全文获取类型
收费全文 | 3588篇 |
免费 | 133篇 |
国内免费 | 102篇 |
专业分类
测绘学 | 81篇 |
大气科学 | 431篇 |
地球物理 | 923篇 |
地质学 | 1126篇 |
海洋学 | 682篇 |
天文学 | 328篇 |
综合类 | 47篇 |
自然地理 | 205篇 |
出版年
2023年 | 13篇 |
2022年 | 23篇 |
2021年 | 55篇 |
2020年 | 57篇 |
2019年 | 66篇 |
2018年 | 145篇 |
2017年 | 142篇 |
2016年 | 154篇 |
2015年 | 101篇 |
2014年 | 184篇 |
2013年 | 243篇 |
2012年 | 153篇 |
2011年 | 219篇 |
2010年 | 201篇 |
2009年 | 206篇 |
2008年 | 182篇 |
2007年 | 189篇 |
2006年 | 157篇 |
2005年 | 143篇 |
2004年 | 116篇 |
2003年 | 113篇 |
2002年 | 111篇 |
2001年 | 84篇 |
2000年 | 92篇 |
1999年 | 61篇 |
1998年 | 52篇 |
1997年 | 47篇 |
1996年 | 28篇 |
1995年 | 40篇 |
1994年 | 21篇 |
1993年 | 19篇 |
1992年 | 21篇 |
1991年 | 19篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 18篇 |
1987年 | 27篇 |
1986年 | 17篇 |
1985年 | 15篇 |
1984年 | 31篇 |
1983年 | 35篇 |
1982年 | 25篇 |
1981年 | 20篇 |
1980年 | 26篇 |
1979年 | 16篇 |
1978年 | 10篇 |
1977年 | 18篇 |
1975年 | 20篇 |
1974年 | 11篇 |
1973年 | 8篇 |
排序方式: 共有3823条查询结果,搜索用时 15 毫秒
71.
Lee W. Casperson 《Astrophysics and Space Science》1977,48(2):389-399
A modification of the rotating-star model has been developed to interpret the periodic energy bursts from pulsars. This new configuration involves -directed oscilltion modes in the stellar atmosphere or magnetosphere, and most aspects of the typical pulse characteristics are well accounted for. Gain is provided by resonant interactions with particles trapped in the stellar magnetic field. The most significant feature is the fact that highly directional beaming of the output energy results as a natural consequence of coherence between the radiation fields emerging from various locations about the pulsar; and a localized radiation origin is not required. 相似文献
72.
It is shown that the new definition1 of strong motion local magnitude M leads to stable estimates of magnitudes for earthquakes in Yugoslavia, with epicentral distances R <100 km and for 2.5 < M < 6.5. Tables with magnitudes computed using this new procedure are presented for all earthquakes contributing to the strong motion accelerogram files in EQINFOS for Yugoslavia.2 The similarity of our findings with the analogous analyses for California suggests new possibilities for relative calibration between various local magnitude scales, which are used in southeastern Europe, and ML in California. 相似文献
73.
A centrifugal mill is a high-power intensity media mill that can be used for ultra-fine grinding, employing centrifugal forces generated by gyration of the axis of the mill tube in a circle. The mill charge motion is quite different depending on the ratio of the gyration diameter to the mill diameter (G / D ratio), varying from a motion similar to that of a conventional tumbling media mill to that of a vibration mill. In this study, a centrifugal mill was constructed with an arrangement where the gyration diameter could be readily adjusted. The batch grinding characteristics of three different minerals (limestone, talc and illite) in water with dispersing agent were investigated at various G / D ratios. It was found that the optimum G / D ratio in terms of the specific energy consumption to give a desired fineness of product was different for the three minerals. This was due to their different reactions to the breakage mechanisms provided by the mill charge motion at varying G / D ratios. The size distributions became progressively narrower at increased grinding times, and particles finer than about 0.1 μm were not detected even for prolonged grinding times. Measurement of specific surface areas indicated that this was not due to an artifact of the size measurements by laser diffractometry. This implies that there is a limitation in which particles finer than 0.1 μm are not produced under the conditions tested in this type of mill, but further investigation is needed for experimental verification of this limit of comminution. 相似文献
74.
Study on the kinetics of iron oxide leaching by oxalic acid 总被引:2,自引:0,他引:2
Suong Oh Lee Tam Tran Yi Yong Park Seong Jun Kim Myong Jun Kim 《International Journal of Mineral Processing》2006,80(2-4):144-152
The presence of iron oxides in clay or silica raw materials is detrimental to the manufacturing of high quality ceramics. Although iron has been traditionally removed by physical mineral processing, acid washing has been tested as it is more effective, especially for extremely low iron (of less than 0.1% w/w). However, inorganic acids such as sulphuric or hydrochloric acids easily contaminate the clay products with SO42− and Cl−, and therefore should be avoided as much as possible. On the other hand, if oxalic acid is used, any acid left behind will be destroyed during the firing of the ceramic products. The characteristics of dissolution of iron oxides were therefore investigated in this study.The dissolution of iron oxides in oxalic acid was found to be very slow at temperatures within the range 25–60 °C, but its rate increases rapidly above 90 °C. The dissolution rate also increases with increasing oxalate concentration at the constant pH values set within the optimum range of pH2.5–3.0. At this optimum pH, the dissolution of fine pure hematite (Fe2O3) (105–140 μm) follows a diffusion-controlled shrinking core model. The rate expression expressed as 1 − (2 / 3)x − (1 − x)2 / 3 where x is a fraction of iron dissolution was found to be proportional to [oxalate]1.5.The addition of magnetite to the leach liquor at 10% w/w hematite was found to enhance the dissolution rate dramatically. Such addition of magnetite allows coarser hematite in the range 0.5–1.4 mm to be leached at a reasonable rate. 相似文献
75.
A. F. Kholtygin G. A. Galazutdinov T. E. Burlakova G. G. Valyavin S. N. Fabrika B. -C. Lee 《Astronomy Reports》2006,50(3):220-231
We present the results of a search for and analysis of line-profile variations in the spectrum of the star ι Her. The observations were acquired with the 1.8 m telescope of the Bohyunsan Optical Astronomy Observatory (Republic of Korea) in May–June 2004. We obtained 69 spectra of the star with signal-to-noise ratios ≈300 and a time resolution of 5–7 min. Profile variability was revealed for six lines of HI, HeI, and SiIII, in the central parts of the lines. The variability amplitude is ≈(1–2)% in units of the intensity of the adjacent continuum. Evidence was found for cyclic variations of the lines, with periods from ≈7h to ≈2.9d. We conclude that ι Her belongs to the group of slowly pulsating stars. 相似文献
76.
77.
Poverty, vulnerability and the impact of flooding
in the Limpopo Province, South Africa 总被引:1,自引:0,他引:1
The article evaluates household vulnerability after the 2000 flood in two poor communities of the Limpopo Province, South
Africa. The study analyses the forms of vulnerability which disasters such as floods present. Using data gathered from a survey
of households, the study presents the impacts, coping and adapting strategies of households after the 2000 flood. The article
argues that beyond the concern over socio-economic circumstances as the major determinant of household vulnerability, the
management of the disaster can serve to perpetuate vulnerability. 相似文献
78.
Victor L. Vinograd Bjoern Winkler Daniel J. Wilson Andrew Putnis Julian D. Gale 《Physics and Chemistry of Minerals》2006,33(8-9):533-544
Static lattice energy calculations, based on empirical pair potentials, were performed for a large set of structures differing in the arrangement of octahedral cations within the garnet 2 × 2 × 2 supercell. The compositions of these structures varied between Ca3Fe2Ge3O12 and Ca4Ge4O12. The energies were cluster expanded using pair and quaternary terms. The derived ordering constants were used to constrain Monte Carlo simulations of temperature-dependent mixing properties in the ranges of 1,073–3,673 K and 0–10 GPa. The free energies of mixing were calculated using the method of thermodynamic integration. The calculations predict a wide miscibility gap between Fe-rich (cubic) and Fe-pure (tetragonal) garnets consistent with recent experimental observations of Iezzi et al. (Phys Chem Miner 32:197–207, 2005). It is shown that the miscibility gap arises due to a very strong cation ordering at the Fe-pure composition, driven by the charge difference between Ca2+ and Ge4+ cations. The structural and thermodynamic analogies between Ca–Ge and Mg–Si systems suggest that a similar miscibility gap should exist between pyrope and Mg–Si-majorite. 相似文献
79.
A short reply to discussion by Chanson. 相似文献
80.