Graphene Based Tungsten/epoxy Composite Films: Design, Fabrication,and Backing Absorber Investigation

Tremendous efforts have been endeavored into the development of backing materials for ultrasonic transducer with suitable acoustic impedance, high attenuation and robustness. Typically, backing materials are composed of a polymer matrix such as epoxy or polyurethane, and inorganic fillers of metallic particles including tungsten, iron, copper, magnesium and aluminum. Epoxy is often used as the polymer matrix in backing material due to its good thermal stability, environmental resistance, and mechanical properties. However, absorption of ultrasound in this kind of polymer is relatively low. To improve the attenuation property of the epoxy composite, rubber modified or surface modified epoxy were used to scatter the acoustic energy. Whereas, the different surface chemistry attribute of inorganic fillers and polymer matrix can cause high interfacial energy, leading to the structural defects including bubbles and cracks, thus showing deteriorated acoustic performance. Therefore, to develop a strategy to solve the problem of interface energy mismatch for high-quality composite film,we designed special structured graphene based tungsten/epoxy composite films, and characterized their structure and sound absorption performance. We have put forward some probable sound absorption mechanism due to their structure, and have acquired a series of important and innovative research results.Graphene oxide consists of a hexagonal ring of carbon network having both sp2 and sp3 hybridized carbon atoms bearing hydroxyl and epoxide functional groups on basal planes, as well as carbonyl and carboxyl groups at the edges of the sheet. It is expected that the amphipathy and flexibility of graphene oxide, and hydrogen bonding interaction between graphene and epoxy might facilitate the co-assembly with W and epoxy. The designed tungsten/graphene oxide/epoxy composite was prepared by a layer-by-layer assembly method, it could help to solve the dispersion of fillers and the optimized content of tungsten was 60 wt.%. Graphene oxide layer acted as an energy converter that could consume sound effectively with high thermal conductivity and its unique thermoacoustic effect, the best content of graphene oxide was 3.5 wt.%. The out-layer epoxy(0.1 wt.%) was designed to prevent graphene oxide nanosheets peeling off from tungsten spheres and was beneficial to guaranteeing the independence and integrity of the composite structure. The impedance of the designed films was 5-6 MRayl, and the optimal attenuation of these films was 22 d B/cm·MHz.To improve the fabrication proceeding, we designed a new assembl ed structure of tungsten/epoxy/graphene/epoxy, the iner-layer epoxy acted as a buffer layer to anchor graphene oxide nanosheets on tungsten spheres and its optimal content was 3.5 wt.%. The designed film had a good flexibility, it was proved by Raman and FTIR spectrum that hydrogen bonds were between the hydroxyl and epoxide functional groups of graphene oxide and the functional residues of epoxy resin. The tighter coating structure improved the acoustic attenuation to 36.58 ± 0.2 d B/cm·MHz.Three dimensional graphene grown by chemical vapor deposition method were used as the skeleton to fabricate the tungsten/epoxy composite films. The enhanced sound absorption property was determined by graphene’s superhigh thermal and electric conductivity, and the films were fabricated by filling tungsten and epoxy composite into the prepared three dimensional graphene skeleton structure. To optimize the film’s fabrication proceeding, 1 ml ethonal and 20 μm tungsten and and 80 wt.% tungsten content were obtained, the best film’s impedance was 11.8 ± 0.1 MRayl and attenuation value was 38.86 ± 1.88 d B/cm·MHz.When centrifugal process was used to help the fabrication of three dimensional graphene based composite film, a sandwich structure with tungsten and epoxy composite ball which was wraped by band shaping epoxy and graphene skeleton was obtained. We could change the condition of solidification to prepare porous films and compact films. It could be proved that the ultrahigh attenuation of the films was derived from the special graphene skeleton structure by comparision of the centrifugal process assisted graphene oxide based films. The impedance of the optimized film was 13.05 ± 0.11 MRayl and the attenuation value reached 110.15 ± 1.23 d B/cm·MHz when 80 wt.% 41 μm tungsten spheres were filled.This paper has applied the two dimensional material graphene into the fabrication field of backing material innovatively, and the superior sound absorption performance made graphene based composite films have high application value.