Search

Ternary half-Heusler (HH) alloys are under intense investigations recently towards achieving high thermoelectric (TE) figure-of-merit (ZT). Of particular interest is the ZrNiPb-based HH alloy, where an optimal value of ZT ~ 0.7 at 773 K has been achieved by co-doping Sn and Bi at Pb site. In this work, we identify an excellent ZT of 1.3 in ZrNi1+xPb0.38Sn0.6Bi0.02 (x = 0.03, at 773 K) composite alloy. This is achieved by synergistic modulation of electronic as well as thermal properties via introduction of minor phase of full-Heusler (FH) in the HH matrix through compositional tuning approach. These Ni-rich ZrNi1+xPb0.38Sn0.6Bi0.02 (0 ≤ x ≤ 0.07) alloys were synthesized via energy efficient and time-curbed techniques that involved Arc melting followed by consolidation via spark plasma sintering. These alloys were characterized by XRD and SEM, which show formation of nanocomposites comprising of HH matrix phase and FH secondary minor phases. Enhancement in ZT is mainly attributed to a synchronized increase in power factor (~42%) and ~25% decrease in its thermal conductivity. Here, TE compatibility factor (S) was also calculated for all samples. The value of |S| ~ 2.7 V−1 (at 773 K) is observed for x = 0.03, which is ~17% higher than bare HH composition (x = 0.0). The theoretically calculated TE device efficiency of best-performing sample ZrNi1.03Pb0.38Sn0.6Bi0.02 is estimated to be η ~ 13.6%. Our results imply that deliberately controlled fine tuning in compositions of HH compounds through compositional tuning approach would lead to novel off-stoichiometric HH phases with enhanced ZT value for efficient TE device fabrication. [ABSTRACT FROM AUTHOR]

The results of in situ studies of the magnetostructural transition occurring in single crystals of an Ni2.19Mn0.81Ga Heusler alloy are presented in this paper. The formation of martensitic twins on exposure to high magnetic fields up to 10 T in the magnetostructural transition regime was observed using an indigenously developed optical microscope. Experiments on magnetization were performed on single crystals of Ni2.19Mn0.81Ga in high magnetic fields, a phase diagram between magnetic field and temperature was constructed, and the isothermal entropy change was estimated. Based on the experimental data and the results from optical microscopic examination, the influence of magnetic field and thermal cycling on martensitic twins and, in turn, on the magnetocaloric effect, is discussed. [ABSTRACT FROM AUTHOR]

First-principles calculations of the stability, electronic, and magnetic properties of full-Heusler compound V2FeSi and Fe2VSi in regular ( L 2 1 ) and inverse ( X A ) structures have been performed using density functional theory within an SCAN meta-GGA functional. It is found that the X A crystal lattice is energetically more favorable for V2FeSi, while Fe2VSi forms the L 2 1 structure. In both cases, the electronic structure of the energetically stable modifications corresponds to half-metallic ferrimagnets. Magnetic properties of energetically favorable structures obey the Slater–Pauling rule. All considered properties of the studied structures are explained within the crystal orbital Hamilton population analysis. [ABSTRACT FROM AUTHOR]

This paper is devoted to the study of magnetocaloric effect (MCE) in Ni-Mn-In-Co Heusler alloys by direct methods in high magnetic fields. Ni-Mn-In-Co Heusler alloys demonstrate the inverse MCE in the magnetostructural transition area. The adiabatic temperature change value ( \Delta T\mathrm {ad} ) is determined by direct extraction method in magnetic fields up to 10 T. It is shown that the value of \Delta T\mathrm {ad} increases with decrease in the difference between the temperature of magnetostructural transition and the Curie point temperatures of the compounds. The amount of isothermal heat emission (sorption) for Ni43Mn37.8In12.2Co7 alloy is determined as a result of magnetization in magnetic field up to 10 T in the magnetostructural transition area. The obtained results are discussed from the viewpoint of magnetic and lattice subsystems influences on the total entropy change ( \Delta St ). [ABSTRACT FROM AUTHOR]

Direct determinations of the isothermal entropy increment, -△ST, in the Heusler alloy Ni50CoMn36Sn13 on demagnetization gave positive values, corresponding to a normal magnetocaloric effect. These values contradict the results derived from heat-capacity measurements and also previous results obtained from magnetization measurements, which indicated an inverse magnetocaloric effect, but showing different values depending on the technique employed. The puzzle is solved, and the apparent incompatibilities are quantitatively explained considering the hysteresis, the width of the martensitic transition and the detailed protocol followed to obtain each datum. The results show that these factors should be analyzed in detail when dealing with Heusler alloys. [ABSTRACT FROM AUTHOR]

Room temperature magnetic refrigeration is an energy saving and environmentally-friendly technology, which has developed rapidly from a basic idea to prototype devices. The performance of magnetic refrigerators crucially depends on the magnetocaloric properties and the geometry of the employed refrigerants. Here we review the magnetocaloric properties of Heusler alloys and related compounds with a high surface to volume ratio such as films, ribbons, and microwires, and compare them with their bulk counterparts. [ABSTRACT FROM AUTHOR]

Structural, electronic, and magnetic properties of Mn2Co1-xVxZ (Z = Ga, Al, x = 0, 0.25, 0.5, 0.75, 1) Heusler alloys were theoretically investigated for the case of L21 (space group Fm 3 ¯ m), L21b (L21 structure with partial disordering between Co and Mn atoms) and XA (space group F 4 ¯ 3m) structures. It was found that the XA structure is more stable at low V concentrations, while the L21 structure is energetically favorable at high V concentrations. A transition from L21 to XA ordering occurs near x = 0.5, which qualitatively agrees with the experimental results. Comparison of the energies of the L21b and XA structures leads to the fact that the phase transition between these structures occurs at x = 0.25, which is in excellent agreement with the experimental data. The lattice parameters linearly change as x grows. For the L21 structure, a slight decrease in the lattice constant a was observed, while for the XA structure, an increase in a was found. The experimentally observed nonlinear behavior of the lattice parameters with a change in the V content is most likely a manifestation of the presence of a mixture of phases. Almost complete compensation of the magnetic moment was achieved for the Mn2Co1-xVxZ alloy (Z = Ga, Al) at x = 0.5 for XA ordering. In the case of the L21 ordering, it is necessary to consider a partial disorder of atoms in the Mn and Co sublattices in order to achieve compensation of the magnetic moment. [ABSTRACT FROM AUTHOR]

Review: 152 refs. [ABSTRACT FROM AUTHOR]

• Demonstration of the new method of studding shape memory alloys in magnetic field. • Stress-temperature-magnetic field dependence for Ni 54 Mn 21 Ga 25 , Ni 50 Mn 41.2 In 8.8 alloys. • Ability of full recoverable deformation of ferromagnetic alloy in magnetic field. • Influence of magnetic field on the martensite transition temperature. • Bilayered functional composite of new type with enhanced properties. Recently the new scheme of layered composite with shape memory effect (SME) for thermal and magnetic actuation was suggested and proved to demonstrate the reversible actuation on micro- and nanoscale. The principles of magnetic field induced martensitic transition (MFIMT) and magnetic field controlled shape memory effect (MFCSME) have the advantages of magnetic-field-controlled actuation at constant temperature, extremely small size of an actuator and compatibility with modern nanotechnologies. The present paper is devoted to experimental study of MFCSME in two ferromagnetic Heusler alloys Ni 54 Mn 21 Ga 25 and Ni 50 Mn 41.2 In 8.8 with positive and negative shift of the martensitic transition temperature in a magnetic field. The three points bending device for dilatometric tests of plate-like samples of the alloys was designed and placed in the field of Bitter coil magnet. The bending deformations versus temperature were measured at various magnetic fields up to 10 T and at mechanical stresses up to 45 MPa. The temperature shifts of thermoelastic martensitic phase transition were found to be 0.55 K/T and −0.95 K/T for Ni 54 Mn 21 Ga 25 and Ni 50 Mn 41.2 In 8.8 respectively. For direct MFCSME study the dependences of the bending deformation on magnetic fields up to 14 T were also obtained at constant temperatures for these alloys. Practically complete recoverable deformation of 0.2% due to MFIMT was obtained at 14 T in Ni 54 Mn 21 Ga 25 alloy in temperature range 316–318 K and stress 8.6 MPa. The new variant of the layered functional composite actuator scheme based on the alloys with MFCSME is suggested. The proposed functional composite combines the layers of the alloys with positive and negative temperature shifts of martensitic transition temperature in magnetic field. It is argued that this combination of the alloys can improve the performance of the composite actuators driven by MFCSME, particularly, it can provide higher generated force, actuation stroke, sensitivity to magnetic field and frequency of actuation. [ABSTRACT FROM AUTHOR]

Large exchange bias (EB) effect is demonstrated in a Ni 56 Mn 21 Al 22 Si 1 polycrystalline ribbon that shows spin-glass behaviour below 68 K and superparamagnetic (SPM) behaviour above this temperature. The average magnetic moment μ of the SPM clusters is estimated to be considerably smaller than that reported for other Heusler alloy systems that demonstrate spin glass behaviour and EB effect. The M – H loops measured in field-cooled (FC) conditions show significant asymmetry that is observed to be strongly field and temperature dependant. The maximum EB field ( H EB ~2.6 kOe at 20 kOe and 2 K) is much larger than that reported for any metallic alloy system with conventional EB effect. The dependence of H EB on temperature shows an exponential decrease. An approach towards enhancement of the EB effect in this conventional EB system can be realized via manipulation of SPM or superferromagnetic (SFM) cluster size with variation of cooling magnetic field. [ABSTRACT FROM AUTHOR]

Hysteresis features of the direct and inverse magnetocaloric effect associated with first-order magnetostructural phase transitions in Ni-Mn-X (X = Ga, Sn) Heusler alloys have been disclosed by differential calorimetry measurements performed either under a constant magnetic field, H, or by varying H in isothermal conditions. We have shown that the magnetocaloric effect in these alloys crucially depends on the employed measuring protocol. Experimentally observed peculiarities of the magnetocaloric effect have been explained in the framework of a model that accounts for different contributions to the Gibbs energy of austenitic gA and martensitic gM phases. Obtained experimental results have been summarized by plotting a phase fraction of the austenite xA versus the driving force gM -- gA. The developed approach allows one to predict reversible and irreversible features of the direct as well as inverse magnetocaloric effect in a variety of materials with first-order magnetic phase transitions [ABSTRACT FROM AUTHOR]

Heusler alloys have attracted much attention due to their low toxicity, mechanical and thermal stability, relative abundance, and potential for thermoelectric power generation. Herein, we demonstrate that TiFe 2 Sn full-Heusler alloy can be obtained by a mechanochemical synthesis of metal precursors followed by spark plasma sintering. It is shown that thus prepared samples exhibit the ordered structure (Heusler phase, MnCu 2 Al-type). The influence of processing parameters on microstructure, phase composition, and thermoelectric properties of TiFe 2 Sn is studied. The thermoelectric figure of merit of the sample fabricated under optimal mechanochemical processing parameters is comparable to that of TiFe 2 Sn synthesized using the conventional route. Thus, our results indicate that the mechanochemical synthesis is a facile, scalable, and cost-effective route for the preparation of TiFe 2 Sn Heusler alloys. • TiFe 2 Sn prepared by mechanochemical synthesis followed by spark plasma sintering. • Mechanochemical synthesis performed at room temperature. • Figure of merit similar to that of TiFe 2 Sn synthesized by a conventional path. [ABSTRACT FROM AUTHOR]