Grain-Size-Dependent Thermal Transport Properties in Nanocrystalline Yttria-Stabilized Zirconia
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GRAIN-SIZE-DEPENDENT THERMAL TRANSPORT PROPERTIES IN NANOCRYSTALLINE YTTRIA-STABILIZED ZIRCONIA Ho-Soon Yang, J.A. Eastman, L.J. Thompson, and G.-R. Bai Argonne National Laboratory, Materials Science Division Argonne, IL 60439
ABSTRACT Understanding the role of grain boundaries in controlling heat flow is critical to the success of many envisioned applications of nanocrystalline materials. This study focuses on the effect of grain boundaries on thermal transport behavior in nanocrystalline yttriastabilized zirconia (YSZ) coatings prepared by metal-organic chemical vapor deposition. A strong grain-size-dependent reduction in thermal conductivity is observed at all temperatures from 6–480 K. The behavior is due primarily to the effect of interfacial (Kapitza) resistance on thermal transport. In response to the application of heat to a material, interfacial resistance results in a small temperature discontinuity at every grain boundary, an effect that is magnified in nanocrystalline materials because of the large number of grain boundaries. The observed behavior in YSZ is compared with predictions derived from a diffuse-mismatch model. Implications for the possible development of improved thermal barriers based on nano-layered structures with large interfacial thermal resistance are discussed. INTRODUCTION The efficiency of gas turbine engines is dictated by the maximum sustained operating temperature of their typically Ni- or Co-based alloy turbine rotors. Recent studies have concluded that significant near-term progress in increasing turbine engine operating temperatures is more likely to come from the development of improved thermal barrier coatings (TBCs), typically yttria-stabilized zirconia, than from the design of new alloys. New processing techniques that result in TBC microstructures with lower thermal conductivity could lead either to higher operating temperatures of turbine engines, resulting in greater efficiency, or thinner coatings for the same operating temperature, which would reduce overall weight. Using nanocrystalline YSZ coatings offers the possibility of lowering thermal conductivity, and may also provide additional benefits for TBC applications because of the possibility of improved toughness and ductility compared to that of coarser-grained ceramics. Recent studies of thermal conductivity in nanocrystalline YSZ coatings have shown a very strong grain size dependence for average grain sizes below approximately 40 nm [1]. Studies of the grain size and temperature dependence of the thermal conductivity in YSZ have been interpreted in terms of the interfacial (Kapitza) resistance to thermal transport [2]. This report summarizes the results of those studies and extends them to include a comparison of the experimental observations with predictions of a diffuse mismatch model. V4.7.1
EXPERIMENTAL PROCEDURES Nanocrystalline YSZ coatings with thicknesses of 500-to-1200 nm were grown on polished polycrystalline α-Al2O3 substrates by metal-organic chemical vapor deposition [1]. Samples with controlled av
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