발간논문

Home > KJMM 논문 > 발간논문

Tetrapod-shaped ZnO crystals were synthesized through a simple oxidation of metallic Zn powder in air without the presence of any catalysts or substrates. X-ray diffraction data revealed that the ZnO crystals had wurtzite structure. It is supposed that the growth of the tetrapod proceeded in a vapor-solid growth mechanism. As the amount of the source powder increased, the size of the tetrapod decreased. The tip morphology of the tetrapod changed from a needle-like shape to a spherical shape with the oxidation time. ZnO crystals with rod shape were fabricated via the oxidation of Zn and Sn mixture. Sn played an important role in the formation of ZnO crystals with different morphology by affecting the growth mode of ZnO crystals. The cathodoluminescent properties were measured for the samples. The strongest green emission was observed for the rod-shaped ZnO crystals, suggesting that the crystals had the high density of oxygen vacancies.

keyword : nanostructured materials, vapor deposition, optical properties, scanning electron microscopy, SEM, ZnO tetrapod

In this paper, the percolation behavior, the critical exponent of conductivity, and the electrical conduction mechanism of polyethylene matrix composites are discussed based on studying the whole change of the resistivity and dielectric constant for carbon black loading. There are two transitions of resistivity for carbon black loading. Below the first transition, resistivity shows an ohmic behavior, and its value is almost the same as that of polyethylene. Between the first and second transitions, the change in resistivity is very sharp, leading to a large critical exponent of conductivity, and a non-ohmic electric field dependence of current has been observed. This electric field dependence has been discussed with a tunneling conduction. Beyond the second transition, the electric field dependence of current shows an ohmic behavior and a gentle change in resistivity has been observed for carbon black loading. The critical exponent of conductivity estimated by using the second transition is satisfied with the universal law of conductivity. The dielectric constant has been observed beyond the first transition and diverges around the second transition. It was concluded that the percolation threshold should be the second transition of resistivity for the polyethylene matrix composites loaded carbon black.

keyword : composites, chemical synthesis, electrical properties, percolation threshold, electrical conductivity