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

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.


Introduction
ZnO-Bi 2 O 3 ceramics are an established class of ZnO-based varistors. It is well known that the nonlinear current-voltage (I-V) characteristics of these ZnO-Bi 2 O 3 varistors are directly dependent on their microstructures, mainly the average grain size and size distribution of ZnO [1]. Typically, a large average grain size of >30 µm is required for low-voltage applications, and a small average grain size of <10 µm is needed for high-voltage applications [2]. Additionally, a narrow grain size distribution is also critical for achieving stability of the electrical field strength of these materials [3].
It has been found that ZnO-V 2 O 5 ceramics also exhibit a nonlinear I-V behaviour comparable to ZnO-Bi 2 O 3 ceramics [4][5][6] and, thus, have the potential to be a new class of varistors [7]. One of the significant advantages of ZnO-V 2 O 5 ceramics is that they can be sintered at a relatively low temperature of~900 • C [8][9][10][11]. This outstanding feature allows these ceramics to be co-fired with Ag (m.p. 961 • C). This enables Ag, instead of expensive Pd or Pt, to be used as inner electrodes for applications in multilayer chip components [12]. Moreover, V 2 O 5 is also a better sintering aid, compared with Bi 2 O 3 for ZnO, enabling ZnO-V 2 O 5 based ceramics to be densified to the same density at a lower temperature than ZnO-Bi 2 O 3 based ceramics [13][14][15][16].
However, ZnO-V 2 O 5 based ceramics suffer from a distinct disadvantage in that they have been shown to exhibit abnormal ZnO grain growth [17]. This is commonly attributed to the high reactivity of the V 2 O 5 -rich liquid phase formed during sintering. This phase assists in the diffusion of Zn 2+ and thus promotes ZnO grain growth, which results in the formation of abnormally grown grains (AGG) of ZnO.
One approach to curb the formation of AGG in ZnO-V 2 O 5 ceramics is to induce the formation of a secondary phase to hinder ZnO grain growth by the addition of a third oxide to the system [18,19]. The two commonly used additives are Cr 2 [20][21][22]. However, these two additives have their own specific limitations. With Cr 2 O 3 , an addition greater than 1 mol% is required to effectively counter the AGG and refine the ZnO grain size in ZnO-V 2 O 5 ceramics. However, at this required addition level, Cr 2 O 3 has also been found to segregate at grain-boundary regions, leading to the deterioration of I-V behaviour due to high leakage current. With Sb 2 O 3 , the addition of up to 2 mol% is needed to control the grain growth of ZnO-V 2 O 5 ; however, the addition of Sb 2 O 3 increases the required sintering temperature for the system. At an Sb 2 O 3 content greater than 0.5 mol%, the sintering temperature could increase to 1200 • C [23], nullifying the advantage of having the ZnO-V 2 O 5 -based system in the first place.
This study investigated the effect of increased additions of PrMnO 3 on the grain growth of ZnO-V 2 O 5 ceramics, with the aim to control the average grain size and size distribution of ZnO to improve their electrical properties as practical varistors.

Sample Preparation
All reagents used in this research were of analytical grade (>99.5% purity) and were supplied by SINOPHARM. PrMnO 3 powder was prepared in-house by mixing Pr 6 O 11 and MnCO 3 at a molar ratio of 1:6 and allowing the mixture to react at 1000 • C for 5 h in air. The formation of the PrMnO 3 phase was confirmed by X-ray powder diffraction (XRD) analysis. Table 1 summarises the nominal compositions of constituent powders used to prepare four ZnO-V 2 O 5 ceramic samples and their designated sample names. The PrMnO 3 powder was introduced with the aim to regulate the grain growth in these ceramics. For each nominal composition, the powder mixture was homogenised by ball milling in absolute alcohol using zirconia balls in a polypropylene container on a planetary mill for 24 h. After milling, the powder mixture was dried at 80 • C for 24 h. The dried powder mixture was then mixed with a 5 wt% polyvinyl alcohol (PVA) binder and pressed into pellets of Φ12 mm × 1 mm under a uniaxial pressure of 130 MPa. The green pellets were first fired at 500 • C for 1 h to remove the binder before being subjected to different sintering temperature/time regimes in an alumina crucible at a heating rate of 4 • C/min. In this study, seven sintered ceramic discs for each nominal composition were produced at 850 • C, 900 • C, and 925 • C for 4 h and at 875 • C for 2, 4, 6, and 8 h. After sintering, the furnace was powered off to allow the sintered discs to cool down naturally within the furnace.

Sample Characterisation
The phase compositions of the sintered and quenched samples were analysed by XRD patterns obtained using a diffractometer (PANalytical X'pert Powder) with Cu K α1 radiation (λ = 0.1541 nm).
The microstructure of the sintered samples was observed and analysed by scanning electron microscopy (SEM) using a ZEISS Supra 55 electron microscope. A polished crosssectional area along the thickness of the sintered disc was prepared and chemically etched in a dilute HCl solution to reveal the grain and phase structures for SEM observation. The average ZnO grain size and grain size distribution were analysed using a representative SEM image of each sintered sample.
To determine the average grain size, the method reported by Mendelson [24] was used. Basically, the average grain-boundary intercept length (L) was first determined by measuring along 10 randomly drawn lines across the SEM image. The average grain size G is then calculated by Equation (1) as follows: To determine the ZnO grain size distribution, the dimensions around 500-800 ZnO grains of each sample were measured from its SEM images using a dedicated image analysis software. The surface area (S) of each grain was then estimated, and the equivalent diameter (d) of each grain was obtained by transforming the surface area of the irregularly shaped grain into a circle of the same area using the method reported by Daneu [25].
The grain growth behaviour was analysed using a phenomenological kinetic grain growth equation established by Nicholson et al., which has been widely used for ZnO-based ceramic systems. The equation is expressed as follows: where G is the average grain size of the ZnO ceramic at time t, G 0 is the initial grain size of the ZnO powder, n is the kinetic grain growth exponent, Q is the apparent activation energy, R is the universal gas constant, T is the absolute temperature, and K 0 is a Tindependent constant. In the case where G 0 is significantly smaller than G, then Equation (2) is simplified to Equation (3) was used throughout this study to analyse the sintering behaviour of the ZnO-V 2 O 5 samples studied in this study.

Sample I-V Characteristics
The DC current-voltage (I-V) behaviour of the sintered samples was measured using a withstand voltage tester (MS2671A, Xi'an Instruments Inc., Xi'an, China). The sintered discs (~Φ8 mm × 1 mm thickness) were painted with a silver paste on both surfaces to enable the measurements. The I-V curve was determined by measuring the current at a stepwise-increased applied voltage until I reached 10 mA. The field strength (E) is defined as the applied voltage per sample thickness, and the current density (J) is the measured current per sample area. The switching field strength E 1mA/cm 2 is the field strength at J = 1 mA·cm −2 . The nonlinear coefficient (α) is calculated by Equation (4) as where E 10mA·cm −2 and E 1mA·cm −2 are the field strengths at 10 Ma·cm −2 and 1 mA·cm −2 , respectively. The higher the value of α is, the better it is for a varistor. Figure 1 shows the XRD patterns of the four ZVCP samples sintered at 875 • C for 4 h. For the ZVC sample, in addition to the main ZnO phase, several minor phases were also detected, including Zn 4 V 2 O 9 , α-Zn 3 (VO 4 ) 2 , and ZnCr 2 O 4 . These minor phases have been commonly observed and reported in sintered ZnO-V 2 O 5 systems. With the addition of PrMnO 3 , a new minor phase of PrVO 4 was also observed in the ZVCP25, ZVCP50, and ZVCP75 samples. Consequently, this experimental result stated that the generation of PrVO 4 could be attributed to the reaction between PrMnO 3 and the V-contained phase. According to previous research studies, the formation of PrVO 4 was reported as a product of phase transition between PrMnO 3 and α-Zn 3 (VO 4 ) 2 with the following reaction:

Phase Compositions and Distributions in Sintered Samples
According to previous research studies, the formation of PrVO4 was reported as a product of phase transition between PrMnO3 and α-Zn3(VO4)2 with the following reaction: ( ) It should be noted that the mentioned phase transition did not produce only the PrVO4 phase. Additionally, the formation of the Mn2O3 phase was obtained in the same reaction. However, few lines of evidence indicating Mn2O3 in this phase observation can be found. As we know, Mn2O3 was helpful to generate the non-Ohm electrical property in ZnO varistor ceramic. Hence, the application of Mn2O3 was widely investigated by researchers. Related experimental results supported that Mn ion could easily be dissolved in ZnO grain. Thus, once the solid solution of ZnO-Mn2O3 formed, the diffraction peaks assigned to Mn2O3 vanished.  Figure 2 shows a typical microstructure of a sintered ZVCP50 sample. As expected, the dominant morphology included grains of the ZnO phase, and the focus here was on the appearance and distribution of the minor phases. As seen in the micrograph, in addition to ZnCr2O4 (ZC) clusters (circled in yellow-dashed lines), PrVO4 (PV) grains (circled in white-dashed lines) were clearly seen to also distribute at the junctions of ZnO grains. In some cases, ZnO grains were observed to grow around either ZnCr2O4 clusters or PrVO4 grains. Figure 3 shows the TEM images, revealed the microstructural differences between PrVO4 and ZnCr2O4 phases. The electron diffraction pattern in Figure 3 further confirmed It should be noted that the mentioned phase transition did not produce only the PrVO 4 phase. Additionally, the formation of the Mn 2 O 3 phase was obtained in the same reaction. However, few lines of evidence indicating Mn 2 O 3 in this phase observation can be found. As we know, Mn 2 O 3 was helpful to generate the non-Ohm electrical property in ZnO varistor ceramic. Hence, the application of Mn 2 O 3 was widely investigated by researchers. Related experimental results supported that Mn ion could easily be dissolved in ZnO grain. Thus, once the solid solution of ZnO-Mn 2 O 3 formed, the diffraction peaks assigned to Mn 2 O 3 vanished. Figure 2 shows a typical microstructure of a sintered ZVCP50 sample. As expected, the dominant morphology included grains of the ZnO phase, and the focus here was on the appearance and distribution of the minor phases. As seen in the micrograph, in addition to ZnCr 2 O 4 (ZC) clusters (circled in yellow-dashed lines), PrVO 4 (PV) grains (circled in white-dashed lines) were clearly seen to also distribute at the junctions of ZnO grains. In some cases, ZnO grains were observed to grow around either ZnCr 2 O 4 clusters or PrVO 4 grains. Figure 3 shows the TEM images, revealed the microstructural differences between PrVO 4 and ZnCr 2 O 4 phases. The electron diffraction pattern in Figure 3 further confirmed the existence of PrVO 4 and ZnCr 2 O 4 . The ZnCr 2 O 4 grains are very small-the grain size is below 0.2 µm-but the PrVO 4 grains have a relatively larger size than ZnCr 2 O 4 , ranging from 0.5 µm to 4 µm. Most grains of ZnCr 2 O 4 and PrVO 4 were found located at the ZnO grain boundaries, and ZnCr 2 O 4 grains tend to aggregate to form a heap of~1 µm in size. This observation suggests that these secondary minor phases could exert a 'pinning' effect, which hindered the migration of ZnO grain boundaries and limited its grain growth. the existence of PrVO4 and ZnCr2O4. The ZnCr2O4 grains are very small-the grain size is below 0.2 μm-but the PrVO4 grains have a relatively larger size than ZnCr2O4, ranging from 0.5 μm to 4 μm. Most grains of ZnCr2O4 and PrVO4 were found located at the ZnO grain boundaries, and ZnCr2O4 grains tend to aggregate to form a heap of ~1 μm in size. This observation suggests that these secondary minor phases could exert a 'pinning' effect, which hindered the migration of ZnO grain boundaries and limited its grain growth.

Average ZnO Grain Sizes in Sintered Samples
For all four sintered samples, the average grain size of each sample under all sintering conditions was determined following the procedure described in Section 2.2, and the results are summarised in Table 2.

Average ZnO Grain Sizes in Sintered Samples
For all four sintered samples, the average grain size of each sample under all sintering conditions was determined following the procedure described in Section 2.2, and the results are summarised in Table 2.

Kinetic Grain Growth Parameters
To reveal the effect of PrMnO 3 addition on the sintering behaviour of the ZnO-V 2 O 5 ceramics, the isothermal grain growth behaviour at 875 • C was first analysed using a rearranged Equation (3), as shown below: Figure 4 illustrates log G vs. log t curves, which show the isothermal grain growth behaviour of the four samples sintered at 875 • C for 2-8 h. A linear fit of the curves enabled the determination of the kinetic grain growth exponent n from the slope of the linear fit for each sample. The n values so determined are also shown in Figure 4. Notably, with the increased addition of PrMnO 3 , in addition to the decrease in the average grain size G, the grain growth rate also decreased, as indicated by the increased n values.

Kinetic Grain Growth Parameters
To reveal the effect of PrMnO3 addition on the sintering behaviour of the ZnO-V2O5 ceramics, the isothermal grain growth behaviour at 875 °C was first analysed using a rearranged Equation (3), as shown below: Figure 4 illustrates log vs. log curves, which show the isothermal grain growth behaviour of the four samples sintered at 875 °C for 2-8 h. A linear fit of the curves enabled the determination of the kinetic grain growth exponent from the slope of the linear fit for each sample. The values so determined are also shown in Figure 4. Notably, with the increased addition of PrMnO3, in addition to the decrease in the average grain size , the grain growth rate also decreased, as indicated by the increased values. To determine the apparent activation energy Q associated with the grain growth, Equation (3) is rearranged as To determine the apparent activation energy Q associated with the grain growth, Equation (3) is rearranged as Q can then be calculated from the slope of the Arrhenius plot of log(G n /t) vs. (1/T). Figure 5 shows the Arrhenius plots of the four samples from the G values obtained at different sintering temperatures shown in Table 2. The apparent activation energy Q values determined from Figure 5 are summarised in Table 3, together with the n values determined from Figure 4. Q can then be calculated from the slope of the Arrhenius plot of log(G n /t) vs. (1/T). Figure 5 shows the Arrhenius plots of the four samples from the G values obtained at different sintering temperatures shown in Table 2. The apparent activation energy values determined from Figure 5 are summarised in Table 3, together with the values determined from Figure 4.  As seen in Table 3, in both this study and other published studies, the basic ZnO-V2O5 binary systems had a kinetic grain growth exponent value spanning 1.5-1.8. This value was lower than the = 3.0 observed for pure ZnO, indicating that the addition  As seen in Table 3, in both this study and other published studies, the basic ZnO-V 2 O 5 binary systems had a kinetic grain growth exponent n value spanning 1.5-1.8. This n value was lower than the n = 3.0 observed for pure ZnO, indicating that the addition of The addition of a third component-e.g., the 0.35 mol% of Cr 2 O 3 in this study or 0.5 mol% Sb 2 O 3 as reported elsewhere-has been shown to increase the n value to 2.9 and 4.0, respectively. This means that the addition of a third component could hinder ZnO grain growth in basic ZnO-V 2 O 5 binary systems. In both cases, this was attributed to the formation of spinel ZnCr 2 O 4 or a ZnSb 2 O 4 secondary phase, which exerts a pinning effect on ZnO grain boundaries and thus hinders ZnO grain growth.
It was clear that the addition of PrMnO 3 to the ZnO + V 2 O 5 (1 mol%) + Cr 2 O 3 (0.35 mol%) ceramics resulted in a further increase in the n value, which increased to 6.1 after a nominal addition of 0.75 mol% of PrMnO 3 for ZVCP75, demonstrating the high effectiveness of PrMnO 3 in suppressing ZnO grain growth. As seen in the XRD and SEM analyses, the addition of PrMnO 3 resulted in the formation of an additional spinel PrVO 4 phase, which was also found to be distributed at the grain boundaries of ZnO similarly to ZnCr 2 O 4 in the sintered ceramics. A similar pinning effect was thus believed to be the main reason for the hindered ZnO grain growth by the addition of PrMnO 3 .
As expected, it is clearly seen in Table 3 that the apparent activation energy values for the grain growth followed an inverse behaviour, compared with the n values. Generally, the higher the activation energy, the higher the n values, the slower the grain growth rate, and the smaller the average ZnO grain sizes. Figure 6 shows SEM images and grain size distribution histograms of the four samples sintered at 875 • C for 4 h. It is clear from the SEM images that with increased additions of PrMnO 3 , in addition to the decrease in the average grain size of ZnO, the size distribution also narrowed significantly.

Grain Size Distribution in Sintered Samples
It can be seen that the ZVC sample without the addition of PrMnO 3 exhibited a broad and bimodal grain size distribution. In the context of this research, larger grains are considered to be the AGG, and the relatively smaller grains are considered as the normally grown grains (NGG). The area fractions of AGG and NGG for each sample are also included in the size distribution histograms in the figure. The addition of PrMnO 3 resulted in the reduction and narrowing of the grain size range, in addition to diminishing the bimodal distribution. For ZVCP75, the grain size distribution became unimodal. Figure 7 shows the average grain sizes of the AGG and NGG vs. the nominal addition of PrMnO 3 . This revealed the impact of PrMnO 3 addition. Notably, the average grain size of NGG did not change much; however, the average grain size of the AGG decreased dramatically. This indicated the effectiveness of PrMnO 3 addition in suppressing the coarsening of ZnO grains in this system. The number fractions of AGG ( f AGG ) and NGG ( f NGG ) were also determined for the sintered samples and are provided in the histograms. Figure 8 shows the switching field strength E 1mA·cm −2 of the four samples sintered at 875 • C for 4 h. For each sample, 10 specimens were prepared, their I-V curves were measured, and their E 1mA·cm −2 values were determined. Figure 8 shows all measured E 1mA·cm −2 data for the four samples. As seen, the basic ZVC sample displayed a very broad range of E 1mA·cm −2 . Clearly, a 0.25 mol% nominal addition of PrMnO 3 resulted in a remarkable narrowing of the E 1mA·cm −2 range for the ZVCP25 sample. Further increasing the PrMnO 3 addition to 0.75 mol% produced a continuous narrowing of the E 1mA·cm −2 range. It can be seen that the ZVC sample without the addition of PrMnO3 exhibited a broad and bimodal grain size distribution. In the context of this research, larger grains are considered to be the AGG, and the relatively smaller grains are considered as the normally grown grains (NGG). The area fractions of AGG and NGG for each sample are also included in the size distribution histograms in the figure. The addition of PrMnO3 resulted in the reduction and narrowing of the grain size range, in addition to diminishing the bimodal distribution. For ZVCP75, the grain size distribution became unimodal. . NGG and f NGG refers to the normally grown grain and its number fraction. AGG and f AGG refer to the abnormally grown grain and its number fraction. Figure 7 shows the average grain sizes of the AGG and NGG vs. the nominal addition of PrMnO3. This revealed the impact of PrMnO3 addition. Notably, the average grain size of NGG did not change much; however, the average grain size of the AGG decreased dramatically. This indicated the effectiveness of PrMnO3 addition in suppressing the coarsening of ZnO grains in this system. The number fractions of AGG ( ) and NGG ( ) were also determined for the sintered samples and are provided in the histograms.  This significant stabilisation of the switching field strength was attributed to the uniformisation of the ZnO grain size as the result of PrMnO3 addition. As seen in Figure 6, a 0.75 mol% addition of PrMnO3 in ZVC effectively eliminated the formation of abnormally grown ZnO grains. The resulting ZVCP75 sample was shown to have a markedly improved stability in its switching field strength. This was also consistent with the results reported by Hng et al. [12], which showed that the presence of just a few abnormally grown ZnO grains could have a pronounced destabilisation effect on the switching field strength of ZnO-V2O5 varistors.   Table 4 summarises the relative density and number fraction of AGG and the average grain size of the sintered samples, together with the nonlinear coefficient and the average and range of the switching field strength of these samples.  This significant stabilisation of the switching field strength was attributed to the uniformisation of the ZnO grain size as the result of PrMnO 3 addition. As seen in Figure 6, a 0.75 mol% addition of PrMnO 3 in ZVC effectively eliminated the formation of abnormally grown ZnO grains. The resulting ZVCP75 sample was shown to have a markedly improved stability in its switching field strength. This was also consistent with the results reported by Hng et al. [12], which showed that the presence of just a few abnormally grown ZnO grains could have a pronounced destabilisation effect on the switching field strength of ZnO-V 2 O 5 varistors.

Electrical Behaviour of Sintered Samples
Furthermore, with a 0.75 mol% addition of PrMnO 3 , there was also a marked increase in the E 1mA·cm −2 value for the ZVCP75 sample. This increase in switch field strength was mostly due to the refined ZnO grain size. Table 4 summarises the relative density and number fraction of AGG and the average grain size of the sintered samples, together with the nonlinear coefficient and the average and range of the switching field strength of these samples. It was noted that all the samples possessed a high sintering density at close to 95% of the theoretical density of ZnO. While the addition of PrMnO 3 had little effect on the sintering density, it effectively reduced and eliminated the fraction of AGG and progressively refined the grain sizes of the resulting ceramics.
The electrical properties are attributed to the developed microstructures. The nonlinear coefficient also increased marginally from 7.2 to 8.9 from ZVC to ZVCP75, respectively, as seen in Table 4.
It was believed that a better sintering ability of ZnO ceramic can be achieved by adding V 2 O 5 addition. During the sintering process, the generated α-Zn 3 (VO 4 ) 2 existed in the form of a liquid phase. Thus, the sintering mechanism was governed by solutionprecipitation process. However, since the absolute homogeneous distribution of V 2 O 5 in raw material was hardly achieved, regions that were rich in V 2 O 5 produced more content of V-enriching liquid. In this case, the abnormal grain growth of ZnO begins in this liquid-phase-rich area, preferentially. To fix this problem, the traditional method needs the introduction of a secondary particle phase to hinder the migration of ZnO grain boundaries, such as the ZnCr 2 O 4 spinel phase. However, a single ZnCr 2 O 4 phase was insufficient to suppress all abnormally grown ZnO grains, as the distribution of the V-enriching liquid phase was still heterogeneous. Comparably, the PrVO 4 particle phase plays a double role in the whole sintering process. Firstly, the PrVO 4 particle hindered the migration of ZnO grain boundaries, similarly to ZnCr 2 O 4 . Additionally, the PrVO 4 particle phase actually was the product of the reaction between PrMnO 3 and α-Zn 3 (VO 4 ) 2 . In a way, the generation of PrVO 4 means the rearrangement of α-Zn 3 (VO 4 ) 2 in the ZnO matrix, which benefits the microstructural homogeneity. Moreover, the Mn 2 O 3 generated by the mentioned reaction could optimise electrical properties. This was the reason why nonlinear coefficients were slightly enhanced by adding PrMnO 3 , even though α-Zn 3 (VO 4 ) 2 was sequentially consumed by added PrMnO 3 . In summary, we recognised that PrMnO 3 addition can be treated as a potential additive in ZnO-V 2 O 5 -based varistor ceramic for its good performance on the repairmen of both grain growth control and electrical properties.

Conclusions
In this study, the effect of PrMnO 3 addition (0.25 to 0.75 mol%) on the grain growth behaviour of ZnO-V 2 O 5 (1 mol%)-Cr 2 O 3 (0.35 mol%) ceramics was systematically investigated during sintering from 850 • C to 925 • C, with the aim to refine the ZnO grain structure. The main findings were as follows: (1) The addition of PrMnO 3 uniformised and refined the ZnO grains in the resulting ceramics. At a 0.75 mol% PrMnO 3 addition, the bimodal grain size distribution was completely eliminated, and the average ZnO grain size was reduced by more than 45%, compared with the sample without PrMnO 3 addition. (2) The addition of PrMnO 3 resulted in remarkably improved stability of the switching field strength, narrowing its range of variation from 1580 V/cm (without PrMnO 3 addition) to only 91 V/cm (with 0.75 mol% PrMnO 3 addition). The homogenisation and reduction in the ZnO grain sizes were responsible for the observed stabilisation of the switching field strength. (3) A phenomenological analysis of the ZnO grain growth kinetics showed that the kinetic grain growth exponent n increased from 2.9 without PrMnO 3 to 6.1 after a 0.75 mol% PrMnO 3 addition, corresponding to an increase in apparent activation energy from 202 ± 29 to 697 ± 66 kJ/mol, respectively. (4) The formation of a PrVO 4 secondary phase as the result of PrMnO 3 addition was responsible for the grain growth behaviour observed. The PrVO 4 phase was found to be mostly located at the ZnO grain boundaries, thus hindering and eventually eliminating the abnormal growth of ZnO grains.