NiMnGa Fibres for use in polymer composites

Dietrich Hinz ,  Nils Scheerbaum ,  Oliver Gutfleisch ,  Karl-Hartmut Müller ,  Ludwig Schultz 

Leibniz-Institute for Solid State and Materials Research, P.O.Box 270116, Dresden D-01171, Germany

Abstract

Bulk single crystals of Ni-Mn-Ga show the strongest magnetic shape memory (MSM) effect. Their use would be advantageous in application as actuators or as mechanical energy absorber. However, especially the high cost for the single crystals is a drawback. Therefore composites made of polymer and MSM particles may also be used for damping applications and as actuators. Ideally, the MSM particles should be, at the operating temperatures, in the martensitic state and single-crystalline when in the austenitic state. We used fibres of 51Ni-27Mn-22Ga (at.%), prepared by crucible melt extraction, as MSM material. The size of the fibres is about 60µm in diameter and 1cm in length. The structural and magnetic properties of the fibres are similar to those of bulk material with the same composition. SEM analysis reveals that the grain size of the melt-extracted fibres is about 5µm. The fibres were annealed at temperatures of 1000-1100C in order to achieve grain growth. After annealing, grain sizes in the order of the diameter of the fibres were observed. The annealing affects also the martensite start temperature. It increases from 30C to 45C. The Curie temperature, determined by DSC and susceptometer measurements to 98C, remains nearly unchanged. XRD analysis revealed that the cubic austenite changes to the tetragonal martensite (5M) with c/a=0.94. The martensite is highly twinned (determined by SEM) and SQUID measurements reveal a magnetisation process at low temperatures, which could be attributed to twin boundary motion. Additionally, the structural properties are strongly affected by a mechanical treatment (milling). The milled fibres no longer show a martensitic transition, unless they were annealed again at 600C. Here XRD analyses showed that milling leads to a higher fraction of the martensite phase compared to the non-milled state. First composites were prepared using different polymers applying various magnetic fields to align the MSM particles.

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Presentation: oral at E-MRS Fall Meeting 2005, Symposium C, by Dietrich Hinz
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-20 14:12
Revised:   2009-06-07 00:44
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