Patent Publication Number: US-11380457-B1

Title: Residue free electrically conductive material

Description:
The present application is a divisional of U.S. patent application Ser. No. 15/986,292, filed May 22, 2018, which claims the benefit of and priority to prior filed Provisional Application Ser. No. 62/510,629, filed 24 May 2017, which is expressly incorporated herein by reference. 
    
    
     RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     FIELD OF THE INVENTION 
     The disclosure is directed to electrically conductive materials, in particular to polymer encapsulated liquids that are electrically conductive, circuit components containing the polymer encapsulated liquids, and methods for making the polymer encapsulated, electrically conductive materials. 
     BACKGROUND AND SUMMARY 
     The recent surge in flexible and stretchable electronics (FSE) research and development has compelled researchers to develop materials and processes that are well-suited toward the advancement of FSE technology. Flexible and stretchable electronics are complete electronic circuits that have the ability to undergo reversible deformation while maintaining their intended functionality. 
     There are numerous commercial applications for FSE circuits. Electronic materials that can be printed with small feature size and highly tunable properties without the requirement of aggressive post-processing are applicable to flexible electronics in the consumer electronics market as well as health and sports health monitoring markets to name a few, where products are required to be printed quickly (roll-to-roll fashion) and in high throughput with reliable functional operation parameters. Printing of electrical contacts pads, flip-chip bump bonding for integrated circuits, flexible integrated antennas, among other uses are quickly identifiable. 
     Current state-of-the-art approaches to fabricating conductors which can survive elastic stretching involve an “island-plus-serpentine” approach wherein rigid (non-stretching) electrical components are connected electrically using meandering (serpentine) solid metal interconnects. By employing this type of curved geometry to fabricate the device interconnects, localized strains are drastically reduced resulting in an overall device elasticity that far exceeds that of the conducting metals in their bulk form. Though the island-plus-serpentine approach has indeed met with a certain degree of success, several limitations have fostered continued research into alternative options. For instance, serpentine patterns require more “real estate” in the device layout, higher stretching can only be achieved where interconnects are not bonded to the substrate, and interconnects are prone to failure after repeated stretching cycles. 
     Conductive liquids offer an unprecedented level of mobility and agility in the field of reconfigurable electronics. There are a variety of conducting fluids available, ranging from highly conductive liquid metals to more resistive metal colloid loaded dielectric fluids, and even include ionic conductive fluids. However, these conductive fluids need to be contained within channels or vascular structures, and individual droplets will coalesce with one another due to surface energy minimization and Laplace pressure gradients, limiting their structurally dictated electromagnetic properties. 
     One of the most useful electrically conductive fluids are gallium based alloys. However, gallium based alloys form a thin oxide shell on the surface of the metal that hinders mechanical reconfigurability. This oxidation creates contamination issues in a device due to the fact that the liquid metal oxide adheres to many surfaces leaving a residue behind. The formation of residue has led to many devices requiring the use of a corrosive (acidic/basic) environments to lessen the effects of the oxide. While this approach has been experimentally proven, the use of acidic/basic environments leads to corrosion of the metallic interfaces as well as alloying between that of the liquid metal and surrounding metallic contacts. As a result, there is a need for a solution that allows liquid metal devices to avoid such effects without compromising the advantage of being reconfigurable. 
     In view of the foregoing, embodiments of the disclosure provide a deformable yet mechanically resilient microcapsule having electrical properties, a method of making the microcapsules, and a circuit component including the microcapsules. The microcapsule containing a gallium liquid metal alloy core having from about 60 to about 100 wt. % gallium and at least one alloying metal, and a polymeric shell encapsulating the liquid core, said polymeric shell having conductive properties. 
     In one embodiment, there is provided a method for making deformable microcapsules containing a gallium liquid metal alloy core. The method includes the steps of (1) providing a double-T-junction apparatus having a first T-junction and a second T-junction; (2) flowing at a rate of about 0.1 mL/Hr a gallium liquid metal alloy emulsion to the first T-junction containing a first aqueous carrier fluid to form first droplets in the first aqueous carrier fluid at a rate of about 100 mL/Hr; (3) flowing at a rate of about 0.1 mL/Hr the first droplets to the second T-junction containing an emulsion of a polymerizable material in a second carrier fluid flowing at a rate of about 100 mL/Hr to form second droplets containing the first droplets as a core and the polymerizable material as a shell; and (4) polymerizing the polymerizable material to provide the deformable microcapsules containing the gallium liquid metal alloy core, wherein the shell of the deformable microcapsules has conductive properties. 
     A further embodiment provides a deformable circuit component having electrical properties. The circuit component has an electrical element containing microcapsules having electrical properties. The microcapsules contain a gallium liquid metal alloy core having from about 60 to about 85 wt. % gallium and at least one alloying metal, and a polymeric shell encapsulating the liquid core, wherein the polymeric shell has conductive properties. 
     In some embodiments, the gallium liquid metal alloy is a gallium and indium alloy. The gallium indium alloy may also contain tin. Accordingly, in some embodiments, the gallium liquid metal alloy may include about 62 wt % to about 95 wt % gallium; about 5 wt % to about 22 wt % indium; and about 0 wt % to about 16 wt % Sn. 
     In other embodiments, the alloying metal of the gallium liquid metal alloy may be selected from tin, silver, gold, thalium, cesium, palladium, platinum, sodium, selenium, lithium, potassium, zinc, copper, cadmium, bismuth, indium, antimony, lead, and combinations of two or more of the foregoing. 
     In some embodiments, the deformable microcapsule has a polymeric shell thickness ranging from about 10 to about 20 microns and a core volume ranging from about 50 to about 200 microliters. The deformable microcapsule may have a mean particle diameter ranging from about 100 μm to about 1 mm. 
     In some embodiments, the polymeric shell of the deformable microcapsule may be coated with an electrically conductive material. 
     In other embodiments, the polymeric shell may be made of a polymerizable material selected from poly(alkyl-methacrylate), polysiloxane, polyurethane, poly(aniline), polypyrrole, polythiophene, poly(ethylenedioxythiophene), and poly(p-phenylene vinylene), wherein the polymerizable material contains a conductive material component. 
     The foregoing embodiments are described in more detail below and may overcome challenges of using flexible circuit materials by encapsulating the liquid metal within a polymer shell. The polymeric shell being elastic will provide mechanical resiliency while providing a protective layer against conducive fluid coalescence. For applications requiring electrically intimate contact, the polymeric shell may be electrically conductive. For applications where the contact is required to be ionically conductive, the elastomer of the polymeric shell may be chosen to be ionically conductive. Most conductive polymers, especially under mechanical stress, exhibit lower conductivity compared with conductive fluids. Therefore, the encapsulation of a liquid conductor within a thin conductive polymeric shell provides a superior material solution to applications that require a mechanical resiliency and conductivity, compared with a conductive elastomeric polymer devoid of a liquid metal core. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a double T-Junction for use in making deformable microcapsule having electrical properties. 
     
    
    
     DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS 
     In hopes of achieving a more elegant, versatile, and robust solution to stretchable conductors, researchers have been examining eutectic blends of Gallium-Indium (eGaIn) and Gallium-Indium-Tin (GaInSn) in earnest as these room temperature liquid metal alloys are inherently stretchable, are non-toxic, and have excellent conductivity. Several groups have already begun to explore the use of Gallium liquid metal alloys (GaLMAs) in multiple FSE applications including strain gauges, stretchable capacitors, reconfigurable antennas, and self-healing circuits. 
     Embodiments of the disclosure provide a method to encapsulate a GaLMA liquid conductor within a conductive elastomeric shell. The elastomeric shell may be formed by means of core-shell microfluidic encapsulation process. The encapsulation process involves linear channels such that one channel will confine the core material (in this case the liquid GaLMA), one channel will contain the shell material (pre-cured elastomer), and another channel will surround the core material with the shell material and carry the core-shell droplet to a curing area. The foregoing procedure is very similar to that shown in  FIG. 1 . 
     In  FIG. 1 , a double T-Junction  10  in operation is viewed from the top, looking down. Channel  12  provides a core material such as GaLMA from a core material source  14  through T-Junction  1  to channel  16  containing a source  18  of aqueous fluid. T-Junction  1  causes the formation of discrete particles  20  of GaLMA to form in the aqueous fluid by adjusting the flow of the aqueous fluid releative to the flow of GaLMA into T-Junction  1 . The outlet of T-Junction  1  provides an aqueous stream containing the discrete particles  20  of GaLMA for continuous flow to T-Junction  2 . The continuous fluid becomes the dispersed fluid as it flows through T-Junction  2  into channel  22 . A source of shell material in an organic carrier fluid is provided from conduit  24  to the channel  22  so that it encapsulates the discrete particles of GaLMA from channel  16  as the combined material emerges as a shell and core material  26  from T-Junction  2  into channel  22 . The shell and core material  26  then flows to a curing area  28  where excess shell material and/organic carrier fluid is removed and the shell of the shell and core material  26  is cured. 
     All three channels  12 ,  16  and  22  can be perpendicular to one another and these channels, with their respective material, can have flow rates that cause the formation of the discrete particles  20  of GaLMA in channel  16  and the encapsulated particles  26  in channel  22 . The flow rates will be determined such that the rate enables the core material (GaLMA)  20  to enter the shell material (polymeric elastomer) channel  22  but only part way so that the core material  20  is sliced into sections that will be surrounded and carried by the shell material  24  through the encapsulating channel  22 . 
     For example, the flow rate of gallium liquid metal alloy forming the core material to T-Junction  1  may range from about 0.05 to about 0.115 mL/Hr. The aqueous carrier fluid  18  for the gallium liquid metal alloy in channel  16  may have a flow rate ranging from about 80 mL/Hr to about 120 mL/Hr. The aqueous carrier fluid may also contain an oxide reducing compound such as NaOH to prevent the formation of an oxide coating on the gallium liquid metal alloy. As set forth above, the oxide coating on the gallium liquid metal alloy may cause corrosion problems if the core material were to escape from the shell material. However, in other embodiments, the gallium liquid metal alloy may also contain an oxide coating. 
     The polymerizable material in channel  22 , forming the shell of the core/shell microcapsule, may be provided in a second carrier fluid to T-Junction  2 . Accordingly, the flow rate of polymerizable material  24  in the second carrier fluid may range from about 80 mL/Hr to about 120 mL/Hr. The second carrier fluid may be selected from a wide variety of organic solvents, including but not limited to, toluene, xylene, ethanol, propanol, and the like. An organic material that is readily evaporated or removed from the cured microcapsules is particularly preferred. 
     The resulting core/shell material  26  and carrier fluid in channel  22  may have a combined flow rate ranging from about 160 mL/Hr to 200 mL/Hr. The curing area  28  may be an extension of the carrier material channel  22  or an external containing area that will also have the carrier material. The curing area  28  may be heated as to thermally cure the elastomeric shell or illuminated with radiation to provide photo-initiated curing. The cured core-shell droplets are collected and further used to make expandable and deformable electrical circuit components. Because of the polymeric shell material, corrosion of metal surfaces and electrical contacts by the oxide coating may be avoided when using the encapsulated GaLMA in electrical devices and circuits. 
     The electrical properties of the GaLMA/microcapsule core/shell materials may be adjusted over a wide range by selecting different conductive polymeric materials used to form the shell of the microcapsule or by coating the microcapsules with conductive or resistive materials. Accordingly, the polymeric shell material may be selected from poly(alkyl-methacrylate), polysiloxane, polyurethane, poly(aniline), plypyrrole, polythiophene, poly(ethylenedioxythiophene), and poly(p-phenylene vinylene). The polymeric shell may have a shell thickness ranging from about 10 to about 20 microns and a mean particle diameter ranging from about 100 μm to about 1 mm. The core volume of the microcapsule may range from about 50 to about 200 microliters. 
     The encapsulation process uses two main components: a flow source such as a syringe pump, peristaltic pump, or other continuous-throughput pumping system and a double-T platform  10  to create core-shell droplets. The platform  10  can be made from a variety of materials. One example is a transparent polymer polydimethylsiloxane (pdms). The channel geometry is made into a mold, from this mold a pdms casting can be made in the form of the negative of the mold. This casting can be bonded to a flat substrate such as glass via oxygen plasma functionalization. The flow sources can then be connected to their respective channels. 
     In order to improve the properties of the GaLMA materials so that they remain electrically conductive, the GaLMA materials are disposed in a polymeric microcapsule that has electrical properties. Accordingly, the cured shell and core material may be coated with a conductive metal such as silver or a conductive material such as carbon nanotubes or silver particles may be incorporated into the polymer that is used to encapsulate the GaLMA particles at T-Junction  2 . 
     When working with GaLMAs, two critical behaviors that must be taken into account: First, in the presence of oxygen (air) GaLMAs spontaneously form a thin (1-2 nm) gallium oxide (GaO) skin on the surface of the metal that largely dictates the mechanical, rheological, and electrical attributes of the material. In the absence of the skin, GaLMAs behave much the same as their toxic counterpart, Mercury, in that they flow readily and spontaneously assume those shapes which minimize free surface energy. While the presence of the oxide film acts to stabilize the LM droplets such that they are able to maintain small (millimeter or less) free-standing structures that stick readily to various substrates, the oxide leaves a residue on critical electrical components. Accordingly, by encapsulating the GaLMA in a polymeric microcapsule, the detrimental effects of an oxide coating on the GaLMA particles may be avoided. Also, since the microcapsule material is deformable and is electrically conductive, there is no formation of oxide residue on the electrical components used with the deformable electrically conductive microcapsules made according to the disclosure. 
     It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the,” include plural referents unless expressly and unequivocally limited to one referent. Thus, for example, reference to “an antioxidant” includes two or more different antioxidants. As used herein, the term “include” and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items. 
     For the purposes of this specification and appended claims, unless otherwise indicated, all numbers expressing quantities, percentages or proportions, and other numerical values used in the specification and claims, are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. 
     While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or can be presently unforeseen can arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they can be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents.