Superior performing nano‐enabled metal oxide varistors

Over voltage is a common phenomenon in various businesses such as wireless communication, computing systems, automotive, entertainment, industrial equipment, and military systems. Even the best‐designed electric system is subject to over voltages. Therefore, various surge protection devices such as surge suppressors (e.g., circuit breakers or fuses, attenuating transients) and surge diverters (e.g, varistor, diverting the surge away from the protected device) are used in the electric systems to suppress it.1 Among them, metal oxide varistors (MOV) have been the number one surge protection component since late 1970s.2,3 Comparatively, MOVs have higher nonlinearity (α value), fast voltage clamping response, and transient surge suppression for wide applications. Metal oxide varistors are a grain boundary‐controlled device, in which the double Schottky barrier has a breakdown voltage of 2‐3.6 V per grain boundary.4,5 The phase composition and thickness of grain boundary, the microstructure uniformity and porosity of a MOV compound directly affect its voltage clamping performance. For example, sintering processes (in air, reduced atmosphere, oxidation) and operating temperature of varistors were found to result in nonlinear properties due to the change of oxidation state of grain boundaries.6‒10 Yet, the conventional thermal sintering process (TSP) is not capable of reaching a full density in a commercial MOV as shown in Figure 1. In addition, sintering at temperatures higher than 1100°C results in nonuniform microstructure with an average grain size of >7 μm, lower α value (<50), low varistor (working) voltage (<200 V/ mm), and higher leakage current (>10 μA/cm2). Researchers Received: 27 May 2019 | Accepted: 11 July 2019 DOI: 10.1002/ces2.10017


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TAN attempted nanopowders as the precursors to lower sintering temperatures expecting better sinterability, capable of cofiring with noble metals, higher homogeneity, and electrical properties. [11][12][13][14][15][16][17][18] However, most of the efforts were made either on ZnO-only or on ZnO-Bi 2 O 3 only, which are not genuine varistor compositions with commercial usefulness. Only limited efforts are carried out on the multiple-ingredient ZnO varistor compositions. Although good results were obtained, there remains necessity to further decrease grain size below 1 μm, sintering temperature below 950°C, leakage current density below 1 μA/cm 2 , and to increase nonlinearity above 60, and working voltage above 1 kV/mm. 17,18 On the other hand, processing methods directly affect the microstructure and properties of varistor compounds ( Figure 2). Conventional pressureless sintering, hot pressing (HP), spark plasma sintering (SPS), and microwave sintering (MWS) were utilized to sinter varistors more or less. SPS and MWS, however, were not well utilized for MOVs in spite of their advantages on lower sintering temperatures and shorter sintering time. [19][20][21][22][23] For example, Lin et al 24 achieved dense ZnO-only compacts using SPS under 850°C without showing varistor characteristics. Macary et al. 25 achieved 0.5 μm grain size in ZnO-Bi 2 O 3 compacts using SPS technique at 600-800°C, but failed to show a meaningful varistor characteristics. Working with nanosized ZnO-Bi 2 O 3 -based multiple-ingredient compositions, Beynet et al. 26 ended up at coarser grain size (3-8 μm) after SPS at 600-870°C, yet delivered electrical properties far from useful nonlinear characteristics. Lin et al 21 only achieved decent varistor characteristics using micron-sized ZnO powders and microwave sintering at 1100°C, which did not leverage the advantages of microwave technique very much. In fact, no reports were found applying SPS and MWS techniques on nanopowder-based varistors to demonstrate useful nonohmic varistor characteristics with high nonlinearity. How good microstructure and electrical properties can be achieved in the meaningful compositions using this field-assisted sintering process should be found out. In this work, we focused on the several meaningful compositions made of nanopowders and investigate their microstructures and electrical properties using low temperature conventional sintering, SPS, and MWS processes. We will demonstrate the superior varistor performance and the capability of these sintering processes.

| EXPERIMENTAL
All oxide ingredients are commercially available from common chemical suppliers such as Horsehead Corporation, Nanostructured and Amorphous Materials Inc., Alpha Aesar, Inc., Sigma-Aldrich, and Spectrum Chemical Mfg. Corp. A mixture is formed from zinc oxide, and additives selected from cobalt, antimony, nickel, and chromium oxide nanopowders as well as bismuth, silicon, and manganese nanopowders in certain ratios. The MOV compositions being studied in this work were based on the previously developed compositions that were characterized for meaningful varistor characteristics. 17,18 They are labeled as MOV-a, MOV-b, MOV-c (this Lab), and MOV-L (commercial) as shown in Table 1. The major ingredients comprise nano-sized ZnO, Sb 2 O 3 , Bi 2 O 3 , and other oxide additives.
The MOV samples are prepared by mixing, calcining, ball milling, and sintering. The precursor mixture is milled in a ball mill for about 6 hours in a ratio materials: ball:isopropyl alcohol = 1:5:2 to form a slurry. The slurry is dried at 100°C. The dried powder is sieved and calcined at 550°C for about 2 hours. The calcined powder is then ball milled for about 4 hours. The powder is then pressed into pellets that are about 25 cm in diameter and 1.5 mm in thickness with a force of about 10 000 pounds for about 1 minute. The pellets are sintered at temperatures from about 850-1050°C.
The SPS unit passes a pulsed DC current through a powder sample contained in a cylindrical graphite punch and dies set while applying a constant pressure. The graphite die used is 30 mm tall, with a 30 mm outer diameter The pressure on the punch and die is increased to 40 MPa. The sample is heated to the sintering temperature for the given MOV powder. The pressure is increased to 60 MPa when the sample has reached its sintering temperature for the specified time.
Microwave sintering of MOV compositions was conducted in a 2.45 GHz microwave furnace of 6 kW in air. The samples loaded in a thermal insulation package received microwave power with proper temperature uniformity that is monitored by an optical pyrometer.
Scanning electron microscopy (SEM) imaging was done using a Zeiss Supra 55VP to show the surface microstructure of MOV components. Focal ion beam (FIB) was used to show the cross-sectional microstructure of MOV components. The DC I-V characteristics were obtained from the measurements of resistance of MOV using an electrical circuit.

| RESULTS AND DISCUSSIONS
Conventional sintering using nanopowder precursors, SPS, and MWS sintering methods are utilized to investigate their effectiveness on microstructural development and varistor performance improvement.

| Nanopowder effect
For different compositions such as MOV-a, lower temperatures are found also feasible for reaching high density. Figure 3 shows the cross-sectional image of the MOV-a sample sintered at 1000°C using a FIB-SEM technique. Comparing with a commercial MOV compound having grain sizes up to 15 μm (Figure 1), the nano-enabled MOV-a has a uniform microstructure with an average grain size being ~3 μm. The electrical performance as shown in Figure 4 also exhibits the advantage of I-V characteristics comparing with the commercial MOV that uses micro-sized oxide powders as precursors. The working voltage of the MOV-a reaches ~0.95 kV/mm, about five times of a commercial MOV's. The nonlinearity value is 20. Sintering at higher temperatures (≥1050°C) does not decrease the working voltage much but increase the nonlinearity to 32. This again shows the advantages of using nanopowder precursors for finer microstructures and varistor behavior in different MOV compositions.

| Sintering additive effect
Adding more sintering additives like 3% Bi 2 O 3 further lowered the sintering temperature down to 850°C as previously reported, which is associated with low-melting eutectic formation. 17,18,27 Recently, we found that sintering MOV-c at higher temperatures (1050°C) still maintains the varistor characteristics with a high nonlinearity of 79 and working electric field of 315 V/mm, which are better than commercial MOVs that are sintered at 1200°C ( Figure 5). The SEM image shows the grain size being no more than 5 microns and the grain boundary phase being uniformly dispersed globally as shown in Figure 6. According to prior grain boundary junction mechanism, the Bi-rich phase developed at the grain boundaries increases the interface acceptor state density and the charge depletion layer width. 4,6 As result, the varistor voltage increases besides the increased number of junctions per unit volume of varistor.

| Spark plasma sintering at lower temperatures
Spark plasma sintering as a typical rate-controlled sintering process functions well with an assistance of a pulse electrical current passing through the compacted powders under a pressure. 28 Because of local heating inside the compact, the interfacial spaces can be eliminated more effectively at relatively low sintering temperatures. 29 As a result, we recently used SPS technique to sinter nano-enabled MOV-a compositions so as to further derive finer microstructure against those from conventional sintering.

| MOV with nanostructure
When the sintering temperature of MOV-a is below 650°C, no high density (<90%) can be reached. When the sintering occurs at above 650°C for 5-15 minutes, high density can be obtained and the powders in the graphite die are converted into dark pellets due to oxygen deficiency as shown in Figure 7A. The microstructure of SPS sample was polished and prepared for the subsequent inspection using SEM and FIB. Figure 7B and C shows the surface fractured image and cross-sectional image, respectively. The grain size is in the range of 0.2-0.6 μm.

| Electrical property
In order to convert the electrically conducting samples to varistor characteristics, all SPS samples were subsequently oxidized in air environment. The oxidation temperature ranges from 550-800°C, and the dwelling time is 1-5 hours. Figure 8A shows the I-V characteristics of a MOV-c composition upon oxidation. The oxidation process is very effective to convert a conducting SPS samples to varistor characteristic performance with a higher nonlinearity (52). This indicates the diffusion of oxygen into grain boundaries to form thick grain boundary junctions as explained in the literature. [16][17][18]30 The insufficient oxygen diffusion in the dense MOV compacts also explains the appearance of high leakage current at low voltages. In order to reach thorough and uniform oxygen diffusion, some MOV samples were only partially compacted using SPS technique, and a subsequent sintering in air was carried out. The I-V characteristics exhibit differently for different starting density after SPS process as shown in Figure 8B. Highly densified MOV-1 only exhibited lower nonlinearity even after further sintering at 900°C for 2 hours in air. A lower density SPS sample exhibits better varistor characteristics after subsequent sintering in air. When a SPS sample is only 45% dense, further sintering at 950°C for 2 hours results in the high nonlinearity (50), low leakage current, and the working electric field. The highest working electric field was achieved for this composition (>2 kV/mm) comparing with the conventionally sintered MOV-a (0.95 kV/mm). The response time of the varistor is also faster and the voltage quickly reaches its flat plateau.

| Microwave sintering for shorter time
Microwave sintering was known to be a noncontact method and to provide heat flow from inside to outside. 31 For  32,33 For nano-sized ZnO smaller than the skin depth, a uniform heating can be expected. Sintering temperature was reduced from 1250 to 1100°C, and much shorter time was used for densification. When it comes to MOV compositions that containing grain boundary phases and nonconductive nano-sized additives, limited information was known. Therefore, we applied microwave sintering technique to MOV-a compositions expecting to understand more about its sintered microstructure and varistor properties.

| Microstructure of nanoenabled MOVs
Sintering of MOV-a pellets was carried at temperature from 750 to 1000°C. The microwave power was provided for 5-20 minutes in various experiments. Figure 9 shows the fracture surface images of three MOV compositions sintered at four different temperatures. The microstructures show limited grain growth but broad grain size distribution with increasing the sintering temperature. At 750°C, the average grain size of MOV-a is about 0.5 micron and the density is relatively low. At 950 or 1000°C, no considerable grain growth occurs and grain size appears in the range of 0.5-2 micron. The grain boundary phase (white areas) appears to be conformally networked at the grain boundaries forming a typical MOV microstructure junctions. Similar microstructures were observed for other two MOV compositions that were sintered at 800-850°C. For example, when sintering MOV-c at 850°C, the grain size is about 1.5 micron and density is moderately high. In comparison with commercial MOV, the grain size is much smaller providing more supply of barrier junctions for varistor performance. However, the unbalanced grain growth can be related to the nonconducting oxide additives that absorbs less microwave power during the sintering.

| Electrical property
The varistor characteristics of the microwave-sintered samples were tested using the steady-state current test setup. Figure 10 shows that the I-V curves for the MOV-a sample after microwave sintering at 950°C for 5 minutes and conventional sintering at 1000°C for 2 hours. The microwave sintering does not result in a high nonlinearity 10 and lower leakage current comparing with that by the conventional sintering. This may be related to the broad grain size distribution and unfavorable grain boundary junction.  for grain growth restriction and sintering temperature reduction. They can sensitively change the sintering process, microstructure, and electrical properties. Use of nanopowders and conventional sintering has resulted in uniform microstructure with smaller average grain size (1-3 μm) and higher nonlinearity (60-130). However, their effect on reaching microstructure below 1 μm is limited if sintering temperature is not lowered than 900°C. 18 Grain sizes of 0.3-1 μm become possible using SPS technique as shown in Figure 11. The shaded bars show the range of sintering temperatures for various MOV compositions. The advantages are attributed to the quick elimination of free surface spaces and retarded grain boundary diffusion due to lowered sintering temperatures. In spite of lower α value, the smaller grain sizes in SPS samples provide the increased number of grain boundaries, which is beneficial for higher working voltage and longer life expectation for MOV. Microwave technique turned out not performing as well as other two techniques in terms of nano-enabled MOV performance. This result is somewhat in agreement with that of MOVs made of micron-sized powder, which exhibits higher nonlinearity but also high leakage current, large grain size distribution in micron range after microwave sintering at 1000-1100°C. 34 This may be related to the use of different compositions and nonuniform electrical conductivity of the oxide mixture. Comparing with conductive ZnO, many other additives are nonconductive and their microwave energy absorption is limited, and the internal heating rate in those regions is less than ZnO region. Inhomogeneous heating and grain growth are then resulted inside the pellets. A recent interesting progress in sintering techniques was the cold sintering process (CSP). 35 Is it possible to use it to further lower the sintering temperature below 250°C and grain size of below 300 nm? This could be an interesting subject of investigation for MOVs.

| CONCLUSIONS
Nano-oxide powders were used as the precursors for several MOV compositions, and superior overall varistor performance was achieved in MOV compositions developed at this laboratory. The conventional sintering process gives rise to homogeneous microstructures, grain size below 3 μm, sintering temperatures 850-1050°C, nonlinearity higher than 60, leakage current of <10 −7 A/cm 2 , and working voltage of >5 times of commercial MOVs. Superior performance was also achieved using field-assisted SPS process and nano-enabled MOV compositions. Submicron grain sizes (0.3-1.5 μm), lower sintering temperature (650-750°C), and higher varistor voltage (2 kV/mm) were shown for a MOV-a composition. The process comprising SPS partial sintering and the subsequent sintering renders more favorable microstructure and varistor performance.
Microwave sintering delivers relatively smaller grain size in shorter sintering time at temperatures of 800-950°C; however, the electrical properties of MOVs being used in this work are not equally good. The internal heating mechanism absorbing microwave power may subject to the nonuniformity issue at conductive ZnO grains and other nonconductive grain boundary phases.