Search

Natural gases have played a significant role in different sectors of the global economy. Recent analyses have shown that the world's gas consumption doubled over the last three decades; further growth of the gas consumption is predicted, rising to be 23%–28% of the total primary energy demand by 2030. Therefore, liquefaction of natural gases rapidly gains global importance. In this context, magnetic refrigeration emerges as a modern energy-saving technique, which is an alternative to the traditional gas-compression refrigeration. This paper is devoted to the study of the magnetocaloric effect in magnetic fields up to 10 T on a representative of the Laves phase alloys, GdNi2, which is considered as a perspective material for liquefaction of natural gases. For a magnetic field change of 10 T, the magnetic entropy change ΔSm ≈ −17 J/kg K and the adiabatic temperature change ΔTad ≈ 6.8 K was attained around Curie temperature TC = 70 K. The maximal value of the adiabatic temperature change measured directly in pulsed magnetic fields up to 50 T is ΔTad ≈ 15 K. [ABSTRACT FROM AUTHOR]

Dysprosium possesses the highest magnetic moment among the rare-earth elements and undergoes a second-order magnetic phase transition to a complex helical antiferromagnetic ordering at Néel temperature TN = 180 K which is followed by a transformation to the ferromagnetic ordering at a lower temperature. Superior magnetic properties and tunability of the magnetic phase transition temperatures make Dy and its solid solutions perspective for the use in technology of magnetic refrigeration at cryogenic temperatures, specifically for the liquefaction of natural gases. In this work we report on the evolution of magnetic properties in solid solutions of Dy100-xYx (x = 0 – 15). It was found that the dilution of Dy magnetic subsystem by a non-magnetic Y has rather a weak impact on the magnetization saturation, which decreases from Ms ∼280 A⋅m2/kg in the x = 0 sample to Ms ∼259 A⋅m2/kg in the x = 15 sample. As a consequence, the isothermal magnetic entropy change was found to decrease slightly in the series of Dy100-xYx (x = 0 – 15) solid solutions. [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]

Composite thermoelectric materials based on skutterudite represent an interesting subject for the synthesis and study of their thermoelectric properties. The introduction of oxide additives typically reduces the cost of the product and can positively affect the mechanical properties and thermal stability of the material. In this work, a high-performance composite skutterudite (InCo 4 Sb 12) + x wt.% (ZnO) (x = 0, 1, 2.7 and 7.4) was obtained by a combination of induction melting and planetary ball milling. Bulk composites with a density of 90 % were obtained by spark plasma sintering. In contrast to the results reported in previously published works, in this case there was an increase in thermal conductivity, which was associated with the formation of agglomerates of highly thermally conductive oxide phases. The figure of merit zT reaches 1.41 at 700 K for the reference sample InCo 4 Sb 12 , and 1.28 for the composite with embedded ZnO inclusions. Despite a 9 % decrease in the zT, the best composite InCo 4 Sb 12 /ZnO demonstrates an increase in hardness of 18 % and reduction in the cost of raw materials. [ABSTRACT FROM AUTHOR]

Skutterudites are one of the most efficient materials for use in thermoelectric applications. From an industrial point of view, along with the high efficiency of thermoelectric materials, it becomes actual to develop a rapid, scalable, and energy-efficient fabrication technique. In this regard, mechanochemical synthesis has gained significant attention as a promising approach. In this work, we demonstrate the applicability and the potential of mechanochemical synthesis for the preparation of p-type skutterudites using Yb0.75Ce0.25Fe3.5Co0.5Sb12 composition as an example. In fact, the time spent on a full cycle of synthesis of bulk samples is less than 10 h, and the highest figure of merit value was 0.62, which is comparable to the values for similar compositions from previously works, where the time for synthesis is tens of times longer. [ABSTRACT FROM AUTHOR]

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]

Oxides are of interest for thermoelectrics due to their high thermal stability, chemical inertness, low cost, and eco‐friendly constituting elements. Here, adopting a unique synthesis route via chemical co‐precipitation at strongly alkaline conditions, one of the highest thermoelectric performances for ZnO ceramics (PFmax=$PF_{\text{max}} =$ 21.5 µW cm−1 K−2 and zTmax=$zT_{\text{max}} =$ 0.5 at 1100 K in Zn0.96Al0.04O${\rm Zn}_{0.96} {\rm Al}_{0.04}{\rm O}$) is achieved. These results are linked to a distinct modification of the electronic structure: charge carriers become trapped at the edge of the conduction band due to Anderson localization, evidenced by an anomalously low carrier mobility, and characteristic temperature and doping dependencies of charge transport. The bi‐dimensional optimization of doping and carrier localization enable a simultaneous improvement of the Seebeck coefficient and electrical conductivity, opening a novel pathway to advance ZnO thermoelectrics. [ABSTRACT FROM AUTHOR]

Review: 152 refs. [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]

The combination of photothermal and magnetic functionalities in one biocompatible nanoformulation forms an attractive basis for developing multifunctional agents for biomedical theranostics. Here, we report the fabrication of silicon–iron (Si-Fe) composite nanoparticles (NPs) for theranostic applications by using a method of femtosecond laser ablation in acetone from a mixed target combining silicon and iron. The NPs were then transferred to water for subsequent biological use. From structural analyses, it was shown that the formed Si-Fe NPs have a spherical shape and sizes ranging from 5 to 150 nm, with the presence of two characteristic maxima around 20 nm and 90 nm in the size distribution. They are mostly composed of silicon with the presence of a significant iron silicide content and iron oxide inclusions. Our studies also show that the NPs exhibit magnetic properties due to the presence of iron ions in their composition, which makes the formation of contrast in magnetic resonance imaging (MRI) possible, as it is verified by magnetic resonance relaxometry at the proton resonance frequency. In addition, the Si-Fe NPs are characterized by strong optical absorption in the window of relative transparency of bio-tissue (650–950 nm). Benefiting from such absorption, the Si-Fe NPs provide strong photoheating in their aqueous suspensions under continuous wave laser excitation at 808 nm. The NP-induced photoheating is described by a photothermal conversion efficiency of 33–42%, which is approximately 3.0–3.3 times larger than that for pure laser-synthesized Si NPs, and it is explained by the presence of iron silicide in the NP composition. Combining the strong photothermal effect and MRI functionality, the synthesized Si-Fe NPs promise a major advancement of modalities for cancer theranostics, including MRI-guided photothermal therapy and surgery. [ABSTRACT FROM AUTHOR]

We report on thermoelectric properties of layered misfit cobaltites Ca 3− x Na x Co 4 O 9− x F x ( x = 0, 0.3, 0.45, 0.6) prepared by a two steps solid state reaction route followed by spark plasma sintering. Structural characterization revealed that a minor amount of Co 3 O 4 is presented in all the samples as an impurity phase. Besides, a trace of NaF phase was detected in the sample with x = 0.6 which can be an indication of the solubility limit of NaF into Ca 3 Co 4 O 9 . Measurements of electrical resistivity and Hall effect showed that double substitution of Ca 2+ by Na + and O 2− by F − results in a decrease of the electrical resistivity which is due to an increase in the carrier concentration. Thermal conductivity of Ca 3− x Na x Co 4 O 9− x F x decreases with increasing temperature which is due to the enhancement of the phonon scattering at high temperatures and slightly increases with x which is caused by the increase in the carrier concentration and the growth of the mean grain size in the doped samples. Seebeck coefficient S of Ca 3− x Na x Co 4 O 9− x F x is positive in the whole temperature range measured. At high temperatures (above 800 K) value of S in the x = 0.3 and x = 0.45 samples exceeds that in the pristine sample. Thermoelectric figure of merit ZT = 0.13 was achieved at 873 K, which is of about 20% higher than that in the pristine Ca 3 Co 4 O 9 . [ABSTRACT FROM AUTHOR]

In this work we report on direct measurements of magnetocaloric effect in plastically deformed Gd-In solid solutions in the shape of foils with concentration of indium up to 3 at.%. When compared to the reference polycrystalline bulk samples, magnetocaloric effect in the cold-rolled foils of Gd 100− x In x ( x  = 0, 1, 3) turned out to be systematically smaller. This was suggested to be due to a cold rolling-induced local magnetic anisotropy which can be diminished by an appropriate thermal treatment of the cold-rolled samples. [ABSTRACT FROM AUTHOR]

Measurements of the adiabatic temperature change (ΔT) and the specific heat transfer (ΔQ) of Ni2.18Mn0.82Ga Heusler alloy were taken in order to quantify the direct giant magnetocaloric effect of the alloy when it is in the vicinity of magneto-structural phase transition (PT) from paramagnetic austenite to ferromagnetic martensite, and their results are presented. A new vacuum calorimeter was used to simultaneously measure ΔT and ΔQ of magnetocaloric materials with a Bitter coil magnet in fields of up to H=140 kOe. Other thermomagnetic properties of this alloy were investigated using standard differential scanning calorimetry and PPMS equipment. The maximal values of magnetocaloric effect in H=140 kOe were found to be ΔT=8.4K at initial temperature 340K and ΔQ=4900 J/kg at 343 K. Using this direct method, we show that the alloy indeed demonstrates the largest value of ΔQ as compared with previously published results for direct measurements of magnetocaloric materials, even though at 140 kOe the magnetic field-induced magnetostructural PT is still not complete. [ABSTRACT FROM AUTHOR]

Half-Heusler compounds exhibit a remarkable variety of emergent properties such as heavy-fermion behaviour, unconventional superconductivity and magnetism. Several of these compounds have been predicted to host topologically non-trivial electronic structures. Remarkably, recent theoretical studies have indicated the possibility to induce non-trivial topological surface states in an otherwise trivial half-Heusler system by strain engineering. Here, using magneto-transport measurements and first principles DFT-based simulations, we demonstrate topological surface states on strained [110] oriented thin films of YPdBi grown on (100) MgO. These topological surface states arise in an otherwise trivial semi-metal purely driven by strain. Furthermore, we observe the onset of superconductivity in these strained films highlighting the possibility of engineering a topological superconducting state. Our results demonstrate the critical role played by strain in engineering novel topological states in thin film systems for developing next-generation spintronic devices. [ABSTRACT FROM AUTHOR]

(Ni48Co6)Mn26Al20 polycrystalline ribbons with B2 structure at room temperature are investigated. Considering the presence of competing magnetic interactions, Arrott-plot analysis gives TN ∼ 170 K. A broad ferromagnetic-paramagnetic transition (TC) is observed at ∼200 K. H-T phase-diagram is used to validate the presence of competing exchange interactions that persist till very close to TC. Based on Néel theory, a cluster model is used to explain the presence of ferromagnetic and antiferromagnetic clusters in the sample. Formation of ferromagnetic clusters can be understood in terms of positive exchange interactions among the Mn atoms that are neighboring Co atoms located at Ni sites. [ABSTRACT FROM AUTHOR]

Herein, we demonstrate that BiCuSeO compound can be formed in bulk directly from the raw materials through reactive spark plasma sintering (RSPS) followed by ball milling and a second short spark plasma sintering step. Compared to BiCuSeO samples obtained by a conventional solid-state reaction, the electrical transport properties of the RSPS bulk were moderately affected by the sintering technique, while the lattice thermal conductivity was almost unaffected, and the figure of merit zT attained a value at 773 K comparable to state-of-the-art BiCuSeO. The results indicate a new scalable method for the preparation of oxyselenides. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Structural, magnetic, and transport properties of Ni52+xMn26-xAl22 (1 ≤ x ≤ 5) melt-spun ribbons have been characterized by a variety of experimental techniques. As the composition changed from x = 1 to x = 5, the martensitic transition temperature T0 [ = (Ms + Af)/2] was found to increase from 277 K to 446 K which was attributed to an increase in the valence electron concentration e/a. In the martensitic state, all the samples demonstrate an anomalous semiconducting behavior of electrical resistivity ρ. This uncommon feature of the transport properties has been ascribed to the existence of a gap (Eg ∼ 0.1 eV) at the Fermi level. A crossover from semiconducting to metallic behavior of ρ observed in the martensitic state of Ni57Mn21Al22 is presumably related to a spin-density wave formation at the Neel temperature TN ≈ 300 K. Analysis of a low-temperature (T < 60 K) part of the resistivity curves and comprehensive magnetic measurements of a Ni57Mn21Al22 (x ≤ 5) sample provide grounds for the conclusion that the splitting of zero-field cooling and field cooling magnetization curves observed at low temperatures is due to a spin-glass state that is formed below the freezing temperature Tf. [ABSTRACT FROM AUTHOR]

ZnO has potential as a thermoelectric material for high-temperature applications, but its performance is limited by the strong interconnection between electrical and thermal transportation. In this study we proved that ZnO bulk samples produced from cost-effective and environmentally friendly nitrates via ultrasonic spray pyrolysis and modified chemical precipitation methods followed by spark plasma sintering, demonstrate significantly higher thermoelectric performance than those fabricated from commercially available ZnO powder. To further explore the potential of the developed chemical precipitation technique, we synthesized a series of Al-doped samples with a nominal composition of Zn 1– x Al x O (x = 0.02, 0.04, 0.06). The substitutuion of Al into the ZnO structure led to two significant effects: (i) a partial substitution of Zn with Al boosts the power factor; (ii) the formation of the ZnAl 2 O 4 spinel phase acting as scattering center for phonons in addition to the mass and strain fluctuations induced by the partial Zn-Al substitution, both leading to the thermal conductivity reduction. As a result, Al doping led to a threefold enhancement of the thermoelectric performance with z T value of ∼0.12 achieved at 1100 K for the Zn 0.94 Al 0.06 O sample. • ZnO ceramics fabricated via ultrasonic spray pyrolysis and chemical precipitation. • Obtained samples exhibit higher degree of disorder and thus increased μ w. • Al doping of ZnO leads to a threefold increase of thermoelectric performance. • A z T value of ∼0.12 achieved for the Zn 0.94 Al 0.06 O sample at 1100 K. [ABSTRACT FROM AUTHOR]

• For the first time, composites made of SiGe and MnTe were investigated. • MnTe enhances the electrical conductivity and reduces the lattice thermal conductivity. • The maximum value of zT for Si 80 Ge 20 B 2 + 4 wt% MnTe was 0.8 at 1000 K. We report on a study of bulk SiGe-MnTe composites obtained via spark plasma sintering of mixed powders composed of SiGe that was fabricated by mechanical alloying and MnTe prepared via solid-state reaction. The presence of MnTe in the SiGe matrix led to an increase in the electrical conductivity by ∼ 50 % and a decrease in the lattice thermal conductivity of the samples by ∼ 28 %, while the Seebeck coefficient was reduced from 252 μV K−1 to 197 μV K−1 at 1000 K. As a result, the peak zT of 0.75 was obtained for pristine Si 80 Ge 20 B 2 , and 0.8 for Si 80 Ge 20 B 2 + 4 wt% MnTe at 1000 K. [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]

Highlights • Magnetocaloric effect (MCE) in cold rolled Dy was studied. • MCE in cold rolled Dy is reduced as compared to the bulk Dy. • Magnetocaloric properties of cold rolled Dy can be restored by annealing. Abstract We report on the magnetic properties and magnetocaloric effect of dysprosium ribbons obtained from a bulk sample by means of cold rolling. A magnetic entropy change Δ S ≈ 5.5 J/(kg·K) and an adiabatic temperature change Δ T ≈ 1.65 K for a bulk sample of dysprosium were observed for in external magnetic field changes of 3 T and 1.9 T, respectively. As compared to the bulk sample, Δ S and Δ T in the cold rolled ribbons is smaller by ∼30% and ∼12%, respectively. These changes in magnetocaloric effect of the cold rolled ribbons are fully reversible and can be restored by additional annealing. [ABSTRACT FROM AUTHOR]

Abstract Bi 1- x Nd x CuSeO (x = 0; 0.05; 0.10; 0.15) was fabricated by the solid-state reaction and the reactive spark plasma sintering for comparative study. It was demonstrated that BiCuSeO could be formed in-situ by the reaction between Bi 2 O 3 , Bi, Cu and Se during the densification process. The effect of Nd doping on the thermoelectric properties has been investigated. The results indicated that for both series of the samples the substitution of Bi3+ by Nd3+ resulted in the increase of charge carrier concentration mainly caused by Bi/Cu deficiencies formation with the substitution. However, the specimens obtained by reactive spark plasma sintering had lower hole concentration which may originate from the high speed of the process resulted in induced point defect and interface scattering and another phase formation mechanism leading to the smaller amount of Bi/Cu deficiencies. Highlights • Nd substituted BiCuSeO oxyselenides were fabricated by reactive spark plasma sintering. • Effect of Bi to Nd substitution on the properties of BiCuSeO has been studied. • By Bi to Nd substitution, Bi and Cu deficiencies formed. • Samples obtained by RSPS exhibited the isotropic behaviour of transport properties. [ABSTRACT FROM AUTHOR]

Highlights • A method of preparation of FeNi L1 0 phase was proposed. • We applied cycling of oxidation/reduction processes at 320 °C of FeNi nanopowder. • This technique excels at up scaling and the ability to control the particle size. • Enhanced diffusion makes the synthesis of Fe-Ni alloys at low temperatures possible. • The influence of the synthesis parameters on physical properties was studied. Abstract FeNi L1 0 (tetrataenite) phase has great perspectives for hard magnetic materials production. In this paper we report on synthesis of this phase in chemically co-precipitated FeNi nanopowder by means of a thermal treatment procedure which includes cycling oxidation and reduction processes at 320 °C. The presence of the FeNi L1 0 phase in the samples was confirmed by magnetic measurements and differential scanning calorimetry analysis. [ABSTRACT FROM AUTHOR]

Abstract: Polymer‐based composites are of high interest in the field of thermoelectric (TE) materials because of their properties: abundance, low thermal conductivity, and nontoxicity. In applications, like TE for wearable energy harvesting, where low operating temperatures are required, polymer composites demonstrate compatible with the targeted specifications. The main challenge is reaching high TE efficiency. Fillers and chemical treatments can be used to enhance TE performance of the polymer matrix. The combined application of vertically aligned carbon nanotubes forest (VA‐CNTF) is demonstrated as fillers and chemical post‐treatment to obtain high‐efficiency TE composites, by dispersing VA‐CNTF into a poly (3,4‐ethylenedioxythiophene) polystyrene sulfonate matrix. The VA‐CNTF keeps the functional properties even in flexible substrates. The morphology, structure, composition, and functional features of the composites are thoroughly investigated. A dramatic increase of power factor is observed at the lowest operating temperature difference ever reported. The highest Seebeck coefficient and electrical conductivity are 58.7 µV K−1 and 1131 S cm−1, respectively. The highest power factor after treatment is twice as high in untreated samples. The results demonstrate the potential for the combined application of VA‐CNTF and chemical post‐treatment, in boosting the TE properties of composite polymers toward the development of high efficiency, low‐temperature, flexible TEs. [ABSTRACT FROM AUTHOR]

We report on thermoelectric properties of low Ge content n -type Si 0.9 Ge 0.1 –Mg 2 Si nanocomposite. Introduction of the Mg 2 Si phase into a SiGe matrix resulted in a dramatic drop of the lattice thermal conductivity beyond the previously reported lowest limit for SiGe alloys due to intensification of phonon scattering on SiGe–Mg 2 Si grain boundaries. For a sample doped with 1 at.% of Mg 2 Si, the peak value of thermoelectric figure of merit ZT reached ~ 0.8 at 800 °C. Sintered nanocomposites still exhibit high thermoelectric performance while being almost two times cheaper than Si 0.8 Ge 0.2 . [ABSTRACT FROM AUTHOR]

We report on the magnetocaloric effect in Gd 100- x B x ( x = 0, 10, 15) cold rolled ribbons. A moderate entropy change Δ S = 5 J/kg·K and adiabatic change of Δ T = 4.8 K were observed for the as-cast materials in an external magnetic field of 3 T which is less by 20% in comparison with the pure gadolinium metal. It was found that a significant (up to 70%) depression of magnetization and magnetocaloric properties developed in the course of plastic deformation can completely be restored by means of a high temperature heat treatment. It is concluded that cold rolling is one promising technique for producing foil shaped magnetocaloric materials suitable for designing heat exchangers of magnetic cooling devices. [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]

We report on thermoelectric properties of p -type boron doped nanostructured bulk Si 80 Ge 20 synthesized via spark plasma technique. We demonstrate that the presence of a limited amount of nanometer-sized SiO 2 inclusions, resulting from the oxidation during processing stages is an effective way to further thermal conductivity reduction in the nanostructured Si 80 Ge 20 alloys. Significant reduction of thermal conductivity and high values of Seebeck coefficient allowed us to reach a peak ZT value of about 0.72 at 800 °C in boron doped Si 80 Ge 20 . [ABSTRACT FROM AUTHOR]

• The effect of an external magnetic field on the dynamic and static properties of a highly anisotropic Ising antiferromagnet is studied. • It has been established that, depending on the magnitude of the magnetic field in the system, the realization of both dipole (FM) and more complex states (QU, LS) is possible. • The types of phase transitions are determined and the phase diagram is constructed for a strongly anisotropic Ising antiferromagnet as a function of the magnitude of the magnetic field. The effect of an external magnetic field on both the static and spectral properties of a highly anisotropic Ising antiferromagnet with magnetic ion spin S = 1 is studied. It is shown that at sufficiently low fields and large values of the single-ion anisotropy constant, a state with a zero value of the average magnetic moment (at the site), which is characterized by tensor order parameters (QU phase), is realized in the system. A further increase in the magnetic field turns the magnet (via a first-order transition) into the "quasi-ferrmagnetic" state. In this state is characterized by the fact that one of the sublattices is magnetized to saturation, whereas in the second the average magnetic moment (per site) is equal to zero. We called this phase the LS (low spin) phase. A further increase in the field leads to the realization of a ferromagnetic phase. All phase transitions are first-order transitions, as evidenced by the behavior of their spectra of elementary excitations. [ABSTRACT FROM AUTHOR]

Aging-induced changes in the features of the calorimetric peak accompanying martensitic transformation were considered in the framework of phenomenological theory. Taking into account two different mechanisms of martensite aging, it was demonstrated that the domination of the internal axial stress contribution narrows the calorimetric peak and elevates its magnitude, while the prevalence of the internal pressure contribution broadens the calorimetric peak and diminishes its magnitude. The developed theoretical approach was applied to modeling of experimentally measured calorimetric peaks of Ni-Mn-Ga alloys. [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]

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]

Among layered oxygen-containing compounds, BiCuSeO is one of the most promising candidates for thermoelectric applications due to its intrinsically low thermal conductivity and good thermal stability. However, the rather poor electrical conductivity of pristine BiCuSeO hinders its potential. Further enhancement of the thermoelectric performance by single doping at Bi site is limited mainly due to dramatic decrease of carrier mobility. Thus, new strategies, such as dual doping or doping with variable-valence elements seem to be promising. Along with that, the development of a fast and scalable synthesis route is essential for the industrial-scale fabrication of thermoelectric materials. Hence, in this paper, Bi 1– x Sm x CuSeO samples (0 ≤ x ≤ 0.08) have been synthesized with a simple and scalable reactive sintering process. For comparison, Bi 1– x Sm x CuSeO oxyselenides were also obtained by the conventional solid-state route. Our results highlight that, Sm for Bi substitution increases the electrical conductivity by 1.5–2 times and decreases the Seebeck coefficient by ~1.4 times at 873 K for both series. Overall, considering the increase of lattice thermal conductivity upon doping and not optimized power factor, the figure of merit z T is reducing upon doping. • Sm-doped BiCuSeO oxyselenides fabricated by reactive sintering. • Sm doping leads to an increase of σ from 39 Ω–1 cm–1 to ~55 Ω–1 cm–1 at 873 K. • Quality factor is reduced upon Sm doping due to increase in κ and decrease of μ w. [ABSTRACT FROM AUTHOR]

The crystal structure, phase transition and exchange bias effect in induction melted polycrystalline ribbons of Ni55Mn19Al24Si2 have been studied using room temperature x-ray diffraction (XRD), differential scanning calorimetry (DSC) and magnetic measurements. The sample was found to show structural transformation temperatures such as austenite start (As)=306K, austenite finish (Af)=316K, martensite start (Ms)=305K and martensite finish (Mf)=294K all above room temperature. The room temperature structure evaluated as orthorhombic 14M with lattice parameters a=4.14Å, b=29.84Å, and c=5.72Å. Importantly at ~2K, the sample showed a large exchange bias field of about 2520Oe, which is the maximum value ever reported among the Heusler alloy samples. [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]

[Display omitted] • Si epitaxial multilayers with embedded CrSi 2 / β-FeSi 2 nanocrystals were grown on Si(1 1 1). • The types and values of CrSi 2 and β-FeSi 2 nanocrystal lattice deformations were defined. • Seebeck coefficient sign change was observed in CrSi 2 (β-FeSi 2)-p/Si-n heterostructure. • The minimum number of holes injected from a single (CrSi 2 /β-FeSi 2) NC was estimated. • Main mechanisms of hole injection from CrSi 2 (β-FeSi 2)-p/Si-n Ncs were established. The possibility of ordered embedding of CrSi 2 and β-FeSi 2 nanocrystals (NCs) into a Si(1 1 1) substrate of n-type conductivity has been demonstrated. The types of NCs and silicon matching were determined, and the types of the crystal lattice deformations of CrSi 2 and β-FeSi 2 NCs were established. For six-layer heterostructures with embedded CrSi 2 and CrSi 2 + β-FeSi 2 NCs, a change in the Seebeck coefficient sign from negative to positive was observed for the first time in a 250–400 K temperature range, which indicates a nanocrystals to silicon hole injection. An estimate was made of the minimum number of holes, injected from a single nanocrystal, which are necessary to compensate for the thermo-emf from an n-type substrate. Band diagrams were constructed for the β-FeSi 2 -p NC/Si-n and CrSi 2 -p NC/Si-n heterojunctions, and the discontinuities in the valence and conduction bands were determined. The minimum barrier (0.065 eV) for holes in the Si-p/NC β-FeSi 2 -p/Si-n system indicates their thermal generation as the main mechanism. The main mechanism of hole injection from Si-p/NC CrSi 2 -p/Si-n system is tunneling through the surface states in the Si band gap due to the large discontinuity (1.075 eV) in the Si valence band. [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]

Recognizing high thermoelectric performance in semiconducting materials is a challenging task. This is because the Seebeck coefficient and electrical conductivity which constitute the thermoelectric power factor are unfavourably coupled. This means decoupling the transport properties of thermoelectric materials to enhance the power factor without compromising the thermal conductivity is essential. Herein we report that the substitution of erbium (Er) within bismuth selenide (Bi 2 Se 3) results in a simultaneous enhancement in Seebeck coefficient and electrical conductivity via effective mass and Fermi energy optimization. The Er-Substitution in Bi 2 Se 3 does not only promote a simultaneous increase in Seebeck coefficient and electrical conductivity but also decreases the thermal conductivity through an enhancement in phonon scattering. Consequently, the optimum composition is found for the Bi 1 · 85 Er 0 · 15 Se 3 sample instigating that, minimal substitution amount is required to optimize the thermoelectric performance. Our numerical calculation also shows that Er substitution alters the Fermi energy of the Bi 2 Se 3 TE materials, thereby enhancing the effective mass. Through Raman and XPS characterization, we also elucidate that Er substitution does not change the chemical structure and chemical bonding of the pristine material appreciably. It thus leads to improvement in the Seebeck coefficient and electrical conductivity via effective mass optimization. This unique work presents a facile, scalable, cost-effective, and controllable synthesis of nanostructured Bi 2 Se 3 toward realizing high-performance thermoelectric devices. Image 1 • A facile synthesis of high purity erbium substituted Bi 2 Se 3 thermoelectric material. • This study elucidates a mechanism of simultaneous enhancement in Seebeck coefficient and electrical conductivity through isovalent substitution. • Numerical discussion of the effect of the erbium substitution on the Fermi energy as evidence of the Seebeck coefficient and electrical conductivity decoupling is shown. • Systematic analysis of the structural, morphology and thermoelectric properties of the Er substituted Bi 2 Se 3 is related. [ABSTRACT FROM AUTHOR]

• Pure BiCuSeO phase doped and undoped prepared by high-energy ball milling. • BiCuSeO phase formation mechanism during high-energy ball milling studied. • BiCuSeO phase was formed 5 times faster than that by a mechanical alloying. • Synthesis performed under ambient atmosphere at room temperature. We report on the direct ultrafast synthesis of Pb-doped BiCuSeO oxyselenides by high-energy ball milling. We show that, contrary to the mechanical alloying sintering routes used in the previous reports which require 7–13 h to fabricate the BiCuSeO phase in powder form, this synthesis technique enables us to obtain pure phase materials at room temperature under air with milling time less than 60 min. [ABSTRACT FROM AUTHOR]

We calculated the electron susceptibility of rare-earth tritelluride compounds RTe3 as a function of temperature, wave vector, and electron-dispersion parameters. Comparison of the results obtained with the available experimental data on the transition temperature and on the wave vector of a charge-density wave in these compounds allowed us to predict the values and evolution of electron-dispersion parameters with the variation of the atomic number of rare-earth elements (R). [ABSTRACT FROM AUTHOR]

We report a numerical study of the field assisted sintering of silicon germanium alloys by a finite element method, which takes into account contact resistances, thermal expansion and the thermoelectric effect. The distribution of electrical and thermal fields was analyzed numerically, based on the experimental data collected from spark plasma sintering (SPS) apparatus. The thermoelectric properties of Si-Ge used within the simulation were considered as the function of density and the sintering temperature. Quantitative estimation of the temperature distribution during the sintering pointed to a significant, up to 60 °C, temperature difference within the specimen volume for the case of the sintering temperature at 1150 °C. [ABSTRACT FROM AUTHOR]