US008893711B2
(12) United States Patent (10) Patent No.: US 8,893,711 B2 Kennedy (45) Date of Patent: Nov. 25, 2014
(54) HIGH TEMPERATURE SOLAR SELECTIVE 4,252,865 A * 2/1981 Gilbert et al...... 428,611 COATINGS (Continued) (75) Inventor: Cheryl E. Kennedy, Lafayette, CO (US) FOREIGN PATENT DOCUMENTS (73) Assignee: Alliance for Sustainable Energy, LLC, CN 2004O10097.125 12, 2004 Golden, CO (US) JP 2004266O23 9, 2004 (*) Notice: Subject to any disclaimer, the term of this WO 2005 121389 12/2005 patent is extended or adjusted under 35 OTHER PUBLICATIONS U.S.C. 154(b) by 1223 days. G.L. Harding, Sputtered metal carbide Solar-selective absorbing Sur (21) Appl. No.: 12/745,319 faces, J. Vac. Sci. Technol., 13, No. 5, 1070 (1976). (22) PCT Filed: Oct. 18, 2007 (Continued) (86). PCT No.: PCT/US2007/081838 Primary Examiner — Avinash Savani S371 (c)(1), (74) Attorney, Agent, or Firm — John C. Stolpa; Michael A. (2), (4) Date: Sep. 1, 2010 McIntyre (87) PCT Pub. No.: WO2009/051595 PCT Pub. Date: Apr. 23, 2009 (57) ABSTRACT Improved Solar collectors (40) comprising glass tubing (42) (65) Prior Publication Data attached to bellows (44) by airtight seals (56) enclose solar US 2010/0313875 A1 Dec. 16, 2010 absorber tubes (50) inside an annular evacuated space (54. The exterior surfaces of the solar absorber tubes (50) are (51) Int. Cl. coated with improved solar selective coatings {48} which F24, 2/50 (2006.01) provide higher absorbance, lower emittance and resistance to F24, 2/48 (2006.01) atmospheric oxidation at elevated temperatures. The coatings F24, 2/05 (2006.01) are multilayered structures comprising Solar absorbent layers F24, 2/46 (2006.01) (26) applied to the meta surface of the absorber tubes (50), GO2B5/28 (2006.01) typically stainless steel, topped with antireflective Savers (28) (52) U.S. Cl. comprising at least two layers 30, 32) of refractory metal or CPC F24J 2/485 (2013.01); Y02E 10/44 (2013.01); metalloid oxides (such as titania and silica) with Substantially G02B5/281 (2013.01); F24J 2/055 (2013.01); differing indices of refraction in adjacent layers. Optionally, F24J 2/4652 (2013.01) at least one layer of a noble metal Such as platinum can be included between some of the layers. The absorbent layers USPC ...... 126/652: 126/651: 126/714; 427/162: cars include cermet materials comprising particles of metal 427/597; 204/19428 compounds is a matrix, which can contain oxides of refrac (58) Field of Classification Search tory metals or metalloids such as silicon. Reflective layers USPC ...... 126/652, 651, 714; 427/162,597; within the coating layers can comprise refractory metal sili 204/19428 cides and related compounds characterized by the formulas See application file for complete search history. TiSi. TiSiC., TiAlSi, TiAN and similar compounds for Zr (56) References Cited and Hf. The titania can be characterized by the formulas TiO, TiOs. TiOx or TiON with X 0 to 1. The silica can be at U.S. PATENT DOCUMENTS least one of SiO, SiO, or SiON with X=0 to 1. 4,098,956 A 7, 1978 Blickensderfer et al. 20 Claims, 23 Drawing Sheets
US 8,893,711 B2 Page 2
(56) References Cited Gibbons, et al Chapter 12: Toward industrial applications, Physical Properties of Quasicrystals, Springer Series in Solid State Sciences, U.S. PATENT DOCUMENTS NY. p. 403 (1999). R.E. Hahn and B.O. Seraphin, Sputtered metal carbide solar-selective 4,312.915 A 1, 1982 Fan 4.321,300 A 3, 1982 Farrauto et al. absorbing surfaces, J. Vac. Sci. Technol. 12,905 (1975). 4.333,448 A 6, 1982 Johnson International Search Report for International (PCT) Application No. 4.334.523 A * 6/1982 Spanoudis ...... 126,652 PCT/US07/81838, dated Oct. 18, 2007. 4.416,916 A 11/1983 Aykan et al. Written Opinion for International (PCT) Application No. PCT/US07/ 4,429,005 A * 1/1984 Penn ...... 428,350 81838, dated.June 30, 2008. 4,437.455 A 3, 1984 Jefferson International Preliminary Report on Patentability for International 4,582,764 A 4, 1986 Allerd (PCT) Application No. PCT/US07/81838, issued Apr. 20, 2010. 4,594.995 A 6, 1986 Garrison C.E. Kennedy, Progress in Development of High-Temperature Solar 5,214,530 A * 5/1993 Coombs et al...... 359,359 5.449,413 A * 9/1995 Beauchamp et al...... 136,257 Selective Coating, Proceedings of ISEC, 2005, Orlando, FL. (Aug. 5,523,132 A 6/1996 Zhang et al. 6-12, 2005). 5,670,248 A 9, 1997 Lazarov et al. Q.C. Zhang and D.R. Mills, Very low-emittance solar selective sur 5,723,207 A 3/1998 Lettington et al. faces using new film structures, J. Appl. Phys. 72 (7), pp. 3013-3021 5,776,556 A 7, 1998 Lazarov et al. (1992). 5,912,045 A 6, 1999 Eisenhammer et al. M. Farooq, A.A. Green, and M.G. Hutchins, Simplified Composite 5,980,977 A 1 1/1999 Deng et al. 6,060,154 A 5, 2000 Adachi et al. Multilayer Selective Solar Absorber Surfacees: A Comparison of 6,650,478 B1 * 1 1/2003 DeBusket al...... 359,585 V: AI2O3 and Ni:SiO2Coatings, Towards a Renewable Future; Silver 2004/O126594 A1 7/2004 Rubbia et al. Jubilee Conference. Conference C73. Proceedings Solar Energy 2005/01895.25 A1 9, 2005 Kuckelkorn et al...... 252/582 Society, Oxford, UK, pp. 297-300 (1999). 2007/0231501 A1* 10/2007 Finley ...... 427,531 R. Blickensderfer, Metal Oxycarbonitride Solar Absorbers, Proc. DOE/DS7 Thermal power Systems Workshop on Selective absorber OTHER PUBLICATIONS coatings, P. Call, SERITP-31-061, Golden, CO, pp. 371-380 (1977). Cankurtaran, et al., “Ultrasonic study of the temperature and pressure R. Blickensderfer, D.K. Deardorff, and R.L. Lincoln, Technical Note: Spectral reflectance of TiNX and Zrnx films as selective solar absorb dependencies of the elastic properties of ceramic dimolybdenum ers, Solar Energy, 19, pp. 429-432 (1977). carbide (O-Mo2C)”, Journal of Materials Science, 2004, vol.39, No. H. Ihara, S. Ebiswa, and A. Itoh, Solar Selective Surface of Zirconium 4, pp. 1241-1248. Carbide Film, Proc. 7th Inc. Vac. Cong. and 3rd Int. Conf. on Solid A.M.D. Sacks, C-A. Wang, A. Yang, and A. Jain, Carbothermal Surfaces, Electrotechnical Lab, Tanashi, Tokyo, Japan, pp. 1813 reduction synthesis of nanocrystalline Zirconium carbide and 1816 (1977). hafnium carbide powders using solution-derived precursors, J. Mat. R. Wuhrer, W.Y. Yeung, M.R. Phillips, and G. McCredie, Study on Sci, 39, 6057-6066 (2004). d.c. magnetron Sputter deposition of titanium aluminum nitride thin A. Sayir, Carbon fiber reinforced hafnium carbide composite, J. Mat. films: effect of aluminum content on coating. Thin Solid Films, Sci., 39, 5995-6003 (2004). 290-291, pp. 339-342 (1996). D.M.Mattox and R.R. Sowell. A Survey of Selective Absorbers and G.B. Smith, P.D. Swift, and A. Bendavid, TiNX films with metallic Their Limitations, Journal de Physique, C1, N1, 42, 19-32 (1981). behavior at highN/Tiratios for better solar control windows, Applied S. Santucci, A.R. Phani, M. DeBiase, R. Alfonsetti, G. Moccia, A. Physics Letters, V 75, No. 5, pp. 630-632 (Aug. 3, 1999). Terracciano, and M. Missori. Thickness dependence of C-54 TiSi2 A. Schuler, V. Thommen, P. Reimann, P. Oelhafen, G. Francz, T. phase formation in TiN/Ti/Si(100) thin film structures annealed in Zehnder, M. Duggelin, D. Mathys, and R. Guggenheim, Structural nitrogen ambient, J. Appl. Phys., 86, 8, 4304-4311 (1999). and optical properties of titanium aluminum nitride films (Til M. Gasch. D. Ellerby, E. Irby, S. Beckman, M. Gusman and S. XAlxN), J. Vac. Sci. Technol. A 19 (3) pp. 922-929 (May/Jun. 2001). Johnson, Processing, properties and arc jet oxidation of hafnium A. Morales and J.I. Ajona, Durability, performance and Scalability of diboride? silicon carbide ultra high temperature ceramics, J. mat. Sci., sol-gel front surface mirrors and selective absorbers, J. Phys. IV 39, 5925-5937 (2004). P. Hvizdos, J. Dusza, W. Steinkellner and K. Kromp, Creep behavior France 9, 3, p. 513-518 (1999). of MoSi2 and MoSi2 + SiO composite, J. Mat. Sci., 39, 4073-4077 P.M. Martin, Adhesion of Thin Films, Vacuum Technology & Coat (2004). ing, pp. 6-12 (Feb. 2004). K. Ito, M. Moriwaki, T. Nakamoto, H. Inui, and M. Yamaguchi, R. Pretorius, Prediction of silicide formation and stability using heats Plastic deformation of single crystals of transition metal disilicides, of formation. Thin Solid Films 290-291, pp. 477-484 (1996). Mat. Sci. Eng., A233, 33-43 (1997). C.E. Kennedy, Advances in Concentrating Solar Power Collectors: C. Schroedter, Evaporation Monitoring System Featuring Software Mirrors and Solar-Selective Coatings. Trigger Points and on Line Evaluation of Refractive Indices, SPIE, C.E. Kennedy, Progress Developing High-Temperature Solar-Selec 652, 15-20 (1986). tive Coatings, Proceedings of Es2007, Energy Sustainability 2007. P.M. Martin, J.W. Johnston, and W.D. Bennet, Multilayer coatings Long Beach, CA, pp. 1-9 (Jun. 27-30, 2007). and optical materials for tuned infrared emittance and thermal coat C.E. Kennedy et al., Performance & Durability of Solar Reflectors & ing, Mat. Res. Soc. Symp Proc. 555, 3-12 (1999). Solar Selective Coatings, Natl Ctir for Photovoltaics and Solar Pro A.E.B. Presland, G.L. Price, and D.L. Trimm, Hillock Formation by gram Review Mtg. 2003. Surface Diffusion on Thin Silver Films, Surf. Sci., 29, 424-434 C.E. Kennedy et al., Progress toward developing a durable high (1972). temperature solar selective coating, 2006 SVC, 49th Annual Techni A.E.B. Presland, G.L. Price, and D.L. Trimm, the role of cal Conf. Proceedings (2005). microstructure and Surface energy in hole growth and island forma C.E. Kennedy et al., Advanced Absorber Materials powerpoint. tion in thin silver films, Surf Sci., 29, 435-446 (1972). C.E. Kennedy, Developing a durable high-temperature Solar selective Eisenhammer, Quasicrystal films: numerical optimization as a Solar coating, 17th Ascel, CSM (Apr. 28, 2006). selective absorber, Thin Solid Films, 270, No. 1, (1995). C.E. Kennedy, Advanced high-temperature Solar selective coatings Haugender, et al., Oxidation of quasicrstalline and crystalline development, PhD proposal powerpoint (Jul. 12, 2005). A1CuFe thin films in air, This Solid Films, 307, No. 120, (1997). C.E. Kennedy et al., Development and testing of high-temperature Eisenhammer, et al., Preparation and properties of Solar selective solar selective coatings, 2004 DOE Solar Energy Technologies Pro absorbers based on A1CeFE and A1CuFeCr thin films: industrial gram Review Meeting. NREL/CP-520-36581, Denver, CO (Oct. aspects, Mat. Res. Soc. Symp. Proc. 553, No. 435 (1999). 25-25, 2004). US 8,893,711 B2 Page 3
(56) References Cited C.E. Kennedy, Progress to Develop an Advanced Solar-Selective Coating, 2008 14th Biennial CSP SolarPaces Symposium, Las Vegas, OTHER PUBLICATIONS NV (Mar. 4-7, 2008). C.E. Kennedy et al., Progress toward developing a durable high C.E. Kennedy, Review of Mid- to High-Temperature Solar Selective temperature solar selective coating, 2006 SVC, Washington, DC Absorber Materials, NREL/TP-250-31267. (Apr. 24-27, 2006) powerpoint. C.E. Kennedy et al., Development and testing of high-temperature Solar selective coatings powerpoint. * cited by examiner U.S. Patent Nov. 25, 2014 Sheet 1 of 23 US 8,893,711 B2
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US 8,893,711 B2 1. 2 HIGH TEMPERATURE SOLAR SELECTIVE designed to be used in evacuated environments, the coatings COATINGS need to be stable in air at elevated temperatures in case the vacuum is breached. Current coatings do not have the stability CONTRACTUAL ORIGIN and performance desired for moving to higher operating tem peratures. For efficient photothermal conversion, solar The United States Government has rights in this invention absorber surfaces must have low reflectance (p=0) at wave under Contract No. DE-AC36-99GO 10337 between the lengths vs2 um and a high reflectance (ps 1) at wa2um. The United States Department of Energy and the National Renew reflectance cutoff may be higher or lower, depending on the able Energy Laboratory (NREL), a Division of the Midwest operating temperature. For parabolic-trough applications, an Research Institute. 10 improved spectrally selective surface should be thermally stable above 450° C., ideally in air, and have a solar absorp TECHNICAL FIELD tance (C) greater than about 0.96 plus a thermal emittance (e) below about 0.07 at 400° C. Achieving the improved proper Solar selective coatings containing multiple components to ties is very important if the parabolic trough systems are optimize the absorption of solar energy in the form of heat are 15 disclosed. By applying Such coatings to Substrates Such as the going to be operable at higher temperatures. receiver tubes of Solar collectors, Solar energy can be At this point, none of the existing prior art coatings used absorbed at high operating temperatures and transferred to commercially have proven to be stable in air at 400° C. heat transfer fluids for the generation of steam or other pur Designing and fabricating a solar-selective coating that is poses. stable in air attemperatures greater than 450° C. requires high thermal and structural stabilities for both the combined and BACKGROUND individual layers, excellent adhesion between the substrate and adjacent layers, Suitable texture to drive the nucleation Trough solar systems use parabolic, single-axis tracking and Subsequent growth of layers with desired morphology, mirrors to concentrate sunlight 30-60 times onto an evacuated 25 enhanced resistance to thermal and mechanical stresses, and receiver tube, which runs the length of the trough at the focal acceptable thermal and electrical conductivities. Other desir line of the reflector, thus heating the heat-transfer fluid (i.e., able properties of good continuity and conformability over synthetic oil) flowing through the receiver. Electricity is gen the tube, as well as compatibility with fabrication techniques. erated by passing the heated oil through a heat exchanger and The material should have a low diffusion coefficient at high using the steam generated to drive a commercial turbine gen 30 temperature and be stable with respect to chemical interac erator. Collectors axe typically aligned on a north-South axis, tions with any oxidation products, including any secondary and the trough tracks the sun from east to west during the day phases present, over long periods of time at elevated tempera to maximize the Sun's energy input to the receiver tube. tures. Selecting materials with elevated melting points and Parabolic-trough solar technology has been demonstrated by large negative free energies of formation may meet these nine utility-scale plants installed between 1984 and 1991 in 35 objectives. Stable nanocrystalline or amorphous materials are California’s Mojave Desert. These plants, referred to as Solar the most desirable (and practical) for diffusion-barrier appli Electric Generating Systems (SEGS), represent 354 mega cations, especially in light of material and process limitations. watts (MW) of installed electric-generating capacity that However, there will likely be trade-offs in the microstructure operate daily, providing power to the electricity grid. The between a highly oxidation-resistant coating (i.e., amorphous SEGS experience has proven the parabolic-trough technol 40 or nanocrystalline) and a Solar-selective coating with both ogy to be a robust and reliable power technology in an indus high absorption (i.e., columnar or porous microstructure) and trial-utility operating environment. Its key advantages are low emittance (i.e., Smooth or highly dense). High thermal proven performance, manufacturing simplicity, use of stan stability is manifested by high melting points, single-com dard equipment and materials, improvement in cost effective pound formation, and lack of phase transformations at ness via incremental steps, and low technical or financial risk 45 elevated temperature. to the investor. Solar-selective “absorber' coatings can be categorized into Experience from the SEGS plants has shown that the reli six distinct types (as shown in FIGS. 2A through 2F below): ability and lifetime of the parabolic-trough collector receiver 2A) intrinsic, 2D) semiconductor-metal tandems, 2C) multi tube or heat-collection element (HCE) is the most significant layer absorbers, 2D) multi-dielectric composite coatings, 2E) operating and maintenance issue for existing and future para 50 textured Surfaces, and 2F) selectively solar-transmitting coat bolic-trough plants. HCE designs presently use an evacuated ing on a black body-like absorber. Reviews of the literature receiver fabricated from stainless-steel tubing with a cermet revealed selected properties of some materials in these cat coating (cermets are highly absorbing metal-dielectric com egories. Intrinsic absorbers use a material having intrinsic posites containing fine metal particles in a dielectric or properties that result in the desired spectral selectivity and ceramic matrix), a Pyrex.R. glass envelope externally and 55 include: metallic W. MoC-doped Mo, Si doped with B, internally coated with an antireflective (AR) coating, and CaF, HfC, ZrB, SnO. In O. EuO, ReO. V.O.s, and conventional glass-to-metal seals. For perspective, each LaB. Semiconductor-metal tandems absorb short-wave receiver tube is usually about 4 m (13.1 ft) long and 70 mm length radiation because of the semiconductor band gap and (2.25 in) in diameter. have low thermal emittance as a result of the metal layer. The overall solar-to-electric efficiencies of parabolic 60 Semiconductors of interest include Si (1.1 eV), Ge (0.7 eV), trough solar power plants can be improved and the cost of and PbS (0.4 eV). Multilayer absorbers use multiple reflec solar electricity could be reduced by improving the properties tions between layers to absorb light and can be tailored to be of the selective coating on the receiver and increasing the efficient selective absorbers. Metal-dielectric composites— solar-field operating temperature to above 400° C. New, called cermets—contain fine metal particles in a dielectric or more-efficient selective coatings will be needed that have 65 ceramic host material. Textured surfaces can produce high both high Solar absorptance and low thermal emittance at Solar absorptance by multiple reflections among needle-like, Such elevated temperatures. Although the present coatings are dendritic, orporous microstructures. Additionally, selectively US 8,893,711 B2 3 4 Solar-transmitting coatings on a black body-like absorber are eters, the microstructure of the oxides can be deposited with also used, but are typically used in low-temperature applica a porous to columnar microstructure, and by codeposition the tions. inclusions or pores can be tilled with metal by evaporation of Multilayer absorbers or multilayer interference stacks can Sputtering. be designed so that they become efficient selective absorbers 5 A double-cermet film structure has been developed (see FIG. 3). The selective effect arises because the multiple through fundamental analysis and computer modeling that reflectance passes through the bottom dielectric layer (E) and has higher photothermal conversion efficiency than Surfaces is independent of the selectivity of the dielectric. A thin semi using a homogeneous cermet layer or a graded film structure. transparent reflective layer (D), typically a metal, separates Solar radiation is effectively absorbed internally and by phase two quarter-wave dielectric layers (C) and (E). The bottom 10 interference in double-cermet solar coatings. Further, it is reflecting layer (D) has high reflectance in the infrared (IR) easier to deposit the double-cermet selective coating than region and is slightly less reflective in the visible region. The graded-cermet layer selective surfaces. The typical double top dielectric layer (C) reduces the visible reflectance. The cermet layer film structure shown in FIG. 5 from surface to thickness of this dielectric determines the shape and position Substrate consists of the following: an AR layer that enhances of the reflectance curve. An additional semitransparent (i.e., 15 Solar absorption; an absorbing layer composed of two thin) metal layer (B) further reduces the reflectance in the homogenous cermet layers, a low-metal-Volume fraction visible region, and an additional dielectric layer (A) increases (LMVF) cermet layer on a high-metal-volume fraction the absorption in the visible region and broadens the region of (HMVF) cermet layer; and a metallic infrared reflector layer high absorption. The basic physics of the multilayer absorber to reduce substrate emittance. Similar to the double-cermet is well understood by those skilled in the art, and computer structure, 4-layer cermets, where the cermet compositional modeling can easily compute the optical properties given by gradient metal volume fractions (VF) vary from about 0.5 to an optimum multilayer design of candidate materials. Multi 0.8 have been made, modeled and prepared with good spec layer interference, stacks provide high Solar absorption and tral selectivity. low thermal emittance, and can be stable at elevated tempera Surface texturing is a common technique used to obtain tures (up to about 400° C.), depending upon the materials 25 spectral selectivity by the optical trapping of Solar energy. used. Several multilayer absorbers using different metals Properly textured Surfaces appear rough and absorb solar (e.g., Mo, Ag, Cu, Ni) and dielectric layers (e.g., Al-O, SiO, energy while appearing highly reflective and mirror-like to CeO, ZnS) are known for high-temperature applications. thermal energy. The emittance values can be adjusted (higher Metal-dielectric composite coatings or absorber-reflector or lower) by modifying the microstructure (microcrystallites) tandems have a highly absorbing coating in the Solar region 30 of the coatings with ion-beam treatments. Even single-mate (i.e., black) that is transparent in the IR, deposited onto a rial surfaces can exhibit selective properties if they have the highly IR-reflective metal substrate (FIGS. 4A through 4C proper roughness, because the selective properties depend on below). The highly absorbing metal-dielectric composite, or the ratios of mean height deviations and the autocorrelation cermet, contains fine metal particles in a dielectric or ceramic distance to the wavelength. Needle-like, dendrite, or porous matrix, or a porous oxide impregnated with metal. These 35 microstructures on the same scale as the wavelength of the films are transparent in the thermal IR region, while they are incident radiation exhibit both wavelength and directional strongly absorbing in the Solar region because of interband selectivity. This geometrical selectivity is not very sensitive, transitions in the metal and the Small particle resonance. however, to the severe environmental effects (i.e., oxidation, When deposited on a highly reflective mirror, the tandem thermal shocks) that can have catastrophic influence on the forms a selective Surface with high Solar absorptance and low 40 lifetime of conventional multilayer selective coatings. thermal emittance. The high absorptance may be intrinsic, In the art of thin-film growth, it is well known that colum geometrically enhanced, or both. The absorbing cermet layer nar microstructure will grow depending on the material itself comprising inherently high-temperature materials can have and the deposition conditions-Substrate temperature, deposi either a uniform or graded metal content. The metal-dielectric tion rate, vacuum pressure, and angle of incidence during concept offers a high degree of flexibility, and the solar selec 45 deposition. It has been determined that evaporation at oblique tivity can be optimized by proper choice of constituents, angles drastically changed the film properties. FIG. 6 illus coating thickness, particle concentration, size, shape, and trates this phenomenon, showing a Substrate upon which a orientation. The solar absorptance can be boosted by suitable film is being deposited by vapor deposition, where angle C. is choices of Substrates and AR layers, which can also provide the flux arrival angle measured from the Substrate normal and protection (for example, from thermal oxidative degrada 50 angle B is the columnar microstructure inclination angle, also tion). A variety of techniques, such as electroplating, anod measured from the substrate normal. It has been found that ization, inorganic pigmentation of anodized aluminum, the direction and magnitude of the magnetic properties wave chemical vapor deposition (CVD), and co-deposition of are dependent upon the angle of incidence, specifically when metal and insulator materials by physical vapor deposition the substrate was tilted through a 45° angle between the (PVD), can produce the composite coatings. A Subclass of 55 evaporation flux and a substrate of PermalloyTM (alloy con this category is a powdered semiconductor-reflector combi taining approximately 79 percent nickel and 21 percent iron) nation, where the Solar-selective properties of semiconductor, on large (3 in substrates and the deposition geometry, as inorganic metal oxides, organic black pigments, and metal shown in FIG. 6. Recently, glancing angle deposition dust-pigmented selective paints can be considered. FIG. 4A (GLAD), illustrated in FIG.7, employing oblique angle vapor shows a metal substrate with attached columns or rods of 60 flux deposition and Substrate motion has been used to engi metal which are encased in a dielectric matrix. neer thin film microstructures on a nanometer scale in three In a graded cermet (FIG. 4B), the reflectance from the dimensions. However, rotating large Substrates such as the cermet is reduced by gradually increasing the metal Volume large receiver tubes used in Solar collectors around their fraction, hence the refractive index of the material, from top to lengths as in this technique is not feasible. bottom, as a function of depth from the surface to the base of 65 The columnar microstructure can be tailored with a wide the film. PVD or CVD techniques can be used to form most range of control dependent on the material and deposition graded cermets. By controlling the PVD deposition param conditions by controlling the Substrate relative to the imping US 8,893,711 B2 5 6 ing flux. The surface of the microstructure must be protected The Solar absorbent materials can comprise oxides, from damage caused by Surface contact orabrasion. Selection nitrides, oxynitrides, or other Suitable compounds of the ofa material having a high intrinsic absorption coefficient can refractory metals above. further optimize the absorptance. Dense arrays of tungsten The IR reflective materials can comprise silicides, borides, whiskers have good selectivity, as do textured Stainless-steel. carbides, and other suitable compounds of the refractory met Textured copper, aluminum, nickel, and chrome were als above. At least one IR reflective layer can also comprise at reported by other sources not to have the necessary thermal least one noble metal selected from the group consisting of stability in air. Metals that are reportedly stable in air at high platinum, palladium, rhodium, ruthenium, indium, gold, and temperatures (>600° C.) include molybdenum, rhodium, osmium. platinum, tungsten, hafnium carbide, and gold. The oxides of 10 nickel and cobalt have been observed to be stable in air above In some embodiments, at least one of the solar absorbent 800° C. Amorphous silicon, germanium, and GaAs have been and IR reflective layers can beformed by depositing materials textured with hydrogen peroxide or by reactive ion or sput to form cermet materials which comprise particles of metals tering etching to produce highly absorbing selective Surfaces. or metal compounds in a matrix of at least one oxide of a The foregoing examples of the related art and limitations 15 refractory metal or metalloid. The metal compounds in Such related therewith are intended to be illustrative and not exclu cermets can comprise a titanium silicide, characterized by the sive. Other limitations of the related art will become apparent formula Ti, Si, where x=1, 3 or 5 and y=1 to 4. Two or more to those of skill in the art upon a reading of the specification layers of Such cermets can be deposited, with each having a and a study of the drawings. different index of refraction and volume fraction of metal (or metal compound) to the oxide of refractory metal or metalloid SUMMARY acting as matrix material. In the coatings comprising cermets, at least one layer of the The following embodiments and aspects thereof are IR reflective material can comprise at least one noble metal as described and illustrated in conjunction with systems, tools listed above. and methods which are meant to be exemplary and illustra 25 The substrates for the methods can be selected from vari tive, not limiting in Scope. In various embodiments, one or ous Suitable IR reflective metals and metal compounds, and more of the above-described problems have been reduced or generally will comprise at least one type of stainless steel. The eliminated, while other embodiments are directed to other Substrates can be planar, curved or tubular, and in the latter improvements. form, can be employed as solar absorber tubes for solar col One embodiment provides methods for manufacturing 30 lector receivers. Solar selective absorbing materials, including the following Further embodiments comprise solar selective coated sub steps: strates prepared by all the methods encompassed in the a) providing a Substrate comprising an infrared reflecting descriptions above. Representative embodiments include: metal; Solar selective coated Substrates comprising an infrared b) applying to the Substrate a layer of an infrared reflecting 35 reflecting Substrate, at least one layer of Solar absorptive material; material and at least one layer of antireflective material, with c) applying to the IR reflecting material at least one layer of the Substrate comprising at least one stainless steel, a plurality a solar absorbent material; of layers of Solar absorptive material comprising at least one d) optionally, applying to the layer of Solar absorbent mate refractory metal oxide, nitride or oxynitride and a plurality of rial another layer of IR reflective material, followed by 40 layers of IR reflective materials comprising at least one another layer of solar absorbent material, which step can be refractory metal silicide, boride or carbide, topped with at repeated, and least one layer of solar antireflective material which com e) applying at least one Surface layer of Solar antireflective prises at least one layer comprising at least one refractory material, to produce a coated Substrate having overall high metal oxide and at least one layer comprising at least one solar absorbance and low IR emittance at elevated tempera 45 metalloid oxide, altogether producing a solar selective coated tures. Substrate having high Solar absorptance and low IR emittance In this exemplary method, the layers of solar absorbent at elevated temperatures. material and IR reflective material can be applied by various The Solar absorbent material can comprise at least one Suitable methods including at least one physical vapor depo titania or silica. The IR reflective materials adjacent to the sition method, at least one chemical vapor deposition method, 50 absorbent layers can comprise a titanium, Zirconium or electron beam methods and various sputtering methods, hafnium silicide, boride or carbide, or at least one ternary which can be ion beam assisted. compound comprising titanium, oxygen and nitrogen. The The surface layer of antireflective material can include at titania can comprise at least one of TiO, TiO, TiO, or least two adjacent thin layers of different oxides or other TiO, N, with X=0 to 1. The silica can comprise at least one Suitable compounds of refractory metals or metalloids such as 55 of SiO, SiO, or SiON, with X=0 to 1. The titanium silicon, the layers having substantially different indices of silicide can comprise at least one of TiSi, Tissi, Tissi, TiSi refraction. The refractory metals for this surface layer can be or TiSi. At least one of the IR reflective layers can comprise selected from the group consisting of Group IVA metals a noble metal selected from the group consisting of platinum, (titanium, Zirconium and hafnium), Group VA metals (vana palladium, rhodium, ruthenium, indium, gold and osmium. dium, niobium and tantalum) and Group VI metals (chro 60 Certain embodiments are resistant to atmospheric oxida mium, molybdenum and tungsten). The topmost layer of tion at temperatures above about 450 deg. C. The coated antireflective material is normally silica, and can be textured Substrates can have at least two layers of Solar absorptive by Suitable physical or chemical means to increase Solar material having low indices of refraction, at least one layer of absorption and minimize surface reflections. In certain Solar absorptive material having a high index of refraction embodiments, the texturing can be performed by depositing 65 adjacent to at least one of the low refractive index layers and antireflective material at an acute angle to the Surface of the at least two layers of the IR reflective material adjacent to the final layer, by physical bombardment, or by chemical etching. layers of absorptive material. US 8,893,711 B2 7 8 The topmost layer of the antireflective material can be tubes in commercial Solar collectors to permit their operation textured to increase Solar absorption and minimize Surface at higher temperatures, facilitating the absorption of more reflection, and can comprise at least one silica having a low Solar energy per unit area of the receiver tubes and increasing index of refraction. Texturing can be accomplished by any the temperature of the heat transfer fluids used so as to pro suitable method, including the deposition of antireflective duce steam at higher temperatures and pressures for the pro material at an acute angle to the Surface of the final layer of the duction of electrical power by turbines. Solar selective coating, bombardment of the Surface or etch In addition to the exemplary aspects and embodiments ing. described above, further aspects and embodiments will At least one of the layers of solar absorbent and IR reflec become apparent by reference to the drawings and by study of tive materials can have the form of a cermet, comprising 10 the following descriptions. particles of at least one refractory metal silicide or a suitable metal in a matrix of at least one refractory metal oxide or BRIEF DESCRIPTION OF DRAWINGS silica, with the refractory metals selected from those listed above. The refractory metal silicide can be a titanium silicide, Exemplary embodiments are illustrated in referenced fig as characterized above. Suitable metals for the particles 15 include the noble metals mentioned above. At least two layers ures of the drawings. It is intended that the embodiments and of Such cermets can be applied as at least one layer of Solar figures disclosed herein are to be considered illustrative rather absorptive material or IR reflective material, with each layer than limiting. having a different index of refraction and volume fraction of FIG. 1 is a side view of a conventional solar collector metal to matrix material. In one embodiment, a layer of cer receiver tube for use in a parabolic-trough Solar power plant; met closest to the substrate has a relatively low index of FIG. 2A is a sectional view of an intrinsic absorber solar retraction and a relatively high, metal Volume fraction, with at coating: least one layer applied above that layer having a relatively FIG.2B is a sectional view of a semiconductor-metal solar high index of refraction and a relatively lower metal volume coating: fraction. In embodiments comprising cermet layers, at least 25 FIG. 2C is a sectional view of a multilayer solar absorbent one layer of the IR reflective material can comprise a noble coating: metal as described above. FIG. 2D is a sectional view of a metal-dielectric cermet An embodiment employing the Solar selective coated Sub Solar absorbent coating: strates described above in Solar energy applications com FIG. 2E is a sectional view of a metal solar absorbent prises a collector receiver tube for a solar parabolic trough 30 coating with Surface texturing; system, comprising a tubular glass envelope having connect FIG. 2F is a sectional view of a Solar-transmitting coating: ing bellows at each end thereof secured by airtight seals and FIG. 3 is a sectional view of a multilayer metal-dielectric means for evacuating the envelope, the envelope concentri cermet Solar absorbent coating: cally enclosing a Solar absorber tube therein, forming an FIG. 4A is a sectional view of a metal-filled dielectric annular space there between which can be evacuated, with the 35 absorber tube being a solar selective coated substrate com cermet coating on a metal Substrate; prising stainless steel and at least one type of Solar selective FIG. 4B is a sectional view of a graded metal-filled dielec coating as described above. A related embodiment comprises tric composite coating on a metal, Substrate; a process of absorbing Solar energy and healing a heat FIG. 5 is a sectional view of a dual layer cermet coating on exchange fluid, comprising steps of passing a heat exchange 40 a metal Substrate, the cermet layers being topped with an fluid through at least one solar absorber tube as described antireflective layer; above while the absorber tube, within a solar collector FIG. 6 is a schematic diagram illustrating the application of receiver tube, is exposed to Sunlight. coatings to a Substrate by employing an oblique incidence of The best mode currently envisioned is solar selective vapor deposition; absorption materials comprising various layered combina 45 FIG. 7 is a schematic diagram illustrating the application of tions of metal infrared reflecting substrates, solar absorbent coatings to a Substrate by vapor deposition using glancing materials, IR reflective materials and solar antireflective (AR) angle deposition (GLAD); materials selected and assembled to provide high Solar FIG. 8 is a graph of percent reflectance vs. wavelength for absorption, low emittance and resistance to atmospheric oxi initial modeling of the Solar selective coating concept and an dation at elevated temperatures. Suitable materials for pre 50 ideal Solar selective coat; paring Solar selective coatings include refractory metal sili FIG. 9 is a graph of percent reflectance vs. wavelength for cides such as TiSi, refractory metal oxides such as TiO, initial modeling of Solar selective coatings using various (titania), silica, noble metals such as platinum and stainless combinations of metal oxides and metals; steel substrates. Optimum results may be obtained by forming FIG. 10 is a graph of percent reflectance vs. wavelength for Some of the layers from cermets, i.e., particles of refractory 55 a TiSi coating; metal, silicides, other metal compounds or refractory or noble FIG. 11 is a graph of percent reflectance vs. wavelength for metals in matrices of Suitable refractory metal oxides such as several versions of modeled TiSi/titania solar selective coat titania or metalloid oxides such as silica. As described below, ings, with and without platinum layers, with 21 or 9 layers; a contemplated use is in tubular Solar receiver tubes in Solar FIG. 12 is a graph of percent reflectance vs. wavelength for collectors for commercial trough solar systems. 60 modeled TiSi+titania/silica solar selective coatings with 9 The selectively solar absorbent coatings applied to sub layers including platinum, after finer modeling; strates in the various embodiments disclosed herein have FIG. 13 is a graph of comparative reflectances and E. many applications for the absorption of Solar energy in the (nom)—the nominal energy versus wavelength for the mod form of heat, which in turn can be transferred via a substrate eled coating compared to a commercial alumina-based cer to a heat transfer fluid for the generation of steam, heating of 65 met coating and an ideal selective coating; water or other fluids, and for many other industrial applica FIG. 14 is a graph of thermal losses with increasing Surface tions. Among present interests is the improvement of receiver temperature for the modeled coating of FIG. 13; US 8,893,711 B2 10 FIG. 15 is a schematic diagram of the operation of an IBAD 20 described as a semiconductor-metal tandem, with a sub electronbeam chamber arranged for codeposition of coatings strate 22 coated with an intrinsically absorbent material 60 as on a flat substrate; in FIG. 2A, typically a semiconductor, with a top AR layer 62, FIG.16 is a schematic diagram of the operation of an IBAD typically a dielectric with a low index, of refraction. Multi electron beam chamber arranged for the deposition of coat layer absorbers, as shown in FIG.2C, can be coated substrates ings on Substrates rotated in a carousel device; 20 comprising a substrate 22, multipleAR layers 62 of dielec FIG. 17 is a schematic diagram of the operation of a tric and a metal layer 64 in between these AR layers. FIG. 2D CFUBS magnetron system; shows a coated Substrate 21 having a metal Substrate 22 FIG. 18 is a schematic diagram of the operation of a topped with a layer of a cermet 68 containing particles 70 of CFUBMS magnetron system with receiver tubes for single 10 at least one metal or metal compound in a matrix 72 of a chamber deposition; dielectric such as alumina. FIG. 2E shows a metal substrate FIG. 19 is a sectional schematic diagram of a multilayer 22 which has been textured on the upper surface to provide Solar selective coating applied to a substrate; projections 66 which can enhance the intrinsic absorbance of FIG. 20 is a sectional schematic diagram of a multilayer the metal and minimize reflectance. Similar texturing can be Solar selective coating comprising cermet materials and 15 employed on various types of coatings applied to metal Sub applied to a substrate; strates (not shown here). FIG. 2F is a coated substrate 20 FIG. 21 is a perspective view of a compact linear Fresnel having a metal Substrate 22 lopped by a thin layer 64 com reflector absorber assembly: prising a black body-like absorber Such as a black enamel, FIG.22 is a side view of a solar power tower tube receiver and a top layer 74 of a selectively transmitting material Such and graphs of temperature vs. locations within the receiver; as a highly doped semiconductor. FIG. 23 is a side view of a solar power tower volumetric FIG.3 illustrates a multilayer absorber or multilayer inter receiver and graphs of temperature vs. location within the ference stack 31 having a bottom dielectric layer E and an receiver; adjacent semi transparent reflective layer D comprising metal FIG. 24 is a graph of atomic concentration (percent) of Ti, or other Suitable materials. Layer D separates two quarter Si and oxygen VS. Sputtering time, based upon XPS spectra; 25 wave dielectric layers, C and D. The bottom-reflecting layer FIG. 25 is a graph of spectral percent reflectance vs. wave D has a high reflectance in the IR region and is slightly less length for deposited and modeled Solar selective coatings; reflective in the visible region, while the lop dielectric layer C FIG. 26 is a bar graph of percent thickness errors for reduces the visible reflectance. Layer B, a semitransparent various Solar selective coatings deposited by compound and metal layer, further reduces reflectance in the visible region, co-deposition; 30 while the top dielectric layer A increases the absorption in the FIG. 27 is a graph of percent reflectance vs. wavelength of visible region and broadens the region of high absorption. co-deposited TiSi individual layers on glass compared with Such multilayer arrays can be applied to various substrates, different Ti and Si composition ratios; and including metal Substrates as discussed in the above coated FIG. 28 is a graph of percent reflectance vs. wavelength for Substrates and shown here as 22. various commercial solar absorbers measured with Surface 35 FIG. 4A illustrates a coated substrate 20 having a metal Optics (SOC)410 or SOC 100 HDR vs. PerkinElmer IR883. Substrate 22 with protruding columns, pins or rods 76, having a top coating of a dielectric 62 which penetrates into the DETAILED DESCRIPTION spaces between rods 76 and provides a top protective coating. This low cost method protects etched substrates of materials Turning briefly to the drawings mentioned above as back 40 Such as anodized aluminum or copper, and also traps incident ground, FIG. 1 shows a commercial solar collector 40 com light. prising a solar absorber tube 50 connected to bellows 44 at FIG. 4B illustrates a cermet coated substrate 21 having a each end by glass-metal seals 56. The solar absorber tube 50 metal substrate 22 covered with a layer of “graded” cermet 68 comprises a glass cylindrical tube 42 enclosing a Solar containing particles of metal 70 in a matrix. 72, with the absorber tube 50 mounted concentrically within tube 42, with 45 concentration or Volume fraction of metal particles in the a vacuum established in the annular space 54 between. Typi matrix decreasing from the bottom of the coating adjacent to cally, the glass tube is made of PyrexTM or similar high tem substrate 22 to the upper surface. perature glass, coated internally and externally with antire FIG. 5 illustrates a "double cermet coated substrate 21, flective coatings (not visible or shown here). This space can built upon a metal substrate 22. Top layer 78 is an antireflec be evacuated by suitable valves or other means 46. On the 50 tive (AR) layer of a dielectric material such as silica. Below surface of the solar absorbertube 50 and within the evacuated this are two homogeneous cermet layers 68A and 68B, which space 54 between the outer tube 42 and the absorber tube 50 can have similar volume fractions of metal to matrix materials are mounted "getters' 52 near each end of collector 40 near or different metal volume fractions, as shown here and bellows 44 to absorb any hydrogen present in this space and described for the variable volume fractions of FIG. 4B. Metal maintain the vacuum. The solar absorber 50 consists essen 55 substrate 22 serves as an infrared reflector layer to reduce tially of a coated substrate 20, typically stainless steel or other Substrate emittance. A separate metal layer 23A Such as a infrared-reflecting metal, with a Solar selective coating 48 on noble metal can be provided between substrate 22 and the first the outer surface. Certain embodiments of solar selective layer of cermet 68B. coatings disclosed and claimed below can be used to enhance FIG. 6 illustrates uprocess of applying a textured or colum the performance of the solar absorbers and thus the solar 60 nar coating 84 on a metal Substrate 22 by depositing the collectors containing them. material via vapor deposition having a flux 80 directed at an Existing Solar selective or absorber coatings are illustrated oblique flux incidence angle C. measured relative to the film in FIGS. 2A through 2F. FIG. 2A shows a coated substrate 20 normal line 82 perpendicular to the substrate 22 surface, comprising a substrate 22 of metal or other Suitable material, producing a columnar film 84 oriented at an angle f3, also with a single layer of material 60 which is intrinsically solar 65 measured relative to the normal line 82. The column inclina absorbent, which can include metals, doped metals, semicon tion angle B falls between the film normal 82 and the vapor ductors, oxides and borides. FIG. 2B shows a coated substrate incidence angle C. The film 84 thus forms in columns, at an US 8,893,711 B2 11 12 angle B to the substrate 22 which improves absorbance and metal carbides, oxides, and nitrides have a high degree of reduces emittance. FIG. 7 illustrates a variation on the proce flexibility, and with further research, multiple layer cermets dure of FIG. 6, known as glancing angle deposition (GLAD). with noble metals could be viable high-temperature absorbers Here, a substrate 22 is rotated about an axis 86 which is tilted for Solar selective coatings. at an oblique angle C. relative to the flux 80 of material 5 Thermodynamically stable quasicrystal-forming alloys provided by vapor deposition. By controlling the angle of (i.e., AlCuFe, AlCuRu, and AlMnPd films) can be used as rotation, the rotation speed and the deposition rate, films with selective absorbers. Quasicrystals exhibit high thermal and symmetrical and highly oblique flux distributions can be pro chemical stability; reflectance from 300 nm to 20 um is about duced including, helical, "Zig-Zag. "staircase' and “post 0.6 and is reportedly nearly independent of wavelength. Qua microstructures. 10 sicrystals themselves show no selective properties, but thin It would be desirable to develop new, more efficient selec (10-12 nm) film stacks on a highly reflective substrate or in a tive coatings which combine relatively high Solar absorbance cermet reportedly show the desired properties. Theoretically, (greater than about 0.96) and relatively low thermal emittance quasicrystals can be expected to achieve high Solar absorp (less than about 0.07) and are thermally stable above 500°C., tance (>0.9) and low thermal emittance (<0.05 at 400° C.) ideally in air; such coatings will thus allow an increase in the 15 with high-temperature stability (500° C.). While more solar fields operating temperatures from about 400°C. to over research is needed on quasicrystal thin films and cermets, 450° C., leading to improved performance and reduced cost these promising materials could also be suitable for Solar of Solar parabolic troughs. Current coatings do not have the selective coatings. stability nor the performance desired for moving to the higher Solar-selective coatings made with multiple cermet layers operating temperatures necessary to reduce the cost of para of quasicrystals or Zirconium, yttrium, hafnium, or titanium bolic trough solar power technology. oxides, nitrides, and silicides that incorporate surface textur Cermets are highly absorbing metal-dielectric composites ing and multiple AR layers are likely to be successful and containing fine metal particles in a dielectric or ceramic meet optical and durability requirements for concentrating matrix, or a porous oxide impregnated with metal. A double solar power (CSP). Potential coatings were modeled with cermet, film structure has been developed that reportedly has 25 specialized optical modeling software and the most promis higher photothermal conversion efficiency than Surfaces ing selective coatings were identified for laboratory prototyp using a homogeneous cermet layer or a graded film structure. ing. Prototype samples are being manufactured on NREL's Optimization studies were done on 4-layer cermets with anti multi-chamber vacuum system, and a limited number of reflection (AR) coatings, where the cermet compositional samples have been optically characterized to optimize the gradient metal volume fractions (VF) varied from 0.5 to 0.8. 30 samples. A number of samples will also be selected for high Independent of material, the 0.7 VF gave the best result, temperature durability testing. resulting in high absorption (C=0.97) and low emittancee Optical Modeling (measured at 100° C.) of 0.13-0.05). Similar to the double As a first step, the modeling program Essential MacleodTM cermet structure, this research has promise for concentrating was used to optically model the potential high-temperature solar power (CSP) applications, but more research is needed. 35 selective surfaces. A. Macleod, Essential Macleod. Optical Combining several concepts, high temperature Solar selec Coating Design Program, 8.14.19, Tucson, Ariz.: Thin Film tive coatings could be developed from materials with intrinsic Center (2007). Essential Macleod was used to synthesize Solar selectivity and high-temperature stability using multiple Solar selective designs, refine existing ones, and extract opti cermet layers along with the appropriate Surface texturing and cal constants of the steel Substrate used in the design. The incorporating multiple anti-reflection (AR) coatings. The 40 program allowed the identification and selection of the coat titanium, Zirconium, or hafnium metal carbides, oxides, and ing thicknesses and compositions required to deposit the most nitrides materials have some of the highest melting points in promising candidates. nature, with HfC having the highest melting point at 331.6°C. Multilayer coatings were modeled because they were sig These materials also have high degrees of spectral selectivity, nificantly easier to model than cermets, with the expectation high hardness, improved wear, corrosion, and oxidation resis 45 of converting the best multilayer designs into cermets later. tance. Solar-selective cermets made with ZrO could be of The material properties of candidate materials for the solar high interest, as ZrO has three phases depending on the selective coating designs were reviewed, and are listed below temperature. The high-temperature tetragonal and cubic in Table I. Materials with low thermal stability and high phases can be stabilized at room temperature by adding dif reactivity were culled, and coatings incorporating materials ferent concentrations of Y.O.s, Al2O, CeO, and other mate 50 with more suitable properties were modeled. The original rials. Cermets made with SiO, and TiO, plus noble metals concept of a ZrO/Pt selective coating did not work as well as such as Au, Pt, and Pd have been found to be solar selective hoped and can be seen in FIG. 8, so other combinations of ZrO and TiO cermets are of interest because of their high oxides and metals were modeled. Combinations including indices of refraction and chemical, mechanical, and thermal HrOYO, ZrO, with Pt, Ta, and W were modeled, as shown stabilities. Titanium, nitride (TiN) and titanium aluminum 55 in FIG. 9 below. Some materials of interest (e.g., AlN, HfN. nitride (Ti Al N), with X=0 to 1, exhibit high hardness, Ta-Os, TiB. TiC,Ti,Al,N, TiO,NYN, ZrB, ZrC, ZrN, and improved wear, corrosion, and oxidation resistance. Ti quasicrystals) have not yet been modeled because optical AlN is oxidation resistant at high temperatures in air (750° constants have not yet been-found. To model, these-coatings, 900° C.), whereas TIN oxidizes at 500° C. The normal emit the dielectrics will need to be deposited, characterized, and tance ofTiN reportedly ranges from 0.40 to 0.14. Single-layer 60 the refractive indexes determined. Six metals were examined, Ti Al N films deposited by reactive magnetron sputtering and in order of reflectivity, Au, Ag, Ta, Mo, W. and Ir had the on copper and aluminum reportedly achieved absorbance highest-reflectance in the near infrared (NIR) to IR, after values (100° C.) of 0.80, but no emittance values were eliminating metals with high mobility. These reflectance val reported. Varying the aluminum and nitrogen content in these ues were essentially uniform from about 2500 nm to 20,000 compounds changes the hardness, color, optical properties, 65 nm, with Ta the highest at 0.98 and Ag lowest at 0.945. composition, microstructure, and pore and grain size. The Incorporating these metals and compounds into cermets optical properties of the titanium, Zirconium, or hafnium would likely improve the absorption properties. US 8,893,711 B2 13 14 A low-emittance, high-temperature material (TiSi) was desired. The addition of AR layers composed of very thin found that, when modeled, gave Solar selective coatings with (quarter-wave) refractory metal-oxygen compounds (or excellent emittance. The reflectance properties of TiSi are dielectric material) with high indices of refraction (TiO) and shown in FIG. 10. low-index of refraction (SiO) improved the absorptance. However, the absorptance of modeled coatings was ini 5 Titanium dioxide (TiO) is one of the most important high tially lower than desired, as shown in FIG. 8. The modeled index materials for optical coatings in the visible and near coating with TiSi improved the optical properties over a infrared, because it is the highest retractive index film mate ZrO2+platinum multilayer coating. Adding an AR layer rial for the visible region and it is hard and stable in combi improved the absorption of the coating with TiSi, but lowered nation with other oxides. Silica is one of the most important the emittance. Titanium silicide in its various Stoichiometrics 10 low index materials for optical coatings in the ultraviolet, (TiSi, TiSi, or TiSi) is an ohmic contact material in Si visible and near infrared wavelength range because it has one technology with a refractive index of 2.21 and resistivity of of the lowest indices of refraction, is very stable, and easy to about 13 to 16 ohm-cm. It is stable to about 450° C., corrosion deposit. In combination they are very important AR coatings; and oxidation resistant, and the material is typically formed at for example, when TiO, is paired with SiO, films, multilayer about 900° C. As shown in FIG. 12 and Table II, with further 15 combinations totaling in excess of 50 layers have been depos refinement, it was possible to simplify the design to nine ited that show excellent durability and little stress. As shown layers and three materials (NREL #6, i.e., TiSi, TiO, and in Table V, additional AR layers further improved the absorp SiO, which were significantly easier to deposit than the tance, but increased the complexity of the design (NRELH4A original 21-layer stack (see Design TiSi TiO-AR4b in and 4B). The difference between the A and B models is that Table III). Including Pt in the stack improves the absorption one layer of the low-emittance, high-temperature TiSimate but also increases the emittance NRELH4B (9L), as may be rial is replaced by a noble metal, Pt, in B. The noble metal has seen in Design TiSi TiO-pt-AR4, table IV. the advantage of raising the absorption, but the disadvantage As shown in FIG. 13, finer modeling refinements of the AR of also slightly raising the emittance. Further modeling layers with a tighter spread between data points between 300 increased the absorptance and reduced the number of layers and 500 nm resulted in absorptance, C-0.959 and emittance, 25 used in the design (NRELi5), to a construction that is signifi e=0.071 at 400° C. for the 9-layer, 4-material stack which cantly easier to deposit. Finer refinements to the design were includes Pt. NREL #6B. Finer refinement was completed developed by modeling the AR layers with a tighter spread for the three-material, nine-layer stack without Pt NREL #6 between data points between 300 and 500 nm and reducing that reduced the thickness of the TiSilayers while retaining the thickness of the primary TiSi reflective layer for the mul the optical properties of NREL #6A of absorptance, C.-0.957 30 tilayer stack (See NREL#6A). The comparison between the and emittance, e=0.061 at 400°C. This basically exceeds the modeled reflectance for NRELH6A and the measured reflec goal specifications by about one percent overall, because one tance of a commercially manufactured improved Al-O- percent in emittance is worth about 1.2 percent in absorp based cermet with an ideal Solar-selective, coating is shown in tance. The key issue becomes the application of optimum FIG. 14, compared to the solar spectrum on the black body coatings. Further improvements can be achieved by incorpo 35 curve at 400 deg. C. The improvement can be seen in the sharp rating improved AR coatings, cermets, and textured Surfaces, knee at the cutoff wavelength for the modeled costing com and using formulations and coating structures to handle trade pared to the commercial coatings sloped curve at the cutoff offs between low emittance and high absorptance for particu wavelength. lar applications. As shown in Table VI, with further refinement, a simple Based upon Applicant's research and review of the relevant 40 multilayer design was found that resulted in absorptance, literature, Solar-selective coatings made with multiple cermet C=0.959 and emittance, e=0.061 at 400° C. for materials with layers of refractory metal compounds that incorporate Surface high thermal stability that should be stable in air up to at least texturing and multiple AR layers are likely to be successful 500° C. but more likely as high as 800°C. In addition, because and meet the CSP optical and durability goals. Designs of only three materials were used, (i.e., TiSi, TiO, and SiO2) Solar-selective coatings with multiple layers were optically 45 that are oxides and silicides of the refractory metal, Ti, or modeled using thin-film design Software. Multilayer coatings silica, the coating should be fairly simple to deposit. As were modeled because they were significantly easier to model shown in FIG. 15, all of the modeled designs using the binary than cermets, with the expectation of later converting the best compound TiSi as the reflective layers had lower radiation multilayer design into a cermet. Materials with low thermal losses than the existing commercial coatings. stability and high reactivity were eliminated, and materials 50 Solar-selective coatings with optical properties exceeding with the most suitable properties were modeled. Modeling Applicant’s present goals (absorptance of at least 0.959 and was necessarily limited to materials with optical constants in emittance of 0.061 or less at 400°C.) were modeled with the the Software's database. Some materials of interest, e.g., the three materials. The modeled performance exceeded the goal refractory metal carbides, borides, and ternary compounds, specification by about one percent overall, as an improvement do not have published optical constants. To have modeled 55 of one percent in emittance is approximately equivalent to a them would have required either finding the constants in the 1.2 percent improvement in absorptance. Further modeling literature or depositing and measuring the optical properties refinements and improvements completed with the software of the compositions of interest, then introducing the optical included eliminating the very thin (<5 nm) layers that are constants into the Software database. The original concept of difficult to deposit. Further planned improvements include a ZrO/Pt multilayer did not work as well as hoped, as can be 60 incorporating a hydrogen permeation layer, and optimizing seen in FIG. 8, because the emittance of the modeled coating the entire HCE (air/glass/AR coating, vacuum, AR coating/ was too high. Solar-selective coating/stainless steel) structure in Solar col A low-emittance, high-temperature refractory metal-sili lector applications. Modeling a cermet Solar-selective coating con binary compound material, TiSi-a, was found for the and incorporating texturing can also be performed to opti primary reflective layer that, when modeled, gave solar-se 65 mize such structures, although depositing a cermet with the lective coatings with excellent emittance. However, the TiSi, TiO, and SiO, may be challenging. Incorporating absorptance of these modeled coatings was lower than improved AR coatings, cermets, and texturing the Surface US 8,893,711 B2 15 16 should further improve the solar-selective coating; however, Deposition: trade-offs exist, between simultaneously obtaining both low The microstructure and the material stability obtained emittance and high absorptance. Preparing commercial coat depend on the technology used to deposit the coating. The ings will require a modest amount of research and develop material properties of deposited thin-film layers of com ment along these lines. pounds and single elements differ substantially from the bulk The primary reflective layers of the solar-selective design properties of the same materials. The choice of coating mate are binary compounds, and the modeling software does not rials is influenced by the various design requirements, includ differentiate between TiSi-a and the different TiSi compound ing the optical, mechanical, chemical, and electrical proper phases (i.e., TiSi, Tissi, Tissia, and TiSi), which have ties desired. Material preparation and form are selected different properties (See FIGS. 10 and 27.) The reflectance 10 according to the available or preferred deposition process(es) values of the binary compounds vary significantly depending and its parameters. The deposition, parameters are respon on the ratios of the two constituent materials. The errors sible for the microstructure, which, in turn, determines the between the modeled properties and those of the deposited optical and mechanical properties of the film layer. coatings can easily be explained by combining the errors in There are a number of available processes which can be layer thickness and the errors in composition (See FIGS. 25 15 used to deposit optical coatings. The most common occur and 26). Any composition errors in the TiSilayer could be under vacuum and are classified as physical vapor deposition resolved by a two-part approach. First, perform a rigorous (PVD) and chemical vapor deposition (CVD). In PVD pro analysis to determine the optimum composition that gives the cesses, the thin film condenses directly into the solid phase highest reflectivity. Second, use the Effective Heat of Forma from the vapor. Physical is intended to indicate the absence of tion model, described by Pretorius and thermal gravimetric chemical reactions in the film formation; but this is an over analysis (TGA) to determine the optimum thin-film phase simplification because chemical reactions are actually formation sequence. Similarly, the present optical modeling involved. CVD is reserved for a family of techniques where software includes only TiO, and SiO, in its database and did the growing film differs Substantially in composition and not include the substoichiometric TiO, and SiO, composi properties front the components of the vapor phase. tions, which have widely different properties. Titanium oxide 25 PVD can be classified based on the methods used to pro can be sputtered or evaporated, where the starting composi duce the vapor and the energy involved in the deposition and tion is either TiOs or TiO. Titanium and oxygen form a growth of the film, but the two main processes are evaporation number of stable phases, the most practical beingTiO, TiO, and sputtering. In thermal evaporation, the material to be TiOs and Ti-Os. However, titanium also forms half a dozen deposited the evaporant is heated to the temperature at which Substoichiometric compounds with oxygen that tend to be 30 it vaporizes. The vapor condenses as a Solid onto the Sub reduced at high temperatures. All of these stoichiometrics can strates that are maintained between ambient and temperatures be evaporated and subsequently oxidized to the final stable below the melting point. Molecules travel virtually in straight phase, TiO. In conventional evaporation processes, SiO, lines between the source and the Substrate, and deposition is grows stoichiometrically if one uses SiO, as the starting line-of-sight; the laws governing the deposition thickness are material, even without an additional oxygen inlet during the 35 similar to the laws governing illumination. The vapor is pro deposition. Only a small amount of oxygen in the 10 mbar duced by heating the evaporant in thermal evaporation and by range is required to avoid absorption losses in the ultraviolet bombarding the evaporant with a beam of electrons in elec wavelength range. Silicon and oxygen form a number of tron-beam evaporation. In sputtering, the vapor is produced stable phases, the most practical being SiO, SiO, and SiO, by bombarding a target with energetic particles, mostly ions, (compounds with X-1.8 are yellow, and with 1.8 R = p. 1 - 2 2 =() where the total volume of the system is the sum of the volume contributions from all the chemical species with the form n=c/v=m-1 K where n is the refractive index, m is the real part of the refractive index (often simply used as the refractive index 10 because m is real in an ideal dielectric material) and K is known as the extinction coefficient, R is the reflectance, and p is the amplitude reflectance coefficient. For example, a The volume fraction can also be expressed in terms of the single layer of SiO, and TiO, multilayer stack, n(SiO)=1.46 numbers of moles by transferring Avogadro's number and n (TiO)=3.02, and using the above formula simplifies to 15 N=6.023x10° between the factors in the numerator. The volume fraction is then defined by 2 3.02+ HE) = 0.121. n; V. d; E V For a double layer of the SiO, and Ti, multilayer stack, where n N1/N is the number of moles of i and V, is the R=(0.121)=0.015. molar volume, and FIG.20 is a sectional schematic diagram of an embodiment 25 of a coated Substrate 21 comprising a Solar selective multi layer coating comprising similar materials to those of the W = Xn, V. coated substrate 20 discussed above and illustrated in FIG. i 19, but containing them in at least one layer in a cermet form, as described above for FIGS. 4B and 5. Substrate 22 can be as 30 described as for FIG. 19, and a thin layer of an IR reflective As with mole fractions, the dimensionless Volume fractions material 24 such as a refractory metal silicide such as TiSi Sum to one by definition, i.e., (23) or a noble metal (23A) such as platinum can optionally be applied first as in FIG. 19. At least two layers of cermet materials can be applied next, each having differing proper 35 Xd, 1. ties to achieve the desired balance between solar absorbance and emittance. As shown, an initial, relatively thick, cermet layer 25 comprises particles (70) of a refractory metal silicide Partial molar volumes are applicable to real mixtures, or noble metal Such as platinum dispersed in a dielectric including solutions, in which the Volumes of the separate, matrix (72) comprising, e.g., an oxide or nitride of a refrac 40 initial components do not sum to the total. For real mixtures tory metal such as Tior metalloid such as Sior other suitable this is generally the case, because there is usually a contrac materials. This particular layer comprises an oxide matrix tion or expansion on mixing due to changes in interstitial having a relatively low index of refraction and a relatively packing and differing molecular interactions in distinction to high Volume fraction of metal silicide particles to matrix, e.g. 45 the paradigm of ideal mixtures. Even so, the total Volume is about two parts particles to matrix. The next cermet layer 27, the sum over the partial molar volumes times the numbers of also relatively thick, can comprise similar materials, but with moles, because the Volume is a homogeneous function of the cermet matrix having a relatively high index of refraction degree one in the amounts of the various chemical species and with a medium Volume fraction of particles to matrix, e.g. present. That is, V(wn win...... n.)- V, where w is the partial molar about one part particles to one part matrix. Another cermet 50 Volume. layer 29, containing similar materials but thinner than the For example, if the amount of everything in the system is previous layers, can optionally be added, comprising a matrix doubled at constant temperature and pressure, the Volume with a relatively high index of refraction and having a rela doubles because all the molecular circumstances remain the tively low volume fraction of metal or metal compound to the same throughout. Thus, oxide matrix, e.g. about one part particles to four parts matrix. 55 Volume fractions (p, are used instead of mole fractions Y, when dealing with mixtures with a large disparity between the W = X in Vi, sizes of the various kinds of molecules (i.e., cermets) because i they express the relative amounts of the various components. 60 In any ideal mixture, the total volume is the sum of the individual Volumes prior to mixing. However, Volumes can in which the partial molar volume is defined as contract or expand upon mixing in non-ideal cases and the additivity of Volume is no longer guaranteed; the molar Vol W;, defe --ÖV ume then becomes a function of both concentration and tem 65 an peratures. If v, is the Volume of one molecule of component i. its volume fraction in the mixture is defined as US 8,893,711 B2 25 26 and n, is the number of moles of componenti. As noted, Tand has a value of 1.05. The addition of a porous SiO, AR layer P are held constant when taking these partial derivatives. can reportedly increase the solar absorptance from 0.90-0.97 These quantities can be measured experimentally, they are to O.98. not constants but vary with the composition of a system. A list of potential dielectrics based on descending order of As with the previous coated substrate 20, this solar selec- 5 refractive index, (based upon numerous sources) includes: tive coating is topped with an antireflective layer 28 compris Si>FeO>CuO>BiO>SiC-CuO>TiO2-MnOTiO2 ing at least one layer each of oxides of a refractory metal Such MnO-MgOTiO2-CrO >Ta-O, MnO>CeO2-MnOWO> as titanium and a metalloid Such as silicon, with the titania FeCrO >MnO>TaO>CuO layer 30 having a high index of refraction and the silica layer WO>AIN>ZrO >ZrOLaO+TiOd-ZrO.PrO+ 10 TiO2-ZrOTiO2-WO>SbO>SiN>Y(MoO)> 32 having a low index of refraction. In this particular embodi SbO>NdO>SiO>SnO2-La-OZrOSO>PrO > ment, the lop silica layer 32 can be applied in a relatively, low HfC)>ScO>BeO>YO>ThC>2MnOSiO>PbF> density, porous fashion. As described above for coated sub BIF>MgOd-MnOSiOd-MgOAlO&AlONZrO.PrO+ strate 20, the top layer of silica or titania can be texturized to improve performance. Al-O->MnOSiOd-Al...O.SiO2Al-O->CeF>TiN>NdF> 15 LaF>BO(AlO)>HfF>SmF>SiO2SiO2-ThF>YbF> In this context, where layers relatively thin compared with YF->LuF>SiOd-BO>SiOF>SrF>CaF·MgF2-LiF) the substrate (which may be a sheet or tube of stainless steel), NaAlF-NaF·AlF-NaF·porous CaF2 porous SiO. are applied, the layers which are relatively thick may be Porous; hybrid compounds (e.g., Al-O-SiO, CuOWO, physically from about one to about 140 times as thick as the MgOAl-O, MgOTiO, MnOSiO, MnOTiO, MnOWO, thinnest layers, e.g. the optional IR reflective layers 24 of 20 Ta-Os-MnO, ZrO2SiO, ZrOTiO, ZrOLaO+TiO, noble metal or refractory metal silicides or the thinnest layers ZrO.PrO+Al-O, or ZrO-PrO+TiO); ternary com of the top antireflective layer 28, which may range in physical pounds (e.g., AlON BO (AlO). FeCr-O, NaAlF. thickness from about 4 to about 85 nm. The thicknesses of SiO.F. Si, N.O., or Y.(MoO.),); and graded refractive these layers may also be compared in terms of optical thick indeX layer systems can be used to provide intermediate ness, defined in comparison to the wavelength of the incident 25 refractive indices that give another degree of freedom in light, as discussed above. The optical thickness of the com designing AR coatings with simple and robust designs. ponent layers of the top antireflective layer can range from Refractory Metal-Oxygen Compounds: about 0.02 to about 0.19 FWOT, for example. The refractive Crystalline TiO, films exist in three phases: anatase (tet indices of the materials in the various layers can vary from ragonal, E. 3.2 eV), rutile (tetragonal, most stable), and about 1.46 to abut 3.02 individually, with the contrasts in 30 brookite (orthorhombic); the physical, chemical, and optical refractive index between adjacent layers being a factor which properties of titania have been modified by additions of V.W. helps to achieve the desired optical effects. Cr, Sb, Ce, and Mn oxides. ZrO is an attractive candidate for Alternate AR and Absorber Layer Materials: high-temperature applications because of its high melting High and low index dielectrics can be used to replace the point and excellent, corrosion resistance. ZrO has three TiO, and SiO, in the AR coating and absorbing layers because 35 phases depending on the temperature; the monoclinic phase is of their flexibility when designing optical coatings. For AR formed from room temperature to 1150° C., the tetragonal coatings, one should ideally have a pair of materials with a phase is stable between 1150-2370°C., and the cubic phase large refractive index contrast, so that fewer layers are is formed at temperatures above 2370° C. Tetragonal ZrO. required. The refractory metal oxide compounds (H?O. can be stabilized into the high-temperature tetragonal and Ta-O, TiO YO, and ZrO) are of interest as the high 40 cubic phase at room temperature with different concentra refractive index materials in the AR coating and absorbing tions of YO, Al-O, CeO, and other additive materials. The layers because of their high indexes of refraction and chemi complete formation of the fluorite (cubic) phase is necessary cal, mechanical, and thermal stability and high melting for Zirconia to be stable at high temperatures; this requires a points. Refractory metal or metalloid oxides (SiO, MgO, minimum of 8 mole percent Y.O. (8YSZ) at high tempera Al-O, and Ta-Os), fluorides (AlF. MgF2, and YF), nitrides 45 tures to 15 mole percent Y.O. (15YSZ) at low temperatures (TiN, TaN), and oxynitride (SiON and AlON) compounds to achieve a 100 percent cubic structure. YSZ (yttria-stabi are of interest for AR coatings because of their low indices of lized Zirconia) is reportedly stable in air in a gas furnace up to refraction and can be used as the high-index of refraction 2200° C. for 26 hours: with repeated cycling. material in both AR coating and absorbing layers. The refrac Refractory Metal-Nitrogen Compounds: tive index of the materials can be varied by changing the 50 Aluminum nitride (AIN) is attractive because of its high coating composition, Stoichiometry, and morphology. For thermal conductivity enabling heat to be removed rapidly, example, the refractive index of various deposited materials good electrical and mechanical properties, and low coeffi can be tailored to any value from 1.47 (SiO) to 2.3 (SiNa) by cient of thermal expansion (CTE) to avoid thermal stresses. controlling the film Stoichiometry via varying the oxygen and Titanium nitride (TiN) can have a variable index of refraction nitrogen partial pressures. Zirconia (ZrO2) and hafnia (HfC)) 55 depending on the composition and the method of deposition. are among the oxide compounds whose tendency for inho TiN forms a single compound and oxidizes at 500° C., but mogeneous index gradients can be reduced by the introduc reportedly does not have good barrier properties. The normal tion of additives such as TiO, SiO, MgO.Y.O. and others to emittance of TiN reportedly ranges from 0.40 to 0.14. Com modify the crystalline growth behavior. The combinations pared to a melting point, of 1668 C. for Ti, Ta is higher at produce improved optical and mechanical properties over the 60 3.017°C. Tantalum nitride (TaN) has a variety of phases that separate pure materials. The addition of a small percentage of all have very high melting points and elevated heats of for YO can lower the refractive index of the combination by mation which demonstrate excellent structural strength at about 0.03, or 1.5 percent below that for pure ZrO; increas elevated temperatures. The stoichiometric TaN (x=1) phase ing amounts of additives will change the refractive index has a melting point T of about 3090°C., a heat of formation more significantly. Porous coatings have lower densities, thus 65 AH) (298 K) of about -60 kcal/mol, and has better barrier also a lower refractive index than dense coatings. For properties; it reportedly fails by diffusion at grainboundaries, example, SiO has a refractive index of 1.46 and porous SiO, but at higher temperatures than TiN. Interstitial nitrogen US 8,893,711 B2 27 28 increases the melting temperature and hardness of C-Hf dynamically stable upon heating at 2200° C. in a reducing single-phase material, and tire brittle-to-ductile transition environment for four hours without any microstructure (BDT) temperature reportedly increases with N concentra changes, phase transformations or reactions, or strength deg tion. radation. Boron carbide (BC) is the third-hardest known Tungsten (W) and its nitrides is another highly refractory material after diamond and cubic boron, nitride, with a melt material with excellent mechanical and physical properties, ing point of 2540° C. It is a p-type semiconductor with a because WN is chemically and thermodynamically stable. bandgap between 1.2 and 1.8 eV, a very low thermal conduc Additionally, it is easily deposited in the amorphous form and tivity, and is thermally stable, oxidation and chemically resis thus has diffusion-barrier stability because potential diffusion tant. The carbides are of interest for CSP applications, espe paths through grain boundaries have been eliminated. Like 10 cially with AR layers added to increase absorption and a TaN Stoichiometry plays an important role in the properties temperature-stable IR reflective layer to improve durability. of tungsten nitride materials. Highly metal-rich amorphous In terms of C. and e6 for the refractory metal carbide com WN (x<0.5) films tend to recrystallize into W and WN pounds, in descending suitability: phases at temperatures as low as 450° C. and nitrogen-rich MoCDTaCeCrC-WC-TiC and in terms of oxidation resis amorphous WN (x>-1) films such as WN possess elevated 15 tance: CrC>ZrC-NbC-VCDTiC>TaC>WC>MoCDH f(r. recrystallization temperatures (about 600° C.), but signifi However, the high emissivity of HfC and ZrC and the lower cantly higher resistivity. Amorphous WN is the phase with oxidation resistance may preclude their use in a selective the best thermal stability, mechanical robustness, conductiv coating. ity, and diffusion barrier properties; however, because of high Ternary alloys or compounds (i.e., titanium, Zirconium, or processing temperatures (>500° C.), its integration into met hafnium metal carbides, oxides, and nitrides) have high allization structures may be limited. degrees of spectral selectivity as absorber layers. The group Substoichiometric compounds of TiN, and ZrN report IV metal compounds are of the general formula MCON, edly had the best combination of high Solar absorptance and with M-Ti, Zr, or Hf, and x-y--Z-2. In tandem absorber low thermal emittance. TiN and ZrN had nearly identical reflector films substoichiometric compounds of ZrC, N, (on optical properties, but the absorptance was lower and the 25 silver) reportedly had the best combinations of high solar emittance higher for HfN. Solar absorbers could be made by absorptance and low thermal emittance. Adding carbon to reactively sputtering the nitrides of Zirconium, yttrium, form zirconium carbonitride (ZrO.N.) increased the solar cerium, thorium, and europium (i.e., ZrN.YN, CeN, ThN, and absorptance by 6 percent. Oxygen, either as Suboxides (TiO, EuN), which are transparent in the infrared, abrasion resis and ZrO) or substituted into the nitrides and carbonitrides tant, inert, very hard, and stable attemperatures in excess of 30 (ZrO, N, or ZrC.O.N.) lowered absorptance and raised emit 500° C. for long periods of time. tance. The selective optical properties of sputtered ZrO, N, on Refractory metal- or Transition metal-carbon compounds aluminum-coated oxidized stainless-steel are reported to be may be of interest for use as the dielectric layers because they thermally stable from room temperature to 600°C. (likely in are extremely hard, have good chemical and mechanical sta vacuum). Sputtered selective absorbers with the structure bility, corrosion resistance, high melting points, high electri 35 Al-O/ZrCN/Ag reportedly have good optical selectivity, cal and thermal conductivity and brittle-to-ductile fracture at with Ol?e, (325°C.)=0.91/0.05 at an operating temperature of high temperatures. It is known that Chromium, iron, molyb 700° C. in vacuum and 175°C. in air. Sputtered ZrO/ZrC/Zr denum, stainless steel, tantalum, titanium, and tungsten car absorbers reportedly have C/e(20° C.)=0.90/0.05 and are bides have been, direct-current (DC) reactively sputtered on thermally stable in vacuum on stainless-steel and quartz. Sub bulk and evaporated copper. The carbides on bulk copper 40 strates up to 600° C. and 800° C., respectively. More recent reportedly have C=0.76-0.81 and e=0.02, and on sputtered research incorporated the selective properties of pores and copper, C-0.84-0.90 and e=0.035-0.06. created a temperature-stable (400° C.), solar-selective coat Molybdenum carbide on bulk-sputtered metal reportedly ing with a void volume of 22-26 percent deposited by reactive had the highest optical properties: C-0.90 and e=0.035. The evaporation of Ti, Zr, and Hf with nitrogen and oxygen onto mechanical properties of MoC change with temperature and 45 copper, molybdenum, or aluminum Substrates with a SiO, it is not as hard as the cubic transition metal carbides (e.g., TiC AR layer. The optical properties of thin surface films of metal and TaC). Titanium carbide (TiC) has a very high melting oxynitrides (niobium, tantalum, Vanadium, Zirconium, tita point (>3000°C.), high hardness, and high electrical conduc nium and molybdenum) made by converting a substrate tivity, but it oxidizes at relatively low temperatures. Selective coated with a metal-halide slurry to metal oxide or oxynitride coatings with TiC were particularly unstable and their appear 50 by heating with oxygen and nitrogen have a high degree of ance changed when stored for a few days at room tempera flexibility, and with further research could be viable-high ture, according to Ritchie. However, bleeding nitrogen in temperature absorbers for the Concentrating Solar Power during the first two minutes of sputtering reportedly improved (CSP) program. the adhesion, friction, and wear properties of the titanium Hybrid compound multilayers (RhC). CuO/Pt) solar-se carbide. 55 lective coatings exhibit high absorptivity, low emissivity, and Zirconium carbide (ZrO) and hafnium carbide (HfC) have resistance to degradation between temperatures of 300° C. high melting points (about 3550° and 3900°C., respectively), and 600° C. A particular solar selective coating consists of a Solid-state phase stability, good thermomechanical and ther hybrid compound absorbing layer with a composition of mochemical properties (high free energy of formation and 55-65 percent Ag., 34.3-44.7 percent CuO, and 0.3-0.7 per low volatility), high hardness and wear resistance, but report 60 cent rhodium oxide (Rh2O); a diffusion layer (between the edly have high emissivity, and high current capacity at absorbing layer and the substrate) of cerium oxide (CeO); elevated temperatures. HfC oxidizes to Hf) and forms a and a metallic or glass Substrate. Films such as these report heterogeneous structure with three distinct layers: a residual edly maintained their solar absorptance of 0.9 and their ther carbide with dissolved oxygen in the lattice; a dense oxide mal emittance of 0.1 for 2000 hours at 500° C. in air. Chang interlayer containing carbon (HfCX/2O) (with, y=2-X) that 65 ing the multilayer Solar-selective coating to an hybrid is an oxygen barrier with a very low diffusion coefficient; and compound absorbing layer with a composition of 50-75 per a porous outer layer of Hf(). HfC is reported to be thermo cent Ag, 9-49.9 percent CuO, 0.1-1 percent Rh/RhO, and US 8,893,711 B2 29 30 0-15 percent Pt (at the expense of the Ag); an interlayer of Ag tures, intrinsic oxidation resistance, relatively low density, or Ag/Pt (between the absorbing layer and the substrate); a high thermal conductivity, and low vapor pressure. The melt metallic or glass Substrate; and at least one AR layer of CeO2, ing temperature is near 2300° C. for the silicides. TiSi has reportedly improved the resistance to degradation. Silver two crystalline phases: the base-centered orthorhombic films agglomerate attemperatures of 300° C. and above, but TiSi phase with the C49-type crystal structure that forms at the addition of an overcoat reportedly stabilizes the silver and 600° C. and whose resistivity is higher i.e., 60-70 micro helps to prevent diffusion. These coatings reportedly have a ohms (LS2); and the face-centered orthorhombic TiSi phase useful operating range of 300° C. to 600° C. and were tested with C54-type crystal structure that forms at higher tempera to about 700° C. in airfor 2845 hours. Solar-selective coatings tures and has lower-resistivity (15-20 uS2). TiSi thin films made with 15-35 percent CuO, 5-15 percent cobalt oxide 10 (CoO), and 60-75 percent manganese oxide (MnO) over with the C54 crystal structure are attractive because they have Pt-coated stainless-steel substrates reportedly have improved the unique physical property of lower resistance with large stability, with absorptance values between 0.88 and 0.92 and thermal stability. By reducing temperature gradients and ther emittance values of 0.06-0.12. They are resistant to degrada mal stresses in the material, high-conductivity materials have tion up to 700° C. for 700 hours in air and have a useful 15 improved thermal-shock resistance. Coatings with higher operating range of 300-600° C. The combinations of prop conductivity or lower resistivity allow more energy to be erties available from the different materials in the hybrid conducted away from the stagnation point where the concen compound absorbing layer make these materials and hybrids trated Solar energy is incident on the tube and allows the of other absorbing materials of interest to the applications energy to be radiated into the lower-temperature regions of involving concentration of Solar power. the tube. TiSi has the highest conductivity, but the lowest Alternate Reflective Layer Embodiments: density (4.10 g cm) and melting temperature (~1540°C.), Refractory and noble metals are of interest based on their in the group IV-VI transition-metal disilicides. Oxidation melting points. Refractory transition metals are those pos resistance at 1200° C. is not as good as for MoSi (the CIIb sessing high melting points and boiling points. A more spe type crystal structure), but is much better than for NbSi and cific definition uses only the melting point as a criterion, with 25 TaSi (the C40-type crystal structure). a cutoff of 1925 deg. C. If this definition were strictly fol TiSi has the highest conductivity, but the lowest density lowed, eleven metals would be called refractory metals from (4.10 g cm) and melting temperature (about 1540° C.), the Groups IV through VIA of the periodic table, including among the group IV-VI transition-metal disilicides. Oxida titanium, Zirconium, hafnium, Vanadium, niobium, tantalum, tion resistance at 1200° C. is not as good as for MoSi (the chromium, molybdenum and tungsten. Sometimes elements 30 CIIb-type structure), but is much better than for NbSi and from Group VIIA, namely rhenium, ruthenium and rhodium TaSi (the C40-type structure). MoSi exhibits the highest are also included, especially those in low formal oxidation oxidation resistance and lowest density, has high stiffness, states. Another, more restrictive definition states that the pure thermal conductivity, and strength at elevated temperature, metal must have a body-centered cubic (BCC) crystal struc but only modest toughness. Alloying MoSi with titanium ture and that the ratio of the melting point of the metallic 35 improves the oxidation resistance attemperatures around 800 oxide to that of the base metal must be less than one; it has also K because titania is a very stable oxide according to its low been suggested that a cutoff melting point temperature of standard-formation free energy. MoSi TiSi pseudobinary 2200 deg. C. be used to define: the “true' refractory metals. alloys reportedly improve the oxidation resistance because Noble metals are any of several metallic chemical elements the scale formed on the Surface is a silica-based oxide and that have outstanding resistance to oxidation, even at high 40 improves the mechanical properties of MoSi-base alloys. In temperatures; the grouping is not strictly defined, but usually addition, composites of a MoSi in SiC grid are reported to is considered to include rhenium, ruthenium, rhodium, palla have increased fracture toughness compared to MoSi. dium, silver, osmium, iridium, platinum and gold; i.e., the WSi closely resembles MoSi with respect to physical metals of Groups VIIb, VIII and Ib of the second and third properties such as melting point (WSi Tabout 2160° C. and transition series of the periodic table. 45 MoSi T about 2020°C.), lattice constant (WSi. C. 0.3211 The metals W. Mo, Ir, Os, and Ta are prime candidates for and c=0.7808 and MoSi, C-0.3202 and c=0.7851), and elec high-temperature applications, but W. Mo, Os, and Ta have tronic structure. Of the hexagonal C40 crystal, structures, very poor oxidation resistance. The metals Ag, Al, Cr, Cu, and CrSi is deformable at the highest temperature, <900° C. Ni while providing low emittance, are not suitable for CSP whereas NbSi and TaSi are deformable at significantly applications because these metals diffuse at fairly low tem 50 lower temperatures. Although NiSi may be of little impor peratures. The diffused metals roughen the film-reflector tance because of its crystal structure phase transformations, interface; a quasi-liquid mixture rises through the pores CoSi melts at 1326°C., has a low density (4.95 g cm), and formed in the interface region and oxidizes on the Surface of its oxidation resistance is as excellent as MoSi. Chromium, the film. This destroys the selective properties and mechani iron, molybdenum, stainless steel, tantalum, titanium, and cal stability of the film. Cermets of dielectrics such as Al-O 55 tungsten silicides were DC reactively sputtered on bulk and and SiO, plus additives of noble metals such as Au, Pt, and Pd evaporated copper by several workers. Titanium silicides have been found to be solar-selective. RF-sputtered reportedly had the best optical performance, with C.-0.87 and Pt—Al-O cermets with graded and uniform Pt compositions e=0.045 and the reflectance of the WSi and TiSi coatings at with Al-O AR layers and Al-O Pt—Al-O compositions 10.6 um was 0.90 or greater. Refractory metal disilicide thin were reported as stable in air at 600° C. with C?e (100° 60 films have been deposited by reactive processing techniques C.)=0.90-0.97/0.08. Cermets made with Au, Pt, Pd, Ir, or V such as DC magnetron sputtering, IBAD, and CVD. In terms and MgO, MgF2. CeO, or SiO cermets are expected to be of C. and ee suitability for the refractory metal disilicide thermally stable over 400° C. compounds: TiSi (C-0.87 and e=0.045)>TaSi> Refractory metal-silicon compounds other than TiSi could CrSi>MoSiWSi and in terms of oxidation resistance: be used as the reflective layer in other embodiments of solar 65 MoSisCoSi>WSi>TiSi (oxidizes at 12000 selective coatings. Refractory metal disilicides are of interest C.>NbSisTaSi>NiSi. The properties of the refractory for selective coatings because of their high melting tempera metal-silicon compounds, alloys, and composites make the US 8,893,711 B2 31 32 ternary Group IV metal silicides with the general formula coating, could be of use for CSP applications. In terms of MSi, with M-Ti, Zr or Hf, of interest to the CSP program. highest to lowest reflectance for the refractory metal diboride Ternary silicon carbide (TiSiC) has an excellent combi compounds: nation of properties in electrical and thermal conductivity, NbB-VB>TiB>LaB>ZrBsTaB, and in terms of high melting point, oxidation resistance, and is reportedly highest to lowest oxidation resistance (temperature): resistant to thermal shock. It also possesses high strength and CeB>HfB>ZrB>TiB>NiB>NiB>NbB>TaB>CoB> toughness, low density, damage tolerance arts stress-strain MgB>CoB>MnB>CrB>MgB>MnB>CrB>FeB>PeB> characteristics. Polycrystalline TiSiC oxidizes in air in the AlB. temperature range from 900 to 1400 deg. C. The scale that is Cermet Structure Embodiments formed is dense, adherent, and layered. The outer layer is pure 10 Multiple cermets based on refractory metal-silicon and TiO, and the inner layer is a mixture of TiO, and SiO. The oxide compounds and other embodiments are of interest combination of properties of TiSiC make the ternary Group because multiple cermets have higher photo-thermal conver IV metal silicon carbides with the general formula M.SiC, sion efficiency and are easier to deposit than graded-cermet with M-Ti, Zr or Hf, of interest to the CSP program. layer selective surfaces. The typical double-cermet layer film Titanium aluminum nitride (Ti Al N) (with Xs1) has 15 structure from surface to substrate consists of the following: high hardness, improved wear, corrosion, and oxidation resis an AR layer that enhances Solar absorption; an absorbing tance. Ti-Al N is oxidation resistant at high temperatures in layer composed of two homogenous cermet-layers, a low air (750°-900° C.). Single-layer Ti Al N films deposited by metal-volume fraction (LMVF) cermet layer on a high-metal reactive magnetron Sputtering on copper and aluminum volume fraction (HMVF) cermet layer; and a metallic infra achieved C. (100° C.)=0.80, but no emittance values were red reflector layer to reduce substrate emittance. Double reported. Higher absorptance can be reached by designing a W—AlN cermet solar coatings with C/e (350° C.)=0.92 multilayer absorber based on Ti Al N films in combination 0.94/0.08–0.10 were deposited by two-target reactive DC with an AR coating, or by using gradient layers. The micro sputtering and are stable at 500° C. in vacuum for 1 hour. structure is columnar, so a cermet made with pores with a Similar to the double-cermet structure, 4-layer cermets, diameter on the order of 30 nm is also passible, Varying the 25 where the cermet compositional-gradient metal Volume frac aluminum and nitrogen content changes the hardness, color, tions (VF) vary, have been modeled and prepared with good optical properties, composition, microstructure, and pore and spectral selectivity. Optimization studies were done on anti grain size. These materials have a high degree offlexibility in reflected 4-layer V Al-O, W Al-O, Co- SiO, the optical properties and, with further research, could be a Cr—SiO, and Ni SiO cermets, where the cermet compo practical high-temperature absorber for the CSP program. 30 sitional-gradient metalVF varies from 0.5 to 0.8. Independent Refractory- or Transition-metal diborides (MB., where of material, a 0.7 VF gives the best result, resulting in C/e M=Hf, Zr, Ti) could be used in solar selective reflective layers (100° C)=0.97/0.13-0.05. because they have melting temperatures greater than 3000°C. To convert the multilayer structure, described in Table II, and enhanced oxidation resistance. HfB and ZrB have the into a multiple cermet will require depositing the TiSi reflec lowest densities (i.e., about 12 and 6 g/cm, respectively) and 35 tive layer into a TiO, or SiO, matrix. This will be difficult and highest melting points (i.e., about 3040° C. mid 3250° C. will require modifications to the deposition process and/or respectively). ZrB also reportedly has excellent resistance to ceramic matrix because it will be difficult to deposit the TiO, thermal shock and oxidation compared to other non-oxide and SiO, without oxidizing the TiSi. This might require sepa ceramics. Composites with SiC have high thermal conductiv rating the oxide and TiSi depositions by baffling. Sputtering ity, good shock resistance, modest thermal-expansion coeffi 40 the TiSifrom a sputtering target with the optimum TiSi com cients, good dimensional Stability, and improved toughness. position and/or depositing the TiO, and SiO, by reactive Incorporation of silicon can aid Hf reactivity by forming a sputtering or evaporation from Substoichiometric oxides liquid phase at 1410° C. or below that can Scavenge excess might also help. Other options include depositing the cermet carbon to form the desired SiC. The major phase detected with different dielectrics as the ceramic matrix instead of the after oxidation at 1627°C. in air on ZrB is ZrO and on HfB; 45 TiO, and SiO, with the TiSi reflective if needed. the addition of 20 vol. percent of TaSi to ZrB reportedly Hydrogen Barrier Embodiments: improves the oxidation resistance of this material. Refractory in Solar collectors, hydrogen permeation and build-up from metal diborides VB, NbB, TaB, TiB, ZrB, and LaB, the heat transfer fluid through the stainless steel tube and solar coatings have been deposited by DC magnetron Sputtering by selective coating into the evacuated receiver annulus can Martin, Vac. Tech. Coat. 6 (April 2004). The coatings have 50 result in Substantial increases in receiver glass temperatures, potential applications as abrasion and chemical protection degradation of the receiver performance, and thermal losses and as Solar thermal control at very high temperatures; The from the receiver. The receiver lifetime can be increased by melting temperatures for bulk NbB, TaB, TiB, and LaB. changing the composition of the steel tube and using addi are 3040° C., 3040°, 3230°, and 2720° C., respectively. The tional hydrogen barrier coatings to reduce hydrogen perme reflectance of the coatings at 10.6 um was 0.90 or greater, 55 ation rates into the vacuum space and immobilizing the except for TaB, which had the lowest reflectance at 0.86. hydrogen. In many designs it is desirable to reduce the per Bulk ZrB films prepared by CVD are reportedly solar-selec meation rate by a factor of over 1000 in comparison to the tive, with C/e (100° C.)=0.67-0.77/0.080.09, ZrB, oxidizes bare metal. Barriers based upon Al-O, SiC. ZrO, TiO, slowly at 400° C. in air, requiring a protective coating at YO, and CrO. SiO, have been demonstrated to provide higher temperatures. SiN. AR coatings increase the Solar 60 permeation reduction factors from 1000 to 1,000,000 in labo absorptance to 0.88-0.93, while only increasing the emittance ratory situations by various workers. The rate controlling at 100° C. from COS to 0.10. High-temperature aging studies mechanism may actually be associated with the character and at 400° C. and 500° C. in air show that SiNZr B coatings population defects rather than the intrinsic character of the are stable up to 1000 hours. Aging studies in air at 600° C. Surface coatings. Dense coatings appear to be relatively show slight increases in the emittance after 300 hours because 65 robust, both mechanically and in the presence of H. The of oxidation of the SiN. ZrB is a stable, high-temperature ternary compound TiAIN coating structure evolves from TiN selective-solar absorber that, with an improved AR protective to AlN when the Al content increases from 0 to 70 percent. US 8,893,711 B2 33 34 The oxidation behavior of deposited TiAIN coatings with in the 425-860° C. range is not recommended if subsequent different Al concentrations oxidation resistance increases and aqueous corrosion resistance is important. Grade 316L is then decreases with increasing Al content; the addition of 40 more resistant to carbide precipitation and can be used in the percent Al reportedly leads to the best oxidation resistance above temperature range. Grade 316H has higher strength at and hydrogen barrier permeation performance at tempera elevated temperatures and is sometimes used for structural tures between 300 and 800° C. Annealing Cr-alloyed steel and pressure-containing applications at temperatures above parts under high vacuum conditions, to produce Cr enriched about 500° C.; it could be used in trough receiver tube. protective surface films without measurable Cr-depleted Grades 321 and 347 are the basic Austenite 18/8 steel Zones next to the film, reportedly increases the hydrogen (Grade 304) stabilized by titanium (321) or niobium (347) permeation and corrosion resistance. Dense coatings of many 10 additions. These grades are used because they are not sensi of the materials of interest as AR and absorber coatings will tive to intergranular corrosion after heating within the carbide also provide hydrogen permeation resistance to the steels. precipitation range of 425-850° C. Grade 321 is the grade of Dense hydrogen permeation coatings should be deposited as choice for applications in the temperature range of up to about a hydrogen permeation coating prior to the deposition of solar 900° C., combining high Strength, resistance to Scaling and selective coatings for Solar collector applications. 15 phase stability with resistance to Subsequent aqueous corro Substrate Materials sion. Grade 321H is a modification of 321 with a higher The Solar selective coating may be deposited on stainless carbon content, to provide improved high temperature steel receiver tubes for solar collectors. It may be deposited on strength. A limitation with 321 is that titanium does not trans stainless steel grades including 316, Ti (321), Kb (347), and fer well across a high temperature arc, so is not recommended alloys (Monel 400) for the use of high temperature molten as a welding consumable. In this case grade 347 is pre salts as heat exchange fluids. The coating may also be depos ferred the niobium performs the same carbide stabilization ited on stainless steel (304), glass, copper, or aluminum tubu task but can be transferred across a welding arc. Grade 347 is lar or flat substrates. therefore the standard consumable for welding 321; because Stainless steel, grade 304L is more readily available in of this 347 would be a better substrate for the receiver tubes. most product forms, and so is generally used in preference to 25 Grade 347 is only occasionally used as parent plate material. 321 if the requirement is simply for resistance to intergranular Like other Austenite grades, 321 and 347 have excellent corrosion after welding. However, 304L has lower hot forming and welding characteristics, are readily brake or roll strength than 321 and so is not the best choice if the require formed and have outstanding welding characteristics. Post ment is resistance to an operating environment over about weld annealing is not required. They also have excellent 500° C., especially for molten salt application. Grade 321 30 toughness, down to cryogenic temperatures. equivalent to Grade 304 in the annealed condition and Supe Monel 400 is a nickel-copper alloy that is resistant to sea rior if a weldment in these grades has not been post-weld water and steam at high temperatures as well as to salt and annealed or if the application involves service in the 425-900 caustic Solutions. The alloy is characterized by good general C. range. It is Subject to pitting and crevice corrosion in warm corrosion resistance, good weldability and moderate to high chloride environments, and to stress corrosion cracking above 35 strength. It has excellent, resistance to rapidly flowing brack about 60°C. It is considered resistant to potable water with up ish water or seawater. It is particularly resistant to hydrochlo to about 200 mg/L chlorides at ambient temperatures, reduc ric and hydrofluoric acids when they are de-aerated. The alloy ing to about 150 mg/L at 60°C. It has good oxidation resis is slightly magnetic at room temperature. The alloy is widely tance in intermittent service to 900° C. and in continuous used in the chemical, oil and marine industries and is corro service to 925°C. These grades perform well in the 425-900° 40 sion resistant in an extensive range of marine and chemical, C. range, and particularly where Subsequent aqueous corro environments; from pure water to nonoxidizing mineral sive conditions are present.321H has higher hot strength, and acids, salts and alkalis. This alloy is more resistant than nickel is particularly Suitable for high temperature structural appli under reducing conditions and more resistant than copper cations. These grades would work better with flat plate col under oxidizing conditions; however, it does show better lectors and hot water applications, but not with molten salt 45 resistance to reducing media than oxidizing. It has good Storage. mechanical properties from Subzero temperatures up to about Grade 316 is the standard molybdenum-bearing grade, sec 480° C. It has good resistance to sulfuric and hydrofluoric ond in importance to 304 amongst the Austenite-stainless acids. Aeration, however, will result in increased corrosion steels. Austenite is the name given to the gamma phase of rates. It may be used to handle hydrochloric acid, but the steel alloys. Generally Austenitie Steel can be defined as steel 50 presence of oxidizing salts will greatly accelerate corrosive with a Face Centered Cubic Structure. Its face-centred cubic attack. Resistance to neutral, alkaline and acid salts is shown, (FCC) structure has more open space than the body-centered but poor resistance is found with oxidizing acid salts such as cubic structure, allowing it to hold a higher proportion of ferric chloride. It has excellent resistance to chloride ion carbon in solution. The molybdenum gives 316 better overall stress corrosion cracking. This grade would work better with corrosion resistance properties than Grade 304, particularly 55 steam applications, but/not with molten salt storage. higher resistance to pitting and crevice corrosion in chloride environments. It has excellent forming and welding charac Alternative Applications Embodiments teristics. It is readily brake or roll formed into a variety of parts for applications in the industrial, architectural, and Depending on the application and with proper optimiza transportation fields. Grade 316L, the low carbon version of 60 tion, the Solar selective coatings may be used for other appli 316, is immune from sensitization (grain boundary carbide cations because of its expected high-temperature and oxida precipitation). Grade 316H, with its higher carbon content, tion resistance properties. Optimizing the design (i.e., cutoff, has applications at elevated temperatures, as does stabilized layer thickness, material Stoichiometry, morphology, and grade 316Ti. The Austenite structure gives these grades excel substrate) could allow the coating to be used in other solar lent toughness, down to cryogenic temperatures. Grade 316 65 applications including Solar heating systems, Volumetric has good oxidation resistance in intermittent service to 870° receivers, and Compact Linear Fresnel Reflector (CLFR) C. and in continuous service to 925°C. Continuous use of 316 absorbers. An example of such an absorber is illustrated in US 8,893,711 B2 35 36 FIG. 21. In this figure, a compact linear Fresnel reflector protect the absorber plate from heat loss. Other collectors absorber 140 is mounted on the ground or other flat surface by include an integrated collector and storage system and the rails 142 which, support reflectors 144. A pair of inverted evacuated tube collector. Integral collector and storage sys cavity receivers 146 are supported above this level by posts tems combine the function of hot water storage and Solar 150 and guy wires 148. energy collection into one unit. Evacuated tube collectors CLFR concepts for large scale solar thermal electricity produce higher temperature water and are more complex than generation plants assume many parallel linear receivers flat plate collectors. Evacuated tube collectors include a series elevated on tower structures that are close enough together for of tubes that contain a heat pipe to absorb solar energy and individual mirror rows to direct reflected solar radiation to transfer it to a liquid medium. The tubes are evacuated two alternative linear receivers on separate towers. This pro 10 vides for more densely packed arrays, where alternating the (vacuum) so that there is very little heat loss from the tube. mirror inclination can eliminate shading and blocking while Most solar collectors are roof-mounted. Solar water heaters maximizing ground coverage. Preferred designs use second are used for domestic hot water, pool heating and space heat ary optics to reduce tower height requirements. Avoiding ing needs. The heat can also be used for industrial applica large mirror row spacings and receiver heights lowers the cost 15 tions or as an energy input for other uses such as cooling of ground preparation, array Substructure cost, tower struc equipment. ture cost, stream line thermal losses, and steam line cost. The The Solar selective coatings can be optimized for higher benefits of the Fresnel approach include small reflector size, temperature use in the solar power tower receiver. A power low structural cost, fixed receiver position without moving tower system uses a large field of mirrors to concentrate joints, and noncylindrical receiver geometry. The illustrated sunlight onto the top of a tower, where the receiver sits. It uses array uses low emittance all-glass evacuated Dewar tubes as an array of flat, movable mirrors, (called heliostats) to focus the receiver elements. the Sun's rays upon a collector tower (the target). The high CLFR systems use an inverted cavity receiver containing a energy at this point of concentrated Sunlight is transferred to waterfstream mixture which becomes drier as the mixture is a substance that can store the heat for later use. This heats pumped through the array. The stream is separated and flows 25 molted salt flowing through the receiver. Then the salts heat through a heat exchanger, where the thermal energy passes to is used to generate electricity through a conventional steam the power plant system. In the initial plant, the reheat cycle generator. Molten salt retains heat efficiently, so it can be only requires steam, at 265° C. The heat-transfer fluid is stored for days before being converted into electricity. That water, and passive direct boiling heat transfer can be used to means electricity can be produced on cloudy days or even avoid parasitic pumping losses and the use of expensive flow 30 several hours after Sunset. That energy can, in turn, be used to controllers. boil water for use insteam turbines. Water had originally been An inverted cavity receiver has been designed using steel used as aheat transfer medium in earlierpower tower versions boiling tubes which can be directly linked with an existing (where the resultant steam was used to power a turbine). This fossil fuel plant steam system. This is much cheaper than system did not allow for storage and power generation during evacuated tubes used in trough plants. Direct steam genera 35 the evening. Examples of heliostat based power plants were tion is much easier with this absorber than with tubular the 10MWe Solar One and Solar Two demonstration projects absorbers in trough collectors. Initially a Chrome Black in the Mojave Desert, which have now been decommissioned. selective coating will be used but new air stable selective The 11 MW PS-10, 20 MW PS-20, and 15 MW Solar Tres coating is being developed for higher temperature operation Power Tower in Spain builds on these projects. required by stand-alone plants (320-360° C.). The optical 40 A metallic-tube receiver was tested in 1985-1986, produc efficiency of the receiver is very high, and uses no auxiliary ing 2.45 kg's hot air at 9.5 bar and 800° C. outlet tempera reflectors. ture. A second panel with ceramic SiC tubes was tested in The individual materials in the coating can be used for a 1987, with a mass flowrate of 0.48 kg's at 9.3 bar and 1000° variety of applications such as antistatic coatings, light-emit C. The high estimated investment costs and the low incident ting, light-detecting, and light-triggered semiconductor 45 solar fluxes permitted by the tubes (lower than 200Wkm) devices, electrical electrodes for flat panel display and elec made it unpractical to pursue the construction of the plant. trochromic devices, Solar cells, heat mirrors, Smart windows, Several Power lower project initiatives have shifted to air resistivity contacts, bipolar interconnect plates in Solid oxide cooled receivers whose outlet temperature is 800° C. Volu fuel cell, oxidation resistant coatings for alloys and steels, and metric receivers use highly porous structures to absorb the for thermal barrier coatings for turbine blades, combustor 50 concentrated solar radiation. The solar radiation is absorbed liners, stator Vanes, pistons, valve heads, exhaust ports, and within the volume of the structure, not on the outer surface. diesel engine cylinders, heat exchangers, burner nozzles, hot The heat transfer medium, i.e., air, is forced through the gas fans and fillers, and pump and valve linings for corrosive porous structure and is heated by convective heat transfer. environments, high temperature aerospace applications Volumetric absorbers show a high open porosity, allowing the including space shuttle tiles, stealth aircraft skin structures, 55 radiation to penetrate deeply into the structure to ensure good and jet engine blades and missile radomes. convective heat transfer. Common volumetric absorbers are The solar selective coatings can be optimized for lower mace from thin heat resistant wires (knitting or layered grids) temperature use in the Solar water heating systems absorber. and metallic or ceramic open-cell matrix structures (reticu Solar beating systems generally include Solar thermal collec lated foams or matrix structures). A good Volumetric absorber tors, a fluid system to move the heat from the collector to its 60 shows the volumetric effect where the absorber temperature point of usage, and a reservoir or tank for heat storage and at the irradiated side is lower than the temperature of the Subsequent use. There are several types of Solar collectors. medium leaving the absorber. Most consist of a flat copper plate, painted black, which has FIG. 22 illustrates schematically the operation of an air water tubes attached to the absorber plate. As solar energy cooled power tower receiver 152 with walls 151 and a central falls on the copper plate and is absorbed, the energy is trans 65 void 153 through which air may flow upward (as shown by ferred to water flowing in the tubes. The absorber plate is arrow 155) as it is heated by absorption of solar energy mounted in a casing that has a clear covering and insulation to impinging on the outer Surfaces of walls 151. The graphs US 8,893,711 B2 37 38 below the schematic show changes in temperature in the walls Mo-Si-B alloys under extreme operating; conditions. 151 and the gas inside at the inlet at the lower end of the uit Many critical gas turbine engine components are currently and the exit at the upper end. made from Ni-base superalloys that are coated with a TBC. FIG. 23 illustrates schematically a volumetric power tower Ceramic-based coatings have enormous commercial receiver 154 having a series of heat resistant wire screens and 5 potential for applications, such as high-temperature coatings metallic or ceramic open cell matrix structures 156 joined for turbine blades, conductive and erosion-resistant coatings together to form a porous structure through which gas 158 can for arc heaters, and protective and wear-resistant coatings for flow from left to right. Solar energy is absorbed by the porous machines and tools. Moreover, they are also used in the elec structure from the left and possibly other surfaces and heats tronic industry (high-temperature sensors), the sports indus the gas as it passes through the structure, as indicated by the 10 try (golf club heads, Snow skis, shoe spikes, etc.), and medical temperature graphs below for the absorber structure and gas equipment. Immediate commercialization applications for passing from inlet to exit. thermal barrier coatings include turbine blades, combustor Many of the materials in the Solar selective coatings are liners, stator Vanes, etc.; aeronautical and land-based turbine based on transparent and semiconducting oxides widely used engines; and pistons, valve heads, exhaust ports, and cylin for a variety of applications such as antistatic coatings, light 15 ders in diesel engines. Other commercial applications include emitting, light-detecting, and light-triggered semiconductor heat exchangers, burner nozzles, hot gas fans and filters, and devices, electrical electrodes for flat panel display and elec pump and valvelinings for corrosive environments, high tem trochromic devices, Solar cells, heat mirrors, and Smart, win perature aerospace applications including space shuttle tiles, dows. The TCO films are wide band-gap semiconductors stealth aircraft skin, structures, plus thermal barrier coatings whose properties depend strongly on the oxidation state (sto in jet engine components including blades and missile ichiometry) and on the nature and quantity of impurities radomes. trapped in the film. The properties of TCO films are very sensitive to the deposition techniques, deposition parameters, EXAMPLES properties of Sputtering targets, and the postdeposition treat ments used. Titanium disilicide (TiSi) is the most widely 25 FIGS 24-28 used silicide for making low resistivity contacts to silicon, in ultra large-scale integration (ULSL) devices. The require Deposition and Testing of Modeled Solar Selective Coatings: ments of low cost and high-temperature corrosion resistance Deposited Solar-selective coatings that reproduce the mod for bipolar interconnect plates in solid oxide fuel cell (SOFC) eled coatings are likely to be successful and meet the optical stacks has directed attention to the use of metallic alloys with 30 and durability requirements for concentrated Solar power oxidation-resistant coatings. Candidate coatings must exhibit applications. In thin-film deposition, a good general rule of chemical and thermal-mechanical stability and high electrical thumb is that the lower the process pressure and the more conductivity during long-term (>40,000 hours) exposure to energetic the process, the more control exists over the film SOFC operating conditions. To increase service lives of aero properties, but the higher the cost of the equipment and the engines, the technology of protective coatings is widely used 35 coating. To begin Solar-selective prototyping, the ion-assisted in the aero engine turbines to improve the high temperature e-beam chamber in NREL's Pernicka multichamber vacuum oxidation resistance of alloys. system was used, instead of the magnetron-sputtering cham Nanomaterials have potential applications in the fields of ber. Although magnetron Sputtering is the process expected to optics, electronics, catalysis and biomedicine because of be used for the final product, the targets for the magnetron unusual chemical and physical properties compared to their 40 sputtering chamber with its three linear arrays of five 1.5 bulk, mainly due to their high surface to volume ratio. Oxide Mini-Mac or two 3'x12' planar magnetron guns are more nanoparticles are of special interest for their application as expensive and less versatile for initial material prototyping. coating materials for improvement of high temperature oxi The e-beam gun can deposit six materials sequentially or dation resistance of various alloys and steel under cyclic co-deposit two different materials simultaneously in 7-cc cru heating conditions. 45 cibles. Dielectrics can be evaporated directly or reactively, High-temperature application of titanium alloys in and the ion gun can be used to improve the quality, compo aeroengines is often limited by their insufficient resistance to sition and density of the coating. By controlling the deposi the aggressive environment. Coatings with excellent oxida tion parameters, the oxides can be deposited with microstruc tion resistance are being developed to increase the maximum tures ranging from porous to columnar to amorphous, and service temperature of conventional titanium alloys from the 50 with co-deposition, a cermet can be made where the dielectric present 520-600° C., the temperature limit set by the is filled with metal. The dielectric chemistry can be controlled mechanical capabilities of most advanced alloys. Compo by activated reactive evaporation (ARE) with the introduction nents that operate under high temperatures and pressures, of gases (e.g., O, N, CO) into the process, Which results in Such as those used in turbines and airplane brakes, require materials with widely different material properties. Introduc materials with excellent chemical and physical properties. 55 ing reactive gases with the evaporation materials selected for Nickel-based alloys (or superalloys) are the current industry this activity provided a great deal offlexibility in the materials standard for these applications. Thermal barrier coats (TBC) that could be prototyped for the high-temperature solar-se allow the final products to withstand temperatures beyond the lective coatings. Initially, the Substrate was glass because melting point of nickel. However, there is a growing need for glass is readily available and lower in cost than a polished products with properties that nickel cannot achieve. Molyb 60 stainless-steel Substrate. In addition, glass is transparent, denum-silicon-borate (Mo-Si B) alloys could potentially allowing the extraction of the refractive indices, of the coat supersede nickel due to their ability to withstand even greater ings from the reflectance and transmittance data with the temperature changes and pressures than existing metals and Essential MacleodTM software. Eventually, the entire solar alloys. However, multilayered coatings that include a diffu selective coatings will be deposited on polished stainless sion barrier layer, an oxidation resistant, layer and an oxida 65 steel, Substrates appropriate to high temperatures. tion barrier layer form a stable gradient of integrated layers AR coatings are typically one-quarter-wavelength stacks that can prevent cracking, peeling and delamination of of dielectrics of alternating high and low refractive indices. US 8,893,711 B2 39 40 Layer thickness control is therefore critical to the optical upgrades/modifications to the deposition system, including properties, and deposition parameters are critical to the the following: installing the co-deposition plate with two microstructure, which defines the mechanical and thermal e-beam guns; as a matter of safety, installing a lift for the properties. Accordingly, pneumatic shutters were installed e-beam deposition plate; installing a second crystal sensor; and wired to the quartz crystal thin-film controller to allow replacing the e-beam Sweep and installing a second e-beam careful control of the critical layer thicknesses, and the thin Sweep; installing shutters for each e-beam gun and the Sub film controller was calibrated. The individual layers for the strate; modifying the associated wiring and plumbing; NRELi6A architecture (see Table VI) were deposited and upgrading the cooling-water manifold; upgrading the their properties were characterized: step profilometry for residual-gas analyzer (RGA) and Software; upgrading/repair thickness; ellipsometry for thickness and index of refraction; 10 ing the gas flow controller, rebuilding a roughing pump; and ultraviotet-visible-near-infrared (UV-VIS-NIR), infrared starting automation of the deposition process. (IR), and Fourier transform infrared (FTIR) spectrophotom After the major modifications were completed, the indi etry for optical properties; scanning electron microscopy vidual layers of the coating architecture were evaporated (SEM) and atomic force microscopy (AFM) for morphology from elemental starting materials by reactive e-beam and Surface roughness, and X-ray photoelectron spectros 15 co-deposition. By manually varying the power and deposition copy (XPS) analysis for stoichiometry. rate of the Ti and Simaterials and/or increasing the amounts The easiest, simplest, lowest-cost solution of evaporating of the reactive gas compounds with various compositions compounds was tried first for several reasons, even though it were produced. The individual layers and the modeled was anticipated that the elements in the compounds could NRELi6A architecture were deposited and optically charac evaporate sequentially in order of lowest melting point. First, terized; the stoichiometry was analyzed with XPS and the many compounds could be deposited very easily in order to results correlated with the IR spectra. The NRELi6A coating screen out materials that did not work as well as modeled with was dark blue and the absorption was improved, but the a small investment of time and materials. Second, several emittance was much poorer compared to the compound stoichiometrics for the TiSi, TiO, and SiO could be depos deposited from substoichiometric materials. Reflectance ited at low cost. Third, depositing compounds of a specific 25 measurements of the optical properties of the co-deposited Stoichiometry by co-deposition would require the installation individual primary reflective layer and the solar-selective of the co-deposition plate with two e-beam guns and the coating were lower than the model and the compound depo purchase of a second e-beam Sweep control, additional sition, as shown in FIG. 25. quartz-crystal sensors, and shutters. Therefore, individual From error analysis, errors in layer thickness led to errors layers in the architecture were reactively evaporated from 30 in absorption, but errors in the stoichiometry led to errors in Substoichiometric starting materials (i.e., TiSi, Tissis, both absorption and emittance. The layer thicknesses for the TiOs TiO, SiO, SiO) with increasing amounts of reactive co-deposited solar-selective coating were incorrect; they gas (i.e. O) and ion assist. It should be noted that the binary were badly overshot by up to 52.7 percent and undershot compounds in the deposited coatings are designated as TiSi down to -10.3 percent on average, shown in FIG. 26. The (stoichiometric) or TiSi (substoichiometric), and likewise 35 primary reflective layers of the solar-selective design are the oxides are TiO, or O-and SiO or SiO. To determine the binary compounds, and the modeling software does not dif appropriate level of ion assist for a dense adherent coating, ferentiate between TiSi-a and the different TiSi compound different levels (high, low, and none) of ion assist were exam phases (i.e., TiSi, Tissis, Tissia, and TiSi), as shown in FIG. ined. SEM and AFM confirmed that the layers morphology 27. See Pretorius, “Prediction of silicide formation...'. The moved from columnar to a dense structure with increasing 40 thicknesses of the individual layers and their chemical com levels of ion assist. The optical properties, Stoichiometry, and position was determined during deposition using a quartz optimum reactive gas flow and partial pressure for the indi crystal microbalance in situ and the Inficon IC/5 deposition vidual layers were determined. XPS verified that stoichiomet controller. In FIG. 28, the reflectance of the binary compound ric oxide layers were deposited when the coating color can be seen to vary significantly depending; on the ratios of became visually transparent. More importantly, the XPS 45 the two constituent materials, titanium and silicon. Two sig results showed that the method of directly evaporating; the nificant results can be observed. First, the titanium rich com TiSibinary-compound reflective layers from substoichiomet positions have a higher reflectivity in the IR-corresponding ric compounds of TiSi or TiSis would not be successful to a low emittance—and a sharper band edge. Second the because the compounds evaporated preferentially into layers deposited coating as measured with the XPS has a poor cor of Ti/mixed TiO, oxides/Si/mixed SiO, oxides from bottom 50 relation with the predicted composition from the IC/5 during to top, as shown in FIG. 24. This result was not unexpected; the deposition run. However, it can be seen from FIG. 28 that but the easiest, lowest cost solution of evaporating com the IR reflectance spectra values measured on the Perkin pounds was tried first before depositing by the more equip Elmer 883 IR spectrophotometer are lower than the spectra of ment intensive and more expensive method of co-deposition. the same samples measured on the newer instruments at Sur While the individual layers were being characterized, the 55 face Optics Corporation (SOC), the SOC 410 hand-held modeled NRELi6A architecture structure was deposited and reflectometer (SOC 410) and the SOC 100 hemispherical characterized. As shown in FIG. 25, the optical performance directional reflectometer. Therefore, the performance of the of the compound NREL#6A was quite encouraging, despite co-deposited NRELi6A coating may not actually be as poor having deposited the coating with known significant errors— as was observed in FIG. 25. Recently, new National Institute including a layered structure that was deposited instead of a 60 of Standards and Technology (NIST) IR standards and a TiSi compound for the reflective layer and layer thicknesses foucusing reflectance attachment were purchased by NREL that were systematically incorrect; they were overshot by up and the spectrophotometer was recalibrated in order to more to 26.1 percent or undershot down to 1.7 percent, as shown in accurately measure the deposited samples with the PE 883 IR FIG. 26. From these results, it was determined that the com spectrophotometer. Until a more accurate instrument (e.g. pound reflective layer and the Solar-selective coating should 65 SOC 410 hand-held reflectometer) can be purchased, mea be deposited by co-deposition to produce a solar-selective surements subcontracted to SOC, or the problem with the low coating closer to that modeled. This required extensive reflectance measurements resolved; the PE 883 IR spectro US 8,893,711 B2 41 42 photometer can be used to evaluate relative improvements in NREL's initial monitoring and control errors greatly the deposited coating but not used to determine the final exceeded 5% for the deposited thin film thickness, therefore optical properties of the coating. the coatings measured performance was as expected interior The performance error between the modeled and deposited to the modeled coating. To resolve the thickness errors, the coatings can easily be explained by examining the errors in 5 deposition monitor and control will be upgraded with the layer thickness and the errors in composition. The composi- addition of an optical monitor, providing positive feedback tion errors can be resolved by performing rigorous analyses to between the quartz crystal monitor and the optical monitor, determine the appropriate compositions that give the highest and completing the automation of the coating process. Auto reflectivity and using the effective heat of formation model by mation will remove human error from the coating deposition Pretorius to determine the thin-film phase-formation 10 process and by providing steering and cutting at sensitive Sequence. turning points, mid-course corrections can be made for any In situ optical monitoring (commonly used in the optical thickness errors in order to deposit a coating that matches the coating industry) or ellipsometry (a very good experimental model. In addition, by varying the ion assist, the optimal technique that is moving into the optical coating industry) coating density will be determined that gives both high allows the stoichiometry to be monitored while the coating is 15 absorption and low emittance with excellent oxidation resis being deposited. The individual layers and the modeled coat- tance. ing were deposited by directly and reactively evaporating the Combining these methods the thickness and compositional compound layers from Substoichiometric compounds and by errors will be eliminated in the final coating. In addition, elemental co-deposition. The deposited archetype proof-of- improvements can be gauged by measuring the deposited concept coating leaves little doubt regarding the coating capa- 20 samples with the PE 883 IR spectrophotometer compared to bility to perform as modeled even though the deposited coat- the new NIST IR standards, but determination of the final ings measured optical performance was lower than the optical properties will require resolving the issue of the lower modeled performance because of thickness and composi- reflectance values when measured with, the PE 883 IR, Sub tional errors known to have occurred during the coating depo- contracting the measurements to Surface Optics (SOC sition. An optical coating is only as good as the optical moni- 25 Corp.), or purchasing an SOC 410. Depositing these opti toring and control during deposition. In most cases to achieve mized individual materials in the Solar-selective coating with high yield for demanding requirements it is necessary to out the thickness and composition errors and measuring them minimize tire optical thickness errors below 1%, particularly accurately should result in deposited Solar-selective coatings at sensitive turning points. As can be seen from FIG. 26. closer to those modeled. TABLE I Material Properties of Some Dielectrics of Interest Compound TiO2 Ta2O5 Z.02 Hf), Sc-03 Y20. Al2O3 SiO2 MgF2 Refractive 3.01 2.1 2.22 1.94 1.9 1.94 1.66 1.46 1.38 Index in (2.4-2.2) (2.4-2.5) (2.3-1.94) (2.36-196) (1.95-1.85) (1.72->1.9) (1.68->1.58) (1.38-1.34) Range 0.4-1.2 0.4-8 O3-8 <0.3->10 O.25-S O3-12 O.3-5 1.2-8 (Lm) Melting 1640 1872 2715 2812 2300 2410 2020 1610 1261 Point Tmp ( C.) Abs Bands O.9 Abs. free 2.9, 6.9 Abs. free Abs. free Abs. free 2%, 3.6 (Lm) Molecular 79.87 441.80 123.22 210.49 137.91 225.81 101.96 60.08 63.30 Weight (g) Crystal TiO4.9 8.2 S.6 9.68 3.86 S.O1 3.97 2.6 Density TiO2 4.2 p (g/cc) rutile, 3.8 Anatase Ti20, 4.6 TiOS 4.6 AGf 834 1487 790 (600K) Hardness Excellent — Exceptional Good Excellent Excellent Deposition Good-Dep Dependent Dependent Adhesion Excellent Excellent Excellent Excellent Good Adh layer Color TiO Gold White White or White White White Clear to Clear Clear TiO, White Black White Ti20 Purple TiOs Black Method Ebeam Ebeam Ebeam Ebeam Ebeam Ebeam Ebeam Ebeam Ebeam Sputter Sputter Sputter Sputter Sputter Evap. TO 1750 -2OOO -2400 -2SOO -2400 --21 OO --950 Temp. TiO, 1640 (° C.) Ti20, 2130 TiOs 1760 Dep. Rate O.2 2-5 1-2 2-4 <5 1-2 2-5 2O US 8,893,711 B2 TABLE I-continued Material Properties of Some Dielectrics of Interest Compound TiO2 Ta2O5 Z.02 Hf), Sc-03 Y20 Al2O3 SiO2 MgF2 Crucible Cu TaGraphite Graphite Cu Graphite Graphite Cu Graphite/Ta Mo 02 Part. 2-20 x 10 2-5 x 10 8 x 10 5-8 x 10 5x1O 5-8 x 10 1 x 10 2 x 106 low 10 Press. (tow) Subs. 250(min.) 175-300 -50-3OO --SO-3OO 200-2SO -50-3OO -200-300 150-2SO Temp (C.) TABLE II Design for 9-Layer Coating Design: TiSi-TO-pt-AR4 (NREL #6B) Reference Wavelength (nm): 510.00 Incident Angle (deg): 0.00 Physical Geometric Void Packing Refractive Extinction Optical Thickness Void Material Layer Material Density Index Coefficient Thickness (FWOT) (nm) Thickness Density Medium Air 1.OOOOO OOOOOO - 1 SO2 OOOOO 1461.78 O.OOOOO O.19436.185 67.81 0.13296.212 Air O.OOOOO 2 T.O. OOOOO 3.01375 O.OOOOO O.O2S89518 4.38 0.00859234 Air O.OOOOO 3 S.O. OOOOO 1461.78 O.OOOOO O.O81972.85 28.60 O.OS6O7728 Air O.OOOOO 4 T.O. OOOOO 3.01375 O.OOOOO O.2124O473 35.94 O.O70478SS Air O.OOOOO 5 Pt OOOOO 2.00403 3.SO668 O.0128.7470 3.28 0.00642442 Air O.OOOOO 6 T.O. OOOOO 3.01375 O.OOOOO O.21091133 35.69 0.06998.302 Air O.OOOOO 7 TSi-A OOOOO 2.21098 2.61715 O.O898.2119 20.72 0.04.062514 Air O.OOOOO 8 S.O. OOOOO 1461.78 O.OOOOO 0.22937677 80.03 0.15691568 Air O.OOOOO 9 TSi-A OOOOO 2.21098 2.61715 8.95563219 2065.77 4.OSO53459 Air O.OOOOO Substrate SS foil (EP+MP) O.OOOOO O.OOOO1 Total Thickness 1.O.O1325078 2342.22 4.5925931S TABLE III Design for 9-Layer Coating Design: TiSi-TO-AR4b (NREL #6A) Reference Wavelength (nm): 510.00 Incident Angle (deg): 0.00 Physical Geometric Void Packing Refractive Extinction Optical Thickness Void Material Layer Material Density Index Coefficient Thickness (FWOT) (nm) Thickness Density Medium Air 1.OOOOO O.OOOOO 1 SO2 1.OOOOO 1461.78 O.OOOOO O.2O614160 71.92 0.14102060 Air O.OOOOO 2 T.O. 1.OOOOO 3.01375 O.OOOOO O.O26O3967 4.41 0.00864029 Air O.OOOOO 3 SO2 1.OOOOO 1461.78 O.OOOOO O.O9028071 31. SO O.O617606S Air O.OOOOO 4 T.O. 1.OOOOO 3.01375 O.OOOOO O.18143749 30.7O 0.06020323 Air O.OOOOO 5 TSi-A 1.OOOOO 2.21098 2.61715 O.O64O2O83 14.77 O.02895592 Air O.OOOOO 6 S.O. 1.OOOOO 1461.78 O.OOOOO O.30O861OO 104.97 O.20581773 Air O.OOOOO 7 TSi-A 1.OOOOO 2.21098 2.61715 O.O6498.043 14.99 0.02938994 Air O.OOOOO 8 SO2 1.OOOOO 1461.78 O.OOOOO O.65398416 228.17 0.44738780 Air O.OOOOO 9 TSi-A 1.OOOOO 2.21098 2.61715 8.08O15197 1863.83 3.65456445 Air O.OOOOO Substrate SS foil (EP+MP) O.OOOOO O.OOOO1 Total Thickness 9.66789786. 2365.2S 4.63774O62 US 8,893,711 B2 47 48 The invention claimed is: antireflective material, to increase Solar absorption and mini 1. A method for manufacturing a Solar selective absorption mize surface reflection and wherein a final layer of the at least material, comprising: one antireflective layer is texturized by a process of deposit providing a Substrate ; ing said at least one antireflective material at an acute angle to applying thereto at least one layer of at least one IR reflec the surface of said final layer. tive material; 8. A Solar selective coated Substrate prepared in accordance applying to said at least one layer of the at least one IR with the method of claim 1. reflective material at least one layer of at least one solar 9. A Solar selective coating for high temperature applica absorbent material including depositing materials to tions, comprising at least three layers in physical contact with form said at least one Solar absorbent material as a cer- 10 one another, wherein at least one layer of the at least three met; and layers comprises a cermet, applying at least one Surface layer of at least one Solar wherein the at least one cermet comprises particles dis antireflective material, to produce a coated Substrate persed within a matrix, having an overall high Solar absorbance and a low IR wherein the particles comprise at least one of a first metal emittance at elevated temperatures, 15 in the form of a metal silicide, an elemental metal, and wherein the at least one Solar absorbent cermet comprising mixtures thereof, the deposited materials comprises particles of at least wherein the first metal comprises at least one of a Group one first metal in the form of a metal silicide, an elemen- IVA metal, a Group VA metal, a Group VI metal, a noble tal metal, and mixtures thereof dispersed in the solar metal, and mixtures thereof, absorbent material, wherein the at least one Solar absor- 20 wherein the matrix comprises at least one of a second metal bent material comprises at least one second metal in the in the form of a metal oxide, a metal silicide, a metal form of a metal oxide, a metal nitride, a metal oxynitride, nitride, a metal oxynitride, a metal boride, a metal car a metal silicide, a metal boride, a metal carbide, a metal bide, a metal ternary compound, and mixtures thereof, ternary compound, and mixtures thereof, wherein the second metal comprises at least one of a Group wherein the at least one first metal of the particles com- 25 IVA metal, a Group VA metal, a Group VI metal, a prises at least one of a Group IVA metal, a Group VA metalloid, a noble metal, and mixtures thereof, and metal, a Group VI metal, a noble metal, and mixtures wherein the Solar selective coating is stable attemperatures thereof, and up to about 450° C. and is characterized by a solar wherein theat least one second metal of the solar absorbent absorptance greater than about 0.96. material comprises at least one of a Group IVA metal, a 30 10. The coating of claim 9, wherein a layer of the at least Group VA metal, a Group VI metal, a metalloid, a noble three layers is a Surface layer comprising a third metal in the metal, and mixtures thereof form of a metal oxide, wherein the third metal comprises at wherein the high Solar absorbance is greater than about least one of a Group IVA metal, a Group VA metal, a Group VI O.96. metal, a metalloid, and mixtures thereof. 2. The method of claim 1 wherein said at least one IR 35 11. The coating of claim 10, wherein the surface layer is reflective material comprises at least one metal in the form of textured. a metal silicide, a metal nitride, a metal oxynitride, a metal 12. The coating of claim 10, wherein the surface layer boride, metal carbide, a metal ternary compound, and mix- comprises at least one of silica and titania. tures thereof, wherein the at least one metal comprises at least 13. The coating of claim 9, wherein at least one layer one of a Group IVA metal, a Group VA metal, a Group VI 40 comprises a fourth metal in the form of a metal oxide, a metal metal, a metalloid, a noble metal, and mixtures thereof. nitride, a metal oxynitride, and mixtures thereof, wherein the 3. The method of claim 1 wherein the second metal, is in the fourth metal comprises at least one of a Group IVA metal, a form of at least one metal oxide. Group VA metal, a Group VI metal, and mixtures thereof. 4. The method of claim 1 wherein said at least one surface 14. The coating of claim 9, wherein at least one layer layer of at least one solar antireflective material is character- 45 comprises at least one of A1CO, AICuRu, AlMnPd, and mix ized by comprising at least two adjacent thin layers of at least tures thereof. one third metal in the form of a metal oxide, a metal nitride, a 15. The coating of claim 9, wherein at least one layer metal oxynitride, a metal fluoride, and mixtures thereof, comprises a fifth metal in the form of a metal silicide, a metal wherein the at least one third metal comprises a Group IVA boride, a metal, carbide, a metal ternary compound, and mix metal, a Group VA metal, a Group VI metal, a metalloid, and 50 tures thereof, wherein the fifth metal comprises at least one of mixtures thereof, wherein said at least two adjacent layers a Group IVA metal, a Group VA metal, a Group VI metal, a have substantially differing indices of refraction such that the noble metal, and mixtures thereof. at least one layer of the at least one antireflective material 16. The coating of claim 9, wherein the second metal of the forms a Surface layer. matrix comprises at least one of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, 5. The method of claim 2 wherein said at least one metal of 55 W, and mixtures thereof. the at least one IR reflective material is selected from the 17. New The coating of claim 9, wherein the metal ternary group consisting of titanium, Zirconium, hafnium, Vanadium, compound is at least one of the second metal is in the form of niobium, tantalum, chromium, molybdenum, tungsten, and a silicide carbide, an aluminum silicide, an aluminum nitride, mixtures thereof. an oxide nitride, and mixtures thereof. 6. The method of claim 1 wherein at least one layer of said 60 18. The coating of claim 9, wherein the particles comprise at least one IR reflective material comprises at least one of at least one titanium silicide in the form of Ti, Si, whereinx platinum, palladium, rhodium, ruthenium, indium, gold, equals 1, 3, or 5, and y equals from 1 to 4 inclusive. osmium, and mixtures thereof, dispersed in a matrix compris- 19. The coating of claim 9, further comprising a stainless ing at least one of a silicide, a boride, a carbide, and a ternary steel substrate. compound. 65 20. The coating of claim 9, comprising a first cermet layer 7. The method of claim 1, further characterized by a step of and a second cermet layer, wherein the first cermet layer and texturizing a final layer of material, comprising at least one second cermet layer are in physical contact, wherein the first US 8,893,711 B2 49 50 cermet layer has a substantially lower metal volume fraction of a particle phase than a Volume fraction of a particle phase in the second cermet layer. k k k k k