全文获取类型
收费全文 | 1623篇 |
免费 | 293篇 |
国内免费 | 343篇 |
专业分类
测绘学 | 81篇 |
大气科学 | 364篇 |
地球物理 | 375篇 |
地质学 | 803篇 |
海洋学 | 143篇 |
天文学 | 153篇 |
综合类 | 160篇 |
自然地理 | 180篇 |
出版年
2024年 | 2篇 |
2023年 | 34篇 |
2022年 | 77篇 |
2021年 | 84篇 |
2020年 | 71篇 |
2019年 | 53篇 |
2018年 | 102篇 |
2017年 | 97篇 |
2016年 | 102篇 |
2015年 | 83篇 |
2014年 | 112篇 |
2013年 | 89篇 |
2012年 | 72篇 |
2011年 | 74篇 |
2010年 | 93篇 |
2009年 | 77篇 |
2008年 | 59篇 |
2007年 | 73篇 |
2006年 | 52篇 |
2005年 | 43篇 |
2004年 | 53篇 |
2003年 | 40篇 |
2002年 | 40篇 |
2001年 | 43篇 |
2000年 | 52篇 |
1999年 | 74篇 |
1998年 | 53篇 |
1997年 | 59篇 |
1996年 | 62篇 |
1995年 | 50篇 |
1994年 | 49篇 |
1993年 | 36篇 |
1992年 | 28篇 |
1991年 | 29篇 |
1990年 | 24篇 |
1989年 | 28篇 |
1988年 | 17篇 |
1987年 | 9篇 |
1986年 | 14篇 |
1985年 | 13篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 3篇 |
1981年 | 7篇 |
1980年 | 3篇 |
1978年 | 3篇 |
1976年 | 3篇 |
1967年 | 1篇 |
1965年 | 1篇 |
1964年 | 2篇 |
排序方式: 共有2259条查询结果,搜索用时 9 毫秒
821.
大堡—灯坪地区铜铅锌矿成因类型主要有热液充填-交代型、氧化淋滤-次生富集型和动力变质分异型,矿石类型主要有构造角砾岩型、碎裂岩型和裂隙充填型;矿化主要赋存于构造硅化碎裂岩带、硅化角砾岩带、裂隙密集带、断层接触的不整合接触带;矿体多呈脉状、囊状、透镜状或似层状产出;铜铅锌矿化具有一定的水平分带和垂直分带特征。区内地层、构造和不整合面与铜铅锌成矿关系密切,尤其是白垩系神皇山组与泥盆-石炭系的角度不整合界面对成矿有利,为区内主要控矿因素。 相似文献
822.
本文以广州市花都区1990年和2008年TM影像作为数据源,在分析花都区2个时期土地利用格局时空特征的基础上,结合人工神经网络与元胞自动机(ANN-CA)模型对研究区的土地利用空间布局进行模拟。研究表明:根据研究需求构建的ANN-CA模型在实际模拟过程中无需人工定义模型结构和转换规则,只需通过3层神经网络即可模拟复杂的城市形态。因此,通过构建ANN-CA模型对土地利用空间布局进行模拟,可以合理预测城市土地利用发展趋势,具有较强的科学性和可靠性。 相似文献
823.
As a hot topic in Landscape Ecology study, ecological connectivity is an important indicator for regional land sustainable use and biological protection. This paper conducted a systematic assessment of ecological connectivity using remote sensing images of Dongshan Island in 1994, 2003 and 2011. Based on least-cost modelling, the method takes into consideration the type of barrier, the distance impact, and the adjacent land use types to obtain the Barrier Effect Index (BEI) and Ecological Connectivity Index (ECI). The application of this method to Dongshan Island showed the ecological connectivity index (ECI) was low in 1994, improved in 2003, and decreased significantly in 2011. The results of the dynamic analysis of landscape structure showed farmland and roads were the main landscape classes that caused the low observed ECI in 1994 and 2003; these tended to divide the landscape and cause fragmentation. Construction land and roads were the main landscape classes resulting in low ECI in 2011, while forest and grassland had a high ECI. Trajectory analysis showed ECI tended to decrease in the low mountain forest zone of the northwestern and southeastern parts of Dongshan Island as well as in the coastal protection forest area. The areas where ECI became high were located in the northeastern part of Dongshan Island where cities and towns are concentrated with high human populations. Therefore, rapid urbanization has been the most important factor driving changes in landscape structure and patterns during the last 17 years on Dongshan Island. The approach not only assists us in revealing the driving mechanism of landscape dynamics from another aspect, but also can assess the impacts of regional and urban plans on landscape structure and function. 相似文献
824.
Chitosan is a natural polysaccharide prepared by the N-deacetylation of chitin. In this paper we have reviewed the methods of preparation of chitosan-based nanoparticles and their pharmaceutical applications. There are five methods of their preparations: emulsion cross-linking, emulsion-droplet coalescence, ionic gelation, reverse micellar method and chemically modified chitosan method. Chitosan nanoparticles are used as carriers for low molecular weight drug, vaccines and DNA. Releasing characteristics, biodistribution and applications are also summarized. 相似文献
825.
介绍如何应用Matlab设计级联梳状(CIC)滤波器、半带滤波器和有限脉冲响应(FIR)滤波器。并设计了一种高效的多级抽取滤波器,将该算法用数字信号处理器(DSP)进行验证,结果表明,该多级抽取滤波器能满足地震数据采集系统的精度要求。 相似文献
826.
介绍一种基于压控振荡器和GPS技术的时间频率基准,实现可编程脉冲、频率输出,达到了长期稳定性与短期稳定性兼优的目的。 相似文献
827.
????2011??????Walfdange????????????????????ECAG??2011?????FG5/232???????????????????????????FG5/232?????????????????????2.5??10
-8 ms -2???????????????н????????? 相似文献
828.
Neuropeptide Y(NPY) is a 36-amino acid peptide of the neuropeptide Y family that plays key roles in the regulation of food intake. In this study,we focused on NPY m RNA expression changes around feeding time and during food deprivation in olive flounder. The olive flounder NPY m RNA levels were analyzed in different tissues and a high level of expression was detected in the brain. We also demonstrated a correlation between NPY expression levels in the brain and feeding schedule. NPY expression levels in olive flounder maintained on a daily scheduled feeding regimen increased shortly before feeding and decreased after the scheduled feeding time. Compared with the-1 h group before feeding,NPY expression in the 3 h group after feeding decreased significantly( P <0.05). Food deprivation led to an 81.7% decrease in NPY m RNA levels in the 24 h fasted group(P <0.05) and a 91.7% decrease in the 48 h fasted group(P <0.05). Therefore,our study demonstrates that NPY expression is associated with food intake in olive flounder. This result reveals the function of NPY in regulating food intake and its potential importance in olive flounder aquaculture. 相似文献
829.
A new species, Clathrocorys gracilis nov. spec., of Tripocyrtidae (Radiolarians: Nassellaria) from surface water in northern South China Sea, is described in this paper. The new species has four radial beams arising from the cephalic base centre, each foot with cup-shaped structure and ramified bar in the proximal section (near cephalic base), and each wing with a large mesh in the middle, a medium mesh and a small mesh on each side respectively. 相似文献
830.
谭术魁 《中国地理科学(英文版)》1999,9(1):1-11
Duringthefloodperiodof1998,someregionsofChinasufferedseriousfloodingandwaterlogging.EspeciallyintheChangjiangRiver,thewholebasinfloodonceemergedin1954occurredagain,andthemostseverefloodinthehistoryalsoemergedintheSonghuaRiverandtheNenjiangRiver,whicharousedmuchattentionacrossthewholecountry.Accordingtoincompletestatistics,till22ndofAugust,29provinces,autonomousregionsandmunicipalitieshadbeenaffectedbythefloodsinvaryingdegrees.ZI.2millionhaoflandwereinundated,223millionPeOPlewereaffeCted,3… 相似文献