(Invited) Graphene Oxide-Based Membranes: Viscoelastic Properties and Application to Broadband Microspeakers

Abstract

Graphene and graphene derivatives have attracted attention as materials for acoustic transduction [1-3]. Graphene oxide (GO) membranes and borate cross-linked graphene oxide (X-GO) membranes combine high stiffness, low mass density, and high loss coefficient. We show [4] that these mechanical properties are ideal for efficient, broadband, electro-acoustic transduction, where the acoustic diaphragm should be light, stiff, and internally damped. We present a quantitative comparison of the figure of merit, (E/ρ 3)1/2, where E is Young's modulus and ρ is the mass density, and the material performance index, tan δ (E/ρ 3)1/2, where tan δ is the loss coefficient. GO and X-GO exhibit high figures of merit, comparable to that of wood and exceeding that of aluminum and thermoplastics. The material performance indices of GO and X-GO exceed that of wood, thermoplastics, and aluminum. The latter are common diaphragm materials in acoustic transducers.For acoustic applications, it is important to understand the frequency dependent viscoelastic response. The complex modulus G = G' + iG'' of GO and X-GO membranes was measured by dynamic mechanical analysis and analyzed by the application of the time-temperature superposition (TTS) principle of polymer rheology. We find that X-GO shows a more than 30% increase in storage modulus G’ over the frequency range of 1 to 10 kHz, exceeding 55 GPa at 10 kHz. There is a reduction in loss coefficient tan δ = G''/G' for X-GO of ≈30% over the same frequency range, reaching tan δ = 0.04 at 10 kHz, which is approximately one order of magnitude larger than that of aluminum.A quantitative comparison of microspeaker performance was conducted at a fixed diaphragm mass m = 15-20 g and 14 mm x 8 mm size, including GO, X-GO, oak wood, polyethylene terephthalate (PET), aluminum, and titanium. Scanning laser measurements confirm pistonic operation of X-GO membranes. Microspeaker sound pressure level response was measured in both time and frequency domains. X-GO diaphragms exhibit 45% higher damping than aluminum membranes in microspeaker assemblies. Consequently, X-GO membranes enable the upshift of loudspeaker breakup frequency by 1/6 to 1/2 octave above speakers assembled with oak wood, PET, aluminum and titanium membranes. GO based materials are thus found to be an exceptional material for electro-acoustic transduction.