Development of novel thermal conductive polymer nanocomposites

Abstract

Developments in the semiconductor industry have revealed thermal management problems causing shrinkage of the components and increased power densities. This tendency in technology has rendered the development of materials, for thermal management, having excellent thermal conductivity, high electrical resistance, and low density important. Especially, materials that are light, cost-effective, easily handled, and suitable component design with desired thermal and electrical properties, namely polymeric matrix nanocomposites, are regarded as new-generation materials for electronic/microelectronic technology components. Among many nano-filling materials used to prepare thermally conductive polymeric nanocomposites, the dielectric ceramic particle, β-phase (hexagonal) silicon nitride (β-Si3N4) (SN), has high thermal conductivity as well as high mechanical strength and high electrical insulating properties where carbon nanotube and graphene cannot provide. Therefore, it attracts attention as a strategic material for electric/electronic applications. In the literature, SN-containing polymeric composite/nanocomposite preparation includes many methods, as SN is added into a commercial thermoplastic polymeric matrix or into thermoset pre-polymer cured later and shaped by operations such as extrusion or injection molding. Although there are improvements in the thermal conductivity of the material with current methods, SN-containing polymeric composites have limitations and problems in their practical applications. One of them is the advanced handling of the polymer nanocomposite, caused by thermoset matrix and strong filling-filling interactions. The other is the low interaction between the filling material and polymeric matrix and the low performance caused by the interface. Therefore, for electronic/microelectronic technology, effective surface modification methods and new composite preparing trends providing improvements in electrical, thermal, and mechanical properties of the nanocomposite and determination of the microelectronic component performance are important. In the design of thermal conductive nanocomposite, the SN, which was prepared in the nanosheet morphology, was not added as a polymer matrix additive, the covalent bond between the modified SNnanosheet and the end-functional polymer matrices was formed and the problems preventing further processing of the material such as filling-filling interaction in nanocomposite production were eliminated. The other unique value of the silicon nitride filled thermal conductive polymer nanocomposites is that they were produced by a synthetic approach that provides structure-property control in the material. In other words, the molecular weight, molecular weight distribution, end-group functionality of the polymer structure to be grown onto the surface of SNNSs can be controlled. In the synthetic approach proposed in this work, in order to increase the interphase interactions between the polymer and the nanoparticle and to provide a homogeneous distribution of the nanofiller in the matrix, the "grafting to" method was implemented. Click chemistry was used as an essential tool to form covalent bond between the filler and matrix. A new polymer nanocomposites consisting of silicon nitride (Si3N4) nanoparticle as a conductive filler and poly(methyl methacrylate) (PMMA), polysulfone (PSU) and poly(ether ether ketone) (PEEK) as a polymer matrices were fabricated using "click" chemistry. The synthesis of these three nanocomposites is a five-step procedure including (i) the preparation of silicon nitride nanosheet (SNNS) by mechanical exfoliation, (ii) the modification of SNNS with successive chemical modification processes, (iii) the synthesizing of polymer matrices with end-functionality, iv) the modification of these end-groups to compatible with click reaction, v) the performing Copper Catalyzed Azide-Alkyne Click reaction (CuAAC) between alkyne functionalized polymer matrices (PMMA-Alkyne, PSU-Alkyne and PEEK-Alkyne) and azide functional SNNS (SNNS-N3). Characterization results confirmed the functionalization of the SNNS surface and the successful "grafting to" method using click reaction. The average crystallite sizes of β-Si3N4 and SNNSs obtained from the mechanical exfoliation process were calculated using the X-ray diffractometry analysis (XRD) results according to the Debye Scherrer equation. The particle size distribution of SN and SNNSs was investigated by photon correlation spectroscopy (PCS) using Zetasizer. The main-chain structures and end-functionalities of polymer matrices were confirmed by proton nuclear magnetic resonance spectroscopy (1H-NMR). The chemical structure of all nanocomposites, their precursors, and the success of click chemistry were verified by Fourier transform infrared spectroscopy (FT-IR). The molecular weights and polydispersities of polymer matrices were calculated using gel permeation chromatography (GPC). The glass-transition temperatures (Tg's) of polymers and nanocomposites were determined by differential scanning calorimetry (DSC). Thermal stabilities and char yield values of nanocomposites and their precursors were performed on a thermogravimetric analyzer (TGA). The surface morphologies and crystal structures of pristine SN and ball-milled SN (SNNS) and the dispersion of SNNSs within the polymer matrices were investigated by scanning electron microscopy (SEM). Electrical resistivities (volume resistivities) of nanocomposites were measured using an electrometer. Viscoelastic behaviors of polymer matrices and nanocomposites were analyzed by a dynamic mechanic analyzer (DMA). The novel thermal conductive polymer nanocomposites were formulated as a thermal interface material (TIM) and their performance as a TIM material was investigated by Thermal Conductivity Analysis (DTC). It has been found that the thermal conductivity of the chemically modified nanocomposites (PMMA-SNNS, PSU-SNNS, and PEEK-SNNS) based TIM is considerably higher than that of TIM prepared with physically blended nanocomposites (PMMA+SNNS, PSU+SNNS, and PEEK+SNNS) having the same composition.