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Comparative study on the effect of particle size on the microstructure of Bi2O3 and Sb2O3 additives in sintered ZnO ceramics

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  • Save American Journal of M aterials Science 2012, 2(5): 153-159 DOI: 10.5923/j.materials.20120205.04 Comparative Studies of The Effect of Particle Size on the Microstructural Characteristics of Bi2O3 and Sb2O3 Additives on Sintered Zno Ceramics Solomon C. Udensi Department of Physics, Federal University of Technology Owerri, P.M .B. 1526 Owerri, Imo State, Nigeria Abstract Two different powder samples of ZnO with minuscule additives, of Bi2O3 and Sb2O3 were prepared through ball milling, in solutions containing deionized water and polyvinyl alcohol (PVA ) using zirconia balls. The powder samples were sieved to obtain particle size ranges of ≤ 38μm (Z, ZB, ZBS) and ≤ 63μm (Z1, ZB1, ZBS1). The sample pellets were sintered at 800℃/ 60mins and 1100 ℃/600mins and their microstructures examined using SEM and X-ray for densification and coarsening kinetics. Grain growths were observed for samples ZB and ZB1 at both temperatures due to the format ion of bismuth-rich liquid phase, whereas for ZBS and ZBS1 grains growths were hindered because of the enclosure of spinel phase which pinned grain boundary motion. SEM micrographs show high densities for particle size range of ≤ 38μm at both temperatures, while for particle size range of ≤ 63μm, densificat ion was highest at 1100℃/ 600mins.The decisive parameters which controlled densification were found to be temperature, sintering time and particle geometry. In the cases where all three parameters were utilized (Z, ZB, ZBS at 1100℃/600mins), denser materials were obtained. Also equal masses of representative samples of ZB and ZB1 were seen to be electrically co mparable fro m the nonlinear p lot. Keywords Ceramics, Particle size, Green body, Grain boundary, Pyrochlore-type, Bismuth-rich 1. Introduction Density controlled materials and components from metals and/or ceramic powder co mpacts are usually prepared by sintering processes, which involve the application of thermal energy. Generally, sintering seeks to produce and reproduce materials with tailored microstructures. Tailoring these microstructures implies controlling the part icle grain sizes, sintered densities, including ‘size d istribution’ of other phases, such as pores. For optimu m sinterability of any green body, the range of the powder particle sizes, as reported widely, should be between 0.1 and 100µm representing a total surface energy range of 500 and 0.5 J/ mole[1] respectively. However, the change in energy during oxide formation ranges between 300 and 1500KJ/ mo le. Therefo re in order to strike a balance between the total surface energy of the powder and that for the o xide format ion, a series of optimization is required so as to control variab les necessary for sintering. Two variables - material and process variables- ensure that these are achieved. Smaller ‘particle grain size’ leads to high density materials[2]. Another important phenomenon * Corresponding author: (Solomon C. Udensi) Published online at Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved which ensures highly desired functional property for sintered materials’ is the milling process. For two powders samples of similar shapes, but different sizes (say with rad ii r1 and r2), sintered by the same sintering mechanisms, Herring scaling law (see equation 1) pred icts that the same degree of densification would be attained[1, 3]. t2 = (ᴧ)α t1 (1) Both radii are connected by the equation, r2 = r1. For the initial stage of sintering (there usually three stages), the exponent, α, are shown to be 3, 4, 1, and 4 respectively for lattice diffusion, grain boundary diffusion, viscous flow, and surface diffusion. Furthermore, capillary pressure difference orig inating fro m d ifference in particle curvatures, diffusivity, viscosity etc., wh ich are temperature dependent variables, had been identified to be important driving forces for sintering[1]. Yttria - stabilized Zirconia (YSZ), Zirconia (ZrO2) and alu mina (Al2O3) balls are known for their “hard-to-wear property” and so are used where less contaminant are estimated during milling[4-6]. For this work Zirconia balls were used. Literatures which have reported studies on sintered ZnO and its derivatives seem, to my knowledge, to have left out comparative investigations on the effect of part icle sizes and/or of temperature on the microstructure of polished-etched surfaces of ZnO. Th is study is necessary, if we want to produce functional technical ceramics with 154 Solomon C. Udensi: Comparative Studies of The Effect of Particle Size on the M icrostructural Characteristics of Bi2O3 and Sb2O3 Additives on Sintered Zno Ceramics exceptional mechanical, electronic, optical, thermal etc. properties, which could be employed in work environ ments (e.g. in electronic, aviat ion and defence industries) where malfunctions/degradations are not be contemplated. In this paper, reports of analytical investigations using scanning electron microscope (SEM ), energy dispersive X-ray (EDX), and X-ray diffraction (XRD) etc. are presented. NaOH. Etch ing reactions were stopped with the use of alcohol to prevent damage being done to the samples. The etched surfaces were sputter-coated with layers of carbon, to aid SEM and EDX studies. The samples were a lso investigated using powder x-ray diffraction for crystalline phase identification. Also the average grain size (G) of each sample was resolved using Mendelson’s intercept method in s tereo lo g y [8]. 2. Experimental 2.1. Materials 150g of three powder samples labelled Z, ZB and ZBS comprising of 100% mo l ZnO (Z), 99% mo l ZnO plus 1% mo l Bi2O3 (ZB) and 98% mo l ZnO p lus 1% mol Bi2O3 plus 1%mo l Sb2O3 (ZBS) respectively were prepared by calculating the proportion by mass of the compounds. Each sample was then mixed with deionized water (little water should be added to avoid sedimentation of the heaviest compound, Sb2O3) and 1% mo l polyvinyl alcohol (PVA), a binder, to increase the mechanical strength of the pellets during uniaxial pressing. The resulting slurries were ball-milled (with Zirconia balls) for 1 hr and dried at a modest temperature of 50℃. The dried samples were then ground using agate mortar/pestle and sieved through a 38µm sieve. The same procedure was followed and finally sieved through a 63µm sieve, in this case the three powder samples were labelled Z1, ZB1 and ZBS1. 1.5g of samples Z, ZB, ZBS, Z1, ZB1, and ZBS1 each was respectively pressed into a disc of diameter 1 centimetre by the application of a uniaxial pressure of 100MPa with a Carver laboratory press. The initial he ight (ho), initia l weight (Wo), and the diameter were recorded before calculating the porosity. Three specimens of each sample were then sintered at 800℃ and 1100℃ for 1 hour and 10 hours in a laboratory oven. The percentage contraction (%∆h), percentage weight loss (%∆W) and final density (% Dens.) were calculated using the data obtained after sintering (i.e. final height (h), final weight (W) and final diameter), with the understanding that the theoretical density of ZnO is 5.78g/c m3[7]. 2.2. Sample Preparation 3. Results and Discussion For a green body to be well sintered, the competit ion between densification rate and coarsening rate should be made to favour the former. In the case of coarsening, particles’ centres are stationary even though, there is reduction in surface area. So the driving mechanis ms for sintering must be lattice diffusion and grain boundary diffusion (once a grain boundary is formed)[1]. If coarsening takes place too rapidly, sintering will not occur because of abrupt reduction in surface area and the consequent increase in convex rad ii (i.e. assuming that the particles are spherically shaped). Therefore care must be taken to control sample variables and heat treatment process in order to favour densification. Tables 1 and 2 show the results obtained fro m sintering samples whose mean part icle sizes were 38μm and 63μm respectively. Their percentage densifications were also shown in the two multip le bar charts below (figures’ 1 and 2). One could see with ease that the densifications of the samples with mean particle sizes’ of 38μm[9, 10], even though sintered at various temperatures and time we re comparatively better than those of 63μm. Furthermore, for both grain sizes, the densifications were greater for those samples sintered at 1100℃ for 10 hours[11] than samples sintered at 800℃. The contractions, samples with 63μm grain sizes had to undergo were also greater than those of 38μm, lending credence to the fact that bigger particle sized samp les would have to travel longer distances fro m the surface to the neck region for densification to occur. Also, in so doing, pores which affect densification were en clo s ed . The samples were of two categories, high density samples (sintered at 1100 ℃ and 800 ℃ with more than 80% densificat ion) and less density samples (less than or equal to 80% densification). Both groups of the samples were b roken into smaller sizes and held firmly in position, in a mould, using an epoxy resin (arald ite and hardener in the ratio of 8:2). Un like the less density samples whose grain boundaries are easily seen, the high density samples were polished on a portable Struers bench polishing machine, using SiC grinding papers and diamond paste of increasing fineness (15, 9, 6, and 1µm) and intermittently cleaned in an ultrasonic bath. The polished surfaces were then etched by dipping them, for 3 minutes, in a solution containing 5M solution of Figure 1. Densifications Z, ZB, ZBS, Z1, ZB1, ZBS1 at 800℃ for 60mins American Journal of M aterials Science 2012, 2(5): 153-159 155 Figure 2. Densifications Z, ZB, ZBS, Z1, ZB1, ZBS1 at 1100 ℃ for 600mins Table 1. Results obtained from sintering samples whose mean particle size was 38μm at 800℃/60mins and 1100℃/600mins Sample (38μm) Z ZB ZBS Z ZB ZBS T(℃)/t (min ) 800/60 1100/600 %∆w -1.5 -3.1 -1.2 -2.4 -5.4 -4.3 %∆h -3.2 -14.2 -0.7 -16.5 -15.4 -16.6 %Dens. 66.4 93.3 60.6 97.9 95.4 96.5 G (µm) 0.5 1.7 0.2 11.9 18.8 11.9 Table 2. Results obtained from sintering samples whose mean particle size was 63μm at 800℃/60mins and 1100℃/600mins Sample (63μm) Z1 ZB1 ZBS1 Z1 ZB1 ZBS1 T(℃)/t (min ) 800/60 1100/600 %∆w -1.8 -2.1 -1.7 -1.4 -5.4 -4.6 %∆h -8.8 -13.7 -1.8 -16.7 -14.7 -16.3 %Dens. 73.7 93.0 30.9 96.7 91.8 96.3 G (µm) 0.84 2.42 0.29 22.3 28.2 22.2 The SEM images of both samp le types and sizes are shown below. There is considerable elimination o f pores. This mainly is due to the application of pressure, during pressing into smaller discs/pellets. However, there are mo re pores in samples with particle sizes of 63μm; this is because of abnormal grain growth which made it difficult to eliminate pores. The samp le of figure 4 (i.e. 63μm) attained higher density than that of figure 3 (i.e. 38μm), even though with higher porosity occasioned by bigger particle size. This is because good densification requires more t ime and high temp eratu res . Figures 5 and 6 are the mic rographs whose samples (ZnO) contain 1% Bi2O3. There was an emergence of ZnO-Bi2O3 liquid phase with eutectic temperature of 740℃[1]. This temperature is well belo w the sintering temperatures, reported herein. The liquid phase was confirmed by EDX (figure 7) to be rich in Bis muth. There was less porosity in sample ZB1 when compared with Z1, because the liquid easily filled the voids through capillary action. These bismuth formations at the grain boundaries also gave the sample its non-ohmic behaviour[12]. Fi gure 3. SEM micrographs of Z sint ered at 800℃ for 60mins Figure 4. SEM micrographs of Z1 sintered at 1100℃ for 600 mins Liquid phase Figure 5. SEM micrographs of ZB sintered at 800℃ for 60 mins Pores Figure 6. SEM micrographs of ZB1 sintered at 1100℃ for 600 mins 156 Solomon C. Udensi: Comparative Studies of The Effect of Particle Size on the M icrostructural Characteristics of Bi2O3 and Sb2O3 Additives on Sintered Zno Ceramics Figure 7. EDX of sample ZB sintered at 800℃ for 60mins Fi gure 8. SEM micrographs of ZBS sint ered at 800℃/60mins Figures 8 and 9 are the micrographs of ZBS and ZBS1 sintered at 800℃ for 60minutes. Their densities fell well below that of ZnO sintered at 800℃/ 60mins but not 1100℃. This was attributed to the hindering effect towards densificat ion by the chemical react ion between oxidized antimony and bismuth oxides, to form an intermediate pyrochlore-type of phase (which usually occurs at temperatures above 750 – 800℃). 4ZnO + 3Bi2O3 + 3Sb2O3 –> 2Zn2Bi3Sb3O14 + 4O2 There was no available Bis muth to form any liquid phase, unlike in the cases of ZB and ZB1. As a result of this, there was reduction in material transfer, leading to s maller grain sizes. This pyrochloric phase[13] was further established by X-ray diffract ion in figure 10. The peaks fro m the graph are attributable to ZnO and Zn2Bi3Sb3O14. Fi gure 9. SEM micrographs of ZBS1 sint ered at 800℃/60mins In the cases of the samples ZBS and ZBS1 sintered for 1100 ℃ /600mins, there were format ion of spinel phase (figure 11) and characterized by EDX to be rich in bismuth (figure 12). These ‘spinel phases’ formed inclusions which pinned the grain boundaries’ motion, and so restricted the ZnO grain growth. ZBS 800/60 1 0,9 ZnO Bi3Sb3Zn2O14 0,8 0,7 0,6 I/I0 0,5 0,4 0,3 0,2 0,1 0 5 15 25 35 45 55 65 75 2θ (º) Fi gure 10. SEM micrographs of Z1 sint ered at 800℃/60mins American Journal of M aterials Science 2012, 2(5): 153-159 157 Spinel phase Figure 11. Micrograph of ZBS showing spinel phase surrounded by Bismuth at 1100℃600mins The electrica l characteristics (i.e. V versus I) of Z, ZB and ZB1, a ll sintered at 1100℃/600mins were plotted, using data obtained from a Keithley mult imeter. For each of the samp le types, whose thickness was approximately 2.00 millimetres with silver e lectrodes, at least three different tests were taken. The voltage was increased in steps of approximately 0.03 volt. The applied electric field (E) and current density (J) were thus calculated, fro m the surface parameters of the s amp les . Figure 13 shows a somewhat linear E versus J relationship for sample Z, and non-linear characteristics for samples ZB and ZB1[14, 15]. Figure 12. EDX showing spinel phase surrounded by Bismuth rich phase Figure 13. Graph of applied electric field versus current density for samples Z, and ZB sintered at 1100 for 600minutes 158 Solomon C. Udensi: Comparative Studies of The Effect of Particle Size on the M icrostructural Characteristics of Bi2O3 and Sb2O3 Additives on Sintered Zno Ceramics The different geomet ries of the grains in ZB and ZB1, and the different orientations of the grain boundaries gave the bulk samp le characteristic junction property, which inhibited current flow at low applied voltages, but broke down beyond a critica l voltage value[16]. The non-linear behaviours in ZB were possible because of the bismuth-rich liquid phase, which afforded it surge proof property when voltages were applied across them. 4. Conclusions characterizat ion my samples and to the technical e xpertise of Augusto Luis Barros Lopes, for h is insightful tutelage in the field of electron microscopy. REFERENCES [1] Suk- Joong L. Kang; Sintering Densification, Grain Growth, and M icrostructure, copyright (2005) Elsevier Butterworth-Heinemann. ISBN 07506 63855 This paper has reported the comparative studies of the effect of part icle size on the Microstructural characteristics of Bi2O3 and Sb2O3 additives on sintered ZnO ceramics. To produce better technical ceramics, h igh density ZnO are preferred to lo w density ZnO. The following conclusions are inferred: • High densifications were recorded fo r samp les whose particles sizes were 38μm (see figure 2, fo r sample sintered at 800℃ for 60 mins, Z) and 63μm (see figure 1, for samp le sintered at 1100℃ for 600 mins, Z1). One could see that for improved tailored materials to be produced fro m sintering processes, one should ensure very small particle sizes of ZnO or start with coarser material but extend heating time, bearing in mind how to reduce the attendant porosity. Sintering these two groups of sample for shorter times left the coarser (63μm) samples with greater porosity, whereas, both particle size ranges (i.e. 38μm and 63μm) attained greater density when sintered for longer t imes (fo r 600 mins herein). For longer times, enough time was available fo r the pores to coalesce and migrate to the surface. • The maximu m grain size for samples sintered at 1100℃ was 22.2µm as against 11.9µm, for those sintered at 800℃ (e.g. for ZBS1 and for ZBS). Th is is because the total number of grain boundaries for the coarser 63μm grains are more than six, a criterion for grain growth. • Varistor characteristics were exh ibited by samples with additives (e.g. ZB and ZB1). This is due to the eme rgence of highly resistive ZnO-Bi2O3 liquid phase in a h ighly conductive ZnO matrix, which is rich in bis muth. These liquid phases separated by grain boundaries form a network of back - to - back p-n junctions of different orientations. Fro m figure 13, one could see that equal mass of arbitrary oriented grains are electrically equivalent. • The densities of ZBS and ZBS1 were below that of ZnO due to the hindering effect of the intermediate pyrochlore – type phase which usually occur between 750 - 800 ℃ • For ZBS and ZBS1 sintered above pyrochloric temperature, (herein at 1100℃) there was an emergence of a bismuth–rich spinel phase, which formed inclusions that restricted grain growth. ACKNOWLEDGEMENTS I am gratefu l to the Depart ment of Ceramics and Glass Engineering of the University of Aveiro, Portugal fo r the [2] M .A. de la Rubia, M . Peiteado, J.F. Fernandez, A.C. Caballero; Compact shape as a relevant parameter for sintering ZnO-Bi2O3 varistors, Journal of the European Ceramic Society 24(6), 1209-1212 (2004) [3] D. Demirskyi, D. Agrawal and A. Ragulya; A scaling law study of the initial stage of microwave sintering of iron spheres, Scripta M aterialia 66 (2012) 323–326 [4] Y. W. Lao, S. T. Kuo, W. H. 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