Paint-on approach for fabrication of electrically active structures

Abstract

A method of making an electrically active structure includes applying a backplane onto a substrate, applying a paint layer onto said backplane, applying a stencil layer on said paint layer, patterning an electrically active structure onto said paint layer through said stencil layer, and removing said stencil layer.

Claims

1. A manufacture comprising a substrate having an electromagnetic metamaterial disposed thereon, wherein said electromagnetic metamaterial comprises a set of stacked paint layers and a plurality of electromagnetic coupling elements, wherein said electromagnetic coupling elements are disposed throughout said set, wherein said set of stacked paint layers comprises a dielectric paint layer and a conductive paint layer, wherein said dielectric paint layer and said conductive paint layer define a stack of two paint layers, wherein said conductive paint layer comprises a conductive patterned structure, wherein said conductive patterned structure comprises an array of said electromagnetic coupling elements, and wherein a paint layer selected from the group consisting of said dielectric paint layer and said conductive paint layer is on said substrate.

2. The manufacture of claim 1, wherein said manufacture is a sensor.

3. The manufacture of claim 1, wherein said dielectric paint layer comprises latex paint.

4. The manufacture of claim 1, wherein said dielectric paint layer comprises an oil-based paint.

5. The manufacture of claim 1, wherein said dielectric paint layer comprises a water-based paint.

6. The manufacture of claim 1, wherein said dielectric paint layer comprises a polymer.

7. The manufacture of claim 1, wherein said substrate comprises a vehicle on which said electromagnetic metamaterial is applied.

8. The manufacture of claim 1, wherein said substrate comprises a curved surface on which said electromagnetic metamaterial is applied.

9. A method specially adapted for the manufacture of manufacture comprising a substrate having an electromagnetic metamaterial disposed thereon, wherein said electromagnetic metamaterial comprises a set of stacked paint layers and a plurality of electromagnetic coupling elements, wherein said electromagnetic coupling elements are disposed throughout said set, wherein said set of stacked paint layers comprises a dielectric paint layer and a conductive paint layer, both of which are paint layers that are elements of said set of stacked paint layers, wherein said conductive paint layer comprises a conductive patterned structure, wherein said conductive patterned structure comprises an array of electromagnetic coupling elements, and wherein a paint layer selected from the group consisting of said of said dielectric paint layer and said conductive paint layer is on said substrate, said method comprising applying a backplane onto said substrate, building a first paint layer on said backplane, said first paint layer being one of said dielectric paint layer and said conductive paint layer, wherein said first paint layer, after having been built, becomes one of said paint layers in said set of stacked paint layers, applying a stencil on said first paint layer, patterning said electrically active element onto said first paint layer through said stencil, and removing said stencil.

10. The method of claim 9, wherein building said first paint layer comprises applying a plurality of coats of paint.

11. The method of claim 9, further comprising building a second paint layer on top of said patterned first paint layer.

12. The method of claim 9, wherein building said first paint layer comprises mixing a reinforcing agent into paint used to build said first paint layer.

13. The method of claim 9, wherein building said first paint layer comprises mixing shredded paper into paint used to build said first paint layer.

14. The method of claim 9, wherein patterning said element comprises applying conductive ink onto said stencil layer.

15. The method of claim 9, further comprising removing said substrate.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is an isometric view of a metamaterial;

(2) FIG. 2 is a cross-section through a unit cell in FIG. 1;

(3) FIG. 3 is a flow-chart for a method of making the metamaterial in FIGS. 1 and 2;

(4) FIG. 4 shows the metamaterial during the various steps in FIG. 3, and

(5) FIG. 5 shows a sensor constructed according to the methods based on the procedure shown in FIG. 3.

DETAILED DESCRIPTION

(6) Referring to FIG. 1, and the cross-section thereof shown in FIG. 2, a metamaterial 10 comprises plural unit cells 12, each of which includes an electromagnetic coupling element 14. In the particular embodiment disclosed, the electromagnetic coupling element 14 is a crossed dipole. This configuration results in a polarization-insensitive metamaterial 10. However, the coupling element 14 can be any suitable structure. Examples of coupling elements 14 include as split ring resonator, electrical split ring resonator, spiral, and log spiral, electrical split ring resonators, double electric split ring resonators, bow tie antennas, coils, spiral antennas, and log periodic antennas

(7) In one embodiment, the crossed dipole is made of strips having an average length of 5.9 mm and an average width of 0.65 mm for use in a unit cell 12 having sides with length 7.2 mm. This is particularly suitable for a metamaterial 10 designed to operate in the X band (8-12 GHz), and in particular, at 9.5 GHz. However, the structure is electromagnetically scalable and can thus be configured to be tuned to any band of frequencies, from the kilohertz range, through the terahertz range and on into the far infrared.

(8) The coupling element 14 is applied by masking a paint layer 18 with a stencil 16. The stencil 16 is temporarily attached to the paint layer 18 with mounting spray. A conductive ink is then applied to form a pattern. Although a variety of conductive inks, such as nickel ink and carbon ink can be used, the preferred ink is a silver ink normally used to prepare specimens in scanning electron microscopy. To achieve the desired thickness, two or more coats of silver ink are applied, with a brief drying time between coats.

(9) In one embodiment, the total thickness of the silver ink is between 12 and 20 micrometers. This can be made by brushing on two coats of ink with a drying time of ten minutes at standard temperature and pressure between coats. This results in the desired thickness while permitting the mask to be easily removed without harming the pattern. The stencil 16 in this embodiment is approximately 3.5 millimeters thick. A suitable material for the stencil 16 is ordinary copy paper. However, the stencil 16 can also be made of textiles.

(10) The paint layer 18 is preferably made of latex paint. However, oil-based paints can also be used, as can polymers that can be brushed onto a substrate and built up layer by layer to a desired thickness. The availability of paint in multiple colors provides the possibility of color matching the metamaterial 10 to its surroundings, thus facilitating its use in, for example, automobiles where aesthetic considerations are important, or as camouflage in military applications.

(11) Other examples of paints include dispersions of different materials with different properties, including colloidal dispersions of nanoparticles, including, without limitation, gold, nickel, iron, cobalt, and various oxides. Other examples are colloidal dispersions of nanowires, including gold, silver, nickel, or other metallic nanowires, and dispersions of nanoplatelets, such as graphene, graphene oxide, reduced graphene oxide, metal oxides such as MoS.sub.2 etc in water or any suitable liquid.

(12) The paint layer 18 in one embodiment is about 1 mm thick. This can be achieved by applying about 30 coats of latex paint. A paint layer 18 of suitable thickness can also be made by painting on Teflon, letting the paint dry, scoring the paint into appropriate sized pieces, peeling them off, and then stacking them to attain the desired thickness, using fresh paint as a glue to hold the structure together. This offers the advantage of speed since one no longer has to wait for each coat of paint to dry.

(13) In some cases, multiple paint layers can be used. For example, one can alternate between dielectric paint layers and conductive paint layers. Or one can pattern electromagnetic coupling elements 14 on a first paint layer and then build a second paint layer on top of the first paint layer, effectively sandwiching the electromagnetic coupling elements between two paint layers. This procedure can be repeated so that the completed metamaterial can have multiple paint layers with various coupling structures at the interfaces formed between them.

(14) To enhance structural integrity, it is useful to include some reinforcing material in the paint used to build a paint layer 18. For example, one can mix shredded paper or some other solid material to form a structurally stronger paint layer 18.

(15) The paint layer 18 is placed on a conductive backplane 20. This conductive backplane 20 can be made by painting silver ink on a substrate 22, such as a PET film. For X band applications, the backplane 20 is about 12-20 micrometers thick. Once the patterning is completed, the backplane 20 can be separated from the substrate 22. This results in a flexible metamaterial that can easily be mounted on a curved surface.

(16) Referring to FIG. 3, a method 23 of making a metamaterial includes applying a conductive backplane to a substrate (step 24). This can be done by painting silver ink on a PET substrate. This is followed by building a paint layer, which includes applying a coat of paint to the backplane (step 26), letting the plaint dry (step 28) and determining if the paint layer is thick enough (step 30). If the paint layer is not thick enough, another coat of paint is applied (step 26) and allowed to dry (step 28). If the paint layer is thick enough, a stencil is applied onto the paint layer (step 32). The paint layer is then patterned with unit cells through the stencil (step 34). This is followed by removal of the stencil from the paint layer (step 36) and removal of the backplane from the substrate (step 38).

(17) FIG. 4 shows the metamaterial in the process of being constructed beginning with a PET used as base material (step 1). This base material is peeled of after fabrication is finished. The PET sheet is painted with three layers of silver conductive paint to a thickness of 20 micrometers (step 2) to form a conductive backplane. Then, a 1 mm thick coat of (latex paint-quick dry, durable, flexible) substrate is used for the absorber fabrication. About 30 coats of paint was applied to reach 1 mm thickness (step 3). Between each coast, the paint was allowed to dry for 10-20 minutes. A paper-based stencil was then laid on (step 4) and conductive silver paint is applied on stencil covered paint substrate. The stencil was then removed after paint was dried for 10-20 mins (step 5).

(18) A metamaterial 10 as described herein is particularly useful for the use of electromagnetic waves in connection with collision avoidance between motor vehicles, and automated maneuvering and piloting of motor vehicles. In such applications, it is desirable to manipulate the surfaces of the vehicles to avoid specular reflections or to otherwise have electromagnetic characteristics that enhance the reliability of radar systems used in such applications.

(19) Metamaterials 10 as described herein are also useful for radar absorption and can therefore be used to reduce radar cross-sections of vehicles that may be targeted by radar, such as drones, military aircraft, ships, and missiles.

(20) Metamaterials 10 as described herein can be made frequency selective. As such, they are suitable for use in radomes and other structures for which it is desirable to pass certain frequencies but to exclude others.

(21) The foregoing methods also permit construction of multi-layered 3D structures with metallic and dielectric patterns of precise dimensions dictated by the size of the stencil. In addition to making antennas and metamaterials, such structures can include devices, sensors, circuits or systems that have traditionally been micro-fabricated or printed.

(22) FIG. 5 shows a chemical gas or vapor sensor 40 formed by painting electrodes 42, 44 from metallic paint, and in between the electrodes 42, 44, painting a sensor region 46 made of a chemically sensitive material on top of a substrate 48.

(23) One example of a chemically sensitive material is a dispersion of carbon nanotubes. Carbon nanotubes or reduced graphene oxides are particularly sensitive to vapors, including those arising from volatile organic compounds.

(24) Instead of micro-patterning in a clean-room, or using a complicated assembly process, one can simply paint these various materials on any arbitrary substrate, including a curved substrate, a flexible substrate, or a hard substrate, to make various electrically active structures.

(25) The sensor 40 shown in FIG. 5 represents just one of many possible structures that could be made using the methods described herein. For example, one can also paint on photovoltaic materials for energy harvesting. Additionally, one could paint a p-type and n-type, stacked side to side or even vertically to make multi junction solar cells. Devices made with a paint having piezoelectric properties can also be used. Such devices could harvest energy from motion.

(26) Yet another application is the painting of an electrode array for biomedical applications. One example of such an electrode arrays is one that is disposed to apply or receive electrical signals. Such arrays can be used for both stimulation and diagnosis. Examples of such arrays are those that interact with the spinal cord in rehabilitation prosthetic devices. Another example of such arrays are those that can be used to record EEG signals from brain. Yet another example of such arrays are those that can be used to record ECG signals from heart.

(27) In addition, one can use the foregoing methods to fabricate conventional electronic devices, such as diodes and transistors. For example, one can form a transistor with metallic paints for gates, source and drain terminals, and then semiconductor paint, such as using the dispersion of semiconducting carbon nanotubes, or graphene, suitably doped, to create the underlying structure.