CA1055744A - Thermal control filter - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Thermal control filter having a substrate capable of transmitting visible energy and having a surface with a coating thereon with the coating being formed of at least five layers identified as first through fifth layers count-ing from the substrate. The first layer is formed of a dielectric material. The second layer is a thin nucleation layer of a material different from the substrate, the third layer is formed of gold. The thin nucleation layer serves as a nucleating layer for the gold layer. The thin nucle-ation layer is thin enough so that it is substantially ineffective in changing the optical properties of the filter. The fourth layer is formed of yttrium oxide and the fifth layer is formed of a dielectric material.

Description

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This invention rela~es to thermal control filters of the type having a transparent substrate with a coating thereon formed of thin films.
In copending application Serial No. 71,009, filed September lO, 1970 and since issued as U.S. Patent No~
3,682,528 dated August 8, 1972, there is disclosed an infra-red interference filter which is of a type intended primarily for the separation of heat from light emanating from incan-descent sources. Specifically, the performance of this filter provides a high transmission in the visual spectrum and high reflection in the infra-red spectrum with a very rapid transition between the visible and the infra-red.
There are, however, other applications where thermal control is desired where the source of radiation is from a much lower temperature source than an incandescent lamp.
Examples of such sources are warm bodies such as ovens and furnaces. In such lower temperature applications, there is - still a need for a filter which will transmit visible llght and which at the same time will reject infra-red energy so that heat which is present on one side of the filter is `
confined to that side of the filter and does not flow to the ~ -other side of the filter. Thus, if the filter is used as a window in a building, the heat from the outside would be ~ ;:. .. .
excluded from the building during the~summertime and in the wintertime heat within the building would not flow out of the building. In the case of an oven or furnace, the heat --would be contained within the oven or the furnace to make the oven or furnace perform more eEficiently and, ln addlt-ionj the outside surface of the filter would be appreciably cooler than a simple uncoated window, and thus, less ;~-.:
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10557~4 dangerous to human beings. The specific filter which is disclosed in the copending application has an additional disadvantage in that it is inadequate for use at very high temperatures. Also, the filter has the disadvantage that it is relatively expensive because it utilizes silver. There is, therefore, a need for a thermal control filter which can be utilized in conjunct;on with low temperature sources, which is relatively inexpensive to produce and has a high operating temperature.
According to the broadest aspect of the invention there is provided, in a thermal control filter, a substrate formed of a material capable of transmitting visible energy, said substrate having a surface and a coating formed on said surface, said coating comprising at least five layers counting from the substrate, said first layer being formed of a dielectric material, said second layer being formed of a thin nucleating layer being thin enough so that it is substantially ineffective in changing the optical properties of the filter, said third layer being formed of gold having a thickness ranging from 2 to 30 millimicrons, said thin nucleating layer serving as a nucleating layer for said gold layer, said fourth layer being formed of yttrium oxide, and -said fifth layer being formed of a dielectric material.
In general, it is an object of the present invention to provide a thermal control filter which is capable of reflecting infra--red energy while transmitting visible energy and which can be utilized in connection with temperature sources . .
which have a temperature substantially lower than that of an incandescent lamp.
Another object of the invention is to provide a :: ~,'' .
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lOSS~44 filter of the above character which can be utilized in a relatively high temperature environment.
Another object of the invention is to provide a filter of the above character which is relatively inexpensive.

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Another object of the invention is to provide a filter of the above character which can be readily produced with a high yield.
Another object of the invention is to provide a filter of the above character that utilizes a bilayer formed of relatively inexpensive materials.
Another object of the invention is to provide a filter of the above character in which the filter is pro-tected by a layer of dielectric material on each side of the bilayer.
Another object of the invention is to provide a filter of the above character which is particularly useul for windows for ovens and furnaces.
Another object of the invention is to provide a ~ -filter of the above character which has low emissivity.
Another object of the invention is to provide a filter of the above character which has a hard, durahle coating which greatly reduces heat flow and surface temper- -ature.
Additional objects and features of the invention will appear from the following description in which the preferred embodiments are set forth ln detail in conjunction with the accompanying drawlng.
- - BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is an isometric view of a portlon of an ,~ oven having a window therein utilizing a thermal control filter incorporating the present invention.
Figure 2 is a cross-sectional view of the window shown in Flgure 1 taken along the line 2-2 o~ Figure 1.
Figure 3 is a graph ~showing the optical ~ -" , ;;~ - - ,', ~ :.

~ -4- ~ ~ -~l)Ss7~4 characteristics of a filter incorporating the present invention.
Fi~ure 4 is a graph showin~ the reflectance and emissivity of a filter incorporating the present invention.
Figure S is an enlarged cross-sectional view similar to Figure 2 showing another embodiment of a filter incorporating the present invention.
In Figure 1 there is shown an oven 11 of a conven-tional type, for example, one which is utilized in the home which is heated either by gas or electricity. As is well -known to those skilled in the art, such an oven is comprised of a housing 12 which is provided with an opening normally closed by a hinged door 13. A window 14 is provided in the door to permit the user of the oven to view the interior of the oven. Conventionally, such windows consist of two spaced panes 16 and 17 of glass which are hermetically sealed 90 as to provide an air pocket therebetween which serves as an insulator. As is well known to those skilled in the art, more recent ovens are provided with a self-cleaning feature in which the ovens are raised to a high temperature. In order to prevent the window 14 from reach-ing an excessively hi~h temperature which could burn a person touching the window, the window 14 is provided with a `
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thermal control filter incorporating the present invention. ~ `
As shown in Figure 2, the thermal control filter ~
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consists of the pane~16 which serves as the substrats for the filter and a coating 18~;which is carried by the sub-strate 16. The substrate 16 is formed of any sultab}e material but it is prsfersbls that it be formed of a ~ ~ ~
~ materisl which;is highly trsnsmitting~in the visible~rsglon ~ ;
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~55~44 of the spectrum. Thus, for example, the substrate 16 can be formed of a glass having an index of refraction of approx-imately 1.517. The substrate 1~ is provided with two parallel surfaces 21 and 22 with the surface 21 being exposed to the outside air or the medium, and the surface 22 having the coating 18 covering the same. The panes 16 and 17 can be referred to as the outer and inner panes if desired.
The coating 18 consists of at least four layers identified as first, second, third and fourth layers 26, 27, .. .. .
28 and 29, respectively, counting from the substrate with the layers 27 and ?8 forming a bilayer and layers 26 and 29 being formed of layers of dielectric material which serve to protect the bilayer and which also add to the optical properties o~ the filter as hereinafter described.
The bilayer consists of a relatively thin layer 27 which is formed of a material di~ferent from the substrate and which serves as a nucleating layer for the thicker metal layer 28. The thin layer 27 is preferably maintained as ; 20 thin as possible so that it will have very little, if any, effect upon the optical characteristics of the filter.
Thus, it has been found that the layer 27 can have a thick-ness ranging from 5 to 29 Angstroms. This thin layer ?7 can be formed of any one of a number of materials. However, it -~
has been found that nickel and chromium are particularly satisfactory. The other metals which also can be utilized are rhodium, paladium, Nichrome, tungsten, etc. In addi-tion, certain dielectric materials such as titanium monoxide, silicon monoxide and aluminum oxide, and metal : ~ -~ 30 oxide mixturles such as those disclosed in U.S. Letters ~-'`; ~ ' " ~
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Patent 3,034,924 also can be utilized to provide a satis-factory pre-coat or nucleating layer for the metal layer 28.
As pointed out above, the thickness of this pre-coat or thin layer is not critical providing it exceeds a certain minimum thickness which is believed to be approx-imately 5 Angstroms. The effect of this pr~o-coat or thin layer appears to be that of a nucleation layer which gives to the thin metal film 23 the optical properties of the bulk metal. In other words, the pre-coat or thin layer 27 is so thin as to be ineffective in changing the optical properties of the filter but causes the subsequent thin metal film to behave as if it were a homogeneous metal slab or, in other words, a slab of bulk metal. ;
The thin metal layer 28 is formed of a metal which makes it possible for the coating to withstand relatively high temperatures as, for example, temperatures ranging from 250C to 450C. To meet these high temperature require-ments, it has been found that it is necessary to select either copper or gold as the metal for this layer. When ` ~ -copper is utilized for this layer as shown in the embodiment ~
disclosed in Figure 2, the copper layer has a thickness of --approximately 13.5 millimicrons. ~owever, the thickness can range from approximately 8 t~ S0 millimicrons. It has been ;-found that the thinner the copper layer, the greater the visual transmission, the greater the emissivity so that~the ~heat flow through the window increases. Conversely, when -the copper layer is thicker, the visual transmisslon decreases-and ~he emissivity decreases so that the heat flow through the glass is decreased. Thus, the coating performs .
more effectively as the thickness of the copper is lncreased.
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~ C~55744 However, this thickness increase has the disadvantage in that as the copper thickness is increased, the visual transmission decreases.
In order to make the bilayer consisting of the layers 27 and 28 into a coating which is hard, durable and stable, it is necessary that the two additional layers 26 -and 29 be provided on opposite sides of the layer. The layers 26 and 29 are formed of a suitable dielectric material such as aluminum oxide and magnesium fluoride for low index materials, and titanium dioxide for a high index material.
In one embodiment of the invention shown in Figure 2, the dielectric layer 26 was formed of aluminum oxide, or sapphire. The layer 26 had a physical thickness of 58 millimicrons and a quarter wave optical thickness of 382.8 millimicrons. These thicknesses can be varied - 30~ while still retaining satisfactory optical characteristics. To -obtain the desired mechanical properties from the layer 26, it is only necessary that it have a thickness which is greater than 100 to 200 Angstroms. - -The layer 29 was formed to have a physical thick- -ness of 37 millimicrons and a quarter wave optical thickness -' :' ' of 244.2 millimicrons. Againj as with the first dielectric -layer 26, the minimum thickness for the layer 29 to obtain the necessary mechanical durability and stability should be from lOO to 200 Angstroms,~ whereas to obtain the desired optical properties, the thiGkness can range to a ~ 30% from the optimum 37 millimicron thickness.
- The l-ayers 26, 27, 28 and 29 forming the coating . ~ :. .
~ 30 ~ 18 can be deposited in a conventional manner. ~The materials ~
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are evaporated sequentially in a vacuum chamber carrying the substrates which are to be coated to form the coating of the present invention thereon. Thus, the coatings 26, 27, 28 and 29 can be deposited in that order in the vacuum chamber.
The first dielectric layer 26 performs an adhesion function for the bilayer as well as protecting the bilayer from air and from the chemicals that are in the glass substrate. The second or outer layer 29 serves as a protect-ive layer for the bilayer and protects it from the atmos-phere. Thus, it can be seen that two dielectric layers stabilize the coating in that they provide mechanical durability as well as protection. I In addition, they serve ~ -an optical function by reducing the reflection from the -~
coating from the side viewed by the viewer as shown in Figure ~.
When the coating is completed, the bilayer pro- -vides the principal properties which are desired from the ;~--filter; however, the two dielectric layers give the coating the desired hardness, durability and stability. It has been found that the copper layer by itself even with the nickel pre-coat layer 27 is unstable. It will oxidize ln the --atmosphere. In addition, it will not pass humidity tests and abrasion tests. By surrounding the bilayer with ~he dielectric layers, the coating will readily pass conven-tional humldity, hardness and abrasion tests for thin fllm coatings. However, the most important characteris~ic is that the two dielectric layers make the coating stable so that the coating will not oxidize and so that it can be utilized in~high temperature environments. ~ ~-~ : .

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Coatings such as that provided in Figure 2 have been capable of withstanding a standard 20 rub abrasion test, a 24 hour humidity test, and a tape test. They have also been able to withstand a high temperature test which consisted of baking the filter at a temperature of 350C in a normal atmosphere for 1000 hours. Such a high temperature test had little or no effect upon the emissivity of the filter.
After the coating 18 has been applied to the substrate 16, the substrate 16 with the coating thereon and the pane 17 are sealed around their outer peripheries and are positioned in such a manner that there is provided a space 31 between the coating and one surface of the pane ].7 which is filled with air normally at atmospheric pressure.
Thus, it can be seen that the coating is provided on the inside of the pane 16 and is exposed to the air in the space 31 between the panes 16 and 17. The assembly can then be incorporated in an oven door as shown in Figure 1. The coating 18 has a very low visual reflection when viewed from the outside as shown in Figure 2. This is a very desirable characteristic. In addition, the reflection, if possible, -..
should be without color, i.e. neutral.
:' -The spectrum performance of a filter such as that shown in Figure 2 is shown in Figure 3. As can be seen, the graph shown in Figure 3 covers the wavelength region from approximately 400 millimicrons to 2500 millimicronsO Four curves are shown in Figure 3 with two of the curves being transmission curves and two of the curves being reflection curves. Two of the curVes deal with an embodiment of the ~-invention hereinafter described, whereas the other two , ~0557~

curves 32 and 33 deal with the embodiment shown in Figure 2.
Curve 32 is the curve which shows the reflection for one design incorporating the present invention and shows that the reflection at 400 millimicrons is 8% and that it is relatively neutral until approximately 600 millimicrons, after which the reflection begins to climb and climbs steadily through 2500 millimicrons where the reflection is approximately 69~. The transmission curve 33 shows that the ~ -transmission starts off at 400 millimicrons at approximately - -63% and then goes up to a peak transmission of approximately 75~ at 600 millimicrons and then falls off throu~h the infra-red region until at 2500 millimicrons that trans-mission is approximately 13%. Thus, it can be seen that the filter does not have neutralness of transmission throughout the visible region. In other words, the transmission is not `
flat across the visual spectrum. --In comparing the characteristics of this filter with the filter which is disclosed in copending application Serial No. 71,009, filed September 10, 1970, since issued on August 8, 1972 as U.S. Patent No. 3,682,528, it can be seen that the present filter utilizing copper does not have the neutralness of transmission in the visible spectrum but has an appearance which is slightly coppery in transmission. In addition, the transmission of the copper design does not fall off as rapidly as does the transmission utilizing silver and for that reason~the silver is more desirable for use with high temperature sources such as a tun~sten light bulb. The reflection of the copper filter of the present invention and the siIver filter in the earlier filed appli~-ation are similar but it can be seen that the copper refle~-. . ':,:
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tion does not increase as rapidly as the reflection of the silver filter. This is relatively unimportant in many applications as, for example, where warm body sources are utilized. In such applications, it is necessary that the high reflection be obtained at wavelengths of 4 microns and greater, in which region the reflection of the copper filter is just as great as the silver filter. Approximately 95~ of the power spectrum of a black body having a temperature of approximately 200C is between 4 to 50 microns with the peak radiation being at approximately 6 microns. Thus, it can be seen for a filter to have a coating which has good emissiv-ity for applications such as for oven doors, it must have low emissivity in the region of 4 to 50 microns. This temperature of 200C for a cool black body is low in com-parison to an incandescent source such as tungsten which has a black body temperature of approximately 3200K. In the case of a black body of this latter temperature, 85% of the energy falls at wavelengths less than 2.5 microns. It is necessary to have good infra-red reflection between 0.7 and 2.5 microns for heat rejection. For applications such as oven doors, it is unnecessary to have high rejection until approximateIy the 4 micron wavelength region.
The filter of Figure 2 has a distinct advantage in that the materials from which it is formed are extremely inexpensive. In addition, the materials are easy to evapor-ate and control so that the yield is very high.
In Figure 4, there lS shown a graph which depicts the heat properties of a filter made in accordance with .
Figure 2. The visual properties of the filter are shown in Figure 3 in which the visual region is represented b~ the ~5S79~4 400 to 700 millimicron region. To determine the heat reflecting properties of the filter, it is necessary to examine the region from ~ to 50 microns. The curve 34 in Figure 4 shows the heat reflectance and the emissivity for the region from 4 to 50 microns and shows that the reflect-ance is approximately 0.85 and the emissivity is 0.15. The emissivity as defined is one minus the reflectance.
When it is desired to utilize the filter in environments having temperatures substantially above 350C., it is desirable that another filter incorporating the invention of the type shown in Figure 5 be utilized. The coating 36 shown therein is formed on the surface 22 of the substrate 16 and consists of at least five layers 37, 38, 39, 41 and 42. Since gold is utilized in place of copper in this design as hereinafter described, and since the gold is ' , '' intrinsically very soft, it is necessary to add to the four layers of the previous embodiment at least one additional layer to obtain the necessary hardness and durability for the coating. Also, because gold has different optical '`
properties from that of copperl it is necessary to utilize different materials and a different design in order to obtain a low visual reflectlon through the glass substrate~
16.
The first la~yer 37~of the coating 36 counting from the substrate is formed of titanium dioxide with an~index of '-` refraction of 2.3. This,layer 37 has a physical thickness ~
of 29.4 m~ microns and a~quarterwave optical thlckness of ,, 270.~8 milllmiorons which~'can~vary + 30~ and~ still obtain satisfactory results. The second or next layer counting~ ~
30 ~ from the substrate is layer 38 which is a thin nucleation ~ ~--

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i5~4 layer formed of a suitable material such as nickel as hereinbefore explained. This thin nucleation layer makes it possible to obtain bulk optical properties from a thin layer of gold. The thin layer 38 can have the same thickness as described in connection with the previous embodiment.
The next layer is layer 39 formed of gold having a physical thickness of 4.4 millimicrons. This is the optimum thickness for the gold layer. However, if desired, the -layer can vary from 2.0 millimicrons to approximately 30 millimicrons in thickness. In the same manner as with the copper layer, the emissivity properties of the coating can be improved by sacrificing visual transmission.
The next layer 41 which is the layer next adjacent to the gold layer is formed of yttrium oxide (Y2O3) Which has an index of refraction of 1.9. This layer of yttrium oxide has a physical thickness of 166 millimicrons and a quarterwave optical thickness of 1260.60 millimicrons. This layer is provided for hardness and gives the desired stabil-ity and hardness to the coating. The optical thickness and physical thickness of this layer can vary within + 204 without too seriously impairing the properties of the coating. ~ ~ ~
The last layer, layer 42, is a dielectric layer -and is utilized for optically~matching the coat1ng to air and consists of a dielectric~layer formed of ma~nesium~
fluoride (MgF2~ which has an index of refraction of 1.38. ~ ;
This magnesium fluoride layer has a physical thickness of 80.15 millimicrons and a quarterwave optical thickness of 442.428 millimicrons. Por the MgF2 the physical~thiokness 3~ and the quarterwave optical thickness oan be varied within , ~ ~ .
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+ 30% while still obtaining acceptable results. The optimum thicknesses set forth give a low visual reflectlon with the most neutral coating. As soon as these optimum thicknesses are changed, the visual reflectance is increased and there is a loss of neutrality in color. In certain applications, it may be desirable to obtain a specific color by reflection and this can be obtained by changing the thicknesses of the dielectric layers.
The layers forming the coating 36 also can be evaporated in a conventional manner in a vacuum chamber by sequentially evaporating the materials onto the substra~e.
The spectral performance of a filter o the type shown in Figure 5 is shown in Figure 3 in which the curve 46 represents the reflection obtained from the gold filter and -the curve 47 represents the transmission ~7hich is obtained ~rom the ~old filter. It can be seen that the transmission by the gold filter is very similar to the transmission by the copper filter. It starts out at 4Q0 millimicrons with a -transmission of approximateIy 60%. It peaks at a trans-mission peak of 75% at 575 to 580 millimicrons and then falls off in the infra-red. Initially, it falls off more rapidly than the copper but then it falls off less rapidly ~ -so that a 2500 millimicrons, the transmission is approx-imately 18~.~ The reflection for the gold starts at 400 ` 25 millimicrons at 16% and decreases to a minimum of 550 millimicrons at approximately 4-1/2%, and then increases until approximately 1000 millimicrons and then decreases until approximateIy 1700 mlllimicrons and then increases to 2500 millimicrons, at which time the reflection is approx-imately 42%. The reflection continues to increase until ~ ' ' '' ` -~ . . ~:
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when 4 microns is reached, which is a critical region, the reflection of the gold is just as good as the copper and ~ -both reflect at 85% as represented by the curve 34 in Figure 4. ~he emissivity is ]ess than .2, i.e., .15.
Thus it can be seen that the gold also provides a very satisfactory thermal con-trol filter and that it can be utilized in the same manner as the filter which is shown in Figure 2. The only difference is that the coating is comprised of more layers which makes it more expensive and, in addition, it requires the use of gold which is more expensive than copper. It, however, does have the advantage in that it can be utilized in much higher temperature environments as, for example, tempexatures ranging from 250C to 550C.
It is apparent from the foregoin~ that there has been provided a new and improved thermal control filter which is particularly adapted for energy emitted from black body sources having temperatures substantially less than the temperatures of incandescent lamps. The filter utilizes a coating which is relatively inexpensive and which can be readily produced with a high yield. It will withstand the conventional humidity and abrasion tests and, in addition, -can withstand high temperatures in excess of 300C for extended periods of time. The coating has low emissivity which makes it possible to obtain a drastic reduction in surface temperature of windows which are exposed to heat. ~ ~-The optical characteristics are determined primarily by the single metal layer that it utilizes which consists of either copper or gold.
Although the principal emphasis on the present . ~ . .

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invention has been directed to the utilization of the filter in conjunction with black body sources having temperatures in excess of 200C, the coatings are also particularly adaptable for use with architectural glass to control the heat input through a window due to solar radiation which is in the region from 400 to 2500 millimicrons and is generally characterized as the shading coefficient of the architect-ural glass. The thermal through-put of heat through the glass due to the temperature difference existing on opposite sides of the glass is determined by the transfer of energy in the region from 4 to 50 microns and is often character-ized as the U-factor of architectural glass. ~s can be seen from the curves shown in Figures 3 and 4, architectural glass with coatings thereon incorporating the present -invention would have excellent shading coefficients and U-factors.

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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a thermal control filter, a substrate formed of a material capable of transmitting visible energy, said substrate having a surface and a coating formed on said surface, said coating comprising at least five layers counting from the substrate, said first layer being formed of a dielectric material, said second layer being formed of a thin nucleating layer being thin enough so that it is substantially ineffective in changing the optical properties of the filter, said third layer being formed of gold having a thickness ranging from 2 to 30 millimicrons, said thin nucleating layer serving as a nucleating layer for said gold layer, said fourth layer being formed of yttrium oxide, and said fifth layer being formed of a dielectric material.

2. A filter as in Claim 1 wherein said first layer is formed of titanium dioxide and said fifth layer is formed of magnesium fluoride.

3. A filter as in Claim 2 wherein said first layer has an optimum physical thickness of 29.4 ? 9 milli-microns and said fifth layer has an optimum physical thick-ness of 80.15 ? 24 millimicrons.

4. A filter as in Claim 1 wherein said gold layer has an optimum thickness of approximately 4.4 milli-microns.

5. A filter as in Claim 1 wherein said yttrium oxide layer has a thickness of approximately 166 ? 34 millimicrons.