Toughening mechanism investigation and ionic conductivity application of zirconia ceramics

• Main Authors: Tsung-Her Yeh, 葉宗和
• Other Authors: none
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/66138988863674010497

博士 === 國立臺灣科技大學 === 機械工程系 === 96 === Toughening mechanism and ionic conductivity enhancement in zirconia based ceramics containing Y2O3 and Nb2O5 as co-dopants to ZrO2 as well as Y2O3 and CeO2 as co-dopants to ZrO2 were investigated by using In-situ compression-X-ray diffraction technique with synchrotron radiation, Raman spectrum and Transmission electron microscopy (TEM). The results demonstrated that the co-doping zirconia system containing cubic and tetragonal mixed phases (Partially-stabilized zirconia: PSZ) exhibit outstanding mechanical properties by adding appropriate amount of dopants. The toughness values of PSZ specimens are 1-3 times higher than that of 3mol% yttria stabilized tetragonal zirconia polycrystal (3Y-TZP). Neither tetragonal-to-monoclinic phase transformation nor tetragonal-to-rhombohedral (t-to-r) nor tetragonal-to-orthorhombic (t-to-o) toughening mechanisms nor existence of ferroelastic domain switching of tetragonal phase were observed. Interestingly, a peculiar phase transformation which correlates cubic (c) and metastable tetragonal phases (pseudo-cubic phase: t’) was observed. The crystallographic relationship between c, t’ and new phase was identified as well as lattice parameters of each phase were calculated using TEM in the present study. Besides, the results of dynamic stress-stain curve and in-situ cyclic compression - X-ray diffraction technique with synchrotron radiation were analyzed to understand the contribution of peculiar phase transformation in toughening mechanism. The effect of thermal annealing on the performance of zirconia based ceramics was also discussed. Electrical properties of zirconia based ceramics demonstrated that the ionic conductivity of zirconia decreases by doping with high lattice binding energy dopants. Oxygen ion migration is found difficult in co-doped zirconia ceramics due to increase in the average binding energy. Moreover, ionic radius of the dopant is identified as another factor that influences the ionic conductivity. When the difference in radius between the dopant and host ions is too large, then a decrease in conductivity was observed due to serious lattice strain in zirconia crystal. The results of the previous investigation concludes that the influencing factors for the enhancement of ionic conductivity in zirconia system were (1) ionic radius of the dopant should be closer to zirconium cation (2) low average binding energy of doped zirconia (3) dopant concentration should not exceed the maximum interfering oxygen vacancies. In the present study, these principles are applied to achieve better ionic conductivity in divalent and trivalent cations co-doped zirconia system. When the amount of dopants exceeds the maximum interfering oxygen vacancies, the low symmetry phase is found. Due to this reason, ionic conductivity of doped zirconia was explained using Raman spectrum and TEM, instead of using general principles like oxygen vacancy clustering and/or occurring of defect association in zirconia specimens. The correlation between outstanding ionic conductivity and local structure of co-doping zirconia system was analyzed by using X-ray absorption spectrum (XAS), near edge X-ray absorption fine structure (NEXAFS) and Fourier transform (FT) method. Finally, novel anodes were developed with the mixture of 40wt% co-doped zirconia material of high ionic conductivity and 60wt% catalytic Ni. The characteristics like Tafel curve, current-overpotential, cyclic voltammetry curves and power density of half cell were studied to understand the mechanisms involved in the present system of specimens. Experimental results shows that the polarization resistance of Ni-8YSZ decreases significantly from 2.32 Ω-cm2 to 1.57 Ω-cm2 (at 800oC) by substituting Zr0.92Y0.155Mg0.005O2.0775 (MgYSZ) instead of 8YSZ, due to enhancement of triple phase boundary (TPB) area. Moreover, a decrease in activation energy for oxygen ion migration in 8YSZ was observed by modifying appropriate elements to 8YSZ, which is good for enhancing the velocity for oxygen ion migration in 8YSZ crystal and electrochemical reaction, containing oxygen ion oxidation and combination of oxygen and hydrogen to become water in modified anode. The catalytic activity of modified anode was correlated with the value of exchange current density (logi0) measured from Tafel plots under 5% H2 condition. The exchange current densities of modified anodes are found to be much higher than that of Ni-8YSZ. The order of catalytic activity in modified anodes follow Ni-MgYSZ> Ni-CaYSZ> Ni-SrYSZ> Ni-YSZ and hence the catalytic activity of anode is observed to depend on the ionic conductivity of co-doping zirconia in modified anode. Besides, the correlation is also found between logi0 and Rp. Increase in exchange current density and decrease in polarization loss were observed due to mass transfer of oxygen ions and easy charge transfer in modified anodes. The electrochemical reactions of anodes were explained using cyclic voltammetry method. Hysteresis loop like curves whose area increases with an increase of catalytic activity of anode is attributed to the easy formation of oxygen ion oxidation (O2-à1/2O2+2e-). The power density of the half cell Ni-MgYSZ/8YSZ with Pt-8YSZ as reference cathode (34.54 mW/cm2) is 23% higher than that of Ni-8YSZ/8YSZ with Pt-8YSZ as reference cathode (28.02 mW/cm2) at 800oC and the tendency of power density in modified anode is similar to that of catalytic activity variation in anode, indicating that the performance of the fuel cell could be enhanced by employing modified anodes with high ionic conductivity zirconia ceramics.