全文获取类型
收费全文 | 53703篇 |
免费 | 21135篇 |
国内免费 | 46268篇 |
专业分类
测绘学 | 4823篇 |
大气科学 | 24070篇 |
地球物理 | 16658篇 |
地质学 | 44838篇 |
海洋学 | 17928篇 |
天文学 | 1601篇 |
综合类 | 6346篇 |
自然地理 | 4842篇 |
出版年
2024年 | 316篇 |
2023年 | 814篇 |
2022年 | 1688篇 |
2021年 | 2149篇 |
2020年 | 2961篇 |
2019年 | 6308篇 |
2018年 | 6655篇 |
2017年 | 6212篇 |
2016年 | 6445篇 |
2015年 | 5812篇 |
2014年 | 5472篇 |
2013年 | 6071篇 |
2012年 | 5837篇 |
2011年 | 5658篇 |
2010年 | 5503篇 |
2009年 | 4773篇 |
2008年 | 3975篇 |
2007年 | 3888篇 |
2006年 | 3443篇 |
2005年 | 3201篇 |
2004年 | 3264篇 |
2003年 | 2986篇 |
2002年 | 2709篇 |
2001年 | 2483篇 |
2000年 | 2145篇 |
1999年 | 2299篇 |
1998年 | 2207篇 |
1997年 | 2208篇 |
1996年 | 1809篇 |
1995年 | 1715篇 |
1994年 | 1543篇 |
1993年 | 1436篇 |
1992年 | 1207篇 |
1991年 | 894篇 |
1990年 | 841篇 |
1989年 | 718篇 |
1988年 | 616篇 |
1987年 | 458篇 |
1986年 | 378篇 |
1985年 | 303篇 |
1984年 | 311篇 |
1983年 | 183篇 |
1982年 | 219篇 |
1981年 | 162篇 |
1980年 | 128篇 |
1979年 | 125篇 |
1978年 | 54篇 |
1977年 | 46篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
目的:观察自拟通窍祛瘀汤治疗外伤性气滞血瘀型脑内血肿的临床疗效。方法:选取外伤性气滞血瘀型脑内血肿患者100例,将其随机分为治疗组(51例)和对照组(49例)。2组均予以基础治疗,对照组在基础治疗上加用西医常规治疗,治疗组在对照组基础上加用自拟通窍祛瘀汤口服治疗。治疗2周后观察2组中医证候疗效,治疗前、治疗1周后、治疗2周后观察2组血肿吸收、水肿吸收、神经功能缺失(NIHSS)评分、中医证候积分情况。结果:总有效率治疗组为94.11%(48/51),明显高于对照组的75.51%(37/49),2组比较,差异有统计学意义(P<0.05)。治疗后2组血肿体积、水肿体积、NIHSS评分、中医证候积分均较治疗前减少,且治疗组与对照组治疗后同时间节点比较,差异均有统计学意义(P<0.05或P<0.01)。结论:外伤性气滞血瘀型脑内血肿患者在常规治疗的基础上使用自拟通窍祛瘀汤治疗可以促进神经功能恢复,改善中医证候,促进血肿和水肿吸收。 相似文献
22.
Past studies of interplanetary magnetic sector boundaries have been based on the assumption that one can determine the field polarities by comparing the field directions with those of the nominal Parker spiral angles. Previous investigators have found evidence for decreases of B, the magnitude of the magnetic fieldB, and increases of , the angle betweenB and the ecliptic plane, at sector boundaries. Others have argued that the characteristic thickness of sector boundaries exceeds that of tangential discontinuities, making sector boundaries a separate class of structures.We use a simple technique for inferring the polarities of interplanetary magnetic fields based on the assumption thatE > 2 keV electrons are always flowing along the magnetic field away from the Sun. Electron data from the UC Berkeley experiment on the ISEE-3 spacecraft are used to examine periods around several apparent sector boundaries in 1978 and 1979. We compare properties of (a) boundaries with field polarity changes and (b) large-angle ( > 60°) directional discontinuities with no field polarity changes. We find no significant differences between the sector boundaries and the directional discontinuities in terms of associated decreases in B or of values of . These results suggest no significant difference between sector boundaries and directional discontinuities other than the change in field polarities. Within limited statistics we find that about half the polarity changes would not have been identified using a requirement that > 90° and that half of the > 120° discontinuities would have been misidentified as polarity changes. 相似文献
23.
We discuss the problems connected with the measurements and evaluation of line-of-sight velocities, obtained with a scanning photoelectric magnetograph using a line-shifter with enhanced sensitivity. We bring arguments for the validity of the results of our photoelectric Doppler velocity recordings. We have found a network of cellularly shaped patterns in the distribution of photo-electrically measured line-of-sight motions, upflowing in the magnetically quiet (blue-shifted) and downflowing in magnetically active (red-shifted) areas of the photosphere, if the mean velocity level is estimated for a sufficiently large measured area. The features of both directions are mutually complementary. We demonstrate the effect of the shift of the reference zero velocity level on the topology of the line-of-sight velocity maps, and the dependence of this level on the size of the area from which it is estimated. 相似文献
24.
We discuss the main mechanisms affecting the dynamical evolution of Near-Earth Asteroids (NEAs) by analyzing the results of three numerical integrations over 1 Myr of the NEA (4179) Toutatis. In the first integration the only perturbing planet is the Earth. So the evolution is dominated by close encounters and looks like a random walk in semimajor axis and a correlated random walk in eccentricity, keeping almost constant the perihelion distance and the Tisserand invariant. In the second integration Jupiter and Saturn are present instead of the Earth, and the 3/1 (mean motion) and v
6 (secular) resonances substantially change the eccentricity but not the semimajor axis. The third, most realistic, integration including all the three planets together shows a complex interplay of effects, with close encounters switching the orbit between different resonant states and no approximate conservation of the Tisserand invariant. This shows that simplified 3-body or 4-body models cannot be used to predict the typical evolution patterns and time scales of NEAs, and in particular that resonances provide some fast-track dynamical routes from low-eccentricity to very eccentric, planet-crossing orbits.On leave from the Department of Mathematics, University of Pisa, Via Buonarroti 2, 56127 Pisa, Italy, thanks to the G. Colombo fellowships of the European Space Agency. 相似文献
25.
26.
27.
28.
29.
本文通过对辽西及邻区中生代盆地火山岩稀土元素配分特征的研究及模拟运算,探讨了其岩浆成因及过程。结果显示南北岩区岩浆成因不同,南岩区岩源为MORB地幔和长期富集型幔熔融混合而成,北岩区为MORB和近期亏损型地幔熔融混合而成。 相似文献
30.
斜坡岩体由小变形到大变形乃至滑坡的发生,实质上是由组成斜坡的各子系统协同作用的结果.将协同学引入斜坡的稳定性预测评价中,并提出了一种新的斜坡失稳时间预测模型──协同预测模型.经实例检验,该模型预测精度较高,可用于滑坡的短期或临滑预报. 相似文献