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Researchers Demonstrate Wideband Metamaterial Absorber Made of 3D Printed Conductive Plastic

Architecture of the wideband MA

There’s been a lot of research into 3D printing metamaterials over the years – due to their unique properties, they’ve been used to make everything from headphones and heart valve models to door locks and acoustic holograms, and maybe someday even our very own invisibility cloaks. But ten years ago, the metamaterial absorber (MA), a type of metamaterial with compact size and thin configuration meant to efficiently absorb electromagnetic radiation, was presented for the first time. Since that time, there have been numerous other MAs, including dual-, triple-, and multiband varieties and the wideband MA.

Because of its high absorption, wideband MAs are highly sought after for applications in sensing, nondestructive detection, and imaging. There are a few ways to increase the absorption bandwidth for wideband MAs, but it’s still tough to manufacture them.

Unit cell of the MA: (a) perspective; (b) layout

A collaborative team of researchers from China’s Hefei University of Technology, the Beijing University of Chemical Technology, and Space Star Technology Co. Ltd. recently published a paper, titled “Wideband Metamaterial Absorbers Based on Conductive Plastic with Additive Manufacturing Technology,” that explains their development of a wideband MA based on 3D printed conductive plastic.

They believe that their new method is the first ever demonstration of a 3D printed wideband MA.

The abstract reads, “This paper proposes a wideband and polarization-insensitive metamaterial absorber (MA) based on tractable conductive plastic, which is compatible with an additive manufacturing technology. We provide the design, fabrication, and measurement result of the proposed absorber and investigate its absorption principle. The performance characteristics of the structure are demonstrated numerically and experimentally. The simulation results indicate that the absorption of this absorber is greater than 90% in the frequency range of 16.3−54.3 GHz, corresponding to the relative absorption bandwidth of 108%, where a high absorption rate is achieved. Most importantly, this additive manufactured structure provides a new way for the design and fabrication of wideband MAs.”

3D printing offers low cost, high efficiency, and convenience, but when it comes to making wideband MAs with the technology, it does lack an appropriately stable and tractable high-resistive film, as the typical materials used for this don’t work with 3D printing. But, the team thought that the absorption bandwidth of the MA could be increased by using highly conductive plastic.

Photographs of the fabrication process: (a) 3D printing PLA material layer; (b) fabricated sample

“The proposed structure provides new opportunities for the design and fabrication of wideband MAs,” the researchers wrote.

The team’s proposed wideband MA is made of a patterned conductive plastic layer embedded in a layer of PLA, the bottom of which is covered with a copper ground film.

“First, a PLA layer with grooves is 3D printed,” the researchers wrote. “Next, the patterned conductive plastics were placed in these grooves, and then the PLA is continually printed above the patterned plastics to seal them. Finally, copper is pasted on the bottom surface of the PLA layer.”

Once they verified that the MA would work, they tested its absorption spectrum, which is greater than 90% from 16.3 to 54.3 GHz. The absorber has a thin thickness and high absorptivity, along with polarization insensitivity. The researchers used numerical simulations of the absorber to demonstrate its mechanism, efficiency, and the surface loss for both the copper ground layer and conductive plastic layer, the latter of which “contributes most power absorption of the absorber for both resonant modes.”

The researchers explained, “Hence, the conductive plastic layer plays an important role in the wideband absorption.”

Measurement setup.

The design was verified in a free space experiment, and the researchers used two horn antennas, connected to a network analyzer, measured the sample’s performance charactertistics in the 18−40 GHz frequency range. This showed that their MA design achieved “a good agreement between the simulated and measured results.”

The research team showed that they could save money and simplify things by 3D printing an effective, high-performance wideband MA based on conductive plastic. Their design strategy also made the 3D printed structure insensitive to wave polarization.

 

 

 

“This study is expected to reveal the potential applications of additive manufacturing technology in the realization of wideband electromagnetic wave absorbers,” the researchers concluded.

Co-authors of the paper are Yujiao Lu, Baihong Chi, Dayong Liu, Sheng Gao, Peng Gao, Yao Huang, Jun Yang, Zhiping Yin, and Guangsheng Deng.

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Could Your Next Headphones Use 3D Printed Metamaterials?

We’ve often seen 3D printing used to fabricate headphones and earbuds. But a team of researchers from the Department of Electrical and Electronic Engineering in the University of Strathclyde in Glasgow are taking this idea to the next level with acoustic metamaterials. Metamaterials, which can morph according to their environment, make up a new class of finely-engineered surfaces that can perform nature-defying tasks like 3D printing holograms and shaping sound waves. Acoustic metamaterials have the unique ability to attenuate sound by breaking the mass-density law, due to properties such as negative effective density and bulk modulus – no easy feat when it comes to small devices since thin walls are easily penetrated by acoustic waves.

The researchers, who are all affiliated in various capacities with the university’s Centre for Ultrasonic Engineering (CUE), recently published a paper, titled “Enhancing the Sound Absorption of Small-Scale 3D Printed Acoustic Metamaterials Based on Helmholtz Resonators,” detailing how their SLA 3D printed acoustic metametarials, based on Helmholtz resonators, can be used for small-scale sound absorption applications.

The abstract reads, “The directional response due to the position of the acoustic source on the sound attenuation provided by the metamaterial is investigated by controlling the location of a loudspeaker with a robot arm. To enhance and broaden the absorption bands, structural modifications are added such that overtones are tuned to selected frequencies, and membranes are included at the base of the resonators. This design is made possible by innovative 3D printing techniques based on stereolithography and on the use of specific UV-curable resins. These results show that these designs could be used for sound control in small-scale electroacoustic devices and sensors.”

To cut a material’s sound transmission by half, you have to double its acoustic frequency, density, or thickness. When an acoustic metamaterial has negative parameters, stop bands will form where the sound is deeply attenuated at certain frequencies.

“Acoustic metamaterials can break the mass-density law by exploiting the stop bands formed in the proximity of the resonance frequencies of their unit cells,” the researchers explain. “These material structures are often based on Helmholtz resonators and membranes. The frequency band that is attenuated by using these kind of unit cells is nevertheless narrow, hence solutions such as coupling of multiple resonances and leveraging the losses of the materials are generally used to make the attenuation broadband.”

Kuka robotic arm configuration.

The team’s paper presents a basic design of these small-scale, sub-wavelength 3D printed acoustic metamaterials, which use Helmholtz resonators arrays to generate stop bands where sound attenuation increases with the number of unit cells. A loudspeaker, guided by a KUKA robot arm through a quarter-hemisphere trajectory, illustrated that an absorption band forms “for every angle of incidence of the impinging sound wave.” A reference microphone was used to measure sound transmission in the air, while the transmission above the sample was measured by a second microphone.

The paper also explains two methods of enhancing the stop band, the first of which requires the resonators’ overtones to be tuned more closely to the fundamental frequency; this causes the band to grow wider. The second method involves printing membranes at the resonators’ base.

“In this work a novel fabrication technique is used, where thin membranes are fabricated inside Helmholtz resonators, the two units consisting of different materials,” the researchers wrote. “The presented manufacturing technique could result in rapid prototyping of metamaterials and contribute to the advancement of this field into the industrial environment. Simple physical models of the presented metamaterials are included in each section, nevertheless this work is mostly based on experimental results and aims at developing metamaterials that could be included in real devices. Potential applications of this work include noise cancellation for devices such as headphones, hearing aids and other sensors.”

Work is continuing to advance acoustic metamaterials in applications like acoustic cloaking, sound focusing and waveguiding, and imaging and computation, but the results are not often integrated into functional devices. But if studies like this one by the University of Strathclyde researchers can be validated, we could be looking at vastly improved headphones in the near future.


Co-authors of the paper are IEEE Student Member Cecilia Casarini, Benjamin Tiller, Carmelo Mineo, Charles N. MacLeod, IEEE Senior Member Professor James F. C. Windmill, and Joseph C. Jackson.

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