Electrically conductive anodized aluminum coatings

Electrolysis: processes – compositions used therein – and methods – Product produced by electrolysis involving electrolytic...

Reexamination Certificate

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C428S427000, C428S935000, C205S173000, C205S174000, C205S118000, C205S121000, C205S105000, C205S106000, C205S224000

Reexamination Certificate

active

06228241

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a porous anodic aluminum oxide coating with enhanced electrical conductivity and more particularly relates to a process for the anodic oxidation of an aluminum alloy substrate.
BACKGROUND OF THE INVENTION
Conventional anodized aluminum coatings contain pores with diameters of 10-20 nm that are present at very high density, ca. 10
10
cm
−2
. The pores are generally aligned normal to the metal surface. These pores extend through the coating thickness, with a thin “barrier” oxide, typically 10-20 nm thick, at the pore base, and, depositing material into the pores of anodic alumina in order to change the coating properties is known in the art. For example, filling with a fluorinated hydrocarbon provides lubricity, and imbibing dye into the pores can make an attractive colored surface. Depositing a small amount of certain metals into each pore creates attractive shades from gold to bronze by a light scattering phenomena. This is widely practiced commercially and is known as electrolytic coloring. This electrolytic coloring process consists generally of two steps: first, dc anodization to grow the porous oxide, for example, in sulfuric acid; and, second, an ac electrolysis in a bath containing the metal cation to be deposited. A general review of electrolytic coloring is given in chapter 8 of Vol. 1 of Wernick, Pinner and Sheasby, “The Surface Treatment and Finishing of Aluminum and its Alloys, 5th ed.”. Moreover, U.S. Pat. No. 3,382,160 issued to T. Asuda on May 7, 1968, and U.S. Pat. No. 4,431,489 issued to B. R. Baker, R. L. Smith and P. W. Bolmer on Feb. 14, 1984 are examples of prior art teachings of electrolytic coloring.
Whether or not a substance is deposited in the coating pores, it is common practice to “seal” the coating by reaction with hot water, or to “cold seal” in certain chemical baths. This step is described in Chapter 11, Vol. 2 of the above referenced work by Wernick, Pinner and Sheasby. These reactions cause the coating to swell into the pores and to make it impervious to penetration by ambient atmosphere and more resistant to corrosion.
In the prior art, the pores have been used as templates to make “nano-wire arrays” by electrolytic deposition of metal or semiconductor into the pores. In this application, the deposit in a pore serves as a “wire” of a length equal to the coating thickness. The coating may either be retained as a support for the deposit or dissolved to expose the nano-wires. This is described in a paper by Routkevitch et al, IEEE Trans. Electr. Dev. 43, 1646-58 (1996).
It has been found difficult to electrolytically deposit another oxide into the pores because this requires anodic conditions which will generally result in further growth of anodic aluminum oxide. For example, Baba, Yoshino and Kono (Adv. Metal Finishing Technology in Japan-1980, p. 129) found that deposition of a small amount of gold into the pores blocked anodic oxidation of aluminum during a subsequent anodic deposition of electrochromic tungsten oxide. In this way they created a layer that changed color in response to a change in voltage polarity. In order to get the strongest color change it would be necessary to fill all, or a majority, of the pores with the electrochromic oxide.
Japanese Patent JP 60,165,391 (Aug. 28, 1985) teaches electrolytically coloring anodized aluminum by directly depositing metal oxides into the pores. This reference also teaches using cathodic dc with solutions containing salts of the metal cation to be deposited, and ac with solutions containing oxyanions of the metal (oxide) to be deposited.
Anodized aluminum is widely used as the exterior surface for spacecraft because it is lightweight, easily fabricated, provides abrasion and corrosion resistance, and can be made to have a range of useful optical properties, described in terms of the coating absorptance and emittance. In a space environment the coating has a typical resistivity of 10
14
ohm cm (negative bias voltage on substrate). This creates a problem during operation because an electrical charge from the space plasma builds up on the surface and cannot bleed off through this highly insulating coating. High voltages (>100 V) may develop across the coating which result in arcing and sporadic discharge with a frequency that depends on details of orbit, bias voltage and location on the spacecraft. The discharges and electrical noise interfere with communication and may cause structural damage.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a coating with enhanced conductivity so that an electrical charge can bleed off through the coating and prevent buildup of excessive voltage.
It is another object of the present invention to provide coatings with a substantial decrease in resistivity.
It is a further object of the present invention to provide a coating with decreased resistivity without degrading other coating properties.
It is also an object of the present invention to provide a coating having the ability to withstand high negative bias voltage in a vacuum plasma without arcing.
It is even a further object of the present invention to provide a coating which has corrosion resistance in ambient earth atmosphere, and suitable optical properties for thermal control in a space environment.
It has been found that the resistivity can be reduced a thousandfold by filling a fraction of the pores with MnO
2
, an electronically conductive oxide. The filled pore fraction is controlled by a prior deposition of metal into the pores. The conditions for metal deposition are adjusted to control both the fraction of the pore population in which metal is deposited and the amount of metal deposited in each pore. These metal “nanoelectrodes” are sites on which MnO
2
can deposit. Only those pores in which metal has deposited can be filled with MnO
2
. The MnO
2
deposit grows from the pore base, and deposition is continued until this deposit reaches the outer surface of the coating. The vacuum plasma can make electrical contact with these conductive channels.
In the use of the terms “MnO
2
” and “manganese dioxide”, these terms are names for the deposit obtained from a manganese salt solution and not meant to specify the stoichiometry. Moreover, the deposit is likely to be a mixture of MnO
2
and suboxides of manganese with the precise composition depending on the process conditions, such as bath temperature, pH and current density.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The particular conditions for metal deposition and MnO
2
deposition are critical for making a successful coating. For efficient electrical coupling with the vacuum plasma, it is necessary to get a uniform dispersion of the MnO
2
filled pores over the coating surface. This requires that the metal sites be uniformly distributed. That is, “uniform” distribution means one for which the spatial distribution of conductive sites approaches a random, also known as a Poisson, distribution. A good distribution is obtained using ac electrolysis for the metal deposition similar to that used for prior art electrolytic coloring. There are two embodiments of the invention. One is to enhance the conductivity of a conventional anodic coating, for example, one grown in sulfuric acid and commonly known as clear anodize, and the other is to make a black anodize coating with enhanced conductivity. The first embodiment is intended to produce enhanced conductivity with minimal increase in coating absorptance, and is achieved by depositing metal into only a fraction of the pores; the amount of metal deposited being too little to impart any color to the coating. The second embodiment makes a coating with increased conductivity and with absorptance near unity, and is achieved by depositing metal into nearly all the pores, and then filling these pores with MnO
2
. In this case, the metal and the conductive oxide strongly absorb solar radiation and impart a deep black coloration to the coating. The pores of conventional black anodize coatings are filled with a black organ

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