Materials of the cylinders of active-piston actuators

1. An actuator including a linear piston in an active material, and an inner sliding cylinder, where the linear piston in an active material includes a multiplicity of sections designed to be controlled so that they can be expanded until they jam in the sliding cylinder and/or lengthen within the sliding cylinder, in such a manner that the piston moves axially in the sliding cylinder by dry friction, including an outer cylinder in which the said sliding cylinder is pre-stressed, with either the outer cylinder or the sliding cylinder being in an anisotropic material with a negative or approximately zero thermal expansion coefficient along at least one axis, and a positive or approximately zero thermal expansion coefficient on at least one other axis, and the other cylinder having a positive or approximately zero thermal expansion coefficient.

2. An actuator according to claim 1 , in which the positive expansion axis of the anisotropic material is directed along a movement axis of the piston in the sliding cylinder.

3. An actuator according to claim 1 , in which the cylinder in anisotropic material is the sliding cylinder, with the outer cylinder being in a material with a positive or approximately zero thermal expansion coefficient.

4. An actuator according to claim 1 , in which the cylinder in anisotropic material is the outer cylinder, with the sliding cylinder being in a material with a positive or approximately zero thermal expansion coefficient.

5. An actuator according to claim 1 , in which the material of the cylinder with a negative or approximately zero thermal expansion coefficient along at least one axis is one of the following materials

α-ZrMo 2 O 8 in triangular structure,

HfMo 2 O 8 ,

β-(Zr 2 O)(PO 4 ) 2

or any composite of these materials.

6. An actuator according to claim 1 , including at least one layer of solid lubricant at the interface between the outer cylinder and the sliding cylinder.

7. An actuator according to claim 6 , wherein the solid lubricant is fluoridated carbon (CF x ) n , where x is a real number which represents the degree of fluoridation and n is a integer number which represents the degree of polymerisation.

8. An actuator according to claim 6 , wherein the solid lubricant is one of the following materials:

hexagonal boron nitride,

MoS 2 ,

WSe 2 ,

WS 2 ,

graphite, interleaved or not interleaved,

tin sulphide (in its SnS, SnS 2 , Sn 3 S 4 forms)

Cerium fluoride (CeF 3 ), or

any mixture of these materials.

9. An actuator according to claim 8 , characterised in that the outer cylinder includes external outgrowths extending radially to the said cylinder.

10. An actuator according to claim 1 , characterised in that one of the two cylinders includes one or more slots, extending radially and longitudinally in the cylinder.