全文获取类型
收费全文 | 2656篇 |
免费 | 131篇 |
国内免费 | 551篇 |
专业分类
测绘学 | 102篇 |
大气科学 | 281篇 |
地球物理 | 1236篇 |
地质学 | 871篇 |
海洋学 | 425篇 |
天文学 | 112篇 |
综合类 | 126篇 |
自然地理 | 185篇 |
出版年
2023年 | 18篇 |
2022年 | 19篇 |
2021年 | 29篇 |
2020年 | 18篇 |
2019年 | 19篇 |
2018年 | 24篇 |
2017年 | 7篇 |
2016年 | 52篇 |
2015年 | 25篇 |
2014年 | 49篇 |
2013年 | 50篇 |
2012年 | 133篇 |
2011年 | 170篇 |
2010年 | 61篇 |
2009年 | 80篇 |
2008年 | 155篇 |
2007年 | 161篇 |
2006年 | 169篇 |
2005年 | 205篇 |
2004年 | 166篇 |
2003年 | 133篇 |
2002年 | 117篇 |
2001年 | 88篇 |
2000年 | 117篇 |
1999年 | 72篇 |
1998年 | 74篇 |
1997年 | 112篇 |
1996年 | 77篇 |
1995年 | 91篇 |
1994年 | 85篇 |
1993年 | 86篇 |
1992年 | 81篇 |
1991年 | 89篇 |
1990年 | 85篇 |
1989年 | 58篇 |
1988年 | 58篇 |
1987年 | 46篇 |
1986年 | 38篇 |
1985年 | 33篇 |
1984年 | 24篇 |
1983年 | 16篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 22篇 |
1978年 | 5篇 |
1976年 | 6篇 |
1974年 | 8篇 |
1948年 | 8篇 |
1947年 | 8篇 |
1943年 | 4篇 |
排序方式: 共有3338条查询结果,搜索用时 15 毫秒
11.
Irvine WM Friberg P Hjalmarson A Ishikawa S Kaifu N Kawaguchi K Madden SC Matthews HE Ohishi M Saito S Suzuki H Thaddeus P Turner BE Yamamoto S Ziurys LM 《The Astrophysical journal》1988,334(2):L107-L111
We report the astronomical identification of the cyanomethyl radical, CH2CN, the heaviest nonlinear molecular radical to be identified in interstellar clouds. The complex fine and hyperfine structures of the lowest rotational transitions at about 20.12 and 40.24 GHz are resolved in TMC-1, where the abundance appears to be about 5 x 10(-9) relative to that of H2. This is significantly greater than the observed abundance of CH3CN (methyl cyanide) in TMC-1. In Sgr B2 the hyperfine structure is blended in the higher frequency transitions at 40, 80, and 100 GHz, although the spin-rotation doubling is clearly evident. Preliminary searches in other sources indicate that the distribution of CH2CN is similar to that for such carbon chain species as HC3N or C4H. 相似文献
12.
Ram S 《Population geography : a journal of the Association of Population Geographers of India》1986,8(1-2):17-26
The author analyzes the spatial distribution of Indians living in Great Britain in 1981. The impact of factors such as demand for labor, location of manufacturing industries, and the attraction of urban areas is examined. 相似文献
13.
Determinants and characteristics of female migration: a case study of Salem City in Tamil Nadu,India
Nangia S Samuel MJ 《Population geography : a journal of the Association of Population Geographers of India》1983,5(1-2):34-43
A random sample of 521 male and 481 female migrants surveyed in Salem city in Tamil Nadu, India, during 1980-1981 was analyzed on the basis of a classification related to reasons (association, marriage, distress, and voluntary) for migration. Women were found to outnumber males in associational migration, both in rural to urban and urban to urban streams. They also formed almost all the marriage migrants. They were more numerous in distress migration as well. There were, however, comparatively few women among voluntary migrants. The incidence of illiteracy among migrant women was generally high, except for voluntary migrants who had completed high school. The share of working women was not negligible, but in most cases present work status was achieved after migrating. 相似文献
14.
An unusual feature has been observed in polychaetes regarding their capacity to accumulate high levels of relatively toxic forms of arsenic in specific tissues. Basal levels of arsenic and distribution of its compounds were investigated in tissues of the Mediterranean polychaete Sabella spallanzanii. Particularly high concentrations were measured in the branchial crown (1036+/-136 microg/g d.w.) and chemical speciation revealed that the predominant form was the relatively toxic dimethylarsinate (DMA). These data suggest a potential role of As as anti-predatory strategy in polychaetes. 相似文献
15.
长江入海沉积物的输送及其入海后的运移 总被引:3,自引:0,他引:3
前言 长江和黄河是世界著名的多沙河流,每年入海泥沙分别为5亿吨和11亿吨,分别占世界的第四位和第二位。千万年来由她们发育了广大的冲积平原,浩浩荡荡东流入海,形成东中国海大陆架沉积物的主要来源。研究长江和黄河沉积物搬运入海的沉积动力过程,不仅对东海大陆架的了解和开发有重要意义,也给陆上经济发展提供科学依据。 相似文献
16.
We have previously demonstrated that medaka CYP3A is associated with metabolism of several endobiotics including steroids and bile acids. In this study, we demonstrate that medaka CYP3A catalysis exhibits unusual kinetic behaviors consistent with allosteric interaction which cannot be described by hyperbolic kinetic models. Using 7-benzyloxy-4-(trifluoromethyl)-coumarin (BFC) and nonylphenol as CYP3A substrates, we describe both homotropic and heterotropic cooperative interactions. Given the role of CYP3A in maintaining the homeostatic balance for numerous endobiotics, enzymatic activation/inhibition by endocrine disruptors (EDCs) represents a putative (non-genomic) mechanism for endocrine disruption. 相似文献
17.
Partitioning of polycyclic aromatic hydrocarbons between water and particles compared to bioaccumulation in mussels: a harbour case 总被引:3,自引:0,他引:3
Hellou J Steller S Leonard J Langille MA Tremblay D 《Marine environmental research》2005,59(2):101-117
Water and particles from three sites located in Halifax Harbour, near and further away from combined municipal sewage effluents (CMSE) were analysed for parental and alkylated polycyclic aromatic hydrocarbons (par and alkPAH). Bioavailability of PAH was compared for inter-tidal mussels collected at the same sites in April and November. The PAH fingerprint determined over 9 sampling times covering a period of 19 months differed more between phases (water and particles) and seasons, than between sites. In the spring, more alkPAH associated with diesel and gasoline were detected in the soluble phase, along with more bioaccumulation of alkPAH in inter-tidal mussels. A broader number of parPAH were detected in mussels collected in the fall. The mean sum of dissolved alkPAH concentrations was higher in water at the site closer to raw CMSE than at the other two sites and particles of that site. However, lowest bioconcentration factors (BCF) were determined in mussels of this more contaminated site. Similar biota-particle accumulation factors (BPAF) were determined for parPAH in mussels from the three sites, all lower than the BCF of alkPAH. The study indicates that sewage treatment plants will reduce the amount of parPAH especially larger than fluoranthene and pyrene from being deposited in the harbour; that alk naphthalenes and fluorenes present in water will continue to be discharged; that the disturbance of sediments can make particle-bound PAH available to mussels. 相似文献
18.
To use two small fish Rivulus marmoratus (Cyprinodontiformes, Rivulidae) and the Japanese medaka Oryzias latipes (Belloniformes) as testing models in molecular ecotoxicology, we have cloned the cytochrome P450 1A (CYP1A) gene after screening of both genomic DNA libraries, and sequenced 11,863 and 7,243 bp including all the exons and introns with promoter regions, respectively. The Rivulus and the medaka CYP1A gene consisted of seven exons (including non-coding exons) with high homology to mammals. In the promoter region, Rivulus CYP1A gene has seven xenobiotic response elements (XREs) and two metal response elements (MREs), while the Japanese medaka CYP1A gene has six XREs and four MREs. Interestingly, medaka CYP1A gene has a number of MREs at the promoter, which may affect its response on metal exposure. We describe here the gene structure of both fish CYP1A genes. 相似文献
19.
缺少密度或孔隙度测定数据的块体堆积速率的计算T.J.S.Sykes等块体堆积速率(MAR)计算已被应用于深海钻探计划(DSDP)和大洋钻探计划(ODP)岩心,以研究大洋盆地的沉积物演化,或把MAR数据分离成岩石学成分以决定方解石补偿深度(CCD)的历... 相似文献
20.
在南半球赤道地区,太平洋东部沿岸的海面温度相对较冷,这些冷水强烈地影响着赤道大陆气候,在由塔西提/达尔文间不规则的高/低气压所定义的南方诗动的减弱价段,这种气候背景型式强烈地受到改变,在南方涛动减弱阶段,厄尔尼诺事件可能会出现,赤道太平洋水团要比通常温暖。 相似文献