For all substances, if you add or subtract heat you will increase or decrease its temperature. This is known as sensible heating/cooling and is true as long as the substance doesn’t change phase, or in other words change from solid to liquid, liquid to gas, solid to gas or the reverse.
Phase change indicates when a material changes from one state to another (solid, liquid, gas) based on external input, typically thermal. Phase change occurs at a set temperature that is characteristic of the material and the phase change that is occurring. This change from one state of matter to another is known as latent heat and requires additional energy. For water, the solid to liquid (melting ice) transition is at 0°C (melting ice) and the liquid to gas transition (boiling) is at 100°C.
Materials that require a lot of energy to change from one phase to another can be used as a type of thermal battery. These materials are commonly referred to as phase change materials or PCMs. Energy can be added to or removed from the materials to get them to change phase. An example of this is putting water (a liquid) in a freezer in order to get it to turn into ice (a solid). This thermal battery, the ice, can then be used to lower the temperature of something warmer than its melting temperature or phase transition temperature (like putting ice in your lemonade on a hot summer day).
Since it requires energy to convert a solid to a liquid without raising temperature, passive devices such as the Arktek passive vaccine storage device use the phase change of a material (ice or PCM) to maintain temperature as the PCM melts. Different temperatures can be maintained by selecting materials that have solid to liquid phase transitions at the desired temperature. By changing the PCM from water to another substance one can tune the temperature inside the Arktek to either higher or lower depending on the PCM. The Deep Freeze Arktek, which was used to support Ebola vaccine trials used a -78°C PCM to keep the vaccines between the required -60° and -80°C temperatures. The hold time of a passive device is dependent upon: the density, phase transition temperature, and heat of fusion on the PCM as well as the insulation of the device and the ambient temperature.