Postgrowth annealing was carried out to investigate the photoluminescence of zinc oxide (ZnO) nanorods synthesized using a thermal chemical vapor deposition method. The observed change in photoluminescence after the annealing processes strongly suggests that positively charged impurity ions or interstitial Zn ions are the recombination centers for green luminescence observed in the present sample. A model based on the interplay between the band bending at the surface and the migration of positively charged impurity ions or Zn ions was proposed, which satisfactorily explains the observed photoluminescence.
We have investigated the Raman scattering of the aligned gallium nitride (GaN) nanorods grown by plasma-assisted molecular-beam epitaxy. It was determined by Raman spectroscopy that the GaN nanorods are relatively strain free. The free carrier concentration, as well as electron mobility of the GaN nanorods, was obtained by the line shape analysis of the coupled A1longitudinal-optical(LO) phonon-plasmon mode. The electron concentration and mobility of electron obtained from line shape analysis are 3.3×1017cm−3 and 140cm2Vs, respectively. The local temperature of the nanorod sample was estimated based on the ratio of Stokes to anti-Stokes Raman peak intensity. Since the position of the LO phonon peak was found to be dependent on both the temperature and the LO phonon-plasmon coupling, it is crucial to consider the temperature effect in determining the frequency of the uncoupled LO phonon mode for the line shape analysis. The frequency of the A1(LO) mode of an undoped bulk GaN was used as a reference to determine the frequency of the uncoupled A1(LO) phonon mode of the GaN nanorods.
In order to optimize casting processes, computational models of solidification have proven to be very valuable to foundrymen. It is experimentally proven that the casting defects are primarily related to mold properties. During the eutectic growth the temperature rises, which is commonly referred to as recalescence. This has a strong effect on the mold walls, and mold wall movement can occur. The huge pressures generated at this time can block voids if mold is rigid. In green sand molds the moisture content will be reduced and mold wall will expand easily. According to previous research results, a distribution of thermophysical properties of the mold in the mold cavity, and the movement (expansion or contraction) of the mold and the metal interface are crucial for formation of many defects. The thermal expansion and bulk density of selected mold materials (olivine sand and zircon sand) and silica sand cores in transient regimes were determined in this study using a computer-controlled dual-pushrod dilatometer.
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