A Dewar flask, often referred to as a vacuum flask, is a specialized container engineered to maintain a substance’s temperature, whether hot or cold. This device is an insulator that slows the transfer of thermal energy between the contents and the surrounding environment. The flask is named after its inventor, the Scottish physicist and chemist Sir James Dewar, who developed the concept around 1892. Dewar created the double-walled vessel for his pioneering work in cryogenics, requiring a container capable of storing liquefied gases like oxygen and hydrogen for extended periods.
The Science of Thermal Insulation
The insulating capability of the Dewar flask stems from its design, which addresses the three fundamental ways heat energy moves: conduction, convection, and radiation. Conduction occurs through direct contact, while convection involves the movement of heated fluids. The flask’s double-walled structure, with the space between the walls evacuated, creates a vacuum that nearly eliminates both of these heat transfer mechanisms.
The vacuum prevents conduction because there are virtually no gas molecules to transfer heat across the gap. Since there is no fluid to circulate, the flask also stops the formation of convection currents. This leaves thermal radiation as the primary remaining pathway for heat exchange.
To combat radiation, the inner surfaces of the double walls are coated with a highly reflective material, often silver or aluminum. This mirror-like coating reflects infrared radiation back toward its source. If the flask holds a cold liquid, the coating reflects external heat away; if it holds a hot liquid, it reflects internal heat back inside. This combination creates effective passive thermal insulation.
Construction and Variations
The basic anatomy consists of an inner vessel, which holds the substance, enclosed by an outer shell, with a vacuum layer separating the two. The inner and outer walls are joined only at the neck, which is often narrow to minimize the area for heat transfer via conduction. Supports between the walls are made from materials with low thermal conductivity to reduce heat leakage.
Dewar flasks vary in material and scale, ranging from small laboratory glass flasks to massive industrial containers. Smaller flasks are often constructed from silvered borosilicate glass, protected by an outer metal housing. These glass Dewars are common in laboratories for temporary storage of cryogens and feature an open top or a loose-fitting stopper to vent pressure.
Larger, robust versions are typically constructed from metals like stainless steel or aluminum, forming cryogenic storage tanks or cryostats. These industrial Dewars can hold hundreds or thousands of liters of liquid gas and are built with complex features, including pressure-relief valves, gauges, and self-pressurizing systems for safe dispensing. Stainless steel offers greater durability for cold applications.
Applications in Science and Industry
The ability of the Dewar flask to maintain stable temperatures makes it a vital tool across modern science and industry, particularly in the field of cryogenics. The most common application involves the storage and transportation of liquefied gases, or cryogens, such as liquid nitrogen (near -196°C) and liquid helium (near -269°C). Without the flask’s insulation, these liquids would rapidly boil away.
In the medical field, Dewar flasks are critical for cryopreservation, safely storing biological materials like sperm, eggs, embryos, blood, and tissue samples for extended periods. They also play a role in advanced medical imaging. Large Dewars are used to cool the superconducting magnets found in Magnetic Resonance Imaging (MRI) machines, which require liquid helium to function. The sustained low temperature allows the magnets to achieve superconductivity, enabling high-resolution imaging.
Beyond medicine, Dewars are utilized in physics research to cool sensitive detectors and components, such as those used in particle accelerators or space telescopes. The flasks are also employed in the electronics industry for processes that require low temperatures, including the testing and production of certain semiconductor components.