Chapter 5. Self-healable and flexible thermoacoustic loudspeakers with AgNW/poly(urethane-hindered
5.1 Introduction
Recently, thermoacoustic (TA) loudspeakers have attracted significant attention as a new type of speaker that does not rely on the vibration of the diaphragm; rather, these speakers depend on the temperature oscillation of the surrounding vibrating air to generate sound.70-72, 236-246 Typical speakers require space for the vibration of the diaphragm, which necessitates a complicated fabrication process and a large volumetric speaker size. However, the TA loudspeaker can be easily fabricated into a thin film-type speaker with a conductive film, thereby producing sounds without mechanical vibration.
Generating sound with the simple thin film-type speaker is a great advantage of TA loudspeaker. It provides several advantages over typical speakers such as simple fabrication process, device- miniaturization, and flexibility for wearable applications. TA loudspeaker generates thermoacoustic sounds under an alternating current (AC) voltage, where periodic Joule heating of a conductive film by AC voltage induces the temperature oscillation that vibrates the surrounding air and generates sounds.71,
72 The first TA loudspeaker was manufactured using a platinum strip clamped to lead electrodes with extremely high heat capacities.236 Conducting nanomaterials reduce the effective heat capacity per unit area (HCPUA) of the TA loudspeaker owing to the lower matter content compared to bulk materials, thereby enhancing heat release into the surrounding air and the sound pressure by air vibration.72 Recently, there have been significant interests regarding the use of various conducting materials such as carbon nanotubes (CNTs),72, 237 graphene,71, 238, 239 graphene oxide aerogel,240 gold,241 copper nanowires (CuNWs),242 polystyrene sulfonate (PEDOT:PSS),243 MXenes,244 and silver nanowires (AgNWs)70, 245 for the fabrication of low heat-capacity films in TA loudspeakers. Although the TA loudspeaker that uses nanomaterials as conductive fillers has significant advantages on sound performance, previously researched TA loudspeakers are vulnerable to external damage, which leads to a disconnection of the conductive network and a reduction in their mechanical durability and flexibility when they are used in wearable electronic devices.71, 72, 245, 246 In addition, transparent TA loudspeakers can offer better appearance and good aesthetic impression in wearable electronic devices.247 Fabricated transparent TA loudspeakers can be applied to smart watches, surfaces of windows, posters, and computer screens without damaging the aesthetic appearance of applied surface.248To apply the TA loudspeaker in wearable devices, the device should endure harsh outside environments to avoid device failure. Thus, developing TA loudspeakers that exhibit transparency, flexibility, long-term stability, outstanding durability, and self-healability is essential.
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Through the application of self-healing polymers, the TA loudspeaker can become fault- tolerant, durable, recyclable, and shape-persistent. Previously, several types of self-healing polymers based on different mechanisms such as covalent bonds (e.g., Diels-Alder reaction and disulfide bonds) and non-covalent bonds (e.g., hydrogen bonds, ionic bonds, π-π interaction, and host-guest interactions) have been demonstrated.249, 250 Recent development of self-healing polymers such as Diels-Alder polymer,66 hydrogen-bonding-based PDMS,251, 252 metal-ligand-based polymers,253 hydrogels,254-256 thiol-based polymer,257 and poly(urethane-hindered urea) (PUHU)258, 259 have enabled further applications to self-healable flexible devices such as capacitive touch sensors, electronic skins, or triboelectric nanogenerators, which maintain their functionality via self-healing processes. Moreover, each self-healing material exhibits pros and cons depending on its chain dynamics and modulus. Self- healing polymers with high modulus such as Diels-Alder polymers and PUHU require high temperature for self-healing. However, they have a solid polymer network and stronger covalent bond such as a C–
C bond in Diels-Alder polymer and a C–N bond in hindered urea bond. Li et al.66 demonstrated a healable capacitive touch screen based on Diels-Alder polymers, where re-establishment of Diels-Alder bonds enables strong bond reconstruction after the healing process. Hydrogels and thiol-based self- healing polymers with low modulus can be healed at a low temperature because of the great chain dynamics of low modulus polymers.260, 261 Self-healing polymers with low modulus is difficult to maintain its original performance. Due to their vulnerability to damages, low modulus limits their usage.
In addition, hydrogels are inapplicable under dry conditions and thiol-based polymers exhibit a yellowish color owing to the presence of sulfur, limiting their use in transparent devices under ambient conditions.
Therefore, in this study, we propose polyurethane bearing bulky urea bonds to address the drawbacks of the previous self-healing polymers because of their transparency in a film state, strong covalent bonding, and fast reaction kinetics between free isocyanate and bulky amine groups that resulted in the rapid healing of surface damages. Transparent and conductive AgNW networks are fabricated on the PUHU film by a simple solution-based process as a highly transparent and self-healing material. The dynamic bonds in PUHU can successfully heal the damages and the AgNW networks covering the PUHU substrate is reconnected under the conditions of 95 °C and 80% relative humidity (RH) within 5 min. The AgNW networks that are completely disconnected are reconnected by reconstructing their conducting pathways after carrying out the self-healing process for 5 min.
Additionally, the AgNW networks on the PUHU can be repetitively healed and their resistance is maintained after 1,000 taping tests and 10,000 bending cycles at a bending radius of less than 1.5 mm.
To prove this concept, a transparent, flexible, and self-healable TA loudspeaker has been proposed, which is based on the AgNW/PUHU self-healable electrodes exhibiting an excellent sound generation
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that is consistent with its initial sound pressure level (SPL). Our self-healable TA loudspeakers can be diversely applied in multifunctional, flexible, and smart electronic devices with improved convenience, appearance, and reusability owing to the excellent transparency, flexibility, and fault tolerance of AgNW/PUHU self-healable electrodes.
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