全文获取类型
收费全文 | 80542篇 |
免费 | 2078篇 |
国内免费 | 842篇 |
专业分类
测绘学 | 2039篇 |
大气科学 | 5957篇 |
地球物理 | 17263篇 |
地质学 | 28758篇 |
海洋学 | 6895篇 |
天文学 | 17332篇 |
综合类 | 406篇 |
自然地理 | 4812篇 |
出版年
2022年 | 421篇 |
2021年 | 756篇 |
2020年 | 825篇 |
2019年 | 887篇 |
2018年 | 2541篇 |
2017年 | 2321篇 |
2016年 | 2602篇 |
2015年 | 1581篇 |
2014年 | 2378篇 |
2013年 | 4078篇 |
2012年 | 2966篇 |
2011年 | 3677篇 |
2010年 | 2843篇 |
2009年 | 3818篇 |
2008年 | 3541篇 |
2007年 | 3304篇 |
2006年 | 3168篇 |
2005年 | 3376篇 |
2004年 | 3417篇 |
2003年 | 2949篇 |
2002年 | 2219篇 |
2001年 | 1932篇 |
2000年 | 1832篇 |
1999年 | 1454篇 |
1998年 | 1536篇 |
1997年 | 1457篇 |
1996年 | 1161篇 |
1995年 | 1216篇 |
1994年 | 1031篇 |
1993年 | 923篇 |
1992年 | 913篇 |
1991年 | 805篇 |
1990年 | 918篇 |
1989年 | 772篇 |
1988年 | 699篇 |
1987年 | 874篇 |
1986年 | 711篇 |
1985年 | 918篇 |
1984年 | 998篇 |
1983年 | 923篇 |
1982年 | 895篇 |
1981年 | 756篇 |
1980年 | 720篇 |
1979年 | 657篇 |
1978年 | 654篇 |
1977年 | 585篇 |
1976年 | 589篇 |
1975年 | 551篇 |
1974年 | 542篇 |
1973年 | 517篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
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
13.
14.
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
15.
16.
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
17.
18.
Isolation of resonance in acoustic backscatter from elastic targetsusing adaptive estimation schemes
It has been shown that at certain frequencies the acoustic backscatter from elastic targets exhibits certain resonance behavior which closely relates to the physical properties of the target such as dimension, thickness, and composition. The purpose of this paper is to develop an automated approach for identifying the presence of resonance in the acoustic backscatter from an unknown underwater target by isolating the resonance part from the specular contribution. An adaptive transversal filter structure is used to estimate the specular part of the backscatter and consequently the error signal would provide an estimate of the resonance part. An important aspect of this scheme lies in the fact that it does not require an underlying model for the elastic return. The adaptation rule is based upon fast recursive least squares (RLS) learning. The approach taken in this paper is general in the sense that it can be applied to targets of unknown geometry and thickness and, further, does not require any a priori information about the target and/or the environment. Test results on acoustic data are presented which indicate the effectiveness of the proposed approach 相似文献
19.
20.