全文获取类型
收费全文 | 8736篇 |
免费 | 1907篇 |
国内免费 | 2856篇 |
专业分类
测绘学 | 822篇 |
大气科学 | 2342篇 |
地球物理 | 2493篇 |
地质学 | 4583篇 |
海洋学 | 1551篇 |
天文学 | 257篇 |
综合类 | 594篇 |
自然地理 | 857篇 |
出版年
2024年 | 67篇 |
2023年 | 189篇 |
2022年 | 384篇 |
2021年 | 391篇 |
2020年 | 425篇 |
2019年 | 521篇 |
2018年 | 432篇 |
2017年 | 436篇 |
2016年 | 452篇 |
2015年 | 483篇 |
2014年 | 626篇 |
2013年 | 672篇 |
2012年 | 629篇 |
2011年 | 627篇 |
2010年 | 511篇 |
2009年 | 664篇 |
2008年 | 624篇 |
2007年 | 712篇 |
2006年 | 633篇 |
2005年 | 494篇 |
2004年 | 521篇 |
2003年 | 427篇 |
2002年 | 390篇 |
2001年 | 307篇 |
2000年 | 272篇 |
1999年 | 259篇 |
1998年 | 227篇 |
1997年 | 208篇 |
1996年 | 189篇 |
1995年 | 145篇 |
1994年 | 150篇 |
1993年 | 88篇 |
1992年 | 65篇 |
1991年 | 65篇 |
1990年 | 42篇 |
1989年 | 33篇 |
1988年 | 32篇 |
1987年 | 37篇 |
1986年 | 12篇 |
1985年 | 14篇 |
1984年 | 7篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1971年 | 3篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 18 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.
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
3.
4.
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
5.
刘峰 《成都信息工程学院学报》2003,18(4):376-380
分组密码是数据通讯中最常用的数据加密方式,以DES为例分析现有分组加密算法的安全隐患,并提出了可变密钥加密和变长密文输出两个新思路,可应用于所有现有分组加密算法以提高安全性,并就该方法的安全性、效率、具体应用做出了分析。 相似文献
6.
THE SIMULATED STORAGE CAPACITY OF FLOOD AND WATERLOGGING IN THE TYPICAL AGRICULTURAL REGION IN JIANGHAN PLAIN 总被引:2,自引:0,他引:2
1INTRODUCTIONJianghanPlainisoneoftheimportantbasesofChina′sagriculturalproductivitylocatedinthemiddleChangjiangRivervalleyofcentralChina.TheJianghanplainapproximately6.6×104km2inareaandover30millioninpopulationistherenowned“countryoffishandrice"in… 相似文献
7.
向文 《大地测量与地球动力学》1997,(2)
为了根据离散观测数据构制连续空间重力变化图像,分析和讨论了3种数值插值方法,计算结果表明多面函数方法插值精度最高。由于逐步回归分析筛选核函数中心点的计算繁琐,文中提出根据分形理论和Shannon取样定理来确定核函数中心点。对滇西试验场进行模拟试算,插值精度可达到4~5(10-8ms-2)。 相似文献
8.
本文提出了激变变星0623十71的4个时间分辨光谱,覆盖轨道周期的73%.其中的3个谱清楚地显示了各种光谱特征随轨道运动的变化,在流量、能谱以及谱线轮廓方面的这种变化正是对一个倾角不太大的激变变星所期望的.然而另一个曝光对分钟的谱,流量和视向速度与预期的相比都太大,很难加以解释,可能它是一次象AEAqr中观测到的跃发事件. 相似文献
9.
Yanjun ZHANG Fenglong ZHANG Full QI Fuquan NI 《东北亚地学研究》2006,9(2):199-203
In view of the situation of excavation of open coal mine for the underground water disaster,we should carry out simulation studies for the numerical value of the water lowering project and improve the accuracy and the level of the water lowering project.On the basis of the hydrological geological conditions of certain open mine digging,a more reasonable seepage numerical model was built according to MODFLOW.It was simulated in advance that the process of the confined water level descending with the time,and combining with the actual observations to test the correctness of the model.The calculation showed that the results coincided well with the results of actual measurement.Based on this,different water lowering numerical simulations were built for the open coal mine digging.It could be simulated and forecast that the changes of the groundwater level in drainage process within and outside the mine pit,and it was quantitatively assessed that the possible water lowering result of the opencast water drainage process,which provide an important basis for the actual water lowering project and the possible project disposal. 相似文献
10.
我们于1989年11月30日晚对PG0027+260进行了时间分辨率为108秒的高速CCD测光,得到了一条完整的光变曲线,从而确认其为激变食变星系统,轨道周期0.146~d 相似文献