J. Mater. Sci. Technol. ›› 2020, Vol. 41: 12-20.DOI: 10.1016/j.jmst.2019.08.055
• Research Article • Previous Articles Next Articles
Edson Cezar Grzebieluckaa*(), João Frederico Haas Leandro Monteiroa, Eder Carlos Ferreira de Souzaa, Christiane Philippini Ferreira Borgesa, André Vitor Chaves de Andradeb, Eloísa Cordoncilloc, Héctor Beltrán-Mirc, Sandra Regina Masetto Antunesa
Received:
2019-07-02
Revised:
2019-08-07
Accepted:
2019-08-20
Published:
2020-03-15
Online:
2020-04-10
Contact:
Cezar Grzebielucka Edson
Edson Cezar Grzebielucka, João Frederico Haas Leandro Monteiro, Eder Carlos Ferreira de Souza, Christiane Philippini Ferreira Borges, André Vitor Chaves de Andrade, Eloísa Cordoncillo, Héctor Beltrán-Mir, Sandra Regina Masetto Antunes. Improvement in varistor properties of CaCu3Ti4O12 ceramics by chromium addition[J]. J. Mater. Sci. Technol., 2020, 41: 12-20.
Fig. 1. (a) Schottky barrier model for atomic defect model on the grain boundary (G.B.) showing different places with negative and positive charge concentration and (b) the similar atomic defect model proposed for CCTO by Cr2O3 addition on the Schottky barrier height (φb) and the space charge layer, adapted from Gupta and Carlson [36].
x mol% in CaCu3Ti4-xCrxO12-δ | Composition | Sample ID |
---|---|---|
0.000 | CaCu3Ti4-xCrxO12-δ | CCTO |
0.025 | CaCu3Ti3.975Cr0.025O12-δ | Cr_025 |
0.050 | CaCu3Ti3.950Cr0.050O12-δ | Cr_050 |
0.075 | CaCu3Ti3.925Cr0.075O12-δ | Cr_075 |
Table 1 Prepared compositions for xmol% addition of chromium in the CaCu3Ti4- xCrxO12-δ and the respective sample identification adopted.
x mol% in CaCu3Ti4-xCrxO12-δ | Composition | Sample ID |
---|---|---|
0.000 | CaCu3Ti4-xCrxO12-δ | CCTO |
0.025 | CaCu3Ti3.975Cr0.025O12-δ | Cr_025 |
0.050 | CaCu3Ti3.950Cr0.050O12-δ | Cr_050 |
0.075 | CaCu3Ti3.925Cr0.075O12-δ | Cr_075 |
Fig. 2. Example image processed by ImageJ software for SEM image of polished and thermally attacked CCTO (a), converted image for binary (b), and (c) whole grains counted without considering pores.
Fig. 3. Rietveld refinement XRD pattern for pure CCTO, sintered at 1070 °C for 12 h, showing Bragg positions for CaCu3Ti4O12 (blue bar), TiO2 (red bar) and CuO (green bar) phases.
Fig. 4. Rietveld refinement XRD pattern for Cr_025 sample, sintered at 1070 °C for 12 h, showing Bragg’s positions for CaCu3Ti4O12 (blue bar), TiO2 (red bar) and CuO (green bar), phases.
Parameters | CaCu3Ti4-xCrxO12-δ Phase | |||
---|---|---|---|---|
CCTO | Cr_025 | Cr_050 | Cr_075 | |
wt% | 90.6 | 91.5 | 88.2 | 90.2 |
a (Å) | 7.38921(4) | 7.39255(6) | 7.39122(4) | 7.39023(4) |
V (ų) | 403.454(4) | 404.001(6) | 403.783(4) | 403.621(4) |
d (g/cm³) | 4.928 | 4.686 | 4.887 | 4.837 |
x | 0.2997(7) | 0.3034(8) | 0.2978(7) | 0.3047(7) |
y | 0.1814(7) | 0.1809(8) | 0.1820(7) | 0.1790(7) |
TiO2 wt% | 4.2 | 3.4 | 4.9 | 4.2 |
CuO wt% | 5.2 | 5.1 | 6.9 | 5.6 |
Rp | 6.24 | 7.12 | 6.13 | 5.76 |
Rwp | 7.95 | 9.61 | 7.82 | 7.69 |
RB | 6.65 | 6.63 | 6.35 | 6.07 |
Rexp | 6.12 | 6.47 | 6.05 | 5.34 |
χ2 | 1.68 | 2.21 | 1.67 | 2.08 |
S | 1.29 | 1.22 | 1.29 | 1.44 |
Table 2 Comparison of measured structural parameters of CCTO and the compositions prepared with different amounts of chromium. CCTO shows space group Im$\bar{3}$; the Wyckoff positions are Ca(0,0,0), Cu(1/2,0,0), Ti-Cr(1/4,1/4,1/4) and O(x,y,0). Numbers put in brackets correspond to standard deviation referenced to the last number for all parameters measured.
Parameters | CaCu3Ti4-xCrxO12-δ Phase | |||
---|---|---|---|---|
CCTO | Cr_025 | Cr_050 | Cr_075 | |
wt% | 90.6 | 91.5 | 88.2 | 90.2 |
a (Å) | 7.38921(4) | 7.39255(6) | 7.39122(4) | 7.39023(4) |
V (ų) | 403.454(4) | 404.001(6) | 403.783(4) | 403.621(4) |
d (g/cm³) | 4.928 | 4.686 | 4.887 | 4.837 |
x | 0.2997(7) | 0.3034(8) | 0.2978(7) | 0.3047(7) |
y | 0.1814(7) | 0.1809(8) | 0.1820(7) | 0.1790(7) |
TiO2 wt% | 4.2 | 3.4 | 4.9 | 4.2 |
CuO wt% | 5.2 | 5.1 | 6.9 | 5.6 |
Rp | 6.24 | 7.12 | 6.13 | 5.76 |
Rwp | 7.95 | 9.61 | 7.82 | 7.69 |
RB | 6.65 | 6.63 | 6.35 | 6.07 |
Rexp | 6.12 | 6.47 | 6.05 | 5.34 |
χ2 | 1.68 | 2.21 | 1.67 | 2.08 |
S | 1.29 | 1.22 | 1.29 | 1.44 |
Fig. 6. SEM images of two regions (a and d) of the Cr_025 sample polished and thermally treated, showing chemical mapping to CuO phase segregation on grain boundary (b) and (e) and showing absence of TiO2 phase segregation on grain boundary (c) and concentration of TiO2 at some grains with a surface texture (f). The elements O, Cr and Ca are homogeneously distributed.
Fig. 7. SEM image features used to measure the average grain size (dg), logarithmic normal grain size distribution and apparent porosity (P) for CCTO (a) and (d), Cr_025 (b) and (e), and Cr_050 (c) and (f).
Fig. 8. Current density (J) versus electrical field (E) for CCTO and for the compositions with different amounts of chromium at (a) 20, (b) 90 and (c) 150 °C.
Fig. 9. Comparison of measured electric breakdown field strength, Eb, (a), nonlinear coefficient, α, obtained between 1 and 10 mA/cm2 (b), measured leakage current, IL, calculated at 70% of the breakdown electrical field (c), and β constant that is proportional to the inverse of the square root of w (d) at the temperature range 20-150 °C for CCTO and for the two compositions studied.
Sample | 1 kHz | 10 kHz | 100 kHz | 1 MHz | ||||
---|---|---|---|---|---|---|---|---|
ε' | tan δ | ε' | tan δ | ε' | tan δ | ε' | tan δ | |
CCTO | 14944 | 0.35 | 12467 | 0.14 | 10562 | 0.15 | 7898 | 0.423 |
Cr_025 | 11352 | 0.20 | 8933 | 0.19 | 6754 | 0.22 | 4514 | 0.45 |
Cr_050 | 11489 | 0.50 | 8939 | 0.21 | 7026 | 0.19 | 5011 | 0.49 |
Table 3 Comparison of dielectric permittivity (ε') and tan δ at 100 °C as a function of frequency for CCTO and for the two compositions studied.
Sample | 1 kHz | 10 kHz | 100 kHz | 1 MHz | ||||
---|---|---|---|---|---|---|---|---|
ε' | tan δ | ε' | tan δ | ε' | tan δ | ε' | tan δ | |
CCTO | 14944 | 0.35 | 12467 | 0.14 | 10562 | 0.15 | 7898 | 0.423 |
Cr_025 | 11352 | 0.20 | 8933 | 0.19 | 6754 | 0.22 | 4514 | 0.45 |
Cr_050 | 11489 | 0.50 | 8939 | 0.21 | 7026 | 0.19 | 5011 | 0.49 |
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