Hexagonal boron nitride (hBN) is a two-dimensional material with exceptional thermal properties, making it a subject of significant interest in the field of thermal management and materials science. Its high thermal conductivity and anisotropic heat dissipation distinguish it from other layered materials. The thermal behavior of hBN is governed by phonon transport mechanisms, isotopic purity, and structural defects, which collectively influence its performance in dissipating heat. Comparisons with graphene and other 2D materials further highlight its unique characteristics.

The thermal conductivity of hBN is among the highest of all insulating materials, with in-plane values reaching up to 400-500 W/mK at room temperature for high-quality single crystals. This places it second only to graphene, which exhibits thermal conductivities exceeding 2000 W/mK. However, unlike graphene, hBN is an electrical insulator, making it particularly valuable for applications where electrical isolation is required alongside efficient heat dissipation. The out-of-plane thermal conductivity of hBN is significantly lower, typically around 2-10 W/mK, reflecting its strong anisotropy. This anisotropy arises from the layered structure of hBN, where in-plane covalent bonds between boron and nitrogen atoms facilitate efficient phonon transport, while out-of-plane interactions are dominated by weak van der Waals forces.

Phonon transport in hBN is primarily governed by acoustic phonons, which are the main heat carriers in non-metallic materials. The high in-plane thermal conductivity is attributed to the large phonon mean free paths and high group velocities of these acoustic modes. Theoretical models based on first-principles calculations and Boltzmann transport equations have shown that the longitudinal acoustic (LA) and transverse acoustic (TA) phonons contribute most significantly to heat conduction. Optical phonons, on the other hand, have limited participation due to their lower group velocities and shorter lifetimes. The phonon dispersion relations of hBN reveal a wide frequency spectrum, with a notable gap between acoustic and optical branches, further influencing thermal transport dynamics.

Isotopic purity plays a critical role in determining the thermal conductivity of hBN. Natural boron consists of two isotopes, boron-10 and boron-11, with a natural abundance ratio of approximately 20:80. The presence of these isotopes introduces mass disorder, which scatters phonons and reduces thermal conductivity. Studies have demonstrated that isotopically pure hBN, where either boron-10 or boron-11 is exclusively used, can exhibit up to a 50% increase in thermal conductivity compared to naturally occurring hBN. This enhancement is due to the reduction in phonon-isotope scattering, allowing for longer phonon mean free paths. Nitrogen, which has only one stable isotope (nitrogen-14), does not contribute to isotopic scattering in hBN.

Defects and grain boundaries also significantly impact the thermal properties of hBN. Point defects, such as vacancies or substitutions, act as scattering centers for phonons, reducing thermal conductivity. Grain boundaries, which are common in polycrystalline hBN, introduce additional phonon scattering due to the mismatch in crystal orientation between adjacent grains. The extent of thermal conductivity reduction depends on the defect concentration and grain size. For instance, nanocrystalline hBN with small grain sizes can exhibit thermal conductivities an order of magnitude lower than single-crystalline hBN. However, even with these imperfections, hBN retains relatively high thermal conductivity compared to many other insulating materials.

Experimental measurements of hBN’s thermal conductivity have employed various techniques, including time-domain thermoreflectance (TDTR), Raman thermometry, and suspended microbridge methods. TDTR, which involves measuring the thermal decay of a metal film deposited on hBN, has been widely used to extract both in-plane and out-of-plane thermal conductivities. Raman thermometry exploits the temperature-dependent shift of Raman peaks to estimate local heating and thermal conductivity. Suspended microbridge methods involve fabricating freestanding hBN flakes and measuring their thermal response to an applied heat flux. These techniques have consistently confirmed the high in-plane thermal conductivity of hBN and its anisotropy.

Theoretical models have complemented experimental efforts by providing insights into the fundamental mechanisms governing thermal transport in hBN. First-principles calculations, coupled with solutions to the Boltzmann transport equation, have been instrumental in predicting phonon lifetimes, scattering rates, and thermal conductivity. These models account for intrinsic phonon-phonon interactions (Umklapp and normal scattering), extrinsic scattering from defects and isotopes, and boundary scattering in finite-sized samples. The agreement between theoretical predictions and experimental data has validated the understanding of hBN’s thermal properties.

When compared to other 2D materials, hBN stands out for its combination of high thermal conductivity and electrical insulation. Graphene, despite its superior thermal conductivity, is electrically conductive, limiting its use in insulating applications. Transition metal dichalcogenides (TMDCs), such as MoS2, exhibit much lower thermal conductivities, typically in the range of 30-100 W/mK, due to their heavier atomic masses and more complex phonon dispersion. Black phosphorus, another 2D material, shows anisotropic thermal conductivity but is chemically unstable under ambient conditions. hBN’s stability, coupled with its thermal performance, makes it a preferred choice for many thermal management applications.

The anisotropic heat dissipation in hBN has important implications for thermal management in layered structures. In heterostructures where hBN is combined with other 2D materials, its high in-plane thermal conductivity can help spread heat laterally, preventing hot spots. The low out-of-plane conductivity, however, means that heat transfer through the thickness of hBN is less efficient. This anisotropy must be carefully considered in the design of multilayer devices to optimize thermal performance.

In summary, hexagonal boron nitride exhibits remarkable thermal properties, characterized by high in-plane thermal conductivity and strong anisotropy. Phonon transport mechanisms, isotopic purity, and structural defects play pivotal roles in determining its thermal behavior. Experimental and theoretical studies have provided a comprehensive understanding of these properties, positioning hBN as a critical material for advanced thermal management. While graphene surpasses hBN in absolute thermal conductivity, hBN’s insulating nature and stability make it indispensable for applications requiring both thermal dissipation and electrical isolation. The continued exploration of hBN’s thermal properties will further unlock its potential in next-generation materials and devices.