全文获取类型
收费全文 | 255682篇 |
免费 | 6392篇 |
国内免费 | 4780篇 |
专业分类
测绘学 | 6989篇 |
大气科学 | 20079篇 |
地球物理 | 54275篇 |
地质学 | 88996篇 |
海洋学 | 21883篇 |
天文学 | 54394篇 |
综合类 | 1770篇 |
自然地理 | 18468篇 |
出版年
2021年 | 2452篇 |
2020年 | 2853篇 |
2019年 | 3064篇 |
2018年 | 3311篇 |
2017年 | 2990篇 |
2016年 | 5775篇 |
2015年 | 4427篇 |
2014年 | 7198篇 |
2013年 | 14474篇 |
2012年 | 6559篇 |
2011年 | 7742篇 |
2010年 | 6825篇 |
2009年 | 9449篇 |
2008年 | 8265篇 |
2007年 | 7660篇 |
2006年 | 9833篇 |
2005年 | 7793篇 |
2004年 | 7669篇 |
2003年 | 7171篇 |
2002年 | 6799篇 |
2001年 | 6096篇 |
2000年 | 6083篇 |
1999年 | 5480篇 |
1998年 | 5483篇 |
1997年 | 5221篇 |
1996年 | 4412篇 |
1995年 | 4520篇 |
1994年 | 4179篇 |
1993年 | 3893篇 |
1992年 | 3621篇 |
1991年 | 3606篇 |
1990年 | 3706篇 |
1989年 | 3376篇 |
1988年 | 3201篇 |
1987年 | 3742篇 |
1986年 | 3262篇 |
1985年 | 4122篇 |
1984年 | 4642篇 |
1983年 | 4310篇 |
1982年 | 4232篇 |
1981年 | 3844篇 |
1980年 | 3593篇 |
1979年 | 3440篇 |
1978年 | 3428篇 |
1977年 | 3218篇 |
1976年 | 2964篇 |
1975年 | 2903篇 |
1974年 | 2866篇 |
1973年 | 3060篇 |
1972年 | 1992篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
21.
Nancy L. Chabot Rachel H. Cueva Andrew W. Beck Richard D. Ash 《Meteoritics & planetary science》2020,55(4):726-743
Some of the defining characteristics of the IIG iron meteorite group are their high bulk P contents and massive, coarse schreibersite, which have been calculated to make up roughly 11–14 wt% of each specimen. In this study, we produced two data sets to investigate the formation of schreibersites in IIG irons: measurements of trace elements in the IIG iron meteorite Twannberg and experimental determinations of trace element partitioning into schreibersite. The schreibersite‐bearing experiments were conducted with schreibersite in equilibrium with a P‐rich melt and with bulk Ni contents ranging from 0 to 40 wt%. The partitioning behavior for the 20 elements measured in this study did not vary with Ni content. Comparison of the Twannberg measurements with the experimental results required a correction factor to account for the fact that the experiments were conducted in a simplified system that did not contain a solid metal phase. Previously determined solid metal/P‐rich melt partition coefficients were applied to infer schreibersite/solid metal partitioning behavior from the experiments, and once this correction was applied, the two data sets showed broad similarities between the schreibersite/solid metal distribution of elements. However, there were also differences noted, in particular between the Ni and P contents of the solid metal relative to the schreibersite inferred from the experiments compared to that measured in the Twannberg sample. These differences support previous interpretations that subsolidus schreibersite evolution has strongly influenced the Ni and P content now present in the solid metal phase of IIG irons. Quantitative attempts to match the IIG solid metal composition to that of late‐stage IIAB irons through subsolidus schreibersite growth were not successful, but qualitatively, this study corroborates the striking similarities between the IIAB and IIG groups, which are highly suggestive of a possible genetic link between the groups as has been previously proposed. 相似文献
22.
Cai Liu Hong-Yan Sun Zhi-Guo Meng Yong-Chun Zheng Yu Lu Zhan-Chuan Cai Jin-Song Ping Alexander Gusev Shuo Hu 《天文和天体物理学研究(英文版)》2019,(5)
The volume FeO and TiO_2 abundances(FTAs) of lunar regolith can be more important for understanding the geological evolution of the Moon compared to the optical and gamma-ray results. In this paper, the volume FTAs are retrieved with microwave sounder(CELMS) data from the Chang'E-2 satellite using the back propagation neural network(BPNN) method. Firstly, a three-layered BPNN network with five-dimensional input is constructed by taking nonlinearity into account. Then, the brightness temperature(TB) and surface slope are set as the inputs and the volume FTAs are set as the outputs of the BPNN network.Thereafter, the BPNN network is trained with the corresponding parameters collected from Apollo, Luna,and Surveyor missions. Finally, the volume FTAs are retrieved with the trained BPNN network using the four-channel TBderived from the CELMS data and the surface slope estimated from Lunar Orbiter Laser Altimeter(LOLA) data. The rationality of the retrieved FTAs is verified by comparing with the Clementine UV-VIS results and Lunar Prospector(LP) GRS results. The retrieved volume FTAs enable us to re-evaluate the geological features of the lunar surface. Several important results are as follows. Firstly, very-low-Ti(1.5 wt.%) basalts are the most spatially abundant, and the surfaces with TiO_2 5 wt.% constitute less than 10% of the maria. Also, two linear relationships occur between the FeO abundance(FA) and the TiO_2 abundance before and after the threshold, 16 wt.% for FA. Secondly, a new perspective on mare volcanism is derived with the volume FTAs in several important mare basins, although this conclusion should be verified with more sources of data. Thirdly, FTAs in the lunar regolith change with depth to the uppermost surface,and the change is complex over the lunar surface. Finally, the distribution of volume FTAs hints that the highlands crust is probably homogeneous, at least in terms of the microwave thermophysical parameters. 相似文献
23.
Subsurface flow measurements using passive flux meters in variably-saturated cold-regions landscapes
To date, passive flux meters have predominantly been applied in temperate environments for tracking the movement of contaminants in groundwater. This study applies these instruments to reduce uncertainty in (typically instantaneous) flux measurements made in a low-gradient, wetland dominated, discontinuous permafrost environment. This method supports improved estimation of unsaturated and over-winter subsurface flows which are very difficult to quantify using hydraulic gradient-based approaches. Improved subsurface flow estimates can play a key role in understanding the water budget of this landscape. 相似文献
24.
Muryshev K. E. Eliseev A. V. Denisov S. N. Mokhov I. I. Arzhanov M. M. Timazhev A. V. 《Izvestiya Atmospheric and Oceanic Physics》2019,55(3):235-241
Izvestiya, Atmospheric and Oceanic Physics - The phase shift between changes in the global surface temperature Tg and atmospheric CO2 content $${{q}_{{{\text{C}}{{{\text{O}}}_{2}}}}}$$ has been... 相似文献
25.
Natural Resources Research - Recognition of reservoir quality is an important objective in reservoir characterization process. By definition, the quality of a reservoir is described by its... 相似文献
26.
27.
We present the finalized catalog of solar energetic proton events detected by the Wind/EPACT instrument over the period 1996?–?2016. Onset times, peak times, peak proton intensity and onset-to-peak proton fluence are evaluated for the two available energy channels, at about 25 and 50 MeV. We describe the procedure utilized to identify the proton events and to relate them to their solar origin (in terms of flares and coronal mass ejections). The statistical relationships between the energetic protons and their origin (linear and partial correlation analysis) are reported and discussed in view of earlier findings. Finally, the different trends found in the first 8 years of Solar Cycles 23 and 24 are discussed. 相似文献
28.
29.
S.?N.?KulichkovEmail author O.?Ye.?Popov K.?V.?Avilov I.?P.?Chunchuzov O.?G.?Chkhetiani A.?A.?Smirnov V.?I.?Dubrovin A.?A.?Mishenin 《Izvestiya Atmospheric and Oceanic Physics》2018,54(3):293-303
Results obtained from simulating the propagation of infrasonic waves from the Chelyabinsk meteoroid explosion observed on February 15, 2013, are given. The pseudodifferential parabolic equation (PDPE) method has been used for calculations. Data on infrasonic waves recorded at the IS31 station (Aktyubinsk, Kazakhstan), located 542.7 km from the likely location of the explosion, have been analyzed. Six infrasonic arrivals (isolated clearly defined pulse signals) were recorded. It is shown that the first “fast” arrival (F) corresponds to the propagation of infrasound in a surface acoustic waveguide. The rest of the arrivals (T1–T5) are thermospheric. The agreement between the results of calculations based on the PDPE method and experimental data is satisfactory. The energy E of the explosion has been estimated using two methods. One of these methods is based on the law of conservation of the acoustic pulse I, which is a product of the wave profile area S/2 of the signal under analysis and the distance to its source E I [kt] = 1.38 × 10–10 (I [kg/s])1.482. The other method is based on the relation between the energy of explosion and the dominant period T of recorded signal E T [kt] = 1.02 × (T [s]2/σ)3/2, where σ is the dimensionless distance determining the degree of nonlinear effects during the propagation of sound along ray trajectories. According to the data, the explosion energy E I,T ranges from 1.87 to 32 kt TNT. 相似文献
30.
Geomagnetism and Aeronomy - Based on data from long-term observations at two geophysical observatories, Borok and College, distantly spaced in latitude and longitude, the results of remote... 相似文献