Tailoring thermal interfaces with nanomaterials

Abstract

Thermal interfaces are key to ensure the reliable performance of many semiconductor, energy and electronic systems. High thermal conductivity (k), low elastic modulus (E) interface materials are required to dissipate heat and relieve thermo-mechanical stresses. The aim of this thesis is to develop compliant, high k nanocomposite materials for thermal interface applications utilizing nanostructured networks.

This thesis further explores the use of external stimuli in the form of magnetic fields to reversibly induce nanoparticle networking and gate heat transport at interfaces, a requirement in many emerging applications. It is demonstrated that magnetic field actuation of ~ 3 - 16 vol. % of magnetite or cobalt-ferrite nanoparticles in a fluid matrix yields ~16 times enhancement of the no field effective thermal conductivity, but only in a gradient magnetic field. Heat transfer modeling shows that the enhancement arises from magnetic field gradient driven bulk convection, rather than the expected nanoparticle network formation.

A forty-fold thermal conductivity increase is obtained by in situ welding of silver nanowire fillers inside polydimethylsiloxane using microwaves. Even for ≤ 0.04 filler volume fractions, welding facilitates nanowire networking that counteracts thermal transport bottlenecks associated with the low polymer thermal conductivity and high polymer-filler interface thermal resistances. The transparency of the polymer to microwaves precludes thermal degradation, and the composites retain high mechanical compliance as indicated by < 1 MPa viscoelastic storage moduli. These findings would be instrumental for controllably realizing high thermal conductivity conformal polymer composites for thermal interface applications.

This thesis also demonstrates novel techniques to create tailored nanowires and networks for high k nanocomposites. Branched Ag nanowires are synthesized via controlled interruptions to microwave-stimulated polyvinylpyrrolidone-directed polyol-reduction of silver nitrate. Microwave exposure results in micrometer-long nanowires passivated with polyvinylpyrrolidone. Cooling the reaction mixture by interrupting microwave exposure promotes nanocrystal nucleation at low-surfactant coverage sites. The nascent nuclei grow into nanowire branches upon further microwave exposure. Dispersions of low fractions of the branched nanowires in polydimethylsiloxane yield up to 60 % higher thermal conductivity than that obtained using unbranched nanowire fillers.

Another important finding of this thesis is that nanowire networks can result in mechanical softening of polymer matrices. It is demonstrated that silver nanowire fillers result in a three-fold decrease in viscoelastic storage modulus of polydimethylsiloxane composites above a low critical filler fraction of ~0.5%, contrary to theoretical predictions presaging a modulus increase. Similar fractions of silver nanocube fillers result in no such observable effects. Rheology measurements and calorimetric kinetics analyses reveal that high surface area nanowire filler percolation networks curtail macromolecular mobility via pre-cure gelation, and hinder crosslinking.

Along with high k, tailoring high thermal contact conductance Gc is crucial for many applications. This thesis reveals a critical correlation between the rheological behavior of a high k gold-nanowire-filled polydimethylsiloxane nanocomposite and its thermal contact conductance with copper. At a critical filler fraction, an abrupt increase in the nanocomposite k is accompanied by a liquid-solid transition and a multifold decrease in Gc. These concurrent changes are attributed to nanowire percolation network formation and pre-cure polymer gelation that inhibits the formation of conformal void-free interfaces. These findings will be important for designing processing sequences to realize heterointerfaces with nanowire filled high k nanocomposite materials.

Realizing high k nanocomposites is a challenge because of difficulties in incorporating high fractions of uniformly dispersed nanofillers and countering low filler-matrix interfacial conductance, while retaining a low elastic modulus. In this thesis, it is demonstrated that these issues are obviated by using < 5 volume % sub-10-nm cold welded gold nanowire fillers to obtain an unprecedented 30-fold increase in polydimethylsiloxane thermal conductivity that is 6-fold higher than previously reported nanocomposites at low nanofiller loadings and exceeds theoretical predictions. The nanowire diameter and aspect ratio are key to obtain cold-welded networks that enhance k at low filler fractions, while fostering low E.