全文获取类型
收费全文 | 25951篇 |
免费 | 5772篇 |
国内免费 | 8398篇 |
专业分类
测绘学 | 4140篇 |
大气科学 | 4102篇 |
地球物理 | 4929篇 |
地质学 | 15402篇 |
海洋学 | 4756篇 |
天文学 | 562篇 |
综合类 | 2545篇 |
自然地理 | 3685篇 |
出版年
2024年 | 196篇 |
2023年 | 578篇 |
2022年 | 1525篇 |
2021年 | 1823篇 |
2020年 | 1503篇 |
2019年 | 1718篇 |
2018年 | 1716篇 |
2017年 | 1510篇 |
2016年 | 1620篇 |
2015年 | 1805篇 |
2014年 | 1831篇 |
2013年 | 2144篇 |
2012年 | 2305篇 |
2011年 | 2289篇 |
2010年 | 2259篇 |
2009年 | 2028篇 |
2008年 | 2015篇 |
2007年 | 1792篇 |
2006年 | 1772篇 |
2005年 | 1381篇 |
2004年 | 1050篇 |
2003年 | 761篇 |
2002年 | 829篇 |
2001年 | 795篇 |
2000年 | 625篇 |
1999年 | 462篇 |
1998年 | 263篇 |
1997年 | 242篇 |
1996年 | 207篇 |
1995年 | 162篇 |
1994年 | 186篇 |
1993年 | 127篇 |
1992年 | 113篇 |
1991年 | 76篇 |
1990年 | 59篇 |
1989年 | 44篇 |
1988年 | 47篇 |
1987年 | 46篇 |
1986年 | 21篇 |
1985年 | 31篇 |
1984年 | 17篇 |
1983年 | 22篇 |
1982年 | 16篇 |
1981年 | 10篇 |
1980年 | 8篇 |
1979年 | 22篇 |
1976年 | 6篇 |
1958年 | 10篇 |
1957年 | 11篇 |
1954年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
121.
Marine biological choruses observed in Xiamen harbour from August 1981 to October 1982 lasted for a few hours each time during the neap tide in spring, summer and autumn, in both the evening and the morning, with molt energy at frequencies between 700 Hz and 1.6 kHz and spectral peaks mainly at 800 Hz, 1 kHz and 1.25 kHz. During choruses, noise spectrum levels rose up to 46 dB higher than the background noise in the sea Mate 0. The statistics of occurrence of the evening choruses indicate that they might be fairly widespread in and near shallow waters of the region. Sources of such choruses are likely to be attributed to Johnnius belengerii and Collichthys lucidut of the family Sciaenidae. 相似文献
122.
Monthly and seasonal panel experiments were carried out in the Yongxing Island from September 1979 to August 1981. Altogether 119 species of fouling organisms are collected, of which, Hydroides elegans, Serpula vermicular is, Scrupocellaria longispinosa, Amathia distant, Enteromorpha spp. and Ostrea spp. are dominant species. Fouling of organisms may occur all the year round with evident seasonal variations, Species succession in the process of community formation is very evident and the climax has arrived after two years. Great variation exists between fouling organisms in different years and at different stations. 相似文献
123.
Using the National Center for Enviromental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and NOAA satellite-obser ved outgoing long-wave radiation (OLR) data, the development of the South China Sea (SCS) summer monsoon and intraseasonal (30-60 d) oscillation (ISO) have been examined. The results show that there exists obvious interannual variability of intraseasonal oscillaiton. Using the 16 a time series of filtered OLR averaged over the SCS, an index is defined to define ““onset events““ over the SCS on the ISO time scales. Of the 16 a examined here, 10 shows a strong ISO signal in the onset of monsoon convection over the SCS. In these cases, the ISO initially suppresses the seasonal development of southwesterly and cyclonic circulation over the SCS before the ISO onset. As the ISO propagates northeastward, the low frequency cyclonic circulation anomaly occurs in the SCS and the low frequency southwesterly wind and convection over here dramatically intensify. The northeast progression of the ISO anomaleis plays a role in the initial suppression and then acceleration of the seasonal cycle of the SCS summer monsoon. 相似文献
124.
125.
对1999年9~10月采自北太平洋亚热带环流区的19份表层海水样品的Ra同位素分析表明。研究海域表层水中的^226Ra、^228Ra放射性比度分别介于0.67~0.92、0.08~0.30Bq/m^3之间,平均值分别为0.74、0.11Bq/m^3.^226Ra/^228Ra)A.R.活度比的变化范围为0.11~0.44,平均值为0.19.上述数值明显低于近岸海域水体的相应值,表现为典型的开阔大洋水的特征.从空间分布的特征看,研究海域Ra同位素含量与^226Ra/^228Ra)A.R.值均呈均匀分布态势.将本研究结果与历史数据进行对比后发现,本研究获得的^226Ra、^228Ra放射性比度比20世纪60~80年代得到的数据来得低,可能与水体层化作用加强导致的Ra补充量的减少以及生物生产力升高导致的Ra迁出量的增加有关.北太平洋亚热带环流区表层水中Ra同位素的时间变化与文献报道的该海域叶绿素a、硅酸盐、磷酸盐含量与初级生产力的历史变化趋势相吻合. 相似文献
126.
127.
The effects of different lateral confinement stress on the fatigue behavior of and cumulative damage to plain concrete are investigated experimentally. Eighty 100 mm x 100 mm x 100 mm specimens of ordinary strength concrete are tested under constant- or variable-amplitude fatigue loading and lateral confinement pressure in two orthogonal directions. A fatigue equation is developed by modifying the classical Aas-Jakobsen S-N equation for taking into account the effect of the confined stress on fatigue strength of plain concrete. The results of variable-amplitude fatigue tests indicate that the linear damage theory proposed by Palmgren and Miner is unreasonable in the biaxial stress state. A nonlinear cumulative damage model that could model the effects of the magnitude and sequence of variable-amplitude fatigue loading and lateral confinement pressure is proposed on the basis of the evolution laws of the residual strains in the longitudinal direction during fatigue tests. The residual fatigue life predic 相似文献
128.
The effect of the cone spacing of a conieal structure on the ice force is studied by model experiments. The ice force reduction coefficient presented in this paper expresses the relationship between the ice force and the arrangement of cones. The experiments prove that the mode of the ice failure before the boundary of upward-downward bending cone (UDBC) is crushing. A conclusion can also be drawn from the experiments that the ice force on the boundary of UDBC is by far less than that on a vertical pile with the same diameter. Moreover, the ice force frequencies on upright and inve-rted cones are obtained under the field condition of the platform JZ20-2, respectively. The results show that the alterna-tion of the ice force on UDBC can hardly induce resonance of platform JZ20-2. 相似文献
129.
Using meteorological data of field observation in 1990 - 2000, especially polar orbit high-resolution NOAA satellite cloud maps received from the Antarctic expedition vessel since 1997, the formation and development of the Prydz Bay cyclone are studied in this paper. Some new viewpoints are suggested such as: when surround-polar cyclone enters the Prydz Bay, it can also intensify and develop in summer; cyclone can also develop in the easterlies in this bay. These view points revise old uncom-plete view point that the Prydz Bay is a burial ground of cyclone, and also further consummate formation-development theory of surround-cyclone in the Antarctic westerlies and cyclone in the Antarctic easterlies. In this paper, the mechanism of ice-air-sea interaction in the Prydz Bay is studied, and the physical process of cyclone formation-development is explained. By use of wholly dynamic transportation method, an energy exchange case of a cyclone, which explosively developed after entering the Prydz Bay, is calcu 相似文献
130.