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   21994篇
  免费   305篇
  国内免费   489篇
系统科学   451篇
丛书文集   352篇
教育与普及   471篇
理论与方法论   118篇
现状及发展   4417篇
研究方法   624篇
综合类   16005篇
自然研究   350篇
  2022年   180篇
  2021年   224篇
  2015年   248篇
  2014年   356篇
  2013年   372篇
  2012年   553篇
  2011年   969篇
  2010年   462篇
  2009年   507篇
  2008年   664篇
  2007年   689篇
  2006年   631篇
  2005年   576篇
  2004年   598篇
  2003年   681篇
  2002年   871篇
  2001年   845篇
  2000年   709篇
  1999年   729篇
  1998年   469篇
  1997年   442篇
  1996年   398篇
  1995年   372篇
  1994年   352篇
  1993年   279篇
  1992年   403篇
  1991年   333篇
  1990年   341篇
  1989年   338篇
  1988年   309篇
  1987年   270篇
  1986年   220篇
  1985年   268篇
  1984年   190篇
  1979年   331篇
  1978年   257篇
  1977年   242篇
  1976年   225篇
  1975年   238篇
  1974年   306篇
  1973年   278篇
  1972年   237篇
  1971年   337篇
  1970年   474篇
  1969年   327篇
  1968年   312篇
  1967年   319篇
  1966年   349篇
  1965年   223篇
  1958年   209篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
本就高温超导体YB2Cu3O6 x的光电导效应的研究现状进行了介绍,并就机理解释及未来的研究工作做了简单的评述。  相似文献   
72.
取17个烃源岩的显微组分,用 ̄13CNMR波谱技术进行了化学结构与成烃潜力的研究,按照脂碳的丰度与油气潜力碳的大小,各组分可按如下顺序排列:藻类体、树脂体、角质体、孢子体、木栓质体、镜质体、惰性体.以惰性碳、油潜力碳与气潜力碳划分显微组分为三组成,并构成三元图,藻类体位于图的上部,相当于I型干酪根;各稳定组分位于图的中部,相应干Ⅱ型干酚根;镜质体与惰质体位于图的左下部,相当于Ⅲ型干酪根.据此,可从烃源岩显微组分的分析组成估算它的生油与生气潜力  相似文献   
73.
武汉机场是国家重点建设项目。本文提出了机场飞行区土方工程的最优调配问题。它是一个规模庞大的大系统最优规划, 变量多达一千四百九十六个, 一般计算机容量已不够。为此, 在工民建施工领域首次提出了大系统土方调配优化数学模型进行优化, 得出的优化方案可节约经费1073441.29元。所建立的大系统土方调配优化数学模型对工民建等大型土方调配优化有普遍的应用价值。  相似文献   
74.
与时俱进是马克思主义的理论品质。在新旧世纪之交的历史时刻,以江泽民为核心的党的第三代领导集体适时地提出了承接“三步走”战略的面向21世纪前五十年的发展战略。形成了具有鲜明时代特征的发展战略思想。  相似文献   
75.
The inherited osteolyses or 'vanishing bone' syndromes are a group of rare disorders of unknown etiology characterized by destruction and resorption of affected bones. The multicentric osteolyses are notable for interphalangeal joint erosions that mimic severe juvenile rheumatoid arthritis (OMIMs 166300, 259600, 259610 and 277950). We recently described an autosomal recessive form of multicentric osteolysis with carpal and tarsal resorption, crippling arthritic changes, marked osteoporosis, palmar and plantar subcutaneous nodules and distinctive facies in a number of consanguineous Saudi Arabian families. We localized the disease gene to 16q12-21 by using members of these families for a genome-wide search for homozygous-by-descent microsatellite markers. Haplotype analysis narrowed the critical region to a 1.2-cM region that spans the gene encoding MMP-2 (gelatinase A, collagenase type IV; (ref. 3). We detected no MMP2 enzymatic activity in the serum or fibroblasts of affected family members. We identified two family-specific homoallelic MMP2 mutations: R101H and Y244X. The nonsense mutation effects a deletion of the substrate-binding and catalytic sites and the fibronectin type II-like and hemopexin/TIMP2 binding domains. Based on molecular modeling, the missense mutation disrupts hydrogen bond formation within the highly conserved prodomain adjacent to the catalytic zinc ion.  相似文献   
76.
非对称极化散射矩阵的目标识别方法研究   总被引:2,自引:0,他引:2  
本文针对单基地测量时线性目标的极化散射矩阵为非对称这一实际情况.根据散射矩阵理论将其进行了对称性修正,在此基础之上,利用本征极化作为雷达目标极化特性的表征并进一步提取了目标的极化特征,同时对五种飞机目标进行了识别实验研究,取得了良好的目标分类或识别效果。  相似文献   
77.
Summary Intraperitoneal injection of allogeneic liver cells from 43-day-old male fetuses into normal 60-day female goat fetuses resulted in persistent hemopoietic chimerism in surviving recipients without clinical evidence of graft-versus-host disease. Transplantation of normal fetal liver cells into preimmunocompetent goat fetuses affected with -D-mannosidosis may provide an alternative strategy for evaluating hemopoietic stem cell transplantation in the treatment of human lysosomal storage diseases.  相似文献   
78.
Summary Genetically-defined rodent strains permit the identification of hippocampal traits which are of functional relevance for the performance of two-way avoidance behavior. This is exemplified here by analyzing the relationship between infrapyramidal mossy fibers (a tiny projection terminating upon the basal dendrites of hippocampal pyramidal neurons) and two-way avoidance learning in about 800 animals. The necessary steps include 1) identification of structural traits sensitive to selective breeding for extremes in two-way avoidance, 2) testing the robustness of the associations found by studying individual and genetical correlations between hippocampal traits and behavior, 3) establishing causal relationships by Mendelian crossing of strains with extreme structural traits and studying the behavioral consequences of such structural randomization, 4) confirming causal relationships by manipulating the structural variable in inbred (isogenic) strains, thereby eliminating the possibility of genetic linkage, and 5) ruling out the possibility of spurious associations by studying the correlations between the hippocampal trait and other behaviors known to depend on hippocampal functioning.In comparison with the classical lesion approach for identifying relationships between brain and behavior, the present procedure appears to be superior in two aspects: it is non-invasive, and it focuses automatically on those brain traits which are used by natural selection to shape behaviorally-defined animal populations, i.e., it reveals the natural regulators of behavior.  相似文献   
79.
Summary The opioid agonists morphine (selective for -receptors) and ethylketocyclazocine (selective for kappa-receptors), at concentrations evoking strong effects in neuronal structures, did not significantly affect the configuration of the intracellularly recorded action potential and the force of contraction in ventricular heart muscle isolated from guinea pigs, rabbits and man. These results suggest that any changes of heart functions in vivo in response to opioid-like drugs are probably not mediated postsynaptically at the myocardial cell membrane but rather presynaptically, influencing the release of noradrenaline and/or acetylcholine from the nerve terminals.  相似文献   
80.
The contribution of anoxygenic photosynthesis to carbon cycling in the Black Sea, the world's largest body of anoxic marine water, has been vigorously investigated and debated for over four decades. Penetration of light into the sulphide-containing deep water may result in a zone of anaerobic primary production by photosynthetic bacteria. We report here the results of analyses of photosynthetic pigments in samples of suspended particulate matter collected from two stations in the western basin of the Black Sea. Our data demonstrate high concentrations of a bacterio-chlorophyll at the chemocline, and thus the potential for anoxygenic photosynthesis as a component of primary production in the carbon cycle of the Black Sea. More than 95% of the pigments in the bacteriochlorophyll-maximum are accounted for by a series of aromatic carotenoids and bacteriochlorophylls-e, including a previously unreported geranyl ester of 4-i-butyl bacteriochlorophyll-e. The distribution of pigments is characteristic of the obligate phototrophs Chlorobium phaeobacteroides and C. phaeovibriodes. Total depth-integrated bacteriochlorophyll at one station exceeded total chlorophyll-a in the overlying oxygenated portion of the euphotic zone. We suggest that anoxygenic photosynthesis is a relatively recent phenomenon in the Black Sea initiated by shallowing of the chemocline over the past decade and development of an anoxic layer devoid of O2 and H2S.  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