Low-emissivity | My Website

Low-Emissivity Materials: Thermal Mirrors for Everyday Life

We all know the feeling: standing near a hot wall in summer, sitting beside a cold window in winter, or wearing clothes that somehow feel too hot outside but not warm enough indoors. A large part of this comfort problem comes from thermal radiation—the invisible infrared light constantly exchanged between our bodies, buildings, and the surrounding environment.

Low-emissivity, or low-E, materials work like thermal mirrors. They do not simply block heat like a thick blanket; instead, they reflect infrared radiation to reduce unwanted heat exchange. This makes them useful for keeping buildings cooler in summer and warmer in winter, and for making clothing more thermally comfortable. My work explores both new and traditional ways to create low-E materials, from smart building coatings to wearable textiles.

Metal-Free Low-E Coatings for Smart Buildings

Conventional low-E coatings usually rely on metals or conductive oxides, which are excellent thermal mirrors but unfortunately also block wireless signals. That is not ideal for modern buildings full of Wi-Fi, 5G, sensors, and smart devices.

To address this trade-off, we developed a metal-free polymer–ceramic coating made from zinc sulfide particles, polyethylene, and tiny air voids. Instead of using free electrons in metals, this coating reflects thermal infrared radiation through multiple scattering—somewhat like how snow looks white because it scatters visible light many times. The result is a low-E coating that provides thermal insulation while still allowing radio-frequency signals to pass through. In simple terms: a wall that reflects heat, but does not kill your Wi-Fi.

All-Polymer Low-E Textiles

Low-E clothing is usually made by adding metallic layers, similar to the shiny emergency blankets used after marathons. These materials can reflect body heat, but they are often shiny, stiff, and vulnerable to washing or mechanical damage.

We developed an all-polymer low-E textile based on polyethylene microfibers. By controlling the molecular structure, crystallinity, and fiber size, the textile can scatter and reflect human-body thermal radiation without using any metal. It looks like an ordinary fabric, but behaves like a soft thermal mirror. This approach opens a route toward durable, washable, and lightweight clothing that can reduce radiative heat gain in hot urban environments and suppress body heat loss in cold conditions.

Radiative Warming and Energy-Harvesting Textiles

In an earlier work, we used a more traditional low-E strategy based on a metallic conductive textile. The metal layer reflects infrared radiation and reduces heat loss from the human body, while a photothermal layer absorbs sunlight and converts it into warmth. We further integrated a triboelectric layer, allowing the textile to harvest small amounts of electricity from body motion.

This textile was designed for cold environments: it keeps the wearer warm through both passive radiative insulation and solar heating, while also offering a simple route toward self-powered wearable electronics. Compared with the newer metal-free studies above, this work used a more conventional low-E mechanism, but combined it with multifunctional textile design.