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   6436篇
  免费   660篇
  国内免费   44篇
系统科学   1099篇
丛书文集   11篇
教育与普及   15篇
理论与方法论   275篇
现状及发展   879篇
研究方法   11篇
综合类   4850篇
  2024年   9篇
  2023年   7篇
  2022年   19篇
  2021年   13篇
  2020年   14篇
  2019年   3篇
  2018年   733篇
  2017年   737篇
  2016年   443篇
  2015年   48篇
  2014年   26篇
  2013年   33篇
  2012年   291篇
  2011年   983篇
  2010年   855篇
  2009年   482篇
  2008年   543篇
  2007年   808篇
  2006年   34篇
  2005年   76篇
  2004年   163篇
  2003年   180篇
  2002年   95篇
  2001年   52篇
  2000年   37篇
  1999年   67篇
  1998年   48篇
  1997年   48篇
  1996年   39篇
  1995年   49篇
  1994年   33篇
  1993年   28篇
  1992年   28篇
  1991年   30篇
  1990年   21篇
  1989年   26篇
  1988年   15篇
  1987年   7篇
  1986年   7篇
  1985年   2篇
  1984年   1篇
  1979年   1篇
  1978年   2篇
  1974年   1篇
  1972年   1篇
  1967年   2篇
排序方式: 共有7140条查询结果,搜索用时 140 毫秒
51.
免疫沉淀法分离纯化感染细胞中的甲肝病毒RNA   总被引:1,自引:0,他引:1  
本研究采用抗血清(多抗血清)沉淀甲肝病毒,并用盐酸胍酸性酚,氯仿一步法分离纯化病毒RNA.此方法能排除非病毒RNA的污染,操作简单,要求条件较低,RNA的纯度和完整性都较好  相似文献   
52.
通过理论分析和实际路面的测量,阐述了直接在汽车车桥上安装传感器测量路面谱的一种简便方法,并通过与用五轮仪测量路面谱方法的结果对比,得出了此种方法在一般汽车性能试验中的实际应用价值.  相似文献   
53.
计算了包含不同自旋取向的单畴粒子具有短程交换相互作用能,各向异性能及长程偶极相互作用能的磁性超晶格的静磁能,利用能面图和局域能量极小模型得到了磁相图和磁滞回线,研究了有限尺寸和温度效应,计算结果较好解释了在超薄磁性薄膜中观察到的两种磁相(磁化强度平行和垂直于薄膜平面)的转变行为。  相似文献   
54.
微波在白血病无损探测中的应用:FDTD分析模型   总被引:3,自引:0,他引:3  
讨论应用微波对白血病进行无损探测的可能性,采用柱坐标系下时域有限差分(FDTD)法分析人体小腿的微波散射特性及白血病的存在对其微波散射特性产生的影响.数值结果表明,通过测量人体小腿上部的微波散射特性,可以反演出骨髓介电特性的变化情况,据此判断白血病存在的可能性及病变的程度.该技术可应用于白血病的无损探测及治疗过程中的监测  相似文献   
55.
论温泉旅游的创新开发   总被引:12,自引:0,他引:12  
近年来,温泉旅游发展迅速,但温泉旅游开发模式和活动项目的雷同化现象十分严重,从社会需求、温泉产品形象、温泉设计理念、温泉经营管理等方面都要求对温泉进行创新开发,温泉旅游开发的创新途径是主题创新、表现形式创新、内容创新和管理创新,只有这样温泉旅游才能可持续发展。  相似文献   
56.
开发了一套基于Web的高校饮食生产管理系统,5大子系统所构成的应用软件主体,几乎覆盖了高校饮食的所有日常生产管理功能.阐述了该系统的设计和实现过程,包括系统的特点,系统的结构,系统的功能设计,数据库结构设计,用户界面设计等内容.  相似文献   
57.
Lightning flash activities on the central Tibetan Plateau have been studied by using the satellite-based Lightning Imaging Sensor(LIS)database from January 1998 to July 2002.The lightning activity shows a clear diurnal variation on the central Plateau.The peak lightning activity appears at about 17:00 which is 3 h earlier than that in Jingxhou,Hubei in the same latitude belt nearby,indicating that the lightning activity is a sensitive indicator of solar heating on the Plateau.The lightning discharge is weaker on the Plateau than Jingzhou.Hubei and other low-altitude continental regions because of the lower convective available potential energy(CAPE)on the Plateau.The CAPE on the Plateau is 12 times lower than that in Jingzhou,Hubei,and 20 times lower than that in the sea-level region,such as Guangzhou and Florida.However,the sensitivity of lightning activity to CAPE changes on the Plateau is up to 30 times more sensitive than other prominent low-altitude regions.  相似文献   
58.
C-isotope composition and significance of the Sinian on the Tarim plate   总被引:9,自引:0,他引:9  
102 samples of limestones,mudrocks shales and tillites from the Sinian in Kuluketage have been analyzed ,Four formations in the Sinian belong to glacial deposits in which the differences in δ^13C are distinct (-14.7‰- 4.2‰),Distinctly negative δ^13C excursions for three times support the viwe of three glacial deposits in this area suggested by previous authors,except for the durations of them ,δ^13C values are generally positive in the Beiyixi Formation of lower part of the Lower Sinian ,representing probably the rock records of a continental rift related to break-up of the Rodinian supercontinent,There are three times of distinct excursions of δ^13C curve in all strata,The first one is from positive values at the bottom to about -5‰ at the top of the Beiyixi Formation ,The second one,from 0‰-3‰ in the Arletonggou Formation of upper part of the Lower Sinian to -3.4‰---14.4‰ in the Teruiaiken Formation.The third one,from positive values in the Zhamoketi Formation of lower part of the Upper Sinian to negative values in glacial varves at the top of the Hankeerqiaoke Formation.  相似文献   
59.
The Yuanmou Basin of Yunnan Province is a hotspot for the research of early homind evolution and its environmental background.During the implementation of the Program“Research of the Origin and Evolution of Early Man and Its Environmental Baqckground” from 1998 to 2001,a huge quantity of hominoid and mammal fossils were collected.The present report systematically describes an Euprox robustus sp.nov.identified during the recent laboratory work on the collected material.Euprox is a group of earliest cervids with true antlers.The new species is the third one of the genus discovered in China.Judged by its morphologic characteristics,the new species feed on juicy and tender leaves of dicotyledon.It implies that the vegetation of its epoch in the Yuanmou Basin is a kind of southern subtropical evergreen forest and the climate is humid and temperate with evident seasonality.The latter is mostly influenced by the monsoon and secondly by the latitude.  相似文献   
60.
Aerosol samples for PM2.5 were collected in Beijing for 38 consecutive days from March to April 2001 using an IMPROVE Sampler. Concentrations of 20 elements in PM2.5 were determined using a PIXE method. Results show that the average mineral dust concentration of PM2.5 was 14.6 Ilg/m3 during the observation period. On the sand-dust event days of March 21 and April 10, dust PM2.5 mass concentrations were 62.4 and 54.1 μg/m^3, respectively.These demonstrate that fine particle pollution by dust event in Beijing was very severe. The enrichment factors of S and Cu reached minimums on the dusty days and were high on the non-dusty days. It is considered that enrichment factors of elements in PM2.5, which are associated with human activities, can probably provide an effective method to distinguish local sources from external sources of dust. Factor analysis on the chemical composition in PM2.5 shows that sources of crustal matters, anthropogenic emission, and oil combustion contributed to PM2.5 levels in air in the springtime of 2001 in Beijing.  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