J. Am. Ceram. Soc., 91 [2] 398–405 (2008) DOI: 10.1111/j.1551-2916.2007.02164.x r 2007 The American Ceramic Society Journal Preparation of High Solids Content Nanozirconia Suspensions Isabel Santacruz,w,z Ketharam Anapoorani, and Jon Binner* IPTME, Loughborough University, Loughborough LE11 3TU, U.K. routes,7–11 such as slip casting, in situ coagulation molding, gel casting, and tape casting, require stable slurries with a high solids loading and a low viscosity and this is difficult to achieve with particles in the sub-100-nm range.12,13 Colloidally stable nanopowder suspensions are known to display a markedly lower volume loading at the same viscosity compared with suspensions with larger particle sizes.13 The dispersion of ceramic powders in either an aqueous or a nonaqueous medium has received considerable attention,14,15 the nature and strength of the interparticle forces and also the quantity, shape, and size of the particles determining the rheological properties of a suspension.16–18 To achieve adequate distance between the particles in ceramic suspensions generally requires the use of surfactants that modify the particle surface. This can be achieved by changing the surface charge,19–22 coating the particles with an organic barrier layer, or the combination of the two, i.e. the use of polyelectrolytes, although the saturation adsorption and dissociation of the latter in an aqueous solution are strongly dependent on the pH of the solution.23,24 It is known that tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide, and tetrapropylammonium hydroxide are all quaternary ammonium surfactants for which the positively charged nitrogen atom can be adsorbed onto particle surfaces, enhancing stabilization in some systems25–28 and also acting as a strong base. The length of the alkyl chain affects the rheology of the suspension, with viscosity, yield stress, and thixotropy all increasing with increasing chain length.27 As a result, the green density decreases as the chain length increases. As a result of these properties, TMAH was studied as a basic agent in the present work. The aim of the present work was to investigate the potential for preparing fully dense, 3-mol% YSZ nanostructured ceramics while retaining a final average grain size of o100 nm. In order to avoid the drawbacks of dry nanopowders, which include uncontrolled agglomeration, the presence of organics (from their synthesis), and potential toxicity due to their ultrafine size, a dilute commercial nanosuspension was used to prepare low viscosity, high solid content nanosuspensions. Optimization of the basic agent required to modify the suspension pH, the dispersant, and the use of ultrasound were all performed. The most promising nanosuspensions were slip cast into green bodies that were subsequently sintered, both the green and sintered bodies being thoroughly characterized in terms of their density and nanostructure, the latter in terms of their homogeneity and grain size. A new colloidal route leading to the production of B99% dense 3-mol% yttria-stabilized zirconia nanostructured ceramics, while retaining a final average grain size of B75 nm, has been developed. The process was based on the production of stable, homogeneous nanosuspensions with solids contents of up to 28 vol% (70 wt%), but viscosities o0.05 Pa . s at any shear rate in the range of study were obtained. The suspensions were formed by the concentration and optimization of precursor, dilute (5.0 vol%) commercial nanosuspensions, the approach requiring a change of pH, from the 2.4 of the as-received suspension to 11.5, and the use of an appropriate anionic dispersant. Exposure of the nanosuspensions to ultrasound also helped to reduce the viscosity further, although it only worked when the dispersant was optimized. The nanosuspension was slip cast to form homogenous green bodies with densities of B55% of the theoretical without agglomeration in the nanostructure; these were subsequently densified using a two-step sintering technique. I. Introduction T HE recent interest in nanocrystalline materials has arisen from their potential to display unusual properties, including higher hardness and strength in both metals and ceramics,1,2 and also lower sintering temperatures, providing the ability to save energy and allowing metals and ceramics to be cofired to a greater extent. If powders can be consolidated into fully dense engineering parts without losing the nanostructure, there is the potential to use the materials for structural, thermal, magnetic, electric, or electronic applications such as capacitors, varistors, electronic substrates, wear, thermal barrier and net shape parts, magnets, and tools.2 Conventional, submicrometer zirconia ceramics are widely used for their excellent mechanical3 and electrical properties4 and hence there is considerable interest in investigating the properties of nanostructured yttria-stabilized zirconia (YSZ) ceramics. Commercial nanopowders can now be produced in relatively large quantities, although to date they are generally strongly agglomerated and/or show large amounts of organics derived from their synthesis.5 However, a major obstacle to the formation of genuinely nanostructured ceramics (average grain size o100 nm) lies in the preparation of homogeneous green bodies. While dry forming via die or isostatic pressing is industry’s generally preferred route, the strong agglomerates that readily form in dry nanopowders, plus the latter’s inability to flow, indicate that wet forming routes are likely to lead to greater success. Colloidal processing generally allows the production of complex-shaped parts with reduced size and number of pores and higher reliability6; however, the majority of wet forming II. Experimental Procedure The precursor, as-received, nanosuspension (MEL Chemicals Ltd., Manchester, U.K.) contained 5.0 vol% of 3-mol% YSZ nanoparticles in deionized water. The solids loading was calculated after drying the suspension in an oven at 601C overnight, followed by calcination at 5001C for 2 h. The density of the dried and calcined powder was measured by He pycnometry (Quantachrome, Fleet, U.K.), resulting in values of 4.88 and 5.55 g/cm3, respectively. All calculations were performed with the powder crystallographic density, 6 g/cm3. A tetragonal/cubic phase content of 92% was observed in the dried nanopowder using XRD, the balance being monoclinic. The as-received suspension, which had a pH of 2.470.1, was G. Franks—contributing editor Manuscript No. 23218. Received May 16, 2007; approved October 2, 2007. Supported by Rolls Royce Fuel Cell Systems Ltd., the PowdermatriX Faraday, and the Engineering and Physical Science Research Council (EPSRC), U.K., under grant No. GR/ S84477/01, and the Spanish Education and Science Ministry under postdoctoral grant No. EX2004-1012. *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: cruz@icv.csic.es z Currently at the Instituto de Cerámica y Vidrio (CSIC), Madrid, Spain. 398 February 2008 Preparation of Nanozirconia Suspensions 399 Fig. 1. Transmission electron microscopy images of the three yttria-stabilized zirconia nanoparticles at different magnifications. Table I. Summary of the Characteristics of the Nanosuspensions Prepared under a Range of Different Conditions also characterized in terms of particle size using an AcoustoSizer II (Colloidal Dynamics, Sydney, Australia) and the dried nanoparticles were examined using transmission electron microscopy (TEM; JEOL 2000FX, JEOL, Tokyo, Japan). The effects of both cationic and anionic dispersants were examined. The former, poly(ethylenimine), PEI (BDH Chemicals Ltd., Poole, U.K.), was appropriate for the as-received acidic suspension,24 while the anionic dispersants required that the pH be modified to the basic region.20,23,24 This was achieved using both 35% ammonia solution (Fisher Scientific, Loughborough, U.K.) and solid TMAH (Aldrich Chemicals Ltd., Dorset, U.K.), the latter having the advantage of not involving the initial further dilution of the precursor nanosuspension. When the ammonia solution was used, the solids content of the nanosuspension declined from 5.0 to 3.3 vol%; with the TMAH, it remained at 5.0 vol% and the suspension displayed superior stability. Additions of 6.7 wt% of TMAH, as a function of the suspension solids content, were found to be required to modify the as-received suspension’s pH from 2.4 up to 11.570.1. The anionic dispersants investigated were Dispex A40, an ammonium polyacrylate-based surfactant, NH4PAA (Ciba Speciality Chemicals, Bradford, U.K.), and triammonium citrate (TAC) (FSA Laboratory, Loughborough, U.K.). The latter was studied because it is a relatively short molecule that was considered to be less likely to be broken by the application of ultrasound, which was used to break down any agglomerates present. The suspensions involving PEI were prepared by mixing 2.0 and 4.0 wt% directly in the as-received suspension; the presence of the strong acid neutralized its basic –NH– groups and conferred a positive charge on the polymer skeleton.19 For the anionic dispersants, 1–4.5-wt% additions of Dispex and TAC were introduced into the pH-modified nanosuspension, Fig. 2. Evolution of the z potential as a function of pH for the diluted, 1.8 vol%, nanosuspension: without a dispersant, with 2.5 wt% of Dispex A40 or triammonium citrate. Fig. 3. Evolution of the particle size and z potential as a function of pH with the addition of tetramethylammonium hydroxide to a 1.8-vol% nanosuspension. Dispersant content Dispersant (wt%) — PEI Dispex A40 0 2.0 2.5 Solids loading (vol%) Basic agent 5.0 — 5.0 — 17 Ammonia 19 TMAH 28 TAC 2.5 24 28 TMAH Viscosity Ultrasound (mPa
s) pH at 100 s 1 70.1 (min) 0 2 0 10 0 1000 2 300 2 260 6 45 Multi700 ultrasound 0 1400 2 75 6 15 2 320 Multi45 ultrasound 2.4 4.6 9.5 9.5 PEI, poly(ethylenimine); TMAH, tetramethylammonium hydroxide; TAC, triammonium citrate. 400 Journal of the American Ceramic Society—Santacruz et al. Vol. 91, No. 2 Fig. 4. Flow curves of dispersed suspensions containing 2.5-wt% Dispex A40 prepared with NH4OH (17.0 vol% solids content) and tetramethylammonium hydroxide (19.0 vol% solids content). preliminary work indicating that 2.5 wt% was the optimum addition. The adsorption of dispersants was achieved by shaking the suspensions on an automatic shaker (KS 260 basic, IKA, Staufen, Germany), in closed plastic bottles for 20 h. Note: in all cases, the amount of the deflocculant addition is expressed in terms of the active matter present in the dispersant with respect to suspension solids loading. z potential and particle/agglomerate size measurements as a function of pH were performed using an AcoustoSizer II. The as-received suspension was diluted to 1.8 vol% using deionized water due to the difficulties found in performing a continuous titration at higher solids loading in the equipment. Titration was performed from acid to base, using 1M NaOH solution (automatic titration) or by manual addition of TMAH. Similar measurements were made on the basic diluted suspension (achieved using ammonia solution) containing 2.5-wt% Dispex A40 or TAC using 1M NaOH and HCl solutions for pH adjustments. The dilute nanosuspensions were subsequently concentrated in a water bath at 501C for between 1 and 4 days, depending on the required final solids loading and, for the anionically dispersed suspensions, whether ammonia solution or TMAH was used. For example, to achieve a solids content of 17 vol% required 4 days for the ammonia solution-based suspension and only 2 days for that produced using TMAH due to the higher solids content at the starting point. Throughout the process, the pH was controlled every 2 h, maintaining it at 9.570.1, and the suspension was stirred constantly. As indicated already, in order to break up any agglomerates present, the suspensions were exposed to ultrasound using a KS150 ultrasound probe (Kerry Ultrasonics Ltd., Skipton, U.K.), with an amplitude of 14 mm and a power of 75 W. In all cases, the ultrasound was applied to 50-mL aliquots of the Fig. 5. Flow curves of a 15.0-vol% suspension dispersed with Dispex A40 after 1 and 2 min of ultrasound. The basic agent was tetramethylammonium hydroxide. Fig. 6. Effect of ultrasound time on the viscosity of a 19.0-vol% suspension dispersed with Dispex A40; (a) flow curves and (b) viscosity at 100 s 1 as a function of ultrasound exposure time. The basic agent was tetramethylammonium hydroxide. different nanosuspensions. A variety of different time periods of ultrasound exposure were investigated, from 0 to 10 min. Note that the ultrasound exposure was performed in 1-min steps, with the suspension being stirred at room temperature for 10 min between each ultrasound application in order to avoid excessive heating of the nanosuspension. A ‘‘multiultrasound’’ approach was also applied to selected suspensions, as distinct from the simple ultrasound method described above. The ‘‘multiultrasound’’ technique involved applying ultrasound to the suspension at several different points in the concentration process until the required solids loading was achieved. For example, the suspension dispersed with Dispex was exposed to ultrasound three times during the evaporation process: 2 min at a solids loading of 15 vol%, and then a further 5 min when it reached 19 vol%, and finally 10 more min at 28 vol%. For the TAC-based suspension, it was found that less ultrasound was required, viz., only two applications, 6 min at 24 vol%, followed by a further 2 min at 28 vol%. In all cases, the suspensions were held in a cold water bath during ultrasonication the suspension heating. Fig. 7. Effect of aging on the viscosity of a 16.0 vol% suspension with Dispex A40 subjected to different ultrasound exposure times. The basic agent was tetramethylammonium hydroxide . February 2008 Preparation of Nanozirconia Suspensions Fig. 8. Average agglomerate size measured by laser scattering for Dispex-based concentrated suspensions with and without ultrasound exposure. The size of any agglomerates present in the nanosuspensions was measured using a Mastersizer 2000 (Malvern Instruments Ltd., Malvern, U.K.). Based on the results, it is believed that the agglomerates formed during the evaporation of the suspension were subsequently broken by the ultrasound, thus allowing more concentrated suspensions to be prepared. The rheological behavior of the suspensions was determined using a Visco 88 BV viscometer (Bohlin Instruments, Cirencester, U.K.) with a C30 concentric cylinder sensor and by varying the shear rate from 0 to 1000 s 1 in 8 min without preshearing; the time taken in obtaining the flow curve being distributed equally across the measurement points. After each exposure to ultrasound, all the suspensions were stirred for 10 min before rheological measurement. To study the effect of aging on the agglomerate size and rheology, the suspensions were left for several days in closed plastic bottles on the shaker before additional measurements were made. 401 Fig. 10. Viscosity versus volume fraction curve for a suspension with tetramethylammonium hydroxide and Dispex A40 using ‘‘multiultrasound.’’ Shear rate: 100 s 1. Ultrasound application at 15.0, 19.0, and 28.0 vol%, the latter being the final solids content. To ensure that comparisons could be made between the different nanosuspensions, the agglomerate size and rheological measurements were all made at pH 9.570.1 at room temperature and the viscosity values presented in the results were all taken from the upward flow curves at a shear rate of 100 s 1. The suspensions with the highest solids content while retaining a low viscosity were subsequently slip cast in plaster of Paris molds to form green bodies measuring 9 mm in diameter by 7 mm in thickness. The resulting green densities of the bodies formed were measured by the Archimedes method using mercury. After the removal of the organic dispersants at 5001C for 2 h, the green samples were sintered in an electrical furnace (UAF 16/10, Lenton Thermal Design, Hope Valley, U.K.) using a two-stage sintering cycle.29 This involved heating the samples to 11501C at 201C/min1 and holding them for 1 min before the temperature was reduced as rapidly as possible to 10001C, where Fig. 9. FEG-SEM micrographs of the fracture surface of green bodies prepared from 15.0-vol% suspensions with tetramethylammonium hydroxide (TMAH) and Dispex after (a) 1 min and (b) 2 min of ultrasound, (c), and (d) green bodies from a dispersed 19.0-vol% suspension (TMAH and Dispex) subjected to 5 min of ultrasound, at different magnifications. 402 Journal of the American Ceramic Society—Santacruz et al. the samples were soaked for 5 or 10 h. The sintered densities of the bodies were measured by the Archimedes method using water. The fracture surfaces of the green and sintered nanozirconia samples were observed by field emission gun scanning electron microscopy (LEO 1530VP, LEO Elektronenmikroskopie GmbH, Oberkochen, Germany). III. Results and Discussions The average particle size of the as-received suspension at pH 2.470.1 was 16 nm70.5%, which correlated well with the particle size observed using TEM (Fig. 1). When PEI was added to the as-received suspension, it resulted in an increase in viscosity under all conditions. Table I shows the viscosity of the suspension containing 2.0-wt% PEI, measured at 100 s 1, and compares it with the as-received suspension. For this reason, and also because at acidic pH the yttria is dissolved, resulting in Y31 cations,30 which would make the processing of concentrated suspensions difficult by promoting coagulation, it was decided to focus on basic pH values. Figure 2 shows the results of the titration of the 1.8-vol% suspension. Note that because 1M NaOH was used, there was a negligible reduction in solids content with pH change. From the figure, it is clear that the isoelectric point (IEP) for the asreceived suspension was just below pH 10; this is higher than generally observed in the literature31,32 and may be due to the presence of residual species, e.g. counter ions, from the preparation of the nanosuspension. At pH values from 2 to 6, the suspension was very stable with a z potential of B60 mV; however, in the basic pH range of 11–12, the maximum absolute value was found to be quite low: B20 mV. Despite this, the particle size of the 1.8 vol%, suspension at pH 11.5 was 31 nm (d50)/53 nm (d85)71%, suggesting that there were no significant agglomerates present, even though the z potential was low. In the same figure, the z potential curves as a function of pH for the suspensions with 2.5-wt% of both Dispex and TAC are also shown. It can be seen that the suspensions with these anionic dispersants had very similar curves with desirably large z potential values over a wide range of pH, from B8 to 12. Figure 3 reveals that the z potential of the as-received nanosuspension changed smoothly from B60 mV through to B 40 mV as the pH was changed from 2.4 to 11.570.1 by the addition of TMAH. The higher absolute value of the z potential at pH 11.5 after the addition of TMAH, compared with that obtained by the addition of NaOH, B 20 mV, confirms that TMAH provides an extra contribution to stability, probably related to the adsorption of N1Me)4 groups,27 which are not available from bases such as NaOH. The change in pH resulted in the formation of agglomerates as the pH passed through the IEP, which occurred at pH B9.5 when no deflocculants were present. Interestingly, the formation of the agglomerates was reversible and hence no significant agglomeration was observed after crossing the IEP (Fig. 3), even though ultrasound was not applied to the suspensions. The flow curves of the nanosuspensions with 2.5-wt% Dispex A40 prepared using TMAH at a solid content of 19.0 vol% and using ammonia solution at a solid loading of 17.0 vol% are shown in Fig. 4. In both cases, 2 min of ultrasound were applied. Both pH agents allowed the formation of moderately concentrated nanosuspensions with low viscosities, although it may be seen that the use of the TMAH allowed slightly higher concentrations to be achieved while retaining a very similar viscosity. This may be due to two reasons: the extra stabilization provided by the TMAH27 and/or the longer evaporation time required when ammonia was added due to the initial suspension dilution, which may contribute to greater agglomeration, thus necessitating longer ultrasound times than were studied in the current work. Whatever the reason, TMAH was selected as the basic agent for further studies because it avoided the initial dilution, Vol. 91, No. 2 hence reducing the processing time, and yielded a slightly lower viscosity for a given solids content. The effect of the ultrasound is plotted in Figs. 5 and 6. These show flow curves for 15.0-vol% solids content-dispersed nanosuspensions after 1 and 2 min of ultrasound exposure (Fig. 5), and flow curves of 19.0 vol% nanosuspensions after 1–6 min of ultrasound (Fig. 6(a)). Dispex A40 was used in both cases. While both curves in Fig. 5 displayed shear thinning behavior without thixotropy, the suspension with 2 min of ultrasound treatment exhibited considerably lower viscosity. A similar outcome is presented in Fig. 6, where longer ultrasound times were required with the higher solids content suspension. Figure 6(b) shows the viscosity of the 19.0 vol% suspensions at 100 s 1 after ultrasound. The stability of the viscosity taken at 100 s 1 during aging of a 16.0 vol% suspension prepared using Dispex A40 after 2, 4, and 8 min of ultrasound is shown in Fig. 7. Once again, this shows that longer periods of ultrasound exposure result in more stable suspensions in the range of study, where samples treated for  4 min were entirely stable for at least 7 days while those treated for only 2 min exhibited a fairly steady increase in viscosity over time. No sedimentation was observed in these suspensions. Stable suspensions with solids loadings up to 19 vol% could be achieved with this approach. Fig. 11. Flow curves of suspensions with tetramethylammonium hydroxide and triammonium citrate, (a) without ultrasound, (b) 24.0 vol% with different ultrasound times, (c) 28.0 vol% without and with a 2-min ultrasound at this solids loading, after a 2-min ultrasound at 24.0 vol% (multiultrasound). February 2008 Preparation of Nanozirconia Suspensions Fig. 12. Viscosity curves for the nanosuspensions: (a) 24.0 vol% using tetramethylammonium hydroxide (TMAH) and triammonium citrate (TAC) after a 6-min ultrasound, (b) 19.0 vol% prepared using TMAH and Dispex A40 after multiultrasound, (c) 28.0 vol% using TMAH and TAC after multiultrasound treatments. Figure 8 reveals the effect of ultrasound on the size of the agglomerates present in the suspension, the d50 value taken from the volume distribution as measured by laser scattering via a Malvern MasterSizer 2000, as a function of solids content. It can be seen how the use of ultrasound significantly reduced the size of the agglomerates present—and also confirms again how at higher solids loadings, longer ultrasound periods were required. The consequences of the presence of the agglomerates on the microstructure of the slip-cast green samples are illustrated by the fracture surfaces shown in Fig. 9. Images (a) and (b) are both from bodies prepared from a 15.0-vol% suspension after 1 and 2 min of ultrasound, respectively (the samples were prepared from the suspensions represented in Fig. 5); it can be seen how the 403 latter is very significantly more homogeneous because the action of the ultrasound broke up the agglomerates present. Figures 9(c) and (d) show the nanostructure obtained by slip casting a 19.0-vol% suspension after 5 min of ultrasound at two different levels of magnification (prepared from suspensions shown in Fig. 6). The homogeneity of the structure is notable. All of the results presented to date were based on the application of ultrasound at the final solid loading. However, this approach suffered from an upper solid loading limit beyond which ultrasound could not be applied because the viscosity was too high and the suspension became a solid even before the application of the ultrasound. This led to the development of the multiultrasound approach described earlier. Figure 10 shows the results achieved for a 28 vol% suspension when this approach was used. While it can be seen that a final viscosity of o1 Pa
s was achieved even at this high solids content (28.0 vol%, 70.0 wt%), unfortunately, it was discovered that the resulting suspensions, which were based on Dispex A40, were unstable. They became a gel over time; the higher the solids content achieved, and hence the more the ultrasound used, the faster this occurred. It is believed that excessive use of ultrasound may have damaged the polymer chain of the dispersant, resulting in an unstable system. This led to the investigation of TAC, a shorter chain anionic dispersant; hence, it is less likely to be broken by the application of ultrasound, because the length of the dispersant chain is a key parameter in the stabilization of nanosuspensions.33,34 Figure 11 reveals the resulting flow curves for the TAC-dispersed suspensions at different solids loadings and with different exposures to ultrasound; note that the concentrated suspensions made with TAC exhibited lower viscosities even without the use of ultrasound than those based on Dispex A40, providing up the potential for achieving even higher solid content suspensions after optimization. Figure 11(a) shows the effect of concentrating the solids loading from the as-received value of 5.0 vol% up Fig. 13. FEG-SEM micrographs of sintered samples prepared from a 19.0-vol% suspension with tetramethylammonium hydroxide (TMAH) and Dispex with soaking times of: (a) 5 h, (b) 10 h, (c) a green sample prepared from a 28.0-vol% suspension with TMAH and triammonium citrate after multiultrasound, and (d) the sample in (c) after sintering (soaking time: 10 h). 404 Journal of the American Ceramic Society—Santacruz et al. to 24.0 vol% without the use of any ultrasound. A significant increase in the viscosity and thixotropy may be observed from 9.0- to 24.0-vol% solids loading. Figure 11(b) reveals how the viscosity and thixotropy decreased by increasing exposure to ultrasound from 2 to 6 min on a 24.0 vol% nanosuspension. A great improvement in viscosity was observed only after 2 min of ultrasound, compared with Fig. 11(a). Figure 11(c) shows the flow curves of two-vol% suspensions, the first after application of just 2 min of ultrasound at 24.0 vol% and the second after 2 min of ultrasound at 24.0 vol%, followed by a further 2 min at 28.0 vol%. When the former curve is compared with that in Fig. 11(b) after 2 min of ultrasound, it can be seen how an increase of just 4 vol% in the solids loading at these high values yields a very considerable viscosity increase. Table I summarizes the effect of the basic agent and anionic dispersant on the viscosity (taken at 100 s 1 from the up-curve) as a function of solids loading with and without the use of ultrasound. The improvement shown by the TMAH/TAC system over the ammonia solution/Dispex A40 system is clearly visible in terms of the lower viscosities observed. Figure 12 shows the viscosity curves of three suspensions in which TMAH was used as the basic agent. It can be seen that even with the application of a single, 6-min ultrasound treatment at the end of the concentration process, the use of TAC results in a 24.0-vol% solids content nanosuspension (Fig. 12(a)), having a lower viscosity than the 19.0-vol% suspension prepared with 2.5-wt% Dispex A40 after the multiultrasound treatment (Fig. 12(b)). When the multiultrasound process is then combined with the TAC, 28.0-vol% suspensions can be seen to display a viscosity o0.05 Pa
s at all shear rates measured (Fig. 12(c)). They were also entirely stable for periods of at least 15 days, confirming the superiority of TMAH over ammonia solution and TAC as a dispersant compared with Dispex A40. These suspensions were slip cast, and green density values of B55% of theoretical (6 g/cm3) were obtained. Figures 13(a) and (b) show the nanostructures of the two-stage sintered, slip-cast samples prepared from a 19.0-vol% suspension prepared with TMAH and Dispex A40 after 5 min of ultrasound; B99% dense ceramics were obtained with an average and uniform grain size of 80 and 90 nm after 5 and 10 h of soaking time, respectively. The equivalent nanostructure for a sample prepared from 28.0-vol% suspension made with TAC and the multiultrasound treatment after a soaking time of 10 h can be seen in Fig. 12(d); its green nanostructure can be seen in Fig. 12(c). The sintered sample has an average grain size of 75 nm, i.e. lower than that obtained from the equivalent sintering cycle for the sample prepared with Dispex A40 (Fig. 12(b)), and is much more uniform due to the greater stability and higher solid loading of the precursor suspension. IV. Conclusions A colloidal route has been developed for the production of B99% dense 3-mol% YSZ nanoceramics with final average grain sizes of B75 nm. It is based on the preparation of 3YSZ nanosuspensions with solids contents up to 28.0 vol% but viscosities as low as 0.05 Pa
s from commercially available, dilute suspensions. The process is based on a series of steps involving initially adjusting the pH to a value of B11.570.1 from the original value of 2.470.1 using (solid) TMAH. This is followed by the addition of anionic dispersants. While both Dispex A40, a commercial surfactant based on ammonium polyacrylate, and TAC work, the latter has been found to be more suitable, offering both the potential to achieve lower viscosities at higher solids contents and also more resistance to the subsequent use of ultrasound. The dilute nanosuspensions can then be concentrated by evaporation at 501C in a water bath. The use of ultrasound energy has been found to be important for breaking any agglomerates that form, thus ensuring that they do not cause a problem in the subsequent nanostructure of the green compo- Vol. 91, No. 2 nents formed. Because of the higher the solid content of the suspension, the more frequent, and the longer periods of ultrasound that were required, an approach based on the use of multiple ultrasound applications for relatively short durations was found to be an appropriate way forward. While this caused problems with nanosuspensions dispersed with Dispex A40, possibly because of damage to the polymer chain, with the shorter chain TAC, high solids content, low viscosity nanosuspensions could be formed that were stable for periods of at least 15 days. This stability range is very high when compared with other nanoparticle suspensions.35 Samples produced by slip casting the most promising nanosuspensions could be sintered using a two-stage sintering process to yield a final average grain size as fine as B75 nm. 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