全文获取类型
收费全文 | 53460篇 |
免费 | 14744篇 |
国内免费 | 18287篇 |
专业分类
测绘学 | 10110篇 |
大气科学 | 7527篇 |
地球物理 | 10000篇 |
地质学 | 35486篇 |
海洋学 | 9901篇 |
天文学 | 750篇 |
综合类 | 4473篇 |
自然地理 | 8244篇 |
出版年
2024年 | 313篇 |
2023年 | 1224篇 |
2022年 | 3409篇 |
2021年 | 4112篇 |
2020年 | 3208篇 |
2019年 | 3783篇 |
2018年 | 3437篇 |
2017年 | 3227篇 |
2016年 | 3336篇 |
2015年 | 3910篇 |
2014年 | 3798篇 |
2013年 | 4617篇 |
2012年 | 5310篇 |
2011年 | 5223篇 |
2010年 | 5177篇 |
2009年 | 4825篇 |
2008年 | 4629篇 |
2007年 | 4315篇 |
2006年 | 4181篇 |
2005年 | 3391篇 |
2004年 | 2488篇 |
2003年 | 1749篇 |
2002年 | 1777篇 |
2001年 | 1704篇 |
2000年 | 1345篇 |
1999年 | 627篇 |
1998年 | 239篇 |
1997年 | 167篇 |
1996年 | 122篇 |
1995年 | 107篇 |
1994年 | 77篇 |
1993年 | 87篇 |
1992年 | 103篇 |
1991年 | 40篇 |
1990年 | 47篇 |
1989年 | 23篇 |
1988年 | 31篇 |
1987年 | 30篇 |
1986年 | 39篇 |
1985年 | 28篇 |
1984年 | 37篇 |
1983年 | 22篇 |
1982年 | 17篇 |
1981年 | 9篇 |
1979年 | 32篇 |
1978年 | 8篇 |
1975年 | 8篇 |
1964年 | 6篇 |
1957年 | 21篇 |
1954年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
利用主成分分析PCA提取2014-08-15~23CG-5相对重力仪静态相对观测的零点漂移和固体潮。结果显示,PCA提取的零点漂移值与最小二乘线性拟合结果非常一致,其差仅在10-2μGal/d量级;PCA估算的固体潮值与CG-5重力仪内部软件提供的固体潮模型计算值统计结果基本一致,两者之差均小于8μGal,均方根均小于5μGal。 相似文献
994.
Lei Yanju Zhang Wenbing Xu Wei Zhang Yanjiao Zhou Huihui Mai Kangsen 《中国海洋大学学报(英文版)》2015,14(3):511-521
The aim of this study was to compare the effects of waterborne copper(Cu) and cadmium(Cd) on survival, anti-oxidative response, lipid peroxidation and metal accumulation in abalone Haliotis discus hannai. Experimental animals(initial weight: 7.49 g ± 0.01 g) were exposed to graded concentrations of waterborne Cu(0.02, 0.04, 0.06, 0.08 mg L-1) or Cd(0.025, 0.05, 0.25, 0.5mg L-1) for 28 days, respectively. Activities of the anti-oxidative enzymes(catalase, CAT; superoxide dismutase, SOD; glutathione peroxidases, GPx; glutathione S-transferase, GST), contents of the reduced glutathione(GSH) and malondiadehyde(MDA) in the hepatopancreas, and metal accumulation in hepatopancreas and muscles were analyzed after 0, 1, 3, 6, 10, 15, 21, 28 days of metal exposure, respectively. Results showed that 0.04 mg L-1, 0.06 mg L-1 and 0.08 mg L-1 Cu caused 100% death of abalone on the 21 st, 10 th and 6th day, respectively. However, no dead abalone was found during the 28-day waterborne Cd exposure at all experimental concentrations. Generally, activities of SOD and GST in hepatopancreas under all Cu concentrations followed a decrease trend as the exposure time prolonged. However, these activities were firstly increased and then decreased to the control level and increased again during Cd exposure. Activities of CAT in all Cu exposure treatments were higher than those in the control. These activities were firstly increased and then decreased to the control level and increased again during Cd exposure. Contents of MDA in hepatopancreas in all Cu treatments significantly increased first and then decreased to the control level. However, the MDA contents in hepatopancreas were not significantly changed during the 28-day Cd exposure. The metals accumulation in both hepatopancreas and muscles of abalone significantly increased with the increase of waterborne metals concentration and exposure time. These results indicated that H. discus hannai has a positive anti-oxidative defense against Cu or Cd. In conclusion, anti-oxidative mechanism in abalone to resist waterborne Cu did not follow the same pattern as that for waterborne Cd. 相似文献
995.
哈西亚图石英闪长岩是东昆仑地区中生代具幔源组分贡献的花岗岩类典型代表,岩体出露于东昆仑中构造带,广泛发育暗色微粒包体。包体为闪长质岩石,并含有一系列岩浆混合成因的证据,如水滴状、长条状塑性流变外形,淬冷边、反向脉等高—中温混合迹象,以及低Mg/(Fe+Mg)、Na/(Ca+Na)值等混合成因特征。包体A/CNK值介于0.77~0.87,属准铝质,富Al2O3、Fe2O3、MgO,贫K2O、Na2O,富集大离子亲石元素(Rb、Ba、K等),同时又具有Ta、Nb、Ti的"TNT"负异常,具有俯冲带幔源岩石的成分特点。依据岩石学、地球化学特征并结合同时期大地构造背景,东昆仑晚古生代—早中生代含暗色微粒包体花岗质岩石是幔源岩浆经历多次熔融、同化、存储和均一(MASH)过程后与壳源岩浆混合的产物。在混合岩浆中,富镁铁质端元是由辉长质岩浆进化而来的闪长质岩浆。 相似文献
996.
Effect of Freeze-Thaw Cycles on Mechanical Properties and Permeability of Red Sandstone under Triaxial Compression 总被引:1,自引:1,他引:0
Geological disasters will happen in cold regions because of the effects of freeze-thaw cycles on rocks or soils, so studying the effects of these cycles on the mechanical characteristics and permeability properties of rocks is very important. In this study,red sandstone samples were frozen and thawed with 0,4, 8 and 12 cycles, each cycle including 12 h of freezing and 12 h of thawing. The P-wave velocities of these samples were measured, and the mechanical properties and evolution of the steady-state permeabilities were investigated in a series of uniaxial and triaxial compression tests. Experimental results show that, with the increasing of cyclic freeze-thaw times, the P-wave velocity of the red sandstone decreases. The number of freeze-thaw cycles has a significant influence on the uniaxial compressive strength, elastic modulus, cohesion, and angle of internal friction. The evolution of permeability of the rock samples after cycles of freeze-thaw in a complete stress-strain process under triaxial compression is closely related to the variation of the microstructure in the rock. There is a highly corresponding relationship between volumetric strain and permeability with axial strain in all stages of the stress-strain behaviour. 相似文献
997.
李海燕 《广东海洋大学学报》2015,(2):60-63
岭南人黄咏梅的小说充满了浓郁的岭南风情。她的小说多以岭南城镇为背景,岭南的各种景观、风俗民情、方言俚语和独特的岭南小吃常常出现在她的笔下,岭南风情在她的作品中得以有力呈现。黄咏梅的岭南风情画满足了读者有关岭南的想象,但过于浓郁的岭南特色却使作品表现出传播范围、作品深度等方面的局限性。 相似文献
998.
设计了一种基于单片机的电容式液位传感器,主要由单片机系统、555定时器、液晶显示屏组成。单片机作为主要控制的部分,控制系统所有的部分,接收555定时器方波信号并读取出其频率,将频率转换成液位高度,显示到LCD1602液晶显示屏幕上,软件计算液位高度,减小了电容与频率转换的线性误差,最终实现算法的设计。 相似文献
999.
1000.