A systematic approach to mass-production of graphene and other 2D materials is essential for current and future technological applications. By combining a sequential statistical design of experiments with in-situ process monitoring, we demonstrate a method to optimize graphene growth on copper foil in a roll-to-roll rf plasma chemical vapor deposition system. Data-driven predictive models show that gas pressure, nitrogen, oxygen, and plasma power are the main process parameters affecting the quality of graphene. Furthermore, results from in-situ optical emission spectroscopy reveal a positive correlation of CH radical to high quality of graphene, whereas O and H atoms, Ar+ ion, and C2 and CN radicals negatively correlate to quality. This work demonstrates the deposition of graphene on copper foil at 1 m/min, a scale suitable for large-scale production. The techniques described here can be extended to other 2D materials and roll-to-roll manufacturing processes.

Large area highly crystalline MoS 2 and WS 2 thin films were successfully grown on different substrates using radio-frequency magnetron sputtering technique. Structural, morphological and thermoelectric transport properties of MoS 2, and WS 2 thin films have been investigated systematically to fabricate high-efficient thermal energy harvesting devices. X-ray diffraction data revealed that crystallites of MoS 2 and WS 2 films are highly oriented in 002 plane with uniform grain size distribution confirmed through atomic force microscopy study. Surface roughness increases with substrate temperature and it plays a big role in electron and phonon scattering. Interestingly, MoS 2 films also display low thermal conductivity at room temperature and strongly favors achievement of higher thermoelectric figure of merit value of up to 1.98. Raman spectroscopy data shows two distinct MoS 2 vibrational modes at 380 cm −1 for E 1 2g and 410 cm −1 for A 1g. Thermoelectric transport studies further demonstrated that MoS 2 films show p-type thermoelectric characteristics, while WS 2 is an n-type material. We demonstrated high efficient pn-junction thermoelectric generator device for waste heat recovery and cooling applications.

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, ultra-oriented fibers made of ultra-high 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 ultra-drawn 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 Wm-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 using a scalable textile manufacturing platform and lays the foundation for exploiting their unique thermomechanical properties for heat spreading functions in flexible/wearable devices.

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With advances in flexible and wearable device technology, thermal regulation will become increasingly important. Fabrics and substrates used for such applications will be required to effectively spread any heat generated in the devices to ensure user comfort and safety, while also preventing overheating of the electronic components. Commercial fabrics consisting of ultra-high molecular weight polyethylene (UHMW-PE) fibers are currently used in personal body armor and sports gear owing to their high strength, durability, and abrasion resistance. In addition to superior mechanical properties, UHMW-PE fibers exhibit very high axial thermal conductivity due to a high degree of polymer chain orientation. However, these materials have not been widely explored for thermal management applications in flexible and wearable devices. Assessment of their suitability for such applications requires characterization of the thermal and mechanical properties of UHMW-PE in the fabric form that will ultimately be used to construct heat spreading materials. Here, we use advanced techniques to characterize the thermal and mechanical properties of UHMW-PE fabrics, as well as other conventional flexible materials and fabrics. An infrared microscopy-based approach measures the effective in-plane thermal conductivity, while an ASTM-based bend testing method quantifies the bending stiffness. We also characterize the effective thermal behavior of fabrics when subjected to creasing and thermal annealing to assess their reliability for relevant practical engineering applications. Fabrics consisting of UHMW-PE fibers have significantly higher thermal conductivities than the benchmark conventional materials while possessing good mechanical flexibility, thereby showcasing great potential as substrates for flexible and wearable heat spreading application.

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