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Application of Impedance Spectroscopy to characterise grain boundary and surface layer effects in el

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Brickwork layer model shows Cgb Cb ... Unpolished. Polished. RT ~ Rgb = 2.04 kW. Cgb = 7.5 nF. Both Rb and Rgb obey the Arrhenius law. ... – PowerPoint PPT presentation

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Title: Application of Impedance Spectroscopy to characterise grain boundary and surface layer effects in el


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 (eoeA)/d
Clear indicates insulating regions Shading
indicates semiconducting regions Semiconductivity
either by chemical doping or oxygen loss.
4
Each region can be represented (to a simple
approximation) as a single parallel RC element
  • For many electroceramics Rgb gtgt Rb and the
    parallel RC elements are connected in series.
    Brickwork layer model shows Cgb gtgt Cb

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 wRC2 1 wRC2

Recall M jwCoZ
M w2CoR2C M
Co wRC 1
wRC2 C 1 wRC2
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
    (gbs)
  • 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
  • Combined Z , M spectroscopic plot
  • Notes
  • Appearance of Debye peaks in the frequency
    window depend on t for the various RC elements.
  • Limits
  • R gt 108 W gt t is high
  • gt wmax lt 1 Hz
  • R lt 102 W gt t is low
  • gt wmax gt 10 MHz

9
The doping mechanism in La-BaTiO3
Rmin - 0.3 -0.5 atom doping (ptcr devices)
heated in air gt 1350 oC followed by rapid
cooling.
Is there a change in doping mechanism with
La-content ?
Low x donor (electronic) doping, La3 e- gt
Ba2 High x Ionic compensation, La3 gt 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 gt 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 gt 1 MW at 25 oC.
RT 675 W 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 gt 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 Rgb for
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 gt 107 W at 25 oC Rb Rinner Router lt
1 kW Cgb 5-6 nF Couter 0.2 nF, Cinner lt 0.2 nF
Air
19
Composition 3 processed in air at 1350 oC
20
Composition 4
Four elements present ? Z fmax lt 10 Hz,
R gt 2 MW M fmax 102 Hz, 0.1 MW, C 7
nF fmax 104 Hz, 1 kW, C 7 nF fmax gt
107 Hz, lt 1kW, C lt 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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