A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of ∼1 nm, the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interface...

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Nanoscale thermal transport. II. 2003–2012

During the last decade a number of ceramic materials, mostly oxides have been suggested as new thermal barrier coating (TBC) materials. These new compositions have to compete with the state-of-the-art TBC material yttria stabilized zirconia (YSZ) which turns out to be difficult due to its unique properties. On the other hand YSZ has certain shortcomings especially its limited temperature capability above 1200 °C which necessitates its substitution in advanced gas turbines. In the paper an overview is tried on different new materials covering especially doped zirconia, pyrochlores, perovskites, and aluminates. Literature results and also results from our own investigations will be presented and compared to the requirements. Finally, the double-layer concept, a method to overcome the limited toughness of new TBC materials, will be discussed.

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Overview on advanced thermal barrier coatings

Rare-earth zirconates have been identified as a class of low-thermal-conductivity ceramics for possible use in thermal barrier coatings (TBCs) for gas-turbine engine applications. To document and compare the thermal conductivities of important rare-earth zirconates, we have measured the thermal conductivities of the following hot-pressed ceramics: (i) Gd 2 Zr 2 O 7 (pyrochlore phase), (ii) Gd 2 Zr 2 O 7 (fluorite phase), (iii) Gd 2.58 Zr 1.57 O 7 (fluorite phase), (iv) Nd 2 Zr 2 O 7 (pyrochlore phase), and (v) Sm 2 Zr 2 O 7 (pyrochlore phase). We have also measured the thermal conductivity of pressureless-sintered 7 wt% yttria-stabilized zirconia (7YSZ)--the commonly used composition in current TBCs. All rare-earth zirconates investigated here showed nearly identical thermal conductivities, all of which were ∼30% lower than the thermal conductivity of 7YSZ in the temperature range 25°-700°C. This finding is discussed qualitatively with reference to thermal-conductivity theory.

Low‐Thermal‐Conductivity Rare‐Earth Zirconates for Potential Thermal‐Barrier‐Coating Applications

Thermal barrier systems have been the subject of vigorous research and development activities over the past few years, driven by the demands for enhanced reliability and substantially higher operating temperatures envisaged for the next generations of gas turbine engines. The menu of candidate materials and architectures has expanded considerably, including numerous concepts based on zirconia as well as radically different materials, multilayers and modulated distributions of porosity and chemical composition. Advances in deposition processes enable increased flexibility for tailoring composition and microstructure to local requirements within the coating system, e.g. for thermal insulation, control of interdiffusion, enhanced resistance against environmental degradation and condition monitoring. Many challenges remain but healthy and growing collaborations between the science and technology communities bode well for future progress in this area.

Emerging materials and processes for thermal barrier systems

Eleven fluorite oxides with five principal cations (in addition to a four-principal-cation (Hf0.25Zr0.25Ce0.25Y0.25)O2-δ as a start point and baseline) were fabricated via high-energy ball milling, spark plasma sintering, and annealing in air. Eight of the compositions, namely (Hf0.25Zr0.25Ce0.25Y0.25)O2-δ, (Hf0.25Zr0.25Ce0.25)(Y0.125Yb0.125)O2-δ, (Hf0.2Zr0.2Ce0.2)(Y0.2Yb0.2)O2-δ, (Hf0.25Zr0.25Ce0.25)(Y0.125Ca0.125)O2-δ, (Hf0.25Zr0.25Ce0.25)(Y0.125Gd0.125)O2-δ, (Hf0.2Zr0.2Ce0.2)(Y0.2Gd0.2)O2-δ, (Hf0.25Zr0.25Ce0.25)(Yb0.125Gd0.125)O2-δ, and (Hf0.2Zr0.2Ce0.2)(Yb0.2Gd0.2)O2-δ, possess single-phase solid solutions of the fluorite crystal structure with high configurational entropies (on the cation sublattices), akin to those high-entropy alloys and ceramics reported in prior studies. Most high-entropy fluorite oxides (HEFOs), except for the two containing both Yb and Gd, can be sintered to high relative densities. These single-phase HEFOs exhibit lower electrical conductivities and comparable hardness (even with higher contents of softer components such as Y2O3 and Yb2O3), in comparison with 8 mol. % Y2O3-stabilized ZrO2 (8YSZ). Notably, these single-phase HEFOs possess lower thermal conductivities than that of 8YSZ, presumably due to high phonon scattering by multiple cations and strained lattices.

High-entropy fluorite oxides

This paper summarizes the basic properties of ceramic materials for thermal barrier coatings. Ceramics, in contrast to metals, are often more resistant to oxidation, corrosion and wear, as well as being better thermal insulators. Except yttria stabilized zirconia, other materials such as lanthanum zirconate and rare earth oxides are also promising materials for thermal barrier coatings.

Ceramic materials for thermal barrier coatings

A series of La 2 O 3 –ZrO 2 –CeO 2 composite oxides were synthesized by solid-state reaction The final product keeps fluorite structure when the molar ratio Ce/Zr ≥ 07/03, and below this ratio only mixtures of La 2 Zr 2 O 7 (pyrochlore) and La 2 O 3 –CeO 2 (fluorite) exist Averagely speaking, the increase of CeO 2 content gives rise to the increase of thermal expansion coefficient and the reduction of thermal conductivity, but La 2 (Zr 07 Ce 03 ) 2 O 7 has the lowest sintering ability and the lowest thermal conductivity which could be explained by the theory of phonon scattering Based on the large thermal expansion coefficient of La 2 Ce 325 O 95 , the low thermal conductivities and low sintering abilities of La 2 Zr 2 O 7 and La 2 (Zr 07 Ce 03 ) 2 O 7 , double-ceramic-layer thermal barrier coatings were prepared The thermal cycling tests indicate that such a design can largely improve the thermal cycling lives of the coatings Since no single material that has been studied so far satisfies all the requirements for high temperature thermal barrier coatings, double-ceramic-layer coating may be an important development direction of thermal barrier coatings

New double-ceramic-layer thermal barrier coatings based on zirconia–rare earth composite oxides

Conventional thermal spray processes as atmospheric plasma spraying (APS) have to use easily flowable powders with a size up to 100 μm. This leads to certain limitations in the achievable microstructural features. Suspension plasma spraying (SPS) is a new promising processing method which employs suspensions of sub-micrometer particles as feedstock. Therefore much finer grain and pore sizes as well as dense and also thin ceramic coatings can be achieved. Highly porous coatings with fine pore sizes are needed as electrodes in solid-oxide fuel cells. Cathodes made of LaSrMn perovskites have been produced by the SPS process. Their microstructural and electrochemical properties will be presented. Another interesting application is thermal barrier coating (TBC). SPS allows the manufacture of high-segmented TBCs with still relatively high porosity levels. In addition to these specific applications also the manufactures of new microstructures like nano-multilayers and columnar structures are presented.

Application of Suspension Plasma Spraying (SPS) for Manufacture of Ceramic Coatings

The spray-drying process of ceramics which are candidate materials for thermal barrier coatings (TBCs), i.e. 3YSZ+0, 2, 4, 6 wt.% Al2O3, is discussed in this paper. The two most important properties of spray-dried powders to determine the coating quality are density and particle size. Polyethyleneimine (PEI) acts as both an organic binder and a dispersant giving low viscosity in the suspension. The optimised suspension composition is: ⩾ 33.6 vol.% powder+1.8 wt.% PEI+ethanol, and operational parameters of the spray-dryer: drying temperature 175°C, feeding rate 55 cm3/min, feeding pressure 1.013×104 Pa.

Detlev Stoever

Papers

Ceramic materials for thermal barrier coatings

New double-ceramic-layer thermal barrier coatings based on zirconia–rare earth composite oxides

Application of Suspension Plasma Spraying (SPS) for Manufacture of Ceramic Coatings

Spray-drying of ceramics for plasma-spray coating

La2(Zr0.7Ce0.3)2O7—A new oxide ceramic material with high sintering-resistance