Application of Impedance Spectroscopy to characterise grain boundary and surface layer effects in electroceramics. - PowerPoint PPT Presentation

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

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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 electroceramics.


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
  1. e vs T for a range of x.
  2. Arrhenius plot of Rb and Rgb for air (1200 C) and
    O2 (1350 C) processed ceramics.
  3. 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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