Effect of intergranular glass films on the electrical conductivity of 3Y-TZP
- PDF / 1,970,424 Bytes
- 13 Pages / 576 x 792 pts Page_size
- 35 Downloads / 187 Views
H. Heinrich, P. Schwander, and G. Kostorz Institut of Applied Physics, ETH-Zurich, CH-8093 Zurich, Switzerland
H. Hofmann and O. Frei Alusuisse Lonza Services AG, CH-8212 Neuhausen, Switzerland (Received 3 June 1993; accepted 11 January 1994)
The electrical conductivity of 3Y-TZP ceramics containing SiO2 and AI2O3 has been investigated by complex impedance spectroscopy between 500 and 1270 K. At low temperatures, the total electrical conductivity is suppressed by the grain boundary glass films. The equilibrium thickness of intergranular films is 1-2 nm, as derived using the "brick-layer" model and measured by HRTEM. A change in the slope of the conductivity Arrhenius plots occurs at the characteristic temperature Tb at which the macroscopic grain boundary resistivity has the same value as the resistivity of the grains. The temperature dependence of the conductivity is discussed in terms of a series combination of RC elements.
I. INTRODUCTION Thin intergranular glass films which are composed of SiO 2 , A12O3, and Y 2 O 3 are formed in Y-TZP ceramics at high sintering temperatures.1"7 They promote sintering of the starting powders as well as grain growth during the post-annealing process and affect the electrical and mechanical properties of Y-TZP ceramics.8"11 It has been shown that the equilibrium film thickness is limited to about 1-2 nm, 4 ' 12 ' 14 and excess amounts of glass segregate in grain triple junctions, in pores, and on the surface of the specimens.4'5'11 The equilibrium width of intergranular films depends on the corresponding interfacial energies and on the viscosity of the glass phase at the temperature of sintering or post-annealing.15'16 A surplus of A12O3 is proposed to act as a scavenger for SiO 2 , removing it from grain boundaries.17-18 The glass phase can also be partially squeezed out from the grain boundaries to three grain pockets by applying a compressive stress.19 Accordingly, in hot-pressed specimens of Y-TZP at 1823 K some "clean" grain boundaries were observed, which did not contain any detectable intergranular phase.16 The Arrhenius plots [Iog(cr7) vs l/T] of the total (dc) conductivity of zirconia-based solid electrolytes often exhibit a change of slope at 800-1200 K, owing to a decrease in activation energy at high temperatures.20"22 Rather controversial explanations for the origin of this curvature of the Arrhenius plots have been given in the literature.23"30 The majority of studies suggests that in the low-temperature region immobile associated complexes, such as [Y Z r Vo]', are formed between extrinsic oxygen 1228
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 5, May 1994
Downloaded: 11 Mar 2015
vacancies and oppositely charged dopant ions. The thermal dissociation of these complexes would require a supplementary energy in addition to the activation energy for the simple migration of "free" charge carriers.23"26 According to this concept, three general stages of the ionic conductivity are proposed to exist in solid electrolytes as a function of temperature.27'
Data Loading...
