Sang Hyo Kweon

Electric field assembled anisotropic alumina composite for thermal dissipation applications

A composite material with anisotropic microstructure was fabricated by DC electric field. Alumina was used as filler for thermal conductivity and polysiloxane resin was used as matrix. According to the alumina morphology and loading amount, various anisotropic microstructures were assembled. The thermal properties of the composite material were investigated parallel to the direction of the applied electric field accordingly. For plate-like alumina with 20 vol% loading, a thermal conductivity of 0.44 W/mK was achieved. With spherically shaped alumina, with an equal loading of 20 vol%, a higher thermal conductivity of 0.46 W/mK was achieved. The increase in the thermal conductivity of the fabricated alumina composites with anisotropic microstructure was attributed to increased filler-to-filler connectivity.

Electrophoretic deposition of Ca2Nb3O10− nanosheets synthesized by soft-chemical exfoliation

Ca2Nb3O10− (CNO−) nanosheets exfoliated from a KCa2Nb3O10 ceramic were deposited on a Pt/Ti/SiO2/Si substrate at room temperature (RT) using the electrophoresis method, and subsequently annealed at various temperatures to remove organic defects. The inter-planar distance of the (001) plane of the CNO film deposited at RT was large (1.68 nm). This distance gradually decreased to 1.44 nm when the annealing temperature was increased to 600 °C due to the removal of organic defects. For CNO films annealed at temperatures higher than 600 °C, a CaNb2O6 secondary phase was formed, indicating that annealing should be conducted at 600 °C to obtain a homogeneous CNO film. Moreover, the CNO film annealed at 600 °C showed a large er value of 65 with a low tan δ of 0.01 at 1.0 MHz. This film also exhibited a high breakdown electric field of 0.43 MV cm−1 and a very low leakage current density of 3 × 10−8 A cm−2 at 0.4 MV cm−1.

Physical Properties of (Na1-xKx)NbO3 Thin Film Grown at Low Temperature Using Two-Dimensional Ca2Nb3O10 Nanosheet Seed Layer

A monolayer Ca2Nb3O10 (CNO) nanosheet was deposited on a Pt/Ti/SiO2/Si substrate using the Langmuir-Blodgett method. This monolayer CNO nanosheet with a (001) surface termination was used as a seed layer to reduce the growth temperature of the crystalline (Na1- xK x)NbO3 (NKN) film. The crystalline NKN film was preferentially grown along the [001] direction at 400 °C. The ferroelectric and piezoelectric properties of this NKN film were influenced by the postannealing atmosphere due to the variations in the amounts of oxygen vacancies in the NKN film. The crystalline NKN film annealed at 300 °C under 50 Torr O2 atmosphere showed promising ferroelectric and piezoelectric properties; εr of 303 and tan δ of 2.0% at 100 kHz, Ps of 15.3 μC/cm2, Pr of 11.7 μC/cm2, and Ec of 78 kV/cm, and d33 of 139 pm/V. This NKN film showed the lowest leakage current, which can be explained by the Schottky emission mechanism. The Schottky barrier heights of the Pt/NKN and NKN/CNO/Pt interfaces were calculated to be 0.97 and 0.28 eV, respectively. The results of this work suggest a new method to grow crystalline thin films at low temperatures by using metal-oxide nanosheets as the seed layer.