Underwater sound absorption properties of polydimethylsiloxane/carbon nanotube composites with steel plate backing
Introduction
Over the last decades, noise has become a serious issue in both air and water. For the latter, the Maritime Strategy Framework Directive adopted in 2008 by the European Union required that each participating member ensures a good marine environment including controlling underwater noise due to its increasing impact by sea transportation [1]. In October 2012, the European Union embarked on the development of the project Achieve QUieter Oceans (AQUO) [2]. One recommendation to reduce underwater noise is the development of materials possessing good underwater sound absorption properties. These materials could then be effectively utilized for civilian maritime systems such as commercial ships and marine platforms. Underwater sound absorption materials are also of interest for stealth marine vessels. This can be achieved by reducing the acoustic reflection from the hull through absorbing the incoming waves from sonar systems [3]. Traditional underwater absorption materials such as rubbers with periodic air pockets have been shown to be able to absorb noise at broad frequency whilst being subjected at low frequency and high pressure [4], [5].
With the development of nanotechnology, nanofillers have been introduced into sound absorption materials [6], [7], [8], especially for applications in air. It is found that carbon nanotubes (CNTs) can improve sound absorption performance especially at low frequency. Verdejo et al. [9] examined polyurethane foams filled with various amounts of oxidized multiwall carbon nanotubes (MWCNTs) from 0.01 to 0.2 wt%. It was concluded that the inclusion of 0.1 wt% MWCNTs significantly increased the sound absorption coefficient at low frequency. Basirjafari et al. [10] fabricated composites of the poly(ether)urethane foam modified by MWCNT-COOH. Up to 14% increase in the average sound absorption coefficient was observed at low loading of 0.05 wt%. Willemsen et al. [11] confirmed that polymeric foams infused with MWCNTs exhibited an increased ratio of sound absorption to density, depending on the weight fraction of carbon nanotubes dispersed within the foam. It was also found that the polyvinylidene fluoride/CNTs membrane had high piezoelectricity which made acoustic foam an efficient sound absorber because of its favorable absorption performance, particularly in the low and middle-frequency regions [12]. However, the sound absorption mechanisms of CNTs are still unclear. Ayub et al. [13] used molecular simulation to find that the acoustic absorption was associated with molecular interactions between acoustic waves and nanomaterials, which were only applicable at very high frequency (in GHz). They also found that a 3 mm thick forest of CNTs can provide up to 10% acoustic absorption within the frequency range 125 Hz-4000 Hz [14]. Currently, most of existing studies were carried out in air, and only few studies were dedicated to investigate the acoustic properties of CNTs for underwater applications [15], [16].
As aforementioned, sound absorption materials in air are usually made out of foams. Different from sound absorption materials in air, rubbers are the common underwater sound absorption materials because of the different acoustic impedances of water and air. PDMS is the most widely used silicon-based rubber because of its low cost, water repellence, durability against strong acid and strong alkali, and repeated machine washes [17]. Also, PDMS is acoustically transparent [18], which means that PDMS has acoustic impedance matched with water. These properties make PDMS suitable for underwater acoustic applications [19], and many studies have chosen PDMS as the matrix material. Sharma et al. [20], [21] investigated sound absorption in water by embedding cylindrical voids in the PDMS rubber. This design showed broadband attenuation of sound. Leroy et al [22], [23] studied the sound absorption property of air bubbles. It is proved that acoustic super-absorption can be achieved by using a meta-screen based on a periodic arrangement of bubbles embedded in the PDMS matrix. Calvo et al. [24] examined the role of disk-shaped air cavities in single and multiple layers of PDMS. An 18 dB transmission loss was achieved. However, to the authors’ knowledge, no publication was dedicated to the investigation of the underwater acoustic properties of MWCNT-COOH modified PDMS.
The aim of this work is to design advanced underwater sound absorption nanocomposites. In this paper, PDMS is adopted as the matrix material, MWCNT-COOH as the nanofiller, and a surfactant is utilized as the dispersant. In order to gain an understanding of the underwater sound absorption, material characterizations of the morphology, compressibility and dynamic mechanical analysis are investigated. The underwater sound absorption coefficient is examined by a designed water-filled impedance tube. Effects of different compositions, concentrations of MWCNT-COOH and hydrostatic pressures on the underwater sound absorption performance are discussed.
Section snippets
Materials and sample fabrication
PDMS was obtained from a commercial supplier (Dow Corning Sylgard 184, USA). MWCNT-COOH was purchased from the US Research Nanomaterials, Inc. of which functionalizing MWCNTs with the carboxylic acid groups enhances both dispersibility and integration of MWCNTs with the material matrix [25], [26]. Specifications from the supplier indicate that the outer diameter and length of MWCNT-COOH are in the range of 10–20 nm and 10–30 µm, respectively. Dispersion-aiding additives based upon solvent-free
Morphological study of PDMS nanocomposites
As described in Section 2.2, the fractured samples were examined using SEM. In Fig. 3, pure PDMS sample without the presence any fillers or cavities can be seen to have a smooth texture. By adding 2 wt% surfactant, many micro-voids were created in the sample of PS as shown in Fig. 3(b). For the PM sample with 2 wt% MWCNT-COOH, the sample has a rough texture with inclusion of agglomerates or clusters of MWCNT-COOH (see Fig. 3(c)). By adding surfactant (2 wt%) in the mixing process of MWCNT-COOH,
Acoustic analysis results
The effects of compositions, MWCNT-COOH concentrations and pressures on the underwater acoustic properties are investigated below.
Discussion
A water-filled impedance tube is an essential facility and designing such a rig possesses significant challenges when compared a conventional air-filled impedance tube. Firstly, air could be easily trapped in the clearance gap (between sample and wall) and around the hydrophones, which can introduce acoustic energy loss and alter the boundary conditions at the tube wall as water fills the tube. To alleviate the problem, the use of distilled water over tap water is preferred and is poured into
Conclusion
In this study, the underwater sound absorption properties of PDMS/MWCNT-COOH nanocomposites have been investigated in the frequency range of 1500 Hz to 7000 Hz under different hydrostatic pressures. It was found that the combination of MWCNT-COOH and the surfactant contributed to a significant enhancement of the underwater sound absorption performance at 0 MPa. The surfactant being introduced into the PDMS not only improved the distribution of MWCNT-COOH but also introduced air voids in the
CRediT authorship contribution statement
Yifeng Fu: Conceptualization, Methodology, Software, Formal analysis, Writing - original draft. Jeoffrey Fischer: Data curation, Software, Writing - review & editing. Kaiqi P an: Validation, Data curation. Guan Heng Yeoh: Resources, Supervision, Writing - review & editing. Zhongxiao Peng: Resources, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors wish to acknowledge access to the characterization facilities of the Australian Microscopy & Microanalysis Research Facilities (AMMRF) node at UNSW Sydney. The authors also wish to acknowledge the scholarship support from the China Scholarship Council (Grant No. 201606950016).
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