High Efficiency Thermoacoustic Loudspeaker Made with a Silica Aerogel Substrate

Abstract

The demonstration in 2008 of a thermoacoustic (TA) loudspeaker made of suspended carbon nanotubes (CNTs) [1] revived the interest in the TA transduction and its application in the audio field. Since then, many solutions have been proposed to improve the efficiency of such devices.Historically, the optimization of the TA loudspeaker technology focused primarily on the reduction of the heat capacity per unit area of the active film, widening the bandwidth of the TA transduction up to 100 kHz and above, [2][3][4][5] well beyond the requirements of the audio applications. So far, the research for efficient active films led to the development of TA loudspeaker made of suspended CNTs, [1,2] single-layer [6] and multilayer [7] graphene, reduced graphene oxide, [8] graphene oxide aerogels, [9] suspended metal wires such as aluminum [10] and gold, [4] Indium-Tin Oxide [5,11] (ITO), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), [12] random networks of metal nanowires such as silver [13] (AgNW) and copper [14] (CuNW), and nanometric metal films of titanium [3] and gold. [15] Recent studies [15] suggest that the TA loudspeaker technology could be further improved engineering the substrate (see Figure S1 in the Supporting Information). In fact, if fabricated using high thermal effusivity materials, such as glass or polymers, the substrate would drain most of the heat generated by the active film, reducing the heat injection into the air.Suspending the active film in air without any support would be the optimal solution, since all the heat generated by the active film would be injected into the air (excluding the losses due to radiative and convective effects). Indeed, devices based on CNTs [2] and metallic nanowires [4,10] fully suspended in air are already reported in literature. Nonetheless, many fabrication processes such as the spray coating, the vapor deposition, and the sputtering require a solid substrate to grow and support the active film. For devices fabricated with such techniques, the optimization of the substrate is extremely important in order to achieve high efficiency.Below the high frequency cutoff, the generated sound pressure is determined by the effusivity ratio. [15] To maximize the efficiency of the TA loudspeaker, the substrate material must have the lowest thermal effusivity possible. Figure 1 shows the effect of the thermal effusivity of the substrate on the theoretical sensitivity The extremely low thermal effusivity of the silica aerogel is exploited to develop a high efficiency thermoacoustic (TA) loudspeaker with solid substrate. The deposition of the electrically conductive, low heat capacity active film on the silica aerogel surface is achieved with both the spray coating of silver nanowires and the sputter coating of gold films. The uniform spray coating of the hydrophobic silica aerogel is enabled by a low pressure plasma treatment, which however impairs its robustness. The spray-coated samples prove to be fragile when subjected to elevated temperatures and thus not suita...

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Highly Transparent, Flexible, and Self-Healable Thermoacoustic Loudspeakers

“…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. − Typical speakers require space for the vibration of the diaphragm, which necessitates a complicated fabrication process and 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.…”

“…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 . Recently, there has been significant interest regarding the use of various conducting materials such as carbon nanotubes (CNTs), , graphene, − graphene oxide aerogels, gold, copper nanowires (CuNWs), polystyrene sulfonate (PEDOT:PSS), MXenes, and silver nanowires (AgNWs) , for the fabrication of low-heat-capacity films in TA loudspeakers. Although the TA loudspeaker that uses nanomaterials as conductive fillers has significant advantages in 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. ,,, In addition, transparent TA loudspeakers can offer a better appearance and a good esthetic impression in wearable electronic devices .…”