TY - JOUR
T1 - Sintering behavior and non-linear properties of ZnO varistors processed in microwave electric and magnetic fields at 2.45 GHz
AU - Badev, Alexandre
AU - Marinel, Sylvain
AU - Heuguet, Romain
AU - Savary, Etienne
AU - Agrawal, Dinesh
N1 - Funding Information:
The authors would like to express their gratitude for the ANR Grant No. 2011 BS08 014 01, which supported this work. The authors are also thankful to the team for giving valuable guidance all along this study in the area of microwave sintering of ceramics, grain growth mechanisms, varistor composition synthesis and electrical measurements. Special thanks to Mr. Jérôme Lecourt and Mr. Jean-François Lefebre from CRISMAT Laboratory, UMR 6508 CNRS-ENSICAEN, for their help and enthusiasm during the experimental parts.
PY - 2013/12
Y1 - 2013/12
N2 - A study of the densification behavior and grain growth mechanisms of ZnO-based varistors composed of 98 mol.% ZnO-2 mol.% (Bi2O 3, Sb2O3, Co3O4, MnO 2) has been carried out. The pressed samples were sintered in microwave electric (E) and magnetic (H) fields using a single-mode cavity of 2.45 GHz. The effect of the sintering temperature (900-1200 C), holding time (5-120 min) and sintering mode (E, H) on the microstructure and electrical properties of the sintered varistor samples were investigated. The grain growth kinetics was studied using the simplified phenomenological equation G n = kte(-Q/RT ). The grain growth exponent (n) and apparent activation energy (Q) values were estimated for both electric and magnetic heating modes and were found to be n = 3.06-3.27, Q = 206-214 kJ mol-1, respectively. The lower value of n estimated in the E field was attributed to a volume diffusion mechanism, whereas the higher n value in the H field sintering was correlated mainly to a combined effect of volume and surface diffusion processes. Samples sintered in the H and E fields showed high final densities. Moreover, the ones sintered in the H field presented slightly higher density values and bigger grains for all sintering temperatures than E field heated ones. The optimal sintering conditions were achieved at 1100 C for a 5 min soaking time for both H and E field processed samples, where respectively densities of 99.2 ± 0.5% theoretical density (TD) and 98.3 ± 0.5% TD along with grain size values of G = 7.2 ± 0.36 μm and G = 6.6 ± 0.33 μm were obtained. Regarding the electrical properties, breakdown voltage values as high as 500-570 V mm-1 were obtained, together with high non-linear coefficients α = 29-39 and low leakage currents (Jl ≈ 5 × 10-3 mA cm-2), respectively, for E and H field sintered varistor samples. Moreover, samples sintered in an H field systematically exhibited higher breakdown voltage values compared to the ones sintered in the E field. This was attributed to an improved coupling between the H field and the present dopants within the ZnO matrix, this latter being mostly semiconductive, thus leading to an enhanced reactivity and improved properties of the electrostatic barrier.
AB - A study of the densification behavior and grain growth mechanisms of ZnO-based varistors composed of 98 mol.% ZnO-2 mol.% (Bi2O 3, Sb2O3, Co3O4, MnO 2) has been carried out. The pressed samples were sintered in microwave electric (E) and magnetic (H) fields using a single-mode cavity of 2.45 GHz. The effect of the sintering temperature (900-1200 C), holding time (5-120 min) and sintering mode (E, H) on the microstructure and electrical properties of the sintered varistor samples were investigated. The grain growth kinetics was studied using the simplified phenomenological equation G n = kte(-Q/RT ). The grain growth exponent (n) and apparent activation energy (Q) values were estimated for both electric and magnetic heating modes and were found to be n = 3.06-3.27, Q = 206-214 kJ mol-1, respectively. The lower value of n estimated in the E field was attributed to a volume diffusion mechanism, whereas the higher n value in the H field sintering was correlated mainly to a combined effect of volume and surface diffusion processes. Samples sintered in the H and E fields showed high final densities. Moreover, the ones sintered in the H field presented slightly higher density values and bigger grains for all sintering temperatures than E field heated ones. The optimal sintering conditions were achieved at 1100 C for a 5 min soaking time for both H and E field processed samples, where respectively densities of 99.2 ± 0.5% theoretical density (TD) and 98.3 ± 0.5% TD along with grain size values of G = 7.2 ± 0.36 μm and G = 6.6 ± 0.33 μm were obtained. Regarding the electrical properties, breakdown voltage values as high as 500-570 V mm-1 were obtained, together with high non-linear coefficients α = 29-39 and low leakage currents (Jl ≈ 5 × 10-3 mA cm-2), respectively, for E and H field sintered varistor samples. Moreover, samples sintered in an H field systematically exhibited higher breakdown voltage values compared to the ones sintered in the E field. This was attributed to an improved coupling between the H field and the present dopants within the ZnO matrix, this latter being mostly semiconductive, thus leading to an enhanced reactivity and improved properties of the electrostatic barrier.
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U2 - 10.1016/j.actamat.2013.09.023
DO - 10.1016/j.actamat.2013.09.023
M3 - Article
AN - SCOPUS:84886240218
SN - 1359-6454
VL - 61
SP - 7849
EP - 7858
JO - Acta Materialia
JF - Acta Materialia
IS - 20
ER -