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Derek C Sinclair Department of Engineering Materials University of Sheffield, UK

Application of Impedance Spectroscopy to characterise grain boundary and surface layer effects in electroceramics. Derek C Sinclair Department of Engineering Materials University of Sheffield, UK. Outline. Introduction Typical electrical microstructures for electroceramics.

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Derek C Sinclair Department of Engineering Materials University of Sheffield, UK

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  1. Application of Impedance Spectroscopy to characterise grain boundary and surface layer effects in electroceramics. Derek C Sinclair Department of Engineering Materials University of Sheffield, UK

  2. Outline • Introduction Typical electrical microstructures for electroceramics. Background to combined Z’’, M’’ spectroscopy. • Example La-doped BaTiO3 ceramics • Conclusions

  3. Typical Electrical Microstructures C = (eoe’A)/d Clear indicates insulating regions Shading indicates semiconducting regions Semiconductivity either by chemical doping or oxygen loss.

  4. For many electroceramics Rgb >> Rb and the parallel RC elements are connected in series. Brickwork layer model shows Cgb >> Cb Each region can be represented (to a simple approximation) as a single parallel RC element Rb Rgb t = RC Cb Cgb

  5. Data analysis using (Z*, M*) works well for series-type equivalent circuits For a single parallel RC element Z* = Z’ - jZ’’ Z’ = R Z’’ = R. wRC 1 + [wRC]2 1 + [wRC]2 Recall : M* = jwCoZ* M’ = w2CoR2C M’’ = CowRC 1 + [wRC]2 C 1 + [wRC]2

  6. Each RC element produces an arc in Z* and M* (or a Debye peak in Z’’ and M’’ spectroscopic plots), however:- Z* (and Z’’ spectra) are dominated by large R (gb’s) M* (and M’’ spectra) are dominated by small C (bulk) Such an approach is useful for studying ceramics with insulating grain boundaries/surface layers and semiconducting grains.

  7. Rb = 20 kW Rgb = 1MW Cb = 60 pF Cgb = 1.25 nF

  8. Notes: • Appearance of Debye peaks in the frequency window depend on t for the various RC elements. • Limits • R > 108W => t is high • => wmax < 1 Hz • R < 102W => t is low • => wmax > 10 MHz Combined Z’’ , M’’ spectroscopic plot

  9. The doping mechanism in La-BaTiO3 Rmin - 0.3 -0.5 atom% doping (ptcr devices) heated in air > 1350 oC followed by rapid cooling. Is there a change in doping mechanism with La-content ? Low x : donor (electronic) doping, La3+ + e- => Ba2+ High x : Ionic compensation, La3+ => Ba2+ + 1/4Ti4+

  10. Phase diagram studies showed that for samples prepared in air ionic compensation was favoured Ba1-xLaxTi1-x/4O3 where 0 ≤ x ≤ 0.25 IS showed all ceramics with x > 0 to be electrically heterogeneous when processed in air and all showed the presence of semiconducting regions. Electrical measurements are inconsistent with the phase diagram results!!

  11. 2 (0.3at%) 3 (3 at%) 4 (20 at%) RT = 675 W at 25 oC RT > 1 MW at 25 oC.

  12. All samples processed at 1350 oC in flowing O2 as opposed to air were insulating at room temperature. Composition 3 ( 3at%) Air (25 C) O2 (25 C) O2 ( 479 C) Cgb ~ 0.12 nF Cb ~ 46 pF

  13. Arrhenius behaviour of Rb and Rgb for Ba1-xLaxTi1-x/4O3 processed in O2 3

  14. Is oxygen loss the source of the semiconductivity in samples processed in air? Ba1-xLaxTi1-x/4O3-d Oox => 1/2O2 + 2Vo.. + 2e’ Samples were processed in Argon at 1350 oC and all were semiconducting at room temperature.

  15. Processing in Ar at 1350 oC Composition 3 (3at%) RT ~ 522 W; Rgb ~ 510 W Rb ~ 12 W, Cgb ~ 2.4 nF

  16. Arrhenius behaviour of Rb and Rgbfor Ba1-xLaxTi1-x/4O3-d processed in Ar at 1350 oC. 4

  17. Return to processing in air at 1350 oC. Composition 3 (3 at%): dc insulator at 25 oC Composition 4 (20 at%): dc insulator at 25 oC

  18. Composition 3 At least three RC elements present. No change in response on polishing the pellets. 3 RT ~ Rgb > 107W at 25 oC Rb ~ Rinner + Router < 1 kW Cgb ~ 5-6 nF Couter ~ 0.2 nF, Cinner < 0.2 nF Air

  19. Composition 3 processed in air at 1350 oC

  20. Composition 4 Four elements present ? Z’’ : fmax < 10 Hz, R > 2 MW M’’ : fmax ~ 102 Hz, 0.1 MW, C ~ 7 nF fmax ~ 104 Hz, ~ 1 kW, C ~ 7 nF fmax > 107 Hz, < 1kW, C < 1 nF Dramatic change on polishing the pellet.

  21. Unpolished Polished RT ~ Rgb = 2.04 kW Cgb = 7.5 nF Both Rb and Rgb obey the Arrhenius law.

  22. Composition 4 (20% La) Ar Ar Air

  23. Conclusions Oxygen loss is responsible for semiconductivity in ‘Ba1-xLaxTi1-x/4O3’ ceramics O2 Ar Air x = 0.03 x = 0.20

  24. Conclusions • IS is an invaluable tool for probing electrical heterogeneities in electroceramics. This is especially true when oxygen concentration gradients are responsible for inducing semiconductivity. • Combined Z’’, M’’ spectroscopic plots are a convenient and efficient method of visually inspecting the data to allow rapid assessment of the electrical microstructure in many electroceramics.

  25. Acknowledgements Finlay Morrison Tony West EPSRC for funding.

  26. Extras • e’ vs T for a range of x. • Arrhenius plot of Rb and Rgb for air (1200 C) and O2 (1350 C) processed ceramics. • Analysis of composition 2.

  27. Excellent dielectrics when processed in O2

  28. Arrhenius plot

  29. Composition 2 ptcr effect RT ~ Rgb Rb ~ 15 W

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