Sintering behavior and non-linear properties of ZnO varistors

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ScienceDirect Acta Materialia 61 (2013) 7849–7858 www.elsevier.com/locate/actamat

Sintering behavior and non-linear properties of ZnO varistors processed in microwave electric and magnetic fields at 2.45 GHz Alexandre Badev a,⇑, Sylvain Marinel a, Romain Heuguet a, Etienne Savary a,b, Dinesh Agrawal c b

a CRISMAT Laboratory, UMR 6508 CNRS-ENSICAEN 6, Boulevard du Mare´chal Juin, 14050 Caen Cedex 4, France LMCPA Laboratory, Universite´ de Valenciennes et du Hainaut-Cambre´sis, ZI du Champ de l’Abbesse, 59600 Maubeuge, France c Microwave Processing and Engineering Centre, The Pennsylvania State University, 107 MRL, University Park, PA, USA

Received 6 May 2013; received in revised form 13 September 2013; accepted 13 September 2013 Available online 4 October 2013

Abstract A study of the densification behavior and grain growth mechanisms of ZnO-based varistors composed of 98 mol.% ZnO–2 mol.% (Bi2O3, Sb2O3, Co3O4, MnO2) 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 Gn = 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 mol1, 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 lm and G = 6.6 ± 0.33 lm were obtained. Regarding the electrical properties, breakdown voltage values as high as 500–570 V mm1 were obtained, together with high non-linear coefficients a = 29– 39 and low leakage currents (Jl  5  103 mA cm2), 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. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Microwave; Varistor; Microstructure; Sintering; Breakdown voltage

1. Introduction Zinc oxide (ZnO) varistors are non-linear, two-terminal, semiconductor voltage-dependent resistors, which are widely used as transient voltage surge suppressors to limit voltage surges in the range from low voltages (5 V) to high ⇑ Corresponding author. Tel.: +33 (0)2 31 45 13 69.

E-mail address: [email protected] (A. Badev).

voltages (up to 1 MV). The non-linear J–E characteristics result from the nature of the grain boundary layer, which mainly consists of ionic additives such as Bi2O3, Sb2O3, Co3O4, etc. Many researchers have reported the sintering behavior of several doped ZnO systems, such as Bi2O3doped ZnO [1], Sb2O3-doped ZnO [2] and Al2O3-doped ZnO [3]. The electrical properties of ZnO varistors are obviously related to the composition and microstructure, such as grain size, density, morphology and the

1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.09.023

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distribution of second phases. As a consequence, the nonohmic properties of these materials are strongly dependent on the fabrication process, which in turn regulates the defect chemistry in the depletion region and in the vicinity of the grain boundary. It is commonly accepted that the microwave sintering process can densify ceramic materials in a very short time and mostly at lower temperatures than conventional sintering methods. Several authors have reported the non-thermal effects induced by the microwave field (E, H) [4], acting as an additional driving force (electromigration, ponderomotive force etc.) for diffusion mechanisms. In 1994, Cherradi et al. [5] were the first to notice that the H microwave field can be used for heating semiconducting materials in a single-mode microwave cavity. The basic experimental setup allowed them to correlate the distribution of the E and H fields inside the cavity with the temperature distribution through a CuO bar, this latter being microwave-heated in either an H field or an E field. This H field component has been successfully used when using a specific assembly for heating up semiconducting materials (mainly oxides). Subsequently, a systematic investigation of the heating profile of various materials in E and H field separation was carried out at Penn State University [6,7] in a single-mode microwave cavity at 2.45 GHz. As stated in one of our previous works based on the sintering behavior of pure ZnO in both E and H fields, the material exhibited denser and more homogeneous microstructure during the sintering in the microwave magnetic (H) field [8]. This study, which had not been carried out previously, was also applied for the investigation of the sintering behavior of ZnO with several dopants added. Indeed, one could expect the specificities of the electric and magnetic field during the microwave selective sintering of ZnO-based compositions. Due to the high absorption of electromagnetic power and the conductive nature of various dopants in ZnO varistor compositions such as Bi2O3 [9], Co3O4 etc., it is expected that the heating will be very high when sintered in a microwave magnetic field. We report the effect of the sintering temperature and soaking time on the microstructure of ZnO-based ceramics heated in microwave E and H fields. The correlation between the microstructure properties and their J–E properties has been also established. For each heating mode, different sintering cycles were used and microstructure and electric characterizations have been carried out. The electrical properties related to the processing conditions are presented and discussed according to the heating conditions. 2. Experimental procedure 2.1. Powder synthesis, characterization and shaping High-purity nano-ZnO powder (Nanogard, 99.99% purity, 60 nm mean particle size) and pure grade dopants in the proportion of 98 mol.% ZnO, 0.5 mol.% Bi2O3,

0.5 mol.% Sb2O3, 0.5 mol.% Co3O4 and 0.5 mol.% MnO2 were used for the preparation of ZnO-based varistors. The mixture was ball-milled in an agate mortar using 5 mm diameter zirconia balls in ethanol media. After drying the slurry at 100 °C for 24 h, an organic binder (Rhodoviol 4%, Prolabo) was introduced to the dry mixture to ensure a good cohesion between particles. The latter was thereafter dried under an infrared lamp in order to evacuate the water content of the binder solution. The shaping of the samples was done by uniaxial pressing at 110 MPa using a cylindrical steel die with a diameter of 8 mm, followed by cold isostatic pressing at 300 MPa. The pellets were then calcined in air at 650 °C for 2 h in order to enhance powder reactivity. The crystalline phases were identified by X-ray diffraction (XRD) using Cu Ka radiation (Phillips X’Pert diffractometer). For the microstructural observations, scanning electron microscopy (SEM) observations were made on 1 lm polished and H3PO4 etched sample surfaces. Bulk densities of the samples were determined using their weights and dimensions. All samples held a green density of 66% of the theoretical density (TD) and weighed 0.5 g. Based on the accounted absolute errors on the samples dimensions and weights, the overall percentage error on the final densities of the samples was estimated to be ±0.5% of the calculated data. Grain size measurements were carried out on the micrographs of the etched samples using the following equation: G ¼ 1:56L

ð1Þ

where G is the average grain size and L is the average grain boundary intercept length of nine random lines on two different micrographs of each sample. Each line accounted for 30 grain interceptions. The percentage error on the grain size values was estimated to be ±5% of the calculated data. 2.2. Microwave furnace The microwave equipment used in this study consists of a 2 kW, 2.45 GHz magnetron source (Sairem GMP20KSM), a water-cooled aluminum circulator, a rectangular twist and a waveguide (WR340) that conducts the microwaves to a tailor-made TE10p single mode cavity with ports for vacuum and gas feed. The operating modes used in the cavity are TE103 (maximum E field region in the center of the cavity where the H field is minimum) and TE102 (maximum H field region in the center of the cavity where the E field is minimum), and were adjusted by tuning the length between the coupling iris and the short circuit piston from both side ends (Fig. 1). The sample was placed in an alumina-silicate box Fiberfax (DURABOARD), which is microwave transparent and ensures a homogeneous thermal insulation of the sample by reducing the thermal radiation from the sample surface (Fig. 1b). The box and sample were placed in the middle of the cavity and the cavity length was tuned to either E or H field mode. An infrared pyrometer was placed above the sample and allowed precise optical temperature

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Fig. 1. Schematic representation of the varistor sample placed in the microwave cavity under microwave radiation.

measurements to be done within the range 348–2500 °C. The heating cycles were interfaced on a computer, which allowed an estimation of the heating rate and reproducibility of the heating cycles. 2.3. Microwave sintering As reported previously [10], the use of microwaves allows the transfer of energy directly into the materials, where it is converted to heat through various absorption mechanisms such as ionic conduction, dipole relaxation, photon–phonon interaction and the Joule effect. Thus, microwave-sintering techniques mostly allow a volumetric heating, providing high heating rates, markedly optimizing the processing conditions as compared to the conventional sintering routes. The microwave sintering was performed in air using a 250 W incident power. Four different temperatures were chosen (900, 1000, 1100, 1200 °C) with four dwell times (5, 30, 60 and 120 min) at each sintering temperature, and the heating ramp was 250 °C min1 for each heating cycle. During the sintering process, the incident power was fixed at 250 W, and in order to increase the temperature inside the sample, the reflected power was adjusted by tuning a stub placed prior to the cavity. In order to compare and analyze the different likely peculiar effects during microwave sintering of ZnO-based varistors, the samples were respectively placed in configurations of maximum electric and magnetic fields distributions at the center of the cavity, corresponding to microwave electric (E) and magnetic (H) sintering modes. During the dwelling step at each sintering temperature, the maximum stable temperature of the sample surface was measured by an infrared pyrometer (Modline 5 model).

2.4. Characterizations of the sintered samples The microstructures of the microwave E and H field sintered samples were examined by SEM (Supra 55, Zeiss). In order to quantify the different phases present in ZnO grains and at the grain boundary regions, a local chemical composition was investigated using energy dispersive spectroscopy (EDS) coupled with SEM, as the microscope is equipped with the EDAX Genesis system. Electric field–current density (E–J) experiments of the sintered pellets were measured at room temperature using a Keithley 237 unit. Before the measurements, the samples were coated with a silver paste at both faces. Ohmic contact was ensured after a heat treatment at 750 °C only for 30 min in order to prevent significant microstructural changes that might occur. The breakdown voltage (EB) was determined for a current density of 1 mA cm2, and the leakage current density (JL) was estimated at 0.8EB. The non-linear coefficient was calculated using the equation:   ðlog J 2  log J 1 Þ a¼ ð2Þ ðlog E2  log E1 Þ with J2 = 10 mA cm2 and J1 = 1 mA cm2. 3. Results and discussions 3.1. Powder synthesis and phase composition of the sintered samples After the ball-milling and calcination processes, ZnObased powder particles are nanosized with a near spherical shape (Fig. 2).

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The average grain diameter (d) value of the powder particles was estimated from the SEM image measurements, and was found to be 65 nm. Then an estimated value of the specific surface area (S) was deduced from d¼

6000 ; Sq

S ¼ 17 m2 g1

process did not change the main phases in our ZnO-based varistor compositions. 3.2. Microwave sintering of ZnO varistors in electric (E) and magnetic (H) fields

ð3Þ

using the theoretical density of the main compound (qZnO = 5.66 g cm3), and assuming spherical and monodispersed powder. The XRD patterns of the samples, previously polished and sintered in different conditions, are shown in Fig. 3. In the XRD diagram are shown the patterns of samples sintered at 1100 °C for a 5 min soaking time. These thermal conditions were estimated to be the optimal ones in terms of densification of the sintered samples. Therefore the properties of microwave E and H field processed samples using the above-mentioned heating cycle will be mainly discussed. Moreover, a set of samples sintered in a conventional furnace were compared in terms of phase composition with the ones heated in both microwave E and H fields, using identical thermal conditions. The XRD patterns of the microwave E and H field sintered samples at 1100 °C revealed crystal structures similar to the conventionally sintered ones. The identified phases were hexagonal ZnO (PDF: 01-089-0511, main phase); Bi2O3 (PDF: 00-002-0988) present for samples calcined at 650 °C; pyrochlore phase (Bi3Zn2Sb3O14), also known as Bi-rich phase, which resulted from the reactivity of the Bi2O3 phase with ZnO and Sb2O3 above 700 °C; and a spinel type phase Zn7Sb2O12 that resulted from the decomposition of the pyrochlore phase at temperatures higher than 1000 °C. As a result, it can be seen that the microwave

The weight losses of ZnO varistors sintered in the range 900–1200 °C for 5–120 min were plotted (Fig. 4). The figure shows that the weight loss evolution with the temperature follows a near linear trend. The weight loss increased with the sintering temperature and soaking time as expected. This is likely due to the evaporation of bismuth for temperatures above 1000 °C, as reported in Ref. [11]. Furthermore, higher sintering temperatures and longer processing time led to a further evaporation of the bismuth phase, as well as some other compounds such as cobalt and antimony. Density measurements were performed on samples sintered in both microwave electric and magnetic fields (Fig. 5a and b). Fig. 5 shows that for both E and H field sintering modes, the maximum density of the samples are obtained at 1100 °C for a 5 min soaking time (98.3 and 99.2 ± 0.5% of the theoretical density respectively). As reported in Ref. [12], the Bi-rich phase that appears at 850 °C homogeneously wets ZnO grains, thereby promoting the densification rate of ZnO varistors by accelerating mass transport, through liquid phase assisted diffusion. Moreover, samples sintered in a microwave H field exhibited slightly higher final densities and larger grain size (Fig. 6b). As mentioned in one of our previous works [8], in semiconductors such as ZnO-based ceramics, it is understood that the H field induces current loops which are responsible for the heating up of the samples. The interaction of the current loops with the H field could give rise to

Fig. 2. SEM image of the varistor powder synthesized by the solid state route.

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Fig. 3. XRD pattern of (a) pure ZnO raw powder, (b) calcined varistor powder, (c) conventionally sintered varistor samples, (d) microwave E field sintered varistor samples, (e) microwave H field sintered varistor samples.

Fig. 4. Weight loss of microwave sintered samples at different temperatures for 5–120 min holding times.

radial Laplace forces applied onto the granular compact, which in turn would lead to a better contact between particles and enhance grain growth and densification. This would also create a particle rearrangement in the material, consequently increasing the kinetics of densification and grain growth. For temperatures above 1100 °C, the weight loss increased and the final relative densities of the samples slightly decreased to 97.7 and 98.7 ± 0.5% TD, respectively

for E and H field heated samples. As reported in the work of Mazaheri [13], this could be related to the antidensification phenomenon that occurs at high temperatures due to the partial evaporation of some dopants and grain growth. The antidensification phenomenon was also observed when the samples were subjected to longer soaking times. As shown in Fig. 5a and b, for soaking times ranging from 5 to 30 min, the relative density of the sintered samples increased when the temperature increased from 900 °C to 1000 °C. Then a decrease of the latter was afterwards observed for temperatures above 1000 °C. For longer soaking times, the important evaporation of bismuth is likely to be responsible for the decrease of the relative density at higher sintering temperatures. An abnormal grain growth was also observed when the sintering temperature was above 1100 °C and for soaking times longer than 60 min, as marked by the extremely higher grain size shown in Fig. 6. As reported in Ref. [9], grain growth and coarsening through coalescence play a major role in the densification process of ZnO varistors. However, when the sintering temperature reached 1200 °C, the grain growth had little effect on the densification since most of the dopants present within the composition began to evaporate, producing larger porosity. In this section, the grain growth kinetics will be discussed in detail.

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Fig. 5. Relative densities of microwave (a) E field, (b) B field sintered ZnO varistors at various temperatures for 5–120 min soaking times.

Fig. 6. Sintering paths for microwave (a) E field, (b) H field sintered ZnO varistors for various soaking times at different temperatures.

3.3. Microstructure characterization and grain growth kinetics

mobility of grain boundaries caused the entrapment of isolated pores and led to larger isolated intergranular pores, as shown in Fig. 7C. Furthermore, the presence of many isolated larger pores present in samples sintered at 1200 °C resulted mostly from the evaporation of the Bi-rich phase along ZnO grains (Fig. 7D). The grain growth kinetics can be determined using the simplified phenomenological kinetics (Eq. (4)). The grain growth exponent (n) can be found at isothermal conditions following the kinetic equation:

Fig. 7 exhibits SEM micrographs of polished and etched varistor samples sintered at 900–1200 °C. It is to be noted that for all sintering temperatures, white spots are seen in SEM photos around ZnO grain boundary regions, corresponding to the Bi-rich (pyrochlore) phase. As the temperature was raised to 1100 °C, this liquid phase spread uniformly throughout the grain boundaries, and the presence of few round shaped small white spots (Fig. 7C) throughout ZnO grains marked the formation of the spinel phase, resulting from the partial decomposition of the pyrochlore phase. For samples sintered at 1200 °C, the quantity of pyrochlore phase present between ZnO grains (Fig. 7D) is less than the one present in the samples sintered at 1100 °C (Fig. 7C). Regarding the porosity, samples sintered at 900 °C presented small isolated pores and few white spots in ZnO grain boundary regions that marked the beginning of liquid Bi-rich phase formation. The presence of a mass transport mechanism through the liquid phase at 1100 °C and higher

n log G ¼ log t þ ½log K 0  0:434ðQ=RT Þ

ð4Þ

where n is the grain growth exponent, t is the sintering time, K0 is a constant, Q is the sintering activation energy, R is the universal gas constant and T is the sintering temperature. This law is valid for ceramic materials holding relative densities above 95%. The n value can be calculated from the slope (1/n) of the log (grain size) vs. log (time) plot. Plots were made for isothermal conditions at each sintering temperatures and the (n) values were calculated from the slopes of the plots

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Fig. 7. SEM micrographs of microwave H field sintered ZnO varistors for a 5 min soaking time at (A) 900 °C, (B) 1000 °C, (C) 1100 °C, and (D) 1200 °C.

constructed. Lin and co-workers have reported the n values for microwave and conventionally sintered ZnO varistors to be 3 and 4, respectively [14]. Gunay et al. have estimated the n value for the ZnO–Sb2O3–CoO system to be 5.2 [12]. They also indicated that the n value in a system was associated with a grain-growth inhibition mechanism criterion. Fig. 8 shows that the n factor decreased mostly with the increase of the sintering temperature for both microwave E and H field sintered samples. The n value for the E field sintered ZnO varistors was found to be 3, which is consistent with a volume diffusion mechanism that takes place during the final stage of the sintering process [14]. This could be explained by the penetration depth of the electric field being within the centimeter range, thus promoting an increment of volume diffusion mechanisms within the ceramic material. On the other hand, the higher n value for microwave H field sintered samples suggests a major contribution of volume diffusion mechanisms with a non-negligible contribution of a surface diffusion mechanism (where the n factor is known to be above 3), which is consistent with the H field penetration depth being within the micrometer range. As the H field would be mainly concentrated on the surface and between ZnO grains, it would promote grain boundary diffusion mechanisms. If Eq. (4) is expressed in the form: n

logðG =tÞ ¼ log K 0  0:434ðQ=RT Þ

ð5Þ

The apparent activation energy (Q) of a grain growth process can be calculated from the Arrhenius plot of log (Gn/t) vs. 1/T (K1). Such plots have been constructed for the microwave E and H field systems, by taking into account the average values of n calculated for each system (Fig. 9). The apparent activation energy of 206 kJ mol1 has been determined for microwave E field sintered samples, which is slightly lower than the one estimated during microwave H field sintering (214 kJ mol1). Lin and coauthors reported the activation energies of 225 kJ mol1 and 363 kJ mol1, respectively, for microwave and conventional sintering systems. The estimated values of the activation energies are in good agreement with the data found in the literature. We can assume that the lower values of n, as well as the lower apparent activation energies obtained through microwave sintering of ZnO varistors, compared to the ones obtained through conventional sintering, are consistent with the often-reported theory of microwave non-thermal effect [7] induced during the microwave sintering. 3.4. Electrical properties Fig. 10 shows the J–E characteristics of the microwave E and H field sintered varistors. As shown in Fig. 10, at low voltage, the current passing through the ceramic is low. As

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Fig. 8. Isothermal grain growth of microwave (a) E field, (b) H field sintered ZnO varistors.

a result, the J–E response is mainly governed by the resistivity of the intergranular or grain boundary phase, which is mainly composed of the pyrochlore (bismuth-rich) phase [15]. For high field regions, the electrical response is mainly governed by the ZnO grains’ resistivity. It can be observed from Fig. 10 that almost all microwave E and H field sintered samples indicate a sharp transition from the low current zone to the non-linear region. Almost all microwave sintered samples exhibit non-linear coefficient values (a) higher than 20, thus presenting varistor type behavior (Table 1). It can be seen from Table 1 that the optimal electrical properties (high a, low Jl, high EB) for both microwave E and H field sintered varistor samples were obtained for a 5 min soaking time at 1100 °C. These results are consistent with the highest values of the relative density obtained for such a heating cycle. For both heating modes (E or H field) and for this optimized thermal cycle, the leakage current is quite low (4–5 lA cm2), which accounts for a high grain boundary resistivity [15], and this is in adequacy with the homogeneous phase distribution obtained within the samples, and previously shown on the microstructures

Fig. 9. Arrhenius plots for the grain growth of microwave (a) E field, (b) H field sintered ZnO varistors.

Fig. 10. J–E characteristics of the sintered varistors for a 5 min soaking time at different temperatures.

(Fig. 7c). Considering the slightly larger grain size (7.2 ± 0.36 lm) of samples sintered in the H field, than the ones sintered in E field (6.6 ± 0.33 lm) (Fig. 6), it is expected a more pronounced decrease of the breakdown

A. Badev et al. / Acta Materialia 61 (2013) 7849–7858 Table 1 Electrical properties of the sintered varistors at different temperatures and for a 5 min soaking time. Tx (°C)

a ± 0.2

Eb ± 2 (V mm1)

Jl ± 0.005 (mA cm2)

Microwave (E field) sintered varistors (5 min at Tx) 900 29.6 376 0.005 1000 29.53 361 0.0068 1100 29.08 503 0.0041 1200 20.87 172 0.07 Microwave (H field) sintered varistors (5 min at Tx) 900 28.13 625 0.0015 1000 37.66 581 0.007 1100 39.05 575.6 0.005 1200 38.9 550 0.0065 The values in bold are the optimal electrical parameters at the optimal sintering conditions.

voltage values for microwave H field processed samples than for those processed in E field. Actually the opposite was observed: for all thermal cycles investigated, the samples sintered in an H field systematically exhibited a higher breakdown voltage values compared to the samples sintered in E field (Table 1). In our previous paper [11], without distinguishing the E and H field mode, and using a ZnO-based susceptor, it was mentioned that the volumetric heating distribution provided by microwaves, promoted the reactivity of dopants with the ZnO matrix, particularly within the grain boundary regions. As a consequence, the properties of the electrostatic barrier were improved, resulting in better non-linear properties as compared with conventional heated samples. In the present work, no susceptor was used (hence it is direct microwave heating) and E and H field modes are compared regarding the microstructure and the non-linear properties of ZnO varistor type component. Although the grain size is bigger for

Fig. 11. Schematic representation of the microwave interaction with the different phases present in ZnO varistors.

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samples sintered in the H field than for those sintered in E field, the breakdown voltage is also significantly higher. This strongly suggests that those samples have an electrostatic barrier higher than the ones sintered in the E field. This is understood by an improved reactivity between dopants (especially Bi2O3, which is highly semiconducting) and ZnO matrix within the grain boundary region. It is believed that in H field mode, the intergranular region, mostly composed of semiconducting type materials (Bi2O3, CoO, MnO etc.), preferentially couples with microwaves, resulting in locally enhanced reactivity. In E field mode, the ZnO matrix, which is mainly insulating, preferentially couples with microwaves, resulting in a lower reactivity within the grain boundary region, where the semiconducting phases are localized. These two distinct behaviors could explain to a large extent the differences observed in breakdown voltages as a function of the E or H field sintering mode. Fig. 11 shows the proposed interactions of the field with the material regarding the abovementioned discussion. 4. Conclusion A study of the densification behavior and grain growth was carried out on varistor samples composed of 98 mol.% ZnO–2 mol.% additives (Bi2O3, Sb2O3, Co3O4, MnO2) using a microwave radiation in E and H field configuration modes. The highest density values, non-linear coefficient and breakdown voltage values were obtained for microwave E and H field sintered samples at 1100 °C for a 5 min soaking time. The lower values of the grain growth exponent (n) and activation energy (Q) estimated during the microwave E field sintering process were attributed to a volume diffusion mechanism, and the higher (n) and (Q) value related to the microwave H field sintering were correlated mainly to a volume diffusion process, with a non-negligible contribution of a surface diffusion. Moreover, the contribution of the magnetic (H) field on the microstructure resulted in enhanced electrical properties of the sintered samples. Following the optimal sintering conditions (1100 °C, 5 min soaking time), samples sintered in a magnetic H field showed slightly higher densities (D = 99.2 ± 0.5% TD) and bigger grains (G = 7.2 ± 0.36 lm) than the ones sintered in electric (E) field (D = 98.3 ± 0.5% TD, G = 6.6 ± 0.33 lm). Regarding the electrical parameters, breakdown voltage values as high as 503–570 V mm1 were obtained together with high non-linear coefficients a = 29–39 and low leakage currents (Jl  5  103 mA cm2), respectively for microwave E and H field heated varistor samples. These values are higher (a = 39, Eb = 570 V mm1) for samples sintered in the H field compared to the ones sintered in the E field (a = 29, Eb = 503 V mm1), which is likely due to an improved coupling between the H field and the different dopants within the grain boundaries region, being of a semiconductive nature. Consequently, the dopants–matrix

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reactivity has been enhanced, together with the properties of the electrostatic barrier. At last but not least, the ability to uncouple the microwave electric field from the magnetic one allowed us to tailor the electrical properties of such functional ceramics, which is a major breakthrough in the microwave sintering process. Acknowledgements 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. Je´roˆme Lecourt and Mr. Jean-Francßois Lefebre from CRISMAT Laboratory, UMR 6508 CNRS-ENSICAEN, for their help and enthusiasm during the experimental parts.

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