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Effect of Dispersant Concentration on Preparation of an Ultrahigh


J. Am. Ceram. Soc., 92 [9] 2168–2171 (2009) DOI: 10.1111/j.1551-2916.2009.03178.x r 2009 The American Ceramic Society

Journal
Effect of Dispersant Concentration on Preparation of an Ultrahigh Density ZnO–Al2O3 Target by Slip Casting
YiHua Sun, WeiHao Xiong, ChenHui Li,w and Lu Yuan
State Key Lab of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China

The rheological behavior of concentrated ZnO–Al2O3 aqueous suspensions has been studied in order to obtain an ultrahighdensity ZnO–Al2O3 composite ceramic target by slip casting. The in?uence of the mass fraction of polyacrylic acid (PAA) on the ?uidity of slurries and the density and strength of the green and sintered bodies was investigated. The slurries exhibited a near-Newtonian ?ow behavior and had a lower viscosity with 0.3 wt% PAA. The excess of PAA enhanced the green strength and the density and strength of the sintered bodies. An ultrahigh density sintered body (499.7% theoretical density) could be obtained after pressureless sintering at 14001C. The Al species were well distributed in the sintered bodies, which showed a homogeneous, defect-free microstructure with no abnormal grain growth.

neity could be achieved. However, the green bodies are often brittle so that an added binder agent is necessary, such as polyethyleneglycol (PEG).4 In our previous study, the well-dispersed ZnO–Al2O3 slurries could be obtained with 0.2 wt% PAA at pH 9 and the PEG could be added up to 0.2 wt%.14 In this paper, the in?uence of the mass fraction of PAA on the dispersion and rheological behavior of slurries and the relationships between the amount of PAA and the density and strength of green and sintered bodies were further studied. The microstructure of sintered bodies and the distribution of aluminum species in the sintered bodies were investigated.

II. Experimental Procedures I. Introduction
casting is one of the most widely used preparation methods in the ceramics industry and it has been proven to be a reliable and simple technology for producing homogeneous and dense green bodies, especially for multicomponent systems or composites, and allowing the manufacture of ceramics with large sizes, high densities, and complex shapes.1–3 Recently, some multicomponent, advanced ceramics have been studied using slip casting, such as transparent yttrium aluminum garnet,4 yttria-stabilized zirconia,5 cordierite6 ceramics, and the like. Nowadays, the use of ZnO–Al2O3 composite ceramics as sputtering targets in commercial magnetron sputtering for the deposition of aluminum-doped zinc oxide (AZO) thin ?lms is gradually increasing. This is attributed to AZO thin ?lms becoming increasingly used as the transparent and conductive electrodes for several optoelectronic devices such as photovoltaic solar cells, liquid crystal displays, or light-emitting diodes.7–9 The high density of ceramic targets plays an important role in the sputtering process and ?nal quality of the thin ?lms.10 In the past years, there were a number of studies addressing the sintering and grain growth of nanocrystalline ZnO for high densi?cation, such as Mazaheri et al.,11 Qin et al.,12 Roy et al.,13 and so on. But high-density ZnO–Al2O3 composites have rarely been studied, especially by colloidal processing. In colloidal processing, particle dispersion is the limiting factor, which affects both the rheology and the homogeneity of suspensions. Polyacrylic acid (PAA) is widely used as a highly effective dispersant that enables the stabilization of most ceramic slurries by ensuring strong electrostatic and steric repulsion over the attractive van der Waals forces. As a result, green bodies with a high density and microstructural homogeLIP

S

R. Moreno—contributing editor

Manuscript No. 25949. Received March 2, 2009; approved April 22, 2009. w Author to whom correspondence should be addressed. e-mail: li_chenhui@sohu.com

Commercial ZnO and a-Al2O3 powders (99.99% purity, Sumitomo, Japan) with speci?c surface areas (BET) of 10.8 and 8.9 m2/g, respectively, and average particle sizes around 260 nm and 320 nm, respectively, were used. A PAA (MW5000, China National Medicines Corporation Ltd., Beijing, China) and a polyethylene glycol (MW40000, Nanjing Chemlin Chemical Co. Ltd., Nanjing, China) were used as the de?occulant agent and binder agent, respectively. Slurries mixed with distilled water, PAA, and mixtures of ZnO and Al2O3 powders (the ratio of mass is 98:2) were prepared with 0.2 wt% PEG at pH 9 and at a solids content of 70 wt% (corresponding to 30 vol%, on the basis of dry powder weight). The pH was adjusted with ammonia (25 wt%). The slurries were treated by ball milling with zirconia grinding media for 36 h and subsequently degassed under vacuum. The apparent viscosity of the slurries was measured using a digital rotational viscometer (Brook?eld RVDV-E, Holtsville, NY) at the shear rates of 9.3, 18.6, 46.5, and 93 s?1 at 251C. The as-prepared slurries were poured into white plaster molds for the production of two kinds of specimens: cylindrical rods for measuring the radial crushing strength and cuboid bars for sintering. After casting, the specimens were dried at 1101C for 2 h and then sintered at 12001, 13001, 14001, and 15001C for 2 h with a heating rate of 51C/min. The polished, sintered specimens were thermally etched at 12001C for 10 min. The densi?cation behavior of green compacts was studied on a dilatometer (DIL 402c, Netzsch, Germany) with a heating rate ?xed at 3001C/h. The green and sintered specimens were evaluated with respect to density (Archimedes’ method). The strength of specimens was investigated by performing crushing or bending tests in a mechanical testing device (Instron3369, Norwood). The values obtained were the average of six measured specimens. The fractured and thermally etched surfaces of sintered specimens were analyzed using a scanning electron microscope (SEM, FEI Quanta 200 microscope, Eindhoven, the Netherlands) equipped with an energy-dispersive spectrometer (EDS). The phase compositions of the green and sintered specimens were identi?ed by X-ray diffraction analysis (XRD,

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X’Pert Pro, PANalytical, Almelo, the Netherlands) with a CuKa ? radiation (l 5 1.5406 A) operated at 40 kV–40 mA.

III. Results and Discussions
In our previous study, the apparent viscosity of ZnO–Al2O3 slurries with some PAA and PEG added had reached a minimum after milling for 36 h. When the amount of PEG added was above 0.2 wt%, the viscosity remarkably increased, which meant that the slurries deteriorated. The viscosity increased as the solids loading increased, but the slurries had good ?uidity expected for slip casting, when the solids loading increased up to 70 wt%. Thus, these conditions in the slurries, 70 wt% solids loading, 0.2 wt% PEG, and milling for 36 h, were selected for further work. The rheological properties of ZnO–Al2O3 slurries are strongly dependent on the amount of dispersant added, as shown in Fig. 1. At low dispersant levels, the slurries show a non-Newtonian behavior. Their viscosities were higher due to insuf?cient adsorption of PAA for separating the particles apart. The viscosity decreased pronouncedly as the PAA additions increased. The slurries with the amount of PAA in the range of 0.15–0.25 wt% exhibited nearly constant viscosity and could be ?tted to Newtonian ?ow behavior with only a very slight degree of shear thinning. The viscosity reached a minimum as the dispersant additions reached 0.2 wt%. The minimum viscosity was approximately 6.6 mPa ? s at a shear rate of 93 s?1. The optimum dispersant concentration was determined at the lowest viscosity, in which the maximum level of particle dispersion was indicated. This result was similar to a reported value for nano-ZnO aqueous suspensions.15 The optimum dispersant level suggested that there was an adequate amount of PAA to cover the particle surface. As a result, the particles maintained far apart from each other by electrosteric stabilization. Then, for 0.3 wt%, the slurries show a near-Newtonian behavior, and the viscosity increased a little, probably because of the presence of excess polyelectrolyte. When the amount of PAA further increased, the viscosity increased dramatically and the slurries became nonNewtonian again because of a decrease in the net negative charge of the mixed powders. At low or high dispersant levels, the shear thinning behavior that was clearly observed might be due to the breakdown of the existence of free surface sites and the possible interaction between the polymer chains, respectively.16 The increased viscosity of the ZnO–Al2O3 slurries might be due to ?occulation. Shrinkage and shrinkage rate curves for the green compacts as a function of temperature are shown in Fig. 2. The green compacts prepared from well-dispersed slurries show the typical shrinkage curve of solid-state reaction sintering and a slow densi?cation rate. The linear shrinkage rate is maximal when the temperature reaches 11651C. The densi?cation rate was sluggish, between 7001 and 9501C, and the ?nal density (95% TD)

Fig. 2. Shrinkage behavior of ZnO–Al2O3 green compacts during sintering.

was not reached until 12501C. After sintering at 12001, 13001, 14001, and 15001C for 2 h, the densities of sintered specimens were 5.24, 5.51, 5.66, and 5.64 g/cm3, respectively. The maximum sintered density was obtained at 14001C, so this condition was selected for further work. Figure 3 shows XRD patterns of the green and sintered specimens. In the green specimen, some small peaks of a-Al2O3 (JCPDS card No. 071-1124, Density: 4.0 g/cm3) were observed. After sintering, the a-Al2O3 peaks disappeared and some small peaks of ZnAl2O4 were detected, which means a-Al2O3 reacted with ZnO to form the ZnAl2O4 spinel phase (JCPDS card No. 073-1961, density: 4.64 g/cm3) during pressureless sintering. All diffraction peaks of the major phases can be indexed as a hexagonal wurtzite ZnO structure (JCPDS card No. 079-0205, Density: 5.72 g/cm3). For 2.0 wt% Al2O3 in the green body, the calculated theoretical density is 5.671 g/cm3. Considering that the solubility limit of Al in Al2O3-doped ZnO sintered body is very small (o0.3 at.%)17 and that Al2O3 reacted with ZnO to form a stable single phase (ZnAl2O4 spinel phase) during sintering,18 and assuming the Al2O3 in the green body was totally transformed into ZnAl2O4 after sintering, the concentration of ZnAl2O4 is 3.59 wt% in the sintered body, and the calculated theoretical density of sintered body is 5.673 g/cm3. So we could reasonably deduce that the actual theoretical density is between 5.671 and 5.673 g/cm3. Table I shows the density and strength of green and sintered bodies produced from slips with different dispersant concentrations. The well-dispersed slurry was achieved for about 0.2 wt% PAA, which resulted in the highest green density value. For 0.3

Fig. 1. Effect of the amount of polyacrylic acid (PAA) on the rheological properties of 30 vol% slurries with 0.2 wt% polyethyleneglycol at pH 9.

Fig. 3. X-ray diffraction patterns of the green and sintered specimens.

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Table I. Comparison of Density and Strength of the Green and Sintered Bodies in Relation to Amounts of PAA
Relative density (% TD) PAA content (wt %) Green body Sintered body Crushing strength Bending strength of green of sintered body (MPa) body (MPa)

0.1 0.2 0.3

54.9 66.7 63.8

99.2 99.7 99.8

2.670.5 12.572.0 18.672.0

92710 145715 148715

wt% PAA, a lesser decrease in the green density was found, in accordance with the lesser increase in the slip viscosity owing to the excess of PAA, but the green body strength increased with an increasing amount of PAA. Although the green density of the former is larger than the latter, the density and strength of the sintered bodies of the latter are larger than the former. In general, it was observed that well dispersed Newtonian slurries gave very homogeneous microstructures, although the sintered specimen derived from a near-Newtonian ?uid also had high density without abnormal grain growth and blowhole development (Figs. 4 and 5). The following is a possible explication : the higher strength of green bodies, owing to the excess of PAA, is advantageous to dehydration and other subsequent processes; the excess of PAA can promote Zn ions to dissolve from the ZnO particles’ surface to the slurries leading to the ZnO particles’ surface with more point defects and dangling bonds in the green bodies, which favors densi?cation during sintering. Figure 4 shows SEM micrographs of fracture surfaces of the sintered specimens for 0.2 and 0.3 wt% PAA. In both specimens, the presence of some well-distributed closed pores entrapped at the grain boundaries and located at triple points with typical sizes of 1.5 mm or less can be seen. It is clearly seen that the fracture occurred through the grains in both specimens, but the fracture surface of the latter is rougher than the former, which produced a longer fracture length and consumed more fracture energy, leading to a higher bending strength (148 MPa). It should be noted that the near-to-theoretical density bodies (45.66 g/cm3) could be obtained after sintering at 1400 1C for 2 h. Their relative densities were con?rmed to be 99.7 and 99.8% TD, respectively, assuming the theoretical density is 5.673 g/cm3. Figure 5 shows ESEM micrographs of the thermally etched surface of the sintered specimen for 0.3% wt% PAA. The average grain sizes of the thermally etched specimen are be-

Fig. 5. ESEM image of the thermally etched surface of ZnO–Al2O3 ceramics: (a) in the secondary electron mold; (b) in the backscattering mold with energy-dispersive spectrometer patterns of off-white regions (c) and dark regions (d).

tween 4 and 8 mm. It is clear that the off-white regions are ZnO phases and the dark regions are ZnAl2O4 spinel phases. The homogeneously distributed ZnAl2O4 phases are clearly visible as dark patches of the size of 1 mm or so at the grain boundaries, and even in the grains, predominantly located at triple points. The results of EDS analyses revealed that most of the Al was distributed between the ZnO phases, while only a smaller share was dissolved in the ZnO phases. The peak for the Al species was found in the grain interior, which means that Al both dissolved into ZnO grains and reacted with ZnO to form ZnAl2O4.17 On the basis of Kim’s research,19 elemental Al exists in the form of a ?ne-grained ZnAl2O4 phase along the grain boundaries, which inhibits the grain growth. The homogeneous distribution of Al species in the microstructure of ZnO–Al2O3 ceramics is of paramount importance to the ?nal quality of sputtering thin ?lms.

IV. Conclusion
At low or high levels of dispersant, the ZnO–Al2O3 slurries are a non-Newtonian ?uid for ?occulation, which occurred because of the insuf?cient adsorption or excess of PAA, respectively. A well-dispersed ZnO–Al2O3 mixture slurry was reached when 0.2 wt% PAA was added. At up to 0.3 wt% PAA, the slurries showed a near-Newtonian behavior. Their viscosity increased slightly but their green body strength also increased. In both cases, the ultrahigh density of ZnO–Al2O3 sintered bodies (B5.66 g/cm3) could be obtained after pressureless sintering at 14001C for 2 h. Once 0.3 wt% PAA is reached, an increase of green body strength for a slight excess of PAA could contribute to the density and strength of the sintered bodies. A maximum sintered density of 99.8% TD can be reached. Furthermore, the microstructure observations reveal that very homogeneous, defect-free materials are obtained without abnormal grain growth. EDS analyses reveal that Al species are well-distributed both in

Fig. 4. Scanning electron microscopic images of the fractured surface of the sintered specimens with 0.2 wt% polyacrylic acid (PAA) (a) and 0.3 wt% PAA (b).

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ZnO grains and at the grain boundaries. Slip casting allows the manufacturing of a defect-free, homogeneous, and ultrahigh density sputtering target of ZnO–Al2O3 composites.

References
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