FIELD OF THE INVENTION

[001] The present invention relates to methods of coating aluminum objects. More particularly, the present invention relates to methods of coating aluminum objects with a coating having an improved conductivity.

BACKGROUND OF THE INVENTION

[002] Anodizing is one of the most commonly used methods of coating and protecting aluminum objects (e.g., machined or cast aluminum parts, aluminum plates, etc.). Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer (i.e., alumina, AI2O3) on the surface of an aluminum object. The outcome of the anodizing process is a porous alumina layer having a typical thickness of several tens of microns.

[003] Some anodized surfaces of aluminum objects, for example, in the semiconductors industry, may accumulate, when in use, electrostatic charges which need to be discharged. Since alumina is an insulator, electrostatic discharging can be achieved by enhancing conductivity to the anodized alumina layer.

[004] Some attempts were made to improve the conductivity of anodized alumina layers. For example, pores of a porous alumina layer were filled with electrically conductive metal oxide nanocomposite. The metal oxide nanocomposite is introduced into the pores of the alumina layer in an electrodeposition process. Another known attempt to increase the conductivity of an alumina layer involved growing carbon nanotubes inside the pores of the alumina layer in a chemical vapor deposition (CVD) process.

[005] However, the methods mentioned hereinabove are typically expensive and are limited in the size of the object that can be treated when applying either of these methods. These methods are unsuitable for treating large objects, for example, of more than 50 cm length and more specifically plates of several meters long. [006] Accordingly, there is a need for a cost-effective method for improving the conductivity of anodized coating.

SUMMARY OF THE INVENTION

[007] Some aspects of the invention may be related to a method of coating an aluminum object, the method may include anodizing at least a portion of a surface of the aluminum object to form an anodized layer of at least 25 pm; and introducing conductive nanoparticles into pores in the anodized layer.

[008] In some embodiments, introducing the conductive nanoparticles into the pores may be conducted by providing kinetic energy to the conductive nanoparticles in a suspension. In some embodiments, providing the kinetic energy to the conductive nanoparticles may be conducted by at least one of: ultrasonic stirring and pressure spraying. In some embodiments, introducing the conductive nanoparticles into the pores is by one of: capillary forces and electric voltage. In some embodiments, introducing the conductive nanoparticles into the pores may be conducted by wetting the porous anodized layer with a suspension comprising the carbon nanoparticles.

[009] In some embodiments, the conductive nanoparticles are selected from: carbon nanoparticles, metal nanoparticles and conductive metal oxides nanoparticles.

[0010] In some embodiments, the method may further include sealing residual porosity, in the anodized layer, left unfilled with the conductive nanoparticles. In some embodiments, sealing the residual porosity is by one of: hot water, sodium dichromate solution, potassium dichromate solution, nickel acetate solution, silicone, and polytetrafluoroethylene.

[0011] In some embodiments, the method may further include introducing a dye into a residual porosity. In some embodiments, introducing the dye is by: mixing the dye with the conductive nanoparticles; and introducing the mixture of the dye and the conductive nanoparticles into the pores in the anodized layer. In some embodiments, the dye is a black dye. [0012] Some additional aspects of the invention may be related to an aluminum coated object. The coted object may include: a body made from aluminum; and a coating covering at least a portion of the surface of the body. In some embodiments, the coating may include, a porous alumina layer; and conductive nanoparticles occupying at least some of the pores of the porous alumina layer.

[0013] In some embodiments, the conductive coating may further include a sealing material applied on top of the porous alumina layer configured to seal residual porosity unoccupied by the conductive nanoparticles. In some embodiments, the conductive coating may further include dye particles located inside at least some of the pores of the porous alumina layer. In some embodiments, the dye particles are black dye particles. In some embodiments, the dye particles are premixed with the conductive nanoparticles. In some embodiments, the dye particles occupy porosity unoccupied by the conductive nanoparticles.

[0014] In some embodiments, the conductive nanoparticles are selected from: carbon nanoparticles, metal nanoparticles and conductive metal oxides nanoparticles. In some embodiments, the sealing material comprises one of: aluminum hydrate, nickel acetate, cobalt acetate, sodium dichromate, potassium dichromate, silicone and polytetrafluoroethylene.

[0015] According to some embodiments of the invention, a method of coating an aluminum object is presented which may include anodizing at least a portion of a surface of the aluminum object to form an anodized layer; introducing nanotubes into pores in the anodized layer; and sealing the anodized layer thereby producing a coated aluminum object.

[0016] In some embodiments, the nanotubes may be introduced by applying, to the anodized layer on the aluminum object, a solution comprising the nanotubes, and placing the aluminum object in an ultrasonic bath.

[0017] In some embodiments, the solution is ultrasonically stirred before the application. [0018] In some embodiments, the solution is applied by a method selected from the group of methods consisting of brushing, polishing, dabbing and spraying. [0019] In some embodiments, the anodized layer further comprises a black dye.

[0020] In some embodiments, the nanotubes are black nanotubes.

[0021] In some embodiments, the thickness of the anodized layer is between about 60- 70 pm.

[0022] In some embodiments, the coated aluminum object has a resistance of about 10 ^L 8- 10 ^L 9W.

[0023] According to some embodiments of the invention an aluminum coated object is provided, which may include a body made from aluminum; and a coating covering at least a portion of the surface of the body. In some embodiments, the coating comprises: a porous alumina layer; and nanotubes occupying at least some of the pores of the porous alumina layer.

[0024] In some embodiments, the coating further comprises a sealing material on top of the porous alumina layer.

[0025] In some embodiments, the aluminum coated object further comprises black dye particles located inside at least some of the pores of the porous alumina layer.

[0026] In some embodiments, the nanotubes are black nanotubes.

[0027] In some embodiments, the thickness of the coating is between about 60-70 pm. [0028] In some embodiments, the coating has a resistance of about 10 ^L 8-10 ^L 9W.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

[0030] Fig. 1 is a flowchart of a method for coating aluminum objects according to some embodiments of the invention; and

[0031] Fig. 2 A is an illustration of a cross-section in an aluminum object coated with a coating according to some embodiments of the invention; and [0032] Fig. 2B is illustration of a microstructure of the coating illustrated in Fig. 2 A according to some embodiments of the invention; and

[0033] Fig. 3 is a flowchart of a method for coating an aluminum object and inserting nanotubes into pores of the object, according to some embodiments of the invention. [0034] It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0035] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention. [0036] Some aspects of the invention may be directed to a novel method of coating an aluminum object. Such a novel method may provide an anodized coating having an improved conductivity in comparison to the known anodized coating. The improved conductivity (reduced resistance) may allow better electrostatic charge evacuation from the anodized surface. In some embodiments, the required surface electrical resistance may be between 10 ⁶ -10 ⁹ Ohm (W), according for example, to ASTM 11.13. Higher or lower surface electrical resistance may result in an uncontrolled discharging of the accumulated electrostatic charge which may harm the electronic components included in a wafer carried or placed the anodized aluminum object (e.g., a plate). In some embodiments the surface electrical resistance may be between 10 ⁸ -10 ⁹ W.

[0037] An alumina coating according to some embodiments of the invention may include a porous alumina layer having a thickness of at least 25 microns and may further include conducting nanoparticles occupying at least some of the pores in the porous alumina layer. In some embodiments the porous alumina layer having a thickness of about 50 microns.

[0038] As used herein the term “an aluminum object” refers to all objects/elements/components made at least partially from aluminum or aluminum alloy (e.g., A1 wrought alloys and/or A1 cast alloys) . For example, an aluminum object may include: an aluminum plate, an aluminum box, or may have any form or shape. An aluminum object may be fabricated by deformation processes, casting, sintering, machining, grinding or any combination thereof.

[0039] As used herein the term “a porous alumina layer” refers to an alumina layer formed on at least one surface of an aluminum object, for example, in an anodizing process. The porous alumina layer may include open nanopores (e.g., having the size of some 10-150 nm in diameter).

[0040] As used herein the term “conducting nanoparticles” refers to any conducting material particles. Such nanoparticles may typically have a diameter of up to 100 nm, for example, 70 nm, 50 nm, 20, nm, 10 nm, 5 nm and 0.4 nm. For example, conducting nanoparticles may include nanocarbon particles (e.g., carbon nanotubes, graphene, graphene-oxide, etc.), nanometal particles (e.g., Au, Ag, Cu, Pt, Pd, Ru, Re, etc.), conducting metal oxides nanoparticles (e.g., RuO _x , etc.) and the like.

[0041 ] Reference is now made to Fig. 1 which is a flowchart of a method 100 for coating an aluminum object according to some embodiments of the invention. Method 100 may include (see in step 110) anodizing at least a portion of a surface of an aluminum object to form an anodized layer of having a thickness of, for example, at least 25 pm, at least 30 pm, 40 pm, 50 pm, 60 pm, 70 pm or more. The layer may be produced using a known anodizing process. A nonlimiting example for an anodization process may include, washing and cleaning at least one surface of the aluminum object, initial etching of the at least one surface in an acidic solution for 20 seconds, and introducing the least one surface into an anodizing bath, e.g., complying with military standard MIL -A-8625F TYPE III, resulting in obtaining a porous alumina layer. [0042] Method 100 may include (see step 120) introducing conductive nanoparticles into pores of the anodized layer. In some embodiments, the conductive nanoparticles may be provided with a kinetic energy in a suspension. The kinetic energy may assist in the insertion of at least some of conductive nanoparticles into the open porosity of the anodized layer (the porous alumina layer). For example, the conductive nanoparticles may be mixed in a liquid medium (e.g., water, alcohol, acetone, or any combination thereof) in an ultrasonic bath to form the suspension. In some embodiments the nanoparticles are nanotubes in a deionized (DI) water solution which is ultrasonically stirred. In some embodiments, the anodized layer of the aluminum object may be introduced into the ultrasonic bath, such that the kinetic energy that was acquired by the conductive nanoparticles from the ultrasonic waves may assist in the insertion of at least some of conductive nanoparticles into the pores of the anodized layer. In some embodiments, the suspension may be pressure sprayed on top of the anodized layer to cause the nanoparticles to be inserted into the pores of the anodized layer. In some embodiments the suspension is applied by at least one of the following methods: brushing, polishing, dabbing or spraying. In some embodiments the suspension (or solution) is applied using fabric. In some embodiments the fabric is clean room fabric. [0043] In some embodiments, the suspension may be introduced into the pores by capillary forces and/or electric voltage. For example, the suspension may wet an absorbing medium (e.g., a sponge) and spread across the porous alumina layer.

[0044] A nonlimiting example may include mixing carbon nanotubes with a liquid containing 90% water and 10% ethanol in an ultrasonic bath, introducing an anodized layer of an aluminum plate (e.g., SAE 6063, SAE 5083 and the like) into the ultrasonic bath and stirring the mixture in the ultrasonic bath for 20 minutes at a temperature of 50°C.

[0045] Another nonlimiting example may include mixing carbon nanotubes with a liquid containing 90% water and 10% ethanol and stirring the mixture in an ultrasonic bath for 5 minutes at a temperature of 36°C to form a suspension. The suspension may then be pressure sprayed on the anodized layer, at 6-8 atm. from a distance of 10 cm. from the surface of the anodized layer.

[0046] In some embodiments, the final microstructure obtained in the process is a composite material that includes a porous alumina layer having at least some of the open porosity filled with conductive nanoparticles., as disclosed and discussed with respect to Figs. 2A and 2B.

[0047] In some embodiments, the method may further include washing the anodized layer following the insertion of the conducting nanoparticles, using any known method. In a nonlimiting example, the washing may include dipping the object into DI water bath 4 times at a temperature of 25-30 °C.

[0048] In some embodiments, method 100 may further include step 130, in which a dye may be introduced into a residual porosity of the anodized layer. In some embodiments, the dye may be any commercial dye, for example, a black pigment. In some embodiments, the dye may be introduced following step 120, using any known method. A nonlimiting example may include, immersing the aluminum object.in a suspension containing water and black pigment, for example, TAC Black -SLH (Black 415) of Okuno Chemical Industries Co., Ltd.

[0049] In some embodiments, the dye may be mixed with the suspension of conductive nanoparticles prior to the introduction of the conductive nanoparticles into the open porosity. Accordingly, the dye particles may be introduced into the open porosity together with the conductive nanoparticles according to any one of the methods disclosed herein above.

[0050] In some embodiments, method 100 may further include step 140, in which the residual porosity in the anodized layer that was left unfilled with the conductive nanoparticles may be sealed. In some embodiments all pores are sealed whether or not they are filled with nanoparticles and/or dye particles. In some embodiments, the sealing may be performed according to any method known in the art. Some nonlimiting examples for sealing porous alumina layers may include applying at least one of: hot water, sodium dichromate solution, potassium dichromate solution, nickel acetate solution, silicone, polytetrafluoroethylene and the like.

[0051 ] Reference is now made to Fig. 2A which is an illustration of a cross-section in an aluminum object coated with an alumina layer, according to some embodiments of the invention. Object 200 may include at least one body 205 made from aluminum or aluminum alloy and a porous alumina coating 210 coating at least a portion of the surface of body 205. In some embodiments, coating 210 may be formed according to steps 110 and 120 method 100 of Fig. 1. In some embodiments, coating 210 may further include a sealing material 220. In some embodiments, sealing material may at least partially penetrate into the open porosity. Additionally, or alternatively, the sealing material may form a sealing layer.

[0052] Body 205 may be made at least partially from aluminum or an aluminum alloy (e.g., A1 wrought alloys and/or A1 cast alloys). For example, body 205 may be or my include: an aluminum plate, an aluminum box, or may have any form or shape or dimensions. Body 205 may be fabricated by deformation processes, casting, sintering, machining, grinding or any combination thereof. Some nonlimiting examples for aluminum plates may include plates having areas of 10mm x 10mm to 4000mm x 4000mm and a thickness of 1 mm to 100mm.

[0053] The microstructure of coating 210 is illustrated in Fig. 2B. as should be understood by one skilled in the art the elements in Figs. 2 A and 2B have not been drawn to scale. For example, the dimensions of some of the elements were exaggerated relative to other elements for clarity.

[0054] In some embodiments, coating 210 may include a porous alumina layer 212 and conductive nanoparticles 214 occupying at least some of the pores of porous alumina layer 212. Porous alumina layer 212 may be formed using any known anodizing method, for example, the methods disclosed with respect to step 110 of Fig. 1. Conductive nanoparticles 214 may include conducting material nanoparticles having a defining size of at most 100 nm, for example, 70 nm, 50 nm, 20, nm, 10 nm, 5 nm and 0.4 nm. As used herein a “defining size” may refer to a dimension defining the functionality and/or properties of the particles, for example, the average diameter of substantially round nanoparticle or the cross-section diameter of a nanotube. For example, conductive nanoparticles 214 may include: nanocarbon particles (e.g., carbon nanotubes, graphene, graphene-oxide, etc.), nanometal particles (e.g., Au, Ag, Cu, Pt, Pd, Ru, Re, etc.), conducting metal oxides nanoparticles (e.g., RuO _x , etc.) and the like.

[0055] In some embodiments, more than one conductive nanoparticle 214 may occupy a single pore. In some embodiments, at least some of the pores may not be occupied by conductive nanoparticles 214.

[0056] In some embodiments, coating 210 may further include dye particles 216 located inside at least some of the pores of porous alumina layer 212. In some embodiments, dye particles 216 may be premixed with conductive nanoparticles 214. In such case one or more dye particles 216 may occupy the same pores together with one or more conductive nanoparticles 214. In some embodiments, dye particles 216 are black dye particles. [0057] In some embodiments, coating 210 may further include sealing material 220 (illustrated in Fig. 2A) applied on top of porous alumina layer 212 and configured to seal residual porosity unoccupied by conductive nanoparticles 214 and optionally also unoccupied by dye particles 216. Some nonlimiting examples for sealing material 220 may include: aluminum hydrate, nickel acetate, cobalt acetate, sodium dichromate, potassium dichromate, silicone, polytetrafluoroethylene and the like. In some embodiments, sealing material 220 may completely penetrate between the open pores in porous alumina layer 212, sealing the upper portion of porous alumina layer 212 (as illustrated by the grey layer penetrating between into porous layer 212). Additionally, or alternatively, sealing material 220 may form a continuous sealing layer on top of porous alumina layer 212.

[0058] Reference is now made to Fig. 3 which is a flowchart of a method 300 for coating an aluminum object and inserting nanotubes into pores of that object, according to some embodiments of the invention.

[0059] Method 300 may include (see in step 310) anodizing at least a portion of a surface of an aluminum object to form an anodized layer of having a thickness of about 50pm. The layer may be produced using a known anodizing process. In some embodiments the process is performed according to BLACK ANODIC HARD COATING PER MIL-A- 8625F TYPE III. A nonlimiting example for an anodization process may include, washing and cleaning at least one surface of the aluminum object, initial etching of the at least one surface in an acidic solution for 20 seconds, and introducing the least one surface into an anodizing bath, resulting in obtaining a porous alumina layer.

[0060] In some embodiments following step 310 the objects are immersed in a DI water bath. For exampled the objected may be immersed for 10 minutes.

[0061] Method 300 may include (see step 320) ultrasonically stirring a solution comprising nanotubes. In some embodiments the solution comprises nanotubes and DI water and is stirred to produce a homogeneous solution. In some embodiments the solution is stirred at a temperature of 20°-60°C. In some embodiments the solution is stirred at room temperature (25 °C). In some embodiments the nanotube concentration is between about 1-10%. In some embodiments the nanotube concentration is between about 2-6%. In some embodiments the nanotube concentration is about 2.5%. In some embodiments the nanotube concentration is about 5%. In some embodiments the nanotubes are black nanotubes.

[0062] Method 300 may include (see step 330) applying the nanotube solution to the anodized layer. The solution may be applied by at least one of the following methods: brushing, polishing, dabbing or spraying. In some embodiments the suspension or solution is applied using fabric. In some embodiments the fabric is clean room fabric. In some embodiments the solution is applied using circular movements.

[0063] Method 300 may include (see step 340) placing the object in an ultrasonic bath, with DI water, thereby introducing the nanotubes into pores of the anodized layer. For example, the objects may be immersed for 30 minutes at a temperature of 55 °C.

[0064] In some embodiments, a vacuum bag may replace the ultrasonic bath for introducing the nanotubes into the pores. For example, the object may be placed in a vacuum bag for 40 minutes. [0065] In some embodiments, the method may further include washing the anodized layer following the insertion of the nanotubes, using any known method. In a nonlimiting example, the washing may include dipping the object into DI water bath 4 times at a temperature of 25-30°C.

[0066] In some embodiments, method 300 may further include introducing a dye into a residual porosity of the anodized layer or into a pore occupied with nanotubes. In some embodiments, the dye may be any commercial dye, for example, a black pigment. In some embodiments, the dye may be introduced during or following step 320, using any known method. In some embodiments the nanotubes are black nanotubes. For example an anodized aluminum part may be inserted into an ultrasonic container with DI water and 5% black nanotubes at 50°C for 10 minutes.

[0067] In some embodiments, the dye may be mixed with the suspension of nanotubes prior to the introduction into the open porosity. Accordingly, the dye particles may be introduced into the open porosity together with the nanotubes according to any one of the methods disclosed herein above.

[0068] Method 300 may include (see step 350), sealing residual porosity in the anodized layer that were left unfilled with the nanotubes (or dye). In some embodiments all pores are sealed whether or not they are filled with nanotubes and/or dye particles. In some embodiments, the sealing may be performed according to any method known in the art. Some nonlimiting examples for sealing porous alumina layers may include applying at least one of: hot water, sodium dichromate solution, potassium dichromate solution, nickel acetate solution, silicone, polytetrafluoroethylene and the like. For example the sealing may be performed in a DI water bath at a temperature of 92 °C for 30 minutes. In some embodiments following the sealing the objects were dried naturally.

[0069] Method 300 may produce a coated aluminum object with a resistance of about 10 ^L 8-10 ^L 9W.

Examples Example I Five (5) Aluminum samples were coated using hardcoat anodize process. The process included dying with black dye. The process was performed according to BLACK ANODIC HARD COATING PER MIL-A-8625F TYPE III (50 pm) CLASS 2 (RoHS COMPATIBLE) - without the final step of sealing. The coating was 50 pm thick. Following the coating, samples 2, 3 and 5 were immersed in a DI water bath for 10 minutes.

A solution comprising DI water and nanotubes (2.5% or 5%), at room temperature (25°C) was ultrasonically stirred. The solution was then manually applied to all samples using a clean room fabric. The 2.5% solution was applied to samples 1, 3, 4 and 5 and the 5% solution was applied to sample 2.

Samples 1, 2 and 5 were then immersed in an ultrasonic bath, with DI water, at a temperature of 55°C for 30 minutes, 10 minutes and 13 minutes (respectively).

Sample 4 was immersed in a vacuum bag for 40 minutes.

All samples were then immersed in a bath, with DI water, at a temperature of 92 °C for 30 minutes for sealing. The samples where then dried in the open air.

A summary of the treatments which each sample received is presented in Table 1 below.

Table 1 : The electrostatic discharge (ESD) was then measured using a TREK METER. Additional measurements were performed a day later, four days later, six days later and 11 days later. The results appear in table 2 below (in W units). The environmental terms four days later were 54% humidity and 20°C and a second sample was washed with IPA (Isopropyl alcohol). The environmental terms six days later were 58% humidity and 19°C. The environmental terms 11 days later were 47% humidity and 20.4°C

Table 2:

As can be seen from table 2, the best resistant results were received from sample 2. The resistance is in the desired range of 10 ^L 8-10 ^L 9, and the resistance is constant over time. Sample 2 was immersed in a DI water bath for 10 minutes. Followed by manually applying a 5% nanotube solution (which was ultrasonically stirred) using a clean room fabric. The sample was then immersed in an ultrasonic bath at a temperature of 55°C for 10 minutes. Followed by immersion in a bath at a temperature of 92°C for 30 minutes for sealing.

Samples 1 and 4 also displayed resistant results within the desired range of 10 ^L 8-10 ^L 9. Samples 3 and 5 displayed resistant results which were beyond the desired range.

[0070] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.