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   33209篇
  免费   1122篇
  国内免费   440篇
测绘学   963篇
大气科学   2767篇
地球物理   7968篇
地质学   12056篇
海洋学   2569篇
天文学   6110篇
综合类   228篇
自然地理   2110篇
  2020年   181篇
  2019年   192篇
  2018年   1035篇
  2017年   940篇
  2016年   798篇
  2015年   553篇
  2014年   624篇
  2013年   1430篇
  2012年   1266篇
  2011年   1313篇
  2010年   840篇
  2009年   1147篇
  2008年   1045篇
  2007年   997篇
  2006年   1025篇
  2005年   1771篇
  2004年   1987篇
  2003年   1568篇
  2002年   921篇
  2001年   687篇
  2000年   677篇
  1999年   608篇
  1998年   574篇
  1997年   591篇
  1996年   490篇
  1995年   473篇
  1994年   452篇
  1993年   417篇
  1992年   388篇
  1991年   347篇
  1990年   400篇
  1989年   311篇
  1988年   349篇
  1987年   385篇
  1986年   330篇
  1985年   495篇
  1984年   536篇
  1983年   540篇
  1982年   434篇
  1981年   427篇
  1980年   429篇
  1979年   376篇
  1978年   396篇
  1977年   346篇
  1976年   367篇
  1975年   340篇
  1974年   378篇
  1973年   368篇
  1972年   231篇
  1971年   183篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
This paper demonstrates the utility, in an applied context, of a standard Teitz and Bart heuristic for solving location-allocation problems. The example discussed is the location of veterinary clinics in Wellington, New Zealand. A distinct ‘service-poor’ area is identified in southern Wellington suggesting that the existing system of clinics, especially the largest multi-veterinary practice, is vulnerable to intrusion by competitors. A variety of strategies, involving new clinics and clinic relocation are analysed to ascertain alternative courses of action for both existing clinic operators and competitors. The appropriateness of this type of analysis as an aid to decision-making is reflected in the locational response of one practice to protect its share of the market for veterinary services.  相似文献   
102.
103.
104.
105.
106.
We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r 2500, where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe,     , and measure a positive cosmological constant,     . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies.  相似文献   
107.
Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope.  相似文献   
108.
Samples of lower Palaeozoic bedrock from the Grand Banks of Newfoundland were examined using reflected light microscopy and Rock-Eval pyrolysis. These samples contained organic material which included bitumen (structureless organic material), acritarchs, chitinozoa, scolecodonts and graptolites. The reflectance of the organic material increased from acritarchs towards graptolites with bitumen showing a wide range of reflectance. Multiple phases of oil migration through one sample were inferred by examination of the bitumens: the lowest reflecting being more recent than the highest reflecting bitumen. Combined reflectance and Rock-Eval analysis indicate that these samples are mature to overmature. The two oldest samples (Arenig-Llanvirn, Ordovician) are assessed as mature and as having potential for generation of liquid hydrocarbons, but must be considered as mainly gas prone.  相似文献   
109.
Three different methods are presented to subtract thermal drifts and low-frequency noise from the signal of infrared array. The first is dead pixels with open Indium bumps, the second is reference output as implemented on the Hawaii2 multiplexer, and the third is dark pixels to emulate reference cells having a capacity connected to the gate of the unit cell field-effect transistor (FET). The third method is the most effective and yields a reduction in readout noise from15.4–9.4 erms. A novel method will be described to extend this readout technique to the Aladdin 1 K × 1 K InSb array. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
110.
Expressions for the mass quadrupole moment tensor in the model for a wide variety of astrophysical objects are shown to be identical in form. This makes it possible to obtain analytical expressions for the gravitational radiation emitted by the sources, as well as the angular distribution, polarization dependence, and the wave forms of the radiation.  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