Transparent Tetragonal Zirconia Ceramics by Colloidal Processing of Nanoparticle Suspension Martin Trunec 1, a * and Oskar Bera 2, b

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1 Advances in Science and Technology Online: ISSN: , Vol. 87, pp doi: / Trans Tech Publications, Switzerland Transparent Tetragonal Zirconia Ceramics by Colloidal Processing of Nanoparticle Suspension Martin Trunec 1, a * and Oskar Bera 2, b 1 CEITEC BUT, Brno University of Technology, Technicka 10, Brno, Czech Republic 2 Faculty of Technology, University of Novi Sad, Bul. Cara Lazara 1, Novi Sad, Serbia a trunec@fme.vutbr.cz, b obera@uns.ac.rs Keywords: nanoparticles, zirconia, suspension, colloidal processing, nanostructure, transparency Abstract. Colloidal processing was applied to a commercial 5 vol% 3Y-ZrO2 nanosuspension with a particle size of nm. The nanosuspension was concentrated by evaporation or by the newly developed method of osmotic dehydration. The viscosity and stability of concentrated suspensions were investigated. The concentrated nanosuspension prepared by osmotic dehydration was consolidated by centrifugation in non-porous moulds. The dried deposit had a relative density of 46% and pores ranged from 4 to 8 nm. This deposit was densified by pressureless presintering to closed porosity, followed by hot isostatic pressing in order to obtain transparent ceramics. After sintering, the tetragonal zirconia retained the nanocrystalline structure with an average grain size of 65 nm and an in-line transmission of 25 % (at 633 nm wavelength and 0.5 mm plate thickness). Introduction Polycrystalline transparent ceramics are important materials for optical applications in extreme conditions where other optical materials do not meet the requirements for high thermo-mechanical properties (polymers, glass) or are difficult and expensive to produce in large and complex shapes (single crystals). Typical applications of these materials include high-temperature windows, transparent armours, missile domes for infrared and combined sensors, infrared emitters, and discharge lamp envelopes [1, 2]. Recently, new applications of transparent polycrystalline ceramics in optics [3], lasers [4] or even dentistry [5] have emerged. Since the discovery of the transformation toughening [6], partially yttria-stabilized tetragonal zirconia has become one of the major high-performance ceramic materials due to its excellent mechanical properties [7, 8], but it has received comparatively little attention with respect to its optical transmission. This is because a reasonable transparency of birefringent tetragonal zirconia ceramics in visible light can only be reached using a fully dense and nanometre-sized microstructure. Klimke et al. [9] showed that a grain size lower than 40 nm is necessary for in-line transmission of 50% if the tetragonal zirconia is fully dense and 1 mm thick. However, the use of nanoparticles represents one of the most challenging tasks in the field of bulk ceramics processing. Due to an inherent tendency of nanoparticles to strong agglomeration, the compacted green bodies exhibit an irregular and loosely packed structure. Such nanoparticle compacts cannot be sintered to full densities while preserving the nanocrystalline structure [10, 11]. Moreover, even a few pores and irregularities in the sintered structure can destroy the in-line transparency due to light scattering at these defects [12]. Colloidal processing of nanoparticles is a useful approach to dispersing and/or removing the agglomerates and obtaining stable and well-dispersed nanoparticle suspensions [13]. To fully exploit the advantages of colloidal processing, ceramic bodies must be formed and consolidated directly from the slurry state [14]. This paper introduces a method for colloidal processing and consolidation of a commercial lowconcentrated nanoparticle suspension for producing transparent tetragonal zirconia ceramics. Experimental Procedure Zirconia nanopowder stabilized by 3 mol% yttria (MELox Nanosize 3Y, Mel, UK), supplied as a stable 5 vol% water nanosuspension, was used for the colloidal processing experiments. The particles with a size of nm (see Fig. 1) were stabilized in the suspension at ph=3.8. Prior to All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-17/09/16,06:20:03)

2 86 13th International Ceramics Congress - Part A consolidation by centrifugation, the zirconia suspension had to be concentrated in order to obtain a reasonably large deposit. Two approaches to dehydrating the nanosuspension were applied. In the first case, the nanosuspension was concentrated by evaporation of the water at a temperature of 45 C, in the other case the suspension was osmotically concentrated in a 20% water solution of polyethylene oxide (Mw=35000) using a tube membrane (SpectraPor 3, Spectrum Laboratories, USA). Two kinds of suspension were concentrated; as supplied untreated nanosuspension and nanosuspension treated with an addition of a dispersant, triammonium citrate (TAC) (Aldrich- Sigma, Germany) [15]. The ceramic suspensions were repeatedly dispersed with ultrasonic homogenizer (U 400-S, IKA Labortechnik, Germany) during the dehydration. The concentrated suspension was poured into a two-part mould-container with rubber insert and then centrifuged (3K30, Sigma, Germany) with centrifugal accelerations of 54500g for 30 min. The mould design enabled removing the deposit from the mould immediately after the centrifugation without any drying. The centrifuged deposit was dried at ambient temperature and controlled humidity for several days, selectively presintered at 300 and 600 C, and finally sintered at C. The transparent bodies were prepared by pressureless presintering in air followed by hot isostatic pressing in a graphite-free press (ABRA Shirp, Switzerland) at 198 MPa of argon. An electro-acoustic technique (Zeta-APS, Matec Applied Sciences, USA) was used to measure the zeta potential of the suspensions. Tetramethylammonium hydroxide and nitric acid were used to control the ph of suspensions. The rheological behaviour of the nanosuspensions was measured in the steady shear mode, using a rotational rheometer (HAKE MARS II, Thermo Scientific, Germany) equipped with a double gap cylinder sensor system. The pore size distribution in green and partially sintered bodies was investigated via mercury intrusion porosimetry (Pascal 440, Porotec, Germany). The densification curves were determined using a high-temperature dilatometer (L75/50, Linseis, Germany). The density of green and sintered bodies was determined by the Archimedes method. The green densities were measured at least three times for each sample and the difference from the mean was less than ±0.2 % t.d. for green bodies and ±0.03% t.d. for sintered bodies. The solid loading of suspensions and the relative density of a deposit were calculated using the value 5.94 gcm -3 (crystallographic density of the powder), whereas the relative density of presintered and sintered ceramic bodies was calculated using the value 6.08 gcm -3 (theoretical density of sintered tetragonal zirconia).the average grain size of sintered bodies was determined using the linear intercept method on at least three SEM micrographs of polished and thermally etched samples. The linear intercept grain size was corrected by a factor of 1.56 to yield the true grain size [16]. Real in-line transmission (RIT) of polished samples was measured with a He-Ne laser (λ = nm), the distance from the sample to the detector was 860 mm (with an opening angle of 0.5 ). Fig. 1 TEM micrograph of zirconia nanoparticles Fig. 2 Zeta potential as a function of ph for 1.5 vol% nanosuspensions with different amounts of the dispersant

3 Advances in Science and Technology Vol Results and Discussions Colloidal treatment and dehydration of nanosuspensions. The effect of the dispersant, triammonium citrate (TAC), on the diluted 1.5 vol% nanosuspension was investigated. Table 1 shows the natural ph and zeta potential of the zirconia nanosuspension after addition of different amounts of the dispersant (the amount of TAC was calculated with respect to zirconia). Fig. 2 shows the zeta potential as a function of ph for nanosuspensions with different amounts of the dispersant. The ph value of an isoelectric point determined from Fig. 2 is also shown in Table 1. It follows from these results that the addition of more than 1.5 wt% TAC changed the charge on the zirconia particles and the nanosuspension became stable in the alkaline area. The ph and zeta potential did not change too much above 2.5 wt% TAC. Therefore, the suspension with 2.5 wt% TAC at ph=11 was chosen for further experiments. The highly alkaline ph ensured that a small ph change did not affect the nanosuspension stability. Table 1 Natural ph, zeta potential and ph of isoelectric point (IEP) of 1.5 vol% zirconia nanosuspensions with different amount of dispersant. TAC [wt%] 0.0% 1.0% 1.5% 2.0% 2.5% 3.0% 3.5% ph Zeta-potential [mv] ph of IEP Fig. 3 shows viscosity changes of nanosuspensions concentrated by evaporation. The untreated nanosuspension gelled after reaching a solid concentration of vol%. On the other hand, the nanosuspension with TAC addition could by concentrated to ~33 vol% of solids at a reasonable viscosity of 190 mpas (at a shear rate of 30 s -1 ). The gelation of the untreated suspension can most probably be explained as a result of increased ion concentration in the suspension during evaporation of water. The high ionic strength eliminated electrostatic stabilization by compressing the electric double layer and thus enabled coagulation of nanoparticles. A similar process could also be detected in the suspension with TAC addition. The zeta potential decreased from -26 mv in the 5 vol% suspension to -11 mv in the 18 vol% suspension. However, large citrate anions (from the dispersant) that were adsorbed on the particle surface acted as a steric hindrance and prevented coagulation. Nevertheless, the steric hindrance of citrate anions is effective over a very short distance and particles could quickly flocculate due to a deep secondary minimum on the curve of total particle interaction energy [17]. This process made the concentrated suspension highly unstable and resulted in a quick and steep viscosity increase after ultrasonic treatment (see Fig. 4). Fig. 3 Viscosity of nanosuspensions in dependence on powder loading (at shear rate of 30 s -1 ) Fig. 4 Viscosity of suspension with an addition of TAC evaporated to 32.8 vol% of solids as a function of time (at shear rate of 30 s -1 )

4 88 13th International Ceramics Congress - Part A Such suspension instability makes regular particle packing and consolidation difficult. Therefore, we have developed a method of osmotic dehydration. In this process, the water was removed from the suspension through a semipermeable membrane in the surrounding polymer solution due to osmotic pressure. During this process the ion concentration can be better controlled and kept almost constant. Although both nanosuspensions, untreated nanosuspension and nanosuspension with TAC, were successfully osmotically dehydrated, the untreated nanosuspensions exhibited extremely high stability. The untreated suspension, osmotically concentrated to 14 vol% of solids, showed similar viscosity as the evaporated suspension with TAC dispersant (Fig. 3), but was stable for 7 days without any viscosity increase. This suspension was successfully consolidated even after a storage of 14 months. Consolidation and sintering. The untreated nanosuspension, osmotically concentrated to 14 vol%, was consolidated by centrifugation in non-porous moulds. A plate (15 x 10 x 2 mm 3 ) with a homogeneous particle packing could be cut from the wet deposit. The dried deposit had a relative density of 46%. The pore size distribution in the dried particle deposit is shown in Fig. 5. The pores were very small with a narrow size distribution (pore size from 4 to 8 nm). The tail of the size distribution at higher pore sizes is not caused by the presence of larger pores but is a result of the compression of adsorbed moisture and other species on the nanoparticle surface. Heating the deposit to 300 or 600 C desorbed the moisture and/or other species and the compressibility disappeared. The removal of the adsorbed layers was connected with particle rearrangement and a slight pore increase. The hypothesis about adsorbed layers was confirmed by measuring the sintering curves of these bodies (see Fig. 6). The moderate shrinkage at temperatures of up to 650 C, found in the dried deposit, was absent in the body preheated to 600 C. In the body preheated to 300 C the adsorbed species were probably not fully removed and therefore a moderate shrinkage in the temperature range C was found that was similar to the shrinkage of the dried deposit in this temperature range. Because no sintering can be expected at temperatures of C, the shrinkage in the dried deposit must have been connected with the removal of adsorbed species. The rest of sintering curves were similar for all bodies. The centrifuged zirconia bodies could be sintered to almost full density (99.8 % t.d.) at a temperature of 1100 C for 1.5 h. The average grain size in the sintered body was 136 nm. The same density could be obtained at a lower sintering temperature of 1050 C and a holding time of 5 hours. In order to obtain transparent tetragonal zirconia ceramics, i.e. highly dense ceramics (almost 100% of theoretical density) with small grain size (<100 nm) and without light scattering defects, hot isostatic pressing was applied to zirconia green bodies. Pressureless presintering of the nanoparticle bodies to a state of closed pores was carried out at 1005 C for 3 h (determined from previous sintering experiments). The bodies were then exposed to hot isostatic pressing at 1010 C Fig. 5 Pore size distribution of dried and preheated nanozirconia deposits Fig. 6 Sintering curves of dried and preheated nanozirconia deposits

5 Advances in Science and Technology Vol Fig. 7 SEM micrograph of transparent nanocrystalline zirconia ceramics Fig. 8 Transparent tetragonal zirconia plate with a thickness of 0.5 mm (30 mm above the text) for 3 h and 198 MPa of argon atmosphere. Fig. 7 shows the nanocrystalline microstructure of final zirconia ceramics. The average grain size determined from the microstructure analysis was 65 nm. The photograph in Fig. 8 is evidence of the transparency of sintered zirconia ceramics. The real inline transmission was about 25% (at 633 nm wavelength and 0.5 mm plate thickness). Such transparency is comparable to the highest in-line transmission value ever reported for tetragonal zirconia ceramics [18]. Summary Colloidal processing was applied to a 5 vol% suspension of yttria stabilized zirconia nanoparticles. The nanosuspension was concentrated by evaporation or by the newly developed method of osmotic dehydration. The untreated suspension gelled during evaporation after reaching 12 vol% solid loading. The nanosuspension treated with a dispersant, triammonium citrate, could be evaporated to 33 vol% solid loading with a reasonable viscosity, but the suspension was unstable. The particles flocculated quickly after ultrasonic dispersion and the suspension viscosity increased. On the other hand, osmotic dehydration enabled preparing very stable concentrated nanosuspensions especially from the untreated suspension. The 14 vol% nanosuspension prepared by osmotic dehydration of untreated suspension was consolidated by centrifugation in non-porous moulds. The dried deposit had a relative density of 46% and pores were in the range from 4 to 8 nm. This deposit was densified by pressureless presintering to closed porosity, followed by hot isostatic pressing at 1010 C for 3 h and 198 MPa of argon atmosphere. After sintering, the tetragonal zirconia ceramics retained the nanocrystalline structure with an average grain size of 65 nm and an in-line transmission of 25 % (at 633 nm wavelength and 0.5 mm plate thickness). Acknowledgement The author (M.T.) gratefully acknowledges the funding provided by the Grant Agency of the Czech Republic under grant No S. This work was realised in CEITEC - Central European Institute of Technology, with research infrastructure supported by the project CZ.1.05/1.1.00/ financed from European Regional Development Fund.

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