Date of Award
Doctor of Philosophy
The magnetic properties of NiMnGe1−xAlx, Ni50Mn35(In1−xBx)15, Ni50Mn35In14.5B0.5 (Bulk, As-Solidified and Annealed melt-spun ribbon) and RE-Infuse Carbon Nanotubes, have been studied by x-ray diffraction, differential scanning calorimetry (DSC), and magnetization measurements. Partial substitution of Al for Ge in NiMnGe1−xAlx results in a ﬁrst-order magnetostructural transition (MST) from a hexagonal ferromagnetic to an orthorhombic antiferromagnetic phase at 186 K (for x = 0.09). A large magnetic entropy change of ∆SM = -17.6 J/kg K for ∆H = 5 T was observed in the vicinity of TM = 186 K for x = 0.09. This value is comparable to those of well-known giant magnetocaloric materials, such as Gd5Si2Ge2, MnFeP0.45As0.55, and Ni50Mn37Sn13. The values of the latent heat (L = 6.6 J/g) and corresponding total entropy changes (∆ST = 35 J/kg K) have been evaluated for the MST using DSC measurements. Large negative values of ∆SM of -5.8 and -4.8 J/kg K for ∆H = 5 T and up to 9T in the vicinity of TC were observed for x = 0.09 and 0.085, respectively. The impact of B substitution in Ni50Mn35In15-xBx Heusler alloys on the structural, magnetic, transport, and parameters of the magnetocaloric effect (MCE) has been studied by means of room-temperature X-ray diffraction and thermomagnetic measurements (in magnetic fields (H) up to 5 T, and in the temperature interval 5-400 K ). Direct adiabatic temperature change (ΔTAD) measurements have been carried out for an applied magnetic field change of 1.8 T. The transition temperatures (T-x) phase diagram has been constructed for H = 0.005 T. The MCE parameters were found to be comparable to those observed in other MCE materials such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X=In, Al, and Ge) Heusler alloys. The maximum absolute value of ΔTAD = 2.5 K was observed at the magnetostructural transition for Ni50Mn35In14.5B0.5. The structural phase transition temperatures, phase structure, and parameters of the magnetocaloric effect (MCE) of Ni50Mn35In14.5B0.5 as Bulk, As-Solidified and Annealed melt-spun ribbon has been studied by means of room-temperature X-ray diffraction and thermomagnetic measurements (in magnetic fields (oH) up to 5 T, and in the temperature interval 5–400 K). Magnetic and structural transitions in Ni50Mn35In14.5B0.5 as ribbons were found to coincide in Ni50Mn35In14.5B0.5 bulk sample, leading to a large magnetocaloric effects associated with the first-order magnetostructural phase transition. In comparison to the bulk Ni50Mn35In14.5B0.5 alloys, both the martensitic transition temperature (TM) and Curie temperature (TC) shifted to lower temperatures. Magnetic measurements revealed that the ribbons undergo a structure transformation similar to the bulk material at the martensitic transformation. The temperature of the transformation depends strongly on lattice parameters of the ribbons. MST shows a weak broad magnetic transition at TCM∼ 160 K, while the Curie temperature of AST TCA is ∼ 297 K. The MCE parameters were found to be comparable to those observed in other MCE materials such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X = In, Al, and Ge) Heusler alloys. These results suggest the possibility to control the martensitic transition in Ni50Mn35In14.5B0.5 through rapid solidification process. A comparison of magnetic properties and magnetocaloric effects in Ni50Mn35In14.5B0.5 alloys as Bulk, As-Solidified and Annealed ribbons is discussed. Carbon nanotube (CNT)/metal-cluster-based composites are envisioned as new materials that possess unique electronic properties which may be utilized in a variety of future applications. Super paramagnetic behavior was reported for CNTs with Gd ions introduced into the CNT openings by internal loading with an aqueous GdCl3 chemical process. In the current work, the magnetic properties of the CNT/Gd composites were obtained by the joining and annealing of Gd metal and CNTs at 850 °C for 48 h. Energy dispersive X-ray analysis shows the presence of Gd intermingled with the CNT walls with maximum and average Gd concentrations of about 20% and 4% (by weight), respectively. The Gd clusters have a non-uniform distribution and are mostly concentrated at the ends of the CNTs. A ferromagnetic-type transition at TC ∼ 320 K, accompanied by jump like change in magnetization and temperature hysteresis typical for the temperature induced first order phase transitions has been observed by magnetization measurements. It was found that Gd infused into the CNTs by the annealing results in a first order paramagnetic-ferromagnetic transition at TC = 320 K.
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