Hexagonal boron nitride (h-BN) is a technologically important layered material used as a dielectric spacer, encapsulant, ultraviolet laser emitter, and hyperbolic material in electronic and photonic applications1,2,3. More recently, h-BN has attracted attention for thermal management of electronics as theoretical calculations4predicted an in-plane thermal conductivity as high as kr~ 550 W m−1 K−1at room temperature, though, highly anisotropic with a two orders of magnitude smaller out-of-plane thermal conductivity (kz~ 5 W m−1 K−1).
The high in-plane thermal conductivity, as well as atomic flatness, makes h-BN an ideal substrate material for next-generation thin-film devices since waste heat can be spread quickly laterally through a large area, avoiding formation of localized hot spots5,6. In addition, h-BN could be a good reinforcing filler for thermal interface and encapsulation composite materials due to its high thermal conductivity and electrical resistivity7,8. Despite its predicted favorable thermal properties, experimental results are few and varied. Reported kr values range from 220 to 420 W m−1 K−1 4,9,10, well below the predicted maximum value. Developing insight into this discrepancy and driving h-BN thermal conductivity to higher values is of great interest both fundamentally and for enabling enhanced thermal engineering.
Quantized lattice vibrations (phonons) in crystals synthesized from elements with natural isotopic concentration scatter due to mass variations of the isotopes in the lattice, thus reducing thermal conductivity11. Enhanced thermal conductivity has been demonstrated in monoisotopic materials (isotopically purified to >99% one isotope), such as in silicon12, germanium13, gallium arsenide14, diamond15, and graphene16. Naturally occurring BN materials are made with two stable B isotopes (19.9% 10B and 80.1% 11B), which present a large mass modulation, and an opportunity to control its thermal conductivity by manipulating the B isotope concentration. Large B isotope effect has been observed in BN nanotubes17, whereas experimental evidence of isotope effects in h-BN has not been possible to date because suitable samples have not been available.
In terms of theoretical predictions, the conventional Callaway approach13,18,19 based on the Boltzmann transport equation (BTE) and formulated within a single-mode relaxation time approximation (RTA) has been widely used to study the isotope effect in numerous material systems, but has challenges in anisotropic layered systems such as h-BN. Often, phonon scattering processes in layered systems cannot be treated as independent resistive processes, an assumption of the RTA20. Ab initio approaches based on full solution of the BTE in combination with first principles density functional theory (DFT) have demonstrated accuracy in describing the thermal conductivity of anisotropic layered materials with natural isotopic concentrations21,22, however, experimental data for monoisotopic layered materials are not available for model validations.
Only recently have isotopically engineered h-BN crystals become available23,24,25. To date, investigations have focused on fundamental isotope effects related to Raman phonon lifetimes and the electronic bandgap23,24. In this work, we experimentally demonstrate the effect of boron isotope concentration on the thermal conductivity of bulk h-BN crystals using a transient thermoreflectance (TTR) technique. The monoisotopic 10B h-BN crystals have in-plane thermal conductivity as high as 585 W m−1 K−1 at room temperature, ~ 80% larger than that of h-BN with disordered isotope concentrations (52%:48% mixture of 10B and 11B). Our measurements are compared with state-of-the-art ab initio thermal conductivity calculations.
Read the Results/Rest of the Study HERE