Carl P. Romao

Negative Thermal Expansion (Thermomiotic) Materials

Whereas most materials expand when heated, some shrink, i.e., show negative thermal expansion (NTE, also known as thermomiotic behavior). The excitement about NTE materials is that, in principle, they could compensate for positive thermal expansion, presenting materials that would not experience thermal stress fracture. We summarize the atomic-level mechanisms of NTE, synthetic routes to NTE materials, experimental techniques to investigate NTE, and types of NTE materials and their applications.

Relationships between elastic anisotropy and thermal expansion in A2Mo3O12 materials

We report calculated elastic tensors, axial Grüneisen parameters, and thermal stress distributions in Al2Mo3O12, ZrMgMo3O12, Sc2Mo3O12, and Y2Mo3O12, a series of isomorphic materials for which the coefficients of thermal expansion range from low-positive to negative. Thermal stress in polycrystalline materials arises from interactions between thermal expansion and mechanical properties, and both can be highly anisotropic. Thermal expansion anisotropy was found to be correlated with elastic anisotropy: axes with negative thermal expansion were less compliant. Calculations of axial Grüneisen parameters revealed that the thermal expansion anisotropy in these materials is in part due to the Poisson effect. Models of thermal stress due to thermal expansion anisotropy in polycrystals following cooling showed thermal stresses of sufficient magnitude to cause microcracking in all cases. The thermal expansion anisotropy was found to couple to elastic anisotropy, decreasing the bulk coefficient of thermal expansion and leading to lognormal extremes of the thermal stress distributions.

Negative stiffness in ZrW2O8 inclusions as a result of thermal stress

Materials with negative stiffness, although inherently unstable in isolation, can be stabilized by external constraints, for example, by inclusion within a material with positive stiffness. We have identified ZrW2O8, a material with negative thermal expansion, as a candidate negative-stiffness material arising from its negative bulk modulus during a ferroelastic cubic–orthorhombic pressure-induced phase transition (PIPT). A hyperelastic constituent equation for this transition was developed and implemented in a finite-element model of ZrW2O8 inclusions in positive stiffness, positive thermal expansion matrices. In these matrices, thermal stress during cooling, originating from thermal expansion mismatch, would be sufficient to initiate the PIPT after small temperature drops. The subsequent progress of the PIPT depends strongly on the thermoelastic properties of the matrix, with stiff, low thermal expansion matrices stabilizing the transition state over broad temperature ranges, indicating that ZrW2O8 or m...

Anisotropic thermal expansion in flexible materials

A definition of the Gruneisen parameters for anisotropic materials is derived based on the response of phonon frequencies to uniaxial stress perturbations. This Gruneisen model relates the thermal expansion in a given direction (

Zero Thermal Expansion in ZrMgMo3O12: NMR Crystallography Reveals Origins of Thermoelastic Properties

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Crystal nucleation and near-epitaxial growth in nacre.

) to one element of the elastic compliance tensor, which corresponds to the Youngs modulus in that direction (

Near‐Zero Thermal Expansion in In(HfMg)0.5Mo3O12

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Thermal, vibrational, and thermoelastic properties of Y2Mo3O12 and their relations to negative thermal expansion

). The model is tested through ab initio prediction of thermal expansion in zinc, graphite, and calcite using density functional perturbation theory, indicating that it could lead to increased accuracy for structurally complex systems. The direct dependence of

The effect of microstructure on thermal expansion coefficients in powder-processed Al2Mo3O12

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Al 2 Mo 3 O 12 /polyethylene composites with reduced coefficient of thermal expansion

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