Arsanjani, Arash

Bartlett, Chad

Bösch, Wolfgang

Höft, Michael

Robins, Luke

Teschl, Reinhard

English

TUGraz OPEN Library

Additive Manufacturing of Non-homogenousDielectric Waveguide Structures and FiltersLuke RobinsInstitute of Microwave andPhotonic EngineeringGraz University of TechnologyGraz, Austrialrobins@tugraz.atReinhard TeschlInstitute of Microwave andPhotonic EngineeringGraz University of TechnologyGraz, Austriareinhard.teschl@tugraz.atArash ArsanjaniInstitute of Microwave andPhotonic EngineeringGraz University of TechnologyGraz, Austriaarash.arsanjani@tugraz.atWolfgang BöschInstitute of Microwave andPhotonic EngineeringGraz University of TechnologyGraz, Austriawbosch@tugraz.atChad BartlettInstitute of Electrical andInformation EngineeringUniversity of KielKiel, Germanychb@tf.uni-kiel.deMichael HöftInstitute of Electrical andInformation EngineeringUniversity of KielKiel, Germanymh@tf.uni-kiel.deAbstract—This paper investigates the use of 3D-printed alu-mina ceramics to achieve variable dielectric constants for novelmicrowave applications. An introduction to lithography-basedceramic manufacturing exhibits the novel process and materialproperties, while detailing printing techniques for achievinggradient and equivalent dielectric constants. Printed samples ofnon-homogeneous dielectrics are presented and briefly discussed.Potential end-use for microwave components are discussed withemphasis on future filter applications.Index Terms—Additive manufacturing, ceramics, dielectrics,dielectric materials, microwave filters, microwave propagation.I. INTRODUCTIONAdditive manufacturing (AM) has become an enablingtechnology in microwave engineering [1]. Components withcomplex and varying design geometries can be prototyped andfabricated quickly at low cost. Metals and plastics (with sub-sequent metal coating) are the most commonly used materialsfor 3D printed microwave components. Reference [2] providesa general comparison of different approaches of manufacturingwaveguides, including a discussion of metalization.High performance ceramics are also widely used to achieveminiaturized, yet low-loss microwave components. They havebeen applied to e.g. dielectric resonators [3] which can be usedto implement larger filter and antenna systems. Traditionally,the ceramics available come in fixed geometries and withpredetermined dielectric constants which have to be chosenupfront to suit the application and frequency range of opera-tion. These dielectrics are often used inside metal structuressuch as waveguides or cavities to implement a desired filteringfunction. Reference [4] provides a comprehensive look at thedifferent AM methods that are used for ceramics, as well ascomparisons of the material properties.As AM enables the manipulation of the geometry at everypoint in a structure with little additional complexity, this paperaims to exploit this characteristic to influence the dielectricconstant throughout structures for future applications in filters.II. THEORETICAL BACKGROUND AND DESIGNCONSIDERATIONSThe AM process made use of throughout this paper isLithography-based Ceramic Manufacturing (LCM). A detailedLithoz LCM process description is provided in [5]. However, itis important to specify some factors of the process and materialas they pose limits on designs and realizable structures, andthese formed part of the investigation areas in this paper. TheLithoz CeraFab 7500 was used for printing, some importantparameters are: Material- Alumina, εr=9.9, Min wall thick-ness= 100 µm, Max wall thickness = 5 mm, Mini layer height= 10 µm, XY pixel size = 40 µm.A. Variable infill dielectricThe core concept being explored in this paper is to makeuse of the ability of AM to dynamically vary the ratio ofdielectric medium to air using a lattice. This enables the print-ing of sections with varying dielectric constants throughoutthe structure. Changing the dielectric constant of a specificceramic has also been shown to be possible by creating non-homogeneous regions by introducing porosity into the finalmaterial. Varying the constant from 12 to 90 of a givenmaterial was successfully realized in [6], [7] without losingthe desirable low-loss characteristics. Through 3D printing,the same result can easily be achieved [8], [9]. A lattice-type structure can be designed to allow for the desired infillpercentage, and through this, a specified dielectric constantcan be obtained. Various models for predicting the dielectricThis manuscript is a reproduction of the accepted version of the paper with the full citation:L. Robins, A. Arsanjani, C. Bartlett, R. Teschl, W. Bösch and M. Höft, "Additive Manufacturing of Non-homogenous Dielectric Waveguide Structures and Filters," 2021 IEEE MTT-S International Microwave Filter Workshop (IMFW), 2021, pp. 326-328, doi: 10.1109/IMFW49589.2021.9642291.https://ieeexplore.ieee.org/document/9642291, Copyright © 2021, IEEEFig. 1: Theoretical effective dielectric constant range vs.porosity.constant of mixtures exist [10], with one of the most commonbeing the Maxwell-Garnett given in (1).εeff = εh + 3fεhεs − εhεs + 2εh − f(εs − εh), (1)where εs is the dielectric constant of the inclusion materialused – in this case air , εh is the host material dielectricconstant of alumina (εr = 9.9) introduced, and f is the fillratio. The models are generally bounded by the upper andlower Wiener bounds which help provide a general expectedrange.B. Dielectric WaveguideTraditional waveguides are hollow structures where thepropagation takes place with air as the dielectric. It is alsopossible to design these structures to use other mediums inplace of air. The higher the dielectric constant, the smaller thewaveguide becomes. The addition of the dielectric medium,however, introduces dielectric losses as the waves propagatethrough. Therefore, it is important to have low-loss dielectricsand take any additional losses introduced into account duringthe design process to meet a desired specification.III. FILTER CONCEPT WITH TEST AREASA stepped impedance waveguide filter was chosen as ademonstrator, as it makes use of the variable dielectric con-cept as well as the ability to be printed without supportingstructures. It consists of alternating sections of high- and low-impedance that correspond to equivalent circuit capacitancesand inductances. Fig. 2 shows the conceptional filter designedand provides four areas that can be validated in smaller stepsbefore being combined into a full filter. The four tested areasare discussed in further detail in this section.1) Porous structures for dielectric variability: The abilityto control the dielectric constant within the structureas well as from print-to-print needed to be verified.Porous cylinders, dielectric wedges with fixed latticesand fixed infill with varying lattice wedges were printedfor characterization. These are shown in Fig. 3.1 23 4Fig. 2: Conceptual prototype filter with numbered verificationareas.Fig. 3: Printed dielectric wedge samples 80-40% porosity (left)and 30% geometry variations (right).2) Dielectric Gradients: Additive manufacturing providesthe unique ability to implement gradients of materialwithout additional complexity. Cosine gradients from 80to 25% infill shown in Fig. 4 were successfully printedand sintered. The samples were measured to achievedielectric constant values of between 8 and 3.8. Thesecan later be incorporated into more advanced designs.3) Bulky Lattice Combinations: Final testing for thispaper was to see how well combinations of bulk materialand the lattice wedges could be printed and sintered.The difference in mass of the lattice structure and solidparts will cause them to sinter at differing rates; this cancause stresses or breakage and could require a redesign ifunstable. These parts shown in Fig. 4 were successfullyprinted and sintered with only minor visible tearing.4) Bulky Parts: As highlighted in design implications, amaximum wall thickness for the Hp360 should not beexceeded before sintering. This provides a challengefor dielectric waveguides or larger components as thedesigner will always be limited to being below that forone of the edges. Test prints as shown in Fig. 4 wereconducted to assess potential solutions.IV. DEMONSTRATOR FILTERFor implementation, a fourth-order stepped impedance filter,centered around 8.8 GHz with a return loss of better than20 dB, was designed. The filter was implemented usingcascaded impedance inverter sections such as in [11]. The nor-malised coupling coefficients, normalised impedance invertervalues, and related high and low impedance lengths(mm) areprovided in the table in Fig. 6. The εr1 value of the latticesection was chosen to be 3.8. This value is low enoughto provide the required large change in impedance betweenthe two sections, but not so low as to make lattice printingunnecessarily difficult. The prototype filter is shown in Fig. 5.Fig. 4: Printed gradient (top), bulky lattice combination (left)and bulky part (right).Fig. 5: Ceramic-based stepped impedance waveguide filterprototypes pre-metalization.A probe feed was integrated into the structure on both sides tofacilitate the RF measurements with SMA-connectors in thefuture. The simulated performance of the filter is shown inFig. 6. Here a dielectric loss tangent of 8×10−5 was assumed,and even though some idealized assumptions were made suchas no air gaps, no misalignment and perfect connections, theresults underline the potential of the approach provided thatthe fabrication is flawless.V. CONCLUSIONBased on theoretical limits, the paper presents practicalconsiderations for the use of ceramic-based AM to achievevariable dielectric constants in monolithic printed structures.A conceptual filter design provided test areas that were in-vestigated and presented. These then fed into the design fora stepped impedance filter prototype to verify the concept ofwhich an initial idealized performance was given.VI. ACKNOWLEDGEMENTThe authors thank Dominik Brouczek, Dr. Martin Schwen-tenwein and the rest of Lithoz for providing the equipment,materials and valuable technical experience enabling the workdetailed in this paper.Fig. 6: Simulated filter idealised performance with designvalues.“This project has received funding from the EuropeanUnion’s Horizon 2020 research and innovation pro-gramme under the Marie Skłodowska-Curie grant agreementNo 811232”.REFERENCES[1] C. Tomassoni, O. A. Peverini, G. Venanzoni, G. Addamo, F. Paonessaand G. Virone, ”3D Printing of Microwave and Millimeter-Wave Filters:Additive Manufacturing Technologies Applied in the Development ofHigh-Performance Filters with Novel Topologies,” in IEEE MicrowaveMagazine, vol. 21, no. 6, pp. 24-45, June 2020.[2] J. M. Jafferson, H. Vinu, and K. Sekaran, “A study of additive manufac-turing technologies and metallizing techniques for microwave waveguidecomponents,” Materials Today: Proceedings, Mar. 2021.[3] P. Boe, D. Miek, F. Kamrath and M. Hoft, ”Hybrid Inline TE/TMMode Dielectric Resonator Filters with Wide Spurious Free Rangeand Controllable Transmission Zeros,” 2020 50th European MicrowaveConference (EuMC), 2021, pp. 555-558,[4] J.-C. Wang, H. Dommati, and S.-J. Hsieh, “Review of additive manu-facturing methods for high-performance ceramic materials,” The Inter-national Journal of Advanced Manufacturing Technology, vol. 103, no.5-8, pp. 2627–2647, 2019.[5] C. Carceller, F. Gentili, D. Reichartzeder, W. Bösch and M. Schwenten-wein, ”Practical considerations in the design of monoblock TM dielec-tric resonator filters with additive manufacturing,” 2017 InternationalConference on Electromagnetics in Advanced Applications (ICEAA),Verona, 2017, pp. 364-367.[6] W. H. She, Z. N. Wing, J. W. Halloran and W. J. Chappell, ”Variabledielectric constants by structured porosity for passive ceramic compo-nents,” IEEE MTT-S International Microwave Symposium Digest, 2005.,Long Beach, CA, 2005, pp. 4 pp.-868.[7] X. Gong et al., ”Tailored and anisotropic dielectric constants throughporosity in ceramic components,” in IEEE Transactions on MicrowaveTheory and Techniques, vol. 53, no. 11, pp. 3638-3647, Nov. 2005[8] E. Massoni et al., ”Characterization of 3D-printed dielectric substrateswith different infill for microwave applications,” 2016 IEEE MTT-SInternational Microwave Workshop Series on Advanced Materials andProcesses for RF and THz Applications (IMWS-AMP), Chengdu, 2016,pp. 1-4.[9] T. Marc, M. Cyrille, D. Nicolas, T. Olivier, R. Maxime and N. Capet,”A Dielectric Resonator Antenna designed with a structured dielectricmaterial,” 2018 IEEE Conference on Antenna Measurements & Appli-cations (CAMA), 2018, pp. 1-2, doi: 10.1109/CAMA.2018.8530519.[10] Liu, X. Da, Hou, Z. L., Zhang, B. X., Zhan, K. T., He, P., Zhang, K.L., & Song, W. L. (2016). A general model of dielectric constant forporous materials. Applied Physics Letters, 108(10).[11] L. Silvestri, E. Massoni, C. Tomassoni, A. Coves, M. Bozzi and L.Perregrini, ”Substrate Integrated Waveguide Filters Based on a DielectricLayer With Periodic Perforations,” in IEEE Transactions on MicrowaveTheory and Techniques, vol. 65, no. 8, pp. 2687-2697, Aug. 2017, doi:10.1109/TMTT.2017.2709745.

Additive Manufacturing of Non-homogenous Dielectric Waveguide Structures and Filters

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Additive Manufacturing of Non-homogenous Dielectric Waveguide Structures and Filters

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