全文获取类型
收费全文 | 1482篇 |
免费 | 254篇 |
国内免费 | 523篇 |
专业分类
测绘学 | 119篇 |
大气科学 | 183篇 |
地球物理 | 351篇 |
地质学 | 844篇 |
海洋学 | 372篇 |
天文学 | 47篇 |
综合类 | 127篇 |
自然地理 | 216篇 |
出版年
2024年 | 16篇 |
2023年 | 51篇 |
2022年 | 90篇 |
2021年 | 90篇 |
2020年 | 84篇 |
2019年 | 93篇 |
2018年 | 122篇 |
2017年 | 92篇 |
2016年 | 92篇 |
2015年 | 100篇 |
2014年 | 97篇 |
2013年 | 90篇 |
2012年 | 112篇 |
2011年 | 94篇 |
2010年 | 93篇 |
2009年 | 95篇 |
2008年 | 89篇 |
2007年 | 74篇 |
2006年 | 67篇 |
2005年 | 61篇 |
2004年 | 63篇 |
2003年 | 64篇 |
2002年 | 58篇 |
2001年 | 49篇 |
2000年 | 55篇 |
1999年 | 49篇 |
1998年 | 45篇 |
1997年 | 36篇 |
1996年 | 26篇 |
1995年 | 29篇 |
1994年 | 24篇 |
1993年 | 14篇 |
1992年 | 7篇 |
1991年 | 2篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 7篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有2259条查询结果,搜索用时 31 毫秒
991.
Youliang Chen Rafig Azzam Ming Wang Shan Xu Lequn Chang 《Environmental Earth Sciences》2011,64(1):29-36
The compressive and tensile strengths of frozen clay are important parameters for frozen wall design in artificial freezing
excavation of tunnels and foundation pits. Up to now, nobody has conducted the compressive and tensile test of frozen clays
in Shanghai area. In this paper, the unconfined compressive and tensile tests of frozen clay specimens drilled from the soil
horizons 3–5 in Shanghai area were conducted in Zwick-Z020kN High-low Temperature Materials Testing Machine and Frozen Soil
Triaxial Testing Machine, the corresponding constitutive equations were suggested; the temperature-unconfined uniaxial compressive
strength relation was discussed; the strain rate–unconfined uniaxial compressive strength and strain rate–uniaxial tensile
strength relations were studied. The relation between moisture content, dry density and unconfined uniaxial compressive strength
was analyzed, too. In addition, the uniaxial compressive elastic modulus of Shanghai frozen clays and its influence factors
were discussed. The research work of the current paper is very helpful for the design and theoretical studies of artificial
freezing excavation in soft soil areas. 相似文献
992.
To study population dynamics of marine ciliates in different artificial seawaters (ASW), the population growth dynamics of a common marine ciliate Euplotes vannus were investigated using beef extract media and rice media for five types of ASW and natural seawater (NSW). The results show that: (1) the population growth rate was in the order of NSW>Flack ASW>Nakamula ASW>Schmadz ASW>Oshima ASW>Subow ASW and was considerably higher in rice media than in beef extract media (apart from Subow ASW); (2) the maximum density of E. vannus in stationary phase in each treatment was ranked as Flack ASW>Nakamula ASW>Schmadz ASW>NSW>Oshima ASW>Subow ASW, and was again higher in rice media than in beef extract media (except for Subow ASW); (3) the exponential and stationary phases were longer in rice media than in beef extract media; (4) strains of E. vannus that had been domesticated for >1 year in ASW grew significantly slower, with lower maximum density and longer stationary phase than those isolated and maintained in NSW. It was demonstrated that: (1) E. vannus may grow well in Flack, Nakamula and Schmads ASW compared with NSW (mainly in terms of growth rate); and (2) Oshima ASW is the preferred choice for stock cultures of E. vannus, but the ASWs Flack, Nakamula and Schmadz are preferred for mass culture. These findings suggest that these three ASWs are effective for the cultivation of marine protozoa for experimental studies on ecology, toxicology and molecular biology. 相似文献
993.
选取栉孔扇贝Chlamys farreri担轮幼虫为材料,采用秋水仙素-低渗-空气干燥法制备染色体标本,应用荧光显带技术,分析了DAPI带和PI带在栉孔扇贝染色体上的分布。DAPI带型结果显示,栉孔扇贝所有染色体上都存在DAPI阳性带,主要分布于传统的着丝粒区和端部区域,另外还存在一些中间区DAPI带及可变带,总带数为62。PI带型结果与DAPI带型结果相似,在所有染色体上都存在PI阳性带。2种带型的阳性带所在位置与异染色质分布区域相吻合。 相似文献
994.
995.
996.
This paper uses Landsat TM images, GIS technology, Digital Elevation and Habitat Assessment Models to assess the habitat suitability of the endangered plant Tetraena mongolica in western Ordos Plateau of China by selecting terrain, soil, climate, and human activity factors as assessment indices. The results are as follows: natural factors such as climate and terrain are not restrictive factors for the survival and development of T. mongolica in the research region, whereas human activity causes habitat quality of T. mongolica to change intensively in quantity and distribution. The area of less suitable habitat increased by 23.87 km2 compared to potential habitat suitability. Thus, in some areas, human activity may be a key factor causing the endangerment of T. mongolica. There were obvious differences of potential and practical habitat suitability between different habitat regions in the study area. The habitat quality was better in Wujiamiao, Dishan and Qipanjing regions, and worse in Wuda and Qianlishan regions. 相似文献
997.
由国家自然基金委员会、教育部主办的中国天体物理暑期讲习班,自2004年首届在上海举办以来,得到了天文界的广泛关注和支持。2007年5月21—30日,由中国科学技术大学天体物理中心承办的第三届中国天体物理暑期讲习班在合肥顺利举行。 相似文献
998.
Gypsum caprocks'' sealing ability is affected by temperature-pressure coupling. Due to the limitations of experimental conditions, there is still a lack of triaxial stress-strain experiments that simultaneously consider changes in temperature and pressure conditions, which limits the accuracy of the comprehensive evaluation of the brittle plastic evolution and sealing ability of gypsum rocks using temperature pressure coupling. Triaxial stress-strain tests were utilized to investigate the differences in the evolution of the confinement capacity of gypsum rocks under coupled temperature-pressure action and isothermal-variable pressure action on the basis of sample feasibility analysis. According to research, the gypsum rock''s peak and residual strengths decrease under simultaneous increases in temperature and pressure over isothermal pressurization experimental conditions, and it becomes more ductile. This reduces the amount of time it takes for the rock to transition from brittle to plastic. When temperature is taken into account, both the brittle–plastic transformation''s depth limit and the lithological transformation of gypsum rocks become shallower, and the evolution of gypsum rocks under variable temperature and pressure conditions is more complicated than that under isothermal pressurization. The sealing ability under the temperature-pressure coupling is more in line with the actual geological context when the application results of the Well #ZS5 are compared. This provides a theoretical basis for precisely determining the process of hydrocarbon accumulation and explains why the early hydrocarbon were not well preserved. 相似文献
999.
By using results from the 2MASS observation, the differences between the X-ray selected BL Lac objects (XBLs) and the radio selected ones (RBLs) in the near infrared are discussed in this paper. It is found that, statistically, the RBLs have redder near infrared colors and steeper spectral indices in the near infrared than XBLs, whereas the XBLs have more influences from their host galaxies than the RBLs. It is also seen that the RBLs have much brighter luminosity in the near infrared than the XBLs. In addition, the high polarization characteristics are found in common for RBLs, but not for XBLs. 相似文献
1000.
We collected 55 galactic extreme carbon stars from the published literature in this paper. Observational data from IRAS, 2MASS
and ISO were analyzed. The results show that the infrared properties of extreme carbon stars are quite different to those
for ordinary visual carbon stars. It is shown from IRAS and 2MASS photometric data that extreme carbon stars have much redder
infrared colors not only in the far infrared, but also in the near infrared, hence they have much thicker ciucumstellar envelopes
and mass loss. It is also indicated from IRAS Low-Resolution Spectra (LRS) and ISO Short Wavelength Spectra (SWS) that they
have much redder infrared spectra from 2 μm to 45 μm. The above results are believed to be the signature of undergoing the
last stages of AGB evolution for extreme carbon stars. 相似文献