全文获取类型
| 收费全文 | 3149篇 |
| 免费 | 619篇 |
| 国内免费 | 713篇 |
专业分类
| 测绘学 | 214篇 |
| 大气科学 | 703篇 |
| 地球物理 | 813篇 |
| 地质学 | 1549篇 |
| 海洋学 | 402篇 |
| 天文学 | 133篇 |
| 综合类 | 358篇 |
| 自然地理 | 309篇 |
出版年
| 2024年 | 5篇 |
| 2023年 | 41篇 |
| 2022年 | 97篇 |
| 2021年 | 139篇 |
| 2020年 | 106篇 |
| 2019年 | 117篇 |
| 2018年 | 128篇 |
| 2017年 | 124篇 |
| 2016年 | 151篇 |
| 2015年 | 108篇 |
| 2014年 | 153篇 |
| 2013年 | 137篇 |
| 2012年 | 166篇 |
| 2011年 | 190篇 |
| 2010年 | 190篇 |
| 2009年 | 193篇 |
| 2008年 | 201篇 |
| 2007年 | 174篇 |
| 2006年 | 167篇 |
| 2005年 | 120篇 |
| 2004年 | 92篇 |
| 2003年 | 103篇 |
| 2002年 | 97篇 |
| 2001年 | 86篇 |
| 2000年 | 115篇 |
| 1999年 | 130篇 |
| 1998年 | 137篇 |
| 1997年 | 147篇 |
| 1996年 | 128篇 |
| 1995年 | 103篇 |
| 1994年 | 123篇 |
| 1993年 | 107篇 |
| 1992年 | 73篇 |
| 1991年 | 52篇 |
| 1990年 | 56篇 |
| 1989年 | 45篇 |
| 1988年 | 46篇 |
| 1987年 | 24篇 |
| 1986年 | 23篇 |
| 1985年 | 17篇 |
| 1984年 | 13篇 |
| 1983年 | 13篇 |
| 1982年 | 14篇 |
| 1981年 | 9篇 |
| 1980年 | 2篇 |
| 1979年 | 3篇 |
| 1978年 | 4篇 |
| 1977年 | 2篇 |
| 1976年 | 4篇 |
| 1973年 | 2篇 |
排序方式： 共有4481条查询结果，搜索用时 46 毫秒
91.
史前人类向青藏高原扩散的过程和适应高海拔缺氧环境的机制是多学科关注的热点科学问题.青海湖盆地是青藏高原旧石器-中石器时代遗址分布最为丰富的区域,对这些遗址出土的石制品原料的分析有助于深入理解青藏高原史前狩猎采集人群的石料开发策略、人群迁徙和交流联系.青海湖盆地151遗址出土的928件石制品的石料研究分析显示,处于末次冰消期的下文化层的石制品以近源的石英和石英岩为主要原料,而处于全新世早中期的上文化层在同类型近源石料仍占主体地位的情况下,开始出现较高比例和多样化的优质硅质石料,并且主要用于生产细石器.野外调查和查阅地质资料均未发现青海湖盆地内有151遗址中出现的同类型优质硅质石料产出,推测其来自远距离搬运.青海湖盆地内其他8个末次冰消期至全新世中期遗址的3269件石制品石料分析结果显示,与151遗址同类型的远源优质硅质石料在全新世早期开始在盆地内的遗址中出现.这一结果表明青海湖盆地末次冰消期古人类活动强度和范围有限,全新世早中期古人类受到全新世大暖期气候变好和周边地区农业人群兴起挤压活动空间的双重影响,在高原上的活动范围和强度大大增加,伴随着开始有意识地开发优质石料,较频繁地进行远距离迁移和人群交流.远源优质硅质石料的产地可能位于北祁连山区和青藏高原上的陆相火山岩区,需要未来更深入的研究揭示.该研究为深入理解青藏高原古人类的高海拔环境适应策略和移动模式提供了重要材料,为理解史前人类向高原扩散的机制提供了重要信息. 相似文献
92.
Based on observations from a dense broadband seismic array located along the northeastern (NE) margin of the Tibetan Plateau in southeastern Gansu Province,we use receiver functions (RFs) to pick the arrival times of P-to-S converted waves and bin the traces in different grids according to the piercing points of the 410 and 660 km discontinuities in the upper mantle.The depths of the two discontinuities are estimated by the ray tracing method with the IASP91 velocity model and a 3-D tomography model.The results indicate the following:(1) The arrival times of the P410s and P660s converted phases are delayed by approximately 1 s than those predicted by the IASP91 model.The mantle transition zone (MTZ) is thicker than that in the global model.(2) The synchronous lags in the P410s and P660s arrival times are consistent with low-velocity anomalies in the upper mantle,which are believed to result mainly from the eastward migration of materials beneath the NE margin of the Tibetan Plateau.(3) Combined with previous tomography results,the depression of the'660'discontinuity and the thickened MTZ are somewhat consistent with the big mantle wedge (BMW) model.However,due to data limitations,more studies are required to explore the BMW in the future. 相似文献
93.
94.
95.
Selection of sand models and identification of parameters using an enhanced genetic algorithm 
下载免费PDF全文
下载免费PDF全文 Numerous constitutive models of granular soils have been developed during the last few decades. As a consequence, how to select an appropriate model with the necessary features based on conventional tests and with an easy way of identifying parameters for geotechnical applications has become a major issue. This paper aims to discuss the selection of sand models and parameters identification by using genetic algorithm. A real‐coded genetic algorithm is enhanced for the optimization with high efficiency. Models with gradually varying features (elastic‐perfectly plastic modelling, nonlinear stress–strain hardening, critical state concept and two‐surface concept) are selected from numerous sand models as examples for optimization. Conventional triaxial tests on Hostun sand are selected as the objectives in the optimization. Four key points are then discussed in turn: (i) which features are necessary to be accounted for in constitutive modelling of sand; (ii) which type of tests (drained and/or undrained) should be selected for an optimal identification of parameters; (iii) what is the minimum number of tests that should be selected for parameter identification; and (iv) what is the suitable and least strain level of objective tests to obtain reliable and reasonable parameters. Finally, a useful guide, based on all comparisons, is provided at the end of the discussion. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
96.
97.
98.
Characterising deep vadose zone water movement and solute transport under typical irrigated cropland in the North China Plain 下载免费PDF全文
下载免费PDF全文 Understanding the dynamics and mechanisms of soil water movement and solute transport is essential for accurately estimating recharge rates and evaluating the impacts of agricultural activities on groundwater resources. In a thick vadose zone (0–15 m) under irrigated cropland in the piedmont region of the North China Plain, soil water content, matric potential, and solute concentrations were measured. Based on these data, the dynamics of soil water and solutes were analysed to investigate the mechanisms of soil water and solute transport. The study showed that the 0–15‐m vadose zone can be divided into three layers: an infiltration and evaporation layer (0–2 m), an unsteady infiltration layer (2–6 m), and a quasi‐steady infiltration layer (6–15 m). The chloride, nitrate, and sulphate concentrations all showed greater variations in the upper soil layer (0–1 m) compared to values in the deep vadose zone (below 2 m). The average concentrations of these three anions in the deep vadose zone varied insignificantly with depth and approached values of 125, 242, and 116 mg/L. The accumulated chloride, sulphate, and nitrate were 2,179 ± 113, 1,760 ± 383, and 4,074 ± 421 kg/ha, respectively. The soil water potential and solute concentrations indicated that uniform flow and preferential flow both occurred in the deep vadose zone, and uniform flow was the dominant mechanism of soil water movement in this study. The piston‐like flow velocity of solute transport was 1.14 m per year, and the average value of calculated leached nitrate nitrogen was 107 kg/ha?year below the root zone. The results can be used to better understand recharge processes and improve groundwater resources management. 相似文献
99.
Macroscopic criteria for Green type porous materials with spheroidal voids: application to double porous materials 下载免费PDF全文
下载免费PDF全文 Combined effects of matrix plastic compressibility and void shape are investigated for ductile porous materials. To this end, a spheroidal volume containing a confocal spheroidal (prolate or oblate) void subjected to uniform strain rate boundary conditions has been first studied. A Green type matrix is chosen as a prototype for investigating effects of plastic compressibility. This is carried out by using a kinematics limit analysis theory from which a closed‐form expression of the macroscopic criterion is established for the considered class of materials. These results are then extended to ductile porous materials made up of a green matrix containing randomly oriented spheroidal voids. In the framework of a two‐step homogenization procedure, the obtained results are implemented to describe the macroscopic behavior of double porous materials involving spherical voids at the microscale and randomly oriented and distributed spheroidal voids at the mesoscale. For validation purpose, the new derived criteria are assessed and validated by comparing their predictions to available upper bounds and numerical data from literature. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
100.