ARTICLE IN PRESS

Infrared Physics & Technology xxx (2006) xxx–xxx www.elsevier.com/locate/infrared

Near-infrared radiative properties of porous zirconia ceramics Leonid A. Dombrovsky a

a,*

, Herve Kamdem Tagne b, Dominique Baillis b, Laurent Gremillard c

Institute for High Temperatures of the Russian Academy of Sciences, Krasnokazarmennaya 17A, NCHMT, Moscow 111116, Russian Federation b Centre de Thermique de Lyon, Institut National des Sciences, Applique´s, Lyon 69621, Villeurbanne Cedex, France c Charge´ de Recherche CNRS GEMPPM, Lyon 69621, Villeurbanne Cedex, France Received 8 May 2006

Abstract Infrared radiative properties of zirconia ceramics of porosity about 16% are studied by means of the measurements of directional– hemispherical reflectance and transmittance in the wavelength range from 2.5 to 9 lm. The recently suggested modified two-flux approximation is examined as a simplified basis of the identification procedure. A comparison with the exact numerical solution confirms a good accuracy of this approach for identification of the absorption coefficient of ceramics. An analysis of the results for transport scattering coefficient showed that scattering is determined by isotropic pores with characteristic average radius about 1 lm. The corresponding approximate theoretical model of radiative properties of ceramics is suggested. The absorption coefficient of bulk zirconia in the semi-transparency range is obtained from the data for porous zirconia ceramics.  2006 Elsevier B.V. All rights reserved. Keywords: Radiation; Infrared; Absorption; Scattering; Ceramics; Zirconia

1. Introduction Zirconia ceramics produced by various plasma-spraying technologies are widely used as thermal barrier coatings to provide thermal insulation and protection to metallic turbine-engine components from the hot gas stream [1,2]. The low thermal conductivity of the ceramics makes it possible to decrease considerably the temperature at the metal–ceramic interface even in the case of small coating thickness (about 0.1–0.5 mm) [3]. There are many other realized and potential industrial applications of yttria-partially stabilized zirconia ceramics [4]. In many cases, one of the more important properties of this material is a very low thermal conductivity. It is known that zirconia is semi-transparent in the near-infrared spectral range. For this reason, the role of thermal radiation in effective thermal conductivity of porous zirconia ceramics is expected to be significant. Note that both absorption *

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and scattering characteristics are important for transient radiative–conductive heat transfer in porous ceramics [5,6]. To the best of our knowledge, there is no experimental data in the literature for absorption and scattering characteristics of porous zirconia ceramics in the near infrared. The theoretical modeling of radiation scattering by pores, grains, and cracks in ceramics is also difficult because of complex microstructure of the material. The objective of the present paper is a complex experimental and theoretical analysis of the near-infrared radiative properties of porous zirconia ceramics. We focus on the main characteristics of the material: the absorption coefficient and transport scattering coefficient which determine the radiation heat transfer in ceramics [7]. It simplifies both the measurements and the mathematics [8]. In the experimental part of the work, we use a formal identification procedure based on the measurements of directional– hemispherical reflectance and transmittance for the samples of ceramics in the wavelength range 2.5 < k < 9 lm and the general model for radiation transfer in a refracting, absorbing and scattering material. The theoretical analysis is based

1350-4495/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2006.11.003

Please cite this article in press as: L.A. Dombrovsky et al., Near-infrared radiative properties of porous zirconia ceramics, Infrared Phys. Techn. (2006), doi:10.1016/j.infrared.2006.11.003