Thermal Conductivity and Cure Kinetics of Epoxy-Boron Nitride Composites—A Review

Abstract

Epoxy resin composites filled with thermally conductive but electrically insulating particles play an important role in the thermal management of modern electronic devices. Although many types of particles are used for this purpose, including oxides, carbides and nitrides, one of the most widely used fillers is boron nitride (BN). In this review we concentrate specifically on epoxy-BN composites for high thermal conductivity applications. First, the cure kinetics of epoxy composites in general, and of epoxy-BN composites in particular, are discussed separately in terms of the effects of the filler particles on cure parameters and the cured composite. Then, several fundamental aspects of epoxy-BN composites are discussed in terms of their effect on thermal conductivity. These aspects include the following: the filler content; the type of epoxy system used for the matrix; the morphology of the filler particles (platelets, agglomerates) and their size and concentration; the use of surface treatments of the filler particles or of coupling agents; and the composite preparation procedures, for example whether or not solvents are used for dispersion of the filler in the matrix. The dependence of thermal conductivity on filler content, obtained from over one hundred reports in the literature, is examined in detail, and an attempt is made to categorise the effects of the variables and to compare the results obtained by different procedures.

Densification: A Route towards Enhanced Thermal Conductivity of Epoxy Composites

“… Thermal conductivity as a function of boron nitride (BN) content (wt%) for epoxy-BN samples prepared under pressure. Dashed lines represent upper, intermediate, and lower trend curves [ 6 ]. Squares represent oriented samples.…”

“…The maximum values of thermal conductivity taken from these references are plotted as a function of the wt% BN in Figure 6, where they are compared with the upper, intermediate, and lower trend curves discussed above. In the compilation of this Figure, the values of the maximum thermal conductivity, together with the corresponding BN content, were taken from more than one hundred references for epoxy-BN composites [6]. In order to simplify and display the overall tendency for the dependence of thermal conductivity on BN content, three trend curves were drawn: an upper trend curve, below which more than 95% of the values fell, including values for both isotropic and anisotropic samples; an intermediate trend curve, which represented an approximation to the upper limit of the isotropic thermal conductivities, thus excluding all those samples for which orientation had been deliberately introduced; and a lower trend curve, below which fewer than 5% of all the values of thermal conductivity fell.…”

“…The dielectric layer must satisfy a number of practical requirements, including good adhesion, ease of processing and low cost, in addition to the physical attributes of electrical insulation and high thermal conductivity. The importance of achieving these objectives can be gauged from the large number of publications devoted to this topic, which have conveniently been collected in several recent comprehensive reviews [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ].…”

Remarkable Thermal Conductivity of Epoxy Composites Filled with Boron Nitride and Cured under Pressure

“…The epoxy resin used was diglycidyl ether of bisphenol-A, DGEBA (Araldite GY240, Huntsman Advanced Materials, Salt Lake City, UT, USA), with a nominal molecular weight per epoxy equivalent (eq) of 182 g/eq, density of 1.17 g/cm 3 and viscosity of 7000 to 9000 mPa.s at 25 • C. The principal cross-linking agent used was a thiol, pentaerythritol tetrakis (3-mercaptopropionate) (Sigma-Aldrich, Saint Louis, MO, USA), with a molecular weight of 488.66 g/mol, density of 1.28 g/cm 3 , and viscosity of 500 mPa.s at 23 • C. In order to initiate the cross-linking reaction of the epoxy with the thiol, a latent initiator, encapsulated imidazole (LC-80, Technicure, A&C Catalysts, Linden, NJ, USA) in the form of powder, was used. In addition, a polyoxypropylene diamine, Jeffamine D-230 (molecular weight 230 g/mol, density 0.948 g/cm 3 , viscosity 9000 mPa.s at 25 • C), and a dicyandiamide, N,N-dimethyl-N-phenyl urea (PDU-250M, Technicure, A&C Catalysts, Linden, NJ, USA), were also used as alternative cross-linking agent and initiator, respectively, for comparative purposes.…”

“…It is widely recognized that the thermal conductivity of epoxy-BN systems increases with increasing BN content [3,4], at least up to about 50 vol.% BN. However, higher BN contents present difficulties for processing on account of their being very stiff pastes, and the thermal conductivity often decreases as a consequence of the presence of voids which are difficult to remove.…”