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   49148篇
  免费   8284篇
  国内免费   11371篇
测绘学   4200篇
大气科学   5107篇
地球物理   9718篇
地质学   31167篇
海洋学   5740篇
天文学   2164篇
综合类   3360篇
自然地理   7347篇
  2024年   264篇
  2023年   723篇
  2022年   1414篇
  2021年   1599篇
  2020年   1664篇
  2019年   1915篇
  2018年   1550篇
  2017年   1796篇
  2016年   2004篇
  2015年   2120篇
  2014年   2583篇
  2013年   2497篇
  2012年   2890篇
  2011年   3044篇
  2010年   2631篇
  2009年   3155篇
  2008年   3102篇
  2007年   3454篇
  2006年   3359篇
  2005年   2928篇
  2004年   2746篇
  2003年   2632篇
  2002年   2412篇
  2001年   2075篇
  2000年   1959篇
  1999年   1778篇
  1998年   1555篇
  1997年   1429篇
  1996年   1288篇
  1995年   1081篇
  1994年   1114篇
  1993年   943篇
  1992年   746篇
  1991年   529篇
  1990年   477篇
  1989年   375篇
  1988年   297篇
  1987年   191篇
  1986年   128篇
  1985年   85篇
  1984年   41篇
  1983年   29篇
  1982年   35篇
  1981年   23篇
  1980年   23篇
  1979年   28篇
  1978年   35篇
  1977年   25篇
  1975年   4篇
  1954年   14篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The effects of natural fish oil,DHA oil and soybean lecithin in microparticulate diets on stress tolerance of larval gilthead seabream(Sparus aurata)were investigated after 15 days feeding trials.The tolerance of larval gilthead seabream to various stress factors such as exposure to air(lack of dissolved oxygen),changes in water temperature(low)and salinity(high) were determined.This study showed that microparticulate diet with natural fish oil and soybean lecithin was the most effective for in-creasing the tolerance of larval gilthead seabream to various stresses,and that microparticulate diet with natural fish oil and palmitic acid(16:0)was more effective than microparticulate diet with DHA oil and soybean lecithin.  相似文献   
2.
It is shown in this paper how to build a canonical transformation of variables, so that the eccentric anomaly becomes the new independent variable. In the case of eccentric elliptical orbits it changes the equations of motion so, that they can be integrated analytically to any order of approximation comparatively easy.  相似文献   
3.
4.
5.
6.
中国古生教授学会微体古生物学分会举行代表大会和学术年会中国古竽物学会微体古物学分会第五次会员代表大分暨第六次不术年会于1996年1月24日到30日在福州市召开。来自全国地质、石油、煤炭的和产、科研和大学的100多位代表参加了这次大会和学术研讨。大会共...  相似文献   
7.
Monocrystalline quartz inclusions in garnet and omphacite from various eclogite samples from the Lanterman Range (Northern Victoria Land, Antarctica) have been investigated by cathodoluminescence (CL), Raman spectroscopy and imaging, and in situ X‐ray (XR) microdiffraction using the synchrotron. A few inclusions, with a clear‐to‐opalescent lustre, show ‘anomalous’ Raman spectra characterized by weak α‐quartz modes, the broadening of the main α‐quartz peak at 465 cm?1, and additional vibrations at 480–485, 520–523 and 608 cm?1. CL and Raman imaging indicate that this ‘anomalous’α‐quartz occurs as relicts within ordinary α‐quartz, and that it was preserved in the internal parts of small quartz inclusions. XR diffraction circular patterns display irregular and broad α‐quartz spots, some of which show an anomalous d‐spacing tightening of ~2%. They also show some very weak, hazy clouds that have d‐spacing compatible with coesite but not with α‐quartz. Raman spectrometry and XR microdiffraction characterize the anomalies with respect to α‐quartz as (i) a pressure‐induced disordering and incipient amorphization, mainly revealed by the 480–485 and 608‐cm?1 Raman bands, together with (ii) a lattice densification, evidenced by d‐spacing tightening; (iii) the cryptic development of coesite, 520–523 cm?1 being the main Raman peak of coesite and (iv) Brazil micro‐twinning. This ‘anomalous’α‐quartz represents the first example of pressure‐induced incipient amorphization of a metastable phase in a crustal rock. This issue is really surprising because pressure‐induced amorphization of metastable α‐quartz, observed in impactites and known to occur between 15 and 32 GPa during ultrahigh‐pressure (UHP) experiments at room temperature, is in principle irrelevant under normal geological P–T conditions. A shock (due to a seism?) or a local overpressure at the inclusion scale (due to expansion mismatch between quartz and its host mineral) seem the only geological mechanisms that can produce such incipient amorphization in crustal rocks. This discovery throws new light on the modality of the quartz‐coesite transition and on the pressure regimes (non‐lithostatic v. lithostatic) during high‐pressure/UHP metamorphism. In particular, incipient amorphization of quartz could favour the quartz‐coesite transition, or allow the growth of metastable coesite, as already experimentally observed.  相似文献   
8.
根据印度洋西北部鸢乌贼(Sym plectoteuthis oualaniensis)作业渔场的现场调查数据.分析了作业水深、作业时段、摄食等级和月相等因子对手钓上钩率的影响。统计表明.晚上、午夜和凌晨3个不同时段和不同作业水深对手钓上钩率均有显著性影响。3个时段中,其摄食等级呈先低后高再低的现象。午夜和凌展,上钩率的变化趋势与摄食等级的变化趋势呈显著的负相关。在月相的影响下.离朔日三四天之前,手钓产量呈上升趋势;离望日五六天之前。手钓产量呈明显的下降趋势。  相似文献   
9.
Stress wave attenuation across fractured rock masses is a great concern of underground structure safety. When the wave amplitude is large, fractures experience nonlinear deformation during the wave propagation. This paper presents a study on normal transmission of P‐wave across parallel fractures with nonlinear deformational behaviour (static Barton–Bandis model). The results show that the magnitude of transmission coefficient is a function of incident wave amplitude, nondimensional fracture spacing and number of fractures. Two important indices of nondimensional fracture spacing are identified, and they divide the area of nondimensional fracture spacing into three parts (individual fracture area, transition area and small spacing area). In the different areas, the magnitude of transmission coefficient has different trends with nondimensional fracture spacing and number of fractures. In addition, the study reveals that under some circumstances, the magnitude of transmission coefficient increases with increasing number of fractures, and is larger than 1. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
10.
Hamiltonian mechanics is applied to the problem of the rotation of the elastic Earth. We first show the process for the formulation of the Hamiltonian for rotation of a deformable body and the derivation of the equations of motion from it. Then, based on a simple model of deformation, the solution is given for the period of Euler motion, UT1 and the nutation of the elastic Earth. In particular it is shown that the elasticity of the Earth acts on the nutation so as to decrease the Oppolzer terms of the nutation of the rigid Earth by about 30 per cent. The solution is in good agreement with results which have been obtained by other, different approaches.  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