Thermal Conductivity of Ultra High Molecular Weight Polyethylene: From Fibers to Fabrics

Unique combinations of properties such as mechanical compliance and chemical stability make polymers attractive for many applications. However, the intrinsic low thermal conductivity of bulk polymers has generally limited their potential for heat dissipation applications, and in fact they are widely used as thermal insulators. But in recent years, gel-spun, ultraoriented fibers made of ultrahigh molecular weight polyethylene (UHMW-PE) have sparked interest in the thermal management community due to their exceptionally high thermal conductivity. These fibers are typically used in commercially produced protective gear such as motorcycle jackets and ballistic vests due to their high mechanical strength, but they have not been widely utilized for heat spreading and thermal management applications. While recent studies have characterized individual fibers and ultradrawn films, the thermal properties of fabrics constructed from these materials remain virtually unexplored. Here, we synthesize plain-weave fabrics from yarns of commercially available gel-spun UHMW-PE and measure the thermal properties of the individual microfibers, yarns, and woven fabrics using an in-house thermal characterization technique based on infrared microscopy. For the woven fabric, we report an effective in-plane thermal conductivity of ∼10 W m−1 K−1 in the direction aligned with the weft yarns, which is 2−3 orders of magnitude higher than conventional textile materials. This work reveals the high thermal conductivity of UHMW-PE fabrics that can be realized by using a scalable textile manufacturing platform and lays the foundation for exploiting their unique thermomechanical properties for heat spreading functions in flexible/wearable devices

Cascaded multi-core vapor chambers for intra-package spreading of high power, heterogeneous heat loads,

A cascaded multi-core vapor chamber (CMVC) is designed for dissipating heat from high-flux hotspots simultaneously with a high-total-power background. Current thermal management strategies rely on spreading high local heat fluxes by conduction in the lid of electronics packages. Embedding vapor chambers within the lid is an attractive option to directly address intra-package hotspots. We investigate the design of intra-lid vapor chambers, for a generic device having a total heat load of 476 W having a background heat flux of 0.75 W/mm2, with hotspots of 8 W/mm2 over a 1 mm2 area. A conventional vapor chamber design, having a single vapor core, will require a thick evaporator wick to avoid the capillary limit for large total power. The necessity for a thick wick then imposes a large thermal conduction resistance when the vapor chamber is exposed to high heat flux hotspots. The proposed CMVC architecture aims to address this limitation. The cascaded architecture comprises a bottom-tier vapor chamber having an array of multiple small vapor cores for spreading heat from the small hotspots. These small vapor cores have short paths of liquid return to the evaporator, such that they can handle their footprint heat load while using thin wicks, resulting in a low hotspot thermal resistance. Furthermore, local dampening of the hotspots by the bottom tier then reduces the thermal conduction resistance across the necessarily thick wick in the top tier. Hence, the cascaded architecture has the potential to significantly reduce the overall thermal resistance, relative to a single tier. To substantiate this design rationale, experiments are performed to illustrate that the resistance of a commercial vapor chamber can be significantly reduced by interfacing the heat source with an intermediate heat spreader. Reduced-order models are then used to understand the effect of the wick properties (porosity and particle size) and geometric parameters on the thermal performance of the CMVC for the representative power map. The optimal CMVC design offers a thermal resistance (0.66 K/W) that is significantly lower compared to a conventional single-core vapor chamber (1.76 K/W) owing to a reduction in the conduction resistances across the internal wicks. That parametric optimization results demonstrate that the thermal resistance of the CMVC is more sensitive to the wick porosity compared to the particle diameter. Furthermore, there exists a wide range of wick properties and vapor core sizes for which near-optimum thermal performance can be attained, which is particularly attractive from the standpoint of flexibility in design and manufacturing