SnO2 nanowhiskers of mass production have been synthesized by evaporating metal tin (Sn) powders at 800 °C. The synthesized products were characterized with scanning electron microscopy, x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Each SnO2 nanowhisker was a tetragonal rutile single crystal, with the diameters ranging from 50 to 200 nm and the lengths extending to tens of micrometres. The SnO2 nanowhiskers exhibited a sensitivity of 23 to 50 ppm ethanol gas at 300 °C. Our results demonstrated that SnO2 nanowhiskers have a promising application for gas sensor fabrication.
Herein we report the fabrication and study of hierarchically structured fibers of a dendronized polymer that were simply prepared by drawing out from their columnar phase in melt. At first, the combined SEM and SAXS results indicate that the fibers contain fibrils that align along the axial direction and have a diameter of ∼200 nm. Further SAXS characterizations confirm an oriented hexagonally packed cylindrical (HPC) structure assembling in the fibrils. At the molecular level, a wormlike single molecular conformation was visualized by AFM, indicating its relatively stiffer structure compared with conventional polymers. Because of this feature, the dendronized polymer can be fabricated into fibers with a highly ordered structure. Therefore, the hierarchical structure that is found in the fibers sheds light on the possible mechanism of the self-organization of the dendronized polymer on different scales.
As public service facilities, urban parks offer many benefits for daily life and social activities for residents. However, the accessibility of public parks to urban residents is often unevenly distributed in spaces that cannot be utilized fully. Here, we used the urban parks in Beijing, China as a case study and examined the relationship between urban park accessibility and population distribution at different administrative levels. Gini coefficient and Lorenz curve were used to evaluate the social equity of urban park accessibility, and the location quotient was used to identify the spatial difference between urban parks and resident population. The results of our study show that the urban park accessibility varies at district and subdistrict levels and that places with more urban parks usually have higher accessibility. Very importantly, the spatial equity is different from the social equity, a mismatch exists between the spatial distribution of urban parks and population, particularly for the elderly residents. These results generate valuable insights, as, in China and many developing countries, current urban public green space planning only uses the ratio of public green space to urban construction land and the per capita public green area.
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