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   59975篇
  免费   630篇
  国内免费   440篇
测绘学   1800篇
大气科学   4637篇
地球物理   11447篇
地质学   20350篇
海洋学   4930篇
天文学   14181篇
综合类   202篇
自然地理   3498篇
  2021年   456篇
  2020年   506篇
  2019年   546篇
  2018年   1214篇
  2017年   1130篇
  2016年   1513篇
  2015年   942篇
  2014年   1540篇
  2013年   3060篇
  2012年   1536篇
  2011年   2084篇
  2010年   1828篇
  2009年   2441篇
  2008年   2171篇
  2007年   2097篇
  2006年   2057篇
  2005年   1810篇
  2004年   1718篇
  2003年   1656篇
  2002年   1602篇
  2001年   1466篇
  2000年   1387篇
  1999年   1287篇
  1998年   1242篇
  1997年   1210篇
  1996年   1063篇
  1995年   982篇
  1994年   930篇
  1993年   858篇
  1992年   788篇
  1991年   786篇
  1990年   856篇
  1989年   767篇
  1988年   727篇
  1987年   849篇
  1986年   733篇
  1985年   942篇
  1984年   1036篇
  1983年   972篇
  1982年   967篇
  1981年   876篇
  1980年   810篇
  1979年   764篇
  1978年   749篇
  1977年   695篇
  1976年   638篇
  1975年   577篇
  1974年   644篇
  1973年   661篇
  1972年   413篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
221.
A finite element model for the computation of thermally stratified flows is described. The model includes a special treatment of the pressure gradient term by splitting it into hydrostatic and hydrodynamic contributions. Four-node elements are used for space-discretization and a semi-implicit scheme for time integration. The computed flow is used to simulate pollutant transport by a finite element model. Numerical results from such simulations, in particular for urban heat islands are described and compared with experimental data.  相似文献   
222.
Revegetation of disturbed land, particularly in arid environments, is often hindered by low seedling establishment. Information on seed biology and germination cues of keystone species is lacking, particularly in arid Australia; a major zone for mining developments. This study investigated seed characteristics and germination of 18 common species required for rehabilitation of disturbed areas at Shark Bay Salt in the Shark Bay World Heritage Area, Western Australia. Untreated seeds of seven species (Aphanopetalum clematideum, Atriplex bunburyana, Austrostipa elegantissima, Melaleuca cardiophylla, Pembertonia latisquamea, Rhagodia baccata, Salsola tragus) exhibited high germination percentages. Seeds of two species (Acacia tetragonophylla, Stylobasium spathulatum) had low imbibition, which increased with hot-water treatment, hence require scarification for germination. Gibberellic acid, smoke water and karrikinolide (a butenolide isolated from smoke) substantially increased germination percentages of three species (Anthocercis littorea, Diplolaena grandiflora, Solanum orbiculatum). Seeds of the remaining six species (Dioscorea hastifolia, Eremophila oldfieldii, Nitraria billardierei, Ptilotus exaltatus, Thryptomene baeckeacea, Zygophyllum fruticulosum) had low germination percentages regardless of treatment. Most species germinated equally well at 26/13 °C and 33/18 °C, however seven species had improved performance at 26/13 °C. This study is significant to land managers and conservation agencies with an interest in optimising germination of arid zone seeds for restoration.  相似文献   
223.

The results of multicolor photometric and polarization observations of the blazar 4C 38.41 (Q 1633+382) performed at St. Petersburg State University, the Pulkovo Astronomical Observatory of the Russian Academy of Sciences, Boston University, and Steward Observatory in 2006–2017 are analyzed. Separate variable sources responsible for the observed activity are distinguished, whose power-lawspectra and high degree of polarization confirm their synchrotron nature. The observed color variability of the object (redder when brighter) can be explained by an increase in the contribution to the total emission of a red component with variable flux and a constant relative spectral energy distribution. A close correlation between the optical and gamma-ray light curves is identified, suggesting the same location of the variable sources responsible for the radiation in these bands.

  相似文献   
224.
225.
Regions of possible motions are established for dynamical systems possessing time-independent Hamiltonians or for systems which are reducible to that form by means of integrals of the motion using only extended point transformations. The method is applied to the problem of three bodies in a plane and surfaces of zero velocity are found. The governing parameters are the energy, angular momentum and the masses of the participating bodies. The analytical and geometrical properties of these surfaces provide qualitative results for given constants of the motion.  相似文献   
226.
In this paper we report on the MEM power spectrum analysis of brightness temperature fluctuations observed at 2.8 GHz during the total solar eclipse of 16 February 1980. The observed periodicities range from 3.5 min to 64 min. These periodicities may arise due to spatial and/or temporal variations in the solar radio emission. The observed periodicities imply presence of scale sizes ranging from 70,000 to 600,000 km assuming that the brightness fluctuations arise because of spatial variation only. On the other hand, if these fluctuations are due to temporal variation, the observed periodicities correspond well to predicted modes of solar global oscillations.  相似文献   
227.
228.
229.
-The hydrodynamic coefficients C_d and C_m are not only dependent on the size of slender cylin-der,its location in water,KC number and Re number,but also vary with environmental conditions,i.e.,in regular waves or in irregular waves,in pure waves or in wave-current coexisting field.In this paper,thenormalization of hydrodynamic coefficients for various environmental conditions is discussed.When aproper definition of KC number and proper characteristic values of irregular waves are used,a unified re-lationship between C_d,C_m and KC number for regular waves,irregular waves,pure waves and wave-cur-rent coexisting field can be obtained.  相似文献   
230.
Glutathione S-transferase activity was measured in the hepatic cytosol front Dissostichus mawsoni and Pagothenia borchgrevinki. Activity measures with 1-chloro-2,4-dinitrobenzene as substrate were 11·2 and 16·7 μmol/min/g tissue respectively. Little or no activity was detected when p-nitrobenzyl chloride or 3,4-dichloro-1-nitrobenzene were used as substrate. The hepatic glutathione S-transferases from D. mawsoni were partially purified using gel filtration and chromatofocusing. Three peaks of activity were resolved. The major isozyme (158-fold purification) eluting at pH7·1 appeared to be catalytically a homodimer. The isozyme was highly inhibited by triphenyltin chloride (IC50 = 0·1 μ) while inhibition constants for Cibicron Blue 3GA, bromosulphophalein and hematin were 1·1, 20 and 34 μ respectively.  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