全文获取类型
收费全文 | 280929篇 |
免费 | 10003篇 |
国内免费 | 9375篇 |
专业分类
测绘学 | 8058篇 |
大气科学 | 24713篇 |
地球物理 | 61086篇 |
地质学 | 101075篇 |
海洋学 | 24186篇 |
天文学 | 56223篇 |
综合类 | 3951篇 |
自然地理 | 21015篇 |
出版年
2022年 | 2284篇 |
2021年 | 3338篇 |
2020年 | 3566篇 |
2019年 | 3908篇 |
2018年 | 4864篇 |
2017年 | 4426篇 |
2016年 | 7134篇 |
2015年 | 5389篇 |
2014年 | 8389篇 |
2013年 | 15595篇 |
2012年 | 7795篇 |
2011年 | 9254篇 |
2010年 | 8269篇 |
2009年 | 10911篇 |
2008年 | 9755篇 |
2007年 | 9080篇 |
2006年 | 10653篇 |
2005年 | 8535篇 |
2004年 | 8386篇 |
2003年 | 7923篇 |
2002年 | 7455篇 |
2001年 | 6721篇 |
2000年 | 6836篇 |
1999年 | 6549篇 |
1998年 | 6301篇 |
1997年 | 6128篇 |
1996年 | 5652篇 |
1995年 | 5286篇 |
1994年 | 4896篇 |
1993年 | 4523篇 |
1992年 | 4167篇 |
1991年 | 3937篇 |
1990年 | 3940篇 |
1989年 | 3618篇 |
1988年 | 3423篇 |
1987年 | 3891篇 |
1986年 | 3396篇 |
1985年 | 4234篇 |
1984年 | 4708篇 |
1983年 | 4375篇 |
1982年 | 4305篇 |
1981年 | 3911篇 |
1980年 | 3624篇 |
1979年 | 3464篇 |
1978年 | 3445篇 |
1977年 | 3231篇 |
1976年 | 2974篇 |
1975年 | 2908篇 |
1974年 | 2876篇 |
1973年 | 3066篇 |
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
121.
122.
运用放射免疫分析法测定了促黄体释放激素类似物 (LHRH A)和人绒毛膜促性腺激素(hCG)对南方鲶血清促性腺激素 (GTH)水平的影响 ,结果表明 :LHRH A和hCG都能提高南方鲶一龄幼鱼血清GTH水平 ,hCG与LHRH A结合注射后血清GTH水平显著高于单独注射hCG或LHRH A后的血清GTH水平。LHRH A和hCG结合注射可诱导性成熟南方鲶排卵 ,同时排卵过程中伴随着血清GTH水平的急剧上升 ,在排卵后迅速下降 ;注射同样剂量的药物而未排卵的血清GTH水平虽有升高 ,但升高的幅度不及排卵鱼。这种血清GTH水平的急剧增高对诱导南方鲶卵母细胞最后成熟和排卵具有重要作用。 相似文献
123.
124.
Earth, Moon, and Planets - 相似文献
125.
126.
127.
PRELIMINARY STUDY ON THE FLOODING AND DROUGHT CLAMITY DURING PAST 1500 YEARS IN THE HAI′AN REGION,JIANGSU PROVINCE 总被引:1,自引:0,他引:1
Collection and arrangement of the historical records of climatic changes and environment evolution,espectial-lyin the aspect of calamities,are made on the history documents of past 1500 years about Haiˊan region,Jiangsu Province.There existed two obvious flooding-drought frequently-occurring periods:one was from 1550 AD to 1850 AD and another was 100 AD to 1200AD.The period of 1550 AD to1850 AD is interrupted by two relatively arid and cold climatic periods:one was from 1630 AD to 1700 AD and another was 1750 AD to 1820 AD.The main characteristic of the calamity periods is that they occurred by turns,and sometimes,both drought and flooding occurred in the same year.The instability of the climatic changes in the Little Ice Age may be the main reason of the frequently-occurring flooding and drought in Haiˊan region.Research results also show that the frequently-occurring periods of flooding and drought is in close relationship with the solar activity,and therefore,occurrence of the flooding and drought may be in relation with the intensity of the solar activity.This hypothesis may need further study in the future. 相似文献
128.
ZHANGPing-yu MAYan-ji YUZhen-han 《中国地理科学(英文版)》2002,12(4):315-320
Manzhouli is the largest land port city on the Sino-Russia border, transit cargo through the land port amount-ed to 5.95 million tons, transit tourists were 304 500 in 2000. It stands at the joint place of China, Mongolia and Russi-a, faces to Siberia area of Russia, receives direct support from the Northeast China and Bohai Sea Rim Area, and possess-es priorities in geographical location, land port infrastructure, water resources, coal resources, tourist resources andgreat potentiality in economic cooperation with Russia. The future urban function is a key port on the First Eurasia Continen-tal Bridge. Manzhouli Port will keep its first place between China and Russia land transport, and it is forecasted that thetransit amount through Manzhouli Port will go up to 10 million tons in 2005 and 20 million tons in 2010. It will be construct-ed to be a trade center of the peripheral area extending to Russia and Mongolia, a key export-oriented processing industri-al zone supported by industries such as export processing industries, export agriculture, trade services, technology trad-ing and the other service industries. It keeps being a well-known touring city for trade, shopping, sightseeing, vocation,local food, recreation and cultural events. To build Manzhouli Export Processing Industry Zone will improve city econom-ic structure, and the main sectors are organic food processing, livestock products processing, garment and furniture indus-try. Moreover, Manzhouli Export Processing Industry Zone will eventually be upgraded to be a border free trade zone.The city functional transition will inevitably affect urban spatial restructure and its expansion. The city space transforma-tion will develop as such: one development axis of No. 301 highway paralleling with Bin - Zhou(Harbin - Manzhouli) rail-way which cuts through central part of Manzhouli City, and links Zhalainuocr District with central city; three urban unitsincluding central city, Zhalainuocr District and Manzhouli Interchange Trade Zone; cohesion with Aoerjin and Cuogangpastures; regional dual-nuclei structure of Hailaer City and Manhzouli City; and the Manzhouli-Zabaykalsk Free TradeZone. 相似文献
129.
The economic development in China and the rising of the living standard need to speed up the urbanisa-tion.The development of small towns is an important way to Chinese urbanization.Land use plays a very important role in the development of small towns.However there are many problems in the development of small towns,esp.in land use.The paper first discusses the land problems in the development of small towns.Such as much cultivated land lies idle,under-utilization and waste of land,increasing illegal use of land,unstable contractural relationship for land use.The relationship between the development of small towns and land use is also studied.Then the guidelines for the land system innovations of small towns are put forward.Namely the sustainability of social and economic development,the bal-ance between land reservation and land utilization,the provision of service to village ,agriculture and farmer,the manage-ment of land resource and land assets,the parsimonious andlegal use of land.The basic framework of land system innova-tions of small towns is put forward finally.It include the land replacement policy for small towns and the permanent tenan-cy of farmland,the overall plan for land uses and other plans in harmony,the establishment of a flexible system of land supplies,using land with payments ,the transfer of agricultural land and refining the land law related to the construction of small towns. 相似文献
130.
A CASE STUDY ON CONCENTRATION AND DECENTRALIZATION:BEHAVIOR AND DYNAMIC MECHANISM OF SPATIAL EVOLUTION IN METROPOLITAN AREA,NANJING,CHINA 总被引:1,自引:1,他引:0
In rapid socio-economic development,the process of concentration and dispersal of various elements tends to be more dramatic,tremendously influencing the shaping and transformation of the space in metropolitan area.Survey of spatial concentration and decentralization has thus become a basic method in examining metropolitan spatial evolution.In this research,three elements were selected as the essential indicators of the process:demographic density distribu-tion,employment density distribution and business office location.Performance of these elements in Nanjing City was exam-ined historically.As Nanjing City could be regarded as a representative of metropolitan areas in China,its situation large-ly suggestes the general characteristics in similar areas of China.Hence based on the investigation of Nanjing City,four general implications were highlighted.First ,metropolitan areas in China are in a violent process and shift of spatial concentra-tion and decentralization.Second,from now to at least the near future,concentration will continue to be the central fea-ture.Third,the landscape of metropolitan areas basically exhibits a dual structure character.The gap in environmental and ecological qualities among different districts will continue for a long time.Fourth,Central Business District (CBD) is playing an important role in helping to convert the traditionally single-centered city structure into a polycentric one. 相似文献