全文获取类型
收费全文 | 97747篇 |
免费 | 1947篇 |
国内免费 | 838篇 |
专业分类
测绘学 | 2296篇 |
大气科学 | 6857篇 |
地球物理 | 19600篇 |
地质学 | 35079篇 |
海洋学 | 8601篇 |
天文学 | 21912篇 |
综合类 | 282篇 |
自然地理 | 5905篇 |
出版年
2022年 | 559篇 |
2021年 | 977篇 |
2020年 | 1097篇 |
2019年 | 1193篇 |
2018年 | 2502篇 |
2017年 | 2392篇 |
2016年 | 2910篇 |
2015年 | 1699篇 |
2014年 | 2867篇 |
2013年 | 5054篇 |
2012年 | 3031篇 |
2011年 | 4154篇 |
2010年 | 3561篇 |
2009年 | 4664篇 |
2008年 | 4252篇 |
2007年 | 4130篇 |
2006年 | 3873篇 |
2005年 | 3016篇 |
2004年 | 2950篇 |
2003年 | 2871篇 |
2002年 | 2613篇 |
2001年 | 2373篇 |
2000年 | 2246篇 |
1999年 | 1851篇 |
1998年 | 1943篇 |
1997年 | 1872篇 |
1996年 | 1550篇 |
1995年 | 1542篇 |
1994年 | 1339篇 |
1993年 | 1214篇 |
1992年 | 1137篇 |
1991年 | 1094篇 |
1990年 | 1187篇 |
1989年 | 1029篇 |
1988年 | 934篇 |
1987年 | 1104篇 |
1986年 | 988篇 |
1985年 | 1240篇 |
1984年 | 1348篇 |
1983年 | 1279篇 |
1982年 | 1229篇 |
1981年 | 1072篇 |
1980年 | 1007篇 |
1979年 | 937篇 |
1978年 | 906篇 |
1977年 | 861篇 |
1976年 | 818篇 |
1975年 | 787篇 |
1974年 | 784篇 |
1973年 | 772篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars. 相似文献
12.
13.
14.
青藏高原隆升的非线性动态有限元仿真研究 总被引:7,自引:4,他引:3
根据青藏高原的地质特征建立分析模型,采用3维动态有限元方法,在计算仿真板块速度场的基础上,计算在青藏高原的隆升过程中该地区地壳岩石的等效应力和位移随时间的变化,计算仿真得到的速度场与1998年GPS观测的速度场吻合良好;与过去一贯的假设相反,计算结果反映出地壳应力场不是静态的,而是此起彼伏,不断变化的,应力值最大且变化最剧烈的地区在克什米尔地区、鄂尔多斯地区和鲜水河-小江断裂带,与地震多发区域吻合。 相似文献
15.
The kinetics of the reactions of C2H radical with ethane (k1), propane (k2), and n-butane (k3) are studied over the temperature range of T = 96-296 K with a pulsed Laval nozzle apparatus that utilizes a pulsed laser photolysis-chemiluminescence technique. The C2H decay profiles in the presence of both the alkane reactant and O2 are monitored by the CH(A2Δ) chemiluminescence tracer method. The results, together with available literature data, yield the following Arrhenius expressions: k1(T) = (0.51 ± 0.06) × 10−10 exp[(−76 ± 30)K/T] cm3 molecule−1 s−1 (T = 96-800 K), k2(T) = (0.98 ± 0.32) × 10−10exp[(−71 ± 60)K/T] cm3 molecule−1 s−1 (T = 96-361 K), and k3(T) = (1.23 ± 0.26) × 10−10 cm3 molecule−1 s−1 (T = 96-297 K). At T = 296 K, k1 is measured as a function of total pressure and has little or no pressure dependence. The results from this work support a direct hydrogen abstraction mechanism for the title reactions. Implications to the atmospheric chemistry of Titan are discussed. 相似文献
16.
17.
Andrew J. Westphal Christopher Snead Janet Borg Eric Quirico Pierre‐Ivan Raynal Michael E. Zolensky Gianluca Ferrini Luigi Colangeli Pasquale Palumbo 《Meteoritics & planetary science》2002,37(6):855-865
Abstract— It has now been about a decade since the first demonstrations that hypervelocity particles could be captured, partially intact, in aerogel collectors. But the initial promise of a bonanza of partially‐intact extraterrestrial particles, collected in space, has yet to materialize. One of the difficulties that investigators have encountered is that the location, extraction, handling and analysis of very small (10 μm and less) grains, which constitute the vast majority of the captured particles, is challenging and burdensome. Furthermore, current extraction techniques tend to be destructive over large areas of the collectors. Here we describe our efforts to alleviate some of these difficulties. We have learned how to rapidly and efficiently locate captured particles in aerogel collectors, using an automated microscopic scanning system originally developed for experimental nuclear astrophysics. We have learned how to precisely excavate small access tunnels and trenches using an automated micromanipulator and glass microneedles as tools. These excavations are only destructive to the collector in a very small area—this feature may be particularly important for excavations in the precious Stardust collectors. Using actuatable silicon microtweezers, we have learned how to extract and store “naked” particles—essentially free of aerogel—as small as 3 μm in size. We have also developed a technique for extracting particles, along with their terminal tracks, still embedded in small cubical aerogel blocks. We have developed a novel method for storing very small particles in etched nuclear tracks. We have applied these techniques to the extraction and storage of grains captured in aerogel collectors (Particle Impact Experiment, Orbital Debris Collector Experiment, Comet‐99) in low Earth orbit. 相似文献
18.
M.P. Leubner 《Planetary and Space Science》2003,51(12):723-729
Clear signature of non-gyrotropic energetic electron distributions was found by ISEE 1 and ISEE 2 spacecrafts just upstream of the Earth's bow shock and recently detected by in situ observations of the WIND plasma experiment. On the other hand, the appearance of non-gyrotropic ion velocity distributions is well established in the magnetotail providing evidence of magnetic reconnection processes. Motivated by these findings we introduce an analytical representation of non-Maxwellian/non-gyrotropic distribution functions, accurately fitting the characteristics of observations, where Maxwellians are recovered as special case of these highly general velocity space distributions. In particular, the analytical distribution function model can serve as basis of detailed wave-particle interaction analysis and of studies of the physical background of the evolution of both, non-gyrotropic electron and ion distributions, which is discussed for various space plasma environments. 相似文献
19.
20.
文中所作的主要工作是为云南天文台十米抛物面设计、制造一付好的对数周期偶极子天线(LPDA)作馈源。工作有:(1)设计一对交叉的LP-DA;(2)分析LPDA特性;(3)估算抛物面的电特性;(4)讨论线极化向园极化转换的问题;(5)天线测量。LPDA的测量结果比较满意。该天线的方向图、平均输入阻抗、三分贝和十分贝波瓣宽度都在从0.5-1.5GHz的频率范围内比较一致和稳定,变化很小。天线理论上的许多成就都是与数子计算机有关的。在文中,我们充分利用了计算机的功能来设计天线,分析天线特性,并从测量所得的所有数据文件中计算天线的参数。所编FORTRAN程序使这一切工作变得容易。最后,我们还讨论了用功率合成器实现线极化向园极化转换的各种情况 相似文献