1.4 Electrical Properties of BaTiO3 Co-Doped with Rare Earth and Mn

LOG CONDUCTIVITY (Ω -1 cm -1 )10-1-21200 o CUn-doped BaTiO 3(Ba 0.99 Y 0.01 )(Ti 0.995 Mn 0.005 )O 3Ba(Ti 0.985 Y 0.01 Mn 0.005 )O 3-yBa(Ti 0.995 Mn 0.005 )O 3-y-3-16 -12 -8 -4 0LOG PARTIAL PRESSURE, Po 2(atm)Fig. 5. Equilibrium electrical conductivity of Y andMn doped BaTiO 3 1200 o Cdue to the electro-migration of oxygen vacancies.The increase in oxygen vacancy concentrationwould give rise to the increase in leakage currents,leading to the degradation of insulating resistance.LOG LEAKAGE CURRENT (A)20-2-4-6-8Un-doped BaTiO 3Ba(Ti 0.99 Y 0.01 )O 2.995Ba(Ti 0.985 Y 0.01 Mn 0.005 )O 3-y(Ba 0.99 Y 0.01 )(Ti 0.995 Mn 0.005 )O 3+yX150 o C,10600V/cmreduced manganese ions, forming a neutral defect••complex ( M n Ti′′ −V). This defect complex couldOprevent oxygen vacancies from migrating towardsthe cathode, resulting in low leakage currents ofMn doped BaTiO3 regardless of the Y replacementof either Ba sites or Ti sites [22-23].The thermally stimulated depolarization current(TSDC) of Ba(Ti 1-x Y x )O 3-0.5x is shown in Fig. 7.The TSDC of specimens increases drastically at thetemperature range above 50 o C and reachesmaximum points, and then gradually decreases.The height of TSDC peak increases with increasingY doping level, and the highest TSDC peakappears at the specimen with 3.0mol% Y.However, the specimen with 5.0mol% Y showslower TSDC peaks than the Ba(Ti 0.97 Y 0.03 )O 2.985 .The main conduction mechanism at thetemperature range studied can be considered as anionic conduction by oxygen vacancy. Thus, theincrease in TSDC of Y-doped BaTiO 3 could be dueto the migration of oxygen vacancies during thepolarization and depolarization process. As shownin Fig. 4 the specimen doped with 5.0mol% Yreveals a microstructure with small grain sizes andrich pores, compared with the specimens dopedwith below 3.0mol% Y. This could be effective forsuppressing the migration of oxygen vacanciesduring the polarization and depolarization process.Thus, the small grain size and the low density inthe sample doped with 5.0mol% Y seem to beresponsible for the decrease in TSDC.-100 1 2 3 4 5LOG TIME (min.)Fig. 6. Time dependence of leakage currents of Yand Mn doped BaTiO 3 at 150 and 10,600 V/cmRödel reported that an increasing amount of Mn 2+is created from Mn 3+ or Mn 4+ during thedegradation process. This seems to occur close tothe anode rather than near the cathode [22]. Asoxygen ions in the lattice move to the anode andare oxidized, doubly ionized oxygen vacancies andcorresponding electrons will be left behind. Thoseelectrons could be consumed for the manganesereduction. This would be expressed as an oxygenvacancy injection from the anode. Consequently,positively charged oxygen vacancies will movetowards the cathode and then be captured by theCURRENT (x10 -8 A)543210Ba(Ti 1-xY x)O 3-0.5x(1) x=0.002(2) x=0.02(3) x=0.03(4) x=0.05T p =150 o Ct p =30 min.V p =200V/cmH. R. =2 o C/min-100 -50 0 50 100 150 200 250TEMPERATURE( o C)Fig. 7. TSDC of Y-doped BaTiO 3TSDC of BaTiO 3 co-doped with Y and Mn isshown in Fig. 8 and Fig. 9. The sharp peaks were(3)(4)(1)(2)27

confirmed around phase transition temperatures ofBaTiO 3 . The TSDC of (Ba 1-x Y x )(Ti 1-y Mn y )O 3decreased at the temperature range above 160 o C,whereas a slight increase in TSDC was observed atthe specimens of Ba(Ti 1-x-y Y x Mn y )O 3 . Hubnerreported that the un-doped BaTiO 3 exhibitstypically three TSDC peaks around the phasetransition temperatures [24]. The sharp peaks ofspecimens may be due to the abrupt depolarizationcurrents accompanied by the phase transition. Theoxygen vacancy of (Ba 0.99 Y 0.01 )(Ti 0.995 Mn 0.005 )O 3specimen can be significantly reduced by excessoxygen with donor (yttrium) doping. AlthoughBa(Ti 0.985 Y 0.01 Mn 0.005 )O 3-y has plenty of oxygen••vacancies, the defect association of ( Mn Ti′′ −V)Ocould effectively suppress the migration of oxygenvacancies during polarization and depolarizationprocess. However, a slight increase in TSDC withBa(Ti0.985Y 0.01 Mn 0.005 )O 3-y above 160 o C implies••that the defect association ( Mn′′ Ti−VO) could bedissociated by reasonable amount of thermalenergy.CURRENT (x10 -9 A)1086420(Ba 0.99Y 0.01)(Ti 0.995Mn 0.005)O 3T p =150 o Ct p =30 min.V p =8000V/cmH. R. =2 o C/min.-100 -50 0 50 100 150 200 250TEMPERATURE ( o C)Fig. 8. TSDC of (Ba 0.99 Y 0.01 )(Ti 0.995 Mn 0.005 )O 3Conclusions(1) As Y content was increased in the systemBa(Ti 1-x Y x )O 3-0.5x , the oxygen vacancyconcentration increased and the conductivityminimum moved to lower oxygen partial pressures.(2) Ba(Ti 0.99 Y 0.01 )O 2.995 showed considerableleakage currents, compared with the un-dopedBaTiO 3 .Current (x10 -9 A)3.02.52.01.51.00.50.0Ba(Ti 0.985Y 0.01Mn 0.005)O 3-yT p =150 o Ct p =30 min.V p =8000V/cmH. R. =2 o C/min.-100 -50 0 50 100 150 200 250TEMPERATURE ( o C)Fig. 9. TSDC of Ba(Ti 0.985 Y 0.01 Mn 0.005 )O 3-y(3) The leakage current ofBa(Ti 0.985 Y 0.01 Mn 0.005 )O 3-y was stable with time andmuch lower than that of the un-doped BaTiO 3 .(4) The lowest leakage current was observed with(Ba 0.99 Y 0.01 )(Ti 0.995 Mn 0.005 )O 3(5) In the system of Ba(Ti 1-x Y x )O 3-0.5x , the heightof TSDC peak increased with increasing Y dopinglevels, and the specimen with 3.0mol% Y showedthe highest TSDC peak, whereas the specimen with5.0mol% Y exhibited lower TSDC peaks thanBa(Ti 0.97 Y 0.03 )O 2.985 . (6) The slight increase inTSDC at the temperature range above 160 o C wasconfirmed with the specimens ofBa(Ti 0.985 Y 0.01 Mn 0.005 )O 3-yAcknowledgementThis research was supported by a grant from theCenter for Advanced Materials Processing(CAMP) of the 21st Century Frontier R&DProgram funded by the Ministry of Science andTechnology, Republic of Korea.References1. R. D. Shannon, Acta Cryst. A32 (1976) 751.2. K. Takada, E. Chang and D. M. Smyth,Advanced in Ceram. 19 (1986) 147.3. J. H. Hwang and Y. H. Han, Solid State Ionics140 (2001) 181.4. L. A. Xue, Y. Chen and J. Brook, Mat. Sci. andEng. B1 (1988) 193.5. H. J. Hagemann and H. Ihrig, Physical ReviewB, 20 (1979) 3871.28

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