Hierarchical Assembly of Tungsten Spheres and Epoxy Composites in

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Hierarchical assembly of tungsten spheres and epoxy composites in three-dimensional graphene foam and its enhanced acoustic performance as a backing material Yunfeng Qiu, Jingjing Liu, Yue Lu, Rui Zhang, Wenwu Cao, and PingAn Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06024 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016

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ACS Applied Materials & Interfaces

Hierarchical assembly of tungsten spheres and epoxy composites in three-dimensional graphene foam and its enhanced acoustic performance as a backing material Yunfeng Qiu#a, Jingjing Liu#a, Yue Lua, Rui Zhangb*, Wenwu Caoc*, and PingAn Hua* a

State Key Laboratory of Robotics and System (HIT), Harbin, 150080, P.R. China.

b

Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin

150080, China. c

Department of Mathematics and Materials Research Institute, The Pennsylvania State

University, University Park, Pennsylvania 16802, USA. KEYWORDS：backing material, three-dimensional graphene, centrifugation-assisted method, acoustic impedance, acoustic attenuation.

Abstract: Backing materials play important role in enhancing acoustic performance of ultrasonic transducer. Most backing materials prepared by conventional methods are failed to show both high acoustic impedance and attenuation, which however determine the band width and axial resolution of acoustic transducer, respectively. In present work, taking advantage of the

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structural feature of 3D graphene foam as confined space for dense packing of tungsten spheres with the assistance of centrifugal force, desired structural requirement for high impedance is obtained. Meanwhile, superior thermal conductivity of graphene contributes to the acoustic attenuation via the conversion of acoustic waves to thermal energy. The tight contact between tungstate spheres, epoxy matrix, or graphene makes the acoustic wave depleted easily for the absence of air barrier. The as-prepared 3DG/W80 wt%/Epoxy film in 1 mm, prepared using ~41 µm W spheres in diameter, not only displays acoustic impedance of 13.05 ± 0.11 MRayl, but also illustrates acoustic attenuation of 110.15 ± 1.23 dB/cm·MHz. Additionally, the composite film exhibits high acoustic absorption coefficient, which is 94.4% at 1 MHz and 100% at 3 MHz, respectively. Present composite film outperforms most of the reported backing materials consisted of metal fillers/polymer blending in terms of acoustic impedance and attenuation.

Introduction Ultrasonic transducers can convert electrical signals to ultrasound or vice versa. There are four basic pressure waves generated by an impulse voltage applied on a piezoelectric core. Reflection of wave amplitude always occurred at the boundaries of the piezoelectric vibrator when it is isolated, leading to “pulse ringing” in a long period. Backing material is one of the major elements in a transducer configuration, and it should possess high acoustic attenuation to adsorb the backward travelling waves generated by piezoelectric core, namely, to minimize the energy returning to the ceramic. Based on this effect, the generation of an artificial echo through uncontrolled supplementary vibrations can be avoided. In order to further enhance the axial resolution of ultrasonic transducer, two critical parameters including wide frequency and narrow pulse should be paid more attention. For this purpose, backing material both with high acoustic


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attenuation and impedance properties can eliminate the internal reverberations of the ceramic after attachment on the back side of piezoelectric ceramic, which is the most effective way to acquire narrow pulse and wide bandwidth. As for acoustic attenuation of backing material, it needs high values in all cases. In contrast, the acoustic impedance of the backing materials can vary from ~3 MRayl to ~30 MRayl depending on the transducer design.1 Basically, the acoustic impedance of a backing material should match that of the piezoelectric core, favorable to eliminating the internal reverberation at the interface between ceramic and air or water. The widely used backing materials possess low acoustic impedance ranging from 2.5 to 3.5 MRay comparing to the piezoelectric core. Such ultrasound transducer has high transmit and receive sensitivities because less acoustic energy is absorbed into the backing layer. However, the lengthened transmit and receive pulses will greatly hinder the axial resolution of ultrasound transducer. Based on the basic principle of optimizing axial resolution, backing material with high acoustic impedance tends to absorb most of the ultrasonic energy from the piezoelectric core due to high boundary transmittance coefficient when the acoustic impedance of a backing material and piezoelectric core are matching. Some strategies including rubber modified2 or surface modified epoxy3 were utilized to enhance scattering of acoustic energy for better attenuation. Particularly epoxy is widely used to prepare backing material4-7 due to its good thermal stability,8 environmental resistance,9 and mechanical properties.10 It should be noted that structural defects including bubbles and cracks always exist in composite film due to unfavorable interfacial energy between inorganic fillers and polymer matrix,11-12 leading to deteriorated acoustic performance such as low acoustic attenuation and impedance. It is easier to fabricate backing materials with low acoustic


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impedance via blending polymer such as epoxy or polyurethane with inorganic fillers including tungsten, iron, copper, magnesium and aluminum particles.13-16 Such backing material also named light impedance backing is ideally used in medical ultrasound transducer for an optimum combination of broad band width, short pulse, and good sensitivity. However, heavy impedance backing (usually higher than 10 MRayl) is highly demanded in non-destructive testing ultrasound transducers, which requires high axial resolutions. To our knowledge, only few strategy on successful fabrication of heavy impedance backing were reported so far, such as porous 3 mol% Yttria-stabilized zirconia17 or hot pressing polymer and metallic fillers composite18. Taken together, dense packing of metal particles proves to be an effective way to enhance the acoustic impedance. In contrast, there are three possible mechanisms for acoustic attenuation, including diffusion attenuation, absorption attenuation, and scattering attenuation. If look more carefully, one may find that the design of a backing material is becoming complicated when taking into account the combination of high acoustic impedance and high acoustic attenuation properties. Furthermore, most the state-of-the-art means for the fabrication of backing materials failed to balance the competition of these two important features. Thus, it is still a great challenge to develop backing materials both possessing high acoustic attenuation and acoustic impedance for optimizing axial resolutions of ultrasound transducer. Graphene-based materials with superior properties, such as high mechanical strength, exceptional electrical transport and thermal conductivity, can provide considerable opportunities for the development of acoustic materials. In this regard, we have previously reported the application of graphene oxide as filler to enhance the acoustic attenuation of backing materials depending on its intriguing thermoacoustic effect.19 Graphene nanomaterials have also illustrated promising application in suppressing electromagnetic waves via polarization relaxation of


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defects or π-electron and synergetic effect with other absorbents20-21. Taking advantage of the thermal properties of graphene, it can act as a stationary heater to produce a time-dependent pressure variation via thermal expansion. Zettl’s group reported that an electrostatic graphene acoustic radio showed ideal equalized response up to ultrasonic region of 0.5 MHz,22-23 which can enable wideband transducers for both sound generation and reception. Although some papers have reported the application of graphene in acoustic field, it is still in an infant stage especially for backing material. Graphene can be manipulated into 3D structure via CVD method using nickel foam as template, serving as unique support for anchoring metal nanoparticles, or providing confined space for the assembly of building blocks in the pores, plus its continuously conductive network or thermal conductivities. The advantages of 3D graphene will make it possible to prepare backing materials with both high acoustic attenuation and high acoustic impedance.24-26 However, to the best of our knowledge the introduction of 3D graphene into backing materials is still unexploited for the purpose of improving acoustic performances. In present work, we successfully fabricated 3DG/W/epoxy composite film via centrifugationassisted method, which shows both superior acoustic attenuation and acoustic impedance. 3DG with few layered structure was synthesized via CVD method using nickel foam as template. Epoxy and tungstate microparticles are hierarchically co-assembled in 3DGF, which not only served as confined assembly space for dense packing of particles for high acoustic impedance, but also acted as an acoustically attenuating material due to porous structure and thermoacoustic effect. Acoustic test was carried out using pulse-echo overlap technique at a frequency of 9 MHz. The composite film in 1 mm displayed acoustic impedance of 13.05 ± 0.11 MRayl, as well as acoustic attenuation of 110.15 ± 1.23 dB/cm·MHz. Meanwhile, the composite film exhibited high acoustic absorption coefficient of 94.4% at 1 MHz and 100% at 3 MHz, respectively.


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Acoustic performances of graphene oxide or reduced graphene oxide/W/epoxy composite film prepared by centrifugation-assisted method, and 3DG/W/epoxy composite film fabricated by natural sedimentation are systematically performed as control groups. The designed 3D graphene based backing materials with such excellent acoustic properties can provide new clues for building ultrasonic transducer with optimal axial resolution. Experimental section Materials and methods Nickel foam was purchased from Aladdin Co. HPLC grade ethanol was bought from SigmaAldrich Co. The commercial epoxy resin (T31) and hardener (W93) were purchased from Aladdin Reagent Company. A stoichiometric resin/hardening ratio of 3:1 by weight was used based on the manufacturer’s information to prepare the epoxy film. Different sized of tungsten powders were ranging from 1-66 µm (99.99%, Aladdin) used as fillers in present work. The morphology of graphene and composite films was investigated by Scanning Electron Microscope (SEM) (SU800, Hitachi, 15 kV) and Transmission Electron Microscope (TEM) (G-20, FEITecnai, 100 kV). Raman spectra were obtained using a Horiba Xplore Raman system with 532 nm excitation. X-ray diffraction measurement was performed using X-ray diffractometer with Cu target (XRD, Empyrean, Panalytical, Netherlands, λ = 0.1542 nm). The thickness was measured by atomic force microscopy (Nanoscope IIIa Vecco). The stress-strain experiments were performed in a single–column testing instrument in tensile mode (Agilent Technologies T150UTM). The specimen was cut into dumbbell shape, and the two ends of it were fixed on a stage. All tensile tests were performed in a controlled force mode with a force ramp rate of 100 mN min-1.


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Preparation of epoxy coated 3DGF. Graphene on nickel foam was synthesized via a modified CVD method, detailed procedures can be found in our previous work.27 Basically, graphene/nickel foam was soaked in 5 wt.% epoxy ethanol solution, then dried at 45 oC for 20 min. This coating procedure was repeated triply to make sure the complete coverage of epoxy layer on graphene. 3D graphene foam (3DGF) stabilized by epoxy was obtained by etching away nickel foam in dilute HCl solution for 12 hours. Preparation of 3DG/W/epoxy or GO/W/epoxy composite films. Natural sedimentation method for the preparation of 3DG/W/epoxy composite film was performed according to the following procedure. W spheres was blending with epoxy according to the weight content, and diluted with different amount of ethanol under stirring. Vacuum force was used to remove the air bubbles in the blending mixture. 3DG/epoxy was cut into 1 × 1 cm piece and put on the bottom of a mould. The blending mixture was decanted into the mould, and cured at 70 oC for 4 h. The top surface of composite film was polished, and the thickness was controlled at 2 mm using vernier calipers. The centrifugation-assisted method was illustrated in Scheme 1. Centrifuge equipped with swing-out rotor was used to prepare 3DG/W/Epoxy composite film. Commercial centrifugal tubes were used as moulds. The bottom was filled with a small amount of epoxy, and a smooth surface was created for placing the film. 3DG/epoxy was cut into circular shape according to the inside diameter of the tubes, and put on the top surface of epoxy in the bottom. Epoxy was diluted with 1 mL ethanol and injected into the tubes, which was centrifuged at 2000 rpm for 10 min. After centrifugation, most W spheres were deposited at the bottom of the tubes. Ethanol was removed at 70 oC overnight. The black part containing W


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phase was separated by cutting machining and polished into desired thickness film using Vernier calipers. The other films with different W contents were fabricated in the similar way. Ultrasonic absorption measurement. The ultrasonic measurements were performed using two immersion-type broadband ultrasonic transducers (Panamatrix V358; Parametric, Waltham, MA) with a central frequency of 9 MHz. The sample was sandwiched between the two immersed transducers, as shown in Figure S1. The velocity and attenuation are derived from the difference of flight and amplitude between the sample and water.28 The attenuation is calculated by taking the transmission coefficients into account at the water-sample and sample-water interfaces,29 and the acoustic impedance of the sample was obtained by the product of the velocity and density.30 The density of each sample was calculated by using the measured weight and volume, with the volume determined by the Archimedes method. Results and discussion 1. Morphology and compositions 3DG was synthesized according to our previous work.27 As illustrated in Scheme S1, ethanol solution serving as carbon source was bubbled into the furnace tube, and graphene growth took place on pretreated nickel foam surface at 1000 oC under Ar/H2 stream. SEM images in Figure S2g to i in ESI illustrated the 3D feature of graphene and layered structure at broken region. Layer number of graphene can be facilely controlled via the variation of H2 stream, as seen in Figure S3a. Double layered graphene determined from the I2D/IG intensity ratio was fabricated at Ar/H2 of 180/20 sccm. Figure S3b illustrated the height of folded graphene layer after etching


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away Ni support by AFM measurement, in which ~2.5 nm thickness of individual folded structure was observed (Figure S3c). It should be noted that the thickness of graphene layer is impossible to directly measure due to the flexible structure of mono- or few-layer graphene and corresponding spontaneous folding after losing the rigid support under ultrasonication. TEM image in Figure S4a is consistent with the observation in AFM. Selected area electron diffraction of one typical fiber indicated good crystalline structure of as-synthesized graphene, and mismatched diffraction points might be caused by multilayered graphene. XRD data in Figure S4c shows one strong peak at 2θ= 26.5

o

attributing to the (002) reflection of graphitic

carbon (JCPDS 75-1621). With the high quality 3DG in hand, we next prepare 3DG/W/Epoxy composite films according to the basic procedure in Scheme 1. Based on our previous experiences on GO/W/Epoxy based backing material, the mismatched interfacial energy among all components always causes structural defects such as cavity or bubble, leading to deteriorated acoustic impedance. Herein, 3DG was obtained after coating a thin layer of epoxy before etching away nickel foam, as shown in Figure S2j to l. As claimed in previous work, 3D graphene network containing even monolayer or double layer graphene can retain intact structure after etching away template. Freestanding pure 3DG can be also prepared after gentle removal of template in present work (Figure S2g). Whereas, considering the processability with epoxy and W sphere under vigorous centrifugation, the protection of epoxy for the integrity of 3DG is quite necessary, not mention to the favorable interface with epoxy matrix. Taking into account the high viscosity of epoxy and hardener mixture, different amounts of ethanol were added into the mixture to facilitate the movement of W spheres inside the 3D pores of 3DG. W spheres would settle gradually on the 3DG in 4 h due to its natural gravity. Figure S5a clearly illustrated the basic structure of


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composite film, wherein 3DG was surrounded by W spheres and epoxy. The hollow structure of 3DG was maintained, and there are some cavities at the interface of epoxy and graphene skeletons as indicated by red arrows in Figure S5b and c. The acoustic waves will be reverberated at the epoxy/air interface due to the mismatched acoustic impedance. Therefore, the acoustic waves can’t reach the surface of 3DG, namely, 3DG is unable to function as effective attenuating component due to the existence of air cavities. According to the structural requirement of a backing material, it is assumed that the 3DG/W/epoxy composite film fabricated by natural sedimentation might possess improved acoustic impedance due to the dense packing of W spheres, however acoustic attenuation property might be not satisfied because of the presence of air cavities. As a-proof-of-concept, 20 µm W spheres was loaded at 90 wt% in epoxy as test films. As shown in Figure 1a, acoustic signal intensity can be effectively suppressed by inserting composite film during the measurement of amplitude comparing to that in water. Figure 1b shows the magnitude spectra obtained from Fourier transforms of the received signals of Figure 1a using different test films or water. As expected, 3DG based composite films obviously displayed the decreased intensity in signal amplitude ranging from the whole frequency up to 12 MHz. Meanwhile, composite film prepared without ethanol showed higher amplitude at ~2 MHz, indicating relatively worse acoustic properties at lower frequency. In order to confirm the positive effect on acoustic properties via the introduction of ethanol during assembly, we prepared more than ten samples to test the changing trend of acoustic impedance and attenuation when adding different amounts of ethanol in Figure 1c and d. The statistical analysis results are summarized in Table S1 to illustrate the average values of all runs, the standard deviations, the minimum or maximum values of all parameters. As shown in Figure 1c, films prepared without ethanol exhibited


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average acoustic impedance of 10.49 ± 0.22 MRayl. In comparison, films prepared with 1 mL and 3 mL ethanol showed 9.57 ± 0.68 and 9.47 ± 0.64 MRayl, respectively, slightly lower than that of the film without using ethanol. Meanwhile, Figure 1d displayed that the acoustic attenuation values are 33.86 ± 3.56, 40.16 ± 4.66, 22.83 ± 3.17 dB/cm*MHz for the films prepared by adding 0 mL, 1 mL, and 3 mL ethanol, respectively. The acoustic absorption coefficients of films prepared by adding 0 mL, 1 mL, and 3 mL ethanol are all above 99.5% at 8 MHz (± 1 MHz). Overall, the addition of 1 mL ethanol could slightly decrease the acoustic impedance with ~8.8%, whereas it results in an increase of acoustic attenuation of 18.8% comparing to the control group without using ethanol. Meanwhile, average acoustic impedance and the acoustic attenuation decreased with ~10% and 33% when using 3 mL ethanol in comparison with the control group without ethanol. SEM images of the 3DG based composite film prepared using 3 mL ethanol at 70 oC are measured in Figure S6. Although the dense packing of W spheres and epoxy can be formed in the pores of 3DG, the air cavities obviously appeared on the surface of epoxy matrix in Figure S6b. According to the calculation equation of Z = ρ ∙ V, wherein Z, V and ρ were the acoustic impedance, velocity and density, respectively, air cavities could decrease the density of composite film leading to deteriorated impedance. Meanwhile, higher volume of ethanol above 1 mL might affect the cross-linking of epoxy, leading to the increased degree of crystallinity of epoxy due to longer aging time, thus causing the worse acoustic attenuation.1 Basically, higher crystallinity of polymers will result in relatively higher stress and lower strain. As shown in Figure S7, the failure strain of 3DG composite films prepared using 1 mL ethanol was close to 6.3%, which was slightly larger than that of composite films using 3 mL ethanol. In contrast, the stress of composite films using 3 mL ethanol is obviously higher than the case using


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1 mL ethanol. Considering all the factors such as the composition of the films, film thickness and size, test setting conditions are almost the same for both composite films, the relatively higher failure stress can be ascribable to the increase of crystallinity during longer aging time. Such phenomena have been confirmed in previous work, for instance, the acoustic loss in polymers is largely dependent on the cross-linkage, that is to say, an increase of crystallinity or cross-linkage of epoxy leads to decreasing attenuation. As a result, 1 mL ethanol was selected to perform the following studies based on the effect on both acoustic impedance and attenuation. Taking into account the brittleness of composite film with high W content and the cost of W spheres, W contents were evaluated using the above mentioned optimal conditions. Acoustic impedance and attenuation are two important judging parameters for the following evaluation. Two sets of experiments were carried out as a function of W contents from 70 to 90%. Figure S8a and b clearly proved that film in 3 mm containing 80 wt% W showed acoustic impedance of 11.8 ± 0.1 MRayl and acoustic attenuation of 38.86 ± 1.88 dB/cm·MHz, respectively, belonging to the group of heavy impedance and high attenuation backing materials. As illustrated in Figure S9a and b, the absorbance coefficients of composite films are 95% at 5 MHz in 3 mm thickness, and 99.96% at 3.5 MHz in 10 mm thickness, respectively. Although the composite film prepared by natural sedimentation showed improved acoustic impedance, attenuation, and absorbance coefficients comparing to most epoxy based backing material and even our previous film using GO as filler, the packing of W spheres are not dense enough due to relatively lower gravity during sedimentation for the requirement of acoustic impedance. Additionally, the air cavities were still existed between graphene and epoxy phase, hindered the acoustic wave transmitting towards graphene. Thus, structure of W/epoxy in the pores of 3DG should be further optimized for enhancing the acoustic properties in terms of acoustic impedance and attenuation.


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Centrifuge equipped with swing-out rotor was used to prepare 3DG/W/Epoxy composite film. Under 2000 rpm, W spheres was driven into the pores by the help of giant centrifugal force, and subsequently assembled during the cross-linking process of epoxy under different temperature. Ethanol can decrease the viscosity of epoxy, and facilitate the movement of W spheres inside the pores of 3DG. On the other hand, the excessive ethanol might evaporate slowly during the crosslinking process of epoxy, and leave some air cavities, which have been confirmed to hinder the acoustic impedance in terms of low density. For example, cross-sectional region of composite film aged at 45 oC was characterized by SEM in Figure S10. 3DG structure was intact after centrifugal treatment, which is important to serve as confined space for W spheres assembly. W spheres were densely packed inside the pores of 3DG, wherein the gaps between epoxy and graphene disappeared after centrifugal treatment. However, Figure S10b to d clearly showed some air cavities on the surface of epoxy coated on graphene, as well as in the W/epoxy mixing phase. As mentioned above, the air cavities could decrease the density of composite film, thus resulting in deteriorated impedance. Therefore, 70 oC was tested to age the assembly process to accelerate the evaporation speed of ethanol for the removal of air cavities. The SEM images of the smooth surface of epoxy phase confirmed the elimination of micro-scaled air cavities in Figure 2c. The randomly chopped surface of composite film in Figure 2e obviously illustrated the successful aging protocol for the preparation of composite without air cavities. Most importantly, the dense packing of W spheres with the assistance of epoxy in the pores of 3DG was successfully maintained as expected in Figure 2b and d, which is mainly formed during the centrifugal process. The function of higher aging temperature was mainly on fast removal of ethanol before the complete solidification of epoxy, and had negligible effect on the formation of dense packing of W/epoxy matrix.


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2. Acoustic performance evaluation. The centrifugation-assisted method can create optimal structure in composite film for the basic requirement of high acoustic impedance and attenuation. In the following part, we systematically evaluated the acoustic performance of composite film prepared by centrifugation-assisted method. Comparison study of 3DG/W/Epoxy composite films contain 80 wt% W spheres aged at 45 and 70 oC was plotted in Figure 3. Both acoustic impedance (Figure 3a) and attenuation (Figure 3b) values of 3DG/W/Epoxy70 are higher than those of 3DG/W/Epoxy45 using 1 to 66 µm W spheres, except the smallest and largest sizes. As discussed above, air cavities left in epoxy at 45 oC will decrease the density of composite film, and weaken the attenuative capacity for acoustic waves. Briefly, for the composite film prepared by centrifugation-assisted method, increasing the particle size can lead to increase the acoustic impedance to 15 MRal at 66 µm. According to previous results, the acoustic attenuation value can reach 110.15 ± 1.23 dB/cm·MHz when using 41 µm W sphere as filler, and the corresponding acoustic impedance is 13.05 ± 0.11 MRayl. Taken together, the acoustic impedance of the composite film prepared by centrifugation-assisted method was comparable to that of the composite film prepared by natural sedimentation method. However, the acoustic attenuation of the former film is about 2.4 times as large as that of the latter film. Pure tungsten possesses ~103 MRayl impedance. Pure epoxy only has ~2.5 MRayl impedance and ~19 dB/cm attenuation at 5 MHz. Lots of work on backing materials have confirmed that the increase of filler contents can effectively improve the density of composite film, thus enhance the acoustic impedance. Present composite films prepared by centrifugation-assisted method used high W content of 80 wt%, and took the advantage of centrifugal force to assist the dense packing of W spheres. When acoustic waves transmit inside the composite film, part of the


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waves will be transmitted through W particles and part of them will be reflected because of the impedance mismatch with its surrounding media of epoxy. Herein, 41 µm W spheres were utilized as the most effective fillers probably because it is close to the size of the acoustic wavelength (500 µm in epoxy) at 5 MHz, and accordingly multiple reflections or scattering might occur. Larger size of W spheres above 41µm gave rise to slightly worse acoustic performance in terms of attenuation might ascribe to the deficient acoustic wave transmit from W to its surrounding media. Inspired from previous work, there was always a decrease in attenuation with an increase of filler content, as confirmed in present experiments in Figure S8b. As compensation, 3DG was introduced into composite film as attenuating component due to its superior thermal conductivity. Furthermore, when the transmitting wave reached the surface of graphene, acoustic energy might cause the vibration of graphene network, namely, the acoustic energy transformed into thermal energy, resulting in effective acoustic attenuation. As displayed in Scheme 2, dense packing W spheres in the pores of 3DG not only provides the ideal structure for the high acoustic impedance, but also create more interfaces between W spheres and epoxy, thus making W spheres as strong scattering sites due to the large impedance mismatch between epoxy and W spheres. Overall, deep understanding on the dependence of acoustic performance of composite film will greatly rely on some numerical analysis of the structure or components in future. In fact, the selection of backing material really depends on the transducer design. It is reasonable that heavy impedance and light impedance are both important for practical application. In order to further prove the universal ability of our centrifugation-assisted method, we used readily available GO as filler to prepare the backing material.31 In our previous work using GO as filler, the acoustic impedance of W/GO/E films was ranging in 5-7 MRayl with the


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increasing GO content. The maximum attenuation value reached 36.58 ± 0.2 dB/cm·MHz at 9 MHz, and the absorption coefficient reaches 96.98 % for 2 mm sample. Herein, we found that composite film containing 80 wt% W sphere in diameter of 34 µm and 3.5 wt% GO illustrated acoustic impedance of 11.23 ± 0.56 MRayl, and acoustic attenuation of 41.01 ± 3.56 dB/cm·MHz in Figure S10. Meanwhile, GO is reduced to rGO using HI method.32 During reduction process, oxygen-containing groups are mostly reduced, namely, sp3 carbon are changed into sp2 carbon for the restoration of conductivity. Therefore, rGO as filler to fabricate composite film can be regarded as randomly oriented graphene as control group. As shown in Figure 3c, the acoustic attenuation properties of rGO based composite film prepared by centrifugation-assisted method are comparable to that of GO based composite film below the frequency of 6 MHz, and the attenuation values are slightly higher above 6 MHz. The slight difference might be due to the better thermal conductivity of rGO after the removal oxygencontaining groups in comparison with GO. Furthermore, 3.5 wt% rGO based composite film containing 80 wt% W sphere in diameter of 34 µm showed acoustic impedance of 11.93 ± 0.36 MRayl, and acoustic attenuation of 42.91 ± 4.67 dB/cm·MHz. Both results solidly confirmed that centrifugation-assisted method was superior to obtain high impedance while maintaining its attenuating ability. We finally compared the acoustic attenuation properties of GO, rGO or 3DG based film prepared by centrifugation-assisted method, and 3DG based film prepared by natural sedimentation method in Figure 3c, it really showed that centrifugation-assisted method can gave rise to the improved attenuation as a function of tested frequency, especially in the case of 3DG based composite film. 1 to 10 mm thickness 3DG/W/Epoxy composite films were prepared at optimal conditions, and corresponding absorption coefficients are shown in Figure 3d. Film in 1 mm can absorb 100% acoustic waves at 3 MHz.


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Based on survey of already published or unclassified information, we summarized the attenuation values and acoustic impedance of our work and previously reported metallic fillers/polymer (most at 5 MHz) in table 1. Basically, backing material with heavy acoustic impedance, namely above 10 MRayl, is seldom reported except Rønnekleiv’s work.35 Such high acoustic impedance was related to the film thickness, W content, or preparation method. Other groups including us were unable to get such high acoustic impedance only using W spheres and epoxy, which might be due to experimental setup or analysis methods. Overall, present 3DG/W/epoxy prepared by centrifugation-assisted method showed ~928 dB/cm attenuation loss, which is about 3.75 and 4.27 times as high as those of W/epoxy and W(Burps)/spurr epoxy, respectively. Other Al2O3 or Si based polymer composite films only showed unsatisfied attenuation values below 200 dB/cm. Some of the reported methods are relying on high temperature of 1100 oC sintering treatment, which hinder the development of practical application of backing materials. Briefly, our centrifugation-assisted method for the preparation of backing materials is more promising and processable taking into account the acoustic impedance and attenuation. Conclusions In summary, our results indicate that 3DG is promising to provide confined space for dense packing of W spheres and epoxy, and its acoustic performance is strongly dependent on both the hierarchical structure of composites and its components. Centrifugation-assisted method was effective to readily obtain 3DG/W/epoxy composite film, in which dense packing of W/epoxy was formed in the pores of 3DG showing improved acoustic impedance comparing to those of composite films prepared by natural sedimentation method or conventional blending method. More importantly, systematic study on acoustic attenuation confirmed that the 3DG based


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composite films present much higher ability to attenuate the acoustic waves via the effective scattering or refection on the dense packing W spheres and the fast conversion of acoustic waves to thermal energy on graphene mediator due to its unique thermal conductivity. In addition, GO as filler to prepare W based composite film via our centrifugation-assisted method exhibited much improved acoustic impedance and attenuation properties comparing to previous simple blending method, indicative of the university of our method for making backing material with high acoustic performance. Based on our finding, a simple and highly effective centrifugationmethod for the preparation of backing material with improved acoustic performance using 3DG as assembly inductor and attenuating agent. It is assumed that present work will facilitate the utilization of 3DG in combination of other metal fillers, which might develop some unexpected heave impedance or high attenuation properties. ASSOCIATED CONTENT Supporting

Information.

Electronic

Supplementary

Information

(ESI)

available:

[Experiments setup; CVD procedure; SEM images of nickel foam, nickel foam/graphene, 3DG, and 3DG/epoxy; I2D/IG ratio, Raman spectra, AFM image, and height profiles of 3DG; TEM image, selected area electron diffraction, Fourier transform, and XRD spectrum of 3DG; SEM image of 3DG/W/epoxy composite film; Statistical analysis; SEM images of composite film using 3 mL ethanol; Stress-strain curves; Acoustic impedance and attenuation of 3DG/W/epoxy composite films; Absorbance coefficients of 3DG/W/epoxy composite films prepared by natural sedimentation; SEM images of 3DG/W/epoxy composite films prepared by centrifugationassisted method; Acoustic impedance and attenuation of GO/W80 wt%/epoxy composite films prepared by centrifugation-assisted method.]. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (P. Hu), [email protected] (R. Zhang), [email protected] (W. Cao), Phone: +86 451 86403583, Fax: +86 451 86403583. Author Contributions Y.F.Q., L.J.J., and P.A.H. conceived the experiments, Y.F.Q. and L.J.J. designed and conducted the co-assembly, characterizations, acoustic experiments, Y.L prepared composite films. Y.F.Q., P.A.H., R.Z., and W.W.C., analyzed the results, Y.F.Q. and P.A.H. wrote the manuscript. All the authors reviewed and approved the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National key Basic Research Program of China (973 Program) under Grant No. 2013CB632900, National Natural Science Foundation of China (NSFC, 61390502, 21373068), Project supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No.51521003) and by Self-Planned Task (NO. SKLRS201607B) of State Key Laboratory of Robotics and System (HIT). Y. Qiu and J. Liu contributed equally to this work. REFERENCES


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1.

Page 20 of 37

Grewe, M. G.; Gururaja, T. R.; Shrout, T. R.; Newnham, R. E., Acoustic Properties of

Particle Polymer Composites for Ultrasonic Transducer Backing Applications. Ieee. T. Ultrason. Ferr. 1990, 37, 506-514. 2.

Nguyen, N. T.; Lethiecq, M.; Karlsson, B.; Patat, F., Highly Attenuative Rubber

Modified Epoxy for Ultrasonic Transducer Backing Applications. Ultrasonics 1996, 34, 669675. 3.

El-Tantawy, F.; Sung, Y. K., A Novel Ultrasonic Transducer Backing from Porous

Epoxy Resin-Titanium-Silane Coupling Agent and Plasticizer Composites. Mater. Lett. 2004, 58, 154-158. 4.

Kirk, K. J.; Schmarje, N., Experimental and Simulated Performance of Lithium Niobate

1-3 Piezocomposites for 2 MHz Non-Destructive Testing Applications. Ultrasonics 2013, 53, 185-190. 5.

Yuli, L.; Gin-Shin, C.; Hsin-Chih, L.; Yu-Cheng, L., Effect of Inter-Rod Coupling on 1-3

Piezocomposite High-Power Ultrasonic Transducers. Adv. Mater. Res. 2012, 476-478, 2105-8. 6.

Tiefensee, F.; Becker-Willinger, C.; Heppe, G.; Herbeck-Engel, P.; Jakob, A.,

Nanocomposite Cerium Oxide Polymer Matching Layers with Adjustable Acoustic Impedance between 4 Mrayl and 7 Mrayl. Ultrasonics 2010, 50, 363-366. 7.

Lionetto, F.; Maffezzoli, A., Polymer Characterization by Ultrasonic Wave Propagation.

Adv. Polym. Technol. 2008, 27, 63-73. 8.

Downey, M. A.; Drzal, L. T., Toughening of Aromatic Epoxy via Aliphatic Epoxy

Copolymers. Polymer 2014, 55, 6658-6663.


20

Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9.

Liu, T.; Nie, Y.; Zhang, L.; Chen, R.; Meng, Y.; Li, X., Dependence of Epoxy Toughness

on the Backbone Structure of Hyperbranched Polyether Modifiers. RSC Adv. 2015, 5, 34083416. 10.

Pan, L.; Lu, S.; Xiao, X.; He, Z.; Zeng, C.; Gao, J.; Yu, J., Enhanced Mechanical and

Thermal Properties of Epoxy with Hyperbranched Polyester Grafted Perylene Diimide. RSC Adv. 2015, 5, 3177-3186. 11.

Hussain, F.; Hojjati, M.; Okamoto, M.; Gorga, R. E., Review Article: Polymer-Matrix

Nanocomposites, Processing, Manufacturing, and Application: An Overview. J. Compos. Mater. 2006, 40, 1511-1575. 12.

Audoly, B., Asymptotic Study of the Interfacial Crack with Friction. J. Mech. Phys.

Solids 2000, 48, 1851-1864. 13.

Zhang, R.; Cao, W.; Zhou, Q.; Cha, J. H.; Shung, K. K.; Huang, Y., Acoustic Properties

of Alumina Colloidal/Polymer Nano-Composite Film on Silicon. Ieee. T. Ultrason. Ferr. 2007, 54, 467-469. 14.

Sugawara, K.; Nishihira, M.; Imano, K., Experimental Study of Acoustic Properties of

(0-3) Composite Materials for Intermediate Layer or Backing of Ultrasonic Transducers. JPN. J. Appl. Phys. 2005, 44, 4347-4349. 15.

Jain, S. K.; Gupta, R.; Chandra, S., Evaluation of Acoustical Characteristics of Ultrasonic

Transducer Backing Materials at High Hydrostatic Pressures. Ultrasonics 1998, 36, 37-40. 16.

Paciorek-Sadowska, J.; Borowicz, M.; Czuprynski, B.; Liszkowska, J., Use of Volcanic

Tuff for Production of Rigid Polyurethane-Polyisocyanurate Foams. Przem. Chem. 2016, 95, 4247.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

17.

Page 22 of 37

Amini, M. H.; Coyle, T. W.; Sinclair, T., Porous Ceramics as Backing Element for High-

Temperature Transducers. Ieee. T. Ultrason. Ferr. 2015, 62, 360-372. 18.

Wu, J.-C.; Cai, H.-H., New Technique for Manufacturing High Impedance Backing

Composites. Technical Acoustics 2008, 2, 017. 19.

Qiu, Y.; Liu, J.; Yang, H.; Gao, F.; Lu, Y.; Zhang, R.; Cao, W.; Hu, P., Graphene Oxide-

Stimulated Acoustic Attenuating Performance of Tungsten Based Epoxy Films. J. Mater. Chem. C 2015, 3, 10848-10855. 20.

Zhang, Y.; Huang, Y.; Zhang, T.; Chang, H.; Xiao, P.; Chen, H.; Huang, Z.; Chen, Y.,

Broadband and Tunable High‐Performance Microwave Absorption of an Ultralight and Highly Compressible Graphene Foam. Adv. Mater. 2015, 27, 2049-2053. 21.

Wang, X.; Qiu, Y.; Cao, W.; Hu, P., Highly Stretchable and Conductive Core–Sheath

Chemical Vapor Deposition Graphene Fibers and Their Applications in Safe Strain Sensors. Chem. Mater. 2015, 27, 6969-6975. 22.

Zhou, Q.; Zheng, J.; Onishi, S.; Crommie, M.; Zettl, A. K., Graphene Electrostatic

Microphone and Ultrasonic Radio. Proc. Natl. Acad. Sci. 2015, 112, 8942-8946. 23.

Zhou, Q.; Zettl, A., Electrostatic Graphene Loudspeaker. Appl. Phys. Lett. 2013, 102,

223109. 24.

Tian, H.; Xie, D.; Yang, Y.; Ren, T.-L.; Wang, Y.-F.; Zhou, C.-J.; Peng, P.-G.; Wang, L.-

G.; Liu, L.-T., Single-Layer Graphene Sound-Emitting Devices: Experiments and Modeling. Nanoscale 2012, 4, 2272-2277. 25.

Tian, H.; Ren, T.-L.; Xie, D.; Wang, Y.-F.; Zhou, C.-J.; Feng, T.-T.; Fu, D.; Yang, Y.;

Peng, P.-G.; Wang, L.-G.; Liu, L.-T., Graphene-on-Paper Sound Source Devices. ACS Nano 2011, 5, 4878-4885.


22

Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


26.

Tian, H.; Li, C.; Mohammad, M. A.; Cui, Y.-L.; Mi, W.-T.; Yang, Y.; Xie, D.; Ren, T.-

L., Graphene Earphones: Entertainment for Both Humans and Animals. ACS Nano 2014, 8, 5883-5890. 27.

Li, X.; Wang, Z.; Qiu, Y.; Pan, Q.; Hu, P., 3d Graphene/Zno Nanorods Composite

Networks as Supercapacitor Electrodes. J. Alloys Compd. 2015, 620, 31-37. 28.

Cobus, L. A.; Ross, K. A.; Scanlon, M. G.; Page, J. H., Comparison of Ultrasonic

Velocities in Dispersive and Nondispersive Food Materials. J. Agric. Food Chem. 2007, 55, 8889-8895. 29.

Treiber, M.; Kim, J.-Y.; Jacobs, L. J.; Qu, J., Correction for Partial Reflection in

Ultrasonic Attenuation Measurements Using Contact Transducers. J. Accoust. Soc. Am. 2009, 125, 2946-2953. 30.

Yao, G.-J.; Cui, Z.-W.; Song, R.-L.; Wang, K.-X., An Ultrasonic Method for

Determination of Acoustic Impedance, Time of Flight, and Attenuation of a Thin LinearViscoelastic Layer. J. Appl. Phys. 2007, 102, 104903. 31.

Song, S. H.; Park, K. H.; Kim, B. H.; Choi, Y. W.; Jun, G. H.; Lee, D. J.; Kong, B. S.;

Paik, K. W.; Jeon, S., Enhanced Thermal Conductivity of Epoxy–Graphene Composites by Using Non-Oxidized Graphene Flakes with Non-Covalent Functionalization. Adv. Mater. 2013, 25, 732-737. 32.

Pei, S.; Zhao, J.; Du, J.; Ren, W.; Cheng, H.-M., Direct Reduction of Graphene Oxide

Films into Highly Conductive and Flexible Graphene Films by Hydrohalic Acids. Carbon 2010, 48, 4466-4474.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

33.

Page 24 of 37

El-Tantawy, F.; Aal, N. A.; El-Daly, A. A.; Abdel-Daiem, A. M.; Bakry, A.; Sung, Y. K.,

A New Phantom Model and Attenuation Backing from Epoxy Resin Nanosized HydroxyapatiteCarbon Black and Multifunctional Agent Composites. Mater. Lett. 2004, 58, 3388-3394. 34.

State, M.; Brands, P. J.; van de Vosse, F. N., Improving the Thermal Dimensional

Stability of Flexible Polymer Composite Backing Materials for Ultrasound Transducers. Ultrasonics 2010, 50, 458-466. 35.

Berg, S.; Rønnekleiv, A., Acoustic Backing in 3-D Integration of Cmut with Front-End

Electronics. Ieee. T. Ultrason. Ferr. 2012, 59, 1537-1549.


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Scheme 1. General procedure for the preparation of 3DG/W/Epoxy composite films. Step 1 to 3: CVD growth of 3DG using nickel foam as template. Step 4: Ethanol facilitates co-assembly of W and epoxy in the confined pores of 3DG. Step 5: Natural sedimentation assembly route. Step 6: Centrifugation-assisted assembly route.


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Figure 1. a) Received acoustic signal from 3DG/W90 wt%/epoxy composite films prepared with the addition of 0, 1, and 3 mL ethanol, respectively, and received acoustic signal in water as a control group. b) Fourier transforms of the received signals in a. c) Acoustic impedance and d) corresponding attenuation values of composite films prepared using 0, 1, and 3 mL ethanol.


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Figure 2. 3DG/W80 wt%/epoxy composite films prepared by centrifugation-assisted method in the presence of 1 mL ethanol at 70 oC. a) Preparation process of centrifugation and solidification. (i) Solid cylinder after the removal of plastic container. (ii) 3DG/W80 wt%/epoxy composite film after cutting and polishing. (iii) Schematic macrostructure of composite film, where red and green colors represent the W spheres and 3DG network, respectively. (iv) Schematic packing manner of W spheres inside the cavity of 3DG network. Low to high magnification SEM images on b, c) top surface and d, e) the interior of composite film. f) Cartoon drawing of dense packing of W/epoxy phase in the pores of 3DG. The cross-sectional surface was cut by doctor blade.


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Figure 3. Acoustic performance of 3DG/W80 wt%/epoxy composite films prepared by different methods. a) Comparison of acoustic impedance and b) acoustic attenuation (at 9 MHz) of composite films prepared at 45 (black curve) and 70 oC (red curve) by centrifugation-assisted method (CAM) as a function of W sphere sizes, respectively. c) Comparison of attenuation values of composite films prepared by CAS using 3DG (black curve), GO (green curve) and rGO (blue curve), and by natural sedimentation (NS) method using 3DG (red curve). d) Absorption coefficients of 3DG/W80 wt%/epoxy composite films prepared by CAM as a function of film thickness.


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Scheme 2. Attenuating pathway of acoustic waves in the hierarchical structure of dense packing of W spheres/epoxy in the pores of 3DG.


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Table 1. Comparisons of our 3DG/W80 wt%/epoxy composite films prepared by centrifugationassisted method with the reported metallic fillers/polymer film at the frequency of 5 MHz. Acoustic Acoustic Frequency attenuation impedance Components References (MHz) (dB/cm) (MRayl) W/epoxy

5

36

~7.2

1

W/polyurethane

5

94

~6.0

1

W/polyethene

5

68

~6.9

1

W/PMMA

5

41

~9.1

1

W/polycarbonate

5

34

~7.8

1

W(Burps)/spurr epoxy

5

217

~8.3

1

W/rubber modified epoxy

5

50

~8.7

2

Ti/Si/epoxy

5

50.34

~8.6

3

HAP-CB

5

60.76

~8.5

33

Stycast1265

5

85

~1.76

34

Polyurethane2305

5

12.5

~1.38

34

Al2O3/Stycast1265

5

159.9

~3.54

34

Al2O3/Polyurethane2305

5

85.6

~2.6

34

W/Polyurethane2350

5

162.5

~3.3

34

W/Al2O3/Polyurethane2350

5

154.2

~3.33

34

W/epoxy

30

247

~19.6

35

W/E/GO/E

5

170

~5.5

19

3DG/W80 wt%/epoxy

5

~928

~13.05

This work


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TOC


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Scheme 1 160x155mm (300 x 300 DPI)


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a) Received acoustic signal from 3DG/W90 wt%/epoxy composite films prepared with the addition of 0, 1, and 3 mL ethanol, respectively, and received acoustic signal in water as a control group. b) Fourier transforms of the received signals in a. c) Acoustic impedance and d) corresponding attenuation values of composite films prepared using 0, 1, and 3 mL ethanol. 165x131mm (300 x 300 DPI)



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Fig.2 165x122mm (300 x 300 DPI)


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Fig.3 160x121mm (300 x 300 DPI)



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Scheme 2 Attenuating pathway of acoustic waves in the hierarchical structure of dense packing of W spheres/epoxy in the pores of 3DG 160x127mm (300 x 300 DPI)


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TOC 254x190mm (96 x 96 DPI)


Hierarchical Assembly of Tungsten Spheres and Epoxy Composites in

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