Isotopic Purity Enhancement of In-Plane Thermal Conductivity of Hexagonal Boron Nitride

Abstract

Hexagonal Boron Nitride (hBN) is a good candidate material for thermal management applications. It is an excellent electronic insulator and has high in-plane thermal conductivity. Because of its similarity to graphite, hBN has attracted interest because it is a good graphene encapsulation material. In the 1970s and 1980s work was done to measure the thermal conductivity of pyrolytic boron nitride. More recently, the effect of modulating the boron isotopic purity on thermal conductivity has been studied for hBN, cubic BN and single walled and multi walled boron nanotubes. The results of these studies into the effect of isotopic modulation show enhanced thermal conductivity making it a better material for thermal management applications. All the studies performed to date were in the range of 100K and above. In this work, I made thermal conductivity measurements on device-like structures at temperatures ranging from 40K to 300K. The aim was to measure the isotopic modulation enhancement of thermal conductivity on thicknesses of hBN similar to those that may be used in electronic substrates and at device-like length scales (ie of the order of 1 micron). We fabricated several specimens from hBN with a natural isotopic abundance of 20% 10B and 80% 11B and several from specimens with a high purity (>99%) of 10B. We measured in-plane thermal conductivities for the natural abundance samples that compare well with measurements taken on pyrolytic boron nitride in the 1980s. This indicates that the defectiveness of samples fabricated using mechanical cleavage and exfoliation may be high. The measurements taken on isotopically pure samples indicated an enhancement in thermal conductivity across the temperature range from 30 to 300K, although the enhancement was particularly marked at temperatures between 100 and 200K. The maximum enhancement in thermal conductivity we measured was 160% (indicating values for the isotopically pure sample that were 2.6 times those for the natural sample). The study concludes that isotopic modulation can significantly enhance the thermal conductivity of hBN at thicknesses suitable for device substrates. We also posit that mechanical exfoliation of relatively thick hBN structures (20-60nm) results in highly defective structures, although this statement needs further qualification.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Konstantin Novoselov (Supervisor) & Andrey Kretinin (Supervisor)