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Additive Manufacturing for RF Materials and Antennas

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: The objective is to develop a multi-material additive manufacturing (AM) capability for fabrication, characterization, and printing of polymer/ceramic composite electromagnetic (EM) and conductive materials for antennas and other radio frequency (RF) devices. AM is a disruptive technology expanding the design space for RF engineers. The result is the ability to design and easily fabricate antennas and other RF devices not realizable via traditional manufacturing methods. 

DESCRIPTION: As the Army moves towards multi-mission platforms, functionality of disparate radio frequency (RF) systems must be integrated into a single system. This requires planar and vertical integration of apertures, substrates, and feed networks to enable multiple modes of operation. Several different antennas and their feed networks consisting of transmission lines, amplifiers, filters, and switches will be incorporated into a single front end across wide bandwidths. The 3D and multi-material approaches needed to achieve these designs makes additive manufacturing (AM) critical to the future of Army RF systems. While strides in AM demonstrate production of robust mechanical parts, less attention is paid to developing and characterizing RF properties of printed materials. Research is also needed in the area of conductive inks for AM. Current processes yield metal layers with low conductivity compared to bulk metal. Increased conductivity of printable inks enhances the power efficiency of RF components. AM allows engineers to re-think the RF design space. Dielectric constants of commercial materials limits current antenna designs. AM facilitates complex designs that required properties not achievable by current manufacturing methods. One example is the Luneberg lens [1] which relies on a graded dielectric constant. AM achieves this previously unrealizable property producing a high gain antenna with a steerable beam and eliminates the large aperture and complicated feed network associated with electrically scanned arrays [2]. Increased versatility of AM in the RF design space requires high dielectric substrates. AM filaments with dielectric constants greater than 4 are not commercially available. However, techniques exist to extrude AM filaments [3] from polymer/ceramic composites with high dielectric constants [4-6]. The process disperses high dielectric ceramic particles into a polymer substrate where the volume fraction of ceramic particles to polymer matrix determines the dielectric constant of the filament [5,7]. These filaments are generally not characterized below X-band frequencies, and the dielectric constant of the printed substrate can deviate from that of the filament. Characterization of complex permittivity of printed dielectric substrates is paramount to this SBIR effort. AM of RF and multi-material structures have hurdles to overcome. One area will be controlling interfaces and bonding between printed layers [8,9]. Another issue is the anisotropic nature due to the orientation of the printed material deposited in each layer which can lead to changes in mechanical properties [10]. Other concerns of AM technologies relate to surface roughness, repetitiveness, and porosity [11-13]. These concerns are being addressed by many research activities [14-17] and should not derail the progression of research in printing multi-material RF structures. A final need for AM of RF components is research into the areas of conductive inks. Conductive inks can achieve conductivity of five to ten times less than bulk copper, but require sintering at temperatures above 175 degrees Celsius [18]. AM substrates printed from polymer based filaments will melt at these temperatures making these methods not viable for AM antennas or other RF components. Alternative methods such as localized laser sintering or flash annealing should be researched to achieve high conductivity in the presence of 3D printed dielectric substrates. 

PHASE I: Phase I shall explore processes for the loading polymers with high dielectric constant particles and the extrusion process for producing high dielectric constant additive manufacturing (AM) filaments that are compatible with an nScrypt 3D printer. Prototype filaments of differing dielectric constants should be produced and a maximum relative permittivity of 15 should be demonstrated. The loss tangent of these filaments should be less than 0.002. Filament diameter should be either 1.75mm (+/- 0.05mm) or 2.85mm (+/- 0.05mm). At the end of Phase I, 3D printed substrates of 8”x8”x0.25” using these filaments will be fabricated and complex permittivity will be measured from 1 GHz to 20 GHz. Differences between the measured permittivity and filaments should be quantified and explained. Furthermore, research into methods for increasing the conductivity of conductive inks printed on composite polymer substrates to 10X less than bulk copper (i.e. 5.8x10^6 S/m) should identify a technique to be demonstrated in Phase II. 

PHASE II: Phase II will demonstrate measured conductivity of sintered conductive ink printed on a polymer substrate reaching 5.8x10^6 S/m or better. Any major deviations identified in Phase I between the complex permittivity of the filament and that of the printed substrate should be accounted for. At the end of Phase II a fully fabricated additively manufactured (AM) antenna structure should be realized (including ground plane, connectors, feed, multiple dielectric substrates, and aperture) in a fully automated process and in a single print utilizing the same machine. Laboratory antenna measurements such as return loss, radiation pattern, and antenna efficiency will be made. A comparison of the AM antenna to the same antenna manufactured by traditional means will be made as well as a comparison to an antenna model utilizing electromagnetic (EM) modeling software such as HFSS or CST. The radiation efficiency of the AM antenna should be within 10% of the same antenna produced by traditional fabrication techniques. The center of the resonance frequency of the AM antenna should vary less than 5% compared to the same antenna produced by traditional fabrication techniques. The AM antenna should also vary less than 5% in resonance frequency and less than 1.0 dB in realized gain across the operational bandwidth of the antenna model. 

PHASE III: Phase III will focus on the commercialization of additive manufacturing (AM) technology for antennas and RF devices. The final AM process should demonstrate the repetitiveness of AM for both military and commercial applications. Commercialization would be of great interest to the radar and wireless sensing community while also providing an innovative technology solution to assist the military reduce logistical burdens for the storing and transporting of antennas and radio frequency (RF) components in the field. Similarly, lightweight AM antennas would be of great interest to the space and satellite communications industry. 

REFERENCES: 

1: R. K. Luneburg & M. Herzberger. Mathematical Theory of Optics. Providence, Rhode Island: Brown University, pp. 189–213, 1944.

2:  D. Roper, B. Good, S. Yarlagadda, & M. Mirotznik, "Fabrication of a Flat Luneburg Lens using Functional Additive Manufacturing", USNC-URSI Radio Science Meeting (Joint AP-S Symposium), 2014.

3:  A. Moulart, C. Marrett & J. Colton, "Polymeric composites for use in electronic and microwave devices", Polymer Engineerung and Science, vol. 44, pgs. 588–597, 2004.

4:  Agarwala M. K. et al., "Structural ceramics by fused deposition of ceramics", Proceedings of Solid Freeform Fabrication Symposium, pgs. 1–8, 1995.

5:  B. Duncan, et al., "3D Printing of Millimeter Wave RF Devices", Workshop on Additive Manufacturing of Antennas and Electromagnetic Structures, MITRE, 2017.

6:  F. Castles, et al., "Microwave Dielectric Characterization of 3D-printed BaTiO3/ABS Polymer Composities", US National Library of Medicine, PMC4778131, 2016. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4778131/#b20

7:  Y. Rao, et al., "Novel polymer-ceramic nanocomposite based on high dielectric constant epoxy formula for embedded capacitor application", Journal of Applied Polymer Science, 2001.

8:  C. Bellehumeur, et al., "Modeling of bond formation between polymer filaments in the fused deposition modeling process", Journal of Manufacturing Processes, vol. 6, iss. 2, pgs. 170-178, 2004.

9:  M. Odell, et al., "Material characterization of fused deposition modeling (FDM) process", Proceedings, Rapid Prototyping and Manufacturing Conference, Society of Manufacturing Engineers, Cincinnati, OH, 2001.

10:  S-H. Ahn, et al., "Anisotropic material properties of fused deposition modeling ABS", Rapid Prototyping Journal, vol. 8, iss 4, pgs. 248-257, 2002.

11:  D. Ahn, et al., "Representation of surface roughness in fused deposition modeling", Journal of Materials Processing Technology, vol. 209, iss. 15-16, pgs. 5593-5600, 2009.

12:  A Sood, et al., "Improved dimensional accuracy of fused deposition modeling processed part using grey Taguchi method", Journal of Material and Design, vol. 30, iss. 10, pgs. 4243-4252, 2009.

13:  K. Ang, et al., "Investigation of the mechanical properties and porosity relationships in fused deposition modeling-fabricated porous structures", Rapid Prototyping Journal, vol. 12, iss. 2, pgs. 100-105, 2006.

14:  K. Thrimurthula, et al., "Optimum part deposition orientation in fused deposition modeling", International Journal of Machine Tools and Manufacturing, vol. 66, iss. 6, pgs. 585-594, 2003.

15:  M. Hossain, et al., "Improved mechanical properties of fused deposition modeling-manufactured parts through build parameter modifications", Journal of Manufacturing Science, vol. 136, iss. 6, 2014.

16:  A. Boschetto, L. Bottini, "Accuracy prediction in fused deposition modeling", International Journal of Advanced Manufacturing Technology, vol. 73, iss. 5, pgs. 913-928, 2014.

17:  K. Tong, et al., "Error compensation for fused deposition modeling (FDM) machine by correcting slice files", Rapid Prototyping Journal, vol. 14, iss. 1, 2008.

18:  [D. Roberson, R. Wicker, E. MacDonald, "Ohmic curing of printed silver conductive traces", Journal of Electronic Materials, vol. 41, iss. 9, pgs. 2553-2566, 2012.

KEYWORDS: Additive Manufacturing, Electromagnetic Materials, Material Characterization, Antennas, RF Devices, Manufacturing Materials, Manufacturing Processes 

CONTACT(S): 

Gregory Mitchell 

(301) 394-2322 

gregory.a.mitchell1.civ@mail.mil 

Dr. Larry Holmes 

(410) 306-4951 

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