The present invention relates to an optical filter configured to withstand a high rate of temperature change and a method of making the same.
Optical filters are currently known and used in thermal photovoltaic applications. Such applications may subject the optical filter to temperatures up to 1000° C. The optical filters have a thin film layer, typically formed of a metallic oxide, disposed on top of a substrate. However, due to the coefficient of thermal expansion differences between the thin film layer and the substrate, the thin film layer may decoupled from the substrate or become physically damaged under such temperatures.
Oblique angle deposition is a self-organizing physical vapor deposition technique that has been used to grow sculpted 3D nanostructures including helices, slanted rods, and zigzag structures. However, oblique angle deposition has been limited to applications in the spectral range measured in nanometers wherein high temperature compatibility and vacuum stability conditions are not addressed. Accordingly, it remains desirable to have an optical filter configured to maintain its intended functional performance under high temperatures, and dramatic temperature changes.
An optical filter is provided. The optical filter includes a substrate and a filter layer disposed on the substrate. The filter layer has a porous columnar micro-structure configured to decouple the thermal expansion stress between the substrate and the filter layer when the optical filter is subjected to high temperature. The filter layer may be formed of a metallic oxide, and the porosity of the columnar micro-structure may be between bulk density and 90%.
A method for making an optical filter having a porous columnar micro-structure configured to decouple the thermal expansion stress between the substrate and the filter layer when the optical filter is subjected to high temperature is also provided. The method includes the step of providing a substrate. The method proceeds to the step of depositing a material onto the substrate utilizing physical vapor deposition and wherein said materials are deposited at an angle with respect to the substrate, between 0 degrees or normal to the substrate surface and 90 degrees or almost completely parallel to the substrate surface. The method may also include the step of depositing a second material on top of the first material, wherein the first and second material are metallic oxides, and the first and second material are different from each other.
An optical filter 10 configured to operate under high temperature and dramatic temperature changes, and a method of making the same is provided. The optical filter 10 includes a substrate 12 and a filter layer 14 disposed on the substrate 12. The filter layer 14 has a porous columnar micro-structure 16 configured to decouple the thermal expansion stress between the substrate 12 and the filter layer 14 when the optical filter 10 is subjected to high temperature.
The filter layer 14 may be formed of a metallic oxide, and the porosity of the columnar micro-structure 16 may be between near bulk density, and about 90%. For use herein bulk density shall mean the density of the metallic oxide in its natural state or as supplied by a manufacturer of said metallic oxide. The filter layer 14 may have a thickness of at least about 100 nanometers, but may be increased based upon the dimensions and operating conditions of the device for which the filter is designed to be used in, such as a thermal photovoltaic power system.
With reference now to
At least one thin film layer 14, also referenced as a “filter layer,” is disposed on the substrate 12.
The thin film layer 14 is formed of a material configured to reject wavelength light from entering the substrate 12. For instance, the thin film layer 14 may be formed of a metallic oxide, to include SiO2, Y2O3, and so forth. It should be appreciated that the metallic oxide is of a material suitable for physical vapor deposition. For use herein, physical vapor deposition is a process to form a thin film, typically in a vacuum, by the condensation of a vaporized form of the desired thin film material. Preferably, the filter layer 14 has a material thickness of at least 100 nanometers, as measured by the distance between the top and bottom surfaces 18, 20 of the filter layer 14.
With reference now to
With reference again to
As used herein the term micro-structure references a size having a volumetric size ranging from 4 nm3 to 5×109 nm3, and the term columnar refers to the tubular dimension of the deposited material. Thus, the term columnar microstructure describes a material having a tubular dimension and of a size having a volumetric size ranging from about 4 nm3 to about 5×109 nm3. The columnar micro-structure 16 is slanted/angled with respect to the substrate 12 along its height. It should be appreciated that the illustration shown in
With reference now to
The method proceeds to the step 120, depositing a material, also referenced below as a “first material,” onto the substrate 12 utilizing physical vapor deposition so as to form a first filter layer 14a. The filter layer 14 is configured to filter light in a predetermined wavelength. The material is deposited at an angle a which may be between about 89° and about 5° with respect to the substrate 12. For use herein, the angle is measured from the planar surface of the substrate 12 as shown in
The first material is of the type that is capable of being deposited using physical vapor deposition, and has a reflective property capable of rejecting long wavelength light. Any such material currently known and used in the art may be adapted for use herein, illustratively include metallic oxides. The vapor deposition is conducted so as to form a layer of the material having a porous columnar micro-structure 16. It should be appreciated that the material itself forms a porous columnar micro-structure 16, and thus the filter layer 14 will include pockets of space 22 adjacent the porous columnar micro-structure 16 which are tubular in dimension, having a length as defined between the top and bottom surfaces of the layer. The term columnar microstructure refers to the tubular dimensions described herein being of a size having a volumetric size ranging volumetric size ranging from 4 nm3 to 5×109 nm3.
The method may further include step 130, wherein a second material is deposited onto the first material, so as to form a second filter layer 14b. The second material is different than the first material. The second material may have a material thickness smaller than that of the first material. The second filter layer 14 is configured filter light in a different wavelength than the first filter layer. Preferably the second material is deposited at an angle β, which may be between 89° and 5° with respect to the substrate 12. Angle β is different than angle α. For use herein, angle β is measured from the planar surface of the substrate 12 as shown in
The deposition may be conducted using an electron beam, wherein the material is cryogenically pumped into the electron beam. The electron beam may be positioned at a distance between 30 cm and 50 cm from the substrate 12, and is angled with respect to the substrate 12 at an angle between 89° and 5° with respect to the substrate 12.
In order to better teach the invention but not to limit its scope in any way, one or more example is provided in conjunction with
In
The red wavelength shift in
The reflectance of the thin films stacks were simulated using the Essential Macleod software. In
The red curve (short dash) shows the same film stack as the black curve but without the native oxide (SiO2) on an Al2O3 (sapphire) substrate 12. Similar to that shown in
Experiments show that the optical filter 10s described herein have been shown to withstand high temperatures (1000° C.) in air for 90 minutes without delamination or significant degradation of their optical properties, even when the substrate 12 underneath the filter is undergoing significant oxidation. The basic spectral characteristics of the reflectance data were preserved after the samples were annealed, it should be understood by those skill in the art that the small shifts seen in the reflectance spectra may be attributed to compaction of the porous filter layer.
It should be appreciated that the optical filter 10 described herein may be suitable for use in: 1) high temperature optical filter 10s in TPV systems; 2) high temperature optical sensors in harsh environments including, but not limited to, a) turbines, jet engines, gas fired power generating turbines, rocket motors, and so forth and b) drilling: oil/gas/mineral exploratory (or active well) monitoring; 3) high temperature oxidation resistant heat shields including, but not limited to a) applications in nuclear power generation systems and b) in turbines/engine/motor heat shielding. The optical filter 10 may be designed to withstand 1000° C., as well as a high rate of temperature change, for example 25° C. to 1000° C. in several seconds. Further, the optical filter 10s that can survive in an oxidizing environment at elevated temperatures, for example temperatures greater than 500° C., greater than 600° C., greater than 700° C., as high as 800° C., as high as 900° C. and even as high as 1000° C. Further, the porous columnar micro-structure 16 decouples the thermal expansion stress between the substrate 12 and filter layers.
It is appreciated that changes, modifications, and so forth can be made by those skilled in the art and still fall within the scope of the present invention. As such, the scope of the invention is provided by the claims and all equivalents thereof.
This patent application claims the benefit of U.S. provisional application 61/907,521 filed on Nov. 22, 2013 (attorney docket no. ARL 13-08P) and titled “High Temperature Optical Filters that can withstand Oxidizing Environments” listing Dr. Thomas Christopher Parker as sole inventor which is hereby incorporated by reference herein including all attachments thereto, including a presentation to the Materials Research Society dated Nov. 28, 2012 titled “Development of High temperature Optical filters”.
The invention described herein may be manufactured, used, and licensed by or for the United States Government.
Number | Date | Country | |
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61907521 | Nov 2013 | US |