Journal of Metals, Materials and Minerals, Vol.20 No.3 pp.127-131, 2010 Densification and Grain Growth in BaO.Bi2O3.ZnO Varistor Ceramics Niti YONGVANICH1,2*, Pranuda JIVAGANONT3, Fariya SAKASUPHALERK1, Porntip HUAYHONGTHONG1 and Weerawat SUWANTEERANGKUL1 1 Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakornpathom 73000 2 Center of Excellence for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, Bangkok 10330 3 National Metal and Materials Technology Center, Pathumthani 12120 Abstract The effect of barium addition on the densification and grain growth of ZnO-based ceramics was investigated. ZnO-based varistors have been employed as surge arrestors to prevent damages from voltage surges in electrical or energy-related equipments such as transformer and consumer products. These devices have been commercially incorporated into energy-supplying grids. The selected composition was ZnO + 0.0025Bi2O3 + (x) BaCO3 where x = 0 and 0.005. Both solid-state processing and precipitation method were investigated. It was shown that the amount of Bi addition was too low to be effective for improving grain growth; however, it was sufficient to enhance densification. When barium was further added, the averaged grain size was more than doubled, possibly due to liquid-phase sintering. This result suggests the possibility of BaO-containing liquid-phase formation along grain boundaries. However, the main drawbacks of barium addition were a slight decrease in density, which was presumed to be caused by closed pores trapped in the grain interior during the sintering process, and a reduction in the threshold voltage. Key words: Grain growth, Varistor, Zinc oxide, Doping Introduction ZnO-based ceramics have been widely used in varistor application due to their suitable electrical properties. The highly nonlinear currentvoltage characteristics and high current handling capability enables them to be efficient overvoltage protectors in electronic circuits, electric power system and equipment for power transmission.(1-5) Different oxide compounds are usually added during the processing to tune various electrical characteristics such as non-linearity and breakdown voltage. Bi2O3 is always included in the composition as its derivatives have been reported to be responsible for the non-ohmic properties. Other well-known dopants and additives include CoO, Nb2O5, Cr2O3, MnO and Sb2O3.(1-5) Among various microstructural aspects, the grain size appears to frequently dictate the breakdown voltage. Several reports have found that high thresholds tend to be associated with small-grained varistor samples.(6-7, 9-10) Control of grain growth is one major subject of research, and wet chemical route has appeared to be one potential solution to tailor the final grain size in the varistor piece.(11-14) Since grain growth is a complex process and can be affected by different dopants and additives in the system, it would be interesting to investigate these compounds in detail. This study examined the influence of barium addition (at 0.5 mol%) on densification and grain growth in ZnO-based varistor ceramics. The starting precursors were prepared from both solid-state processing and chemical precipitation method in order to investigate the effect of the processing condition on the final microstructure. Materials and Experimental Procedures The chemicals for the solid-state processing were reagent-grade ZnO, Bi2O3 and BaCO3. They were weighed according to the composition: ZnO + 0.0025Bi2O3 + (x) BaCO3 where x = 0 and 0.005. The mechanical mixture was ball-milled with alumina balls in ethanol for 24 hours. For the precipitation method, reagent-grade ZnCl2, Bi(NO3)3.5H2O and C4H6BaO4 were dissolved in distilled water (1M). The pH of the system, using a 1M NaOH solution, was adjusted to be 14. After one-hour of continuous stirring, the solutions were dried at 120°C and washed thoroughly to remove chloride ions. Thermogravimetric analysis (TGA, 7 Perkin-Elemer) was employed to determine the *Corresponding author E-mail: niti.yongvanich@gmail.com 128 YONGVANICH, N. et al. Results and Discussion The XRD patterns of the calcined nanopowders are shown in Figure 1. TGA showed complete decomposition of organic residues at 400°C (data not shown) which was chosen to be the calcination temperature. No impurity peaks were present within the detection limit of XRD. The averaged crystallite sizes calculated by the Scherrer’s formula are approximately in between 45 and 50 nm for both x = 0.00 and x = 0.005 samples. No preferential orientation was observed, and all major peaks can be indexed as a zincite phase according to the JCPDS database (75-0576). The morphology of the crystals (Figure 2) displays both equiaxed and slightly elongated character. The latter might be yielded from the hexagonal structure of ZnO. Agglomeration of the particles is clearly seen as evidenced by high specific surface area and grain coalescence at the calcination temperature of 400°C. Figure 2. TEM image of the ZnO nanoparticles calcined at 400°C for 6 hours. Figure 3 shows the relationship between bulk density and sintering temperature of all samples. Similar densities were obtained from both solid-state processing and precipitation method. The undoped pellets display a continuous increase in density from 66% (900°C) to the maximum of 94% (1300°C) where as the density values of the (Ba,Bi)-doped pellets never exceeded 90%. In fact, they fluctuate in the range between 84% and 90%. The greatest improvement in sintering was observed in the samples of Bi-doped compositions among which a relative density of 94% was readily acquired at 1100°C. 100 Relative RelativeDensity Density (%) (% calcination temperature. The precipitates were calcined at 400°C for 6 hours to remove inorganics. The morphology of the nanosized powders was examined through Transmission Electron Microscope (TEM, Philips EM201C). The powders were pressed into disks with a diameter of 8 mm and a thickness of ~ 2 mm. These disks were sintered in air for 6 hours at 900, 1000, 1100, 1200, 1300 and 1400°C, with heating and cooling rates of 5°C/min. Sinterability was characterized by the geometric (bulk) density measurement. Scanning electron microscopy (SEM, Hitachi S-3000N) was used to examine the microstructure. The averaged grain size was calculated by the lineintercept method. Phase formation was confirmed by X-rays diffraction (XRD, D/MAX 2000 Rigaku). The non-linear response was measured by a simple variac and multimeters. 95 90 85 80 75 70 65 60 900 Bi-doped (Ba,Bi)-doped Undoped 1000 1100 1200 1300 1400 Sintering Temperature (C) Figure 3. Relationship between density and sintering temperature of the undoped, Bi-doped and (Ba,Bi)-doped samples. Similar trends were observed for both solid-state processed and precipitated samples. Figure 1. XRD patterns of the ZnO nanopowders (undoped and doped) calcined at 400°C for 6 hours. The sintered samples were crushed and x-rayed to examine phase purity. Figure 4 shows the XRD pattern of the (Ba,Bi)-doped sample (x = 0.005) sintered at 1300°C for 6 hours. The major peaks belong to a zincite phase with very small traces of secondary peaks. The intensities of these 129 Denssification andd Grain Groowth in BaO.B Bi2O3.ZnO Varistor V Ceraamics mine minor peakks are too loow to accurately determ their correspponding phaases. Fig gure 6. SEM M image of tthe (Ba,Bi)-d doped sample sinteered at 1300°C for 6 hourrs. (a) Solidstate processing. (bb) Precipitatio on method. Figure 4. XRD X pattern of the (Ba,Bi)--doped sample (x = 0.005) sinteered at 1300°°C. The impuurity peeaks are denotted as asteriskks. Thhe microstrucctures of the sintered s sampples (1300°C, 6 hours) weree shown in Figures F 5 andd 6. Most grainns appeared to be equiaaxed with soome being a litttle elongatedd. There seemed to be no significant intergranularr pores. Thee averaged grrain sizes of thee undoped annd Bi-doped samples are 4.0 μm for sollid state proocessing andd 15.9 μm for precipitation. A dramaatic change in grain size, s however, was w observved in the (Ba,Bi)-doped sample (10.5 μm for soolid-state proocessing and 6.6 μm for preccipitation). Note N that diffferent trends in grain size upon u addition of barium were obtainned: an increasee in the soolid-state proocessing buut a decrease in the precipittation methodd. Figure 6 also a displays a number of small pores trapped witthin these large (Ba,Bi)-dopeed grain; theese intragranuular pores couldd be responssible for a loower densityy in this (Ba,Bii)-doped sam mple. Also,, the Bi-doped precipitatedd sample (Figure 5(b)) displays sm mall, discrete graains of approoximately 1 to 2 μm aloong grain bounddaries. Figure 5. SE EM image off the Bi-dopedd sample sinteered att 1300°C for 6 hours. (a) Soliid-state processsing. (bb) Precipitationn method. Figure 7 shows thee microstrucctures of the (Baa,Bi)-doped precipitated p saample sintereed at 1300°C C forr 6 hours att different m magnification ns. A greatt deg gree of poroosity can be seen with siizes varying fro om approxim mately 5 to 220 μm. Accu umulation off porres to form macroscopic m vvoids (larger than 50 μm) occcurred extensively, confirm ming sluggish h sinterabilityy as evidenced byy the density data in Figu ure 3. Hence, thee (Ba,Bi)-dopping tended too yield both intergranularr and d intragranullar pores. Fiigure 7(b) also shows a miccrostructure at a higher m magnificatio on. It is veryy inteeresting to see s smaller grains trapp ped within a larg ge grain (in the middle oof the figuree). This type of grain entrapm ment was observed locatin ng randomlyy thrroughout thee sample. Itt is believed d that suchh occcurrence might be caussed by abno ormal grainn gro owth typicallly associated with multticomponentt sysstems withinn which addditives posssessing low w meelting temperatures or aare capable of inducing forrmation of a liquid phase with other oxides. o Fig gure 7. SEM image i of the (Ba,Bi)-doped d precipitatedd samplle sintered att 1300°C forr 6 hours att differeent magnificattions. Increasses in both ddensity and grain g growthh at the temperaatures higherr than 1200°C for bothh Bi--and (Ba,Bii)-doped sam mples could d likely be exp plained by liquid phasse formation n; differentt sysstems would yield different mechanisms of suchh forrmation. Adddition of Bi2O3 has been n reported to ressult in drastic improvem ment in densiification andd graain growth kinetics. k Thee sintering temperatures t bettween 1100°°C and 1300°C were much m higherr thaan the eutecctic temperaature (~750°°C) for the ZnO-Bi2O3 sysstem.(1, 3) Thhe formed liquid l phase 130 YONG GVANICH, N. N et al. enhances diffusion d durring sinteringg. Nevertheless, this study shows thaat there waas virtually no difference in i grain sizee between thhe undoped and Bi-doped samples. s Succh finding could c likely be explained by b the amounnt of Bi addiition which was w very low in i this studyy. Previous studies usually employed Bi B addition of o more than 0.5 mol%.(3, 8-9) In addition, a very small amount of Bii2O3 (0.04 mool%) has been staated to yield microstructura m al inhomogenneity with noticeable porosityy.(2) Hence, there t seemedd to exist a criticcal amount of o Bi additionn beyond whhich significant grain g growthh could occurr. milarly, the inncrease in graain size of more m Sim than two-ffold in the (Ba,Bi)-dooped solid-state sample shoould not be correlated to Bi addittion alone. Nevvertheless, thhe theory of o liquid phhase sintering coould still be valid v in spitee of the abseence of observabble intergranuular liquid films in Figurre 6. The larger ionic radiuss of Ba2+ (0.134 nm) woould likely segreegate along grain bounddaries due too its insolubilityy with the sm maller Zn2+ ioon (0.074 nm m).(4) The BaO liqquid phase, along a with poossible Ba-Znn-O compoundss, was reporrted to help enhance grrain growth ratee.(4) A typical t I-V characteristic is shownn in Figure 8 forr (Ba,Bi)-doped solid-statee sample sinteered at 1300°C for 6 hourss. Other sam mples displaayed similar I-V format. No significant difference d in the threshold voltage v was obtained o bettween the soolidstate and precipitatedd samples. The threshhold voltages at 0.1 0 mA/cm2 are approxim mately 120 V/cm V for the Bi-dooped sampless and 90 V/cm m for the (Ba,B Bi)doped sampples. These relatively r low w values of the threshold vooltage mightt be explaineed by ineffecttive barrier alonng the grain boundary duue to inefficiient amount of bismuth-cont b taining liquidd phase. Co onclusions The unndoped pelleets display a continuous inccrease in density to thhe maximu um of 94% (13 300°C). On the contraryy, the densitty values off thee (Ba,Bi)-dooped pellets only fluctu uated in the ran nge between 84% and 90%. The best sinterabilityy waas observed in the Bi--doped samp ples with a relaative densityy of 94% readdily obtained d at 1100°C. The averaged grain g sizes w were found to t be similarr bettween the undoped and Bi-doped samples. s Ann inccrease of moore than twoo-fold in graain size was obsserved in thee (Ba,Bi)-dopped sample. Differences in both densiffication and grain growtth could be exp plained by the mechaanism of liiquid phase forrmation as well as poossible inho omogeneous disstribution off additives. The amou unt of Biadd dition of onnly 0.5 mool% in this study was suffficient to grreatly improvve sintering but did nott seeem to be hiigh enough to producee a requiredd am mount of liqquid phase. 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