Seungsu Baek

Ambient-dried silica aerogel doped with TiO2 powder for thermal insulation

Two step-derived SiO2 sol doped with TiO2 powder dissolved in ethanol was prepared. After aging and washing of wet gel, surface modification with TMCS (trimethylchlorosilane) was followed to prevent additional condensation reaction during drying. Ambient-dried SiO2-TiO2 gel was heat-treated at various temperatures. The effects of the heat-treated temperature on the chemical bonding state of SiO2-TiO2 gel were investigated by means of FTIR and XRD analyses. After heat-treatment at 350 °C, crack-free SiO2-TiO2 gel was synthesized with density of 0.14 g/cm3, porosity of 94%, and specific surface area of 613 m2/g. Ambient-dried SiO2-TiO2 gel heat-treated at 350 °C exhibited thermal conductivities of 0.0136 W/m·K and 0.0284 W/m·K at room temperature and 400 °C, respectively.

Dimensionally stable and light-colored polyimide hybrid reinforced with layered silicate

Light-colored polyimide hybrids with a quaternary alkylammonium modified montmorillonite, Cloisite® 20A (C20A) were successfully synthesized via a solution intercalation method. The structural and morphological features of the PI/C20A hybrids were characterized by Fourier transform infrared (FTIR) spectroscopy, wide-angle X-ray diffraction (WAXD), field emission scanning electron microscopy (FE-SEM), and spherical aberration correction scanning transmission electron microscope (STEM). The dimensional stabilities were strongly dependent upon the chemical and morphological structures which were influenced by the Cloisite® 20A loading content. A critical Cloisite® 20A concentration was obtained for the evolution of both the structural and physical properties of the PI/C20A hybrids. The dynamic stress behaviors were investigated in situ by using the wafer bending method during thermal imidization of the softbaked PI/C20A hybrid precursor and thermally cured hybrid films using a thin film stress analyzer. The PI/C20A hybrid films exhibited stability at 510 °C, high glass transition and relatively low coefficient of thermal expansion, and an improved solvent resistance capacity.

Fabrication and Properties of Reactively Hot Pressed HfB₂-HfC Ultra-High Temperature Ceramics

HfB₂-HfC composites were prepared by reactive hot pressing using Hf and B₄C at temperatures of 1800 and 1900℃ for 60 min under 32 ㎫ in an Ar atmosphere. The reaction sequences of the HfB₂-HfC composite were studied through series of pressureless heat treatments ranging from 800 to 1600℃. The effect of size reduction of the starting powders on densification was investigated by vibration milling. Fully dense HfB₂-HfC composites were obtained by size reduction of the starting powders via vibration milling. The oxidation behaviour of the HfB₂-HfC composites at 1500℃ in air showed formation of a non-protective HfO₂ scale with linear mass gain. Examination of the mechanical properties showed that particle size reduction via vibration milling also led to improved flexural strength, hardness and fracture toughness.

A study of the annealing atmosphere effects on the electrical properties of graphene-incorporated direct-patternable ZnO thin films

An improvement in the electrical properties of the films is absolutely required for the application of ZnO thin film in electrical devices. And the conventional dry etching process is accompanied by the generation of physical defects, degradation of the properties. The electrical property of direct-patternable, graphene-incorporated ZnO thin film prepared by photochemical solution deposition was improved by N2 atmosphere annealing. Direct-patterning of graphene-incorporated ZnO thin films by photochemical solution deposition was performed without a photoresist or etching process. The transmittance of graphene-incorporated ZnO thin films was similar to ZnO thin film. The transmittance of ZnO thin film was unaffected by annealing atmosphere such as air or N2. The incorporation of graphene slightly decreased the crystallinity of the ZnO thin films and annealing atmosphere did not affect to the crystallinity of ZnO thin films. The resistivity was decreased due to the mobility enhancement because of the π-bond nature of the graphene surface. In addition, N2 atmosphere annealing decreased the resistivity of ZnO thin film than O2 atmosphere annealing due to decreasing of oxidation on the surface. These results suggest that a micro-patterned system can be simply fabricated at low cost and the electrical properties of ZnO thin films can be improved by graphene incorporation and N2 atmosphere annealing.

Effect of surface modification of SiO2 nanoparticles on the mechanical properties of ambient pressure dried silica aerogels

In this study, we synthesised silica aerogel composites using SiO2 nanoparticles and studied the effects of surface modification of silica nanoparticles on the properties of silica aerogel composites. The surface condition of the silica nanoparticles was controlled to be either hydrophobic or hydrophilic. Accordingly, the effect of the surface chemical state of silica nanoparticles on the network structure of hybridised silica aerogels and finally on their mechanical properties could be investigated. The microstructural and mechanical properties of silica aerogels were strongly dependent on the nature of the surface of hybridised silica nanoparticles.

Structure of mesoporous Al 2 O 3 thin film obtained by surfactant templating

These Ordered mesoporous aluminum oxide films with porosity ranges of 5 to 37% were synthesized by evaporation-induced self-assembly (EISA) using surfactant templating. To investigate the effects of mesoporous structure on mechanical properties, changes in pore structure properties including pore distribution and porosity were monitored as functions of surfactant concentration (Pluronic P-123, PEO20PPO70PEO20). The structural properties were analysed using powder X-ray diffractometer and grazing-incidence small-angle x-ray scattering in PLS 4C2 beamline. These alumina films had body-centered cubic pore structure or random-oriented pore structure depending on the surfactant concentration used, and different mechanical properties were obtained depending on the pore structure. Therefore, a control of pore structure in mesoporous materials is the important factor to strengthen the porous structural materials.