Abstract Film-type shape-configurable speakers with tunable sound directivity are in high demand for wearable electronics. Flexible, thin thermoacoustic (TA) loudspeakers—which are free from bulky vibrating diaphragms—show promise in this regard. However, configuring thin TA loudspeakers into arbitrary shapes is challenging because of their low sound pressure level (SPL) under mechanical deformations and low conformability to other surfaces. By carefully controlling the heat capacity per unit area and thermal effusivity of an MXene conductor and substrates, respectively, it fabricates an ultrathin MXene-based TA loudspeaker exhibiting high SPL output (74.5 dB at 15 kHz) and stable sound performance for 14 days. Loudspeakers with the parylene substrate, whose thickness is less than the thermal penetration depth, generated bidirectional and deformation-independent sound in bent, twisted, cylindrical, and stretched-kirigami configurations. Furthermore, it constructs parabolic and spherical versions of ultrathin, large-area (20 cm × 20 cm) MXene-based TA loudspeakers, which display sound-focusing and 3D omnidirectional-sound-generating attributes, respectively.

Professor Hyunhyub Ko and his research team in the School of Energy and Chemical Engineering at UNIST, in collaboration with Dr. Ki-Seok An's team at the Korea Research Institute of Chemical Technology (KRICT), has led a groundbreaking research effort in the development of film-type shape-configurable speakers. These speakers, based on the unique properties of MXene, offer tunable sound directivity and hold immense promise for the rapidly growing field of wearable electronics.

Figure 1. A shape-configurable MXene-based TA loudspeaker with directivity-tunable sound generation. a) Schematic illustrating the working mechanism of the flexible MXene-based TA loudspeaker. b) Conceptual sound-pressure/distance plots and sound directivities of unidirectional thick (top) and bidirectional ultrathin substrates (middle), and schematics depicting 3D sound distributions of differently configured ultrathin TA loudspeakers (cylindrical, uniaxial kirigami, parabolic, and spherical) (bottom). c) Conceptual plots of intrinsic variables (HCPUA and thermal effusivity [top]) and extrinsic variables (input power and distance [bottom]). d) Photographs showing an MXene-based TA loudspeaker with an ultrathin (0.8-µm-thick) parylene substrate (left; scale bar, 2 cm); deformable and conformal contact with a fabric electrode having a line width of 150 µm (middle; scale bar, 500 µm); and a large-area (20 cm × 20 cm) specimen (right; scale bar, 5 cm).

Traditional loudspeakers with bulky vibrating diaphragms face limitations in integrating with wearable devices due to their bulky vibrating diaphragms. However, the team's ultra-thin thermoacoustic (TA) loudspeakers, which are free from such limitations, demonstrate remarkable potential in this domain. The challenge lies in configuring these speakers into arbitrary shapes due to their low sound pressure level (SPL) under mechanical deformations and limited conformability.

To overcome these challenges, the research team focused on controlling the heat capacity per unit area of the MXene conductor and the thermal effusivity of the substrates. Through this approach, they successfully developed an ultrathin MXene-based TA loudspeaker that exhibited a high SPL output of 74.5 dB at 15 kHz and maintained stable sound performance over a duration of 14 days.

Figure 2. Effects of substrate thickness on the performance of the MXene-based TA loudspeaker.

A key breakthrough of this research is the ability to generate bidirectional and deformation-independent sound in various bent, twisted, cylindrical, and stretched-kirigami configurations using the parylene substrate. The parylene substrate, with a thickness smaller than the thermal penetration depth, enabled the production of speakers capable of sound output in multiple directions, regardless of their shape.

In addition, the researchers extended their innovation to the creation of parabolic and spherical versions of ultrathin, large-area (20 cm × 20 cm) MXene-based TA loudspeakers. These speakers demonstrated sound-focusing and 3D omnidirectional-sound-generating attributes, respectively, opening up new possibilities for immersive audio experiences.

Dr. Jinyoung Kim, co-first author of the study who is currently affiliated with the prestigious Georgia Institute of Technology, highlighted the exceptional performance of the MXene-based speakers. The combination of MXene's two-dimensional conductive nature and parylene's controllable thickness enables the production of thermophonic speakers with superior performance. The ultra-thin speaker production technique developed in this study has the potential to become a key technology for various wearable devices and shape-deformation-induced sound control.

Figure 3. Sound-focusing and omnidirectional-sound-generating capabilities of the 3D MXene-based TA loudspeakers.

Professor Ko emphasized the wide-ranging applications of this thermal sound speaker technology. He noted, "The ability to customize device structure design and achieve shape-shifting and directional-adjustable speakers opens up possibilities for portable and home audio systems, active noise control, flexible active displays, and immersive entertainment systems."

Supported by the National Research Foundation (NRF) of Korea and the National Research Council of Science & Technology (NST) grant by the Korea government (MSIT), the findings of this research were officially published in Advanced Materials on November 16, 2023.

Journal Reference
Jinyoung Kim, Geonyoung Jung, Seokhee Jung, et al., "Shape-Configurable MXene-Based Thermoacoustic Loudspeakers with Tunable Sound Directivity," Adv. Mater., (2023).