Fabrication and effect of applied field stresses on stability of ZnO ceramics with CaMnO₃ as additive for varistor application
The major challenge in the continuing development of varistor has been to reduce the energy loss which is due to temperature and applied field stresses and to attain a good stability. The aims of this study are; to study the effect of CaMnO3 (CMO) contents on nonlinearity coefficient (α) enhancem...
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| Format: | Thesis |
| Language: | English |
| Published: |
2015
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/68166/1/FS%202015%2078%20IR.pdf |
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| Summary: | The major challenge in the continuing development of varistor has been to reduce the
energy loss which is due to temperature and applied field stresses and to attain a good
stability. The aims of this study are; to study the effect of CaMnO3 (CMO) contents on
nonlinearity coefficient (α) enhancement of ZnO based varistor ceramics; and secondly
to study the nonlinear stability of this ceramic against DC electrical field and thermal
stresses as function of CMO contents. The first aim has been carried out by synthesis
of ZnO + xCMO ceramics by citrate gel coating technique, where x ranging from 3 – 7
mol% and sintered at various sintering times from 1 – 3 hrs. The prepared samples
were characterized by using TG/DTA, EDX, SEM, XRD, and I-V measurements. The
second aim was carried out by choosing the samples with best electrical properties
subjecting them simultaneously to stresses of specific temperature and DC voltage over
a certain period of time to study the degree of degradation. XRD analysis shows that
the main phase in the material systems was ZnO while CMO as the secondary phase
increase gradually with the increase of mol%, x. The TG/DTA analysis shows that,
good quality of ZnO-CMO can be achieved at calcination temperature above 650 °C.
Average density of the ceramics in all systems has the same trend where it decreases
with the increase of doping level and sintering time. The maximum grain size was
found to be 23.4 μm at the highest sintering time at 3 hrs at 7 mol% CMO doping level,
as Mn is a strong grain enhancer. The SEM and EDX results verified that Ca and Mn
were distributed within the grain as well as in the grain boundaries and triple point
junctions. The value of α was found to increase with amount of CMO up to 5 mol%
concentration. An optimum sintering time of 3 hrs for 5 mol% doping level gave the
best α, attaining a value of 42.3 which represent a high protective function of ZnO
varistor. The DC stress has caused a decrease in varistor voltage, α, and also an
increase in leakage current. After exposure to DC and heat stresses for 18 hours, the
varistor ceramics experienced a slight drop in nonlinearity. Thus, the varistor ceramic
samples exhibited a high level of leakage current compared to samples before
degradation which indicated they have been degraded or deteriorated. The degradation
percentage was calculated in terms of changes in varistor parameters before and after
stress. It showed that sample with 3 mol% CMO possess lowest degradation percentage. From this result, it was found that samples fabricated at 3 hrs sintering
time have the best microstructure and electrical properties. |
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