全文获取类型
收费全文 | 10328篇 |
免费 | 2191篇 |
国内免费 | 3124篇 |
专业分类
测绘学 | 1171篇 |
大气科学 | 3290篇 |
地球物理 | 2661篇 |
地质学 | 4614篇 |
海洋学 | 1726篇 |
天文学 | 687篇 |
综合类 | 599篇 |
自然地理 | 895篇 |
出版年
2024年 | 40篇 |
2023年 | 111篇 |
2022年 | 292篇 |
2021年 | 364篇 |
2020年 | 406篇 |
2019年 | 556篇 |
2018年 | 427篇 |
2017年 | 495篇 |
2016年 | 477篇 |
2015年 | 544篇 |
2014年 | 732篇 |
2013年 | 727篇 |
2012年 | 709篇 |
2011年 | 776篇 |
2010年 | 619篇 |
2009年 | 840篇 |
2008年 | 824篇 |
2007年 | 939篇 |
2006年 | 799篇 |
2005年 | 615篇 |
2004年 | 619篇 |
2003年 | 526篇 |
2002年 | 468篇 |
2001年 | 384篇 |
2000年 | 366篇 |
1999年 | 345篇 |
1998年 | 327篇 |
1997年 | 264篇 |
1996年 | 228篇 |
1995年 | 170篇 |
1994年 | 181篇 |
1993年 | 110篇 |
1992年 | 87篇 |
1991年 | 68篇 |
1990年 | 43篇 |
1989年 | 33篇 |
1988年 | 30篇 |
1987年 | 24篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 9篇 |
1983年 | 2篇 |
1982年 | 9篇 |
1981年 | 10篇 |
1980年 | 2篇 |
1979年 | 6篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1971年 | 3篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
A constitutive model that captures the material behavior under a wide range of loading conditions is essential for simulating complex boundary value problems. In recent years, some attempts have been made to develop constitutive models for finite element analysis using self‐learning simulation (SelfSim). Self‐learning simulation is an inverse analysis technique that extracts material behavior from some boundary measurements (eg, load and displacement). In the heart of the self‐learning framework is a neural network which is used to train and develop a constitutive model that represents the material behavior. It is generally known that neural networks suffer from a number of drawbacks. This paper utilizes evolutionary polynomial regression (EPR) in the framework of SelfSim within an automation process which is coded in Matlab environment. EPR is a hybrid data mining technique that uses a combination of a genetic algorithm and the least square method to search for mathematical equations to represent the behavior of a system. Two strategies of material modeling have been considered in the SelfSim‐based finite element analysis. These include a total stress‐strain strategy applied to analysis of a truss structure using synthetic measurement data and an incremental stress‐strain strategy applied to simulation of triaxial tests using experimental data. The results show that effective and accurate constitutive models can be developed from the proposed EPR‐based self‐learning finite element method. The EPR‐based self‐learning FEM can provide accurate predictions to engineering problems. The main advantages of using EPR over neural network are highlighted. 相似文献
3.
自从在活动星系核NGC4945的视线方向上发现第一个河外H2O超脉泽源以来,迄今为止已发现了19个河外H2O超脉泽源,对与活动星系核成协和河外H2O超脉泽源及分子谱线的观测和研究是探测和研究活动星系核核区中央源,拱核气体和尘埃环性质的非常有效的工具,主要评述对河外H2O超脉泽源及与其相关分子谱线的搜索,观测和理论研究现状。 相似文献
4.
本文提出了一种把经纬度网格点上的气象要素值转化到正方形网格点上的多项式插值方案。对插值方案作了稳定性讨论和定量及定性分析。结果表明,这种插值方案方便可行,而且有广泛适用性。 相似文献
5.
Yunfei Bao Shengbo Chen 《东北亚地学研究》2006,9(1):100-108
IntroductionThe radiance leaving the earth-atmosphere sys-tem which can be sensed by a satellite borne ra-diometer is the sum of radiation emission fromtheearth surface and each atmospheric level that aretransmittedtothe top of the atmosphere.The radia-tion emissionfromthe earthsurface andthe radianceof each atmospheric level can be separated fromtheradiance at the top the atmospheric level a satellitemeasured.Thus,the earth surface parameters willbe retrieved from the surface radiance after a… 相似文献
6.
7.
Due to the geological time scales required for observation of catchment evolution, surrogates or analogues of field data are necessary to understand long‐term processes. To investigate long‐term catchment behaviour, two experimental model catchments that developed without rigid boundaries under controlled conditions are examined and a qualitative and quantitative analysis of their evolution is presented. Qualitatively, the experimental catchments have the visual appearance of field scale data. Observation demonstrates that changes in catchment shape and network form are conservative. Quantitative analysis suggests that the catchments reach an equilibrium form while a reduction in the channel network occurs. While the catchments are laboratory scale models, the results provide insights into field scale behaviour. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
8.
Estela M. Reynoso † Simon Johnston Anne J. Green Bärbel S. Koribalski 《Monthly notices of the Royal Astronomical Society》2006,369(1):416-424
We have observed the supernova remnant (SNR) G290.1−0.8 in the 21-cm H i line and the 20-cm radio continuum using the Australia Telescope Compact Array (ATCA). The H i data were combined with data from the Southern Galactic Plane Survey to recover the shortest spatial frequencies. In contrast, H i absorption was analysed by filtering extended H i emission, with spatial frequencies shorter than 1.1 kλ. The low-resolution ATCA radio continuum image of the remnant shows considerable internal structure, resembling a network of filaments across its 13-arcmin diameter. A high-resolution ATCA radio continuum image was also constructed to study the small-scale structure in the SNR. It shows that there are no structures smaller than ∼17 arcsec, except perhaps for a bright knot to the south, which is probably an unrelated object. The H i absorption study shows that the gas distribution and kinematics in front of SNR G290.1−0.8 are complex. We estimate that the SNR probably lies in the Carina arm, at a distance 7 (±1) kpc. In addition, we have studied nearby sources in the observed field using archival multiwavelength data to determine their characteristics. 相似文献
9.
10.
1 INTRODUCTIONIn the previous paper (on et al. 1999, hereafter Paper I), we investigated the wavelength-dependence of four colltribution functions (CFs) derived from dmerent formal solutions andreferring to different emergellt quantities in the unpolarized case. Because one cannot generallyassign a single formation region to the whole line band in a real stellar atmosphere, e.g., the solaratmosphere, instead, the line formation region can be defined as the layers deviating farthestfrom t… 相似文献