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Dr. Bogdan POPESCU

Scientific Researcher III

Laboratory of Magnetism and Superconductivity

bogdan[DOT]popescu[AT]infim[DOT]ro

Education

2013 - PhD in Physics at Bucharest University, Department of Physics .

Expertize

-materials synthesis: arc melting, solid state reaction, sintering, vapor deposition, metal flux growth, powder metallurgy;

-properties characterization:  X-ray diffraction, electric and thermal transport, magnetic properties, particle size analysis, IR thermographic analysis;

Patents

A000734/16.11.2020

U00038/13.11.2019

Publications

1. Influence of synthesis method and processing on the thermoelectric properties of CoSb3 skutterudites
Authors: Kumar, MU; Swetha, R; Sahana, BV; Kuri, RS; Popescu, B; Assahsahi, I; Kumari, L

Published: MAR 2024, JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 35, 514, DOI: 10.1007/s10854-024-12277-1

In the present work, single phase CoSb3 skutterudite nanomaterials were prepared via a ball milling method, and their thermoelectric characteristics were compared with the samples synthesized by the solvo-hydrothermal method. Thermoelectric transport properties were recorded in the temperature regime of 300-830 K. Both samples exhibit p-type conduction behavior with a positive sign of Seebeck coefficient. CoSb3 ball mill sample exhibit higher resistivity of 127 x 10(-5)Omega.m at 300 K as compared to CoSb3 solvo-hydrothermal sample with 4.85 x 10(-5)Omega.m. However, CoSb3 ball mill sample show enhanced Seebeck coefficient of 183 mu V/K at 473 K where CoSb3 solvo-hydrothermal sample depict 98 mu V/K at 650 K. Furthermore, the total thermal conductivity of sintered CoSb3 ball mill and CoSb3 solvo-hydrothermal samples was found to be 3.02 Wm(-1)K(-1) and 3.23 W m(-1)K(-1) at room temperature and reaches a significantly lower value of 2.47 W m(-1)K(-1) and 2.46 W m(-1)K(-1) at 580 and 670 K, respectively. The dimensionless figure of merit ZT values of CoSb3 ball mill and CoSb3 solvo-hydrothermal obtained are 0.053 and 0.060 at 650 and 700 K, respectively. A systematic tuning of processing parameters by ball milling method can lead to better thermoelectric efficiency.
2. Kinetics and the Effect of Thermal Treatments on the Martensitic Transformation and Magnetic Properties in Ni49Mn32Ga19 Ferromagnetic Shape Memory Ribbons
Authors: Tolea, F; Popescu, B; Bartha, C; Enculescu, M; Tolea, M; Sofronie, M

Published: JAN 2023, MAGNETOCHEMISTRY, 9, 7, DOI: 10.3390/magnetochemistry9010007

In our work, the kinetics of martensitic transformations and the influence of thermal treatments on martensitic transformations, as well as the related magnetic properties of the Ni49Mn32Ga19 ferromagnetic shape memory melt-spun ribbons, have been investigated. Thermal treatments at 673 K for 1, 4 and 8 h can be considered an instrument for fine-tuning the performance parameters of alloys. One-hour thermal treatments promote an improvement in the crystallinity of these otherwise highly textured ribbons, reducing internal defects and stress induced by the melt-spinning technique. Longer thermal treatments induce an important magnetization rise concomitantly with a slight and continuous increase in martensitic temperatures and transformation enthalpy. The activation energy, evaluated from differential scanning calorimeter experimental data with a Friedman model, significantly increases after thermal treatments as a result of the multi-phase coexistence and stabilization of the non-modulated martensitic phase, which increases the reverse martensitic transformation hindrance.
3. Kinetics and the Effect of Thermal Treatments on the Martensitic Transformation and Magnetic Properties in Ni49Mn32Ga19 Ferromagnetic Shape Memory Ribbons
Authors: Tolea, F; Popescu, B; Bartha, C; Enculescu, M; Tolea, M; Sofronie, M

Published: JAN 2023, MAGNETOCHEMISTRY, 9, 7, DOI: 10.3390/magnetochemistry9010007

In our work, the kinetics of martensitic transformations and the influence of thermal treatments on martensitic transformations, as well as the related magnetic properties of the Ni49Mn32Ga19 ferromagnetic shape memory melt-spun ribbons, have been investigated. Thermal treatments at 673 K for 1, 4 and 8 h can be considered an instrument for fine-tuning the performance parameters of alloys. One-hour thermal treatments promote an improvement in the crystallinity of these otherwise highly textured ribbons, reducing internal defects and stress induced by the melt-spinning technique. Longer thermal treatments induce an important magnetization rise concomitantly with a slight and continuous increase in martensitic temperatures and transformation enthalpy. The activation energy, evaluated from differential scanning calorimeter experimental data with a Friedman model, significantly increases after thermal treatments as a result of the multi-phase coexistence and stabilization of the non-modulated martensitic phase, which increases the reverse martensitic transformation hindrance.
4. Thermoelectric properties of p-type Mg2Si0.3Sn0.7 doped with silver and gallium
Authors: Assahsahi, I; Popescu, B; El Bouayadi, R; Zejli, D; Enculescu, M; Galatanu, A

Published: MAY 25 2023, JOURNAL OF ALLOYS AND COMPOUNDS, 944, 169270, DOI: 10.1016/j.jallcom.2023.169270

Structure, composition, and transport properties of Mg2-xAgx(Si0.3Sn0.7)1-yGay (x = {0, 0.021, y = {0, 0.02, 0.04, 0.061) solid solutions produced by melting followed by spark plasma sintering are investigated. The preparation method is adjusted to control sample stoichiometry and phase composition. Doping with two types of dopants at different sites, while employing synthesis methods which generate a small amount of secondary phase, is an uncommon approach in this materials, expected to enhance their thermoelectric performance. An enhanced carrier concentration but diminished mobility is observed in the samples with higher amounts of dopant, which leads to the highest values of the power factor, for Mg1.98Ag0.02Si0.27Sn0.67Ga0.06 in a narrow temperature range (575-675 K) around the peak value, of 9.10-4 Wm-1 K-2 at 625 K. The two types of dopants have opposing effects on the thermal conductivity, with Ag promoting strong phonon scattering and decreasing its values while Ga increases them because of its en-hanced carrier concentration. The rather high thermal conductivity values of the double doped compounds produce low values of the ZT without exceeding 0.29 at 627 K for Mg1.98Ag0.02Si0.3Sn0.7 sample.(c) 2023 Elsevier B.V. All rights reserved.
5. The inclusion of ceramic carbides dispersion in In and Yb filled CoSb3 and their effect on the thermoelectric performance
Authors: Popescu, B; Galatanu, M; Enculescu, M; Galatanu, A

Published: FEB 10 2022, JOURNAL OF ALLOYS AND COMPOUNDS, 893, DOI: 10.1016/j.jallcom.2021.162400

In this work, thermoelectric properties of the nanocomposite materials produced via ball milling followed by spark plasma sintering from In0.2Yb0.2Co4Sb12 double filled skutterudite (SKT) with silicon and tungsten carbide inclusions are investigated. The nanocomposites with low volume ratios of beta-SiC and '-WC have a significantly increased power factor, while the lattice thermal conductivity is lowered only for beta-SiC composites. The power factor enhancement in '-WC/SKT composite compensates the increase of its thermal conductivity, and as a consequence, the maximum value of the figure of merit, 0.97, is attained for 0.33 v% 'WC at 450 degrees C, with 15% higher than that of the simple skutterudite sample. (c) 2021 Elsevier B.V. All rights reserved.
6. Influence of the synthesis parameters on the transport properties of Mg2Si0.4Sn0.6 solid solutions produced by melting and spark plasma sintering
Authors: Assahsahi, I; Popescu, B; Enculescu, M; Galatanu, M; Galca, AC; El Bouayadi, R; Zejli, D; Galatanu, A

Published: APR 2022, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 163, DOI: 10.1016/j.jpcs.2021.110561

In this work, the influence of the preparation route on the structural, morphological, and thermoelectric properties of the Mg2Si0.4Sn0.6 solid solutions is investigated. The synthesis based on melting the constituent elements in a closed graphite crucible followed by spark plasma sintering allows mixing elements with a large difference of their melting temperatures and a good control of sample stoichiometry. The optimized synthesis route is validated by the doped V and Sb samples, which yield good thermoelectric performance. The n-type doping leads to two orders of magnitude increase of the carrier concentration, and thus a subsequent increase of the electrical conductivity, which, in turn, augments greatly the power factor of the Mg1.98V0.02Si0.385Sn0.6Sb0.015 to 42.61 x10(-4) Wm(-1)K 2 at 650K. Although doping slightly enlarges the thermal conductivity, a peak value of the figure of merit ZT similar to 1.15 is obtained at 723K, which is 20 times higher than the ZT of un-doped material.
7. MICROSTRUCTURE, MAGNETIC AND MAGNETOSTRICTIVE BEHAVIOUR IN RAPIDLY QUENCHED OFF-STOICHIOMETRIC Ni-Mn-Ga FERROMAGNETIC SHAPE MEMORY ALLOYS
Authors: Sofronie, M; Tolea, F; Enculescu, M; Pasuk, I; Popescu, B

Published: 2022, ROMANIAN REPORTS IN PHYSICS, 74, 503, DOI:

This work reports the effect of the rapid solidification technique and thermal treatment on the martensitic transformation (MT), magnetic and magnetostrictive properties on the off-stoichiometric Ni49Mn31Ga20 and Ni51Mn28Ga21 ferromagnetic shape memory ribbons. The samples were investigated by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, magnetic and magnetostrictive measurements. The temperature dependence of the X-ray phases analysis shows the presence of martensite structures, both tetragonal and monoclinic, at room temperature and allowed to study their evolution through MT. The thermal treatment induces changes in the microstructure with implications in MT and Curie temperatures evolution. The competition between the magnetization orientation and twin boundary motion within martensitic variants under magnetic field evidenced in the magnetic-strain curves was discussed and correlated with the magnetic data.
8. Enhanced magnetocaloric properties of La0.8K0.2-xPbxMnO3 nanoparticles by optimizing Pb doping concentrations
Authors: Bouzid, SA; Essoumhi, A; Rostas, AM; Kuncser, AC; Negrila, CC; Iacob, N; Galatanu, A; Popescu, B; Sajieddine, M; Galca, AC; Kuncser, V

Published: JUN 15 2022, CERAMICS INTERNATIONAL, 48, DOI: 10.1016/j.ceramint.2022.02.239

Polycrystalline La(0.8)K(0.2-x)Pb(x)MnO3 (x = 0.05, 0.10, 0.15, 0.20) ceramics were successfully prepared by flash combustion route and their structural, morphological, magnetic and magnetocaloric properties were investi-gated. Structural analyses using X-ray diffraction reveal that all samples are crystallized in the rhombohedral structure and belong to R <(3) over bar > c space group. The increase of Pb doping does not modify the crystalline structure but changes the grain size and lattice parameters. X-ray photoelectron spectroscopy (XPS) fitting results of Mn 2p peaks confirmed the coexistence of Mn3+ and Mn4+ ions which contribute to the double exchange interactions improving the ferromagnetic order in the samples. The magnetization's temperature and magnetic field de-pendences indicate a second-order ferromagnetic-paramagnetic transition of the ceramics. A significant mag-netic entropy change near room temperature was observed for La0.8K0.1Pb0.1MnO3, showing considerable magnetocaloric properties. Furthermore, electron paramagnetic resonance spectroscopy (EPR) was also used to examine the ferromagnetic-paramagnetic transition.
9. Magnetic and Magnetostrictive Properties of Ni50Mn20Ga27Cu3 Rapidly Quenched Ribbons
Authors: Sofronie, M; Tolea, M; Popescu, B; Enculescu, M; Tolea, F

Published: SEP 2021, MATERIALS, 14, DOI: 10.3390/ma14185126

The influence of the rapid solidification technique and heat treatment on the martensitic transformation, magnetic properties, thermo- and magnetic induced strain and electrical resistivity is investigated for the Cu doped NiMnGa Heusler-based ferromagnetic shape memory ribbons. The martensitic transformation temperatures are unexpectedly low (below 90 K-which can be attributed to the disordered texture as well as to the uncertainty in the elements substituted by the Cu), preceded by a premartensitic transformation (starting at around 190 K). A thermal treatment slightly increases the transformation as well as the Curie temperatures. Additionally, the thermal treatment promotes a higher magnetization value of the austenite phase and a lower one in the martensite. The shift of the martensitic transformation temperatures induced by the applied magnetic field, quantified from thermo-magnetic and thermo-magnetic induced strain measurements, is measured to have a positive value of about 1 K/T, and is then used to calculate the transformation entropy of the ribbons. The magnetostriction measurements suggest a rotational mechanism in low fields for the thermal treated samples and a saturation tendency at higher magnetic fields, except for the temperatures close to the phase transition temperatures (saturation is not reached at 5 T), where a linear volume magnetostriction cannot be ruled out. Resistivity and magnetoresistance properties have also been measured for all the samples.
10. Structural, magnetic and magnetostrictive properties of the ternary iron-palladium-silicon ferromagnetic shape memory ribbons
Authors: Sofronie, M; Popescu, B; Enculescu, M

Published: FEB 5 2021, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 127, DOI: 10.1007/s00339-021-04315-0

The influence of the partial substitution of Fe by Si and thermal treatments on the structural, magnetic and magnetostrictive properties of the Fe67.5Pd30.5Si2 rapidly solidified ribbons has been investigated. A remarkable decrease in the martensite transformation temperature, with similar to 65 K lower than that of the Fe-Pd archetype alloy, is observed in the as-prepared ribbons. The thermal treatments shift the martensite transformation temperatures upward, with approximately 13 K for the higher thermal treatment. Also, these induce an improvement in the crystallinity in these ribbons with high texture and an increase in the crystallite size as a result of reducing the internal defects and stress. The thermodynamic considerations discussed in the frame of the Clapeyron-Clausius relation by using the calorimetric and thermomagnetic measurements (up to 7 T) reveal a weak influence of the magnetic fields on the martensitic transformation temperatures (similar to 0.5 K/T). The magnetostriction decrease with temperature under small magnetic fields was discussed, beside an unusual behaviour in the technically saturated domain. This behaviour is based on the coexistence of the ordinary and forced magnetostrictions, the last one increasing faster with the temperature decreasing.
11. The Effect of the In-Situ Heat Treatment on the Martensitic Transformation and Specific Properties of the Fe-Mn-Si-Cr Shape Memory Alloys Processed by HSHPT Severe Plastic Deformation
Authors: Gurau, C; Gurau, G; Tolea, F; Popescu, B; Banu, M; Bujoreanu, LG

Published: AUG 2021, MATERIALS, 14, DOI: 10.3390/ma14164621

This work focuses on the temperature evolution of the martensitic phase epsilon (hexagonal close packed) induced by the severe plastic deformation via High Speed High Pressure Torsion method in Fe57Mn27Si11Cr5 (at %) alloy. The iron rich alloy crystalline structure, magnetic and transport properties were investigated on samples subjected to room temperature High Speed High Pressure Torsion incorporating 1.86 degree of deformation and also hot-compression. Thermo-resistivity as well as thermomagnetic measurements indicate an antiferromagnetic behavior with the Neel temperature (T-N) around 244 K, directly related to the austenitic gamma-phase. The sudden increase of the resistivity on cooling below the Neel temperature can be explained by an increased phonon-electron interaction. In-situ magnetic and electric transport measurements up to 900 K are equivalent to thermal treatments and lead to the appearance of the bcc-ferrite-like type phase, to the detriment of the epsilon(hcp) martensite and the gamma (fcc) austenite phases.
12. Influences of Dispersions' Shapes and Processing in Magnetic Field on Thermal Conductibility of PDMS-Fe3O4 Composites
Authors: Stancu, V; Galatanu, A; Enculescu, M; Onea, M; Popescu, B; Palade, P; Aradoaie, M; Ciobanu, R; Pintilie, L

Published: JUL 2021, MATERIALS, 14, DOI: 10.3390/ma14133696

Composites of magnetite (Fe3O4) nanoparticles dispersed in a polydimethylsiloxane (PDMS) matrix were prepared by a molding process. Two types of samples were obtained by free polymerization with randomly dispersed particles and by polymerization in an applied magnetic field. The magnetite nanoparticles were obtained from magnetic micrograins of acicular goethite (alpha-FeOOH) and spherical hematite (alpha-Fe2O3), as demonstrated by XRD measurements. The evaluation of morphological and compositional properties of the PDMS:Fe3O4 composites, performed by SEM and EDX, showed that the magnetic particles were uniformly distributed in the polymer matrix. Addition of magnetic dispersions promotes an increase of thermal conductivity compared with pristine PDMS, while further orienting the powders in a magnetic field during the polymerization process induces a decrease of the thermal conductivity compared with the un-oriented samples. The shape of the magnetic dispersions is an important factor, acicular dispersions providing a higher value for thermal conductivity compared with classic commercial powders with almost spherical shapes.
13. Multifunctional GaFeO3 Obtained via Mechanochemical Activation Followed by Calcination of Equimolar Nano-System Ga2O3-Fe2O3
Authors: Diamandescu, L; Tolea, F; Feder, M; Vasiliu, F; Mercioniu, I; Enculescu, M; Popescu, T; Popescu, B

Published: JAN 2021, NANOMATERIALS, 11, DOI: 10.3390/nano11010057

The equimolar oxide mixture beta-Ga2O3-alpha-Fe2O3 was subjected to high-energy ball milling (HEBM) with the aim to obtain the nanoscaled GaFeO3 ortho-ferrite. X-ray diffraction, Fe-57 Mossbauer spectroscopy, and transmission electron microscopy were used to evidence the phase structure and evolution of the equimolar nano-system beta-Ga2O3-alpha-Fe2O3 under mechanochemical activation, either as-prepared or followed by subsequent calcination. The mechanical activation was performed for 2 h to 12 h in normal atmosphere. After 12 h of HEBM, only nanoscaled (similar to 20 nm) gallium-doped alpha-Fe2O3 was obtained. The GaFeO3 structure was obtained as single phase, merely after calcination at 950 degrees C for a couple of hours, of the sample being subjected to HEBM for 12 h. This temperature is 450 degrees C lower than used in the conventional solid phase reaction to obtain gallium orthoferrite. The optical and magnetic properties of representative nanoscaled samples, revealing their multifunctional character, were presented.
14. Martensitic Transformation and Magnetic Properties of Ni57Fe18Ga25 Shape Memory Alloy Subjected to Severe Plastic Deformation
Authors: Popescu, B; Gurau, C; Gurau, G; Tolea, M; Sofronie, M; Tolea, F

Published: OCT 2021, TRANSACTIONS OF THE INDIAN INSTITUTE OF METALS, 74, 2498, DOI: 10.1007/s12666-021-02293-8

The effects of severe plastic deformation (SPD) process via high-speed high-pressure torsion technique on martensitic transformation of Ni-Fe-Ga Heusler shape memory alloy are the subject of this work. The results show that moderate degrees of deformation lead to a decrease in the martensitic transformation temperatures, while the heat of reaction is enhanced only for the sample processed with the lowest degree of deformation. The results are explained by the interplay between the constituent tetragonal L10 and the cubic gamma crystal structures and the evolution of the samples morphology with the severity of deformation. The reduction in the samples granulation due to the progressive increase in the SPD is reflected by the magnetic properties of the samples with decreasing coercivity and Curie temperatures. At the highest applied degree of deformation, sample nanostructuring and a possible amorphization might explain the vanishing of MT.
15. Magnetic and magnetostrictive properties of the ternary Fe67.5Pd30.5Ga2 ferromagnetic shape memory ribbons
Authors: Sofronie, M; Tolea, F; Tolea, M; Popescu, B; Valeanu, M

Published: JUL 2020, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 142, DOI: 10.1016/j.jpcs.2020.109446

Specific functional characteristics of the ferromagnetic shape memory alloy Fe67.5Pd30.5Ga2 prepared as ribbons by rapid quenching technique are reported. The shift of the martensitic transformation temperature induced by the applied magnetic field was determined from the thermomagnetic measurements at various fields up to 5T being proved to be in good agreement with the result obtained from calorimetric data via the Clapeyron - Claussius relation. Magnetostriction measurements reveal a pure spin rotation mechanism under low applied magnetic fields, allowed by the reduced magnetic anisotropy. In high magnetic fields and at temperatures close to martensitic transformation, the magnetostriction has a linear increase up to the maximum considered magnetic induction of 3T. This behavior has been discussed in connection with the forced magnetostiction aspects.
16. Development of W-monoblock divertor components with embedded thermal barrier interfaces
Authors: Galatanu, M; Cioca, M; Ighigeanu, A; Ruiu, G; Enculescu, M; Popescu, B; Galatanu, A

Published: SEP 2019, FUSION ENGINEERING AND DESIGN, 146, 1354, DOI: 10.1016/j.fusengdes.2019.02.074

In the case of DEMO fusion reactor, the divertor should be able to extract a steady heat flux of about 10 MW/m(2). A promising concept is the W-monoblock which should be connected to a CuCrZr or an advanced Cu ODS alloy pipe passing through the W component. Taking into account the optimum operating temperature windows for W and existing Cu-based alloys and the thermal expansion coefficients mismatch of these two materials, a "thermal barrier" interface material is inserted in between in order to mitigate the thermal stresses and to optimize the heat flow through divertor components. In this work we investigate the feasibility to realize such divertor components using materials produced by FAST (field assisted sintering technology). This powder metallurgy technique was used firstly to produce W or W-based composites and the thermal barriers in an almost final shape and then to join the materials in realistic divertor mock-ups. The thermal barrier materials are various Cu-based composites which are included both as single material or as functionally graded components. The interface quality between different materials is investigated by scanning electron microscopy and the heat flow through components is evaluated using simulations.
17. Specific Changes in the Magnetoresistance of Ni-Fe-Ga Heusler Alloys Induced by Cu, Co, and Al Substitutions
Authors: Tolea, F; Tolea, M; Sofronie, M; Popescu, B; Crisan, A; Leca, A; Valeanu, M

Published: APR 2017, IEEE TRANSACTIONS ON MAGNETICS, 53, DOI: 10.1109/TMAG.2016.2628386

In this paper, we investigate the influence of Cu, Co, and Al substitutions on the transport properties, and in particular, the magnetoresistive effect in Ni-Fe-Ga ferromagnetic shape memory alloys (FSMAs) prepared as ribbons by melt spinning method and subjected to different thermal treatments. X-ray diffraction, differential scanning calorimetry, magnetometry, and magnetoresistive characterizations were performed. In the range of the martensitic transformation (MT), different FSMA compositions show a rich spectrum of different behaviors. For one of the compositions (Ni52Fe20Co2Ga23Al3), the magnetoresistance (MR) showed a local minimum or, on the contrary, a local maxima of reduced amplitude on cooling, in the range of the MT, depending on the performed thermal treatments. In the same composition, by replacing one Al atom with a Co one, no local extremes are seen, the alloy having a concomitant magneto-structural transition. When the magnetic field was varied, the MR showed a nonmonotonic variation in the martensite phase for some compounds, possibly due to the dynamics of the martensite variants realignment. From the studied compositions, the highest MR found on the MT of -9% for 5 T is for Ni50Fe20Ga27Cu3.
18. Cu-based composites as thermal barrier materials in DEMO divertor components
Authors: Galatanu, M; Enculescu, M; Ruiu, G; Popescu, B; Galatanu, A

Published: NOV 2017, FUSION ENGINEERING AND DESIGN, 124, 1134, DOI: 10.1016/j.fusengdes.2017.02.031

For DEMO fusion reactor an expected heat flux of about 10MW/m(2) should be extracted by the divertor which will have, most likely, an armour part made of W and a following heat sink part made of Cu, or ODS Cu alloy. Unfortunately, for these materials the optimum operating temperature windows do not overlap. Thermal barrier materials are interface materials included in such components, aiming to keep the temperatures of both armour and heat sink parts in the corresponding operating windows, and to mitigate the effects of their different thermomechanical properties. Here we propose a simple spark plasma sintering route to create Cu-based composites with a high content (10-40 vol%) of various dispersed materials (Al or Y oxides, C, SiC), allowing a fine tuning of the content and a large pool of predefined shapes and dimensions. The resulting specimens can be further joined to armour and heatsink components via a similar electrical field assisted technology. Micro-structural and thermal properties are investigated for these materials allowing to select the most suited materials in view of their thermal conductivity and thermal expansion coefficients. (C) 2017 The Authors. Published by Elsevier B.V.
19. Magnetoelastic properties in polycrystalline ferromagnetic shape memory Heusler alloys
Authors: Sofronie, M; Tolea, F; Crisan, AD; Popescu, B; Valeanu, M

Published: 2016, 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), 2, 1537, DOI: 10.1016/j.prostr.2016.06.194

The influence of the heat treatments on the martensitic transformation, magnetic properties and thermo- and magnetic induced strain on Ni50Fe20Ga27Cu3 ferromagnetic shape memory alloy prepared as ribbons by melt spinning technique are investigated. The degree of atomic order as effect of different thermal treatments produces important changes in the magneto-crystalline anisotropy of the martensite phase. The anomalies evidenced in the thermo-and magnetic-strain curves are discussed and correlated with the thermo-magnetic data. The transformation-induced strains with and without magnetic field have been measured, the results setting out the influence of the pre-martensitic transformation. Copyright (C) 2016 The Authors. Published by Elsevier B.V.
20. Effect of thermal treatments in Ni-Fe-Ga with Co substitutions and Ni-Mn-Ga melt spun ribbons
Authors: Tolea, F; Sofronie, M; Crisan, AD; Popescu, B; Tolea, M; Valeanu, M

Published: 2016, 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), 2, 1480, DOI: 10.1016/j.prostr.2016.06.187

The effect of "in situ" thermal treatments (by DSC measurements) on the martensitic transformation in two representative Ni-Fe-Ga and Ni-Mn-Ga alloys has been studied and discussed by correlating the structural and magnetic properties. The alloys were prepared from high purity elements, by arc melting under argon protective atmosphere as bulk and also as melt-spun ribbons - an alternative preparation route that also allows to assess the influences of grains size and strain induced by this processing method. All samples presented reversible thermo-elastic transformations. The thermal treatments promote a reduction of the martensitic transformation temperatures in the Ni-Fe-Ga investigated samples, as opposed to the stoichiometric Ni2MnGa where the temperatures increase with increasing the annealing temperatures. Interestingly however, the off-stoichiometric Ni-Mn-Ga with increased Ni content recovers the behaviour with reduction of transformation temperatures by thermal treatments. The precipitation of the secondary FCC (gamma) phase is inherently found in Ni-Fe-Ga alloys with Ga <= 27% at, and also-although in lower amounts- in the off-stoichiometric Ni-Mn-Ga. The gamma phase is considered to contribute to the decrease of the MT temperatures (via valence electrons concentration depletion of the main matrix) and of the transformation heat as well as to the final structural degradation if the temperature of the thermal treatments is further increased. In addition, this phase, located mainly at the grain boundaries, is responsible for the improved ductility of Ni-Fe-Ga based alloys. Changes in the transformation heat due to thermal treatments are observed and discussed in both types of alloys, the maxima of the transformation heat being associated with the highest atomic order. Thermo-magnetic measurements show that Ni-Fe-Ga alloys have close magnetic and structural transitions temperatures, with promising applications for magnetic refrigeration. Copyright (C) 2016 The Authors. Published by Elsevier B.V.
21. Direct sintering of SiC-W composites with enhanced thermal conductivity
Authors: Galatanu, M; Popescu, B; Enculescu, M; Tiseanu, I; Craciunescu, T; Galatanu, A

Published: OCT 2013, FUSION ENGINEERING AND DESIGN, 88, 2602, DOI: 10.1016/j.fusengdes.2013.05.036

Different types of SiC-W composites have been realized by spark plasma sintering in a single step process starting with beta SiC nanometric powder, W nanometric and micrometric sized powders, and W foils. SEM, EDX, XRD and X-ray tomography have been used to analyze the sample morphology while the thermal properties of the resulting materials have been investigated up to 1000 degrees C using a LFA thermal analyzer. The results show the possibility to produce dense W-SiC composites, with enhanced thermal conductivity using a relatively simple route. (C) 2013 Elsevier B.V. All rights reserved.
22. Solid-state synthesis and spark plasma sintering of SrZrO3 ceramics
Authors: Popescu, B; Enache, S; Ghica, C; Valeanu, M

Published: JUN 2 2011, JOURNAL OF ALLOYS AND COMPOUNDS, 509, 6399, DOI: 10.1016/j.jallcom.2011.03.009

SrZrO3 powders are obtained by solid state reaction from SrCO3 and ZrO2 precursors, without involving intermediate calcination and grinding steps. The resulted powders are essentially within a single phase, with sub-micron average crystallite size. Pellets of these powders show a relatively poor sintering behavior, when fired up to 1600 degrees C. Alternatively, spark plasma sintering technique is used in order to obtain nearly 100% dense samples at the expense of excessive grain coarsening (i.e., up to 5 mu m in diameter). Crystalline structure, composition and morphology of the specimens obtained in this work are investigated by X-ray diffraction, scanning and transmission electron microscopy together with energy dispersive X-ray spectroscopy. (C) 2011 Elsevier B.V. All rights reserved.
23. CATALYTIC COMBUSTION OF METHANE OVER UNSUPPORTED AND gamma-Al2O3 SUPPORTED Sr2FeTaO6 and Sr2Fe0.7Co0.3TaO6 DOUBLE PEROVSKITES
Authors: Popescu, I; Redey, A; Marcu, IC; Popescu, B; Mako, E; Sandulescu, I

Published: NOV-DEC 2009, REVUE ROUMAINE DE CHIMIE, 54, +, DOI:

Sr2FeTaO6 and Sr2Fe0.7Co0.3TaO6 double perovskites, unsupported and supported on gamma-Al2O3 were prepared, characterized and studied in the catalytic combustion of methane, as a test reaction for volatile organic compounds (VOCs) destruction. These mixed oxides, with double perovskite structure, present good catalytic activities, and after their dispersion - 5 % (wt.) - on gamma-Al2O3 support, the reaction rates increased compared with the unsupported samples.
24. Low temperature magnetic and transport properties in Ce(Ag, Ni)Sb-2 compounds
Authors: Popescu, B; Royanian, E; Michor, H; Hilscher, G; Bauer, E; Galatanu, A

Published: APR 1 2008, PHYSICA B-CONDENSED MATTER, 403, 939, DOI: 10.1016/j.physb.2007.10.287

Structural, thermodynamic and,transport, properties of CeAg1-xNixSb2 compounds are investigated. The Ni substitution of Ag decreases the magnetic ordering temperature from approximate to 9.5 to approximate to 6 K, while the characteristic temperatures, considered as the temperature of the resistivity maximum, increase from 26 to 65 K. (c) 2007 Elsevier B.V. All rights reserved.
25. Shape memory and associated properties in Fe-Mn-Si-based ribbons produced by melt-spinning
Authors: Valeanu, M; Filoti, G; Kuncser, V; Tolea, F; Popescu, B; Galatanu, A; Schinteie, G; Jianu, AD; Mitelea, I; Schinle, D; Craciunescu, CM

Published: JUL 2008, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 320, E167, DOI: 10.1016/j.jmmm.2008.02.105

Four Fe-Mn-Si alloys, Fe(62)Mn(32)Si(6), Fe(62)Mn(20)Si(5)Cr(8)Ni(5), Fe(62)Mn(16)Si(5)Cr(12)Ni(5) and Fe(65)Mn(9)Si(7)Cr(10)Ni(9), were obtained by the melt-spinning method. The samples were structurally, magnetic and shape memory effect (SME) investigated, both ''as quenched'' and thermally treated. The Mn-rich compositions show different phase, magnetic behavior and SME in comparison with Mn-poor compositions. The thermal treatments generate transformation between the two existing majority phases (alpha and gamma), related magnetization and SME behavior. The features are derived from the corroboration of structural, magnetic interaction and magnitude of SME data. (c) 2008 Elsevier B.V. All rights reserved.
26. Physical properties of the new ferromagnetic Kondo system CeAg1-xNixSb2
Authors: Popescu, B; Birsan, A; Galatanu, A; Royanian, E; Hilscher, G; Bauer, E

Published: JUL 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 1629, DOI:

Structural, magnetic and transport properties of CeAg1-xNixSb2 compounds are investigated on polycrystalline samples. The experimental results show the formation of ferromagnetic compounds with sharp transitions decreasing from 9.5 K in the 100% Ag compound to 8.1 K for 33% Ni substitution, 7 K for 67% Ni substitution to 6 K for the 100% Ni compound. The characteristic temperatures, considered as the temperature of the resistivity maximum To increases from 26 K to 44 K, 54 K and 65 K, respectively. These results indicate Ce(AgNi)Sb2 compounds as a ferromagnetic Kondo system.
27. Pressure response of Ce(Ag, Ni)Sb(2) compounds
Authors: Khan, RT; Galos, R; Bauer, E; Popescu, B; Galatanu, A

Published: JUL 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 1615, DOI:

The pressure response of CeAg(1-x)Ni(x)Sb(2) is Studied by means of electrical resistivity measurements. The ferromagnetic ordering temperatures T(c) of this series decrease as pressure is applied, except CeNiSb(2), showing a pressure driven continuous increase of T(c). This refers to crystalline electric field effects originating distinct different magnetic ground states. Concomitantly, the pressure response of various material dependent parameters also behave in the opposite direction.
28. The magnetic effects in Sr2FeMoO6 perovskite type compound obtained by soft chemistry
Authors: Aldica, G; Plapcianu, C; Badica, P; Valsangiacom, C; Popescu, B

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 463, DOI:

Recently, a double perovskite oxide, Sr2FeMoO6, has been reported to show a very sharp magnetoresistance (MR) response at relatively small-applied fields and at high temperatures. The half-metallic state of this compound ensures a theoretical magnetic moment of 4 mu(B) per formula unit. Ordering control is expected to be influenced by the synthesis route. In this regard, in this article, perovskite samples were obtained from the liquid phase, using oxalic acid as complexing agent. Application of a field above 0.08 T increases the zero-field-cooled (ZFC) magnetization. Below this field, for the ZFC-curves measured from 2 K up to 400 K, magnetization increases, reaching a peak at T-cups(ZFC), before decreasing cups gradually to a paramagnetic state. Saturation moment is relatively low (2.1-2.2 mu(B)), probably due to anti-site defects, as suggested in the literature.
29. Mossbauer spectroscopy and magnetic measurements on Fe-Ni-Co-Ti shape memory alloys
Authors: Tolea, F; Schinteie, G; Popescu, B

Published: AUG 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1506, DOI:

Fe-Ni-Co-Ti shape memory alloys were prepared by various techniques. A comparison between samples prepared by classical metallurgy and by melt spinning technique is performed in respect to the microscopic mechanisms responding for the shape memory effects. X-ray diffraction, thermomagnetic measurements and Mossbauer spectroscopy were applied for a complete structural and magnetic characterization. Only samples supporting an aging treatment gave evidence for shape memory effects and correlated structural transformations.
30. Effects of the Severe Plastic Deformation on the Magnetic Properties of Zr13Co87 Ribbons
Authors: Popescu, B; Palade, P; Sofronie, M; Kuncser, A; Gurau, C; Gurau, G; Tolea, F

Published: , METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, DOI: 10.1007/s11661-021-06507-y

The influence of the severe plastic deformation via high-speed high-pressure torsion (HSHPT) on the structural and magnetic properties of the Zr13Co87 alloys is investigated. Moderate applied deformation promotes the growth of the rhombohedral hard magnetic phase leading to the increase of the sample's hardness and magnetic coercivity. A higher degree of deformation affects the samples morphology leading to a critical value of the grain size under which the exchange coupling of the soft phase is less effective. Additionally, it produces a random alignment of the anisotropy axes, which are both detrimental to the hard magnetic properties.
31. MICROSTRUCTURE, MAGNETIC AND MAGNETOSTRICTIVE BEHAVIOUR IN RAPIDLY QUENCHED OFF-STOICHIOMETRIC Ni-Mn-Ga FERROMAGNETIC SHAPE MEMORY ALLOYS
Authors: Sofronie, M; Tolea, F; Enculescu, M; Pasuk, I; Popescu, B

Published: , ROMANIAN REPORTS IN PHYSICS, 2022, DOI: 503

This work reports the effect of the rapid solidification technique and thermal treatment on the martensitic transformation (MT), magnetic and magnetostrictive properties on the off-stoichiometric Ni49Mn31Ga20 and Ni51Mn28Ga21 ferromagnetic shape memory ribbons. The samples were investigated by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, magnetic and magnetostrictive measurements. The temperature dependence of the X-ray phases analysis shows the presence of martensite structures, both tetragonal and monoclinic, at room temperature and allowed to study their evolution through MT. The thermal treatment induces changes in the microstructure with implications in MT and Curie temperatures evolution. The competition between the magnetization orientation and twin boundary motion within martensitic variants under magnetic field evidenced in the magnetic-strain curves was discussed and correlated with the magnetic data.
32. Martensitic Transformation and Magnetic Properties of Ni57Fe18Ga25 Shape Memory Alloy Subjected to Severe Plastic Deformation
Authors: Popescu, B; Gurau, C; Gurau, G; Tolea, M; Sofronie, M; Tolea, F

Published: , TRANSACTIONS OF THE INDIAN INSTITUTE OF METALS, DOI: 10.1007/s12666-021-02293-8

The effects of severe plastic deformation (SPD) process via high-speed high-pressure torsion technique on martensitic transformation of Ni-Fe-Ga Heusler shape memory alloy are the subject of this work. The results show that moderate degrees of deformation lead to a decrease in the martensitic transformation temperatures, while the heat of reaction is enhanced only for the sample processed with the lowest degree of deformation. The results are explained by the interplay between the constituent tetragonal L10 and the cubic gamma crystal structures and the evolution of the samples morphology with the severity of deformation. The reduction in the samples granulation due to the progressive increase in the SPD is reflected by the magnetic properties of the samples with decreasing coercivity and Curie temperatures. At the highest applied degree of deformation, sample nanostructuring and a possible amorphization might explain the vanishing of MT.
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