U.S. patent application number 15/520026 was filed with the patent office on 2017-11-23 for enhanced thermal stability on multi-metal filled cermet based spectrally selective solar absorbers.
This patent application is currently assigned to University of Houston System. The applicant listed for this patent is Massachusetts Institute of Technology, University of Houston System. Invention is credited to Feng Cao, Gang Chen, Daniel Kraemer, Zhifeng Ren.
Application Number | 20170336102 15/520026 |
Document ID | / |
Family ID | 55858152 |
Filed Date | 2017-11-23 |
United States Patent
Application |
20170336102 |
Kind Code |
A1 |
Ren; Zhifeng ; et
al. |
November 23, 2017 |
Enhanced Thermal Stability on Multi-Metal Filled Cermet Based
Spectrally Selective Solar Absorbers
Abstract
A spectrally selective solar absorber is described and comprises
a substrate, double cermet layers comprising multi-metal
nanoparticles embedded in a dielectrics matrix, and double
antireflection layers deposited on cermet layers. The tungsten or
titanium or tantalum infrared reflector layer suppressing the
diffusion of substrate elements and multi-metal nanoparticles in
the cermet are disclosed.
Inventors: |
Ren; Zhifeng; (Houston,
TX) ; Cao; Feng; (Houston, TX) ; Kraemer;
Daniel; (Cambridge, MA) ; Chen; Gang;
(Cambridge, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Houston System
Massachusetts Institute of Technology |
Houston
Cambridge |
TX
MA |
US
US |
|
|
Assignee: |
University of Houston
System
Houston
TX
Massachusetts Institute of Technology
Cambridge
MA
|
Family ID: |
55858152 |
Appl. No.: |
15/520026 |
Filed: |
September 29, 2015 |
PCT Filed: |
September 29, 2015 |
PCT NO: |
PCT/US2015/052952 |
371 Date: |
April 18, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62072124 |
Oct 29, 2014 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F24S 70/16 20180501;
F24S 70/30 20180501; F24S 70/12 20180501; Y02E 10/40 20130101; F24S
70/225 20180501 |
International
Class: |
F24J 2/46 20060101
F24J002/46; F24J 2/48 20060101 F24J002/48 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This work was partially supported by "Concentrated Solar
Thermoelectric Power (CSP)", DOE SunShot CSP grant, under award
number DE-EE0005806 and "Solid State Solar-Thermal Energy
Conversion Center (S.sup.3TEC)," an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Science, Office
of Basic Energy Science under award number
DE-SC0001299/DE-FG02-09ER46577 (GC and ZFR).
Claims
1. A method of fabricating a solar absorber, comprising: disposing
a first layer in contact with a substrate; disposing a second layer
in contact with the first layer; disposing a third layer in contact
with the second layer; disposing a fourth layer in contact with the
third layer; and disposing a fifth layer in contact with the fourth
layer, wherein disposing the fifth layer forms a solar absorber
comprising an absorbance within a first predetermined range and an
emittance within a second predetermined range.
2. The method of claim 1, wherein the substrate comprises at least
one of stainless steel, tantalum (Ta), titanium (Ti), copper (Cu),
aluminum (Al), silicon (Si), quartz, and combinations thereof.
3. The method of claim 1, wherein the first layer comprises at
least one of tungsten (W), tantalum (Ta), titanium (Ti), and
combinations thereof, and wherein a thickness of the first layer is
from about 10 nm to about 300 nm.
4. The method of claim 1, wherein the first layer comprises
Tungsten (W), Tantalum (Ta), Titanium (Ti), Molybdenum (Mo),
Chromium (Cr), Vanadium (V), Niobium (Nb), Zirconium (Zr), or
combinations thereof and wherein the thickness of the first layer
is from about 100 nm to about 200 nm.
5. The method of claim 1, wherein the second layer and the third
layer each comprise at least one of Nickel (Ni), Cobalt (Co), Iron
(Fe), Tungsten (W), Tantalum (Ta), Titanium (Ti), Molybdenum (Mo),
Chromium (Cr), Vanadium (V), Niobium (Nb), Zirconium (Zr), and
Al.sub.2O.sub.3.
6. The method of claim 1, wherein the third layer has a lower metal
volume fraction than the second layer.
7. The method of claim 1, wherein the fourth layer comprises at
least one of Al.sub.2O.sub.3, MgO, TiO.sub.2, V.sub.2O.sub.3, ZrO,
or combinations thereof.
8. The method of claim 1, wherein the fifth layer comprises
SiO.sub.2.
9. The method of claim 1, wherein disposing the second layer on the
first layer bonds the second layer to the substrate.
10. A solar absorber comprising: a reflector layer disposed in
contact with a substrate; a first cermet layer disposed in contact
with the reflector layer; a second cermet layer disposed in contact
with the first cermet layer; and at least two anti-reflective
coating (ARC) layers, wherein at least one ARC layer is disposed in
contact with the second cermet layer.
11. The solar absorber of claim 10, wherein the substrate comprises
at least one of stainless steel, tantalum, titanium, copper,
aluminum, silicon, quartz, and combinations thereof.
12. The solar absorber of claim 10, wherein the reflector layer
comprises at least one of tungsten (W), tantalum (Ta), titanium
(Ti), and combinations thereof, and wherein a thickness of the
reflector layer is from about 10 nm to about 300 nm.
13. The solar absorber of claim 10, wherein the first cermet layer
and the second cermet layer each comprise at least one of Nickel
(Ni), Cobalt (Co), Iron (Fe), Tungsten (W), Tantalum (Ta), Titanium
(Ti), Molybdenum (Mo), Chromium (Cr), Vanadium (V), Niobium (Nb),
Zirconium (Zr), and Al.sub.2O.sub.3, and wherein the second cermet
layer has a lower metal volume fraction than the first cermet
layer.
14. The solar absorber of claim 10, wherein the first layer of the
at least two ARC layers comprises at least one of Al.sub.2O.sub.3,
MgO, TiO.sub.2, V.sub.2O.sub.3, ZrO, or combinations thereof.
15. The solar absorber of claim 10, wherein the second layer of the
at least two ARC layers comprises SiO.sub.2.
16. The solar absorber of claim 10, wherein the reflector layer is
a bonding layer between the substrate and the first cermet
layer.
17. The solar absorber of claim 10, wherein the reflector layer
comprises a plurality of separately deposited reflector layers.
18. A solar absorber comprising: a reflector layer disposed in
contact with a substrate; a first cermet layer disposed in contact
with the reflector layer, wherein the reflector layer comprises at
least one of tungsten (W) or nickel (Ni).
19. The solar absorber of claim 17, further comprising a second
cermet layer disposed in contact with the first cermet layer and at
least two anti-reflection (ARC) layers, wherein at least one ARC
layer is disposed in contact with the second cermet layer.
20. The solar absorber of claim 17, wherein the substrate comprises
at least one of stainless steel, tantalum, titanium, copper,
aluminum, silicon, quartz, and combinations thereof.
21. The solar absorber of claim 17, wherein a thickness of the
first cermet layer is from about 10 nm to about 300 nm.
22. The solar absorber of claim 17, wherein the first cermet layer
and the second cermet layer each comprise at least one of Nickel
(Ni), Cobalt (Co), Iron (Fe), Tungsten (W), Tantalum (Ta), Titanium
(Ti), Molybdenum (Mo), Chromium (Cr), Vanadium (V), Niobium (Nb),
Zirconium (Zr), and Al.sub.2O.sub.3, wherein the second cermet
layer comprises a lower metal volume fraction than the second
layer.
23. The solar absorber of claim 17, wherein the first layer of the
at least two ARC layers comprises at least one of Al.sub.2O.sub.3,
MgO, TiO.sub.2, V.sub.2O.sub.3, ZrO, or combinations thereof.
24. The solar absorber of claim 17, wherein the second layer of the
at least two ARC layers comprises SiO.sub.2.
25. The solar absorber of claim 17, wherein the reflector layer
bonds the substrate and the first cermet layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a 35 U.S.C. .sctn.371 national stage
application of PCT/US2015/052952 filed Sep. 25, 2015, and entitled
"Enhanced Thermal Stability on Multi-Metal Filled Cermet Based
Spectrally Selective Solar Absorbers," which This patent
application claims priority to and incorporates in its entirety
U.S. Provisional Patent Application 62/072,124 filed Oct. 29, 2014,
and entitled "Enhanced Thermal Stability on Multi-Metal Filled
Cermet Based Spectrally Selective Solar Absorbers," filed Oct. 29,
2014 each of which are hereby incorporated herein by reference in
their entirety for all purposes.
BACKGROUND
Background of the Technology
[0003] Solar thermal technologies such as solar hot water and
concentrated solar power trough systems employ spectrally-selective
solar absorbers. These solar absorbers are designed to efficiently
absorb the sunlight while suppressing re-emission of infrared
radiation at elevated temperatures.
BRIEF SUMMARY OF THE DISCLOSURE
[0004] In an embodiment, a method of fabricating solar absorbers
comprising: disposing a first layer in contact with a substrate;
disposing a second layer in contact with the first layer; disposing
a third layer in contact with the second layer; disposing a fourth
layer in contact with the third layer; and disposing a fifth layer
in contact with the fourth layer, wherein disposing the fifth layer
forms a solar absorber comprising an absorbance within a first
predetermined range and an emittance within a second predetermined
range.
[0005] In an embodiment, a solar absorber comprising: a reflector
layer disposed in contact with a substrate; a first cermet layer
disposed in contact with the reflector layer; a second cermet layer
disposed in contact with the first cermet layer; and at least two
anti-reflective coating (ARC) layers, wherein at least one ARC
layer is disposed in contact with the second cermet layer.
[0006] In an embodiment, a solar absorber comprising: a reflector
layer disposed in contact with a substrate; a first cermet layer
disposed in contact with the reflector layer, wherein the reflector
layer comprises at least one of at least tungsten (W) or nickel
(Ni).
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] For a detailed description of the exemplary embodiments
disclosed herein, reference will now be made to the accompanying
drawings in which:
[0008] FIG. 1 is a schematic of a spectrally selective solar
absorber configuration according to certain embodiments of the
present disclosure.
[0009] FIG. 2 illustrates the bidirectional reflectance spectra of
solar absorbers before and after annealing that were fabricated
according to certain embodiments of the present disclosure.
[0010] FIGS. 3A and 3B illustrate the surface roughness of a solar
absorber upon annealing where the solar absorber was fabricated
according to certain embodiments of the present disclosure.
[0011] FIGS. 4A-4D are AFM images illustrating the morphology
change of a single cermet layer before and after annealing
according to certain embodiments of the present disclosure.
[0012] FIG. 5 illustrates XRD patterns for pristine and annealed
cermet coatings where the coatings were fabricated according to
certain embodiments of the present disclosure.
[0013] FIG. 6 is a chart of Raman spectra of cermet coatings before
and after annealing where the coatings were fabricated according to
certain embodiments of the present disclosure.
[0014] FIG. 7 illustrates the spectral bidirectional reflectance
response of solar absorbers that were fabricated according to
certain embodiments of the present disclosure.
[0015] FIG. 8 illustrates XRD patterns of solar absorbers
fabricated according to certain embodiments of the present
disclosure.
[0016] FIG. 9 illustrates the experimental set up and results for
steady state calorimetric measurements of samples fabricated
according to certain embodiments of the present disclosure.
[0017] FIG. 10 illustrates the spectral properties of a plurality
of solar absorbers that were fabricated according to embodiments of
the present disclosure.
DISCUSSION OF DISCLOSED EXEMPLARY EMBODIMENTS
[0018] The paper by F. Cao, D. Kraemer, G. Chen, and Z. Ren,
entitled Enhanced Thermal Stability of W--Ni--Al2O3 Cermet-based
Spectarlly Selective Solar Absorbers with W Infrared Reflector, is
incorporated by this reference in its entirety.
[0019] Efforts for the development of thermally stable selective
solar absorbers may focus on spectral selectivity and thermal
stability for high temperature applications. Selective solar
absorbers, which may be referred to as "solar absorbers" herein,
were fabricated according to certain embodiments of the present
disclosure and may be based on two cermet layers and were
fabricated using a magnetron sputtering technique on mechanically
polished stainless steel substrates. Cermets are composite
materials comprising metallic and ceramic materials that may
therefore comprise desirable properties of both ceramics and
metals. For example, cermets may be resistant (to loss of
properties and deformation) to high temperatures like a ceramic,
and may be able to undergo plastic deformation like metallic
materials. Cermets may be used in both electronic and mechanical
applications including in solar applications and for cutting and
machining tools that may also experience high temperature.
Reflector layers provide solar reflectance by reflecting
wavelengths in various wavelength ranges, including the visible,
infrared, and ultraviolet ranges, in order to reduce the heat
transferred to the surface of an apparatus employing the reflector
layer. In some embodiments herein, infrared reflector layers may be
employed in solar absorbers. Wavelength ranges may comprise
infrared wavelengths above 700 nm (10.sup.-9 m) to about 1 mm,
visible wavelengths may range from about 400 nm to about 700 nm,
ultraviolet light wavelengths may range from less than 400 nm
(e.g., shorter wavelengths than visible light) to about 10 nm,
x-rays may range from less than about 10 nm (e.g., shorter than
ultraviolet light) to about 10 pm (picometers, 10.sup.-12 m), and
gamma rays may be less than about 10 pm, that is, shorter than
x-ray wavelengths.
[0020] In various contexts and applications, the emittance
(emissivity) of a surface may be considered because a low emittance
may indicate that the solar absorber wastes less energy through
emitting thermal radiation than materials with a high emittance.
The same principle may apply, for example, in insulation
applications where it may be desirable for a window to retain heat
using a coating or a film. In an embodiment, an operating
temperature where selective solar absorbers may be desired is from
about 500 to about 600.degree. C. Nickel and tungsten were employed
in certain embodiments for the infrared reflector layer in
selective thermal absorbers discussed herein, the results of those
experiments are discussed herein, including one in which a stable
solar absorptance of about 0.90 and total hemispherical emittance
of 0.15 at 500.degree. C. was obtained using tungsten as the
infrared reflector layer. While the infrared reflector layer may be
referred to in some embodiments as "a layer," the infrared
reflector layer may be a plurality of individual (separate) layers
which may be of the same or differing layer types and/or varying
thicknesses, or combinations or the same type of material and
different types of material with the same or varying thicknesses
depending upon the embodiment.
[0021] In one embodiment, a spectrally selective solar absorber
comprises a substrate (stainless steel, tantalum, titanium, copper,
aluminum, nickel, silicon, quartz, and combinations thereof), an
infrared reflector layer or bonding layer (tungsten, tantalum,
titanium, nickel, silver, gold, aluminum, and combinations
thereof), a first and a second cermet layer which may comprise
multi-metal nanoparticles in dielectric matrix and two
anti-reflection coatings. The term "selective" may be used to
describe the manner in which the solar absorber is fabricated so
that the solar absorber provides an absorbance within a first
predetermined wavelength range and an emittance within a second
predetermined wavelength range. Herein, a "cermet layer," s a
combination of two or more metals and a ceramic, and in some
embodiments, a combination of at least two layers cermet1 ("C1")
and cermet 2 ("C2") may be employed in a solar absorber, where each
of C1 and C1 comprises a combination of any two or more metals,
including but not limited to Nickel (Ni), Cobalt (Co), Iron (Fe),
Tungsten (W), Tantalum (Ta), Titanium (Ti), Molybdenum (Mo),
Chromium (Cr), Vanadium (V), Niobium (Nb), Zirconium (Zr), and at
least one of Al.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2,
Ta.sub.2O.sub.5, AlN, or other dielectric materials as appropriate
for the end application's desired absorbance and emittance
ranges.
[0022] The introduction of multi-metal nanoparticles in cermet, as
discussed in certain embodiments of the present disclosure, as
compared to the use of solely single-metal nanoparticles provides
additional tuning parameters (e.g., the metal/ceramic
concentrations and different component selections) with which to
tailor the optical properties of the cermet absorption layers. The
anti-reflection coatings ("ARC"), also referred to as "ARC layers"
discussed herein may comprise Al.sub.2O.sub.3, MgO, TiO.sub.2,
V.sub.2O.sub.5, Ta.sub.2O.sub.5, ZrO.sub.2, SiO.sub.2, and other
oxide layers that may be appropriate for various desired ranges of
emittance and reflectance in solar absorbers. In some embodiments,
the stable infrared reflector layer suppresses the diffusion of the
substrate elements into the cermet layer and results in enhanced
thermal stability of the solar absorber at elevated temperature.
The metal infrared reflector layer also improves to some extent the
spectral selectivity of the solar absorber due to its low infrared
emittance.
[0023] Introduction
[0024] Sunlight may be converted into a useful terrestrial heat
source by employing sunlight absorbing surfaces in the form of
solar absorbers. Solar absorbers may be employed in solar thermal
systems such as solar hot water systems and concentrated solar
power (CSP) trough systems, as well as in emerging technologies
such as solar thermoelectric, solar thermo-photovoltaic, and solar
thermionic generators. The solar thermal receiver efficiency may
depend on the optical properties of the solar absorber. To maximize
the efficiency of a solar absorber, it may be desirable for a solar
absorber to comprise a near-blackbody absorptance (.alpha.) in the
solar spectrum range while retaining a low emittance (.di-elect
cons.) in the infrared (IR) range, and be thermally stable at their
operational temperatures. The solar absorbers discussed herein may
be employed in processes, methods, and products to convert received
wavelengths into energy sources.
[0025] Discussed herein is a spectrally-selective solar absorber
("solar absorber") and methods of fabricating the solar absorber
comprising a layer disposed between a substrate and an absorber
coating that demonstrates a long-term stability at high
temperatures (T>400.degree. C.) as well as a stable solar
absorptance of about 0.90 and a hemispherical emittance of 0.15. As
used herein, a "spectrally selective" solar absorber may be defined
by the range of wavelengths it is designed to reflect and/or
absorb.
[0026] As for mid-temperature (about 100.degree. C.<T<about
400.degree. C.) and high-temperature (T>about 400.degree. C.)
applications, cermet-based coatings may be employed and comprise
ceramic metallic composites which may be good candidates for
inclusion in the solar absorber due to their high solar
absorptance, low emittance, and good thermal stability. These
desirable properties may be attributed to the high temperature
stable ceramic host. Cermet-based spectrally selective solar
absorbers may present and be employed as single, double, and triple
cermet layers. The thin cermet layer is typically in contact with a
metallic surface for high solar absorptance that is transparent to
IR radiation. The absorption of solar radiation in the cermet layer
may be due to interbank transitions in the metal and small particle
plasmonic resonances.
[0027] A "graded metal volume fraction" is the term used herein to
describe a combination of two or more cermet layers comprising
different metal volume fractions (weight of metallic/(weight of
metallic+ceramic combined)). The graded metal volume fraction
between and within the cermet layers gives it a gradual increase in
the refractive index from surface to the substrate, which reduces
reflection compared with single cermet layer absorbers that often
use black metals such as black chrome, black nickel, or black
tungsten as their metal fillers. Solar absorbers fabricated
according to embodiments of the present disclosure that comprise
cermet multilayers (C1 and C2 in this example) with different metal
volume fractions introduces a stepwise change in the refractive
index that may result in a low reflection of visible light due to
interference effects.
[0028] In some embodiments, additional anti-reflection coatings may
be applied to the solar absorbers to further reduce reflection
losses. Consequently, cermet-based solar absorbers have a tunable
parameter space (range) based upon their constituents, coating
thicknesses, particle concentration, size, shape, and orientation
to optimize their spectral selectivity. Various combinations of
host materials such as Al.sub.2O.sub.3, AlN, and SiO.sub.2 with
metallic filler atoms such as Ni, Co, Ti, Mo, W, Pt, Stainless
steel (SS), Cu, Ag, Au have been investigated in terms of their
respective effectiveness for the optical performance and thermal
stability of the cermet surfaces. These combinations of host
materials have ceramic host materials in common that possess high
temperature stability, and are therefore complimentary. The metal
filler atoms may be chosen for their high melting point and their
resistance to both nitriding and oxidation, in order to enhance and
ensure thermal stability.
[0029] In an embodiment, in the case of solar absorbers with
mid-temperature applications, the cermet layers may be deposited on
metal substrates such as polished aluminum or copper due to their
low IR emittance and high thermal conductivity. In an embodiment, a
diffusion barrier between the substrate and the cermet layer was
introduced with a spontaneously formed Fe.sub.2O.sub.3 layer by
annealing the stainless steel substrate at 500.degree. C. in air.
However, the surface roughness of the substrate changes when
forming an Fe.sub.2O.sub.3 layer, which eventually affects the
surface roughness of solar absorber and then increases the
emittance. Also, the Fe.sub.2O.sub.3 layer on the back side of the
stainless steel may introduce another thermal resistance layer in a
solar absorber, which will decrease the heat transport efficiency
from the absorber to the thermal system. Surface smoothness may be
a desirable property in solar absorbers, so the impact of annealing
was evaluated and is discussed herein.
[0030] The embodiments herein discuss depositing, for example, a
nickel (Ni) or tungsten (W) layer that may be referred to as an
inter-reflector (IR) layer onto a mechanically polished substrate
that may comprise stainless steel. Depending upon the embodiment
and the substrate material employed, the substrate may not be
polished. This IR layer may act note only to bond the substrate to
other layers but also as a diffusion barrier and as a low IR
emittance coating to improve spectral selectivity. The performance
of the metal IR reflector layer with a double-layer cermet
structure and two antireflection coatings (ARCs) is discussed
herein. In contrast to cermet structures that may be filled with
particles of one metal type, the cermet layers based on an
Al.sub.2O.sub.3 ceramic host material may be filled with high
temperature stable Ni--W alloy prepared by co-sputtering.
Therefore, the cermet layers may each comprise not only the metal
volume fraction in each cermet layer but also the volume fraction
of the individual constituent which may be adjusted to tailor the
optical properties of a solar absorber depending upon the end
application, subsequent processing, or customer specifications.
[0031] In one example experiment, a plurality of individual layers
of the solar absorbers were deposited using a magnetron sputtering
technique. The spectral bidirectional reflectance responses of the
fabricated solar absorbers were measured at room temperature before
and after annealing at 600.degree. C. for 7 days. The solar
absorptance and total hemispherical emittance were measured at
elevated temperatures of up to 500.degree. C.
[0032] In an embodiment, the spectrally-selective solar absorbers
may be deposited in contact with a substrate, for example, a
mechanically polished stainless steel substrate. The deposition may
be performed using a commercial magnetron sputtering equipment (AJA
international, Inc.). For the thickness measurement of the C1 and
C2 layers, the materials may be simultaneously deposited on Si
wafer substrates partly covered by a mask. Prior to the deposition
process, the chamber may be evacuated to lower than
4.times.10.sub.-7 Torr. The deposition targets are high purity
nickel (99.999%, 2'' Dia.), tungsten (99.95%, 3'' Dia.),
Al.sub.2O.sub.3 (99.98%, 2'' Dia.), and SiO.sub.2 (99.995%, 3''
Dia.). DC power is supplied to the metal targets (Ni, W) to deposit
the metal layer and for the metal particle. The dielectric layer is
deposited by RF magnetron sputtering. Co-sputtering may be employed
to deposit more or one dielectric layers, such as the C1 and C2
layers. The metal fill fractions of the cermet layers may be
controlled by independent input power control to the corresponding
targets. The complete deposition process may be performed in an
argon plasma environment at a pressure of 3 mTorr. The detailed
preparation parameters are summarized in Table 1 herein.
[0033] Regarding the thermal stability, the solar absorbers
fabricated according to embodiments of the present disclosure are
characterized in terms of their phase, morphology, and optical
properties both before and after annealing the samples at
600.degree. C. for 7 days at a vacuum pressure of about
5.times.10.sup.-3 Torr using a tubular furnace. The X-ray
diffraction (XRD) patterns were obtained using a PANalytical
multipurpose diffractometer with an X'Celerator detector and Cu
K.alpha. radiation (.lamda.=1.54056 .ANG.) operating at 45 kV and
40 mA. Raman scattering spectra measurements were carried out on a
T64000 Raman system (Horiba Jobin Yvon) at room temperature. The
excitation source is the 514 nm laser line of an air cooled Ar-ion
laser.
[0034] The thickness of the cermet films were measured with an
Alpha-step 200 Profilometer (Tencor). The growth rates of metal and
dielectric layers comprising the cermet layers (films) were
measured by a quartz crystal monitor equipped in the sputtering
system. The morphology and roughness of the films were measured
with a Veeco Dimensions 3000 Atomic Force Microscope (AFM). The
spectral bidirectional reflectance was measured at room temperature
with a Spectrophotometer by Varian (Cary 500i, angle of incidence
8.degree., absolute spectral reflectance accessory) covering the
wavelength range of 0.3-1.8 .mu.m, and with an FT-IR Spectrometer
by Thermo Scientific (Nicolet 6700, angle of incidence 12.degree.)
covering the wavelength range of 1.8-20 .mu.m. The latter (relative
measurement) requires a reference with known spectral reflectance
which is chosen to be a specular gold mirror (Thorlabs).
[0035] FIG. 1 is a schematic of a spectrally selective solar
absorber configuration according to certain embodiments of the
present disclosure. It is to be appreciated that, while different
patterns are used to distinguish the layers, these indications are
not necessarily indicative of differences in the layers that are
visible to the naked eye, and it is also to be understood that the
relative thickness of layers may vary between embodiments. The
spectrally selective solar absorbers fabricated according to
certain embodiments of the present disclosure for mid- and
high-temperature applications are based on a double cermet layer
configuration with two ARC layers and a metal layer with high IR
reflectance as diffusion barrier. The two ARC layers ARC1 and ARC2
may also be Al.sub.2O.sub.3 and SiO.sub.2 thin films,
respectively.
[0036] In alternate embodiments, the ARC1 layer may comprise MgO,
TiO.sub.2, V.sub.2O.sub.3, ZrO, or combinations thereof. In order
to investigate the effect of the ARC layers, the solar absorber
multilayer structures were fabricated according to certain
embodiments of the present disclosure with tungsten, optically
thick nickel, or very thin nickel layer as and IR reflector or
bonding layer. The detailed parameters are summarized in Table 1.
In the embodiment in Table 1, the substrate may comprise a metal
layer, for example, nickel having a DC power density of 12.3
W/cm.sup.2 or tungsten having a DC power density of 2.2 W/cm.sup.2
for tungsten. The C1 layer may comprise W+Ni+Al.sub.2O.sub.3 with a
DC power density of 0.33 W/cm.sup.2 for tungsten and 0.99
W/cm.sup.2 for nickel, and a RF power density of 9.9 W/cm.sup.2 for
Al.sub.2O.sub.3. The cermet2 layer may comprise
W+Ni+Al.sub.2O.sub.3 with a DC power density of 0.26 W/cm.sup.2 for
tungsten, and 0.74 W/cm.sup.2 for nickel, and a RF power density of
9.9 W/cm.sup.2 for Al.sub.2O.sub.3. The ARC1 layer may comprise
Al.sub.2O.sub.3 with a RF power density of 9.9 W/cm.sup.2 and the
ARC2 layer may comprise SiO.sub.2 with a RF power density of 4.4
W/cm.sup.2.
TABLE-US-00001 TABLE 1 Substrate Bonding layer/IR C1 thickness C2
thickness ARC1 ARC2 Sample material layer thickness/type (nm) (nm)
(nm) (nm) C1 SS 10 nm Ni 180 N/A N/A N/A C2 SS 10 nm Ni N/A 28 25
55 S-SS SS 10 nm Ni 11 28 25 55 S-Ni/SS SS 300 nm Ni 11 28 25 55
S-W/SS SS 300 nm W 11 28 25 55 S-W/SS-2 SS 200 nm W 11 28 25 55
S-W/SS-3 SS 100 nm W 11 28 25 55 S-W/SS-4 SS 50 nm W 11 28 25 55
S-W/SS-5 SS 10 nm W 11 28 25 55
[0037] The multilayer stack that makes up the spectrally selective
solar absorbers fabricated according to certain embodiments of the
present disclosure may comprise one bonding or IR reflector layer,
double cermet absorption layers and double ARC layers which further
reduce reflection in the visible range. In some embodiments,
multiple IR-reflector layers of the same or differing compositions
and/or concentrations (metal fractions) may be used in different
arrangements in a solar absorber. The use of mechanically polished
stainless steel as the substrate may provide high temperature
stability and may be cost-effective, which can promote large scale
deployment as a potential solar absorber candidate in high
temperature solar receivers. It has been shown that elemental
diffusion of iron and carbon from a stainless steel into the cermet
layer can be detrimental for the optical properties, and a
diffusion barrier may be employed to combat this diffusion. Thus,
the thermal stability of optimized coatings was evaluated on
stainless steel with a 10 nm thin nickel bonding layer which may
act as a diffusion barrier (the S-SS sample). Details about
multilayer stack composition and preparation parameters are
summarized in Table 1 above.
[0038] FIG. 2 illustrates the bidirectional reflectance spectra of
the pristine (where "pristine" is the term used to describe a
condition before annealing) and annealed solar absorbers fabricated
according to certain embodiments of the present disclosure. The
reflectance of the pristine sample is close to zero in the visible
range, which is expected for a double-cermet-absorption-layer
combined with a double-ARC-layer due to the intrinsic absorption of
the double-cermet layer and the reflectance reducing interference
effects. The sharp transition wavelength range from low reflectance
to high reflectance appears to be from about 1 to about 3 .mu.m,
which can result in promising spectral selectivity even at high
temperatures. However, the degraded optical properties of the solar
absorber upon annealing at 600.degree. C. for 7 days show a
detrimental effect on spectral selectivity. The spectral
reflectance below about 1.1 .mu.m increases while it decreases
above about 1.1 .mu.m which results in a broadening of the
transition wavelength range and ultimately decreases the solar
absorptance and increases the IR emittance.
[0039] FIGS. 3A and 3B illustrate the surface roughness of the
absorber subsequent to annealing. No significant surface roughness
change upon sample annealing is observed, indicating that the
annealing process does not significantly (e.g., to where it would
be noticeable or negatively impact functionality) degrade the
surface roughness. FIG. 3A is an atomic force microscopy ("AFM")
image of an S-SS solar absorber with a 10 nm thick nickel layer
before annealing and FIG. 3B is an AFM image of the S-SS solar
absorber the 10 nm thick nickel layer after annealing at about
600.degree. C. for 7 days. The sample retains the groove structure
created by the mechanical polishing process applied to the
stainless steel substrate. The root mean square roughness (Rq) of
the sample before and after annealing is calculated to be 6-8 nm
using a NanoScope Analysis software.
[0040] FIGS. 4A-4D are AFM images illustrating the morphology
change of a single cermet layer before and after annealing. FIGS.
4A-4D are AFM images of the morphology changes of a single cermet
layer deposited on a mechanically polished stainless steel
substrate coated with a 10 nm Ni layer without any ARC layer after
annealing at 600.degree. C. for 7 days. FIG. 4A illustrates the
morphology of a cermet1 layer with a high metal volume fraction in
Al.sub.2O.sub.3 before annealing and FIG. 4B illustrates the
morphology of the same sample after annealing. In an embodiment, a
"high metal volume fraction" refers to a metal volume fraction
above about 62% and a "low metal volume fraction" refers to a metal
volume fraction below about 56%. FIG. 4C illustrates the morphology
of a cermet2 layer with a low metal volume fraction in
Al.sub.2O.sub.3 before annealing and FIG. 4D illustrates the
morphology of the same sample after annealing.
[0041] In another embodiment, two cermet samples (C1 and C2) were
fabricated without being disposed in contact with anti-reflective
coating ("ARC") layers, and were evaluated in terms of their phases
and morphology before ("pristine") and after annealing. The
multilayer stacks deposited onto the stainless steel substrates
consist of a 10 nm nickel bonding layer and a single cermet layer
with the only difference between the two samples being the metal
particle concentration in the cermet layers and their respective
thicknesses (C1 and C2 as detailed in Table 1). Both samples C1
(FIG. 4B) and C2 (FIG. 4D) show significant changes in their film
morphology upon annealing. Similar to the previous sample (S-SS),
the C1 and C2 samples start out with a groove surface structure;
however, the annealing process leads to a rapid growth of the Ni--W
alloy within the cermet layer from diameters of about 80 nm to
about 300 nm or, in some embodiments, about 400 nm. And the
roughness increases from about 6-8 nm to about 47-50 nm. The
difference in the metal volume fraction and layer thickness between
sample C1 and C2 does not affect the particle growth and roughness
change. However, the unchanged roughness of the previous sample
(S-SS) with double ARC and much thinner double-cermet layer may
indicate that the ARC layers suppress the particle growth within
the cermet or the particle growth is much less pronounced in
significantly thinner cermet layers.
[0042] FIG. 5 illustrates XRD patterns for pristine and annealed
cermet coatings. FIG. 5 illustrates the phase analysis before and
after annealing for cermet coatings with 10 nm nickel layers
disposed on stainless steel for both cermet1 and cermet2 as noted,
this phase analysis was conducted using X-ray diffraction and shows
the sharp peaks for the stainless steel substrate and the Ni--W
alloy in the single-cermet layers. No diffraction peaks are
observed for the dielectric Al.sub.2O.sub.3 even after annealing at
600.degree. C. for 7 days due to its stable amorphous nature.
However, X-ray diffraction spectra show an additional monoclinic
FeWO.sub.4 phase after sample annealing. Iron atoms diffuse at high
temperatures from the stainless steel substrate into the cermet
layer and may react with tungsten and residual oxygen to form the
observed FeWO.sub.4 phase.
[0043] FIG. 6 is a chart of Raman spectra of pristine and annealed
cermet coatings. In particular, FIG. 6 illustrates Raman
measurements showing two distinct peaks located at 882 cm.sup.-1
and 691 cm.sup.-1 for the annealed samples which can be traced back
to A.sub.g modes of FeWO.sub.4. Also, the solar absorber with thin
nickel layer (S-SS) after annealing displays a very low reflectance
in mid-IR range compared to that before annealing as shown in FIG.
2, which may indicate a destruction of IR reflector and a formation
of nonmetallic phase between substrate and coatings. Thus, the
degradation of the optical properties for the solar absorber sample
(S-SS) may be the formation of FeWO.sub.4 phase in the cermet
layers at high temperature.
[0044] FIG. 7 illustrates the spectral bidirectional reflectance
response of solar absorbers fabricated according to certain
embodiments of the present disclosure. The solar absorber samples
(indicated by S--Ni/SS and S--W/SS in Table 1) were fabricated with
300 nm thick metal layers as the diffusion barrier between the
stainless steel substrate and the double cermet layer. Nickel and
tungsten were employed as indicated as the diffusion barrier metals
due to their high melting point and low IR emittance which improves
the spectral selectivity of the solar absorber compared to the
previous sample S-SS with a very thin nickel layer. Both thick
metal layers in the samples S--Ni/SS and S--W/SS significantly
increased the spectral reflectance in the mid-IR range without
altering the spectral response below 2.5 .mu.m.
[0045] FIG. 8 illustrates XRD patterns of solar absorbers
fabricated according to certain embodiments of the present
disclosure. FIG. 8 illustrates that the sample with the thick
nickel layer (S--Ni/SS) shows two nickel peaks which disappear
after sample annealing, indicating that the nickel reacts with iron
atoms from the SS substrate. The sample with a thick tungsten layer
(S--W/SS) did not appear to be affected by the sample annealing,
thus demonstrating a stable tungsten layer which prevents the iron
diffusion.
[0046] FIG. 9 illustrates the experimental set up and results for
steady state calorimetric measurements of samples fabricated
according to certain embodiments of the present disclosure. FIG. 9
illustrates both the solar absorptance and total hemispherical
emittance of a fabricated solar absorber (S--W/SS) was directly
measured at elevated temperatures (up to 500.degree. C.) using
simple steady state calorimetric methods. Samples were attached to
a heater assembly and suspended in a vacuum chamber. The electrical
heater power input employed was directly related to the radiation
heat loss from the sample surface. Thus, the total hemispherical
emittance can be calculated with the electrical heater power inputs
and the measured sample and surrounding temperatures. The solar
absorptance was measured at elevated temperatures using a solar
simulator. The sample/heater assembly is suspended in the vacuum
chamber facing a viewport allowing the solar simulator beam to
irradiate the sample surface. The solar absorptance can be obtained
by varying the incident radiation power onto the sample and
measuring the corresponding electric heater power adjustments to
maintain the sample surface at a constant temperature. The near
normal solar absorptance and total bidirectional emittance are
calculated from the spectral reflectance data, indicating that the
developed spectrally selective solar absorber with tungsten
infrared reflector layer can be a good candidate for high
temperature solar thermal applications (See Table 2 below).
TABLE-US-00002 TABLE 2 Before Annealing After Annealing Sample
Absorptance Emittance Absorptance Emittance S-SS 91.7% 8.63% 90.66%
15.99% S-Ni/SS 93.20% 5.46% 91.38% 14.10% S-W/SS 92.2% 5.65% 90.77%
5.7%
[0047] The near-normal solar absorptance (divergence half angle of
about 15.degree.) is close to independent of temperature with a
value of about 0.9 which is in good agreement with the calculated
solar absorptance from the spectral data. It has been theoretically
shown that cermet-based solar absorbers exhibit a solar absorptance
with only weak angle dependence. Thus, only little deviation from
here demonstrated solar absorptance should be expected even for
concentrated solar power applications with a large range of
incident angles. However, future research efforts could
experimentally investigate the angle dependence of the solar
absorptance to quantify the effect. The total hemispherical
emittance shows the typical temperature dependence of a spectrally
selective solar absorber with approximately 0.09 at 100.degree. C.
and 0.15 at 500.degree. C.
[0048] FIG. 10 illustrates the spectral properties of a plurality
of solar absorbers fabricated according to embodiments of the
present disclosure. The tungsten metal layer thickness may be
optimized to keep the production cost minimal without losing the
low emittance and long term thermal stability of the solar
absorber. A plurality of solar absorbers with tungsten layer
thicknesses of 10, 50, 100, and 200 nm were fabricated as indicated
in Table 1 and their spectral properties were compared before and
after the annealing at 600.degree. C. for 7 days. The curves in
FIG. 10 of wavelength v. % reflectance are in the following order,
and the corresponding compositions/configurations are listed in
order below in Table 3, which comprises the same values for each
composition/configuration as Table 1.
TABLE-US-00003 TABLE 3 Ordered Results from FIG. 10 Bonding
layer/IR C1 C2 Substrate layer thickness thickness ARC1 ARC2 Sample
material thickness/type (nm) (nm) (nm) (nm) S-W/SS-3 SS 100 nm W 11
28 25 55 S-W/SS-2 SS 200 nm W 11 28 25 55 S-W/SS-3 (annealed) SS
100 nm W 11 28 25 55 S-W/SS-4 SS 50 nm W 11 28 25 55 S-W/SS-2
(annealed) SS 200 nm W 11 28 25 55 S-W/SS-4 (annealed) SS 50 nm W
11 28 25 55 S-W/SS-5 SS 10 nm W 11 28 25 55 S-W/SS-5 (annealed) SS
10 nm W 11 28 25 55
[0049] For the pristine (as-made, prior to annealing if annealing
is performed) samples, the tungsten layer thickness only affects
the spectral reflectance at wavelength larger about 2 The annealing
process, however, alters the spectral response in the complete
wavelength range with the largest effect at wavelengths longer than
about 1.2 .mu.m. The spectral reflectance increases and the thermal
stability improves with increasing tungsten layer thickness. A
tungsten layer thickness of 100 nm (as in examples S--W/SS-3) is
sufficient to provide good (commercially scalable and usable)
thermal stability and to act as a low emittance coating on
stainless steel at high temperatures.
CONCLUSIONS
[0050] Iron atoms diffusing from the stainless steel substrate into
the cermet layer may not have a desirable effect on the optical
properties of a selective solar absorber. The spectrally selective
solar absorbers fabricated according to certain embodiments of the
present disclosure may be based on double cermet layers
(W--Ni--Al.sub.2O.sub.3 cermet) with double antireflection layers
on a mechanically polished stainless substrate fabricated according
to embodiments of the present disclosure. In some embodiments, a
100 nm thick tungsten layer may be employed to suppress the
degradation of the optical properties at high temperatures and to
lower the emittance relative to the stainless steel substrate,
which improves the spectral selectivity of the solar absorber, for
example, in applications where Ni may not be as effective an
Fe-diffusion barrier and IR reflector. Using the materials,
apparatus, systems and methods discussed herein, a solar absorber
was fabricated with a solar absorptance of about 0.9 and total
hemispherical emittance of about 0.15 at an operating temperature
of 500.degree. C. In alternate embodiments, this layer may comprise
Tantalum (Ta), Titanium (Ti), Molybdenum (Mo), Chromium (Cr),
Vanadium (V), Niobium (Nb), Zirconium (Zr), or combinations
thereof.
[0051] Exemplary embodiments are specifically disclosed and
variations, combinations, and/or modifications of the embodiments
and/or features of the embodiments made by a person having ordinary
skill in the art are within the scope of the disclosure.
Alternative embodiments that result from combining, integrating,
and/or omitting features of the embodiments are also within the
scope of the disclosure. Where numerical ranges or limitations are
expressly stated, such express ranges or limitations should be
understood to include iterative ranges or limitations of like
magnitude falling within the expressly stated ranges or limitations
(e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater
than 0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a
numerical range with a lower limit, R.sub.l, and an upper limit,
R.sub.u, is disclosed, any number falling within the range is
specifically disclosed. In particular, the following numbers within
the range are specifically disclosed:
R=R.sub.1+k*(R.sub.u-R.sub.l), wherein k is a variable ranging from
1 percent to 100 percent with a 1 percent increment, i.e., k is 1
percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . , 50
percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97
percent, 98 percent, 99 percent, or 100 percent. Moreover, any
numerical range defined by two R numbers as defined in the above is
also specifically disclosed. Use of broader terms such as
"comprises," "includes," and "having" should be understood to
provide support for narrower terms such as "consisting of,"
"consisting essentially of," and "comprised substantially of."
Accordingly, the scope of protection is not limited by the
description set out above but is defined by the claims that follow,
that scope including all equivalents of the subject matter of the
claims. Each and every claim is incorporated into the specification
as further disclosure, and each claim is an exemplary embodiment of
the present invention.
[0052] While exemplary embodiments of the invention have been shown
and described, modifications thereof can be made by one skilled in
the art without departing from the scope or teachings herein. The
embodiments described herein are exemplary only and are not
limiting. Many variations and modifications of the compositions,
systems, apparatus, and processes described herein are possible and
are within the scope of the invention as claimed. Accordingly, the
scope of protection is not limited to the embodiments described
herein, but is only limited by the claims that follow, the scope of
which shall include all equivalents of the subject matter of the
claims. Unless expressly stated otherwise, the steps in a method
claim may be performed in any order. The recitation of identifiers
such as (a), (b), (c) or (1), (2), (3) before steps in a method
claim are not intended to and do not specify a particular order to
the steps, but rather are used to simplify subsequent reference to
such steps.
* * * * *