2.4 Conclusion
3.3.4 Microstructural View of Point
Figure 3.7 shows that MZY05 exhibited elongated grains of mullite, while mullite grains in MZY30 and MZY80 were equiaxed. From the SEM micrograph of MZY05 [Fig. 3.7(a)], it was concluded that PSZ was located at the grain boundaries of mullite and was isolated by mullite grains. It was inferred that the conductivity was still dominated by mullite as mentioned in previous section, although the grain conductivity in MZY05 was increased because of the addition of PSZ. Figure 3.7(b) reveals that some PSZ particles became interconnected in MZY30. The conductivity would be
significantly affected by PSZ. Figure 3.7(c) showed that mullite grains were surrounded by PSZ grains in MZY80. The rapid path of interconnected PSZ for electrical conductivity was formed in MZY80, and the grain and grain-boundary conductivities in MZY80 were completely controlled by PSZ.
3.4 Conclusions
1. The electrical conductivities of mullite/PSZ composites with PSZ content were measured by the ac impedance spectroscopy. The impedance spectra of monolithic mullite showed only one semicircle, while the monolithic PSZ and mullite/PSZ composites showed two semicircles.
2. The conductivities of mullite/PSZ composites increased with the PSZ content, but no percolation relationship was observed. The real parts of conductivities measured at 1 MHz and 1 KHz were in good agreement with the Lichtenecker’s rule and the mixing rule, respectively.
3. The activation energies of electrical conduction for mullite/PSZ composites were different in the high-frequency and low-frequency regions, depending on the PSZ content. The activation energies of grain conductivities in mullite and PSZ were about 65 and 79 kJ/mol, respectively, while those in the composites were calculated in between these two values. Furthermore, the activation energies sharply increased at 10 to 20 vol% PSZ in the low-frequency region.
4. The electrical conductivities of mullite/PSZ composites were effectively increased by the incorporation of PSZ so that high-PSZ composites
could be used for electrochemical purposes such as a gas sensor at high temperatures.
Table 3.1 Designations, compositions, hot pressing conditions, relative densities, and x-ray phases of various mullite/PSZ composites
* M = mullite; Z = 3 mol% Y2O3-ZrO2; t-Z = tetragonal ZrO2; m-Z = monoclinic ZrO2
Designation Composition* Hot Pressing
Condition# Relative
Density (%) XRD Phases*
M 100 v/o M 1675oC/45 min 97.8 M
MZY05 95 v/o M + 5 v/o Z 1600oC/45 min 97.1 M, t-Z, m-Z MZY10 90 v/o M + 10 v/o Z 1600oC/45 min 96.7 M, t-Z, m-Z MZY20 80 v/o M + 20 v/o Z 1600oC/45 min 95.4 M, t-Z, m-Z MZY30 70 v/o M + 30 v/o Z 1600oC/45 min 96.7 M, t-Z, m-Z MZY40 60 v/o M + 40 v/o Z 1600oC/45 min 97.5 M, t-Z, m-Z MZY50 50 v/o M + 50 v/o Z 1600oC/45 min 97.1 M, t-Z, m-Z MZY60 40 v/o M + 60 v/o Z 1600oC/45 min 97.8 M, t-Z, m-Z MZY80 20 v/o M + 80 v/o Z 1600oC/45 min 97.7 M, t-Z, m-Z Z 100 v/o Z 1600oC/45 min 99.9 t-Z, m-Z
# All samples were fabricated under the pressure of 30 MPa in Ar.
HP 4194A
Furnace
Pt plate Sample Pt wire
Al2O3 plate
Fig. 3.1 Impedance-measuring system.
0 1 2 3 4 5 6 7
Fig. 3.2 Typical impedance spectra. (a) For MZY10 at temperatures ranging from 400 to 1000oC; (b) For MZY80 at temperatures ranging from 200 to 450oC. The enlarged views at low impedances are shown in the inset.
0 1 2 3 4 5 6
Fig. 3.3 Impedance spectra of the composites with various PSZ contents: (a) M, MZY05, MZY10 and MZY20 at 700oC;
(b) MZY30, MZY40, MZY50, MZY60, MZY80 and Z at 300oC. The enlarged views at low impedances are shown in the inset.
0.5 1.0 1.5 2.0 2.5 3.0
Fig. 3.4 Arrhenius plots of conductivities determined (a) in the high-frequency region; (b) in the low-frequency region of impedance spectra.
0 20 40 60 80 100 60
70 80 90 100
activation energy (kJ/mol)
zirconia (vol%)
high frequency region low frequency region monolithic mullite
Fig. 3.5 The activation energy versus PSZ content curves of mullite/PSZ composites in the high-frequency and low-frequency regions, respectively, of the impedance spectra.
0 20 40 60 80 100 -11
-10 -9 -8 -7 -6
ln σ' (ohm-1 cm-1 )
(a)
zirconia (vol%)
0 20 40 60 80 100
-18 -16 -14 -12 -10 -8 -6
ln σ' (ohm-1 cm-1 )
(b)
zirconia (vol%)
Fig. 3.6 The real part of conductivity versus PSZ content curve at the frequencies of (a) 1 MHz and (b) 1 KHz at 600°C. The fitting lines in (a) and (b) were determined by Lichtenecker’s rule and the general mixing equation, respectively.
( c ) ( b )
( a )
Fig. 3.7 Scanning electron micrographs of mullite/PSZ composites: (a) MZY05; (b) MZY30; (c) MZY80. The dark phase is mullite and the bright phase is PSZ.
Chapter 4
Oxygen Diffusivities and Surface Exchange Coefficients in Porous Mullite/Zirconia Composites Measured by the Conductivity Relaxation Method
4.1 Introduction
Mullite is a frequently used material for applications at high temperatures due to its advantageous properties, including good creep resistance, excellent chemical stability, and suitable high-temperature strength.1 In addition, mullite is a potential substrate material because it has a favorable dielectric constant and thermal expansion coefficient.58 To further improve its mechanical properties (e.g., fracture toughness), ZrO2 particles and/or SiC whiskers have been incorporated in mullite, as indicated in previous studies.5,
9, 59, 60
One can modify the properties of composite materials by combining two or more components. The properties of composites are highly dependent upon the size, shape and content of the individual components. Previous studies57, 61 investigated the oxygen diffusivities and electrical conductivities of mullite/PSZ composites with various PSZ contents. The percolation phenomenon for oxygen diffusion in mullite/PSZ composites was observed at 30-40 vol% PSZ, while no such a phenomenon was observed for electrical conduction.61 The electrical conductivities of mullite/PSZ composites followed Lichtenecker’s rule at high frequencies and the general mixing equation at low frequencies.
Recently, the mechanical properties of porous mullite/ZrO2 composites have been investigated.62, 63 Haslam and Lange62 developed a processing method to strengthen porous mullite/ZrO2 composites without shrinkage using evaporation/condensation sintering in an HCl atmosphere. Latella and Mehrtens63 indicated that the mullite/ZrO2 composites with 62% porosity showed no strength degradation at temperatures ranging from 25 to 1200oC.
Because porous composites could be used in hot gas filtration environments, the influence of ZrO2 content on diffusion and/or surface exchange rate in porous mullite/ZrO2 composites is an important subject.
To date, little research has been conducted on the character of mass transfer in porous mullite/ZrO2 composites. In a previous study,64 Ganeshananthan and Virkar measured the oxygen surface exchange coefficients of porous La0.6Sr0.4CoO3-δ using the conductivity relaxation method. In this study, the conductivity relaxation method was used to measure the diffusivities and surface exchange coefficients of porous mullite/PSZ composites with various PSZ contents. The effects of PSZ content and oxygen partial pressure on the diffusivities and surface exchange coefficients of porous mullite/PSZ composites were explored.
4.2 Experimental Procedures
Based upon a previous study on the percolation phenomenon, mullite/PSZ composites containing more than 40 vol% PSZ were defined as "high-PSZ composites"; otherwise, the composites were categorized as "low-PSZ composites."
The composites in this study were fabricated by sintering mixtures of mullite (KM-mullite, 71.86 wt% Al2O3, 28.07 wt% SiO2, 0.03 wt% Fe2O3, 0.03 wt%
NaO2 and 0.01 wt% MgO2, 0.2 µm on average, Kyoritsu Ceramic Materials Co., Nagoya, Japan), 3 mol% Y2O3-stabilized ZrO2 or 3Y-ZrO2 powder (TZ-3Y, 94.75 wt% ZrO2, 5.21 wt% Y2O3, 0.005 wt% Al2O3, 0.005 wt%
SiO2, 0.002 wt% Fe2O3 and 0.022 wt% NaO2, 0.3 µm on average, Toyo Soda Mfg., Co., Tokyo, Japan), and carbon (Vulcan XC72, 0.03 µm on average, Cabot Co., Billerica, MA), wherein 30 vol% carbon was used as the pore-forming agent.
The starting powders were first dispersed in alcohol. The pH value was adjusted to 10 using NH4OH as an electrolyte, and then the powder mixtures were dried on a hot plate. Subsequently, they were uniaxially pressed at 63 MPa for a few minutes. The cold-pressed samples were heated to burn out carbon at 600°C for 30 min and then sintered at 1200-1550°C for 2-3 h depending upon the composition of the powder mixture.
The densities of the sintered bodies were determined by the Archimedes method using de-ionized water as an immersing medium, and the relative densities were then calculated. The designations, compositions, sintering conditions, and relative densities of these composites are listed in Table 4.1.
The sintered composites were cut into pieces about 3.5 × 3.5 × 0.8 mm in size. Samples were ground and polished with a precision polishing machine (Model Minimet 1000, Buehler Ltd, Lake Bluff, IL) using standard procedures as described previously.57
The conductivity relaxation method was used to measure the electrical
resistivities of bulk samples under different oxygen partial pressures.
Figure 4.1 shows a schematic diagram of the conductivity-measuring system.
The specimen together with two Pt electrodes was clipped by two plates of Al2O3. After being connected to an electric potentiometer, the specimen was placed in an Al2O3 tube furnace. The measuring temperatures were less than the sintering temperature by at least 200°C to avoid densification during the measurements. When the electric conductivity was saturated at a certain oxygen partial pressure (Po21), a gas mixture with a decreased oxygen partial pressure (Po22) was introduced into the chamber. While the oxygen partial pressure changed from Po21 to Po22, the variation of electrical resistance was recorded with a multimeter (Model 2000, Keithley Instruments Inc, Cleveland, OH). This procedure was repeated using a stepwise decrease in the oxygen partial pressure after the electric conductivity was saturated again.
It was noted that when the ratio of initial to final Po2 is larger than 20, linear exchange kinetics are no longer valid.65 Thus, a narrow Po2 change was conducted to make certain that systematic error in measuring the diffusivities and surface exchange coefficients could be avoided. The oxygen partial pressures chosen in this study were 20.2, 14.1, 10.1, 6.07 and 2.02 kPa in sequence. Variation of the oxygen partial pressure was achieved by different flow rates of oxygen and argon.