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   2064篇
  免费   10篇
  国内免费   33篇
系统科学   51篇
丛书文集   15篇
教育与普及   3篇
理论与方法论   53篇
现状及发展   871篇
研究方法   184篇
综合类   886篇
自然研究   44篇
  2019年   12篇
  2018年   21篇
  2017年   23篇
  2016年   20篇
  2015年   17篇
  2014年   17篇
  2013年   42篇
  2012年   75篇
  2011年   99篇
  2010年   31篇
  2009年   14篇
  2008年   57篇
  2007年   55篇
  2006年   83篇
  2005年   58篇
  2004年   61篇
  2003年   96篇
  2002年   129篇
  2001年   147篇
  2000年   100篇
  1999年   68篇
  1997年   12篇
  1992年   27篇
  1991年   11篇
  1990年   20篇
  1989年   19篇
  1988年   21篇
  1987年   26篇
  1986年   25篇
  1985年   22篇
  1984年   22篇
  1983年   16篇
  1982年   12篇
  1981年   15篇
  1980年   24篇
  1979年   37篇
  1978年   35篇
  1977年   36篇
  1976年   29篇
  1975年   37篇
  1974年   28篇
  1973年   31篇
  1972年   43篇
  1971年   43篇
  1970年   31篇
  1969年   32篇
  1968年   24篇
  1967年   30篇
  1966年   34篇
  1965年   21篇
排序方式: 共有2107条查询结果,搜索用时 31 毫秒
941.
从特征标诱导对应的角度讨论了π-块覆盖的Clifford定理,推广了p-块论中的相关结论。  相似文献   
942.
1Introduction In custom manufacturing time-to-market is of overriding importance. Thus, asymmetric hydrogenation will be applied only if an appropriate catalyst can be found in a very short period of time. For this reason we have build up a full-scale capability for High Throughput Experimentation, encompassing several robot systems. As ligand synthesis was the last remaining barrier we have explored, together with the University of Groningen, the use of simple monodentate ligands, such as MonoPhosTM. These ligands can be synthesised in two chemical steps, from BINOL, PCl3 and an amine.Surprisingly, very high enantioselectivities can be obtained in the asymmetric hydrogenation of α-and β-dehydro-amino acids and esters, aromatic enamides, itaconic acid and ester[1]. See Scheme 1 and 2(at page 830).  相似文献   
943.
Insulin is necessary to produce an increase of plasma fibrinogen in rats submitted to tissue injury or ACTH administration. This increase is more significant when endogenous or exogenous excess of insulin is present, while in uninjured rats the absence or excess insulin does not modify plasma fibrinogen.  相似文献   
944.
One thing about technical artefacts that needs to be explained is how their physical make-up, or structure, enables them to fulfil the behaviour associated with their function, or, more colloquially, how they work. In this paper I develop an account of such explanations based on the familiar notion of mechanistic explanation. To accomplish this, I (1) outline two explanatory strategies that provide two different types of insight into an artefact’s functioning, and (2) show how human action inevitably plays a role in artefact explanation. I then use my own account to criticize other recent work on mechanistic explanation and conclude with some general implications for the philosophy of explanation.  相似文献   
945.
论述了改进自由动态子结构方法,并通过对十层井塔的分析计算,论述了该方法的适用条件及优越性。  相似文献   
946.
针对湿法同时脱硫脱硝工艺 ,提出通过构造一个液相反应过程 ,将 NOx 和 SO2 溶解在溶液中生成的 NO2 -2 和 SO2 -3 经过一系列连续的液相离子反应生成有利用价值的 NH+4 ,实现资源的合理利用。在 35°C~ 74°C,离子强度为 0 .5 5 mol/L时对羟胺二磺酸钾磺化反应动力学进行了研究 ,得出反应的动力学方程为 :- dcHIDSdt =kc HON( SO3) 2 -2 · c HSO-3,其中 k=5 .5 0 9× 1 0 8· e-6.154× 10 4 / RT;在5 0°C、离子强度为 0 .31 mol/L~ 0 .65 mol/L时 ,研究了该反应的盐效应 ,结果表明该反应呈负盐效应 ,离子强度为零时 ,有最大的速率常数 k0 =0 .0 2 4 7L /( mol· min)  相似文献   
947.
Familial combined hyperlipidemia (FCHL, MIM-144250) is a common, multifactorial and heterogeneous dyslipidemia predisposing to premature coronary artery disease and characterized by elevated plasma triglycerides, cholesterol, or both. We identified a mutant mouse strain, HcB-19/Dem (HcB-19), that shares features with FCHL, including hypertriglyceridemia, hypercholesterolemia, elevated plasma apolipoprotein B and increased secretion of triglyceride-rich lipoproteins. The hyperlipidemia results from spontaneous mutation at a locus, Hyplip1, on distal mouse chromosome 3 in a region syntenic with a 1q21-q23 FCHL locus identified in Finnish, German, Chinese and US families. We fine-mapped Hyplip1 to roughly 160 kb, constructed a BAC contig and sequenced overlapping BACs to identify 13 candidate genes. We found substantially decreased mRNA expression for thioredoxin interacting protein (Txnip). Sequencing of the critical region revealed a Txnip nonsense mutation in HcB-19 that is absent in its normolipidemic parental strains. Txnip encodes a cytoplasmic protein that binds and inhibits thioredoxin, a major regulator of cellular redox state. The mutant mice have decreased CO2 production but increased ketone body synthesis, suggesting that altered redox status down-regulates the citric-acid cycle, sparing fatty acids for triglyceride and ketone body production. These results reveal a new pathway of potential clinical significance that contributes to plasma lipid metabolism.  相似文献   
948.
This paper discusses an application of the CRASP model of Action Research (AR) on Operations Research (OR). The CRASP model portrays AR as a style of staff development that would lead to professionalism. It also advocates social responsibility, committed and informed action, and a critical approach. All of these concerns are inherent to the process of reflection and development characteristic of OR. As an OR practitioner I participate in this process of reflection and development and applied the CRASP model of AR in performing one application novel to OR, namely, OR for street children. Reflecting on this application of AR on OR for street children, some similarities between AR and OR surface, as well as some difficulties in applying AR on OR, thereby enriching both AR and OR. It is hoped that through collaborative reflection with other AR and OR practitioners, this paper will further our professional development.  相似文献   
949.
950.
Inhaled allergens from house dust, mites or animal danders activate human complement in vitro by engaging the C1-component through a non-antibody-dependent mechanism. These earlier findings are extended by showing that the allergenic components in extracts of Parietaria pollen are almost equally potent complement activators as those from house dust or mites. Spectroscopic evidence indicates that haemolytic complement consumption by the Parietaria allergens and their enzymic fragments is most likely related to post-translational side-chains comprising flavonoid derivatives. These adsorbed and/or peptide-bound tannin-like structures may also explain the exceptional stability of the high- and low-molecular mass allergenic components in Parietaria pollen extracts. Received 12 November 1996; received after revision 12 December 1996; accepted 17 December 1996  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