全文获取类型
收费全文 | 5188篇 |
免费 | 143篇 |
国内免费 | 240篇 |
专业分类
系统科学 | 168篇 |
丛书文集 | 190篇 |
教育与普及 | 156篇 |
理论与方法论 | 24篇 |
现状及发展 | 21篇 |
研究方法 | 24篇 |
综合类 | 4987篇 |
自然研究 | 1篇 |
出版年
2024年 | 28篇 |
2023年 | 76篇 |
2022年 | 100篇 |
2021年 | 115篇 |
2020年 | 79篇 |
2019年 | 28篇 |
2018年 | 48篇 |
2017年 | 52篇 |
2016年 | 67篇 |
2015年 | 117篇 |
2014年 | 181篇 |
2013年 | 160篇 |
2012年 | 173篇 |
2011年 | 196篇 |
2010年 | 185篇 |
2009年 | 218篇 |
2008年 | 235篇 |
2007年 | 243篇 |
2006年 | 172篇 |
2005年 | 174篇 |
2004年 | 139篇 |
2003年 | 110篇 |
2002年 | 93篇 |
2001年 | 152篇 |
2000年 | 138篇 |
1999年 | 254篇 |
1998年 | 266篇 |
1997年 | 279篇 |
1996年 | 266篇 |
1995年 | 205篇 |
1994年 | 213篇 |
1993年 | 140篇 |
1992年 | 149篇 |
1991年 | 120篇 |
1990年 | 86篇 |
1989年 | 99篇 |
1988年 | 103篇 |
1987年 | 55篇 |
1986年 | 40篇 |
1985年 | 13篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有5571条查询结果,搜索用时 78 毫秒
851.
ZHANG Qi LI JingMing NIU LiHong CHEN ZiHui YANG Lin ZHANG ShiMan CAO LiangCai ZHANG FuShi 《科学通报(英文版)》2013,58(1):74-78
A novel diarylethene,namely 4,5-(2,5-dimethyl thiophene) phthalimide,was synthesized and successfully introduced to rewritable holographic data storage.Upon the alternative illumination of UV and visible light(>400 nm),this compound underwent rapid,reversible inter-conversion between colorless open-ring isomer and yellow-green ring-closed form in both solution and polymethyl methacrylate(PMMA) film.Subsequently,we investigated the characteristics of volume homographic recording of the diarylethene-doped PMMA film(1 m thick).The maximum refractive index modulation(0.87‰) of the film during recording could be reached within just 120 s which gave the ability of fast recording.The high quality reconstruction after 50 write/erase cycles demonstrated its excellent fatigue-resistance and high resolution.All those results indicated that this molecule was a reliable fast write/erase holographic storage material. 相似文献
852.
一种融合 UKF 和EKF 的粒子滤波状态估计算法 总被引:1,自引:0,他引:1
在扩展卡尔曼滤波算法(Extended Kalman Filter,EKF)和不敏卡尔曼滤波算法(Unscented Kalman Filter,UKF)的基础上,提出一种基于融合的粒子滤波算法(Fusion based particle filter, FPF)。该算法首先利用EKF 与UKF 分别预测粒子状态,然后通过融合算法得到粒子的重要性建议分布,实现粒子状态更新。因为充分利用了量测信息,因而能有效提高状态估计精度。仿真中通过实例将该算法与已有的粒子滤波算法(Particle filter algorithm,PF)进行比较,结果表明该算法各方面性能都有较大改进。 相似文献
853.
Internet security problems remain a major challenge with many security concerns such as Internet worms, spam, and phishing attacks. Botnets, well-organized distributed network attacks, consist of a large number of bots that generate huge volumes of spam or launch Distributed Denial of Service (DDoS) attacks on victim hosts. New emerging botnet attacks degrade the status of Internet security further. To address these problems, a practical collaborative network security management system is proposed with an effective collaborative Unified Threat Management (UTM) and traffic probers. A distributed security overlay network with a centralized security center leverages a peer-to-peer communication protocol used in the UTMs collaborative module and connects them virtually to exchange network events and security rules. Security functions for the UTM are retrofitted to share security rules. In this paper, we propose a design and implementation of a cloud-based security center for network security forensic analysis. We propose using cloud storage to keep collected traffic data and then processing it with cloud computing platforms to find the malicious attacks. As a practical example, phishing attack forensic analysis is presented and the required computing and storage resources are evaluated based on real trace data. The cloud-based security center can instruct each collaborative UTM and prober to collect events and raw traffic, send them back for deep analysis, and generate new security rules. These new security rules are enforced by collaborative UTM and the feedback events of such rules are returned to the security center. By this type of close-loop control, the collaborative network security management system can identify and address new distributed attacks more quickly and effectively. 相似文献
854.
A non-cooperative game model based on brittleness entropy is formulated for preventing cascading failure of complex systems.Subsystems of a complex system are mapped to the players of the game.The influence of collapsed subsystems to other subsystems is also taken into account in the definition of payoff function except for their own entropy increase.This influence is named brittleness entropy.Each player has two optional strategies:rational for negative entropy and irrational for negative entropy.The model is designed to identify the players who select an irrational strategy for negative entropy.The players who select the irrational strategy for negative entropy continue to compete for negative entropy after the recovery of ordered state and make other subsystems can' t get enough negative entropy to reduce entropy increase.It leads to cascading failure of the complex system in the end.Genetic algorithm is used to seek the solution of game model,and the simulation result verifies the effectiveness of the proposed model.The model provides a new way to prevent cascading failure of complex systems. 相似文献
855.
《元刊》中单音节动词与单纯趋向动词的组合搭配占有统领性地位,这与汉语双音节化的特点有密切联系。在与动词组合的过程中,趋向动词本身的活跃程度也决定了与它组合的普通动词数量上的优势。从语法功能上看,《元刊》中的趋向动词常作谓语或谓语中心语,在句法结构中,作补语的趋向动词"来、去"等的位置比较灵活,可置于宾语前,也可置于宾语后,但"出、入、上、下、起、到、开"等,它们作补语时只能放在宾语前。 相似文献
856.
857.
858.
主要研究了北京市密云水库沉积物中重金属的总量以及形态分布特征,采用风险评价准则法(RAC)和Tucker3模型探讨沉积物中重金属的潜在危害性.结果表明,各重金属元素的平均含量都超过北京市土壤背景值,重金属元素有一定的富集.Cu有机结合态约占总量的3%~68%;Mn可交换态、碳酸盐结合态约占总量的1%~18%、7%~31%;Pb、Zn的铁锰氧化物结合态分别占总量的12%~53%、4%~27%;其它重金属均以铁锰氧化物结合态和残渣态为主,平均含量占总量的80%以上.根据RAC法得出,Mn(除采样点18)处于中等风险甚至高风险等级;Pb、Zn、Cu属从低风险到中等风险级;Ni、V、Cr、Ti的潜在生态风险较低.该结论将为北京市密云水库保护、人体健康安全保护措施的制定提供科学依据. 相似文献
859.
本文对268株结核分枝杆菌临床分离株(包括213株利福平耐药株、55株利福平敏感株)rpoB基因RRDR区进行测序并比较分析.结果表明,在213株利福平耐药株中有200株(93.90%)在rpoB基因RRDR区发生了突变,在55株利福平敏感株中RRDR均未发生突变.突变频率最高的类型依次为:531位点(55.87%),526位点(16.43%),516位点(10.33%)和511位点(8.92%),以上四个位点的突变菌株数占所有利福平耐药株的91.55%.四川地区利福平耐药株中511位点突变频率高于全国其他地区. 相似文献
860.
宁乡青铜文化是在中原青铜文化的影响和刺激下发展起来的,但其青铜铸造技术并非直接来源于中原,而是来源于虎方.虎方青铜铸造技术直接来源于二里岗时期居住在湖北黄陂盘龙城的中原人.宁乡青铜文化起源于武丁时期,因武丁南征而兴起.古三苗国以宁乡炭河里为中心从殷墟二期开始创造了灿烂的青铜文明. 相似文献