Thermoses, also known as vacuum flasks, are designed to maintain the temperature of their contents. Whether keeping beverages steaming hot during a chilly morning commute or refreshing drinks cold on a warm afternoon, these vessels significantly slow down temperature changes.
Key Materials and Their Roles
Thermoses are constructed from several materials, chosen for their insulating properties. The outer casing often consists of durable stainless steel or various plastics. Stainless steel, particularly types like 304 (18/8 or 18/10), is a popular choice due to its resistance to corrosion, resilience against impacts, and hygienic properties. Plastic provides a lighter and often more economical alternative, protecting internal components.
The inner flask can be made from either glass or stainless steel. Glass, particularly borosilicate glass, was traditionally favored for its insulating qualities and its non-reactive surface. Modern designs often use double-walled stainless steel for the inner container, offering superior durability and shatter resistance compared to glass. Stainless steel is also hygienic and non-reactive.
The reflective coating applied to the surfaces facing the vacuum gap minimizes heat transfer. This coating, often made of silver, reflects thermal radiation, preventing heat from radiating. This mirrored surface is strategically placed on the inner surface of the outer wall and the outer surface of the inner wall. The stopper or lid, typically made from materials like cork, rubber, or various plastics, forms a tight seal at the flask’s opening. These materials are selected for their poor heat conductivity, to prevent heat loss through the top.
How Thermoses Maintain Temperature
Thermoses minimize heat transfer, slowing the rate at which contents change temperature. This is achieved by addressing the three main methods of heat transfer: conduction, convection, and radiation. The design features work in combination to create an effective thermal barrier.
Conduction, the transfer of heat through direct contact, is significantly reduced by the vacuum layer between the inner and outer walls of the flask. As a vacuum lacks matter, few molecules transfer heat. This airless space acts as an effective insulator, preventing heat from being conducted between the inner container and the exterior.
Convection, which involves heat transfer through the movement of fluids like air or liquid, is also eliminated by the vacuum. With no air or other fluid in the space between the walls, convection currents cannot form and circulate heat. The tightly sealed stopper or lid prevents convection by trapping the air inside the flask and stopping warm air from escaping or cold air from entering.
Radiation, the transfer of heat through electromagnetic waves, is mitigated by the reflective surfaces within the thermos. The silvered or mirrored coatings on the inner walls reflect infrared radiation. This reflection redirects heat waves back towards the contents if they are hot, or away from them if they are cold, minimizing heat transfer by radiation. The combination of these design elements allows the thermos to maintain the temperature of its contents for an extended period.