Browsing by Author "Tsai, Chih-Long"

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Anode-pore tortuosity in solid oxide fuel cells found from gas and current flow rates
(2008) Schmidt, V. Hugo; Tsai, Chih-Long
The effect of solid oxide fuel cell (SOFC) anode thickness, porosity, pore size, and pore tortuosity on fuel and exhaust gas flow is calculated. Also determined is the concentration of these gases and of diluent gases as a function of position across the anode. The calculation is based on the dusty-gas model which includes a Knudsen (molecule–wall) collision term in the Stefan–Maxwell equation which is based on unlike-molecule collisions. Commonly made approximations are avoided in order to obtain more exact results. One such approximation is the assumption of uniform total gas pressure across the anode. Another such approximation is the assumption of zero fuel gas concentration at the anode–electrolyte interface under the anode saturation condition for which the SOFC output voltage goes to zero. Elimination of this approximation requires use of a model we developed (published elsewhere) for terminal voltage V as a function of electrolyte current density i. Key formulae from this model are presented. The formulae developed herein for gas flow and tortuosity are applied to the results of a series of careful experiments performed by another group, who used binary and ternary gas mixtures on the anode side of an SOFC. Our values for tortuosity are in a physically reasonable low range, from 1.7 to 3.3. They are in fair agreement with those obtained by the other group, once a difference in nomenclature is taken into account. This difference consists in their definition of tortuosity being what some call tortuosity factor, which is the square of what we and some others call tortuosity. The results emphasize the need for careful design of anode pore structures, especially in anode-supported SOFCs which require thicker anodes.
Ceramic processing and electrochemical analysis of proton conductive solid oxide fuel cell
(Montana State University - Bozeman, College of Letters & Science, 2010) Tsai, Chih-Long; Chairperson, Graduate Committee: V. Hugo Schmidt
Ba(Zr 0.8-xCe xY 0.2)O 3-delta (0 < or = x < or = 0.4) (BZCYs) powders were successfully fabricated by both solid state reaction and glycine-nitrate process. Lithium fluoride (LiF) was selected as a liquid phase sintering additive to lower the sintering temperature of BZCYs. Using LiF as an additive, high density BZCYs ceramics can be obtained at sintering temperatures 200~300 °C lower than the usual 1700 °C with much shorter soaking time. Nuclear reaction investigations showed no lithium and a small amount of fluorine reside in the sample which indicates the non-concomitant evaporation of lithium and fluorine during the sintering process. Scanning electron microscopic investigations showed the bimodal structure of BZCY ceramics and grain growth as Ce content increases. In a water saturated hydrogen containing atmosphere, BZCY ceramics have higher conductivity when LiF is used in the sintering process. LiF-added BZCY electrolyte-supported fuel cells with different cathodes were tested at temperatures from 500 ~ 850 °C. Results show that Pt cathode gives much higher power output than ceramic cathodes, indicating much larger polarization from ceramic cathodes than Pt. Ba(Zr 0.6Ce 0.2Y 0.2)O 3-delta anode supported proton conductive solid oxide fuel cells (H-SOFCs) show low power output due to its low proton conductivity. Ba(Ce 0.8Y 0.2)O 3-delta anode supported H-SOFCs show excellent power output. Different H 2 and O 2 partial pressures were used for fuel and oxidative gas, respectively, to obtain information for V(i) modeling. Different thicknesses of supporting anode were used to obtain saturation current densities of H-SOFC. Using the dusty-gas model which includes Stefan-Maxwell equation and Knudsen terms, the calculation gave tortuosity of our supporting anode 1.95 ± 0.1. The gas concentrations across the anode were also calculated by knowing the tortuosity of the supporting anode. An electrochemical model of H-SOFC was developed. The excellent agreement between model and experimental data implies that our model is close to the true physical picture of H-SOFC. The more accurate prediction of our model, based on a physical picture of electrochemical processes, also provides a replacement for using the Butler-Volmer equation in SOFC modeling. In the parametric analysis, our model shows that ohmic polarization and cathodic polarization limit the performance of H-SOFC. Research for improving H-SOFC performance should be focused on reducing electrolyte thickness, increasing proton conductivity of electrolyte and finding a compatible cathode material.
Dielectric, hypersonic, and domain anomalies of (PbMg1/3Nb2/3O3)1-x(PbTiO3)x single crystals
(2001) Tu, Chi-Shun; Tsai, Chih-Long; Schmidt, V. Hugo; Luo, H.; Yin, Z.
Dielectricpermittivities,Brillouinbackscattering spectra, polarization-electric field hysteresis loops, and domain structures have been measured as a function of temperature in relaxor-based ferroelectricsingle crystals(PbMg1/3Nb2/3O3)1−x(PbTiO3)x(PMN-xPT)(PbMg1/3Nb2/3O3)1−x(PbTiO3)x(PMN-xPT) for x=0.24x=0.24 and 0.34. For PMN-24%PT, a diffuse phase transition which is associated with a broad frequency-dependent dielectric maximum was observed near 380 K. As the temperature increases, the PMN-24%PT crystal gradually develops cubic regions and is fully converted into the cubic state near 375 K. An extra dielectric anomaly appears at 370 K, possibly due to the percolating polar cluster induced by an external electric field. PMN-34%PT exhibits a nearly normal ferroelectric phase transition near 445 K from the tetragonal to the cubic phase. In addition, a weak diffuse phase transition observed near 280 K may result from partial conversion of rhombohedral phase to tetragonal phase. The dielectric thermal hysteresis confirms that the transitions near 280 and 445 K are diffuse first order and first order, respectively. The dielectricpermittivities of PMN-24%PT and PMN-34%PT obey the relation, ε′m/ε′(f,T)=1+[T−Tm(f )]γ/2δ2γ,εm′/ε′(f,T)=1+[T−Tm(f )]γ/2δγ2, above the temperature of permittivity maximum Tm.
Effect of lithium fluoride on thermal stability of proton-conducting Ba(Zr0.8-xCexY0.2)O2.9 ceramics
(2010) Tu, Chi-Shun; Huang, C.-C.; Lee, S.-C.; Chien, R. R.; Schmidt, V. Hugo; Tsai, Chih-Long
In-situ X-ray diffraction (XRD) and micro-Raman scattering have been used to study the thermal stability of lithium fluoride (LiF)-added (7% weight ratio) Ba(Zr0.8−xCexY0.2)O2.9 (BZCY: x = 0.1 and 0.2) proton-conducting ceramic powders as a function of temperature in 1 atm of flowing CO2. This work reveals that LiF-addition can reduce the thermal stability of Ba(Zr0.8−xCexY0.2)O2.9 in CO2 and cause decomposition to BaCO3, and possibly Ba3Ce2(CO3)5F2 (or CeCO3F), and Y2O3-like compound after exposure to CO2 from high temperatures. LiF-related compounds can be removed after calcining (or sintering) in air above 1200 °C, but a minor amount of a Y2O3-like compound could remain after calcining at 1400 °C in air.
Hypersonic and dielectric anomalies of (Pb(Zn1/3Nb2/3)O3)0.905(PbTiO3)0.095 single crystal
(1999-09-01) Tu, Chi-Shun; Chao, F.-C.; Yeh, C.-H.; Tsai, Chih-Long; Schmidt, V. Hugo
Both the longitudinal (LA) Brillouin backscattering spectra and dielectric permittivities along the [001] direction have been measured as a function of temperature for relaxor-ferroelectric single-crystal (Pb(Zn1/3Nb2/3)O3)0.905(PbTiO3)0.095 (PZN-9.5%PT). A sharp ferroelectric phase transition (which is associated with a sharp Landau-Khalatnikov-like phonon damping maximum) was observed near 460 K. As the temperature decreases, a diffuse phase transition (which is associated with a broad acoustic phonon damping maximum) was detected near 340 K. This broad damping evolution is attributed to dynamic order-parameter fluctuations. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 340 and 460 K) are diffuse first order and first order, respectively. The frequency-dependent dielectric data ɛ′′c(f,T) prove the existence of an electric dipolar relaxation process below 360 K. The activation energy, the Vogel-Fulcher temperature, and attempt frequency corresponding to this relaxation process are also calculated.
Low temperature sintering of Ba(Zr0.8‑xCexY0.2)O3-δ using lithium fluoride additive
(2010) Tsai, Chih-Long; Kopczyk, Michael; Smith, R. J.; Schmidt, V. Hugo
Lithium fluoride (LiF) was selected as a liquid phase sintering additive to lower the sintering temperature. The effects of LiF on the sinterability, microstructure, and electrochemical properties of Ba(Zr0.8 − xCexY0.2)O3 − δ (0 ≤ x ≤ 0.4) (BZCYs) ceramics were investigated. Using LiF as an additive, high density BZCYs ceramics can be obtained at sintering temperatures 200–300 °C lower than the usual 1700 °C with much shorter soaking time. Nuclear reaction investigations showed no lithium and a small amount of fluorine reside in the sample which indicates the non-concomitant evaporation of lithium and fluorine during the sintering process. Scanning electron microscopic investigations showed the bimodal structure of BZCYs ceramics and grain growth as Ce content increases. In a water saturated hydrogen containing atmosphere, BZCYs ceramics have higher conductivity when LiF is used in the sintering process. LiF-added BZCYs electrolyte-supported fuel cells with platinum electrodes were tested at temperatures from 500 to 850 °C. Results show that LiF is an excellent sintering additive for lowering the sintering temperature of BZCYs.
Orientation dependence and electric-field effect in the relaxor-based ferroelectric crystal (PbMg1/3Nb2/3O3)0.68(PbTiO3)0.32
(2002-03-01) Tu, Chi-Shun; Tsai, Chih-Long; Chen, J.-S.; Schmidt, V. Hugo
Dielectric permittivities, polarization–electric-field hysteresis loops, and domain structures have been measured as a function of temperature in relaxor-based ferroelectric single crystals (PbMg1/3Nb2/3O3)0.68(PbTiO3)0.32 (PMN-32%PT) for ⟨110⟩cub and ⟨211⟩cub orientations. Contrary to the pure PbMg1/3Nb2/3O3 (PMN), PMN-32%PT exhibits apparent crystallographic orientation dependences of dielectric permittivities, polarizations, domain structures and phase transitions. With a prior field-cooled treatment, a field-induced state, perhaps of orthorhombic symmetry, is evidenced and coexists with the rhombohedral symmetry in the low-temperature region. This field-induced phase is manifested by an extra dielectric peak observed near 373 K for the ⟨211⟩cub orientation. A relaxation mechanism which is responsible for the so-called diffuse phase transition crosses a wide temperature region of ∼340–400 K and results from fluctuations between rhombohedral and tetragonal states. In order of increasing temperature (without a prior field-cooled treatment), PMN-32%PT undergoes successive phase transformations: rhombohedral phase→coexistence of rhombohedral and tetragonal phases→tetragonal phase→coexistence of tetragonal and cubic phases→cubic phase.
Phases and domain structures in relaxor-based ferroelectric (PbMg1/3Nb2/3O3)0.69(PbTiO3)0.31 single crystal
(2001) Tu, Chi-Shun; Chen, L. -F.; Schmidt, V. Hugo; Tsai, Chih-Long
The Brillouin back-scattering spectra, dielectric permittivities, polarization–electric field (P–E) hysteresis loops and domain structures have been measured as a function of temperature in a relaxor-based ferroelectric single crystal (PbMg1/3Nb2/3O3)0.69(PbTiO3)0.31 (PMN–31%PT). In order of increasing temperature, PMN–31%PT undergoes succes-sive phase transitions: rhombohedral phase (below ∼ 370 K) → coexistence of rhombohedral and tetragonal phases (between ∼ 370 and ∼ 380 K) → tetragonal phase (between ∼ 380 and ∼ 400 K) → coexistence of tetragonal and cubic phases (betwee ∼ 400 and ∼ 420 K) → cubic phase (above ∼ 420 K). An extra ferroelectric anomaly of the dielectric per-mittivity appears at 370 K possibly due to the percolating polar cluster induced by an external electric field. It was found that different individual domain regions have different transition temperatures. This phenomenon suggests an inhomogeneous dis-tribution of Ti4+ concentration in the PMN–31%PT crystal. The dielectric permittivity ε' of PMN–31%PT obeys the empirical relation, ε'm/εγ ( f, T ) = 1 +[T − Tm( f )]'/2δγ2 , above the temperature of permittivity maximum Tm.
Thermal stability of Ba(Zr0.8-xCexY0.2)O2.9 ceramics in carbon dioxide
(2009) Tu, Chi-Shun; Chien, R.R.; Schmidt, V. Hugo; Lee, S.-C.; Huang, C.-C.; Tsai, Chih-Long
In situx-ray diffraction spectra (25–1000 °C) have been measured as a function of temperature for proton-conducting Ba(Zr0.8−xCexY0.2)O2.9 (x=0.0–0.4)ceramics in CO2 atmosphere. Atomic vibrations before and after exposure to CO2 were obtained by using the micro-Raman scattering (150–1600 cm−1). Ba(Zr0.8Y0.2)O2.9 and Ba(Zr0.6Ce0.2Y0.2)O2.9 reveal a promising thermal stability in CO2 without apparent decomposition up to 1000 °C. However, Ba(Zr0.5Ce0.3Y0.2)O2.9 and Ba(Zr0.4Ce0.4Y0.2)O2.9 exhibit thermally stable below 550 °C and then proceed an obvious chemical decomposition of BaCO3 and Zr0.8−xCexY0.2O2 above 550 °C, which were clearly evidenced by the Raman vibrations of 1057 and 466 cm−1, respectively. A first-order orthorhombic-hexagonal structure transition was confirmed in BaCO3 in the region of 810–850 °C upon heating. This study suggests that the Ba(Zr0.8−xCexY0.2)O2.9ceramics with x≤0.2 are promising candidates for proton-conducting applications in CO2-containing environment.
Tortuosity in anode-supported proton conductive solid oxide fuel cell found from current flow rates and dusty-gas model
(2011) Tsai, Chih-Long; Schmidt, V. Hugo
Ba(Ce0.8Y0.2)O3−δ anode-supported proton conductive solid oxide fuel cells were fabricated and tested. By changing the H2 partial pressure at the anode side, the effect of anodic concentration polarization on open-circuit voltage of the cell was observed. Saturation current densities under concentration polarization were obtained from different anode thickness cells and were used for tortuosity calculation. The calculation is based on the dusty-gas model which includes Knudsen diffusion and Stefan–Maxwell equation terms. The tortuosity value for our supporting anode is 1.55 ± 0.1 which is in a physically reasonable range for modern porous anode materials. The tortuosity that we found is independent of the cell testing temperature and anode thickness, which is consistent with the fact that tortuosity is a geometric factor of the anode structure. The derived equation also can be used for predicting the effect of varying the anode thickness, porosity and pore size. Also, the concentration of the gases as a function of position across the anode is determined.