Thermal and Dielectric Behaviour of Polymer Composites with Hybrid Fillers

Abstract

This thesis reports on the analytical and experimental study on thermal and dielectric behaviour of hybrid filler polymer composites. The objective is to explore the possibility of using multiple ceramic fillers in polymers to make composites suitable for microelectronic applications. The first part of the report is on the development of theoretical heat conduction models based on which mathematical correlations have been proposed for estimation of effective thermal conductivity of polymer composites with single as well as hybrid fillers. The second part has provided the description of the materials used, routes adopted to fabricate the various epoxy and polypropylene composites and the details of the experiments that are conducted during this research. It also presents the test results in regard to the physical, micro-structural and mechanical characteristics of all the epoxy and polypropylene based composites filled with single filler i.e. micro-sized Aluminium nitride (AlN)/ Aluminium oxide (Al2O3). A comparative evaluation of the effects of premixing of solid glass microspheres with micro-sized AlN/Al2O3 on the different physical and mechanical properties of composite systems is also reported. The last part has emphasized on the thermal and dielectric characteristics of the composites under this investigation. It includes an assessment of the effective thermal conductivity of these composites using the proposed theoretical models. Effects of inclusion of various combinations of single/hybrid fillers on the effective thermal conductivity (keff), glass transition temperature (Tg), coefficient of thermal expansion (CTE) and dielectric constant (ɛc) of the composites are presented.
Analytical models developed in this work for evaluating effective thermal conductivity of single/hybrid filler reinforced polymer composites are based on the principle of law of minimal thermal resistance and equal law of specific equivalent thermal conductivity. The values obtained from the theoretical model for single filler polymer composites are in close approximation with the corresponding measured values up to percolation threshold. For hybrid filler model, the calculated values are in good approximation for the entire range of filler content as no percolation is seen for hybrid composites. Percolation is the phenomenon which occurs when the content of conductive filler in matrix becomes substantially high so as to form thermal bridges across the planes throughout the system resulting in a sudden improvement of conductivity. The volume fraction of filler at which sudden jump in the composite effective thermal conductivity occurs is called the percolation threshold of that filler-matrix combination. This phenomenon however has not occurred for hybrid filler composites.
The present research also shows that the selected aluminum based ceramic powders have the potential to be successfully used as functional filler materials in both thermoset and thermoplastic polymers. It is also noticed that the epoxy based composites have higher void fraction compared to that in the polypropylene based composites. Inclusion of spherical particles in these polymeric resins has not resulted in any improvement in the load bearing capacity (tensile strength). On the other hand, hardness and compressive strength values have been found to have improved invariably for all the composites.
Inclusion of single filler i.e. micro-sized AlN/Al2O3 appreciably enhances the effective thermal conductivity of polymers. Other thermal properties like CTE and Tg also get modified accordingly. But, with addition of these fillers, little increase in the value dielectric constant is noted. The polymer composite fabricated in present work must possess low dielectric constant which does not get completely fulfilled with single fillers. So SGM is introduced as a secondary filler to overcome this problem. With the addition of SGM in combination with AlN/Al2O3 modifies various physical, mechanical and thermal properties. But most importantly, a noticeable change is observed in case of dielectric constant value. With SGM as the secondary filler, much lower value of dielectric constant is obtained which is almost around that of the neat polymer. It is seen that apart from the effective thermal conductivity, all the other properties shows positive modification for hybrid filler composites as compared to single filler composites as far as their applications in microelectronics are concerned.
The particulate filled polymer composites developed for this investigation are expected to have adequate potential for a wide variety of applications particularly in microelectronic industries. With enhanced thermal conductivity, improved glass transition temperature, reduced thermal expansion coefficient and modified dielectric characteristics, the epoxy and polypropylene composites with appropriate proportions of fillers can be used in microelectronic applications like electronic packaging, encapsulations, printed circuit board substrates etc.