全文获取类型
收费全文 | 19631篇 |
免费 | 3031篇 |
国内免费 | 3971篇 |
专业分类
测绘学 | 1295篇 |
大气科学 | 3265篇 |
地球物理 | 4710篇 |
地质学 | 9683篇 |
海洋学 | 2285篇 |
天文学 | 1576篇 |
综合类 | 1618篇 |
自然地理 | 2201篇 |
出版年
2024年 | 91篇 |
2023年 | 317篇 |
2022年 | 882篇 |
2021年 | 999篇 |
2020年 | 847篇 |
2019年 | 955篇 |
2018年 | 1150篇 |
2017年 | 1003篇 |
2016年 | 1115篇 |
2015年 | 893篇 |
2014年 | 1186篇 |
2013年 | 1218篇 |
2012年 | 1122篇 |
2011年 | 1180篇 |
2010年 | 1175篇 |
2009年 | 1111篇 |
2008年 | 1006篇 |
2007年 | 885篇 |
2006年 | 801篇 |
2005年 | 692篇 |
2004年 | 559篇 |
2003年 | 508篇 |
2002年 | 441篇 |
2001年 | 487篇 |
2000年 | 518篇 |
1999年 | 698篇 |
1998年 | 597篇 |
1997年 | 559篇 |
1996年 | 543篇 |
1995年 | 408篇 |
1994年 | 391篇 |
1993年 | 361篇 |
1992年 | 309篇 |
1991年 | 222篇 |
1990年 | 179篇 |
1989年 | 159篇 |
1988年 | 133篇 |
1987年 | 113篇 |
1986年 | 106篇 |
1985年 | 86篇 |
1984年 | 77篇 |
1983年 | 71篇 |
1982年 | 71篇 |
1981年 | 48篇 |
1980年 | 41篇 |
1979年 | 49篇 |
1978年 | 28篇 |
1977年 | 26篇 |
1976年 | 25篇 |
1975年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
231.
232.
233.
养殖对虾与海捕对虾鉴别方法初探 总被引:1,自引:1,他引:1
通过对养殖中国对虾(Penaeus orientalis)和海捕中国对虾肌肉煮熟后的失水率,提取物中的甜菜碱和氧化三甲胺含量,以及虾肉脂质中所含脂肪酸组成和含量进行了分析比较,得到下列结果:(1)两类对虾鲜样煮熟后失水率相差6%以上;(2)海捕对虾肌肉中的甜菜碱和氧化三甲胺含量比养殖对虾的高,两者有显著差别;(3)养殖对虾肌肉所含C18:2ω6为17.3±1.5%,海捕对虾为2.0±0.8%。从以上结果本文推荐以脂肪酸C18:2ω6含量来鉴别养殖对虾与海捕对虾。 相似文献
234.
Yang Zhengji Zuo Qihua
Senior Engineer River Harbour Department Nanjing Hydraulic Research Institute Nanjing Engineer River Harbour Department Nanjing Hydraulic Research Institute Nanjing 《中国海洋工程》1991,(2)
The wave forces on 1, 2 and 4 vertical circular piles under the action of several wave trains of the same wave parameters, such as Hs,Tp and Mo, but different wave group factor GF are measured in the laboratory. After comparing these forces, it may be concluded that the mean and significant wave forces are almost of no difference for the wave trains with different GF. When GF is larger, the one-tenth of the wave froce extreme is slightly increased and the maximum wave force is much larger than the ones with smaller GF, to which attention must be paid in engineering practice. 相似文献
235.
Bin Zhao Yong Yang Gaowen He Heng Zhang Weilin Ma 《Marine Georesources & Geotechnology》2020,38(9):1037-1045
AbstractCobalt-rich crust has attracted increased attention due to their economic value. Studies have indicated that seamounts in the Western Pacific Ocean are rich in cobalt-rich crust resources. The Caiwei Guyot in Western Pacific Ocean is one of the cobalt-rich crust exploration areas contracted between China and the International Seabed Authority. A large volume of research has been conducted to elucidate the tectonic evolution, basement type, sediment type, gravity and magnetic anomaly characteristics, and geochemical characteristics of shallow surface sediments at Caiwei Guyot. However, a research gap exists on the sedimentary strata below the pelagic deposits and above the volcanic basement of the Caiwei Guyot. This paper summarizes that two main types of sediments existing on the top of Caiwei Guyot. The deposition thickness on the top of Caiwei Guyot is characterized by three sedimentary centers and exposed periphery. Pelagic sediments are difficult to form at the northeastern portion and edge area of Caiwei Guyot due to the strong bottom current environment, which makes these areas suitable for crust growth. This paper delineated three cobalt-rich crust prospective areas at the Caiwei Guyot with of significant implications for exploration and mining-lease-block selections at Caiwei Guyot. 相似文献
236.
黄、东海海域水团发育对底质沉积物分布的影响 总被引:6,自引:0,他引:6
通过对黄、东海海域底质沉积物中的细粒级粘土矿物组合特征和全粒级碳酸盐含量分析、对比上覆水团和悬浮体资料,探讨研究区水团发育消长对底质沉积物分布的影响作用。结果表明研究区可以划分为长江冲淡水影响的长江型物质沉积区;黄海水影响的黄河型物质沉积区和黑潮水以及东海次表层水影响的残留型物质沉积区。而水团发育消长可直接影响下伏底质的分布特征。黄河型物质沉积区与残留型物质沉积区的分界相应于黄海冷水向东南扩展与黑潮次中层混合水向陆架爬升互为消长的长期综合平衡位置,而黄海冷水向东南的扩展范围代表着黄河物质在陆架向东南扩展沉积的最大范围. 相似文献
237.
Background signal of the scintillation detector routinely used for atmospheric Kr-85 counting follows fluctuations, and their connection with cosmic ray variations is searched for. A strong correlation between the 27-day moving average of sunspot numbers and the background is obtained from the 1988 data. 相似文献
238.
Taking into account the effect of roughness (or local departures from sphericity) of the emitting layers in the chromosphere-corona transition zone (CCT) allows one to determine the optical depths of layers responsible for resolved structures in Cii, Ciii, Oiv, and Ovi lines. The result, at the top of the irregularities, is of the order of respectively 1 3.5,2.0, 1.6,0.5, and for the bottom of these irregularities, 2 = 0.7, 0.4, 0.3, 0.25. The characteristic angle of these irregularities is, respectively, of the order of 35 °, 33 °, 35 °, and 41 °. For unresolved structures of Civ and Ovi (already analyzed in the spherical symmetry hypothesis in Paper III), one finds 1 0.6; 0.9 and 2 0.12; 0.2 in the case of quiet areas; in the case of active areas, the range is broader for Civ and Ovi, from 1.0 to 1.7 for 1 and from 0.2 to 0.9 for 2. The values obtained from Ovi are in reasonable agreement with each other for resolved and unresolved structures. And the obtained values of 1 and 2 correspond not too badly with the determinations made in Paper III, by methods not exceedingly influenced by the spherical symmetry hypothesis. 相似文献
239.
The light curved in the CM field 总被引:1,自引:0,他引:1
In this paper we introduce the CM field in Sections 2 and 3 based on the paper by Wang and Peng (1985), and calculate the light curved in the CM field in Section 4. The result shows thatP makes CM larger than C at
, and smaller at
. Under a special circumstance which source, CM lens, and observer are in the same line, if we get |
0=0
,
and |
=/2
, we can determine theP(M) andQ(M) of the CM lens,M is the mass of the CM lens. 相似文献
240.
本文以《徐霞客游记》为依据,对明末南方的社会经济生活状况进行了初步归纳和分析,以揭示徐霞客对我国人文地理学的突出贡献。 相似文献