全文获取类型
收费全文 | 1215篇 |
免费 | 22篇 |
国内免费 | 27篇 |
专业分类
教育 | 781篇 |
科学研究 | 207篇 |
各国文化 | 6篇 |
体育 | 79篇 |
综合类 | 69篇 |
文化理论 | 3篇 |
信息传播 | 119篇 |
出版年
2023年 | 3篇 |
2022年 | 32篇 |
2021年 | 40篇 |
2020年 | 47篇 |
2019年 | 28篇 |
2018年 | 31篇 |
2017年 | 41篇 |
2016年 | 19篇 |
2015年 | 57篇 |
2014年 | 81篇 |
2013年 | 137篇 |
2012年 | 73篇 |
2011年 | 92篇 |
2010年 | 87篇 |
2009年 | 72篇 |
2008年 | 68篇 |
2007年 | 66篇 |
2006年 | 57篇 |
2005年 | 38篇 |
2004年 | 37篇 |
2003年 | 28篇 |
2002年 | 25篇 |
2001年 | 15篇 |
2000年 | 15篇 |
1999年 | 6篇 |
1998年 | 8篇 |
1997年 | 9篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1992年 | 2篇 |
1988年 | 2篇 |
1981年 | 2篇 |
1971年 | 1篇 |
1967年 | 1篇 |
1963年 | 1篇 |
1957年 | 1篇 |
1943年 | 1篇 |
1937年 | 1篇 |
1920年 | 1篇 |
1889年 | 1篇 |
1885年 | 3篇 |
1882年 | 6篇 |
1880年 | 5篇 |
1877年 | 1篇 |
1872年 | 1篇 |
1871年 | 1篇 |
1867年 | 2篇 |
1860年 | 1篇 |
1843年 | 1篇 |
排序方式: 共有1264条查询结果,搜索用时 0 毫秒
41.
The earliest ovules in the Late Devonian (Famennian) are surrounded by a cupule that is involved in both protection and pollination, and generally have free integumentary lobes. Here we report a new taxon from the Famennian of China, Guazia dongzhiensis gen. et sp. nov. The terminally borne ovule is apparently acupulate (without cupule) and has four radially arranged wing-like integumentary lobes that are extensively fused, and folded lengthwise and inwards. Guazia provides evidence that not all Devonian seeds possess a cupule and it increases their diversity in integumentary lobes. This genus also suggests that the integuments develop new functions, probably including wind dispersal at the expense of the cupules. 相似文献
42.
43.
开关电源实验教学探讨 总被引:1,自引:2,他引:1
开关电源采用交—直—交—直变换技术,是一种组合变流电路,包含整流、逆变、变压、滤波等电路,融汇了电力电子技术及其课程教学的主要内容,对学生提炼电力电子技术课程教学的关键知识点有很大的作用。通过该实验,帮助学生深入学习与理解电力电子技术的基本知识,培养和提高实验技能以及分析问题和解决问题的能力,取得了良好的实验教学效果。 相似文献
44.
Le YING De-dong KONG Yuan-yuan GAO Feng YAN Yue-fei WANG Ping XU 《Journal of Zhejiang University. Science. B》2018,(3)
目的:探讨不同提取方法所得的漳平水仙饼茶中提取物的主要化学成分、体外抗氧化作用和对小鼠乳腺癌4T1细胞增殖和转移的抑制作用。创新点:首次对漳平水仙饼茶中不同提取物的主要化学成分、体外抗氧化作用和对小鼠乳腺癌4T1细胞增殖和转移的抑制作用进行了研究。方法:以漳平水仙饼茶为原料,利用乙酸乙酯、正丁醇和水分别进行萃取,得到三种提取物。对提取物的茶多酚、黄酮、原花青素、总糖、蛋白质、氨基酸、儿茶素单体、茶黄素、咖啡碱等主要化学成分进行检测。利用DPPH、ABTS和FRAP三种不同方法进一步检测了三种提取物的体外抗氧化性,并评价了提取物对小鼠乳腺癌4T1细胞的增殖、细胞周期、迁移、侵袭等的影响作用。同时,采用皮尔森系数分析和聚类分析两种分析方法探讨了提取物中的化学成分与其抗氧化活性和对4T1细胞作用的相关性。结论:本实验结果显示,漳平水仙饼茶乙酸乙酯层提取物具有最高的茶多酚、黄酮、原花青素和儿茶素单体的含量(表1~3,图1),并且在三种不同的抗氧化体系中均显示出最强的抗氧化性能(表4,图2),同时对小鼠乳腺癌4T1细胞的增殖、细胞周期、迁移、侵袭有非常明显的抑制作用(图3~6)。乙酸乙酯层提取物之所以具有最强的生物活性主要与其含有较多的茶多酚等活性成分有关(表5,图3)。 相似文献
45.
Florence Le Hebel Pascale Montpied Andrée Tiberghien Valérie Fontanieu 《International Journal of Science Education》2013,35(4):468-487
ABSTRACTThe understanding of what makes a question difficult is a crucial concern in assessment. To study the difficulty of test questions, we focus on the case of PISA, which assesses to what degree 15-year-old students have acquired knowledge and skills essential for full participation in society. Our research question is to identify PISA science item characteristics that could influence the item’s proficiency level. It is based on an a-priori item analysis and a statistical analysis. Results show that only the cognitive complexity and the format out of the different characteristics of PISA science items determined in our a-priori analysis have an explanatory power on an item’s proficiency levels. The proficiency level cannot be explained by the dependence/independence of the information provided in the unit and/or item introduction and the competence. We conclude that in PISA, it appears possible to anticipate a high proficiency level, that is, students’ low scores for items displaying a high cognitive complexity. In the case of a middle or low cognitive complexity level item, the cognitive complexity level is not sufficient to predict item difficulty. Other characteristics play a crucial role in item difficulty. We discuss anticipating the difficulties in assessment in a broader perspective. 相似文献
46.
本文基于产业结构偏离度指标分析得出:抚州市产业结构和就业结构发展不协调,就业结构滞后于产业结构,第一产业剩余劳动力有待转出;第二产业吸纳劳动力的潜力较大;第三产业存在着过剩的劳动力,劳动生产率较低,产业内劳动力有面临着向其他产业转移的压力。进而提出优化工业内部结构,拓宽就业主渠道;优先发展第三产业中前向、后向关联效应大,对就业带动效应较大的行业;调整农业内部结构,扩大第一产业内部就业等政策建议。 相似文献
47.
Nam Cao Hoai Le Murat Gel Yonggang Zhu Jian Wang Helen Dacres Alisha Anderson Stephen C. Trowell 《Biomicrofluidics》2014,8(6)
Bioluminescence resonance energy transfer (BRET) is a form of Förster resonance energy transfer. BRET has been shown to support lower limits of detection than fluorescence resonance energy transfer (FRET) but, unlike FRET, has not been widely implemented on microfluidic devices for bioanalytical sensing. We recently reported a microscope-based microfluidic system for BRET-based biosensing, using a hybrid, high quantum-efficiency, form of BRET chemistry. This paper reports the first optical fiber-based system for BRET detection on a microfluidic chip, capable of quantifying photon emissions from the low quantum-efficiency BRET2 system. We investigated the effects of varying core diameter and numerical aperture of optical fibers, as well as varying microfluidic channel design and measurement conditions. We optimized the set-up in order to maximize photon counts and minimize the response time. The optimized conditions supported measurement of thrombin activity, with a limit of detection of 20 pM, which is lower than the microscope-based system and more than 20 times lower than concentrations reported to occur in plasma clots. 相似文献
48.
麦全安 《北京体育大学学报》1999,(3)
运用长期追踪实验法对足球运动员的一般耐力训练作了研究,探讨了三种负荷方法对男子足球运动员肺通气功能的影响,试图为足球运动员一般耐力训练提供一种有效的实用方法。 相似文献
49.
50.
Le Cai Wanzhen He Xudong Xue Jianyao Huang Ke Zhou Xiahong Zhou Zhiping Xu Gui Yu 《国家科学评论(英文版)》2021,8(12)
Intrinsic graphene features semi-metallic characteristics that limit its applications in electronic devices, whereas graphene nanoribbons (GNRs) are promising semiconductors because of their bandgap-opening feature. However, the controllable mass-fabrication of high-quality GNR arrays remains a major challenge. In particular, the in situ growth of GNR arrays through template-free chemical vapor deposition (CVD) has not been realized. Herein, we report a template-free CVD strategy to grow large-area, high-quality and self-aligned GNR arrays on liquid copper surface. The width of as-grown GNR could be optimized to sub-10 nm with aspect ratio up to 387, which is higher than those of reported CVD-GNRs. The study of the growth mechanism indicates that a unique comb-like etching-regulated growth process caused by a trace hydrogen flow guides the formation of the mass-produced self-aligned GNR arrays. Our approach is operationally simple and efficient, offering an assurance for the use of GNR arrays in integrated circuits. 相似文献