全文获取类型
收费全文 | 16547篇 |
免费 | 3222篇 |
国内免费 | 4959篇 |
专业分类
测绘学 | 1710篇 |
大气科学 | 3689篇 |
地球物理 | 3744篇 |
地质学 | 9136篇 |
海洋学 | 2598篇 |
天文学 | 571篇 |
综合类 | 1515篇 |
自然地理 | 1765篇 |
出版年
2024年 | 79篇 |
2023年 | 249篇 |
2022年 | 745篇 |
2021年 | 916篇 |
2020年 | 743篇 |
2019年 | 954篇 |
2018年 | 930篇 |
2017年 | 838篇 |
2016年 | 973篇 |
2015年 | 974篇 |
2014年 | 1084篇 |
2013年 | 1164篇 |
2012年 | 1150篇 |
2011年 | 1163篇 |
2010年 | 1106篇 |
2009年 | 1162篇 |
2008年 | 1046篇 |
2007年 | 973篇 |
2006年 | 912篇 |
2005年 | 796篇 |
2004年 | 608篇 |
2003年 | 580篇 |
2002年 | 513篇 |
2001年 | 521篇 |
2000年 | 514篇 |
1999年 | 630篇 |
1998年 | 440篇 |
1997年 | 456篇 |
1996年 | 408篇 |
1995年 | 363篇 |
1994年 | 292篇 |
1993年 | 255篇 |
1992年 | 213篇 |
1991年 | 170篇 |
1990年 | 144篇 |
1989年 | 149篇 |
1988年 | 120篇 |
1987年 | 75篇 |
1986年 | 66篇 |
1985年 | 48篇 |
1984年 | 47篇 |
1983年 | 28篇 |
1982年 | 19篇 |
1981年 | 25篇 |
1980年 | 17篇 |
1979年 | 16篇 |
1978年 | 8篇 |
1973年 | 7篇 |
1958年 | 11篇 |
1954年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
71.
摄食水平和性别对稀有鮈鲫生长和能量收支的影响 总被引:7,自引:0,他引:7
于1995年10月采集本所自繁有ju鲫(约1.03g/ind),在30℃水温下进行摄食-生长实验,研究了从饥饿到饱食5个摄食水平对不同性别稀有ju鲫的生长和能量收支的影响,结果表明,鱼体干物和能量含量不受摄食率和性别的影响,随着摄食率的增加,雌鱼的湿重,干重和能量特定生长率均呈双对数增加,雄鱼湿重特定生长率呈双对数增加,而干物质和能量特定生长率呈指增加,雄鱼生长速度及摄食率低于雌鱼,随着摄食率的增加,食物能损失于粪便中的比例均呈上升趋势,损失于排泄物中的比例变化不大,呈下降趋势,用于代谢的比例呈下降趋势,用于生长的比例呈上升趋势,除饥饿组外,在各个摄食水平,雌鱼食物能用于代谢的比例较雄鱼低,而用于生长的比例较雄鱼高。 相似文献
72.
台湾海峡蓝圆鲹的食性研究 总被引:2,自引:0,他引:2
本文材料系1965年,1977年,1988年至1989年台湾海峡灯光围网、拖山网的渔获物。采用个体百分比(N),重量百分比(W),出现频率(F),出现频率百分比,胃饱满系数,类群更替率等指标进行分析,并根据3个指标,计算了相对重要性指标。蓝圆鲹食性广泛,主要食料有桡足矣、小型鱼类、糠虾类、长尾类和短尾类等。幼鱼食料以桡足类最为重要,成鱼食料则以鱼类为重要。蓝圆鲹摄食强度有明显的季节变化。周年摄食强度呈双峰型,高峰期正好比两个产卵期迟1个月,说明蓝图鲹生殖后强烈摄食。幼鱼摄食强度昼夜变化明显,以黄昏和黎明摄食较强烈。蓝图鲹主要饵料生物的高生物量区,可作为寻找中心渔场的良好生物学指标之一,这在生产实践中有重要意义。 相似文献
73.
74.
75.
During the period August 1985 to May 1986, phytoplankton in the southern Taiwan Strait was collected and studied for distributional variability in relation to hydrography. The results indicated that maximum standing crops of phytoplankton occurred in October and May due to the outgrowth of certain species of diatoms and blue-green algae. The majority of phytoplankton appeared in the water in the top 25 m and occurred in distinct clusters under the influence of water movement. Multivariate analysis indicated that hydrographic parameters, which accounted for the variability of phytoplankton distribution, varied seasonally. Vertical, spatial and temporal variabilities were also apparent. The close relationship between hydrography and algal distribution justifies the use of variations in the phytoplankton population as a useful tracer of water movement. 相似文献
76.
The VOF method and the k–ε model, combined with the equation of state of air at constant temperature, have been used to calculate the total horizontal wave force caused by monochromatic waves acting on a perforated caisson with a top cover. From comparison of various parameters, such as the total horizontal force, the pressure difference on the front wall, the pressure on the back wall and the pressure on the top cover, between the numerical results and test data, it can be seen that the numerical results agree well with the test data. It is concluded that the method described in this paper can be utilized to calculate wave forces acting on perforated caissons with a top cover in the case of nonovertopping, nonbreaking waves. A simplified method to calculate the total horizontal force has been developed, based on test data, using a least-squares method. A comparison between the numerical results and the values calculated from the simplified equations shows good agreement. Therefore the simplified equations can be used in engineering applications to evaluate the total horizontal force on a perforated caisson with a top cover. 相似文献
77.
Li Yucheng He Ming Professor Dalian University of Technology Dalian Former master student Dalian University of Technology Dalian 《中国海洋工程》1994,(3)
-Based on the extended Morison Equation and model tests, the in-line forces on small square cylinders caused by waves (regular and irregular) and currents are analyzed in detail in this paper. The hydrodynamic coefficient CD and Cu related to KC number and the effect of direction of wave incidence are also given, which can be used in engineering practice. 相似文献
79.
Tan Jiahua Ma Wei Pan Bin Associate Professor Shanghai Jiao Tong University Shanghai Lecturer shanghai Jiao Tong University Shanghai 《中国海洋工程》1992,(1)
The basis, process and results of the demonstration of the main dimensions of a 75000t floating production and storage vessel are discussed in this paper. A simple but reliable orthogonal design method is applied in the main dimension optimization. The ideas of gradual approximation and feedback from various aspects are put into effect. During the demonstration, in order to make the model tally with the actual situation, the draft design is closely related to the computational analysis, so that the demonstration model can be verified at any time; the handling of the overall system is closely related to the research of each item, which is beneficial not only to the mastery of various regularities, but also to the balance of decisions. Finally, according to the computational results and the regularities obtained from analysis, the main dimensions are determined. 相似文献
80.
The impact of several chemical poisons on the Arca granosa absorbing 45Ca was studied. The results showed that the absorption of 45Ca in the blood and the soft tissue by Arca granosa was inhibited by the inorganic compounds of Cu, Zn, Pd, Cd, Hg and so on, among which the inhibition of Cd was the strongest, next was that of Cu and Hg. But the inhibition of the absorption of 45Ca in the shell of Arca granosa was little. The impact of three kinds of energy metabolism inhibitory substances on the absorption of 45Ca by the Area granosa showed that the absorption was an active transfer process. 相似文献