In one or more embodiments of the present disclosure, the method further includes disposing a liquid infusion tube between the base plate and the top plate, and vacuuming the first space through the liquid infusion tube. The liquid infusion tube has a first opening and a second opening. The second opening is opposite to the first opening. The first opening is located in the first space. The second opening is exposed outside the main body. The method further includes introducing the working fluid into the first space through the liquid infusion tube; and sealing up the liquid infusion tube.

In one or more embodiments of the present disclosure, the method further includes fixing the main body in a mold and forming a plastic outer frame by injection molding in the mold. The plastic outer frame includes a first frame body and an encapsulating structure. The first frame body surrounds to define a second space. The main body is at least partially located in the second space and abuts against the first frame body. The encapsulating structure is connected with an inner side of the first frame body. A portion of the liquid infusion tube exposed outside the main body is embedded inside the encapsulating structure.

In one or more embodiments of the present disclosure, the plastic outer frame further includes two second frame bodies. The second frame bodies are separated from and opposite with each other. The second frame bodies define the second space therebetween. The second frame bodies are respectively connected with the first frame body. The encapsulating structure is connected with one of the second frame bodies. Each of the second frame bodies and the first frame body form a stair structure.