inner electrodes for multilayer varistors

Inner Electrodes for Multilayer Varistors

Shu-Ting Kuo and Wei-Hsing Tuan*

Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan

Yeh-Wu Lao, Chung-Kai Wen, and Huey-Ru Chen

Walsin Technology Corporation, Kaohsiung 700, Taiwan

In the present study, Pt or AgPd metal is used as the inner electrode for Bi2O3-doped ZnO multilayer varistors (MLV).The growth of the ZnO grains is constrained by the presence of the inner electrodes. The Pt inner electrodes are chemicallyinert to Bi2O3-doped ZnO. The Bi2O3 could react with Pd to form PdBi2O4. The Bi2O3-rich liquid also tends to wet theAgPd electrode. The size of ZnO grains in the MLV/AgPd specimen is larger. The ZnO grains in the MLV/AgPd specimencan even grow to a size larger than the layer thickness at the expense of electrode continuity.

Introduction

Many passive components nowadays are in a mul-tilayered structure due to the demands of miniaturiza-tion and integration. In order to increase the volumeefficiency, the layer thickness has decreased from tens ofmicrometers to several micrometers in the last decade.1–3

The interactions between ceramic layers and inner elec-trodes thus become important for the performance ofmultilayered components.

The ZnO-based ceramics exhibit unique nonlinearcurrent–voltage (I–V) characteristics, being commonlyused as varistors to protect electronic devices againstvoltage surges.4–7 Because many 3C electronic gadgetsare now operated under a low voltage through a re-chargeable battery, multilayer varistors (MLV) are beingdeveloped for such low-voltage applications.

MLVs are composed of ZnO-based ceramic layersand inner electrodes. The basic requirement for the in-ner electrode is its ability to cofire with the ceramic toelevated temperatures. Precious Pt has been used as in-ner electrodes for some time; it has been proved to besuitable as an inner electrode for ZnO-based MLV.8

However, the cost of Pt is around three times that of the70Ag/30Pd alloy. Because of this cost issue, AgPd alloysare now considered as candidate materials for innerelectrodes. Previous studies on multilayer ceramic ca-pacitors indicated that reactions could take place be-tween AgPd and Bi2O3.3,9–11 Because Bi2O3 is essentialfor the preparation of ZnO-based varistors, the interac-tion between the Bi2O3-doped ZnO and AgPd alloys atan elevated temperature is a topic of increasing techno-logical importance.

In the present study, a ‘‘simulated’’ multilayerstructure is prepared. A model varistor composition,5 wt% Bi2O3-doped ZnO, is used as the ceramic partfor the MLV. Both Pt and AgPd (70 wt%/30 wt%)

Int. J. Appl. Ceram. Technol., 6 [2] 223–230 (2009)DOI:10.1111/j.1744-7402.2008.02269.x

Ceramic Product Development and Commercialization

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r 2008 The American Ceramic Society

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metals are used as the inner electrodes. The metallicelectrodes and ceramic layers of various thicknesses arethen laminated into a multilayer structure. The phaseand microstructure evolution during cofiring are theninvestigated.

Experimental Procedure

To prepare the 5 wt% Bi2O3-doped ZnO greentape, several solvents and binders were first mixed withthe ceramic powders. The tape, with a thickness of20 mm was then cast. Both Pt and 70%Ag–30%Pdcoprecipitate powders were used for the preparation ofthe metallic pastes. The pastes were deposited onto thegreen tape by screen printing. A multilayer structurecontaining seven layers with thicknesses from 20 to140 mm was made by laminating 1–7 green tapes.To facilitate comparison, the layers with different thick-nesses were all built into one component. The laminateswere then cut into a size of 1.85 mm (length)�0.95 mm (width)� 0.75 mm (thickness). All specimenswere firstly fired from room temperature to 4001C in airfor 60 min using a heating rate of 11C/min to removethe organics. After the binder burnout stage, firing wasperformed in air at 900–12001C for 60 or 1000 min,with heating and cooling rates of 51C/min. In order toprevent the vaporization of Bi2O3 at elevated tempera-tures, the specimens were sintered in a powder bedcomposed of ZnO and 5 wt% Bi2O3.

The phases of the multilayer specimens were char-acterized with a synchrotron X-ray beam (Beam line17B1, National Synchrotron Radiation Research Cen-ter, Hsinchu, Taiwan). In order to carry out the phaseanalysis on multilayer specimens, several multilayerspecimens were mounted together into resin and thenground to expose the cross section of the specimens. Thesynchrotron X-ray beam was then spotted at the crosssection of the specimens to carry out the phase analysis.For microstructure observation, the specimens wereground with SiC abrasive papers and polished with0.05 mm Al2O3 particles. The specimens were etchedwith dilute hydrochloric acid. The microstructures wereobserved by a scanning electron microscope (XL-30,Philips, Eindhoven, The Netherlands). The composi-tion analyses were carried out by using energy-dispersiveX-ray spectroscopy (EDX, LEO Instrument, Cambridge,U.K.) or an electron probe microanalyzer (EPMA,Model JAX-8200, JEOL, Tokyo, Japan). The size of

the ZnO grains was determined by applying an imageanalysis technique. By assuming that each grain is spher-ical in shape, the size of the ZnO grains was estimated.The current–voltage (I–V) characteristics of the speci-mens were measured using a dc current method at cur-rents ranging from 1 mA to 0.05 A. The nonlinearcoefficient (a-value) was determined by measuring thevoltage at the applied currents of 1 and 10 mA.

Results

Figure 1 shows the X-ray diffraction (XRD) pat-terns of the MLV/Pt and MLV/AgPd specimens aftersintering at 11001C for 60 min. Apart from ZnO, Pt, orAgPd, a Bi2O3-rich phase is also present in both theMLV specimens. No reaction phase is found in theMLV/Pt specimens. However, a new phase, PdBi2O4, isfound in the MLV/AgPd specimen.

Figure 2 shows the microstructures of the MLVspecimens; these specimens are sintered at 12001C for60 min. There are eight layers of inner electrodes in thecenter of the MLV specimens; the particle packing inthe middle part of the green specimen is not the same asthat in the end parts. The shrinkage is larger in themiddle part of the MLV specimens. This larger shrink-age in the middle part may also induce some openingin the center of the MLV/Pt specimens, Fig. 2a.Because the metallic Pt is relatively soft and ductile,

Fig. 1. X-ray diffraction patterns for multilayer varistors (MLV)/Pt and MLV/AgPd specimens after sintering at 11001C for 1 h.

224 International Journal of Applied Ceramic Technology—Kuo, et al. Vol. 6, No. 2, 2009

the Pt electrodes are deformed and filled into most ofthe opening after grinding and polishing. Nevertheless,the Pt electrodes remain in their original positions aftersintering at 12001C. However, some AgPd electrodesdisappear after sintering at 11001C for 1000 min andat 12001C for 60 min. A typical example is shown inFig. 2b. In the figure, there is only one thick electroderemaining in the MLV/AgPd specimen. The remainingelectrode is the one that was close to the thinnest ce-ramic layer before sintering. For the MLV/AgPd spec-imens sintered either at a lower temperature or witha shorter time, the AgPd electrodes are intact aftersintering.

Figure 3 shows the detailed microstructures for thefirst, second, and third layers of the specimens shown inFig. 2. The ZnO grains are elongated along the direc-tion parallel to the Pt inner electrode in the MLV/Ptspecimen. The Bi2O3-rich phase tends to be perpendic-ular to the Pt electrode. In the MLV/AgPd specimens,

the ZnO grains are more or less equiaxed. An isolatedAgPd alloy grain is located between ZnO grains. Thereaction phase, PdBi2O4 phase, and Bi2O3-rich liquidphase are in the form of sequential dendrites or layers,suggesting that the following reaction has taken placeduring the cooling stage:

Pdþ Bi2O4 ! PdBi2O4 ð1Þ

Figure 4 shows the size of ZnO grains within Pt orAgPd inner electrodes as a function of layer thickness.As the sintering temperature is lower than 10001C, thesize of the ZnO grains is smaller than the layer thick-ness. Furthermore, the size of ZnO grains shows littledependence on the layer thickness. As the sintering tem-perature is higher than 11001C, the size of ZnO grainsincreases with an increase in layer thickness. Comparingthe specimens treated at the same sintering temperature,

Fig. 2. Cross section of (a) multilayer varistors (MLV)/Pt and (b)MLV/AgPd specimens after sintering at 12001C for 60 min.Detailed microstructures in the squares are shown in Fig. 3.

Fig. 3. Microstructures within the first to third layer for (a)multilayer varistors (MLV)/Pt and (b) MLV/AgPd specimens aftersintering at 12001C for 60 min.

www.ceramics.org/ACT Inner Electrodes 225

the size of ZnO grains in the MLV/AgPd specimens islarger than that in the MLV/Pt specimens.

The composition mapping as determined by theEDX analysis in an MLV/Pt specimen is shown in Fig. 5.The Pt electrode is continuous within the MLV/Pt spec-imen after sintering at 11001C for 1000 min, Fig. 5d.The Bi2O3-rich phase is mainly located between theZnO grains, Fig. 5d. There is hardly any Bi2O3-richphase present at the ZnO/Pt interface. Figure 6 showsthe composition distribution in the MLV/AgPd speci-men as determined by the EPMA analysis. It is worthnoting that Ag is no longer continuous in the MLV/AgPd specimen. At the discontinuous points between theAgPd electrodes (Fig. 6e), the Bi intensity is high (Fig.6c). A small amount of Pd is also found at the discon-tinuous points (Fig. 6f). This indicates that both theBi2O3-rich phase and PdBi2O4 are located at the dis-continuous points of the AgPd electrodes. Furthermore,the Bi2O3-rich phase is found to surround the AgPdelectrode (Fig. 6c).

The I–V curves for the MLV/Pt and MLV/AgPdspecimens are shown in Fig. 7. The breakdown voltageof MLV/Pt specimens decreases with an increase insintering temperature, demonstrating that the size ofthe ZnO grains increases with an increase in the sinte-ring temperature. Apart from the MLV/AgPd specimenprepared by sintering at 12001C for 60 min, the break-down voltage of the MLV/AgPd specimen is lower thanthat of the MLV/Pt specimens. This is mainly because

the size of ZnO grains in the MLV/AgPd specimen islarger. For the MLV/AgPd specimens sintered in thetemperature range from 9001C to 11001C, the break-down voltage of the MLV/AgPd shows little depen-dence on the sintering temperature. The breakdownbehavior of varistor is dominated by the largest grainbetween inner electrodes.8 It thus indicates that someZnO grains are relatively large in the MLV/AgPd spec-imens even when the sintering temperature is low. Asthe MLV/AgPd specimen is prepared by sintering at12001C for 60 min, the breakdown voltage then jumpsto a very high value.

Discussion

A novel multilayer structure is adopted in the pres-ent study. The structure allows us to investigate theeffect of layer thickness on the stability of the innerelectrode. Metallic Pt is an inert metal. It is stable atelevated temperatures in terms of its physical and chem-ical characteristics. The shape of the Pt electrode re-mains the same even when the sintering temperature isas high as 12001C. The Pt does not react with eitherZnO or Bi2O3-rich phases at elevated temperatures.Furthermore, the Pt electrodes act as a diffusion barrierto mass transportation. The size of ZnO grains is there-fore small within the Pt inner electrodes, as comparedwith the grains outside the inner electrodes (see Fig. 4).As the size of ZnO grains approaches the layer thickness,the ZnO grain can thus grow only in the direction par-allel to the direction of the inner electrodes. To shortenthe interface area, the Bi2O3-rich phase tends to be per-pendicular to the inner electrodes. A columnar structureis thus formed after sintering at a temperature above11001C.

However, the 70%Ag–30%Pd electrode is not asstable as we would expect. Because a powder bed con-taining sacrificing Bi2O3 is used in the present study, theBi2O3 remains in the specimen after sintering (see Fig. 1).The presence of Bi2O3 encourages the formation ofPdBi2O4 during the cooling-down stage, as demonstratedby reaction (1). The PdBi2O4 and Bi2O3-rich phases areformed in sequence. A dendrite or a layer structure is thusformed (see Fig. 3b). Because part of the Pd in the AgPdalloy is consumed by the reaction between Pd and Bi2O3

and some Ag may vaporize above 10001C,12 the com-position ratio of Ag to Pd in the AgPd electrode is notconstant during sintering. However, the AgPd inner

Fig. 4. Average size of ZnO grains in the multilayer varistors(MLV)/Pt and MLV/AgPd specimens as a function of layerthickness.


electrodes can still act as a diffusion barrier to masstransportation as the sintering temperature is low.The columnar structure is also observed in the MLV/AgPd specimen after sintering at 11001C for 100 min,Fig. 6a. However, the AgPd inner electrodes are notcontinuous after firing at 11001C for 1000 min orat 12001C for 60 min. It thus indicates that thestability of the AgPd electrode is a thermal-assistedprocess.

The AgPd electrode is initiated with mixtures offine Ag and Pd particles. The AgPd alloy is formedduring the heating-up stage. After sintering, the AgPd

alloy is still polycrystalline in nature. The EPMA anal-ysis demonstrates that the wetting of the Bi2O3-richphase on AgPd grain is good (see Fig. 6c). The wettingof the Bi2O3-rich liquid on AgPd alloy grains is so goodthat the liquid phase can even penetrate into the AgPdelectrodes. Then, the ZnO grains grow into the neigh-boring ZnO layer after the lead of the Bi2O3-rich liquidphase. The continuity of the AgPd electrode is thus de-stroyed. The ZnO grains are larger in the thicker layer,Fig. 4. The grain boundaries in the thicker layer canmove more easily across the electrode due to less con-straint exerted by the inner electrodes. Therefore, the

Fig. 5. (a) Microstructure of multilayer varistors/Pt specimen after sintering at 11001C for 1000 min and the corresponding energy-dispersiveX-ray spectroscopy mapping of (b) Zn, (c) O, (d) Bi, and (e) Pt.


only electrode left behind is the one near the thinnestceramic layer.

As far as the electrical performance is concerned,the Bi2O3-rich boundary layer acts as an electrostaticbarrier to the transportation of the electron. The break-down voltage for each boundary phase is more or less aconstant. The breakdown of an MLV component istherefore determined by the number of the Bi2O3-richboundaries within the inner electrodes. The size of theZnO grains increases with an increase in the sinteringtemperature; the breakdown voltage is thus decreasedwith an increase in the sintering temperature. However,as the sintering temperature is increased to 12001C, thebreakdown voltage of the MLV/AgPd jumps to a veryhigh value. This further confirms that the AgPd elec-trodes are no longer continuous.

Apart from the MLV specimens sintered at12001C, the nonlinear coefficient varies from 4 to 7for the MLV/Pt specimens and from 2 to 3 for the

MLV/AgPd specimens. The a-values of the MLV/Ptspecimens are around two times that of the MLV/AgPdspecimens. The newly formed PdBi2O4 phase is a semi-conducting phase;3 its presence may lead to the decreaseof the a-value.13

The wetting of the Bi2O3-rich phase on the innerelectrode plays an important role in the microstructurestability of MLV. The wetting of the Bi2O3-rich liquidon Pt is poor. Because the energy of the ZnO–Bi2O3

boundary phase is low,14 a very thin Bi2O3-rich bound-ary phase always exists between the ZnO grains. Thewetting of the Bi2O3-rich liquid on AgPd is also good.The Bi2O3-rich liquid can thus wet the AgPd grains.Each AgPd grain then changes slowly to a sphericalshape to decrease its surface area. The Bi2O3-rich phasecan also move across the discontinuous AgPd grains.The AgPd grains then become isolated from each otherwithin the Bi2O3-doped ZnO matrix. The breakdownvoltage of the MVL/AgPd is then increased.

Fig. 6. (a) Microstructure of multilayer varistors/AgPd specimen after sintering at 11001C for 100 min and electron probe microanalyzermapping of (b) Zn, (c) Bi, (d) O, (e) Ag, and (f) Pd.


From a practical point of view, the choice of innerelectrodes is important for the performance of MLV.Because the size of ZnO grains is larger in the MLV/AgPd specimens, the use of an AgPd inner electrode canproduce MLV with a lower breakdown voltage. How-ever, there are limits for the sintering temperature andsintering time. The sintering temperature has to be lowenough to limit the wetting of the Bi2O3-rich phase onthe AgPd grains. According to the previous study,15 thepresence of a small amount of Ag dopant (o0.76 mol%)promotes the grain growth of ZnO. The degree of theXRD peaks of ZnO in the MLV/AgPd specimen isslightly lower than that of the peaks of ZnO in theMLV/Pt specimen (see Fig. 1), indicating that the latticeparameters of ZnO are larger in the MLV/AgPd spec-imen. Because the Ag1 ion (0.115 nm)16 is larger than

the Zn21 ion (0.075 nm), it thus confirms the dissolu-tion of some Ag into ZnO during cofiring.

Conclusions

In the present study, the feasibility of using anAgPd alloy as the inner electrodes for MLV is evaluated.The Pd in the AgPd alloy could react with Bi2O3 inthe varistor composition. A small amount of Ag candissolve into ZnO to promote its growth. The MLV/AgPd specimen can only be produced by sintering at atemperature lower than 11001C and a time shorter than1000 min. The key is the wetting of the Bi2O3-rich liq-uid on the AgPd alloy during cofiring. The sinteringtemperature has to be low enough to produce a stablemicrostructure.

Acknowledgment

The present study is supported by the NationalScience Council, Taiwan, through the contract numberNSC95-2221-E-002-083.

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Fig. 7. I–V curves for (a) multilayer varistors (MLV)/Pt and (b)MLV/AgPd specimens. The specimens were sintered at the indicatedtemperatures for 60 min.


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inner electrodes for multilayer varistors

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