Investigation of thermal transport in polymer composites with percolating networks of silver thin films by the flash diffusivity method

Abstract

The flash diffusivity method, also known as laser flash analysis (LFA), is commonly used to obtain the thermal diffusivity (a) and thermal conductivity (j) of materials, due to its relative simplicity, rapid measurements, small sample size requirement, and standardized commercially available instruments. In this work, an epoxy adhesive was filled with a large fraction of homogeneous micron-sized polymethylmethacrylate spheres coated with thin silver films, such that a percolating metallic network that dominated the electric and thermal transport formed through the polymer at <3 vol. % silver. Specific heat capacity (C p) was measured from the LFA measurements by a comparative method and from the total and reversible heat flow signals of modulated differential scanning calorimetry (MDSC). j was estimated as the product of a, C p , and density (q) and was found to vary significantly with the method to find C p. The electron contribution was found from the electrical conductivity by the Wiedemann-Franz law and was used to elucidate the thermal transport mechanisms in the composite. A theoretical background for the various methods is included.

Thermal transport in silver-coated polymer sphere composites by the bidirectional 3ω method

“…The thermal transport by electrons is limited to occur within the silver thin films or by tunneling across ultrathin epoxy layers between two AgPS. The thermal transport of the AgPS-ICA system has been discussed previously in the context of LF measurements 22 , which concluded that the thermal transport in the AgPS is dominated by the electronic heat transfer in the Ag layer. We believe a minor thermal transport mechanism deserves mention, which is the possibility of electronphonon interactions across the Ag-polymer interfaces.…”

“…For the 45 and 50AgPS sample, the SS measurements give thermal conductivities that are 46 % and 38 % above the 3ω measurements, respectively. Pettersen et al 22 did LF measurements of AgPS-ICAs, and showed that the technique for measuring the specific heat had a significant effect on the measured thermal conductivity for this sample system. However, this effect alone can not explain the large discrepancies observed between the transient and steady state methods.…”

“…So even if the LF measurement itself can be performed within seconds for most solids, the required additional measurements adds up to significantly more. Additionally, the separate measurements of ρ and C p may result in larger errors 22 . Depending on the accuracy needed, reference measurements of samples with similar C p and ρ can be used, omitting the measurements of these for the specific sample.…”

“…This material system, hereafter referred to as AgPS-ICA, has a large advantage over the conventional ICAs due to the significant reduction in costly silver. AgPS-ICAs have yielded thermal conductivities similar to the conventional ICAs at Ag filler fractions of around 3 vol.% measured using both steady state (SS) and LF techniques 21,22 . For both methods, a clear correlation was observed between AgPS volume fraction and thermal conductivity.…”

“…This discrepancy is possibly due to the volumetric constraint exerted on the sample during curing and measurement by the SS method. However, the thermal transport mechanisms of this novel AgPS-ICA needs further investigation 22 , and the best suited thermal characterization technique is yet to be found.…”