Title: Application of Impedance Spectroscopy to characterise grain boundary and surface layer effects in electroceramics.
1Application of Impedance Spectroscopy to
characterise grain boundary and surface layer
effects in electroceramics.
- Derek C Sinclair
- Department of Engineering Materials
- University of Sheffield, UK
2Outline
- Introduction
- Typical electrical microstructures for
electroceramics. - Background to combined Z, M spectroscopy.
- Example
- La-doped BaTiO3 ceramics
- Conclusions
3Typical Electrical Microstructures
C (eoeA)/d
Clear indicates insulating regions Shading
indicates semiconducting regions Semiconductivity
either by chemical doping or oxygen loss.
4Each 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.
7Rb 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
9The 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
12All 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
13Arrhenius behaviour of Rb and Rgb for
Ba1-xLaxTi1-x/4O3 processed in O2
3
14Is 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.
15Processing in Ar at 1350 oC
RT 522 W Rgb 510 W Rb 12 W, Cgb 2.4 nF
16Arrhenius behaviour of Rb and Rgb for
Ba1-xLaxTi1-x/4O3-d processed in Ar at 1350 oC.
4
17Return 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
18Composition 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
19Composition 3 processed in air at 1350 oC
20Composition 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.
21Unpolished
Polished
RT Rgb 2.04 kW Cgb 7.5 nF Both Rb and Rgb
obey the Arrhenius law.
22Composition 4 (20 La)
Ar
Ar
Air
23Conclusions
- Oxygen loss is responsible for semiconductivity
in Ba1-xLaxTi1-x/4O3 ceramics
O2
Ar
Air
x 0.03
x 0.20
24Conclusions
- 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.
25Acknowledgements
- Finlay Morrison
- Tony West
- EPSRC for funding.
26Extras
- 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.
27Excellent dielectrics when processed in O2
28 29Composition 2
ptcr effect
RT Rgb
Rb 15 W