White Carbon Materials for Advanced Heat Management

Yoke Khin YapDr. Yoke Khin Yap, professor in the Michigan Tech Department of Physics at Michigan Technological University (Michigan Tech), has invented a novel class of boron nitride (BN) nanomaterials for advanced heat management. BN phases are structurally similar to those of carbon solids. We have hexagonal phase-BN (h-BN), cubic phase-BN (c-BN), BN nanotubes (BNNTs), BN nanosheets (BNNSs, mono- and few- layered h-BN sheets). These BN structures are analogous to graphite, diamonds, carbon nanotubes (CNTs), and graphene, respectively [1]. Therefore, BN materials can be referred as “white carbon” as they are white in appearance due to their large band gap (~6eV).

Despite the structural similarity, the properties of BN materials are different from those of carbon solids. For example, graphite is electrically conducting while h- BN is insulating due to their large band gap. A common property among the BN and carbon materials is their high heat conductivity that hold potential applications for advanced heat management. BN nanostructures are predicted to have a thermal conductivity, as high as 2000 W/m-K, about 10-times higher than that of metals [2]. Therefore, BN materials can be in contact with active electrical components to dissipate heat without the risk of an electrical short circuit.

Dr. Yap is a leading expert in BN nanomaterials, specializing in the technology of direct synthesis of BNNTs and wavy BNNSs on substrates. BNNTs developed by Dr. Yap are of high purity and high quality, two desirable attributes for applications in electronic devices. The wavy BNNSs are unique in that they have full surface contact with the substrates. They also have wavy edges that stick out from the substrate surface to enhance the contact area with the surrounding cool air/environment. Michigan Tech demonstrates that the coatings of BNNTs and wavy BNNSs can both enhance the heat dissipation rate of hot Silicon chips by as much as 250% in static ambient air.

Figure 1 shows the appearance of BNNTs (top row) and the wavy BNNSs (bottom row) under a scanning electron microscope. As shown, BNNTs are long in length (~40 microns), offering a large contact surface area with air, an important feature to accelerate heat dissipation. However their small diameter (20-50nm), results in a very small contact area with the hot substrate surface.

YKYap boron nitride nanomaterials

In contrast, the wavy BNNSs offer a much larger surface area to contact with the hot substrate surface. Their wavy edges also provide an enhanced contact area with the surrounding cool air but smaller than that offered by BNNTs. The Yap research group have combined the benefits of both materials by growing BNNTs on top of the wavy BNNSs. Results indicate that such uniquely combined BNNT/BNNS structures in the presences of gas flows promote cooling better than BNNTs and BNNSs alone.

Finally, the Michigan Tech team has also demonstrated that these BNNSs and BNNTs can be transferred to desired surfaces. They found that BNNTs and BNNSs grown on Si substrates can be peeled and transferred on to fresh Si substrates. This suggests that these novel BN nanomaterials can be transferred on to hot surfaces of electrical and electronic devices to promote cooling. Michigan Tech has filed a utility patent application and is seeking industry partners to help commercialize the technology. Please contact Michael Morley (mcmorley@mtu.edu) for further information.

 

References

[1]. Y. K. Yap, “B-C-N Nanotubes, Nanosheets, Nanoribbons, and Related

Nanostructures,” http://www.azonano.com/article.aspx?ArticleID=2847

[2]. T. Ouyang, Y. P. Chen, Y. Xie, K. K. Yang, Z. G. Bao, J. X. Zhong, “Thermal

Transport in Hexagonal Boron Nitride Nanoribbons,” Nanotechnology 21, 245701

(2010).