全文获取类型
收费全文 | 21545篇 |
免费 | 4637篇 |
国内免费 | 6434篇 |
专业分类
测绘学 | 2656篇 |
大气科学 | 3485篇 |
地球物理 | 4767篇 |
地质学 | 12685篇 |
海洋学 | 3098篇 |
天文学 | 738篇 |
综合类 | 2289篇 |
自然地理 | 2898篇 |
出版年
2024年 | 151篇 |
2023年 | 463篇 |
2022年 | 1316篇 |
2021年 | 1502篇 |
2020年 | 1213篇 |
2019年 | 1359篇 |
2018年 | 1559篇 |
2017年 | 1305篇 |
2016年 | 1460篇 |
2015年 | 1498篇 |
2014年 | 1597篇 |
2013年 | 1692篇 |
2012年 | 1779篇 |
2011年 | 1775篇 |
2010年 | 1708篇 |
2009年 | 1518篇 |
2008年 | 1355篇 |
2007年 | 1237篇 |
2006年 | 1105篇 |
2005年 | 884篇 |
2004年 | 640篇 |
2003年 | 576篇 |
2002年 | 568篇 |
2001年 | 552篇 |
2000年 | 515篇 |
1999年 | 553篇 |
1998年 | 389篇 |
1997年 | 369篇 |
1996年 | 310篇 |
1995年 | 280篇 |
1994年 | 276篇 |
1993年 | 225篇 |
1992年 | 171篇 |
1991年 | 131篇 |
1990年 | 103篇 |
1989年 | 92篇 |
1988年 | 77篇 |
1987年 | 61篇 |
1986年 | 49篇 |
1985年 | 51篇 |
1984年 | 20篇 |
1983年 | 22篇 |
1982年 | 23篇 |
1981年 | 11篇 |
1980年 | 13篇 |
1979年 | 17篇 |
1978年 | 5篇 |
1958年 | 15篇 |
1957年 | 8篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
The colony-forming Phaeocystis species are causative agents of dense bloom occurrences in coastal waters worldwide. It is difficult to separate them because of the different morphologies associated with their colonial stages. In this study we applied molecular approaches to analyze the genetic variation of Phaeocystis globosa and Phaeocystis pouchetii from several geographic regions, and to assist in tracing the dispersal of bloom-forming Phaeocystis species in coastal waters of China. The sequences of the internal transcribed spacers (ITS1 and ITS2) of rDNA and the 5.8S ribosomal RNA gene of Phaeocystis strains were determined. Sequence comparison shows that P.globosa was the most divergent to P. pouchetii, exhibiting sequence divergence higher than 0.08. However, lower genetic divergences existed between strains of P.globosa. The sequence comparison of the Phaeocystis rDNA ITS clearly shows that the species isolated from the southeast coast of China is identified as P.globosa rather than P. cf. pouchetii or other species. Furthermore, the significance of rDNA variation in distinct global populations of P.globosa suggested it might have had sufficient time to accumulate detectable mutations at the rDNA locus, supporting the hypothesis of ancient dispersal of P.globosa to many areas, meaning that P.globosa blooms in the coastal waters of China are endemic rather than a newly introduced species or a foreign source. Finally, based on the high divergent region of rDNA ITS, a pair of species-specific primers for P.globosa were designed, they could be useful to detect the presence of this species in mixed plankton assemblages or flagellate stages that are recognized with diffculties by means of conventional microscopy. 相似文献
92.
93.
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
94.
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
95.
96.
97.
Discussion on Coastal Erosion Near Xizhuang, Penglai, China 总被引:1,自引:0,他引:1
Wu Guiqiu Li Wei Professor First Institute of Oceanography State Oceanic Administration Qingdao Associate Researcher 《中国海洋工程》1994,(2)
-In this work, on the basis of the characteristics of coastal erosion near Xizhuang (Penglai, Shandong, China) and the in-situ measured data and theoretical calculation, the causes of coastal erosion there are obtained: (1) natural erosion, (2) beach sand borrowing, and (3) sand borrowing at the Dengzhou shoal near Xizhuang, which results in the loss of the function of the wave force resistance of the shoal. Since the wave energy is increased, the longshore sediment transport rate is increased. Coastal erosions are more and more serious. A scientific basis to reduced the calamity of coastal erosion in local area is presented. Some experiences are accumulated for studying coastal erosion. 相似文献
98.
作者用聚类分析法划分浅海水团,目的在于克服大洋水团分析法进行分析所遇到的一些困难,它在根据不同海区特点确定临界值与检验值时有较大的灵活性。本文讨论了浅海变性水团与中心渔场的关系。结果表明,东海底层冷水、东海表层水和大陆沿岸水对渔业生产有重要影响。 相似文献
99.
在浅层工程折射地震测量中 ,通常采用相遇观测系统采集数据和相应的解释方法进行解释 ;对于单边观测采集方法由于与其相应的解释方法较少而少被采用。本文提出 1种方法简单、精度可靠、可直接利用同一点相邻 2排列 2次观测的初至折射时间并适合于海洋走航式单边连续观测系统的折射初至资料解释方法。其精度基本与同观测段的采用相遇观测的差异时距方法解释的精度十分接近。该方法即适合于人工解释又适合于计算机自动解释。 相似文献
100.
报道一种新的有效催化剂-碘-钒盐和在一定条件下合成四溴邻苯二甲酸酐的方法。探讨催化剂的组成配比、用量、温度及废酸循环使用等因素对合成方法的影响。该催化剂的催化效果和常用的碘、碘-铁催化剂相同,适用于四溴邻苯二甲酸酐的合成。 相似文献