Table II. Average Density of YSZ Processed Using Three Sintering Techniques
Technique Temperature (oC)
Average Density (g/cm3)
No Dwell Dwell *
Microwave 1400 4.11 5.32
1450 4.76 5.70
Fast-Fire 1400 4.83 5.28
1450 5.49 5.61
Traditional 1400 5.52 5.70
* Dwell times were 15 minutes for microwave and fast-fire; 2 hours for conventional
10000 20000 30000 40000 50000 60000 70000 0
-10000 -20000
0 -50 -100
Z" (Ohms)
100 200 300
0 -1 -2
0 2 4 6
Z' (Ohms) a) Collected at 300 oC
b) Collected at 550 oC
c) Collected at 900 oC
grain grain
boundary electrode
electrode
electrode Rg + Rgb
Rg + Rgb
Rg + Rgb
Figure 20. Representative impedance plots collected at a) 300oC, b) 550oC, c) 900oC for 8 mol% YSZ.
Figure 21 - 23 are plots of the conductivity data for a set of YSZ samples sintered from 1350oC - 1450oC using the three different sintering methods. Figure 24 shows the conductivity data for the 1400oC samples that included the faster heating profile for the microwave sintered samples and the samples that included a dwell time. For ionic conductors, the conductivity is expected to depend on temperature as
( )
k TT o EA 1 ln
lnσ = σ − (15)
where σ is conductivity, σo is a constant, T is measurement temperature in Kelvin, Ea is activation energy, and k is Boltmann’s constant. Values for the activation energies and σo were extracted from the plots of ln (σT) vs. 1000/T and are summarized in Table II, along with the statistics of the linear least-squares fit to the data.
-2 -1 0 1 2 3 4 5 6
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4
1000/T ln (σ T)
Fast-Fire Traditional Microwave
Figure 21. Conductivity as a function of reciprocal temperature for samples processed at 1350oC with no dwell time.
-2 -1 0 1 2 3 4 5 6
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4
1000/T ln (σ T)
Fast-Fire Traditional Microwave
Figure 22. Conductivity as a function of reciprocal temperature for samples processed at 1400oC with no dwell time.
-2 -1 0 1 2 3 4 5 6
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4
1000/T ln(σ T)
Fast-Fire Traditional Microwave
Figure 23. Conductivity as a function of reciprocal temperature for samples processed at 1450oC with no dwell time.
-4 -3 -2 -1 0 1 2 3 4 5 6 7
0.6 0.8 1 1.2 1.4 1.6
1000/T ln (σ T)
Trad 1400D MW 1400s MW 1400sD FF 1400s
Figure 24. Conductivity as a function of reciprocal temperature for samples processed at 1400oC. Note “s” indicates faster microwave heating rate profile, “D” indicates dwell.
For samples processed with all three processing temperatures, there was a linear relationship of ln (σT) vs. 1000/T, indicating that the mechanism of conduction does not change over the measured temperature range. The only significant difference was that the conductivity of the fast-fire samples produced at 1350oC was slightly lower than that for both the microwave and conventional sintered samples. This difference is due to the larger number of grain boundaries, which results from the fast heating rate and the temperature being under the manufacturer’s recommended sintering temperature. This did not occur for the microwave processed samples because the grain size in the center of the samples were larger than the surface, therefor the amount of grain boundaries were reduced.
For each sintering temperature and technique, the activation energy was determined from equation (12). The calculated activation energies range from 0.86 to 0.95 eV and are given in Table III. For a given sintering method, the activation energy increases with decreasing sintering temperature, which is attributed to the larger number of grain boundaries in the samples processed at lower sintering temperatures. Also, the microwave and conventionally sintered samples showed similar activation energies, both less than the fast-fired samples, showing that there is no significant difference in the mechanism of conduction for the conventional and microwave processed samples.
Table III shows the comparison of the activation energy for the samples processed using dwell times. The activation energy is significantly higher for the faster microwave heating profile, which can be explained by the higher number of grain boundaries and lower density. With the addition of a dwell time, the activation energy decreases as is expected. However, the activation energies of the microwave dwell time samples are still higher than the slower heating profile samples of the same temperature. The conventional sample showed an increase in activation energy with the addition of the dwell time, which was unexpected. The activation energy for this sample is similar to the lowest sintering temperature without the inclusion of a dwell. Nightingale et al. showed that testing the conductivity via DC four-probe method showed the microwave sintered samples showed slightly lower, but comparable, electrical activation energy when analyzed against conventional methods.3
Table III. Summary of Electrical Conductivity Parameters for YSZ Sintered Using the Three Techniques
Sintering Method
Sintering Temp.
(oC)
σo (S/cm) EA (eV) R2
1350 7.5 x 105 0.93 0.999
1400 6.9 x 105 0.89 0.999
1450 6.4 x 105 0.88 0.995
Traditional (no dwell)
1500 6.7 x 105 0.86 0.999
1350 6.5 x 105 0.92 0.995
1400 8.4 x 105 0.92 0.997
Microwave (Profile A)
(no dwell) 1450 6.0 x 105 0.87 0.999
1350 6.7 x 105 0.95 0.999
1400 8.5 x 105 0.95 0.993
Fast-fire (Profile A)
(no dwell) 1450 8.4 x 105 0.92 0.991
1400 8.4 x 105 0.97 0.999
Microwave (Profile B) (no dwell)
1450 1.1 x 106 0.96 0.999
Traditional (15 m dwell)
1400 1.4 x 106 0.93 0.998
1400 1.2 x 106 0.93 0.999
Microwave (Profile B) (15 m dwell)
1450 1.2 x 106 0.93 0.999
Fast Fire (Profile B) (15 m dwell)
1400 1.3 x 106 0.96 0.999
*Note: Conductivity can be calculated as σ = (σo/T) exp (EA/kT)
Figure 25 shows the conductivity of each sample prepared by the different sintering techniques as a function of the average grain size. The conductivity of 8 mol%
YSZ increases with increasing grain size, reflecting the significant contribution of grain boundaries to sample resistance. The conductivity-vs.-grain-size relationship is similar for samples processed using all three methods, which suggests that the local electrical properties of the grain and grain-boundary regions do not depend on the sintering method used. The microwave and conventionally sintered samples produced by Li et al. shows the same general relationship for ZrO2 co-doped with CaO and Y2O3.28
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
0 0.2 0.4 0.6 0.8 1 1.2
Average Grain Size (µm)
Conductivity
Traditional Fast-Fire Microwave
Figure 25. Graph of the conductivity as a function of average grain size.
V SUMMARY AND CONCLUSIONS
The microstructure and electrical properties of 8 mol% YSZ sintered using three different techniques were compared. The density and grain size was highly dependent on the sintering method and sintering temperature used. At higher sintering temperatures, however, the relationship between the density and grain size as a function of sintering temperature of the microwave and conventional processes were similar. This indicates that the microwave-processed samples can achieve comparable density / grain size relationships, but the time required for the microwave samples is significantly less.
The relationship between the density and electrical properties to the average grain size for all three sintering techniques were similar. This can be inferred that the local electrical properties of 8 mol% YSZ is not dependent on the sintering method used but only on the microstructures produced. This also implies that any of the three techniques can be used to achieve 8 mol% YSZ with desired characteristics, but that the time required to achieve the desired microstructure will be different.
VI FUTURE WORK
Recent work has shown that the properties of less pure YSZ can be significantly improved using different heating profiles.29,30 The current study was conducted using relatively high-purity samples. Future work, involving less pure YSZ powders, should be conducted to determine whether microwave sintering can provide any benefits for sintering 8 mol% YSZ.
The addition of a binder and other processing aids for producing a higher starting green density will aid in achieving a higher final density material.
The ability to co-fire 8 mol% YSZ with cathode and anode materials would be beneficial. Studies should be conducted to determine the interaction of different materials to microwave energy, and the interface between the electrolyte and the electrodes.
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8. J. William D. Callister, Materials Science and Engineering an Introduction, 4th ed.; pp. 395-6. John Wiley & Sons, New York, 1997.
9. F.C. Fonseca, E.N.S. Muccillo, and R. Muccillo, "Analysis of the Formation of ZrO2:Y2O3 Solid Solution by the Electrochemical Impedance Spectroscopy Technique," Solid State Ionics, 149 [3-4] 309-18 (2002).
10. D.D. Upadhyaya, A. Ghosh, K.R. Gurumurthy, and R. Prasad, "Microwave Sintering of Cubic Zirconia," Ceram. Int., 27 415-8 (2001).
11. M.N. Rahaman, Ceramic Processing and Sintering; pp. 374-83. Marcel Dekker, New York, 1995.
12. P. Boch and N. Lequeux, "Do Microwaves Increase the Sinterability of Ceramics?"
Solid State Ionics, 101-103 1229-33 (1997).
13. W.H. Sutton, "Microwave Processing of Ceramics - An Overview." Mater. Res.
Symp. Pro., 190, 276-83 (1992).
14. W.H. Sutton, "Microwave Processing of Ceramic Materials," Am. Ceram. Soc.
Bull., 68 [2] 376-86 (1989).
15. Z. Xie, J. Yang, X. Huang, and Y. Huang, "Microwave Processing and Properties of Ceramics with Different Dielectric Loss," J. Eur. Ceram. Soc., 19 [3] 381-7 (1999).
16. D.E. Clark, D.C. Folz, and J.K. West, "Processing Materials with Microwave Energy," Mater. Sci. Eng., A287 153-8 (2000).
17. J. Wilson and S.M. Kunz, "Microwave Sintering of Partially Stabilized Zirconia,"
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18. C. Zhao, J. Vleugels, C. Groffils, P.J. Luypaert, and O.V.D. Biest, "Hybrid Sintering with a Tubular Susceptor in a Cylindrical Single-Mode Microwave Furnace," Acta Mater., 48 [14] 3795-801 (2000).
19. M.A. Janney and H.D. Kimrey, "Diffusion-Controlled Process in Microwave-Fired Oxide Ceramics," Mater. Res. Symp. Pro., 189, 215-27 (1991).
20. S.A. Nightingale, H.K. Worner, and D.P. Dunne, "Microstructural Development during the Microwave Sintering of Yttria - Zirconia Ceramics," J. Am. Ceram. Soc., 80 [2] 394-400 (1997).
21. A. Goldstein, N. Travitzky, A. Singurindy, and M. Kravchik, "Direct Microwave Sintering of Yttria - Stabilized Zirconia at 2.45GHz," J. Eur. Ceram. Soc., 19 [12]
2067-72 (1999).
22. A. Pawlowski, M.M. Bucko, and Z. Pedzich, "Microstructure Evolution and
Electrical Properties of Yttria and Magnesia Stabilized Zirconia," Mater. Res. Bull., 37 [3] 425-38 (2002).
23. J. Samuels and J.R. Brandon, "Effect of Composition on the Enhanced Microwave Sintering of Alumina-Based Ceramic Composites," J. Mater. Sci., 27 [12] 3259-65 (1992).
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Edited by J.R. Macdonald. John Wiley & Sons, New York, 1987.
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Mater. Sci. Eng., B278 [56] 140-4 (2000).
29. L. J-H, T. Mori, L. J-G, T. Ikegami, M. Komatsu, and H. Haneda, "Improvement of Grain-Boundary Conductivity of 8 mol% Yttria-Stabilised Zirconia by Precursor Scavenging of Siliceous Phase," J. Electro. Soc., 147 [7] 2822-9 (2000).
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APPENDIX
Figure 26 a-n: Impedance spectra for traditionally processed samples sintered at 1400oC, for temperatures ranging from 200oC – 1000oC.
-4.00E+06
-3.00E+06
-2.00E+06
-1.00E+06
0.00E+00
0 1000000 2000000 3000000 4000000
Z'
Z"
26 a) Impedance spectra taken at 200oC.
-50000
-40000
-30000
-20000
-10000
0
0 10000 20000 30000 40000 50000
Z'
Z"
26 b) Impedance spectra taken at 300oC.
-4000
-3000
-2000
-1000
0
0 1000 2000 3000 4000
Z'
Z"
26 c) Impedance spectra taken at 400oC.
-1000
-750
-500
-250
0
0 250 500 750 1000
Z'
Z"
26 d) Impedance spectra taken at 500oC.
-750
-500
-250
0
0 250 500 750
Z'
Z"
26 e) Impedance spectra taken at 550oC.
-400
-300
-200
-100
0
0 100 200 300 400
Z'
Z"
26 f) Impedance spectra taken at 600oC.
-200
-150
-100
-50
0
0 50 100 150 200
Z'
Z"
26 g) Impedance spectra taken at 650oC.
-100
-75
-50
-25
0
0 25 50 75 100
Z'
Z"
26 h) Impedance spectra taken at 700oC.
-30
-20
-10
0
5 15 25
Z'
Z"
35
26 i) Impedance spectra taken at 750oC.
-12.5
-10
-7.5
-5
-2.5
0
5 7.5 10 12.5 15 17.5
Z'
Z"
26 j) Impedance spectra taken at 800oC.
-5
-4
-3
-2
-1
0
4 5 6 7 8 9
Z'
Z"
26 k) Impedance spectra taken at 850oC.
-3
-2
-1
0
3 4 5 6
Z'
Z"
26 l) Impedance spectra taken at 900oC.
-2.5
-2
-1.5
-1
-0.5
0
2 2.5 3 3.5 4 4.5
Z'
Z"
26 m) Impedance spectra taken at 950oC.
-2
-1.5
-1
-0.5
0
1.5 2 2.5 3 3.5
Z'
Z"
26 n) Impedance spectra taken at 1000oC.
Figure 27 a-m: Impedance spectra for microwave processed samples sintered at 1400oC, for temperatures ranging from 200oC – 1000oC.
-4000000
-3000000
-2000000
-1000000
0
0 1000000 2000000 3000000 4000000
Z'
Z"
o
-75000
-50000
-25000
0
0 25000 50000 75000
Z'
Z"
27 b) Impedance spectra taken at 300oC.
-6000
-5000
-4000
-3000
-2000
-1000
0
0 1000 2000 3000 4000 5000 6000
Z'
Z"
27 c) Impedance spectra taken at 400oC.
-1000
-750
-500
-250
0
0 250 500 750 1000
Z'
Z"
27 d) Impedance spectra taken at 500oC.
-750
-500
-250
0
0 250 500 750
Z'
Z"
27 e) Impedance spectra taken at 550oC.
-750
-500
-250
0
0 250 500 750
Z'
Z"
27 f) Impedance spectra taken at 600oC.
-300
-200
-100
0
0 100 200 300
Z'
Z"
27 g) Impedance spectra taken at 700oC.
-100
-75
-50
-25
0
0 25 50 75 100
Z'
Z"
27 h) Impedance spectra taken at 750oC.
-25
-20
-15
-10
-5
0
5 10 15 20 25
Z'
Z"
30
27 i) Impedance spectra taken at 800oC.
-25
-20
-15
-10
-5
0
0 5 10 15 20 25
Z'
Z"
27 j) Impedance spectra taken at 850oC.
-7.5
-5
-2.5
0
2.5 5 7.5 10
Z'
Z"
27 k) Impedance spectra taken at 900oC.
-4
-3
-2
-1
0
2 3 4 5 6
Z'
Z"
27 l) Impedance spectra taken at 950oC.
-4
-3
-2
-1
0
1 2 3 4 5
Z'
Z"
27 m) Impedance spectra taken at 1000oC.
Figure 28 a-n: Impedance spectra for fast-fire processed samples sintered at 1400oC, for temperatures ranging from 200oC – 1000oC.
-5000000
-4000000
-3000000
-2000000
-1000000
0
0 1000000 2000000 3000000 4000000 5000000 6000000
Z'
Z"
28 a) Impedance spectra taken at 200oC.
-75000
-50000
-25000
0
0 25000 50000 75000
Z'
Z"
28 b) Impedance spectra taken at 300oC.
-5000
-4000
-3000
-2000
-1000
0
0 1000 2000 3000 4000 5000
Z'
Z"
28 c) Impedance spectra taken at 400oC.
-750
-500
-250
0
0 250 500 750
Z'
Z"
28 d) Impedance spectra taken at 500oC.
-400
-300
-200
-100
0
100 200 300 400 500
Z'
Z"
28 e) Impedance spectra taken at 550oC.
-500
-400
-300
-200
-100
0
0 100 200 300 400 500
Z'
Z"
28 f) Impedance spectra taken at 600oC.
-400
-300
-200
-100
0
0 100 200 300 400
Z'
Z"
28 g) Impedance spectra taken at 650oC.
-200
-150
-100
-50
0
0 50 100 150 200
Z'
Z"
28 h) Impedance spectra taken at 700oC.
-30
-20
-10
0
10 20 30 40
Z'
Z"
28 i) Impedance spectra taken at 750oC.
-10
-7.5
-5
-2.5
0
7.5 10 12.5 15 17.5
Z'
Z"
28 j) Impedance spectra taken at 800oC.
-5
-4
-3
-2
-1
0
5 6 7 8 9
Z'
Z"
10
28 k) Impedance spectra taken at 850oC.
-3
-2
-1
0
3 4 5 6
Z'
Z"
28 l) Impedance spectra taken at 900oC.
-2
-1.5
-1
-0.5
0
2.5 3 3.5 4 4.5
Z'
Z"
28 m) Impedance spectra taken at 950oC.
-1.25
-1
-0.75
-0.5
-0.25
0
2 2.25 2.5 2.75 3 3
Z'
Z"
.25
28 n) Impedance spectra taken at 1000oC.
Figure 29 a-c: Representative SEM images of the traditional, fast-fire, and microwave samples sintered at 1350oC.
1 µm
29 a) SEM image of a traditionally sintered sample.
1 µm
29 b) SEM image of a fast-fired sintered sample.
1 µm
29 c) SEM image of a microwave sintered sample.
Figure 30 a-g: Representative SEM images of the traditional, fast-fire, and microwave samples, with and without the inclusion of a dwell time, sintered at 1400oC.
1 µm
30 a) SEM image of a traditionally sintered sample without a dwell time.
1 µm
30 b) SEM image of a traditionally sintered sample with a dwell time.
1 µm
30 c) SEM image of a fast-fired sintered sample without a dwell time.
1 µm
30 d) SEM image of a fast-fire sintered sample with a dwell time.
1 µm
30 e) SEM image of a microwave sintered sample without a dwell using the
“microwave” heating profile.
1 µm
30 f) SEM image of a microwave sintered sample without a dwell time using the “microwave s” heating profile.
1 µm
30 g) SEM image of a microwave sintered sample with a dwell time using the
“microwave s” heating profile.
Figure 31 a-f: Representative SEM images of the traditional, fast-fire, and microwave samples, with and without the inclusion of a dwell time, sintered at 1450oC.
1 µm
31 a) SEM image of a traditionally sintered sample.
1 µm
31 b) SEM image of a fast-fire sintered sample without a dwell time.
1 µm
31 c) SEM image of a fast-fire sintered sample with a dwell time.
1 µm
31 d) SEM image of a microwave sintered sample without a dwell using the
“microwave” heating profile.
1 µm
31 e) SEM image of a microwave sintered sample without a dwell time using the “microwave s” heating profile.
1 µm
31 f) SEM image of a microwave sintered sample with a dwell time using the
“microwave s” heating profile.
Figure 32 a-d: Representative SEM images of the fast-fire and microwave cross sections.
1 µm
32 a) SEM image of a fast-fire sintered sample at the edge of the cross section.
1 µm
32 b) SEM image of a fast-fire sintered sample at the center of the cross section.