Grain and Grain-Boundary Electric Resistance Characterization via Impedance Spectroscopy M. A. L. Nobre a* , F. R. Praxedes a, F. S. Bellucci b, S. Lanfredi a a Faculdade
de Ciências e Tecnologia – FCT, Univ Estadual Paulista – UNESP, Laboratory of Composites and Ceramic Functional - LaCCeF – Presidente Prudente, 19060-900, SP, Brazil b Ministério da Ciência e Tecnologia- MCTI - Brasília – DF, 70067-900, Brasil. *
[email protected]
OBJECTIVES
INTRODUCTION
Investigation
by
impedance
EXPERIMENTAL PROCEDURE Nb2O5.nH2O
Fe2O3
K2CO3
SrCO3
spectroscopy of the resistivity and
Thermistor
electric conductivity parameters as a function of temperature of the KSr2FeNb4O15- ceramic.
Dielectrical Properties
Investigation
of
the
behavior
with
thermistor negative
temperature coefficient
high energy ball milling
dry in a grove box
Sintering 1523 K/2h
Calcination 1373 K/10h
Uniaxilly pressed
(NTC) of
the KSr2FeNb4O15- ceramic.
Electrical Characterization
Ferroelectric properties
RESULTS AND DISCUSSION 10
10 4
Rgb = 4.86x10
4
5
1.8
-Im(Z) (10 .cm)
2.4
603 K 623 K 668 K 703 K
2
1.2
3 3
0.6
3
0.0 0.0
1
2 2
0.6
1.2
1.8
2.4
3.0
10 -7
-9
Cgb = 2.25x10 F
Cg = 1.02x10 F
1.6
Experimental Fitting
3
2
0.8 4
1
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Re(Z) (10 .cm)
-4.5
log dc (.cm)
Model: ExpDec -2.0x10 -2.2x10 -2.4x10
-2
-2
Chi^2 R^2
= 1.5666E-9 = 0.99988
-5.0
500 K
1500
KSr2(FeNb4)O15-
1200
0.78 eV
900
413 K
0.57 eV 600 KSr2Nb5O15
1.0 1.2 1.4 1.6 1.8 2.0
1000/T (K)
0
200
600
800
1000
Temperature (K)
-1
Figure: Arrhenius’ type diagram of the d.c. conductivity
400
Figure: Dielectric permittivity curves of the KSr2FeNb4O15- and of the KSr2Nb5O15, as a function of temperature.
REFERENCES S. Lanfredi, C. Darie, F.S. Bellucci, C. V. Colin and M. A. L. Nobre, Dalton Trans., 2014, 43, 10983-10998. S. Lanfredi, A. R. F. Lima, M. R. Besse and M. A. L. Nobre, Appl. Math. Sci., 2015, 9, 2015, 5839 – 5869.
In the temperature range at 668 K, the classical beta parameters of a thermistor is investigated. The beta parameter of grain-boundary is derived being equal to 1.03 x 104 K, while the beta one of grain is equal to 0.54 x 104 K, in the temperature range from 298 K to 963 K. The theoretical adjust (dashed curve) of data via exponential decay gives an excellent match with experimental data. Arrhenius’ type diagram of the d.c. conductivity dc. It seems that the curve exhibits a set of distinct regions. An anomaly occurs at around 668 K. This anomaly can be assigned to a phase transition phenomenon. Permittivity value (2000) of the KSr2FeNb4O15- is twice higher than the permittivity one of KSr2Nb5O15. The permittivity curve of the KSr2Nb5O15 shows a strong and broad polarization peak of high intensity at 413 K, with permittivity value equal to 1800.
CONCLUSION
300
Temperature (K)
Figure: Negative temperature coefficient at grain boundary. Square dot represents experimental data and dashed line represent theoretical adjust.
Figure: Grain and grain boundary resistance as a function of temperature.
-5.5
-6.5
1000
1800
0.99 eV
-6.0
800
Temperature (K)
1.09 eV
-7.5 650 700 750 800 850 900 950
600
2100
-7.0
-2
400
-2
-1
-1
NTCR coefficient (K )
Grain boundary
-1.8x10
GB
10 200
4.0
-2
-2
5
10
-4.0
-1.6x10
10
4
Figure: (a) Impedance diagrams of KSr2FeNb4O15- at several temperatures; (b) impedance diagram at 780 K, inset show its equivalent electric circuit derived from theoretical adjust of data..
-2
6
5
Re(Z) (10 .cm)
-1.4x10
668 K
3
6
-1.2x10
7
10
10
780 K
Electrical response of KSr2FeNb4O15- at 780 K is well described by electric circuit compose of two parallel RC circuits in series.
gb
8
0.0
3.6
10
2.4
0.0
g
9
(.cm)
(a) KSr (FeNb )O 2 4 15-
(b) Rg = 6.22x10
6
-Im(Z) (10 .cm)
3.0
Bulk properties are only dominant up to 668 K, considering the frequency range investigated. At temperatures higher than 668 K, grain boundary properties emerges exhibiting semiconductor features with negative temperature coefficient resistance.
ACKNOWLEDGMENTS
