Effect of La 2 O 3 addition and O 2 atmosphere on the electric properties of SnO 2 TiO 2

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1 Materials Chemistry and Physics 90 (2005) Effect of La 2 O 3 addition and O 2 atmosphere on the electric properties of SnO 2 TiO 2 L.G.P. Simões a, M.R. Cássia-Santos a, M.M. Oliveira a, E. Longo a,, J.A. Varela b a Centro Multidisciplinar de Desenvolvimento de Materiais Cerâmicos (CMDMC)/LIEC, Departamento de Química, UFSCar, C.P. 676, São Carlos, SP, Brazil b Centro Multidisciplinar de Desenvolvimento de Materiais Cerâmicos (CMDMC)/LIEC, Instituto de Química, UNESP, C.P. 355, Araraquara, SP, Brazil Received 7 May 2003; received in revised form 22 October 2003; accepted 7 November 2003 Abstract The electric properties of (Sn, Ti)O 2 doped with 1.00 mol% CoO, 0.05 mol% Nb 2 O 5 and x mol% La 2 O 3 (0.25 x 1.00) have been studied. Sn 0.25 Ti 0.75 Co 0.01 Nb doped with 0.50 mol% La 2 O 3 has a nonlinearity coefficient of 6. An increase in the concentration of La 2 O 3 raised its resistivity, thereby altering the electric properties of the material. A thermal treatment in oxygen atmosphere increased the nonlinearity coefficient to a value of Elsevier B.V. All rights reserved. Keywords: Varistor; Electrical properties; Oxidizing atmosphere 1. Introduction Tin dioxide is an n-type semiconductor with a crystal structure of the rutile type [1]. This oxide is used as sensors for gas, photovoltaic cells and electronic devices, due to nondensifying mechanisms such as evaporation condensation and diffusion through the SnO 2 lattice [2 4]. Recently, it was demonstrated that small additions of CoO and MnO 2 promote the densification of SnO 2 [5]. Pianaro et al. [6] studied the effect of Cr 2 O 3 on the SnO 2 CoO Nb 2 O 5 system (1.00 mol% CoO and 0.05 mol% Nb 2 O 5 ). They obtained a high nonlinearity coefficient (α = 40), using a concentration of 0.05 mol% Cr 2 O 3. Oliveira et al. [7] studied the effect of La 2 O 3 addition. They showed that concentrations of 0.05 mol% La 2 O 3 increase the resistivity of the SnO 2 CoO Nb 2 O 5 system and obtained a nonlinearity coefficient of 9.2. TiO 2 and SnO 2 have a tetragonal structure of the rutile type, being a semiconductor of the n-type; both form solid solutions in great extension [8]. TiO 2 differs from SnO 2 in the sintering behavior because it does not need dopants for densification. However, TiO 2 ceramics doped with the same elements as those used for SnO 2 varistors present nonohmic properties [9]. Corresponding author. Tel./fax: address: dels@power.ufscar.br (E. Longo). Atashbar and Sun [10] studied thin films of TiO 2 prepared by the sol gel method and doped with Nb 2 O 5 to use as sensor for O 2. This doping increases the sensitivity to O 2 detection to 40%. Bueno et al. [11] showed that the addition of TiO 2 to SnO 2 improves the densification, due to substitution of Sn 4+ for Ti 4+ in the structure, forming a solid solution and the nondensifying mechanisms that increase the densification. Radeca et al. [12] analyzed the solid solution of (Sn, Ti)O 2 in thin films. It was proposed that the detection of hydrogen in the air involves a pre-adsorption of oxygen species (O 2,O and O 2 ), being the variation of the electric resistance, under exhibition of H 2, high and reproducible, indicating that this material is a better sensor than pure SnO 2. Small additions of TiO 2 to SnO 2 favor the sensitivity and selectivity in the gas detection, what promotes the increase of the density of active superficial centers, favoring the chemiadsorption. However, small additions of SnO 2 to TiO 2 decrease the sensitivity. Recently, the electric properties and the microstructure of material based on (Sn, Ti)O 2 have been studied [13]. These polycrystalline ceramics with different compositions present promising applications as sensor, capacitor, and varistor. In this present work, we studied the electric properties of (Sn, Ti)O 2 -based materials doped with 1.00 mol% CoO, 0.05 mol% Nb 2 O 5 and x mol% La 2 O 3 (0.25 x 1.00). We analyzed the influence of La 2 O 3 on the densification process and the microstructure of Sn 0.25 Ti 0.75 Co 0.01 Nb /$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /j.matchemphys

2 L.G.P. Simões et al. / Materials Chemistry and Physics 90 (2005) Experimental details The powders were prepared using the ball milling process in an alcohol medium. The oxides used were SnO 2 (Cesbra), TiO 2 (Merck), CoO (Aldrich), Nb 2 O 5 (CBMM) and La 2 O 3 (Merck). The composition (mol%) of the material studied was the following: (SnO x) TiO 2, being x equal to 1.00CoO Nb 2 O 5 + yla 2 O 3, with y equal to 0.25, 0.50, 0.75 and 1.00 mol%. The powders were pressed into pellets (11 mm 1 mm) by uniaxial pressing (20 MPa) followed by isostatic pressing at 210 MPa. The pellets were sintered at 1450 C for 2 h, with heating and cooling rates of 10 K min 1. The microstructure was characterized by scanning electron microscopy (SEM; ZEISS model DMS 940 A) and energy-dispersive spectroscopy (EDS). The relative density was calculated by the Archimedes method. The presence of a solid solution was proved by X-ray diffraction (XRD; SIEMENS model D-5000, Cu K radiation). For the electrical measurements, silver contacts were deposited on the sample surface, after witch the pellets were heat-treated at 400 C for 30 min. Current voltage measurements were taken using a high-voltage unit (KEITHLEY model 237). The samples were subjected to treatments in O 2 atmosphere (under a flux of 10 l min 1 )at900 C for 30 min. 3. Results and discussion The Sn 0.25 Ti 0.75 O 2 -based materials had a relative density of 96% (theoretical density 100%). The addition of just 0.25 mol% La 2 O 3 decreased the density to 21.6% (Table 1). The formation of a second phase was not observed in the X-ray diffraction patterns (Figs. 1 and 2). By means Fig. 1. (a) XRD pattern, (b) E J curves, (c) SEM image of the Sn 0.25 Ti 0.75 CoNbLa x material sintered at 1450 C with a cooling rate of 10 K min 1 and 0.25 mol% La 2 O 3, and (d) mapping of the elements in the ceramic.

3 236 L.G.P. Simões et al. / Materials Chemistry and Physics 90 (2005) Table 1 Relative densities of Sn 0.25 Ti 0.75 CoNbLa x with different molar fractions of La 2 O 3 La 2 O 3 (mol%) Density (%) of these figures, just the rutile structures of SnO 2 and TiO 2 are observed. Fig. 1c and d shows SEM images of Sn 0.25 Ti 0.75 CoNb doped with 0.25 mol% La 2 O 3 and the mapping of the elements in the ceramic, respectively. Larger concentrations of Co and La precipitated at the grain boundary. It can be supposed that there is the formation of the La 2 CoO 4 phase observed by standard data (JCPDS card No ). The microstructures present a larger ho- mogeneity if compared with ZnO-based varistor materials. The solid solution formation of the substitutional type was discussed by Bueno et al. [13]. Reddy et al. [14] studied SnO 2 -based sensors doped with La 2 O 3 (1 10 mol%). They observed in SnO 2 doped with 2 mol% La 2 O 3, the formation of the La 2 Sn 2 O 7 phase with pirochlore structure. This phase is important in the control of the grain size, and it can destabilize the SnO 2 TiO 2 solid solution. The precipitation of Co can indicate a decrease in the density of the ceramic, because Co is responsible for the densifying of the system. A decrease in the mean grain size was observed and in SnO 2 CoO Nb 2 O 5 materials doped with La 2 O 3 and Cr 2 O 3 [5 7]. According to these works, Cr 3+ segregates at the grain boundary, promoting an increase of the resistivity of the system and of its nonlinear coefficient (α), due to the absorption of oxygen species at the grain boundary. The α value raises from 8 in the absence of Cr 3+ to 41 for the material doped with only 0.05 mol% Cr 2 O 3 due to Fig. 2. (a) XRD pattern, (b) E J curves, (c) SEM image of the Sn 0.25 Ti 0.75 CoNbLa x material sintered at 1450 C with a cooling rate of 10 K min 1 and 0.50 mol% La 2 O 3, and (d) mapping of the elements in the ceramic.

4 L.G.P. Simões et al. / Materials Chemistry and Physics 90 (2005) the interaction between Cr 3+ and oxygen species (O 2,O 2, O,O ). Leite et al. [15] and Cassia-Santos et al. [16] showed, for SnO 2 CoO varistor systems doped with Nb 2 O 5,Ta 2 O 5 and Cr 2 O 3, the influence of thermal treatments in oxidizing and inert atmospheres after sintering. They observed that thermal treatments in N 2 atmosphere promote a decrease of the nonlinear coefficient values. Such an effect is reversed when thermal treatments are made in an O 2 -rich atmosphere. Oliveira et al. [17] and Bacelar et al. [18] also observed the same behavior of SnO 2 -based varistor systems doped with other oxides (La, Ce, Al, Mn and Co). All these studies showed the importance of these ions that interact with different oxygen species. Thereby, the potential barrier value is increased for the creation of active sites for the adsorption of these oxygen species. The changes in the potential barrier lead to better varistor characteristics. Similarly to Cr 2 O 3, the reactions of the defects promoted by La 2 O 3 are described by the following reactions: La 2 O 3 2La Ti + V O + 3O x o (1) La Sn,Ti + O 2 La Sn,Ti O 2(ads) (2) La Sn,Ti O 2(ads) La x Sn,Ti O 2(ads) + La Sn,Ti La x Sn,Ti O 2(ads) (3) 2La x Sn,Ti (O (ads) ) (4) La x Sn,Ti (O (ads) ) + La Sn,Ti La x Sn,Ti (O structural ) 2 + La x Sn,Ti (5) The electric field (E) as function of the current density (J) is presented in Figs. 1 and 2. Samples with small concentrations of La 2 O 3 (0.25 mol%) show the E J characteristic of a linear system. An increase of the La 2 O 3 concentration to 0.50 mol% promotes the formation of a potential barrier of varistors systems with a nonlinearity coefficient of 6 and a breakdown field (E) of 1000 V cm 1. It is observed that an increase of the La 2 O 3 concentration increases the resistivity of Sn 0.25 Ti 0.75 NbCo and decreases the mean grain size. A concentration of 0.50 mol% La 2 O 3 increased the resistivity drastically, what disabled the calculation of the nonlinearity of this system, within the experimental conditions of this study. This increase of the resistivity can be associated to the increase of the height of the potential barrier and the second phase, precipitated in the area of the grain boundary, rich in Ti 4+,La 3+ and Co 2+. The thermal treatment of Sn 0.25 Ti 0.75 NbCoLa in oxygen atmosphere increased the nonlinearity coefficient to 3 with a breakdown field of 260 V cm 1. For Sn 0.25 Ti 0.75 NbCoLa , there was an increase in the nonlinearity coefficient from 6 to 9 and the breakdown field increased from 1000 to 1500 V cm 1. This increase in the nonlinearity coefficient and breakdown field should be related to the adsorption of the oxygen species (O 2,O 2,O Table 2 Nonlinear coefficient α and breakdown electric field E under different atmospheres a System α E (V cm 1 ) Density (%) WT O 2 WT O 2 Sn Co 1.00 Nb 0.05 [15] Sn Co 1.00 Nb 0.05 La 0.3 [7] Sn Ti Co 1.00 Nb 0.05 La Sn Ti Co 1.00 Nb 0.05 La a Without thermal treatment (WT) and with thermal treatment in oxygen atmosphere at 900 C for 1 h after sintering (O 2 ). and O ) at the grain boundary [16]. In terms of the proposed mechanism for the formation of the potential barrier, it is supposed that the transition metals, precipitated in the grain boundary region, oxidize when they are treated in an O 2 atmosphere and the interface regions become rich in oxygen species and trapped electrons [16,17]. Cassia-Santos and co-workers [13] observed that undoped Sn 0.25 Ti 0.75 O 2 shows a characteristic electric behavior of resistor systems. However, when being doped with just 0.05 mol% Nb 2 O 5 (Sn 0.25 Ti 0.75 Nb ), it presents electric characteristics of a varistor system of low voltage, with a nonlinearity coefficient 12 and of a breakdown field of 420Vcm 1. The addition of 1.00 mol% CoO to the material doped with Nb 2 O 5 (Sn 0.25 Ti 0.75 O 2 Nb ), promoted the increase resistivity, making unfeasible the nonlinearity calculation [13]. The increase of the resistivity should be associated to the second phase precipitated at the grain boundary of La 2 CoO 4. The amount of Nb 2 O 5 is not enough to promote the conductivity in the grains. Nb 2 O 5 when added to Sn 0.25 Ti 0.75 O 2 promoted the conductivity, because it substitutes Sn 4+ or Ti 4+ for Nb 5+ ions, according to the following reaction: 2Nb 2 O 5 4Nb (Sn,Ti) + V Nb + 10Ox O (6) Table 2 shows the values of the nonlinearity coefficient and breakdown field for different varistor systems on the basis of SnO 2 doped with 1.00 mol% CoO and 0.05 mol% Nb 2 O 5. This system has a high nonlinearity coefficient of 28 and a breakdown field of 5300 V cm 1. However, when La 2 O 3 is added to this system, the values of the nonlinearity coefficient and breakdown field decrease, showing that the precipitation of La 2 CoO 4 promotes a decrease of the potential barrier. 4. Conclusions Sn 0.25 Ti 0.75 O 2 -based materials doped with 1 mol% CoO and 0.05 mol% Nb 2 O 5 and additionally doped with La 2 O 3 (0.25 and 0.50 mol%) show varistor properties. The varistor property depends on the La 2 O 3 concentration because an increase in the La 2 O 3 concentration in the Sn 0.25 Ti 0.75 O 2 -based material resulted in a high resistivity, what disabled the calculation of the α and E values.

5 238 L.G.P. Simões et al. / Materials Chemistry and Physics 90 (2005) Thermal treatments in an O 2 atmosphere provide better varistor responses, and increase the nonlinear coefficient. All these results show that the defects originated by the presence of dopants with valences of +3 and +2 and the oxygen species (O 2,O 2,O,O ) present in the grain boundary region are responsible for the formation of the potential barrier, and consequently for the varistor property of the material. Acknowledgements The financial support of this research project by the Brazilian research funding agencies FINEP/PRONEX, CNPq and FAPESP/CEPID is gratefully acknowledged. References [1] Z.M. Jarzebbski, J.P. Marton, J. Electrochem. Soc. 129 (1976) 299C. [2] K.L. Chopra, S. Major, P.K. Panya, J. Electrochem. Soc. 102 (1983) 1. [3] J.G. Duhn, J.W. Jou, B.S. Chiou, J. Electrochem. Soc. 136 (1989) [4] J.G. Fagan, V.R.W. Amarakoon, Am. Ceram. Soc. Bull. 72 (1993) 119. [5] P.R. Bueno, S.A. Pianaro, E.C. Perreira, L.O.S. Bulhões, E. Longo, J.A. Varela, J. Appl. Phys. 84 (1998) [6] S.A. Pianaro, P.R. Bueno, E. Longo, J.A. Varela, J. Mater. Sci. Lett. 14 (1995) 692. [7] M.M. Oliveira, P.R. Bueno, M.R. Cassia-Santos, E. Longo, J. Eur. Ceram. Soc. 21 (2001) [8] H.P. Naidu, A.V. Virkar, J. Am. Ceram. Soc. 81 (1998) [9] P.R. Bueno, E. Camargo, E. Longo, E.S.A. Leite, S.A. Pianaro, J.A. Varela, J. Mater. Sci. Lett. 15 (1996) [10] M.Z. Atashbar, H.T. Sun, Thin Solid Films 326 (1997) 953. [11] P.R. Bueno, E.R. Leite, E. Longo, L.O.S. Bulhões, C.O. Paiva-Santos, J. Eur. Ceram. Soc. 23 (2003) 887. [12] M. Radeca, K. Zakrzewska, M. Rekas, Sens. Actuators B 46 (1998) 194. [13] P.R. Bueno, M.R. Cassia-Santos, L.G.P. Simões, J.W. Gomes, E. Longo, J.A. Varela, J. Am. Ceram. Soc. 85 (2002) 282. [14] G.C.V. Reddy, S.V. Manorama, V.J. Rao, J. Mater. Sci. 35 (2000) [15] E.R. Leite, A.M. Nascimento, P.R. Bueno, E. Longo, J.A. Varela, J. Mater. Sci. 4 (1999) 321. [16] M.R. Cassia-Santos, P.R. Bueno, E. Longo, J.A. Varela, J. Eur. Ceram. Soc. 21 (2001) 161. [17] M.M. Oliveira, P.R. Bueno, E. Longo, J.A. Varela, Mater. Chem. Phys. 74 (2002) 150. [18] W.K. Bacelar, M.M. Oliveira, V.C. Souza, E. Longo, E.R. Leite, J. Mater. Sci., submitted for publication.