Caption of Table and Figures
3. Results and Discussion
Fig. 1(a) shows the X-ray diffraction patterns of (ZrO2)0.92(Y2O3)0.075(MgO)0.005 (YSZM) and (Bi2O3)0.75(Y2O3)0.25 (YSB) after pre-calcined at 1350oC for 2 hrs and at 800oC for 20 hrs, respectively. The cubic phase and δ structure were found in YSZM and YSB specimens. There are no second phases observed in the X-ray diffraction patterns.
The total bulk conductivity vs. 1/T plot of (ZrO2)0.92(Y2O3)0.08-x(MgO)x system was shown in Fig. 1(b). The total conductivities of (ZrO2)0.92(Y2O3)0.08-x(MgO)x system with
x=0.005 and 0.01 are found to be higher than (ZrO
2)0.92(Y2O3)0.08The X-ray diffraction patterns of 1 mol% YSB-doped YSZ (1YBZ) after sintered at temperatures between 1100
(8YSZ) matrix, therefore addition of YSB into co-doped zirconia YSZM should enhance the ionic conductivity and decrease the sintering temperature of zirconia.
oC and 1300oC for 5 hrs as shown in Fig. 2(a), the results reveals co-existence of the monoclinic-ZrO2 and the cubic-ZrO2 in the specimens.
Formation of m-ZrO2 is attributed to the dissolution of Y2O3 from YSZ to δ-YSB to decrease the Y/Zr ratio of YSZ, and therefore c-to-m phase transformation was induced during sintering process. The amounts of m-ZrO2, δ-YSB and c-ZrO2 in YSB-doped YSZ and YSB-doped YSZM at temperatures between 1100oC and 1300oC for 5 hrs were calculated and listed in Table 1. A decreasing trend of m-ZrO2 and increasing amount of c-ZrO2 with increase of sintering temperatures were observed. It is ascribed to that the occurrence of serious Bi2O3 evaporation and yttria of YSB diffused to the solid solution of Y2O3-ZrO2. Similar phenomenon was observed in the specimens of 2 mol%
YSB-doped YSZ (2YBZ) and 3 mol% YSB-YSB-doped YSZ (3YBZ) as shown in Fig. 2(b) and Fig.
2(c). Residual Bi2O3 is observed in 3YBZ at the temperatures below 1200oC, indicating that it is difficult for the solid solution of Bi2O3-Y2O3-ZrO2 to form at lower sintering temperatures, due to lower solid solution rate of Bi2O3 in YSZ.
Fig. 2(d) to Fig. 2(f) shows the X-ray diffraction patterns of YBZM specimens after sintered at 1100oC to 1300oC for 5 hrs. The specimens of YSB doped YSZM (YBZM) display smaller amount of m-ZrO2, than YBZ specimens at the same Bi2O3 addition and sintering conditions. It is well known that the melting temperature of YSB at around 1000oC, δ-YBZ phase might be formed as a thin Bi2O3 -rich liquid film at grain boundary as a diffusion path for material sintering. The diffusion velocity of Mg2+, Y3+ and Bi3+ in zirconia matrix are different, and the solid solution limit of Bi2O3-ZrO2 is much lower than that of MgO-ZrO2 and Y2O3-ZrO2. The element distribution of Bi3+, Mg2+ and Y3+
is effective in changing the mircostructural morphologies of YBZ and YBZM. However, the experimental results demonstrated that MgO might be a better stabilizer than Y2O3 in Y2O3-Bi2O3-ZrO2 ternary system to form the c-ZrO2 structure. The amount of monoclinic-ZrO2 also decreases with an increase of YSB content, due to existence of larger amount of Y2O3
The relationship between specimen density and sintering temperatures within 1200
stabilizer.
oC and 1300oC of YBZM sintered at the same soaking time of 5 hrs was listed in Table 2. The density of YBZM specimens depend on sintering temperature, because of the density of specimen have been affected by the evaporation of δ-YSB and the capillary action of liquid phase at grain boundary [13-17]. The density of specimen sintered at 1200oC, which closes to the theoretical density of 94.83%, is higher than that of specimen
sintered at 1300oC. The decreasing of specimen density at 1300oC is due to serious δ-YSB evaporation at higher sintering temperature. Besides, Specimen density of YBZM is observed to increase with an increase of YSB amount attributed to that the infiltration of liquid YSB phase fills into grain boundaries and pores during sintering process.
Figure 3 displays the scanning electron images of surface morphology in YSB-doped YSZ system after sintered at 1200oC and 1300oC for 5 hrs. The grain size of the specimens is found to increase with an increase of YSB content. This morphology may be attributed to formation of large amount of oxygen vacancy by dissolving Y2O3 and Bi2O3 into YSZ. Oxygen vacancy provides diffusion path and enhances diffusional velocity of defects and impurities. Besides, the grain size of YSB-doped YSZ specimens increases with increase of sintering temperatures, due to enhancement of the diffusional velocity of Y2O3 and Bi2O3 to form large amount of oxygen vacancy in solid solution of Bi2O3-Y2O3-ZrO2 ternary system. Mixture of small and large grains in microstructures may be contributed by the segregation of Bi2O3 and Y2O3
The same trend for grain growth in YSB-doped YSZ (YBZ) specimens was also found in the specimens of YSB-doped YSZM (YBZM) system as shown in Fig. 4. The growth rate of grain size in YBZM system is significantly faster than in YBZ system, because MgO and Y
in YSB-doped YSZ specimens.
2O3 co-doped zirconia system possesses large radius of oxygen vacancy to migrate oxygen ion, defects and impurities easily, compared with YSZ specimen. The element segregation in the specimens was determined using SEM-EDS analysis. EDS results for small and large grains of 3 mol% YSB-doped YSZM (3YBZM) specimens after sintered at 1200oC for 5 hrs are shown in Table 3. The smaller grain at grain boundary shows higher amount of Y, Bi and Mg elements than the large grain. It
might be due to that a thin Bi2O3-rich liquid film exists at the grain boundary as a diffusion path for the sintering, and Y, Bi as well as Mg are easily segregated into grain boundaries. Intergranular fracture was found in the fracture surface of YSB-doped YSZM system as shown in Fig. 5(a). Because the δ-YSB phase appeared as a thin Bi2O3 -rich liquid film at the grain boundary and the mechanical properties of thin Bi2O3 -rich liquid film at grain boundary is worse than zirconia matrix.
We compare the impedance patterns at 800oC for two specimens 8YSZ sintered at 1500oC and 3YBZM at 1200oC shown in Fig. 5(b). The semicircles of 3YBZM are much smaller than that of 8YSZ, indicating that the ionic conductivity of intragrain, grain boundary and total bulk of 3YBZM are all higher than that of 8YSZ. The relationship between the conductivities of intragrain and grain boundary in 3YBZM with different temperatures resulted in abrupt increment of the grain boundary conductivity, due to the segregation of YSB with high conductivity at grain boundaries. The resistance of grain boundary significantly decreased with increase of operation temperatures and disappeared while testing temperatures higher than 700oC as shown in Fig. 6(a), compared to the resistance of intragrain in 3YBZM specimen. Arrhenius plots for the total bulk conductivities of 8YSZ sintered at 1500oC and YSZM sintered at 1500oC, compared to that of YBZM system sintered at temperatures between 1200oC and 1300o
The ionic conductivities of 1200
C were shown in Fig. 6(b) and Fig. 6(c). It is interesting that 3YBZM exhibits the best results. The ionic conductivities of total bulk in YBZM system was demonstrated to be enhanced by adding YSB from 1 mol% to 3 mol%.
oC sintered specimens of YBZM system are all higher than YBZM specimens sintered at 1300oC, due to high density of specimens.
Moreover, if the sintering temperature is too high to induce evaporation of YSB, ionic conductivity of specimen would be significantly reduced as shown in 1300oC sintered specimens of YBZM. The tendency of ionic conductivity values in YBZM specimens, which was 3YBZM > 2YBZM >1YBZM, is related to the increasing amount of YSB content, which were 46% higher than that of 8YSZ as shown in Fig. 6(d). Accordingly, adding appropriate amounts of MgO and YSB could reduce the sintering temperature of YSZ system from 1500oC to 1200oC and suppress the formation of m-ZrO2 to produce highly dense specimen for enhancing the ionic conductivity of YSZ matrix from 0.01 S/cm to 0.02 S/cm at 800oC.
4. Conclusions
The specimens of YSB doped YSZM (YBZM) display smaller amount of m-ZrO2, than YBZ specimens at the same Bi2O3 addition and sintering conditions, indicating that MgO might be a better stabilizer than Y2O3 in Y2O3-Bi2O3-ZrO2 ternary system to form the c-ZrO2 structure. Besides, the results demonstrated that the 3YBZM specimen sintered at 1200oC exhibits the best ionic conductivity and highest specimen density.
Decrease of specimen density at 1300o
The grain size of the specimens is found to increase with an increase of YSB content, attributed to formation of large amount of oxygen vacancies by dissolving Y
C is due to serious δ-YSB evaporation at higher sintering temperature. Specimen density of YBZM is observed to increase with an increase of YSB amount, due to infiltration of liquid YSB into the pores of specimens during sintering process.
2O3 and Bi2O3 into YSZ. Oxygen vacancy provides diffusion path and enhances diffusional
velocity of defects and impurities. The grain size of YSB-doped YSZ specimens is also observed to increase with increase of sintering temperatures. Growth rate of grain size in YSB-doped YSZM system is significantly faster than YSB-doped YSZ system, because of MgO and Y2O3 co-doped zirconia system possesses large radius of oxygen vacancy to migrate oxygen ion, defects and impurities easily, compared with YSZ specimen. Thin Bi2O3-rich liquid film at grain boundary as a diffusion path for the sintering, therefore Y, Bi and Mg elements are easily segregated into grain boundary. The intergranular fracture was found in the fracture surface of YSB-doped YSZM system.
The tendency of ionic conductivity values in YBZM specimens, which are 3YBZM
> 2YBZM >1YBZM, is related to the increasing amount of YSB content, which were 46% higher than that of 8YSZ. Accordingly, adding appropriate amounts of MgO and YSB could reduce the sintering temperature of YSZ system from 1500oC to 1200oC and suppress the formation of m-ZrO2 to produce highly dense specimen for enhancing the ionic conductivity of YSZ matrix from 0.01 S/cm to 0.02 S/cm at 800o
1. H. Yahiro, Y. Eguchi, K. Eguchi and H. Arai: J. Appl. Electrochem, Vol. 18 (1988) 527.
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Acknowledgements
Financial support from the National Science Council of Taiwan, Republic of China through project numbers NSC 92-2216-E-011-044 and NSC 93-2216-E-011-031 is gratefully acknowledged by the authors.
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