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   1688篇
  免费   68篇
  国内免费   62篇
系统科学   23篇
丛书文集   53篇
教育与普及   16篇
理论与方法论   4篇
现状及发展   19篇
综合类   1702篇
自然研究   1篇
  2024年   6篇
  2023年   11篇
  2022年   25篇
  2021年   36篇
  2020年   28篇
  2019年   23篇
  2018年   16篇
  2017年   23篇
  2016年   39篇
  2015年   35篇
  2014年   73篇
  2013年   46篇
  2012年   94篇
  2011年   118篇
  2010年   72篇
  2009年   86篇
  2008年   76篇
  2007年   93篇
  2006年   105篇
  2005年   103篇
  2004年   101篇
  2003年   82篇
  2002年   70篇
  2001年   63篇
  2000年   58篇
  1999年   42篇
  1998年   31篇
  1997年   35篇
  1996年   50篇
  1995年   29篇
  1994年   31篇
  1993年   29篇
  1992年   20篇
  1991年   15篇
  1990年   14篇
  1989年   13篇
  1988年   11篇
  1987年   7篇
  1986年   4篇
  1985年   4篇
  1955年   1篇
排序方式: 共有1818条查询结果,搜索用时 140 毫秒
101.
接力切换是TD-SCDMA系统的重要组成部分,但是由于技术的不成熟和种种原因,接力切换并未在现有 网络系统中完全实现。提出了一种基于硬切换并带有接力切换部分特点的切换模式,该模式基于硬切换的演变而 来,同时又借用智能天线产生的定位信息,不仅缩短了切换延时而且相对接力切换减少了信令交互的复杂度。基 于3GPP的各项标准,按照这种切换模式分析了同步同频和异步异频时的切换具体时延,为将来的TD-SCDMA网 络规划和优化工作奠定了基础。  相似文献   
102.
A new type of triploid hybrids (TC) was produced by crossing red crucian carp (♀) with allotetraploid hybrids (♂). This type of triploids with no barbel was spindle-shaped and gray in color. Compared with their parents, some data of the morphological traits in the triploid hybrids were intermediate to those of their parents, and some were beyond their parents with heterozygous traits. Under the same culture conditions, the growth rate of the new type triploids was faster than their maternal red crucian carp (RC). The gonadal development of triploids was obviously slower than that of diploids, and the germ cells in them were degenerated. Compared with another type of triploids produced by Japanese crucian carp (♀) with allotetraploid hybrids (♂), the new type triploids not only kept faster growth rate and sterility, but also had no barbel that was similar to the common carp. The absence of the barbel in the new type of triploids has the important significance in both the inheritance and fish breeding.  相似文献   
103.
以70%乙醇浸提商陆浆果红色素,采用吸光度值的变化和光谱扫描法研究了光照、温度和pH值对色素稳定性的影响,测定了商陆红色素的还原力、清除DPPH.和OH.的能力.结果表明:商陆红色素在pH3~9较稳定,对光和高温不稳定,因此可以作为冷饮食品的着色剂开发;商陆红色素有一定的还原力和抗氧化性,值得深入研究.  相似文献   
104.
一类转库问题流向优化问题的模型与解法   总被引:1,自引:0,他引:1  
转库是大型企业物流管理工作中的重要环节·针对企业决策支持系统的子系统转库作业日计划问题进行了分析,为一类转库流向问题建立了优化模型具有特殊约束0-1整数线性规划问题(0-1ILP)·分析了具体问题的性质·为求解这类NP-难问题,给出了一种在实际中行之有效的求解问题的算法降维替换算法·以SAS语言为环境,用实际问题作为计算算例,对这种算法的优点进行了总结:该算法在实际应用中是切实可行的,在时间上是节约的,尤其适合于大规模的问题  相似文献   
105.
Carbon isotopic composition of pedogenic carbonate can be used to estimate the proportion of C4 and C3 plants. Here we present carbon isotopic data of carbonate in a red earth section at Xifeng, central Loess Plateau. Results show that C4 vegetation increased in ∼4.4 Ma B.P., stabilized between 4.0 and 3.0 Ma B.P. The character and timing of C4 expansion on the Loess Plateau are similar, but different with other localities, e.g. Pakistan and Africa, implying that regional climate changes were main factors driving the expansion of C4 plants. This event is comparable in timing with increased aridity evidenced by Xifeng grain size and North Pacific eolian dust records. Therefore we argue that the Pliocene expansion of C4 plants in northern China might have been caused by the increased aridity, which in turn might be related to rapid uplift of the Tibetan Plateau.  相似文献   
106.
The results of paleogeomagnetics and geochemistry of the coral reef in well Nanyong 2 of Nansha Islands showed that the bottom of the black sedimentary interlayer corresponds to the conversion boundary line between Brunhes Postive Polarity and Matuyama Reversed Polarity (B/M) and the cold/warm (19/20) climatic conversion bounds on δ18O curve, 0.78 Ma ago; and the red sedimentary interlayer corresponds to the ReunionⅠpolarity excursion (reversion) of the geomagnetic field, 2.01—2.04 Ma ago. Comparing with the normal light-coloured coral reef rock, the magnetic susceptibility (c), residual magnetization intensity (Mr) and the content of MnO and Fe2O3 of the black sedimentary layer appeared obviously positive abnormity. The magnetic susceptibility (c), residual magnetization intensity (Mr) and the content of Fe2O3 of the red sedimentary layer also appeared positive abnormity. Combining with the analyzing results of paleontology, we hold that this pair of special and typical sedimentary interlayers was relative to the sudden change of paleoclimate, i.e. the global climatic change and its incidental polarity reversal of the geomagnetic field directly affected the living environment of the paleontological species as well as the dispersion and enrichment of some chemical elements, especially the elements sensitive to redox such as Fe and Mn. For example, the elements Fe and Mn concentrated in the glacial period would be largely oxidized and diluted when the climate warmed up suddenly, and the originally oxidized high valence Fe would be condensed again when the climate cooled suddenly. This is possibly one of the important reasons of appearing and disappearing of the red and black sedimentary events of coral reef in well Nanyong 2.  相似文献   
107.
The faunal ecological type and sequence in the Tertiary red clay indicate that Hipparion fauna during the period 7.4-6.1 Ma in the middle reaches of the Yellow River is composed mainly of dorcadoides and mixed types. This represents semiarid veld environment with small fluctuation. The fossil assemblages characterized by the abundance of gaudryi types and even the occurrence of some Hominoid and Chalicotherium indicate an obviously warm and humid period since 5.3 Ma. The fossil assemblages during 3.4 Ma show the typical steppe fauna representing dry climate. It is indicated by the changes of ecological sequence that the climatic conditions in Pliocene are drier than that in late Miocene. The clay-skin content of red clay, which has a synchronous change with faunal features, is a rather good replacing palaeoclimatic index.  相似文献   
108.
The faunal ecological type and sequence in the Tertiary red clay indicate that Hipparion fauna during the period 7.4–6.1 Ma in the middle reaches of the Yellow River is composed mainly of dorcadoides and mixed types. This represents semiarid veld environment with small fluctuation. The fossil assemblages characterized by the abundance of gaudryi types and even the occurrence of some Hominoid and Chalicotherium indicate an obviously warm and humi period since 5.3 Ma. The fossil assemblages during 3.4 Ma show the typical steppe fauna representing dry climate. It is indicated by the changes of ecological sequence that the climatic conditions in Pliocene are drier than that in late Miocene. The clay-skin content of red clay, which has a synchronous change with faunal features, is a rather good replacing palaeoclimatic index.  相似文献   
109.
坚硬煤层注水预裂水压力计算   总被引:1,自引:0,他引:1  
主要运用有效应力原理及断裂力学推导了煤层注水所需的最小注水压力,初步探讨了多组裂隙同时存在对煤层最小注水压力影响的计算方法,并分析了影响倾斜坚硬煤层注水压力的因素,指出了注水压力与煤层倾角及埋藏深度之间的关系,其理论结果与现场相符。  相似文献   
110.
中国南方红土的研究进展   总被引:10,自引:0,他引:10  
中国南方的红土是晚新生代以来重要堆积物,为特定的气候环境下的产物,蕴藏着丰富的环境演化信息,为全球变化的 良好信息载体,作者对红土的成因、发育期次、年代学、古气候学、生物学、磁学性质等方面的不同观点作了较为详细的论述,从而提出了目前红土研究中存在的问题及解决的方法。  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