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   482篇
  免费   8篇
  国内免费   8篇
系统科学   7篇
丛书文集   10篇
教育与普及   1篇
理论与方法论   2篇
现状及发展   84篇
研究方法   24篇
综合类   369篇
自然研究   1篇
  2022年   3篇
  2021年   8篇
  2020年   5篇
  2019年   4篇
  2017年   9篇
  2016年   6篇
  2015年   10篇
  2014年   20篇
  2013年   20篇
  2012年   26篇
  2011年   25篇
  2010年   9篇
  2009年   17篇
  2008年   19篇
  2007年   25篇
  2006年   27篇
  2005年   15篇
  2004年   20篇
  2003年   23篇
  2002年   18篇
  2001年   21篇
  2000年   14篇
  1999年   19篇
  1998年   7篇
  1997年   15篇
  1996年   11篇
  1995年   5篇
  1994年   4篇
  1993年   4篇
  1992年   4篇
  1991年   3篇
  1990年   6篇
  1989年   4篇
  1988年   4篇
  1987年   6篇
  1986年   4篇
  1985年   2篇
  1981年   2篇
  1980年   6篇
  1979年   8篇
  1978年   4篇
  1977年   3篇
  1976年   5篇
  1975年   3篇
  1974年   4篇
  1973年   4篇
  1971年   2篇
  1970年   2篇
  1969年   3篇
  1967年   3篇
排序方式: 共有498条查询结果,搜索用时 14 毫秒
291.
设计一款煤矿井下突水监测及预警系统.系统以STM32单片机作为主控芯片,搭载嵌入式实时操作系统uC/OS-Ⅲ,通过各传感器对井下的水温、水压、水流量、围岩应力等信息进行实时感知和处理;利用LoRa智能网关将异常水文信息上传到地面远程监控预警平台,预警突水事故;监控服务端将预警结果下发到井下监测站,便于井下工作人员实时准确了解工作面的水情安全状态.系统测试结果表明,该系统水文参数采集精度均在1.5% 内,通信丢包率不超过5%,平均功耗较低.  相似文献   
292.
利用过氧化物过氧化二异丙苯(DCP)和多官能团化合物三烯丙基异氰尿酸酯(TAIC),对聚甲基乙撑碳酸酯(PPC)进行了交联研究。研究发现,DCP和TAIC的用量对PPC的交联均有较大影响,PPC交联后,其凝胶含量、机械性能、弹性模量、玻璃化转变温度均有所增加,1%的DCP和4%的TAIC用量可得到较理想的交联PPC材料。  相似文献   
293.
构建大学生社会实践新组织模式的思考   总被引:4,自引:0,他引:4  
赵婵 《山西科技》2006,(3):25-26
近些年来,大学生社会实践活动日益蓬勃发展,但其组织模式还不能适应高等教育培养高素质人才的发展要求。文章分析了当前大学生社会实践存在的问题,提出从组织机构、参与主体、指导教师及评价体系四方面着手,构建更为科学、合理、有效的大学生社会实践组织模式。  相似文献   
294.
实验羊草种群幼苗对不同梯度盐碱胁迫的生理响应   总被引:11,自引:1,他引:11  
在盐胁迫、碱胁迫及混合盐碱胁迫条件下,测定了实验羊草种群幼苗的含水率、叶片叶绿素含量和细胞膜透性等胁变指标,结果表明:含水率和叶绿素含量随胁强的增大总体呈下降趋势,细胞膜透性随胁强的增大而增加;叶绿素含量和细胞膜透性与胁强之间关系可以用直线方程y=α bx表达出来,羊草幼苗对不同胁迫的敏感性顺序为:Na2CO3胁迫、混合盐碱胁迫、NaCl胁迫。  相似文献   
295.
固定化啤酒废酵母对Pb2+的吸附   总被引:1,自引:0,他引:1  
用2 %海藻酸钠与1 %明胶混合为包埋剂固定啤酒酵母废菌体,研究固定化啤酒废酵母对Pb2 的吸附行为.利用原子吸收光谱法测定Pb2 含量.结果表明,固定化啤酒废酵母吸附Pb2 受吸附时间、吸附温度、溶液pH值、酵母添加量和Pb2 起始浓度等因素影响.实验确定了固定化啤酒废酵母对Pb2 的最佳吸附条件.即:pH为3.0~5.0,Pb2 浓度为100 mg·L-1,酵母添加量1.2 g·L-1,吸附温度25 ℃,吸附时间120 min.在一定的浓度范围内啤酒废酵母对Pb2 的吸附符合Langmuir吸附模型和Freundlich吸附模型,但符合Freundlich吸附模型的程度更优.  相似文献   
296.
玉米淀粉的热力学性质与消化性   总被引:4,自引:0,他引:4  
对4种不同直链/支链淀粉含量的玉米淀粉(蜡质玉米淀粉、普通玉米淀粉、Hylon Ⅴ和Hylon Ⅶ)的热力学性质及体外消化性进行测定,进一步分析了淀粉热力学性质与消化性的关系.结果表明:高支(蜡质和普通)玉米淀粉与高链玉米淀粉(Hylon Ⅴ和Hylon Ⅶ)的热力学性质存在显著差异;高支玉米淀粉的起糊温度在70℃左右...  相似文献   
297.
α-溴代醛是一种有用的有机合成中间体,主要用于制备α、β-不饱和醛和定向合成的光学活性异构体.本文从易得的原料开始,制备一种使用方便的溴化剂-二溴麦尔多姆酸,使脂肪醛发生α-溴化反应不仅条件温和、操作简便,而且收率高,后处理方便.所合成的一系列α-溴代醛经沸点.1HNMR和GC—MS所确证.  相似文献   
298.
Mutationally activated kinases define a clinically validated class of targets for cancer drug therapy. However, the efficacy of kinase inhibitors in patients whose tumours harbour such alleles is invariably limited by innate or acquired drug resistance. The identification of resistance mechanisms has revealed a recurrent theme—the engagement of survival signals redundant to those transduced by the targeted kinase. Cancer cells typically express multiple receptor tyrosine kinases (RTKs) that mediate signals that converge on common critical downstream cell-survival effectors—most notably, phosphatidylinositol-3-OH kinase (PI(3)K) and mitogen-activated protein kinase (MAPK). Consequently, an increase in RTK-ligand levels, through autocrine tumour-cell production, paracrine contribution from tumour stroma or systemic production, could confer resistance to inhibitors of an oncogenic kinase with a similar signalling output. Here, using a panel of kinase-'addicted' human cancer cell lines, we found that most cells can be rescued from drug sensitivity by simply exposing them to one or more RTK ligands. Among the findings with clinical implications was the observation that hepatocyte growth factor (HGF) confers resistance to the BRAF inhibitor PLX4032 (vemurafenib) in BRAF-mutant melanoma cells. These observations highlight the extensive redundancy of RTK-transduced signalling in cancer cells and the potentially broad role of widely expressed RTK ligands in innate and acquired resistance to drugs targeting oncogenic kinases.  相似文献   
299.
Asthma,rhinitis and eczema(allergic or non-allergic)have increased throughout the world during the last decades,especially among children.Changes in the indoor environment are suspected to be important causes.China has experienced a dramatic change in indoor environmental exposures during the past two decades.However,such changes and their associations with children’s asthma and other health aspects have not been thoroughly studied.China,Children,Homes,Health(CCHH),Phase I,was a cross-sectional questionnaire survey of 48219 children 1–8 years old in 10 Chinese cities during 2010–2012.The questionnaire includes the International Study of Asthma and Allergies in Childhood(ISAAC)core health questions and additional questions regarding housing,life habits and outdoor environment.In health analyses,children aged 3–6 years old were included.The prevalences of doctor diagnosed asthma varied from 1.7%to 9.8%(mean 6.8%),a large increase from 0.91%in 1999 and 1.50%in2000.The prevalence of wheeze,rhinitis and atopic eczema(last 12 months)varied from 13.9%to 23.7%,24.0%to 50.8%and4.8%to 15.8%,respectively.Taiyuan had the lowest prevalences of all illnesses and Shanghai the highest,except for wheezewhere the highest value was for Urumqi.We found(1)no obvious association between disease prevalences and ambient PM10concentrations and(2)higher prevalences of disease in humid climates with hot summers and cold winters,but with no centrally heated buildings.Associations between the diseases and economic status as indexed by Gross Domestic Product(GDP)requires further study.  相似文献   
300.
The activated B-cell-like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL) remains the least curable form of this malignancy despite recent advances in therapy. Constitutive nuclear factor (NF)-κB and JAK kinase signalling promotes malignant cell survival in these lymphomas, but the genetic basis for this signalling is incompletely understood. Here we describe the dependence of ABC DLBCLs on MYD88, an adaptor protein that mediates toll and interleukin (IL)-1 receptor signalling, and the discovery of highly recurrent oncogenic mutations affecting MYD88 in ABC DLBCL tumours. RNA interference screening revealed that MYD88 and the associated kinases IRAK1 and IRAK4 are essential for ABC DLBCL survival. High-throughput RNA resequencing uncovered MYD88 mutations in ABC DLBCL lines. Notably, 29% of ABC DLBCL tumours harboured the same amino acid substitution, L265P, in the MYD88 Toll/IL-1 receptor (TIR) domain at an evolutionarily invariant residue in its hydrophobic core. This mutation was rare or absent in other DLBCL subtypes and Burkitt's lymphoma, but was observed in 9% of mucosa-associated lymphoid tissue lymphomas. At a lower frequency, additional mutations were observed in the MYD88 TIR domain, occurring in both the ABC and germinal centre B-cell-like (GCB) DLBCL subtypes. Survival of ABC DLBCL cells bearing the L265P mutation was sustained by the mutant but not the wild-type MYD88 isoform, demonstrating that L265P is a gain-of-function driver mutation. The L265P mutant promoted cell survival by spontaneously assembling a protein complex containing IRAK1 and IRAK4, leading to IRAK4 kinase activity, IRAK1 phosphorylation, NF-κB signalling, JAK kinase activation of STAT3, and secretion of IL-6, IL-10 and interferon-β. Hence, the MYD88 signalling pathway is integral to the pathogenesis of ABC DLBCL, supporting the development of inhibitors of IRAK4 kinase and other components of this pathway for the treatment of tumours bearing oncogenic MYD88 mutations.  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