Colloidal processing, surface characterization, and sintering of nano ZrO 2 powders
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Yu Jia, Yuji Hotta,a) Kimiyasu Sato, and Koji Watari National Institute of Advanced Industrial Science and Technology, Advanced Sintering Technology Group, Advanced Manufacturing Research Institute, Anagahora 2266-98, Shimoshidami, Moriyama-ku, Nagoya, 463-8560, Japan (Received 18 November 2004; accepted 18 February 2005)
Colloidally stable suspensions are required to fabricate dense samples with uniform microstructure by colloidal processing methods, which necessitate dispersion of ceramic powders in a liquid medium. Aqueous nano ZrO2 suspensions were prepared using polyethylenimine (PEI) as a dispersant. PEI adsorption on nano ZrO2 surfaces was promoted with increasing initial PEI content and suspension pH. Isoelectric point was shifted from pH 7 at 0 wt% PEI to pH 10.4 at 3 wt% PEI. Stable suspensions had mean particle sizes in the range of 100 to 150 nm and sedimentation rates less than 0.4 mm/h, as compared to 2–5.5 m and 10–50 mm/h for unstable suspensions. Samples with 98% relative density were fabricated after sintering at 1300 °C for 4 h from colloidally stable suspensions. I. INTRODUCTION
Dispersion of ceramic powders in a liquid medium is required in colloidal processing methods such as slip casting and tape casting. These methods have been shown to be superior to conventional dry pressing in terms of controlling density and uniform microstructure evolution in the green and sintered states.1,2 A homogeneous mixture of solid and polymer in a liquid medium can be obtained in this method, but if the undesired aggregates form, suspension stability and subsequently sintered properties are severely degraded. Therefore, aggregates must be prevented during the colloidal processing stage.3 Nano particles, in particular, have a tendency to form aggregates in a solution due to strong omnipresent van der Waals attractive forces at short interaction distances.4 Therefore, sufficiently large stabilizing forces such as electrical double-layer repulsion or steric interactions should be applied to provide an energy barrier against aggregation.3 It was shown that stabilization of nano particles with polyelectrolytes has resulted in better rheological results than electrostatic stabilization alone due to steric effects of the polymer.5 Polyethylenimine (PEI) has been used as a dispersant for various ceramic powders and shown to enhance stability of ceramic powders in water.4–9 When
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0168 1348
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 5, May 2005 Downloaded: 12 Mar 2015
it is dissolved in neutral or acidic solution, the adsorption of protons results in the protonation of the amine groups. Therefore, positively charged PEI easily adsorbs on the negatively charged ceramic surfaces, which provides an electrosteric effect preventing aggregation of the ceramic powders.5 In this study, dispersion of nano ZrO2 powders in water was carried out as a function of PEI content and pH. It was aimed to investigate both PE
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