U.S. patent application number 15/638440 was filed with the patent office on 2018-01-04 for solar energy converter and related methods.
The applicant listed for this patent is Carnegie Mellon University. Invention is credited to Pengfei Li, Baoan Liu, Sheng Shen.
Application Number | 20180006166 15/638440 |
Document ID | / |
Family ID | 60807190 |
Filed Date | 2018-01-04 |
United States Patent
Application |
20180006166 |
Kind Code |
A1 |
Shen; Sheng ; et
al. |
January 4, 2018 |
Solar Energy Converter and Related Methods
Abstract
A solar thermal energy device is provided. Also provided is a
method of making a solar thermal energy device.
Inventors: |
Shen; Sheng; (Pittsburgh,
PA) ; Li; Pengfei; (Shen Zhen, CN) ; Liu;
Baoan; (Pittsburgh, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carnegie Mellon University |
Pittsburgh |
PA |
US |
|
|
Family ID: |
60807190 |
Appl. No.: |
15/638440 |
Filed: |
June 30, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62493365 |
Jun 30, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02E 10/40 20130101;
F24S 70/30 20180501; F24S 70/225 20180501; H01L 31/02168 20130101;
Y02E 10/50 20130101; H01L 31/18 20130101; F24S 70/16 20180501; F24S
70/25 20180501 |
International
Class: |
H01L 31/0216 20140101
H01L031/0216; F24J 2/46 20060101 F24J002/46; F24J 2/48 20060101
F24J002/48; H01L 31/18 20060101 H01L031/18 |
Goverment Interests
STATEMENT REGARDING FEDERAL FUNDING
[0006] This invention was made with government support under Grant
No. CBET-1253692 awarded by the National Science Foundation. The
government has certain rights in the invention.
Claims
1. A solar device, comprising, a metallic substrate comprising a
plurality of protuberances each having a base having an edge or a
diameter, and an apex, and tapering from the base to the apex,
wherein the distance between the base of adjacent protuberances is
less than 200 nm, and the diameter and/or length of a side of the
base ranges from 100 nm to 1000 nm.
2. The device of claim 1, further comprising an antireflective
coating having a refractive index greater than one and less than
the maximum refractive index of the metallic substrate at the base
of the protuberances.
3. The device of claim 2, wherein the antireflective coating
comprises an oxide of aluminum, hafnium, titanium, and/or
zirconium.
4. The device of claim 2, wherein the antireflective coating
comprises a metal oxide or a silicon nitride.
5. The device of claim 2, wherein the antireflective coating has a
thickness of from 50 nm to 1 .mu.m in thickness.
6. The device of claim 2, wherein the antireflective coating
comprises an aluminum oxide.
7. The device of claim 1, wherein the base of the protuberance is a
square or rectangle.
8. The device of claim 1, wherein the metallic substrate comprises
nickel, copper, gold, silver or an alloy thereof.
9. The device of claim 1, wherein the protuberances are pyramidal,
conical, frusto-pyramidal, or frusto-conical.
10. The device of claim 9, wherein the protuberances are pyramidal,
having a square or rectangular base wherein a plane containing a
side of the protuberance is at an angle of 54.7 degrees from the
plane of the major surface of the contiguous portion of the
metallic substrate from which the protuberance extends.
11. The device of claim 1, wherein the distance between the base of
adjacent protuberances is less than 50 nm.
12. The device of claim 1, wherein the protuberances are
uniformly-spaced on the substrate.
13. A template for producing a solar device, comprising a substrate
having a major surface comprising a plurality of indentations, each
of the plurality of indentations having an opening at the major
surface, wherein the openings are spaced less than 200 nm apart,
and the opening of each of the plurality of indentations having a
side length or a diameter ranging from 100 nm to 1000 nm.
14. The template of claim 13, wherein the template is a silicon
substrate having a major surface comprising a silica coating, and
the indentations are pyramidal indentations with a square or
rectangular opening at the major surface.
15. The template of claim 13, wherein the openings are
regularly-spaced.
16. A method of making a solar thermal absorbing device,
comprising, a. depositing a metal layer onto a template, comprising
a substrate having a major surface comprising a plurality of
indentations, each of the plurality of indentations having an
opening at the major surface (that is, coplanar with the major
surface), wherein the openings are spaced less than 200 nm apart,
and the opening of each of the plurality of indentations having a
side length or a diameter ranging from 100 nm to 1000 nm, wherein
the metal is deposited in an amount to fill in the indentations of
the template and to produce a contiguous metal layer, optionally
having a thickness of from 1 .mu.M to 100 .mu.M over at least a
portion of the major surface of the template comprising the
indentations; and b. releasing (e.g., peeling or delaminating) the
deposited metal layer from the template to produce a metallic
substrate having a major surface, comprising a plurality of
protuberances on the major surface, each of the plurality of
protuberance having a base having an edge or a diameter, and an
apex, and tapering from the base to the apex, wherein the distance
between the base of adjacent protuberances is less than 200 nm, and
the diameter or an edge of the base ranges from 100 nm to 1000
nm.
17. The method of claim 16, wherein the template is a silicon
substrate having a major surface comprising a silica coating, and
the indentations are pyramidal indentations with a square or
rectangular opening at the major surface.
18. The method of claim 16, further comprising depositing an
antireflective coating over at least a portion of the metallic
substrate including the plurality of protuberances.
19. The method of claim 18, wherein the antireflective coating
comprises a metal oxide or a silicon nitride.
20. The method of claim 18, wherein the antireflective coating has
a thickness of from 50 nm to 1 .mu.m.
21. The method of claim 18, wherein the antireflective coating
comprises an oxide of aluminum, hafnium, titanium, and/or
zirconium.
22. The method of claim 18, wherein the antireflective coating
comprises an aluminum oxide.
23. The method of claim 16, wherein the metallic substrate is
nickel, copper, silver, gold, or an alloy thereof.
24. The method of claim 16, wherein the protuberances are
pyramidal, conical, frusto-pyramidal, or frusto-conical.
25. The method of claim 24, wherein the protuberances are
pyramidal, having a square or rectangular base and wherein a plane
containing a side of the protuberance is at an angle of 54.7
degrees from the plane of the major surface of the contiguous
portion of the metallic substrate from which the protuberance
extends.
26. The method of claim 24, wherein the protuberances are
frusto-conical or frusto-pyramidal.
27. The method of claim 16, wherein the distance between the base
of adjacent protuberances is less than 50 nm.
28. The method of claim 16, wherein the protuberances are
uniformly-spaced on the substrate.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 62/493,365, filed Jun. 30, 2016, which is
incorporated herein by reference in its entirety.
[0002] Provided herein are solar thermal energy converter devices
and methods of making and using the devices.
[0003] At present, the conversion of solar energy into electricity
mainly relies on two approaches: solar photovoltaics that convert
solar photons directly into electricity, and solar thermal energy
conversion in which solar photons are first converted into thermal
energy, then converted to electricity. Compared to traditional
photovoltaics, one major advantage of solar thermal energy
conversion is the utilization of nearly the entire solar spectrum,
allowing for higher energy conversion efficiency. In solar thermal
technologies, such as concentrated solar power, solar
thermophotovoltaic, solar thermionic, and solar thermoelectric
systems, solar absorbers are crucial components that absorb and
convert sunlight into thermal energy. In order to achieve the
maximum conversion efficiency of incident solar flux to heat, one
important strategy is to employ spectrally selective solar
absorbers that exhibit a near-blackbody absorption in the solar
spectrum while suppressing infrared emission at elevated
temperatures. However, developing cost-effective and large-scale
solar selective absorbers with both high conversion efficiency and
high temperature stability remains a challenge.
[0004] For mid- and high-temperature operations of solar thermal
systems, especially concentrated solar power systems, the
development of high-performance solar selective absorbers has been
challenging due to a variety of factors such as spectral
performance, material stability at high temperatures, and
manufacturing cost. Metal-dielectric multilayer structures can
achieve a good spectral selectivity because of the interference
effect. However, the use of these multilayer structures in practice
is limited by both high-temperature instability and high-cost
fabrication such as sputter deposition and electron beam
evaporation. Semiconductor-metal structures utilize semiconductors
with proper bandgaps to absorb solar radiation, and an underlying
layer of metal to provide high infrared reflectance. A selective
coating with Si--Ge nanoparticles has recently been demonstrated to
have a measured solar absorptivity .about.90% and an infrared
emissivity <30% (J. Moon, et al., High performance multi-scaled
nanostructured spectrally selective coating for concentrating solar
power Nano Energy 2014, 8:238-246). The main drawback of the
semiconductor-metal composition is the non-flexible spectral
selectivity due to the intrinsic bandgaps of semiconductors.
Cermet-based coatings made of ceramic-metal composites have also
been developed and studied for use in solar selective absorbers.
Thin single or multiple cermet layers are typically deposited on a
metal surface for high solar absorptance while being transparent to
infrared radiation. Although various combinations of host ceramics
such as Al.sub.2O.sub.3, AlN, and SiO.sub.2 with metal particle
fillers including Ni, Co, Mo, W, Au, Ag, etc., have been
extensively investigated in terms of spectral performance and
thermal stability, it is still quite difficult to develop
high-performance cermet-based absorbers stable at >700.degree.
C. Nanophotonic structures, e.g., 1D, 2D, and 3D photonic crystals,
have been explored for use as solar selective absorbers, but these
structures are far from meeting the goal of .about.100% absorptance
for the broad-band solar spectrum. Moreover, these nanophotonic
structures are fabricated through high-cost and complex processes,
such as reactive ion etching (RIE), atomic layer deposition (ALD),
chemical mechanical planarization (CMP), and cannot be easily
scaled up.
[0005] In view of the above, an easily and inexpensively-made,
thermally-stable device having high visible-light absorptance and
high IR reflectivity is desirable.
SUMMARY
[0007] Metal-based wafer-scale nanophotonic solar selective
absorbers with excellent solar selective absorptivity and thermal
stability are provided. Also provided is a template (mold)
stripping method which can drastically increase throughput and
decrease fabrication cost of the absorbers. The novel solar
selective absorbers with three-dimensional (3D) nanophotonic
structures can significantly facilitate transformative advancements
in the design and performance of solar thermal systems.
[0008] A solar device is provided, comprising, a metallic substrate
comprising a plurality of protuberances each having a base having
an edge or a diameter, and an apex, and tapering from the base to
the apex, wherein the distance between the base of adjacent
protuberances is less than 200 nm, and the diameter and/or length
of a side of the base ranges from 100 nm to 1000 nm.
[0009] A template for producing a solar device also is provided,
comprising a substrate having a major surface comprising a
plurality of indentations, each of the plurality of indentations
having an opening at the major surface (that is, coplanar with the
major surface), wherein the openings are spaced less than 200 nm
apart, and the opening of each of the plurality of indentations
having a side length or a diameter ranging from 100 nm to 1000
nm.
[0010] Further, a method of making a solar thermal absorbing device
is provided. The method comprises, depositing a metal layer onto a
template, comprising a substrate having a major surface comprising
a plurality of indentations, each of the plurality of indentations
having an opening at the major surface (that is, coplanar with the
major surface), wherein the openings are spaced less than 200 nm
apart, and the opening of each of the plurality of indentations
having a side length or a diameter (ranging from 100 nm to 1000 nm,
wherein the metal is deposited in an amount to fill in the
indentations of the template and to produce a contiguous metal
layer, optionally having a thickness of from 1 .mu.M to 100 .mu.M
over at least a portion of the major surface of the template
comprising the indentations; and releasing (e.g., peeling or
delaminating) the deposited metal layer from the template to
produce a metallic substrate having a major surface, comprising a
plurality of protuberances on the major surface, each of the
plurality of protuberance having a base having an edge or a
diameter, and an apex, and tapering from the base to the apex,
wherein the distance between the base of adjacent protuberances is
less than 200 nm, and the diameter or an edge of the base ranges
from 100 nm to 1000 nm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a schematic cross-sectional view of a portion of
one aspect of a solar device as described herein.
[0012] FIG. 2 depicts various exemplary shapes of protuberances,
with the two-dimensional bases on the left (cross-section at A of
FIG. 1), and three-dimensional depictions of protuberances having
the depicted base to the right.
[0013] FIG. 3 is a schematic cross-sectional view of a portion of
one aspect of a template showing a single indentation, for use in
production of a solar device as described herein.
[0014] FIGS. 4A and 4B. Solar selective absorber in a solar thermal
system. (FIG. 4A) Schematic of the solar thermal system. (FIG. 4B)
The left area (with peak less than 1 .mu.M) and right area (with
peak at approximately 3 .mu.M) indicate the solar spectrum and the
blackbody radiation spectrum at 1000 K, respectively. The dashed
line is the spectral absorptivity of an ideal solar absorber.
[0015] FIGS. 5A-5C. Fabrication of nanopyramid arrays as a solar
selective absorber. (FIG. 5A) Fabrication flow of the solar
selective absorber. (FIG. 5B) SEM image of the fabricated
nanopyramid structures. Scale bar: 3 .mu.m. (FIG. 5C) A photograph
of the flexible solar absorber.
[0016] FIGS. 6A-6E. Optimization of solar absorber geometries.
(FIG. 6A) Schematic of nickel nanopyramid arrays with an ARC layer.
Maximized overall efficiencies at concentration factors (FIG. 6B)
C=1, (FIG. 6C) C=100, and (FIG. 6D) C=1000. (FIG. 6E) Averaged
absorptivity spectrum at different incident angles 8 for 1=500 nm,
d=100 nm, and h=75 nm.
[0017] FIGS. 7A and 7B. Spectral absorptance of nanophotonic solar
selective absorbers. (FIG. 7A) Spectral absorptance from both
experiment and simulation. The solid curves are the measured
results from experiments, and the dashed curves are from
simulations. (FIG. 7B) Measured spectral absorptance for a bare
nickel plate, P600 sample with/without ARC, and P800 sample
with/without ARC.
[0018] FIGS. 8A and 8B. Thermal stability test at high temperature.
(FIG. 8A) Solar selective absorptance before (solid curve) and
after 800.degree. C. annealing in vacuum for >5 hours (dashed
curve). (FIG. 8B) SEM image of the nanoprymid arrays after
800.degree. C. annealing. Scale bar: 3 .mu.m.
DETAILED DESCRIPTION
[0019] As used herein, all numbers expressing dimensions, physical
characteristics, processing parameters, quantities of ingredients,
reaction conditions, and the like, used in the specification and
claims are to be understood as being modified in all instances by
the term "about". Accordingly, unless indicated to the contrary,
the numerical values set forth in the following specification and
claims may vary depending upon the desired properties sought to be
obtained by the present invention. At the very least, and not as an
attempt to limit the application of the doctrine of equivalents to
the scope of the claims, each numerical value should at least be
construed in light of the number of reported significant digits and
by applying ordinary rounding techniques. Moreover, all ranges
disclosed herein are to be understood to encompass the beginning
and ending range values and any and all subranges subsumed therein.
For example, a stated range of "1 to 10" should be considered to
include any and all subranges between (and inclusive of) the
minimum value of 1 and the maximum value of 10; that is, all
subranges beginning with a minimum value of 1 or more and ending
with a maximum value of 10 or less, e.g., 1 to 3.3, 4.7 to 7.5, 5.5
to 10, and the like.
[0020] The terms "formed over", "deposited over", or "provided
over" mean formed, deposited, or provided on but not necessarily in
contact with the surface. For example, a coating layer "formed
over" a substrate does not preclude the presence of one or more
other coating layers or films of the same or different composition
located between the formed coating layer and the substrate.
Likewise, the terms "under" or "between" in the context of
specified coating layers does not preclude the presence of one or
more other coating layers or films of the same or different
composition located between the recited layers. The term "film"
refers to a coating region of a desired or selected coating
composition. A "layer" can comprise one or more "films", and a
"coating" or "coating stack" can comprise one or more "layers".
[0021] The terms "visible region" or "visible light" refer to
electromagnetic radiation having a wavelength in the range of 380
nm to 800 nm. The terms "infrared region" or "infrared radiation"
refer to electromagnetic radiation having a wavelength in the range
of greater than 800 nm to 100,000 nm. The terms "ultraviolet
region" or "ultraviolet radiation" mean electromagnetic energy
having a wavelength in the range of 300 nm to less than 380 nm.
[0022] As used herein, spatial or directional terms, such as
"left", "right", "inner", "outer", "above", "below", and the like,
relate to the invention as it is shown in the drawing figures.
However, it is to be understood that the invention can assume
various alternative orientations and, accordingly, such terms are
not to be considered as limiting. Depictions of various aspects of
the invention and elements thereof in the figures, such as coating
layers, are not necessarily to scale, but are drawn in a manner to
better illustrate and to facilitate description of the described
structure.
[0023] According to one aspect of the invention, provided herein is
a solar device that comprises a metallic substrate comprising a
plurality of protuberances each having a base having an edge or a
diameter, and an apex, and tapering from the base to the apex,
e.g., the area of a cross section of the protuberance parallel to
the base decreases from the base to the apex. The device is
exceptionally efficient, with a theoretical solar-to-electrical
conversion efficiency (e.g., .eta..sub.m, as described below) of at
least 30% for non-concentrated light, and at least 50% for
concentrated solar light (e.g., 100-fold, or 1000-fold, for
example, as in solar concentrator or reflector arrays). The device
performs as a near perfect reflector in the infrared range, with
reflectivity close to 100% (i.e., .about.0% emissivity), e.g.,
greater than 90% at wavelengths greater than 1 .mu.m, while being
an excellent absorber for visible light with an average
absorbitivity larger than 95%.
[0024] In various aspects, the distance between the base of
adjacent protuberances is less than 200 nm, and the diameter and/or
length of a side of the base ranges from 100 nm to 1000 nm. In one
aspect, the device further comprises an antireflective coating
having a refractive index greater than one and less than the
maximum refractive index of the metallic substrate at the base of
the protuberances.
[0025] In aspects, the metal substrate and protuberances are a
unified structure that can be prepared from any heat-stable metal
and/or alloy or combination of metals and/or alloys, stable at
operational temperatures of the device, e.g., at 800.degree. C. As
described herein, nickel and alloys thereof are suitable metals and
alloys. In other aspects, copper and alloys thereof are suitable
metals and alloys. Examples of other suitable metals include:
silver, gold, and alloys thereof. The substrate and protuberances
may comprise the same metallic substance, or can be of different
metallic substances. As described below, the method of
manufacturing of the device allows for use of different metallic
substances for the substrate and the protuberances.
[0026] Metal layers, such as the metallic substance used to produce
the device described herein manufactured, e.g., on a silicon
template as described below, can be deposited by any useful
physical or chemical deposition methods, as are broadly-known in
the semiconductor and optical arts, such as, without limitation
physical vapor deposition (PVD), chemical vapor deposition (CVD),
sputtering, chemical plating, electroplating, plasma-enhanced CVD
(PECVD) and electron beam physical vapor deposition (EBPVD). Often,
the method used to deposit the metallic substance will vary
depending upon the composition of the metallic substance, but an
appropriate deposition method can be determined depending upon the
metallic substance to be deposited. In one example, the metal layer
is deposited on a template by a combination of sputtering, followed
by electroplating (applying a charge to a metallic "electrode"
prepared by sputtering or any deposition process). Other layers,
such as the antireflective coating layer, also may be deposited by
any useful physical or chemical deposition method, e.g., as
described above for the metallic layer, including PVD and CVD
methods.
[0027] In aspects, the protuberances are arranged in any pattern on
the substrate, for example, as depicted in FIGS. 5B and 6A, below.
The protuberances are typically arranged on the substrate as an
array, and depending on the design and desired separation between
protuberances, e.g., ranging from 0 nm to 200 nm apart, an array (a
regular pattern) of protuberances may be utilized. In one aspect,
the array of protuberances is periodic, in that the protuberances
are arranged in a repeating, e.g., uniform, pattern, for example,
on a grid, or other geometric pattern. As indicated below, the
structure absorbs optimally as the distance between protuberances
approaches zero, and, thus, the protuberances can be tiled on the
substrate in any periodic or aperiodic tesselation pattern.
[0028] Antireflective coatings are broadly-known in the optical and
photonics fields, and are applied to reduce surface reflections.
Antireflective coatings include ceramics, cements, dielectric
films, and other substances, such as, without limitation metal
oxides and metal nitrides. Non-limiting examples of metal oxides
useful for inclusion in the antireflective coating include oxides
nitrides and fluoride of aluminum, hafnium, titanium, and/or
zirconium, such as, without limitation: Al.sub.2O.sub.3, AlN,
SiN.sub.x (with x being representative of the various oxidation
states of silicon), MgF.sub.2, TiO.sub.2, ZrO.sub.2, HfO.sub.2, and
SiO.sub.2, optionally with metal particle fillers including Ni, Co,
Mo, W, Au, Ag, etc., added by sputtering. In one aspect, the
antireflective coating is aluminum oxide. The antireflective
coating may have any effective thickness. Useful thicknesses for
the antireflective coating ranges from 10 nm to 200 nm, for
example, from 50 nm to 100 nm, and in one aspect, from 65 nm to 80
nm, for example 70 nm or 75 nm. Other antireflective coatings
useful in the device are coatings that are heat-stable at
800.degree. C.
[0029] Other coatings, such as a protective coating able to
withstand operational temperatures of the device, e.g., at least
800.degree. C., may be deposited over the substrate and, when
present optionally over the antireflective coating. Nevertheless,
for purposes herein, the dimensions, and tapered shape of the
protuberances must be substantially maintained in order to optimize
solar absorption, such that any coatings over the metallic
substrate of the protuberances are optimally less than 200 nm,
e.g., less than 100 nm in total thickness.
[0030] According to one aspect of the invention, a portion of solar
device 10 is depicted schematically in FIG. 1. Device 10 comprises
a plurality of protuberances 20 (only one is depicted in FIG. 1)
having a base 22, and a substrate 30, having a major surface 32
from which protuberances 20 extend. The base 22 of protuberance 20
is located at the plane of major surface 32 from which protuberance
20 extends. The base 22 of protuberance can be any shape, though
the shape of the base 22, and protuberance 20 is often dictated by
the crystal structure of a template, e.g., a silicon template as
depicted herein, in which case the base 22 is square or
rectangular.
[0031] FIG. 2 depicts different shapes for the base (cross-section
at A of FIG. 1), including convex square, circle, hexagon, and
rectuagular shapes, and also depicts three-dimensional
protuberances that in the case of the square, circle, and hexagon
are depicted both as pyramidal or conical and frusto-pyramidal, or
frusto-conical. In FIG. 2, the protuberance having a concave
crescent base is only depicted as a frusto-pyramid, and the
protuberance having a rectangular shape is only depicted as a
pyramid.
[0032] The base of a protuberance may be any two-dimensional closed
shape, such as a polygon, e.g., a regular polygon, or curved
(circular, oval, etc.), closed shape that can be convex or concave.
In one aspect, due to the use of etched crystalline, e.g., silicon,
templates for the substrate, the closed shape is a convex polygon,
and may be a regular polygon, e.g., a square or rectangle, and the
protuberance is a pyramid or frusto-pyramid. An effective diameter
of the base is the length of a line segment passing thorough the
centroid of the base of the protuberance and having end points on
the boundary of the two-dimensional closed shape of the base of the
protuberance, and for concave shapes, the end points of the segment
are points on the boundary most distal to (farthest from) the
centroid. The base of the protuberance is coplanar with a surface
of the contiguous portion of the metallic substrate from which the
protuberance extends, and the apex of the protuberance is a point
or two-dimensional shape most distal to the base of the
protuberances. A cone or conical shape has a curved base that can
be circular, oval, elliptical, or any curved shape.
[0033] In aspects, heat generated by the solar thermal device
described herein can be used for any purpose. In one aspect, the
device is used as a general purpose thermal device to heat air or
water for use in thermal collection and distribution systems, e.g.,
for architectural heating, residential or commercial property
heating or water heating. Alternatively, the heat is used for
heating of materials for use in industrial processes, such as for
heating water or other substances for use in chemical reactions,
for heating furnaces, for solar desalinization (e.g., solar
stills), etc. In another aspect, the device is used as part of a
heat engine to generate work e.g., in the production of
electricity, etc. as is broadly-known, such as for boiling water
for use in a steam generation system.
[0034] In one aspect, a method of making a solar thermal absorbing
device is provided. The method comprises depositing a metal layer
onto a template, comprising a substrate having a major surface
comprising a plurality of indentations, each of the plurality of
indentations having an opening at the major surface (that is,
coplanar with the major surface). In the template, the openings are
spaced less than 200 nm apart, and the opening of each of the
plurality of indentations having a side length or a diameter of the
opening at the major surface ranging from 100 nm to 1000 nm,
wherein the metal is deposited in an amount to fill in the
indentations of the template and to produce a contiguous metal
layer, optionally having a thickness of from 1 .mu.M to 100 .mu.M
over at least a portion of the major surface of the template
comprising the indentations; and stripping (e.g., releasing,
peeling, or delaminating) the deposited metal layer from the
template to produce a metallic substrate having a major surface,
comprising a plurality of protuberances on the major surface, each
of the plurality of protuberance having a base having an edge or a
diameter, and an apex, and tapering from the base to the apex,
wherein the distance between the base of adjacent protuberances is
less than 200 nm, and the diameter or an edge of the base ranges
from 100 nm to 1000 nm. In one aspect, the template is a silicon
substrate having a major surface comprising a layer of native oxide
or silica coating, and the indentations are pyramidal indentations
with a square or rectangular opening at the major surface. The
shape and height of the indentations are a consequence of the
process of etching the silicon blank with KOH, resulting in a
pyramidal indentation having sides at an angle of 54.7.degree.
relative to the plane of the major surface of the silicon template.
The metal layer may be deposited onto the template by any useful
deposition method, e.g., as described herein, for example, by
sputtering the metal onto the major surface of the substrate to
fill in the indentations and to produce a thin coating of the metal
over the major surface, followed by electrodeposition (e.g.,
electroplating) of the metal, or a metal that can be the same or a
different metal or alloy as the sputtered metal, onto the sputtered
metal. In aspects, the deposited metal is nickel, copper, gold,
silver, or an alloy thereof, e.g., an alloy with tungsten, such as
a nickel-tungsten alloy.
[0035] According to one aspect of the invention, in reference to
FIG. 3, a template 150 is provided. The template 150 comprises a
substrate 160 having an oxide coating 162. In one example, the
substrate 160 is a silicon substrate, and the coating 162 is
silica. FIG. 3 depicts only a portion of a template 150, depicting
a single indentation 170, while a complete template would include a
plurality of indentations, e.g., in an array. The template 150 has
a major surface 180, and the indentation 170 has an opening 172,
with the dotted line indicating the plane of the major surface
180.
[0036] The method optionally further comprises depositing an
antireflective coating over at least a portion of the metallic
substrate including the plurality of protuberances. The
antireflective coating is as described herein, and may be a metal
oxide or a silicon nitride, such as aluminum oxide
(Al.sub.2O.sub.3) or silicon nitride (SiN.sub.x), deposited over
the metallic substrate to a thickness of from 50 nm to 1 .mu.m,
e.g., from 65 nm to 80 nm, for example, 70 nm or 75 nm.
[0037] In aspects, the protuberances are pyramidal, conical,
frusto-pyramidal, or frusto-conical, and in one aspect, the
protuberances are pyramidal, having a square or rectangular base,
and wherein a plane containing a side of the protuberance is at an
angle of 54.7 degrees from the plane of the major surface of the
contiguous portion of the metallic substrate from which the
protuberance extends. In one aspect, the distance between the bases
of adjacent protuberances is less than 50 nm. In another aspect,
the protuberances are uniformly or periodically spaced on the
substrate to form an array.
Example 1--Design of Solar Absorber
[0038] An optimal solar absorber needs to maximize the
solar-to-electrical energy conversion. The solar-to-electrical
conversion efficiency .eta. of a solar thermal system (FIG. 4A) can
be calculated by multiplying the solar-to-thermal conversion
efficiency .eta..sub.solar-th, which is exclusively determined by
the properties of the solar absorber, and the thermal-to-electrical
energy conversion efficiency .eta..sub.th-e. The theoretical
maximum of .eta..sub.th-e is the Carnot efficiency, namely
.eta..sub.th-e=1-T.sub.0/T.sub.A, where T.sub.0 is the ambient
temperature and T.sub.A is the working temperature of the solar
absorber. As a result, one can quantitatively characterize the
performance of the solar absorber by the overall efficiency of the
solar thermal system:
.eta. = .eta. solar - th .times. ( 1 - T 0 T A ) , ( 1 )
##EQU00001##
where .eta..sub.solar-th is a function of the frequency-dependent
absorptivity .alpha.(.lamda.) and the working temperature of the
solar absorber. Based on energy balance equation, n.sub.solar-th
can be explicitly expressed as:
.eta. solar - th = C .times. .intg. d .lamda. .alpha. ( .lamda. ) E
solar ( .lamda. ) - .intg. d .lamda. .alpha. ( .lamda. ) E B (
.lamda. , T A ) C .times. .intg. d .lamda. E solar ( .lamda. ) , (
2 ) ##EQU00002##
where E.sub.solar is the spectral solar irradiation, E.sub.B
(.lamda., T.sub.A) is the blackbody radiation at temperature
T.sub.A, and C is the concentration factor that is usually on the
order of 10.about.1000. It is worth noting that if the absorber is
placed in air, the convection loss through air also needs to be
considered in addition to radiation loss.
[0039] Ideally, solar absorbers should have a step-function like
spectral selection with 100% absorption for the solar spectrum and
0% emission for the infrared range, where the "cut-off" wavelength
is located at the intersection of C.times.E.sub.solar and E.sub.B,
as illustrated in FIG. 4B. However, in reality, there exists no
optical structure reaching this ideal performance. For any
realistic structure, detailed spectral absorptivity/emissivity
(.alpha.(.lamda.)) needs to be considered for both sunlight and
thermal infrared radiation (at T.sub.A). In Equation (1),
increasing T.sub.A leads to a higher Carnot efficiency, but
simultaneously reduces .eta..sub.solar-th due to an increasing
radiation loss. Thus, for a given solar absorber, considering its
specific structure (e.g., multilayer or 3D photonic crystals) and
material, and the concentration factor C, one can find its
optimized geometry and working temperature T.sub.A by maximizing
the overall conversion efficiency .eta..
[0040] The solar devices described herein are solar selective
absorbers based on light trapping principles. In aspects of the
invention, metals are chosen as the absorber base materials because
they are usually stable at high temperatures and have a high
reflectance (thus, low emittance) in the infrared range. For the
infrared light whose wavelength is much larger than the size of
nanopyramids, the nanopyramid structure performs as a perfect
reflector with reflectivity close to 100% (i.e., .about.0%
emissivity). On the other hand, the nanopyramid structure behaves
like a perfect solar absorber in the solar spectrum due to the
tapered subwavelength geometry of the nanopyramids, which generally
exhibits broadband anti-reflection performance. The nanopyramids
enhance solar absorption by matching the wave impedance between air
and the metal substrate because the effective refractive index
gradually changes from n=1 at the top to n=n.sub.metal at the
bottom. To achieve an even better impedance match, a thin layer of
an antireflective material, such as aluminum oxide
(Al.sub.2O.sub.3) and silicon nitride (SiN.sub.x), can be coated
onto the nanopyramids, which increases the absorption by further
mitigating the abrupt change of the effective refractive indexes at
the top and bottom of the nanopyramids.
Example 2--Manufacture of Solar Absorber
[0041] Large-scale nickel nanopyramid structures were manufactured
using a template stripping method, as shown in FIG. 5A. The entire
fabrication process starts with patterning a thin silicon nitride
layer on a regular (100) silicon wafer. Using laser interference
lithography, nanohole arrays are patterned onto the silicon nitride
layer. This layer serves as a mask for anisotropic KOH etching,
which results in inverted nanopyramids on the silicon wafer. After
the etching step, the silicon nitride layer is removed with HF
etching. The silicon wafer is then used as a template for rapid
replication of the nanopyramids with nickel. A 100 nm thick nickel
film is first sputtered on the silicon master template as a seed
layer for electrochemical deposition. The replicating nickel layer
can reach any desired thicknesses with electrochemical deposition.
Due to water-assisted subcritical debonding between nickel and
native oxide on the silicon wafer, the nickel layer with
nanopyramid arrays can be readily peeled off from the silicon
master template. The facets of the resulting nickel nanopyramids
are atomically smooth and free of contamination (FIG. 5B inset).
The stripped functional sheet can be attached to either flexible or
rigid substrates of various materials depending on their target
applications (FIG. 5C). Compared with the previous works using
nanophotonic structures as selective surfaces, the large-scale
nanofabrication process developed here is particularly
cost-effective and robust because a variety of low-cost metals such
as nickel, copper, and their alloys can be used.
[0042] After the peeling step, the silicon templates or molds
retain their original conditions with little contamination or
damage. Therefore, they can be reused many times, leading to a
low-cost and high-throughput fabrication process.
Example 3--Refinement of Structure of Solar Absorber
[0043] In order to maximize the energy conversion efficiency .eta.,
the aforementioned universal principle (Equations 1 and 2) was
applied to optimize the design of nickel nanopyramid structures as
solar selective absorbers. FIG. 6A shows the schematic of a nickel
nanopyramid array produced on a silicon substrate. The shapes of
all the pyramids are uniquely determined by the KOH etching of the
(100) silicon wafer, where the angle equals 54.7 degrees. The
selective absorption of nickel nanopyramid arrays is determined by
the following parameters: (1) length of the pyramid edge l; (2)
distance between adjacent pyramids d; and (3) thickness of the
anti-reflection coating (ARC) h, all of which can be tuned in our
fabrication process. The best design by maximizing .eta. was
numerically searched from all the possible cases with different
combinations of l, d and h. For each case, direct numerical
simulation was first performed to obtain the spectral absorptivity
.alpha.(.lamda.). Then, the maximum overall efficiency .eta..sub.m
for this case is solved together with the optimized working
temperature T.sub.A based on Equations 1 and 2. As a result, the
best design of nickel nanopyramid arrays is determined by the case
where .eta..sub.m reaches a global maximum.
[0044] According to the numerical search results (FIGS. 6B-6E), the
optimized design of the nickel nanopyramid arrays as selective
solar absorbers needs to satisfy the following three criteria: (1)
the length of pyramid edge l is about 500 nm; (2) the distance
between adjacent pyramids d should be as small as possible; (3) the
thickness of the anti-reflection coating h is around 70 nm as long
as the pyramidal shapes are preserved after coating the pyramids
with Al.sub.2O.sub.3. For the third criterion, the anti-reflection
coatings with thickness h.di-elect cons.[70 nm, 300 nm] achieve
very similar performance. However, we choose h.about.75 nm as the
optimal design in order to lower fabrication cost. Furthermore,
thick Al.sub.2O.sub.3 coatings will increase the thermal emission
in the infrared range due to its optical phonon resonance. While
our numerical search is based on solar concentration factor C=1, it
turns out that the optimal design is insensitive to the variation
of C. FIG. 6B-6D illustrates the maximized overall
solar-to-electrical conversion efficiency .eta..sub.m for different
combination of 1 and d at h=75 nm, for C=1,100,1000, respectively.
The optimal dimensions for different C remain almost the same. For
C=1000, TA=1000.degree. C., the theoretical overall conversion
efficiency .eta..sub.m with the optimized nanopyramid structures
can be as large as 68%.
[0045] The angular dependency of the absorption spectrum of nickel
nanopyramid arrays also was investigated. For the case of 1=500 nm,
d=100 nm and h=75 nm, the averaged absorptivity spectrum (the
average of s-polarization and p-polarization contributions) was
numerically evaluated for the incident angle .theta. up to
70.degree. (FIG. 6E). The result shows that the selective
absorption maintains almost the same for the angles in the range of
.+-.50.degree., which indicates the omnidirectional absorption of
the nickel nanopyramid arrays.
[0046] To characterize the selective absorption properties of the
functional nickel sheet, two spectral measurement systems were used
to cover the visible and infrared spectra with corresponding
wavelength ranging from 300 nm to 10 .mu.m. A Perkin Elmer LAMBDA
950 UV/Vis/NIR Spectrophotometer with a 150 mm diameter integrating
sphere is used to measure the total absorptivity spectrum in the
visible and near-infrared range. For the mid-infrared absorption
measurement, a Thermo Scientific Nicolet iS50R FT-IR spectrometer
is used with a gold mirror as a reference. Because the samples are
all opaque to the electromagnetic waves in the aforementioned
visible and infrared spectra, the absorptivity (A=1-R) of the
samples was obtained by subtracting the measured reflectance (R).
The spectral selective absorptance of the functional nickel sheets
are shown in FIG. 4 with the spectrum spanning from visible to
infrared range. Two types of samples were measured, one type has a
nanopyramid pitch size of 800 nm ("P800"), the other type has a
pitch size of 600 nm ("P600"). The P800 and P600 samples have base
widths 1=600.+-.50 nm and 1=500.+-.30 nm, respectively. The
thicknesses of Al.sub.2O.sub.3 ARC coatings are .about.75 nm for
both cases. The finite-difference time-domain (FDTD) simulated
absorptivity of the nickel nanopyramid structures agrees well with
our measurements (FIG. 7A). As shown in FIG. 7B, the absorption of
bare nickel nanopyramids can be further enhanced by adding the ARC
layer. The nanopyramids arrays with ARC layer are intrinsically
complex. The small bumps and dips observed in their absorption
spectrum may be caused by the combined photonic phenomena, such as
interference, plasmonics and resonance, and the system errors of
measurement instruments. The spectral absorptance of the functional
nickel sheet clearly shows a highly selective characteristic over
the measurement range. For the wavelength below about 1.3 .mu.m,
the average absorptance is -0.95, which is almost two-fold higher
than that of a flat nickel surface (FIG. 7B). Beyond a narrow
transition range from 1 .mu.m to 2.5 .mu.m, the absorptance of the
nickel sheet falls to a value as low as 0.1 for wavelengths >2.5
.mu.m, which is remarkably beneficial for the low thermal emission
in the infrared range.
[0047] The stability of solar selective absorbers at high operating
temperatures is of great importance for their practical
application. A thermal annealing test is conducted using the
samples fabricated with the aforementioned process. The fabricated
solar absorbers are annealed at a temperature of 800.degree. C. in
vacuum (<1.5.times.10-5 Torr) for >5 hours. The spectral
absorptance after annealing is slightly degraded compared to the
original samples (FIG. 8A). In the wavelength range between 500 nm
and 1 .mu.m, a .about.8% drop in absorption is observed, whereas a
.about.10% increase in absorption is in the infrared range (5-15
.mu.m). This is probably due to phase transition or crystal growth
during the annealing process which causes a slight change in the
surface topography, or the oxidation of nickel in the medium high
vacuum for annealing. Further investigation using scanning electron
microscopy shows that the surface of nanopyramids becomes rougher
after annealing, but the overall nanopyramid structures are still
well maintained, as shown in FIG. 8B. The thermal stability test
demonstrates that the nickel nanopyramids based solar absorbers
have great potential for high temperature applications.
[0048] In summary, large-scale low-cost nanophotonic solar
absorbers have been developed based on nickel nanopyramid
structures. The fabrication process mainly exploits cost-effective
materials and technologies such as template stripping and
electroplating. The measured absorptivity/emissivity demonstrates
excellent spectrum selection with .about.95% solar absorptivity and
.about.10% emissivity in the infrared range. Due to the 3D design,
the excellent selective absorption maintains for the incident
angles in the range of .+-.50.degree., which indicates the
omnidirectional absorption of the solar absorbers. The thermal
annealing tests indicate that the nickel nanophotonic absorbers are
stable at 800.degree. C. The combined spectrum selection, high
temperature stability, and omnidirectional absorption demonstrated
in our work are unprecedented compared to existing solar absorber
structures/materials. Other than solar-to-electricity energy
conversion, the high-performance solar selective absorbers can be
equally applied to other solar thermal systems, such as solar water
heaters and solar fuel production.
[0049] The following clauses outline various illustrative aspects
of the invention: [0050] 1. A solar device, comprising, a metallic
substrate comprising a plurality of protuberances each having a
base having an edge or a diameter, and an apex, and tapering from
the base to the apex, wherein the distance between the base of
adjacent protuberances is less than 200 nm, and the diameter and/or
length of a side of the base ranges from 100 nm to 1000 nm. [0051]
2. The device of clause 1, further comprising an antireflective
coating having a refractive index greater than one and less than
the maximum refractive index of the metallic substrate at the base
of the protuberances. [0052] 3. The device of clause 1, wherein the
base of the protuberance is a square or rectangle. [0053] 4. The
device of clause 2, wherein the antireflective coating comprises a
metal oxide or a silicon nitride. [0054] 5. The device of clause 4,
wherein the antireflective coating has a thickness of from 50 nm to
1 .mu.m in thickness. [0055] 6. The device of any one of clauses 1
to 5, wherein the metallic substrate comprises nickel, copper,
gold, silver or an alloy thereof, e.g., an alloy with tungsten,
such as a nickel-tungsten alloy. [0056] 7. The device of clause 2,
wherein the antireflective coating comprises an aluminum oxide.
[0057] 8. The device of any one of clauses 1 to 7, wherein the
protuberances are pyramidal, conical, frusto-pyramidal, or
frusto-conical. [0058] 9. The device of clause 8, wherein the
protuberances are pyramidal, having a square or rectangular base
wherein a plane containing a side of the protuberance is at an
angle of 54.7 degrees from the plane of the major surface of the
contiguous portion of the metallic substrate from which the
protuberance extends. [0059] 10. The device of any one of clauses 1
to 9, wherein the distance between the base of adjacent
protuberances is less than 50 nm. [0060] 11. The device of any one
of clauses 1 to 10, wherein the protuberances are uniformly-spaced
on the substrate. [0061] 12. The device of clause 2, wherein the
antireflective coating comprises an oxide of aluminum, hafnium,
titanium, and/or zirconium. [0062] 13. A template for producing a
solar device, comprising a substrate having a major surface
comprising a plurality of indentations, each of the plurality of
indentations having an opening at the major surface (that is,
coplanar with the major surface), wherein the openings are spaced
less than 200 nm apart, and the opening of each of the plurality of
indentations having a side length or a diameter ranging from 100 nm
to 1000 nm. [0063] 14. The template of clause 13, wherein the
template is a silicon substrate having a major surface comprising a
silica coating, and the indentations are pyramidal indentations
with a square or rectangular opening at the major surface. [0064]
15. The template of clause 13, wherein the openings are
regularly-spaced. [0065] 16. A method of making a solar thermal
absorbing device, comprising, [0066] a. depositing a metal layer
onto a template, comprising a substrate having a major surface
comprising a plurality of indentations, each of the plurality of
indentations having an opening at the major surface (that is,
coplanar with the major surface), wherein the openings are spaced
less than 200 nm apart, and the opening of each of the plurality of
indentations having a side length or a diameter ranging from 100 nm
to 1000 nm, wherein the metal is deposited in an amount to fill in
the indentations of the template and to produce a contiguous metal
layer, optionally having a thickness of from 1 .mu.M to 100 .mu.M
over at least a portion of the major surface of the template
comprising the indentations; and [0067] b. releasing (e.g., peeling
or delaminating) the deposited metal layer from the template to
produce a metallic substrate having a major surface, comprising a
plurality of protuberances on the major surface, each of the
plurality of protuberance having a base having an edge or a
diameter, and an apex, and tapering from the base to the apex,
wherein the distance between the base of adjacent protuberances is
less than 200 nm, and the diameter or an edge of the base ranges
from 100 nm to 1000 nm. [0068] 17. The method of clause 16, wherein
the template is a silicon substrate having a major surface
comprising a silica coating, and the indentations are pyramidal
indentations with a square or rectangular opening at the major
surface. [0069] 18. The method of clause 16 or clause 17, further
comprising depositing an antireflective coating over at least a
portion of the metallic substrate including the plurality of
protuberances. [0070] 19. The method of clause 18, wherein the
antireflective coating comprises a metal oxide or a silicon
nitride. [0071] 20. The method of clause 18, wherein the
antireflective coating has a thickness of from 50 nm to 1 .mu.m.
[0072] 21. The method of clause 18, wherein the antireflective
coating comprises an oxide of aluminum, hafnium, titanium, and/or
zirconium. [0073] 22. The method of clause 18, wherein the
antireflective coating comprises an aluminum oxide. [0074] 23. The
method of any one of clauses 16-22, wherein the metallic substrate
is nickel, copper, silver, gold, or an alloy thereof, e.g., an
alloy with tungsten, such as a nickel-tungsten alloy. [0075] 24.
The method of any one of clauses 16-23, wherein the protuberances
are pyramidal, conical, frusto-pyramidal, or frusto-conical. [0076]
25. The method of clause 24, wherein the protuberances are
pyramidal, having a square or rectangular base and wherein a plane
containing a side of the protuberance is at an angle of 54.7
degrees from the plane of the major surface of the contiguous
portion of the metallic substrate from which the protuberance
extends. [0077] 26. The method of any one of clauses 16 to 25,
wherein the distance between the base of adjacent protuberances is
less than 50 nm. [0078] 27. The method of any one of clauses 16-26,
wherein the protuberances are uniformly-spaced on the substrate.
[0079] 28. The method of clause 24, wherein the protuberances are
frusto-conical or frusto-pyramidal.
[0080] Having described this invention, it will be understood to
those of ordinary skill in the art that the same can be performed
within a wide and equivalent range of conditions, formulations and
other parameters without affecting the scope of the invention or
any embodiment thereof.
* * * * *