Haruhiko Morito

Magnetic-field-induced shape recovery by reverse phase transformation

Large magnetic-field-induced strains have been observed in Heusler alloys with a body-centred cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field. These materials have attracted considerable attention as potential magnetic actuator materials. Here we report the magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy. Stresses of over 100 MPa are generated in the material on the application of a magnetic field of 70 kOe; such stress levels are approximately 50 times larger than that generated in a previous ferromagnetic shape-memory alloy. We observed 3 per cent deformation and almost full recovery of the original shape of the alloy. We attribute this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase at 298 K in the Ni45Co5Mn36.7In13.3 single crystal.

Metamagnetic shape memory effect in a Heusler-type Ni43Co7Mn39Sn11 polycrystalline alloy

Shape memory and magnetic properties of a Ni43Co7Mn39Sn11 Heusler polycrystalline alloy were investigated by differential scanning calorimetry, the sample extraction method, and the three-terminal capacitance method. A unique martensitic transformation from the ferromagnetic parent phase to the antiferromagneticlike martensite phase was detected and magnetic-field-induced “reverse” transition was confirmed in a high magnetic field. In addition, a large magnetic-field-induced shape recovery strain of about 1.0% was observed to accompany reverse martensitic transformation, and the metamagnetic shape memory effect, which was firstly reported in a Ni45Co5Mn36.7In13.3 Heusler single crystal, was confirmed in a polycrystalline specimen.

Magnetic and martensitic phase transitions in ferromagnetic Ni–Ga–Fe shape memory alloys

Ferromagnetic shape memory alloys with a body-centered-cubic ordered structure in a Ni–Ga–Fe system have been developed. The alloys with the composition range of Ni 27 at. % Ga (20–22 at. %)Fe exhibit a thermoelastic martensitic transformation from a B2 and/or an L21 parent to a martensite phase, with a seven-layer modulated (14M) and a five-layer modulated (10M) structure, in the ferromagnetic state. The parent phase transforms from the B2 to the L21 structure at about 970 K during cooling, and the degree of the L21 order in the parent phase is increased by annealing at 773 K, resulting in the increase of both the martensite starting and the Curie temperatures. The ductility of these alloys is improved by introducing of a small amount of a γ-phase solid solution. Consequently, we can conclude that the present alloys are promising for ferromagnetic shape memory alloys.

Magnetocrystalline anisotropy in single-crystal Co–Ni–Al ferromagnetic shape-memory alloy

The magnetocrystalline anisotropy in a single-crystal Co37Ni34Al29 ferromagnetic shape-memory alloy has been investigated. The prestrain was applied to the parent phase in order to nucleate the specific variant in the sample cooled down through the martensitic transformation temperature. The applied magnetic field facilitates the growth of variants parallel to the applied magnetic field in analogy with the prestrain. From these results of selective nucleation of variants, the magnetocrystalline anisotropy energy in the single crystal Co37Ni34Al29 β′ martensite phase is estimated to be 3.9×106 erg/cm3. In the single crystal, the observed magnitude of the reversible magnetic-field-induced strains is 0.06%.

Magnetic-field-induced strain of Fe-Ni-Ga in single-variant state

The magnetic-field-induced strain (MFIS) and the magnetocrystalline anisotropy in Fe19.3Ni54.2Ga26.5 ferromagnetic shape memory alloy have been investigated in a single-variant state. From the magnetization curves, the magnetocrystalline anisotropy constant K in the single crystal Fe19.3Ni54.2Ga26.5 β′ martensite phase is estimated to be 1.8×106 erg/cm3 at 5 K. In the single-variant martensite phase, the reversible MFIS of 0.02% is observed, and the value of K is reduced with increasing temperature. On the other hand, the magnitude of MFIS increases up to 100 K, and then decreases with increasing temperature. Finally, no MFIS is observed above 150 K. From these data, the condition of K for the MFIS can be confirmed at low temperatures.

Stress-assisted magnetic-field-induced strain in Ni–Fe–Ga–Co ferromagnetic shape memory alloys

To obtain a large strain for Ni–Fe–Ga ferromagnetic shape memory alloys, the Curie temperature was increased by adding Co, and the magnetic-field-induced strain (MFIS) has been investigated under static stresses. The magnetocrystalline anisotropy constant K is increased by the addition of Co, and Ni49Fe18Ga27Co6 alloy gives a relatively large value of 1.15×106erg∕cm3 at 300K. From the stress-strain curves for this alloy, the twinning stress is estimated to be 8–9MPa. Consequently, the Ni49Fe18Ga27Co6 alloy exhibits a large MFIS of about 8.5% at room temperature under a static compressive stress of about 8MPa.

Li-Rich Li-Si Alloy As A Lithium-Containing Negative Electrode Material Towards High Energy Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are generally constructed by lithium-including positive electrode materials, such as LiCoO2, and lithium-free negative electrode materials, such as graphite. Recently, lithium-free positive electrode materials, such as sulfur, are gathering great attention from their very high capacities, thereby significantly increasing the energy density of LIBs. Though the lithium-free materials need to be combined with lithium-containing negative electrode materials, the latter has not been well developed yet. In this work, the feasibility of Li-rich Li-Si alloy is examined as a lithium-containing negative electrode material. Li-rich Li-Si alloy is prepared by the melt-solidification of Li and Si metals with the composition of Li21Si5. By repeating delithiation/lithiation cycles, Li-Si particles turn into porous structure, whereas the original particle size remains unchanged. Since Li-Si is free from severe constriction/expansion upon delithiation/lithiation, it shows much better cyclability than Si. The feasibility of the Li-Si alloy is further examined by constructing a full-cell together with a lithium-free positive electrode. Though Li-Si alloy is too active to be mixed with binder polymers, the coating with carbon-black powder by physical mixing is found to prevent the undesirable reactions of Li-Si alloy with binder polymers, and thus enables the construction of a more practical electrochemical cell.

Synthesis and crystal structures of Ca4SiN4 and new polymorph of Ca5Si2N6.

Single crystals of Ca4SiN4 were found in the product prepared by heating Ba, Ca, Si, NaN3, and Na at 900 °C. Ca4SiN4 [space group P2(1)/c (No. 14), Z = 4, a = 9.1905(4) Å, b = 5.9775(3) Å, c = 11.0138(7) Å, β = 116.4054(17)°] is isotypic with Ca4GeN4 and K4SiO4. Isolated [SiN4](8-) tetrahedra were identified in the structure by single-crystal X-ray diffraction. After reheating the product at 900 °C, a new polymorph of Ca5Si2N6 crystallized. The space group of the polymorph [C2/m (No. 12), Z = 4, a = 6.2712(5) Å, b = 10.0175(8) Å, c = 12.0287(8) Å, β = 99.303(2)°] is different from C2/c previously reported for Ca5Si2N6, while both polymorphs are composed of Ca(2+) and edge-sharing double tetrahedra [Si2N6](10-).

Preparation of Bulk β-FeSi2 Using a Na–Si Melt

Polycrystalline bulk β-FeSi2 was prepared by heating a compact body of Fe powder in a Na–Si melt at 1173 K for 24 h. The relative density of the bulk sample was 78% of the theoretical density of β-FeSi2. Mn-doped and Co-doped bulk samples were also prepared from compact bodies of Fe powder mixed with Mn and Co powders. The Seebeck coefficients measured for the Mn- and Co-doped samples in the temperature range between 400 and 1000 K were +125 to +270 µV/K and -120 to -180 µV/K, respectively.

Crystal structures and magnetization distributions in the field dependent ferromagnetic shape memory alloy Ni54Fe19Ga27

The mesoscopic and microscopic mechanisms giving rise to shape memory behaviour in single crystals of the ferromagnetic alloy Ni54Fe19Ga27 have been investigated using polarized and unpolarized neutron diffraction. The measurements confirm that the Curie temperature TC coincides with the martensitic phase transition at TM = 296 K. At room temperature the crystal, as grown, had the tetragonal L10 structure with c/a≈1.20. It transformed to the cubic Heusler L21 structure at ≈330 K. In subsequent heating and cooling cycles the transition took place at TM≈295 K and it was found that applying a magnetic field raised TM by ≈0.3 K T−1, making the material attractive for applications. The tetragonal structure has space group I4/mmm and is related to the cubic parent phase by a Bain transformation. The change in cell volume at the transition is only ≈1%, suggesting that the atomic moments are unchanged, although the magnetization drops significantly. The polarized neutron results show that in the cubic phase the magnetic electrons at the iron-rich sites have predominantly eg symmetry (60(3)%), a distribution similar to that observed in Fe3Al and Fe3Si. A small transfer of magnetization from Fe to Ni is associated with the martensitic transition, but no significant redistribution of magnetic electrons between orbitals whose degeneracy is lifted, such as that predicated by the band Jahn–Teller mechanism, was observed.