi universities will be moving data and programs from local servers to the internet, thereby providing users with the ability to access and share information at any time from multiple devices. The migration to cloud-based IT resources is not yet widespread in Saudi universities due to several challenges including security, legal policies and implementation. At present, there is lack of research and guidance for Saudi universities on how to overcome these challenges and how contextual factors can influence the successful migration to the educational clouds.This research presents a framework for the successful migration to cloud technology in the Saudi Arabian universities. In this research, a set of key critical success factors (CSFs) were identified by synthesizing components from studies concerned with the migration of cloud for higher education and factors identified from the successful implementation of WBL (Web Based Learning) and ERP (Enterprise Resource Planning) on higher education in Saudi Arabia. Based on this knowledge, the proposed framework was evaluated via expert review and a survey by IT specialists from the Saudi universities. The initial CSFs were updated based on the expert reviews and the results were analysed. Based on the findings at this stage, additional CSFs were added to the framework as suggested by the experts. Subsequently, in order to confirm the reviewed CSFs, additional investigation via a structured online questionnaire was conducted and the outcome was analysed via one-sample t-test with the data integrity analysed via Cronbach’s alpha. The outcome indicated the majority of CSFs to be statistically significant except the Physical Location CSF. Potential future study and contributions are discussed. 相似文献
75.
The cloud point temperature, the temperature at which the first wax crystals appear in solution, is one of the most important specifications associated with the low temperature behaviour of a fuel. A database of about 80 fuels and fuel blends was collected and used to assess the performance of the three predictive local composition models (Wilson, NRTL and UNIQUAC) for cloud point prediction. The results indicates that Predictive UNIQUAC can predict the cloud points of the fuels within the experimental uncertainty of the measurements. It could thus be a useful tool in the production of fuels both in the design of refining process and the blend of fuels to meet the low temperature specifications. 相似文献
76.
77.
78.
79.
针对传统的三维人脸识别算法成本较高且不能很好地处理带有光照、表情等变化人脸识别的问题,设计了低分辨率Kinect传感器采集三维点云的鲁棒人脸识别系统。首先,通过鼻尖检测、人脸剪裁、姿势校正、对称填充及平滑采样得到规范的纹理图像;然后,在纹理图像上运用判别色彩空间变换,从而最大化类与类之间的分离性;最后,利用多模态稀疏编码有效地重建误差以得到查询图像与训练集之间的相似度,并利用Z-得分技术完成最终的人脸识别。在通用人脸数据库CurtinFaces、PIE及AR上的识别率可高达96.7%,实验结果表明,相比其它几种人脸识别算法,本文算法取得了更好的识别效果。 相似文献
80.