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   57575篇
  免费   4384篇
  国内免费   2398篇
化学   29956篇
晶体学   474篇
力学   1869篇
综合类   83篇
数学   4352篇
物理学   11855篇
无线电   15768篇
  2024年   128篇
  2023年   883篇
  2022年   1101篇
  2021年   1620篇
  2020年   1535篇
  2019年   1435篇
  2018年   1197篇
  2017年   1115篇
  2016年   1943篇
  2015年   1893篇
  2014年   2227篇
  2013年   3472篇
  2012年   3960篇
  2011年   4124篇
  2010年   2792篇
  2009年   2867篇
  2008年   3571篇
  2007年   3303篇
  2006年   3112篇
  2005年   2910篇
  2004年   2357篇
  2003年   2051篇
  2002年   1935篇
  2001年   1499篇
  2000年   1274篇
  1999年   1067篇
  1998年   802篇
  1997年   769篇
  1996年   821篇
  1995年   677篇
  1994年   626篇
  1993年   571篇
  1992年   536篇
  1991年   468篇
  1990年   393篇
  1989年   323篇
  1988年   291篇
  1987年   221篇
  1986年   169篇
  1985年   256篇
  1984年   201篇
  1983年   158篇
  1982年   172篇
  1981年   139篇
  1980年   139篇
  1978年   135篇
  1977年   156篇
  1976年   134篇
  1975年   148篇
  1973年   128篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
951.
952.
953.
We introduced an easy approach to realize the Laplacian first-order and second-order differential of the object wave in transverse direction by use of one in-line digital hologram. It can be used for pinpoint the position of the objects.  相似文献   
954.
Blockcopolymer (BCP) lithography is an emerging nanolithography technique for fabrications of various nanoscale devices and materials. In this study, self-assembled BCP thin films having cylindrical nanoholes were prepared on gold by surface neutralization using self-assembled monolayer (SAM). Oxygen plasma treatment was investigated as a way to enhance the functionality of Au surface toward SAM formation. After surface neutralization, well-ordered nanoholes with 9 to 20 nm diameters were formed inside BCP thin films on Au surfaces through microphase separation. The effects of oxygen plasma treatment on the formation of BCP nanopattern were investigated using surface analysis techniques including X-ray photoelectron spectroscopy (XPS) and water contact angle measurement. Au nanodot arrays were fabricated on gold film by utilizing the BCP nanotemplate and investigated by atomic force microscopy (AFM).  相似文献   
955.
基于近红外光谱的人参与西洋参的快速鉴别研究   总被引:5,自引:0,他引:5  
基于近红外光谱分析技术结合模式判别方法建立了一种人参和西洋参鉴别的新方法。收集根状、根须和粉末状的样品共90份,在有聚乙烯包装袋的情况下直接采集近红外光谱,去除原始光谱中包装袋的显著吸收后进行了MSC与一阶导数处理,然后采用移动窗口偏最小二乘法选择了建模光谱区间,分别建立了PLS-DA,PCA-DA和SVM判别模型,并对3种模型作了对比分析,结果表明SVM判别效果最优,其对预测集的正确判别率为100%。该方法准确、便捷,可实际应用于企业原料药材的质量控制,实现对原料药材的快速筛查。  相似文献   
956.
The purpose of this study was to characterize the neurochemical profiles of various parts of the canine brain using proton nuclear magnetic resonance spectroscopy, tissue extraction, and external simulated phantom concentration quantification. The occipital, frontal, and temporal lobes, thalami, cerebellar cortices, and spinal cords of five pure bred adult beagles were collected, and heavy water solutions for the nuclear magnetic resonance sample were prepared using the methanol–chloroform–water extraction method. The metabolite concentrations in canine brain tissues were measured and compared with those found in human and rat brain tissues. In addition, the cross peaks of Lac, Glu/Gln, and mIns were identified using two-dimensional correlation spectroscopy in the canine frontal cortex. The present study demonstrated the absolute quantification of canine neuronal parts using in vitro high-resolution magnetic resonance spectroscopy, with tissue extraction used to accurately measure metabolite concentrations, thus providing valuable metabolic information regarding the various canine neuronal regions.  相似文献   
957.
958.
Thermoluminescent dosimeters are widely applied in the measurement of radiation dose. Since the luminescent centers are not lost during the reading process, radiophotoluminescent glass dosimeters (RPLGDs) can be repeatedly readout. The aims of this study were to make RPLGD and to analyze its physical and chemical properties. This study used chemical compounds such as Na3PO4, AlPO4, AgCl, AgNO3, B2O3, H3PO4 and P2O5 as the main ingredients. Depending on the self-fabricated glass compounds, the melting temperatures were between 1100 and 1500 °C. When the 0.1 mol% of silver was added, the radiation detection of self-fabricated RPLGD reached to the highest sensitivity. However, when silver concentration was larger than 0.1 mol%, the radiation detection sensitivity plummets rapidly. Excess amounts of silver and aluminum decreased the penetrate capability of ultraviolet spectrum. Based on our study, we found that chemical compound concentrations and melting process, which all affect radiation sensitivity and stability of self-fabricated RPLGD.  相似文献   
959.
Nitrogen-doped titania (TiO2−xNy) nanoparticles with monoclinic phase were successfully prepared by a microwave-assisted hydrothermal process. The obtained TiO2−xNy powders showed high specific surface area, fine particle size, and excellent photocatalytic ability for the oxidative destruction of nitrogen monoxide under irradiation of monochromatic LED light with various wavelengths.  相似文献   
960.
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