Magnetocaloric effect: From materials research to refrigeration devices

Magnetic Heusler alloys which undergo a martensitic transition display interesting functional properties. In the present review, we survey the magnetocaloric effects of Ni-Mn-based Heusler alloys and discuss their relation with the magnetic shape-memory and magnetic superelasticity reported in these materials. We show that all these effects are a consequence of a strong coupling between structure and magnetism which enables a magnetic field to rearrange martensitic variants as well as to provide the possibility to induce the martensitic transition. These two features are respectively controlled by the magnetic anisotropy of the martensitic phase and by the difference in magnetic moments between the structural phases. The relevance of each of these contributions to the magnetocaloric properties is analysed.

https://iopscience.iop.org/article/10.1088/0953-8984/21/23/233201/pdf

Magnetocaloric effect and its relation to shape-memory properties in ferromagnetic Heusler alloys

Thermoelectric generators: A review of applications

Shape memory alloys (SMAs) with high transformation temperatures can enable simplifications and improvements in operating efficiency of many mechanical components designed to operate at tem...

High temperature shape memory alloys

This paper provides a detailed overview of developments in transducer materials technology relating to their current and future applications in micro-scale devices. Recent advances in piezoelectric, magnetostrictive and shape-memory alloy systems are discussed and emerging transducer materials such as magnetic nanoparticles, expandable micro-spheres and conductive polymers are introduced. Materials properties, transducer mechanisms and end applications are described and the potential for integration of the materials with ancillary systems components is viewed as an essential consideration. The review concludes with a short discussion of structural polymers that are extending the range of micro-fabrication techniques available to designers and production engineers beyond the limitations of silicon fabrication technology.

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New materials for micro-scale sensors and actuators An engineering review

Crystal structure of 7M modulated Ni–Mn–Ga martensitic phase

Portable thermoelectric power generation based on catalytic combustor for low power electronic equipment

Structural, magnetic and anisotropic properties of Ni2MnGa melt-spun ribbons

Phase transition and mechanical damping properties: A DMTA study of NiTiCu shape memory alloys

Ferromagnetic materials based on the Heusler alloy Ni 2 MnGa have been recognized as interesting actuating materials. Large strains can be obtained by applying an external magnetic field acting on the reorientation of the mesoscopic structure of martensite. Even if a great part of the investigation has been performed on single crystals, an interest exists for the development of polycrystalline technology. It has been recognized that, in order to balance the strong uncoupling effect due to the large number of fixed interfaces (e.g. the grain boundaries), the polycrystalline sample should have some texture. This should result in some pre-orientation of the twinned martensitic structure and, if possible, in a modification of the random distribution of the easy axis orientation within the material. Strong material texturing can be obtained basically by using directional solidification, rapid solidification techniques or plastic deformation. The effects of directional solidification, and of melt spinning have been evaluated in the past. In the present work, we report on successfully plastic deformation of polycrystalline NiMnGa alloys. In literature, there are few (if any) papers devoted to this topic. The great brittleness of the alloy at room temperature is well known. The rare attempts performed to deform the material are reported as failures. The approach used here is based on the hot working of NiMnGa ingots in special metallic cans filled with Argon and sealed. Hot deformation of a 7×7×40 mm 3 small slab down to 1.25×11.9×130 mm 3 was successful. Even after this severe plastic deformation, it was possible to easily remove the deformed material from the special can. The structural and functional characterization of the final product, which maintained the martensitic transformation, will be presented and discussed.

Francesca Passaretti

Papers

Crystal structure of 7M modulated Ni–Mn–Ga martensitic phase

Portable thermoelectric power generation based on catalytic combustor for low power electronic equipment

Structural, magnetic and anisotropic properties of Ni2MnGa melt-spun ribbons

Phase transition and mechanical damping properties: A DMTA study of NiTiCu shape memory alloys

Plastic deformation of NiMnGa polycrystals