(Co, Nb, Sm)-Doped Tin Dioxide Varistor Ceramics Sintered UsingNanopowders Prepared by Coprecipitation Method

Hajieh Bastami and Ehsan Taheri-Nassajw

Department of Materials Science and Engineering, Tarbiat Modares University, 14115-143 Tehran, IranQ2

Philippe F. Smet, Katleen Korthout, and Dirk Poelman

LumiLab, Department of Solid State Sciences, Ghent University, 9000 Gent, Belgium

(Co, Nb, Sm)-doped SnO2-based ceramic with varistor charac-teristics was successfully fabricated by a conventional sinteringof nanosized (Co, Nb, Sm)-doped SnO2 powders prepared bycoprecipitation method. Values of aB38 and EB5 1430 V/mmwere obtained for the varistor doped with 0.05 mol % Sm2O3,sintered at 12001C for 1 h. Microstructure development wasstudied by scanning and transmission electron microscopy, ele-mental analysis, and X-ray diffraction. The mean grain size ofthe SnO2-based varistor was 0.877 lm. No other phase besidesSnO2 was observed in the varistor doped with 0.05 mol %Sm2O3. The presence of Nb, Co, and Sm inside the SnO2 grainsconfirmed the formation of solid solution. No segregation of Nbwas observed and Nb atoms were homogeneously distributed.The excess amounts of Co were segregated at triple-junctionsbetween SnO2 grains. A Sm-rich region with a typical thicknessof 65 nm was observed at the grain boundary of the varistor.

I. Introduction

VARISTOR ceramics are technologically important for theirhighly nonlinear electrical characteristics. They are used as

reversible solid state switches with large-energy-handling capa-bilities.1–5 Recently, many papers have been published on newSnO2-based varistors.6–9 Tin oxide is an n-type semiconductorwith rutile crystalline structure and has many interesting elec-tronic properties. It has very low densification rate due to itshigh surface diffusion at low temperatures and high pressure athigh temperatures. Dense SnO2-based ceramics can be achievedby introducing dopants such as CoO and MnO2

10 or by hotisostatic processing.11 Pianaro et al.12 found SnO2-based ceram-ics doped with 1.00 mol% CoO and 0.05 mol% Nb2O5 aspromising varistor materials (aB8 and EB 5 187 V/mm). In thisvaristor system, niobium increases the electronic conductivity ofgrains12 and cobalt is extremely active in the promotion of tinoxide densification.10,12 Addition of trivalent transition oxidessuch as Fe2O3,

13 Cr2O3,12,14 or rare earth oxides improves

significantly the electrical characteristics of SnO2-based va-ristors.5,15–19 For example, a significant increase of a and EB

to 41 and 400 V/mm, respectively, is possible by addition of 0.05mol% Cr2O3.

12,14 The grain-boundary region has a ‘‘p-typesemiconductor nature’’ (due to transition metal atoms segre-gated at the grain boundary), while the bulk has an ‘‘n-typesemiconductor nature.’’ This configuration enables electrons tolocalize on the grain surfaces, giving rise to a negative surface(negative interfacial states). Consequently, electron depletionlayers are formed that act as potential barriers.12,14,20–22 The

basic concept underlying varistor action is that the I–V charac-teristics are controlled by the electrostatic barriers existing at theinterface between the grains. The potential barriers at the grainboundary come from the interface charge due to lattice mis-match, defects and dopants at the grain boundary. The elec-tronic charge changes the Fermi level near the grain boundary,resulting in band bending. The electronic charges stored in aninterface represent a repulsive potential for the majority carriers(the electrons in the case of an n-type semiconductor) across theinterface.3,4,20–22 However, the nonohmic properties are con-trolled by grain–grain junctions with transition metals segre-gated at the grain boundary.20,21

Interest in the synthesis and sintering of nanocrystalline ce-ramics has recently grown due to significant improvement intheir properties as compared with the conventional coarser graincompacts.23 Duran et al.24,25 reported that the breakdown volt-age of the varistors increases with the reduction in the grain size.It has been largely accepted that the nonohmic properties ofvaristors are controlled by grain-boundary features and distri-bution of transition metal oxides along the grain boundary ap-pears to be very important in varistor ceramics.25,26 A moreuniform distribution of additives improves the electrical nonlin-earity of varistors. Nanosized powders are obtained by a varietyof synthesis techniques such as polymeric precursor,27 hydro-thermal,28 sol–gel,29 and coprecipitaion (chemical) routes.30,31

Coprecipitation method has been known as a simple way forpreparing homogenous nanostructured powders.28,30–33

In this work, the influence of samarium oxide on the micro-structure development and electrical properties of (Co, Nb)-doped SnO2-based varistors fabricated by synthesized nanosizedpowers prepared by coprecipitation method were investigated.

II. Experimental Procedures

(1) Powder Synthesis and Characterization

Nanocrystalline-doped SnO2 powder was prepared by coprecip-itation method. The authors have reported previously the detailsof powder synthesis and characterization.30 For the purpose ofidentification, the sample was given the following specific name:SCNSm: 98.90 SnO211.00 CoO10.05 Nb2O510.050 Sm2O3

(mol%).The specific surface area of the powder was determined by the

Brunauer–Emmett–Teller (BET) method. The particle size (D)of the powder was calculated from the following equation basedon the assumption that the particles have a spherical shape34

DBET ¼6

DthSBET(1)

where DBET is the average particle size, SBET is the BET surfacearea, and Dth is the theoretical density of the powder.

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L. Levinson—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: taheri@modares.ac.irManuscript No. 28477. Received August 25, 2010; approved January 28, 2011.

Journal

J. Am. Ceram. Soc., ]] []]] 1–7 (2011)

DOI: 10.1111/j.1551-2916.2011.04467.x

r 2011 The American Ceramic Society

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(2) Compaction, Sintering, and Microstructure Development

The varistor was fabricated using nanosized (Co, Nb, Sm)-doped SnO2 powder prepared by coprecipitation method asfollows: the synthesized powder was first wet milled in ethanolusing zirconia balls for 1 h. After drying at 601C, the obtainedpowder was granulated with PVA binder and then uniaxiallypressed (15 MPa) into a disk with 12 mm in diameter and1.0 mm in thickness followed by cold isostatic pressing (240MPa). After burning out the PVA binder at 6501C, the disk wasplaced in a zirconia crucible and fully surrounded with the pow-ders of matching composition to avoid cobalt loss during thesintering and to ensure the desired composition. The disk wassintered by conventional method at 12001C for 1 h in air, andthen cooled down freely to room temperature. Silver paste wasapplied on the surfaces of the sintered disks to form electrodesby firing at 6001C for 10 min.

The apparent density of the sintered sample was estimated bythe Archimedes method and related to theoretical density 6.95 g/cm3 of SnO2. The crystal structure of the ceramic was charac-terized by XRD with CuKa radiation (Philips X-pertQ3 ). The crys-tallite size of the products was determined from the X-raybroadening line using Scherrer’s equation. The microstructureof the sintered pellet was observed using a TESCAN micro-scope. The grain size was calculated by Mendelson equation.35

Scanning transmission electron microscopy (STEM, JEM-2200FS, JEOL Ltd., Tokyo, Japan, equipped with Cs correc-tor, operated at 200 keV) was used for characterization of themicrostructure.

I–V characteristic was measured using high voltage measureunit (Haefely Technology, model CS 200Q4 ). The electric break-down field, EB, was measured at 1 mA/cm2 current density andthe nonlinear coefficient, a, was estimated from room temper-ature J–E curves by the following equation:

a ¼ 1

logðE10mA=E1mAÞ(2)

where E10 mA and E1 mA are the electric fields corresponding tothe current densities of 10 and 1 mA/cm2, respectively.

III. Results and Discussion

(1) Powder Characterization

As discussed in our earlier report30 the XRD pattern of thenanosized powder doped with 0.05 mol% (SCNSm), calcined at7001C for 2 h displayed only tetragonal rutile structure. TheXRD pattern of the varistor doped with 0.05 mol% Sm2O3,sintered at 12001C for 1 h also confirmed the existence of SnO2

(see Fig. 1). Since the XRD pattern of the synthesized powderwas similar to that of the sintered sample, thus to avoid repe-

tition, its XRD pattern is not shown here (see Fig. 1). The crys-tallite size of 98.90 SnO211.00 CoO10.05 Nb2O510.050 Sm2O3

(mol%) system calcined at 7001C for 2 h was about 10 nm.Figure 2 shows the SEM micrograph of SCNSm calcined at

7001C for 2 h. The particles were spherical in shape and in the sizerange of 45–70 nm.30 The average particle size of the nanopowderwas 30 nm calculated by substitution of the BET data into Eq.(1). The above-mentioned particle sizes achieved by SEM andBET method were in good agreement with the crystallite sizecalculated from the half-width of the XRD peaks of (110) planesusing Scherrer’s equation (10 nm), taking into account that someclustering of the 10 nm crystallites probably occurred, leading toa larger apparent particle size in SEM and BET.

(2) Microstructure Development and Electrical Propertiesof (Co, Nb, Sm)-Doped SnO2

As mentioned in the earlier section, no other phase besides SnO2

was observed in the XRD pattern of the SCNSm sintered at12001C for 1 h (see Fig. 1). This is in agreement with otherstudies.5–9,12–17,19–22,36–43 The relative density of this sample was99.42% of theoretical density. The SEM micrograph of theSCNSm sintered at 12001C for 1 h is shown in Fig. 3. The mean

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Fig. 1. The XRD pattern (with CuKa radiation) of the varistor dopedwith 0.05 mol% Sm2O3, sintered at 12001C for 1 h displaying only te-tragonal structure of SnO2. The XRD pattern of the nanosized powderdoped with 0.05 mol% Sm2O3, calcined at 7001C for 2 h revealed onlySnO2, as well.

Fig. 2. SEM micrograph of the nanosized powder doped with0.05 mol% Sm2O3, calcined at 7001C for 2 h showing the sphericalmorphology and the size range of 45–70 nm.

Fig. 3. SEM micrograph of the sample sintered at 12001C for 1 hshowing only SnO2 grains.

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grain size calculated from Mendelson equation was 0.887 mm.The microstructure was simple and showed only SnO2 grains.Therefore, no secondary phase precipitated at the grain bound-ary was observed by the XRD and SEM observations. Figure 4shows the STEM image and corresponding elemental mappingsfor Sn, Nb, Co, Nb, and Sm. These observations micrograph(Fig. 4(a)) confirmed that no liquid phase was formed duringsintering. Therefore, the sintering seems to be controlled bydiffusion in the solid state. This is in accordance with the find-ings of other studies.5–9,12–17,19–22 X-ray mapping was conductedto determine the composition distribution of the SCNSm sin-tered at 12001C for 1 h. As shown in Fig. 4(c), the distribution ofNb was steady and its mapping was very similar to that of Sn(Fig. 4(b)), indicating the formation of a solid solution. Al-though the Nb concentration is close to the detection limit, theabsence of segregated Nb supports the conclusion of the for-mation of a solid solution. For Sm (Fig. 4(d)), we also had theindication of the formation of a solid solution, although thiscould not be proven unambiguously due to the very low Smconcentration. In addition, Sm atoms were segregated in someregions outside the grains (at the grain boundary) and formedSm-rich regions over there. However, the segregation of Sm wasnot a steady one so that at some regions outside the grains (atsome grain boundaries), there was a very dense segregation ofSm (Fig. 4(d)). Co showed a similar behavior as samarium. Thepresence of Co inside the grains also indicates the formation ofsolid solution (Fig. 4(e)). Here, similar to Sm (see Fig. 4(d)), thesegregation of Co was not steady so that it is very dense in somegrain boundaries. However, since the mol% of CoO used in theSCNSm (1 mol%) was higher than its solubility (0.5 mol%CoO), the excess amounts of Co seemed to be segregated at thegrain boundary, especially in triple-junctions (Fig. 4(d)), similarto the finding of other researchers.13,16–18,27,36 Varela et al.36

confirmed the presence of Co2SnO4 adjacent to the triple-junc-tions using TEM coupled to EDS analysis as well as electrondiffraction patterns. However, the X-ray mapping for Sn, Nb,Co, Nb, and Sm indicated the formation of a solid solution ofNb, Co, and Sm in the matrix of SnO2 and also showed thatsome of the grain boundaries were Co-rich and Sm-rich.

The STEM image, X-ray mappings (of Nb, Sm, and Coatoms) and also selected area diffraction (SAD) of a Co-richregion are shown in Fig. 5. As clearly shown in Figs. 5(f) and (g),electron diffraction patterns of well crystallized structure for thepoints 1 and 2 were achieved. They confirmed a rutile-typestructure corresponding to SnO2 (Figs. 5(f) and (g)). The SADof the Co-rich region (point 3, Fig. 5(h)) could not be matchedwith Co compounds, like cobalt oxide or Co2SnO4. Therefore,through segregation at the grain boundary, Co caused the for-mation of a separate crystalline structure, which is not identifiedby the JCPDS records.

Bright and dark field TEM images and SAD patterns of theSm-rich region are shown in Fig. 6. As shown in Fig. 6(c), thethickness of the Sm-rich region at the grain boundary in point 2was 65 nm. The SAD patterns of the points 1 and 3 both cor-responded to the rutile-type structure of SnO2 (see Figs. 6(e) and(g)), although they had a different crystallographic orientation.According to the SAD patterns of points that were rich in Sm, awell-crystallized structure was formed at the grain boundary butno match could be found with the standard Sm compounds,such as samarium oxide. Further investigations revealed that theSm-rich region observed in the sample doped with 0.05 mol%Sm2O3 was structurally similar to Sm2GeO5, although it is nota priori clear whether Sm2SnO5, which has not been describedbefore, can actually be formed.

Microstructure observations revealed that samarium cationswere mainly segregated at the grain boundaries. Solute segrega-tion occurs when there are lower energy sites at the grain boundaryfor the solute atoms than in the bulk. When the grain-boundarymigrates, the solute atoms segregated at the grain boundary are aptto remain attached to the grain boundary that provides them withlow energy sites. In other words, the solutes have a tendency todiffuse along with the moving boundary and this solute diffusionacts as a drag force against the boundary movement.44 A secondfactor leading to boundary segregation is the reduction in the elas-tic strain energy of the crystal lattice due to size difference betweenthe solute atoms and the host atoms for which it substitutes.45,46

The threshold electric field versus the current density of theSnO2-based varistors doped with 0.05 mol% Sm2O3 sintered at

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Fig. 4. Scanning transmission electron microscopy (STEM) image (a) and X-ray mappings for (b) Sn, (c) Nb, (d) Sm, and (e) Co atoms, for the sampledoped with 0.05 mol% Sm2O3 sintered at 12001C.

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12001C for 1 h is illustrated in Fig. 7. The nonlinear coefficientand the breakdown voltage of (Co, Nb, Sm)-doped SnO2-basedvaristors were 38 and 1430 V/mm, respectively.

The improvement of nonlinear characteristics can be due tothe segregation of samarium cations at the grain boundary,probably inducing electronic interface states that can trap elec-trons at the SnO2–SnO2 interface and thus lead to a significantincrease of the a value similar to the findings of other

studies.12,14–22,27 The sensitivity of (Co, Nb)-doped SnO2-basedvaristors properties to the small concentration of Sm2O3 issimilar to that of Cr2O3.

12,14 It seems that samarium is presentat the grain boundary, probably altering the potential barrier atthe grain boundary and affecting the varistor properties.20–22

Because the nonlinear current–voltage characteristics of va-ristor ceramics are controlled by grain-boundary features,1,3,4,20–22

it is necessary to point out that in the present work, we found

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Fig. 6. (a) Bright-field TEM image from Sm-rich region; (b) and (d) dark field TEM images from the points 1 and 3, respectively, (c) dark field TEMimage from the point 2 rich in Sm (SAD) patterns for the points (e) 1, (f) 2, and (g) 3, respectively, for the sample doped with 0.05 mol% Sm2O3 sinteredat 12001C.

Fig. 5. (a) STEM from Co-rich region; X-ray mapping for (b) Sn, (c) Nb, (d) Sm, (e) Co atoms; selected area selected area electron diffraction (SAD)patterns for (f) point 1, (g) point 2, and (h) point 3, for the sample doped with 0.05 mol% Sm2O3 sintered at 12001C.

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several types of junctions in the microstructure of (Co, Nb, Sm)-doped SnO2-based varistor: direct SnO2–SnO2 junction, triplejunction rich in cobalt and grain-boundary phase rich in Sm.The direct SnO2–SnO2 junction with transition metal segregatedin it suggests this is the main kind of junction controlling theoverall nonohmic properties.17,21,37 It has been reported that theelectrically active barriers are, for the most part, present in suchkind of junction. However, a grain boundary is treated as a junc-tion with a different Fermi level of the bulk.20–22

According to the model proposed by Bueno et al.,20,21 thenonohmic properties of varistors may be improved by enrichingthe grain boundary with oxygen species. The transition metalsegregated at the grain boundary is the origin of the potentialbarriers since it is the main cause of the formation of an oxideregion (i.e., metal oxide region) with a ‘‘p-type semiconductornature,’’ which means a metal deficient and oxygen rich regionor phase. Oxidation of this region depends on the features of themetal oxide segregation and also on the thermodynamic equi-librium between the oxygen species in the grain and the grainboundary.20–22 Oliveira et al.17 investigated the grain-boundarysegregation and precipitates in La2O3- and Pr2O3-doped SnO2

and CoO-based varistors. They found two kinds of precipitatephases (Co2SnO4 and Pr2Sn2O7) at the grain boundary and dis-cussed the effect of precipitates on the electrical properties ofSnO2-based varistors. These precipitate phases are mainlylocated in the triple-junctions of SnO2 grains. Oliveira et al.17

reported that despite of microstructure heterogeneities observed,the most probable determinant factor for the existence of aSchottky-type barrier that controls the nonlinear electrical prop-erties is the presence of an oxide layer (oxidation of the segre-gated transition metals) at the grain–grain junction type.

Considering that the ionic radius of Co21 (0.072 nm) is closeto that of Sn41 (0.071 nm), substitution of Sn41by Co21 can bewritten10,12

CoO �!SnO2Co00Sn þ V��O þ

1

2O2 (3)

The cobalt oxide could influence the adsorption of oxygenspecies at the grain boundary as represented by the followingequations38:

Co00Sn þ1

2O2 �!

SnO2Co00Sn:OðadsÞ (4)

Co00Sn:O2ðadsÞ �!SnO2

Co0Sn:O02ðadsÞ (5)

Co0Sn:O02ðadsÞ �!

SnO2CoXSn:ðO0Þ2ðadsÞ (6)

2CoXSn:ðO0Þ2ðadsÞ �!SnO2

2CoXSn:ðO00Þ2ðstructuralÞ þ CoXSn (7)

The introduction of Nb2O5 to the SnO2 lattice should lead tothe following reactions12,14,39:

2Nb2O5 �!SnO2

4Nb�Sn þ V0000Sn þ 10OXO (8)

V0000Sn þ 2V��O ! 2V�O þ V00Sn (9)

Nb2O5 �!SnO2

Nb�Sn þ 2e0 þ 1

2O2 þ 4OX

O (10)

V��O þ e0 ! V�O (11)

Nb2O5 increases the electrical conductivity of SnO2 and actsas electron donor.12,14,20,21 The electrons produced by Eq. (10)may be involved in the transformation of V��O into V�O.

34

The substitution of Sn41 by Sm31 as the following reactionleads to distortion of the SnO2 lattice because of the differencebetween the ionic radius of Sm31 (0.096 nm) and that of Sn41

(0.071 nm)30:

Sm2O3 �!SnO2

2Sm0Sn þ 3OXO þ V��O (12)

Much more Sm2O3 will reside at the SnO2 grain boundarydue to the partial substitution of Sn41 by Sm31. The transitionmetal atoms segregated at the SnO2 grain boundary are the or-igin of the potential barrier.20,21 Chromium oxide and rare earthoxide-doped SnO2-based varistors show excellent nonlinear elec-trical properties. They create oxygen vacancies to promote theformation of adsorbed oxygen species, which are responsible forthe barrier formation near the grain boundary. In polycrystal-line semiconductors, the trapping of electrons at the grainboundary has a decisive influence on the electrical transportproperties through the formation of electrostatic potentialbarriers.20,21,38

Similar to Cr2O340–43 and La2O3

15 samarium could react withoxygen species according to the following equations:

Sm0Sn �!SnO2

Sm0Sn:O2ðadsÞ (13)

Sm0Sn:O2ðadsÞ �!SnO2

SmXSn:O

02ðadsÞ (14)

SmXSn:O

02ðadsÞþSm0Sn �!

SnO2 ðSmXSnÞ2:O002ðadsÞ (15)

ðSmXSnÞ2:O002ðadsÞ �!

SnO22ðSmX

Sm:O02ðadsÞÞ (16)

SmXSm:O

02ðadsÞ þ Sm0Sn �!

SnO2 ðSmXSnÞ2:O002ðstructuralÞ (17)

Due to distortion of the SnO2 lattice induced by the substi-tution of Sm for Sn, the Sn ions at the distorted region readilyescape from original site to become an interstitial Sn����i leavinga vacancy VSn

0000 at its original site according to

SnXSn: �!SnO2

Sn����i þ V0000Sn (18)

The role of CoSn00 , SmSn

0 , VSn00 00, and VSn

00 defects (due to thereduction of Sn41 into Sn21 in the interstitial sites) would berelated to the increase of the oxygen species adsorbed at thegrain-boundary interface, causing a decrease in the conductivityof the grain boundary by the donation of electrons to the O2

adsorbed at the grain boundary. The adsorbed oxygen at thegrain boundary captures electrons from the negatively chargeddefects at the grain boundary and thus stays at the interface.3,4,33

Based on the models for ZnO-based varistors proposed byGupta and Carlson,47 Bueno et al.48 proposed a model for

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Fig. 7. The threshold electric field versus the current density of the SnO2-based varistors doped with 0.05 mol% Sm2O3, sintered at 12001C for 1 h.

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SnO2-based varistors. In this model the donors (positivecharges) are distributed on both sides of the grain boundaryand are compensated by the acceptors (negative charges) at theinterface of the grain boundary. Considering the microstructureof (Co, Nb, Sm)-doped SnO2-based varistor and the presence ofthe Sm-rich region at some grain boundary, the defect barriermodel proposed by Bueno and colleagues could be modified asshown in Fig. 8. Glot and colleagues studied the nonohmic con-duction in SnO2-based varistors. They found high values of theactivation energy of electrical conduction and the barrier height inSnO2-based varistors. They also reported that the conduction pro-cess is thermally activated not only at low electric field but also athigh electric field. This allows a fairly simple explanation of con-duction in SnO2-based varistors as thermoionic emission acrossthe barrier with the barrier height dependent on electric field.49,50

IV. Conclusions

Through the current study on the microstructure developmentsand electrical properties of (Co, Nb, Sm)-doped SnO2-basedvaristor prepared by coprecipitation method we can draw thefollowing conclusions:

(1) Values of aB38 and EB 5 1430 V/mm for the varistordoped with 0.05 mol% Sm2O3 sintered at 12001C were obtained.High threshold voltage and large nonlinear coefficient qualifythe Sm-doped SnO2 as ceramic varistors.

(2) As no segregation of Nb was observed in the micro-structure of the SnO2-based varistor, Nb atoms were presum-ably homogeneously distributed inside the SnO2 grains.

(3) The presence of Co inside the SnO2 grains of the varistorwas attributed to the formation of a solid solution. The excessamounts of Co were segregated at the triple-junction of theSnO2-based varistor.

(4) Sm-rich regions formed at the grain boundaries. Thistype of segregation of Sm suggests that it plays an importantrole in developing the Schottky barriers.

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15I. P. Silva, A. Z. Simoes, F. M. Filho, E. Longo, J. A. Varela, and L. Perazolli,‘‘Dependence of La2O3 Content on the Non-Linear Electrical Behavior of ZnO,CoO and Ta2O5 Doped SnO2 Varistors,’’ Mater. Lett., 61, 2121–5 (2007).

16M. M. Oliveira, P. R. Bueno, M. R. Cassia-Santos, E. Longo, and J. A.Varela, ‘‘Sensitivity of SnO2 Nonohmic Behavior to the Sintering Process and tothe Addition of La2O3,’’ J. Eur. Ceram. Soc., 21, 1179–85 (2001).

17M. M. Oliveira, P. C. Soares Jr., P. R. Bueno, E. R. Leite, E. Longo, and J. A.Varela, ‘‘Grain-Boundary Segregation and Precipitates in La2O3 and Pr2O3 DopedSnO2.CoO-Based Varistors,’’ J. Eur. Ceram. Soc., 23, 1875–80 (2003).

18R. Parra, Y. Maniette, J. A. Varela, and M. S. Castro, ‘‘The Influence of Yt-trium on a Typical SnO2 Varistor System: Microstructural and Electrical Fea-tures,’’ Mater. Chem. Phys., 94, 347–52 (2005).

19P. Qi, J. F. Wang, W. B. Su, H. C. Chen, G. Z. Zhang, C. M.Wang, and B. Q.Ming, ‘‘(Er, Co, Nb)-Doped SnO2 Varistor Ceramics,’’ Mater. Chem. Phys., 92,578–84 (2005).

20P. R. Bueno, M. R. Cassia-Santos, E. R. Leite, E. Longo, J. Bisquert, Garcia-Belmont, and F. Fabregat-Santiago, ‘‘Nature of the Schottky-Type Barrier ofHigh Dense SnO2 Systems Displaying Nonohmic Behavior Q5,’’ J. Appl. Phys., 88,6545–8 (2000).

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22P. R. Bueno, J. A. Varela, and E. Longo, ‘‘Admittance and Dielectric Spectros-copy of Polycrystalline Semiconductors,’’ J. Eur. Ceram. Soc., 27, 4313–20 (2007).

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27R. Parra, L. A. Ramajo, M. S. Goes, J. A. Varela, and M. S. Castro, ‘‘FromTin Oxalate to (Fe, Co, Nb)-Doped SnO2: Sintering Behavior, Microstructuraland Electrical Features,’’ Mater. Res. Bull., 43, 3202–11 (2008).

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38M. R. C. Santos, P. R. Bueno, E. Longo, and J. A. Varela, ‘‘Effect of Oxidingand Reducing Atmosphere on the Electrical Properties of Dense SnO2-BasedVaristors,’’ J. Eur. Ceram. Soc., 21, 161–7 (2001).

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Fig. 8. The proposed atomic defect model for Schottky barrier at thegrain boundary of (Co, Nb, Sm)-doped SnO2-based varistor.

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