APPLIED PHYSICS LETTERS
VOLUME 79, NUMBER 1
2 JULY 2001
Role of oxygen at the grain boundary of metal oxide varistors: A potential barrier formation mechanism P. R. Bueno,a) E. R. Leite, M. M. Oliveira, M. O. Orlandi, and E. Longo LIEC–Interdisciplinary Laboratory of Electrochemistry and Ceramics, Department of Chemistry, ˜ o Carlos, C. Postal 676, 13565-905 Sa ˜ o Carlos, SP, Brazil UFSCar–Federal University of Sa
共Received 27 November 2000; accepted for publication 18 April 2001兲 A model is proposed here to explain how the chemical features of metal oxide varistors can alter their nonohmic physical behavior, based on nonohmic similarities in the electrical properties of ZnO- and SnO2-based varistors. The proposed model explains the electrical properties of ZnO- and SnO2-based varistors before and after thermal treatments in oxygen- and nitrogen-rich atmospheres, which cause similar changes in the nonohmic feature of these polycrystalline ceramics with greatly differing chemical compositions and microstructures. The model is based on the key role that oxygen plays in varistor grain boundaries, independently of the type of ceramic system 共ZnO-, SnO2-or even SrTiO3-based varistors兲 involved. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1378051兴 Our research group has recently discovered a class of highly dense nonohmic polycrystalline ceramics based on tin dioxide1 and, since this discovery, we have been engaged in studying this class of nonohmic ceramic materials.1–5 High densification can be achieved by the addition of CoO,6 – 8 MnO2 , or even of ZnO9 to SnO2, allowing for nonohmic properties such as those reported in Refs. 1–5 to be obtained. The nature of the potential barrier was characterized in SnO2-based varistor systems and found to be Schottky-like, similarly to that frequently reported for the traditional ZnObased varistor system, despite the fact that the SnO2-based varistor differs microstructurally from the ZnO-based varistor.3 The nonohmic electrical behavior in traditional ZnOand SrTiO3-based ceramics, as discussed in Refs. 10 and 11, is linked to oxygen, which plays a key role in the grain boundaries of such ceramics. An absorbed layer of bismuth with a thickness of about ⬃5 Å in a ZnO-based varistor is necessary to create potential barriers at the grain boundaries, and the height of these potential barriers largely depends on the excess amount of oxygen present at the interface between grains in ZnO varistors.10 However, very little is known about the chemical nature of grain boundary interfaces in metal oxide varistors and its relationship to nonohmic electrical behavior. Therefore, the main purpose of this letter is to show that the chemical origins of the potential barrier in polycrystalline ceramics 共whatever the n-type semiconductor matrix兲 depend on the amount of oxygen present at grain boundary interfaces. Furthermore, the main role of transition metal oxide as a dopant is to control the oxygen concentration at the grain boundary interface. Based on this evidence, a chemical barrier formation mechanism is proposed to explain the physical origins of interfacial trapping states. Thermal treatments in oxygen- and nitrogen-rich atmospheres at 900 °C prove that the nonlinear coefficient 共␣兲 values in SnO2-based varistors are dependent on thermal treatment under different atmospheres.4 The ␣ values are siga兲
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nificantly lower when the material is subjected to a nitrogenrich atmosphere. Moreover, they can be restored close to almost their original values when they are subjected to thermal treatment in an oxygen-rich atmosphere. This reversible behavior is shown in Table I 共the preparation and sintering conditions of SnO2-based systems have already been described in previous papers1–5 and shall be withheld here. The mean grain size, dc and ac electrical measurements were taken in the same way as described in Ref. 3兲. The variations in the electrical properties with atmosphere are due only to the oxidizing effect at the grain boundary, as discussed in Ref. 4 共no differences has been observed in the samples’ average grain size with thermal treatments at 900 °C兲.4 Figure 1 illustrates the response of impedance spectroscopy at 300 °C of the as-sintered samples and the samples treated in N2-rich and O2-rich atmospheres, which to prove this statement. This figure shows that the total electrical resistance of the as-sintered sample 共which is approximately the grain boundary resistance兲 decreases to one half of its original values and is totally recovered after heat treatment in an O2-rich atmosphere. All the semicircular arcs in the complex plane in Fig. 1 yield to an arc, with the center displaced below the real axis, because of the presence of distributed elements and a relaxation process resulting from the trapped states. However, grain boundary resistance is totally recovered after thermal treatment in an O2-rich atmosphere and the trapped states are differently distributed, as shown in Table II. This table presents the ⌽ b 共barrier height兲, N d 共the donor concentration兲, N IS 共negative states at the interface between the SnO2 grains兲 and ␦ 共barrier width兲 values, calculated, as suggested in Ref. 3, for a back-to-back Schottky-type potential TABLE I. ␣, E b , b mean grain size 共d兲 and the relative densities ( r ) for SnO2CoO-based varistor systems doped with Nb2O5 and La2O3. Treatment As-sintered N2-rich atmosphere O2-rich atmosphere
␣
E b /V cm⫺1
v b /V grain⫺1
d/m
r /%
32 16 26
10 513 9564 9343
1.98 1.82 1.78
1.91 1.92 1.92
⬃94 ⬃95 ⬃95
0003-6951/2001/79(1)/48/3/$18.00 48 © 2001 American Institute of Physics Downloaded 02 Jul 2007 to 200.136.226.189. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
Bueno et al.
Appl. Phys. Lett., Vol. 79, No. 1, 2 July 2001
FIG. 1. Impedance responses of a SnO2CoO-based varistor system in complex plane and J – E characteristic behavior 共inset兲, showing the reversibility of grain boundary resistance based on heat treatments under different atmospheres. 共䊏兲 As-sintered with ␣⬃32, 共䉱兲 N2-rich with ␣⬃16, and 共䊊兲 O2-rich atmosphere with ␣⬃26.
barrier of SnO2 CoO-based varistor systems. Based on these values, it can be concluded that thermal treatment in a N2-rich atmosphere causes a decrease, mainly in the N IS states and ⌽ b values, while thermal treatment in an O2-rich atmosphere causes a significant increase in the N d and, particularly, in the N IS states in a thinner region of the grain boundary. This suggests that such treatments change mainly the electronic states of the grain boundary region. This behavior is better explained in association with a precipitate phase that is rich in Co atoms and oxygen species in SnO2CoO-based varistors and in Mn atoms, as well as oxygen species in the SnO2MnO-based system.12 The precipitate can be viewed through high-resolution transmission electron microscopy 共HRTEM兲 micrographs 关The microstructures presented here were characterized by an energy dispersive spectroscopy 共EDS兲 stage attached to the scanning electron microscopy 共SEM兲 and by a high-resolution scanning transmission electron microscopy 共HRSTEM兲—model VGHB603兴, as depicted in Fig. 2 for the SnO2MnO-based varistor. Similar results can also be obtained in the SnO2CoO-based system.7 The precipitate observed at the grain boundaries is rich in Mn and oxygen, presenting a thickness of about 5–7 nm 共Fig. 2兲. The mapping of the oxygen element by EDS stage attached to SEM 共Fig. 3兲 shows that the grain boundary region is richer in oxygen than the grain is. The behavior depicted in Table I can be ascribed to the oxidizing effect of CoO during heating up to 600 °C, followed by the reducing effect at temperatures above 1000 °C. Equations 共3兲 through 共5兲 in Ref. 4 show CoO oxidation and
49
FIG. 2. 共Color兲 HRTEM micrographies showing the grain boundary of the SnO2MnO-based varistor system and a precipitate rich in Mn and O elements and about 5–7 nm of thickness.
reduction reactions at different temperatures and how the thermal treatment at 900 °C in an oxygen-rich atmosphere affects the oxygen content in the grain boundary interfaces. The reactions described in Ref. 4 also explain the influence of the postsintering cooling rate on the ␣ values.4,5 SnO2MnO-based systems behave as illustrated in Table I when treated in oxygen- and nitrogen-rich atmospheres, undergoing similar reactions to those13 exemplified in Eqs. 共3兲– 共5兲 in Ref. 4. A thermal gravimetric analysis of CoO used in the preparation of the SnO2CoO-based varistor system indicates a gain in mass which corresponds to the transformation of CoO into Co3O4, a finding that reinforces our conclusions. Moreover, behavior resembling that illustrated in Table I and Fig. 1 for the SnO2CoO-based system also occurs in the ZnO-based varistor, as demonstrated by Sonder et al.14 共Fig.
TABLE II. ⌽ b , N d , N IS , and values for a back-to-back Schottky-type potential barrier of SnO2CoO-based varistor systems doped with Nb2O5 and La2O3. These calculations take into consideration the average number of grains between electrodes. Treatment
⌽ b /eV
N d (⫻1024)/m⫺3 N IS(⫻1016)/m⫺2 ␦/nm
FIG. 3. 共Color兲 Mapping of Sn, Mn and O elements of SnO2MnO-based varistor systems by EDS stage attached to SEM. 共a兲 Typical micrograph of the SnO2MnO-based varistor with partial precipitation of the Mn in the grain boundary region; 共b兲 mapping of Sn, indicating that the precipitated region is poor in Sn, but rich in 共c兲 Mn and 共d兲 oxygen. Downloaded 02 Jul 2007 to 200.136.226.189. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
As-sintered 1.11⫾0.06 N2-rich atmosphere 0.68⫾0.05 O2-rich atmosphere 0.71⫾0.04
1.05 1.14 4.51
4.25 3.46 7.01
20.2 15.2 7.8
50
Bueno et al.
Appl. Phys. Lett., Vol. 79, No. 1, 2 July 2001
3 of Ref. 14兲 and by Stucki et al.10 共Fig. 1 of Ref. 10兲. Thus, annealing ZnO-based varistors in reducing atmospheres increases their leakage current10,14 共as shown in Fig. 1 for SnO2-based varistors兲 and, in some cases, destroys their nonlinear electrical behavior. These effects can be reversed by annealing in air or oxygen.14 Nakano et al.11 have also demonstrated that the same behavior occurs in SrTiO3-based varistors 共Fig. 4 of Ref. 11兲. A large body of evidence indicates that the similarities between all these ceramics are associated with the amount of oxygen in the grain boundary and dopants in the form of metal oxide precipitate phases with a ‘‘p-type semiconductor nature’’ 共metal deficient and oxygen rich phase兲 or that are good ionic conductors. The studies of Stucki and Greuter,10 for example, have revealed that the concentrations of oxygen and bismuth at the grain boundaries are affected differently by postsintering thermal treatment in vacuum and air in ZnO-based varistors. Earlier reports have also suggested that varistor characteristics are related to the particular crystalline form that the Bi2O3 takes on10,14 with oxygen at the grain boundary surface.15 The role of Bi as the ‘‘grain boundary activator’’ may, in the simplest scenario, be limited to supplying excess oxygen to the grain boundaries.10,15 The bismuth-rich phase is interconnected and continuous along the three- and four-grain junctions throughout the microstructure. This topology is considered by many to be crucial for the transport of oxygen into the material during postsintering annealing. Some authors also state that nonohmic properties can be enhanced using ‘‘highly oxygenated’’ Bi2O3 as a starting material.16 Our preliminary studies of SnO2-based systems point to a similar conclusion. Going back to the traditional ZnO-based varistor system, it is known that the addition of transitional elements, such as Co and Mn, is necessary to increase varistor nonlinearity. Both of the oxides formed by Co and Mn are acceptors and their valence state may change in the vicinity of a grain boundary, particularly with local changes in the oxygen potential. Egashira et al. report on other important findings that reinforce our conclusions.17 Their study demonstrate that porous ZnO- and SnO2-based ceramics have nonlinear I–V characteristics at high temperatures 共400– 600 °C兲 and that, similarly to dense ZnO, SnO2, and SrTiO3-based nonohmic ceramics, these characteristics are dependent on oxidizing gases. They conclude that the gas-sensing mechanism of ZnO- and SnO2-based varistor type sensors is triggered by negatively charged chemisorbed species, which determine the height of the Schottky barrier at grain boundaries. It would be difficult to interpret all these very similar behaviors if there were no similarities in the formation of the potential barrier in polycrystalline ceramics having very different chemical compositions and microstructures. Thus, based on this evidence and similarities we propose a phenomenological model for barrier formation in metal oxide varistors that takes such similarities into account despite the composition. The model considers that varistor materials with optimal electrical properties contain an excess of both oxygen and acceptor metal atoms 共precipitated on the grain boundary surface兲, such as Bi in ZnO-based varistors or Mn and Co in SnO2-based varistors. These metal atoms are mainly transition metal oxides that generally have several
oxidation states, which cause the amount of oxygen to increase at the grain boundary. Therefore, the transition metal precipitated at the grain boundary becomes more oxidized when treated in an O2-rich atmosphere 共because of the ease with which its valence state changes兲, causing the electrontrapping interfacial region to become richer in oxygen species. It is proposed that the grain boundary region has a ‘‘p-type semiconductor nature’’ 共due to Co3O4-, Mn3O4-like phases precipitated at the grain boundary兲, while the bulk has an ‘‘n-type semiconductor nature’’ 共SnO2-, ZnO-, and SrTiO3-based varistor matrix兲. This configuration enables electrons to become localized on the surfaces, giving rise to a negative surface 共negative interfacial states兲. To maintain local electrical neutrality, the charges are compensated by ionized shallow donors and bulk electron traps. As a result electron depletion layers are formed and act as potential barriers. The potential barriers have a Schottky-like nature due to negative interfacial states, the selfsame nature often found in all metal oxide varistors and in most metal oxide gas sensors17 at higher temperatures. Considering such a grain boundary configuration, annealing under a reducing atmosphere 共N2, Ar, or vacuum兲 eliminates excess oxygen, allowing the metal atoms to remain and, thereby, decreasing the nonlinear electrical properties of the material. Therefore, the physical origin of the interfacial states is not an intrinsic one caused by lattice mismatch at the boundary, but an extrinsic one resulting from metal atoms precipitated at the grain boundaries. An important aspect to be investigated in further studies is to determine the type of oxygen species. Some evidence for O2⫺ and O⫺ has also been suggested in Refs. 2 and 17. The authors are grateful to Professor Dr. Martin P. Harmer from Lehigh University for supplying the STEM facility. This work was supported by the Brazilian research funding agencies CNPq and FAPESP. 1
S. A. Pianaro, P. R. Bueno, E. Longo, and J. A. Varela, J. Mater. Sci. Lett. 14, 692 共1995兲. 2 ˜ es, E. Longo, and P. R. Bueno, S. A. Pianaro, E. C. Pereira, L. O. S. Bulho J. A. Varela, J. Appl. Phys. 87, 3700 共1998兲. 3 P. R. Bueno, M. R. Cassia-Santos, E. R. Leite, E. Longo, J. Bisquert, G. Garcia-Belmonte, and F. Fabregat-Santiago, J. Appl. Phys. 88, 6545 共2000兲. 4 M. R. Cassia-Santos, P. R. Bueno, E. Longo, and J. A. Varela, J. Eur. Ceram. Soc. 21, 161 共2001兲. 5 E. R. Leite, A. M. Nascimento, P. R. Bueno, E. Longo, and J. A. Varela, J. Mater. Sci.: Mater. Electron. 10, 321 共1999兲. 6 J. A. Cerri, E. R. Leite, D. Gouvea, and E. Longo, J. Am. Ceram. Soc. 79, 804 共1996兲. 7 J. A. Varela, J. A. Cerri, E. R. Leite, E. Longo, M. Shamsuzzoha, and R. C. Bradt, Ceram. Int. 25, 256 共1999兲. 8 M. S. Castro and C. M. Aldao, J. Eur. Ceram. Soc. 18, 2233 共1998兲. 9 W. Yongjun, W. Jinfeng, C. Hongcun, Z. Weilie, Z. Peilin, D. Houmin, and Z. Lianyi, J. Phys. D 33, 99 共2000兲. 10 F. Stucki and F. Greuter, Appl. Phys. Lett. 57, 446 共1990兲. 11 Y. Nakano and N. Ichinose, J. Mater. Res. 5, 2910 共1990兲. 12 M. O. Orlandi, P. R. Bueno, E. R. Leite, and E. Longo 共unpublished兲. 13 C. N. Turton and T. I. Turton, The Oxide Handbook 共IFI/Plenum, New York, 1973兲, pp. 378 –379. 14 E. Sonder, M. M. Austin, and D. L. Kinser, J. Appl. Phys. 54, 3566 共1983兲. 15 E. Olsson, G. L. Dunlop, and R. Osterlund, J. Appl. Phys. 66, 5072 共1989兲. 16 T. R. N. Kutty and S. Ezhilvalavan, Mater. Chem. Phys. 38, 267 共1994兲. 17 M. Egashira, Y. Shimizu, T. Takao, and S. Sako, Sens. Actuators B 35–36, 62 共1996兲.
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