The modern thermos flask was invented in 1892 by Scottish scientist Sir James Dewar.
At the time, he was conducting research on liquefying gases. To liquefy gases at low temperatures, a container that could isolate the gas from the external temperature was first needed. He commissioned glassmaker Burgh to blow-blow a double-walled glass container, coating the inner walls of both layers with mercury, and then evacuating the air between the layers to create a vacuum. This vacuum flask, also called a "Dewar flask," can maintain the temperature of the liquid inside for a certain period, regardless of whether it is cold or hot. Because thermos flasks are mainly used in the home to keep hot water warm, they are also called hot water bottles.
The structure of a thermos flask is not complicated. It consists of a double-walled glass liner, with a vacuum between the two layers, and the liner is plated with silver or aluminum. The vacuum prevents heat convection, and since glass itself is a poor conductor of heat, the silver plating reflects heat radiated from the inside of the container back into the container. Heat transfer mainly occurs through three methods-conduction, convection, and radiation. When the space between the two container walls is a vacuum, the absence of air molecules naturally blocks heat conduction and convection. Adding a reflective layer to the inner wall further reduces heat radiation, resulting in a highly efficient and inexpensive insulated container. Conversely, if the bottle contains a cold liquid, it prevents external heat from radiating into the interior. The stopper of an insulated bottle is typically made of cork or plastic, neither of which are good conductors of heat.