The effect of the starting powders particle size on the electrical properties of sintered Co doped ZnO varistors
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Date
2015
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Publisher
Springer
Abstract
Pure and Co doped zinc oxide nanopowders
have been synthesized by sol–gel route. The obtained
powders, after calcination in air at 550, 650, 750 and
850 C, were consolidated and sintered using microwaves.
The effect of sintering temperature on the density and
electrical properties was investigated. The best characteristics
are obtained using a sintering temperature equal to
1,075 C. The powders and sintered samples are characterized
by X-ray diffraction, microstructure images, density
and electrical measurements. The studied nanopowders
have: a density of 5.22 (g/cm3), a breakdown voltage of
446.5 V/mm and a coefficient of non-linearity of 11.48 for
ZnO doped with 7 mol% Co, and a density of 5.19 (g/cm3),
a breakdown voltage of 292.5 V/mm and a non-linearity
coefficient of 11.62 for ZnO doped with 5 mol% Co. The
XRD results indicate that pure and Co doped ZnO powders
are solid solutions crystallizing in pure wu¨ rtzite structure,
and consisted of a mixture of nanoparticles with an average
grain size between 36 and 210 nm. The grain size
decreases with increasing Co concentration and reaches its
lowest value at 7 mol% Co and increases with increasing
temperature. On the other hand the increase of Co concentration
leads to a decrease of the porosity and an
increase of the density of samples sintered at 1,070 C and
leads also to an increase of the electric field (E) in the nonlinear
area. The samples of 5 mol% Co sintered at
1,075 C, show that the non-linear coefficient a decreases
by increasing of calcined temperature, therefore increases
with decreasing of grain and particle sizes.
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Keywords
Co doped ZnO, Grain size, Varistor, Electrical properties, Microwave, Porosity, Density