Influence of processing on microstructure and electrical characteristics of multilayer varistors

The paper reports on the influence of processing on microstructure and electrical properties of multilayer varistors based on zinc oxide doped with Bi2O3, Sb2O3, Co2O3, MnO, Cr2O3, B2O3, and SiO2. 0.5–1 wt% of AlF3-CaB4O7 was used as a new effective sintering aid. The behavior of green laminates during heating was characterized using differential thermal analysis and a heating microscope. As revealed by XRD, SEM, and EDS methods, the varistor layers are composed of ZnO grains of 1–5 µm size, submicrometer spinel and pyrochlore grains situated at the ZnO grain boundaries, and nanometric Bi2O3-rich films surrounding ZnO grains. Complex impedance studies carried out in the frequency range of 0.01 Hz-2 MHz at temperatures changing from −30 to 150 °C imply the formation of semiconducting grains and insulating grain boundaries. Frequency dependence of dielectric permittivity shows a high plateau at lower frequencies, typical for barrier layer capacitance effect. The fabricated multilayer varistors show nonlinear current-voltage characteristics with a high nonlinear coefficient of 26–38. The breakdown voltage was found to decrease within the range of 66–130 V with sintering temperature increasing from 1000 to 1100 °C. Good surge current capability of the varistors was confirmed by the tests using 8/20 µs pulses.

Tailoring of the properties of the potential barrier is a complex interplay of the composition and level of dopants, as well as oxygen content related to thermal treatment conditions. Each of the dopant oxides has its essential contribution to the creation of the demanded microstructure and varistor nonlinear characteristics.
The key role is attributed to those additives which form double Schottky barriers at grain boundaries: Bi 2 O 3 , PrO x , Co 2 O 3 , SnO 2 , NiO. In the most popular ZnO-Bi 2 O 3 varistors, intergranular layers responsible for creation of double Schottky barriers are formed during sintering by the Bi 2 O 3 -rich liquid phase containing dissolved other dopants. Very important dopants responsible for ensuring good nonlinear properties are transition metal oxides, such as Co 2 O 3 , MnO 2 , Cr 2 O 3 , TiO 2 . Some dopants as MnO 2 , SiO 2 serve as sintering aids, and others, like Sb 2 O 3 , Al 2 O 3 , MgO, TiO 2 , Cr 2 O 3 , Fe 2 O 3 , inhibit excessive grain growth. Some additives have simultaneously a few functions, e.g., SiO 2 can both lower sintering temperature and decrease grain size of ZnO, as well as increase the potential barrier height which leads to the improved nonlinear characteristics [9].
In general, larger ions of Bi, Ba, Sr, Pr show tendency to segregate at grain boundary region. Smaller ions of Mn, Co, Ni, Cr, Al can create trap states both in ZnO grain interiors and at grain boundaries. Dopants entering ZnO grains can form shallow donor levels with the activation energies of about 0.05 eV or deep donor levels with the activation energies higher than 1.65 eV [10]. Bulk traps in ZnO are also attributed to intrinsic defects, such as doubly ionized zinc interstitial Zn i A very important factor influencing the barrier height and consequently proper varistor properties is oxygen content in the grain boundary region [19]. In the degraded ZnO varistors, a significant oxygen deficit was observed. Furthermore, thermal treatment in O-rich atmosphere was found to result in recovery of the degraded varistors [20]. During sintering diffusion of oxygen proceeds easily in Bi 2 O 3 rich phase surrounding ZnO grains since Bi 2 O 3 is a good ionic conductor with a high oxygen vacancy concentration.
Growing application range of low voltage devices and strong demand for miniaturization and enhanced integration scale in electronics stimulate development of multilayer varistors . These elements destined for surge protection of integrated circuits, hybrid circuits, and surface mount circuits, are fabricated by co-sintering of several tape cast ceramic foils with screen printed internal electrodes connected in parallel. The dense, fine-grained, and uniform microstructure is crucial for good properties of multilayer varistors. Uniform dopant distribution inside the entire varistor is known to improve significantly leakage current and nonlinear properties of a varistor [22]. Furthermore, broad grain size distribution has been reported to cause poor performance in ESD conditions and shorter life time of multilayer varistors [23]. Pr doped ZnO multilayer chip varistors were found to exhibit better ESD withstand capability than those based on Bi doped ZnO [24].
The paper describes processing, microstructure, composition, electrical characteristics, and surge current capability of multilayer varistors based on doped zinc oxide. In the present investigation, the materials with the composition and level of dopants optimized basing on our previous studies [33,34] were utilized for preparation of green varistor tapes. The literature devoted to LTCC processing of multilayer varistors and characterization of green varistor tapes is scarce. This paper was focused on LTCC process of multilayer varistor fabrication comprising tape casting, lamination, and cosintering as well as on optimization of thermal treatment conditions by means of differential thermal analysis and heating microscope studies. Furthermore, the influence of sintering temperature and the effect of a sintering aid AlF 3 -CaB 4 O 7 on microstructure and nonlinear behavior of the fabricated varistors were investigated.

Experimental
Multilayer varistors were fabricated in a process which included several operations: preparation of slurry, tape casting of doped ZnO ceramic foils, laser cutting of green sheets, screen printing of internal electrodes, stacking of green sheets, isostatic lamination, dicing into individual varistors, firing of multilayer varistors, deposition, and firing of Ag external connections of internal electrodes.
For preparation of a slurry, the powder obtained by the coprecipitation method based on ZnO doped with Bi 2 O 3 , Sb 2 O 3 , CoO, MnO, Cr 2 O 3 , B 2 O 3 , and SiO 2 was ball-milled with organic additives. The composition of the inorganic part of the slurry is shown in Table 1. This composition was optimized basing on the results of our previous studies [33,34], where the use of different levels of Pr 2 O 3 , CoO, and Sb 2 O 3 dopants has led to worse electrical characteristics. The organic components applied in the slurry comprised polyvinyl butyral as a binder, fish oil as a dispersant, polyethylene glycol and dibutyl phthalate as plasticizers, toluene and isopropyl alcohol as solvents. The composition of the slurry is given in Table 2.
In an additional version of the basic varistor composition, in order to decrease the sintering temperature, 0.5-1 wt% of a low melting mixture AlF 3 -CaB 4 O 7 (1:1 molar ratio) was used as a sintering aid.
Tape casting (TTC-1200, Mistler, USA) was applied for obtainment of ceramic flexible tapes. After casting, the green tapes were dried at room temperature and at 50 ℃ for a few hours. Then, sheets with desired dimensions and holes for positioning were cut by a laser (E-355-3-G-OA, Oxford Lasers, UK). For basic composition, internal electrodes made of a commercial thick film Pt paste were deposited using screen printing technique. For the composition with AlF 3 -CaB 4 O 7 , an Ag commercial paste was utilized. After stacking of 5-10 green ceramic sheets with electrodes separated by 1-3 layers of blank sheets, isostatic lamination was performed under a pressure of 30 MPa at 70 ℃ (IL-4008PC, Pacific Trinetics Corporation, USA).
The observation of green laminates in a Leitz heating microscope provided information about optimal sintering range of ceramics and proper quality of lamination process. Images which illustrate changes of the shape and  dimensions of green laminate samples were registered during heating up in the temperature range of 20-1350 ℃. Differential thermal analysis and thermogravimetric studies of green varistor tapes were performed using a Netzsch simultaneous thermal analyzer in order to determine the burnout range of organic components of the tape. Green laminates were diced into individual varistors and subsequently fired according to the carefully adjusted profile with a maximum temperature of 1000-1100 and 920-950 ℃ held for 2 h for the basic varistor composition and the composition with AlF 3 -CaB 4 O 7 addition, respectively. A final operation was deposition of external connections made of Ag paste and their firing at 850 ℃. Test varistor samples used for electrical characterization were 1 mm thick with the electrode surface area of 0.8 cm 2 .
The microstructure studies and elemental microanalysis of multilayer varistors were carried out using an FEI scanning electron microscope and the EDAX Genesis EDS system. The phase composition of the varistor ceramic layers was detected by a Philips X'Pert diffractometer.
Impedance spectroscopy method was used to investigate dielectric response of the varistors in the temperature range from -30 to 150 ℃ at frequencies ranging from 0.01 Hz to 2 MHz (Atlas Sollich, Poland and Quadtech, USA).
A Keithley SourceMeter was applied for currentvoltage measurements of the fabricated multilayer varistors in a voltage range of 0-110 V. Surge current capability of multilayer varistors was tested using a standard impulse current with waveform 8/20 μs for short-circuited output. The surge tests were performed using UCS 500-M Ultra Compact Simulator manufactured by EM Test. Voltage waveform was monitored using high voltage probe and current shape was observed as voltage drop on shunt resistor, both recorded on a Keysight MSO-X 3014T Oscilloscope. Initial and after surge characterization of the tested devices were performed using Source Measure Unit.

Results and discussion
The obtained varistor tapes were smooth and flexible with a good mechanical strength and uniform thickness. The thickness of the tape after drying was about 80 μm. In Fig. 1, the results of thermogravimetric studies and differential thermal analysis (TG, DTG, and DTA curves) are presented for a green varistor laminate. In the temperature range 200-450 ℃, a distinct exothermic effect was detected in the DTA curve, accompanied by a rapid mass loss in the TG curve. There occur an inflection point at 308 ℃ and two local small maxima at 353 and 420 ℃. These exothermic effects are related to burnout of organic components from the green tape.
Changes in dimensions and shape of green varistor samples were observed in a heating microscope (Fig.  2). The sample with the composition given in Table 1 starts to shrink at about 900 ℃. A significant shrinkage takes place above 1000 ℃. No melting or softening of the sample occurs up to 1350 ℃. The temperature range of 1050-1100 ℃ was chosen as optimal for effective sintering. The applied firing profile ensures slow burnout of organic components from the green tape and from the electrode paste (in the temperature range of 300-450 ℃) and subsequent good densification of ceramic layers as a result of high temperature sintering process. The use of 0.5 wt% of AlF 3 -CaB 4 O 7 sintering aid enabled lowering the sintering temperature from 1050 to 920 ℃ due to formation of liquid phase based on this additive. This effect of improved sinterability is illustrated in Fig. 2(b) which shows changes of the sample dimensions for the basic composition with 0.5 wt% of AlF 3 -CaB 4 O 7 addition. The shrinkage onset was found to decrease from 900 ℃ for the basic varistor composition to 830 ℃ for the AlF 3 -CaB 4 O 7 enriched composition. Figures 3(a)-3(c) compare SEM images of ceramic layers in multilayer varistors sintered at different temperatures. The images reveal the characteristic, dense, and fine-grained varistor microstructure composed of relatively big ZnO grains, thin Bi 2 O 3 -rich films surrounding these grains or segregated at triple grain junctions, and small grains rich in antimony and/or bismuth pinned at grain boundaries. The compatibility of platinum based electrodes with the ceramic layers is very good, as illustrated in Fig. 3(d). The thicknesses of the ceramic layers made of a single varistor green tape are 65, 52, and 45 μm for the multilayer samples sintered at 1000, 1050, and 1100 ℃, respectively. In general, the microstructure is similar for samples sintered at 1000, 1050, and 1100 ℃. However, a moderate growth of ZnO grains and a decrease in uniformity of ZnO grain sizes, along with a better densification degree can be observed for the varistors fired at 1050 and 1100 ℃. The average grain sizes are 2, 2.8, and 3.3 μm for the varistors sintered at 1000, 1050, and 1100 ℃, respectively. The varistor composition applied in this study prevents excessive grain growth, thus for well densified samples, which were sintered at 1050 and 1100 ℃, the maximal grain sizes do not exceed 5 and 7 μm, respectively, and a narrow grain size distribution range is maintained. Kuo et al. [28] proved that in a multilayer varistor structure composed of a few Bi 2 O 3 doped ZnO layers with different thicknesses ranging from 20 to 140 μm, the grain growth was constrained by the inert Pt inner electrodes. Both the grain size and the grain size variation decreased with diminishing thickness of the ceramic layer [28]. In this work, no differences were observed between grain sizes in the ceramic layers of 50-250 μm thick, separated by the electrodes in a multilayer structure, and those in  Table 3. Dominant grains of 1-5 μm size (represented by the point 1 in Fig. 4(b) and Table 3), correspond to ZnO. The relatively big ZnO grains are surrounded by thin nanometric layers rich in bismuth (point 2 in Fig. 4(b) and Table 3) which form at grain boundaries insulating potential barriers responsible for the creation of varistor properties.   Table 3 Results of EDS analysis in the points marked in Fig. 4 Smaller and brighter grains of 0.3-1 μm size, containing elements with higher atomic number are situated at boundaries of ZnO grains (points 3, 4, and 5 in Fig. 4(b) and Table 3). The EDS spectra determined in these points indicate the presence of zinc, antimony, bismuth, and chromium. These are presumably grains of spinel Zn 7 Sb 2 O 12 or pyrochlore Bi 3 Sb 3 Zn 2 O 14 enriched in other dopant elements, such as Cr, Co, Mn. The spinel grains are known to play a crucial role in inhibition of growth of ZnO grains. Spinel phase was reported to undergo reversible transformation into more conductive pyrochlore phase during cooling step [31]. As shown in Fig. 5, the presence of small amounts of spinel and pyrochlore phases was revealed in the XRD pattern besides the prevailing ZnO phase. Figure 6 shows SEM images of the fractured cross-section of the multilayer varistor with 0.5 wt% of AlF 3 -CaB 4 O 7 addition sintered at 920 ℃. Owing to lowering of the sintering temperature, the use of cheap silver internal electrodes was possible. As illustrated in Fig. 6(a), the cooperation between Ag conductive layer and varistor ceramic layer is very good. The comparison of Fig. 3(a) and Fig. 6 due to the sintering aid presence. The sample with AlF 3 -CaB 4 O 7 is better densified at lower temperature and preserves the characteristic varistor microstructure. Because of lower sintering temperature mean ZnO grain size for a sample sintered at 920 ℃ is slightly lower that that at 1050 ℃. Furthermore, from Fig. 6(c) it follows that for the AlF 3 -CaB 4 O 7 enriched varistor thin insulating films at ZnO grain boundaries responsible for barrier layer effect could not be so easily recognized as for the same composition without the applied sintering aid (Fig. 4(b)). The leakage current of the fabricated varistors with the basic composition decreases with increasing sintering temperature. The values of this parameter determined at 80% U 1mA are 124, 117, and 81 μA/cm 2 for the varistors sintered at 1000, 1050, and 1100 ℃ , respectively.
In Figs. 7(a) and 7(b) the current-voltage characteristics are depicted for the varistors sintered at 1000, 1050, and 1100 ℃. Above the breakdown voltage V b , a rapid increase in current and transition to the region of nonlinear characteristic of the varistor occurs which results from an abrupt decrease in the varistor resistance. There is a strong dependence of the breakdown voltage on the sintering temperature. The breakdown voltages were found to be 130, 80, and 66 V for varistors sintered at 1000, 1050, and 1100 ℃, respectively. The observed decrease in breakdown voltage with the rising sintering temperature is consistent with the results of several authors [11,16,18,23,25]. Lower values of the breakdown voltage for the varistors sintered at higher temperatures are due to the grain growth and consequently a decrease in the grain boundary number between the adjacent electrodes. The electrostatic barrier created at Bi-rich boundaries of ZnO grains was estimated to be at a level of about 3 V [2,27]. The breakdown voltage of a multilayer varistor increases linearly with increasing number of grain boundaries between inner electrodes [27]. Generally, application of ceramic layers with an enhanced thickness leads to an increase of the number of grain boundaries and consequently to a higher breakdown voltage. However, this effect can be to some extent suppressed by facilitated grain growth due to a larger space between internal electrodes [27]. In this work, it was stated that both the breakdown voltage and the nonlinear coefficient are higher when the thickness of the varistor layers is increased. Since for multilayer varistors the breakdown voltage is proportional to the total number of active grain boundaries between the inner electrodes, this crucial varistor parameter can be easily tailored by changing the thickness of the varistor layer as well as to some extent by changing the sintering temperature.
The nonlinear coefficients α determined from the slope of the logI = f(logU) plots presented in Fig. 7(b) are 38, 26, and 37 for the sintering temperatures of 1000, 1050, and 1100 ℃, respectively. The effect of the sintering temperature on this parameter is not clear. The results obtained by other authors are also ambiguous and strongly dependent on the specific varistor composition and the considered sintering temperature range. In general, it seems that nonlinear coefficient grows with sintering temperature up to attaining optimal sintering range corresponding to the proper microstructure and then drops with further increasing of this temperature. Numerous and often contradictory temperature dependent factors are supposed to have impact on the dependence of nonlinear coefficient on sintering temperature, such as bismuth loss due to volatilization, uniform distribution of additives, solubility of additives in bismuth-rich phase, secondary phase formation at grain boundaries, oxygen content, changes in the grain boundary resistivity, fraction of electrically active grain boundaries. For rare-earth doped ZnO varistors, lowering of nonlinear coefficient with increasing sintering temperature was observed and assigned to changes in grain boundary resistivity [11]. A decrease in nonlinear coefficient with sintering temperature, attributed to the diminished grain boundary number, was also reported for the multilayer varistor based on ZnO doped with Al, Mn, Co, Sb, and Bi oxides [25]. For ceramic varistors based on pure ZnO, nonlinear coefficient grew with sintering temperature changing in the range 800-1100 ℃ , remained constant for 1100 and 1200 ℃, and significantly decreased when the sintering temperature was raised to a high temperature of 1400 ℃ [18].  grains can be detected only at low temperatures and high frequencies. In the inset of Fig. 8(a), a small depressed arc attributed to this response is visible at -30 and 0 ℃. At higher temperatures, the nonzero intercept with the real axis implies the existence of a www.springer.com/journal/40145 high frequency arc beyond the measurement range, as illustrated in the inset of Fig. 8(b). The resistance of grains assessed at a level of 3 Ω is low and slightly temperature dependent. The ratio of the resistivities of grain boundaries and grains is high. This leads to creation of potential barriers at grain boundaries responsible for the varistor behavior.
In Fig. 8(c), the dielectric permittivity of the varistor ceramic is plotted as a function of frequency in the range 10-2×10 6 Hz for a few temperatures ranging from -30 to 150 ℃. High (600-1200) and slightly frequency dependent values of dielectric permittivity at low frequency region followed by an abrupt drop at higher frequencies above (10 5 Hz) are characteristic for the barrier layer capacitance effect. Dielectric permittivity for grain boundary barrier layer capacitors, to which ZnO varistors belong, is proportional to dielectric permittivity of the insulating barrier at grain boundaries and the grain size, and inversely proportional to the thickness of this barrier layer (ε ≈ ε gb d g /d gb , where ε gb is the dielectric permittivity of the grain boundary layer, d g is the grain size, and d gb is the thickness of the grain boundary). Raising of the sintering temperature does not change markedly dielectric permittivity of the insulating barrier layer, but results in grain growth and an increase in the insulating layer thickness, which can lead to a decrease of the dielectric permittivity [11]. Figure 9 presents the current-voltage characteristics for the multilayer varistor with 0.5 wt% of AlF 3 -CaB 4 O 7 . Although SEM observations (Fig. 6) indicated that the microstructure of a varistor ceramic doped additionally with 0.5 wt% AlF 3 -CaB 4 O 7 as a sintering aid remains dense and similar to that for the basic material without this additive, the varistor properties were found to be worse. The breakdown voltage of the multilayer varistor with the single ceramic layer thickness of 170 μm remains at a high level of 280 V ( Fig. 9(a)), but the nonlinear coefficient decreases to a lower value of 9 ( Fig. 9(b)). At sintering temperature of 920 ℃ , AlF 3 -CaB 4 O 7 addition forms a liquid phase and enters the grain boundary region. This effect facilitates sintering process, does not decrease breakdown voltage, although it diminishes the nonlinear coefficient. This implies that proper functioning of the blocking layers at the grain boundaries was degraded probably due to a disadvantageous change in the composition of these regions. Elucidation of this effect needs further studies. Figure 10(b) shows the current-voltage characteristics for the basic varistor composition before and after surge testing using the pulse shape 8/20 μs for short-circuit current presented in Fig. 10(a). The ability of the multilayer varistor to withstand these pulses is very good. The changes in the course of the I = f(U) characteristics after applying of 10 pulses are small, at a level of about 1%.

Conclusions
The composition of green varistor tapes based on ZnO doped with Bi, Sb, Co, Mn, Cr, B, and Si oxides as well as processing procedure of multilayer varistors were optimized basing on heating microscope and DTA studies. A proper, uniform, and fine-grained varistor microstructure was attained that resulted in the formation of effective potential barriers at ZnO grain boundaries. The size of ZnO grains was not dependent on the thickness of the ceramic layers ranging from 50 to 250 μm. The developed multilayer varistors exhibited good nonlinear electric properties. The dominant dielectric response represented by a big arc in the complex impedance plots was related to grain boundaries. The breakdown voltages of the varistors were 66-130 V and decreased with increasing sintering temperature due to a moderate grain growth. The nonlinear coefficients were relatively high, at a level of 26-38. After surge current tests the current-voltage characteristics were found to remain almost unchanged. Effective lowering of the sintering temperature to a level of 920 ℃ enabling the use of cheap Ag internal electrodes due to a small addition of AlF 3 -CaB 4 O 7 was accompanied by inferior nonlinear varistor behavior as compared with the composition without this additive. This experiment proved that the varistor composition is a delicate balance and the desired properties of grain boundary barriers can be easily degraded.
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