Role of Substrate Surface Morphology on the Performance of Graphene Inks for Flexible Electronics

Silver Nanoparticle-Assisted Electrochemically Exfoliated Graphene Inks Coated on PVA-Based Self-Healing Polymer Composites for Soft Electronics

Smart sensors with self-healing capabilities have recently aroused increasing interest in applications in soft electronics. However, challenges remain in balancing the sensors' self-healing and compatibility between their sensing and substrate layers. This study evaluated several self-healing polymer substrates and graphene ink-based strain-sensing coatings. The optimum electrochemically exfoliated graphene (e-graphene)/silver nanoparticle-coated tannic acid (TA)/superabsorbent polymer/graphene oxide (GO) blended poly(vinyl alcohol) polymer composites exhibited improvements of 47.1 and 39.2%, respectively, for the healing efficiency in a substrate crack area and in the graphene-based sensing layer due to conductive layer adhesion. While TA was found to improve healing efficiency on the coating surface by forming hydrogen bonds between the sensing and polymer layers, GO healed the polymer surface due to its ability to form bonds in the polymer matrix. The superabsorbent polymer was found to absorb excess water in e-graphene dispersion due to its host-guest interaction, while also reducing the coating thickness.

Sustainable Upcycling of Spent Electric Vehicle Anodes into Solution-Processable Graphene Nanomaterials

A major transition to electric vehicles (EVs) is underway globally, as countries target reductions in greenhouse gas emissions from the transport sector. As this rapid growth continues, significant challenges remain around how to sustainably manage the accompanying large volumes of waste from end-of-life lithium-ion batteries that contain valuable rare earth and critical materials. Here, we show that high-shear exfoliation in aqueous surfactants can upcycle spent graphite anodes recovered from an EV into few-layer graphene dispersions. For the same hydrodynamic conditions, we report a process yield that is 37.5% higher when using spent graphite anodes as the precursor material over high-purity graphite flakes. When the surfactant concentration is increased, the average atomic layer number reduces in a similar way to that of high-purity precursors. We find that the electrical conductance of few-layer graphene produced using the graphite flake precursor is superior and identify the limitations when using aqueous surfactant solutions as the exfoliation medium for spent graphite anode material. Using these nontoxic solution-processable nanomaterial dispersions, functional paper-based electronic circuit boards were fabricated, illustrating the potential for end-to-end, environmentally sustainable upcycling of spent EV anodes into new technologies.

Graphene Flake Self-Assembly Enhancement via Stretchable Platforms and External Mechanical Stimuli

While the green production and application of 2D functional nanomaterials, such as graphene flakes, in films for stretchable and wearable technologies is a promising platform for advanced technologies, there are still challenges involved in the processing of the deposited material to improve properties such as electrical conductivity. In applications such as wearable biomedical and flexible energy devices, the widely used flexible and stretchable substrate materials are incompatible with high-temperature processing traditionally employed to improve the electrical properties, which necessitates alternative manufacturing approaches and new steps for enhancing the film functionality. We hypothesize that a mechanical stimulus, in the form of substrate straining, may provide such a low-energy approach for modifying deposited film properties through increased flake packing and reorientation. To this end, graphene flakes were exfoliated using an unexplored combination of ethanol and cellulose acetate butyrate for morphological and percolative electrical characterization prior to application on polydimethylsiloxane (PDMS) substrates as a flexible and stretchable electrically conductive platform. The deposited percolative free-standing films on PDMS were characterized via in situ resistance strain monitoring and surface morphology measurements over numerous strain cycles, with parameters extracted describing the dynamic modulation of the film's electrical properties. A reduction in the film resistance and strain gauge factor was found to correlate with the surface roughness and densification of a sample's (sub)surface and the applied strain. High surface roughness samples exhibited enhanced reduction in resistance as well as increased sensitivity to strain compared to samples with low surface roughness, corresponding to surface smoothing, which is related to the dynamic settling of graphene flakes on the substrate surface. This procedure of incorporating strain as a mechanical stimulus may find application as a manufacturing tool/step for the routine fabrication of stretchable and wearable devices, as a low energy and compatible approach, for enhancing the properties of such devices for either high sensitivity or low sensitivity of electrical resistance to substrate strain.

Enhanced Thermal Conductivity of Epoxy Composites Filled with Al2O3/Boron Nitride Hybrids for Underfill Encapsulation Materials

In this study, a thermal conductivity of 0.22 W·m-1·K-1 was obtained for pristine epoxy (EP), and the impact of a hybrid filler composed of two-dimensional (2D) flake-like boron nitride (BN) and zero-dimensional (0D) spherical micro-sized aluminum oxide (Al2O3) on the thermal conductivity of epoxy resin was investigated. With 80 wt.% hybrid Al2O3-BN filler contents, the thermal conductivity of the EP composite reached 1.72 W·m-1·K-1, increasing approximately 7.8-fold with respect to the pure epoxy matrix. Furthermore, different important properties for the application were analyzed, such as Fourier-transform infrared (FTIR) spectra, viscosity, morphology, coefficient of thermal expansion (CTE), glass transition temperature (Tg), decomposition temperature (Td), dielectric properties, and thermal infrared images. The obtained thermal performance is suitable for specific electronic applications such as flip-chip underfill packaging.