Materials with viscoelastic stiffness greater than diamond, T. Jaglinski, D. Kochmann, D. S. Stone, R. S. Lakes, Science315, 620-622, Feb. 2 (2007) Abstract
We show that composite materials can exhibit a viscoelastic (Young's) modulus far higher than that of either constituent. The modulus (but not strength) was observed to be substantially greater than that of diamond. These composites contain barium titanate inclusions, which undergo a volume change phase transformation if not constrained. In the composite the inclusions are partially constrained by the surrounding metal matrix. The constraint stabilizes the negative bulk modulus (inverse compressibility) of inclusions. This negative modulus arises from stored elastic energy in inclusions, in contrast to periodic composite metamaterials which exhibit negative refraction via inertial resonant effects. Conventional composites with positive stiffness constituents have aggregate properties bounded by a weighted average of constituent properties; their modulus cannot exceed that of the stiffest constituent.
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Here is a related paper which provides analysis of stability.
W. Drugan
, "Elastic composite materials having a negative stiffness can be stable" Physical Review Letters98, 055502, (2007)
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Materials with viscoelastic stiffness greater than diamond