Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
8 BAC~CCROUND OF THE I~VE~ITION
g The ~ficiency of conversion of pho~on energy _nto ehermal
energy is dependent upon the relationship of the portion of the photon
11 energy absorbed to the portion of heat that l5 emitted or reflected.
12 Metals which have good ther~al properties, absorb or are non-transparent
13 at essentially all wavelengths, while at the same time t:~ey also reflect
14 much of the energy to which they are exposed. Generally, highly reflec-
tive surfaces have both low absorbtivity znd low emissivity. Since
16 absorbtivity and emissivity are interrelated, the art thus far has
1, developed along the lines of multilayered structures wherein one layer
18 has one desirable property and another layer another desirable property.
19 An e~ample of such structure is shown in U. S. Pa~ent 3,920,413. Such
structures however, are subject to structural limitations in that the
21 effect of one layer may interfere with the optimum benefit from another.
22 Further, the manufacture of multilayered structures frequently involves
23 many processing considerations in fabrication.
24 DESCRIPTION OF THE INVE~TION
The invention involves a reflectiviey control surface region
26 for photon absorbing materials such that the reflection rrom tne
27 photon absorbing material sur ace is attenuated and reflected by the
ZR operatio~ of the c:iteria selected or the rzdiation control surface region
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tour so thac the net reflection ~f~ece is sharply curtailed.
~nodically oxidized tungsten wich a particular type o rough surface
can meee ehe criceria of ehe inv~nelon so thae ~ superior phoeon absorber
and a superior con~er~er of solar energy lneo heat results.
REFERE~CE T0 REL~IED APPLIC~II0~
In U.S. Patent No. 4,005,698, issued February 1, 1977
to J.J. Cuomo, a new surface is provided which is a geometric maze
of aligned needle-like protrusions with dimensions and spacing
related to visible light wavelength. The material of the U.S.
Patent No. 4,005,698 provides a more efficient photon energy
absorber than has been seen heretofore in ehe art and when the refleceivity
control surface region is applied thereeo an even more improved phoeon
energy absorber is produced that can absorb 99.94Z of incident lighe at
a particular wavelength.
Mbre p~icularly, there is providRd:
A photothermal absorblng member having a minimi~ed total
reflectivity comprising in combination:
a body of photon absorbing material having a surface contour
insuring multiple refleccions of incident light from the surface
thereof with a contour conforming reflection control surface region
associated with said body, said region exhibiting a first reflect-
ivity and the interface of said body and said region exhibiting a ;
second reflectivity;
said region being of a material with an index of refraction of
a specific magnitude and ehe material of said body of photon absorb-
ing material having an index of refraction and an extinction coef-
ficient each of a specific magnitude such that in combination the
three specific magnitudes operate to subseantially equate said first
and said second reflectivity.
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DESCRIPTION OF IHE DR~WINGS
FIG. 1 is a schematic view of che opcical operation o the
invention.
FIG. 2 is a photomicrogr3ph of a hillock-type of tungscen
surface.
FIG. 3 is a photomicrograph of a dendrieic-type of tungsten
surface.
FIG. 4 is a plot of light wavelength vs. reflectance
lllustrating the effect of the invention on three types of surfaces.
FIG. 5 is a ploc of light wavelength vs. reflectance
illustrating the effect of the invention on reflectance for several
angles of lncitence of the light.
FIG. 6 is a plot of thic~ness of surface region of
tungsten oxide on tungsten vs. wavelength at the maximum of the
absorption, and appears out of sequential order on the same sheet
as FIG. 1.
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1 ` DETAILED DESCRIPTION OF THE INVENTION
2 The efficiency of conversion of light energy lnto heat may
3 be expressed as:
4 Equation 1 ~fficiency 5 Energy absorbed - Ener~y reradiated
Referring to FIG. 1, a schematic view is presented showing
6 the effect of the invention on the absorption and reflection of light.
7 In FIG. 1, a radiation control surface region 1 is shown as an optically
8 transpaIe~t material for the desired wavelength having a surface 2
9 parallel to the surface 3 of the photon absorbing material and having athickness 4 related to the wavelength of the incident light. The optical
11 and physical specifications of the reflection control surface region are
12 interrelated as is set forth below.
13 For purposes of definltion reflection means energy that impinges
14 and is returned without entering the material in conerast to reradiation
whe~e the energy enters the material and by virtue of a change in tempera-
16 ture of the material energy is emitted by the material.
17 In FIG. 1 the light striking the surface 2 has an inltial
18 reflectivity component 5 ant a series of decreasing subsequent components,
19 three of which are illustrated as elements 6, 7, and 8. In operation,
the light reflectet from the surface 3 is enhanced or diminished by
21 interference with the light returning from surface 2 from a previous ~ -
22 reflection.
23 The following description is set forth using an oxide of a
24 metal photon absorber material as an illustration although it will be
apparent in the light of the principles descrlbet that coatings other
26 than oxides as well as materials other than the composition of the base
27 metal may be provided to achieve the desired properties.
28 ; In FIG. 1 the first reflection coefficient (element c) may be
l, 29 expressed as follows:
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, Equati~n 2 Initial retlectl3n co~ttlcl~nt (elemen~ 5) - (rl) ~ 1 + N
2 where rl is air-co-o~id~ retlectivit~
3 and N ls the lnde~ ot r~frace~on of the o~ld~.
4 Similarly the reflectlon coefficient of (element 6) ~ay be
expressed by Equation 3.
6 Equation 3 Reflection coefflci~nt of telement 6) - ~r2~1/2 N~ No i ~ m
Nm No i k
7 where r2 is oxide-~o-~eeal reflectivity.
8 Nm is index of re~raccion of mecal.
9 . Km i5 extlnction côefficient of the o~ide.
i is ~
ll Thus the relaeionship of reflection coefficiene3 for the compo-
12 nent9 5, 6, 7 and 8, etc. is as follows:
13 5 - r
14 6 ~ r2 (l-rl ) 2
7 ~ r22rl (l-rl )2
16 8 - r23rl2 (l-rl ) 2
17 Hence the reflectivity of che control surlace 1 is gg expressed
18 in Equation 4 d/ 2 2
19 Equation 4 RTOTAL ~ ¦rl + r2 e ( 1)
l~rlr2
where d is the thickness 4
21 ant ~ is the wavelengCh.
22and ¦ ¦ indicate absolute values.
23Equation S l~rl ~r2 1 ~1 ~ DESIRED
-rlr2 I REFLECTIVITY AT ~IN.
24This is approximately
25Equation 6 j~rl¦ ¦r2lj < DESIRED
~EFLECTIVITY AT ~IN. ,~
26 For applications involving the conversion of solar energy
into heat the desired reflectivity at minimum wavelengeh (~ ) should
28 be less than O.OS.
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The desirable goal is 'or ~ OT~L to be 3S small as possible and
~ the reflectivity of the surface 2 is ne3rly equ31 to che ~flectivity of
3 the surface 3.
4 The criteria for a radlation control surface ~egion 1 for a J
desired wavelength may be e~pressed as follows:
6 Equation 7
7 ~ o¦ ¦Nm~No~i Km ¦¦ ~ DESIRED
+ Nol INn-No+i Km 11 ~EFLECTIVITY AT MIN.
In essence as may be seen from Equation 7, the criteria of the
9 reflection control surface region of the inveneion OperatQ to e~uate the
effect of the reflection components of surface 3 wieh that of the initial
11 reflection of the incident light from surface 2.
12 The thic~ness d in (elemenc 4 ln FIG. 1) enters in two ways. rt
; 13 is part of the calculations of Equation 4 establishlng the desired reflec-
14 tivity at che wavelength of the minimum, and then as will be described in
FIG. 6, it permits movement of the wavele~gth minimum.
16 In such a relationship it is apparent thac a desirable goal is
17 to absorb all ratiation in the desired band of wavelength co reflect all
; 18 undeslred wavelengths~ and to keep the de-cired wavelength energy that is
19 reflected to a minimum. This is done in accordance with the.invention by .
providing wavelength selective reflection control surface region at the
21 surface of a photon absorbing macerial such thac the air-to-region 1
22 reflectivity, thic~ness, and co~tour; the inde~ of ref~action of region 1;
73 and the index of and extinction coefficient of ehe photon absorber material
24 all interact to.curtail the light reflected from ehe photon absGrbing
material.
26 The surface contour is best chosen to be rough or te~tured such
,
.- 27 that light which is incident normal to the surface musc substantially
,: ,
28 undergo more than one reflection before it can escape from che surface.
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1 This roughened or ~e~eured surface, in combinaclon wieh the reflection
2 control layer, produces an absorptance which is greacer and which covers
3 a larger band o~ ~avelength than a simple anti-refleccive coating on a
4 smooth metal. For e~ample, an anti-reflective coating on a smooth metal
S has a reflectance af RToTAL which varies with wavelengch, while an anti-
6 refleceive coacing on a roughened or te~tured surface, in which lighe suffers
7 two bounces before being returned, has a reflectance of ~ OTAL which is
8 less than ~ OTAL
9 The reflection control region may be contrastedrwith passivating
coatings by the fact that in the passivating coating ehe primary concern
11 is chemical proteceion of inertness and therefore che choice of materials
12 is tirected at this purpose.
13 Referring next to FIGS. 2 and 3 there are shown phoeomicrographs
14 of eungsten surfaces having respectively lncreasing degrees o absorptivity.
The surface of FIG. 2 is known as a hillock surface well known in the
16 art and the surface of FIG. 3 is known as a dendritic surface as see forth
17 in the referenced U.S. Patent No. 4,005,698. Both surfaces are prepared by
18 tl : technique of Chemical Vapor Deposition, well known in the art. The
hillock structure is much thinner than the dendritic structure and hence
20, is less expensive. The degree of magniflcation is shown on the photo-
21 ~icrograph. The radiation control surface region of the invention when
, .
22 fabricated in connection with surfaces such as the surfaces of the type
23 of FIGS. 2 and 3 and with a flat surface not illustrated results in an
24 abrupt decrease in total reflectivity for a particular_wavelength which
; 25 is selectable in accortance with the criteria set forth above.
26 This is illustrated in ;he graph of FIG. 4 wherein Total ~eflec-
27 tance for normally incident light is plotted against wavelength in microns.
28 Three curves are shown. ;A dotted curve is shown for flat tungsten, a dashed
29 curve is for hillock material in-FIG. 2 and the solid cur~e is for the
dendritic material in FIG. 3. It should be noced chat che radia~ion con~rol
31 surface region in accordance with the invention produces a peak in absorpeicn
32 in the ViCiQicy o~ 0.62 microns. This wavelength is acce?ced in che art
as being at or near the pea~c in solar emissivity. From the
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logarithmic scale of FIG. 4 it may be seen that the dendritic material
2 of FIG. 3 when provided wi~h the radiation control surface region of the
3 invention absorbs 99.94% of the incident light at 0.55 microns.
4 Referring next to FIG. 5 the effect of the invention for vary-
S ing directlons of incident light on a dendritic surface is shown.
6 In the graph of FIG. 5 total reflectivity is plotted against
7 wavelength in nanometers for 0, 20, 40, 60 and 80 angle of incidence
8 of light. ~n each instance the absorption peak appears at approximately
9 the same wavelength.
In accordance with the invention, fabrication of the reflection
11 control surface region 1 of FIG. 1 is accomplished by providing the region
12 1 material contoured to the surface configuration of the photon absorbing
13 material, with the desired parameters which are: the reflection co-
14 efflcient from the surface 2 of the region 1 material is approximately
equal to the reflection coefficient of the interface 3 between the photon
16 absorbing material and region 1. These reflectivity coefficients are
17 related to the index of refraction of region 1 material the index of
18 refraction of the photon absorbing material, and the extinction coefficlent
19 of the photon absorbing material. These are well establishet parameters
ln the art and are available in most standard handbooks. In order to
21 enable one skilled in the art to minimize experimentation however, a set
22 of specific values for equations 2-7 are provided in the Table 1 for the
material W03 as the radiation control surface region 1 on dendritic W as
24 shown in FIG. 3.
~:;! 25 TABLE 1 '
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26 W W03 W - W03
27 n 3.43 2.26 ___
-! 28 k 2.96 0.0 ___
29 Irll ___ _ _ 0.386
Ir I 2 ___ ___ 0.496
31 Irll-~r2l ___ ___ 0.012
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The fabrication of the radiation control surface region 1 is
2 particularly adaptable to processes that for~ chemical compounds of the
3 photon absorber material. Such processes use the material of the photon
4 absorber materlal as one component, form in a conformal contour with the
surface and are generally easily controllable for the desired thickness
6 range of the surface control region. Some examples of such processes are
7 anodization or oxidation, nitridation and carburization. One particularly
8 controllable fabrication approach is the technique of anodization where
9 the material of the photon absorber and the region formed so permit in
accordance with the criteria of the invention set forth above. In thi~
11 technique an oxide is frequently formed that limits current flow so that
12 thickness of the region is precisely correlated with voltage. Some metals
13 forming advantageous oxides useful in accordance with the invention are
14 W, Mo, Hf, V, Ta and Nb.
Again in order to facilitate the practice of the invention,
16 Table 2 sets forth the relationship between the thickness dimen~ion 4 of
17 FIG. 1 and anodi~ation voltage for the material WO3 on W.
18 TABLE 2
VOLTAGE THICKNESS
In Volts In ~m
' 20 0.035
0.045
0.055
0.065
0.075
19 As an illustration of the spectacular advantages of the
invention the following test results of a particular embodiment are
21 provided.
22 A hillock tungsten surface as illustrated in FIG. 2 was anodized
23 in a phosphorlc acid bath at a voltage of 30V. In this technique the WO3
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region stops the anodic reaction at a specific thickness which is
2 controlled by the applied voltage. The ratio of "absorptivity" to
3 incident radlation "over" "hemispherical emissivity", in other words,
4 (a/~) for thls surface at 150C is 3.9. In the following table the
efficiency as computed by Equation 1 for this surface is compared to that
6 of a standard blackbody for varying temperature.
!
TABLE 3
RERADIATION IN WATTS
EFFICIENCY IN ~ PER SQ. CM
T TUNGSTEN BLACKBODY TUNGSTEN BLACKBODY
50C 80% 32% 0.015 0.063
75C 75% 12% 0.020 0.083
100C 68% 0 0.027 0.1125
150C 51% 0 0.044 > 0.1
200C 26% 0 0.069 > 0.1
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8 From the table it may be seen that efficiencies of greater than
9 50% are realized for temperatures up to 150C.
One ma;or benefit is that the technlque of the invention now
11 ~nke~ possible a new photon absorbing material in that antireflective
12 coating benefits may now be imparted to substrates having photon absorbing
13 properties derlved from surface irregularities.
14 For most solar energy conversion applications it i9 desirable
to have photon absorbers which absorb greater than 90~ of the solar spectrum.
16 Neither flat metal, rough metal nor simple antireflective coatings thereon
17 can achieve this result. However, in combination with the reflection
18 control surface region of the invention applied to particular types of
19 texturet or rough metal surfaces such as tungsten low reflectance over a
broad spectral region can be achieved. Textured or roughened surfaces,
21 which normal incident light experiences multiple reflections off the surface
'l
' 22 of the reflection control layer, have been found to yield the desired ab-
;,l 23 sorptance for the solar spectrum. In contrast antireflection coatings on
24 smooth metals have an absorptance which covers only a small portion of the
2-5 solar spectrum.
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1 While the invention has been shown in connection with a specific
2 embodiment of anodized tuhgsten it will be apparent to one skilled in
3 the art that in th~ light of the principles set forth many speclfic
4 embodiments can be realized.
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