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   3588篇
  免费   710篇
  国内免费   827篇
测绘学   212篇
大气科学   711篇
地球物理   600篇
地质学   1255篇
海洋学   919篇
天文学   588篇
综合类   250篇
自然地理   590篇
  2024年   19篇
  2023年   50篇
  2022年   124篇
  2021年   164篇
  2020年   143篇
  2019年   178篇
  2018年   136篇
  2017年   147篇
  2016年   135篇
  2015年   179篇
  2014年   187篇
  2013年   263篇
  2012年   176篇
  2011年   153篇
  2010年   159篇
  2009年   222篇
  2008年   214篇
  2007年   288篇
  2006年   267篇
  2005年   264篇
  2004年   238篇
  2003年   189篇
  2002年   210篇
  2001年   162篇
  2000年   158篇
  1999年   147篇
  1998年   123篇
  1997年   72篇
  1996年   63篇
  1995年   37篇
  1994年   52篇
  1993年   41篇
  1992年   38篇
  1991年   25篇
  1990年   28篇
  1989年   28篇
  1988年   17篇
  1987年   8篇
  1986年   5篇
  1985年   2篇
  1984年   4篇
  1983年   2篇
  1982年   2篇
  1981年   1篇
  1980年   1篇
  1979年   2篇
  1978年   1篇
  1977年   1篇
排序方式: 共有5125条查询结果,搜索用时 15 毫秒
61.
鮸鱼胚胎发育的研究   总被引:2,自引:0,他引:2  
研究了人工繁殖时鮸鱼(Miichthys miiuy)胚胎的发育过程。鮸鱼受精卵呈圆球形,浮性,卵径为0.99~1.15 mm,中央大多具有1个油球,少数为2~3个或较多个油球。在水温为24.5~24.7℃,盐度为24的天然海水中,鮸鱼胚胎发育历时约21h 48 min孵化出膜。整个胚胎发育分为5个阶段,共21个发育期。  相似文献   
62.
A novel method for prediction of the load carrying capacity of a corroded reinforced concrete beam (CRCB) is presented in the paper. Nine reinforced concrete beams, which had been working in an aggressive environment for more than 10 years, were tested in the laboratory. Comprehensive tests, including flexural test, strength test for corroded concrete and rusty rebar, and pullout test for bond strength between concrete and rebar, were condueted. The flexural test results of CRCBs reveal that the distribution of surface cracks on the beams shows a fractal behavior. The relationship between the fractal dimensions and mechanical properties of CRCBs is then studied. A prediction model based on artificial neural network (ANN) is established by the use of the fractal dimension as the corrosion index, together with the basic intbrmation of the beam. The validity of the prediction model is demonstrated through the experimental data, and satisfactory resuits are achieved.  相似文献   
63.
长竹蛏人工育苗技术的初步研究   总被引:4,自引:0,他引:4  
王云 《台湾海峡》2001,20(1):27-31
本文对长竹蛏繁殖习性、胚胎发育和人工育苗技术关键进行研究.结果表明亲贝经暂养可促进性成熟;幼虫对温、盐度的适应较广;变态附着采用缩光培育,未经缩光的附着时间延长,存活率降低;幼虫长至约240μm变态附着,附着量在(0.6~10)×106个/m  相似文献   
64.
基于EMD与神经网络的机械故障诊断技术   总被引:2,自引:0,他引:2  
经验模式分解 (EMD)是分析非线性、非平稳信号的有力工具 ,它将信号分解为突出了原信号的不同时间尺度的局部特征信息的内在模函数 (IMF)分量。本文通过将各 IMF分量输入到 BP网络中进行训练学习和故障诊断 ,比直接输入原信号可以提高 BP网络对故障诊断的准确率 ,而且减少了训练时间。  相似文献   
65.
Dynamics and Variability of Terra Nova Bay Polynya   总被引:1,自引:0,他引:1  
Abstract. We present a process study on the dynamics and variability of the Terra Nova Bay polynya in the western sector of the Ross Sea. The air-sea heat exchange is known to be particularly large in polynya during the winter, when differences between air and sea temperatures are large. We apply a 1-D model (Pease, 1987; Van Woert, 1999a, 1999b), which is modified in the latent heat parameterisation in order to account for time-dependent relative humidity and cloud coverage. Furthermore, the Ice Collection Depth is correlated linearly with a variable wind speed. The model is forced with two different meteorological data sets: the operational analysis of the European Center for Medium Range Weather Forecasts atmospheric data set and the meteorological parameters measured by an Automatic Weather Station located on the coast of Terra Nova Bay. The results are compared in terms of polynya extension, ice, and High Salinity Shelf Water production. According to the two different wind velocities, the results obtained from the different data sets clearly differ. Qualitatively, however, the results are in good agreement.  相似文献   
66.
从1989年4月~1990年10月,我科在69例病人中使用CHIMEHERB人工皮,覆盖治疗了供皮区,Ⅱ°烧伤创面、脱痂创面的残余肉芽创面.临床应用表明:CHIMEHERB人工皮能加快创面的愈合,是目前较好的一种创面覆盖物,用人工皮覆盖创面2~3天后,人工皮干燥,不需再换药,可减轻病人的痛苦.  相似文献   
67.
Storms and shoreline retreat in the southern Gulf of St. Lawrence   总被引:1,自引:0,他引:1  
Storms play a major role in shoreline recession on transgressive coasts. In the southern Gulf of St. Lawrence (GSL), southeastern Canada, long-term relative sea-level rise off the North Shore of Prince Edward Island has averaged 0.3 m/century over the past 6000 years (>0.2 m/century over 2000 years). This has driven long-term coastal retreat at mean rates >0.5 m/a but the variance and details of coastal profile response remain poorly understood. Despite extensive sandy shores, sediment supply is limited and sand is transferred landward into multidecadal to century-scale storage in coastal dunes, barrier washover deposits, and flood-tidal delta sinks. Charlottetown tide-gauge records show mean relative sea-level rise of 3.2 mm/a (0.32 m/century) since 1911. A further rise of 0.7±0.4 m is projected over the next 100 years. When differenced from tidal predictions, the water-level data provide a 90-year record of storm-surge occurrence. Combined with wind, wave hindcast, and sea-ice data, this provides a catalogue of potentially significant coastal storms. We also document coastal impacts from three recent storms of great severity in January and October 2000 and November 2001. Digital photogrammetry (1935–1990) and shore-zone surveys (1989–2001) show large spatial and temporal variance in coastal recession rates, weakly correlated with the storm record, in part because of wave suppression or coastal protection by sea ice. Large storms cause rapid erosion from which recovery depends in part on local sand supply, but barrier volume may be conserved by washover deposition. Barrier shores with dunes show high longshore and interdecadal variance, with extensive multidecadal healing of former inlet and overwash gaps. This reflects recovery from an episode of widespread overwash prior to 1935, possibly initiated by intense storms or groups of storms in the latter half of the 19th century. With evidence from the storms of 2000–2001, this points to the importance of storm clustering on scales of weeks to years in determining erosion vulnerability, as well as the need for a long-term, large-scale perspective in assessing coastal stability. The expected acceleration in relative sea-level rise, together with projections of increasing storm intensity and greatly diminished winter ice cover in the southern GSL, implies a significant increase in coastal erosion hazards in future.  相似文献   
68.
69.
海中悬移质是决定海洋光学性质、海洋水质,河口海岸带演变动力过程的重要环境参数。本文利用模拟遥感反射比数据集建立人工神经网络反演悬移质浓度,并利用东中国海现场同步数据对该算法进行验证。  相似文献   
70.
On the Polyps of the Common Jellyfish Aurelia aurita in Kagoshima Bay   总被引:1,自引:0,他引:1  
There is the natural habitat of polyps of the common jellyfish Aurelia aurita in the Taniyama area, Kagoshima Bay. We examined the attachment substrata, density, colony structure and strobilation of the polyps. The polyps were observed only on the horizontal undersurface of floating piers. They attached specifically to Mytilus shells, solitary ascidians, calcareous polychaete tubes, muddy amphipod tubes and the gap space that fouling animals peeled off the substrata. The polyp colonies were distributed in patches. Spatial distribution patterns of the polyps within their colonies were uniform. Strobilation occurred during late December to March, when water temperatures were 16–17°C, and a large number of ephyrae were released. An increase in man-made structures such as floating piers in coastal areas may lead to bloomings of Aurelia aurita medusae. This revised version was published online in July 2006 with corrections to the Cover Date.  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