Isotropic thermal conductivity achieved by anisotropic solid fillers

Electronic products play a fundamental role in facilitating technological advances across different fields. For decades, one major inherent issue that has faced the design and fabrication of these products is effective thermal management. Since electronics components are highly susceptible to excess heat, it is important to remove the unwanted heat generated by the components to prevent them from getting damaged and malfunctioning. A number of materials, especially those made up of organic polymer matrices and inorganic particles, have been deemed suitable for applications as thermal interface materials (TIMs). Although polymers possess desirable properties, their low thermal conductivity has become a serious issue.

In most cases, thermally conductive inorganic fillers have been added to compensate for the low thermal conductivity of these polymers. Among them, layered solid particles such as graphite and hexagonal boron nitride are commonly used as thermally conductive fillers in many polymer composites. This can be attributed to their relatively higher in-plane thermal conductivity even though they exhibit much lower thermal conductivity in the cross-plane direction. Consequently, these fillers are usually oriented via different processes like filtration to endow the composites with anisotropic thermal conductivity.

Despite numerous research efforts devoted to fabricating polymer composites with bolded thermal conductivity in the desired direction, inadequate anisotropic thermal conductivity for heat dissipation has been reported in some cases. This shows the significance of preparing polymer composites with superior isotropic thermal conductivity taking advantage of the high thermal conductivity of the layered solid fillers. Therefore, developing simple and scalable strategies for effective usage of the layered fillers to prepare isotropic TIMs is highly desirable.

To this note, Dr. Kimiyasu Sato, Dr. Yuichi Tominaga, Dr. Yuji Hotta and Dr. Yusuke Imai from the National Institute of Advanced Industrial Science and Technology (AIST) developed a new and simple method for preparing layered solid fillers-based polymer composites with isotropic thermal conductivity. This approach firstly involved producing spherical composite particles followed by compression forming of the produced particles. This was achieved via agitation-driven layer-by-layer assembly of the solid fillers. The effectiveness of this approach was validated. Their work is currently published in the journal, Composites Part A.

The research team showed that integrating the spherical particles with higher symmetric properties resulted in the preparation of layered solid fillers and polymer composites with desirable isotropic thermal conductivity. The resulting composites exhibited a solid loading exceeding 60 vol%. Consequently, their thermal conductivity was about 28 – 29 Wm^-1K^­-1 for graphite/epoxy specimen samples and about 8 Wm^-1K^-1 for hexagonal boron nitride/epoxy composite samples. The configuration of the internal solid fillers obtained by integrating the spheres was characterized using X-ray diffraction. For the spherical particles, their internal structures were observed to become symmetric in relation to their central point. For the spheres, the radial directions were always perpendicular to the basal planes of the solid filler particles.

In summary, AIST scientists presented a novel approach for fabricating isotropic thermal conductivity employing the anisotropic solid fillers to realize the current capability requirements of TIMs used in various applications. The main benefits of this novel approach included thermal isotropy, superior thermal conductivity and ease of pouring the composite spheres that are highly compatible with most manufacturing and industrial processes. In a statement to Advances in Engineering, the authors explained their new method would contribute to the design of high-performing TIMs with extended application scope in various industries.