US20160108534A1

US20160108534A1 - Aluminum deposition devices and their use in spot electroplating of aluminum

Info

Publication number: US20160108534A1
Application number: US14/516,608
Authority: US
Inventors: Sheng Dai; Xiao-Guang Sun; Charles L. Hussey; Li-Hsien Chou
Original assignee: UT Battelle LLC; University of Mississippi
Current assignee: UT Battelle LLC; University of Mississippi
Priority date: 2014-10-17
Filing date: 2014-10-17
Publication date: 2016-04-21

Abstract

A method for spot electroplating aluminum onto a metallic substrate without submersion or dipping of the metallic substrate in an electroplating bath, the method comprising: (i) spot coating said metallic substrate with an aluminum ion-containing electrolyte contained within a protective structure possessing at least one aperture, and releasing said electrolyte from said at least one aperture onto said metallic substrate to form a coating of said electrolyte thereon, wherein said electrolyte is in contact with an anode; and (ii) applying a voltage potential between the anode and metallic substrate polarized as cathode when the aluminum ion-containing electrolyte is released from said aperture and forms a coating on the metallic substrate, to produce a coating of aluminum on the substrate. Devices, such as brush and ball pen plating devices, for achieving the above-described method are also described.

Description

This invention was made with government support under Prime Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to methods and aluminum-containing electrolytes useful in the electroplating of aluminum, and more particularly, to devices useful in spot plating aluminum onto various metallic substrates.

BACKGROUND OF THE INVENTION

Metal surface coating has played an important role in extending the life cycle of structural materials commonly used in large rugged equipment for use on land, air, and sea. Aluminum and its many versatile alloys are routinely used as surface coatings for the corrosion protection of many metals, offering both barrier and sacrificial protection. In addition, aluminum and its alloys are being considered as favorable alternatives for cadmium coatings on the protective shells of electrical connectors in military ground systems in view of the known toxic and carcinogenic nature of cadmium and hexavalent chromium materials.
Currently, there are various methods for aluminum deposition, such as hot dipping, thermal spraying, sputter deposition, vapor deposition, and electrodeposition. However, a particularly attractive method for depositing aluminum and its alloys is isothermal electrodeposition, either by tank or brush plating. Electrodeposition is an attractive technique because it generally leads to thin, economical coatings that are usually adherent and do not affect the structural and mechanical properties of the substrate. Moreover, the thickness and quality of the deposits can be controlled by adjustment of the deposition rate by tuning such experimental parameters as overvoltage, current density, electrolyte composition, and temperature.
Unfortunately, neither aluminum nor its alloys can be electrodeposited from aqueous solutions because hydrogen is evolved before aluminum can be plated. Thus, it is necessary to employ non-aqueous solvents (both molecular and ionic) for this purpose. On a commercial basis, aluminum is plated by using the well known SIGAL® process. Although known to be very effective, the SIGAL® process requires a plating bath composed of alkyl aluminum fluorides dissolved in toluene. Not surprisingly, the technique raises a number of environmental and safety objections because the alkyl aluminum compounds are pyrophoric and toxic, and the toluene solvent is flammable and can lead to volatile organic compound (VOC) emissions. The inefficiency of aqueous electroplating also makes it a major energy consumer. For example, in electrolytic hard chrome plating, only 10-20% of the power supplied is used for actual deposition; the remaining power is consumed through hydrogen generation and other losses.
More recently, aluminum-containing ionic liquids (i.e., aluminum-containing molten salts) have gained increasing prominence as substantially improved electrolytes for the deposition of aluminum. The ionic liquids possess an advantageous combination of physical properties, including non-flammability, negligible vapor pressure, high ionic conductivity, and high thermal, chemical, and electrochemical stability. Therefore, they are amenable for the electroplating of reactive elements, which is impossible using aqueous or other organic solvents. Thus far, the ionic liquids used for the electrodeposition of aluminum has focused on chloroaluminate anions, which are typically obtained by mixing anhydrous AlCl₃with an organic chloride salt, such as 1-ethyl-3-methyl imidazolium chloride (EMImCl), 1-(1-butyl)pyridinium chloride (N-BPCl), or other related salt. However, because of the hygroscopic nature of AlCl₃and the resulting chloroaluminate, the electroplating generally must be performed in an inert gas atmosphere, which significantly increases cost and complexity of the process.

SUMMARY OF THE INVENTION

By use of novel applicator devices and methods for their use, the instant disclosure overcomes the persistent problem in the art of having to implement costly precautions against moisture during aluminum electroplating. The invention achieves this by employing applicator devices that include a protective structure within which an aluminum ion-containing electrolyte is incorporated, with the provision of apertures in the protective structure to permit release of the electrolyte onto a metallic substrate. The applicator device can be, for example, a polymer membrane, a brush plating device, or a ball pen plating device in which the electrolyte is (or can be) impregnated or incorporated.
In the electroplating method, the electrolyte is made to be in contact with an anode in the device at the time the electrolyte is released from the protective device and applied as a coating on the substrate. A voltage potential is then applied between the anode and the substrate (polarized as cathode) in order to produce a coating of aluminum within an area bounded by the coating of the electrolyte. A further advantage of the methods described herein is the ability to spot electroplate metallic substrates that are generally too large or cumbersome to electroplate by immersing or dipping into a bath.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. A general schematic depicting an exemplary electroplating process using a polymer membrane, such as an ionogel or polymer gel membrane, impregnated with an aluminum-containing electrolyte.

FIG. 2. A general schematic depicting a portable plating brush for spot electroplating aluminum.

FIGS. 3A, 3B. Graphs showing cyclic voltammograms of a) AlCl₃-EMIC (1.5:1) and b) AlCl₃-4-propylpyridine (1.4:1) molten mixtures on a Pt electrode (2 mm in diameter) under a scan rate of 100 mV/s at room temperature with Al wire used as the counter and the reference electrode, wherein EMIC is an acronym for ethylmethylimidazolium chloride.

FIGS. 4A, 4B. Graphs showing cyclic voltammograms of mixtures of AlCl₃and acetamide at a) 1:1 and b) 1.2:1 ratios on a Pt working electrode (2 mm in diameter) with Al wire as the counter and reference electrode. The scan rate was 10 mV/s.

FIGS. 5A, 5B, 5C, and 5D. Graphs showing cyclic voltammograms of a mixture of AlCl₃and 4-propylpyridine (1.5:1) in a) no solvent; b) dichloromethane (DCM); c) acetonitrile (AN), and d) tetrahydrofuran (THF) on a Pt working electrode (2 mm in diameter) with Al wire as counter and reference electrode. The scan rate was 100 mV/s.

FIG. 6. Photo showing an acrylamide polymer membrane containing 60 wt % of AlCl₃-EMIC (1.5:1).

FIG. 7. Graph showing cyclic voltammograms of the polymer membrane containing 60 wt % of 1.5:1 AlCl₃-EMIC molten mixture at 40° C. at a scan rate of 100 mV/s. Cu and Al plates were used as working and counter electrode, respectively. The area of the working electrode was 3.75 cm².

FIGS. 8A, 8B. Graphs showing cyclic voltammograms of a polymer membrane containing 60 wt % of a) AlCl₃-4-propylpyridine (1.4:1) and b) AlCl₃-acetamide (1.2:1) at 40° C. at a scan rate of 100 mV/s. Cu and Al plates were used as working and counter electrode, respectively. The area of the working electrode was 1.68 cm².

FIG. 9. Photo showing a portable plating brush electroplating a coating of aluminum on a copper substrate.

FIGS. 10A, 10B. Photos showing (a) macroscopic view of the aluminum coating produced by the portable plating brush shown in FIG. 9 using AlCl₃-EtMeImCl ionic liquid electrolyte, and (b) optical micrograph (500× ) of the same aluminum coating, wherein EtMeImCl refers to 1-ethyl-3-methylimidazolium chloride.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect, the invention is directed to devices useful for spot electroplating a metallic substrate (i.e., “substrate”) with an aluminum coating. The devices include a protective structure in which the aluminum ion-containing electrolyte (i.e., “electrolyte”) is contained. The term “protective”, as used herein, indicates an ability of the protective structure to provide substantial protection from air, and particularly, moisture, as commonly found in air. Thus, to be optimally protective, the protective structure should ideally be capable of substantially or completely surrounding or encasing the electrolyte housed therein, except that the protective structure includes at least one aperture to permit release of the electrolyte onto a substrate. For optimal effect, the number of apertures are ideally limited to the extent possible while permitting suitable release of the electrolyte. Typically, the electrolyte will remain contained within the protective structure unless an action effecting release is taken. The action effecting release may be, for example, the application of pressure to the device as provided by, for example, pressing or other means for applying of pressure. The action effecting release may alternatively be provided by including a releasing (i.e., transfer) feature in the device, wherein the releasing feature serves to transfer the electrolyte from inside of the protective structure to the substrate by, for example, capillary action or other spreading mechanism. The device, when ultimately assembled, necessarily includes electrical wiring means to permit a voltage potential and current to be transmitted between the anode and substrate polarized as cathode.
The protective structure can be made of any material non-reactive with the electrolyte. Some materials suitable for the protective structure include, for example, plastic, metal, glass, or ceramic, provided that the material is appropriate for the intended means for release.
The device, when ultimately assembled, also includes an anode located in a position suitable for contact with the electrolyte when the electrolyte is incorporated into the device. The anode can be any of the anodes well known in the art for electroplating aluminum. In one embodiment, the anode is an aluminum anode. In another embodiment, the anode is an inert anode, such as a porous or non-porous graphite, titanium-containing, tantalum-containing, or platinum-containing anode.
In a first embodiment, the protective structure is a porous polymer membrane. the pores in the membrane serve as the at least one aperture described above. The term “membrane”, as used herein, refers to a shape having two of its dimensions significantly larger (typically, at least 10, 20, 50, or 100 times) than the third dimension, which can be referred to as the thickness. Thus, the term “membrane” may adopt the shape of a film or a sheet. The membrane can have any suitable thickness. In different embodiments, the thickness is precisely, about, at least, greater than, up to, or less than, for example, 1, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, or 1000 microns (i.e., 1000 μm, where 1000 μm is equivalent to 1 mm). The thickness may also be significantly larger than 1 mm, such as 2, 3, 4, 5, 6, 7, 8, 9, or 10 mm (1 cm). The thickness may also be within a range bounded by any two of the foregoing values. The term “about”, as used herein, generally indicates no more than ±10, ±5, ±2, or ±1% from an indicated value. In some embodiments, the polymer membrane is part of a layered or laminate structure in which the polymer membrane is in contact with or bonded to an anode layer, such as a layer (e.g., sheet or foil) of aluminum, aluminum alloy, or inert anode material. A portion of the polymer membrane should be left uncovered to permit the uncovered portion of the polymer membrane to make contact with the substrate. For example, the anode layer can be in contact with or bonded with one side of the polymer membrane with the other side of the polymer membrane uncovered.
The pores in the polymer membrane are of suitable size to release the electrolyte material at an acceptable rate. In a first embodiment, the polymer membrane includes macropores, which are typically pores having a size (typically diameter, for circular pores) of above 50 nm. In different embodiments, the macropores have a size of precisely, about, at least, or greater than 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, 225 nm, 250 nm, 275 nm, 300 nm, 325 nm, 350 nm, 375 nm, 400 nm, 425 nm, 450 nm, 475 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, 1000 nm (1 μm), 2 μm, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 100 μm, 200 μm, 300 μm, 400 μm, or 500 μm, or a particular size, or a variation of sizes, within a range bounded by any two of the foregoing values. In a second embodiment, the polymer membrane includes mesopores, which are typically pores having a size of at least 2 nm and up to 50 nm. In different embodiments, the mesopores have a size of precisely or about 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm,6 nm, 6.5 nm, 7 nm, 7.5 nm, 8 nm, 8.5 nm, 9 nm, 9.5 nm, 10 nm, 11 nm, 12 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, or 50 nm, or a particular size, or a variation of sizes, within a range bounded by any two of the foregoing values. In a third embodiment, the polymer membrane includes micropores, which are typically pores having a size of less than 2 nm. In different embodiments, the micropores have a size of precisely, about, up to, or less than 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or 1.9 nm, or a particular size, or a variation of sizes, within a range bounded by any two of the foregoing values. In some embodiments, only one type of pore is included in the polymer membrane, or one or more types of pores may be excluded. In other embodiments, two or three of any of the types of pores are included, e.g., macropores and mesopores; or mesopores and micropores; or macropores, mesopores, and micropores. When two or more pore types are present, the pore sizes may be distributed within an overlapping range bounded by any two of the exemplary pore sizes provided above. Moreover, the distribution of pore sizes may be within a single pore size distribution (i.e., monomodal) or be within more than one pore size distribution (e.g., bimodal or trimodal), as typically characterized by a pore volume vs. pore size distribution plot.
The polymer membrane can have any polymeric composition, provided that it is substantially unreactive with all components of the electrolyte and can function to retain the electrolyte and release the electrolyte as intended. For most applications, the polymer is preferably flexible to the extent that it can closely follow and make consistent contact with the contours of the substrate surface on which it is being applied. In some embodiments, the polymer membrane may be required to bend or wrap around non-planar portions of the substrate. The polymer membrane can be constructed of, for example, a vinyl-addition polymer, polyalkylene oxide (e.g., polyethylene oxide or polypropylene oxide or co-polymer thereof), polyurethane, polyester, or polyurea. In other embodiments, the polymer may include a substantial inorganic component, such as found in the sol gels, polysiloxanes (e.g., polyorganosiloxanes), and hybrid organic-inorganic materials. In yet other embodiments, the polymer may be an ionogel, including those based on ionic liquid hybrid materials, as described, for example, in Chen et al., Applied Materials & Interfaces, vol. 6, pp. 7840-7845, 2014; Le Bideau, et al., Chem. Soc. Rev., 40(2):907-25, Feb. 2011; Neouze et al., Chem. Mater., 18(17), pp. 3931-3936, 2006; and U.S. Pat. No. 8,163,834, the contents of which are herein incorporated by reference in their entirety.
In particular embodiments, the polymer membrane is constructed, at least partially, of a vinyl-addition polymer. The term “vinyl-addition polymer”, as used herein, refers to any of those polymers, known in the art, derived from the addition polymerization of unsaturated monomers. Some examples of vinyl-addition polymers include, polyacrylamide, polyethylene, polypropylene, polyvinylpyridine, polyacrylate, polymethacrylate, polybutadiene, polyacrylonitrile, polystyrene, and fluorinated versions (e.g., polyvinylidene fluoride and polyhexafluoropropylene). In some embodiments, a homopolymer is used, while in other embodiments a copolymer is used, wherein the copolymer can include two, three, or more different monomers, and can be arranged, for example, as a block, alternating, graft, or periodic copolymer.
Numerous porous polymer membranes are known in the art, with many of them suited to function as materials that can be impregnated with aluminum-containing electrolytes for purposes of the instant invention. Alternatively, the porous polymer membrane can be prepared by methods well known in the art, such as by polymerization in the presence of a templating agent or volatile or porogenic substance. Drying and/or heating may be employed to aid in the production of pores. In some embodiments, the electrolyte is incorporated into the porous polymer membrane by impregnating the porous polymer membrane with the electrolyte, e.g., by absorption, which typically involves capillary action. In other embodiments, the electrolyte is incorporated into the porous polymer membrane by forming the porous polymer membrane from a reaction solution that includes the monomers and the electrolyte. In the latter case, the electrolyte becomes entrapped within spaces of the polymer as the polymer is being formed from the monomers.
In another embodiment, the protective structure is an applicator device that includes (i) a compartment in which the aluminum ion-containing electrolyte and the anode is contained, and (ii) transferring means for transferring the electrolyte from the compartment through the at least one aperture onto the metallic substrate. The compartment can be of any suitable size and shape, provided that it can hold a reservoir of the electrolyte. The compartment can be made of any suitable material, such as any of those described above for the protective structures. The transferring means is any physical feature incorporated into or integrated with the compartment that can transfer electrolyte from within the compartment to an area external from the compartment and onto a substrate. Since a voltage potential needs to be established across the substrate to the anode via the coating of electrolyte, the transferring means should be capable of transferring the electrolyte by a process in which the transferring means makes direct contact with the substrate on which the electrolyte is being applied. If the transferring means works by indirect application (e.g., by spraying), a voltage potential cannot be made between the cathodic substrate and anode. For this reason, spraying may not be considered. However, spraying may be considered if the coating of electrolyte made by spraying is subsequently made to be in electrical communication with the anode so that a voltage potential between the anode and cathodic substrate can be established. In some embodiments, the electrolyte may be rendered highly viscous (e.g., by inclusion of viscosity enhancing agents or hardening agents) so as to form an adherent coating (i.e., retained film) that maintains its shape with minimal spreading over time, and the adherent coating subsequently contacted with an anode in interconnection with the cathodic substrate to form a coating of aluminum. Such adherent coating may be applied by, for example, spraying or painting (e.g., brushing or rolling). The coating may also include a component (e.g., photoresponsive ionogel or crosslinkable agent) that may function to harden the coating upon exposure to a stimulus, such as by irradiation or chemical treatment.
In one embodiment, the transferring means works by transferring the electrolyte by capillary action. Such transferring means may include, for example, fibers (e.g., filaments or strands), which may or may not be hollow. When the electrolyte is placed in the compartment, one end of the fibers should be in contact with the electrolyte, with the remaining length of the fibers traversing the aperture and extending to a region outside of the compartment. In another embodiment, the transferring means may be a soft foam material capable of becoming impregnated or saturated with the electrolyte. As the electrolyte can be transferred onto a substrate by brushing with fibers or a foam saturated with electrolyte, the above-described applicator device can be referred to as a “brush plating device”. In some embodiments, the applicator device relies only on the passive transferring means (e.g., brushing) to transfer the electrolyte. In other embodiments, the applicator device further includes an active transferring means to aid in transfer of the electrolyte through the passive transferring means. The active transferring means can be, for example, a pumping element that serves to apply pressure on the electrolyte reservoir to encourage movement of the electrolyte into the passive transferring means.
In another embodiment, the transferring means works by transferring the electrolyte by active spreading. The active spreading is achieved by using mechanical action to move a transferring element in such a manner that movement of the transferring element transfers electrolyte from the compartment reservoir to an area outside the compartment. The spreading means can be, for example, a rotatable ball traversing an aperture in the compartment, wherein the rotatable ball is in contact with electrolyte in the compartment and can transfer electrolyte to a region outside of the compartment by being rotated, e.g., as in a ball point pen. The ball may be non-porous, in which case only the surface of the ball functions to spread the electrolyte. Alternatively, the ball may be porous, in which case the interior and surface of the ball function to transfer and spread the electrolyte. As the electrolyte can be transferred onto the substrate by contacting the ball with the substrate and employing ball rotation (e.g., by friction), the above-described applicator device can be referred to as a “ball pen plating device”. In another embodiment, the transferring means may be a roller, with a mechanism simulating a paint roller, instead of a ball. The roller may be porous or non-porous, as described above for the rotatable ball. In the case of a roller, as the electrolyte can be transferred onto the substrate by contacting the roller with the substrate and rolling the roller (e.g., by friction), the above-described applicator device can be referred to as a “roll plating device”. As in the case of the brush plating device, the ball pen or roll plating device may or may not work in concert with an active transfer element, such as a pumping element, to improve transfer of the electrolyte to the ball or roller element.
The aluminum ion-containing electrolyte can be any of the liquid aluminum-containing electrolytes known in the art useful in electroplating a layer of aluminum onto a metallic substrate. In order to conduct electrical current between the anode and substrate, the electrolyte should be suitably conductive. In a first embodiment, the electrolyte includes aluminum ions and counterions dissolved in a non-aqueous solvent, e.g., an alkyl aluminum or aluminum halide compound dissolved in an organic (non-aqueous) solvent, such as an alkyl aluminum fluoride dissolved in toluene (as in the SIGAL® process) or use of another organic solvent, such as benzene, cyclohexane, tetrahydrofuran, or dimethyl sulfide. Such electrolytes are described, for example, in U.S. Pat. Nos. 4,003,804, 4,032,413, 4,071,415, 4,126,523, 4,152,220, 4,379,030, 4,721,656, and U.S. Application Publication No. 2011/0253543, the contents of which are herein incorporated by reference in their entirety. In a second embodiment, the electrolyte includes an aluminum-containing ionic liquid (i.e., as an aluminum-containing molten salt or solution of ionic liquid in a solvent). The aluminum-containing ionic liquid can be those, well known in the art, which include chloroaluminate anions, such as those obtained by mixing anhydrous AlCl₃with an organic chloride salt, such as 1-ethyl-3-methyl imidazolium chloride (EMImCl), 1-(1-butyl)pyridinium chloride (N-BPCl), or other related salt. Such ionic liquid electrolytes are described, for example, in U.S. Pat. Nos. 2,446,331, 5,041,194, 5,827,602, 7,915,426, 8,778,163, U.S. Application Publication No. 2012/0189778, and Q. Liao, et al., I. Electrochem. Soc., vol. 144, no. 3, March 1997, the contents of which are herein incorporated by reference in their entirety.
In particular embodiments, the electrolyte is exclusively or includes an ionic liquid composition containing a trihalo aluminum (III) species complexed with at least one organic uncharged (neutral) ligand (also referred to as “ligand”). The halogen atoms in the trihalo aluminum (III) species can be selected from any of the halogens, i.e., fluorine, chlorine, bromine, and iodine, which respectively correspond to aluminum fluoride (AlF₃), aluminum chloride (AlCl₃), aluminum bromide (AlBr₃), and aluminum iodide (AlI₃), and multiples thereof, such as the dimer Al₂Cl₆. Thus, the ionic liquids described above can be conveniently described according to the general stoichiometric formula AlX₃.L_n, where X is a halogen atom, L is an organic uncharged ligand, and n is an integer of at least 1, typically 1, 2, or 3. Molecules of solvation (i.e., adducts) may or may not also be included in the formula. Multiples of the foregoing general formula (e.g., Al₂X₆.L_2n) are also embraced by the general formula. The term “complex” or “complexed”, as used herein, indicates a bonding interaction between the neutral organic ligand and the aluminum ion. The association between the aluminum ion and ligand in the above-described ionic liquid is typically a dative covalent interaction, generally between the electron-deficient aluminum ion and electron-donating heteroatom in the ligand. As the ligand considered in the above-described ionic liquid is uncharged, there is no ionic bonding between the aluminum ion and the ligand. There is, however, an ionic association between the aluminum ion and the halide atoms, which provides the ionic character of the composition.
The organic uncharged ligand particularly considered herein is or includes a ring structure having at least three ring carbon atoms and at least one ring heteroatom selected from nitrogen and sulfur. In different embodiments, the ring heteroatoms may be selected from only nitrogen atoms, or only sulfur atoms, or a combination of nitrogen and sulfur atoms, or a combination of nitrogen and oxygen atoms, or a combination of sulfur and oxygen atoms. The ring structure may be unsaturated (e.g., aliphatic or aromatic) or saturated. The ring structures generally contain a total of five, six, or seven ring atoms (i.e., five-, six-, or seven-membered rings), at least three of which are ring carbon atoms and at least one of which is a heteroatom. Generally, the ring structure includes one, two, or three ring heteroatoms.
Some examples of five-membered unsaturated rings containing at least one ring nitrogen atom include pyrrole, imidazole, pyrazole, oxazole, isoxazole, thiazole, and the triazole rings (i.e., 1,2,3-triazole and 1,2,4-triazole). Some examples of six-membered unsaturated rings containing at least one ring nitrogen atom include pyridine, pyrazine, pyrimidine, pyridazine, 1,3,5-triazine, and oxazine rings. Some examples of seven-membered unsaturated rings containing at least one ring nitrogen atom include azepine and the diazepine rings (e.g., 1,2-diazepine, 1,3-diazepine, and 1,4-diazepine).
Some examples of five-membered saturated rings containing at least one ring nitrogen atom include pyrrolidine, imidazolidine, oxazolidine, and thiazolidine rings. Some examples of six-membered saturated rings containing at least one ring nitrogen atom include piperidine, piperazine, morpholine, and thiomorpholine rings. Some examples of seven-membered saturated rings containing at least one ring nitrogen atom include azepane and diazepane rings.
Some examples of unsaturated rings containing at least one ring sulfur atom include thiophene, thiazole, isothiazole, and thiadiazole rings. Some examples of saturated rings containing at least one ring sulfur atom include tetrahydrothiophene and thiopyran rings.
The ring structure containing the at least one heteroatom may or may not also be fused to another ring, thereby resulting in a fused ring structure. Some examples of fused ring structures include indole, purine, quinoline (benzopyridine), isoquinoline, benzimidazole, benzoxazole, benzothiazole, benzoxazoline, benzothiophene, benzoxazine, and phenoxazine.
In some embodiments, the ring structure of the uncharged ligand includes at least one alkyl substituent (i.e., alkyl group) containing at least one carbon atom. The alkyl substituent can improve the properties of the ionic liquids, particularly by decreasing their melting points, and preferably making them room temperature ionic liquids. In different embodiments, the alkyl group can include precisely or at least one, two, three, four, five, six, seven, eight, nine, ten, eleven, or twelve carbon atoms, or a number of carbon atoms within a range bounded by any two of the foregoing numbers. The alkyl group can be straight-chained or branched. Some examples of straight-chained alkyl groups include methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, and n-dodecyl groups. Some examples of branched alkyl groups include isopropyl, isobutyl, sec-butyl, t-butyl, isopentyl, neopentyl, 2-methylpent-1-yl, 3-methylpent-1-yl, isohexyl, isoheptyl, and isooctyl groups.
In other embodiments, the ring structure of the uncharged ligand includes at least one alkenyl substituent (i.e., alkenyl group) containing at least two carbon atoms and the presence of at least one carbon-carbon double bond. The alkenyl substituent can also improve the properties of the ionic liquids, particularly by decreasing their melting points, and preferably making them room temperature ionic liquids. In different embodiments, the alkenyl group can include precisely or at least two, three, four, five, six, seven, eight, nine, ten, eleven, or twelve carbon atoms, or a number of carbon atoms within a range bounded by any two of the foregoing numbers. The alkenyl group can be straight-chained or branched. Some examples of straight-chained alkenyl groups include vinyl, propen-1-yl (allyl), 3-buten-1-yl (CH₂═CH—CH₂—CH₂—), 2-buten-1-yl (CH₂—CH═CH—CH₂—), butadienyl, 4-penten-1-yl, 3-penten-1-yl, 2-penten-1-yl, 5-hexen-1-yl, 6-hepten-1-yl, and the like. Some examples of branched alkenyl groups include propen-2-yl, 1-buten-3-yl (CH₂═CH—CH.—CH₃), 1-buten-2-yl (CH₂═C.—CH₂—CH₃), 1-penten-4-yl, 1-penten-3-yl, 2-penten-4-yl, 2-penten-3-yl, and 1,4-pentadien-3-yl.
In some embodiments, the at least one alkyl or alkenyl group attached to the ring structure is composed of only carbon and hydrogen atoms. In other embodiments, the alkyl or alkenyl group may include one or more heteroatoms, such as one or more selected from oxygen, nitrogen, sulfur, and halogen atoms. A particular example of an alkyl group substituted with at least one heteroatom is an alkyl group containing at least one oxygen atom, e.g., a hydroxy group (OH), or ether group (—O—) as found in the alkoxides (i.e., —OR, where R is an alkyl group with or without further heteroatom substitution) or groups of the general formula —(CH₂)_s—(O—CH₂CH₂)_tH, where s is 0 or an integer from 1 to 12 and t is 0 or an integer from 1-12. A protic group, such as OH, should not be present in the ionic liquid if it becomes deprotonated by other groups in the ionic liquid or by another component in contact with the ionic liquid. In some embodiments, the alkyl group may be a partially or completely fluorinated alkyl group, such as CF₃, or CF₂CF₃, or a fluorinated sulfone, such as —SO₂F or —SO₂CF₃.
The at least one alkyl or alkenyl substituent can be included on the ring structure provided that it does not result in a charged ligand. For example, in the case of an unsaturated ring, the alkyl or alkenyl substituent must not be located on a ring nitrogen atom if the nitrogen atom is part of an unsaturated bond, since this would result in a positively charged ring nitrogen atom (i.e., the alkyl or alkenyl substituent can only be located on a ring carbon atom in that case). If the ring nitrogen atom is not part of an unsaturated bond (either in an unsaturated or saturated ring), then the ring nitrogen atom can bear a single alkyl or alkenyl substituent while remaining uncharged, as long as the ring nitrogen atom is not part a fused side of a fused ring system. The alkyl or alkenyl substituent must not be located on a ring sulfur atom since this would result in a positively charged ring sulfur atom.
In particular embodiments, the ionic liquid is or includes an alkyl-substituted or alkenyl-substituted pyridine or imidazole ring, wherein the alkyl or alkenyl substituent is on a ring carbon atom of the pyridine or imidazole ring. The alkyl-substituted pyridine ligand can be, for example, a 2-alkyl-pyridine, 3-alkyl-pyridine, 4-alkyl-pyridine, 2,3-dialkyl-pyridine, 2,4-dialkyl-pyridine, 3,4-dialkyl-pyridine, 2,3,4-trialkyl-pyridine, 3,4,5-trialkyl-pyridine, or 2,3,5-trialkyl-pyridine, wherein it is understood that the number designating the alkyl group is relative to the location of the ring nitrogen atom, where the ring nitrogen atom is designated as position 1 (thus, a 4-alkyl-pyridine contains the alkyl group in a position directly opposite from the ring nitrogen atom in the pyridine ring). In similar fashion, the alkyl-substituted imidazole ligand can be, for example, a 2-alkylimidazole, 4-alkylimidazole, 2,4-dialkylimidazole, 4,5-dialkylimidazole, or, 2,4,5-trialkylimidazole, wherein it is understood that the number designating the alkyl group is relative to the location of the ring nitrogen atoms, which occupy positions 1 and 3 on the imidazole ring. The alkyl group in any of the above exemplary alkyl-substituted pyridine or imidazole ligands can be replaced with an alkenyl group to provide an equal number of exemplary alkenyl-substituted pyridine and imidazole ligands. The ring may also include a combination of alkyl and alkenyl groups. In some embodiments, the alkyl-substituted ring contains no substituent other than one or more alkyl and/or alkenyl substituents, i.e., remaining positions on the ring are occupied by hydrogen atoms.
The ionic liquid described herein is typically a liquid at room temperature (e.g., 15, 18, 20, 22, 25, or 30° C.) or lower. However, in some embodiments, the ionic liquid may not be a liquid at room temperature, but becomes a liquid at a higher temperature than 30° C. if it is used at an elevated temperature that melts the compound to be an ionic liquid. Thus, in some embodiments, the ionic liquid may have a melting point of up to or less than 100, 90, 80, 70, 60, 50, 40, or 35° C. In other embodiments, the ionic liquid may be a liquid at a temperature of or less than 100, 90, 80, 70, 60, 50, 40, or 35° C. In other embodiments, the ionic liquid is a liquid at or below 10, 5, 0, −10, −20, −30, or −40° C. The term “liquid”, as used herein, indicates an ability of the substance to readily flow, typically no more than about 1,000 centipoise (1,000 cP). In different embodiments, the viscosity of the ionic liquid is up to or less than, for example, 1,000, 800, 700, 600, 500, 400, 300, 200, 100, 50, 25, 10, 5, or 1 cP, or a viscosity within a range bounded by any two of these values.
The ionic liquids described above are generally prepared by combining and mixing an aluminum trihalide (e.g., AlCl₃) and the organic neutral ligand in the liquid state in a molar ratio that produces a composition that behaves as an ionic liquid at a desired temperature, such as room temperature. In some embodiments, the mixture is heated to ensure dissolution of the aluminum trihalide in the organic neutral ligand. In particular embodiments, the ratio of aluminum trihalide to organic neutral ligand is precisely or about, for example, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, or 2:1, or a ratio within a range bounded by any two of the foregoing values.
In one embodiment, the electrolyte (electroplating solution) contains the ionic liquid in the substantial or complete absence of a solvent, i.e., typically as a molten form of the ionic liquid. In another embodiment, the electrolyte contains the ionic liquid in admixture with one or more solvents. The solvent may function, for example, to help solubilize other components in the electrolyte (e.g., an electrolyte salt), improve wettability, or improve qualities of the aluminum deposit. The one or more solvents can be selected from any of the organic and inorganic solvents known in the art, provided that the solvent or solvent mixture does not adversely react or interact with the ionic liquid or the plating process. The solvent or solvent mixture should be completely miscible with the ionic liquid and any other components that may be included in the electrolyte.
The organic solvent can be ionic or non-ionic. In the case of an ionic solvent, the ionic solvent can be any of the ionic liquids of the art or as described herein. In the case of a non-ionic solvent, the non-ionic solvent can be, for example, a hydrocarbon, alcohol, ketone, carbonate, sulfone, siloxane, ether, nitrile, sulfoxide, or amide solvent, or a mixture thereof. Some examples of hydrocarbon solvents include hexanes, cyclohexane, benzene, toluene, decalin, and xylenes, or halogenated versions of hydrocarbons, e.g., methylene chloride, trichloroethylene, or perchlorethylene. Some examples of alcohol solvents include methanol, ethanol, n-propanol, isopropanol, n-butanol, and isobutanol, and the diols, such as ethylene glycol, diethylene glycol, and triethylene glycol. Some examples of ketone solvents include acetone and 2-butanone. Some examples of carbonate solvents include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC), and fluorocarbonate solvents (e.g., fluoroethylene carbonate and trifluoromethyl propylene carbonate). Some examples of sulfone solvents include methyl sulfone, ethyl methyl sulfone, methyl phenyl sulfone, methyl isopropyl sulfone (MiPS), propyl sulfone, butyl sulfone, tetramethylene sulfone (sulfolane), and phenyl vinyl sulfone. Some examples of siloxane solvents include hexamethyldisiloxane (HMDS), 1,3-divinyltetramethyldisiloxane, the polysiloxanes, and polysiloxane-polyoxyalkylene derivatives. Some examples of ether solvents include 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, diglyme, triglyme, 1,3-dioxolane, and the fluorinated ethers (e.g., mono-, di-, tri-, tetra-, penta-, hexa- and per-fluoro derivatives of any of the foregoing ethers). Some examples of nitrile solvents include acetonitrile, propionitrile, and butyronitrile. Some examples of sulfoxide solvents include dimethyl sulfoxide, ethyl methyl sulfoxide, diethyl sulfoxide, methyl propyl sulfoxide, and ethyl propyl sulfoxide. Some examples of amide solvents include formamide, N,N-dimethylformamide, N,N-diethylformamide, acetamide, N,N-dimethylacetamide, N,N-diethylacetamide, gamma-butyrolactam, and N-methylpyrrolidone. Other organic solvents include hexamethylphosphoramide (HMPA) and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU). In the case of an inorganic solvent, the inorganic solvent is other than water, such as carbon disulfide or supercritical carbon dioxide. In some embodiments, any one or more of the above classes or specific types of solvents are excluded from the electroplating solution.
If a solvent is included, the one or more ionic liquids can be included in any suitable amount, typically at least 10 wt % by weight of solvent and ionic liquid. In different embodiments, the ionic liquid is included in an amount of precisely, about, at least, or above, for example, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 95, 98, or 100 wt % by weight of the ionic liquid plus solvent, or an amount within a range bounded by any two of the foregoing exemplary values.
In some embodiments, one or more salts of an alkali or alkaline earth metal is included in the electrolyte to increase the conductivity of the electrolyte or to improve aspects of the aluminum deposit. The salt should be completely dissolved in the electrolyte at the temperature employed for electroplating. The salt can be, for example, a halide of an alkali or alkaline earth metal. Some examples of alkali halides include lithium chloride, lithium bromide, sodium fluoride, sodium chloride, sodium bromide, potassium chloride, and potassium bromide. Some examples of alkaline earth halides include magnesium chloride, magnesium bromide, and calcium chloride. The salt can be included in any desired amount in the electrolyte to suitably adjust the conductivity of the electrolyte or other aspects of the process. In some embodiments, the salt is included in an amount of 0.1, 0.5, 1, 2, 5, 10, 15, or 20 wt % by weight of the electrolyte, or in an amount within a range bounded by any two of the foregoing values.
In another aspect, the instant disclosure is directed to methods for electroplating aluminum onto a metallic substrate by use of any of the spot plating applicator devices described above. The term “spot electroplating” (or equivalently, “spot plating”), as used herein, is meant to indicate a process of electroplating in which the substrate is not submerged or dipped into an electroplating bath, and instead, electroplated in one or more regions of the substrate outside of an electroplating bath. The one or more regions of the substrate being plated each typically define a surface area less than the total surface area of the substrate. However, the instant invention contemplates the possibility where a substantial or total platable surface area of a substrate is aluminum plated by the spot plating process disclosed herein, with a result similar or commensurate to what would be provided by the substrate being submerged in an aluminum electroplating bath.
In the method, any of the applicator devices described above, charged with aluminum-containing electrolyte, is manipulated to release the electrolyte to form a coating of the electrolyte on the metal substrate. Before, during, or after a coating of the electrolyte is formed on the substrate, a suitable voltage potential is maintained across the anode and the substrate polarized as cathode in order to convert the electrolyte coating into a coating of aluminum. To achieve this, the anode should be in contact with the electrolyte (typically, in contact with the electrolyte reservoir, but possibly in direct contact with the coating). In some embodiments, as applied to those devices containing compartments, the anode is at least partially submerged within the electrolyte contained in the compartment.
In the case of an electrolyte-impregnated polymer membrane, the polymer membrane may be contacted with the substrate and suitably pressed to release the electrolyte from pores therein to form a coating on the substrate. In some embodiments, the polymer membrane includes fastening means (e.g., tape, or hook and loop fastener, such as Velcro®) to keep the polymer membrane firmly applied onto the substrate (e.g., by wrapping onto itself) and to compress the polymer membrane to encourage egress of the electrolyte. In some embodiments, the polymer membrane has a sticky or tacky quality that keeps it firmly affixed to the substrate.
Before or after a coating has been formed, and with the polymer membrane still in contact with the electrolyte coating, a voltage potential is applied between an aluminum anode (e.g., aluminum foil), which is in contact with the polymer membrane (typically, the side opposite to the side in contact with the substrate), and the metallic substrate polarized as cathode to form a coating of aluminum on the substrate. The aluminum coating is necessarily within or defined by the area bounded by the coating of electrolyte. The polymer membrane can then be removed to reveal the freshly coated layer of aluminum. If necessary, the polymer membrane can be recharged with electrolyte before being used to spot plate a different, overlapping, or same section of the substrate, or before spot plating a different substrate.
In the case of a brush plating device, the fibers of the brush, once saturated with electrolyte by capillary action or by application of pressure (e.g., by a pump on the electrolyte reservoir), are contacted with the substrate to deposit a coating of electrolyte on the substrate. With the fibers in contact with the electrolyte coating, a voltage potential is applied between the anode (in contact with the electrolyte reservoir) and the metallic substrate polarized as cathode to form a coating of aluminum on the substrate.
Similarly, by use of a ball pen or roller plating device, electrolyte is made to coat or saturate the ball or roller along with suitable mechanical action to transfer electrolyte from the ball or roller onto the substrate to deposit a coating of electrolyte on the substrate. With the ball or roller in contact with the electrolyte coating, a voltage potential is applied between the anode (in contact with the electrolyte reservoir) and the metallic substrate polarized as cathode to form a coating of aluminum on the substrate.
The metallic (conductive) substrate can have any composition for which deposition of aluminum may be desired. The metallic substrate may include, for example, one or more metals selected from titanium, tantalum, iron, cobalt, nickel, copper, and zinc, and thus, may be a substantially pure metal or a binary, ternary, or higher alloy. In particular embodiments, the metallic substrate is iron, or an iron-containing alloy, such as a steel.
The electroplating process can employ any of the conditions (e.g., temperature, concentration, voltage, current density, etc.) commonly used in the art of aluminum electroplating, provided that the conditions are suitably adjusted and modified, if necessary, to accommodate the novel electroplating processes and/or aluminum-containing electrolytes described herein. The conditions can be as disclosed, for example, in U.S. Pat. Nos. 4,003,804, 4,071,415, 4,126,523, 4,152,220, 4,379,030, and 5,041,194, the contents of which are herein incorporated by reference in their entirety.
In some embodiments, the spot electroplating process is conducted in air without alteration of the atmosphere. In other embodiments, the electroplating process is conducted under a modified atmosphere, which can be partially or completely composed of an inert gas. The inert gas may be, for example, nitrogen or argon. The use of an inert gas may be helpful in preventing or lessening exposure of the electrolyte to moisture and oxygen. In the case of a brush, ball pen, or roller plating device, in some embodiments the compartment housing the electrolyte is initially or repetitively flushed with an inert gas to further ensure protection of the electrolyte from air.
In some embodiments, the electroplating process is conducted with the electrolyte being at or below room temperature, e.g., a temperature of about, up to, or less than 15, 20, 25, or 30° C. In other embodiments, the electroplating process is conducted with the electrolyte being at an elevated temperature, such as a temperature of about, at least, or above 40, 50, 60, 70, 80, 90, 100, 110, or 120° C. In other embodiments, the electroplating process is conducted with the electrolyte being at temperature within a range bounded by any two of the foregoing exemplary temperatures. Any of the applicator devices described above can be configured to include a heating or cooling element either inside or outside of the compartment (and optionally, a temperature measuring device) to achieve an electrolyte temperature lower or higher than ambient temperature. Alternatively, or in addition, the substrate may be suitably heated or cooled to a desired temperature before, during, or after coating the substrate with the electrolyte.
The electroplating process may use direct or pulse current. Any suitable current density may also be used, such as a current density of at least 0.01, 0.05, 0.1, 0.5, or 1 A/dm²and up to 2, 5, 10, 15, 20, 25, 30, 40, or 50 A/dm². The electroplating time may be suitably varied and used in conjunction with a particular current density and temperature to achieve a desired thickness of the aluminum coating. The electroplating time may be, for example, 1, 5, 10, 20, 30, 40, 50, 60, 90, or 120 minutes depending on the current density and temperature to achieve a desired thickness. The thickness of the aluminum coating for the initial (first plate) or final (i.e., total of one or successively layered plates) may be precisely, about, at least, above, up to, or less than, for example, 1, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, or 300 microns, or a thickness within a range bounded by any two of the foregoing values.
Examples have been set forth below for the purpose of illustration and to describe certain specific embodiments of the invention. However, the scope of this invention is not to be in any way limited by the examples set forth herein.

EXAMPLES

Introduction

By one methodology, the moisture sensitivity of chloroaluminate-based ionic liquids was significantly reduced by forming polymer gel electrolytes (polymer membranes), either by impregnating liquid electrolytes into preformed membranes, or by co-casting polymer and liquid electrolytes, or by copolymerization of monomers in the presence of plasticizers. For more practical applications, the polymer gel membranes can be cast directly onto aluminum foil, and the resulting layered composite wrapped around a substrate to perform the plating. In this configuration, the aluminum foil serves as the anode and the substrate as the cathode during the electroplating process. A general depiction of the process using a polymer gel electrolyte is provided in FIG. 1.
By another methodology, the moisture sensitivity of the chloroaluminate-based ionic liquids was significantly reduced by sealing the ionic liquid electrolyte inside the reservoir of a portable plating brush. The device can be conveniently used when plating is desired at a particular location of a large substrate. A depiction of the process using a portable plating brush is provided in FIG. 2.
The above two methodologies, and variants thereof, can facilitate the electroplating process particularly in situations where the substrate is either too large to fit into a conventional plating bath or the configuration of the substrate prohibits such a plating process. These portable plating devices for electrodeposition of Al will benefit coating manufacturers and electronics assembly by reducing or eliminating the use of pyrophoric and/or toxic materials while being convenient and integratable into conventional manufacturing processes.

Experimental Section

Preparation of a room temperature molten mixture: Ethylmethylimidazolium chloride (EMIC) was purified by recrystallization and vacuum drying before use. 4-propylpyridine was purified by distillation. Acetamide and AlCl₃were purified by sublimation. The AlCl₃-containing mixtures were prepared by slowly adding AlCl₃to the imidazolium chloride (or the organic solvent) inside an argon-filled glove box. The molar mixing ratio for EMIC was fixed at an AlCl₃: EMIC ratio of 1.5:1, while that for 4-propylpyridine was fixed at an AlCl₃:4-propylpyridine ratio of 1.4:1. For acetamide, the mixing ratio was fixed at AlCl₃:Acetamide=1.2:1.
Preparation of a self-standing membrane using AlCl₃-based molten mixture: To a cooled solution of AlCl₃in dichloromethane was slowly added an equal molar amount of acrylamide with stirring. After addition, the mixture changed to a clear yellow solution. Under the protection of nitrogen, a calculated amount of AlCl₃-based molten mixture was added to this solution, followed by addition of the initiator AIBN (1 wt % of monomer). Finally, the solution was cast into a Teflon®-coated aluminum Petri dish and evaporated at room temperature to result in a self-standing membrane after 24 hrs.
Preparation of portable plating brushes for the electrodeposition of Al: The portable plating brush was made by inserting a short length of fiberglass rope into the end of plastic or glass tubing so as to provide a brush-type tip. Lodging a ceramic ball into the end of the tube provided a ball pen. A suitable amount of the AlCl₃-EtMeImCl ionic liquid was added into the barrel/reservoir of the brushes, which soaked into the rope by capillary action or was retained as a liquid in the ballpoint pen. Leaking of ionic liquid from the brush was avoided by using a high-density rope, with care taken to not add an excessive amount of electrolyte. For the ball pen, the ceramic ball was retained loosely to permit it to rotate and provide fresh electrolyte. However, a plug of potting epoxy was cast into the tube barrel and drilled with a small orifice to allow the ionic liquid to flow slowly around the ball tip without draining quickly from the reservoir.
The anode used was a large spiral of high purity 1-mm diameter aluminum wire or other source of pure Al. To incorporate the anode, the anode was forced into the end of the rope inside the barrel of the brush or immersed in the liquid contents of the ball pen. Because the devices were being tested under ambient conditions in the open atmosphere, they were flooded with an inert shielding gas (dry nitrogen or even very dry air is sufficient) to exclude moisture.
Electrochemical measurement: Cyclic voltammetry (CV) was performed inside an argon-filled glove box under different scan rates. For the three-electrode system, Pt was used for the working electrode. Al was used as counter and reference electrode. For the polymer gel membranes, a two-electrode system was used system in which copper (Cu) plate was used as the working electrode and Al plate as the anode. Platinum was treated by polishing with Al₂O₃, followed by washing with deionized water and drying. The copper electrode was treated by polishing with sand paper, followed by degreasing in acetone under ultrasonic exposure for 15 minutes, and then activated in 5 wt % HCl aqueous solution for two minutes to remove any oxide layer that may have formed. Finally, the copper electrode was rinsed thoroughly with deionized water and degreased in dichloromethane for 10 minutes to remove organic impurities and form a chloride layer resistant to oxide formation. The Al electrode was treated by polishing with sand paper, followed by activation in an acidic solution composed of 1% HNO₃, 65% H₃PO₄, 5% acetic acid, and water for 5 minutes. The Al electrode was then rinsed thoroughly with deionized water and degreased in acetone for 5 minutes. Controlled-current electrolysis experiments were performed with a simple adjustable DC power supply.

Results and Discussion

Cyclic voltammetry of molten mixtures: FIGS. 3A and 3B show the cyclic voltammograms of a) AlCl₃-EMIC (1.5:1) and b) AlCl₃-4-Propylpyridine (1.4:1) molten mixtures on a Pt electrode (2 mm in diameter) under a scan rate of 100 mV/s at room temperature. In both mixtures, the reduction peaks attributed to Al deposition and the oxidation peaks attributed to the stripping of Al were observed. The overpotential for deposition of Al in EMIC mixture was observed to be −100 mV while that in 4-propylpyridine was observed to be −125 mV, which indicates that the former (EMIC mixture) is more kinetically favorable for aluminum deposition. In addition, by comparing these two CVs, it is found that the current densities of the EMIC-based mixture are significantly higher than the current densities of the 4-propylpyridine-based mixture, which indicates a much higher ionic conductivity in the former solution.
FIGS. 4A and 4B show the CVs of the mixtures of AlCl₃and acetamide at different ratios (1:1 and 1.2:1 AlCl₃to acetamide) on a Pt working electrode (2 mm in diameter) under a scan rate of 10 mV/s. Al wire was used as the counter and reference electrode. Under similar conditions as used for the EMIC- and 4-propylpyridine-based mixtures, the deposition and stripping of Al were observed for these two mixtures. However, the current densities were found to be much higher for the 1.2:1 mixture than for the 1:1 mixture, which indicates more cation complex [AlCl₂(Acetamide)₂]⁺in the former solution. In addition, the overpotential for Al deposition was found to be only −80 mV for the 1.2:1 mixture while it was found to be −180 mV for the 1:1 mixture, suggesting that a higher amount of AlCl₃is more favorable for Al deposition.
In order to cast the polymer gel electrolyte membrane, a suitable or optimal solvent was first sought. To do this, different solvents were added to the mixture of AlCl₃-4-propylpyridine (1.5:1), and the CV was scanned at 100 mV/s at room temperature. FIGS. 5A-5D show the comparison of the CVs. The results obtained without solvent (FIG. 5A) indicate a well defined Al deposition and stripping peak. With addition of dichloromethane (DCM), as shown in FIG. 5B, the current densities became significantly increased, which indicates a much improved ionic conductivity due to reduced viscosity. However, when acetonitrile (AN) was added to the mixture, as shown in FIG. 5C, it became separated into two layers, which indicates that the coordination between AN and AlCl₃is much stronger than between AN and propylpyridine, and as a result no reversible Al deposition/stripping was observed. The same result was observed when tetrahydrofuran (THF) was used as solvent, as shown in FIG. 5D. Thus, when preparing polymer gel membranes, DCM was used as a solvent.
Self-standing polymer gel membranes containing AlCl₃-based molten mixture: When AlCl₃was mixed with acrylamide directly, a solid was formed, indicating that a polymerization reaction occurred due to the exothermal reaction. To avoid such side reaction, AlCl₃was added to acrylamide/DCM solution at 0° C. using an ice bath. After mixing them at 0° C., AICl₃-containing mixtures were added, followed by addition of AIBN as initiator and polymerization at room temperature for 24 hours. FIG. 6 shows a typical picture of the polymer gel membrane containing 60 wt % of AlCl₃-EMIC (1.5:1) mixture.
FIG. 7 shows the CVs of the membrane containing 60 wt % of AlCl₃-EMIC (1.5:1) mixture with Cu plate as the working electrode and Al plate as the counter electrode. Generally, the membrane exhibited good electrochemical behavior for the deposition and stripping of Al. It was noticed that the current densities increased with increasing scan cycles, which indicates an activation process, probably due to the residual surface oxide on the Al plate. Nevertheless, this was the first example of the membrane showing the deposition and stripping of Al. An additional reduction peak was observed at 0.1 V, which may be due to the reduction of residual double bond (acryl group) within the membrane.
Polymer gel membranes containing 60 wt % of AlCl₃-4-propylpyridine (1.4:1) and AlCl₃-acetamide (1.2:1) were also prepared. The corresponding CVs are provided in FIGS. 8A and 8B, respectively. The deposition and stripping peaks of Al were observed in both membranes. However, the current densities were much smaller than that based on the AlCl₃-EMIC mixture, mainly due to the intrinsic lower ionic conductivities of the latter two membranes.
Aluminum deposition using portable plating brush: FIG. 9 shows a photo of the portable brush-type pen during plating of aluminum on a copper substrate. When the brush was pressed against the substrate, a small amount of ionic liquid was deposited onto the substrate. When the power supply was active, the electrodeposition of Al began. The Al film was produced in a very short amount of time (about 5 seconds), and the total plating process lasted about 10 minutes. FIG. 10A provides a photo of Al deposited on a Cu coupon with the portable plating brush. An optical microscope image of the film is shown in FIG. 10B. Visually, the Al films deposited on Cu have a specular appearance.

SUMMARY

In summary, polymer gel membranes containing AlCl₃-EMIC (1.5:1), AlCl₃-acetamide (1.2:1), and AlCl₃-4-propylpyridine (1.4:1) were successfully prepared for the first time. These polymer gel membranes exhibited good electrochemical behavior for the deposition and stripping of Al. It has been shown that the selection of solvent in the process of preparation of the polymer gel membrane affects the electrochemical properties of the membrane, and that the intrinsic ionic conductivity of the ionic liquid plays a key role in the performance of the final polymer gel membrane. Another way to increase the electrochemical performance of the polymer gel membrane is to use a higher temperature. For practical applications, other polymer matrixes such as poly(vinylidene fluoride-hexafluoropropylene) (PVdF(HFP)), polyvinylpyridine, polyacrylate, polymethacrylate, polyacrylonitrile (PAN), polyethylene oxide (PEO), polyethylene, polypropylene membranes, and the like, can also be used.
In addition, a portable plating brush using an Al-based ionic liquid (AlCl₃-EtMeImCl) electrolyte was also successfully prepared. The process was used to successfully plate Al on Cu or steel substrates. Al films can be produced in a very short time (˜5 seconds), and after 5 minutes of plating time, a dense specular film of Al was obtained on Cu or steel. The portable plating brush can find a wide variety of applications in various industries, including the defense industry. Moreover, the plating process can be extended to aluminum alloy plating, such as Al—Mn, Al—Nb, Al—W plating, by controlling the composition of the ionic liquid and with addition of the appropriate alloying metals and other components.
While there have been shown and described what are at present considered the preferred embodiments of the invention, those skilled in the art may make various changes and modifications which remain within the scope of the invention defined by the appended claims.

Claims

What is claimed is:

1. A method for spot electroplating aluminum onto a metallic substrate without submersion or dipping of the metallic substrate in an electroplating bath, the method comprising:

(i) spot coating said metallic substrate with an aluminum ion-containing electrolyte contained within a protective structure possessing at least one aperture, and releasing said aluminum ion-containing electrolyte from said at least one aperture onto said metallic substrate to form a coating of said aluminum ion-containing electrolyte onto said metallic substrate, wherein said aluminum ion-containing electrolyte is in contact with an anode; and

(ii) applying a voltage potential between the anode and said metallic substrate as cathode when said aluminum ion-containing electrolyte is released from said aperture and forms a coating on said metallic substrate, wherein said coating of electrolyte conducts electrical current between said anode and metallic substrate, to produce a coating of aluminum on said metallic substrate within an area bounded by the coating of said aluminum ion-containing electrolyte.

2. The method of claim 1, wherein said protective structure is a polymer membrane in which said aluminum ion-containing electrolyte is impregnated, wherein said polymer membrane contains pores for releasing said aluminum ion-containing electrolyte onto the substrate.

3. The method of claim 2, wherein said polymer membrane is laminated with aluminum foil, serving as anode, on a side of the polymer membrane not in contact with the metallic substrate.

4. The method of claim 2, wherein said polymer membrane is constructed of a vinyl-addition polymer.

5. The method of claim 2, wherein said polymer membrane is removed to reveal a freshly coated layer of aluminum within its original bounds.

6. The method of claim 1, wherein said aluminum ion-containing electrolyte comprises a non-aqueous solvent in which aluminum ions and counterions are dissolved.

7. The method of claim 1, wherein said aluminum ion-containing electrolyte comprises an aluminum-containing ionic liquid.

8. The method of claim 1, wherein said protective structure is an applicator device comprising (i) a compartment in which said aluminum ion-containing electrolyte and said anode is contained, and (ii) transferring means for transferring said aluminum ion-containing electrolyte from the compartment through said at least one aperture onto the metallic substrate, wherein said transferring means is in contact with the metallic substrate during electrolyte application and voltage application.

9. The method of claim 8, wherein said applicator device is a brush plating device in which said transferring means comprises fibers.

10. The method of claim 8, wherein said applicator device is a ball pen plating device in which said transferring means is a rotatable ball.

11. The method of claim 8, wherein said anode is at least partially submerged within the aluminum ion-containing electrolyte.

12. The method of claim 8, wherein said compartment is flushed with an inert gas.

13. The method of claim 8, wherein said aluminum ion-containing electrolyte comprises a non-aqueous solvent in which aluminum ions and counterions are dissolved.

14. The method of claim 8, wherein said aluminum ion-containing electrolyte comprises an aluminum-containing ionic liquid.

15. The method of claim 1, wherein said metallic substrate is comprised of at least one metal selected from titanium, iron, cobalt, nickel, copper, and zinc.

16. A device useful for spot electroplating a metallic substrate with aluminum, the device comprising a protective structure suitable for housing an aluminum ion-containing electrolyte, wherein said protective structure possesses at least one aperture suitable for release of the aluminum ion-containing electrolyte, and wherein said protective structure includes an anode in a location suitable for contacting said aluminum ion-containing electrolyte.

17. The device of claim 16, wherein said protective structure houses said aluminum ion-containing electrolyte.

18. The device of claim 16, wherein said protective structure is a polymer membrane in which said aluminum ion-containing electrolyte is impregnated, wherein said polymer membrane contains pores for releasing said aluminum ion-containing electrolyte onto the substrate.

19. The device of claim 18, wherein said polymer membrane is laminated with aluminum foil, serving as anode, on a side of the polymer membrane, with a portion of the polymer membrane not laminated with aluminum foil, said portion of polymer membrane suitable for contact with the metallic substrate to deposit a coating of aluminum thereon.

20. The device of claim 18, wherein said polymer membrane is constructed of a vinyl-addition polymer.

21. The device of claim 16, wherein said protective structure is an applicator device comprising (i) a compartment suitable for housing said aluminum ion-containing electrolyte and containing said anode, and (ii) transferring means for transferring said aluminum ion-containing electrolyte from the compartment through said at least one aperture onto the metallic substrate by direct contact of the transferring means with the metallic substrate.

22. The device of claim 21, wherein said applicator device is a brush plating device in which said transferring means comprises fibers.

23. The device of claim 21, wherein said applicator device is a ball pen plating device in which said transferring means is a rotatable ball.