Manipulating thermal transport in soft materials
Keywords: Electrical Engineering.

Electronics are ubiquitous, playing significant roles in modern work, entertainment, medicine, and myriad other aspects of our lives. As the new applications have grown, so too has the variety of chips. Unique product requirements accompany each particular application, but the demand for simultaneous protection from and interaction with the outside environment has driven the growth of a specialized field within electronics: packaging. The package protects the chip from damage during assembly and operation, makes electrical connections between the chip and the exterior electronics (often a printed circuit board), accommodates stresses due to thermal expansion, and facilitates heat removal from the chip. High thermal conductivity and mechanical compliance are both desirable in electronics packaging materials but tend to be mutually exclusive in naturally occurring materials. The development of highly conductive and compliant materials has long focused on the addition of inorganic fillers to an organic matrix, compromising some of the compliance of the polymer in exchange for some of the conductivity of the filler. Recent research has demonstrated the possibility for greater progress by approaching the problem from different directions: (1) increasing the thermal conductivity of the polymer without any filler material by manipulating the inter- and intra-molecular interactions, and (2) increasing the compliance of thermally conductive inorganic materials through nanoscale porosity. This work incorporates contributions to both of these directions. First, several experiments on spin coated polystyrene are presented. The macromolecular alignment due to the shear forces from spin coating was hypothesized to increase thermal conductivity in the direction of the alignment. The measurements in the vertical, radial, and azimuthal direction of the films suggest that the spin coating process did not significantly alter the thermal conductivity relative to the bulk polymer. This was found to be true even for bottlebrush polystyrene samples, with long side chains elongating the polymer to a gyration radius of ~100 nm. In the process of executing this measurement, multiple novel fabrication processes were developed to avoid damaging the sensitive polymer samples with caustic fabrication steps intended for more robust, inorganic materials. In particular, two different processes are presented in order to fabricate metal lines with sub-micron widths on polymer samples to perform 3-omega thermal conductivity measurements. The first, a nanostencil process, almost completely decouples lithography and etching from the sample by patterning membranes with desired features that are then used as shadow masks. The second uses room-temperature lithography to avoid thermal expansion mismatch between metal and polymer layers. Second, bifurcated 3-omega measurements of polystyrene (PS) and poly(4-chlorostyrene) (P4ClS) are performed, offering insight into the role of electrostatic intermolecular interactions in thermal conduction. The enhanced dipole moment in P4ClS was hypothesized to increase the thermal conductivity by strengthening the intermolecular interactions, but the difference in thermal conductivity between PS and P4ClS was found to be within the uncertainty. Finally, a novel technique to prepare nickel and gold films with tunable pore size and porosity is presented. The pore sizes range from 30 nm to 45 nm and the porosity from 35% to 65%. The electrical resistivities are also measured and found to be in agreement with a percolation-based effective medium theory including size effects. Each of these experiments individually addresses a specific question regarding material preparation, device fabrication, or thermal transport. In aggregate, the goal of this work is to inform the design of high thermal conductivity compliant materials. Further enhancements in packaging materials following this work and others will continue to make modern electronics more efficient and bring unprecedented electronic applications to reality.