Thermoelectric properties of Fe-doped SnO2 ceramics

Masahiro YASUKAWA＊

Abstract
The electrical conductivity σ and Seebeck coefficient S of Fe-doped SnO2 ceramics with nominal atomic
ratios of x = Fe/(Sn+Fe) = 0.01, 0.05, and 0.10 were measured at high temperatures between 773 and 1073 K
in air to elucidate their electrical conduction behavior and thermoelectric properties. The results show that the
Fe-doped SnO2 ceramics are n-type semiconductors that exhibit thermally activated conductivities and that the
number of electron carriers increases with increasing Fe content. The n-type semiconducting behavior is
attributed to electron hopping from Fe2+ to Fe3+, where the doped Fe3+ ions are partially reduced to Fe2+ ionsat high temperatures. Among the investigated compositions, the ceramic with x = 0.05 exhibits the largest
thermoelectric power factor S2σ; the maximum value is 8. 4 × 10-6 W･m-1･K-2 at 1073 K, which isapproximately two orders of magnitude lower than the target value (1 × 10-3 W･m-1･K-2) for practical
thermoelectric materials.

1. Introduction
The continued development of high-performance thermoelectric materials is important because they canconvert waste heat energy into useful electric energy directly. The thermoelectric performance of a material isevaluated by the dimensionless figure of merit ZT, where Z is the figure of merit and T is the averagetemperature between the ends of the material having a temperature difference. The figure of merit Z is
estimated using the equation Z = S2σ
κ-1, where S is the Seebeck coefficient, σ is the electrical conductivity,and κ is the thermal conductivity. According to this equation, achieving higher Z values and better
thermoelectric performance for a material requires the simultaneous realization of larger S, higher σ, andlower κ values. The S2σ value, also known as the thermoelectric power factor, can be used to estimate thethermoelectric performance of a material without the κ value. For a thermoelectric material to have practicalapplications, a ZT value of 1 or greater is needed; for example, values of S2σ 􂉥 10-3 W･m-1･K-2 for κ = 1
W･m-1･K-1 at T = 1000 K are desired.
Because oxide materials are generally stable at high temperatures in air, thermoelectric properties have been
investigated for numerous metal oxides. Although good thermoelectric properties have been reported forseveral oxides containing, for example, Ti, Co, or Zn [1-9], further exploration of thermoelectric oxides withenhanced performance is needed to realize thermoelectric oxides suitable for use in practical applications. Tin(IV) oxide, SnO2, is a thermally stable monoxide having a rutile-type structure; its characteristics include
nontoxicity and easy handling. SnO2 becomes an n-type conductor if it contains oxygen vacancies orpentavalent cations doped into the Sn sites; thus, the thermoelectric properties have been investigated for* Department of Social Design Engineering, National Institute of Technology, Kochi College, 200-1 Monobe, Nankoku
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various SnO2-based bulk materials [10-17]. Yanagiya et al. [14] reported a power factor of 1.2×10-4 W･

m-1･K-2 at 1072 K for Sb- and Zn-doped SnO2 ceramics sintered by the spark plasma sintering (SPS) method.
Vo et al. [15] and Rubenis et al. [16] reported the thermoelectric properties of Ta-doped SnO2 and Sb-doped
SnO2 ceramics, respectively, both of which were prepared by coprecipitation with subsequent SPS and werecomposed of nanosized grains. Tsubota et al. [17] prepared Sb-doped SnO2 ceramics by firing mixturescontaining CuO and reported a ZT value of 0.29 at 1073 K for the Sn0.985 Cu0.005 Sb0.01 O2 ceramic. Thisprevious work suggests that such SnO2-based ceramics are promising thermoelectric oxides [17].
We have reported the preparation of dense SnO2-based ceramics by a simple method of adding Fe2O3 at Fe/(Sn+Fe) atomic ratios of 0.10 or less [18]. We described that doping Fe3+ into the Sn4+ sites causes charge
compensation to form oxygen vacancies, which promote material migration and grain growth, leading todensification of the bulk samples. We also found that the prepared Fe-doped SnO2 ceramics are electricallyinsulating because they lack charge carriers at room temperature. However, the electrical conduction behavior
and thermoelectric properties at high temperatures have not been reported and remain unclear. Therefore, thehigh-temperature thermoelectric properties of Fe-doped SnO2 ceramics should be investigated further.
In this study, the electrical conductivity and Seebeck coefficient of Fe-doped SnO2 ceramics are measured athigh temperatures in air to elucidate their electrical conduction behavior and thermoelectric properties. The
ceramics are found to be n-type semiconductors, likely because of electron hopping from Fe2+ to Fe3+ at theoctahedral sites of the rutile-type structure.

2. Experiments
Fe-doped SnO2 ceramics were prepared by the solid-state reaction method reported previously [18].
Powdered reagents of SnO2 (purity 􂉥99.9%, Kojundo Chemical) and Fe2O3 (purity 􂉥99.9%, Wako Pure
Chemical) were weighed stoichiometrically with atomic ratios x = Fe/(Sn+Fe) = 0.00, 0.01, 0.05, and 0.10
and mixed thoroughly with a small amount of ethanol in an agate mortar. The mixed powder was dried andmolded into a disk with a diameter of 15 mm. Each compact was placed in an alumina boat and heated to 1673
K over a period of 5 h, maintained at 1673 K for 1 h, and then cooled to room temperature under flowing air.
The fired disk with x = 0.00 did not shrink; however, those with x = 0.01, 0.05, and 0.10 shrank dramaticallyafter the firing, indicating that the Fe2O3-added SnO2 disks were sintered.
The sintered disks with x = 0.01, 0.05, and 0.10 were cut into rectangular-shaped bars with dimensions of ~5mm × ~5 mm × ~10 mm for measurements of their electrical conductivities and Seebeck coefficients. The
bulk density of the rectangular-shaped bar was estimated using the weight and the volume calculated from the
bar's dimensions. The densities were 6.43, 6.40, and 6.50 g･cm-3 for the bar samples with x = 0.01, 0.05, and0.10, respectively, all of which are greater than 90% of the theoretical density (7.00 g･cm-3) calculated using
the unit-cell volume for a SnO2 crystal [19]. The microstructure and crystalline phases were examined using
the remaining part cut from the sintered disk. The microstructure for the fractured surface of the sinteredsamples was observed by scanning electron microscopy (SEM; JEOL, JSM-6610LA). The fractured samples
were washed in deionized water using an ultrasonic washer to remove debris and were then dried before being
observed by SEM. The crystalline phases in the sintered samples were identified by powder X-ray diffraction(XRD) measurements with Cu K radiation (Rigaku, RINT-Ultima III). The XRD pattern for the fired sample
with x = 0.00 was also measured for comparison.
The electrical conductivity σ and Seebeck coefficient S of the rectangular-shaped bars with x = 0.01, 0.05,
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Thermoelectric properties of Fe-doped SnO2 ceramics

and 0.10 were measured at several steady temperatures from 1073 to 773 K in air using laboratory-madeequipment. Au electrodes were sputtered onto the surface of the bar sample. The σ values were measured
using the standard direct-current four-probe method. The thermoelectromotive force ΔV and temperature
difference Δ T between the two ends of the bar sample were measured using Pt wires and Pt/Pt-Rh
thermocouples, respectively; the S values were calculated by correcting the linear gradient of ΔV/ΔT for thethermopower of Pt [20]. The ΔT was varied within a range of ~5 K by cooling one end of the bar sample in acontrolled airflow; approximately 10 sets of ΔV and ΔT data were acquired.

3. Results and discussion
Figure 1 shows SEM images of the fractured surfaces of the sintered samples with x = 0.01, 0.05, and 0.10.
All the samples have well-densified microstructures consisting of grains grown to sizes ranging from a few
micrometers to ~10 µm but include numerous small pores, which lower the bulk density of the samples.
However, no substantial differences were observed in the microstructures among the three ceramics.

Fig. 1 SEM images of the fractured surfaces of Fe2O3-added SnO2 ceramics with atomic ratios x = Fe/(Sn+Fe) = 0.01, 0.05, and 0.10, fired at 1673 K for 1 h in flowing air.
Figure 2(a) shows XRD patterns for the fired samples with x = 0.00, 0.01, 0.05, and 0.10. All the XRD
patterns are indexed with diffraction peaks for the tetragonal rutile-type SnO2 [19]; however, very weak peaks(marked by asterisks) are also observed in the patterns of the x = 0.05 and 0.10 samples. These weak peaks are
ascribed to Sn-doped Fe2 O3 (Fe2-y Sny O3) [21], which is likely a metastable phase that remained in theceramics in small amounts after the rapid sintering process. Figure 2(b) shows a magnification of the XRD
patterns in the 2θ range 61-67°, which includes three diffraction peaks-310, 112, and 301-of the tetragonal
rutile-type structure. Although no shifts are observed for the three diffraction peaks among the samples with
0.00 􂉤 x 􂉤 0.10, the diffraction peaks of the samples with Fe2 O3 added are broadened. In addition, theextremely weak diffraction peaks of the secondary phase Fe2-ySnyO3 even in the pattern of the ceramic with x= 0.10 suggest that Fe ions are successively doped into the rutile-type structure with increasing Fe content to x
= 0.10. These XRD results are consistent with those reported in a previous paper [18].
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Fig. 2 (a) XRD patterns for Fe2O3-added SnO2 samples with atomic ratios x = Fe/(Sn+Fe) = 0.00, 0.01,
0.05, and 0.10, fired at 1673 K for 1 h in flowing air. Asterisks denote diffraction peaks of Fe2−ySnyO3[21]. (b) Magnification of the XRD patterns in the 2θ range 61–67°. The indices 310, 112, and 301 denotediffraction peaks from the tetragonal rutile-type structure.
Figure 3(a) shows the temperature dependence of the electrical conductivity σ in the temperature range
773-1073 K for the Fe-doped SnO2 ceramics with x = 0. 01, 0. 05, and 0. 10. The σ values increase
monotonously as the temperature increases for all the ceramics; the values also increase with increasing Fe
content. Figure 3(b) shows plots of ln σ as a function of T-1. Good linear relations of ln σ versus T-1 areobserved for all the ceramics, indicating that the electrical conductivity exhibits Arrhenius-type thermally
activated behavior expressed by σ=σ0exp(-ΔE/kT), where σ0 is the constant, ΔE is the activation energyof electrical conductivity, and k is the Boltzmann constant. The activation energy of the electrical conductivity
is estimated to be 0.91, 0.72, and 0.70 eV for x = 0.01, 0.05, and 0.10, respectively, and decreases graduallywith increasing Fe content.

Fig. 3 (a) Temperature dependence of the electrical conductivity σ for Fe-doped SnO2 ceramics withnominal atomic ratios x = Fe/(Sn+Fe) = 0.01, 0.05, and 0.10. (b) The reciprocal temperature dependence
of ln σ for Fe-doped SnO2 ceramics with nominal atomic ratios x = Fe/(Sn+Fe) = 0.01, 0.05, and 0.10.
44Thermoelectric properties of Fe-doped SnO2 ceramics

Figure 4 shows the temperature dependence of the Seebeck coefficient S in the temperature range 773－
1073 K for the Fe-doped SnO2 ceramics with x = 0.01, 0.05, and 0.10. The S values range from －730 to －410 µV･K-1, and all the values are negative, indicating that the ceramics are n-type conductors because ofelectron carriers. Notably, the absolute values of S are smaller for the ceramics with larger x, indicating thatthe ceramics with greater Fe contents have more electron carriers.

Fig. 4 Temperature dependence of the Seebeck

Fig. 5 Temperature dependence of the thermoelectriccoefficient S for Fe-doped SnO2 ceramics withpower factor S2σ for Fe-doped SnO2 ceramics

nominal atomic ratios x = Fe/ (Sn + Fe) = 0. 01,

with nominal atomic ratios x = Fe/(Sn+Fe) = 0.01,

0.05, and 0.10.

0.05, and 0.10.

The electrical conduction behavior of the Fe-doped SnO2 ceramics is discussed in this paragraph. It was
described in a previous paper [18] that the Fe ions in Fe-doped SnO2 ceramics are doped as Fe3+ at the Sn4+
sites of the rutile-type structure and oxygen vacancies are formed in the structure because of charge
compensation as a result of the firing. Therefore, the chemical formula of the Fe-doped SnO2 can be written as
Sn1-xFexO2-x/2, where x is the atomic ratio of the Fe3+ ions doped for the Sn4+ sites in the structure. Because ofthis charge compensation, the Fe-doped SnO2 phase does not have any electron carriers. In fact, the Fe-doped
SnO2 ceramics are electrical insulators at room temperature. However, It is considered that more oxygen
vacancies are generated at high temperatures, accompanied by the reduction of Fe3+ to Fe2+ according to thefollowing reactions:O2－ → 1/2O2 + 2e－ ,2Fe3+ + 2e－ → 2Fe2+ .
Thus, Fe3+ and Fe2+ exist in a mixed valence state in the Fe-doped SnO2 phase at high temperatures. Because
Fe2+ has one more electron than Fe3+, this extra electron of the Fe2+ can migrate to the nearest Fe3+, leaving
Fe3+ at the former site. This electron transfer process is hopping conduction. Therefore, a possible reason for
the n-type electrical conduction observed at high temperatures for the present Fe-doped SnO2 ceramics is the
hopping conduction of electrons from Fe2+ to Fe3+. It is also considered that more Fe2+ ions－that is, more
hopping electron carriers－are formed with increasing Fe content in the Fe-doped ceramics. The decrease in
the activation energy ΔE of the electrical conductivity with increasing Fe content might also reflect a decreasein the average distance between Fe2+ and Fe3+ in the rutile-type structure.
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Figure 5 shows the temperature dependence of the thermoelectric power factor S2σ in the temperature range
773－1073 K for the Fe-doped SnO2 ceramics with x = 0.01, 0.05, and 0.10. The S2σ values increase withincreasing temperature for all the ceramics. The S2σvalues are highest for the ceramic with x = 0.05 and thehighest value is 8.4×10-6 W･m-1･K-2 at 1073 K. This value is low compared with that reported for theSn0.985Cu0.005Sb0.01O2 ceramic (4.8×10-4 W･m-1･K-2 at 1073 K) [17], where the electron carriers generated
from Sb move through the conduction band. The lower S2σ values for the present Fe-doped SnO2 ceramics are
attributed to lower σ values, which are likely due to fewer electron carriers and lower electron mobilities of
the hopping conduction. Therefore, the electrical conductivity σ needs to be substantially improved toachieve a significant improvement in the power factor.

4. Conclusions
The electrical conductivity σ and Seebeck coefficient S were measured in the temperature range 773－
1073 K for Fe-doped SnO2 ceramics with nominal atomic ratios of x = Fe/(Sn+Fe) = 0.01, 0.05, and 0.10.
The Fe-doped SnO2 ceramics are n-type semiconductors at high temperatures, and the number of electron
carriers increases as the Fe content increases from x = 0.01 to x = 0.10. The n-type semiconducting behavior is
attributable to the hopping of electrons from Fe2+ to Fe3+ at the octahedral sites of the rutile-type structure,
where the Fe2+ ions are partially generated by reduction of Fe3+ ions at high temperatures. The S2σ values arehighest for the ceramic with x = 0.05, and the maximum value is 8.4×10-6 W･m-1･K-2 at 1073 K.

Acknowledgments
This research did not receive any specific grant from funding agencies in the public, commercial, or not-forprofit sectors.

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