High modulus, high thermal conductivity radiative passive coolant
A polymer bilayer includes a layer of a porous fluoropolymer directly overlying a layer of polyethylene. The polyethylene layer may be porous or dense and may include an ultra-high molecular weight polymer. The polymer bilayer may be co-integrated with structures (e.g., wearable devices) exposed to high thermal loads (>0-1000 W/m 2 ) and provide passive cooling thereof. For instance, passive cooling of AR/VR glasses under different solar loads may be achieved by a polymer bilayer that is both highly reflective across solar heating wavelengths and highly emissive in the long-wavelength infrared. The high reflectance decreases energy absorption across the solar spectrum while the high emissivity promotes radiative heat transfer to the surroundings.
1. A layered structure comprising:
an optically transparent layer comprising ultra-high molecular weight polyethylene; and
an IR reflective layer overlying the optically transparent layer, wherein a thickness of the optically transparent layer ranges from approximately 10 micrometers to approximately 1 mm and a thickness of the IR reflective layer ranges from approximately 0.2 mm to approximately 1 mm, the IR reflective layer comprises an IR reflective PVDF foam, and the layered structure has a short wavelength (0.25<λ<5 μm) infrared reflectance of at least approximately 10%, and a long wavelength (8<λ<14 μm) infrared reflectance of less than approximately 10%.
2. The layered structure of claim 1 , wherein the optically transparent layer has a thermal conductivity of at least approximately 5 W/mK.
3. The layered structure of claim 1 , wherein the optically transparent layer has an elastic modulus of at least approximately 2 GPa and a tensile strength of at least approximately 0.7 GPa.
4. The layered structure of claim 1 , wherein the IR reflective layer comprises an IR reflective paint overlying the optically transparent layer.
5. The layered structure of claim 4 , wherein the IR reflective paint comprises an organic dye or nanoscale particles of a metal oxide.
6. The layered structure of claim 1 , wherein the PVDF foam comprises a PVDF-based polymer selected from the group consisting of PVDF-CTFE, PVDF-HFP, PVDF-TFE, PVDF-TrFE, PVDF-TrFE-TFE, and combinations thereof.
7. The layered structure of claim 1 , wherein the PVDF foam has a porosity of at least approximately 15 vol. %.
8. The layered structure of claim 1 wherein the PVDF foam comprises pores having an average pore size of at least approximately 100 nm.
9. The layered structure of claim 1 , wherein a thickness of the optically transparent layer is less than a thickness of the IR reflective layer.
10. A device comprising the layered structure of claim 1 , wherein the device is selected from the group consisting of smart watches, and virtual reality (VR), augmented reality (AR), and mixed reality (MR) glasses and headsets.
11. A layered structure comprising:
an optically transparent layer of ultra-high molecular weight polyethylene; and
an IR reflective layer directly overlying the optically transparent layer, wherein a thickness of the optically transparent layer ranges from approximately 10 micrometers to approximately 1 mm and a thickness of the IR reflective layer ranges from approximately 0.2 mm to approximately 1 mm, the optically transparent layer has a thermal conductivity of at least approximately 5 W/mK and an elastic modulus of at least approximately 2 GPa, and the IR reflective layer comprises an IR reflective PVDF foam.
12. The layered structure of claim 11 , wherein the layered structure has a short wavelength (0.25<λ<5 μm) infrared reflectance of at least approximately 10%, and a long wavelength (8<λ<14 μm) infrared reflectance of less than approximately 10%.
13. The layered structure of claim 11 , wherein the IR reflective layer comprises an IR reflective paint.
14. A method comprising:
forming an optically transparent layer of ultra-high molecular weight polyethylene; and
forming an IR reflective layer over the optically transparent layer to create a layered structure having a short wavelength (0.25<λ<5 μm) infrared reflectance of at least approximately 10% and a long wavelength (8<λ<14 μm) infrared reflectance of less than approximately 10% a thickness of the optically transparent layer ranges from approximately 10 micrometers to approximately 1 mm and a thickness of the IR reflective layer ranges from approximately 0.2 mm to approximately 1 mm, and, wherein forming the IR reflective layer comprises forming a PVDF foam layer by inversion phase separation of a polymer solution in a non-solvent bath.
15. The method of claim 14 , wherein forming the IR reflective layer over the optically transparent layer comprises laminating the IR reflective layer to the optically transparent layer.
16. The method of claim 14 , further comprising forming a layer of a pressure sensitive adhesive or a layer of an optically clear adhesive between the optically transparent layer and the IR reflective layer.
17. The method of claim 14 , wherein forming the optically transparent layer comprises vacuum compression molding a fibrous polyethylene mat.