Chapter 4 Results and Discussions
4.2 Properties of LSM Powders
4.2.4 Electrical Properties of LSM Powders
4.2.4 Electrical Properties of LSM Powders
In the application of SOFC, the ohmic polarization from electrolyte had been
reduced by decreasing the thickness of electrolyte[Chen et al., 2006] or using the alternative
material, such as doped ceria[Zhang et al., 2007]. As a result, the polarization contributed from
electrodes becomes an important issue that determines the performance of SOFC[Chen et
al., 2007]. Therefore, the electrical property of the interface between LSM and YSZ was
analyzed in this section.
The synthesized and commercial LSM (H. C. Starck) powders were coated on
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sintered YSZ electrolyte by spray-coating and thermal treated at 1200oC for 1 hr before
measurement. The detail thermal treatment procedures of every LSM powders are
shown in Fig. 3.5.
Fig. 4.24 shows the fracture surfaces of the interfaces between LSM and YSZ
thermal treated at 1200oC. After thermal treatment, the thickness of LSM thin films was
6 μm for all the synthesized LSM layers. The microstructure of the fracture surface of
P-LSM porous layer is also shown in Fig. 4.25. The thickness of the porous layer from
H-LSM is shown in Fig. 4.24(c), which is only 3 μm. In addition, the porosity of two
synthesized LSM thin films was obviously greater than H-LSM thin layer from the
SEM microstructures. To calculate the length density (LA) of TPB between LSM and
YSZ, Saltykov equation[Harrigan et al., 1984] is introduced,
L
number of objects or interceptions per unit length. The TPB zone is counted based on
the corner effects of conducting flux of LSM materials in contact with YSZ[Yu et al., 2004].
The results of the measured TPB are shown in Table 4.4. It can be seen that the
synthesized LSM films showed similar LA of TPB, around 1.5 μm/μm2. H-LSM film
showed 30% higher LA of TPB (2 μm/μm2) than synthesized LSM powders, implying
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the possibility of a lower activation polarization for H-LSM powder.
The residual carbon content was also analyzed to identify the effect on electrical
conductivity. The carbon contents of all the LSM powders are listed in Table 4.5. The
residual carbon content of A-LSM powders was much greater than P-LSM powders
after 900oC thermal treatment. With carefully thermal treatment control, the residual
carbon content in P-LSM powders could be reduced to half (from 470 ppm to 212 ppm).
A-LSM powders, however, the thermal treatment of slow heating didn’t result in an
obvious reduction of residual carbon content.
The interface resistance between LSM and YSZ was measured by 3-terminal
electrical measurement. The electrodes and attachment configurations had been
mentioned in Section 3.5.8. In practice, the voltage signal needed a few min to reach
stable since the constant current was inputted. One example is shown in Fig. 4.26(a).
Therefore, to get the stable voltage signal, the voltage value was obtained from the
average of the voltage signal from 200 s to 400 s. As the increase of the input current,
the measured voltage increased, as shown in Fig. 4.26(b). The interface resistance was
calculated from the slope in the linear region of I-V curve.
The area of working electrode was kept at 0.38 cm2 in this study. Table 4.6 lists the
interfacial ASR of the specimens of P-LSM, P-LSM-S, A-LSM, A-LSM-S, and H-LSM
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layers. The Arrhenius plot is shown in Fig. 4.27. The resistance from LSM film itself
was neglected in this study. According to Hsu’s research[Hsu, 2003], the electrical
conductivity of LSM thin film with PAA/LSM = 2 was 2.1 S/cm at 1000oC. In other
word, the ASR of the LSM thin film with 6 μm in thickness was only 0.3 mΩ, which
could be ignored if comparing with the interfacial resistance of a few Ω in this study.
For all the LSM layers, the ASR decreased as the increasing of testing temperature from
~10 kΩ at 300oC down to ~5 Ω at 800oC. From the ASR results, there was no apparent
difference in all the LSM layers. The reasons for the similar electrical properties can be
several folds. Similar chemical composition, close residual carbon content in the LSM
films and porous microstructures are considered.
In order to offer the appropriate interface strength between LSM grains and YSZ
electrolyte plate, the thermally treated temperature must be higher than 1200oC, as
shown in Fig. 4.28. The P-LSM-S film thermally treated at 1100oC was easily removed
from YSZ plate by external physical force.
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Fig. 4.18 DTA/TG results of P-LSM powders. The temperature ramp rate was 10oC/min.
0 200 400 600 800 1000
Temperature (oC)
-100 0 100 200 300 400
DTA (μV)
-60 -40 -20 0
TG (wt%)
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Fig.4.19 XRD patterns of P-LSM powders (a) as-prepared, and by different thermal treatment at the temperature of (b) 300oC, (c) 400oC, (d) 500oC, (e) 600oC, (f) 700oC, (g)
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Fig. 4.20 DTA/TG results of A-LSM powder. The temperature ramp rate was 10oC/min.
0 200 400 600 800 1000
Temperature (oC)
-20 0 20 40
DTA (μV)
-60 -40 -20 0
TG (wt%)
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20 30 40 50 60 70 80
2θ (degree)
Intensity
Fig.4.21 XRD patterns of A-LSM powders (a) as-prepared, and by different thermal treatment at temperature of, (b) 300oC, (c) 400oC, (d) 500oC, (e) 600oC, (f) 700oC, (g)
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Fig. 4.22 TEM micrographs showing (a) BF and (b) negative CDF of 800oC thermal treated P-LSM powders, and (c) BF and (d) negative CDF of A-LSM 800oC thermal treated powders.
(a) (b)
(d) (c)
300 nm 300 nm
300 nm 300 nm
(012) (110)
(202)
(012)
(110) (202)
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Table 4.2 Quantitative analysis of P-LSM and A-LSM
P-LSM A-LSM
P-LSM 24.7%±0.5% 13.1%±0.8% 62.2%±0.3%
A-LSM 22.0%±0.9% 14.8%±1.3% 61.2%±0.7%
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Table 4.3 BET specific surface area of P-LSM and A-LSM thermal treated at various temperatures
T (oC) 500 600 700 800 900
BET (m2/g) P-LSM 36.8 28.4 16.5 4.34 1.37
A-LSM 26.0 18.1 15.9 12.8 4.05
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Fig. 4.23 Plot of specific surface area against thermal treated temperature of P-LSM and A-LSM.
500 600 700 800 900
Thermal Treated Temperature (oC)
0 10 20 30 40 50
Specific Surface Area (m 2 /g )
P-LSM A-LSM
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Fig. 4.24 SEM microstructures showing fracture surfaces of sintered (a) P-LSM-S, (b) A-LSM-S, and (c) H-LSM layers on YSZ substrates thermal treated at 1200oC for 1 hr.
(a) (b)
(c)
2μm 2μm
2μm
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Fig. 4.25 SEM micrograph showing fracture surface of porous P-LSM layer on YSZ electrolyte thermally treated at 1200oC for 1 hr.
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Table 4.4 Characteristic length of TBP of YSZ and all the LSM thin layers calculated according to Saltykov equation
H-LSM P-LSM P-LSM-S A-LSM A-LSM-S
PL (1/μm) 1.29 1.07 0.914 0.914 0.876
LA ( μm/μm2) 2.03 1.66 1.44 1.44 1.38
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Table 4.5 Carbon content of LSM powders by the treatment in specified conditions 900oC thermal treatment
P-LSM P-LSM-S A-LSM A-LSM-S H.C. Starck
average C (ppm) 470 212 923 825 N/A
1200oC thermal treatment
P-LSM P-LSM-S A-LSM A-LSM-S H.C. Starck
average C (ppm) 103 182 108 193 134
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Fig. 4.26(a) Voltage curve profile with 7.5 mA current input and (b) linear I-V curve of 1200oC thermally treated A-LSM-S/YSZ half cell at 500oC.
0 100 200 300 400 500
Time (sec)
0 1 2 3
Voltage (V)
Current Input (a)
(b)
0 0.004 0.008
Current (A)
0 1 2 3 4
Vo lt ag e ( V )
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Table 4.6 ASR results of all the LSM layers on YSZ electrolyte thermal treated at 1200oC ASR
(Ω cm2) 300oC 400oC 500oC 600oC 700oC 800oC
H-LSM 13.5k 688 82.7 28.8 7.25 N/A
P-LSM 7.12k 500 76.8 18.8 9.47 5.25
P-LSM-S 13.4k 514 84.0 16.1 11.0 6.36
A-LSM 9.28k 690 132 24.6 10.0 5.43
A-LSM-S 7.50k 516 110 15.0 9.07 5.82
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Fig. 4.27 Arrhenius plot of log(T/ASR) of various LSM layers prepared by spray coating, then thermal treating at 1200oC for 1 hr.
0.0008 0.001 0.0012 0.0014 0.0016 0.0018
1/T (1/K)
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Fig. 4.28 SEM micrographs showing fracture surfaces of sintered P-LSM-S layers and YSZ thermally treated at (a) 1100oC and (b) 1200oC for 1 hr.
(a)
(b) 1 μm
1 μm
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