Single-crystalline θ-TaN breaks thermal conductivity ceiling for metallic materials!

High-performance electronic devices have become more densely integrated and power-intensive, exposing a hard limit in thermal management, where conventional high-thermal-conductivity metals such as copper and silver have approached an effective ceiling of about 400 W m^-1 K^-1. Transition-metal nitrides have been predicted to offer much higher heat transport, however, producing high-quality, low-defect single crystals has been challenging.

A multidisciplinary team led by researchers at Tsinghua University, Southern University of Science and Technology and Eastern Institute of Technology report the successful growth of metallic θ-TaN single crystals with exceptionally low defect concentration via a high-temperature (T) and high-pressure (P) method. This approach uniquely enables the growth of high-quality crystals with exceptionally low defect concentration. With combinational characterizations, the team determined the as-synthesized θ-TaN single crystals exhibit excellent crystallinity with a remarkably low average nitrogen vacancy concentration of merely 1%. It fully demonstrates the superiority of the high-T and high-P method in preparing phase-pure, low-defect transition metal nitride single crystals that have long been challenging to fabricate via conventional synthesis routes.

Time-domain thermoreflectance (TDTR) measurements show room-temperature thermal conductivity significantly exceeding the traditional metallic limit and well above copper and silver. The synthesized θ-TaN’s thermal conductivity decreases with temperature and the value rivals high-conductivity nonmetallic materials such as boron arsenide above 450 K, making θ-TaN promising for high-temperature heat-spreading applications. Electrical measurements confirm that θ-TaN retains typical metallic conductivity, enabling the application for device integration. Moreover, the synthesized θ-TaN show the largest Lorenz number among reported metallic materials due to the dominant phonon contributions to the total thermal conductivity.

The team combined thermal measurements, first-principles calculations, and neutron powder diffraction to explain the exceptional thermal transport. In θ-TaN the acoustic and optical phonon branches are well separated with a large energy gap, which effectively suppresses phonon-phonon scattering rate. In addition, electronic carriers show extremely low density of states near the Fermi level, leading to weak phonon-electron scattering. These combined factors minimize the resistance for phonon-mediated heat transfer. Theoretical calculations indicate an ideal, defect-free θ-TaN crystal can approach thermal conductivity near 1000 W m^-1 (in the same order of magniture with diamond). Nitrogen vacancies are the dominant limiting factor, where even 0.005% nitrogen vacancy concentration can reduce thermal conductivity by around 40%. In the present crystals, the high-pressure and high-temperature synthesis procedure induced vacancy concentration largely accounts for the gap from the theoretical limit.

θ-TaN shows excellent thermal stability, which resists oxidation in air far exceeding copper and many carbon-based heat-spreading materials. The combined excellent thermal and mechanical properties make θ-TaN suitable for potential applications include thermal interface materials, heat spreaders for high-power chips, aerospace high-temperature components, and other advanced thermal-management demands. The researchers believe their findings “pave the way for a new generation of high-performance thermal management materials.”

---

---