Thermal Transport Properties of Ultra-High-Temperature Ceramic Superlattices

Inorganic Non-Metallic Materials Nanoscience Chemistry Materials Science
ultra-high-temperature ceramics superlattice thermal conductivity interfacial thermal resistance thermal stability

Academic Background

In the design of high-temperature thermal insulation and thermoelectric materials, reducing thermal conductivity is a critical goal. The superlattice (SL) structure, formed by alternating layers of different materials, can effectively suppress phonon thermal transport, thereby significantly reducing the thermal conductivity of materials. This property makes superlattices highly promising for applications in thermal barrier coatings (TBCs) and thermoelectric materials. However, the thermal transport properties of ultra-high-temperature ceramics (UHTCs) under extreme conditions and the design of their superlattice structures remain poorly understood. Transition metal carbides, such as HfC and TaC, are ideal candidates for high-temperature applications due to their high melting points and structural stability. Yet, research on the thermal conductivity and interfacial thermal resistance of HfC/TaC superlattices remains limited.

This study aims to experimentally investigate the thermal transport properties of HfC/TaC superlattices, particularly the influence of interface spacing on thermal conductivity, and to reveal the contributions of phonons and electrons to thermal transport. Additionally, the study evaluates the thermal stability of superlattices in high-temperature environments and proposes an anti-oxidation protective layer design, providing new insights for the development of ultra-high-temperature thermal insulation and thermoelectric materials.

Source of the Paper

This paper was co-authored by Xin Liang and Shuhang Yang, affiliated with the College of Materials Science and Engineering, Beijing University of Chemical Technology, the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, and the School of Nanoscience and Engineering, University of Chinese Academy of Sciences. The paper was published on April 15, 2025, in the journal Applied Physics Letters, titled Thermal Transport Properties of Ultra-High-Temperature Ceramic Superlattices, with the DOI 10.1063⁄5.0263593.

Research Process and Results

1. Preparation and Structural Characterization of Superlattice Samples

The study first designed HfC/TaC superlattice samples, with periodic units formed by alternating layers of HfC and TaC of equal thickness. By adjusting the thickness of each layer, the interface spacing (dsl) ranged from 9.5 nm to 84.5 nm. The total thickness of all samples was kept constant (approximately 600 nm) to exclude the influence of sample boundary scattering effects on thermal conductivity measurements. The samples were grown on silicon substrates using non-reactive magnetron sputtering, and their structures were characterized in detail using X-ray diffraction (XRD) and transmission electron microscopy (TEM).

XRD analysis showed that both HfC and TaC single-layer films have a face-centered cubic (FCC) structure, with preferred growth along the (111) plane. TEM images and elemental distribution analysis further confirmed the periodic stacking of the superlattice structure, with interface spacing largely consistent with the design values. Additionally, high-resolution TEM images revealed defects at the superlattice interfaces and grain sizes of only a few nanometers.

2. Thermal Conductivity Measurements and Interfacial Thermal Resistance Analysis

The study used time-domain thermoreflectance (TDTR) to measure the cross-plane thermal conductivity (κ) of the superlattices. The results showed that the thermal conductivity exhibited a crossover dependence on interface spacing: as the interface spacing decreased from 84 nm to 31 nm, the thermal conductivity gradually decreased; however, when the interface spacing further decreased, the thermal conductivity increased instead. At an interface spacing of 31 nm, the phonon thermal conductivity reached a minimum value of 0.84 W/m·K, demonstrating the potential of HfC/TaC superlattices for thermal insulation applications.

Furthermore, the study prepared a single-interface bilayer sample to determine the interfacial thermal resistance (Rk) of the HfC/TaC interface and decomposed the contributions of phonons and electrons to the interfacial thermal resistance. The results indicated that the phonon thermal resistance at the HfC/TaC interface was significantly higher than the electron thermal resistance, primarily due to strong phonon scattering caused by interface defects.

3. High-Temperature Stability and Anti-Oxidation Protective Layer Design

To evaluate the stability of the superlattices in high-temperature environments, the study subjected the samples to heat treatment at 1200°C in air. The results showed that the thermal conductivity of unprotected samples increased significantly after heat treatment, and the superlattice structure gradually disappeared, forming a brick-like structure. XRD and STEM-EDS analysis confirmed significant oxidation reactions within the samples.

To inhibit oxidation, the study designed a protective layer of approximately 20 nm thick HfO2. Experiments demonstrated that the HfO2 protective layer effectively prevented oxygen diffusion, significantly reducing the oxygen concentration within the samples and preserving the low thermal conductivity of the superlattices. After heat treatment, the thermal conductivity of the protected samples increased only from 1.88 W/m·K to 2.71 W/m·K, close to the level of the as-grown samples.

Conclusions and Significance

This study experimentally revealed the thermal transport properties of HfC/TaC superlattices and their dependence on interface spacing, reporting for the first time the phenomenon of superlattices exhibiting higher thermal conductivity than their constituent materials at small interface spacings. The study also quantitatively determined the interfacial thermal resistance of the HfC/TaC interface and revealed the different contributions of phonons and electrons to thermal transport. Additionally, the designed HfO2 protective layer effectively improved the high-temperature stability of the superlattices, providing important references for the development of ultra-high-temperature thermal insulation and thermoelectric materials.

Research Highlights

  1. Interface Spacing-Dependent Thermal Conductivity: First discovery of the crossover dependence of thermal conductivity on interface spacing in HfC/TaC superlattices, revealing the underlying phonon transport mechanisms.
  2. Quantitative Analysis of Interfacial Thermal Resistance: Experimental determination of the interfacial thermal resistance of the HfC/TaC interface, decomposing the contributions of phonons and electrons, providing new perspectives for understanding the thermal transport mechanisms of superlattices.
  3. High-Temperature Stability and Anti-Oxidation Design: The designed HfO2 protective layer effectively inhibited oxidation of superlattices in high-temperature environments, offering a feasible solution for ultra-high-temperature applications.

Other Valuable Information

The study also provided detailed experimental data and structural characterization results, including XRD, TEM, and STEM-EDS analyses, offering rich references for subsequent research. Additionally, the paper discussed the potential applications of superlattices in thermoelectric materials, pointing the way for further research.

Through this study, not only has the understanding of the thermal transport mechanisms of superlattices been deepened, but new ideas and methods have also been provided for the design of high-temperature thermal insulation and thermoelectric materials.