The difference in strength between the covalent bonds within the hexagonal planes and Van der Waals forces between the planes leads to anisotropy of many of hBN’s properties. One example is thermal conductivity: the typical in-plane thermal conductivity is 300 W/mK, because the covalent bonds transfer thermal energy very efficiently, whereas through-plane thermal conductivity is typically only 30 W/mK. This is critical when formulating BN-filled polymer composites for thermal interface materials; if platelets are used by themselves, they tend to align in the direction of flow of the polymer. As such, this makes for effective heat spreading but can limit the through-plane thermal conductivity of the compound. Strategies to keep platelets in random orientations, such as agglomeration, combining various sizes to improve packing, or mixing platelets with other particles to disrupt alignment, can help make thermal conductivity more isotropic.
Platelets alone tend to align in the polymer flow direction, creating thermal pathways that are more effective for heat spreading.
Using hBN agglomerates helps to randomly orient the platelets, creating more isotropic thermal conductivity.
Combining BN platelets with non-acicular particles will disrupt alignment, improving through-plane thermal conductivity.
The anisotropic properties at the platelet level are also seen, though to a lesser degree, in our hot pressed solids. Our hot pressing method uses uniaxial pressure, which makes it favorable for the platelets to grow perpendicular to the pressing direction. In this case, the orientation of a machined part to the pressing direction should be considered based on property needs.
Platelet faces align parallel to the press (perpendicular to the pressing direction)
Platelet alignment should be considered when machining components