Passive Thermal Switch Coating for Solar Reflection and Emissivity Switching
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Solution Overview
Problem
Existing thermal coatings face challenges in achieving high thermal emissivity contrast and efficient radiative cooling, particularly due to high solar absorptance in the visible to near-infrared region and limited performance in the mid-infrared region, which affects applications such as solar cells and batteries.
Innovation Solution
A passive thermal switch coating is developed using a double-layer thermochromic oxide structure with an infrared-transparent dielectric material, incorporating high contrast gratings and Fabry-Perot dielectric thin films to achieve high reflectance in the visible-to-near-infrared region and high emissivity in the mid-infrared region, utilizing vanadium dioxide (VO2) for phase transition and optimized grating arrangements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If thermochromic oxide layers are used to achieve high thermal emissivity at hot temperatures, then thermal emission switching capability is improved, but absorption in the visible to near-infrared region increases
Solution Approach 1:
The coating is divided into multiple functional layers: a first thermochromic oxide layer, an infrared-transparent dielectric material layer, and a second thermochromic oxide layer. Each layer serves a specific function - the thermochromic layers provide temperature-dependent emissivity switching, while the dielectric layer provides broadband reflection in the visible-to-near-infrared region and transparency in the mid-infrared region. This segmentation allows independent optimization of each layer's properties to resolve the contradiction between high thermal emissivity and low visible-NIR absorption.
Solution Approach 2:
The patent employs a composite structure combining thermochromic oxide materials (such as VO2) with infrared-transparent dielectric materials. This composite approach enables the coating to simultaneously achieve high thermal emissivity at hot temperatures through the thermochromic layers while maintaining low absorption in the visible-to-near-infrared region through the dielectric layer's broadband reflection properties.
2Object-affected harmful factors
If broadband reflection in visible-to-near-infrared region is implemented to block sunlight, then solar absorption is reduced, but thermal emission switching capability may be compromised
Solution Approach 1:
The coating structure separates the solar reflection function (performed by the dielectric material layer with high contrast grating structures) from the thermal emission switching function (performed by the thermochromic oxide layers). This segmentation allows the dielectric layer to provide broadband reflection in the visible-to-near-infrared region to block sunlight, while the thermochromic layers independently provide temperature-dependent emissivity switching in the mid-infrared region.
Solution Approach 2:
The infrared-transparent dielectric material layer acts as an intermediary between the sunlight blocking function and the thermal emission switching function. It provides broadband reflection in the visible-to-near-infrared region to prevent solar absorption, while simultaneously maintaining transparency in the mid-infrared region to allow the thermochromic layers to perform thermal emission switching without interference.
3Temperature
If layer thickness and grating parameters are optimized for enhanced thermal performance, then emissivity contrast is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent systematically optimizes key parameters including the thickness of each layer (first thermochromic oxide layer, dielectric material layer, second thermochromic oxide layer) and the grating parameters (period, depth, duty cycle) to achieve maximum emissivity contrast. By carefully controlling these parameters, the coating achieves record-high emissivity contrast exceeding 0.5, with the dielectric layer thickness optimized for both broadband solar reflection and mid-infrared transparency.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The coating achieves a record-high emissivity contrast of 0.9 to 0.1 between hot and cold states, reducing solar absorptance and enhancing thermal emission switching, suitable for self-cooling of solar cells, batteries, and electrical devices.
Implementation Method 1
insulator-to-metal temperature phase transition vanadium dioxide (VO2) can enable radiative property switching in the mid- to far-infrared wavelengths
Implementation Method 2
identification of a monolithic high-performance turn-down thermal emittance coating can be no more than 2 μm thick, consisting of a VO2 sub-wavelength nanowire grating array on an index-matched Fabry-Perot dielectric thin film
Implementation Method 3
The sunlight absorption can be prevented by implementing the structure that creates high contrast in refractive index using Si and Ge. The near-wavelength high contrast grating and prism array provides less solar absorption, but full transparency in mid-infrared region
Implementation Method 4
this coating enables responsive passive radiative cooling at high temperatures exceeding transition
Data Source
AI summary
A passive thermal switch coating can include: a first thermochromic oxide layer; a layer of an infrared-transparent dielectric material; and a second thermochromic oxide layer. The infrared-transparent dielectric material can be disposed between the first thermochromic oxide layer and the second thermochromic oxide layer. The thermal switch coating can also include a substrate, and the second thermochromic oxide layer can be disposed on the substrate.