Precursor scavenging of the resistive grain-boundary phase in 8 mol% yttria-stabilized zirconia: Effect of trace concent
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John Drennan Centre for Microscopy and Microanalysis, The University of Queensland, St Lucia, Brisbane, Qld 4072, Australia
Doh-Yeon Kim Center for Microstructure Science of Materials, School of Materials Science and Engineering, Seoul National University, Seoul 151-742, South Korea (Received 12 February 2001; accepted 29 May 2001)
The influence that trace concentrations of SiO2 have on improving grain-boundary conduction via precursor scavenging using additional heat treatment at 1200 °C for 40 h before sintering was investigated. At a SiO2-impurity level (SIL) 艋160 ppm by weight, the grain-boundary resistivity (gb) decreased to 20% of its value, while no improvement in grain-boundary conduction was found at a SIL 艌 310 ppm. The correlation between the resistance per unit grain-boundary area, gb, and average grain size indicated that the inhomogeneous distribution of the siliceous phase in the sample with a SIL 艌 310 ppm hampered the scavenging reaction.
I. INTRODUCTION
It is generally acknowledged that the siliceous phase at the grain boundary is detrimental to the ionic conduction of stabilized zirconia.1– 4 So far, Al2O3 is known as the most effective additive for scavenging the resistive siliceous phase.4 –8 However, as the sintering temperature increases to 艌1600 °C in a YSZ–Al2O3 composite, the grain-interior conductivity of 8 mol% yttria-stabilized zirconia (8YSZ) decreases due to the dissolution of Al2O3 into the 8YSZ lattices.4,5,9 As a consequence, the improvement in grain-boundary conduction is sometimes nullified by a deterioration of the grain-interior one. In a previous report,10 we suggested a new method of improving 8YSZ grain-boundary conduction via a twostage sintering process. The reaction between the precursor and trace siliceous impurity was suggested as one possible route for scavenging the siliceous phase. It was referred to as “precursor scavenging” from its scavenging not by the additive but by the precursor itself. Precursor scavenging could markedly improve grainboundary conduction without affecting the grain interior even at the high sintering temperatures.
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Address all correspondence to this author. Current address: School of Materials Science and Engineering, Seoul National University, Seoul 151-742, South Korea. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 8, Aug 2001
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In this contribution, the influence of a trace SiO2 concentration on precursor scavenging was investigated in 8YSZ by varying the SiO2 concentrations from 100 to 1000 ppm by weight. The results were correlated with complex impedence spectra and microstructure to investigate the scavenging mechanism and determine the optimal siliceous concentration window that is valid for precursor scavenging. II. EXPERIMENTAL PROCEDURES
High-purity 8YSZ powder (TZ-8Y, Tosoh Corporation, Tokyo, Japan), SiO2 sol (MA-ST-M, solvent CH3OH, 40.3 wt% SiO2, Nissan Chemical Co., Tokyo, Japan), and Na2CO3 ⭈ 10H2O (GR grade, Kanto Chemical Co., Tokyo, Japan) were
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