全文获取类型
收费全文 | 140897篇 |
免费 | 2089篇 |
国内免费 | 1705篇 |
专业分类
测绘学 | 4005篇 |
大气科学 | 9769篇 |
地球物理 | 27168篇 |
地质学 | 53222篇 |
海洋学 | 11747篇 |
天文学 | 28481篇 |
综合类 | 2404篇 |
自然地理 | 7895篇 |
出版年
2022年 | 686篇 |
2021年 | 1236篇 |
2020年 | 1319篇 |
2019年 | 1492篇 |
2018年 | 8037篇 |
2017年 | 7112篇 |
2016年 | 6081篇 |
2015年 | 1986篇 |
2014年 | 3280篇 |
2013年 | 5856篇 |
2012年 | 4674篇 |
2011年 | 7791篇 |
2010年 | 6791篇 |
2009年 | 8276篇 |
2008年 | 7020篇 |
2007年 | 7752篇 |
2006年 | 4606篇 |
2005年 | 3769篇 |
2004年 | 3811篇 |
2003年 | 3637篇 |
2002年 | 3399篇 |
2001年 | 2740篇 |
2000年 | 2642篇 |
1999年 | 2099篇 |
1998年 | 2178篇 |
1997年 | 2032篇 |
1996年 | 1722篇 |
1995年 | 1705篇 |
1994年 | 1485篇 |
1993年 | 1440篇 |
1992年 | 1328篇 |
1991年 | 1324篇 |
1990年 | 1313篇 |
1989年 | 1208篇 |
1988年 | 1117篇 |
1987年 | 1301篇 |
1986年 | 1149篇 |
1985年 | 1414篇 |
1984年 | 1587篇 |
1983年 | 1508篇 |
1982年 | 1381篇 |
1981年 | 1353篇 |
1980年 | 1167篇 |
1979年 | 1128篇 |
1978年 | 1071篇 |
1977年 | 982篇 |
1976年 | 946篇 |
1975年 | 920篇 |
1974年 | 906篇 |
1973年 | 986篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Solution for a plane strain rough‐walled hydraulic fracture driven by turbulent fluid through impermeable rock 下载免费PDF全文
The impact of turbulent flow on plane strain fluid‐driven crack propagation is an important but still poorly understood consideration in hydraulic fracture modeling. The changes that hydraulic fracturing has experienced over the past decade, especially in the area of fracturing fluids, have played a major role in the transition of the typical fluid regime from laminar to turbulent flow. Motivated by the increasing preponderance of high‐rate, water‐driven hydraulic fractures with high Reynolds number, we present a semianalytical solution for the propagation of a plane strain hydraulic fracture driven by a turbulent fluid in an impermeable formation. The formulation uses a power law relationship between the Darcy‐Weisbach friction factor and the scale of the fracture roughness, where one specific manifestation of this generalized friction factor is the classical Gauckler‐Manning‐Strickler approximation for turbulent flow in a rough‐walled channel. Conservation of mass, elasticity, and crack propagation are also solved simultaneously. We obtain a semianalytical solution using an orthogonal polynomial series. An approximate closed‐form solution is enabled by a choice of orthogonal polynomials embedding the near‐tip asymptotic behavior and thus giving very rapid convergence; a precise solution is obtained with 2 terms of the series. By comparison with numerical simulations, we show that the transition region between the laminar and turbulent regimes can be relatively small so that full solutions can often be well approximated by either a fully laminar or fully turbulent solution. 相似文献
12.
Abstract— Active capture is a new process for the incorporation of large quantities of heavy noble gases into growing surfaces. Adsorption in the conventional sense involves surface bonding by polarization (Van der Waals forces). What is referred to as “anomalous adsorption” of heavy noble gases involves chemical bonds and can occur when other (more chemically active) species are not available to preempt sites with unfilled bonds. Anomalous adsorption has been observed under conditions of fracture, vacuum deposition and ionizing radiation. Active capture depends upon anomalous adsorption to retain noble gases on a surface long enough to be captured in a growing surface film as it is deposited. The fundamental principle may be the impingement onto the growing film with sufficient energy to liberate surface electrons (work function energy of a few electronvolts) so that they are retained by anomalous adsorption long enough to be entrapped in the growing surface. Trapping efficiencies of ?1% have been observed for Kr and Xe in laboratory experiments, implying a fundamentally new mechanism for the incorporation of heavy noble gases onto surfaces. It may play a role in explaining the large concentrations of planetary noble gases contained in phase‐Q. 相似文献
13.
We present our observations of the galaxy UGS 5600 with a long-slit spectrograph (UAGS) and a multipupil field spectrograph (MPFS) attached to the 6-m Special Astrophysical Observatory telescope. Radial-velocity fields of the stellar and gaseous components were constructed for the central region and inner ring of the galaxy. We proved the existence of two nearly orthogonal kinematic subsystems and conclude that UGC 5600 is a galaxy with an inner polar ring. In the circumnuclear region, we detected noncircular stellar motions and suspected the existence of a minibar. The emission lines are shown to originate in H II regions. We estimated the metallicity from the intensity ratio of the [N II]λ6583 and Hα lines to be nearly solar, which rules out the possibility that the polar ring was produced by the accretion of gas from a dwarf companion. 相似文献
14.
15.
16.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars. 相似文献
17.
The magnetic-field distribution outside a flat, infinitely conductive unbounded disk in the field of a point magnetic dipole is determined. A relationship is established between the problem of magnetic-field determination and the problem of the flow of an ideal incompressible fluid around an infinitely thin disk. 相似文献
18.
本文综述了活动星系核 ,特别是blazar天体的研究现状 ,对blazar天体的多波段与多波段能谱特性研究进行了较为详细的评述。主要的研究工作包括以下内容 :(一 )γ噪blazar天体的短时标光变研究 ,通过对 1 6个γ噪blazar天体 (其中包括全部已证认和观测到VHEγ射线爆发的可能甚高能γ射线源 )自 1 998年的光学观测及光变分析研究表明 ,短时标光变 (小时量级 )是GeVγ噪blazar天体的普遍特性 ,光变幅度通常可达 0 .6mag/h ,对PKS 1 51 0 -0 89类星体的观测发现在一个小时内对象变暗 2个星等 ,对如此激烈的光变变暗目前的理论还不能很好的解释 ,但它同样反映了辐射区域的内部结构 ;而对TBLs的监测表明 ,其光学波段的短时标光变没有其他对象激烈 ,出现的频度和振幅变化都较小 ;(二 )在研究γ噪blazar天体光变时 ,研究了寄主星系对光变的影响 ,得到了 1ES 2 3 44 + 51 4的光变与PSF的FWHM的关联 ,表明随着大气视宁度的下降 (即FWHM变大 ) ,对象变暗 ,即由于寄主星系的影响从而导致假光变的产生 ;(三 )引进两个多波段复合谱指数 ,αxox=αox-αx 及αoro=αor-αo。对样本的统计研究表明 ,RBLs是能谱特性界于XBLs和OVVs之间的一类中间态 ,所得结果支持了Sambrunaetal.( 1 996)大样本多波段能谱分布特性的统计研 相似文献
19.
20.