Nanocoating individual cohesive boron nitride particles in a fluidized bed by ALDIntroductionExperimentalCoating different sites on BN plateletsEx situ FTIR SpectroscopyDynamic aggregation of BN Particles during fluidizationApplication of coated BN as a Filler MaterialConclusionsAcknowledgementsReferencesNanocoating individual cohesive boron nitride particlesin a fluidized bed by ALDJeffrey R. Wank, Steven M. George, Alan W. Weimer*Department of Chemical and Biological Engineering, University of Colorado, Engineering Center, ECCH 111,Campus Box 424, Boulder, CO 80309-0424, USAReceived 20 June 2003; received in revised form 13 January 2004Available online 18 May 2004AbstractExperiments are conducted with alumina (Al2O3) deposition on a wide size range of hexagonal boron nitride (BN) platelet-likeparticles. Successful deposition of alumina films on these particles, with film thickness controllable at the Angstrom level, is observedbased upon TEM imaging, ICP-AES, particle size distributions, and surface area analysis. While fluidizing, fine BN particles aggregatein the bed. The aggregates are the entities fluidizing, not the primary particles. However, individual particles are coated using AtomicLayer Deposition (ALD), not aggregates. Since ALD is a surface chemistry phenomenon, the films grow uniformly on every primaryparticle. BN particles are small platelets with different functional groups on the basal planes and edge planes. A small exposure toreagents [2.5106Langmuir (L) per reagent per cycle], will only coat the edge planes of uncoated BN particles. A larger dose of1108L will coat the entire uncoated BN particle (edge and basal planes). After 10 ALD cycles of the 1108L dose, the exposurescan be reduced to 1106L as the film is then growing on alumina and not BN. Peel strength data indicate that adhesion between thecoated particles and a cured epoxy in a filled composite is f 25% stronger than that of uncoated particles and the epoxy. The overallthermal conductivity drops f 17% for an identical filler loading as expected due to the additional thermal resistance added by the film.However, the viscosity of an epoxy resin loaded with coated BN is as much as five times lower than that of the resin loaded with thesame amount of uncoated BN. These results indicate that the loading of Al2O3nanocoated BN particles in an epoxy matrix can besubstantially increased relative to that of uncoated particles. The thermal conductivity of the more highly filled composite will beincreased without adversely impacting filled resin viscosity or the peel strength of the cured material. This is the first reported studyindicating that cohesive primary particles that fluidize as aggregates in a fluidized bed can be individually coated with a nano-thickceramic film using ALD.D 2004 Elsevier B.V. All rights reserved.Keywords: Atomic Layer Deposition; ALD; Fluidized bed; Boron nitride; Nanolayer; Thin film; Coating; Particle; Platelet1. IntroductionEpoxy resins are widely used in the packaging ofmicroelectronic circuits. The resins by themselves havepoor heat transfer properties. Fillers are typically used toenhance thermal conductivity (Tc). Viscosity effects arevery important as well. Generally, if solids can be addedto the resin and the viscosity of the mixture does notincrease much, then thin bondlines and full area coverageduring processing can be maintai ned. Using fillers in thismanner is well studied, but far from optimized [1–7].There are many different types of filler material s. Forinstance, Hatta et al. [3] used SiO2,Al2O3fibers, BNparticles, and Si3N4whiskers. This group found that theorientation of the filler has a strong effect on the overallthermal conductivity of the composite. This has applica-tion for BN particles because of their platelet nature. Hilland Supancic [8] also studied platelet-shaped particles ofsimilar size and shape. They concluded that flat plateletsprovide higher than predicted heat transfer as fillermaterial because of high surface area contacting whenthe BN plates ‘‘stack’’ upon one another. Hexa gonal BNhas an additional advantage because of its softer nature,0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2004.03.010* Corresponding author. Tel.: +1-303-492-3759; fax: +1-303-492-4341.E-mail address: [email protected] (A.W. Weimer).www.elsevier.com/locate/powtecPowder Technology 142 (2004) 59 –69which leads to deformation of the particles and increasedpacking density. This suggests that coating the edges ofthe platelet-like particles without coating the basal planescould be advantageous over coating the entire particle.Since BN has the highest thermal conductivity of anyfiller used [1,9,10], it has the bes t potential as a fillermaterial. The main challenge with BN is that the surfaceis very inert. This leads to poor interfacial adhesion betweenBN and epoxy resins [11]. One way to counter this effect isto coat the surface of the BN particles with an ultrathin layerof a material that adheres well to the polymer. Any filmgrown on a BN particle must be extremely thin because theTc through the BN particle/film composite will be reducedwith the added thermal resistance of the film material. In thecase of ALD-grown alumina, Tc c 20 W/m K. When thefilm reaches 2 nm, the overall thermal conductivity willhave decreased by f 50%. A graphical representation ofthis phenomenon is shown in Fig. 1.Xu and Chung [12] found that by processing the particleswith silane, they could increase the loading of BN into anepoxy matrix. The film that this group produced is not wellcharacterized. The only analysis is that there is a 2.4% byweight increase (after silane treatment). They also noti cedthat by incre asing the weight percent of the deposit above 2.4wt.%, the Tc of the composite started to decrease. This wasmost likely due to the interlayer becom ing a thermal barrier.The ability to coat fine particles with thin layers of othersubstances has recently been given considerable attention[13–17]. These coatings allow altering of the surfaceproperties of the particles without affecting the bulk prop-erties. Atomic Layer Deposition (ALD) [18,19] provides aunique method for performing this task, because the filmsgenerated using this method are conformal and can becontrolled down to the atomic level.ALD is similar to chemical vapor deposition (CVD),except that the binary CVD reaction is split into two half-reactions