Analysis of structural defects and their influence on red-emitting γ-Al2O3:Mn4+,Mg2+ nanowires using positron annihilation spectroscopy.

The present paper reported on the analysis of structural defects and their influence on the red-emitting γ-Al2O3:Mn4+,Mg2+ nanowires using positron annihilation spectroscopy (PAS). The nanowires were synthesized by hydrothermal method and low-temperature post-treatment using glucose as a reducing agent. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), and photoluminescence excitation (PLE) were utilized, respectively, for determining the structural phase, morphology and red-emitting intensity in studied samples. Three PAS experiments, namely, positron annihilation lifetime (PAL), Doppler broadening (DB), and electron momentum distribution (EMD), were simultaneously performed to investigate the formations of structural defects in synthesized materials. Obtained results indicated that the doping concentration of 0.06% was optimal for the substitution of Mn4+ and Mg2+ to two Al3+ sites and the formation of oxygen vacancy (VO)-rich vacancy clusters (2VAl + 3VO) and large voids (~0.7 nm) with less Al atoms. Those characteristics reduced the energy transfer between Mn4+ ions, thus consequently enhanced the PL and PLE intensities. Moreover, this optimal doping concentration also effectively controlled the size of nanopores (~2.18 nm); hence, it is expected to maintain the high thermal conductivity of γ-Al2O3 nanowire-phosphor. The present study, therefore, demonstrated a potential application of γ-Al2O3 nanowire-phosphor in fabricating the high-performance optoelectronic devices.

Evaluation of energy intensity of transport service sectors in Vietnam.

This research analyses energy intensity of transport service sectors in Vietnam and its changing trend in the past years using IO tables and LMDI decomposition method. Energy consumption of 38 economic sectors in 2007, 2012 and 2018 is determined, of which transport service sectors were the second largest energy consumer (17.71 Mtoe), occupied 18.5% of total energy consumed in Vietnamese economy in 2018. In terms of energy intensity, a rising trend is seen in all transport service sectors, of which four most important transport services including bus and other road passenger transport, freight transport service by road and pipeline, waterway shipping freight and aviation passenger reached 0.62 kgoe/USD, 0.72 kgoe/USD, 0.60 kgoe/USD and 0.62 kgoe/USD in 2018, respectively. The ineffective structural change and ineffective energy intensity change are the reasons behind the upward trend in these sectors. Using Leontief inverse, the study also unveils how demanded on transport services by other economic sectors in terms of energy and how much energy embodied in all inputs of any economic sector. In order to keep the energy intensity stable and gradually decreasing, the recommendations are focused on effectiveness in structural changes and improvements in energy efficiency.