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Graphene-Based Composite EMI Shielding for RF Device Protection

Description:

TECHNOLOGY AREA(S): Electronics, 

OBJECTIVE: Develop a graphene-based composite EMI shielding material capable of replacing metal shielding in IC packages and printed circuit board components. 

DESCRIPTION: As soldier electronics and their components operate at faster speeds, smaller size, and in closer confinements a substantial increase in Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI) can lead to system failures. This effort supports the FREEDOM ERP as it enables enhanced technologies to protect next generation of highly mobile RF communications for battlefield dominance in the broad bandwidth frequencies X-band (8-12 GHz) to the Ku-band (12-18 GHz). Metal EMI shields in IC packages and printed circuit board components have limitations in poor chemical resistance, oxidation in long term harsh environments, high density, flexibility, and form factor. Current strategies to obtain the desired EMI shields mainly rely on increasing the material's thickness to prolong the EM wave absorption routes or loading large amounts of fillers in order to increase its electrical conductivity [1]. However, these factors inevitably increase the production cost and limit scalability. Generally, conductive fillers have high aspect ratio and large specific surface area, such as carbon nanofibers (CNF), multi-wall carbon nanotubes (MWCNT), stainless-steel fibers (SSF), and graphene layers offer advantages without the limitations imposed by pure metals. Fillers are preferred because they can be dispersed in lightweight polymers to establish efficient conductive paths in the composites and form sufficient interfaces with the polymer matrices, leading to enhanced electrical conductivity and interfacial polarization that are beneficial to the EMI shielding performance. Among the conductive fillers, graphene has the best conductivity, lowest density, and highest thermal conductivity [2]. Silver and cooper have excellent conductivity but the aging stability of metallic nanostructures is big concern for long term storage of electronics in systems for harsh environments. The development of graphene filler ink formulation offers a major opportunity for improving the deposition of composite shielding materials. Graphene filler ink materials can be dispensed economically by drop casting or using printing technologies with controlled patterning capabilities leading to new technologies, and applications. The inks can be deposited on substrates by methods such as drop casting, inkjet printing, and aerosol-jet printing [3-5]. A path forward lies in the improvement of ink formulation. Ink formulations rely on factors including selection of flake size, solvents, and surfactants that provide the best combination for direct exfoliation of pristine graphene. Several factors limit achieving the high theoretical conductivity 10exp8 S/cm of graphene, which is 3 orders of magnitude higher than highly conductive metals such as copper. These factors include inter-flake percolation, type of binder that hold the fillers together, and post decomposition by annealing. Also, there is a lack of “pristine” graphene flakes in the market [6] that can be improved. The ultimate goal is to deposit a shielding material with properties that would be capable of replacing metal shielding fully in printed circuits. 

PHASE I: The responsive proposal shall develop a shielding material. Identify an ink formulation and description of above mentioned factors (i.e. graphene content, inter-flake percolation, surfactants, selection of ink binder, drying agents, and annealing process for binder decomposition) with a printing approach that forms a composite shield on a substrate. For all solutions the ability to block the greatest amount of incident EMI waves by the deposited shielding material is a driver. METRICS: Graphene-based composite material (less than 1mm thick) on Kapton or Mylar substrate: -Electrical conductivity greater than 100,000 S/m -Substrate or annealing temperature less than 250°C -Good flexibility and film adhesion to substrate after manual bending, and no micro cracking. -EMI shielding efficiency (EMI SE) greater than 50dB across broadband frequencies (8-18 GHz) During the phase-I the contractor shall report and deliver on the following: (1) Report on the material selection, process development, challenges, and accomplishments. (2) Document the method of deposition, and minimum size linewidth achieved. (3) Characterize the composite structural properties, composition and size of the nanostructures. (4) Document the binder, surfactants, solvents used including the effects of annealing/sintering temperature or post chemical treatment on the conductivity of the deposited composite. (5) Conduct EMI shielding efficiency studies, record the scattering parameters using a vector analyzer with two port measurement techniques or an equivalent method in the frequency range 8-18 GHz to validate performance claims. (6) Delivery of deposited material samples on Mylar or Kapton substrate with pattern size 0.5”x1.5” for validation: (a) Deposition single layer of uniform film on sample as a baseline. Include data of the measured film uniformity, roughness, thickness, resistivity, conductivity, and EMI SE. (b) Deposition multiple layers (up to a maximum total thickness of 1mm) that demonstrates best achieved conductivity and EMI SE. Include data of the measured film uniformity, roughness, thickness, resistivity, conductivity, and EMI SE. (7) Propose improvements for phase-2, challenges, solutions, and applications. 

PHASE II: This phase addresses a current needs relevant to the US military: a lightweight and durable EMI shield on IC packages and printed circuit boards components that protect against radio frequency interference (RFI). Develop a scale-up demonstration/validation program, on an application relevant to the US military based upon the previous phase I requirements and on-going proprietary developments. METRICS: -Ability to the composite film adhere to variety of electronic substrates and packaging (Mylar, Kapton, Si, SiO2, glass, ceramic, paper), determine the limitations. -Repeatability of printing square shield patterns (1 mm, 1 cm, 5 cm, 10cm), determine the limitations. -Reliability of formulated solution for printing without clogging the printing head, determine the number of cycles. -Storage of ink formulation beyond 3 months without observable segregation and settling, determine the maximum time within contract period. -Stability of composite film at mil specification temperatures -65 degrees F to 165 degrees F. This phase is intended produce a full scale 3D prototype EM shielding. The full scale system will also address key factors in maintaining an effective shield, such as conformal coating that allows for continuity of conductivity over integrated circuits. The main technical objectives: (1) Demonstrate/validate a system that is capable of repetitive or continuously coating. (2) Fabricate a full-scale 3D prototype of the shielding on IC’s and PCB components. (3) Validate that the EMI shielding performance meets the specified requirements. 

PHASE III: Implement a business case and partner with a DOD supply chain to commercialize the EMI shielding material system to a TRL 7 System prototype demonstration in field environment. We envision use of shielding in military applications such as solider radios, prognostics/diagnostics, unmanned air vehicles (UAV’s), drones, unattended ground sensors, security access/entry, RF ID tags, and air defense systems. For commercial applications in cell phones, computers, and medical equipment. 

REFERENCES: 

1: M. Verma, et al. "Graphene nanoplatelets/carbon nanotubes/polyurethane composites as efficient shield," Composites Part B 120 (2017) 118. http://dx.doi.org/10.1016/j.compositesb.2017.03.068

KEYWORDS: Shielding, Electromagnetic Interference (EMI), Radio Frequency (RF), Flexible Shielding Effectiveness (SE) Conformal 

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