WO2017013607A1 - Method for structural colouration of anodised aluminium by forming photonic crystals by means of current pulses - Google Patents

Abstract

The invention relates to a structural colouration method for colouring an aluminium substrate, which comprises forming a nano structure of one-dimensional photonic crystals by means of a process of periodic current pulse anodisation, the colour obtained depending on the application time of the maximum and/or minimum current density of the pulse, and the colour varying with the angle of observation.

Description

 METHOD OF STRUCTURAL COLORATION OF ALUMINUM ANODIZED BY FORMATION OF PHOTONIC CRYSTALS BY PULSES OF

STREAM

Invention Area

The present invention relates to the area of structural coloration of a substrate and more specifically to the structural coloration of an aluminum substrate by the formation of one-dimensional photonic crystals through a pulse anodization.

Background of the Invention

The coloration of aluminum by means of anodizing is widely used in applications of both structural, profiling, and even design objects.

Anodizing is a process of surface electrochemical oxidation that is carried out by electrolysis in an acid medium using the metal to be anodized as an anode. In the case of aluminum, when the process is carried out using sulfuric acid as an electrolyte, the generated oxide acquires a nanoporous structure. This nanoporous structure has pores of cylindrical geometry whose section size depends on the current densities and application times.

At present, the techniques to color these anodized aluminum are mainly based on two technologies:

one . Pigmentation coloring

2. Electrocolor by electrodeposition of metals

i In pigmentation coloration, the pigments (such as anilines) diffuse into the pores of the alumina thus providing the desired color.

On the other hand, in electrocolor coloring, metals are electrodeposted at the bottom of the pores, and the tone of the final color, produced by interference of the light reflected in the base aluminum and in the deposited metal, depends on the amount of said deposited metal. Few electrodeposltado metals produce light colors and more produce dark colors.

However, a third technique of structural coloring of aluminum obtained by anodlzado by means of current pulses generating structures known as "photon crystals" has been reported. This application incorporates notable innovations and improvements to these methods. Unlike the techniques by pigmentation or electrocolor by electrodeposition of metals, this technique does not require the use of additives to obtain color, since it arises from the interaction of light with the structure of photon crystals, also called " photon structure ". In said structure the index of refraction is modulated by porosity difference in at least two repetitive alternating layers. Photon crystals, for example 1-D or 1-D photon crystals, are structures with periodic structural and constitutive properties (spatial periods and refractive indices) that modify the propagation of light Incident in the structure, generating a phenomenon of destructive interference for a given Interval of wavelengths, or more specifically, colors. The reflectivity in this Wavelength Range is maxlmlza, thus generating a prohibited band of the photon crystal (BPCF). Due to this principle, the color obtained is brighter at a lower cost. This technique has a marked difference in the range of colors that can be obtained in comparison with the electrodeposition of metals (electrocolor), which usually results in opaque colors in the range of brown / gold tones.

These materials colored by structural coloring have various applications in optoelectronics, liquid and gas sensors, and diffraction networks, flat screens, etc. When anodizing aluminum is carried out by pulses of variable current, a structure of these characteristics is obtained, thus coloring the material in the visible spectrum.

Additionally, the interference of the light that is reflected from the photonic structure can produce colors that vary according to the angle of observation. This phenomenon is due to the difference in optical paths that light travels as it travels at different angles through the material. As the angle of the light beam increases, with respect to the normal surface, the prohibited band of the photonic crystal (BPCF) shifts towards shorter wavelengths, altering the observed color. This can be understood as a shift towards the blue color as the angle of the light beam increases.

This property is difficult to obtain by other coloring methods and is decoratively attractive, especially in design applications such as eyeglass frames, card holders, aerosol packaging applications, etc.

The methods for obtaining such a structural coloration in aluminum substrates are known in the art. The scientific document "Structural coloring oí aluminum" published by Elsevier in 201 1, in the name of LiuYisen, Chang Yi, LingZhiyuan, HuXing, and Li Yi, discloses a procedure for the structural coloring of an aluminum substrate by anodizing by pulses In this process, the substrate is anodized with a constant current density followed by a pulse anodization within an acidic medium, where the time of each pulse is linearly variable in time. The structural coloration obtained on the aluminum substrate depends on the angle of observation. However, the times of each pulse and the amount of pulses needed to obtain a certain color with this method are very high, taking between 28 and 43 hours to color an article, according to the desired color. Additionally, to avoid distortions of the color obtained due to variations in the thickness and refractive index of the photonic layers by the acid attack of the medium, the method requires a compensation system that regulates the time of each pulse based on the depth of the anodizing . Finally, the method requires the use of high purity aluminum (99.99%). These particularities limit industrial applicability and raise the cost of the method.

US patent application US 2013/0299353 A1 in the name of Catcher Technology Co., Ltd., published on 1/14/2013 discloses a method for obtaining structural coloration on an aluminum substrate whose observed color depends on the angle of the observer. However, said request does not disclose a direct relationship between the formation of a photonic structure and the color obtained, nor the sharpness of the resulting color. Additionally, said method does not use a multilayer photonic structure.

There is then a need for a process for the structural coloring of an aluminum substrate that produces a bright and crisp color, with variable color depending on the angle of observation, and that can be carried on an industrial scale using commercially available materials and with a smaller application time required.

Brief Description of the Invention

It is then an object of the present invention to provide a method for the structural coloration of an aluminum substrate of industrial purity grade by the formation of a multidimensional photon structure multilayer through an anodlzaclón by current in pulses and later electrodeposlclón of a metal like the nickel to obtain a bright and clear coloration that depends on the angle of observation. The method of coloring of the present invention requires a relatively short process time and does not require compensatory methods or complex adjustments.

Thus, the present invention provides a quick and economical method to produce a clear structural coloration in an aluminum substrate comprising the formation of a one-dimensional photon crystal structure in said aluminum substrate, which provides coloration by light interference without the need for pigments. , and that uses commercial grade electrolytic materials and baths.

More specifically, the method of structural coloring of anodised aluminum of the present invention comprises the steps of:

- a first process of anodlzaclón at constant potential, to form a transparent protective layer,

- a second process of anodlzaclón by current in the form of periodic pulses, to form a photon nanostructure,

- a third electrodeposlclon process of a metal, such as tin, nickel, cobalt, copper, etc. at the bottom of nanopores; wherein the profile of the periodic pulses used for the second anodlzaclon process is shaped as a rectangular current density profile, formed by a first maximum current density value (l _max ), applied during a maximum current density time (t _max ), followed by a second minimum current density value (l _m¡n ), applied for a period of minimum current density (t _min ), where the maximum current density time and the minimum current density time determine the color obtained.

In a preferred embodiment of the present invention, the maximum current density corresponds to l _max values between 20-70 mA / ^cm2 and the minimum current density _m¡n l corresponds to between 2-40 mA / cm ^2.

In a preferred embodiment of the present invention, the second anodizing process uses an amount of pulses not less than 30 periodic pulses.

In another preferred embodiment of the present invention, the aluminum substrate is a commercial alloy.

In yet another preferred embodiment of the present invention, the first constant potential anodizing process is performed between 10-25 V, for 5-30 min.

In a preferred embodiment of the present invention, the electrolytic bath of the first constant potential anodizing process comprises an acid selected from sulfuric, phosphoric or oxalic acid with a concentration between 5-25% w / v.

In a preferred embodiment of the present invention, the electrolytic bath of the second current anodization process in the form of periodic pulses uses the same electrolytic bath as the first anodization process at constant potential.

In a preferred embodiment of the present invention, the electrodeposited metal during the third electrodeposition process of a metal is nickel. In a preferred embodiment of the present invention, the third electrodeposition process of a metal is carried out using concentrated solutions of salts thereof, such as tin, nickel, cobalt, copper salts, etc.

In a preferred embodiment of the present invention, the third electrodeposition process of a metal is an electrodeposition by current cycles.

In a preferred embodiment of the present invention, between 250 and 1000 current cycles are used for electrodeposition of the metal whose current density profile is of the rectangular type and comprises:

- a zero resting current density l _rep during a resting time t _rep , between 25-100 ms,

- a negative depolarization current density value l of _S poi between 0.01 -0.05 mA / cm ² during a depolarization time t of _S poi of 1 -15 ms,

- a positive deposition current density value l _dep os of 0.01 - 0.05 mA / cm ² during a deposition time t _of pos of 5-20 ms, and

- a null resting current density l _rep for a resting time t _re between 25-100 ms.

In a preferred embodiment of the present invention, prior to the first anodization process at constant potential, the aluminum substrate is subjected to a wash with acetone or water and subsequently dried with a stream of nitrogen or dry air.

In a preferred embodiment of the present invention, prior to the second current anodizing process in the form of periodic pulses, the aluminum substrate is subjected to a wash with deionized water. In a preferred embodiment of the present invention, after electrodeposition of the metal, the aluminum substrate is subjected to a wash with deionized water and subsequently dried with a stream of nitrogen or dry air.

In a more preferred embodiment of the present invention, the desired color is obtained by setting the maximum current density time and regulating the minimum current density time in the second current anodization process in the form of periodic pulses.

In a more preferred embodiment of the present invention, the desired color is obtained by setting the minimum current density time and regulating the maximum current density time in the second current anodizing process in the form of periodic pulses.

Brief description of the drawings

Figure 1 illustrates the photonic multilayer nanostructure produced by the structural coloring method of the present invention.

Figure 2 illustrates an example of current density pulse profile for current anodization in the form of periodic pulses according to the structural coloring method of the present invention.

Figure 3 illustrates an example of current density pulse profile for metal electrodeposition by current cycles according to the structural coloring method of the present invention.

Figure 4 shows the variation of the color obtained as a function of the thickness of one of the photonic crystal layers of the photonic structure and the observation angle according to an assay of the method of structural coloring of the present invention. Figures 5a and 5b illustrate the diffuse reflectance spectra of the photon structures of the colors obtained according to the method of structural coloring of the present invention.

Detailed description of the invention

The structural coloring method of the present invention is described in detail below with reference to the accompanying drawings.

The structural coloring method of the present invention is a method for obtaining the controlled formation of a structure of one-dimensional photonic crystals on an aluminum substrate. The purpose of this photonic structure is to obtain a color without pigmentation that has a variable color depending on the angle of the observer.

The objective of the method of the present invention, which is detailed below, is to obtain a coloring structure as illustrated by way of example in Figure 1, wherein said structure comprises:

one . A transparent protective layer

The function of this protective layer is to prevent the dissolution of the first layers of the photonic structure at the time of current anodizing in the form of periodic pulses; since alumina is soluble in sulfuric acid. This protective layer does not modify the colors generated by the photonic structure, since it is formed so that its thickness does not interfere with the wavelengths reflected by the photonic crystal. The size of the pore sections of this protective layer varies between 13 and 50 nm. 2. A photonic structure.

The photonic structure itself is a one-dimensional periodic porous layer, product of an anodizing process, which defines the final color of the substrate. In the coloring method of the present invention, the photonic structure is obtained by anodizing the aluminum substrate with a periodic pulse current, which is to say, with a current that varies periodically between a maximum current density and a density of minimum current Said pulse current anodizing process produces a photonic nanostructure consisting of a plurality of nanometric porous layers disposed one above the other, corresponding to the anodizing pulses. As mentioned earlier, these types of structures are known in the art as one-dimensional photonic crystals.

The final color obtained depends on the index of refraction and the thickness of each layer. These two parameters of structure formation can be adjusted by controlling the density of the maximum and minimum currents in the anodizing pulses and the application time of said maximum currents and in order to obtain the desired color.

3. A dark metallic background

In the last stage of manufacturing the coloring layer, an electrodeposition of a metal, preferably nickel, is performed to give the structure a dark background. Said dark background, in conjunction with the photonic structure allows to obtain a color of greater brightness. This principle is based on the fact that the light that is reflected from the structure not only corresponds to the light reflected by the prohibited band of the photonic crystal (BPCF) but also by spurious light coming from the reflection in the aluminum, which alters the final color of the substrate. The dark background product of the Metal electrodeposition absorbs the wavelengths of these unwanted reflections, allowing to obtain a more defined and brighter color.

Execution Example

To produce this coloring structure, formed by the protective layer, the photonic structure and the dark metal background, in an embodiment of the present invention by way of example the structural coloring method of the present invention comprises the steps of:

one . A first wash and degreasing of the surface to be colored,

 2. A first process of anodizing at constant potential to form the protective layer,

 3. A second process of current anodization in the form of periodic pulses to form the photonic nanostructure,

 4. A second surface wash,

 5. A metal electrodeposition, and

 6. A third wash.

These steps are described in detail below.

1 - First wash

The substrate to be colored is washed and degreased by an acetone bath and subsequently dried in a stream of nitrogen or dry air.

2- First anodizing process - formation of the Transparent Protective layer

The protective layer is formed by anodizing the aluminum substrate with direct current. This anodizing process is typically performed. maintaining a constant potential (10-25 V) for a predefined time, (5-30 min) in a solution of sulfuric acid with a concentration of 5-25% m / V.

3- Second anodizing process - formation of the photonic structure

The photonic nanostructure is the layer of material that provides the structural color to the aluminum substrate. Said structure is formed by anodizing the aluminum substrate by means of a current in the form of periodic pulses. This anodizing process is carried out by applying numerous current pulses, preferably more than 30, in the same electrolytic bath used in the previous stage. The expression "pulse" current density profile rectangular current formed by a peak value l _max, followed by a minimum current value or valley _m¡n l, more commonly application time to the maximum value. The color obtained will depend on the maximum and minimum current density of each pulse and the application time of each one. In the method of structural coloration of the present invention, a rectangular current density pulse profile is established consisting of a positive constant maximum current density value between 20-70 mA / cm ² , applied during a density time maximum current, followed by a positive minimum current density value between 2-40 mA / cm ² , applied during a minimum current density time, as shown in the diagram in Figure 2. Varying the application time From these current densities in relation to each other, different colors can be obtained on the surface.

In this method, the amount of pulses does not affect the color obtained, but the reflectivity of the photonic crystal in the zone of the prohibited band of the photonic crystal (BPCF). 4- Second wash

Prior to the metal electrodeposlclone stage, the substrate is washed with delifted water.

5- Metal electrodeposition

The aluminum substrate is subjected to an electrodeposition of a metal such as nickel, tin, cobalt or copper with a cycle current. Said metal electrodeposition is preferably performed by applying between 250-1000 current cycles according to the current density profile illustrated in Figure 3. Preferably, the electrodeposited metal is nickel.

6- Third wash

At the end of the coloring process, the substrate is washed again with deionized water and dried with a stream of nitrogen or dry air.

The resulting nanostructure is illustrated in Figure 1 typically with micrometric thicknesses and results in a structural, bright and crisp coloration, the color of which varies according to the angle of observation. The total time of application of the method is approximately 30 minutes, unlike the techniques known in the art, whose processes can take several hours, which provides a great economy of resources with the consequent reduction of costs.

To obtain different colors, it is necessary to modify some of the parameters that define the color to be obtained, that is the refractive index and the thickness of the layers of the photonic structure. The variation of the applied current density modifies both parameters, while the time modifies mainly the thickness. Due to these relationships, it is substantially simpler to modify only the current application time and maintain the maximum and minimum current densities at respective fixed values. More specifically, the maximum and minimum current density values (l _max , l _m¡n ) are set, the application time of one of the maximum current density or the minimum current density is set and the various colors are tuned varying the time of application of the other. Consequently, it is possible to synthesize different nanostructures that reflect colors throughout the visible spectrum.

This type of color control obtained is simple to implement and does not require any sophisticated control or compensation system, and does not require any adjustment during the anodizing process.

Example of Obtaining Specific Colors

A series of tests were carried out using this color tuning methodology, where the application time of one of the maximum or minimum currents is set and the application time of the other is varied. For these tests, 50 x 28 x 2 mm samples of 6061 aluminum for industrial use were used, with a purity of between 95.85 and 98.61%. The electrolytic bath corresponds to a solution of sulfuric acid in a concentration between 5-25% w / v and an Autolab PGSTAT30 potentiostat was used as the source.

The use of industrial grade aluminum, simple power equipment, commercially available bathrooms and the reduced total time of the process substantially minimize the cost of its industrial application, both small and large scale.

Colors were obtained by reflection in the shades of violet, blue, green, orange, brown or brown, etc. varying the pulse times of l _max and l _m ¡„between 5 and 50 seconds. In each case you can set the time t _max , for example. in 5 seconds and vary the time t _m ¡„between 5 and 50 seconds. On the other hand, in a second In carrying out the method, the application time of the minimum current density l _min in 20 seconds is set and the application time of the maximum current l _max is varied to obtain the desired color.

It can be seen that the color obtained is variable according to the angle of observation as seen in Figure 4. As mentioned above, the color change with the angle is a characteristic property of photonic crystals, due to the difference in optical paths that light travels as it travels at different angles through the material. As the angle of the light beam increases, with respect to the normal one, the photonic band gap shifts towards shorter wavelengths, that is, color shift towards blue.

To determine the wavelengths corresponding to each of the colors obtained, diffuse reflectance spectra of the photonic structures were performed as seen in Figures 5a and 5b. Forbidden bands corresponding to each color are shown in these figures. It is observed that it is feasible to easily obtain alumina substrates on colored aluminum throughout the entire visible spectrum, which results in the design of structures with a wide variety of colors and of varying sizes.

Claims

Claims one . A method of structural coloring of anodized aluminum by periodic pulses to color an aluminum substrate, characterized in that it comprises the steps of - a first anodization process at constant potential, to form a transparent protective layer, - a second process of current anodization in the form of periodic pulses, to form a photonic nanostructure, - a third electrodeposition process of a metal, such as tin, nickel, cobalt and copper, at the bottom of the nanopores, where the profile of the periodic pulses used for the second anodizing process is shaped as a density profile of rectangular current, formed by a first maximum current density value, applied during a maximum current density time, followed by a second minimum current density value, applied during a minimum current density time, where the time of Maximum current density and the minimum current density time determine the color obtained. 2. The method according to claim 1, characterized in that the maximum current density is 20-70 mA / cm ² and the minimum current density is 2-40 mA / cm ² . 3. The method according to any of the preceding claims, characterized in that the second process of current anodizing in the form of periodic pulses uses not less than 30 periodic pulses. 4. The method according to any of the preceding claims, characterized in that the aluminum substrate is a commercial alloy. 5. The method according to any of the preceding claims, characterized in that the first anodlzaclon process at constant potential is performed at 10-25 Volts for 5-30 minutes. 6. The method according to any of the preceding claims, characterized in that the electrolytic bath of the first constant anodlzaclone process comprises an acid selected from sulfuric, phosphoric or oxalic acid with a concentration between 5-25% w / v. 7. The method according to any of the preceding claims, characterized in that the electrolytic bath of the second periodic pulse anodlzaclone process uses the same bath as the first constant potential anodlzaclon process. 8. The method according to any of the preceding claims, characterized in that the electrodeposted metal during the third electrodeposlclone process of a metal is nickel. 9. The method according to any of the preceding claims, characterized in that the third electrodeposlclone process of a metal is carried out using concentrated solutions of salts thereof, such as tin, nickel, cobalt, or copper salts. 10. The method according to any of the preceding claims, characterized in that the third electrodeposlclone process of a metal is an electrodeposlclón by current cycles. eleven . The method according to claim 10, characterized in that the electrodeposlclón of the metal uses 250-1000 current cycles whose current density profile is of the rectangular type and comprises: - a zero resting current density l _rep during a resting time t _rep , between 25-100 ms, - a value of current density of negative depolarization l _deS poi 0.01 -0.05 mA / cm ² for a time t spoi depolarization of 1 - 15 ms, - a positive deposition current density value l _dep os of 0.01 - 0.05 mA / cm ² during a deposition time t _dep os of 5-20 ms, and - a null resting current density l _rep for a resting time t _rep between 25-100 ms.  12. The method according to any of the preceding claims, characterized in that prior to the first anodizing process at constant potential, the aluminum substrate is subjected to a wash with acetone or water and subsequently dried with a stream of nitrogen or dry air. 13. The method according to any of the preceding claims, characterized in that prior to the second current anodizing process in the form of periodic pulses, the aluminum substrate is subjected to a wash with deionized water. 14. The method according to any of the preceding claims, characterized in that after the metal electrodeposition, the aluminum substrate is subjected to a wash with deionized water and subsequently dried with a stream of nitrogen or dry air. 15. The method according to any of the preceding claims, characterized in that the desired color is obtained by setting the maximum current density time and regulating the minimum current density time in the second current anodizing process in the form of periodic pulses . 16. The method according to any of the preceding claims, characterized in that the desired color is obtained by setting the minimum current density time and regulating the maximum current density time in the second anodlzaclon process by current in the form of periodic pulses .