全文获取类型
收费全文 | 39807篇 |
免费 | 6915篇 |
国内免费 | 9458篇 |
专业分类
测绘学 | 2984篇 |
大气科学 | 8549篇 |
地球物理 | 9725篇 |
地质学 | 19017篇 |
海洋学 | 4667篇 |
天文学 | 1875篇 |
综合类 | 4368篇 |
自然地理 | 4995篇 |
出版年
2024年 | 131篇 |
2023年 | 505篇 |
2022年 | 1670篇 |
2021年 | 1896篇 |
2020年 | 1521篇 |
2019年 | 1873篇 |
2018年 | 2101篇 |
2017年 | 1893篇 |
2016年 | 2246篇 |
2015年 | 1954篇 |
2014年 | 2349篇 |
2013年 | 2363篇 |
2012年 | 2185篇 |
2011年 | 2298篇 |
2010年 | 2361篇 |
2009年 | 2328篇 |
2008年 | 2101篇 |
2007年 | 2009篇 |
2006年 | 1604篇 |
2005年 | 1546篇 |
2004年 | 1149篇 |
2003年 | 1161篇 |
2002年 | 1112篇 |
2001年 | 1173篇 |
2000年 | 1402篇 |
1999年 | 1916篇 |
1998年 | 1548篇 |
1997年 | 1491篇 |
1996年 | 1382篇 |
1995年 | 1208篇 |
1994年 | 1027篇 |
1993年 | 928篇 |
1992年 | 730篇 |
1991年 | 597篇 |
1990年 | 454篇 |
1989年 | 426篇 |
1988年 | 380篇 |
1987年 | 214篇 |
1986年 | 213篇 |
1985年 | 141篇 |
1984年 | 133篇 |
1983年 | 93篇 |
1982年 | 84篇 |
1981年 | 75篇 |
1980年 | 49篇 |
1979年 | 42篇 |
1978年 | 16篇 |
1977年 | 14篇 |
1976年 | 13篇 |
1958年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
The masers of E-type methanol in orion KL and SGR B2 总被引:2,自引:0,他引:2
Using a simplified model the statistical equilibrium and radiative transfer equations of E-type-CH3OH are solved for Orion KL and SgrB2. According to our calculation results and the observation data taken by Matsakiset al. (1980) and Morimotoet al. (1985a, b), the physical conditions of both sources are estimated. In theJ
2-J
1 E methanol maser region of Orion KL, the density, kinetic temperature, dust temperature, and the fractional abundance are 0.8–2×106 cm–3, 150, 30–90 K, 0.8–8×10–6. In the 4–1-30 E and 5–1-40 E methanol maser region of Sgr B2 the correspondance physical conditions above are 104 cm3, 45, 23 K, and 7×10–7, respectively. 相似文献
132.
133.
134.
A mesoscale planetary boundary layer (PBL) numerical model has been developed to study airflow over complex topography. Turbulence closures using the turbulent kinetic energy (TKE) and dissipation () equations are investigated in combination with the level 2.5 scheme of Mellor and Yamada (1982) to determine eddy diffusivities for momentum and heat. This modified E- closure is simpler than the level 3 one which requires more prognostic equations for moist turbulent transport.One-dimensional (1-D) model results show that the PBL mean flows under various stability conditions are not significantly sensitive to the modified Blackadar and Kolmogorov eddy mixing-length formulations used in this E- model, although the latter yields excessively large mixing lengths in the entrainment region of the upper PBL. Eddy mixing lengths in the Kolmogorov-type formulation can be better defined by introducing background dissipation. Using the same prognostic TKE equation, the 1-D model results are not significantly affected by different diagnostic formulations in the closures. The simulated results compare well with large-eddy simulations and those obtained using higher-order closure schemes including the level 3 one. The results are found to be insensitive to eddy Prandtl number, in contrast to the 2-D model results (see Part II). 相似文献
135.
TL ages of 56 loess samples collected from Xinjiang and the middle reaches of the Yellow River were determined by using fine-grained quartz and man-made light source bleaching techniques.The results indicate that :1.The well-preserved loess sections in Xinjiang began depositing 400000 a ago.The loess/paleosol series may be comparable with that observed in the middle reaches of the Yellow River but the depositing time was probably delayed by 30000-50000a.2.The bottom boundary age of the Malan loess varies from one place to another.The TL dates can be divided into two group:70000-130000 a and 30000-50000 a.The TL data are con-sistent with the appearance of two warmer climate periods since the Pleistocene,indicating obvious regional differences in stratigraphic development of loess,i.e.,paleosol occurred in the warm climate period in the same location,but loess was deposited in other locations.The initial age of loess deposition tends to become older from west to east and from south to north .In contrast ,the cold cli-matic belt showed an opposite trend.The TL ages of loess provide strong evidence for the Quaternary stratigraphy throughout geological times and the shift tendency of climatic belt.3.The depositing rate of the Malan loess along the middle reaches of the Yellow River varies from place to place but the TL age shows a linear relationship with the depositing thickness.The de-positing rate of loess in the north is higher than that in the south,suggesting that loess materials are derived from desert areas of northern China. 相似文献
136.
提出川滇地洼系“四层楼”铜矿床序列的形成与陆壳演化的成生联系,是与本区陆壳由前地槽—地槽—地台—地洼演化各阶段与之相匹配的成矿作用的产物.与此同时,并总结了本区“四层楼”铜矿床序列的成矿作用具有明显的继承性、新生性、旋回性及层控性四大特点和多因复成矿床的成矿模式. 相似文献
137.
Gang Zhao Yonggheng Zhao Jtasheng Huang Lonlong Feng Keliang Huang Qiuhe Peng Tan Lu 《Astrophysics and Space Science》1988,141(1):169-176
In this paper, the neutrino mass has been determined from SN1987a observation in a manner that the simultaneity of neutrino emission is not regarded as the starting point, but is itself defined through the analysis by Monte-Carlo simulation. The result is that the neutrino mass lies in 3–4 eV, possiblym
v
3.6 eV. Neutrino luminosity variation and neutrino spectrum are also obtained. Comparison with theories gives further support to the mass determination, and also predicts the mass of progenitor star of SN1987a to be in the range of 12–25M
.The project supported by the National Natural Science Foundation of China. 相似文献
138.
We found that the structure of the Universe can be characterized by a set of actions
s
. This means that some discontinuous phenomena in the Universe can be considered as large-scale quantum effects. The behaviour of matter on a typical scale is determined by the behaviour of matter on other scales through the interactions. 相似文献
139.
Deng Zu-Gan Zhou You-Yuan Liu Yong-Zhen Dai He-Jun 《Astrophysics and Space Science》1986,126(2):301-312
In former works (Zhouet al., 1983, 1985), a quantitative method have been developed to take the selection effects in the identification of emission lines of quasars into account. It was proved that these selection effects may be the cause of the unevenness in the redshift distribution of quasars. The present work is a continuation and development of former works. We use results given by the surveys with same limit-apparent magnitude and choose the quasars whose absolute magnitudes are within a specific range as the analysing samples. Using the method given in the former papers we may find out the evolutionary parameter in an evolutionary law with form of (1+z)
y
from the best fitting between the calculative and observational redshift distribution. The result of analysis shows that the evolutionary law of quasars selected by slitless technique isp =p
0(1 + z)6.5 ± 1 up toz=2.8. This result coincides with and generalizes the earlier result given by other authors. 相似文献
140.
This paper is the second one of a series of papers on the redshift distribution of QSOs. In this paper, we shall study the influence of the selection effect in the identification of emission lines on the redshift distribution of QSOs more thoroughly than the previous paper (Zhouet al., 1983). If we assume that the QSO's redshift is cosmological, adopt the standard model, and consider the selection effect due to the redshift identification, the limiting apparent magnitude in the observation and the evolutionary effect of QSOs, we can compute the emission line redshift distribution for the so-called optically selected QSOs discovered by objective prism, grating prism technique alone, the QSOs discovered by positional methods or by colour technique and for whole QSOs, respectively (see Figures 6, 11, 12). The results of computation agree with the observations very well, especially for optically selected QSOs; the computational distribution has almost the same shape with the observational one. For this kind of the QSOs the computational distribution may give the positions and heights of all these observed peaks. The correlation coefficient between the calculated and observed distributions is larger than 0.95. It shows that (a) the peaks and dips in the redshift distribution of QSOs are mainly caused by the selection effect in the redshift identification, and (b) the redshift of QSOs is cosmological. 相似文献