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The degradation behavior of high-voltage SnO2 based varistors sintered at different temperatures
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The degradation behavior of high-voltage SnO2 based varistors sintered at different temperatures Mehdi Abdollahia, Mohammad Reza Nilforoushana,∗∗, Mohammad Maleki Shahrakib,∗, Mehdi Delshad Chermahinia, Majid Moradizadeha a b
Department of Materials Engineering, Faculty of Engineering, Shahrekord University, Shahrekord, Iran Department of Materials Engineering, Faculty of Engineering, University of Maragheh, Maragheh, 55181-83111, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: SnO2 Varistor Degradation Sintering temperature
In this research, for the first time, the stability of SnO2 based varistor ceramics sintered in the range of 1250–1350 °C against DC-accelerated aging and impulse surge current tests, was systematically studied. Microstructural study of the sintered samples by XRD and FESEM indicated that the sintering temperature only affects densification and grain size, while phase composition remains intact. With the increase of sintering temperature from 1250 °C to 1350 °C, the mean grain size increased from 1.6 to 8 μm. The maximum nonlinear coefficient of 50 and the minimum leakage current density of 1.5 μA/cm2 were obtained in the sample sintered at 1300 °C. The breakdown electric field decreased from 800 V/mm to 270 V/mm, when sintering temperature increased from 1250 °C to 1350 °C. The samples sintered at 1250 °C did not show stability against neither of DCaccelerated aging and impulse current tests. The varistors sintered at 1300 °C exhibited the excellent resistance to DC-accelerated aging degradation, while ceramics sintered at 1350 °C showed the best resistance to impulse current degradation.
1. Introduction Today, with the rapid development in electric and electronic devices, the safe protection of power systems and electronics devices against electrical transient surges, over-voltages, and noises is a main subject in electronic area [1]. Having the non-linear characteristic in electric field-current density curve (E-J), the metal oxide varistors (MOVs) are connected in parallel to electric and electronic systems to discharge the transient surge and limit the voltage to a harmless value that is safe for the electrical devices [2]. A MOV is characterized by nonlinear coefficient (α), leakage current density (JL), and breakdown electric felid (Eb). These parameters are affected by microstructural changes of MOV which itself is tuned by the formulation and the processing procedure [3]. MOVs are classified by their breakdown electric to low, high, and ultra-high voltage varistors [4]. To select a suitable MOV for a specific device, the operating conditions including the operating voltage (commonly 0.8 Eb), temperature, magnitude and number of transient surges must be considered [5]. These parameters are important in electrical degradation of a MOV and directly affect the MOV service life. The degradation of a MOV is defined as the
∗
deterioration of its main electrical parameters i.e. leakage current density, nonlinear coefficient and breakdown electric field, under various applied stresses and external factors [6]. When a MOV is electrically degraded, its protection performance will be very poor. Two types of long-term and short-term degradation induced by respectively continuous working voltage and transient surges (lightening type) are observed in MOVs [7–11]. In long-term degradation, leakage current density increases gradually when MOV is operated within continues operating voltage. The continues working voltage gradually induce irreversible asymmetric reduction in Schottky barriers height and as a result of that the leakage current of the varistor increases with time. When the leakage current reaches a critical value, thermal runaway phenomenon occurres [12]. A common practically procedure to test the long-term degradation is DC-accelerated aging test in which the evaluation of electrical properties is performed after applying the working voltage with elevated temperature for a long period of time (aging condition) [13]. In short-term degradation, electrical parameters are degraded by very intense currents in a short interval (impulse currents or transient surges) [14]. An ideal MOV should clamped transient surges without any electrical degradation, however this capability
Corresponding author. Corresponding author. E-mail addresses: [email protected] (M.R. Nilforoushan), [email protected] (M. Maleki Shahraki).
∗∗
https://doi.org/10.1016/j.ceramint.2020.01.186 Received 7 December 2019; Received in revised form 5 January 2020; Accepted 17 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mehdi Abdollahi, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.186
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a 100-mesh screen. Green pellets with the dimension of 13 mm in diameter and 1 mm in thickness were obtained after pressing of granulated powders by a hydraulic press at a pressure of 250 MPa. Then, samples were heated at 600 °C for 2 h for burning out all the PVA binder. The green samples were sintered in an electric muffle furnace at 1250 °C, 1300 °C and 1350 °C for 2 h at a heating rate of 5 °C/min. The cooling rate was 2 °C/min from sintering temperature to 900 °C and then pellets were naturally cooled to the room temperature in furnace.
depends on the number, shape, magnitude, and time duration of transient surges [15]. To examine this kind of degradation, the standard shape of impulse currents (8/20 μs) with different magnitudes are applied to MOVs and afterwards changes in electrical parameters are evaluated [16]. A desirable MOV presents an appropriate resistance against both types of degradation. Since 1972, MOVs based on ZnO were commercialized in high- and low-voltage applications due to their high nonlinearity and suitable energy absorption [17]. One of unsolved problems in these MOVs is degradation issue. The study of electrical degradation mechanism and also the effort to improve the electrical degradation resistance are the main topics of recent research in ZnO based varistors area and in this regard, some research has been done [18]. It was reported that doping suitable oxides to ZnO varistors could improve both DC-accelerated aging degradation and impulse degradation [19]. Furthermore, the acceptable mechanism for degradation phenomenon in this type of varistors has been proposed to be the defects migration to grain boundaries which leads to the asymmetric distortion in double Schottky barriers [20]. SnO2 based varistors are a modern type of MOVs that present unique electrical and microstructural features compared to the traditional ones (ZnO based varistors) [21–24]. The composition of SnO2 with the addition of minor content of some oxides (1 mol% of CoO, 0.05 mol% of Cr2O3 and 0.05 mol% of Nb2O5) is a well-known system for the highvoltage applications [25]. It was reported the above-mentioned composition sintered at 1300 °C presents excellent electrical properties including Eb = 4 kV/cm and α = 41. Besides super features such as simple microstructure and high value of nonlinearity, SnO2 varistors sintered at 1300 °C exhibit excellent stability against long-term degradation [26–28]. Despite outstanding resistance to DC-accelerated aging degradation, however, these varistors show low resistance against short-term degradation and suffer from low stability against impulse surges [29–31]. In order to the increase of stability against impulse surges, Metz et al. [32] increased sintering from 1300 to 1350 °C and observed that surge capability of SnO2 varistors improved. However, the long-term degradation of sintered samples was not evaluated in this work. In another work, it is reported that samples sintered at temperatures lower than 1150 °C present a weak stability in DC-accelerated aging tests and impulse surge tests [33]. Although the effect of sintering temperature on other electrical properties of SnO2 based varistors such as nonlinear coefficient, leakage current density and breakdown electric field was studied in literature [34,35], there is no a comprehensive study on degradation behavior of SnO2 varistors sintered at different temperatures and therefore, it is necessary to clarify the effect of sintering temperature on the degradation phenomenon in SnO2 varistors. With all mentioned above, this research is a comprehensive study to investigate the degradation behavior of SnO2 based varistors sintered at different temperatures against both DC-accelerated-aging test and impulse surge current test.
2.2. Microstructure analysis and measurement The crystalline phase composition of sintered samples was studied by X‐ray diffraction (XRD Philips X'Pert System) using the Co Kα radiation. The XRD patterns were acquired in 2Ɵ range of 20–80° at ambient temperature by selecting a step size of 0.02° and the time per step of 1 s. The apparent density of sintered samples was calculated by the Archimedes method. The relative density of specimens was obtained from the ratio of apparent density to the theoretical density of SnO2 that is 6.95 g/cm3. The surface morphology of the sintered samples was examined using a field emission scanning electron microscope (FESEM, TESCAN). 2.3. Electrical measurement Before electrical measurement, all as-sintered samples were polished to acquire smooth surfaces. Then, silver paste was coated on both faced surfaces of sintered samples and then heated up to 550 °C for 10 min. The low current region of E-J curve of the sintered specimens was characterized by a source meter of Keithley 2410. The breakdown electric field (Eb), leakage current density (JL) and nonlinear coefficient (α) of samples was calculated in this region. The breakdown electric field (Eb) was measured at the applied electric field corresponded to the standard current density of 1 mA/cm2. The nonlinear coefficients of (α) were calculated in the standard range of 1–10 mA/cm2, following the equation of:
( ) log ( ) log
α=
J2
E2
J1
E1
Eq. 1
Where E2 and E1 are the electric fields at the standard current densities of J2 = 10 mA/cm2 and J1 = 1 mA/cm2, respectively. The leakage current density (JL) was also measured at 0.8Eb. To characterize the behavior of varistors in the high current region, impulse current surges with 8/20 μs waveform were applied by using a current generator and its response was recorded by a Rigol DS5022 M digital oscilloscope. 2.4. DC-accelerated aging test The DC-accelerated aging test of the sintered samples was performed at continuous operating electric field of 0.85EB for 24 h at the temperature of 150 °C. The leakage current density was simultaneously monitored at 30 min intervals during this test by a source meter unit (Keithley 2410). After DC-accelerated aging test, the E-J characteristics and electrical parameters of samples were again examined at room temperature.
2. Experimental procedures 2.1. Sample preparation Varistors were prepared by conventional ceramic processing, which begins with weighing, mixing, and milling of oxide powders in ball mills. According to Pianaro varistor composition for high-voltage applications, reagent-grade raw materials were utilized in the molar proportion of 98.9% SnO2 (Merck, 99%), 1.00% CoO (Usnano, 99%), 0.05% Nb2O5 (Merck, 99%), and 0.05% Cr2O3 (Iolitech, 99.7%). The weighted reagent-grade oxides were homogenized in deionized water using a high-power sonication. Then the obtained slurry was mixed by zirconia balls and milling media for 5 h using a planetary ball milling. The resultant was dried at 120 °C for 2 h, and the dried mixture was then mixed with polyvinyl alcohol solution as binder (2 wt% based on powder weight). Granulated powders was produced by sieving through
2.5. The impulse current aging test The impulse current aging test was performed at the surge currents of 250, 500, 1000 A/cm2 with 8/20 μs waveform (continuously 3 times with the time interval of 2 min for each surge current) using a surge generator. The applied impulse surge current tests are in the strongest conditions according to surge withstanding capability of SnO2 based varistors. The time interval between each surge current cycle was 2
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that there exist no secondary phases which is in accordance with the XRD results. Moreover, it can be seen that the sample sintered at 1300 °C possesses minimum porosity among all samples which confirms the results acquired from the calculated relative densities. Noting to these images, it can be concluded that the noticeable influence of sintering temperature is the growth of grains so much so that the mean grain size increments from 1.6 μm to 8 μm with the increase of sintering temperature from 1250 °C to 1350 °C (Fig. 2). This phenomenon indicates that the mobility of grain boundaries is high at the sintering temperature of 1350 °C. It is worth to note that the sample sintered at 1350 °C experiences an abnormal grain growth so that some grains are about 18 μm in size. The exact mechanism of this abnormal growth is not clear. Already, the abnormal grain growth has been observed in SnO2 based varistors doped with Bi2O3 and also with CuO [2,23]. Furthermore, in the sample sintered at 1350 °C, the twin boundaries phenomenon is found in abundance. It was reported Nb2O5 effectively contributes to the formation of the twin boundaries. However, at low temperatures, niobium oxide loses its ability to produce twin boundaries [36]. Fig. 4 illustrates the E-J curves of the sintered samples in different sintering temperatures before and after both degradation tests including DC-accelerated-aging test and impulse surge current test. The electrical parameters of samples before degradation tests such as α, Eb, and JL are depicted in Table 1. From this table, the maximum value of nonlinear coefficient is 50 for the sample sintered at 1300 °C indicating this fact that Schottky barriers with the maximum height were formed in this sample. Moreover, the breakdown electric field drops off monotonically from 800 to 270 V/mm with the increase in sintering temperature from 1250 °C to 1350 °C. This is due to the inverse relation of breakdown electric field with the grain size: increase in sintering temperature results in grain size increase which causes a reduction in electric breakdown field. In addition, decline in the Schottky barrier height is another factor which contributes to the decrease in electric breakdown field in the lower sintering temperature. For example, considering the lower grain size of sample sintered at 1250 °C compared to the sample sintered at 1300 °C, the electric breakdown field did not increase by the same proportion with the grain size decreasing which is due to the lower height of Schottky barrier in the sample sintered at 1250 °C. Also, the leakage current density is its minimum in the sample sintered at 1300 °C. The behavior of SnO2 varistors after DC-accelerated aging test is presented in Fig. 4. From this figure, the E-J curves of samples sintered at 1250 °C and 1350 °C, especially at pre-breakdown region, are drifted
Fig. 1. XRD patterns of samples sintered at different sintering temperatures.
10 min. After applying the respective surge current, the E-J characteristics and electrical parameters of samples were examined at room temperature. 3. Results and discussion Fig. 1 shows the XRD spectrum of sintered samples at different temperature. X-ray data for all samples present identical behavior and exhibit the cassiterite tin oxide structure without any secondary phases within the XRD detection limit. With a meticulous observation, it can be seen that the increase in sintering temperature from 1250 °C to 1350 °C results in a partial shift of the main diffraction peak corresponding to (110) plane from 31.08° to 31.02°. Considering the composition of samples which is equal in all samples, this shift is attributed to the increase in the lattice parameter with the increase in the sintering temperature. Fig. 2 presents the variation of relative density and mean grain size of samples with changes in sintering temperature. The maximum density of 99.5% is achieved at the sintering temperature of 1300 °C. For the temperature more than 1300 °C, due to the additive loss, the relative density diminishes, however, in the sintering temperature less than 1300 °C, densification is not completed and the relative density is low [35]. FESEM images of the sintered samples, presented in Fig. 3 indicate
Fig. 2. Relative density and grain size of SnO2 based varistors versus sintering temperature. 3
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Fig. 3. FESEM images of samples sintered at a)1250 °C b) 1300 °C and c) 1350 °C. (inset: BSE mode of FESEM in a higher magnification).
through the degradation test is attributed to the migration of positive charge defects to grain boundary and compensation with other negative charge defects at grain boundary [27,28]. This compensation causes a reduction in the Schottky barrier height located at grain boundary and consequently results in the increase in leakage current density and also decline in breakdown electric field and nonlinear coefficient. Considering the degradation behavior of samples in DC-accelerated aging test, it can be concluded that in the sample sintered at 1300 °C, defects could not migrate toward grain boundary, while in the sample sintered at 1350 °C, this process occurs more easily. Bearing in mind the pro0 posed Schottky barrier model, the defects of Sni00 , Vo00 and NbSn can be considered as positive charge defects which may be the main stimulus of degradation [37]. Seemingly, these defects do not have the ability to migrate in the sample sintered at 1300 °C for two plausible reasons: firstly, the concentration of these defects at 1300 °C is too low; secondly, the activation energy for the diffusion of these defects to the grain boundary is too high. Nevertheless, with the increase in the sintering temperature, both the concentration of these defects and the diffusion ability of these defects enhance. The increment in concentration of defects with the increase in sintering temperature, spe0 cially NbSn is discussed in the following. The increase in lattice parameter with the rise in sintering temperature which was proved by XRD spectrums is the reason of development of defects diffusion ability. An interesting phenomenon seen in E-J curves before degradation test is the extending of upturn region of these curves to higher values of
to the right-side that is a clear sign of degradation phenomenon. While, the sample sintered at 1300 °C experiences a little change. The electrical parameters of samples after applying DC-accelerated aging test are presented in Table 1. After this degradation test, the nonlinearity of the sample sintered at 1250 °C has disappeared. Also, the nonlinear coefficient of the sample sintered at 1350 °C has dropped from 38 to 28. While, a slight decrease in the nonlinearity of the sample sintered at 1300 °C is observed and it reduces from 50 to 47. Considering other data such as leakage current density and electric breakdown field, it can be concluded that the sample sintered at 1300 °C possesses the best stability against the DC-accelerated aging test. This result is in agreement with the results of other researches presented in Table 2 which did not find any degradation in the SnO2 based varistors sintered at 1300 °C [27,28]. The changes in electric current density over time in DC-accelerated aging test are presented at Fig. 5. Clearly, thermal runaway occurs at the sample sintered at 1250 °C during the degradation test, as a result of Joule self-heating phenomenon which is due to the high applied electric field at high temperature of the degradation test [33]. The variation in electric current density with time in the sample sintered at 1300 °C could be ignored since the electric current density is almost constant throughout the applied test. The current density increases from 0.48 mA/cm2 to 0.57 mA/cm2 after 24 h. However, for the sample sintered at 1350 °C, the current density with a steady trend enhances from 0.67 mA/cm2 to 1.78 mA/cm2. The proposed mechanism for gradual increase of electric current density in a varistor 4
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Fig. 4. E-J curves before and after DC and impulse degradation tests a) 1250 °C, b) 1300 °C, and c) 1350 °C (insets: the response of samples to last applied impulse current of 1000 A/cm2 in impulse degradation test).
This behavior shows this fact that the sample sintered at 1250 °C does not meet the requirements for applying the impulse degradation test. Already, Metz and coworkers [32] have observed this phenomenon and linked it to the electrical resistance of grains in SnO2 based varistors. Fig. 5b shows the electrical resistivity of grains versus sintering temperature acquired from the slope of E-J curve at the upturn region. From this figure, the electrical resistivity of grains decreases sharply with increasing the sintering temperature. Considering the significant role of Nb2O5 in the electrical resistance of the grains, it seems that Nb2O5 is more soluble at higher sintering temperatures and its concentration increases in tin oxide with the increase in sintering tem0 perature [21,38]. This increase in NbSn concentration in higher sintering temperature may reduce stability of SnO2 varistors against DCaccelerated aging degradation. Furthermore, it was reported that the energy absorption capability of varistors has a direct relationship with the grain electric resistivity and a lower value of this parameter results in a higher energy absorption in varistors [39]. Therefore, due to highest grain electric resistivity observed in the sample sintered at 1250 °C, this sample takes a low-energy absorption capability and could not withstand against the intense impulse currents. In the insets of Fig. 4b and c, the response of SnO2 varistor to the last impulse of 1 kA/ cm2 applied in impulse degradation test is presented. It can be observed that the response of sample sintered at 1300 °C has partly fluctuation and it occurs at more time in 8/20 μs? This response comes from electrical degradation [40]. Though, the varistor sintered at 1350 °C has a response without any fluctuation and is similar to the applied impulse. This results in Table 1 and E-J curves in Fig. 4 indicate that the sample sintered at 1300 °C was degraded but the sample sintered at 1350 °C had a great stability against impulse degradation test. Based on Table 1, the electrical parameters of sample sintered at 1300 °C are associated with a sharp drop after impulse degradation test so that α drops off from 50 to 6, and Eb declines from 485 to 261 V/mm. It was reported that the degradation mechanism in impulse degradation test is similar to the DC-accelerated aging test. However, in the impulse degradation test, much more energy in a shorter time is applied to the
Table 1 The electrical parameters of sample before degradation test and their changes after applying degradation tests.
1250°C 1300°C 1350°C
Before degradation
After DC-accelerated aging test
After impulse current aging test
α
Eb (V/ mm)
JL (μA/ cm2)
α
Eb (V/ mm)
JL (μA/ cm2)
α
Eb (V/ mm)
JL (μA/ cm2)
26 50 38
800 485 270
8 1.5 4.3
1.3 47 28
92 482 207
800 4 250
– 6 35
– 261 267
– 700 7
Table 2 The comparison of stability against long- and short-term degradation of some reported sintered SnO2 based varistors with this work. Sintering temperature
1300 1300 1300 1300 1150 1350 1250 1300 1350
°C °C °C °C °C °C °C °C °C
Stability against Long-term degradation
Short-term degradation
Strong Strong – – Weak – Weak Strong Good
Weak – Weak – Weak – Weak Weak Good
Maximum of surge withstand current
Reference
– –
[26,27] [28] [29] [30] [33] [31] This work This work This work
1000 A/cm2 10 A/cm2 1000 A/cm2 200 A/cm2 1300 A/cm2 2500 A/cm2
electric current density without any physical damage to the samples with the increase in sintering temperature. The end of the curve or surge-withstand current in sample sintered at 1250 °C is 200 A/cm2. For the samples sintered at 1300 °C and 1350 °C, the maximum of surgewithstand current are 1300 and 2500 A/cm2 and respectively (Table 2).
5
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Fig. 5. a) Current density versus time of different sintered samples during DC-accelerated aging test, b) Grain electric resistivity versus sintering temperature.
1. At the sintering temperature of 1250 °C, SnO2 based varistors had no resistance to degradation tests. The Joule heating phenomenon caused by the high breakdown voltage of these varistors has made this sample to be less resistance to DC-accelerated aging test. However, the high value of grain electric resistivity in SnO2 based varistors resulted in a poor stability of this sample against impulse currents. 2. The sample sintered at 1300 °C showed a decent resistance to the long-term degradation test, but showed poor stability against shortterm degradation. The low concentration of the positive charge defects or their low diffusion rate in the test conditions are the plausible reasons which justify this behavior. However, its poor degradation behavior against the impulse degradation test is due to the high electric resistivity of the grains, causing the temperatures of sample to rise and in this condition, defects move across the boundary due to the energy of impulse current and they weaken the double Schottky barriers. 3. Samples sintered at 1350 °C showed stability in the DC-accelerated aging test to an acceptable level but showed just a good resistance to impulse degradation test. At 1350 °C the positive charge defects have a higher concentration and/or diffusivity, whereas the grain electric resistivity of these samples is in a lowest state.
varistor compared to the DC-accelerated test and this energy is generally sufficient to move the defects to the grain boundary [6]. The energy of electrical impulses turns into the heat form in the varistor. If the heat generation rate exceeds the dissipation heat of device, the temperature of the varistors rises. Therefore, in subsequent repetitive impulses, the degradation phenomenon occurs more severely due probably to the migration of defects to the grain boundary and the reduction of the Schottky barrier. The energy of an impulse can be calculated from the following equation [17]: t
E=
t
∫ VI dt = ∫ I 2Rdt 0
0
Eq. 2
Considering the above equation, it can be concluded that in a same impulse current, samples with higher grain electric resistance (R) receive more electrical energy. Thus, it can be said that in the sintered samples at 1300 °C, due to higher electrical resistivity of the grains compared to the sintered samples at 1350 °C (Fig. 5b), more electrical energy is absorbed and converted into heat which increases the temperature and facilitates the movement of defects to the grain in subsequent pulses during the degradation test. Consequently, the sample sintered at 1350 °C, due to its lower grain resistivity, has a higher withstanding against impulse degradation test and the sample sintered at 1250 °C has a very low surge withstanding capability as a result of its very high grain resistivity and is mechanically broken down at 200 A/ cm2 of impulse current. The comparison of stability against long- and short-term degradation of some reported sintered SnO2 based varistors with this work is presented Table 2. It is worth to note that although the sample sintered at 1300 °C possesses a decent stability against DC-accelerated aging test, it has a poor resistance to the impulse degradation test. While, the samples sintered at 1350 °C have an appropriate stability against impulse degradation test and relatively sufficient resistance to DC-accelerated aging test. Thereby, to evaluate the lifetime of a varistor, stability against both DC degradation and impulse degradation should be taken into account, hence, SnO2 varistors that have been sintered at 1350 °C present a better performance. However, improving the degradation resistance of these varistors against DC-accelerated aging test could be a stimulating subject for further research. In addition, the precise determination of the defects or ions which are the main factor of degradation and the degradation mechanisms in the SnO2 based varistor can also be an interesting topic for research.
Declaration of competing interest The authors declare that no conflict of interest exists. Acknowledgements This work supported by Iran National Science Foundation (Grant Number: 97006409). References [1] H. Bastami, E. Taheri-Nassaj, P.F. Smet, K. Korthout, D. Poelman, (Co, Nb, Sm)‐doped tin dioxide varistors ceramics sintered using nanopowders prepared by coprecipitation method, J. Am. Ceram. Soc. 94 (2011) 3249–3255. [2] P. Mahmoudi, A. Nemati, M. Maleki Shahraki, Grain growth kinetics and electrical properties of CuO doped SnO2-based varistors, J. Alloy. Comp. 770 (2019) 784–791. [3] P. Meng, X. Zhao, X. Yang, J. Wu, Q. Xie, J. He, J. Hu, J. He, Breakdown phenomenon of ZnO varistors caused by non-uniform distribution of internal pores, J. Eur. Ceram. Soc. 39 (2019) 4824–4830. [4] A. Izoulet, S. Guillemet-Fritsch, C. Estournès, J. Morel, Microstructure control to reduce leakage current of medium and high voltage ceramic varistors based on doped ZnO, J. Eur. Ceram. Soc. 34 (2014) 3707–3714. [5] M. Maleki Shahraki, P. Mahmoudi, M. Abdollahi, T. Ebadzadeh, Fine-grained SnO2 varistors prepared by microwave sintering for ultra-high voltage applications, Mater. Lett. 230 (2018) 9–11. [6] J. He, Metal Oxide Varistors: from Microstructure to Macro-Characteristics, WileyVCH Verlag GmbH & Co. KGaA, Boschstr, 2019, pp. 407–439. [7] J. Lin, S. Li, J. He, L. Zhang, W. Liu, J. Li, Zinc interstitial as a universal microscopic origin for the electrical degradation of ZnO-based varistors under the combined DC and temperature condition, J. Eur. Ceram. Soc. 37 (2017) 3535–3540.
4. Conclusions In this study, the effect of sintering temperature on the degradation behavior of SnO2 based varistors for high-voltage applications against both DC-accelerated aging test and impulse degradation test was studied and the following results were achieved: 6
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[24] H. Bastami, E. Taheri-Nassaj, Synthesis of nanosized (Co, Nb, Sm)-doped SnO2 powders using co-precipitation method, J. Alloy, Comp 495 (2010) 121–125. [25] S.A. Pianaro, P.R. Bueno, E. Longo, J.A. Varela, Microstructure and electric properties of a SnO2 based varistor, Ceram. Int. 25 (1999) 1–6. [26] M.A. Ponce, M.A. Ramírez, R. Parra, C. Malagú, M.S. Castro, P.R. Bueno, J.A. Varela, Influence of degradation on the electrical conduction process in ZnO and SnO2 –based varistors, J. Appl. Phys. 108 (2010) 074505. [27] M.A. Ramírez, R. Tararam, Z. Simões, A. Ries, E. Longo, J.A. Varela, Degradation analysis of the SnO2 and ZnO-based varistors using electrostatic force microscopy, J. Am. Ceram. Soc. 96 (2013) 1801–1809. [28] M.A. Ramírez, M. Cilense, P.R. Bueno, E. Longo, J.A. Varela, Comparison of nonOhmic accelerated ageing of the ZnO- and SnO2-based voltage dependent resistors, J. Phys. D Appl. Phys. 42 (2009) 015503. [29] M.A. Ramírez, W. Bassi, P.R. Bueno, E. Longo, J.A. Varela, Comparative degradation of ZnO- and SnO2-based polycrystalline non-ohmic devices by current pulse stress, J. Phys. D Appl. Phys. 41 (2008) 122002. [30] M.A. Ramírez, W. Bassi, R. Parra, P.R. Bueno, E. Longo, J.A. Varela, Comparative electrical behavior at low and high current of SnO2- and ZnO-based varistors, J. Am. Ceram. Soc. 91 (2008) 2402–2404. [31] Z.Y. Lu, Z. Chen, J.Q. Wu, SnO2-based varistors capable of withstanding surge current, J. Ceram. Soc. Jpn. 117 (2009) 851–859. [32] R. Metz, J. Morel, M. Houabes, J. Pansiot, M. Hassanzadeh, High voltage characterization of tin oxide varistors, J. Mater. Sci. 42 (2007) 10284–10287. [33] M. Maleki Shahraki, Mehdi Delshad Chermahini, S. Alipour, P. Mahmoudi, I. Safaee, M. Abdollahi, Ultra-high voltage SnO2 based varistors prepared at low temperature by two-step sintering, J. Alloy. Comp. 805 (2019) 794–801. [34] J.A. Aguilar-Martínez, A.B. Glot, A.V. Gapono, M.B. Hernández, J. Guerrero-Paz, Current-voltage characteristics of SnO2–Co3O4–Cr2O3–Sb2O5 ceramics, J. Phys. D Appl. Phys. 42 (2009) 205401. [35] P.A. Santos, S. Maruchin, G.F. Menegoto, A.J. Zara, S.A. Pianaro, The sintering time influence on the electrical and microstructural characteristics of SnO2 varistor, Mater. Lett. 60 (2006) 1554–1557. [36] S. Tominc, A. Rečnik, Z. Samardžija, G. Dražić, M. Podlogar, S. Bernik, N. Daneu, Twinning and charge compensation in Nb2O5–doped SnO2–CoO ceramics exhibiting promising varistor characteristics, Ceram. Int. 44 (2018) 1603–1613. [37] W.X. Wang, J.F. Wang, H.C. Chen, W.B. Su, G.Z. Zang, Effects of Cr2O3 on the properties of (Co, Nb)-doped SnO2 varistors, Mater. Sci. Eng. B 99 (2003) 470–474. [38] Q.Y. Wei, J. He, J. Hu, Dependence of residual voltage ratio behavior of SnO2-based varistors on Nb2O5 addition, Sci. China Technol. Sci. 54 (2011) 1415–1418. [39] Q. Wei, J. He, J. Hu, Y. Wang, Influence of Cr2O3 on the residual voltage ratio of SnO2-based varistor, J. Am. Ceram. Soc. 94 (2011) 1999–2002. [40] Z.Y. Lu, A.B. Glot, A.I. Ivon, Z.Y. Zhou, Electrical properties of new tin dioxide varistor ceramics at high currents, J. Eur. Ceram. Soc. 32 (2012) 3801–3807.
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The degradation behavior of high-voltage SnO2 based varistors sintered at different temperatures Mehdi Abdollahia, Mohammad Reza Nilforoushana,∗∗, Mohammad Maleki Shahrakib,∗, Mehdi Delshad Chermahinia, Majid Moradizadeha a b
Department of Materials Engineering, Faculty of Engineering, Shahrekord University, Shahrekord, Iran Department of Materials Engineering, Faculty of Engineering, University of Maragheh, Maragheh, 55181-83111, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: SnO2 Varistor Degradation Sintering temperature
In this research, for the first time, the stability of SnO2 based varistor ceramics sintered in the range of 1250–1350 °C against DC-accelerated aging and impulse surge current tests, was systematically studied. Microstructural study of the sintered samples by XRD and FESEM indicated that the sintering temperature only affects densification and grain size, while phase composition remains intact. With the increase of sintering temperature from 1250 °C to 1350 °C, the mean grain size increased from 1.6 to 8 μm. The maximum nonlinear coefficient of 50 and the minimum leakage current density of 1.5 μA/cm2 were obtained in the sample sintered at 1300 °C. The breakdown electric field decreased from 800 V/mm to 270 V/mm, when sintering temperature increased from 1250 °C to 1350 °C. The samples sintered at 1250 °C did not show stability against neither of DCaccelerated aging and impulse current tests. The varistors sintered at 1300 °C exhibited the excellent resistance to DC-accelerated aging degradation, while ceramics sintered at 1350 °C showed the best resistance to impulse current degradation.
1. Introduction Today, with the rapid development in electric and electronic devices, the safe protection of power systems and electronics devices against electrical transient surges, over-voltages, and noises is a main subject in electronic area [1]. Having the non-linear characteristic in electric field-current density curve (E-J), the metal oxide varistors (MOVs) are connected in parallel to electric and electronic systems to discharge the transient surge and limit the voltage to a harmless value that is safe for the electrical devices [2]. A MOV is characterized by nonlinear coefficient (α), leakage current density (JL), and breakdown electric felid (Eb). These parameters are affected by microstructural changes of MOV which itself is tuned by the formulation and the processing procedure [3]. MOVs are classified by their breakdown electric to low, high, and ultra-high voltage varistors [4]. To select a suitable MOV for a specific device, the operating conditions including the operating voltage (commonly 0.8 Eb), temperature, magnitude and number of transient surges must be considered [5]. These parameters are important in electrical degradation of a MOV and directly affect the MOV service life. The degradation of a MOV is defined as the
∗
deterioration of its main electrical parameters i.e. leakage current density, nonlinear coefficient and breakdown electric field, under various applied stresses and external factors [6]. When a MOV is electrically degraded, its protection performance will be very poor. Two types of long-term and short-term degradation induced by respectively continuous working voltage and transient surges (lightening type) are observed in MOVs [7–11]. In long-term degradation, leakage current density increases gradually when MOV is operated within continues operating voltage. The continues working voltage gradually induce irreversible asymmetric reduction in Schottky barriers height and as a result of that the leakage current of the varistor increases with time. When the leakage current reaches a critical value, thermal runaway phenomenon occurres [12]. A common practically procedure to test the long-term degradation is DC-accelerated aging test in which the evaluation of electrical properties is performed after applying the working voltage with elevated temperature for a long period of time (aging condition) [13]. In short-term degradation, electrical parameters are degraded by very intense currents in a short interval (impulse currents or transient surges) [14]. An ideal MOV should clamped transient surges without any electrical degradation, however this capability
Corresponding author. Corresponding author. E-mail addresses: [email protected] (M.R. Nilforoushan), [email protected] (M. Maleki Shahraki).
∗∗
https://doi.org/10.1016/j.ceramint.2020.01.186 Received 7 December 2019; Received in revised form 5 January 2020; Accepted 17 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mehdi Abdollahi, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.186
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a 100-mesh screen. Green pellets with the dimension of 13 mm in diameter and 1 mm in thickness were obtained after pressing of granulated powders by a hydraulic press at a pressure of 250 MPa. Then, samples were heated at 600 °C for 2 h for burning out all the PVA binder. The green samples were sintered in an electric muffle furnace at 1250 °C, 1300 °C and 1350 °C for 2 h at a heating rate of 5 °C/min. The cooling rate was 2 °C/min from sintering temperature to 900 °C and then pellets were naturally cooled to the room temperature in furnace.
depends on the number, shape, magnitude, and time duration of transient surges [15]. To examine this kind of degradation, the standard shape of impulse currents (8/20 μs) with different magnitudes are applied to MOVs and afterwards changes in electrical parameters are evaluated [16]. A desirable MOV presents an appropriate resistance against both types of degradation. Since 1972, MOVs based on ZnO were commercialized in high- and low-voltage applications due to their high nonlinearity and suitable energy absorption [17]. One of unsolved problems in these MOVs is degradation issue. The study of electrical degradation mechanism and also the effort to improve the electrical degradation resistance are the main topics of recent research in ZnO based varistors area and in this regard, some research has been done [18]. It was reported that doping suitable oxides to ZnO varistors could improve both DC-accelerated aging degradation and impulse degradation [19]. Furthermore, the acceptable mechanism for degradation phenomenon in this type of varistors has been proposed to be the defects migration to grain boundaries which leads to the asymmetric distortion in double Schottky barriers [20]. SnO2 based varistors are a modern type of MOVs that present unique electrical and microstructural features compared to the traditional ones (ZnO based varistors) [21–24]. The composition of SnO2 with the addition of minor content of some oxides (1 mol% of CoO, 0.05 mol% of Cr2O3 and 0.05 mol% of Nb2O5) is a well-known system for the highvoltage applications [25]. It was reported the above-mentioned composition sintered at 1300 °C presents excellent electrical properties including Eb = 4 kV/cm and α = 41. Besides super features such as simple microstructure and high value of nonlinearity, SnO2 varistors sintered at 1300 °C exhibit excellent stability against long-term degradation [26–28]. Despite outstanding resistance to DC-accelerated aging degradation, however, these varistors show low resistance against short-term degradation and suffer from low stability against impulse surges [29–31]. In order to the increase of stability against impulse surges, Metz et al. [32] increased sintering from 1300 to 1350 °C and observed that surge capability of SnO2 varistors improved. However, the long-term degradation of sintered samples was not evaluated in this work. In another work, it is reported that samples sintered at temperatures lower than 1150 °C present a weak stability in DC-accelerated aging tests and impulse surge tests [33]. Although the effect of sintering temperature on other electrical properties of SnO2 based varistors such as nonlinear coefficient, leakage current density and breakdown electric field was studied in literature [34,35], there is no a comprehensive study on degradation behavior of SnO2 varistors sintered at different temperatures and therefore, it is necessary to clarify the effect of sintering temperature on the degradation phenomenon in SnO2 varistors. With all mentioned above, this research is a comprehensive study to investigate the degradation behavior of SnO2 based varistors sintered at different temperatures against both DC-accelerated-aging test and impulse surge current test.
2.2. Microstructure analysis and measurement The crystalline phase composition of sintered samples was studied by X‐ray diffraction (XRD Philips X'Pert System) using the Co Kα radiation. The XRD patterns were acquired in 2Ɵ range of 20–80° at ambient temperature by selecting a step size of 0.02° and the time per step of 1 s. The apparent density of sintered samples was calculated by the Archimedes method. The relative density of specimens was obtained from the ratio of apparent density to the theoretical density of SnO2 that is 6.95 g/cm3. The surface morphology of the sintered samples was examined using a field emission scanning electron microscope (FESEM, TESCAN). 2.3. Electrical measurement Before electrical measurement, all as-sintered samples were polished to acquire smooth surfaces. Then, silver paste was coated on both faced surfaces of sintered samples and then heated up to 550 °C for 10 min. The low current region of E-J curve of the sintered specimens was characterized by a source meter of Keithley 2410. The breakdown electric field (Eb), leakage current density (JL) and nonlinear coefficient (α) of samples was calculated in this region. The breakdown electric field (Eb) was measured at the applied electric field corresponded to the standard current density of 1 mA/cm2. The nonlinear coefficients of (α) were calculated in the standard range of 1–10 mA/cm2, following the equation of:
( ) log ( ) log
α=
J2
E2
J1
E1
Eq. 1
Where E2 and E1 are the electric fields at the standard current densities of J2 = 10 mA/cm2 and J1 = 1 mA/cm2, respectively. The leakage current density (JL) was also measured at 0.8Eb. To characterize the behavior of varistors in the high current region, impulse current surges with 8/20 μs waveform were applied by using a current generator and its response was recorded by a Rigol DS5022 M digital oscilloscope. 2.4. DC-accelerated aging test The DC-accelerated aging test of the sintered samples was performed at continuous operating electric field of 0.85EB for 24 h at the temperature of 150 °C. The leakage current density was simultaneously monitored at 30 min intervals during this test by a source meter unit (Keithley 2410). After DC-accelerated aging test, the E-J characteristics and electrical parameters of samples were again examined at room temperature.
2. Experimental procedures 2.1. Sample preparation Varistors were prepared by conventional ceramic processing, which begins with weighing, mixing, and milling of oxide powders in ball mills. According to Pianaro varistor composition for high-voltage applications, reagent-grade raw materials were utilized in the molar proportion of 98.9% SnO2 (Merck, 99%), 1.00% CoO (Usnano, 99%), 0.05% Nb2O5 (Merck, 99%), and 0.05% Cr2O3 (Iolitech, 99.7%). The weighted reagent-grade oxides were homogenized in deionized water using a high-power sonication. Then the obtained slurry was mixed by zirconia balls and milling media for 5 h using a planetary ball milling. The resultant was dried at 120 °C for 2 h, and the dried mixture was then mixed with polyvinyl alcohol solution as binder (2 wt% based on powder weight). Granulated powders was produced by sieving through
2.5. The impulse current aging test The impulse current aging test was performed at the surge currents of 250, 500, 1000 A/cm2 with 8/20 μs waveform (continuously 3 times with the time interval of 2 min for each surge current) using a surge generator. The applied impulse surge current tests are in the strongest conditions according to surge withstanding capability of SnO2 based varistors. The time interval between each surge current cycle was 2
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that there exist no secondary phases which is in accordance with the XRD results. Moreover, it can be seen that the sample sintered at 1300 °C possesses minimum porosity among all samples which confirms the results acquired from the calculated relative densities. Noting to these images, it can be concluded that the noticeable influence of sintering temperature is the growth of grains so much so that the mean grain size increments from 1.6 μm to 8 μm with the increase of sintering temperature from 1250 °C to 1350 °C (Fig. 2). This phenomenon indicates that the mobility of grain boundaries is high at the sintering temperature of 1350 °C. It is worth to note that the sample sintered at 1350 °C experiences an abnormal grain growth so that some grains are about 18 μm in size. The exact mechanism of this abnormal growth is not clear. Already, the abnormal grain growth has been observed in SnO2 based varistors doped with Bi2O3 and also with CuO [2,23]. Furthermore, in the sample sintered at 1350 °C, the twin boundaries phenomenon is found in abundance. It was reported Nb2O5 effectively contributes to the formation of the twin boundaries. However, at low temperatures, niobium oxide loses its ability to produce twin boundaries [36]. Fig. 4 illustrates the E-J curves of the sintered samples in different sintering temperatures before and after both degradation tests including DC-accelerated-aging test and impulse surge current test. The electrical parameters of samples before degradation tests such as α, Eb, and JL are depicted in Table 1. From this table, the maximum value of nonlinear coefficient is 50 for the sample sintered at 1300 °C indicating this fact that Schottky barriers with the maximum height were formed in this sample. Moreover, the breakdown electric field drops off monotonically from 800 to 270 V/mm with the increase in sintering temperature from 1250 °C to 1350 °C. This is due to the inverse relation of breakdown electric field with the grain size: increase in sintering temperature results in grain size increase which causes a reduction in electric breakdown field. In addition, decline in the Schottky barrier height is another factor which contributes to the decrease in electric breakdown field in the lower sintering temperature. For example, considering the lower grain size of sample sintered at 1250 °C compared to the sample sintered at 1300 °C, the electric breakdown field did not increase by the same proportion with the grain size decreasing which is due to the lower height of Schottky barrier in the sample sintered at 1250 °C. Also, the leakage current density is its minimum in the sample sintered at 1300 °C. The behavior of SnO2 varistors after DC-accelerated aging test is presented in Fig. 4. From this figure, the E-J curves of samples sintered at 1250 °C and 1350 °C, especially at pre-breakdown region, are drifted
Fig. 1. XRD patterns of samples sintered at different sintering temperatures.
10 min. After applying the respective surge current, the E-J characteristics and electrical parameters of samples were examined at room temperature. 3. Results and discussion Fig. 1 shows the XRD spectrum of sintered samples at different temperature. X-ray data for all samples present identical behavior and exhibit the cassiterite tin oxide structure without any secondary phases within the XRD detection limit. With a meticulous observation, it can be seen that the increase in sintering temperature from 1250 °C to 1350 °C results in a partial shift of the main diffraction peak corresponding to (110) plane from 31.08° to 31.02°. Considering the composition of samples which is equal in all samples, this shift is attributed to the increase in the lattice parameter with the increase in the sintering temperature. Fig. 2 presents the variation of relative density and mean grain size of samples with changes in sintering temperature. The maximum density of 99.5% is achieved at the sintering temperature of 1300 °C. For the temperature more than 1300 °C, due to the additive loss, the relative density diminishes, however, in the sintering temperature less than 1300 °C, densification is not completed and the relative density is low [35]. FESEM images of the sintered samples, presented in Fig. 3 indicate
Fig. 2. Relative density and grain size of SnO2 based varistors versus sintering temperature. 3
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Fig. 3. FESEM images of samples sintered at a)1250 °C b) 1300 °C and c) 1350 °C. (inset: BSE mode of FESEM in a higher magnification).
through the degradation test is attributed to the migration of positive charge defects to grain boundary and compensation with other negative charge defects at grain boundary [27,28]. This compensation causes a reduction in the Schottky barrier height located at grain boundary and consequently results in the increase in leakage current density and also decline in breakdown electric field and nonlinear coefficient. Considering the degradation behavior of samples in DC-accelerated aging test, it can be concluded that in the sample sintered at 1300 °C, defects could not migrate toward grain boundary, while in the sample sintered at 1350 °C, this process occurs more easily. Bearing in mind the pro0 posed Schottky barrier model, the defects of Sni00 , Vo00 and NbSn can be considered as positive charge defects which may be the main stimulus of degradation [37]. Seemingly, these defects do not have the ability to migrate in the sample sintered at 1300 °C for two plausible reasons: firstly, the concentration of these defects at 1300 °C is too low; secondly, the activation energy for the diffusion of these defects to the grain boundary is too high. Nevertheless, with the increase in the sintering temperature, both the concentration of these defects and the diffusion ability of these defects enhance. The increment in concentration of defects with the increase in sintering temperature, spe0 cially NbSn is discussed in the following. The increase in lattice parameter with the rise in sintering temperature which was proved by XRD spectrums is the reason of development of defects diffusion ability. An interesting phenomenon seen in E-J curves before degradation test is the extending of upturn region of these curves to higher values of
to the right-side that is a clear sign of degradation phenomenon. While, the sample sintered at 1300 °C experiences a little change. The electrical parameters of samples after applying DC-accelerated aging test are presented in Table 1. After this degradation test, the nonlinearity of the sample sintered at 1250 °C has disappeared. Also, the nonlinear coefficient of the sample sintered at 1350 °C has dropped from 38 to 28. While, a slight decrease in the nonlinearity of the sample sintered at 1300 °C is observed and it reduces from 50 to 47. Considering other data such as leakage current density and electric breakdown field, it can be concluded that the sample sintered at 1300 °C possesses the best stability against the DC-accelerated aging test. This result is in agreement with the results of other researches presented in Table 2 which did not find any degradation in the SnO2 based varistors sintered at 1300 °C [27,28]. The changes in electric current density over time in DC-accelerated aging test are presented at Fig. 5. Clearly, thermal runaway occurs at the sample sintered at 1250 °C during the degradation test, as a result of Joule self-heating phenomenon which is due to the high applied electric field at high temperature of the degradation test [33]. The variation in electric current density with time in the sample sintered at 1300 °C could be ignored since the electric current density is almost constant throughout the applied test. The current density increases from 0.48 mA/cm2 to 0.57 mA/cm2 after 24 h. However, for the sample sintered at 1350 °C, the current density with a steady trend enhances from 0.67 mA/cm2 to 1.78 mA/cm2. The proposed mechanism for gradual increase of electric current density in a varistor 4
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Fig. 4. E-J curves before and after DC and impulse degradation tests a) 1250 °C, b) 1300 °C, and c) 1350 °C (insets: the response of samples to last applied impulse current of 1000 A/cm2 in impulse degradation test).
This behavior shows this fact that the sample sintered at 1250 °C does not meet the requirements for applying the impulse degradation test. Already, Metz and coworkers [32] have observed this phenomenon and linked it to the electrical resistance of grains in SnO2 based varistors. Fig. 5b shows the electrical resistivity of grains versus sintering temperature acquired from the slope of E-J curve at the upturn region. From this figure, the electrical resistivity of grains decreases sharply with increasing the sintering temperature. Considering the significant role of Nb2O5 in the electrical resistance of the grains, it seems that Nb2O5 is more soluble at higher sintering temperatures and its concentration increases in tin oxide with the increase in sintering tem0 perature [21,38]. This increase in NbSn concentration in higher sintering temperature may reduce stability of SnO2 varistors against DCaccelerated aging degradation. Furthermore, it was reported that the energy absorption capability of varistors has a direct relationship with the grain electric resistivity and a lower value of this parameter results in a higher energy absorption in varistors [39]. Therefore, due to highest grain electric resistivity observed in the sample sintered at 1250 °C, this sample takes a low-energy absorption capability and could not withstand against the intense impulse currents. In the insets of Fig. 4b and c, the response of SnO2 varistor to the last impulse of 1 kA/ cm2 applied in impulse degradation test is presented. It can be observed that the response of sample sintered at 1300 °C has partly fluctuation and it occurs at more time in 8/20 μs? This response comes from electrical degradation [40]. Though, the varistor sintered at 1350 °C has a response without any fluctuation and is similar to the applied impulse. This results in Table 1 and E-J curves in Fig. 4 indicate that the sample sintered at 1300 °C was degraded but the sample sintered at 1350 °C had a great stability against impulse degradation test. Based on Table 1, the electrical parameters of sample sintered at 1300 °C are associated with a sharp drop after impulse degradation test so that α drops off from 50 to 6, and Eb declines from 485 to 261 V/mm. It was reported that the degradation mechanism in impulse degradation test is similar to the DC-accelerated aging test. However, in the impulse degradation test, much more energy in a shorter time is applied to the
Table 1 The electrical parameters of sample before degradation test and their changes after applying degradation tests.
1250°C 1300°C 1350°C
Before degradation
After DC-accelerated aging test
After impulse current aging test
α
Eb (V/ mm)
JL (μA/ cm2)
α
Eb (V/ mm)
JL (μA/ cm2)
α
Eb (V/ mm)
JL (μA/ cm2)
26 50 38
800 485 270
8 1.5 4.3
1.3 47 28
92 482 207
800 4 250
– 6 35
– 261 267
– 700 7
Table 2 The comparison of stability against long- and short-term degradation of some reported sintered SnO2 based varistors with this work. Sintering temperature
1300 1300 1300 1300 1150 1350 1250 1300 1350
°C °C °C °C °C °C °C °C °C
Stability against Long-term degradation
Short-term degradation
Strong Strong – – Weak – Weak Strong Good
Weak – Weak – Weak – Weak Weak Good
Maximum of surge withstand current
Reference
– –
[26,27] [28] [29] [30] [33] [31] This work This work This work
1000 A/cm2 10 A/cm2 1000 A/cm2 200 A/cm2 1300 A/cm2 2500 A/cm2
electric current density without any physical damage to the samples with the increase in sintering temperature. The end of the curve or surge-withstand current in sample sintered at 1250 °C is 200 A/cm2. For the samples sintered at 1300 °C and 1350 °C, the maximum of surgewithstand current are 1300 and 2500 A/cm2 and respectively (Table 2).
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Fig. 5. a) Current density versus time of different sintered samples during DC-accelerated aging test, b) Grain electric resistivity versus sintering temperature.
1. At the sintering temperature of 1250 °C, SnO2 based varistors had no resistance to degradation tests. The Joule heating phenomenon caused by the high breakdown voltage of these varistors has made this sample to be less resistance to DC-accelerated aging test. However, the high value of grain electric resistivity in SnO2 based varistors resulted in a poor stability of this sample against impulse currents. 2. The sample sintered at 1300 °C showed a decent resistance to the long-term degradation test, but showed poor stability against shortterm degradation. The low concentration of the positive charge defects or their low diffusion rate in the test conditions are the plausible reasons which justify this behavior. However, its poor degradation behavior against the impulse degradation test is due to the high electric resistivity of the grains, causing the temperatures of sample to rise and in this condition, defects move across the boundary due to the energy of impulse current and they weaken the double Schottky barriers. 3. Samples sintered at 1350 °C showed stability in the DC-accelerated aging test to an acceptable level but showed just a good resistance to impulse degradation test. At 1350 °C the positive charge defects have a higher concentration and/or diffusivity, whereas the grain electric resistivity of these samples is in a lowest state.
varistor compared to the DC-accelerated test and this energy is generally sufficient to move the defects to the grain boundary [6]. The energy of electrical impulses turns into the heat form in the varistor. If the heat generation rate exceeds the dissipation heat of device, the temperature of the varistors rises. Therefore, in subsequent repetitive impulses, the degradation phenomenon occurs more severely due probably to the migration of defects to the grain boundary and the reduction of the Schottky barrier. The energy of an impulse can be calculated from the following equation [17]: t
E=
t
∫ VI dt = ∫ I 2Rdt 0
0
Eq. 2
Considering the above equation, it can be concluded that in a same impulse current, samples with higher grain electric resistance (R) receive more electrical energy. Thus, it can be said that in the sintered samples at 1300 °C, due to higher electrical resistivity of the grains compared to the sintered samples at 1350 °C (Fig. 5b), more electrical energy is absorbed and converted into heat which increases the temperature and facilitates the movement of defects to the grain in subsequent pulses during the degradation test. Consequently, the sample sintered at 1350 °C, due to its lower grain resistivity, has a higher withstanding against impulse degradation test and the sample sintered at 1250 °C has a very low surge withstanding capability as a result of its very high grain resistivity and is mechanically broken down at 200 A/ cm2 of impulse current. The comparison of stability against long- and short-term degradation of some reported sintered SnO2 based varistors with this work is presented Table 2. It is worth to note that although the sample sintered at 1300 °C possesses a decent stability against DC-accelerated aging test, it has a poor resistance to the impulse degradation test. While, the samples sintered at 1350 °C have an appropriate stability against impulse degradation test and relatively sufficient resistance to DC-accelerated aging test. Thereby, to evaluate the lifetime of a varistor, stability against both DC degradation and impulse degradation should be taken into account, hence, SnO2 varistors that have been sintered at 1350 °C present a better performance. However, improving the degradation resistance of these varistors against DC-accelerated aging test could be a stimulating subject for further research. In addition, the precise determination of the defects or ions which are the main factor of degradation and the degradation mechanisms in the SnO2 based varistor can also be an interesting topic for research.
Declaration of competing interest The authors declare that no conflict of interest exists. Acknowledgements This work supported by Iran National Science Foundation (Grant Number: 97006409). References [1] H. Bastami, E. Taheri-Nassaj, P.F. Smet, K. Korthout, D. Poelman, (Co, Nb, Sm)‐doped tin dioxide varistors ceramics sintered using nanopowders prepared by coprecipitation method, J. Am. Ceram. Soc. 94 (2011) 3249–3255. [2] P. Mahmoudi, A. Nemati, M. Maleki Shahraki, Grain growth kinetics and electrical properties of CuO doped SnO2-based varistors, J. Alloy. Comp. 770 (2019) 784–791. [3] P. Meng, X. Zhao, X. Yang, J. Wu, Q. Xie, J. He, J. Hu, J. He, Breakdown phenomenon of ZnO varistors caused by non-uniform distribution of internal pores, J. Eur. Ceram. Soc. 39 (2019) 4824–4830. [4] A. Izoulet, S. Guillemet-Fritsch, C. Estournès, J. Morel, Microstructure control to reduce leakage current of medium and high voltage ceramic varistors based on doped ZnO, J. Eur. Ceram. Soc. 34 (2014) 3707–3714. [5] M. Maleki Shahraki, P. Mahmoudi, M. Abdollahi, T. Ebadzadeh, Fine-grained SnO2 varistors prepared by microwave sintering for ultra-high voltage applications, Mater. Lett. 230 (2018) 9–11. [6] J. He, Metal Oxide Varistors: from Microstructure to Macro-Characteristics, WileyVCH Verlag GmbH & Co. KGaA, Boschstr, 2019, pp. 407–439. [7] J. Lin, S. Li, J. He, L. Zhang, W. Liu, J. Li, Zinc interstitial as a universal microscopic origin for the electrical degradation of ZnO-based varistors under the combined DC and temperature condition, J. Eur. Ceram. Soc. 37 (2017) 3535–3540.
4. Conclusions In this study, the effect of sintering temperature on the degradation behavior of SnO2 based varistors for high-voltage applications against both DC-accelerated aging test and impulse degradation test was studied and the following results were achieved: 6
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