首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   278933篇
  免费   24261篇
  国内免费   11651篇
电工技术   17150篇
技术理论   12篇
综合类   18241篇
化学工业   39067篇
金属工艺   16824篇
机械仪表   16774篇
建筑科学   18644篇
矿业工程   7195篇
能源动力   7298篇
轻工业   18838篇
水利工程   6039篇
石油天然气   10618篇
武器工业   2221篇
无线电   33132篇
一般工业技术   36150篇
冶金工业   10558篇
原子能技术   2699篇
自动化技术   53385篇
  2024年   2998篇
  2023年   3970篇
  2022年   6850篇
  2021年   9270篇
  2020年   7194篇
  2019年   5320篇
  2018年   19826篇
  2017年   19882篇
  2016年   15693篇
  2015年   9488篇
  2014年   11271篇
  2013年   13189篇
  2012年   17952篇
  2011年   24758篇
  2010年   22390篇
  2009年   18864篇
  2008年   19979篇
  2007年   20251篇
  2006年   11714篇
  2005年   10374篇
  2004年   7427篇
  2003年   6596篇
  2002年   5864篇
  2001年   4780篇
  2000年   3949篇
  1999年   3239篇
  1998年   2136篇
  1997年   1735篇
  1996年   1619篇
  1995年   1360篇
  1994年   1110篇
  1993年   703篇
  1992年   576篇
  1991年   417篇
  1990年   332篇
  1989年   270篇
  1988年   193篇
  1987年   136篇
  1986年   115篇
  1985年   57篇
  1984年   46篇
  1981年   49篇
  1980年   50篇
  1968年   50篇
  1966年   47篇
  1965年   45篇
  1959年   46篇
  1958年   38篇
  1955年   63篇
  1954年   68篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
A novel boned perovskite manganese oxide magnetoresistant material was prepared using La0.7Sr0.3MnO3 (LSMO) as the precursor powders and metal tin (Sn) as the binder. The microstructure and phase characteristics, low-field transport properties were studied. Sn segregated at the grain boundaries of LSMO grains. The insulator–metal (IM) transition and enhanced LFMR are only observed with a low content of Sn, due to grain boundary disorder and spin polarized tunneling between grain boundaries. The Sn addition induced resistivity decreasing dramatically. In the high temperature PM region, the resistivities for samples with low Sn content follow the adiabatic small-polaron-hopping model.  相似文献   
962.
The present issue of Journal of Superconductivity and Novel Magnetism collects the papers presented at the International Conference on “Quantum Phenomena in Complex Matter.” Superstripes 2010 held in Erice, Italy, from July 19 to 25, 2010, at the Ettore Majorana Foundation and Centre for Scientific Culture. In this preface, I discuss the scientific scenario of the search for the mechanism of the macroscopic quantum coherent phase at high temperature “superconductivity in complexity.”  相似文献   
963.
Antioxidant enzymes protect muscle tissues against oxidation from reactive oxygen species and increase the stability of the muscle. Phospholipid hydroperoxide glutathione peroxidase (GPx4) is an important antioxidant enzyme in muscle tissue. So the aim of this study was to identify the effects of mRNA expression level of GPx4 gene on meat quality properties in Longissimus dorsi of three pig breeds. Total 36 pigs (12 pigs for each breed) were used to measure the meat quality properties and to analyze the expression levels of GPx4 gene by quantitative real-time PCR. The results showed that meat quality, GPx4 activity, and GPx4 mRNA levels were affected by pig breeds. There were the lowest expressible moisture, drip loss, and cooking loss in Laiwu Pig among the three breeds, and also there were the highest GPx4 activity and the expression levels of GPx4 gene in Laiwu Pig compared with the other two pig breeds. Analysis of meat samples divergent for water-holding capacity identified highly negative associations between expressions levels of GPx4 gene and expressible moisture, drip loss and cooking loss. These results suggest that the higher mRNA expression level of GPx4 gene up-regulate the water-holding capacity and the activity of GPx4, and the expression of GPx4 gene plays an important role in physiological changes related to the meat quality properties and the mechanisms of the antioxidant ion of meat.  相似文献   
964.
This study used in vitro and in vivo experimental models to investigate the anti-inflammatory potential of the butanol fraction of red bean ethanol extract (BF-RBEE), which contains the biologically active molecule catechin-7-β-d-glucopyranoside. Treatment with BF-RBEE inhibited nitric oxide production in lipopolysaccharide (LPS)-stimulated macrophages through suppression of extracellular signalregulated kinase and IκBα activation. In an in vivo murine sepsis model, oral administration of BF-RBEE improved mouse survival. Specifically, the survival rate of mice injected with LPS was 0 and 40% in ICR and BALB/c mice, respectively, whereas the survival of mice co-treated with BF-RBEE was 100% in both mouse types. This increase in survival with the BF-RBEE administration was correlated with decreased tumor-necrosis factor-α and interferon-γ and increased interleukin-10. Oral administration of BF-RBEE also inhibited total and ovalbumin-specific IgE production in experimental mice. These results suggest the possible usefulness of red beans in the treatment of inflammatory diseases.  相似文献   
965.
通过对广东省化纤发展现状和有利因素进行分析,指出了主要存在问题,提出未来发展建议.  相似文献   
966.
967.
Smallholder farmers in dryland areas, including the Arab region, face many risks, including those relating to weather and markets. While risk taking is necessary for economic development, many risks are so severe that responses to them entail long-term losses. Social risk management is a family of tools that facilitates efficient risk management and can help eliminate risk-related poverty traps. Safety nets form an important part of the social risk management toolkit; it is therefore important to evaluate their effectiveness. This review has identified several considerations for the design and implementation of safety nets to improve food security among small-scale producers in dryland areas. The main conclusions are: safety nets must be part of an integrated risk management strategy; they should be designed to contribute to long-term agricultural growth; and more effort is needed to establish efficient risk management mechanisms prior to crises.  相似文献   
968.
969.
In this issue     
  相似文献   
970.
Coordination across sectors to address undernutrition with its varied underlying causes remains a challenge in developing countries. In Afghanistan, harmonization of health and agriculture policies and actions to reduce endemic undernutrition occurred during 2002-2007. This qualitative case study explores the forms of harmonization, enabling features and constraints in the policy process. We interviewed 57 stakeholders (39 central level, 18 provincial) involved in public nutrition or food security issues. Forms of harmonization included written policies, reseach, training and advocacy. Important features of the policy process included: policy entrepreneurs with operational and strategic capacity, consensus-building using a shared causal framework, working groups and strategic alliances. This case presents an interesting alternative to a national nutrition coordinating body, an approach that has met with mixed results, often due to the lack of authority, budget, and operational capacity of such a body to oversee ministries and enforce national nutrition objectives. Mid-level professionals mobilized into task forces achieved much with the support of national and international partners. This study highlights the importance of building capacity for sustaining change through local institutions.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号