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Physical Vapor Deposition (PVD) as a Method to produce High Aspect Ratio Conductive Flakes for Advanced Bispectral or Infrared (IR) Obscuration

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: To develop a low-cost manufacturing process for the production of metal composite flakes/discs for use as visible and infrared obscurants. Develop and demonstrate a PVD method to produce highly electrically conductive flakes/discs with optimum dimensions in the for IR obscuration. These PVD produced flake/disc materials shall have an electrical conductivity on the order of iron, although a conductivity on the order of copper is preferred. Additionally, the PVD produced material should be appropriately chosen so as to provide attenuation in the visible region of the spectrum, i.e. via absorption. Also, the PVD produced material must not be ‘pyrophoric’ in nature and must be disseminated via hot air turbine smoke generators and explosively disseminated via explosive central burster munitions. Aluminum is an example of material that meets conductivity requirements, is efficiently manufactured via PVD processes but is ‘pyrophoric’ in nature and creates a flaming hazard when disseminated via explosive central burster grenade or hot air turbine smoke generator. Higher density materials may have an advantage over low density materials for volumetric dissemination purposes. Finally, the PVD produced Flake material should provide a means of mitigating particle agglomeration, so that aerosolization is maximized during the dissemination process. Dissemination approaches for the newly developed material shall include pneumatic (e.g. smoke generator) or explosive (e.g. grenade) techniques. There are two essential dimensional requirements for the flakes produced. First, the length requirement is vital for achieving the desired electromagnetic properties. The distribution must be relatively narrow with a major lateral dimension of about 3 µm (+/- 1 µm) in order to produce a strong resonance within the FIR atmospheric transmission window (8 to 12 µm). Second, flake thicknesses should be as thin as possible within the constraints of flake production. This may prove to be in the vicinity of 10-30 nm, although an ideal thickness of 1-2 nm is desired. A realistic goal of this effort is to produce an IR obscurant with extinction coefficients in the 8-20 m2/gram range. 

DESCRIPTION: Smoke and obscurants play a crucial role in protecting the Warfighter by decreasing the electromagnetic signature that is detectable by various sensors, seekers, trackers, optical enhancement devices and the human eye. Recent advances in materials science now enable the production of precisely engineered obscurants with nanometer level control over particle size and shape. Numerical modeling and many measured results on metal flakes affirm that more than order of magnitude increases over current performance levels are possible if high aspect-ratio conductive flakes/discs can be effectively disseminated as an un-agglomerated aerosol cloud. CURRENT STATUS: Aluminum has been demonstrated as a PVD produced material that has high extinction cross-section/unit mass characteristics on the order of 10 m2/gram. Despite its high extinction, aluminum PVD flakes are too pyrophoric and too low in packing density to be practical for dissemination in munitions. Previous efforts with copper PVD processes were unable to produce desired particle dimensions. Novel approaches to generating metal/ disk shapes with the required dimensions is one possible approach that may be integrated into the PVD process. For example, patterning a substrate with a photolithographic technique prior metal deposition is one possible approach to achieving disc/flake shapes. Currently, the best obscurants for IR attenuation are comprised of brass flakes, which have an extinction cross-section/unit mass of 1.4 m2/g. 

PHASE I: Demonstrate with samples an ability to produce PVD produced flakes with major dimensions of 3 µm (+/- 1 µm) microns in length, thicknesses of 10-30 nm, and conductivity of iron or better (10^5 mho/cm). (5) 10-gm samples shall be provided to ECBC for evaluation. 

PHASE II: Demonstrate that the process is scalable by providing 5 1-kg samples with no loss in performance from that achieved with the small samples. In Phase II, a design of a manufacturing process to commercialize the concept should be developed. 

PHASE III: The techniques developed in this program can be integrated into current and future military obscurant applications. Improved grenades and other munitions are needed to reduce the current logistics burden of countermeasures to protect the soldier and associated equipment. This technology could have application in other Department of Defense interest areas including high explosives, fuel/air explosives and decontamination. Improved separation techniques can be beneficial for all powdered materials in the metallurgy, ceramic, pharmaceutical and fuel industries. Industrial applications could include electronics, fuel cells/batteries, furnaces and others. 

REFERENCES: 

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2:  Huffman, D.R.

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5:  Takayuki, Nakao, Metal pigment flakes and method for producing metal pigment flakes, PCT Int. Appl. (2015), WO 2015146977 A1 20151001

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9:  Bujard, Patrice

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14:  Takayuki, Nakao, Method for producing metallic flake pigment, PCT Int. Appl. (2016), WO 2016047253 A1 20160331

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KEYWORDS: Physical Vapor Deposition, Metals, Infrared Obscuration 

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