1. Have enough strength, stiffness, small mass and light weight to ensure the minimum inertial force. 

2. Good thermal conductivity, high temperature resistance, high pressure, corrosion resistance, sufficient heat dissipation capacity, and small heating area. 

3. Small friction coefficient between the piston and the piston wall. 

4. When the temperature changes, the size and shape change are small, and the minimum gap are maintained with the cylinder wall. 

5. The thermal expansion coefficient is small, the specific gravity is small, and it has good wear reduction and thermal strength.