Lun Howe Mark’s research while affiliated with University of Toronto and other places

An ideal dielectric material for microelectronic devices requires a combination of high anisotropic thermal conductivity and low dielectric constant (ɛ′) and loss (tan δ). Polymer composites of boron nitride nanotubes (BNNTs), which offer excellent thermal and dielectric properties, show promise for developing these dielectric polymer composites. Herein, a simple method for fabricating polymer/BNNT composites with high directional thermal conductivity and excellent dielectric properties is presented. The nanocomposites with directionally aligned BNNTs are fabricated through melt‐compounding and in situ fibrillation, followed by sintering the fibrous nanocomposites. The fabricated nanocomposites show a significant enhancement in thermal properties, with an in‐plane thermal conductivity (K‖) of 1.8 Wm⁻¹K⁻¹—a 450% increase—yielding a high anisotropy ratio (K‖/K⊥) of 36, a 1700% improvement over isotropic samples containing only 7.2 vol% BNNT. These samples exhibit a 120% faster in‐plane heat dissipation compared to the through‐plane within 2 s. Additionally, they display low ɛ′ of ≈3.2 and extremely low tan δ of ≈0.014 at 1 kHz. These results indicate that this method provides a new avenue for designing and creating polymer composites with enhanced directional heat dissipation properties along with high K‖, suitable for thermal management applications in electronic packaging, thermal interface materials, and passive cooling systems.

We generated carbon aerogel with a high thermal stability and a wider operational range by carbonizing resorcinol-formaldehyde (RF) aerogels. The carbonization process was conducted using a gas mixture that consist...

The creation and application of PET nanofibrils for PP composite reinforcement were studied. PET nanofibrils were fibrillated within a PP matrix using a spunbond process and then injection molded to test for the end-use properties. The nanofibril reinforcement helped to provide higher tensile and flexural performance in solid (unfoamed) injection molded parts. With foam injection molding, the nanofibrils also helped to improve and refine the microcellular morphology, which led to improved performance. Easily and effectively increasing the strength of a polymeric composite is a goal for many research endeavors. By creating nanoscale fibrils within the matrix itself, effective bonding and dispersion have already been achieved, overcoming the common pitfalls of fiber reinforcement. As blends of PP and PET are drawn in a spunbond system, the PET domains are stretched into nanoscale fibrils. By adapting the spunbonded blends for use in injection molding, both solid and foamed nanocomposites are created. The injection molded nanocomposites achieved increased in both tensile and flexural strength. The solid and foamed tensile strength increased by 50 and 100%, respectively. In addition, both the solid and foamed flexural strength increased by 100%. These increases in strength are attributed to effective PET nanofibril reinforcement.

Open‐cell foams are foams with porous, interconnected cellular structure. This unique structure has given open‐cell foams the versatility to be used in different types of applications including acoustics, filtration, membranes, and bioscaffolds. Conventional open‐cell polymer foams comprise mainly thermosetting and/or crosslinked materials; such materials are difficult to process and have limited end‐product recyclability. This article presents a commercially viable molding process to fabricate open‐cell foams using noncrosslinked thermoplastic polypropylene (PP). Highly open‐cell and/or reticulated structures at an open‐cell content of 84% are attained. The strategies employed in molding of PP foam with high open‐cell degree include: 1) use of mold‐opening method to achieve high void fractions; 2) reduction of cell‐wall thickness through increasing the cell densities via controlled crystallization; and 3) use of low viscosity or low melt‐strength polymer resins to promote cell‐wall opening. The molding process proposed may be extended to other semicrystalline thermoplastic materials for fabricating recyclable open‐cell foams.

Manufacture of thermoplastic foams with a fine cellular structure (a higher expansion ratio, a higher cell density, and smaller cell sizes) is challenging work due to the weak viscoelastic behavior and the unsuitable crystallization behavior of common thermoplastic materials. In this work, a novel method of making microcellular foams with micro‐/nano‐fibrillar reinforced polymeric composites (M/NFC) is introduced, which shows various advantages compared to conventional foams. The M/NFC foams have improved cellular structures, excellent mechanical properties, and enhanced thermal insulation properties, which make them popular candidates for structural applications and insulative products. Various methods to manufacture of M/NFC foam are summarized. To understand the fundamental mechanisms of the foaming enhancement by incorporating micro‐/nano‐size fibrils, the rheological and crystallization behavior of the M/NFC are analyzed. It is shown that the micro‐/nano‐fibrils can strengthen the melt strength, induce faster crystallization, and increase the number of crystals. Due to the improvement of the cell morphology and the stiffness of the cell walls, the reinforced foams have superior mechanical properties. A hierarchically porous structure in high expansion ratio reinforced foams has also been developed. It is believed that the nano‐size holes in the cell walls can further reduce the thermal conductivity of the foams.

In this work, the structural evolution of conductive polymer composites (CPCs) in response to mechanical deformation (uniaxial and biaxial compressive and tensile strains) is theoretically modeled and experimentally verified. The structural responses in mechanically deformed CPCs were simulated by incorporating the corresponding topological changes in representative volume element (RVE) and embedded filler networks. The percolating filler networks were then modeled as an equivalent electrical circuit consisting of tunneling and intrinsic resistances to examine the effect of deformation on the percolation threshold and the effective electrical conductivity. The results revealed that the filler alignment caused by strain changed both the vertical and lateral percolation thresholds, albeit with different trends. With an increase of uniaxial tensile (or equivalently, biaxial compressive) strain, applied on the vertical direction, the vertical percolation threshold initially reached a minimum value before rising, while the lateral percolation threshold monotonically increased. On the other hand, following incremental uniaxial compression (or equivalently, biaxial tension), the lateral percolation threshold reached a minimum value before increasing, while the vertical percolation threshold monotonically increased. The same relationship was observed in CPCs containing 1D fillers with different aspect ratios. The validity of the theoretical models was verified by comparing the predicted electrical conductivity values with the experimentally observed data obtained from polypropylene - multiwalled carbon nanotube nanocomposites.