全文获取类型
收费全文 | 28860篇 |
免费 | 1114篇 |
国内免费 | 2131篇 |
专业分类
测绘学 | 1876篇 |
大气科学 | 2749篇 |
地球物理 | 5412篇 |
地质学 | 14267篇 |
海洋学 | 1661篇 |
天文学 | 1776篇 |
综合类 | 2542篇 |
自然地理 | 1822篇 |
出版年
2024年 | 48篇 |
2023年 | 143篇 |
2022年 | 326篇 |
2021年 | 343篇 |
2020年 | 281篇 |
2019年 | 287篇 |
2018年 | 5026篇 |
2017年 | 4274篇 |
2016年 | 2836篇 |
2015年 | 505篇 |
2014年 | 370篇 |
2013年 | 304篇 |
2012年 | 1208篇 |
2011年 | 2967篇 |
2010年 | 2272篇 |
2009年 | 2528篇 |
2008年 | 2083篇 |
2007年 | 2542篇 |
2006年 | 197篇 |
2005年 | 349篇 |
2004年 | 540篇 |
2003年 | 583篇 |
2002年 | 554篇 |
2001年 | 285篇 |
2000年 | 258篇 |
1999年 | 215篇 |
1998年 | 137篇 |
1997年 | 86篇 |
1996年 | 78篇 |
1995年 | 77篇 |
1994年 | 73篇 |
1993年 | 58篇 |
1992年 | 57篇 |
1991年 | 28篇 |
1990年 | 31篇 |
1989年 | 18篇 |
1988年 | 16篇 |
1987年 | 12篇 |
1986年 | 12篇 |
1985年 | 12篇 |
1984年 | 9篇 |
1983年 | 2篇 |
1982年 | 8篇 |
1981年 | 27篇 |
1980年 | 22篇 |
1977年 | 1篇 |
1976年 | 6篇 |
1958年 | 4篇 |
1957年 | 1篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Here we report iron (Fe) isotopic data of three pure Fe solution standards (IRMM-014, GSB Fe, and NIST 3126a) and five widely used geological reference materials (RMs) from the United States Geological Survey and Geological Survey of Japan obtained on a Neptune Plus multi-collector–inductively coupled plasma–mass spectrometer (MC-ICP-MS) in our laboratory over the past 3 years. The instrumental mass bias was corrected by three independent methods: sample-standard bracketing (SSB), Ni doping?+?SSB, and 57Fe–58Fe double spike?+?SSB. Measurements reveal that both the Ni doping and double spike methods helped calibrate short-term fluctuations in mass bias. Collectively, almost all measurements of RMs yielded δ56Fe within?±?0.05 of recommended values, provided that each sample was measured four times on MC-ICP-MS. For the first time, new recommended values for NIST SRM3126a are reported (δ56Fe?=?0.363?±?0.006, 2SE, 95% CI; and δ57Fe?=?0.534?±?0.010, 2SE). 相似文献
52.
W. J. Zhang M. Zhang F. Xiao L. P. Fang D. S. Wang 《International Journal of Environmental Science and Technology》2014,11(3):731-738
Pretreatment of waste emulsions with high organic content by a combined process of vibratory shear enhanced process and Fenton’s oxidation prior to biological treatment was investigated. Vibrating membrane had shown good performance in chemical oxygen demand and oil removals and the mitigation of concentration polarization. However, the permeate after filtration processing still contained high content of organics. Thus, additional Fenton oxidation was applied to reduce the organic loading, and improve the biodegradability of the wastewater. The optimal molar ratio of ferrous iron to hydrogen peroxide was 0.05 obtained from the jar-test experiments. Removal of organics was enhanced by increasing hydrogen peroxide dosage, while efficiency of hydrogen peroxide reached maximum of 1.11(w/w) at the hydrogen peroxide dosage of 6.8 g/L. Furthermore, the biological experiments indicated that the high concentration of organics could inhibit microbial activity, which decreased the chemical oxygen demand degradation rates. The adaptive period of the microbe was greatly shortened using Fenton’s reagent at the low dosages. The improvement of the biodegradability could be explained by partial mineralization and chemical transformation of parent organic compounds after Fenton oxidation. 相似文献
53.
Calcite was synthesized by four methods, and the luminescence decay-time was measured for nine samples before and after heating
hydrothermally in the temperature range 200–400°C. Decay-time data were collected between room temperature and approximately
15 K. The decay time at room temperature is approximately 50 ms, with little difference between a given calcite before and
after hydrothermal treatment. The decay time at 15 K is always greater than at room temperature as the effect of thermal quenching
diminishes. Differences in decay time before and after heating are more apparent at low temperature owing to this reduction
in thermal quenching. The decay time decreased significantly in two samples, and an increase in decay time was observed in
the remaining seven samples following heating. Among the latter group, the change in decay time was insignificant in three
samples. The results are compared with previous data in which it was shown that the effect of heating is to increase the intensity
of luminescence. 相似文献
54.
Citizens as sensors: the world of volunteered geography 总被引:62,自引:0,他引:62
Michael F. Goodchild 《GeoJournal》2007,69(4):211-221
In recent months there has been an explosion of interest in using the Web to create, assemble, and disseminate geographic information provided voluntarily by individuals. Sites such as Wikimapia and OpenStreetMap are empowering citizens to create a global patchwork of geographic information, while Google Earth and other virtual globes are encouraging volunteers to develop interesting applications using their own data. I review this phenomenon, and examine associated issues: what drives people to do this, how accurate are the results, will they threaten individual privacy, and how can they augment more conventional sources? I compare this new phenomenon to more traditional citizen science and the role of the amateur in geographic observation. 相似文献
55.
Despite a recent increase in the number of vulnerability analyses there has been relatively little discussion of vulnerability
assessment of social–environment system, especially when they face multiple hazards. In this study, we developed an applicable
and convenient method to assess vulnerability of social–environment system at a regional scale. Vulnerability is quantified
by measuring three critical elements (i.e. hazards, sensitivity, and resilience) through some key variables. The results showed
that vulnerability is high in Miaofeng Mountain in Mengtougou District, the hills of Pinggu County and the riparian zones
of the lower courses of the Beiyun and Yongding Rivers; but low in the city of Beijing and the southwestern part of the Fangshan
District. Areas of very high, high, medium, and low-vulnerability account for 6.19, 25.48, 33.06, and 35.27% of the total
area, respectively. The degree of vulnerability decreases in a northwest direction in mountainous areas and declines from
watercourses to riparian zones along a lateral direction in the plain. Some adaptive strategies are also proposed. 相似文献
56.
选取123例乳腺病理性矿化样品,利用光镜、扫描电镜对乳腺矿化进行了微形貌分类研究,并利用统计学方法探讨了乳腺矿化集合体类型与患者年龄、乳腺病变类型和癌变阶段的相关性。研究发现,根据乳腺矿化在光学显微镜下的可分辨形态,将其分为两种类型:粒状集合体与块状集合体;其中粒状集合体为小于200μm的规则粒状钙化,少部分有环带结构,和有机组织结合紧密;而块状集合体则为大于200μm的片状钙化,形状不规则。统计分析数据显示,随年龄增加,块状集合体出现病理比例增加,粒状集合体出现病理比例减少,敏感年龄分区界限为50岁;粒状集合体在50岁以上和50岁以下患者中单独出现的概率分别为24%和53%;块状集合体在乳腺原位癌(71. 4%)患者中占主导,粒状集合体在乳腺纤维腺瘤(64. 5%)患者中占主导。该研究可为未来乳腺癌的诊断提供基于乳腺矿化精细矿物学特征的辅助新方法。 相似文献
57.
Mark Anglin Harris 《Environmental Geology》2009,56(8):1517-1522
Several years (7) after incidental lithification by added gypsum, the effect of crushing and phyto-organics on some physical
properties of red mud wastes (RMW) was examined under laboratory conditions. It was hypothesized that mechanically breaking
up the hardened masses would increase their permeability. However, on the contrary, crushing these rock-hard masses into <2-mm
particles did not increase permeability. A phyto-organic treatment on the other hand greatly improved permeability of the
crushed entities such that infiltration rates increased from 0 to 10 cm/h. After 24 weeks of incubation, the structure of
the newly formed phyto-organically treated gypsum-hardened RMW aggregates was also substantially improved over that of the
controls, as a >3-fold increase in resistance to disintegration during wet sieving was exhibited by the phyto-organically
treated samples. The increase in water infiltration by combining crushing with phyto-organic applications was ascribed to
a decrease in particle dispersion, creating more stable clay clusters, which in turn stabilized channels and pores. Conversely,
it was concluded that the singular crushing of gypsum-hardened bauxite waste in the waste dumps is potentially wasteful and
ineffective. 相似文献
58.
在简要介绍预选区水文地质条件基础上,主要通过压水试验、抽水试验等水文地质试验得出各种水文地质参数从而分析预选区含水介质渗透性变化特征以及地层渗透性变化规律。 相似文献
59.
本文提出了用NOAA气象卫星AVHRR的定量资料计算冬小麦种植面积的方法,即绿度指数法和单通道法。绿度指数法是根据冬小麦在特定生育期内绿度值基本上保持为常数的特点,用几块巳知样地的种植面积,推算整体种植面积;单通道法是只使用AVHRR第二通道的反照率来计算冬小麦种植面积。这两种方法的优点在于不需考虑大气削弱的订正处理问题,从而使计算得以简化。 相似文献
60.