The Grain Growth Control of ZnO-V2O5 Based Varistors by PrMnO3 Addition

Abstract

In this study, the grain growth behaviour of ZnO-V2O5-based ceramics with 0.25–0.75 mol% additions of PrMnO3 was systematically investigated during sintering from 850 °C to 925 °C. with the aim to control the ZnO grain size for their application as varistors. It was found that with the increased addition of PrMnO3, in addition to the decrease in the average grain size, the grain size distribution also narrowed and eventually changed from a bimodal to unimodal distribution after a 0.75 mol% PrMnO3 addition. The grain growth control was achieved by a pinning effect of the secondary ZnCr2O4 and PrVO4 phases at the ZnO grain boundaries. The apparent activation energy of the ZnO grain growth in these ceramics was found to increase with increased additions of PrVO4, hence the observed reduction in the ZnO grain sizes.

Effects of Dopants and Processing Parameters on the Properties of ZnO-V2O5-Based Varistors Prepared by Powder Metallurgy: A Review

“…After mixing the MOV powders with a liquid medium and a small amount (e.g., 0.5–5 wt.%) of organic additive (binders, dispersing agents, among others), drying and sieving them with a typical mesh size of ≤149 μm, the granulation of the MOV composite powders takes place. Binders are selected from polyvinyl alcohol (PVA) [ 15 , 16 , 25 , 38 , 83 , 128 , 160 ], polyvinyl butyral (PVB) [ 27 , 35 , 36 , 124 , 161 , 162 , 163 ], ethyl cellulose (EC) [ 37 ], acrylic latex polymers [ 164 , 165 ], etc. Dispersing agents are chosen from polyurethane polymers [ 108 , 109 ], carboxylic anionic surfactants (sodium or ammonium polyacrylate polymers), phosphate anionic surfactants, cationic surfactants (quaternary ammonium salts), nonionic surfactants [ 166 ], etc.…”

“…The grain growth kinetics during LPS of MOVs from the ZnO-based systems is determined by Equation (11) [ 21 , 83 , 177 , 178 ]: where G is the mean ZnO grain size at the sintering time t , G 0 is the initial ZnO grain size, n is the kinetic grain growth exponent, Q is the apparent activation energy for grain growth, K 0 is the preexponential constant of the ceramic material, R is the universal gas constant, and T is the absolute temperature (T = 300 K).…”

“…Usually, G 0 = 0 when G >> G 0 , and the kinetic grain growth exponent ( n ) is estimated from the slope of the plots of lg G versus lg t , which is equal to 1/n [ 21 , 83 , 177 , 178 ]: …”

“…The apparent activation energy for grain growth ( Q ) is set from Equation (13) [ 21 , 83 , 177 , 178 ] ( Q is estimated from the slope of Arrhenius plots of lg( G n /t ) versus 1/ T ): …”

“…Most commercial ZnO-based varistors are composed of ZnO-Bi 2 O 3 systems with up to 8–12 MO dopants and typically yield ZnO grain sizes (d ZnO ) of 10–100 μm and a depletion length of the doped GBs of 50–100 nm, a breakdown field (E B ) of 2–3 kV/cm, and a nonlinear coefficient of about 40–50 [ 52 , 56 , 59 , 80 , 81 , 82 ]. MOVs with coarse-grained microstructures (mean d ZnO > 30 µm) and lower E B values are used for low-voltage (LV) applications, while MOVs with fine-grained microstructures (mean d ZnO < 10 µm) are required for high-voltage (HV) and medium-voltage (MV) applications [ 59 , 83 ].…”