Preparation of dense SnO2-based ceramics by Fe2O3 addition

Masahiro YASUKAWA*1 and Naoya MATSUOKA*2*1

Department of Social Design Engineering, National Institute of Technology, Kochi College, 200-1 Monobe,
Nankoku 783-8508, Japan*2

Department of Materials Science and Engineering, National Institute of Technology, Kochi College, 200-1
Monobe, Nankoku 783-8508, Japan

Abstract
Dense SnO2-based ceramics have been prepared by firing Fe2O3-added SnO2 compacts with atomic ratiox = Fe/(Sn+Fe) = 0.005 ­ 0.10 at 1673 K. The densification is remarkably promoted even with the addition ofa small amount of x = 0.005. The XRD results suggest that Fe3+ ions are doped into Sn4+ sites of the rutiletype structure, forming oxygen vacancies due to charge compensation. The oxygen vacancies promotematerial migration and coalescence of the particles, leading to the grain growth and densification. Coadditions with Fe2O3 and M2O5 (M = Nb, Sb, Ta) have also been performed to explore preparation ofelectroconductive and dense SnO2-based ceramics for targeted compositions of Sn0.98−yFe0.02MyO2 with y =0.00 ­ 0.04. The co-additions with y < 0.02 lead to dense and insulating ceramics by firing at 1673 K while
those with y 􂉥 0.02 lead to non-densified bulks. These results can be explained by charge compensation
between Fe3+ and M5+ (M = Nb, Sb, Ta) ions co-doped into the Sn4+ sites of the rutile-type structure.

1. Introduction
Tin dioxide, SnO2, is a wide-gap insulator with a rutile-type structure and becomes an n-type conductor
when oxygen vacancies or doped pentavalent cations such as Sb5+ are included in the structure [1]. The SnO2based semiconducting materials are widely used as transparent conducting films or porous gas sensors. The
ceramic form has been expected for applications to electrodes, varistors, or thermoelectric materials at hightemperatures as described below.
It is well known that pure SnO2 powder is hard to be densified by normal firing method. Various SnO2based ceramics were prepared by firing with an additive such as CuO [2-5], CoO [6-13], MnO2 [7], and ZnO[14] as a sintering agent for applications to electrodes or varistors. Several ceramics were also prepared by coaddition of CoO and Fe2O3 [15, 16] or ZnO and Fe2O3 [17]. The densification was promoted by a liquid phaseformed during the firing with CuO [2-5] or by oxygen vacancies formed due to charge compensation when the
Sn4+ sites were doped with lower valency cations such as Co2+ [6, 7, 9, 12], Mn2+ [7], Zn2+ [14], or Fe3+
together with Co2+ [16] or Zn2+ [17]. To investigate the thermoelectric properties [18-24], several SnO2 *1*2

Corresponding author.
Graduated in 2020.
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based ceramics were prepared by spark plasma sintering (SPS) method [21-23] or by firing with an additive
CuO as a sintering agent [24]. The SPS method is effective for rapid preparation of dense ceramics but it needs
the exclusive equipment.
On the other hand, the firing of compacts with a sintering agent is a simple and convenient method toprepare dense ceramics. As described above, although several additives were used as sintering agents to
prepare dense SnO2-based ceramics, use of Fe2O3 as a single additive has not been reported. Since Fe2O3 is acommon oxide and easily handled, it is worth investigating the effect of Fe2O3 as a single additive to prepare
dense SnO2-based ceramics. Chemani et al. [25] reported the influence of Fe2O3 additive on the
microstructural development of SnO2, but dense SnO2-based ceramics were not prepared owing to lower firing
temperatures of 1273 K ­ 1473 K.
In this study, Fe2O3-added SnO2 compacts have been fired at high temperatures of 1473 K ­ 1673 K toinvestigate the densification of the samples. It is reported that the firing with Fe2 O3 addition brings about
doping of Fe3+ ions into Sn4+ sites of the rutile-type structure to form oxygen vacancies due to charge
compensation and leads to grain growth and densification of the compacts. Co-additions with Fe2O3 and M2O5
(M = Nb, Sb, Ta) have also been performed aiming to prepare electroconductive and dense SnO2 -based
ceramics.

2. Experiments
Powder reagents of SnO2 (purity 􂉥 99.9 %, Kojundo chemical), Fe2O3 (purity 􂉥 99.9 %, Wako Pure
Chemical), Nb2O5 (purity 􂉥 99.9 %, Wako Pure Chemical), Sb2O5 (purity 􂉥 99.9 %, Kojundo chemical),and Ta2O5 (purity 􂉥 99.9 %, Wako Pure Chemical) were used. For preparation of Sn1−xFexO2, the SnO2 andFe2O3 powders were weighed stoichiometrically with x = 0.00, 0.001, 0.005, 0.01, 0.02, 0.03, 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 cylinder with 5 mm or 15 mm in diameter. Each compact was fired at 1473 K ­ 1673 K for 1 h
in air-flow on an alumina boat. For preparation of Sn0.98−yFe0.02MyO2 (M = Nb, Sb, Ta), the SnO2, Fe2O3, andM2O5 powders were weighed stoichiometrically with y = 0. 00, 0. 01, 0. 02, 0. 03, and 0. 04, and mixed
thoroughly with a small amount of ethanol in an agate mortar. The mixed powder was dried and molded into acylinder with 5 mm in diameter, and each compact was fired at 1673 K for 1 h in air-flow on an alumina boat.
The bulk density of the fired samples was estimated from the weight and the volume which was
calculated from the diameter and the height. The microstructure for the fractured surface of the fired samples
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 dried before the SEM
observations. The crystalline phases in the fired samples were identified by powder X-ray diffraction (XRD)
measurements with CuKα radiation (Rigaku, RINT-Ultima III), and the lattice parameters a and c of the rutiletype structure were calculated by the least-squares method using the observed diffraction angles. The electricalresistance for the fired samples was measured between two points with a distance of about 1 mm using a
multimeter (Custom Corporation, CDM-03D).

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Preparation of dense SnO2-based ceramics by Fe2O3 addition

3. Results and discussion
Figure 1 shows firing temperature dependence of the bulk density for the Fe2O3-added SnO2 samples
with x = 0.00, 0.05, and 0.10. The Fe2O3-added samples with x = 0.05 and 0.10 are densified by firing at eachtemperature between 1473 K and 1673 K, and the bulk density becomes higher with increasing firing
temperature. On the other hand, the pure SnO2 compact without Fe2O3 addition is not densified by firing at
1673 K and the relative density after the firing is about 50 % for the theoretical density (7.01 g･cm­3) [26].
Any weight loss indicating volatilization of the constituent elements is not observed for these samples with x =0.00, 0.05, and 0.10 even after the re-firing at 1673 K for 1 h in air-flow.

Figure 2 Plots of the bulk density on the atomic
ratio x = Fe/(Sn+Fe) for Fe2O3-added SnO2
samples fired at 1673 K for 1 h in air-flow.

Figure 1 Firing temperature dependence of thebulk density for Fe2O3-added SnO2 samples withatomic ratio x = Fe/(Sn+Fe) = 0.00, 0.05, and 0.10.

Figure 2 shows plots of the bulk density on the atomic ratio x in the range of x = 0.00 ­ 0.10 for the Fe2O3added SnO2 samples fired at 1673 K for 1 h in air-flow. The samples with x = 0.005 ­ 0.10 are well densified

by the firing, exhibiting the bulk densities higher than 6.3 g･cm­3. It should be noted that the sample evenwith x = 0.005 is well densified by the firing, but that with x = 0.001 is not densified. The color of thedensified bodies with x = 0.005 ­ 0.10 is dark brown, but the pulverized powders are pale grey to dark greywith increasing x. On the other hand, the colors of the fired bodies with x = 0.00 and 0.001 are white andreddish white, respectively. All the fired bodies with x = 0.00 ­ 0.10 are electrically insulating at room
temperature, because the electrical resistances are beyond the detection limit (40 MΩ) of the multimeter.
Figure 3 shows SEM images for the fractured surfaces of the samples with x = 0.00 ­ 0.10 after firing at
1673 K for 1 h in air-flow. The sample with x = 0.00 has a porous and non-densified microstructure consistingof small particles with sub-micrometers to a few micrometers in size, whereas the samples with x = 0.005 ­0.10 have well-densified microstructures consisting of grains with a few micrometers to about 10 micrometersin size. The sample with x = 0.001 is not densified and is porous but consists of grains combined each other.
The small holes observed for the samples with x = 0.02 ­ 0.10 are considered to be traces of Sn-doped Fe2O3(Fe2­zSnzO3) particles probably removed by washing using the ultrasonic washer before SEM observations.
The Fe2­zSnzO3 phase is confirmed by XRD results described in the next paragraph.

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Figure 3 SEM images for the fractured surfaces of Fe2O3-added SnO2 samples with atomic ratio x = Fe/(Sn+Fe) = 0.00, 0.001, 0.005, 0.01, 0.02, 0.03, 0.05, and 0.10 fired at 1673 K for 1 h in air-flow.

Figure 4 (a) XRD patterns for Fe2O3-added SnO2 samples with atomic ratio x = Fe/(Sn+Fe) = 0.00,
0.005, 0.01, 0.02, 0.03, 0.05, and 0.10 fired at 1673 K for 1 h in air-flow. Asterisks denote diffraction
peaks from Fe2−z SnzO3 [27]. (b) Magnification of the XRD patterns to 2θ range of 30° – 45°. (c)Magnification of the XRD patterns to 2θ range of 53° – 60°. The indices 220 and 002 denote diffraction
peaks from the tetragonal rutile-type structure. (d) Plots of the full width at half maximum (FWHM)
for the diffraction peaks 220, 002, and 321 (2θ ~ 78.72°) on the atomic ratio x = Fe/(Sn+Fe).
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Preparation of dense SnO2-based ceramics by Fe2O3 addition

Figure 4(a) shows XRD patterns for the samples with x = 0.00 ­ 0.10 after firing at 1673 K for 1 h in airflow. All the XRD patterns are indexed with diffraction peaks for the rutile-type SnO2 [26], but extremely
weak peaks from Sn-doped Fe2O3 (Fe2­zSnzO3) [27] are additionally observed at the 2θ angles marked byasterisks for the samples with x = 0.02 ­ 0.10 as shown in Figure 4(b). This indicates that extremely slightamount of Fe2­zSnzO3 exists locally as another stable phase besides the main rutile-type phase after the firing.
It is supposed that the small holes observed in the SEM images for the samples with x = 0.02 ­ 0.10 are thetraces of small particles of the Fe2­zSnzO3 phase. The residue of the Fe2­zSnzO3 phase may be due to rapid
densification process during the firing. Figure 4(c) shows a magnification of the XRD patterns to a 2θ range
of 53°­ 60°, which exhibits two diffraction peaks 220 and 002 for the rutile-type structure. It should be notedthat any peak shifts are not observed for the both diffraction peaks with increasing x from x = 0.00 to x = 0.10.
Indeed, the calculated lattice parameters a and c of the tetragonal rutile-type structure are constant for all thesamples with x = 0.00 ­ 0.10, indicating a = 4.7359(3) Å and c = 3.1855(3) ­ 3.1860(3) Å, respectively.
Although the lattice parameters are constant for all the samples, it is considered that the Fe ions are doped into
the rutile-type structure successively with increasing x because the peak intensities of the Fe2­zSnzO3 phaseare extremely low even for the samples with x = 0.05 and 0.10. Based on the ionic radius [28], it can beconsidered that the Sn4+ ions (ionic radius: 0.69 Å) are substituted with Fe3+ ions with a high spin state (ionicradius: 0.645 Å) rather than Fe3+ with a low spin state (ionic radius: 0.55 Å). The previous paper on 57Fe
Mössbauer spectra of the rutile-type Sn0.95Fe0.05O2 sample fired at 1423 K reported that the Fe ions are doped
into the Sn sites as high spin Fe3+ [29]. Figure 4(d) shows plots of the full width at half maximum (FWHM)
for the diffraction peaks 220, 002, and 321 on the atomic ratio x. Although there is some dispersion for thevalues, the tendency of the increase in the FWHM with increasing x may suggest distortion of lattice planes
due to doping of the Fe ions into the Sn sites.
The densification mechanism for the Fe2O3 -added SnO2 is briefly described in this paragraph. The
doping of Fe3+ ions into the Sn4+ sites brings about the formation of oxygen vacancies to compensate the lackof positive charges in the structure. The oxygen vacancies formed during the firing promote diffusion of oxideions and material migration leading to coalescence of the particles and grain growth to densify the Sn1­xFexO2
bulks. The similar mechanism was described in the previous papers for the other SnO2 -based ceramics
prepared with a sintering additive such as CoO [6, 7, 9, 12], MnO2 [7], or ZnO [14] and with two additives
such as CoO and Fe2O3 [16] or ZnO and Fe2O3 [17].

Figure 5 Plots of the bulk density on the M
content y in targeted compositions Sn0.98−yFe0.02
MyO2 (M = Nb, Sb, Ta). The samples co-addedwith Fe2O3 and M2O5 (M = Nb, Sb, Ta) were
fired at 1673 K for 1 h in air-flow.
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In order to explore preparation of electroconductive and dense SnO2-based ceramics, co-addition of Fe2O3
and pentavalent metallic oxide of Nb2O5, Sb2O5, or Ta2O5 to SnO2 has been performed expecting that the Fe2O3
and M2O5 (M = Nb, Sb, Ta) work as the sintering agent and the electron dopant, respectively. Figure 5 shows
plots of the bulk density for the samples fired at 1673 K on the M content y in the targeted compositions of
Sn0.98­yFe0.02MyO2 (M = Nb, Sb, Ta; y = 0.00 ­ 0.04). It is clearly seen for M = Nb, Sb, Ta that the samples

with y = 0.00 and 0.01 are densified exhibiting the bulk densities higher than 6.0 g･cm­3 whereas those with y􂉥 0.02 are not densified exhibiting the bulk densities lower than 3.9 g･cm­3. Any weight loss indicating
sublimation of the constituent elements is not observed after the re-firing at 1673 K for the fired bodies of M =Nb, Sb, Ta. The electrical resistances measured using the multimeter are beyond the detection limit (40 MΩ)for all the fired samples except for Sn0.98­yFe0.02SbyO2 with y = 0.03 and 0.04. The XRD patterns indicate that
all the fired samples of M = Nb, Sb, Ta are composed of the rutile-type single phase except for the samples with
y = 0.03 and 0.04 of M = Ta, which include small amounts of Ta2O5 not dissolved in the rutile-type phase. They dependence of the bulk density shown in Figure 5 can be explained by charge compensation mechanism
between Fe3+ and M5+ ions co-doped into the Sn4+ sites of the rutile-type structure. For the fired samples with
y = 0.00 and 0.01 of M = Nb, Sb, Ta, oxygen vacancies are generated but electron carriers are not generated
because the number of Fe3+ ions doped in the structure is larger than that of M5+ ions, leading to the formationof the densified and highly resistive ceramics. For y = 0.02 of M = Nb, Sb, Ta, neither oxygen vacancies nor
electron carriers are generated in the structure because the number of doped Fe3+ ions is equal to that of M5+ions, leading to the formation of the non-densified and highly resistive bulks. For y > 0.02 of M = Nb, Sb, Ta,
oxygen vacancies are no longer generated, leading to the formation of the non-densified bulks. Therefore, thepreparation of electroconductive and dense SnO2-based ceramics has not been successful by the present coadditions of Fe2O3 and M2O5, owing to charge compensation between Fe3+ and M5+ ions doped in the Sn4+
sites. However, these results are consistent with the aforementioned result that the Fe ions are doped as Fe3+ inthe Sn4+ sites forming oxygen vacancies, leading to the densification of the Fe-doped SnO2.

4. Conclusions
Dense SnO2-based ceramics have been prepared by firing Fe2O3-added SnO2 compacts with atomic ratiox = Fe/(Sn+Fe) = 0.005 ­ 0.10 at 1673 K for 1 h in air-flow. The successive doping of Fe3+ ions into the Sn4+
sites of the rutile-type structure with increasing x has been deduced on the basis of the XRD patterns, the
FWHM of the diffraction peaks, and the comparison of the ionic radii. The doping of Fe3+ ions into the Sn4+
sites results in oxygen vacancies to compensate the lack of positive charges in the structure, and the oxygen
vacancies promote diffusion of oxide ions and material migration, leading to coalescence of particles, grain
growth, and densification of the fired bodies. The firing of SnO2 compacts co-added with Fe2O3 and M2O5 (M= Nb, Sb, Ta) also brings about charge compensation between Fe3+ and M5+ (M = Nb, Sb, Ta) doped in therutile-type structure, preventing simultaneous realization of densification and electron doping.

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