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Investigation of electrical conductivity and seebeckcoefficient of Ca-and Sr-doped LaCrO3Gro Stakkestad a , Sonia Faaland b & Tove Sigvartsen aa Protech AS , Fantoftveien 38, N-5036, Fantoft, Bergen, Norwayb Norwegian Institute of Technology , S. Scelands vei 9, 7034, Trondheim, NorwayPublished online: 27 Sep 2006.

To cite this article: Gro Stakkestad , Sonia Faaland & Tove Sigvartsen (1996) Investigation of electrical conductivityand seebeck coefficient of Ca-and Sr-doped LaCrO3 , Phase Transitions: A Multinational Journal, 58:1-3, 159-173, DOI:10.1080/01411599608242400

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Phase Transirions, 1996. Vol. 58. pp. 159-173 Reprints available directly from the publisher Photocopying permitted by license only

0 1996 OPA (Overseas Publishers Association) Amsterdam B.V. Published in The Netherlands under license by Gordon and Breach Science Publishers SA

Printed in Malaysia

INVESTIGATION OF ELECTRICAL CONDUCTIVITY AND SEEBECK

COEFFICIENT OF Ca- AND Sr-DOPED LaCr03

GRO STAKKESTAD,' SONIA FAAL,AND2 and TOVE SIGVARTSEN'

'Protech AS, Fanroftveien 38, N-5036 Fantoj?, Bergen, Norway 'Norwegian Institute of Technology, S. Sadands vei 9,

7034 Trondheim, Norway

(Received 21 February 1996)

The electrical conductivity and the Seebeck coefficient of one self produced Ca-doped LaCr03 interconnector plate and two commercial Ca- and Sr-doped LaCr03 interconnector plates for solid oxide fuel cells have been studied by 4 point DC measurements from oxidizing to reducing atmospheres. The microstructure was studied on both as-sintered samples and samples exposed to a COz rich atmosphere. The commercial Sr-dopedLaCr03 has a higher electrical conductivity than the self produced Ca-doped LaCr03, while the latter seemed to be most resistant to COz rich atmosphere.

1 INTRODUCTION

The high electrical conductivity and the thermal expansion coefficient matching that of Y-stabilized Zr02, make doped LaCrO3 suitable as interconnector in solid oxide fuel cells (SOFC) (Schafer and Schmidberger, 1987; Sakai, Kawada, Yokokawa, Dokiya and Iwata, 1990; Yasuda and Hikita, 1991). The interconnector must be chemically and electrically stable in both oxidizing and reducing atmospheres.

Much work has been done to study the effect of different dopants and sintering procedures on the microstructure and conductivity properties of LaCr03 (e.g., Koc and Anderson, 1990; Sakai, Kawada, Yokokawa and Dokiya, 1991; Nasrallah, Carter, Anderson and Koc, 1991). The long time stability of Ca- and Sr-doped LaCrO3 during SOFC operating conditions has been questioned (Sakai, Kawada, Yokokawa and Dohya, 1993), since

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secondary phases (Cam (CrO4),, , SrCrO4) segregate to free surfaces through the grain boundaries.

In this work we have focused on the transport properties of Ca- and Sr-doped LaCrO3 both in oxidizing and reducing atmosphere mainly at 1000°C. The compounds have also been exposed to C02 rich atmosphere for -500 hrs. Microstructure studies have been carried out both on the as-sintered and exposed samples. This is to get a better understanding of the effect on different doping on the sintering properties, electrical properties, and stability of the LaCr03 interconnect plate being used in SOFC.

2 EXPERIMENTAL

2.1 Materials

The materials that have been investigated are listed in Table 1.

2.2 Methods

Electrical conductivity (DC) and the Seebeck coefficient were measured on rectangular specimens on which 4 Pt-PtlORh thermocouples were wound. The conductivity cell containing the specimen, was inserted into a tube furnace. The atmosphere in the conductivity cell was achieved by using flowing gas mixtures of 02/Ar and CO/CO2. The flow of each gas was controlled by Bronkhorst mass flow meters and controllers. The temperature was kept at 1000°C. Electrical conductivity measurements were performed using a 4 point DC technique. The electrical conductivity as a function of

TABLE 1 Characteristics of the samples.

Sample Nominal Sintering temperature (" C )

1700 1600 1400

1750-1 800 1750-1800

*Plates produced at Prototech AS ** Commercial plates, Japan.

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SEEBECK COEFFICIENT OF LaCr03 161

p02 was measured for all the materials. The electrical conductivity as a function of temperature was measured for the LC20CO(B) and LS2OCO in pure oxygen. The Seebeck coefficients were determined by measuring temperature gradients and thermal emf.

Scanning electron microscopy (SEM). Fracture surfaces of the different samples were examined by SEM (JEOL JSM-6400) using secondary electron imaging. For elemental analysis a Tracor X-ray system (EDS-system, series 11) was used. The LC20CO(B), LClOCO and LS2OCO samples were studied by SEM as-sintered and after exposure to CO/CO2 rich atmosphere for 530 hours at 1000°C.

Transmission electron microscopy (TEM). Microstructure of LC20CO(B) and LS2OCO were studied by TEM (Philips EM 400T( 120 kV) and Philips CM30(300 kV)). The latter was connected to a Philips EDAX (X-ray analysis system) and an EELS (electron energy loss spectrometer).

The following procedure were used to prepare the samples for TEM. The samples were cut into thin slices of -200 pm by using a diamond saw. Disks of 3 mm in diameter were cut from these slices with an ultrasonic disc-cutter (Gatan, 601). The disks were then ground to a thickness of 100-120 pm by the use of diamond papers with grain sizes of 30, 9, 3, and 1 pm. A dimple machine was used (Gatan, dimple-grinder, 656) to make the disks thin enough for ion-etching. The dimpled disks had a center thickness of 30-50 pm. Ion-etching with Ar-ions was then performed (Gatan, dual ion-mill, 600) for about 24 hours to make the samples thin enough for TEM.

3 RESULTS AND DISCUSSION

3.1 Microstructure

Examination of the electron diffraction patterns from the TEM studies reveals that the perovskite-like phase in the LC20CO(B) and LS2OCO materials has orthorhombic symmetry, cf. Figure 1. Table 2 shows the lattice parameters of these samples. The unit cell contains four LaCrO3 formula units.

The volume of the unit cell, V, expands after exposure to the reducing CO/CO2 atmosphere, cf. Table 2. For LC20CO(B) this expansion is -4.2%, and for the LS20CO sample -5.7% expansion is observed. The unit cell in the Sr-doped LaCrO3 material expands more than the Ca-doped LaCr03.

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162 G. STAKKESTAD et al.

TABLE 2 Lattice parameters and relative densities of the Ca- and Sr-doped LaCr03 samples. (I) as-sintered and iII) after exposure to CO/COz (PO* = 10;;; ) atmosphere for 530 hours at 1000" c .

Relative Sample u (A) b ( h c(A) V (A3) density* (9%)

LC20CO(A) 92** LC20CO(B) I 5.46 7.68 5.47 229.58 92

I1 5.55 7.83 5.50 239.36 LC20CO(C) 70** LS20CO I 5.49 7.76 5.44 23 1.70 96

I1 5.57 7.96 5.52 244.96

*Relative density is d/do, where d is the hulk density of the sample and do is the theoretical density determined from the lattice parameters and the chemical formula of each sample.

**The calculated relative density is based on the lattice parameters of the LCZOCO(B) sample.

FIGURE 1 Diffraction pattern of LS20CO as-sintered, showing the orthorombic (La,Sr)Cr03 phase in the [010] direction.

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SEEBECK COEFFICIENT OF LaCrOg 163

TABLE 3 Average grain size (pm) and average pore diameter (pm) of the Ca- and Sr-doped LaCr03 as-sintered and after exposure to CO/COz (pOz = 10;:) for 530 hours at 1000°C.

( I ) as sintered Average Average (11) exposed to grain size pore diameter

Sample CO/C02 ( p m ) (w) LC20CO(B) I I 40

I1 10 40 LClOCO I 25 3

I1 25 3 LS’LOCO I 15 3

I1 15 2

This has also been observed by others (Hendriksen, Carter and Mogensen, 1995).

Table 3 shows the average grain and pore size of the samples observed from SEM. Figures 2 ,3 and 4 show images of the as-sintered materials. From Table 3 and Figure 2 we observe that the pore diameter in the LC20CO(B) sample is much larger than for the LClOCO and LS2OC0, Figures 3 and 4. The two latter samples, however, have a significantly higher pore density than LC20CO(B). Examination of the samples by optical microscopy after exposure to reducing atmosphere, showed that the Sr-doped LaCr03 material has a lot of cracks, cf. Figure 5, while the two Ca-doped materials are not much different from the as-sintered samples.

3.2 Electrical Properties

Figure 6 shows the electrical conductivity of LC2OCO(B), LClOCO and LS2OCO at 1000°C as a function of the oxygen partial pressure. At high oxygen partial pressure the electrical conductivity is 30.9,22.4 and 39.8 S/cm for the LC2OC0, LClOCO and LS2OCO samples, respectively.

Doping the lanthanum chromite with strontium seems to give a little higher electrical conductivity at high oxygen partial pressures than doping with calcium. This is not consistent with work done by others (Mori, Yamamoto, Itoh, Abe, Yamamoto, Takeda, and Yamamoto, 1994) where Ca-doped LaCrO3 has been found to give a higher electrical conductivity

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164 G. STAKKESTAD et al.

FIGURE 2 SEM image of LC20CO(B) fracture surface as-sintered.

FIGURE 3 SEM image of LClOCO fracture surface as-sintered.

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SEEBECK COEFFICIENT OF LaCr03 165

FIGURE 4 SEM image of LS2OCO fracture surface as-sintered.

FIGURE 5 Optical microscopy image ( x25) of the LS20CO sample exposed to COz rich atmosphere for 530 hours at 1000°C.

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166 G. STAKKESTAD et al.

0

O t

a 0

v . .

0

0

0 0

0 .

0 LC1OCO

-0,3 -20 -1 5 -10 -5 0

log p02(atm)

FIGURE 6 Electrical conductivity (S/cm) at 1000°C for LC20CO(B), LClOCO and LS2OCO as a function of PO* (atm).

LC20CO(A)

g 3 j p 0 03 1 1 5 2 2,s

lOOO/T (1IK)

FIGURE 7 Arrhenius plot of log(aT)(SWcm) as a function of temperature (l/K) for LC20CO(A) and LS2OCO at PO? = 1.

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SEEBECK COEFFICIENT OF LaCrOg 167

than Sr-doped LaCrO3. Table 2 shows that LS2OCO has a higher relative density than the LC2OCO material, whlch may explain the higher electrical conductivity of the Sr-doped LaCr03 at high oxygen partial pressures. In the reducing atmosphere, however, the conductivity of Ca- and Sr-doped LaCr03 is approximately the same.

The results show that the conductivity increases with increasing Ca dopant concentration. This has also been observed by Mori et al. (1994).

The electrical conductivity can to a first approximation be interpreted in terms of a simple defect model comprising the acceptor dopants (Ca or Sr substituting La), oxygen vacancies and electron holes as the dominating defects. Adopting the Kroger-Vink terminology (Kroger and Vink, 1965) and using Ca as example, the electroneutrality of the material can be approximated as

2[Vo ..I + p = [ CalLa ] = constant (1)

At high p02, the electron holes are the dominating positive defects ( p >> 2[V0 -1):

p = [ Ca’La] = constant (2)

Under these conditions the p-type electronic conductivity is thus independent of p02, and the temperature dependence is given by the change in the mobility. For small polaron hopping conduction, we thus have (Karim and Aldred, 1979)

(3 ) cp = e[ Ca’La ] p p = e[ Ca’La ]pPT-’ exp(E,/kT)

where e is the elementary charge, p: is a charge carrier and material dependent mobility pre-exponential, and Ep is the activation energy of migration of electron holes. By plotting the conductivity at high p02 as log(aT) vs. 1/T, Ep was found to be 0.13 and 0.15 eV, respectively, for the LC20CO(A) and LS2OCO samples, roughly in agreement with literature values (Yasuda and Hikita, 1991; Karim and Aldred, 1979).

Below a certain p02 the conductivities decrease with decreasing p02, with slopes in the log-log plot of 0.18-0.20. This is interpreted as a region where oxygen vacancies have taken over as the dominating positive defects

0

(2[VO ..I >> p ) :

(4) 2[v0 ..I = [ Ca’La ] = constant

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168 G. STAKKESTAD et al.

Under these conditions it can be shown that the minority concentration of electron holes, p , is proportional to ~0;’~. The observation of slope less than 0.2 rather than the predicted 0.25 has been suggested to be due to deviations from the ideal defect structure and dilute solution of defects (Van Roosmalen and Cordfunke, 1991; Anderson, 1993).

3.3 Seebeck Measurements

Measurements of Seebeck coefficients as a function of oxygen partial pressure of the materials LC20CO(Aj and LS2OCO gave positive value, cf. Figure 8. This confirms that the materials are p-type conductors.

Figure 9 shows the Seebeck coefficients of LS2OCO as a function of temperature. This type of temperature dependence has been observed for (La,Sr)(Cr,Mn)Os by Koc et al. (1989) and has been explained to be due to mobility of the charge carriers.

Heikes (1961) has shown that the Seebeck coefficient of small polaron hopping conductors can be expressed as

Q = k/ze[ln(l - cj/c - s / k ]

where k is Boltzmann’s constant, e is elementary charge, z expresses the sign of the charge carrier, c is the fraction of occupied hopping sites and s is a vibrational entropy due to relaxation of the lattice around the charge carrier. This equation leads to a temperature independent Seebeck coefficient. Usually, the entropy s is small enough to be neglible, therefore the Seebeck coefficient depends only on the concentration term. Using the Seebeck data and Heike’s formula, the fraction of the hopping sites were calculated for LC20CO(Aj, and LS2OCO at 1000°C and p0z = 1, cf. Table 5.

TABLE 5 Seebeck coefficients (pV/K), calculated fraction of occupied charge carrier sites and the mobility (cm’Nsec) for the Ca- and Sr-doped LaCr03 samples at 1000°C and pol = 1.

~~ ~

Seebeck coeficient, Fraction of occupied Mobility,

Sample Q (PV/KJ hopping sites, c. p (cm’Nsec)

LCZOCO(A) 210.80 0.08 0.13 LS2OCO 205.49 0.08 0.16

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SEEBECK COEFFICIENT OF LaCrOg

450 ~

400

350 B

0 0

YO:: 5 250

x 200 al P g 150

cn 100

50

0 .

169

--

-- 0

m

I

8 1 0 0

0 0 0 u s --

--

--

--

FIGURE 8 Seebeck coefficients (pV/K) as a function of pOz (atm) for LC20CO(A) and LS2OCO at 1000°C.

0 0.5 1 1 3 2 25

1OOO/T (1IK)

FIGURE 9 Seebeck coefficients (pV/K) as a function of temperature (UK) for LS2OCO at p O r = l .

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170 G. STAKKESTAD et al.

O J 0 0 3 1 1.5 2 2,s

1 OOO/T(llK)

FIGURE 10 Arrhenius plot of log(~T)(cm2KNsec) as a function of temperature (UK) for LS2OCO at PO? = 1.

The mobility of charge carriers can be determined by combining the Seebeck coefficient and electrical conductivity data

where c is the electrical conductivity, N v is the total number of hopping sites, e is the elementary charge, Q is the Seebeck coefficient and k is Boltzmann constant. The total number of hopping sites is given as the Cr concentration, which equals the inverse of the volume of a formula unit (Table 2). The values of the total number of hopping sites used in our calculations is 1.72* and 1.74 * cmP3 for the LC2OCO and LS2OCO samples, respectively. Using the values for the total number of hopping sites, the Seebeck coefficients and the electrical conductivity, the mobility of charge carriers as a function of temperature were calculated from Equation 6 for LS2OCO. The mobility of the charge carriers was also calculated for LC2OCO at 1000°C and p02 = 1, cf. Table 5.

All in all, the results indicate that the charge carrier density is the same in both the Ca- and Sr-doped sample.

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The finding of charge carrier densities of 0.08 may be compared with the nominal doping level which should give 0.2 holes per mole of LaCrO3 perovskite. If these holes are distributed among the Cr-ions (as Cr4+) the charge carrier density would be expected to be 0.2 to a first approximation. In a letter to the editor Doumerc (1994) points out that the symmetric Chailun-Beni formula (Ckaikin and Beni, 1976), sometimes used for holes

Q = k/lel ln[2(1 - c)/c] (7)

is relevant to O - ( 2 p 5 ) holes distributed among 02- ions. The formula differs from Helke’s with a factor 2, which will give us charge carrier densities of 0.16. This indicates that the holes in the perovskite can be viewed as located on oxygen ions rather than chromium ions.

Electrical conductivity as a function of oxygen partial pressure has been measured for Ca doped (20%) LaCr03 sintered at different temperatures, of Figure 1 1. The conductivity increases significantly with increasing sintering temperatures up to 1600°C. A further increase in sintering temperature ( 17OO0C), does not affect the conductivity behaviour. The increasing conduc- tivity with increasing sintering temperature can be explained by the increase in density of the materials, cf. Table 2. From these experiments a sintering temperature of 1600°C is recommendable.

1.6 1

4

b

__

E E m 4

A * * n

4 .

m *

0

8 . A m

b

4 LCPOCO(B, 1600 C)

P

-20 -15 -1 0 -5 0

log pO2 (atrn)

FIGURE 11 Electrical conductivity (S/cm) at 1000°C as a function of PO;? (atm) for samples LC2OCO(A, B and C) sintered at given temperature.

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172 G. STAKKESTAD et 01.

4 CONCLUSION

Structure studies revealed that the investigated Ca- and Sr-doped LaCr03 materials had orthorombic symmetry. The self produced Ca-doped material seemed to be most resistant to the C02 rich atmosphere. Electrical conductiv- ity and Seebeck coefficients indicate that all materials are p-type conductors. It has been found that the commercial Sr-dopedLaCrO3 has a higher electrical conductivity than the self produced Ca-doped LaCr03. This may be explained by the hgher relative density. The charge carrier density was found to be the same in both materials, but less than the nominal doping level. Which may indicate that the holes are distributed both on the Cr- and 0-ions.

Acknowledgements

Gro Stakkestad is indebted to the Norwegian Research Council for financial support.

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