Title of Invention

A PROCESS FOR THE PREPARATION OF SOLAR SELECTIVE ABSORBER COATINGS OVER METALS AND ALLOYS

Abstract

The present invention provides a process for the preparation of solar selective absorber coating over metals and alloys useful for harnessing solar energy. In the present invention no deposition of materials different from the substrate takes place. It converts the substrate surface itself into selective absorbers by plasma oxidation and oxygen ion implantation. Unlike the chemical conversion techniques, the present invention is clean and environmentally friendly and uses oxygen or mixture of the other gases bearing oxygen and gases or such as N2 for the conversion process with the help of plasma to obtain solar selective absorber coatings which is stable at high temperature up to 400°C and has good mechanical properties.

Full Text

The present invention relates to a process for the preparation of solar selective absorber coatings over metals and alloys useful for harnessing solar energy. The present invention particularly relates to a process for the preparation of solar selective absorber coatings by using plasma oxidation and oxygen ion implantation. Solar selective absorber coatings are useful in harnessing solar energy for various applications in domestic, industrial sectors. These coatings should have high solar absorbivity and low emissivity in order to obtain coatings of high solar selectivity ratio. Further these coatings should be thermally stable at higher temperature applications. Solar selective absorber coatings are normally prepared either by depositing a suitable material on suitable substrates by different methods. These include chemical methods such as chemical vapor deposition (CVD), electro-chemical methods or by physical vapor deposition (PVD) such as evaporation and by plasma assisted deposition such as sputtering. In these methods a coating that is generally made up of a different composition from the substrate is deposited on the substrate. Here, the quality and thermal stability of deposited coating rely on the properties of the new material deposited on the substrate and may suffer from poor adhesion to the substrate. Coatings prepared by these processes are prone to wear, delamination and degradation. In chemical method, the surface itself is converted to a selective absorber by chemical / electrochemical means by treating the surfaces in suitable chemical baths. These baths contain chemicals, which are generally not environment friendly and cause ecological damage. Thus, while the chemical conversion methods are ecologically unfriendly, coating techniques depend on the deposition of an over layer, which may have adhesion problems and degrade at higher temperatures. Some of the PVD/CVD techniques generating complex multi layer structures are not simple to produce and require accurate control of thickness and composition. Reference may be made to US patent 430926, wherein a graded surface coating is reactively sputtered onto a tubular substrate by advancing the substrate in an axial direction through a cylindrical sputtering chamber in the presence of a sputter supporting gas. The sputtering chamber includes a cathode liner from which metal is sputtered onto the substrate. A reactive gas is directed into the sputtering chamber from a feed point outside of the chamber, whereby reactive sputtering occurs within the chamber. In yet another form of selective absorber formation, oxides of transition metals are applied with suitable binders (organic/inorganic) by thermal or plasma spray technique and fired at high temperatures. It is claimed that these absorbers are stable up to 550° F. However, this method also relies on formation/deposition of an over layer coating of different chemical nature than the substrate. Reference may be made to US patent 4268319, wherein improved coatings for high temperature solar collectors are deposited by selecting thermally stable inorganic oxides of ferrites, metal oxides and mixtures thereof having solar absorptances greater than 0.9 at wavelengths ranging from about 0.35 to 3.0 microns. The coatings can be applied to a heat transfer surface by painting an organic silicate dispersion of the optically active material onto the substrate and thereafter curing the silicate binder Hence, the main object of the present invention is to provide a process for the preparation of solar selective absorbers, which obviates the drawbacks of above-mentioned methods. Another object of the present invention is to provide a process for surface modification of metals and alloys for the manufacture of solar selective absorber coatings useful for harnessing solar energy. Yet another object? of the present invention is to provide an ecologically safe and environment friendly process of making solar selective absorbers. Yet another object of the present invention is to provide an ecologically safe and environment friendly process of making solar selective absorbers. Yet another object of the present invention is to provide a selective absorber that is stable at higher temperature. Still another object is to provide a selective absorber that has better mechanical properties such as hardness. Still another object is to provide a process for making the solar selective absorber on non-planar shapes. Accordingly, the present invention provides a process for the preparation of solar selective absorber coatings over metals and alloys useful for harnessing solar energy, which comprises the steps of: polishing and cleaning the substrate, a) polishing and cleaning the substrate, b) positioning the above said substrate inside a vacuum chamber preferably on a high voltage electrode, c) creating a vacuum in the above said vacuum chamber to a base pressure of d) introducing a process gas in the said evacuated vacuum chamber to obtain a process pressure in the range of 10"4 to 10-2 Torr, and generating a plasma of the process gas, e) applying negative high voltage pulses of 10 to 40 kilovolts to the above said substrate, at a temperature in the range of 150-600°C and f) removing the desired solar selective absorber substrate from the vacuum chamber. In an embodiment of the present invention the substrate is cleaned outside the vacuum chamber in organic solvents and inorganic chemicals. In another embodiment, the substrate used is polished by silicon carbide cloth of at least 800 grit. In yet another embodiment of the present invention, the substrate used is stainless steel. In yet another embodiment, the substrate used is alloy of iron with Cr, Ni, Mo, Mn such as SS304, 316 with other minor alloying elements. In yet another embodiment, the substrate used is alloy of iron with Cr with concentration of 5-30wt%. In yet another embodiment, the radio frequency power supply used for the generating plasma operates at about 13.56MHz. In yet another embodiment, the plasma generating system used is a DC power supply. In yet another embodiment, the radio frequency power used is coupled with process gas by inductive coupling, by placing an antenna inside or outside the vacuum chamber In yet another embodiment, the RF power used is coupled with process gas by capacitive coupling, by placing an electrode inside the vacuum chamber In yet another embodiment, the process gas used is oxygen or oxygen bearing molecules such as NO, NO2, CO or C02- In yet another embodiment, the process gas used is mixture of oxygen and other gases such as argon and helium. In yet another embodiment, the substrate used is heated to a temperature above 300°C before the plasma is applied. In a typical process, the substrates are cleaned in suitable solvents, dried and placed inside the vacuum chamber preferably on an electrically insulated substrate holder. The vacuum chamber is then pumped to a base pressure, less than the process pressure, typically less than 10~5 Torr. If necessary the substrates can be heated using the substrate heater or the chamber heater. After reaching the required base pressure, the substrates may be cleaned by mild argon ion sputtering for about 15-20 minutes. Then the process gas (oxygen) is admitted into vacuum chamber at appropriate flow rates and the chamber pressure is controlled by using a suitable control mechanism. The process pressure may vary between 10-4 to 5x10-3 Torr. The plasma is then generated using a radio frequency power resource preferably operating at 13.56 MHz. After the plasma stabilizes, high voltage pulses of negative polarity were applied to the substrate at selected "ON" time and frequency and voltage. The voltage can vary from 10 to 40 kilovolts and the frequency from few hertz to several kilohertz and the "ON" time from 5 microseconds to several tens of microseconds. During the implantation and oxidation, the temperature of the substrate increases and reaches a steady value depending on the voltage parameters and other operating conditions such as gas pressure and RF power. If the required temperature is not reached the chamber/substrate heaters may be switched on to get desired temperature. This temperature is generally in the range 150 to 600°C. The implantation time is calculated from the time the desired temperature was reached. Generally, the time varies from 30 min to 1 hr depending on the process conditions chosen. After the process, the substrates can be removed from the vacuum chamber. Selective absorbers absorb solar energy in the solar spectrum, particularly in the range of 0.3 to 1.5 µm, where nearly 90 % of solar energy is concentrated and emit in the long wavelength part of the spectrum depending on the temperature of the emitting surface. Therefore, it is desirable to have high absorptivity in the main absorption band, typically in excess of 0.8 and low reflectivity in the region and low emissivity, in the emission band, typically less than 0.2. The selective property of various selective coatings and absorbers has been attributed to the chemical composition and its distribution in the near surface region of the absorbers, typically in less than 0.5 (am thickness. Various models of this structure include a top surface of dielectric layers, mainly oxides of metals, and a gradient of this oxide towards the inner bulk with bulk being metallic. Apart from this simple model, the more sophisticated models include distribution of nano sized metallic particles in an oxide/dielectric matrix with the oxide fraction increasing towards surfaces. Another model comprises multiple layer coatings with varying metallic and dielectric layer thickness. The absorbers act in the following way. The dielectric part absorbs the radiation and the metallic part emits. By combination of metallic to dielectric thickness, maximum absorptivity is obtained coupled with low emissivity. In case of conversion coatings, the substrate, mainly metals and alloys, oxide formation takes place by chemical means by dipping/immersing the substrates in suitable chemicals and controlling the thickness by controlling the time and temperature of the bath. The chemicals are generally hazardous and pose environmental damage. The present invention overcomes the above problems associated with the coatings prepared by chemical conversion means by using only oxygen, a major constituent of air and freely available in the atmosphere. It also overcomes the problem of deposition techniques mainly chemical and electrochemical in the same manner. It does not involve the control of the thickness and composition of materials different from the substrate and has no adhesion or delamination problem as in the PVD/CVD techniques where precise control of layer thickness is also difficult to achieve by normal/ simple means. In the present invention no deposition of materials different from the substrate takes place. It converts the substrate surface itself into selective absorbers by plasma oxidation and oxygen ion implantation. Unlike the chemical conversion techniques, the present invention is clean and environmentally friendly and uses oxygen or mixture of the other gases bearing oxygen and gases or such as N2 for the conversion process with the help of plasma. During this novel conversion process, oxygen plasma containing ionized, neutral and excited oxygen atoms is formed by the dissociation of the feed gas along with the molecular oxygen in the feed gas. Under ionic bombardment on the substrate, caused by the applied high voltage pulses and also by ions in the plasma itself, oxygen ions are implanted into the substrate. During this process the substrate is also heated. If sufficient heating is not provided in the process, external heating can also be used to get the desired temperature. When the voltage pulse is not there or during the "OFF" period of high voltage pulse, the neutral, excited oxygen atoms and molecular oxygen can react with the substrate atoms to form various oxides of the substrate atoms. Due to implantation, the concentration of oxygen in the implanted region can exceed the solid solution limit and that assists in stoichiometric oxide formation. Since the energy of the ions is not uniform and also multiply with changed ions, such as dimer and polymeric ions, those are present in the absence of mass selection using magnetic, electric field, the implanted profile will be complex/This complex profile is further pushed into the interior of the surface by temperature assisted diffusion process, resulting in gradation in the oxide thickness and composition and oxygen concentration. Further, the oxidation of the sample during the absence of high voltage pulse helps to maintain supply of oxygen. This results in a graded oxide / oxygen enriched layer in the substrate surface giving rise to high absorptivity. With control of this dielectric layer thickness and composition by controlling the process parameters, high absorptivity with low emissivity of the substrate, a selective absorber is formed. The following examples are given by the way of illustration and therefore should not be construed to limit the scope of the invention in any manner. Example 1 In one example of the present invention, the substrate chosen is SS304. It was polished with a 1000grit size silicon carbide cloth and ultrasonically cleaned in acetone and methanol and dried. After cleaning, the substrate was placed on the substrate holder, namely the high voltage electrode, and the chamber evacuated to a base pressure of Example 2 The substrate SS304 was subjected to same treatment as given in example 1 except that in the present case the pressure was 1.3 mT and the substrate temperature was 350°C for 1 hours. Measured values of a and e are 0.784 and 0.13 respectively. The samples were heated at 400°C for 3 hours in an oven at atmospheric presstire. After cooling to room temperature, a and £ values were again measured and found as 0.788 & 0.13 respectively. Example 3 After cleaning, the substrate SS304 was kept on the high voltage electrode. The substrate was cleaned inside the vacuum chamber by argon ion sputtering and subjected to implantation and oxidation. The process gas pressure was 1.3 mT and the flow rate was 16 sccm oxygen. The plasma was created by an RF power of 120 watts. The pulse voltage parameters are - 20kV, 40 µsec ON time at a repetition rate of 1300 Hz. the substrate temperature was 380°C. The process was carried out for 45 minutes. After cooling, the sample was removed from the vacuum chamber and values of a & £ were found as 0.911 and 0.14 respectively. Example 4 Type of substrate selected was SS316. The flow of oxygen was 18 seem and the pressure was 1.8 mT. Plasma was generated by 120 W RF power. The voltage pulse was 22 kV with 40 µsec and 900 Hz repetition rate and the temperature was 400°C. The process was carried out for 45 minutes. After cooling, the values of a and ε for the coated substrate were 0.841 & 0.17 respectively. Example 5 Type of substrate selected was SS304. The substrate was oxidized and implanted at 390°C the process gas pressure was 1.3mT with 19 seem flow of oxygen. RF power was 120 W and applied pulsed voltage was 19kV with 40 microsecond duration at 1200 Hz repetition rate. The measured a and £ values are 0.896 and 0.14, respectively. Example 6 The selected substrate is SS304. The process pressure was 2.3 mT and the flow was 26secm oxygen and the substrate temperature was 410°C and the RF power was 120 W. The applied pulse voltage was 18kV with 40 microseconds ON time and 1200-repetition rate. The measured a and e values are 0.814 & 0.12 respectively. Advantages of the present invention are as follows: 1. Process used for the manufacture of solar selective absorbers is environment friendly. 2. Manufacturing process provides thermally stable coatings at high temperature in order of 400°C. 3. Process does not use hazardous chemicals. 4. Process provides a coating that has good adhesion to substrate surface and don't have problem of delamination. 5. Present invention produces a solar selective absorber with good hardness and adhesion to the substrate. 6. The present invention process has further advantage of converting non- planar surfaces such as cylinders, outside of tubes into selective absorbers, which are used in industrial heat exchangers. We claim: 1. A process for the preparation of solar selective absorber coatings over metals and alloys useful for harnessing solar energy, which comprises the steps of: a) polishing and cleaning the substrate, b) positioning the above said substrate inside a vacuum chamber preferably on a high voltage electrode, c) creating a vacuum in the above said vacuum chamber to a base pressure of d) introducing a process gas in the said evacuated vacuum chamber to obtain a process pressure in the range of 10-4 to 10-2 Torr, and generating a plasma of the process gas, e) applying negative high voltage pulses of 10 to 40 kilovolts to the above said substrate, at a temperature in the range of 150-600°C and f) removing the desired solar selective absorber substrate from the vacuum chamber. 1. A process as claimed in claim 1, wherein the substrate is cleaned outside the vacuum chamber in organic solvents and inorganic chemicals. 2. A process as claimed in claims 1&2, wherein the substrate is polished by silicon carbide cloth of at least 800 grit. 3. A process as claimed in claims 1-3, wherein the substrate is stainless steel. 4. A process as claimed in claims 1-4, wherein the substrate is alloy of iron with Cr, Ni, Mo, Mn such as SS304, 316 with other minor alloying elements. 5. A process as claimed in claims 1-5, wherein the substrate is alloy of iron with Cr with concentration of 5-30wt%. 6. A process as claimed in claims 1-6, wherein radio frequency power supply for the generating plasma operates at about 13.56MHz. 7. A process as claimed in claims 1-7, wherein the plasma generating system is a DC power supply. 8. A process as claimed in claims 1-8, wherein the radio frequency power is coupled with process gas by inductive coupling, by placing an antenna inside or outside the vacuum chamber 9. A process as claimed in claims 1-9, wherein the RF power is coupled with process gas by capacitive coupling, by placing an electrode inside the vacuum chamber 10. A process as claimed in claims 1-10, wherein the process gas is oxygen or oxygen bearing molecules such as NO, NO2, CO or CO2. 11. A process as claimed in claims 1-11, wherein the process gas is a mixture of oxygen and other gases such as argon and helium. 12.A process as claimed in claims 1-12, wherein the substrate is heated to a temperature above 300°C before the plasma is applied. 13.A process as claimed in claims 1-13, wherein the solar selective absorber coating obtained is stable up to a temperature of about 400°C.

Documents:

2629-DEL-2005-Abstract-(26-06-2012).pdf

2629-del-2005-abstract.pdf

2629-DEL-2005-Claims-(26-06-2012).pdf

2629-del-2005-claims.pdf

2629-DEL-2005-Correspondence Others-(26-06-2012).pdf

2629-del-2005-correspondence-others.pdf

2629-DEL-2005-Description (Complete)-(26-06-2012).pdf

2629-del-2005-description (complete).pdf

2629-del-2005-form-1.pdf

2629-del-2005-form-18.pdf

2629-del-2005-form-2.pdf

2629-DEL-2005-Form-3-(26-06-2012).pdf

2629-del-2005-form-3.pdf

2629-del-2005-form-5.pdf