Diffused Lattice Vibration and Ultralow Thermal Conductivity in the Binary Ln–Nb–O Oxide System

Abstract

In the pursuit of low thermal conductivity materials for thermal management, one always tries to increase the material entropy by increasing the number of components in the materials to scatter heat‐carrying phonons. However, it also drastically increases the technological complexity to synthesize materials with the target complex composition. Here, a material family is presented with simple composition Ln3NbO7, which only contains binary oxides of Ln2O3 (Ln = Dy, Er, Y, Yb) and Nb2O5. The thermal conductivities approach the theoretical minimum limit, where the large chemical inhomogeneity due to the charge disorder and fluctuation of bonding length in Ln3NbO7 plays a major role. Despite the simple composition, Ln3NbO7 demonstrates an unprecedentedly high scattering rate of vibration states, as confirmed by the highest elastic constant/thermal conductivity ratio, as well as the diffused wavevector‐frequency dispersion. In contrast to the conventional wisdom that low thermal conductivity materials should be explored in the pool of “complex” multiple‐component materials, this work points out an avenue to look into materials with simple composition but large internal chemical inhomogeneity, which would be of both scientific and technological significance in the fields of thermal barrier coating, thermoelectric materials, etc.

Advanced Materials for High‐Temperature Thermal Transport

“…b) Temperature‐dependent κ of the Ln 3 NbO 7 oxides compared with typical TBC materials. Reproduced with permission . Copyright 2019, Wiley‐VCH.…”

“…Different from aforementioned studies which focused on increasing the number of components to enhance phonon scattering and reduce κ, Yang et al developed another material family, Ln 3 NbO 7 , containing only binary oxide Ln 2 O 3 and Nb 2 O 5 . As shown in Figure b, the fours type of materials all showed low κ below 1.5 W m −1 K −1 at room temperature, and thermally outperforms a variety of TBC materials in the entire temperature range from room temperature to 1000 °C.…”

Mechanical properties, oxygen barrier property, and chemical stability of RE3NbO7 for thermal barrier coating

“…It was found that, as shown in Figure , the crystal structures of RE 3 NbO 7 are fluorite structure (cubic) with space group Fm‐3m where RE and Nb atoms randomly occupy the positions of 4 a cations by the ratio of 3:1, and 1/8 of the oxygen sites on the 8 b position is vacant to keep the chemical balance. Especially, those compounds show pronounced disordering of cations and the random distribution of oxygen vacancies over anion sites, which was confirmed by both Raman spectrum and high‐resolution transmission electron microscope (HRTEM) analysis …”

“…In RE 3 NbO 7 compound, there are mainly two types of ionic bonding, RE 3+ –O 2‐ and Nb 5+ –O 2‐ . The large difference in length and strength of these two bonding leads to a large chemical inhomogeneity . This finally results in softening of the lattice and reduced elastic constant.…”

“…The dense rare earth niobates RE 3 NbO 7 (RE = Dy, Y, Er, Yb) were synthesized by the solid reaction which was described in detail in our previous work . The relative density of all specimens is larger than 98% which is favorable for testing the intrinsic properties of the materials.…”