Young-Hoon Seong

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.

연료전지용 LSGM 페로브스카이트계 전해질의 합성 및 특성 연구

The family of (Sr,Mg)-doped LaGaO₃ compounds, which exhibit high ionic conductivity at 600-800℃ over a wide range of oxygen partial pressure, appears to be promising as the electrolyte for intermediate temperature solid oxide fuel cells. Conventional synthesis routes of (Sr,Mg)-doped LaGaO₃ compounds based on solid state reaction have some problems such as the formation of impurity phases, long sintering time and Ga loss during high temperature sintering. Phase stability problem especially, the formation of additional phases at the grain boundary is detrimental to the electrical properties of the electrolyte. From this point of view, we focused to synthesize single phase (Sr,Mg)-doped LaGaO₃ electrolyte at the stage of powder synthesis and to apply relatively low heattreatment temperature using novel synthesis route based on combustion method. The synthesized powder and sintered bulk electrolytes were characterized by XRD, TG-DTA, FT-IR and SEM. AC impedance spectroscopy was used to characterize the electrical transport properties of the electrolyte with the consideration of the contribution of the bulk lattice and grain boundary to the total conductivity. Finally, relationship between synthesis condition and electrical properties of the (Sr, Mg)-doped LaGaO₃ electrolytes was discussed with the consideration of phase analysis results.

Mechanical properties of C–SiC composite materials fabricated by the Si–Cr alloy melt-infiltration method:

Although carbon fiber-reinforced silicon carbide matrix composites fabricated using the liquid silicon infiltration method exhibit high thermal and oxidation resistances, their physical characteristics are limited because of the presence of unreacted, free Si within the materials. To resolve this problem, ingots prepared by alloying Cr with Si in ratios of 0, 5, 10, 25, and 50 at% were melted and made to infiltrate the composite, resulting in the formation of CrSi2 in the unreacted, free Si region without degrading the composite’s properties. The CrSi2 in the composite material reduced the amount of free Si and caused minimal variation in the flexural strength while significantly improving the fracture toughness of the composite. The results of scanning electron microscopy and transmission electron microscopy analyses indicated that the improvement in the fracture toughness was due to the presence of an amorphous interlayer between the Si and CrSi2 phases, as well as because of a stress field surrounding the CrSi2 phase.

Electrical properties of cellulose-based carbon fibers investigated using atomic force microscopy

Natural fibers have been actively investigated for the reinforcement of biocomposite, absorbent, and functional fibers, because of their environmentally friendly properties. We manufactured carbon materials based on two different natural fibers by heat treatment at 700, 900, and 1100 °C, respectively. The thermal behavior, surface morphology, and electrical properties of these carbon materials were investigated by thermogravimetric analysis (TGA) and atomic force microscopy (AFM); specifically, the electrical properties were studied through three-dimensional current images, and the current-voltage curve (I–V) characteristics using current-atomic force microscopy (c-AFM) on a nano-ampere scale. The thermal stability, roughness, and local current of the carbon materials were improved by increasing the carbonization temperature, and carbonized filter paper (FP) showed higher values in all characterization analyses than carbonized henequen (HQ) due to the higher content of cellulose in FP. Moreover, the currentvoltage characteristics of carbonized FP at 1100 °C and commercial carbon fiber were similar at high voltages (−5 or 5 V). Therefore, we expected that carbon materials from FP would be more suitable than HQ for use as environmentally friendly electrode materials.