37 research outputs found
Crystal Structures of Modulated Martensitic Phases of FSM Heusler Alloys
No full textMultifunctional ferromagnetic shape memory Heusler alloys are frequently characterized by structural modulation in martensitic phases. In particular, modulated martensitic phases, showing the higher magnetic field induced strain (MFIS) performance, are the most promising candidates for technological applications. Depending on the composition, as well as pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along defined crystallographic directions, accompanies the martensitic transformation. Over the years, different Ni-Mn-Ga modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding ideal nM superstructure (where n indicates the number of basic unit cells constituting the superlattices). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure has been fully determined by applying superspace formalism. The results, obtained by structure refinements on powder diffraction data, suggest a unified crystallographic description of the modulated martensitic structures, here presented, where every different “nM” periodicity can be straightforwardly represented
Influence of the transition width on the magnetocaloric effect across the magnetostructural transition of Heusler alloys
No full textWe report a complete structural and magneto-thermodynamic characterization of four samples of the Heusler alloy Ni-Co-Mn-Ga-In, characterized by similar compositions, critical temperatures and high inverse magnetocaloric effect across their metamagnetic transformation, but different transition widths. The object of this study is precisely the sharpness of the martensitic transformation, which plays a key role in the effective use of materials and which has its origin in both intrinsic and extrinsic effects. The influence of the transition width on the magnetocaloric properties has been evaluated by exploiting a phenomenological model of the transformation built through geometrical considerations on the entropy vs. temperature curves. A clear result is that a large temperature span of the transformation is unfavourable to the magnetocaloric performance of a material, reducing both isothermal entropy change and adiabatic temperature change obtainable in a given magnetic field and increasing the value of the maximum field needed to fully induce the transformation. The model, which is based on standard magnetometric and conventional calorimetric measurements, turns out to be a convenient tool for the determination of the optimum values of transformation temperature span in a trade-off between sheer performance and amplitude of the operating range of a material
Evidence for in-plane tetragonal c-axis in Mn<sub>x</sub>Ga<sub>1-x</sub> thin films using transmission electron microscopy
No full textTetragonal MnxGa1-x (x = 0.70, 0.75) thin films grown on SrTiO3 substrates exhibit perpendicular magnetic anisotropy with coercive fields between 1 and 2 T. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) reveal that 40 nm samples grown at 300-350 degrees C lead to films with the tetragonal c-axis oriented primarily perpendicular to the film plane but with some fraction of the sample exhibiting the c-axis in the film plane. This structure results in an undesirable secondary magnetic component in the out of plane magnetization. Growth at 300 degrees C with a reduced thickness or Mn concentration significantly decreases the tetragonal c-axis in the film plane. (C) 2015 Elsevier Ltd. All rights reserved
On the broadening of the martensitic transition in Heusler alloys: from microscopic features to magnetocaloric properties
No full textWe report on the sharpness of the magnetostructural transition in a Ni-Co-Mn-Ga-In Heusler alloy, which has a key role in defining the magnetocaloric effect exploitable in efficient and environmentally friendly magnetic refrigerators. Magnetic and calorimetric analysis together with optical microscopy in phase-imaging mode are compared to correlate the transition broadening with microscopic features of the sample. The three techniques give the same temperature behavior of the martensitic transition. Microscopy analysis of different sample areas and during cycles in temperature reveals the spatial homogeneity of the transition width and its reproducibility in subsequent transformations
Ferromagnetic shape memory Ni-Mn-Ga alloys: a new synergy between structure and properties
No full textThe family of Ni-Mn based Heusler alloys provides an extended playground of physical properties. The interplay between a reversible martensitic transformation (MT) and magnetically ordered states gives rise to a series of functional properties that can be exploited for developing innovative devices [1] which originate from the possibility to dramatically change the materials properties by an applied external stimulus, such as magnetic field, stress of pressure. One of the most interesting property rely on the occurrence of large strains activated by external magnetic field. This phenomenon which takes the name of Magnetic Shape Memory (MSM) is mainly related to the particular crystalline structure assumed by the martesntic phase characterizing such family of this alloy. The martensitic transformation induced by the temperature change (corresponding to TM) consists of a deformation of the lattice of the austenitic phase having a structure type L21 with cubic symmetry. In Ni-Mn-Ga alloys the austenite is associated to a ferromagnetic state which is retained during the martensitic transformation. Upon the application of a magnetic field the martensite is magnetized by sliding the twinning boundaries typically featuring the product phase. The macroscopic effect of this process is represented by giant strains and this special synergy between the crystal structure of martensite and magnetic properties is at the basis of several international studies focused on the possible technological applications especially in the field of micromechanics. The cooperative aspect between crystal structure and magnetism is also manifested in the giant magnetocaloric effect. This property has triggered attention to the possible applications in the refrigeration industry (green technology). Basically, the observed magnetocaloric effect is closely related to the large difference in entropy between the martensitic and austenitic phase. In this paper we present the main physical properties of the Ni-Mn-Ga Heusler alloys potentially suitable for the design of a new generation of multifunctional smart devices
Incommensurate and Commensurate Structural Modulation in Martensitic Phases of FSMA
No full textMagnetic and structural properties in multifunctional FSMA (Ferromagnetic Shape Memory Alloys) belonging to Heusler family are frequently related to the occurrence of structural modulation in martensitic phases. The highest MFIS (Magnetic Field Induced Strain) effect has been observed in Ni-Mn-Ga alloys showing martensitic modulated structures. Depending on the composition, pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along specific crystallographic directions, accompanies the martensitic transformation. Over the years, different modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding superstructure (nM with n=3, 5, 6, 7, 12 etc). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure can be solved by applying superspace approach. The crystallographic representation of different modulated structures, obtained by structure refinement on powder diffraction data, suggests a unified description where every different “nM” periodicity can be straightforwardly represented. It will be presented an overview illustrating structural features of several displacive modulated martensitic lattices. For a specific Ni-Mn-Ga composition, the evolution of structural modulation upon temperature change will be illustrated
Charge order and tilt modulation in multiferroic (KxM(II)xM(III)1-xF3)(0.4 < x < 0.6) transition metal fluorides with tetragonal tungsten bronze structure
No full textTransition metal fluorides with tetragonal tungsten bronze (TTB) structure, having general formula (KxMxM1-xF3)-M-II-F-III with 0.4 < x < 0.6 and M belonging to transition metals, have been studied over the past decades because of the frustrated magnetic ordering arising at low temperature. We investigated the structural properties of TTB fluorides by transmission electron microscopy (TEM). The results, coupled with an accurate structure analysis performed by single-crystal XRD on K0.53FeF3, revealed ferroelectricity and a unified structural description of these materials, where charge order and tilt modulations coexist, each one with its own periodicity differently commensurate to the conventional TTB cell. Whereas the charge order is peculiar of the mixed valence fluorides,the nature of the tilt modulation is clearly associable to the structural features of TTB niobates, suggesting also for the fluoride bronzes the existence of a generalized ferroelectric-ferroelastic behavior at room temperature
Structural properties and multiferroic phase diagram of K0.6Fe0.6(II)Fe0.4(III)F3 fluoride with TTB structure
No full textEpitaxial Ni–Mn–Ga/MgO(100) thin films ranging in thickness from 10 to 100nm
No full textThin films of Ni–Mn–Ga alloy ranging in thickness from 10 to 100 nm have been epitaxially grown on MgO(1 0 0) substrate. Temperature-dependent X-ray diffraction measurements combined with room-temperature atomic force microscopy and transmission electron microscopy highlight the structural features of the martensitic structure from the atomic level to the microscopic scale, in particular the relationship between crystallographic orientations and twin formation. Depending on the film thickness, different crystallographic and microstructural behaviours have been observed: for thinner Ni–Mn–Ga films (10 and 20 nm), the L21 austenitic cubic phase is present throughout the temperature range being constrained to the substrate. When the thickness of the film exceeds the critical value of 40 nm, the austenite-to-martensite phase transition is allowed. The martensitic phase is present with the unique axis of the pseudo-orthorhombic 7M modulated martensitic structure perpendicular to the film plane. A second critical thickness has been identified at 100 nm where the unique axis has been found both perpendicular and parallel to the film plane. Magnetic force microscopy reveals the out-of-plane magnetic domain structure for thick films. For the film thickness below 40 nm, no magnetic contrast is observed, indicating an in-plane orientation of the magnetization
Studio delle proprietà meccaniche e smorzanti dei compositi in fibra di carbonio nano-rinforzati.
No full textNella tesi in esame ci si pone come obiettivo principale quello di indagare il comportamento meccanico e dinamico di un materiale composito nano-rinforzato. Il composito posto in esame risulta essere un materiale innovativo in quanto unisce un prepreg in fibra di carbonio con fibre unidirezionali e membrane di nano-fibre in gomma. Negli ultimi anni si è notata una crescente esigenza nel settore industriale volta a diminuire le vibrazione in sistemi meccanici sottoposti a sollecitazioni periodiche, come ad esempio, mandrini per macchine automatiche o rulli per il settore tissue o covering, tutto questo giustifica la ricerca portata avanti in questa tesi che consiste nello sviluppare un materiale composito innovativo in grado di attenuare le vibrazioni senza d’altra parte aumentarne il peso. Sono da tempo riconosciute le alte capacita smorzanti della gomma, ma spesso l’integrazione nei materiali compositi avviene attraverso l’inserimento di una strato spesso di gomma tra due
layer rigidi (struttura a sandwich), questo porta ad un aumento considerevole del peso e dello spessore, caratteristiche che possono rendere un materiale composito meno attraente in alcuni campi di applicazione. Lo smorzamento o damping risulta essere un fenomeno impegnativo da studiare nei materiali metallici perché esso è influenzato da innumerevoli fattori. Per quanto
riguarda i materiali compositi la sfida diventa ancora più impegnativa data la loro natura intrinsecamente eterogenea.
Concludendo, dalle prove sperimentali, è emerso un incremento notevole delle prestazioni smorzanti dei provini nano-modificati, lasciando inalterate le prestazioni meccaniche
