The phase - change material (PCM) is used as a thermo - mechanical actuator in thermostats. It uses the structure of n - alkanes. The volume difference between its solid and liquid states is large, and its melting point is affected by the chain length. After being sealed in a movable actuator device, temperature changes cause it to expand or contract, thereby adjusting the performance of external systems such as hydraulic, heating, and cooling. The principle is simple, relying on material properties and mechanical integrity to provide a stable and reliable temperature control and adjustment function.