Radiative Cooling Panels For Spacecraft

Abstract

A self-adjusting passive radiative cooling panel for spacecraft including a dielectric (e.g., HfO.sub.2) layer sandwiched between a mirror layer and spaced-apart thin-film phase-change (e.g., thermochromic) material islands disposed in a grating pattern having a lattice constant in the 2 to 10 μm range, depending on expected spacecraft operating temperatures. At low temperatures the phase-change material islands enter dielectric state phases that prevent generation of guided modes in the dielectric layer resulting in zero or low mid-IR emission. At high temperatures the phase-change material islands enter a metal state phase that couples mid-IR (thermal) radiation to guided mode resonances resulting in high mid-IR emission. The thermal emission can be tuned by the lattice constant of the grating pattern to peak at a target mid-IR wavelength (e.g., 8 μm), thereby significantly increasing the thermal emission contrast between the low and high temperature states resulting in the minimization of system-wide thermal transients.

Claims

1. A radiative cooling panel configured to selectively emit thermal radiation having a peak mid-IR wavelength approximately equal to an associated blackbody spectrum peak wavelength corresponding to an anticipated operating temperature of said panel, said panel comprising: a mirror layer; a dielectric layer disposed over an upper surface of the mirror layer and consisting essentially of a low-loss dielectric material; and a plurality of phase change islands disposed on an upper surface of the dielectric layer and arranged in an array pattern having a lattice constant in the range of 2 to 10 μm, wherein said plurality of phase change islands comprise a phase change material that transitions between a first phase and a second phase in response to an external stimulus, and wherein dielectric layer and said plurality of phase change islands are operably configured such that: when said phase change material is in said first phase, said phase change material decreases coupling of guided modes in said dielectric layer, whereby said radiative cooling panel emits a minimal amount of said thermal radiation, and when said phase change material is in said second phase, said phase change material increases coupling of guided modes in the said dielectric layer, and causes said guided modes to achieve a peak absorption/emission resonance approximately equal to said peak mid-IR wavelength, whereby said radiative cooling panel emits relatively large amounts of said thermal radiation.

2. The radiative cooling panel according to claim 1, wherein said plurality of phase change islands occupy a fill factor portion of a total area of said upper surface of said dielectric layer, said fill factor portion being in the range of 20% and 80%.

3. The radiative cooling panel according to claim 1, wherein said mirror layer comprises at least one of silver, aluminum and gold.

4. The radiative cooling panel according to claim 1, wherein said dielectric layer comprises one of Hafnium Dioxide (HfO.sub.2) and Potassium Bromide (KBr).

5. The radiative cooling panel according to claim 1, wherein said dielectric layer has a thickness smaller than said target nominal wavelength divided by a refractive index of said dielectric material at said anticipated operating ambient temperature and said peak mid-IR wavelength.

6. The radiative cooling panel according to claim 1, wherein said phase change material comprises a thermochromic material having a transition temperature, and wherein said external stimulus comprises an ambient temperature of said radiative cooling panel, whereby said thermochromic material transitions from said second phase to said first phase when said ambient temperature decreases from above to below said transition temperature, and said thermochromic material transitions from said first phase to said second phase when said ambient temperature increases from below to above said transition temperature.

7. The radiative cooling panel according to claim 6, wherein said thermochromic material comprises at least one of VO.sub.2, V.sub.2O.sub.3, V.sub.2O.sub.5, V.sub.6O.sub.13 and Ti.sub.nO.sub.2n+1.

8. The radiative cooling panel according to claim 1, wherein each said phase change island comprises a thin-film structure comprising Vanadium Dioxide (VO.sub.2) and having a thickness in the range of 20 nm and 100 nm.

9. The radiative cooling panel according to claim 8, wherein each said thin-film structure further comprises a dopant incorporated into said VO.sub.2 such that said transition temperature of said doped VO.sub.2 is in the range of 20° C. and 30° C.

10. The radiative cooling panel according to claim 1, wherein said dielectric layer comprises Hafnium Dioxide (HfO.sub.2), and wherein each said phase change island comprises a thin-film Vanadium Dioxide (VO.sub.2) structure that is either embedded into or disposed on top of said upper surface of said dielectric layer.

11. The radiative cooling panel according to claim 10, wherein each said thin-film VO.sub.2 structure comprises Vanadium Dioxide (VO.sub.2) doped with Tungsten (W).

12. The radiative cooling panel according to claim 1, further comprising one or more outer layers disposed over the plurality of phase change islands, said one or more outer layers comprising at least one of a conductive material layer, a solar reflective material layer and a protective material layer.

13. A self-adjusting passive radiative cooling panel configured to generate relatively low thermal radiation emissions when subjected to ambient temperatures below a predetermined median operating temperature, and to generate substantially higher thermal radiation emissions having a mid-IR peak wavelength when subjected to ambient temperatures above said predetermined median operating temperature, said panel comprising: a mirror layer comprising a reflective metal; a dielectric layer disposed over an upper surface of the mirror layer; and a plurality of spaced-apart phase change islands disposed in an array pattern having a lattice constant on an upper surface of the dielectric layer, wherein said dielectric layer comprises a low mid-IR loss dielectric material having a thickness that is less than said mid-IR peak wavelength, wherein each island of said plurality of phase change islands includes a thin-film structure consisting of one or more thermochromic materials configured to change from a dielectric state to a metal state when said ambient temperatures increases from below said predetermined median operating temperature to above said predetermined median operating temperature, and wherein said lattice constant of said plurality of phase change islands is set such that, when said thermochromic material is in a metal state, said plurality of phase change islands cause guided modes is said dielectric layer to achieve a peak resonance approximately at said mid-IR peak wavelength.

14. The radiative cooling panel according to claim 13, wherein said plurality of phase change islands occupy a fill factor portion of a total area of said upper surface of said dielectric layer, said fill factor portion being in the range of 20% and 80%.

15. The radiative cooling panel according to claim 13, wherein said mirror layer comprises at least one of silver, aluminum and gold.

16. The radiative cooling panel according to claim 13, wherein said dielectric layer comprises one of Hafnium Dioxide (HfO.sub.2) and Potassium Bromide (KBr).

17. The radiative cooling panel according to claim 13, wherein the thin-film structure of each said phase change island has a thickness in the range of 20 nm and 100 nm.

18. The radiative cooling panel according to claim 17, wherein each said thin-film structure further comprises a dopant incorporated into said thermochromic material such that said doped thermochromic material has a transition temperature in the range of 20° C. and 30° C.

19. The radiative cooling panel according to claim 13, further comprising one or more outer layers disposed over the plurality of phase change islands, said one or more outer layers comprising at least one of a conductive material layer, a solar reflective material layer and a protective material layer.

20. A self-adjusting passive radiative cooling panel comprising: a mirror layer; a dielectric layer disposed over an upper surface of the mirror layer and comprising Hafnium Dioxide (HfO.sub.2) having a nominal dielectric thickness; a plurality of spaced-apart phase change islands disposed on an upper surface of the dielectric layer and arranged in an array pattern having a lattice constant, wherein each island of said plurality of phase change islands consists of a thin-film structure comprising a thermochromic material selected from the group including VO.sub.2, V.sub.2O.sub.3, V.sub.2O.sub.5, V.sub.6O.sub.13 and Ti.sub.nO.sub.2n+1, wherein said nominal dielectric thickness of said dielectric layer and said lattice constant of said plurality of phase change islands are configured such that, when said thermochromic material is in a metal state, said plurality of phase change islands cause guided modes is said dielectric layer to achieve resonance at a peak mid-IR wavelength.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where:

[0022] FIG. 1 is a perspective top view showing a radiative cooling panel according to a simplified embodiment of the present invention;

[0023] FIG. 2 is an exploded perspective view showing the radiative cooling panel of FIG. 1;

[0024] FIGS. 3(A) and 3(B) are simplified cross-sectional side views depicting the radiative cooling panel of FIG. 1 during operation;

[0025] FIGS. 4(A), 4(B), 4(C) and 4(D) are a graphs respectively depicting modeled emission spectrums for exemplary radiative cooling panels;

[0026] FIG. 5 is perspective top view showing a radiative cooling panel according to an alternative simplified embodiment of the present invention; and

[0027] FIG. 6 is perspective top view showing a radiative cooling panel according to another alternative simplified embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0028] The present invention relates to improved passive cooling panels for spacecraft. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, directional terms such as “upper”, “top”, “lower”, and “bottom” are intended to provide relative positions for purposes of description, and are not intended to designate an absolute frame of reference. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.

[0029] FIG. 1 depicts a simplified radiative cooling panel 100 of the present invention. In operation, radiative cooling panel 100 is physically mounted on and operably thermally coupled to an underlying support surface 90 (e.g., an outer surface or other structure through which heat energy E.sub.T is operably transmitted to or from a spacecraft), and generally functions to selectively emit thermal radiation E.sub.R having a peak mid-IR wavelength λ.sub.R that is approximately equal to an associated blackbody spectrum peak wavelength λ.sub.P corresponding to an anticipated operating ambient temperature in which panel 100 is expected to operate. For example, as indicated in FIG. 1, when a host spacecraft is deployed in sunlight, radiative cooling panel 100 receives solar radiation E.sub.S having normal solar radiation wavelengths λ.sub.S(i.e., having a peak radiation level centered around 550 nm), which generally produces higher ambient operating temperatures than when the spacecraft is deployed out of direct sunlight.

[0030] For purposes of explaining associated details, panel 100 is not drawn to scale in the associated figures, and it is understood that actual panels would be much larger than that shown in the figures.

[0031] FIGS. 1 and 2 depict exemplary radiative cooling panel 100 in assembled and exploded views, respectively, and show that panel 100 essentially comprises a three-layer structure including a continuous bottom flat mirror layer 110, an intermediate planar dielectric layer 120, and an upper layer nominally defined by a patterned array of spaced-apart phase change islands 130. As indicated in FIG. 1, in practical applications mirror layer 110 is physically mounted on underlying support surface 90. As depicted in FIG. 2, dielectric layer 120 is disposed directly onto an upper surface 112 of mirror layer 110, and phase change islands 130 are embedded into an upper surface 122 of dielectric layer 120 such that uppermost surfaces 132 of phase change islands 130 are co-planar with upper surface 122 (i.e., such that panel 100 has a planar outer surface).

[0032] According to an aspect of the invention, mirror layer 110 includes a continuous (sheet-like) film comprising a material exhibiting total or near total reflectance of incident solar radiation E.sub.S. In a presently preferred embodiment, mirror layer 110 comprises a reflective metal (e.g., a pure metal such as silver, aluminum or gold, or an alloy thereof) having a suitable thickness t.sub.M (e.g., 200 nm), whereby mirror layer 110 exhibits both good solar reflection properties as well as good thermal conduction of heat energy E.sub.T to/from underlying support surface 90. Suitable metal films are fabricated, for example, using known integrated circuit fabrication processes (e.g., by way of sputter deposition of metal).

[0033] Referring to FIG. 1, dielectric layer 120 is preferably implemented using a low-IR loss dielectric material that exhibits a moderate refractive index in the mid-IR range (i.e., 7 to 13 μm), and is formed with a nominal thickness t.sub.S that is smaller than a targeted nominal emission wavelength (e.g., 8 μm) divided by the refractive index n of the dielectric material. As used herein, a low loss dielectric material is a dielectric material having a magnetic/electric loss tangent value of 0.1 or lower, and “exhibiting a moderate refractive index in the mid-IR range” is intended to mean a refractive index value in the range of 1.5 and 3. In a presently preferred embodiment, dielectric layer 120 is implemented using Hafnium Dioxide (HfO.sub.2) having a thickness t.sub.S that that facilitates the generation of guided resonances having target mid-IR peak wavelengths in the manner described below. To facilitate these resonances, the HfO.sub.2 layer is formed with a nominal thickness t.sub.S that is less that the target mid-IR peak wavelengths divided by the refractive index of the dielectric material (utilized to implement dielectric layer 120 at the spacecraft's anticipated operating ambient temperature and the target mid-IR wavelength, e.g., 800 nm). Although HfO.sub.2 having a thickness—determined by one of the above methods and more accurately by full-wave electromagnetic simulations—is currently believed to optimize the optical characteristics of panel 100, other dielectric materials and other dielectric thicknesses may be utilized.

[0034] Referring again to FIG. 1, spaced-apart phase change islands 130 are disposed on upper surface 122 of dielectric layer 120 and arranged in an array pattern having a lattice constant L that facilitates the generation of guided resonances in dielectric layer 120 having a peak resonance wavelength that is approximately equal to the target mid-IR wavelength. For most practical embodiments, utilizing the materials described herein, lattice constant L is in the range of 2 to 10 microns. Referring briefly to FIG. 2, note that the size (i.e., area defined by width dimensions W.sub.P) of each phase change island 130 is less important than lattice constant distance L, which defines the periodic spacing of islands 130 in the grating pattern, and in a presently preferred embodiment phase change islands 130 are preferably formed with areas that produce a fill factor (i.e., area covered by islands 130 divided by exposed dielectric layer surface 122) of approximately 50% (i.e., in the range of 20% and 80%).

[0035] According to an aspect of the invention, phase change islands 130 comprise a thermochromic (phase change) material (e.g., VO.sub.2, V.sub.2O.sub.3, V.sub.2O.sub.5, V.sub.6O.sub.13 and Ti.sub.nO.sub.2n+1) that transitions between a dielectric state (first phase) and a metal state (second phase) in response to an external stimulus (e.g., in response to a change in ambient temperature). That is, these thermochromic materials are characterized by a transition temperature, and change between the metal and dielectric states when heated above or cooled below the transition temperature.

[0036] As explained below, by selecting a thermochromic material whose transition temperature coincides, e.g., with an average spacecraft system operating temperature, the dielectric state at ambient temperatures below the transition temperature, phase change islands 130 effectively function to “turn off” thermal radiation emissions by way of decreasing or preventing guided modes in the dielectric material, and by entering the metal state at high temperatures, phase change islands effectively function to “turn on” thermal radiation emissions by way of exciting guided modes in the dielectric layer. Moreover, by forming phase change islands 130 at a lattice constant spacing L according to the methods described herein, phase change islands further function to promote the generation of guided resonances in dielectric layer 120 at or near peak blackbody emission frequencies that maximize cooling efficiency at a given operating temperature.

[0037] Referring to FIG. 2, according to a presently preferred embodiment, phase change islands 130 are implemented using thin-film Vanadium Dioxide (VO.sub.2) structures having a thickness t.sub.TFI in the range of 20 nm and 100 nm. As set forth above, VO.sub.2 has the beneficial characteristic of transitioning between a dielectric state (first phase) when subjected to ambient temperatures below its nominal transition temperature (i.e., approximately 66° C.) to a metal state (second phase) at ambient temperatures above its transition temperature. Moreover, VO.sub.2 has the added beneficial characteristic of having a transition temperature that can be adjusted by way of adding a suitable dopant during the thin-film formation process. For example, the transition temperature of VO.sub.2 is adjusted downward (e.g., into the range of 20° C. and 30° C.) by way of a tungsten dopant.

[0038] FIGS. 3(A) and 3(B) respectively depict panel 100 at two different time periods associated with two different ambient temperatures. Specifically, FIG. 3(A) depicts panel 100 at a time t0 when subjected to an ambient temperature T.sub.P0 that is below transition temperature T.sub.T of phase change islands 130, whereby panel 100(t0) is in a non-emitting mode. In contrast, FIG. 3(B) depicts panel 100 at a time t1 when subjected to an ambient temperature T.sub.P1 that is above transition temperature T.sub.T, whereby panel 100(t1) in a thermal radiation emission (cooling) mode.

[0039] Referring to FIG. 3(A), when phase change islands are at ambient temperature T.sub.P0 below transition temperature T.sub.T(i.e., T.sub.P0<T.sub.T), phase change islands 130 are in the dielectric state (i.e., first phase, indicated by “130(P1)” in FIG. 3(A)). When VO.sub.2 is in its dielectric state, its refractive index nearly matches that of HfO.sub.2, making the structure weakly absorbing, preventing generation of guided modes GM.sub.1 in dielectric layer 120, whereby panel 100 emits a minimal amount of thermal radiation (i.e., E.sub.R1(λ.sub.2R)˜0). Note also that almost all incident solar radiation E.sub.S1 passes through the dielectric material and is reflected by mirror layer 110. By Kirchhoff's law, absorptivity is equal to emissivity, so the emission is very low.

[0040] FIG. 3(B) shows panel 100 when phase change islands 130 are at ambient temperature T.sub.P1 above transition temperature T.sub.T (i.e., T.sub.P0>T.sub.T), and therefore transitioned to their metal state (i.e., second phase, indicated by “130(P2)” in FIG. 3(B)). In this state, the metal state of phase change islands 130 interacts with dielectric layer 120 in a way that produces substantial coupling of mid-IR guided mode resonances GM.sub.2 in dielectric layer 120. In addition, these guided resonances are “tuned” (i.e., their frequency is controlled) by the grating pattern of the phase change islands 130, and more particularly, by lattice constant L at which islands 130 are spaced, whereby the guided resonances achieve a peak resonance that approximately equals the targeted mid-IR peak wavelength (e.g., 8 μm). The guided modes in the dielectric layer are coupled to free space FS via the grating pattern utilized to form phase change islands 130. Accordingly, panel 100(t1) is in a thermal radiation emission mode that effectively draws thermal heat E.sub.T2 from underlying structures (not shown), and converts the thermal heat to thermal radiation E.sub.R2 that is emitted into free space FS.

[0041] FIGS. 4(A) to 4(D) are graphs showing modeled optical response characteristics for radiative cooling panels generated in accordance with the specific embodiments described above, where the respective graphs are modeled for different lattice constants L of 3 μm (FIG. 4(A)), 4 μm (FIG. 4(B)), 5 μm (FIG. 4(C)) and 6 μm (FIG. 4(D)) using a suitable dielectric thickness. Note that emissions are modeled for a temperature above the phase change material's transition temperature (i.e., 100° C.) to illustrate panel operating characteristics when in the emission (cooling) mode (i.e., as described above with reference to FIG. 3(B)), and for a temperature below the phase change material's transition temperature (i.e., 30° C.) to illustrate panel operating characteristics when in the non-emitting mode (i.e., as described above with reference to FIG. 3(A)). As indicated in these graphs, the respective peak mid-IR emission wavelengths λ.sub.R1 to λ.sub.R4 can be tuned and positioned at a desired emission wavelength by changing the lattice constant L. FIGS. 4(A) to 4(D) also demonstrate that panel emissions can be modulated from around 10% at low temperatures to about 90% at high temperatures.

[0042] FIG. 5 is perspective top view showing a radiative cooling panel 100A according to an alternative embodiment of the present invention in which phase change islands (e.g., thin-film VO.sub.2 pads) 130A are patterned on upper surface 122A of dielectric layer 120A (i.e., instead of embedded inside the dielectric material forming the dielectric layer, as described above with reference to FIGS. 1 and 2). As mentioned above, each thin-film VO.sub.2 structure 130B optionally includes a dopant (e.g., Tungsten) to adjust the associated transition temperature. Other structures of panel 100A are the same as those mentioned above.

[0043] In accordance with another exemplary alternative embodiment shown in FIG. 6, a radiative cooling panel 100B includes an additional (outer) layer 140B disposed over phase change islands 130B and exposed portions of upper surface 122B of dielectric layer 120B. Outer layer 140B (e.g., indium tin oxide) forms one or more of a conductive material layer, a solar reflective material layer and a protective material layer that provide added robustness, higher surface conductivity, and/or further increase solar reflectance of panel 100b. A sufficiently thin outer layer of this type will either not affect the optical performance, or the structural parameters can be adjusted to compensate for the presence of such a coating. Other structures of panel 100B are the same as those mentioned above.

[0044] Although the present invention has been described with respect to an exemplary specific embodiment, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well, all of which are intended to fall within the scope of the present invention. For example, mirror layer 110 (FIG. 1) may be implemented using a stack of alternating non-metal materials, but a reflective metal is preferred for performance and cost purposes. In addition, dielectric layer 120 may be implemented using dielectric materials other than HfO.sub.2 (e.g., Potassium Bromide (KBr)), but is preferably formed using a low mid-IR loss (i.e., having a magnetic/electric loss tangent value of 0.1 or lower). In addition, phase change islands 130 may be implemented using thermochromic materials other than those specified above, or using other types of phase change materials controlled by external stimuli other than ambient temperature (e.g., passively in response to another environmental condition such as ambient light, or actively in response to a stimulus generated by a control circuit). In addition, although the phase change islands are depicted in the figures as square-shaped pads, the phase change islands may have any operable shape (e.g., rectangle or circle).