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   74014篇
  免费   9101篇
  国内免费   9051篇
系统科学   1206篇
丛书文集   711篇
教育与普及   103篇
理论与方法论   161篇
现状及发展   17843篇
研究方法   1634篇
综合类   69882篇
自然研究   626篇
  2020年   1086篇
  2019年   3626篇
  2018年   3793篇
  2017年   4015篇
  2016年   4196篇
  2015年   3910篇
  2014年   4027篇
  2013年   3540篇
  2012年   3209篇
  2011年   3595篇
  2010年   2853篇
  2009年   2255篇
  2008年   2066篇
  2007年   2252篇
  2006年   1702篇
  2005年   1779篇
  2004年   2318篇
  2003年   1857篇
  2002年   1665篇
  2001年   1711篇
  2000年   1549篇
  1999年   1225篇
  1997年   614篇
  1994年   695篇
  1993年   596篇
  1992年   1233篇
  1991年   1018篇
  1990年   935篇
  1989年   834篇
  1988年   854篇
  1987年   760篇
  1986年   826篇
  1985年   909篇
  1984年   633篇
  1979年   1304篇
  1978年   1009篇
  1977年   946篇
  1976年   757篇
  1975年   781篇
  1974年   1091篇
  1973年   944篇
  1972年   989篇
  1971年   1079篇
  1970年   1479篇
  1969年   1155篇
  1968年   1074篇
  1967年   1079篇
  1966年   1002篇
  1965年   755篇
  1958年   722篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
制备介孔复合环境材料的酸洗刻蚀工艺   总被引:2,自引:1,他引:1  
探索了一种制备介孔复合环境材料的方法--破坏式造孔与有机复合相结合,使最终产品可大量吸附污染水体中的各种污染物并具有一定的杀菌功能.该方法通过酸洗刻蚀,达到了去除原材料中的杂质和刻蚀孔径的目的,所制备的中间产品24h吸湿率为原材料的2倍,染料吸附量为原材料的1.6倍.  相似文献   
42.
极低硫钢的精炼脱硫动力学模型   总被引:2,自引:1,他引:1  
通过对影响脱硫反应限制性环节的分析,将持续接触和瞬时接触两种反应模式综合考虑,建立了精炼脱硫动力学模型,并通过实验室"极低硫钢精炼脱硫"实验数据对模型进行验证.结果表明,本模型的预测结果与实验结果比较吻合.  相似文献   
43.
As the structural analogues of (bacterio)chlorophylls in photosynthetic organisms, porphyrins play an important role in building up artificial photosynthetic systems[13]. In natural photosynthetic pigment-protein complexes,certain amino acid residuals modulate the reactions ofcharge transfer and energy transfer via coordination andother types of interactions with chlorophyll molecules.For instance, in higher-plant photosynthetic reaction cen-ter PSⅡ, the oxidization-reduction system consistin…  相似文献   
44.
45.
Obese postmenopausal female volunteers were given timed daily oral dosages of bromocriptine, and tested for reduction of body fat stores. This dopamine agonist has been shown to reset circadian rhythms that are altered in obese animals and to reduce body fat levels in several animal models. The participants were instructed not to alter their existing exercise and eating behavior during treatment. Skinfold measurements were taken on 33 subjects as indices of body fat. The measurements (e.g., suprailiac) were reduced after six weeks by about 25%, which represents a reduction of 11.7% of the total body fat. These dramatic decreases in body fat, which are equivalent to that produced by severe caloric restriction, were accompanied by more modest reductions of body weight (2.5%), indicating a possible conservation of protein that is usually lost as a consequence of such caloric restriction. The effects of bromocriptine treatment on body fat and hyperglycemia were also examined in non-insulin dependent diabetics being treated with oral hypoglycemics (7 subjects) or insulin (7 subjects). Total body fat was reduced by 10.7% and 5.1% in diabetics on oral hypoglycemics and insulin, respectively, without any significant reductions in body weight. Hyperglycemia was reduced in most of the 15 diabetic subjects treated leading to euglycemia and even cessation of hypoglycemic drugs in 3 of the 7 subjects during 4-8 weeks of bromocriptine treatment. These findings support the hypothesis that obesity and type II diabetes may be treated effectively with bromocriptine when administered at the proper times and dosages.  相似文献   
46.
A Kowluru  R A Kowluru 《Experientia》1992,48(5):486-488
Urinary excretion of glycated albumin was quantitated in genetically hyperglycemic mice (C57BL-Ks-J, db/db mice), a model for non-insulin-dependent diabetes mellitus, and compared with their non-diabetic littermates. The data indicated a preferential excretion of glycated albumin in non-diabetic mice. This phenomenon of 'editing' of glycated albumin is decreased significantly in diabetic mice. Quantitative measurements of overall excretion of glycated albumin suggested that the loss of editing in diabetic mice is due to the dilution of glycated albumin by the unmodified albumin which is excreted in large amounts in diabetic mice. Therefore, the loss of editing observed in this model resembled the one we characterized in insulin-dependent diabetic humans and a streptozotocin-diabetic rat model.  相似文献   
47.
提出了利用上位机参与实时控制,综合分析在线参数、离线参数和间接参数,辅助发酵过程工艺研究和优化控制,并给出了上位机软件包的部分源程序及框图。该软件包对发酵工艺人员进一步了解菌种的遗传特性、细胞代谢调节和反应器工程特性方面有较大帮助。  相似文献   
48.
微分代数控制问题的数值计算方法   总被引:3,自引:0,他引:3  
描述许多轨道控制问题的方程通常构成非线性半显式的微分代数系统。本文提出一些数值方法来计算这些控制问题的控制规律。对于一个模型问题,本文进行了稳定性分析,并且给出了稳定性区域。文中估计了这些方法的全局误差,它们给出控制误差与计算步长的关系。  相似文献   
49.
E Littler  A D Stuart  M S Chee 《Nature》1992,358(6382):160-162
Human cytomegalovirus (HCMV, a betaherpes virus) is the cause of serious disease in immunologically compromised individuals, including those with acquired immunodeficiency syndrome. One of the compounds used in the chemotherapy of HCMV infections is the nucleoside analogue 9-(1,3-dihydroxy-2-propoxymethyl)-guanine (ganciclovir). The mechanism of action of this drug is dependent on the formation of the nucleoside triphosphate, which is a strong inhibitor of the viral DNA polymerase. Thymidine kinase, which is encoded by many of the herpesviruses, catalyses the initial phosphorylation of ganciclovir. But there is no evidence for the coding of this enzyme by HCMV, and DNA sequence analysis of the HCMV genome has shown that there is no open reading frame characteristic of a herpesvirus thymidine kinase. Here we present biochemical and immunological evidence that the HCMV UL97 open reading frame codes for a protein capable of phosphorylating ganciclovir. This protein seems to be responsible for the selectivity of ganciclovir and will be useful tool in the understanding and refinement of the antiviral activity of new selective anti-HCMV compounds.  相似文献   
50.
The characteristic circular dichroism of bilirubin bound to human serum albumin undergoes a remarkable sign inversion on addition of halothane, chloroform and other volatile anesthetics. This sign inversion, which is completely reversed by removal of the anesthetic, reflects a pronounced conformational change of the bound ligand; probably a complete inversion of chirality. The observation suggests that association of volatile anesthetics with proteins can markedly alter the internal topography of receptor sites and potentially influence the stereoselectivity of ligand binding.  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