首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   246篇
  免费   11篇
  国内免费   11篇
测绘学   12篇
大气科学   19篇
地球物理   55篇
地质学   98篇
海洋学   27篇
天文学   24篇
综合类   6篇
自然地理   27篇
  2024年   1篇
  2023年   3篇
  2022年   6篇
  2021年   8篇
  2020年   3篇
  2019年   13篇
  2018年   13篇
  2017年   9篇
  2016年   10篇
  2015年   10篇
  2014年   13篇
  2013年   12篇
  2012年   11篇
  2011年   13篇
  2010年   18篇
  2009年   12篇
  2008年   10篇
  2007年   12篇
  2006年   14篇
  2005年   4篇
  2004年   9篇
  2003年   9篇
  2002年   8篇
  2001年   4篇
  2000年   2篇
  1998年   1篇
  1997年   3篇
  1996年   5篇
  1995年   7篇
  1994年   3篇
  1993年   2篇
  1992年   6篇
  1991年   1篇
  1990年   2篇
  1989年   2篇
  1988年   2篇
  1986年   1篇
  1980年   1篇
  1979年   1篇
  1978年   1篇
  1974年   1篇
  1969年   1篇
  1960年   1篇
排序方式: 共有268条查询结果,搜索用时 15 毫秒
201.
202.
Propylene glycol alginate sodium sulfate-loaded nanoparticles(PSS-NP) has been shown potential to prevent the microvascular endothelial injuries caused by diabetic cardiomyopathy(DCM). In this study, we aimed to investigate the effects of PSS-NP on the dysfunction of cardiac microvascular endothelia in streptozotocin(STZ)-induced DCM rat model. Echocardiographic measurements showed a significant improvement of cardiac function in the PSS-NP-treated group. Our results revealed that the abnormalities of cardiac systolic and diastolic functions were suppressed by the treatments of prostaglandin E1(PGE1), low molecular weight heparin(LMWH), PSS and PSS-NP. Our comparison analysis indicated that PSS-NP showed the strongest inhibitory effects on microvascular endothelial injuries. Transmission electron microscopy analysis demonstrated that PSS-NP protected the cardiac microvascular endothelium and further improved endothelium dysfunction in DCM rats. ELISA and Western blot assays further showed a high efficiency of improving cardiac microvascular endothelial dysfunction with PSS-NP. Our results demonstrated that PSS-NP increased the protein expression of phosphatidylinositol 3-kinase(PI3K)-p85 and vascular endothelial growth factor(VEGF)-A, and the phosphorylation of protein kinase B(Akt) and endothelial nitric oxide synthase(eNOS), which were involved in the amelioration of cardiac microvascular endothelial dysfunction. These data suggest that PSS-NP may be a novel approach to the treatment the coronary microcirculation disorder diseases such as DCM.  相似文献   
203.
ABSTRACT

Due to increased attention on environmental issues, industrial sustainable restructuring is high on the research agenda. The article adds to the literature through an analysis of a restructuring process within a Norwegian process industry cluster. By introducing an analytical framework combining the concepts of entrepreneurial discovery process and path dependency theory, the article offers a deeper understanding of the role of various types of actors in processes of restructuring. The authors categorise the actors as either firm-level entrepreneurs or system-level entrepreneurs on the basis of their motivation. While the former are motivated mainly by firm success, the latter find their motivation mainly in developing systemic factors. The analytical framework suggests a stepwise analysis of the restructuring, and in each step, the significance and interplay between the two types of entrepreneurs are discussed. Finally, the authors discuss the potential for new path development following from this process. The case study illustrates the important role of system-level entrepreneurs as facilitators and of firm-level entrepreneurs as utilisers of opportunities. The empirical case demonstrates that the two types of entrepreneurs are highly interactive throughout the restructuring process. The article contributes to existing literature by advancing the actor perspective in new path development.  相似文献   
204.
ABSTRACT

With combined insights from evolutionary economic geography and transition studies, the article examines the engagement of different regions in Norway in the innovation networks created within the European Union’s environmental programmes. The aim is to explore the programmes’ potential for supporting green economy and economic restructuring through branching and new path creation. The authors assess which regions participate in the programmes, which international networks they build, and which organisations participate in different regions. They compare three regions with different restructuring needs and research capacity – the counties of Rogaland, Hordaland, and Sør-Trøndelag (now part of the county of Trøndelag). They find that overall, Norwegian organisations participate relatively frequently in the programmes, but private firms play a marginal role. Their partners are mainly in core EU regions. Regional participation in the programmes is a function of research capacity as well as oil dependence. The authors conclude that in research-oriented regions, research establishments tend to dominate participation, creating potential for restructuring mainly through path creation. In oil-dependent regions, private firms account for a higher share of participants, enhancing the potential for branching. As the former regions participate more, the programme can mainly stimulate path creation.  相似文献   
205.
Bioassay of water from different parts of the Peter the Great Bay using microalgae P. tricornutum revealed that water quality influences not only the growth but also the physiological state of the microalgae. There was no difference in the cell size between control and experimental cultures. Inhibition of microalgae growth was revealed in water with low phosphates concentration, for example, in the Marine Reserve. High phosphates concentration and moderate organic pollution caused the stimulation of miñroalgae growth in such sites as the Tumen, the Partizanskaya and the Vtoraya rechka rivers mouths.  相似文献   
206.
Larval fish community structure was studied in the northeastern Aegean Sea (NEA) over an area influenced by the advection of Black Sea water (BSW). Sampling was carried out in early summer during a period of 4 years (2003–2006). Taxonomic composition and abundance presented high variability in space that remained relatively constant among years. Tow depth and indicators of trophic conditions in the upper water column (i.e., zooplankton displacement volume, fluorescence) explained significantly the structure of larval assemblages during all surveys. The northern continental shelf (Thracian and Strymonikos shelf), where a large amount of enriched, low salinity BSW is retained, was dominated by larvae of epipelagic species, mainly anchovy (Engraulis encrasicolus). Interannual changes in horizontal extension of the BSW seemed to match closely observed changes in the distribution of anchovy larvae. Mesopelagic fish larvae were particularly abundant beyond the continental shelf (over the North Aegean Trough) where a strong frontal structure is created between the low salinity waters of BSW origin and the high salinity waters of the Aegean Sea. Larvae of certain mesopelagic species (e.g., Ceratoscopelus maderensis) may occasionally be transported inshore when the prevailing current meanders towards the coast or feeds anticyclonic gyres over the continental shelf.  相似文献   
207.
为了探讨地震和地裂缝耦合作用下位于地裂缝上盘的地铁隧道顶部地表沉降规律及其对附近建筑物的影响,以邻近穿越地裂缝场地的西安地铁3号线为工程背景,利用FLAC3D有限差分软件,结合理论分析,对西安人工地震波、El Centro波和Kobe波三种不同地震波作用下邻近地裂缝带地铁隧道建设场地地表沉降问题进行了研究。结果表明:地震作用下,隧道顶部一定范围内的地层沉降量显著大于其周围地层,形成宽度约9~16 m的沉降凹槽;El Centro波作用下沉降凹槽的宽度最大,约15.9 m,超越概率为10%的西安人工合成地震波次之,约11.6 m,而Kobe波作用下沉降凹槽的宽度最小,约9.5 m;隧道上覆地层沉降凹槽的沉降规律符合peck公式;隧道顶部约20 m范围内场地地表受地震和地裂缝耦合作用影响最强烈,沉降最大。   相似文献   
208.
Texture or spatial arrangement of neighborhood objects and features plays an important role in the human visual system for pattern recognition and image classification. The traditional spectral–based image processing techniques have proven inadequate for urban land use and land cover mapping from images acquired by the current generation of fine–resolution satellites. This is because of the high frequency spatial arrangements or complex nature of urban features. There is a need for an effective algorithm to digitally classify urban land use and land cover categories using high–resolution image data. Recent studies using wavelet transforms for texture analysis have generally reported better accuracy. Based on a high–resolution ATLAS image, this study illustrates four different wavelet decomposition procedures – the standard, horizontal, vertical, and diagonal decompositions – for urban land use and land cover feature extraction with the use of 33×33 pixel samples. The standard decomposition approach was found to be the most efficient approach in urban texture analysis and classification. For comparison purposes and to better evaluate the accuracy of wavelet approaches in image classification, spatial autocorrelation techniques (Moran's I and Geary's C ) and the spatial co–occurrence matrix method were also examined. The results suggest that the wavelet transform approach is superior to all other approaches.  相似文献   
209.
The interannual variability associated with the El Ni?o/Southern Oscillation (ENSO) cycle is investigated using a relatively high-resolution (T42) coupled general circulation model (CGCM) of the atmosphere and ocean. Although the flux correction is restricted to annual means of heat and freshwater, the annual as well as the seasonal climate of the CGCM is in good agreement with that of the atmospheric model component forced with observed sea surface temperatures (SSTs). During a 100-year simulation of the present-day climate, the model is able to capture many features of the observed interannual SST variability in the tropical Pacific. This includes amplitude, lifetime and frequency of occurrence of El Ni?o events and also the phase locking of the SST anomalies to the annual cycle. Although the SST warming during the evolution of El Ni?os is too confined spatially, and the warming along the Peruvian coast is much too weak, the patterns and magnitudes of key atmospheric anomalies such as westerly wind stress and precipitation, and also their eastward migration from the western to the central equatorial Pacific is in accord with observations. There is also a qualitative agreement with the results obtained from the atmospheric model forced with observed SSTs from 1979 through 1994. The large-scale dynamic response during the mature phase of ENSO (December through February) is characterized by an eastward displacement and weakening of the Walker cell in the Pacific while the Hadley cell intensifies and moves equatorward. Similar to the observations, there is a positive correlation between tropical Pacific SST and the winter circulation in the North Pacific. The deepening of the Aleutian low during the ENSO winters is well captured by the model as well as the cooling in the central North Pacific and the warming over Canada and Alaska. However, there are indications that the anomalies of both SST and atmospheric circulation are overemphasized in the North Pacific. Finally, there is evidence of a coherent downstream effect over the North Atlantic as indicated by negative correlations between the PNA index and the NAO index, for example. The weakening of the westerlies across the North Atlantic in ENSO winters which is related to a weakening and southwestward displacement of the Icelandic low, is in broad agreement with the observations, as well as the weak tendency for colder than normal winters in Europe. Received: 31 October 1995 / Accepted: 29 May 1996  相似文献   
210.
正1 Introduction The salt lakes of the Crimea contain the practically inexhaustible sources of salts of sodium,magnesium,bromine and other chemical elements(Ponizovskii,1965),being the potential powerful raw materials base for the  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号