全文获取类型
收费全文 | 4714篇 |
免费 | 715篇 |
国内免费 | 1545篇 |
专业分类
测绘学 | 328篇 |
大气科学 | 143篇 |
地球物理 | 675篇 |
地质学 | 4025篇 |
海洋学 | 585篇 |
天文学 | 10篇 |
综合类 | 304篇 |
自然地理 | 904篇 |
出版年
2024年 | 16篇 |
2023年 | 56篇 |
2022年 | 160篇 |
2021年 | 209篇 |
2020年 | 222篇 |
2019年 | 224篇 |
2018年 | 232篇 |
2017年 | 185篇 |
2016年 | 201篇 |
2015年 | 234篇 |
2014年 | 333篇 |
2013年 | 347篇 |
2012年 | 316篇 |
2011年 | 355篇 |
2010年 | 309篇 |
2009年 | 334篇 |
2008年 | 341篇 |
2007年 | 334篇 |
2006年 | 386篇 |
2005年 | 318篇 |
2004年 | 300篇 |
2003年 | 247篇 |
2002年 | 191篇 |
2001年 | 168篇 |
2000年 | 176篇 |
1999年 | 145篇 |
1998年 | 101篇 |
1997年 | 85篇 |
1996年 | 95篇 |
1995年 | 84篇 |
1994年 | 67篇 |
1993年 | 40篇 |
1992年 | 30篇 |
1991年 | 40篇 |
1990年 | 23篇 |
1989年 | 17篇 |
1988年 | 12篇 |
1987年 | 13篇 |
1986年 | 3篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有6974条查询结果,搜索用时 15 毫秒
101.
We estimate the response of chl-a (mg · m–3) to changes in concentrations of total phosphorus (TP) by calculating the slopeS = chl-a/TP in chl-a =f(TP) graphs. Results show that in years where algae are P-limited oligotrophic lakes respond less (median slope 0.21) to changes in nutrient concentrations than eutrophic lakes, (median slope 0.31) and these again less than hypereutrophic lakes, (median slope 1.02). We find no saturation value for the slope within the TP range considered (6–480 mg · m–3). Chl-a in eutrophic lakes responds more frequently to non-nutrient factors than oligotrophic and hypereutrophic lakes. Results obtained by replacing TP with a new nutrient parameter, TP = 0.056 · TP · IN0.226, in which inorganic nitrogen, IN, is factored in, suggest that nitrogen has an influence on chl-a in oligotrophic lakes. Blue-green algae respond less to changes in TP than other algal species, e.g., diatoms. 相似文献
102.
饱和软土压缩试验时,经常出现压缩曲线“反常”、压缩系数“倒大”的现象。它是土的原始结构发生破坏前后的不同压缩性的客观反映。饱和软土的结构力很微弱,唯质量好的原状土才能见到这种“反常”。应从成孔、取样、测试及资料整理等多个环节保证其工程意义。 相似文献
103.
Periodicities in natural remanent magnetization and palaeoclimatic records detected by walsh spectra
The time series consisting of nonsinusoidal Natural Remanent Magnetization (NRM) and palaeoclimatic variations for the past two million years have been spectrally examined by using a new Walsh transform technique. The results show statistically significant periodicities (at 95% confidence level) of approximately 92,000 years; 43,000 years and 21,000 years in the ensemble spectra of the NRM intensity (mineralogic) variations. These NRM periodicities are remarkably close to the well-known Milankovitch cycles. The ensemble spectra of palaeoclimatic records reveal only 41,000 years statistically significant (95%) periodicity corresponding to the obliquity cycle. The study suggests that NRM variations in deep sea sediments probably are more sensitive recorders of palaeoclimatic memory than the oxygen isotope variations. 相似文献
104.
105.
106.
107.
108.
109.
多数岩质边坡稳定性分析已基本解决了常走向单坡面临空的边坡平面滑动问题。然而,在路堑边坡工程和自然边坡中有很多边坡坡面并不是平面,它包含多个坡面。多坡面边坡包含两个或多个不同走向的坡面。多坡面边坡几何特征不同于单坡面边坡,所以滑动条件也不一样。在赤平投影图上,多坡面的滑动包络线是组成边坡的每个坡面单独投影包络线的组合。为了探讨方便,只讨论了双坡面临空岩质边坡的滑动破坏模式,并将其分为双坡面临空下的平面破坏和楔形体破坏。在赤平投影图上,双坡面滑动区域定义为双坡面边坡滑动包络图中两侧坡面的真倾线之间的面积。如果有一两个节理面真倾向线落在滑动区内,滑动破坏才可能发生。 相似文献
110.
Edward Derbyshire 《中国地球化学学报》2006,25(B08):219-219
Fine atmospheric dust includes mineral particles and aggregates, fibrous minerals and fibrous organic material. Generation, dislodgement and transport (deflation) of natural dust with the finer (〈4 microns) components suspended as silt-size aggregates, is widespread in and adjacent to the world's drylands, as well as deriving from volcanic vents. Silica is a highly fibrogenic agent in lung tissue. Long-term inhaling of siliceous dusts can lead to a number of fibrotic lung diseases, including natural (non-occupational) pneumoconioses (notably silicosis, but including asbestosis and others). Different polymorphs of silica show different levels of toxicity in interaction with lung tissue. Particles with highly active surfaces may release radicals, causing cell damage. Some types of inhaled particulates are degraded by macrophages, but many are highly resistant and persist in the lungs, some stimulating fibroblastic cells to deposit collagen. Silicosis is an inflammation of the lung commonly caused by silicate mineral particles, leading to fibrosis. Three types are recognized: nodular pulmonary fibrosis (simple or chronic silicosis), acute silicosis, and accelerated silicosis. Generally, finer particulates have greater oxidative capacity than the coarser fractions. They contain more reactive oxygen species, their greater bioreactivity making them more toxic to pulmonary tissue. Nevertheless, inhalation of large dust particles (〉 10μm) may constitute a health risk if the mineralogy is toxic, regardless of where the grains lodge in the respiratory system. Dust may absorb harmful gases, disease-generating bacteria and carcinogenic hydrocarbon compounds. Silica-related respiratory disease may also an exacerbate cardiac problem, and epidemiology suggests a link with tuberculosis. Quantification of dust loading and exposure requires study of spatial and temporal patterns, supported by meteorological analysis, airflow modeling and satellite-borne imagery. Some acute, short-term health impacts have been assessed using atmospheric and health records both before and after a dust storm or by comparison of populations within and outside such events. Analysis of the size, shape, mineralogy and geochemistry of ambient dust particulates provides information on natural dust sources, dust concentrations, and potential particulate toxicity, as well as providing a datum for assessment of human exposure levels. 相似文献