全文获取类型
收费全文 | 40901篇 |
免费 | 729篇 |
国内免费 | 184篇 |
专业分类
系统科学 | 1205篇 |
丛书文集 | 534篇 |
教育与普及 | 70篇 |
理论与方法论 | 390篇 |
现状及发展 | 17141篇 |
研究方法 | 1481篇 |
综合类 | 20460篇 |
自然研究 | 533篇 |
出版年
2018年 | 770篇 |
2017年 | 808篇 |
2016年 | 555篇 |
2012年 | 733篇 |
2011年 | 1938篇 |
2010年 | 1011篇 |
2009年 | 612篇 |
2008年 | 1120篇 |
2007年 | 1545篇 |
2006年 | 665篇 |
2005年 | 700篇 |
2004年 | 770篇 |
2003年 | 818篇 |
2002年 | 711篇 |
2001年 | 1143篇 |
2000年 | 1079篇 |
1999年 | 761篇 |
1994年 | 369篇 |
1992年 | 729篇 |
1991年 | 544篇 |
1990年 | 620篇 |
1989年 | 612篇 |
1988年 | 560篇 |
1987年 | 650篇 |
1986年 | 594篇 |
1985年 | 758篇 |
1984年 | 603篇 |
1983年 | 480篇 |
1982年 | 435篇 |
1981年 | 453篇 |
1980年 | 546篇 |
1979年 | 1115篇 |
1978年 | 929篇 |
1977年 | 880篇 |
1976年 | 751篇 |
1975年 | 784篇 |
1974年 | 983篇 |
1973年 | 879篇 |
1972年 | 919篇 |
1971年 | 1020篇 |
1970年 | 1308篇 |
1969年 | 990篇 |
1968年 | 997篇 |
1967年 | 937篇 |
1966年 | 864篇 |
1965年 | 593篇 |
1959年 | 311篇 |
1958年 | 539篇 |
1957年 | 358篇 |
1956年 | 304篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
991.
992.
This paper proposes a knowledge-scientific approach to evaluation of community service systems from the viewpoints of knowledge creation, consciousness reform, and value co-creation. A concrete example of the community service system treated here is an education program for old men to find their reason for living after the retirement. After introducing this program and the traditional evaluation methods for such a program, the paper emphasizes the necessity of developing new evaluation methods for such a community service system based on knowledge science. The paper proposes a new evaluation framework and reports an actual evaluation result using the interview data from participants in that program. 相似文献
993.
Nicole Note 《Foundations of Science》2016,21(2):311-314
This final comment provides, a theoretical framework on how to conceive the self as presented in the key-note paper ‘Meaningfulness, volunteering and being moved. The event of wit(h)nessing’. This is deemed requisite to achieve a full understanding of how depth in meaningfulness comes about. 相似文献
994.
995.
Traditional processes for treating vanadium slag generate a huge volume of solid residue and a large amount of harmful gas, which cause serious environmental problems. In this study, a new process for the comprehensive utilization of vanadium slag was proposed, wherein zeolite A and a V2O5/TiO2 system were synthesized. The structural properties of the as-synthesized zeolite A and the V2O5/TiO2 system were characterized using various experimental techniques, including X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and infrared spectroscopy. The results reveal that zeolite A and the V2O5/TiO2 system are successfully obtained with high purity. The results of gas adsorption measurements indicate that the prepared zeolite A exhibits high selectivity for CO2 over N2 and is a candidate material for CO2 capture from flue-gas streams. 相似文献
996.
To reveal the impact of the composite agglomeration process (CAP) on the reduction disintegration properties of TiO2-rich ironmaking burden for a blast furnace, the reduction disintegration indices (RDIs), mineral constituents, and microstructure of the products prepared by the CAP and the traditional sintering process (TSP) were investigated. The results showed that, compared to the sinter with a basicity of 2.0 prepared by the TSP, the RDI+6.3 and the RDI+3.15 of the CAP product with the same basicity increased by 28.2wt% and 13.7wt%, respectively, whereas the RDI-0.5 decreased by 2.7wt%. The analysis of the mineral constituents and microstructure of the products indicated that the decreasing titanohematite content decreased the volume expansion during reduction. Meanwhile, the decreasing perovskite content decreased its detrimental effect on the reduction disintegration properties. In addition, the higher silicoferrite of calcium and aluminum (SFCA) content improved the strength of the CAP product. Together, these factors result in an improvement of the RDI of the CAP products. In addition, compared to the sinter, the reduced CAP products clearly contained fewer cracks, which also led to mitigation of reduction disintegration. 相似文献
997.
In recent years,bank credit business is booming with the increasing borrowing intention of China’s listed companies,and debt financing has become the major approach among listed companies’financing strategies.As a series of institutional arrangements about rights,responsibilities and benefits between different shareholders,corporate governance mechanism has a significant influence on the cost of debt financing.This paper employs variable coefficient panel data model to investigate the relationship of the listed company’s debt financing costs and corporate governance mechanism in terms of structural characteristics and time series characteristics.The results show that optimizing the structure of both Board of Directors and Board of Supervisors,establishing a reasonable management incentive system and reducing the concentration of ownership properly can directly contribute to a lower company’s debt financing costs.Meanwhile,property rights have an interactive influence on corporate governance from four aspects,which indirectly effect in company’s debt financing costs. 相似文献
998.
Takayuki Fujita Masanari Umemura Utako Yokoyama Satoshi Okumura Yoshihiro Ishikawa 《Cellular and molecular life sciences : CMLS》2017,74(4):591-606
As one of the most important second messengers, 3′,5′-cyclic adenosine monophosphate (cAMP) mediates various extracellular signals including hormones and neurotransmitters, and induces appropriate responses in diverse types of cells. Since cAMP was formerly believed to transmit signals through only two direct target molecules, protein kinase A and the cyclic nucleotide-gated channel, the sensational discovery in 1998 of another novel direct effecter of cAMP [exchange proteins directly activated by cAMP (Epac)] attracted a great deal of scientific interest in cAMP signaling. Numerous studies on Epac have since disclosed its important functions in various tissues in the body. Recently, observations of genetically manipulated mice in various pathogenic models have begun to reveal the in vivo significance of previous in vitro or cellular-level findings. Here, we focused on the function of Epac in the heart. Accumulating evidence has revealed that both Epac1 and Epac2 play important roles in the structure and function of the heart under physiological and pathological conditions. Accordingly, developing the ability to regulate cAMP-mediated signaling through Epac may lead to remarkable new therapies for the treatment of cardiac diseases. 相似文献
999.
Xin Meng Jack Clews Vasileios Kargas Xiaomeng Wang Robert C. Ford 《Cellular and molecular life sciences : CMLS》2017,74(1):23-38
The cystic fibrosis transmembrane conductance regulator (CFTR) is responsible for the disease cystic fibrosis (CF). It is a membrane protein belonging to the ABC transporter family functioning as a chloride/anion channel in epithelial cells around the body. There are over 1500 mutations that have been characterised as CF-causing; the most common of these, accounting for ~70 % of CF cases, is the deletion of a phenylalanine at position 508. This leads to instability of the nascent protein and the modified structure is recognised and then degraded by the ER quality control mechanism. However, even pharmacologically ‘rescued’ F508del CFTR displays instability at the cell’s surface, losing its channel function rapidly and it is rapidly removed from the plasma membrane for lysosomal degradation. This review will, therefore, explore the link between stability and structure/function relationships of membrane proteins and CFTR in particular and how approaches to study CFTR structure depend on its stability. We will also review the application of a fluorescence labelling method for the assessment of the thermostability and the tertiary structure of CFTR. 相似文献
1000.
This paper primarily aims to test a Pleistocene refuge-type scenario, as previously proposed for the gonyleptid Geraeocormobius sylvarum, a semi-deciduous forests dweller in subtropical Argentina, Brazil and Paraguay. Palaeodistributional models of this species were built using MaxEnt for two Last Glacial Maximum (LGM = 21,000 years ago) simulations – Community Climate System Model (CCSM) and Model for Interdisciplinary Research on Climate (MIROC) – and for 6000 years ago (?6k = HCO, the Holocene climatic optimum). Both LGM models retrieved a fragmented pattern. For CCSM, range was split into multiple, scattered fragments. MIROC resulted in very few patches, with a decided range reduction because of a strong humidity drop. Models for ?6k recovered a moderate range expansion. No past connection between the core area and the yungas was predicted. Analysis of variables importance showed that two precipitation predictors (bc18, precipitation warmest quarter; bc14, precipitation driest month) and two temperature predictors (bc7, temperature annual range; bc9, mean temperature driest quarter) scored as the most influencing overall. The Limiting Factor analysis recognized them as limiting too, in different parts of the species range. LGM palaeomodels of G. sylvarum are compatible with the refuge hypothesis invoked in previous molecular analyses, to explain the high genetic diversity found in the core area. Additionally, the results reinforced the hypothesis of the recent anthropogenic origin of the yungas disjunct populations. 相似文献