全文获取类型
收费全文 | 5509篇 |
免费 | 683篇 |
国内免费 | 161篇 |
专业分类
化学 | 1080篇 |
晶体学 | 30篇 |
力学 | 96篇 |
综合类 | 86篇 |
数学 | 520篇 |
物理学 | 1328篇 |
无线电 | 3213篇 |
出版年
2024年 | 11篇 |
2023年 | 74篇 |
2022年 | 245篇 |
2021年 | 242篇 |
2020年 | 200篇 |
2019年 | 135篇 |
2018年 | 128篇 |
2017年 | 216篇 |
2016年 | 240篇 |
2015年 | 206篇 |
2014年 | 387篇 |
2013年 | 326篇 |
2012年 | 347篇 |
2011年 | 387篇 |
2010年 | 239篇 |
2009年 | 306篇 |
2008年 | 314篇 |
2007年 | 314篇 |
2006年 | 291篇 |
2005年 | 299篇 |
2004年 | 229篇 |
2003年 | 231篇 |
2002年 | 177篇 |
2001年 | 169篇 |
2000年 | 100篇 |
1999年 | 114篇 |
1998年 | 70篇 |
1997年 | 76篇 |
1996年 | 84篇 |
1995年 | 43篇 |
1994年 | 35篇 |
1993年 | 29篇 |
1992年 | 21篇 |
1991年 | 10篇 |
1990年 | 11篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 14篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有6353条查询结果,搜索用时 15 毫秒
41.
对JPEG(joint photographic experts group)图像实施篡改往往会产生双重JPEG(double JPEG,DJPE) 压缩痕迹,分析该痕迹有助于揭示图像压缩历史并实现篡改区域定位。现有算法在图像尺寸较小和质量因子(quality factor,QF) 较低的时候性能不佳,对两个QF的组合情况存在限制。本文提出了一种端到端的混合QF双重JPEG压缩图像取证网络,命名为DJPEGNet。首先,使用预处理层从图像头文件中提取表征压缩历史信息的量化表 (quantization table,Qtable) 特征,将图像从空域转换至DCT(discrete cosine transform)域构造统计直方图特征。然后,将两个特征输入到由深度可分离卷积和残差结构堆叠而成的主体结构,输出二分类结果。最后,使用滑动窗口算法自动定位篡改区域并绘制概率分布图。实验结果表明,在使用不同Qtable集生成的小尺寸数据集上,DJPEGNet所有指标均优于现有最先进的算法,其中ACC提高了1.78%,TPR提升了2.00%,TNR提升了1.60%。 相似文献
42.
Depth-Image-Based Rendering (DIBR) is one of the main fundamental techniques for generating new viewpoints in 3D video applications such as multi-viewpoint video (MVV), free viewpoint video (FVV) and virtual reality (VR). Due to the imperfections of color images, depth maps or texture restoration techniques, several types of distortions occur in synthesized views. However, most of related works evaluated the quality of DIBR-synthesized views by only detecting a specific type of distortion, such as stretching, black holes, blurring, etc., which were unable to accurately evaluate the quality of DIBR-synthesized views. In this paper, a new no-reference image quality assessment method is proposed to evaluate the quality of DIBR-synthesized images by combining multi-layer and multi-scale features of images. To be specific, the distortions introduced by different stages of virtual viewpoint synthesis are first analyzed, and then multi-layer and multi-scale features are extracted to estimate the degree of texture and structure distortions. As a result, individual quality scores associated with two types of distortions (e.g., structural distortion and texture distortion) are aggregated to an overall image quality. Experimental results on two publicly available DIBR datasets show that the method has better performance than the state-of-the-art models.Index Terms: image quality assessment, DIBR-synthesized image, distortion correction, BIQA. 相似文献
43.
Xuan Xie Linyu Huang Chengwen Tang Qian Ning 《International Journal of Satellite Communications and Networking》2023,41(5):427-440
The low-earth orbit (LEO) satellite network, composed of a large number of satellite nodes, is a hot research topic at present. Due to the characteristics of the large-scale LEO satellite network, such as many satellite nodes, short orbit period, large dynamic change of topology, and unstable link-state, its communication quality of service (QoS) requirements are difficult to meet. Aiming at this problem, various factors that may affect data transmission are first analyzed. The network link selection problem is modeled as a multi-constraint optimization decision problem, a routing mathematical model based on linear programming (LP) is designed, and its solution is solved. Aiming at the problem of limited onboard computing resources, a multi-object optimization Dijkstra algorithm (MOODA) is designed. The MOODA finds the optimal path according to the comprehensive performance of the link. It solves the problems of poor comprehensive QoS performance and the low degree of load balancing of the paths found by the Dijkstra algorithm. The simulation results show that the paths found by the two algorithms have good QoS, robustness, and load balancing performance. 相似文献
44.
Considering the high requirements for omnidirectional video compression, we propose an objective quality evaluation method to assess quality loss in encoding omnidirectional videos. According to characteristics of 360° videos, we consider multi-space signal characterization (MSSC) to fully characterize the distortions of video signals from spatial/image domains to frequency domains and from image content to motion information, and further consider multi-channel information aggregation (MCIA) to fuse scores from multiple projection planes and temporal divided groups. The main innovation of our method is to establish a universal framework in bridging the connection between typical quality assessment and 360° quality assessment to measure 360° video quality effectively and efficiently. Experimental results show that our method outperforms state-of-the-art 2D quality metrics and quality metrics for omnidirectional images. 相似文献
45.
Rapid dissolution procedure for base-metals-bearing complex materials for quality control assessment
A less labour-intensive method, involving a mixture of 1.0 g MnO2 + 20 mL HCl (11), is proposed for decomposing seven ores and six metallurgical products for accurate and precise estimation of copper, lead, zinc, cobalt and nickel for quality control assessment. Twentyone international reference materials were also analysed. Results of regression analyses are presented and intermethod comparison studies reveal that the probability of results being different compared with a reference method was less than 1%. Sample decomposition is straightforward and the method has been found to be very simple, rapid and easily adaptable, as it involves no separation of the analyte from the matrix elements. 相似文献
46.
Kaj Heydorn 《Mikrochimica acta》1991,105(1-3):1-10
In scientific research laboratories it is rarely possible to use quality assurance schemes, developed for large-scale analysis. Instead methods have been developed to control the quality of modest numbers of analytical results by relying on statistical control:
Analysis of precision serves to detect analytical errors by comparing thea priori precision of the analytical results with the actual variability observed among replicates or duplicates. The method relies on the chi-square distribution to detect excess variability and is quite sensitive even for 5–10 results.
Interference control serves to detect analytical bias by comparing results obtained by two different analytical methods, each relying on a different detection principle and therefore exhibiting different influence from matrix elements; only 5–10 sets of results are required to establish whether a regression line passes through the origo.
Calibration control is an essential link in the traceability of results. Only one or two samples of pure solid or aqueous standards with accurately known content need to be analyzed.
Verification is carried out by analyzing certified reference materials from BCR, NIST, or others; their limited accuracy of 5–10% make them less suitable for calibration purposes. 相似文献
47.
Study of Tea Digitized Chromatographic Fingerprint Spectra Using Micellar Electrokinetic Chromatography 总被引:1,自引:0,他引:1
IntroductionTeaisthemostwidelyconsumedbeverageinpeople’slife .Theheightenedpopularityofthisbeveragethroughouttheworldinrecentyearsmaybedueinparttotheevidenceofarelationshipbetweenteaconsumptionandpreventionofcertainformsofhumandisease .Themaincomponentsofteaarepolyphenoliccompounds ,commonlyknownascatechins ,whichrepresentagroupofcom poundsbelongingtotheflavonoidfamily .Thesecom poundsarewidelydistributedinthetealeavesandconsti tuteupto 30 %ofthedryleafweight.1Muchinteresthasbeenfocusedoncat… 相似文献
48.
Stephen de Mora Jean-Pierre Villeneuve Eric Wyse 《Accreditation and quality assurance》2007,12(11):587-592
Aspects of the International Atomic Energy Agency (IAEA) Analytical Quality Control Services (AQCS) for marine environmental
studies are discussed, focusing on recent laboratory performance studies (LPS) and the production of reference materials for
trace metals and organic compounds in various marine matrices. The IAEA has organized seventeen global interlaboratory studies
for a range of organic contaminants. Of note has been the inclusion of numerous polychlorinated biphenyl congeners (PCBs)
and some sterols of anthropogenic origin. Concurrently, there have been eleven worldwide intercomparison exercises for trace
metals in the marine environment, most of which included methylmercury. Although such interlaboratory studies can help improve
performance in individual laboratories and regional laboratory networks, the results reveal that problems remain in the determination
of some metals and many organic contaminants. 相似文献
49.
张德礼 《中国科学B辑(英文版)》1991,(8)
In this paper, superhigh reproductive rate strains of MEV with titre more than HA8192* or TCID50 log9.7 10 have been achieved both by cultivation in cell lines with different susceptibility to MEV and by isolating and identifying in field by the author. The systematic tests proved that S18 and L12 strains of MEV are the best strains for vaccine preparation. In this study, the best means for the tissue cultivation of MEV and the most advanced technological process for the production and detection of serum-free cell-cultured MEV fluids with super-high HA titre in batches in large quantities have been established for the first time. Optimum conditions for MEV inactivation were determined, and safe and effective inactivated vaccines with mineral oil or A1(OH)3 gel adjuvant were successfully prepared with serum-free cell-cultured MEV fluids. Both vaccines with different adjuvants can be manufactured in batches in large quantities and have been widely used all over China since 1986. The change laws of the imm 相似文献
50.