全文获取类型
收费全文 | 65440篇 |
免费 | 31044篇 |
国内免费 | 17292篇 |
专业分类
测绘学 | 12990篇 |
大气科学 | 9701篇 |
地球物理 | 13321篇 |
地质学 | 47867篇 |
海洋学 | 13417篇 |
天文学 | 1012篇 |
综合类 | 5892篇 |
自然地理 | 9576篇 |
出版年
2025年 | 140篇 |
2024年 | 2537篇 |
2023年 | 2873篇 |
2022年 | 4517篇 |
2021年 | 5235篇 |
2020年 | 4519篇 |
2019年 | 5027篇 |
2018年 | 4472篇 |
2017年 | 4231篇 |
2016年 | 4369篇 |
2015年 | 5099篇 |
2014年 | 4777篇 |
2013年 | 5657篇 |
2012年 | 6265篇 |
2011年 | 5974篇 |
2010年 | 5962篇 |
2009年 | 5669篇 |
2008年 | 5824篇 |
2007年 | 5424篇 |
2006年 | 5083篇 |
2005年 | 4361篇 |
2004年 | 3299篇 |
2003年 | 2308篇 |
2002年 | 2388篇 |
2001年 | 2158篇 |
2000年 | 1807篇 |
1999年 | 964篇 |
1998年 | 433篇 |
1997年 | 310篇 |
1996年 | 303篇 |
1995年 | 202篇 |
1994年 | 205篇 |
1993年 | 194篇 |
1992年 | 206篇 |
1991年 | 114篇 |
1990年 | 128篇 |
1989年 | 75篇 |
1988年 | 56篇 |
1987年 | 62篇 |
1986年 | 72篇 |
1985年 | 60篇 |
1984年 | 50篇 |
1983年 | 48篇 |
1982年 | 41篇 |
1981年 | 34篇 |
1980年 | 16篇 |
1979年 | 30篇 |
1978年 | 21篇 |
1957年 | 22篇 |
1954年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
选取pH、DO、CODCr、TN、TP等水质指标近10a的监测数据,采用单因子指数及内梅罗指数法,对东昌湖六湖区水质动态变化进行了分析研究。结果表明:pH、DO单因子指数在调查年度内呈波动中下降趋势,且均满足IV类水水质标准;TN、TP、CODCr三项指标单因子指数较大,对湖区水体质量影响较大,是造成东昌湖水体质量变化的主要原因,特别是TN、TP;内梅罗指数呈波动中下降趋势,表明东昌湖水质近10a内在逐步改善。最后在认真分析东昌湖水质动态变化状况的基础上,进一步提出了针对性的防治建议。 相似文献
22.
One of the problems in signal processing is estimating the impulse response function of an unknown system. The well-known Wiener filter theory has been a powerful method in attacking this problem. In comparison, the use of stochastic approximation method as an adaptive signal processor is relatively new. This adaptive scheme can often be described by a recursive equation in which the estimated impulse response parameters are adjusted according to the gradient of a predetermined error function. This paper illustrates by means of simple examples the application of stochastic approximation method as a single-channel adaptive processor. Under some conditions the expected value of its weight sequence converges to the corresponding Wiener optimum filter when the least-mean-square error criterion is used. 相似文献
23.
An experimental study on reduction of U (Ⅵ) by anaerobic bacteria, Shewane//a putrefaciens, is first reported here in China. The experimental conditions were: 35℃ and pH =7.0-7.4, corresponding to a physicochemical environments in which the sandstone-hosted interlayer oxidation-zone type uranium deposit formed in Northwest China's Xinjiang. Bacteria adopted in the present experiment, Shewanella putrefaciens, occur extensively in natural environment. Our study shows that nano-crystal precipitates of uraninite quickly occurred on the surface of the cells within one week. It was found that the pitchblende was characterized by a random arrangement of uraninite nanocrystals (2-4 nm) in it, significantly different from natural pitchblende in which uraninite nanocrystals are arranged in order. Finally, a possible mechanism of uranium biomineralization by microorganisms in the deposits is discussed. Our investigation may supply a technical train of thoughts for bioremediation of nuclear-contaminated water environments and for underground dissolving extraction of the sandstone-hosted uranium ores. 相似文献
24.
SHA Liqing ZHENG Zheng TANG Jianwei Wang Yinghong ZHANG Yiping CAO Min WANG Rui Liu Guangren WANG Yuesi SUN Yang 《中国科学D辑(英文版)》2005,48(Z1)
With the static opaque chamber and gas chromatography technique, from January 2003 to January 2004 soil respiration was investigated in a tropical seasonal rain forest in Xishuangbanna, SW China. In this study three treatments were applied, each with three replicates: A (bare soil), B (soil+litter), and C (soil+litter+seedling). The results showed that soil respiration varied seasonally, low from December 2003 to February 2004, and high from June to July 2004. The annual average values of CO2 efflux from soil respiration differed among the treatments at 1% level, with the rank of C (14642 mgCO2· m-2. h-1)>B (12807 mgCO2· m-2. h-1)>A (9532 mgCO2· m-2. h-1). Diurnal variation in soil respiration was not apparent due to little diurnal temperate change in Xishuangbanna. There was a parabola relationship between soil respiration and soil moisture at 1% level. Soil respiration rates were higher when soil moisture ranged from 35% to 45%. There was an exponential relationship between soil respiration and soil temperature (at a depth of 5cm in mineral soil) at 1% level. The calculated Q1o values in this study,ranging from 2.03 to 2.36, were very near to those of tropical soil reported. The CO2 efflux in 2003was 5.34 kgCO2· m-2. a-1 from soil plus litter plus seedling, of them 3.48 kgCO2· m-2. a-1 from soil (accounting for 62.5%), 1.19 kgCO2· m-2. a-1 from litter (22.3%) and 0.67 kgCO2·m-2. a-1 from seedling (12.5%). 相似文献
25.
XU Jianming XU Xiangde LIU Yu DING Guoan CHEN Huailiang HU Jiangkai ZHANG Jianchun WU Hao LI Weiliang HE Jinhai YANG Yuanqin WANG Jiahe 《中国科学D辑(英文版)》2005,48(Z2)
Based on analysis of the air pollution observational data at 8 observation sites in Beijing including outer suburbs during the period from September 2004 to March 2005, this paper reveals synchronal and in-phase characteristics in the spatial and temporal variation of air pollutants on a city-proper scale at deferent sites; describes seasonal differences of the pollutant emission influence between the heating and non-heating periods, also significantly local differences of the pollutant emission influence between the urban district and outer suburbs, i.e. the spatial and temporal distribution of air pollutant is closely related with that of the pollutant emission intensity. This study shows that due to complexity of the spatial and temporal distribution of pollution emission sources, the new generation Community Multi-scale Air Quality (CMAQ) model developed by the EPA of USA produced forecasts, as other models did, with a systematic error of significantly lower than observations, albeit the model has better capability than previous models had in predicting the spatial distribution and variation tendency of multi-sort pollutants. The reason might be that the CMAQ adopts average amount of pollutant emission inventory, so that the model is difficult to objectively and finely describe the distribution and variation of pollution emission sources intensity on different spatial and temporal scales in the areas, in which the pollution is to be forecast. In order to correct the systematic prediction error resulting from the average pollutant emission inventory in CMAQ, this study proposes a new way of combining dynamics and statistics and establishes a statistically correcting model CMAQ-MOS for forecasts of regional air quality by utilizing the relationship of CMAQ outputs with corresponding observations, and tests the forecast capability. The investigation of experiments presents that CMAQ-MOS reduces the systematic errors of CMAQ because of the uncertainty of pollution emission inventory and improves the forecast level of air quality. Also this work employed a way of combining point and area forecasting, i.e. taking the products of CMAQ for a center site to forecast air pollution for other sites in vicinity with the scheme of model products "reanalysis" and average over the "area". 相似文献
26.
我们搜集了国际国内地震台网资料、宏观考察资料、地震专题研究报告,对1900—1949年5级以上地震全部进行了核查,重新进行定位处理,并结合宏观考察资料和前人工作成果进行分析研究,得到较为准确的震中位置。通过这项研究,增补了原目录中遗漏的地震,并对目录中的错误进行了修正,从而得到一份较为完整可靠的地震目录。 相似文献
27.
云南西部施甸地区早石炭世早期地层称香山组,分上、下两段。下段建立四射珊瑚Rotiphyllum yudongens-Commutia crassoseptata-Antikinkaidia typica组合带。上段下部建立Thuriantha sinensis-Tachylasma shidianense组合带.本文共描述四射珊瑚11个属,14个种和2个未定种(其中包括2个新属,12个新种)。 相似文献
28.
辽宁抚顺下第三纪含煤地层发育,其中古城子组主煤层以盛产琥珀闻名中外,琥珀中含有大量昆虫化石,本文重点记述产自该组琥珀中的同翅目扁蚜科的一个新属—乐氏蚜属。 相似文献
29.
West Sarawak,which located in the northwest of South Kalimantan Diwa Region,underwent geosynclinal period (An C-T),platform period (J1-J2) and diwa period (J3-).In geosynclinal evolution period,the crust was thickened and accreted by sedimentary accumulating,orogenic movement and metamorphism.In the different kinds of volcanic-sedimentary rocks,content of gold is general higher than the mean value of upper crust and its enrichment factor ranges from 0.94 to 4.7.The basement tectonics formed in geosynclinal fold inversionstage strike northeast,which controlled the distribution ofmagmatic activities and gold mineralization belts of the west part of west Sarawak. 相似文献
30.
This paper discusses the strategy for successfully predicting the location of potential hidden ore bodies in aged ore field,and presents the result of location prediction of hidden ore bodies in Fenghuangshan ore field,Tongling.Innovative conceptual targeting procedures based on a genetic understanding of mineralization systems,carefully geological investigation and correct deduction,together with new geochemical and geophysical technology and integrating of comprehensive information are all very important for the successful prediction.In the aged Fenghuangshan ore field,through researching by application of the metallogenic theory of polygenetic compound ore deposits and triple-frequency induced polarization method and exploration tectono-geochemical method,we predicted location and quality of hidden ore bodies.According to the prediction,hidden high quality Cu-Au ore bodies of skarn type and porphyry type have been discovered. 相似文献