|Title of Invention||
"AN IMPROVED PROCESS FOR MANUFACTURING SOLAR SELECTIVE BLACK CHROMIUM COATING HAVING INCREASED THERMAL STABILITY"
|Abstract||The effect of heat treatment on the absorptance and emittance of black chromium coatings has shown that by annealing for a shorter duration at temperatures around 400°C can improve the solar selectivity (a/s ratio) of the order of 5 and also long term thermal stability. This finding is very useful for getting coatings with increase thermal stability for applications like solar steam generation.|
|Full Text||The present invention relates to an improved process for manufacturing solar selective black chromium coating having increased thermal stability.
The present inventions particularly relates to an improved process relating to higher thermal stability and better solar selectivity of the black chromium coating by a short duration- thermal stabilization treatment.
In recent years, greater attention is shown in harnessing alternative sources of energy like solar energy in domestic and industrial application to save fossil fuels like petroleum, diesel and coal. Use of flat plate and concentrating type solar collectors for high temperature applications like solar steam generation requires large area of absorbers to run turbines or for industrial applications. Main usage of the present invention is in the field of harnessing solar energy for water heating and steam production.
In the prior art, conventionally black chromium is deposited by electrolyzing an aqueous solution of sulphate-free chromic acid containing certain additives known as 'catalysts' at low temperature and at very high current density.
In our U.S.A patent (5019223), Canada patent (2006130), Australian patent (611957), European patent (033683) and Indian patent (167895), we have described and claimed a room temperature black chromium plating bath and process for depositing solar selective coatings.
The black chrome coatings deposited from chromic acid bath containing inorganic additives has shown better thermal and UV stability. However, the thermal stability of such coatings is limited to 200-300°C for long term applications. In solar steam generation, where the solar selective coating applied on the absorber tube kept in the focus of the concentrating collectors has to have higher thermal stability.
Black chromium is a graded composite of metallic chromium particles in a dielectric matrix of chromium compounds. The substrate / coating interface is rich in metallic component and the surface is rich in dielectric compounds. Morphology and chemical composition together determine the optical properties of the black chrome coating. So, for the stability of the coating, both these characteristics should not change. The optical properties of black
chromium coating such as absorptance (a) and emittance (s) change with time at higher temperature. It should be minimal at and above 300°C. But these values decrease steadily with time at higher temperatures resulting in decreased solar selectivity (α/ε ratio). It is important to have higher absorptance above 0.90 at higher temperatures.
An attempt has been made to codeposit Mo from the proprietary 'Chromonyx' black chrome plating bath by Smith and Ignatiev (Solar Energy Materials 1981, 4, 119-133). This method is a costly method and basically 'Chromonyx' bath contained organic additives has poor thermal and UV stability.
With reference to USA Patent 4174265, 4196063, they do not deal with solar selective black chromium plating. The USA Patent 4894125 discusses black chromium plated pliable foils useful for space environment and use to a temperature of about 450°C. The absorptance value of this coating was 0.95 and emittance value was 0.8. The emittance value has come down to 0.4 due to heating. This patent does not discuss about steam generation and the long term thermal stability. It claims that black chromium is plated on a composite ductile foil for entirely different application. The substrates used for this application does not include copper.
Main object of the present invention is to provide improved thermal stability of the coating plated from black chromium bath (USA Patent 5019223) by a process of thermal stabilization.
Another object of the invention is achieving a simple short-term thermal treatment process for the black chromium coated plates to enhance solar selectivity (α/ε ratio).
Yet another object of the invention is achieving a solar selective coating suitable for high temperature applications like steam generation.
Still another object of the invention is to achieve higher thermal stability and also better solar selectivity for the solar selective coating by a thermal stabilization process.
Accordingly the present invention provides, an improved process for manufacturing solar selective black chromium coating having increased thermal stability which comprises, subjecting a substrate to be coated as a cathode in a black chromium electroplating bath
•characterized in that the said electroplating is carried out at a current density in the range of 2000 - 3000 A/m2 for a time duration of 60-180 sec, at room temperature, to obtain a black chromium coated substrate, heat treating the said coated substrate at a temperature in the range of 350 - 450°C in the presence of air for a period in the range of 30-120 min.
In an embodiment of the present invention, the black chromium plating bath is such as an aqueous solution of sulphate free chromium acid containing 2-20 gpl of alkali metalnitrate, 3 -40 gpl of boric acid and 0.2 -5 gpl of fluosilicate ions or 0.5 - 5 gpl of floride ions.
In another embodiment of the present invention, the substrate such as copper, nickel, steel and stainless steel has been coated with black chromium coatings.
In an another embodiment of the present invention, the electroplating effect is carried out at a current density in the range 2000 - 3000 A/m2 for duration in the range of 60-180 sec.
In still another embodiment of the present invention, the black chromium alloy coating has increased thermal stability of order of 2 - 5 times at a temperature of order of 300-400°C.
In further embodiment of the present invention, heat treating the said-coated substrate is at a temperature in the range of 350 - 450°C in the presence of air for a period in the range of 30 -120 min.
This invention is related to the improvement in thermal stability and solar selectivity of the electroplated black chromium coatings by carrying out a short duration heat treatment in the temperature range of 350°C to 450°C.
Black chromium is electrodeposited from an aqueous solution of sulphate-free chromic acid containing certain additives known as 'Catalysts' at low temperatures and at high current densities. A room temperature black chromium plating bath as per our earlier USA Patent (5019223) has been used to deposit solar selective coatings. The values of absorptance (a)
and emittance (s) are above 0.92 and below 0.2 respectively. Black chromium emittance is observed to drop upon initial heating which is accompanied with evaporation of water vapour from the coating causing some porosity in the coating. The reduction in the absorptance value is caused by the oxidation of metallic particles. Therefore, retarding the oxidation of metallic particles can enhance thermal stability of the black chrome coating and also a modification of the non-metallic part of the film may be beneficial.
Black chromium coating from the patented bath (USA Pat 5019223) was plated onto different substrates like copper, nickel plated copper and electroformed nickel plates to carry out the thermal aging study. This patented bath formulation has many advantages like giving coherent and adherent solar selective coatings at room temperature and at a current density of 1000 -3000 A/dm2. The bath can work with a trivalent concentration as low as 1gpl and can tolerate upto 16 gpl.
Black chromium coating must have higher absorptance (a) for solar radiation and low emittance in the thermal range. The coating obtained from the above mentioned bath is capable of giving high absorptance (above 0.92) and low emittance (below 0.2). However when these coatings are heated due to the absorption solar radiation, they loose their high absorbing capability due to change in the coating's morphology and composition. This thermal degradation is worse in the case of coatings obtained from baths containing organic chemicals as additives due to the carbon incorporation in the deposit causing poor thermal stability and UV stability. This problem is not there for coatings plated from USA 5019223 bath since only inorganic chemicals have been used as additives. We have found out from our studies that by giving a short duration heating between 350 to 450°C the coating could be stabilized resulting in the improvement of solar selectivity (a/s ratio) and also thermal stability.
The novelty of present invention resides in providing increased thermal stability of order of 2 -5 times at a temperature of order of 300-400°C. The above said novelty has been achieved by non-obvious inventive steps of providing a thicker coating and thermal stabilization by heating the coated substrate at 350 to 450°C in the presence of air for the duration of 30-120 min.
The process of the present invention is illustrated by the following examples however these examples should not be construed to limit the scope of the present invention.
Black Chromium plated electroformed nickel plate specimens kept inside borosil glass petri dishes were heated for different duration in an air circulated oven at 400°C. The effect of heat treatment on the values of a and e are given in the Tablel.
It is clear from the Tablel that, the solar selectivity of the coating has improved three times by a simple heat treatment for 2 hrs. at 400°C. Similarly, depending upon the temperature of the stabilization step, the annealing duration varies. Higher the temperature, lower will be the duration.
Results of long duration heat treatment (thermal aging) at 400°C are given in the following Table2. As can be seen from the table that the time taken for 5% degradation in a is more than 100 hours except for the coating having initial e value around 0.11.
On the basis of this invention, a thermal stabilization step at 400°C for 2 hrs. has shown a lot of improvement in the thermal stability and also resulted in the enhancement of solar selectivity. Optimum benefit is obtained for the black chromium coating with an initial s value around 0.41.
Researchers and inventors have observed the degradation of the black chromium coating due to heat treatment and analysed the cause. However, they have not utilized short-term heat treatment method as a stabilization of black chromium coating. In the initial stages the black chromium coatings undergo morphological and compositional changes. There is also substrate diffusion into the coating and migrating to the surfaces, affecting the optical properties. In our method, we deliberately give a thicker coating i.e. having high emittance and then stabilize the coating by a short duration heat treatment to get the low emittance and high solar selectivity. In this way, the obtained thermally stabilized coating shows higher thermal stability. Since we have started with a thicker coating, the degradation due to substrate migration is also reduced considerably. This sort of thermal stabilization treatment for solar selective black chromium coating has not been reported by anyone earlier.
The present invention is a cost effective one and the benefits in terms of higher solar selectivity and long term thermal stability at higher temperatures (300 -400°C) are considerable. While black chromium plating, the plating duration was doubled to get e = 0.43 as compared to s = 0.11 coating and a thermal stabilization step at 350 - 450°C. Depending upon the initial emittance value, the heating duration was chosen. In the case of ε ~ 0.4
coating, the emittance has reduced to 0.12 in 2 hrs. time at 400°C. These type of coatings will find a lot of applications in the solar steam generation and automobile exhausts. The colour of the coating will not change with time.
The main advantages of the present invention:
1. Increased service life of the solar collectors of the order of 2 - 5 times.
2. Reliability of the solar selective coatings has been improved.
3. Useful for high temperature applications like solar steam generations.
4. Useful for coatings automobile components like exhaust pipe and silencers.
1. An improved process for manufacturing solar selective black chromium coating having increased thermal stability which comprises, subjecting a substrate to be coated as a cathode in a black chromium electroplating bath characterized in that the said electroplating is carried out at a current density in the range of 2000 - 3000 A/m2 for a time duration of 60 - 180 sec, at room temperature, to obtain a black chromium coated substrate, heat treating the said coated substrate at a temperature in the range of 350 -450°C in the presence of air for a period in the range of 30-120 min.
2. An improved process as claimed in claim 1, wherein black chromium plating bath is such as an aqueous solution of sulphate free chromium acid containing 2-20 gpl of alkali metalnitrate, 3 - 40 gpl of boric acid and 0.2 -5 gpl of fluosilicate ions or 0.5-5 gpl of floride ions.
3. An improved process as claimed in claim 1-2,wherein the substrate such as copper, nickel, nickel-plated copper or steel, steel and stainless steel has been coated with black chromium coatings.
4. An improved process as claimed in claim 1-5, wherein the black chromium coating has solar selectivity of the order of 2- 5 times or higher at a temperature of the order of 300-400°C.
5. An improved process for manufacturing solar selective black chromium coating having increased thermal stability as herein described with reference to the examples.
|Indian Patent Application Number||748/DEL/2003|
|PG Journal Number||31/2009|
|Date of Filing||29-May-2003|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG NEW DELHI 110001 INDIA.|
|PCT International Classification Number||C25D 3/08|
|PCT International Application Number||N/A|
|PCT International Filing date|