Fire-Retardant-Functionalized MXene-Mulberry Paper for Lightweight Foldable Thermoacoustic Speakers

This work reports fire-retardant$\text{Ti}_{3} \mathrm{C}_{2} \mathrm{T}_{\mathrm{x}}$MXene-functionalized on-paper speakers for lightweight, foldable, and shape-configurable electronics. The speaker, without any moving components, converts electrical signals into periodic thermal oscillations, which induce acoustic pressure waves in air, referred to as thermoacoustic (TA) phenomena. Mulberry paper, a lightweight, porous, and chemical/thermal-resistant cellulose substrate, was dip-coated with$\text{Ti}_{3} \mathrm{C}_{2} \mathrm{T}_{\mathrm{x}}$MXene dispersion and subsequently spray-coated with ammonium dihydrogen phosphate$\left(\text{NH}_{4} \mathrm{H}_{2} \text{PO}_{4}\right)$to provide electrical conductivity and fire retardancy, respectively. These solution-based coatings allowed the speakers to minimize mass increase, thereby maintaining low heat capacity advantageous for efficient TA operation. Therefore, the speakers preserve the desired mechanical properties of paper, including lightweight, foldability and shape configurability (attachable to furniture), while emitting considerably high acoustic waves with$\sim 67 \text{dB}$sound pressure level (SPL) without burning and oxidation. Furthermore, practical applicability was validated by real-time music streaming playback, demonstrating stable sound reproduction.