Huesup Song

Stable sealing glass for planar solid oxide fuel cell

Abstract The thermal and chemical stability of glasses in the BaO–Al2O3–La2O3–B2O3–SiO2 system were investigated as well as bonding characteristics and wetting behavior to yttria stabilized zirconia (YSZ) electrolyte, to develop a suitable sealing glass for planar solid oxide fuel cell operating at 800–850 °C. The thermal properties such as glass transition temperature, softening temperature and thermal expansion coefficient were found to depend on the B2O3:SiO2 ratio in glass composition; thus the bonding characteristics of the glass to YSZ were also influenced by this ratio. The glass having a minimum thermal expansion mismatch with YSZ showed an excellent endurance during thermal cycling. No interface reaction was observed for all the glass/YSZ specimens heat-treated at 800–850 °C up to 100 h.

Electrical conductivity and defect structure of CeO2-ZrO2 mixed oxide

The electrical conductivity of CeO2-ZrO2 system was measured as functions of the temperature and oxygen partial pressure and of the composition. The ionic conduction was prevailing in the ZrO2 rich phase due to the increase of ionic defect concentration via homovalent doping effect. The enhancement of n-type electronic conductivity was observed in intermediate and CeO2 rich phase compared with pure CeO2, which originated either from homovalent doping effect or increase of electronic mobility due to the change of transport mechanism.

Synthesis and properties of cubic zirconia-alumina composite by mechanical alloying

Abstract Synthesis of 8mol%Y 2 O 3 –ZrO 2 (8YSZ) powder with Al 2 O 3 by mechanical alloying (MA) and the sintering behavior of this powder were investigated. The solubility of Al 2 O 3 in 8YSZ was 1.15 wt.% after MA for 48 h. Large agglomerated particles formed during MA resulted in a slow densification rate of the compact. After eliminating the agglomerates by sedimentation, the relative density of 97% was obtained by sintering the compact at 1400°C for 2 h. Sintered microstructure of mechanically alloyed 3wt.%Al 2 O 3 –8YSZ (3A8YSZ) consisted of 8YSZ grains in the size range of 0.5–1.2 μm and 20–400 nm size Al 2 O 3 particles dispersed uniformly in the 8YSZ. Fracture toughness and hardness of the composite was improved due to intragranular dispersion of Al 2 O 3 particles. Electrical conductivity of the composite was 6.4×10 −5 S cm −1 at 400°C and also exhibited significant increase over monolithic 8YSZ. Improvement in lattice conductivity was unique in this composite and attributed to the role of intragranular Al 2 O 3 particles, which provided strong lattice distortion of 8YSZ matrix.

Electrical conductivity and defect structure of yttria-doped ceria-stabilized zirconia

AbstractThe electrical property and defect structure of 12 mro CeO –ZrO , which has been known for good mechanical and 22 thermal properties, were studied in order to investigate the possibility of its use as a solid electrolyte for electrochemicaldevice, like electrolyser, sensor and fuel cell. The electrical conductivity of 12 mro CeO –ZrO was greatly enhanced and 22 electrolytic domain boundary EDB was also extended to lowŽ. P with yttria doping, which were readily comparable with O 2 those of commercial electrolyte, YSZ, CSZ. Unlike normal stabilized zirconia, saturation point of electrical conductivityappeared at higher doping concentration 12–15 mŽ.ro yttria and the activation energy was not also changed seriously withdoping till 15 mro yttria doping. q2001 Elsevier Science B.V. All rights reserved. Keywords: Zirconia; Ceria; Yttria; Electrical conductivity; Defect structure 1. IntroductionIn the last few decades, zirconia-based ceramichas been studied with great interests for many appli-cation fields due to its good mechanical, chemicaland electrolytic properties. Especially, doped zirco-nia with a lower valence cation like Ca

Variation of contact angles with temperature and time in the Al-Al2O3 system

The contact angles of liquid Al on polycrystalline Al2O3 determined by the conventional sessile drop method were obtuse (∼120 deg) up to 900 °C but decreased rapidly at 1000 °C. When the molten Al was squeezed through a narrow orifice and dropped onto the substrate, the contact angle at 900 °C was 77 deg and decreased linearly with temperature. At 1000 °C and 1100 °C, the contact angles decreased slowly with holding times up to 50 and 6 hours, respectively. At 1200 °C, the contact angle also decreased with holding time up to 40 minutes, after which it oscillated, resulting in a ring pattern on the substrate. The structural change of the Al2O3 substrate surface is suggested to be an important variable that determines the wetting behavior of the Al-Al2O3 system.

Pore structure evolution during solvent extraction and wicking

Pore structure evolution and binder distribution during both solvent extraction and wicking were investigated for plastic-formed ceramic bodies. The solvent extraction rate had a t−12 dependence, while the wicking rate varied depending on the degree of saturation. There existed a debinding front separating the debinded region (with pendular state) from the undebinded region (with fluid state) during solvent extraction, while the green body went through sequential transitions of capillary structure as a whole during wicking with the help of the rapid re-distribution of the binder system. In fact, the debinding process by wicking could be divided into four regimes: (i) the rapidly falling rate regime up to capillary state, (ii) a constant rate regime up to the funicular state, (iii) the first slowly falling rate regime to the pendular state, and (iv) the second slowly falling rate regime by evaporation and diffusion.

The effect of input gas ratio on the growth behavior of chemical vapor deposited SiC films

In an effort to protect a RBSC (reaction-bonded silicon carbide) reaction tube, SiC films were chemically vapor deposited on RBSC substrates. SiC films were prepared to investigate the effect of the input gas ratios (dilute ratio, α = PH2/PMTS = QH2/QMTS) on the growth behavior using MTS (metyltrichlorosilane, CH3SiCly3) as a source in hydrogen atmosphere. The growth rate of SiC films increased and then decreased with the decrease of the input gas ratio at the deposition temperature of 1250°C. The microstructure and preferred orientation of SiC films were changed with the input gas ratio; Granular type grain structure exhibited the preferred orientation of (111) plane in the high input gas ratio region (α = 3–10). Faceted columnar grain structure showed the preferred orientation of (220) plane at the low input gas ratios (α = 1–2). The growth behavior of CVD SiC films with the input gas ratio was correlated with the change of the deposition mechanism from surface kinetics to mass transfer.

Effect of minor binder on capillary structure evolution during wicking

Abstract The wax-based binder system showed discrete transition behavior in capillary structure during debinding due to the rapid redistribution of major binder, e.g. paraffin wax. Depending on the minor binder added, the transition behavior in capillary structure was significantly altered presumably due to the difference in chemical compatibility between major and minor binders. Limited chemical compatibility between major and minor binders might promote the funicular to pendular transition because of the segregation of polar minor binder in the nonpolar paraffin wax matrix. It was also demonstrated that the thermal pyrolysis should be initiated with increased temperature just past the funicular to pendular transition point in order to reduce the total debinding time. ©

Performance and Reliability Improvement of Planar SOFC Stack with Advanced Design of Unit Cell and Sealing

Commercialization of SOFC technology requires the development of high performance SOFC stack, combined with an engineering focus on stack reliability and cost. Recently KIST was successful to greatly improve the performance and reliability of SOFC stack based on newly developed 10x10 cm2 anode- supported cells and glass-based compression-seal gaskets. In this study, a novel design of SOFC unit cells with higher thermo-mechanical stability and better electrochemical performance has been introduced and its evaluation results are discussed in terms of its electrical property, structural property and unit cell performance. We also address the key factors and the latest progress in sealing technique in a view point of sealing capability and thermal-cycle stability.

Effect of Carbon Black Addition on Reaction-Bonded Silicon Carbide Ceramics

High strength reaction-bonded silicon carbide ceramics was successfully produced by reducing the amount of residual silicon and the silicon pocket size with carbon black as an additional carbon source. A prototype of wafer carrier was also produced in near-net dimension by planar contact infiltration of molten silicon into a preform joined with six pieces of simple shape by eliminating process shrinkages. Forming shrinkages were decreased to a negligible level by compression molding, while sintering shrinkage was eliminated by reactive infiltration of molten silicon.