Na2CaV4O12: A low-temperature-firing dielectric with lightweight, low relative permittivity, and dielectric anomaly around 515 C

A low temperature fired Na2CaV4O12 ceramic was synthesized via a solid-state reaction route at a temperature range of 350–550 °C. The Thermal analysis confirmed the densification and melting temperature of Na2CaV4O12 to be 530 °C and 580 °C, respectively. Dielectric properties together with the electrical conductivity were characterized at a broad frequency and temperature range. A super-low relative permittivity of εr = 7.72 and loss tangent of tanδ = 0.06 were obtained at 1 MHz at room temperature. A dielectric anomaly peak took place around 515 °C, which was associated with the phase transition from P4/nbm to P 4‾ b2. Ac impedance spectrum coupled with complex modulus plots unveiled the electrical conduction mechanism, which was dominated by the short-range movement of the charge carriers at low temperatures (T ≤ 220 °C) however long-range migration of charge carriers emerged at higher temperatures

Measuring spatio-temporal dynamics of impervious surface in Guangzhou, China, from 1988 to 2015, using time-series Landsat imagery

This study evaluated the spatio-temporal change characteristics of urban development at different scales with time-series impervious surface fractions. Landsat-5 Thematic Mapper (TM) and Landsat-8 Operational Land Imager (OLI) images were used to extract impervious surface fractions using a modified linear spectral mixture analysis method in Guangzhou from 1988 to 2015. The results indicated that the impervious surface area has substantially increased, from 70.3 km(2) in 1988 to 580.5 km(2) in 2015. In 2015, the impervious surfaces were distributed almost throughout the whole region of the study area, except in the forest region. Next, impervious surface weighted mean centre (ISWMC) and the standard deviational ellipse (SDE) methods were used to systematically analyse the principle orientation, direction, spatio-temporal expansion trends, and the distribution differences of impervious surfaces at the whole and local region scales from 1988 to 2015. The spatio-temporal dynamics of ISWMC exhibited different expansion directions and intensities of impervious surfaces at the whole and local region scales. On a whole region scale, the principle expansion direction of impervious surfaces was northward. However, the expansion trend of impervious surfaces in the different districts was significantly different from other trends at the local region scale. The parameters of SDE were used to investigate the orientation and the clustering or dispersion degree of impervious surface at different scales. The results from SDE analysis indicated that the impervious surfaces exhibited uncertainty in the expansion direction at the whole region scale; in contrast, they had a distinct preferred orientation and expansion direction at the local region scale. The analysis revealed that urban expansion exhibited different change characteristics in various directions at the local region scale. In summary, the results at the local region scale can better reflect the change trajectory of spatio-temporal dynamics of urban development and its fine spatial structure than at the whole region scale. (C) 2018 Elsevier B.V. All rights reserved

Phase transformation and ionic conductivity mechanism of a low-temperature sintering semiconductor Na2CaV4O12

Alkaline earth metal vanadates have drawn attention because of their potential applications in electrochemical devices. Here, Na2CaV4O12 was prepared at extremely low temperatures (350-550 oC) and showed a semiconductor behavior with a bandgap of 2.92 eV. A phase transition from P4/nbm to P ̅ b2 occurred at 510 oC was identified by an in-situ XRD upon heating, where the 16 n site for oxygen atoms in the P4/nbm phase evolves into two distinguishable 8i sites in the P ̅ b2 phase. Ionic conduction in Na2CaV4O12 at elevated temperatures was reported for the first time in the present work. A strong correlation between ionic conductivity and phase structure of Na2CaV4O12 is observed. The charged carriers are mainly sodium ions for the low-temperature P4/nbm phase, while mixed conduction contributed by sodium ions and oxide ions happened in the transformed phase. Bond valence-based energy landscape calculations disclosed a two-dimensional interstitial diffusion mechanism for Na+ ions in the Na2Ca-layers, as well as a two-dimensional diffusion mechanism for oxide ions in the V4O12-layers. The novel semiconductor ceramic would have potential applications in all-solid sodium ions batteries or solid oxide fuel cells as electrolytes

Long-Term Spatiotemporal Characteristics and Impact Factors of Land Surface Temperature of Inhabited Islands with Different Urbanization Levels

Surface thermal environment (STE) is closely related to the comfort and health of residents, affecting regional livability, and its spatial and temporal changes are deeply affected by the urbanization process. Considering there is a lack of effective comparative analysis on STE in different urbanized inhabited islands, the special geographical unit and vital human settlement environment, long-term spatiotemporal characteristics and impact factor quantitative analyses were performed in two inhabited islands via the RS and GIS methods. The results suggest that the surface heat amplitude of the highly urbanized Xiamen Island decreases, with the surface heat intensity continuing to increase from 2000 to 2020, while that of the lowly urbanized Kinmen Island is reversed. Although the land surface temperature (LST) of the two inhabited islands shows similar spatial distribution characteristics with evident cold/hot spots, the geographical distribution characteristics of high LST zones are significantly different, and the thermal landscape of Xiamen Island is more fragmented, discrete, and simple in shape, as revealed by the landscape metrics. We demonstrate that the area proportion between cooling land (water body and greenland) and warming land (bare land and impervious surface) is the most influential factor of LST in the two islands while the marine environment is a unique contributor to STE of inhabited islands compared with inland cities, where the seawater around the island can reduce LST over a range of distances, and the influence of elevation on LST is mostly indirect. These results provide a scientific basis and case support for understanding the STE situation of inhabited islands with different urbanization levels

Using the Eco-Erosion Index to assess regional ecological stress due to urbanization – a case study in the Yangtze river delta urban agglomeration

Urban agglomeration (the spread of cities into large agglomerations) has become the main form of urbanization in China, and natural ecosystems surrounding the urban areas are becoming degraded and fragmented as a result. Although ecological indicators have been widely used to assess the regional ecological stress resulting from urbanization, few of them consider spatial adjacency relationships between urban and natural landscapes. From this perspective a novel ecological indicator, the Eco-Erosion Index (EEI), was developed and applied to assess the regional ecological stress caused by urban agglomeration development across 26 cities in the Yangtze River Delta, China (YRD). We analyzed: i) temporal change in land use and land cover (LULC) and ecosystem services value (ESV) in YRD from 1990 to 2010, ii) spatiotemporal dynamics of EEI of YRD at different scales: provincial, municipal, and 5 km-grid, iii) inter-relationships between EEI and LULC and ESV to explore its effectiveness as an indicator. The results showed that urban agglomeration in YRD has led to increasing regional ecological stress from 1990 to 2010. EEI values increased from 0.197 in 1990 to 0.321 in 2010. The closer to Shanghai City, the greater the EEI values of the cities become. EEI is highly related with LULC and ESV but integrates these two variables as it represents both the spatial occupation and landscape adjacency effects. The EEI values demonstrate some scale effects, and EEI at fine scale provides useful information to guide sustainable urban landscape management

Long-Term Spatiotemporal Characteristics and Impact Factors of Land Surface Temperature of Inhabited Islands with Different Urbanization Levels

Surface thermal environment (STE) is closely related to the comfort and health of residents, affecting regional livability, and its spatial and temporal changes are deeply affected by the urbanization process. Considering there is a lack of effective comparative analysis on STE in different urbanized inhabited islands, the special geographical unit and vital human settlement environment, long-term spatiotemporal characteristics and impact factor quantitative analyses were performed in two inhabited islands via the RS and GIS methods. The results suggest that the surface heat amplitude of the highly urbanized Xiamen Island decreases, with the surface heat intensity continuing to increase from 2000 to 2020, while that of the lowly urbanized Kinmen Island is reversed. Although the land surface temperature (LST) of the two inhabited islands shows similar spatial distribution characteristics with evident cold/hot spots, the geographical distribution characteristics of high LST zones are significantly different, and the thermal landscape of Xiamen Island is more fragmented, discrete, and simple in shape, as revealed by the landscape metrics. We demonstrate that the area proportion between cooling land (water body and greenland) and warming land (bare land and impervious surface) is the most influential factor of LST in the two islands while the marine environment is a unique contributor to STE of inhabited islands compared with inland cities, where the seawater around the island can reduce LST over a range of distances, and the influence of elevation on LST is mostly indirect. These results provide a scientific basis and case support for understanding the STE situation of inhabited islands with different urbanization levels