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   92373篇
  免费   1249篇
  国内免费   880篇
测绘学   2643篇
大气科学   6732篇
地球物理   17238篇
地质学   35048篇
海洋学   7680篇
天文学   20602篇
综合类   396篇
自然地理   4163篇
  2022年   472篇
  2021年   807篇
  2020年   842篇
  2019年   937篇
  2018年   4182篇
  2017年   3864篇
  2016年   3608篇
  2015年   1481篇
  2014年   2448篇
  2013年   4068篇
  2012年   3262篇
  2011年   5215篇
  2010年   4573篇
  2009年   5460篇
  2008年   4586篇
  2007年   5140篇
  2006年   3032篇
  2005年   2621篇
  2004年   2494篇
  2003年   2464篇
  2002年   2269篇
  2001年   1850篇
  2000年   1761篇
  1999年   1463篇
  1998年   1479篇
  1997年   1387篇
  1996年   1232篇
  1995年   1195篇
  1994年   1069篇
  1993年   919篇
  1992年   873篇
  1991年   886篇
  1990年   931篇
  1989年   787篇
  1988年   773篇
  1987年   888篇
  1986年   761篇
  1985年   953篇
  1984年   1043篇
  1983年   957篇
  1982年   915篇
  1981年   799篇
  1980年   777篇
  1979年   712篇
  1978年   704篇
  1977年   618篇
  1976年   560篇
  1975年   552篇
  1974年   548篇
  1973年   600篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Rapid industrial growth and increasing population has resulted in the discharge of wastes into the ocean, wastes which ultimately reach the seabed and contaminate the marine sediments. The soil-contaminants interaction, and their associated physico chemical properties with sediments control the behavior of marine clays. Marine clay deposits of low strength and high compressibility are located in many coastal and offshore areas. There are several foundation problems encountered in these weak marine clays. In this study, experimental work was carried out in the laboratory to stabilize soft marine clays using the lime column technique. Also the lime-induced effects on the physical and engineering behavior of marine clays in sulfate-contaminated marine environment was investigated. Consolidation tests indicate that compressibility of the lime-treated samples was reduced to 1/2-1/3 of the virgin soil after 45 days treatment. The test results also suggest that the lime column technique can be conveniently used to improve the behavior of contaminated marine clay deposits.  相似文献   
962.
963.
Acoustic behavior of gas-bearing sediments is significantly different from that of gas-free sediments. In situ velocity profiles and acoustic signal characteristics in gas-bearing sediments of the upper several meters of the sea floor in Kiel Bay are presented in this study. Observed velocities in gas-bearing sediments are both higher and lower than those of the gas-free sediments. Small amounts of gas appear to cause signal reverberation without much attenuation. whereas large amounts of gas cause substantial attenuation.  相似文献   
964.
965.
966.
Metallothionein (MT) is a low-molecular-weight, cysteine-rich, metalbinding protein found in all vertebrates. The primary structure of the protein is evolutionary conserved, especially the location of cysteine-residues. There are nonetheless significant differences between MTs from different vertebrate classes, notably in major antigenic epitopes. Previous work has shown that MT from different fish species also differ. The objective of the present work was to characterize MT from temperate, sub-tropical and tropical fish species by their relative reactivities with two anti-fish MT antisera. One or both antisera cross-reacted with MT from all fish species investigated. Surprisingly, the relative reactivities of the two antisera with MT from species belonging to different taxonomic groups appeared to reflect differences at the level of family rather than differences between major phylogenetic lineages.  相似文献   
967.
968.
A procedure for the four-dimensional (4D) analysis of the hydrophysical fields in the Black Sea with an assimilation of the temperature and salinity (T, S) data was realized on the basis of a numerical model which involves the primitive equations of motion, and the heat and salt advection equations. Two experiments were carried out which differed by the observation data assimilation procedure. Analysis has shown that the observation data assimilation procedure realized using the energy-balanced model allows the reproduction of some synoptic features of the circulation in the Black Sea. A comparison of two computations demonstrates the efficiency of assimilating the measurement data on the basis of the 4D analysis as compared with the sequential objective analysis.Translated by Mikhail M. Trufanov.  相似文献   
969.
钝顶螺旋藻部分原生质体及单细胞的制备与培养   总被引:21,自引:0,他引:21  
于1992年1月-1994年4月,进行超声波处理方法制备钝顶螺旋藻部分原生质体以作为基因工程受体,以及制备单细胞用于固体平板克隆化培养的研究。研究结果表明:超声波以20kHz频率、15W功率作用30s,可使藻丝体断裂成15.0±1.6个细胞长度;延长作用时间,至2-6个细胞长度时,细胞壁结构遭到破坏,形成部分原生质体;继续作用,可形成少量原生质体和大量单细胞。断裂藻丝体、部分原生质体、单细胞以及原生质体均可涂布于固体培养基上再生或生长。以一定密度涂布单细胞与原生质体,能够形成彼此分开的单个克隆,可用于筛选及遗传分析。本文提供了一种节省溶菌酶的制备螺旋藻透性体的方法,超声波作用利于外源基因的导入,而涂布培养利于进一步的筛选和形成克隆。  相似文献   
970.
Our intent has been to develop rapid and sensitive bioassay procedures for laboratory and field applications based on hemolymph factors in animals such as the bay mussel, Mytilus edulis. We found that mussel hemolymph could be easily and repeatedly sampled from individual animals in sufficient amounts to allow analyses for lysozyme, proteolytic activity, total protein and hemocyte counts. In static tests there seemed to be graded responses indicating release of increasing amounts of the above factors into the hemolymph in roughly direct proportion to Cu2+ concentration at lethal and sublethal levels. We interpreted this to be evidence for increased lysosomal and cellular disruption as cupric ion exposure was increased or prolonged. At the same time, Cu2+ concentrations in the mussel hemolymph increased up to an order of magnitude above ambient in the range of 26 to 267 ppb. We felt that this was evidence for the release of Cu-binding proteins into the mussel hemolymph. Simultaneously with the above events the serum granulocytes increased in numbers above control values by factors of 3 or 4 in response to copper challenge while macrophage (terminology of Moore & Lowe, 1977) counts, including controls, declined precipitously.  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