FIELD OF THE INVENTION;
This invention relates to a super black coating for use as a superior optical absorber
This invention further relates to a method for fabrication of a good absorber material to be used as super black coating (SBC) or Ultra Black Coating (UBC) The UBC is 25 times blacker than the conventional black paint The SBC is made up from an alloy of Ni and P is extremely stable even under the very low temperature even at cryogenic temperature For its stability at very low temperature the present invention showed its potential applications in space technology and optical devices
BACKGROUND OF THE INVENTION:
Optical detector and conversion systems require a good absorbing surface with low emittance to convert the total optical radiation into a sensible heat The converted heat is transferred into the active media for performing preferred activities Thus, optical detector/devices require a good absorbing surface for fabricating an efficient detector The device/ detector efficiency depends on various factors like quality of absorber materials, surface quality of the absorber, stability of the surface quality and heat transfer capability Integration of the above items contributes to a good quality of device/detector
All over the world a number of industries are involved in manufacturing solar thermal and optical devices The quality of the device surface varies from manufacturer to manufacturer At the present moment the collector materials are fabricated from Cr based materials and are mainly composed of Cr2O3 This coating is manufactured using electroplating

technique Large area Cr2O3 based device fabrication suffers from various limitations mainly in performance and stability Moreover, this type of coating has no downstream applications Thus, industries are looking for good quality collector with high absorption coefficient, low emissivity, high heat transfer coefficient and large downstream applications The main aim of the present invention is to develop a Super Black Coating (SBC) which has number of applications ranging from solar thermal conversion to opto-electronic detection
OBJECTS OF THE INVENTION;
It is therefore an object of this invention to provide a super black coating for use as an optical absorber, which is blacker than conventional black paints
It is a further object of this invention to provide a super black coating for use as an optical absorber, which is extremely stable under low temperatures and even at cryogenic temperature
Another object of this invention is to provide a super black coating for use as an optical absorber, which has high absorption coefficient, low emissivity
Yet another object of this invention is to provide a super black coating for use as an optical absorber, which has a large number of downstream applications
A further object of this invention is to provide a super black coating for use as an optical absorber, which is cost-effective

These and other objects and advantages of the invention will be apparent to a reader on reading the ensuing description
BRIEF DESCRIPTION OF THE INVENTION:
Thus according to this invention is provided a super black coating for use as a superior optical absorber
In accordance with this invention, the Super Black Coating or Ultra Black Coating (SBC) a superior optical absorber has been fabricated over copper sheet as well as aluminum sheet by electroplating and electroless plating technique and subsequent etching technique The plating consists of deposition of nickel and phosphorus composite The composite is then etched in oxidizing media for converting it into super black with very absorbing capacity with very low emissing characteristics
The technology for fabrication and processing of SBC is as follows

The substrates over which the SBC is made require cleaning and pre¬conditioning for successful plating of Ni-P composite For quality and effective black coating about 65-70 µm of Ni-P composite was required on the basic substrates like Cu or Al depending upon the requirement After successful plating the coating is rinsed in distilled water and immersed in etching bath for blackening the plated surface The black surface is then dried and performance was tested and quantified.

The coating fabrication consists of two fold process In the first fold about 60-70 µm thick Ni-P coating was deposited both on copper (Cu) and aluminum (Al) substrate by electroplating as well as electroless plating techniques For electroplating purpose the composition of plating bath is presented in Table I and for electroless plating purpose the bath composition is presented in Table II

In electroplating process a Ni plate was used as the anode and the deposit developed over the desired conducting substrates like Cu or Al

and DC electricity was supplied from external sources In electroless process the substrate was first cleaned and preconditioned with sensitizing and activating through sensitizing and activating solution presented in Table III
Table III
Sensitizing & Activation Bath

The sensitized and activated surface is then placed in the electroless solution presented in Table II After 2-3 hours of deposition a 60-75 µm thick Ni-P was obtained on the basic substrates The deposit obtained by the above two process is then placed in oxidizing/etching bath for blackening purpose The composition of oxidizing/etching bath is presented in Table IV After oxidizing/etching the black surface was water rinsed and dried for quality testing
Table IV
Etching / Oxidation Bath & Processing Condition


The invention will now be explained m greater details with the help of the following non-limiting example
EXAMPLE;
An aluminium sheet was cleaned and pre-conditioned and immersed in an electroplating bath in which the electroplating solution containing hypophosphorus acid (30gms/Liter), hypophosphoric acid (35 gms/Liter), nickel carbonate (25gms/Liter), Nickel chloride (45 gms/Liter) and Nickel sulphate (150 gms/Liter) The pH was around 1 2 to 2 1 and a temperature of about 60-70°C was maintained during plating A nickel plate was used as the anode and the deposition took place on the Al substrate DC was supplied from an external source
The electroplated surface was then immersed in a sensitizing bath containing about 80gms/Liter tin chloride and 200 ml/Liter hydrochloric acid, at room temperature This was followed by dipping the sensitized substrate in an activation bath containing palladium chloride (450mg/Liter) and hydrochloric acid (10ml/hter) The bath was maintained at about 50°C to obtain the sensitized and activated surface, which was then immersed in an electroless solution containing 30gms/Liter Nickel chloride, 15gms Liter sodium hypophosphite, 50gms/Liter ammonium chloride and 96gms/Liter sodium citrate. The pH was maintained at about 8.5 to 9 5 and plating was carried out at about 95°C for about 3 hours, at the end of which a 65 µm N1-P coating was deposited on the surface The coated surface was then placed in an etching bath containing sulphuric acid (5 moles/Liter) and sodium nitrate 4 moles 1 Liter), at a temperature of about 50°C for 4 minutes to obtain a blackened surface. The surface was rinsed thoroughly to make it free from adhering chemicals, and dried

WE CLAIM:
1 A process for the preparation of a super-black coating comprising
the steps of cleaning and pre-conditioning a substrate, followed by
subjecting the same to electroplating to obtain an electroplated
surface, followed by electroless plating to obtain a plated surface
coated with a Nickel-Phosphorous composite,
rinsing the plated surface and immersing the same into an etching bath to obtain a surface with a super-black coating
2 The process as claimed in claim 1, wherein said substrate is copper or aluminium sheet
3 The process as claimed in claim 1, wherein the Nickel-phosphorous composite is about 65-70 µm thick
4. The process as claimed in claim 1, wherein electroplating is carried out at a pH of 1 2 to 2.1
5 The process as claimed in claim 1, wherein for the step of electroplating, the preconditioned substrate is immersed in an electroplating bath comprising
Hypo phosphorus Acid (H3PO3) 25-35 gm/ Liter
Hypo phosphoric Acid (H3PO4) 30-40 gm/ Liter
Nickel Carbonate (NiCO3) 20-30 gm/ Liter
Nickel Chloride (NiCl2) 40-50 gm/ Liter
Nickel Sulphate (NiSO4) 140-165 gm/ Liter

6 The process as claimed in claim 4, wherein electroplating is carried out at 60 to 70°C
7. The process as claimed in claim 2, wherein for the step of electroless plating, the electroplated surface is subjected to sensitization in a sensitizing bath followed by activation in an activation bath, to obtain a sensitized and activated surface, prior to the step of electroless plating
8 The process as claimed m claim 7, wherein the sensitization bath
comprises
Tin Chloride (SnCl2) 75-85 gm/ Liter
Hydrochloride Acid (HC1) 150-250 ml / Liter
9 The process as claimed in claim 7, wherein the activation bath
comprises
Palladium Chloride (PdCl2) 400-500 mg/Liter
Hydrochloride Acid (HCl) 8-10 ml/Liter
10 The process as claimed m claim 1, wherein electroless plating is carried out at a pH of 8 5 to 9 5
11 The process as claimed in claim 1, wherein electroless plating is carried out at 90 to 95°C.
12 The process as claimed in claim the preceding claims, wherein for the step of electroless plating, the sensitized and activated substrate is immersed in an electroless plating bath comprising

Nickel Chloride (N1Cl2) 25-35 gm/ Liter
Sodium Hypoposphite (Na2HPO2) 10-20 gm/ Liter
Ammonium Chloride (NH4Cl) 45-50 gm/ Liter
Sodium Citrate 95-100 gm/ Liter
13 The process as claimed in claim 2, wherein the plated surface is immersed in an etching bath at a temperature of 45 to 65°C
14 The process as claimed in claim 2, wherein etching is carried out for 2 to 4 minutes
15 The process as claimed in claim 2, wherein said etching is carried out in an etching bath comprising
Sulphuric Acid (H2SO4) 4 5-65 Mole/ Liter
Sodium Nitrate (NaNO3) 3.0 - 5.0 Mole/Liter
16 A super black coating for use as a superior optical absorber, obtained
by the process as claimed in claim 1