CERAMICSINTERNATIONAL

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http://dx.doi.org/0272-8842/& 20

nCorrespondinE-mail addre

y.jiang.1@bham1Tel.: þ86 23

(2014) 14405–14412

Ceramics International 40 www.elsevier.com/locate/ceramint

Rheological characterization and shape control in gel-castingof nano-sized zirconia powders

Gang Liua,b,1, Moataz M. Attallahb, Yun Jiangb,n, Tim W. Buttonb

aFaculty of Materials and Energy, Southwest University, 400715 Chongqing, PR ChinabSchool of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

Received 23 March 2014; received in revised form 5 June 2014; accepted 6 June 2014Available online 14 June 2014

Abstract

In this study, a water soluble resin based gel system was developed incorporating nano-sized zirconia powders. The effects of the resin contentand the solid loading on the rheological properties, density, mechanical properties and shape control have been investigated. All the slurries showtypical shear-thinning behaviour. Higher resin-content leads to higher viscosity and larger green density. The density of the green componentobtained from a slurry with 30 wt% resin can reach 2.85 g/cm3 (nearly 50% of the theoretical density). However, when the resin content isbeyond 25 wt%, the sintered density decreased. Higher density, solid-loading and applying external force are helpful to improve the shapecontrol.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Rheological property; Gel-casting; Nano-sized zirconia; Shape control

1. Introduction

Nano-sized powders have attracted much attention in thepast decades due to the significant improvement in keyproperties. Zirconia nano-particles have thus become one ofthe most important ceramic materials, for example, they canprovide an effective way to lower thermal conductivity inthermal barrier coatings [1]. Moreover, ZrO2 is a prevalentbiomaterial in dentistry and dental implantology.

However, microstructural control is essential if specificrequirements need to be fulfilled. The fabrication process playsan important role in the control of the microstructure as well asthe shape [2,3]. Gel casting [4], a commercial colloidal process[5], is relatively new and very attractive for manufacturinghigh-quality ceramics with homogeneous microstructures andcomplex shapes. It is actually a moulding technique using an

10.1016/j.ceramint.2014.06.03614 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author.sses: liugangxjtu@gmail.com (G. Liu),.ac.uk (Y. Jiang).68254376; fax: þ86 23 68254376.

in-situ polymerisation process to immobilise the ceramicpowders and to allow them to hold the shape of the mould[6]. Different gel casting systems have been studied since itwas first developed in Oak Ridge National Laboratory in the1990s [7,8]. However, each system has its own drawbacks, forexample, the free radical polymerisation of monomer and crosslinker will be inhibited by the oxygen in the frequently usedsystem, leading to surface-exfoliation of the green compact [9].Although the processing systems based on coordinative orbiopolymer chemistry could be carried out in air, the resultingslurries and samples show high viscosity and poor greenstrength, respectively [5], limiting their further application.In recent years, a ring-open polymerisation mechanism basedon the poly-addition reaction between a water-soluble epoxyresin and amine hardener has been introduced in thegel casting technique, allowing the gel to be processed in air[10,11]. However, this technique is still in its early stageand few studies are incorporated nano-sized powders. There-fore, the purpose of the present investigation is to developa water-soluble epoxy resin based gel system suitable fornano-sized ZrO2 powders.

Fig. 1. Secondary electron SEM image of the zirconia powders used inthis study.

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2. Experimental

2.1. Raw materials

A commercial yttria-stabilized zirconia powder (Tosoh-zirconia, Japan) was used as the ceramic powder. Ethyleneglycol diglycidyl ether (EGDE, Sigma-Aldrich, Germany) andbis(3-aminopropyl) amine (Sigma-Aldrich, Germany) wereused as epoxy resin (monomer) and hardener, respectively.Specially, the weight ratio of the resin content and the hardenercontent was fixed at 1:0.23 for all slurries. 0.86 ml ammoniumpolyacrylate (NH4PAA) solution (Allied Colloids, Bradford,UK) per 100 g powder was added as the dispersant. Distilledwater (17 MΩ cm) was used as the vehicle.

2.2. Processing

An aqueous solution containing EGDE was prepared bydissolving the epoxy resin into the mixture of distilled waterand dispersant prior to the preparation of slurries. Afterwards,slurries with various solids loading (30 vol%, 35 vol%, 40 vol%, 45 vol%) were prepared by adding zirconia powders intothe premixed solution under constant stirring. Slurries wereball milled for 24 h with zirconia milling media. Hardener wasadded into the slurries and mixed. The resulting slurries werethen poured into suitable silicone moulds with desired shapesand degassed in a vacuum system for 1 min to aid mouldingand de-airing. After drying at room temperatures for 24 h,demoulding was carefully done and the green bodies weretransferred into a 40 1C oven for further drying for next 24 h.Sintering was carried out in air. A slow ramp of 1 1C/min to600 1C was conducted to burn out the polymer followed by5 1C/min ramp to 1550 1C, where it was held for 2 h. Sampleswere ground and polished for observation and measurement.

2.3. Characterization

The rheological behaviour was characterised with an AR500 rheometer (TA instruments, USA) with a cone and planesystem (Truncation: 59 mm; Core: 4 cm, 21). A solvent trapwas coupled to the measuring unit to prevent evaporation. Thegelation behaviour of the zirconia slurries with varying resincontents was studied by monitoring the evolution of theviscosity under a constant shear of 0.1 s�1 at 20 1C. Thecriterion used for the idle time measurement is based on theclassical theory that one of the attributes of the gel point is aninfinite steady-shear viscosity [12]. The flow behaviour of theslurries was measured at 20 1C under the continuous shearmode within the shear rate range of 600 s�1 (from 0.1 up to600 s�1) in 90 s. All the rheological characterization startedimmediately after the addition of hardener. All the densities inthis study were measured by Archimedes method. The Vickersmicrohardness test was performed on the sintered sample usinga Mitutoyo MVK-H1 microhardness tester (1 kg load for 10 s)(Mitutoyo Ltd, UK). An optical digital camera (OlympusSP350, Olympus Co (UK) Ltd) was used to record theshape of the green components. Scanning Electron Microscopy

(SEM) (JSM 7000, Jeol, Tokyo, Japan) was employed toobserve the microstructure of green and sintered samples.

3. Results and discussion

3.1. Influence of experimental variables on properties

A secondary electron SEM micrograph of the zirconiapowder used in this investigation is shown in Fig. 1 and itcan be seen that the powder has a near-spherical morphologywith a primary particle size around 100 nm.Fig. 2(a) shows the effect of the various resin contents on

the rheological properties of the slurries with 35 vol% solidsloading. All the slurries showed shear thinning behaviourwithin the measurement range. As the resin content increasedfrom 15 wt% to 30 wt%, the viscosities of zirconia suspensionsincreased at the same shear rate, especially when the shear rateis low (below 40 s�1). The viscosity of the resin used in thisstudy at 20 1C is 20 mPa s (from the product information),which is much larger than that of water (1.002 mPa s). So, theviscosity of the slurry system including water, powder,dispersant and resin, will arise with the increase of the resincontent. Meanwhile, according to our observation during thehardener mixing and slurry casting process, the hardener isvery helpful to lower the viscosity of this slurry system (thefluidity became better after adding hardener). As mentioned inthe experimental section, the ratio of resin (monomer) tohardener (coupling reagent) was 1:0.23 (wt%). More resinadded means more hardener needed. It is probably that theeffect of resin content on viscosity dominated when the resincontent increased from 15 wt% to 20 wt%, so the viscosity ofthe slurry system increased. However, the effect of hardeneraddition on viscosity had a peak value under this fixed solidsloading of 35 vol%. Therefore, when the resin contentincreased from 20 wt% to 25 wt%, the effect of hardener onviscosity gradually played an important role (actually under thetwo action mechanisms just mentioned), so the viscosity of

Fig. 2. Rheological properties of the slurries: (a) typical rheological flow behaviour of the 35 vol% zirconia slurries with various resin contents; (b) viscosity of35 vol% zirconia slurries with different resin contents as a function of time; (c) rheological flow behaviour of the zirconia slurries with various solids loading.

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20 wt% curve is close to that of 25 wt% curve. With furtheraddition of resin content to 30 wt%, the effect of resin contenton viscosity started to dominate again. Hence, the slurry with30 wt% resin content showed the highest viscosity.

The idle time is defined as the time after which the gelationbegins. It is an important parameter in the gelcasting process.If the idle time is too short, i.e. r10 min, the gelation willoccur when mixing hardener in the ceramic slurry; if it is toolong, i.e. Z70 min, it is not feasible from the economicalpoint of view [13]. Therefore, the idle time should beoptimized. Fig. 2(b) shows the viscosity as a function of timefor the 35 vol% slurries with various resin contents. It is foundthat all the curves possess a period with constant viscosity,then the viscosities in all curves start to increase gradually untilthey begin to increase rapidly towards infinity. The period withconstant viscosity is defined as the idle time in the polymer-ization process, which is the available time that can be used forcasting [14]. As shown in Fig. 2(a), the idle time of the slurriesdecreased from about 1350 s to 900 s when the resin contentincreased from 15 wt% to 30 wt%. The decrease of the idletime was mainly attributed to the “container” effect of theslurry in the presence of ceramic powders according to Dong’swork [15]. For the slurry gelation, polymer chain lines are

needed to connect the ceramic particles and fill the intersticesof the particles in the slurry, for which a certain period of timeis required (the idle time). The idle time depends on the size ofthe “container”: the smaller the “container”, the shorter timethe polymer chain line is needed to connect the particles. Whenthe resin content is increased, the space between the particlesin the slurry (the vehicle (water) content is fixed) is reducedleading to a reduction of the “container” size, which finallyresults in the decrease of the idle time. Therefore, higher resincontent leads to shorter idle time, which is in agreement withOlhero’s investigation [16]. Moreover, one interesting phe-nomenon is that Fig. 2(b) shows a neat difference in the idletime between these systems with different resin contents (15–20 wt% vs. 25–30 wt%). Probably, for this epoxy-hardenersystem, when the resin content is over 20 wt%, the ‘container’size will dramatically decrease, finally resulting in the neatdifference.Fig. 2(c) shows the effects of solid loadings on the

rheological properties of the slurries with 15 wt% resincontent. All the slurries tested here showed typical shearthinning behaviour within the measurement range. Viscositiesincreased dramatically with increase of solid loadings from30 vol% to 45 vol%. Fig. 3 show the dispersion status of

Fig. 3. The dispersion status of zirconia powders in these slurries with different solids loadings (a) 35 vol% with 15 wt% resin; (b) 40 vol% with 15 wt% resin; (c)45 vol% with 15 wt% resin. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1Physical property of the green and sintered samples.

Solid-loading[vol%]

Resincontent[%]

Greendensity[g/cm3]

Sintereddensity[g/cm3]

Microhardness[GPa] aftersintering

35 15 2.69 5.28 13.3470.5035 20 2.66 5.16 12.4570.5735 25 2.84 5.40 14.0870.3835 30 2.85 5.36 13.7670.6240 15 2.75 5.30 13.8470.2145 15 3.02 5.45 14.1870.43

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zirconia powders in these slurries. Due to the relatively lowsolids loading (30 vol%), the powders in Fig. 3(a) exhibits agood dispersion status. However, evident aggregates can beobserved, especially when the solids loading increased to45 vol%, as shown in Fig. 3(b) and (c) (red circles). That isbecause significant increase of the attractive potential energydue to long-range van der Waals interactions between particleswhen the solids loading arises especially using this nano-sizedzirconia powders. Therefore, the presence of attractive inter-particle interactions and the formation of particle aggregatesresorts into higher values and stronger concentration depen-dence of viscosity. Similar results can be found in Tan’s andChen’s work [17,18].

Table 1 summarizes the physical properties of the green andsintered components consolidated by this aqueous gel castingtechnique. For the 35 vol% slurry, with increase of the resincontent from 15 wt% to 30 wt%, the density of the greencomponent increased. This behaviour occurs because a 3Dnetwork of fine pores will be generated during gelation, whichcan exert high suction pressure on drying resulting in theshrinkage of gelcast samples [16]; higher resin content leads toenhanced fineness of the 3D network pores, which gives

stronger capillary forces making the particles approach eachother more upon drying. The increased fraction of the resinfilling the interstitial pores particles also contributes to theincrease of the green density with increase of the resin content.These two effects result in the increase of the green density.Fig. 4(a)–(d) shows the secondary electron SEM micro-

graphs of the green samples with different amounts of epoxyresin. Due to the small change of the densities as shown in

Fig. 4. Secondary electron SEM micrographs of the fracture surface of the green gelcast bodies obtained from different slurries (a) 35 vol% with 15 wt% resin; (b)35 vol% with 20 wt% resin; (c) 35 vol% with 25 wt% resin; (d) 35 vol% with 30 wt% resin; (e) 40 vol% with 15 wt% resin;(f) 45 vol% with 15 wt% resin. (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1, the variation of the porosity is not obvious in theseSEM images. The sintered densities showed a different trendcompared with the green densities. When the added amount ofthe resin was more than 25 wt%, the sintered density started todecrease, indicating that high resin content over 25 wt% isdetrimental to the sintering behaviour. When the resin content

increased from 15 wt% to 30 wt%, the microhardness showeda very similar trend to the sintered density, as shown inTable 1.The effect of solid-loading has also been investigated as

shown in Table 1. With increase of the solid-loading from35 vol% to 45 vol% with 15 wt% resin content, the density of

Fig. 5. ((a) to (c)) Secondary electron SEM images of sintered samples obtained from slurries with different solid-loading but with the same resin content of 15 wt%(a) 35 vol%; (b) 40 vol%; (c) 45 vol%.

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the green component increased from 2.69 g/cm3 to 3.02 g/cm3.Moreover, the sintered density and the microhardness reportedin Table 1 for these samples showed the same trend when thesolid loading increased. The sintered density increased from5.28 g/cm3 (89% relative density) to 5.45 g/cm3 (92% relativedensity) and the microhardness increased from 13.3 GPa to14.2 GPa, similar to that reported in Tulliani’s work [19](14.3 GPa, 95% relative density). A gradual decrease of thedensity of pores was found, as can be seen in Fig. 5(a)–(c),which shows the microstructure of the fracture surface of thesesamples. Accordingly, with increase of the solid loading, adenser microstructure could be observed. Therefore, highersolid loading is helpful to improve the density which would beexpected to improve the mechanical properties, which isconsistent with Chen’s investigation [18].

Fig. 6. Green components obtained from slurry of 35 vol% solid-loading and15 wt% resin: (left) with bending, (right) without bending. (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of this article.)

3.2. Influence of experimental variables on shape control

Two microgears in the green state fabricated from slurrieswith 35 vol% solid-loading and 15 wt% resin content areshown in Fig. 6. It can be seen that the right microgear showsmore defects than the left one, although the left one does have

several defects. The only difference between the two is thePDMS mould used for manufacturing the gear shown in Fig. 6(a) was bent slightly after casting. Bending the PDMS mould

Fig. 7. Green components obtained from 35 vol% solid-loading slurries withdifferent resin contents 30 wt% (left hand side), 25 wt% (middle), 15 wt% resin(right hand side). (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

Fig. 8. Green components obtained from slurries with 40 vol% solid-loadingand 15 wt% resin content. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

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slightly allows the slurry to fill the mould fully and reproducethe very fine structure of the mould accurately.

Several long bars in the green state produced from slurrieswith 35 vol% solid-loading and different resin contents areshown in Fig. 7. Bending the PDMS moulds by hand wasapplied here. It can be observed that with increase of the resincontent from 15 wt% (right hand side) to 25 wt% (middle one),the surface of the samples becomes much smoother and theedges become much sharper. This is mainly because thesample with 15 wt% resin content has the lower density (asshown in Table 1), resulting in lower mechanical strength ofthe green component. However, the long bars obtained fromslurries with 25 wt% and 30 wt% (left hand side) resin contentdo not show much difference. This is because the two greencomponents show very similar densities. Thus, higher resin

content (no more than 30 wt%) is needed for preparing greencomponent with few drawbacks.Based on the above results, bending the mould and adopting

less than 30 wt% resin content are applied in the followingwork. One microgear and spanner in the green state fabricatedfrom slurry with 40 vol% solid-loading and 15 wt% resincontent are shown in Fig. 8. It can be seen clearly that thesurface of these samples becomes much smoother and theedges become much sharper compared to that of the sampleshown in Fig. 6. According to Table 1, the density of thesample obtained from 40 vol% slurry is bigger than that ofsample obtained from 35 vol% slurry. Thus, high solid-loadingis very helpful to improve the shape control, leading to muchbetter green components.

4. Conclusions

In summary, a water-soluble epoxy resin based gel systemwas developed incorporating nano-sized zirconia powders anddifferent shapes were produced by using such system. Withincrease of the resin content from 15 wt% to 30 wt%, theviscosity of the slurry increased and the idle time becameshorter from about 1350 s to 900 s. Higher resin content ishelpful to improve the green density and also the shapecontrol, but not suitable for casting process. Moreover, thesintered density decreased when the resin content was beyond25 wt%. High solid-loading leads to high green and sintereddensity, which is also better for the shape control. In additionan external force, such as bending by hand, is necessary forshape control in order to reproduce the fine structure ofthe mould.

References

[1] K. Jiang, S.B. Liu, C.C. Li, Nano–nano composite powders of lanthanumzirconate and yttria-stabilized zirconia by spray pyrolysis, J. Am. Ceram.Soc. 96 (10) (2013) 3296–3303.

[2] C. Gomez-Yanez, H. Balmori-Ramirez, F. Martinez, Colloidal processingof BaTiO3 using ammonium polyacrylate as dispersant, Ceram. Int. 26 (6)(2000) 609–616.

[3] G. Liu, D. Zhang, T.W. Button, Preparation of concentrated bariumtitanate suspensions incorporating nana-sized powders, J. Eur. Ceram.Soc. 30 (2) (2010) 171–176.

[4] W.M. Sigmund, N.S. Bell, L Bergstrom, Novel powder-processingmethods for advanced ceramics, J. Am. Ceram. Soc. 83 (7) (2000)1557–1574.

[5] J.A. Lewis, Colloidal processing of ceramics, J. Am. Ceram. Soc. 83 (10)(2000) 2341–2359.

[6] R. Gilissen, J.P. Erauw, A. Smolders, E. Vanswijgenhoven, J. Luyten,Gelcasting, a near net shape technique, Mater. Des. 21 (4) (2000)251–257.

[7] O.O. Omatete, M.A. Janney, R.A. Strehlow, Gelcasting—a new ceramicforming process, Am. Ceram. Soc. Bull. 70 (10) (1991) 1641 (&).

[8] A.C. Yong, O.O. Omatete, M.A. Janney, P.A. Menchhofer, Gelcasting ofalumina, J. Am. Ceram. Soc. 74 (3) (1991) 612–618.

[9] J.S. Ha, Effect of atmosphere type on gelcasting behavior of Al2O3 andevaluation of green strength, Ceram. Int. 26 (3) (2000) 251–254.

[10] X.J. Mao, S.Z. Shimai, M.J. Dong, S.W. Wang, Gelcasting of aluminausing epoxy resin as a gelling agent, J. Am. Ceram. Soc. 90 (3) (2007)986–988.

G. Liu et al. / Ceramics International 40 (2014) 14405–1441214412

[11] X.J. Mao, S.Z. Shimai, M.J. Dong, S.W. Wang, Investigation of newepoxy resin for the gelcasting ceramics, J. Am. Ceram. Soc. 91 (4) (2008)1354–1356.

[12] M.A. Rao, Rheology of Fluid and Semisolid Foods, Springer, 2007.[13] M. Szafran, P. Wiecinska, A. Szudarska, T. Mizerski, New multi-

functional compounds in gelcasting process-introduction to their synth-esis and application, J. Aust. Ceram. Soc. 49 (1) (2013) 1–6.

[14] X.G. Xu, Z.Y. Wen, X.W. Wu, J. Lin, X.Y. Wang, Rheology andchemorheology of aqueous γ-LiAlO2 slurries for gel-casting, Ceram. Int.35 (2009) 2191–2195.

[15] M.J. Dong, X.J Mao, Z.Q. Zhang, Q. Liu, Gelcating of SiC using epoxyresin as gel former, Ceram. Int. 35 (4) (2009) 1363–1366.

[16] S.M. Olhero, L. Garcia-Gancedo, T.W. Button, F.J. Alves, J.M.F. Ferreira, Innovative fabrication of PZT pillar arrays by a colloidalapproach, J. Eur. Ceram. Soc. 32 (5) (2012) 1067–1075.

[17] Q.Q. Tan, Z.T. Zhang, Z.L. Tang, S.H. Luo, K.M. Fang, Influence ofpolyelectrolyte dispersant on slip preparation of nano-sized tetragonalpolycrystals zirconia for aqueous-gel-tape-casting process, Mater. Chem.Phys. 80 (3) (2003) 615–619.

[18] B.Q. Chen, D.L. Jiang, J.X. Zhang, M.J. Dong, Q.L. Lin, Gel-casing ofbeta-TCP using epoxy resin as gelling agent, J. Eur. Ceram. Soc. 28 (15)(2008) 2889–2894.

[19] J.M. Tullinai, C. Bartuli, Bemporad, V. Naglieri, M. Sebastinai, Prepara-tion and mechanical characterization of dense and porous zirconiaproduced by gel casting with gelatin as a gelling agent, Ceram. Int. 35(6) (2009) 2481–2491.