全文获取类型
收费全文 | 21240篇 |
免费 | 4112篇 |
国内免费 | 4281篇 |
专业分类
测绘学 | 1161篇 |
大气科学 | 1792篇 |
地球物理 | 4916篇 |
地质学 | 12916篇 |
海洋学 | 3207篇 |
天文学 | 79篇 |
综合类 | 1642篇 |
自然地理 | 3920篇 |
出版年
2024年 | 64篇 |
2023年 | 304篇 |
2022年 | 657篇 |
2021年 | 961篇 |
2020年 | 858篇 |
2019年 | 928篇 |
2018年 | 793篇 |
2017年 | 907篇 |
2016年 | 843篇 |
2015年 | 960篇 |
2014年 | 1280篇 |
2013年 | 1533篇 |
2012年 | 1276篇 |
2011年 | 1358篇 |
2010年 | 1304篇 |
2009年 | 1309篇 |
2008年 | 1336篇 |
2007年 | 1377篇 |
2006年 | 1426篇 |
2005年 | 1248篇 |
2004年 | 1185篇 |
2003年 | 1073篇 |
2002年 | 988篇 |
2001年 | 868篇 |
2000年 | 822篇 |
1999年 | 700篇 |
1998年 | 582篇 |
1997年 | 493篇 |
1996年 | 396篇 |
1995年 | 378篇 |
1994年 | 328篇 |
1993年 | 270篇 |
1992年 | 199篇 |
1991年 | 144篇 |
1990年 | 102篇 |
1989年 | 102篇 |
1988年 | 61篇 |
1987年 | 59篇 |
1986年 | 34篇 |
1985年 | 36篇 |
1984年 | 20篇 |
1983年 | 15篇 |
1982年 | 7篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1979年 | 3篇 |
1978年 | 13篇 |
1977年 | 4篇 |
1973年 | 3篇 |
1971年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
A new computational framework is developed for the design and retrofit of building structures by considering aseismic design as a complex adaptive process. For the initial phase of the development within this framework, genetic algorithms are employed for the discrete optimization of passively damped structural systems. The passive elements may include metallic plate dampers, viscous fluid dampers and viscoelastic solid dampers. The primary objective is to determine robust designs, including both the non‐linearity of the structural system and the uncertainty of the seismic environment. Within the present paper, this computational design approach is applied to a series of model problems, involving sizing and placement of passive dampers for energy dissipation. In order to facilitate our investigations and provide a baseline for further study, we introduce several simplifications for these initial examples. In particular, we employ deterministic lumped parameter structural models, memoryless fitness function definitions and hypothetical seismic environments. Despite these restrictions, some interesting results are obtained from the simulations and we are able to gain an understanding of the potential for the proposed evolutionary aseismic design methodology. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
992.
吉林省发展矿泉水产业的战略思考 总被引:1,自引:0,他引:1
本文在分析吉林省矿泉水资源特征和矿泉水事业发展现状的基础上,结合国内外矿泉水产业的发展趋势,指出了当前吉林省矿泉水产业发展中存在的问题,提出了快速发展矿泉水产业的对策和建议。 相似文献
993.
994.
城市缺水是目前带有普遍性的重大问题 ,但在城市建设过程中 ,宝贵的地下水资源遭到浪费的现象又大量存在 ,解决好施工降水与环境保护问题是关系到城市发展的重要问题之一 ,结合郑州市的工程实践 ,谈谈施工降水中的环境保护及水资源合理利用问题。 相似文献
995.
库水位变化对库岸边坡稳定性的影响 总被引:3,自引:0,他引:3
在假定坡体孔隙水水位为水平线且不考虑渗透作用影响的基础上,基于极限平衡法考察了水位上升及下降的快慢对边坡安全系数的影响。对比计算表明:在水位缓慢变化即坡体内外水位线等高的条件下,边坡的安全系数随着水位坡高比的增大先略减小后急剧增大,且在水位坡高比为0.3处取得最小值,在边坡完全淹没于水中时取得最大值。当边坡完全淹没于水中后,水位高于坡顶的多少对边坡安全系数没有影响;在水位骤降或陡升条件下,相同库水位对应的边坡安全系数基本上均小于水位缓慢变化情况下的安全系数,故工程实际中无论是排水还是蓄水,都应尽量保持水位缓慢变化,这样才能使边坡处于较安全的状态。 相似文献
996.
997.
为探明宁东煤田枣泉矿井首采区煤层自然范围及充水性,根据采区内地球物理物征,采用高精度磁法,直流电测深法和激发极化法进行综合勘探。在磁法勘探中结合测区曲线特征,建立数据体模型,正演计算烧变岩边界;电法勘探利用烧变岩含水后电阻率降低的特征确定其含水范围及富水性。 相似文献
998.
The aim of this study is to shed light on the hydrogeochemical characteristics of karst underground waters at shallow depth in Guiyang City, Guizhou Province with an emphasis on the geochemistry of major elements. Guiyang City bears abundant underground waters and it is also an important representative of the karst areas throughout the world. Ca^2 and Mg^2 are the dominant cations, accounting for 81% -99.7% of the total, and HCO3^- and SO4^2- are the dominant anions. Weathering of limestones and dolostones is the most important factor controlling the hydrogeochemistry of underground waters, and weathering of sulfate and evaporite rocks is less important. Moreover, the precipitation and human activities also have a definite influence on the hydrogeoehemistry of underground waters in the region studied. 相似文献
999.
1000.
Under Rayleigh equilibrium condition, stable isotopic ratio in residual water increases with the decrease of the residual water proportion f exponentially, and the fractionation rate of stable isotopes is inversely proportional to temperature. However, under kinetic evaporation condition, the fractionation of stable isotopes is not only related to the phase temperature but also influenced by the atmospheric humidity and the mass exchange between liquid and vapor phases. The ratio δ in residual water will not change with f after undergoing evaporation of a long time for great relative humidity. The rate that the evaporating water body reaches isotopic steady state is mainly dependent on the relative humidity in atmosphere. The analysis shows that the actual mean linear variety rates, about -30.0, of the δ18O in residual water versus the residual water proportion at Nagqu and Amdo stations are consistent with the simulated process under temperature of 20 oC and relative humidity of 50%. The distillation line simulated under Rayleigh equilibrium condition is analogous to the global meteoric water line (MWL) as the temperature is about 20 oC. Under non-equilibrium condition, the slope and constant values of distillation line are directly proportional to temperature and relative humidity. According to the basic data, the simulated distillation line is very consistent with the actual distillation line of Qinghai Lake. 相似文献