首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8708篇
  免费   1265篇
  国内免费   1654篇
测绘学   349篇
大气科学   1627篇
地球物理   2173篇
地质学   3899篇
海洋学   1131篇
天文学   680篇
综合类   841篇
自然地理   927篇
  2024年   24篇
  2023年   127篇
  2022年   329篇
  2021年   353篇
  2020年   335篇
  2019年   305篇
  2018年   420篇
  2017年   404篇
  2016年   503篇
  2015年   354篇
  2014年   453篇
  2013年   471篇
  2012年   419篇
  2011年   469篇
  2010年   499篇
  2009年   509篇
  2008年   412篇
  2007年   412篇
  2006年   362篇
  2005年   285篇
  2004年   272篇
  2003年   301篇
  2002年   259篇
  2001年   265篇
  2000年   266篇
  1999年   359篇
  1998年   346篇
  1997年   323篇
  1996年   279篇
  1995年   270篇
  1994年   254篇
  1993年   197篇
  1992年   179篇
  1991年   105篇
  1990年   101篇
  1989年   79篇
  1988年   62篇
  1987年   44篇
  1986年   29篇
  1985年   30篇
  1984年   35篇
  1983年   20篇
  1982年   24篇
  1981年   17篇
  1980年   15篇
  1979年   15篇
  1978年   5篇
  1977年   4篇
  1976年   4篇
  1958年   4篇
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
101.
Recently, Zhang slightly modified the standard big bang theory and developed a new cosmological model called black hole universe, which is consistent with Mach’s principle, governed by Einstein’s general theory of relativity, and able to explain all observations of the universe. Previous studies accounted for the origin, structure, evolution, expansion, and cosmic microwave background radiation of the black hole universe, which grew from a star-like black hole with several solar masses through a supermassive black hole with billions of solar masses to the present state with hundred billion-trillions of solar masses by accreting ambient matter and merging with other black holes. This paper investigates acceleration of the black hole universe and provides an alternative explanation for the redshift and luminosity distance measurements of type Ia supernovae. The results indicate that the black hole universe accelerates its expansion when it accretes the ambient matter in an increasing rate. In other words, i.e., when the second-order derivative of the mass of the black hole universe with respect to the time is positive $\ddot{M}(t) > 0$ . For a constant deceleration parameter $q = -M(t) \ddot{M}(t)/\dot{M}(t) \sim-0.6$ , we can perfectly explain the type Ia supernova measurements with the reduced chi-square to be very close to unity, χ red~1.0012. The expansion and acceleration of black hole universe are driven by external energy.  相似文献   
102.
The Plastic Scintillator Detector(PSD) onboard the DArk Matter Particle Explorer(DAMPE)is designed to measure cosmic ray charge(Z) and to act as a veto detector for gamma ray identification.To fully exploit the charge identification potential of PSD and to enhance its capability to identify gamma ray events, we develop an alignment method for the PSD. The path length of a given track in the volume of a PSD bar is derived taking into account the shift and rotation alignment corrections. By examining energy spectra of corner-passing events and fully contained events, position shifts and rotations of all PSD bars are obtained, and are found to be on average about 1 mm and 0.0015 radian respectively. To validate the alignment method, we introduce artificial shifts and rotations of PSD bars into the detector simulation.These shift and rotation parameters can be recovered successfully by the alignment procedure. As a result of the PSD alignment procedure, the charge resolution of the PSD is improved from 4% to 8%, depending on the nuclei.  相似文献   
103.
Through the water areas extracted from remote sensing images and the combination of the methods for establishing the formula for calculating tidal influx with tidal data, the tidal influx of the Haikou Bay, Hainan Province was found to be 5.14×107m3 in 1990, 5.80×107m3 in 1984 and 5.05×107m3 in 1965, respectively.After the analysis of the morphological and tidal range factors which determine tidal influx, this paper presents the trend of the changes in tidal influx caused by the changes in the morphological factors of the Haikou Bay.It is found that a decreasing trend was shown with a depressive rate of 2×10-3during the period from 1965 to 1984, and an increasing trend with an incremental rate of 1×10-3 during the period of 1984-1990.The main reason for the appearance of the decreasing trend before 1984 is the natural deposition and silting-up of the bay sediments; after 1984, the dredging and expansion of the Haikou Port and the Haikou New Port which caused an increase in water area at the mean low tide are the leading factor which causes the increase in tidal influx.  相似文献   
104.
Field observations of suspended particulate matter (SPM) in the Bohai Bay, China have not been widely reported. The aim of this paper is to describe the horizontal and vertical distribution of mass and volume concentrations of SPM, respectively, based on observed data at 312 stations in the northern Bohai Bay during summer of 2006. A numerical model ECOMSED coupled with a sediment transport module was also established to further discuss the mechanism of the thermocline effect on the vertical distribution of SPM. The mass concentrations of SPM exhibited high inshore values and low offshore values in the horizontal distribution; while in the vertical direction, characteristics of the volume concentration of SPM can be divided into two types: one with a sharp peak at depth of 10–15 m and another without. The peak value at the depth of the thermocline was resulted from concentrated phytoplankton. A numerical experiment further displayed that the thermocline can also prevent particles from being resuspended upward.  相似文献   
105.
Based on the equations of the self-generated magnetic field in the q-distributed plasmas, the studies show that the magnetic field is modulationally unstable by the perturbation method and the equations have self-similar collapse solution. The anomalous magnetic viscosity of accretion disks generates from highly spatially intermittent flux of the self-generated magnetic field. In addition, the anomalous viscosity coefficient is 8 orders more than the molecular viscosity and is modified by the adjustable index q, which may preferably explain the observations.  相似文献   
106.
We describe how a local plasma structure can be changed by a transverse shear flow using numerical simulation to investigate the disturbance process near the magnetopause. The results show that magnetic field lines are bent by transverse shear flow disturbance near the current sheet region. There are multiple bipolar structures of the normal magnetic field in the numerical simulation. We term this new feature as K-point magnetic reconnection, realistic for discussing space observations.  相似文献   
107.
108.
Comparison of quasar (QSO) absorption-line spectra with laboratory spectra provides a precise probe for variability of the fine-structure constant, α , over cosmological time-scales. We constrain variation in α in 21 Keck/HIRES Si  iv absorption systems using the alkali-doublet (AD) method in which changes in α are related to changes in the doublet spacing. The precision obtained with the AD method has been increased by a factor of 3:     . We also analyse potential systematic errors in this result. Finally, we compare the AD method with the many-multiplet method, which has achieved an order of magnitude greater precision, and we discuss the future of the AD method.  相似文献   
109.
110.
When μ is smaller than Routh’s critical value μ 1 = 0.03852 . . . , two planar Lyapunov families around triangular libration points exist, with the names of long and short period families. There are periodic families which we call bridges connecting these two Lyapunov families. With μ increasing from 0 to 1, how these bridges evolve was studied. The interval (0,1) was divided into six subintervals (0, μ 5), (μ 5μ 4), (μ 4μ 3), (μ 3μ 2), (μ 2μ 1), (μ 1, 1), and in each subinterval the families B(pL, qS) were studied, along with the families B(qS, qS′). Especially in the interval (μ 2μ 1), the conclusion that the bridges B(qS, qS′) do not exist was obtained. Connections between the short period family and the bridges B(kS, (k + 1)S) were also studied. With these studies, the structure of the web of periodic families around triangular libration points was enriched.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号