Magnetocaloric Properties of Fe and Ge Doped Ni2Mn1−xCuxGa

The magnetocaloric properties of Fe and Ge doped Ni2Mn0.75Cu0.25Ga Heusler alloys have been investigated. Using Ni2Mn0.75Cu0.25Ga as the parent material, the Fe doped system (Ni2Mn1−x(Cu–Fe)xGa) and a Ge doped system (Ni2Mn1−xCuxGa1−xGex) were studied. The manipulation of the Mn–Cu subsystem with Fe doping results in a decrease of the first order magnetostructural transition temperature, whereas the substitution of Ge for the Mn–Cu–Ga subsystems results in an increase of the magnetostructural transition temperature. In both cases the giant magnetocaloric effect is successfully preserved

Intermartensitic Transformations in Ni2Mn1−xCoxGa Heusler Alloys

Ni2MnGa that possesses a Heusler L21 structure undergoes a martensitic transformation from the parent cubic (austenitic) phase to a low temperature complex tetragonal structure at TM = 202 K and has a Curie temperature of TC = 376 K. Some research groups have observed an intermartensitic transformation at a temperature TI\u3cTM, in Ni2MnGa single crystals. In this work detailed studies on the influence of substitution in the Mn subsystem by magnetic Co on the intermartensitic transformation properties of Ni2Mn1−xCoxGa compounds have been done by magnetization (5–400 K) and thermal expansion (80–300 K) measurements. The samples Ni2Mn0.92Co.08Ga, Ni2Mn0.91Co.09Ga, and Ni2Mn0.90Co.10Ga were studied. The intermartensitic transformation is observed in all of these alloys. As the Co concentration increases, the transformation becomes more pronounced and the temperature range for which the alloys stay in the intermartensitic state decreases as Co concentration increases. Similarities were observed between the magnetization and thermal expansion curves. The results are discussed in terms of the internal stress produced as a result of the Co substitution

Inverse Magnetocaloric Effect in Ferromagnetic Ni50Mn37+xSb13−x Heusler Alloys

A study of the magnetocaloric effect (MCE) in the ferromagnetic Heusler alloys Ni50Mn37+xSb13−x (x = 0,0.5,1) has been carried out through magnetization measurements. An inverse magnetocaloric effect was observed in the vicinity of the first order martensitic transition. A maximum positive magnetic entropy change of ΔSm ≈ 19 J/kg K at approximately 297 K for a magnetic field change of 5 T was observed. It is demonstrated that the martensitic transformation temperature, and the corresponding ΔSm, can be tuned through a slight variation in composition

The Delivery of Post Natal Care and its Association with Newborn Survival and Feeding: A focus on Africa

The overall goal of this dissertation is to understand how postnatal care (PNC), a relatively new intervention, functions in the context of sub-Saharan Africa with respect to improving neonatal survival and newborn feeding. Further, this dissertation also evaluates if a quality improvement intervention is able to improve PNC. Paper one examines the distribution of PNC and if it is associated with neonatal mortality in 17 countries in sub-Saharan Africa. Findings from descriptive analysis show that PNC coverage is low, often lower than skilled delivery, and that newborns from wealthier households are more likely to receive PNC. Using multi-level modelling to examine if PNC is associated with neonatal mortality, findings show that PNC is associated with saving newborn lives. PNC provided at home and facilities show about the same level of neonatal mortality reductions. Finally, the association of PNC with neonatal mortality is different in East and Southern Africa compared with the West and Central African countries we studied. These findings show the potential of PNC as a mechanism to improve newborn survival. In paper two, we examine if PNC is associated with improved newborn feeding practices. Using multi-level models, we examine if PNC is associated with breastfeeding within one day of birth and with prelacteal feeds (PLFs), which refer to liquids other than breastmilk that are given to newborns before breastfeeding is established. The results indicate that PNC is associated with early breastfeeding though not with PLFs. These findings may indicate that the implementation of PNC must better be tailored to promote the reduction of PLFs. The third paper examines if a quality improvement (QI) intervention in Ghana can improve PNC for the mother and newborn. Using three waves of household survey data, the analysis shows that PNC is associated with improvements in PNC for the mother though not for the newborn. The intervention was associated with significant reductions in PNC within one week for either the newborn or mother. These findings suggest that the program may have emphasized maternal health but not newborn health improvements. Additionally, intervention and comparison areas used overlapping approaching for programming though the intervention area had a strong community component to implementation. The results suggest that community implementation of the QI intervention may have indeed assisted in improvements in PNC. Overall, these results show promise that firstly, national implementation of PNC can save lives and improve newborn feeding though additional work is needed to supplement existing approaches to newborn feeding. Then, QI interventions may be a useful mechanism to improve PNC coverage. These findings are useful to monitor national outcomes as well as examine small-scale studies which contribute to the overall knowledge base on PNC implementation.Doctor of Philosoph

Phase Transitions and Corresponding Magnetic Entropy Changes in Ni2Mn0.75Cu0.25−xCoxGa Heusler Alloys

Detailed studies of room temperature crystal structures, phase transitions, and related magnetic entropy changes (ΔSm) in shape memory alloys Ni2Mn0.75Cu0.25−xCoxGa (x = 0.0, 0.01, 0.02,0.025 0.03, 0.05) have been carried out by x-ray diffraction, magnetization, and thermal expansion measurements in magnetic fields of up to 5 T and in a temperature interval of 5–400 K. The high temperature austenitic cubic phase passes through a magnetic transition to ferromagnetic state and a structural transition to martensitic phase at the same temperature for all samples of the Ni2Mn0.75Cu0.25−xCoxGa system. The first order magnetostructural transition temperature increases from 308 to 345 K with increasing Co concentration. All of the alloys in the Ni2Mn0.75Cu0.25−xCoxGa system were found to possess large magnetic entropy changes. The maxima in the magnetic entropy changes ranged from ΔSmmax = −48 J/kg K to −64 J/kg K in a temperature range of 308–345 K

Exchange Bias in Bulk Mn Rich Ni–Mn–Sn Heusler Alloys

An experimental study on the exchange bias properties of bulk polycrystalline Ni50Mn50−xSnx Heusler alloys has been performed. Martensitic transformations have been observed in the alloys for some critical Sn concentrations. The alloys, while in their respective martensitic phases, are found to exhibit exchange bias effect. Shifts in hysteresis loops of up to 225 Oe were observed in the 50 kOe field cooled samples. The observed exchange bias behavior in Ni50Mn50−xSnx is attributed to the coexistence of antiferromagnetic and ferromagnetic exchange interactions in the system

The Structural and Magnetic Properties of Ni2Mn1−xMxGa (M = Co, Cu)

In Ni2MnGa (cubic structure of L21 type) a first order martensitic structural transition, from the parent cubic (austenitic) phase to a low temperature complex tetragonal structure, takes place at TM = 202 K, and ferromagnetic order in the austenitic phase sets at TC = 376 K. In this work, the Mn sites in Ni2MnGa have been partially substituted with magnetic Co and nonmagnetic Cu, and the influence of these substitutions on the structural and magnetic properties of Ni2Mn1−xMxGa (M = Co and Cu) have been studied by XRD and magnetization measurements. X-ray diffraction patterns indicate that the Co doped system possess a highly ordered Heusler alloy L21 type structure for 0.05\u3cx\u3c0.12, and the Cu doped compounds possess L21 structure for 0.05\u3cx\u3c0.10. The ferromagnetic ordering temperature increases with increasing Co concentration for this system, and rapidly decreases with increasing Cu concentration. Both systems show the increase in TM with increasing Co and Cu concentration. (T-x) phase diagrams have been plotted. The results are discussed in terms of 3d-electron concentration variation

Exchange Bias Behavior in Ni–Mn–Sb Heusler Alloys

The authors report the observation of exchange bias in bulk polycrystalline Ni50Mn25+xSb25−x Heusler alloys. Shifts in hysteresis loops of up to 248 Oe were observed in the 5 T field cooled samples. The observed exchange bias behavior in Ni50Mn25+xSb25−x is attributed to the coexistence of antiferromagnetic and ferromagnetic exchange interactions in the system. Such behavior is an addition to the multifunctional properties of the Ni50Mn25+xSb25−x Heusler alloy system