摘要：
In an aspect, a sports ball includes a core including a negative Poisson's ratio (NPR) foam material, in which the core has a Poisson's ratio of between 0 and -1, and in which the NPR foam material includes an NPR polymer foam; and a cover layer surrounding the core. In an aspect, a golf club includes an elongated shaft; a grip disposed at a first end of the elongated shaft; and a head connected to a second end of the elongated shaft, the head having a flat face configured for contact with a golf ball, in which the flat face is formed of a negative Poisson's ratio (NPR) foam material, in which the head has a Poisson's ratio of between 0 and -1, and in which the NPR foam material includes an NPR metal foam, an NPR ceramic foam, or an NPR-PPR composite foam.
权利要求：
What is claimed is:
1. A sports ball comprising: a core comprising a negative Poisson’s ratio (NPR) foam material, in which the core has a Poisson’s ratio of between 0 and -1, and in which the NPR foam material comprises an NPR polymer foam; and a cover layer surrounding the core.
2. The sports ball of claim 1, in which the NPR foam material comprises an NPR rubber foam.
3. The sports ball of claim 2, in which the NPR rubber foam comprises one or more of butadiene, polybutadiene, or styrene-butadiene.
4. The sports ball of claim 1, in which the NPR foam material comprises a thermoplastic polymer NPR foam or a viscoelastic elastomer NPR film.
5. The sports ball of claim 1, in which the core has a Poisson’s ratio of between 0 and -0.8.
6. The sports ball of claim 1, in which the NPR foam material is composed of a cellular structure having a characteristic dimension of between 0.1 pm and 3 mm.
7. The sports ball of claim 1, in which the core comprises a composite material comprising the NPR foam material and a positive Poisson’s ratio (PPR) material.
8. The sports ball of claim 1, comprising a middle layer surrounding the core and disposed between the core and the cover.
9. The sports ball of claim 8, in which the middle layer comprises second NPR foam material.
10. The sports ball of claim 1, in which the cover is formed of a third NPR foam material.
11. The sports ball of claim 1, in which the ball comprises a golf ball.
12. The sports ball of claim 1, in which the ball comprises a baseball or a softball.
13. The sports ball of claim 1, in which the ball comprises a cricket ball.
14. A method of making a sports ball, the method comprising: forming a core of a sports ball from a negative Poisson’s ratio (NPR) foam material, in which the NPR foam material comprises an NPR polymer foam, and in which the core has a Poisson’s ratio of between 0 and -1; disposing a cover to surround the core of the sports ball.
15. The method of claim 14, in which forming the core from an NPR material comprises heating and compressing a positive Poisson’s ratio (PPR) foam material to form the NPR material.
16. The method of claim 14, in which forming the core from an NPR material comprises forming the core from nano- or micro-structured PPR materials.
17. The method of claim 14, in which forming the core from an NPR material comprises forming the core using an additive manufacturing technique.
18. A golf club comprising: an elongated shaft; a grip disposed at a first end of the elongated shaft; and a head connected to a second end of the elongated shaft, the head having a flat face configured for contact with a golf ball, in which the flat face is formed of a negative Poisson’s ratio (NPR) foam material, in which the head has a Poisson’s ratio of between 0 and -1, and in which the NPR foam material comprises an NPR metal foam, an NPR ceramic foam, or an NPR- PPR composite foam.
19. The golf club of claim 18, in which the head is formed of an NPR metal foam, an NPR ceramic foam, or an NPR-PPR composite foam.
20. The golf club of claim 18, in which the flat face is integral with the head.
21. The golf club of claim 18, in which dimples are defined on one or more surfaces of the golf club.
22. The golf club of claim 21, in which the flat face does not have dimples.
23. The golf club of claim 21, in which dimples are defined on a surface of the elongated shaft of the golf club.
24. The golf club of claim 21, in which dimples are defined on the head of the golf club.