全文获取类型
收费全文 | 23607篇 |
免费 | 3839篇 |
国内免费 | 4398篇 |
专业分类
测绘学 | 846篇 |
大气科学 | 1251篇 |
地球物理 | 4835篇 |
地质学 | 11614篇 |
海洋学 | 2638篇 |
天文学 | 6133篇 |
综合类 | 1035篇 |
自然地理 | 3492篇 |
出版年
2024年 | 99篇 |
2023年 | 289篇 |
2022年 | 752篇 |
2021年 | 780篇 |
2020年 | 800篇 |
2019年 | 952篇 |
2018年 | 765篇 |
2017年 | 792篇 |
2016年 | 804篇 |
2015年 | 937篇 |
2014年 | 1105篇 |
2013年 | 1096篇 |
2012年 | 1216篇 |
2011年 | 1257篇 |
2010年 | 1050篇 |
2009年 | 1719篇 |
2008年 | 1512篇 |
2007年 | 1614篇 |
2006年 | 1687篇 |
2005年 | 1447篇 |
2004年 | 1438篇 |
2003年 | 1427篇 |
2002年 | 1154篇 |
2001年 | 1022篇 |
2000年 | 988篇 |
1999年 | 864篇 |
1998年 | 941篇 |
1997年 | 547篇 |
1996年 | 484篇 |
1995年 | 460篇 |
1994年 | 409篇 |
1993年 | 318篇 |
1992年 | 277篇 |
1991年 | 169篇 |
1990年 | 148篇 |
1989年 | 126篇 |
1988年 | 91篇 |
1987年 | 100篇 |
1986年 | 63篇 |
1985年 | 34篇 |
1984年 | 12篇 |
1983年 | 13篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 8篇 |
1979年 | 12篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1954年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
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
2.
3.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars. 相似文献
4.
The magnetic-field distribution outside a flat, infinitely conductive unbounded disk in the field of a point magnetic dipole is determined. A relationship is established between the problem of magnetic-field determination and the problem of the flow of an ideal incompressible fluid around an infinitely thin disk. 相似文献
5.
6.
The Cassini spacecraft, en route to Saturn, passed close to Jupiter while the Galileo spacecraft was completing its 28th and 29th orbits of Jupiter, thus offering a unique opportunity for direct study of the solar wind-Jovian interaction. Here evidence is given of response of the Jovian magnetopause and bow shock positions to changes of the north-south component of the solar wind magnetic field, a phenomenon long known to occur in equivalent circumstances at Earth. The period analyzed starts with the passage over Cassini of an interplanetary shock far upstream of Jupiter. The shock's arrival at Galileo on the dusk-flank of the magnetosphere caused Galileo to exit into the solar wind. Using inter-spacecraft timing based on the time delay established from the shock arrival at each spacecraft, we point out that Galileo's position with respect to the Jovian bow shock appears to correlate with changes in the disturbed north-south reversing field seen behind the shock. We specifically rule out the alternative of changes in the shape of the bow shock with rotations of the interplanetary magnetic field as the cause. 相似文献
7.
平衡剖面的制作流程及其地质意义 总被引:9,自引:0,他引:9
平衡剖面技术是地质思维和计算机技术的结晶,使对断层构造的研究提高到定量阶段,其依据是在垂直构造走向的剖面上,地层长度和面积(2D)或体积(3D)是均衡的。在此原理基础上利用数学手段对盆地的构造发育史进行正演和反演模拟,直观地再现地下构造的原始几何形态,迅速提供地震剖面的构造解释方案,并对解释结果进行检验(不平衡的剖面其解释一般有问题),为深刻认识构造发育史、分析油气运移及聚集规律提供依据,提高了工作效率。其结果也为盆地模拟、油藏模拟、定量计算构造伸缩量等地质研究打下了坚实的基础[1]。 相似文献
8.
9.
1 IntroductionAccordingtogeologicalstructure ,theGansu Ningxia Qinghaiareabelongstothenortheastmar ginofQinghai Tibetblock .Thisareahasbeenpaidmuchattentionby geo specialistsinChinaandabroadbecauseofitssignificanttectonicmovement,itsintensiveseismicity ,anditsimportanceinearth quakehazardmitigation .IntheDevelopmentPro gramonNationalKeyBasicResearchesundertheProject“MechanismandPredictionofContinentalStrongEarthquakes”,themechanismsofcontinen talstrongearthquakesarestudied ,usinghypo… 相似文献
10.