3D Printing and Metalization Methodology for High Dielectric Resonator Waveguide Microwave Filters

Enrico Massoni, Matteo Guareschi,

Maurizio Bozzi, Luca Perregrini

University of Pavia, Dept. of Electrical, Computer and Biomedical Engineering,

Pavia, Italy

Umberto Anselmi Tamburini

University of Pavia, Dept. of Chemistry,

Pavia, Italy

Gianluca Alaimo, Stefania Marconi,

Ferdinando Auricchio

University of Pavia, Dept. of Civil Engineering

and Architecture, Pavia, Italy

Cristiano Tomassoni

University of Perugia, Dept. of Engineering,

Perugia, Italy

Abstract—This paper presents a novel approach to manufacture and metalize a standard waveguide microwave filter, with a high dielectric resonator inserted in it. As a first step, a set of 3D-printed ABS plastic filaments have been measured, to establish their electrical properties and suitability for microwave design. Subsequently, a new commercial silver conductive lacquer has been tested to metalize a standard hollow WR-90 waveguide, printed by Fused Deposition Modeling (FDM) technique. This lacquer furnishes acceptable conductivity and proper shielding of the electromagnetic fields inside the structure, operating among microwave frequencies. In parallel, a special thin pellet, chemically synthesized, has been investigated and fabricated. This small cylindrical rod behaves like a high dielectric resonator, that has been employed to design a highly selective waveguide microwave filter, paving the road to the implementation of a novel class of microwave components and systems.

Keywords—3D printing; filter; high dielectric resonator; metalization; silver lacquer; waveguide.

I. INTRODUCTION Additive manufacturing (3D printing) has already shown a

high potential for fast and inexpensive designing and prototyping of unconventional components and non-planar structures [1]-[3]. While many devices have been printed and tested, there has not yet been an extensive study on the plastic filaments available on the market for general purposes, applied to microwave use. Despite this consideration, a set of samples has been printed, in order to retrieve their electromagnetic characteristics. In this paper a commercial filament of Acrylonitrile Butadiene Styrene (ABS), with additive blue colorant, has been investigated and measured, providing dielectric permittivity constant εr and dielectric loss tangent factor tanδ, among microwave frequencies. Two methodologies have been adopted to gather those values: the former consists of a standard metallic WR-90 waveguide set-up, with block samples inserted in it, providing accurate but narrow-band results in X-band (from 8.2 GHz to 12.4 GHz). The latter uses two microstrip lines of different lengths, exhibiting values in a larger frequency span, nominally from 2 GHz to 20 GHz [4]-[7]. In parallel, a deep investigation

on metalization procedures has been set up. A commercial silver based lacquer has been tested and validated in this work, providing experimental results for a 3D-printed hollow rectangular waveguide operating in the X-band. Those considerations pave the way to the future realization of the conductive parts related to components and systems operating at microwave frequencies. In addition to this, a new chemically grown material has been synthesized to create a high dielectric resonator of cylindrical shape. This pellet will be employed for the design, simulation and experimental validation of a high selective waveguide filter, as a first technological demonstrator for a future class of microwave components with geometry similar to that in [8]-[9].

II. FABRICATION PROCESS The Fused Deposition Modeling (FDM) technique

represents one of the leading solutions adopted to assembly microwave devices and systems [4],[5]. This approach is perfectly suited for printing polymeric materials like Acrylonitrile Butadiene Styrene (ABS). In this work, a standard commercial ABS, with additional blue colorant, has been printed and measured, thus enabling to retrieve the electromagnetic characteristics of the material itself. For those reasons, two different methodologies have been adopted. The former consists of a standard rectangular WR-90 waveguide based set-up (w=22.86 mm and h=10.16 mm). From the half-wavelength resonances of the 3D-printed samples, with length L=20 mm, inserted in the waveguide, it is possible to retrieve the electromagnetic properties in the X-band (ranging from 8.2 GHz to 12.4 GHz). Results provided values of εr=2.80 for the dielectric permittivity constant and tan δ=0.008 for the dielectric loss tangent factor. The latter, instead, implements a microstrip line set-up, that uses two different microstrip lines of lengths L1=60 mm and L2=100 mm, respectively. This second solution allows for an electromagnetic characterization of the material on a larger bandwidth spanning from 2 GHz to 20 GHz, providing values of εr=2.80 and tan δ=0.009. The two methodologies furnished comparable values for the electromagnetic properties of the ABS filament employed, that will be used in the simulations presented in this work.

IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes (IMWS-AMP 2017), 20-22 September 2017, Pavia, Italy

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Fig. 1. Photograph of the high dielectric resonator pellet with dimensions of the radius r=4.4 mm and height h=1.7 mm.

In parallel, a large variety of different commercial conductive products has been investigated, with the aim of obtaining reasonable conductivity among microwave frequencies. Furthermore, when dealing with additive manufacturing techniques, it is also of crucial importance being able to metalize the final 3D-printed dielectric parts. Those reasons pushed the authors towards finding a good commercial product. The final candidate was a commercial silver based lacquer, provided by RS Components company. This lacquer requires to be deposited on the surfaces of interest of the printed device to be metalized. Subsequently, in order to favor the silver crystals to melt and create an effectively conductive layer, it is important to treat the cured device with an oven at a temperature of 100° Celsius for a time interval comprises between 10 and 15 minutes. Afterwards, this silver based paint requires to cool down for a time interval of 5 hours, in order to provide the surface resistance and the conductivity properties stated in the data sheet.

Simultaneously, a novel high dielectric resonator has been designed and created by the Department of Chemistry of the University of Pavia. It consists of a cylindrical small pellet of diameter d=8.8 mm and height h=1.7 mm, shown in Fig. 1. The chosen material is 80% of TiO2 plus 2% of CuO. Measured values provided εr=80 and tan δ=0.0002. This pellet will be used in the implementation of the filter presented in this work.

III. 3D-PRINTED HOLLOW RECTANGULAR WAVEGUIDE

This section is entirely dedicated to the design, simulation and experimental validation of a 3D-printed hollow rectangular waveguide, with L=50 mm, w=22.86 mm and h=10.16 mm, thus operating in the X-band, ranging from 8.2 GHz to 12.4 GHz. The manufactured prototypes are depicted in Fig. 2 (starting from left, upper view and through view before metalization and on the right side the device after the deposition of the silver based lacquer, ready for being measured). Moreover, it is notable to remark that the external skeleton of the hollow rectangular waveguide has been realized with the ABS filament via the FDM technology, with a nominal minimum thickness of 0.5 mm, thus maintaining a cut-off frequency similar to the standard classical WR-90. In addition to this, for measurement purposes and to avoid lateral electromagnetic waves leakage, a proper mechanically stable set-up has been arranged. In Fig. 3 depicts the photographs of the implemented measurement set-up system.

Fig. 2. Photographs of the 3D-printed hollow rectangular waveguide prototype (before and after metalizing the sidewalls), with relative physical lengths: L=50 mm, w=22.86 mm, h=10.16 mm.

Fig. 3. Photographs of the measurement set-up for the 3D-printed hollow rectangular waveguide prototype.

Fig. 4. Scattering parameters curves versus frequency of the 3D-printed hollow rectangular waveguide with sidewalls metalized by silver lacquer: |S11| simulated (black continuous line), |S11| measured (black dashed line), |S21| simulated (grey continuous line), and |S21| measured (grey dashed line).

With this solution, it has been possible to directly measure the prototype. The graph highlighted in Fig. 4 displays the results. The cut-off frequency of the fundamental mode is identified around f=6.7 GHz, like in the standard WR-90. Transmission (|S21| curve) is around -0.25 dB along the entire frequency band and reflection (|S11| curve) is maintained well below -10 dB allover the entire operative bandwidth. Moreover, at the exact center of the X-band, thus for f=10.3 GHz, the measured |S21| is -0.19 dB and the measured |S11| is -20.5 dB. Reported values comprise the effects of both the flanges used in the measurement set-up system.

IV. HIGH DIELECTRIC RESONATOR WAVEGUIDE FILTER

3D printing via FDM technology of ABS filaments, silver based lacquer and chemically grown high dielectric resonator pellets are together merged to design, simulate and validate a first prototype of a high dielectric resonator waveguide filter.

IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes (IMWS-AMP 2017), 20-22 September 2017, Pavia, Italy

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Fig. 5. Photograph of the 3D-printed hollow rectangular waveguide microwave filter (metalized on the left side and before metalization on the right side).

Fig. 6. Depiction of the 3D-printed hollow rectangular waveguide microwave filter with the high dielectric resonator pellet inserted in it (dimensions and physical parameters: w=22.86 mm, h=10.16 mm, L1=10.5 mm, L2=8 mm, v=9.2 mm and angle of 135°).

Fig. 7. Scattering parameters curves versus frequency of the 3D-printed hollow rectangular waveguide microwave filter with the high dielectric resonator inserted in it: |S11| simulated (black continuous line), |S11| measured (black dashed line), |S21| simulated (grey continuous line), and |S21| measured (grey dashed line).

Photographs of the printed prototypes (before and after metalization) are reported in Fig. 5. The structure itself (highlighted in Fig. 6) consists of two segments of length L1=10.5 mm, width w=22.86 mm and height h=10.16 mm, totally equal to the 3D-printed hollow rectangular waveguide discussed in Sec. III. Instead, the central portion, of width v=9.2 mm and length L2=8 mm, represents a waveguide segment under cut-off, in which inserting the high dielectric resonator pellet, rotated by an angle of 135°, as shown in the right portion of Fig. 6. The tab inside the central part will pick up the frequencies of the evanescent modes of the guide under cutoff and, by making them resonate inside it, will amplify them, allowing for the transmission of the electromagnetic waves through the entire device. Simulated and measured scattering parameters are reported in Fig. 7. Experimental results show the agreement between simulations and measurements. The simulated central frequency f0=7.17 GHz is verified by the measured one. Moreover, the quality factor of

this filter, evaluated at -3 dB points, provides simulated values of Qs=130, confirmed by the measured one of Qm=120. High selectivity is hereby performed resulting in a 0.83% fractional band, good feature for a high dielectric resonator waveguide filter, paving the way to more complex filtering structures operating at microwave frequencies.

V. CONCLUSIONS AND FUTURE PERSPECTIVES

This paper presented the electromagnetic characterization of a commercial filament of Acrylonitrile Butadiene Styrene (ABS), employed for the implementation of 3D-printed microwave devices via the Fused Deposition Modeling (FDM) technique. In addition to this, when dealing with additive manufacturing technologies, the possibility of metalizing complex structures provides a good improvement to realize microwave systems. A commercial silver based lacquer has been tested to metalize a 3D-printed hollow WR-90 waveguide, providing the proper shielding of the fields inside the structure itself. Scattering parameters curves have been measured and highlighted in this work. Moreover, a novel composite material pellet has been designed and chemically grown, to synthesize a small cilinder, that represents a high dielectric resonator. Exaclty this resonator, has been measured, retrieving its electromagnetic properties in order to design a high dielectric resonator waveguide microwave filter. Experimental results are hereby discussed, yielding the validation of the measurement with respect to the simulations. The structure is highly selective in term of percent bandwidth, due to the properties of the high dielectric resonator pellet and pave the road to the future implementation of promising microwave filters and systems.

REFERENCES

[1] E. MacDonald et al., “3D Printing for the Rapid Prototyping of Structural Electronics,” IEEE Access, Vol. 2, pp. 234-242, Dec. 2014.

[2] H. Lipson and M. Kurman, Fabricated: The New World of 3D Printing, John Wiley & Sons, 2013.

[3] J. Veres et al., “Additive manufacturing for electronics “Beyond Moore”," IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, USA, December 3–7, 2016.

[4] S. Moscato et al., “Additive Manufacturing of 3D Substrate Integrated Waveguide Components,” IET Electronics Letters, Vol. 51, No. 18, pp. 1426-1428, Sept. 2015.

[5] E. Massoni et al., “3D-Printed Substrate Integrated Slab Waveguide for Single-Mode Bandwidth Enhancement,” IEEE Microwave and Wireless Components Letters, Vol. 27, No. 6, pp. 536-538, June 2017.

[6] S. Moscato et al., “Infill Dependent 3D-Printed Material Based on NinjaFlex Filament for Antenna Applications,” IEEE Antennas and Wireless Propagation Letters, Vol. 15, No. 1, pp. 1506–1509, 2016.

[7] E. Massoni et al., “Characterization of 3D-Printed Dielectric Substrates with Different Infill for Microwave Applications,” IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (IMWS-AMP 2016), Chengdu, China, July 20-22, 2016.

[8] C. Tomassoni, S. Bastioli, and R. V. Snyder, “Compact Mixed-Mode Filter Based on TE101 Cavity Mode and TE01δ Dielectric Mode,” IEEE Transactions on Microwave Theory and Techniques, Vol. 64, No. 12, pp. 4434–4443, Dec. 2016.

[9] C. Tomassoni, S. Bastioli, and R. V. Snyder, “Propagating Waveguide Filters Using Dielectric Resonators,” IEEE Transactions on Microwave Theory and Techniques, Vol. 63, No. 12, pp. 4366–4375, Dec. 2015.

IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes (IMWS-AMP 2017), 20-22 September 2017, Pavia, Italy

978-1-5386-0480-9/17/$31.00 ©2017 IEEE

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