全文获取类型
收费全文 | 1985篇 |
免费 | 152篇 |
国内免费 | 151篇 |
专业分类
测绘学 | 144篇 |
大气科学 | 184篇 |
地球物理 | 447篇 |
地质学 | 801篇 |
海洋学 | 162篇 |
天文学 | 317篇 |
综合类 | 56篇 |
自然地理 | 177篇 |
出版年
2023年 | 19篇 |
2022年 | 45篇 |
2021年 | 73篇 |
2020年 | 48篇 |
2019年 | 54篇 |
2018年 | 85篇 |
2017年 | 81篇 |
2016年 | 113篇 |
2015年 | 69篇 |
2014年 | 95篇 |
2013年 | 121篇 |
2012年 | 95篇 |
2011年 | 118篇 |
2010年 | 90篇 |
2009年 | 116篇 |
2008年 | 102篇 |
2007年 | 93篇 |
2006年 | 81篇 |
2005年 | 55篇 |
2004年 | 37篇 |
2003年 | 42篇 |
2002年 | 32篇 |
2001年 | 43篇 |
2000年 | 30篇 |
1999年 | 38篇 |
1998年 | 32篇 |
1997年 | 34篇 |
1996年 | 28篇 |
1995年 | 32篇 |
1994年 | 25篇 |
1993年 | 20篇 |
1992年 | 19篇 |
1991年 | 29篇 |
1990年 | 14篇 |
1989年 | 16篇 |
1988年 | 14篇 |
1987年 | 13篇 |
1986年 | 16篇 |
1985年 | 18篇 |
1984年 | 15篇 |
1983年 | 14篇 |
1982年 | 24篇 |
1981年 | 12篇 |
1980年 | 19篇 |
1979年 | 11篇 |
1978年 | 11篇 |
1974年 | 11篇 |
1973年 | 13篇 |
1972年 | 16篇 |
1971年 | 14篇 |
排序方式: 共有2288条查询结果,搜索用时 382 毫秒
1.
Studying relativistic compact objects is important in modern astrophysics to understand several astrophysical issues. It is therefore natural to ask for an internal structure and physical properties of specific classes of compact stars from astrophysical observations. We obtain a class of new relativistic solutions with anisotropic distribution of matter for compact stars. More specifically, stellar models, described by an anisotropic fluid, establishing a relation between metric potentials and generating a specific form of mass function, are explicitly constructed within the framework of General Relativity. New solutions can be used to model compact objects, which adequately describe compact strange star candidates like SMC X-1, Her X-1 and 4U 1538-52, with observational data taken from Gangopadhyay et al. (Mon. Not. R. Astron. Soc. 431:3216, 2013). As a possible astrophysical application the obtained solutions could explain the physics of selfgravitating objects, and might be useful for strong-field regimes where data are currently inadequate. 相似文献
2.
U. C. Joshi A. K. Sen M. R. Deshpande J. S. Chauhan 《Journal of Astrophysics and Astronomy》1992,13(3):267-277
Photopolarimetric observations of comet Austin with the IAU/IHW filter system were obtained on the 2.34 m Vainu Bappu Telescope
(VBT) of the Indian Institute of Astrophysics, at Kavalur, India, during pre-perihelion phase on February 20,1990 and on the
1.2 m telescope of the Physical Research Laboratory at Gurusikhar, Mount Abu during postperihelion phase on May 2 and 4, 1990.
The comet appeared bluer than a solar analog during post-perihelion phase on May 2 and 4. The percent polarization shows a
sharp increase towards the red on May 2 and 4. The dominant sizes of the dust particles appear to lie in a narrow range of
0.1 to 0.5 Μm. Regarding the molecular band emission, CN and C2 bands are quite strong; C3 emission was also found to be strong though the observations on May 2 and 4 show significant variation as compared to C2 emission. Molecular band polarization for CN, C3, C2 and H2 O+ have been calculated. It has been found that emission polarization in CN, C2 and C3 is between 1–7% (phase angle between 107.4–109 degrees). For CN and C2 the polarization values are close to the theoretically predicted values, but for C3 the polarization value falls much below the theoretically predicted value. A similar result was found for comet Halley. 相似文献
3.
4.
The most general accurate solutions for the Buchdhal fluid sphere were obtained and matched with the Schwarzchild's exterior
solution at the pressure free interface. Various parameters of the solutions were so adjusted that the energy density, pressure
and temperature were positive and decreasing away from the centre, and the velocity of sound was less than unity throughout
the spheroid model. Using this procedure, the maximum mass of the fluid sphere with a surface density of2 × 1014gm cm-3 was determined to be 3.82MΘ and 4.57 MΘ for strong and weak energy conditions respectively. PACS number: 0402, 0402J, 0440D,
95301
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
This paper presents an on-line trained neural net work controller for ship track-keeping problems. Following a brief review of the ship track-keeping control development since the 1980's, an analysis of various existing backpropagation-based neural controllers is provided. We then propose a single-input multioutput (SIMO) neural control strategy for situations where the exact mathematical dynamics of the ship are not available. The aim of this study is to build an autonomous neural controller which uses rudder to regulate both the tracking error and heading error. During the whole control process, the proposed SIMO neural controller adapts itself on-line from a direct evaluation of the control accuracy, and hence the need for a “teacher” or an off-line training process can be removed. With a relatively modest amount of quantitative knowledge of the ship behavior, the design philosophy enables real time control of a nonlinear ship model under random wind disturbances and measurement noise. Three different track-keeping tasks have been simulated to demonstrate the effectiveness of the training method and the robust performance of the proposed neural control strategy 相似文献
6.
The new calculations of the Oiv temperature-sensitive EUV line ratios are presented and compared with previous results. 相似文献
7.
宣森 《广东海洋大学学报》1993,(1)
将专家系统技术与常规控制、模糊控制、超前控制、仿人智能操作控制以及计算机控制技术相结合,设计了一种新的基于专家系统决策的多模式控制(MCDBES)。经试验和实际运行结果表明,MCDBES系统对未知数学模型的被控对象,具有良好的控制性能。本文提出的MCDBES已成功地应用于燃油陶瓷窑炉计算机控制系统。 相似文献
8.
In many astrophysical problems, the study of the stability of an atmosphere in the presence of a magnetic field is of importance. In most cases the MHD instabilities of atmospheres are studied by energy principle of Bernsteinet al. (1958). In this paper, a general method for studying the stability of a system subject to MHD equations of conditions has been proposed. This is based on the local potential concept put forward by Glansdorff and Prigogine (1964). The scheme for securing stability criteria has been demonstrated in two particular cases. 相似文献
9.
10.
The flow of laser radiation in a plane-parallel cylindrical slab of active amplifying medium with axial symmetry is treated as a problem in radiative transfer. The appropriate one-dimensional transfer equation describing the transfer of laser radiation has been derived by an appeal to Einstein'sA, B coefficients (describing the processes of stimulated line absorption, spontaneous line emission, and stimulated line emission sustained by population inversion in the medium) and considering the rate equations to completely establish the rational of the transfer equation obtained. The equation is then exactly solved and the angular distribution of the emergent laser beam intensity is obtained; its numerically computed values are given in tables and plotted in graphs showing the nature of peaks of the emerging laser beam intensity about the axis of the laser cylinder. 相似文献