全文获取类型
收费全文 | 91733篇 |
免费 | 1432篇 |
国内免费 | 721篇 |
专业分类
测绘学 | 2383篇 |
大气科学 | 6163篇 |
地球物理 | 18593篇 |
地质学 | 32700篇 |
海洋学 | 8198篇 |
天文学 | 21074篇 |
综合类 | 257篇 |
自然地理 | 4518篇 |
出版年
2022年 | 654篇 |
2021年 | 1042篇 |
2020年 | 1149篇 |
2019年 | 1249篇 |
2018年 | 2723篇 |
2017年 | 2531篇 |
2016年 | 2951篇 |
2015年 | 1508篇 |
2014年 | 2736篇 |
2013年 | 4790篇 |
2012年 | 3078篇 |
2011年 | 3895篇 |
2010年 | 3524篇 |
2009年 | 4456篇 |
2008年 | 3931篇 |
2007年 | 4065篇 |
2006年 | 3757篇 |
2005年 | 2594篇 |
2004年 | 2549篇 |
2003年 | 2346篇 |
2002年 | 2356篇 |
2001年 | 2209篇 |
2000年 | 2024篇 |
1999年 | 1627篇 |
1998年 | 1626篇 |
1997年 | 1706篇 |
1996年 | 1350篇 |
1995年 | 1314篇 |
1994年 | 1263篇 |
1993年 | 1073篇 |
1992年 | 1002篇 |
1991年 | 1008篇 |
1990年 | 1053篇 |
1989年 | 953篇 |
1988年 | 891篇 |
1987年 | 991篇 |
1986年 | 857篇 |
1985年 | 1105篇 |
1984年 | 1208篇 |
1983年 | 1131篇 |
1982年 | 1115篇 |
1981年 | 997篇 |
1980年 | 973篇 |
1979年 | 850篇 |
1978年 | 887篇 |
1977年 | 786篇 |
1976年 | 717篇 |
1975年 | 724篇 |
1974年 | 688篇 |
1973年 | 749篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
81.
Similarity solutions, describing the flow of a perfect gas behind spherical shock waves, are investigated including the radiation heat flux. The shock is assumed to be propagating in a medium at rest. Shock radius varies exponentially with time and density is inversely proportional to fifth power of the shock radius immediately ahead of the shock front. 相似文献
82.
83.
Rostov State Pedagogical Institute; All-Union Scientific Research Institute Gradient. Translated from Astrofizika, Vol. 33, No. 3, pp. 379–393, November–December, 1990. 相似文献
84.
Faith Vilas David J. Tholen Larry A. Lebofsky Humberto Campins Glenn J. Veeder Richard P. Binzel Alan T. Tokunaga 《Icarus》1985,63(2):201-205
Visual and infrared observations were made of Amor asteroid 1982 DV during its discovery apparition. Broadband visual and near-infrared photometry shows that it is an S-class asteroid. Narrowband spectrophotometry shows an absorption feature due to olivine or pyroxene or both centered at 0.93 μm. Applying a nonrotating thermal model to 10-μm photometry, the geometric albedo is calculated to be approximately 0.27. The geometric albedo for a slowly rotating, rocky surface was calculated for 1 night to be 0.15, consistent with S-class asteroid albedos. Thus, 1982 DV is either one of the most reflective S-class asteroids known, or a significant amount of bare rock is exposed on the asteroid's surface. For the nonrotating model, ellipsoidal dimensions for 1982 DV are 3.5 × 1.4 × 1.4 km. 相似文献
85.
Shelf-mounted Ocean Thermal Energy Conversion (OTEC) plants require installation of cold-water pipes (CWP) on slopes of40deg to depths of 1000 m. In addition, tower platforms containing OTEC power systems may be located on lesser sloped terrain near shore and exposed to special environmental loading problems affecting foundation design. Shelf-mounted installations require careful attention to site selection and geotechnical considerations for foundation integrity on sloped surfaces. This paper primarily discusses research associated with cold-water pipe and foundation installations on steep slopes, although research continues on tower platforms located on the shelf. At least five nations are in various stages of development of OTEC systems for island applications. Each of their systems is either shelf mounted or land based and requires that a large diameter cold-water pipe be installed on a steep slope to provide cold water from 1000-m depths. In addition to the installation and deployment of the large cold-water pipe, the most significant problem is the design and installation of suitable foundations that will last for several decades. To date there is very little experience in the offshore industry for large installations on steep slopes. A major scale-model research project is underway on the slopes of the island of Hawaii. A section of pipe 2.4 m in diameter and 24 m long was installed using combination concrete foundations and joints. The pipe and foundations are fully instrumented to measure environmental loading forces due principally to currents and waves. Environmental measurements will also be taken in the test area. The measurement data will be used to validate available analytical models for subsequent use in aiding industry in providing more cost-effective designs for OTEC pipes and foundations. 相似文献
86.
87.
Electron impact excitation rates for transitions in the S v ion, calculated with theR-matrix code, are used to derive the electron temperature sensitive emission line ratiosR
1 =I(854.8 Å)/I(786.9 Å),R
2 =I(852.2 Å)/I(786.9 Å),R
3 =I(849.2 Å)/I(786.9 Å), andR
4 =I(1199.1 Å)/I(786.9 Å), which are found to be significantly different from previous estimates. A comparison of the present results with observational data for a sunspot obtained with the Harvard S-055 spectrometer on boardSkylab reveals generally good agreement between theory and experiment, except in the case ofR
1, which is probably due to blending in the 854.8 Å feature. The possible effects of Lyman continuum absorption on the observed line ratios is briefly discussed. 相似文献
88.
89.
Range of validity of seismic ray and beam methods in general inhomogeneous media – I. General theory
Summary. The limitations of asymptotic wave theory and its geometrical manifestations are newly formalized and scrutinized. Necessary and sufficient conditions for the existence of acoustic and seismic rays and beams in general inhomogeneous media are expressed in terms of new physical parameters: the threshold frequency ω0 associated with the P/S decoupling condition, the cut-off frequency ωc associated with the radiation-zone condition, the total curvature of the wavefront and the Fresnel-zone radius.
The analysis is facilitated with the introduction of a new ancillary functional – the hypereikonal which is capable of representing ordinary as well as evanescent waves. The hypereikonal is the natural extension of the eikonal theory.
With the aid of the above new parameters, simple conditions are obtained for the decoupled far field, the decoupled near field, two point dynamic ray tracing, paraxial wavefields and Gaussian beams. 相似文献
The analysis is facilitated with the introduction of a new ancillary functional – the hypereikonal which is capable of representing ordinary as well as evanescent waves. The hypereikonal is the natural extension of the eikonal theory.
With the aid of the above new parameters, simple conditions are obtained for the decoupled far field, the decoupled near field, two point dynamic ray tracing, paraxial wavefields and Gaussian beams. 相似文献
90.