Testing of Thermochromic Materials with Passive Radiative Cooling Regulation

open_access
Open Access
Author:
Shi, Yifan
Area of Honors:
Mechanical Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
Linxiao Zhu, Thesis Supervisor
Yuan Xuan, Thesis Honors Advisor
Keywords:
Thermochromic film
Mechanical Engineering
VO2
Abstract:
This thesis investigates into the change of optical properties of a thermochromic film consisting of vanadium dioxide (VO2) due to temperature change. Thermochromic film is a multilayer coating made of material which will change phase depending on the temperature. Beyond a transition temperature, VO2 is metallic with a rutile phase (R). On the other hand, below the transition temperature, VO2 is insulating with a monoclinic phase (M). Without doping, the transition temperature between monoclinic phase and rutile phase is around 68 °C. Doping VO2 with W, Ti, Mg, and Al can lower the transition temperature toward room temperature. In our simulation and experimental studies, we used tungsten doped vanadium dioxide (W-VO2) and assume its transition temperature is 30 °C. We took two methods to analyze the optical properties of the thermochromic film - MATLAB simulation for VO2/MgF2/Al multilayer coating, and experimental attempts for VO2/PMMA/Si fabrication and optical properties measurement. Both MgF2 and PMMA are largely lossless in the mid infrared wavelengths. Silicon (Si) and aluminum (Al) serve as substrates. Aluminum has highly reflective in the infrared, which is used as substrate for our battery thermal management application. Si is transparent in the mid infrared, and we use it as a substrate for thermochromic window application. In MATLAB simulation, we optimize the thickness of VO2 such that the emissivity above transition temperature is close to 1 and emissivity below transition temperature is close to 0. The thickness of MgF2 is judicially chosen to form a Fabry-Perot resonance at 10 μm. Next, we obtained the emissivity of the thermochromic film in both high emissivity case (εhigh) and low emissivity (εlow) case. Based on the emissivity profiles, we calculate the radiation power emitted ii out from battery to surrounding Pbattery, the power absorbed by the battery from the ambient Pambient. Under static air condition, we calculated the state steady temperature of the battery. By comparing the dynamic thermal management provided by the thermochromic film with static coatings, we show that the thermochromic film provides dynamic thermal management for the battery. In experimental attempts, we fabricated the VO2/PMMA/Si sample using spin coating. To examine the emissivity, change of the thermochromic film in hot and cold ambient temperatures, we placed the sample on heat plate, heated it up and captured its temperature under 8-13 μm through infrared camera. We cannot observe clear change of the slope of the signal across a large temperature range. It might be because the coating consisting of W-VO2 and PMMA is not uniform. This research discussion concludes by confirming the advantage of using thermochromic film for dynamic battery thermal management through MATLAB simulation. The overall goal is to contribute to test the optical properties of W-VO2 in various potential applications. The findings of this thesis can be useful for developing dynamic photonic structures for thermal management applications.