Keywords: Nanocomposites, Thermal conductivity, Boron nitride, Boron nitride nanosheets, Aerogels, Bidirectional freeze casting, Centrosymmetric structure

With ever increasing power densities and decreasing form factors of modern electronic devices it is necessary to develop new materials that can dissipate the generated heats in more than one direction. Many 3D structures have been explored for this purpose but none of them conduct heat equally in two or three directions. The aim of this project is to fabricate such composites that show heat conductance in two directions and demonstrate electrical insulation so that they can be applied for relevant thermal management. h-BN is used as a precursor material to prepare amino-functionalized nanosheets (f-BNNS) after exfoliation. BNNS have a...[ Read more ]

Keywords: Nanocomposites, Thermal conductivity, Boron nitride, Boron nitride nanosheets, Aerogels, Bidirectional freeze casting, Centrosymmetric structure

With ever increasing power densities and decreasing form factors of modern electronic devices it is necessary to develop new materials that can dissipate the generated heats in more than one direction. Many 3D structures have been explored for this purpose but none of them conduct heat equally in two or three directions. The aim of this project is to fabricate such composites that show heat conductance in two directions and demonstrate electrical insulation so that they can be applied for relevant thermal management. h-BN is used as a precursor material to prepare amino-functionalized nanosheets (f-BNNS) after exfoliation. BNNS have as high thermal conductivity (TC) of around 1700-3000 W/mK [1].

Solutions of f-BNNS were prepared using water-soluble polyvinyl alcohol (PVA) which is used as a binding agent to allow the nanosheets to form stable 3D aerogels. Initially, the solution is poured into a custom-made mold and freeze-casted, followed by freeze-drying for 48 hr under vacuum. After freeze-drying a highly aligned radially centrosymmetric-bidirectional structured (BD) aerogel is obtained while a unidirectionally aligned aerogels (UD) is also prepared for comparison. After obtaining the aerogels, a PDMS matrix was infiltrated into the aerogel via vacuum infiltration which is then cured to obtain a flexible composite.

The thermal conductivities of the radially aligned BD-centrosymmetric and UD aligned composites are measured in both the axial and radial directions on a hot-disk machine. For BD composites, the axial thermal conductivity (ATC) is found to be higher than the radial thermal conductivity (RTC), while the difference between the ATC and RTC become smaller with increasing BNNS content, indicating its tendency towards an isotropic composite. On the contrary, the UD composite presents a larger difference between the ATC and RTC, showing a more anisotropic structure with increasing BNNS content.

In the BD aligned composites, the crevices are more prominent and the aerogel walls are less interconnected in the radial direction than in the axial direction, hence the lower RTC at low BNNS contents. However, as the BNNS content is increased the crevices are filled up leading to high interconnectivity and the high RTC. In the UD composites, in contrast, the difference between the ATC and RTC get larger making the composite more anisotropic with increasing BNNS content. This observation is because the BNNS planes are aligned only in the axial direction and are not interconnected transverse to it as the PDMS matrix is filled between them, thus the matrix with a low TC of 0.18 W/mK contributes most to the composite RTC.

The BD composite delivers the highest ATC of 1.38 W/mK and the corresponding RTC of 1.03 W/mK when the f-BNNS content is 8.38 wt%. In comparison, the UD composite presents an ATC of 0.718 W/mK and an RTC of 0.296 W/mK at a f-BNNS content of 9.80 wt%. These values represent TC enhancements of 667 and 472% in the axial and radial directions respectively of the BD composite whereas they are 300 and 64% in the axial and radial directions respectively, for the UD composite.

The scanning electron microscopy of BD aerogels confirms the alignment. and the top view indicates that the central part is much denser than the peripheral part with wider crevices. During freeze casting, the individual freezing planes grow from the mold wall towards the center where they all meet making the center dense. The electrical conductivity measured at 20 Hz is about 10^-14 S/cm, indicating the composites an insulator.

In summary, highly thermally conductive and electrically insulating interconnected 3D axial/radial BD composites are fabricated via a freeze-casting method. They possess a novel structure showing a similar TCs in the two directions at a high f-BNNS content of 9.80 wt%.