Metamaterials simulation for thermal diffusers

The heat extraction efficiency of a cylindrical diffuser can be optimized by applying differential geometry [J.-P. Huang, Theoretical Thermotics: Transformation Thermotics and Extended Theories for Thermal Metamaterials (Springer, 2020)], in order to find a metamaterial design. That can be done by coupling a thermally insulating material (polytetrafluoroethylene) with a high thermal conductivity material (copper) where the heat flow is directed. By controlling the distance between the isothermal contours, to extract the heat while maintaining a constant temperature gradient along the diffuser avoiding heat accumulation

Elastocaloric effect in Ti-Ni shape-memory wires associated with the B2 ↔ B19' and B2 ↔ R structural transitions

"We have studied the elastocaloric properties of Ti-Ni shape-memory wires subjected to specific heat treatments to decouple the B2 R transition from the B2 B19' one. The entropy values at moderate stresses (similar to 170 MPa) for the B2 B19' transition are remarkably high (in the range 60-80 J/kg K). However, in spite of the B2 R transition exhibits significantly lower entropy changes (similar to 12 J/kg K), the much smaller hysteresis of this transition gives rise to a larger reversible elastocaloric effect for low applied stresses. Therefore, the reversible elastocaloric strength associated with the B2 R is larger than the elastocaloric strength associated with the B2 B19' transition.

Caloric effects induced by magnetic and mechanical fields in a Ni50Mn25-xGa25Cox magnetic shape memory alloy

"We have studied the elastocaloric effect in a Co-doped Ni-Mn-Ga magnetic Heusler shape memory alloy in the vicinity of its martensitic transition. Measurements of the length change as a function of temperature have been carried out across the transition under applied compression stresses and magnetic fields. The isothermal stress-induced entropy changes have been computed from the experimental data. Results evidence a significant elastocaloric effect associated with the large entropy change of the structural phase transition. The alloy also exhibits a magnetocaloric effect at low applied magnetic fields. It is shown that application of a magnetic field below 1 T increases the estimated elastocaloric relative cooling power by about 20%. A comparison of elasto-and magnetocaloric properties indicates that a similar relative cooling power is reached under application of 10 MPa or 0.8 T.

Metamaterials simulation for thermal diffusers

The heat extraction efficiency of a cylindrical diffuser can be optimized by applying differential geometry [J.-P. Huang, Theoretical Thermotics: Transformation Thermotics and Extended Theories for Thermal Metamaterials (Springer, 2020)], in order to find a metamaterial design. That can be done by coupling a thermally insulating material (polytetrafluoroethylene) with a high thermal conductivity material (copper) where the heat flow is directed. By controlling the distance between the isothermal contours, to extract the heat while maintaining a constant temperature gradient along the diffuser avoiding heat accumulation

Synthesis and characterization of nickel ferrite-barium titanate ceramic composites

Magnetoelectric composites were synthesized from piezoelectric BaTiO3 and ferrimagnetic NiFe2O4. These two phases are mixed, mechanically milled and heat treated. A characterization is made by means of electronic microscopy and x-ray diffraction. The results of the magnetic, electric, ferroelectric and piezoelectrical response of two different compositions are obtained. The composites are superparamagnetic in all the measured temperature range. It was observe a magnetic change induced by a structural transition of the ferroelectric phase. The composites behaves as an acceptable mechanical resonator when the content of the ferroelectric phase is equal or greater than 60 wt.%.The authors thank to CICYT (Project MAT2004-04843-C02-02) for financial support.Peer reviewe