Characterization of Single crystal KNN Ceramics (K0.5Na0.5NbO3)

1. Characterization of KNN
Single Crystals by Slow-
Cooling Technique
Presented by:
Syed Ali Afzal 1

2. Introduction
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3. Introduction 1/3
 PZT = Toxic Pb
(Applications: Sensor,
Actuators and
Transducers).
 Saito et al. [1]= Textured
KNN-based ceramics,
d33 = 416 pC/N.
 Most research work on
KNN with MPB (x=0.5),
to improve densification
and piezoelectric props.
Densification = High
volatility of K and Na.
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4. Introduction 2/3
 (K0.5Na0.5)NbO3 by Solid-state reaction using KTaO3
as seed [8]. Size = 20µm, no piezoelectric properties.
 0.95(K0.5Na0.5)NbO3-0.05LiNbO3 single crystal [9], d33
= 405 pC/N. However, hysteresis loop suggested
large leakage current.
 Mn doped KNN crystal [10]: Leakage current was
suppressed by Mn doping.
 Earlier work: (KxNa1-x)NbO3 single crystals by slow
cooling by raw material (K0.5Na0.5)NbO3.
◦ However, x < 15 mol% by XRF
◦ Reasons:
 High volatility of K+ and Na+
 Radius of K > Na, not easy to enter in KNN lattice
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5. Introduction 3/3
 This paper:
◦ Starting material: 0.8(K0.5Na0.5)NbO3-0.2K2CO3
◦ Resulting material: (K0.56Na0.44)NbO3
 We investigated:
◦ Phase structure
◦ Dielectric properties
◦ Piezoelectric properties
◦ Domain structure of KNN single crystal
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6. Experimental
Procedure
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7. Experimental Procedure 1/3
Crystal growth process by slow cooling
Sample preparation:
(40 g KNN powder put into Pt crucible, sealed and put into Al2O3 crucible)
Calcination:
(At 800°C for 2h)
Ball milling:
(weighed a/c to 0.8(K0.5Na0.5)NbO3-0.2K2CO3 and ball milled for 5 h)
Raw materials:
(99.99% pure powders of Na2CO3, K2CO3 and Nb2O5)
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8. Experimental Procedure 2/3
0
200
400
600
800
1000
1200
1400
0 10 20 30 40 50 60 70 80
Temperature(°C)
Time (h)
Temperature profiles for KNN crystal growth
(1) Room temp to 1000°C
Rate = 100°C/h
(2) Heating at 50°C/h to
1250°C
Soak for 6 h
(3) Slow cooling from 1250°C
to 1150°C
Rate = 5°C/h (4) Then to 1000°C @ 10°C/h
(6) Then to 600°C @ 50°C/h
Soaking for 2 h
(7) Finally to room temp
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9. Experimental Procedure 3/3
 Crystal structure analysis: XRD (RIGAKU D/MAX-
2400)
 Composition analysis: XRF Spectrometer
(S4PIONEER/4KW)
 For electrical measurements:
◦ Dimensions: 2 x 2 x 0.3 mm3
◦ Ag electrodes painted on both sides
◦ Poled under 2KV/mm dc field @ 120°C for 10 min in Si oil
 Dielectric measurements: Multi-frequency LCR meter
(HP 4284 A), from -150°C to 500°C. Heating rate =
3°C/min
 Temperature dependence of piezoelectric constant
d33: Piezo- d33 meter (ZJ-3A) with heater
 Domain structures: Polarizing light microscope 9

10. Result and
Discussion
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11. 1. Crystal size
 Spontaneous nucleation at critical temperature.
 Cuboid shape: Achromous and semi-transparent.
 Largest size: 4 x 4 x 8 mm3 at top part.
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12. 2. Phase structure 1/2
 XRD patterns of KNN single crystal powder (Fig. 2).
 Orthorhombic phase: (110)/(001) peak splitting around 22° and
(220)/(002) peak splitting around 45°. Orthorhombic perovskite
structure.
 (220)/(002) peak splitting of top and middle part, difference in lattice
parameters.
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13. 2. Phase structure 2/2
 XRD of smooth surfaces in top part, orthorhombic (001) face confirmed.
 Intensity of (001) and (002) face ~ 109 and 1014. Very high crystallization
and crystal orientation.
 K/Na ratio by XRF = 0.56/0.44; [001]-oriented (K0.56Na0.44)NbO3 crystals
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14. 3. T dependence of the Dielectric
Permittivity 1/2
 03 dielectric permittivity peaks at different frequency are observed in
Fig. 4(a).
 A/c to KN-NN, phase transition sequence is R→O→T→C.
 Fig. 4(a); consistent with KN-NN
 R→O = -100°C
 O→T = 214°C
 T→C = 433°C
Figure 4 (a). Temp dependence of
dielectric permittivity
KN-NN phase 14

15. Inset of Fig. 4(a) shows R→O phase transition and Curie-Weiss 1/ ε
3. T dependence of the Dielectric
Permittivity 2/2
 R→O phase transition peaks, ε frequency dispersion is shown:
◦ ε peak value temperature is nearly the same for different frequency; relaxation behavior
need to be studies
◦ Phase transition peaks for O→T and T→C are much sharper.
 Relationship b/w ε and T (above TC) by a Curie-Weiss law:
◦ 1/ ε = (T-To)/C, where C = 2.44 x 105K and Curie-Weiss temp To = 647K (374°C)
◦ Therefore, KNN crystals are normal ferroelectrics with first order phase transition
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16. 4. T dependence of the Dielectric
loss
 Falls remarkably b/w 1kHz to 100 kHz especially at T > 250°C.
 Significant rising at 1 kHz above 250°C implies that defects and their
movements inc. with inc. temp.
 Confirmed: Mn in KNN is effective for suppressing leakage current and
decreasing the loss [10, 11].
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Figure 4 (b). Temperature dependence of dielectric loss for [001]-oriented KNN single crystal at
different frequency

17. 5. Piezoelectric coefficient d33 for
[001]-KNN single crystal
 d33 is b/w 112pC/N and 147 pC/N.
 Avg = 130 pC/N at room temp. Max = 220 pC/N is at around 130°C.
 d33 dec. in the range 210~230°C corresponding to depolarization i.e.
O→T phase transition temp (Fig.4).
 Although Curie ~400°C, highest application temp ~200°C.
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18. 6. Domain structure
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 Region A = Laminar; Region B = Spindlelike.
 Domain width (region A) = 5µm - 20µm.
Figure 6. (a) The orthorhombic domain structure of KNN single crystal. (b)
The magnified image of region A