(48d) Designed Boron Nitride Filler Particles for Thermally Conductive Composites

Boron nitride (BN) platelet particles are used as a filling material to increase the thermal conductivity of electronic plastic packages. The effectiveness of the filler material is limited by: (1) poor surface wetting of the BN particles with the resin, which results in high viscosity and limited loadings, and (2) poor interfacial adhesion of the BN particles to the polymer in the cured composite BN/epoxy matrix, which limits peel strength and thermal conductivity. Both of these limitations arise as a result of the inert BN surface, which is extremely difficult to modify by conventional Chemical Vapor Deposition (CVD) and wet chemical methods. A more desirable designed BN filler particle is one in which the high bulk thermal conductivity is maintained while the BN particle surface is modified (coated) to allow for improved wetting and interfacial adhesion within polymer systems. Individual primary BN particles should be coated as opposed to coating BN agglomerates. Selectively coating only the edges (not the basal planes) of BN particles is desirable for increased wettability and interfacial adhesion of the BN edges within a polymer matrix while maintaining a high thermal conductivity via direct BN basal plane stacking. This project provides for the development of improved boron nitride (BN) filler materials for electronic thermal management applications. Novel Atomic Layer Deposition (ALD) nanocoating is used to selectively functionalize edges only and edges/basal planes to improve wetting of BN platelets with resin encapsulants. The improved wetting allows for significantly reduced viscosity (~ 5 times less) of BN/resin mixtures during processing and improved interfacial adhesion in the cured composite. These improvements are realized by placing an ultra-thin (nm thick), conformal, pin-hole free, chemically bonded SiO₂ nanofilm on individual BN particles. The SiO₂ nanocoating is applied to the BN particles using the ALD process. The ALD process involves using two self-limiting sequential surface chemical reactions. The deposition of silica onto BN particles is split into two half reactions. Two precursors are required to generate the silica nanolayer: trisdimethylaminosilane and hydrogen peroxide (50% in water). The precursors are used individually in the two separate half reactions. The ALD coating process takes place in a vibrating fluidized bed under vacuum at 500 °C. A mass spectrometer is used to monitor and study the two half reactions. The use of the mass spectrometer will allow for the identification of the chemical mechanism for each half reaction, as this particular chemistry has not been previously demonstrated. Individual fine sized BN platelet particles have been selectively nanocoated (edges only or edges/basal planes) with chemically bonded SiO₂ films with thickness ranging from ~ 0.8 ? 50 nanometers. The nanocoated BN was blended at a 40 volume % loading in a liquid encapsulant mixture, cured, and tested for thermal conductivity, peel strength, and viscosity.