全文获取类型
收费全文 | 118923篇 |
免费 | 7210篇 |
国内免费 | 8624篇 |
专业分类
测绘学 | 4853篇 |
大气科学 | 12696篇 |
地球物理 | 24798篇 |
地质学 | 46396篇 |
海洋学 | 11576篇 |
天文学 | 22044篇 |
综合类 | 3864篇 |
自然地理 | 8530篇 |
出版年
2023年 | 679篇 |
2022年 | 1983篇 |
2021年 | 2606篇 |
2020年 | 2338篇 |
2019年 | 2645篇 |
2018年 | 4043篇 |
2017年 | 3754篇 |
2016年 | 4547篇 |
2015年 | 3102篇 |
2014年 | 4571篇 |
2013年 | 6383篇 |
2012年 | 4585篇 |
2011年 | 5504篇 |
2010年 | 5196篇 |
2009年 | 6064篇 |
2008年 | 5298篇 |
2007年 | 5257篇 |
2006年 | 4665篇 |
2005年 | 3765篇 |
2004年 | 3507篇 |
2003年 | 3339篇 |
2002年 | 3222篇 |
2001年 | 2980篇 |
2000年 | 3003篇 |
1999年 | 3096篇 |
1998年 | 2870篇 |
1997年 | 2690篇 |
1996年 | 2397篇 |
1995年 | 2232篇 |
1994年 | 1954篇 |
1993年 | 1750篇 |
1992年 | 1558篇 |
1991年 | 1463篇 |
1990年 | 1428篇 |
1989年 | 1217篇 |
1988年 | 1152篇 |
1987年 | 1166篇 |
1986年 | 1033篇 |
1985年 | 1224篇 |
1984年 | 1350篇 |
1983年 | 1217篇 |
1982年 | 1128篇 |
1981年 | 1083篇 |
1980年 | 953篇 |
1979年 | 911篇 |
1978年 | 871篇 |
1977年 | 748篇 |
1976年 | 708篇 |
1975年 | 711篇 |
1973年 | 716篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
The effects of natural fish oil,DHA oil and soybean lecithin in microparticulate diets on stress tolerance of larval gilthead seabream(Sparus aurata)were investigated after 15 days feeding trials.The tolerance of larval gilthead seabream to various stress factors such as exposure to air(lack of dissolved oxygen),changes in water temperature(low)and salinity(high) were determined.This study showed that microparticulate diet with natural fish oil and soybean lecithin was the most effective for in-creasing the tolerance of larval gilthead seabream to various stresses,and that microparticulate diet with natural fish oil and palmitic acid(16:0)was more effective than microparticulate diet with DHA oil and soybean lecithin. 相似文献
22.
Mars Global Surveyor accelerometer observations of the martian upper atmosphere revealed large variations in density with longitude during northern hemisphere spring at altitudes of 130-160 km, all latitudes, and mid-afternoon local solar times (LSTs). This zonal structure is due to tides from the surface. The zonal structure is stable on timescales of weeks, decays with increasing altitude above 130 km, and is dominated by wave-3 (average amplitude 22% of mean density) and wave-2 (18%) harmonics. The phases of these harmonics are constant with both altitude and latitude, though their amplitudes change significantly with latitude. Near the South Pole, the phase of the wave-2 harmonic changes by 90° with a change of half a martian solar day while the wave-3 phase stays constant, suggesting diurnal and semidiurnal behaviour, respectively. We use a simple application of classical tidal theory to identify the dominant tidal modes and obtain results consistent with those of General Circulation Models. Our method is less rigorous, but simpler, than the General Circulation Models and hence complements them. Topography has a strong influence on the zonal structure. 相似文献
23.
24.
25.
Grid method is employed for sampling covering soil at the test field,which is reclamation area filled by coal mining wastes for cropland in the Fushun coal mine,Liaoning Province,the Northeast China.The soil samples are taken at different locations,including three kinds of covering soil,three different depths of soil layers and four different covering ages of covering soil.The spatial-temporal variation of heavy metal element content in reclamation soil is stud-ied.The results indicate that the content of heavy metal elements is decreasing year after year,the determinant reason why the content of heavy metal elements at 60cm depth layer is higher than that at 30cm depth layer and surface is fertiliz-er and manure application;the metal elements mainly come from external environment;there is no metal pollution coming from mother material (coal mining wastes)in plough layer of covering soil. 相似文献
26.
Zsolt Sándor Bálint Érdi Carl D. Murray 《Celestial Mechanics and Dynamical Astronomy》2002,84(4):355-368
The dynamics of co-orbital motion in the restricted three-body problem are investigated by symplectic mappings. Analytical and semi-numerical mappings have been developed and studied in detail. The mappings have been tested by numerical integration of the equations of motion. These mappings have been proved to be useful for a quick determination of the phase space structure reflecting the main characteristics of the dynamics of the co-orbital problem. 相似文献
27.
28.
29.
对青藏东北缘现今块体划分、运动及变形的初步研究 总被引:19,自引:9,他引:10
利用2维非连续变形分析方法(DDA),以位移代替围压作为边界约束力,研究青藏东北缘现今块体划分及其运动变形。根据该地区地质构造及地震活动,以GPS点测量位移作为模拟结果约束点,得出了较合理的块体划分模型和随时间演化的主应变分布图,并把应变高值区与近几年来发生的5级以上地震作对比,得出了研究区内地震危险性可能较大的区域。另外,对模拟的甘青块体与阿拉善块体的边缘带断裂左旋运动做了大概计算。 相似文献
30.
油田抽油机的抽油杆幌动幅度过大,是引起抽油杆断裂的一个重要因素,检测这种幌动幅度是防止抽油杆断裂的一种有效手段。设计了一种基于面阵CCD和普通半导体激光器(LD)测量这种幌动的悬点投影测量方法,通过数字卷积滤波,达到了范围为0-40mm,误差<0.2mm的技术检测指标。 相似文献