全文获取类型
收费全文 | 1932篇 |
免费 | 191篇 |
国内免费 | 29篇 |
专业分类
化学 | 150篇 |
力学 | 58篇 |
综合类 | 54篇 |
数学 | 675篇 |
物理学 | 122篇 |
无线电 | 1093篇 |
出版年
2024年 | 1篇 |
2023年 | 17篇 |
2022年 | 37篇 |
2021年 | 79篇 |
2020年 | 58篇 |
2019年 | 55篇 |
2018年 | 56篇 |
2017年 | 66篇 |
2016年 | 84篇 |
2015年 | 88篇 |
2014年 | 143篇 |
2013年 | 175篇 |
2012年 | 139篇 |
2011年 | 117篇 |
2010年 | 100篇 |
2009年 | 109篇 |
2008年 | 128篇 |
2007年 | 144篇 |
2006年 | 120篇 |
2005年 | 108篇 |
2004年 | 73篇 |
2003年 | 58篇 |
2002年 | 50篇 |
2001年 | 32篇 |
2000年 | 27篇 |
1999年 | 20篇 |
1998年 | 2篇 |
1997年 | 8篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 5篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1979年 | 3篇 |
排序方式: 共有2152条查询结果,搜索用时 15 毫秒
41.
42.
运用大数据挖掘原理,对海量3G终端MR数据进行深入研究。从解决用户感知、用户体验方面入手,寻找网络质量规律,并在优化实例中予以应用。丰富了优化手段,取得良好效果,提高了优化效率,为深度优化提供了新思路。 相似文献
43.
ToxML,a data exchange standard with content controlled vocabulary used to build better (Q)SAR models
M. Ali M. Patel D. Wilkinson P. Judson K. Cross D. Bower 《SAR and QSAR in environmental research》2013,24(6):429-438
Development of accurate quantitative structure–activity relationship (QSAR) models requires the availability of high quality validated data. International regulations such as REACH in Europe will now accept (Q)SAR-based evaluations for risk assessment. The number of toxicity datasets available for those wishing to share knowledge, or to use for data mining and modelling, is continually expanding. The challenge is the current use of a multitude of different data formats. The issues of comparing or combining disparate data apply both to public and proprietary sources. The ToxML project addresses the need for a common data exchange standard that allows the representation and communication of these data in a well-structured electronic format. It is an open standard based on Extensible Markup Language (XML). Supporting information for overall toxicity endpoint data can be included within ToxML files. This makes it possible to assess the quality and detail of the data used in a model. The data file model allows the aggregation of experimental data to the compound level in the detail needed to support (Q)SAR work. The standard is published on a website together with tools to view, edit and download it. 相似文献
44.
根据微孔磷酸铝合成数据的特点,针对合成数据库中合成参数过多和交叉描述等问题,利用改进的遗传编程算法对具有(6,8)元环的微孔磷酸铝合成参数进行特征提取,优化出新的复合特征来更好地描述磷酸铝合成中溶剂和模板剂的特征,并通过参数进化过程的研究,考察了模板剂和溶剂对产物生成的具体影响,从而指导具有特定结构磷酸铝的定向合成. 相似文献
45.
Natural products (NPs) are evolutionarily optimized as drug-like molecules and remain the most consistently successful source of drugs and drug leads. They offer major opportunities for finding novel lead structures that are active against a broad spectrum of assay targets, particularly those from secondary metabolites of microbial origin. Due to traditional discovery approaches’ limitations relying on untargeted screening methods, there is a growing trend to employ unconventional secondary metabolomics techniques. Aided by the more in-depth understanding of different biosynthetic pathways and the technological advancement in analytical instrumentation, the development of new methodologies provides an alternative that can accelerate discoveries of new lead-structures of natural origin. This present mini-review briefly discusses selected examples regarding advancements in bioinformatics and genomics (focusing on genome mining and metagenomics approaches), as well as bioanalytics (mass-spectrometry) towards the microbial NPs-based drug discovery and development. The selected recent discoveries from 2015 to 2020 are featured herein. 相似文献
46.
Ntombizethu Nokuphiwa Msomi Tiara Padayachee Nomfundo Nzuza Puleng Rosinah Syed Justyna Dorota Kry Wanping Chen Dominik Gront David R. Nelson Khajamohiddin Syed 《Molecules (Basel, Switzerland)》2021,26(6)
The impact of lifestyle on shaping the genome content of an organism is a well-known phenomenon and cytochrome P450 enzymes (CYPs/P450s), heme-thiolate proteins that are ubiquitously present in organisms, are no exception. Recent studies focusing on a few bacterial species such as Streptomyces, Mycobacterium, Cyanobacteria and Firmicutes revealed that the impact of lifestyle affected the P450 repertoire in these species. However, this phenomenon needs to be understood in other bacterial species. We therefore performed genome data mining, annotation, phylogenetic analysis of P450s and their role in secondary metabolism in the bacterial class Gammaproteobacteria. Genome-wide data mining for P450s in 1261 Gammaproteobacterial species belonging to 161 genera revealed that only 169 species belonging to 41 genera have P450s. A total of 277 P450s found in 169 species grouped into 84 P450 families and 105 P450 subfamilies, where 38 new P450 families were found. Only 18% of P450s were found to be involved in secondary metabolism in Gammaproteobacterial species, as observed in Firmicutes as well. The pathogenic or commensal lifestyle of Gammaproteobacterial species influences them to such an extent that they have the lowest number of P450s compared to other bacterial species, indicating the impact of lifestyle on shaping the P450 repertoire. This study is the first report on comprehensive analysis of P450s in Gammaproteobacteria. 相似文献
47.
由于目前在小型无人机执行器故障诊断中存在着智能化程度较低,容易受到人为因素干扰,从而出现故障漏检等问题,难以满足小型无人机对飞行安全的要求。为此,本文提出一种基于多维数据关联规则挖掘(Multidimensional Data Association Rules Mining: MDARM)和VxWorks操作系统的小型无人机执行器故障诊断方法,通过建立执行器内部传感器测量的温度、压力、流速、力矩等相关变量的历史数据库,并对这些数据进行预处理,以避免带来噪声污染,并利用可测量参数与不可测量参数之间的关联性,建立故障诊断知识库,避免了诊断过程中的人为因素干扰,实现小型无人机执行器故障的精准测量。实验结果证明,这种方法能够有效地提高故障准确率64.7%,对小型无人机执行器的智能诊提供有效指导,应用前景广阔。 相似文献
48.
A data mining method based on constructive neural networks 总被引:1,自引:0,他引:1
~~A DATA MINING METHOD BASED ON CONSTRUCTIVE NEURAL NETWORKS[1] Jiawei Han, Micheline Kamber. Data Mining Concept and Techniques. Beijing, Higher Education Press, chapters 1 and 7.
[2] Simon Haykin. Neural Networks: A Comprehensive Foundation. 2nd ed. Beijing, Tsinghua University Press, chapters 1, 4, 12 and 14.
[3] Zhang Ling, Zhang Bo. A geometrical representation of McCulloch-Pitts neural model and its applications. IEEE Trans, on Neural Networks, 10(19… 相似文献
49.
Ewan Couic Alicia Tribondeau Vanessa Alphonse Alexandre Livet Noureddine Bousserrhine 《Molecules (Basel, Switzerland)》2022,27(6)
Understanding ecological trajectories after mine site rehabilitation is essential to develop relevant protocols adapted for gold mining sites. This study describes the influence of a range of mine site rehabilitation and revegetation protocols on soil physicochemical parameters and microbial activities related to carbon, nitrogen and phosphorus cycles. We sampled soil from six rehabilitated mining sites in French Guiana with different plant cover (herbaceous, Cyperaceous, monoculture of Clitoria racemosa and Acacia mangium and association of C. racemosa and A. mangium). We measured the mineralization potential of organic matter by estimating the mineralization of carbon, nitrogen and phosphorus and the microbial catabolic diversity balance. The results showed an improvement in the quality of organic matter on revegetated sites with tree cover. On restored sites with fabaceous species, the microbial biomass is three times higher than non-restored sites, improving the rates of organic matter mineralization and restoring the catabolic diversity to the level of natural Guyanese soils. These results confirm that the establishment of fabaceous species under controlled conditions significantly improves the restoration of microbial communities in mining soils. 相似文献
50.
Javier Esteban-Escao Berta Castn Sergio Castn Marta Chliz-Ezquerro Csar Asensio Antonio R. Laliena Gerardo Sanz-Enguita Gerardo Sanz Luis Mariano Esteban Ricardo Savirn 《Entropy (Basel, Switzerland)》2022,24(1)
Background: Electronic fetal monitoring (EFM) is the universal method for the surveillance of fetal well-being in intrapartum. Our objective was to predict acidemia from fetal heart signal features using machine learning algorithms. Methods: A case–control 1:2 study was carried out compromising 378 infants, born in the Miguel Servet University Hospital, Spain. Neonatal acidemia was defined as pH < 7.10. Using EFM recording logistic regression, random forest and neural networks models were built to predict acidemia. Validation of models was performed by means of discrimination, calibration, and clinical utility. Results: Best performance was attained using a random forest model built with 100 trees. The discrimination ability was good, with an area under the Receiver Operating Characteristic curve (AUC) of 0.865. The calibration showed a slight overestimation of acidemia occurrence for probabilities above 0.4. The clinical utility showed that for 33% cutoff point, missing 5% of acidotic cases, 46% of unnecessary cesarean sections could be prevented. Logistic regression and neural networks showed similar discrimination ability but with worse calibration and clinical utility. Conclusions: The combination of the variables extracted from EFM recording provided a predictive model of acidemia that showed good accuracy and provides a practical tool to prevent unnecessary cesarean sections. 相似文献