NIMP| Mihai-Alexandru GRIGOROSCUTA, Author at NIMP

Education

PhD. at University "Politehnica" of Bucharest, Faculty of Materials Science and Engineering

Positions

Researcher

Research interests

Processing of different composites (Spark Plasma Sintering; intense ultrasonication, magneto-texturing): MgB2, other borides and rare earths.

Measurements of different properties: magnetism, superconductivity (VSM, SQUID, PPMS); mechanical properties (hardness, flexure, compression, INSTRON equipment); crystal growth (TiO2, LASCO)

Expertize

Processing by Spark Plasma Sintering of various borides and boride composites.

Crystal growth of superconductors (LASCO).

Superconductor, mechanical, physico-chemical properties of composites.

Patents

1. Method for making multifunctional medical devices, intended to immobilize joints or fractures, involves making design of functionalized porous system by three-dimensional (3D) printing using biocompatible poly lactic acid (PLLA) filament, loading with PVP polymer and magnesium diboride powder. 2022-47568N. RO135565(A0)

2. Preparation of antibacterial product used as mouthwash, involves mixing powder material containing main phase containing magnesium boride and secondary phase containing magnesium oxide and magnesium, with solution containing chlorhexidine. 2021-36330A. RO134808(A0)

3. Processing of magnesium-dibromide-based superconductive strip/wire for e.g. power machine, involves plastic deforming and electric-field-assisted sintering powder made of hexagonal boron nitride and magnesium dibromide in light metal sheath. 2021-02305V. RO134662(A2)

4. Obtaining optical material by sintering mixture of ceria, ytterbium oxide and erbium(III) oxide powders to obtain target material to deposit on silicon substrate to produce thin layer based on ceria, doped with ytterbium and erbium oxides. 2021-02305L. RO134631(A0)

5. Manufacturing superconductive solid bodies of magnesium diboride used in e.g. current limiters, by subjecting dense superconductive bodies to sintering assisted by intense electric field, using fast heating. 2020-C1185G. RO134559(B1)

6. Preparing magnesium boride-based material with antimicrobial activity used for protecting surface against bacterial adherence, by mixing magnesium boride powder with polyvinylpyrrolidone, dissolving in ethanol, and applying on surfaces. 2020-27602Q. RO133974(A0)

7. Processing superconductor strip or wire comprising magnesium diboride-based core, involves performing plastic deformation and intense electric field-assisted sintering, immersing strip/wire under vacuum and heating strip/wire. 2019-457114. RO133106(A2)

Resume

2021 - Present: Researcher at Laboratory of Magnetism and Superconductivity in NIMP, Romania

2016- 2021: PhD. studies at University ”Politehnica” of Bucharest , Faculty of Material Science and Engineering, Coordinator: Badica Petre, Cojocaru Mihai Ovidiu. Subject: Boron and rare earth based materials for different applications

2015-2021: Research assistant at Laboratory of Magnetism and Superconductivity in NIMP, Romania

May-2019, November 2018: Research internship at NIMS, Tsukuba, Japan

2014-2016: MSc degree at University ”Politehnica” of Bucharest , Faculty of Material Science and Engineering, Biomaterials. Thesis title: Spectral conversion in photovoltaic applications by using Yb and Er codoped CeO₂ thin films

May-August 2013: Research internship at IPCMS, Strasbourg, France. Subject: Structural and optical properties of CeO₂ thin films doped with rare-earth ions for photovoltaic applications

2010-2014: Bachelor degree at University ”Politehnica” of Bucharest , Faculty of Material Science and Engineering. Subject: Structural and optical properties of CeO₂ thin films doped with rare-earth ions for photovoltaic applications

Publications

1. Functionalization of graphene oxide surface by conjugation with glucosamine and analysis of interactions occurring in nanoceramic-graphene heterostructures

Authors: Kukielski, M; Kasprzak, A; Zurowski, R; Tanska, J; Wiecinska, P; Wieclaw-Midor, A; Zygmuntowicz, J; Grigoroscuta, MA; Wiecinski, P

Published: JAN 1 2024, POWDER TECHNOLOGY, 431, 119089, DOI: 10.1016/j.powtec.2023.119089

The paper describes the chemical modification of the graphene oxide (GO) surface with glucosamine, which was conjugated to GO via a carbodiimide-mediated amidation-type reaction. The efficiency of the reaction was confirmed by FT-IR and Raman spectroscopy and elemental analysis. The elaborated functionalization significantly reduces the agglomeration of graphene oxide platelets. The interactions at the interface of GO and two types of ceramic nanoparticles (ZrO2 and Al2O3 with particle sizes of 58 nm and 158 nm, respectively) were examined by zeta potential measurements at different pH values. The viscosity of ZrO2 suspensions (35 vol% of solid loading) at 1.5 s- 1 increases from 24.8 Pas to 170.7 Pas and 142.7 Pas with the addition of GO and f-GO, respectively. The interfaces at ceramic-graphene heterostructures obtained by slip casting method have been analyzed by SEM. The investigations revealed the presence of multilayer graphene oxide and flexible single-layer f-GO within the ceramic matrix.

2. Multi-Parametric Exploration of a Selection of Piezoceramic Materials for Bone Graft Substitute Applications

Authors: Nedelcu, L; Ferreira, JMF; Popa, AC; Amarande, L; Nan, B; Balescu, LM; Geambasu, CD; Cioangher, MC; Leonat, L; Grigoroscuta, M; Cristea, D; Stroescu, H; Ciocoiu, RC; Stan, GE

Published: FEB 2023, MATERIALS, 16, 901, DOI: 10.3390/ma16030901

This work was devoted to the first multi-parametric unitary comparative analysis of a selection of sintered piezoceramic materials synthesised by solid-state reactions, aiming to delineate the most promising biocompatible piezoelectric material, to be further implemented into macro-porous ceramic scaffolds fabricated by 3D printing technologies. The piezoceramics under scrutiny were: KNbO3, LiNbO3, LiTaO3, BaTiO3, Zr-doped BaTiO3, and the (Ba0.85Ca0.15)(Ti0.9Zr0.1)O-3 solid solution (BCTZ). The XRD analysis revealed the high crystallinity of all sintered ceramics, while the best densification was achieved for the BaTiO3-based materials via conventional sintering. Conjunctively, BCTZ yielded the best combination of functional properties-piezoelectric response (in terms of longitudinal piezoelectric constant and planar electromechanical coupling factor) and mechanical and in vitro osteoblast cell compatibility. The selected piezoceramic was further used as a base material for the robocasting fabrication of 3D macro-porous scaffolds (porosity of similar to 50%), which yielded a promising compressive strength of similar to 20 MPa (higher than that of trabecular bone), excellent cell colonization capability, and noteworthy cytocompatibility in osteoblast cell cultures, analogous to the biological control. Thereby, good prospects for the possible development of a new generation of synthetic bone graft substitutes endowed with the piezoelectric effect as a stimulus for the enhancement of osteogenic capacity were settled.

3. Measurements of Surface Impedance in MgB2 in DC Magnetic Fields: Insights in Flux-Flow Resistivity

Authors: Alimenti, A; Torokhtii, K; Garcia, PV; Silva, E; Grigoroscuta, MA; Badica, P; Crisan, A; Pompeo, N

Published: JAN 2023, MATERIALS, 16, 205, DOI: 10.3390/ma16010205

We present the multifrequency measurements of the surface resistance of spark-plasma-sintered MgB2 performed through a dielectric loaded resonator operating at 16.5 and 26.7 GHz. By normally applying magnetic fields <= 1.2 T to the sample surface, we drove it in the mixed state. By means of data-rooted analysis, we found that the sample vortex dynamics could be fully described within a single-component approach. Pinning phenomena were present and characterized by a depinning frequency smaller than the measurement ones. The multiband nature of the superconductor emerged in the flux-flow resistivity, whose field dependence could be interpreted well within theoretical models. By exploiting them, the upper critical field was extracted in the low-temperature range, which exhibited a consistent temperature trend with the values obtained at the onset of the resistive transition near Tc, and was well in line with literature data on other polycrystalline samples.

4. Efficient Sintering of Mo Matrix Composites-A Study of Temperature Dependences and the Use of the Sinter Additive Ni

Authors: Solodkyi, I; Petrusha, V; Grigoroscuta, MA; Schmelzer, J; Hasemann, G; Betke, U; Badica, P; Krüger, M

Published: OCT 2023, METALS, 13, 1715, DOI: 10.3390/met13101715

Mo matrix composites (MMC) with Mo-9Si-8B inclusions were fabricated by pressure-less sintering (PLS) and spark plasma sintering (SPS) techniques at temperatures between 1200-1500 degrees C using 1 wt.% Ni sinter additive. The positive impact of the addition Ni addition on the sinterability and formation of a continuous Mo matrix of MMC with randomly distributed Mo3Si and Mo5SiB2 inclusions was determined. The Ni addition increased the shrinkage of MMC during PLS by almost a third. The continuous Mo matrix of MMC and a relative density of more than 98% was obtained after SPS at 1400-1500 degrees C. The composite with the maximum relative density of 98% showed a Vickers hardness of 482 +/- 9 (HV20). The potential of using Ni-activated PLS and SPS to produce high-density MMC is shown.

5. Measurements of Surface Impedance in MgB₂ in DC Magnetic Fields: Insights in Flux-Flow Resistivity

Authors: Alimenti, A; Torokhtii, K; García, PV; Silva, E; Grigoroscuta, MA; Badica, P; Crisan, A; Pompeo, N

Published: JAN 2023, MATERIALS, 16, 205, DOI: 10.3390/ma16010205

6. Mesoporous Composite Bioactive Compound Delivery System for Wound-Healing Processes

Authors: Purcareanu, B; Ene, MD; Morosan, A; Mihaiescu, DE; Florea, MA; Ghica, A; Nita, RA; Drumea, V; Grigoroscuta, MA; Kuncser, A; Badica, P; Olariu, L

Published: SEP 2023, PHARMACEUTICS, 15, 2258, DOI: 10.3390/pharmaceutics15092258

Currently, the treatment of wounds is still a challenge for healthcare professionals due to high complication incidences and social impacts, and the development of biocompatible and efficient medicines remains a goal. In this regard, mesoporous materials loaded with bioactive compounds from natural extracts have a high potential for wound treatment due to their nontoxicity, high loading capacity and slow drug release. MCM-41-type mesoporous material was synthesized by using sodium trisilicate as a silica source at room temperature and normal pressure. The synthesized mesoporous silica was characterized by using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), N2 absorption-desorption (BET), Dynamic Light Scattering (DLS) and Fourier transform infrared spectroscopy (FT-IR), revealing a high surface area (BET, 1244 m2/g); pore diameter of approx. 2 nm; and a homogenous, ordered and hexagonal geometry (TEM images). Qualitative monitoring of the desorption degree of the Salvia officinalis (SO) extract, rich in ursolic acid and oleanolic acid, and Calendula officinalis (CO) extract, rich in polyphenols and flavones, was performed via the continuous recording of the UV-VIS spectra at predetermined intervals. The active ingredients in the new composite MCM-41/sage and marigold (MCM-41/SO & CO) were quantified by using HPLC-DAD and LC-MS-MS techniques. The evaluation of the biological composites' activity on the wound site was performed on two cell lines, HS27 and HaCaT, naturally involved in tissue-regeneration processes. The experimental results revealed the ability to stimulate collagen biosynthesis, the enzymatic activity of the main metalloproteinases (MMP-2 and MMP-9) involved in tissue remodeling processes and the migration rate in the wound site, thus providing insights into the re-epithelializing properties of mesoporous composites.

7. MgB2-based biodegradable materials for orthopedic implants br

Authors: Badica, P; Batalu, ND; Balint, E; Tudor, N; Barbuceanu, F; Peteoaca, A; Micsa, C; Eremia, AD; Trancau, OI; Burdusel, M; Grigoroscuta, MA; Aldica, GV; Radu, D; Porosnicu, I; Tiseanu, I

Published: SEP-OCT 2022, JOURNAL OF MATERIALS RESEARCH AND TECHNOLOGY-JMR&T, 20, DOI: 10.1016/j.jmrt.2022.07.164

Bulk high density MgB2 and a composite material made of a PLA matrix and MgB2 powder inclusions were in vivo tested as candidates for biodegradable materials for orthopedic implants. A rat model was used. Implants were introduced into femoral bone, in transversal and longitudinal directions. Assessment of the implant-tissue interaction was performed by X-ray imaging, X-ray computer tomography, electron microscopy, cytology, and histopathology on samples at 40 and 90 days after surgery. Both materials are biocompatible, bone and adjacent soft tissue showing good tolerance of implants. Biodegradation of MgB2 is faster than for PLA-MgB2 composite, but in both cases, it is accompanied by bone regeneration. Results suggest that use of MgB2-containing composites can promote space and time control of degradation and promotes MgB2 as a promising material for fracture repair. (c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

8. MgB2 with Addition of Cubic BN and Ge2C6H10O7 Obtained by Spark Plasma Sintering Technique

Authors: Ionescu, AM; Aldica, G; Popa, S; Enculescu, M; Sandu, V; Pasuk, I; Burdusel, M; Grigoroscuta, MA; Miu, L; Badica, P

Published: JUL 2022, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, DOI: 10.1007/s10948-022-06350-1

High density (94-98% of the theoretical density) MgB2 samples added with C6H10Ge2O7 and cubic BN with compositions (MgB2)(1-x)(Ge2C6H10O7)(0.0028)(cBN)(x) (x = 0.003, 0.005, 0.007, 0.01) and (MgB2)(1-y)(Ge2C6H10O7)(y)(cBN)(0.005) (y = 0.0014, 0.0028, 0.005, 0.0075) were obtained by spark plasma sintering technique. For optimum doped samples with x = 0.005-0.007 and y = 0.0028-0.005, a weak enhancement of zero-field critical current density J(c0), irreversibility field H-irr, and volume pinning force F-p,F-max was determined. This behavior is very different from similar samples added with a single additive for which H-irr has a large enhancement. Consequently, it suggests the presence of opposite structural and microstructural effects induced by the additives. These effects, on the one hand, are discussed to decrease the sensitivity of MgB2 superconducting properties in the co-added samples comparative to samples added with C6H10Ge2O7, and, on the other hand, they contribute to anomalies that were found when assessing the pinning force-related parameters by the universal scaling law.

9. Microwave and Terahertz Properties of Spark-Plasma-Sintered Zr0.8Sn0.2TiO4 Ceramics

Authors: Nedelcu, L; Burdusel, M; Grigoroscuta, MA; Geambasu, CD; Enculescu, M; Badica, P; Banciu, MG

Published: FEB 2022, MATERIALS, 15, 1258, DOI: 10.3390/ma15031258

Zr0.8Sn0.2TiO3 (ZST) powders synthesized by solid-state reaction were subject to processing by spark plasma sintering (SPS). A single-phase ceramic with a high relative density of 95.7% and 99.6% was obtained for sintering temperatures of 1150 degrees C and 1200 degrees C, respectively, and for a dwell time of 3 min. In order to reduce the oxygen vacancies, as-sintered discs were annealed in air at 1000 degrees C. The dielectric loss of the annealed samples, expressed by the Q x f product, measured in the microwave (MW) domain, varied between 35 THz and 50 THz. The intrinsic losses (Q x f ~ 60 THz) were derived by using terahertz time-domain spectroscopy (THz-TDS).

10. Full Length Article Towards high degree of c-axis orientation in MgB2 bulks

Authors: Grigoroscuta, MA; Aldica, GV; Burdusel, M; Sandu, V; Kuncser, A; Pasuk, I; Ionescu, AM; Suzuki, TS; Vasylkiv, O; Badica, P

Published: AUG 2022, JOURNAL OF MAGNESIUM AND ALLOYS, 10, DOI: 10.1016/j.jma.2021.10.013

The paper presents fabrication and characterization of spark plasma sintered textured (001) MgB2 with a record degree of orientation of about 40% and 16% by high-energy ultra-sonication and slip casting in high magnetic field (12 T) and 0 T magnetic field, respectively. Structural characterization was performed by X-ray diffraction, and electron microscopy. The analysis revealed unexpected preferred orientation also in the MgO secondary phase due to the epitaxial growth of (111) MgO on (001) MgB2. The influence of oriented microstructure on the superconducting characteristics expressed by critical current density (Jc), irreversibility field ( H irr), and on the pinning properties were assessed. High anisotropy versus sample orientation in applied magnetic field, H , was observed for Jc, Hirr, pinning activation energy ( U *) extracted from relaxation measurements. The zero-field critical current, Jc0 and Fp,maxare weakly or not dependent on the direction of H , while the other indicated parameters are significantly influenced. Results enable control of superconducting parameters by further optimization of microstructure through MgB2 texturing as a novel and viable strategy for development of bulk MgB2 with enhanced properties when taking advantage of its anisotropy.(c) 2021 Chongqing University. Publishing services provided by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ) Peer review under responsibility of Chongqing University

11. Screening of magnetic fields by superconducting and hybrid shields with a circular cross-section

Authors: Gozzelino, L; Fracasso, M; Solovyov, M; Gomory, F; Napolitano, A; Gerbaldo, R; Ghigo, G; Laviano, F; Torsello, D; Grigoroscuta, MA; Aldica, G; Burdusel, M; Badica, P

Published: APR 1 2022, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 35, 044002, DOI: 10.1088/1361-6668/ac4ad0

The use of superconducting (SC) materials is crucial for shielding quasi-static magnetic fields. However, the need for space-saving solutions with high shielding performance requires the development of a three-dimensional (3D) modelling procedure capable of predicting the screening properties for different orientations of the applied field. In this paper, we use a 3D numerical model based on a vector potential formulation to investigate the shielding ability of SC screens with cylindrical symmetry and a height/diameter aspect ratio close to unity, without and with the superimposition of a ferromagnetic (FM) circular shell. The chosen materials were MgB2 and soft iron. First, the outcomes of the calculations were compared with the experimental data obtained with different shielding arrangements, achieving a notable agreement in both axial field (AF) and transverse field (TF) orientations. Then, we used this validated modelling approach to investigate how the magnetic mitigation properties of a cup-shaped SC bulk can be improved by the superimposition of a coaxial FM cup. Calculations highlighted that the FM addition is very efficient in enhancing the shielding factors (SFs) in the TF orientation. Assuming a working temperature of 30 K and using a layout with the FM cup protruding over the SC one, SFs up to eight times greater than those with a single SC cup were attained at applied field up to 0.15 T, reaching values equal to or higher than 10(2) in the inner half of the shield. In the AF orientation, the addition of the same FM cup incurs a modest worsening at low fields, but at the same time it widens the applied field range where SF > 10(4) occurs near the close extremity of the shield to over 1 T.

12. Antimicrobial Activity of MgB2 Powders Produced via Reactive Liquid Infiltration Method

Authors: Padhi, SK; Baglieri, N; Bonino, V; Agostino, A; Operti, L; Batalu, ND; Chifiriuc, MC; Popa, M; Burdusel, M; Grigoroscuta, MA; Aldica, GV; Radu, D; Badica, P; Truccato, M

Published: AUG 2021, MOLECULES, 26, DOI: 10.3390/molecules26164966

We report for the first time on the antimicrobial activity of MgB2 powders produced via the Reactive Liquid Infiltration (RLI) process. Samples with MgB2 wt.% ranging from 2% to 99% were obtained and characterized, observing different levels of grain aggregation and of impurity phases. Their antimicrobial activity was tested against Staphylococcus aureus ATCC BAA 1026, Enterococcus faecalis ATCC 29212, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, and Candida albicans ATCC 10231. A general correlation is observed between the antibacterial activity and the MgB2 wt.%, but the sample microstructure also appears to be very important. RLI-MgB2 powders show better performances compared to commercial powders against microbial strains in the planktonic form, and their activity against biofilms is also very similar.

13. Sintered and 3D-Printed Bulks of MgB2-Based Materials with Antimicrobial Properties

Authors: Badica, P; Batalu, ND; Chifiriuc, MC; Burdusel, M; Grigoroscuta, MA; Aldica, GV; Pasuk, I; Kuncser, A; Popa, M; Agostino, A; Operti, L; Padhi, SK; Bonino, V; Truccato, M

Published: OCT 2021, MOLECULES, 26, DOI: 10.3390/molecules26196045

Pristine high-density bulk disks of MgB2 with added hexagonal BN (10 wt.%) were prepared using spark plasma sintering. The BN-added samples are machinable by chipping them into desired geometries. Complex shapes of different sizes can also be obtained by the 3D printing of polylactic acid filaments embedded with MgB2 powder particles (10 wt.%). Our present work aims to assess antimicrobial activity quantified as viable cells (CFU/mL) vs. time of sintered and 3D-printed materials. In vitro antimicrobial tests were performed against the bacterial strains Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Staphylococcus aureus ATCC 25923, Enterococcus faecium DSM 13590, and Enterococcus faecalis ATCC 29212; and the yeast strain Candida parapsilosis ATCC 22019. The antimicrobial effects were found to depend on the tested samples and microbes, with E. faecium being the most resistant and E. coli the most susceptible.</p>

14. In Vitro Evaluation of MgB2 Powders as Novel Tools to Fight Fungal Biodeterioration of Heritage Buildings and Objects

Authors: Gheorghe, I; Avram, I; Corbu, VM; Marutescu, L; Popa, M; Balotescu, I; Blajan, I; Mateescu, V; Zaharia, D; Dumbrava, AS; Zetu, OE; Pecete, I; Cristea, VC; Batalu, D; Grigoroscuta, MA; Burdusel, M; Aldica, GV; Badica, P; Datcu, AD; Ianovici, N; Bleotu, C; Lazar, V; Ditu, LM; Chifiriuc, MC

Published: JAN 13 2021, FRONTIERS IN MATERIALS, 7, DOI: 10.3389/fmats.2020.601059

The 17th-19th century wooden and stone churches are an iconic symbol for the Romanian national heritage, raising urgent needs for the development of efficient and ecofriendly restoration and preservation solutions. Nanotechnology has a great but largely unexplored potential in this field, providing new tools and methods to achieve higher consolidation and protection efficiency, mainly due to the ability of nanoparticles to inhibit the growth and metabolic activity of different biodeteriorating agents, including fungi. The purpose of the present study was to report for the first time on the efficiency of MgB2 materials, mainly prized for their practical superconducting properties, against a large collection of filamentous fungal strains recently isolated from biodeteriorated wooden and stone heritage objects. Four types of MgB2 powders, with a crystallite size of 42-113 nm, were tested by qualitative (on 149 strains) and quantitative (on 87 strains) assays. The cytotoxicity was evaluated by the microscopic analysis of SiHa cells morphology and Hep2 cell cycle analysis and the ecotoxicity by the Allium test. The tested filamentous fungal strains belonged to 11 different genera, and those isolated from mural paintings and wooden objects exhibited the best capacity to colonize the inert substratum. All MgB2 powders exhibited similar and relatively low minimal inhibitory concentrations (MIC) values against the Aspergillus and Penicillium isolates, which were predominated among isolates. From the tested powders, PVZ and CERAC proved to be more efficient against the strains isolated from stone and wood materials, while LTS was active against the fungal strains colonizing the mural paintings and museum objects. The cytotoxicity results indicated that the tested powders are toxic for the human cells at concentrations higher than 50 mu g/ml, but, however, the very short lifetime of these NPs prevents their accumulation in the natural environment and, thus, the occurrence of toxic effects. The tested powders proved to be ecofriendly at the active antifungal concentrations, as suggested by the phytotoxicity test results. Taken together, our results suggest the potential of the MgB2 materials for the development of environmentally safe antifungal substances, which can be used in the control of the material cultural heritage biodeterioration process.

15. MgB2 powders and bioevaluation of their interaction with planktonic microbes, biofilms, and tumor cells

Authors: Badica, P; Batalu, ND; Chifiriuc, MC; Burdusel, M; Grigoroscuta, MA; Aldica, G; Pasuk, I; Kuncser, A; Enculescu, M; Popa, M; Marutescu, LG; Gheorghe, I; Thamer, O; Bleotu, C; Pircalabioru, GG; Operti, L; Bonino, V; Agostino, A; Truccato, M

Published: MAY-JUN 2021, JOURNAL OF MATERIALS RESEARCH AND TECHNOLOGY-JMR&T, 12, 2184, DOI: 10.1016/j.jmrt.2021.04.003

Commercial nanopowders of MgB2 were characterized from the viewpoint of granulometric distribution, structure, microstructure, and pH behavior in water. The powders are very different: a higher amount of the MgB2 phase with a lower tendency for agglomeration determines a higher rate of pH-increase. A higher rate of pH-increase usually produces a stronger antimicrobial activity against Staphylococcus aureus, Enterococcus faecium, Escherichia coli, Pseudomonas aeruginosa, Candida albicans, and Candida parapsilosis reference strains. The variation of the pH-increase rate suggests the possibility of temporo-spatial control of MgB2 bioactivity, although the contribution of other factors should not be neglected. Remarkably, the efficiency of the MgB2 powders is higher against biofilms than on microbes in the planktonic state. Further, our experiments confirm the antimicrobial efficiency of MgB2 in the in vitro tests against 29 methicillin resistant clinical S. aureus isolates and 33 vancomycin resistant E. faecium/faecalis strains, but in this case the biofilms are more resistant than planktonic cells. The MgB2 treatment of infected mice led to a significant decrease of E. coli colonization in liver, spleen and peritoneal liquid and it also caused changes in the intestinal microbiota. The activity of powders on HeLa and HT-29 tumor cell lines was assessed by inverted microscopy, flow cytometry, and evaluation of the cellular cycle. MgB2 inhibits tumor cell growth influencing DNA synthesis (S-phase). The obtained results indicate that the tested powders could provide promising solutions for the development of large-spectrum multifunctional antimicrobial and anti-biofilm agents, and/or for anti-cancer therapies. (C) 2021 The Authors. Published by Elsevier B.V.

16. Kaolin clay pottery discovered in the Roman city of Romula (Olt County, Romania)

Authors: Badica, P; Alexandru-Dinu, A; Grigoroscuta, M; Locovei, C; Kuncser, A; Bartha, C; Aldica, G; Negru, M; Batalu, D; Cruceru, N; Savulescu, I

Published: APR 2021, JOURNAL OF ARCHAEOLOGICAL SCIENCE-REPORTS, 36, DOI: 10.1016/j.jasrep.2021.102899

Pottery vessels made of kaolin clay from the Roman Period (2nd, 3rd centuries CE) found in Romula (Re?ca village, Olt County, Romania) from Dacia Inferior (Malvensis) were investigated by petrographic, X-ray diffraction, X-ray fluorescence, thermal analysis, electron microscopy, and mechanical tests. Our results are compared with available data on kaolin clays and pottery vessels from other sites located along the lower course of Danube river and near the Black Sea, namely in Moesia Superior, Moesia Inferior, and Thracia. Archeological and geographical contexts are addressed. Results of our analysis suggest a local production of ceramics in Romula, by using raw materials from the north of Lower Danube, in opposition to the idea that kaolin ware was imported from the provinces south of the Danube.

17. Partially-oriented MgB2 superconducting bulks with addition of B4C and cubic BN obtained by slip casting under high magnetic field and spark plasma sintering

Authors: Grigoroscuta, MA; Aldica, G; Pasuk, I; Burdusel, M; Sandu, V; Kuncser, A; Suzuki, TS; Vasylkiv, O; Badica, P

Published: FEB 2021, MATERIALS RESEARCH BULLETIN, 134, DOI: 10.1016/j.materresbull.2020.111103

Partially-oriented MgB2 bulk discs (13 and 9 %) with the starting compositions of (MgB2)(0.99)(B4C)(0.01) and (MgB2)(0.99)(c-BN)om were fabricated by slip casting under an H-0 = 12 T magnetic field (perpendicular to the disc surface) and subsequent spark plasma sintering. The maximum critical current density and irreversibility field are for H//H-0 (H=applied field). These values are higher or similar to the randomly-oriented samples with the same composition. The maximum volume pinning force (F-p) is lower in the partially-oriented ones than in the randomly-oriented samples. The pinning-force-related parameters depend on the additive and orientation. Assessment of the major pinning mechanism within the scaling and percolation models considering these parameters shows significant limitations. A method to scale F-p is proposed; for the randomly and partially-oriented samples (that show an extra peak in F-p), the single and double Gaussian functions fit well. The results suggest an anisotropic influence of carbon substituting for boron in the MgB2.

18. Antibacterial composite coatings of MgB2 powders embedded in PVP matrix

Authors: Badica, P; Batalu, ND; Burdusel, M; Grigoroscuta, MA; Aldica, G; Enculescu, M; Pircalabioru, GG; Popa, M; Marutescu, LG; Dumitriu, BG; Olariu, L; Bicu, A; Purcareanu, B; Operti, L; Bonino, V; Agostino, A; Truccato, M; Chifiriuc, MC

Published: MAY 5 2021, SCIENTIFIC REPORTS, 11, DOI: 10.1038/s41598-021-88885-2

Three commercial powders of MgB2 were tested in vitro by MTS and LDH cytotoxicity tests on the HS27 dermal cell line. Depending on powders, the toxicity concentrations were established in the range of 8.3-33.2 mu g/ml. The powder with the lowest toxicity limit was embedded into polyvinylpyrrolidone (PVP), a biocompatible and biodegradable polymer, for two different concentrations. The self-replenishing MgB2-PVP composite materials were coated on substrate materials (plastic foil of the reservoir and silicon tubes) composing a commercial urinary catheter. The influence of the PVP-reference and MgB2-PVP novel coatings on the bacterial growth of Staphylococcus aureus ATCC 25923, Enterococcus faecium DMS 13590, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, in planktonic and biofilm state was assessed in vitro at 6, 24, and 48 h of incubation time. The MgB2-PVP coatings are efficient both against planktonic microbes and microbial biofilms. Results open promising applications for the use of MgB2 in the design of anti-infective strategies for different biomedical devices and systems.

19. On the pinning force in high density MgB2 samples

Authors: Sandu, V; Ionescu, AM; Aldica, G; Grigoroscuta, MA; Burdusel, M; Badica, P

Published: MAR 15 2021, SCIENTIFIC REPORTS, 11, DOI: 10.1038/s41598-021-85209-2

An analysis of the field dependence of the pinning force in different, high density sintered samples of MgB2 is presented. The samples were chosen to be representative for pure MgB2, MgB2 with additives, and partially oriented massive samples. In some cases, the curves of pinning force versus magnetic field of the selected samples present peculiar profiles and application of the typical scaling procedures fails. Based on the percolation model, we show that most features of the field dependence of the critical force that generate dissipation comply with the Dew-Hughes scaling law predictions within the grain boundary pinning mechanism if a connecting factor related to the superconducting connection of the grains is used. The field dependence of the connecting function, which is dependent on the superconducting anisotropy, is the main factor that controls the boundary between dissipative and non-dissipative current transport in high magnetic field. Experimental data indicate that the connecting function is also dependent on the particular properties (e.g., the presence of slightly non-stoichiometric phases, defects, homogeneity, and others) of each sample and it has the form of a single or double peaked function in all investigated samples.

20. Investigation of flux jumps during pulsed field magnetization in graphene-added MgB2 bulks

Authors: Yokoyama, K; Oka, T; Berger, K; Dorget, R; Koblischka, M; Grigoroscuta, M; Burdusel, M; Batalu, D; Aldica, G; Badica, P; Sakai, N; Muralidhar, M; Murakami, M

Published: 2020, 14TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS2019), 1559, DOI: 10.1088/1742-6596/1559/1/012080

MgB2 bulk superconductors are expected to be utilized as rare-earth-free and lightweight trapped field magnets. However, the flux jumps frequently happen during the magnetizing processes, and heavily degrade the field-trapping performances. We have investigated the effect of additives to the MgB2 bulk samples prepared by spark plasma sintering process, and observed various flux jumps during the pulsed field magnetizing processes, which were carried out at 14 K which was obtained by the 2-stage GM cryocooler. The authors classified the flux motions as three categories as "no flux flow", "fast flux flow", and "flux jump" regions, and investigated the conditions where the flux jumps happen. We observed some drastic flux jumps in the pristine and clarified the effect of graphene addition to the flux jumps. The experimental results showed us a possible expansion of no flux jump region, and suggested us the improvement of field trapping capability.

21. High magnetic shielding properties of an MgB2 cup obtained by machining a spark-plasma-sintered bulk cylinder

Authors: Gozzelino, L; Gerbaldo, R; Ghigo, G; Torsello, D; Bonino, V; Truccato, M; Grigoroscuta, MA; Burdusel, M; Aldica, GV; Sandu, V; Pasuk, I; Badica, P

Published: APR 2020, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 33, DOI: 10.1088/1361-6668/ab7846

Superconductors are key materials for shielding quasi-static magnetic fields. In this work, we investigated the shielding properties of an MgB2 cup-shaped shield with small aspect-ratio of height/outer radius. Shape and aspect-ratio were chosen in order to address practical requirements of both high shielding factors (SFs) and space-saving solutions. To obtain large critical current densities (J(c)), which are crucial for achieving high magnetic-mitigation performance, a high-purity starting MgB2 powder was selected. Then, processing of the starting MgB2 powder into high density bulks was performed by spark plasma sintering. The as-obtained material is fully machinable and was shaped into a cup-shield. Assessment of the material by scaling of the pinning force showed a non-trivial pinning behaviour. The MgB2 powder selection was decisive in enlarging the range of external fields where efficient shielding occurs. The shield's properties were measured in both axial- and transverse-field configurations using Hall probes. Despite a height/outer radius aspect ratio of 2.2, shielding factors higher than 10(4) at T = 20 K up to a threshold field of 1.8 T were measured in axial-field geometry at a distance of 1 mm from the closed extremity of the cup, while SFs > 10(2) occurred in the inner half of the cup. As expected, this threshold field decreased with increased temperature, but SFs still exceeding the above mentioned values were found up to 0.35 T at 35 K. The shield's shape limits the SF values achievable in transverse-field configuration. Nevertheless, the in-field J(c) of the sample supported SFs over 40 at T = 20 K up to a field of 0.8 T, 1 mm away from the cup closure.

22. Enhanced critical current density at high magnetic fields in MgB2 with Ga/In acetylacetonate processed by spark plasma sintering

Authors: Batalu, D; Aldica, G; Burdusel, M; Grigoroscuta, M; Pasuk, I; Kuncser, A; Ionescu, AM; Badica, P

Published: MAY-JUN 2020, JOURNAL OF MATERIALS RESEARCH AND TECHNOLOGY-JMR&T, 9, 3733, DOI: 10.1016/j.jmrt.2020.01.109

Dense samples (94-96%) with starting composition (MgB2)(0.99)(X-acac)(0.01) (X-acac denotes Ga or In acetylacetonate) were obtained by spark plasma sintering. The resulting material is a superconducting composite, where carbon substitutes for boron in the crystal structure of MgB2. Added samples show enhanced critical current density at high magnetic fields and this is reflected in high values of irreversibility field (H-irr) at temperatures below 25 K when compared to a pristine sample. More efficient is In-acac addition and it promotes a H-irr of similar to 12.4T (100 A/cm(2) criterion) at 5 K. Carbon substitution for boron in the crystal structure of MgB2 has a strong influence on the pinning force and its related parameters and promotes in the added samples a grain boundary pinning mechanism as the dominant one, whereas the pristine sample with a low amount of carbon shows a major mechanism of point pinning type. However, our analysis indicates on the synergetic effects of the carbon substituting for boron and of the microstructural details on pinning and critical current density. The result strongly emphasizes the significantly different behavior of the additive during processing of the MgB2 samples, although thermal analysis experiments on both additives show very similar decomposition patterns. (C) 2020 The Authors. Published by Elsevier B.V.

23. Reproducibility of small Ge2C6H10O7-added MgB2 bulks fabricated by ex situ Spark Plasma Sintering used in compound bulk magnets with a trapped magnetic field above 5T

Authors: Badica, P; Aldica, G; Grigoroscuta, MA; Burdusel, M; Pasuk, I; Batalu, D; Berger, K; Koblischka-Veneva, A; Koblischka, MR

Published: JUN 29 2020, SCIENTIFIC REPORTS, 10, DOI: 10.1038/s41598-020-67463-y

Bulk discs (20 mm diameter and 4.3 mm thickness) of MgB2 added with Ge2C6H10O7 were obtained by Spark Plasma Sintering. Six samples with composition Mg B 2 (Ge2C6H10O7)(0.0014) and one undoped sample were fabricated under similar conditions and were magnetically characterized in order to determine the scattering of properties and reproducibility. The main source of the scattering of the properties is the decomposition of the additive due to elimination of the organic part in gas form, which occurs stepwise with intensive vacuum drops at around similar to 560 and similar to 740 degrees C. A third drop, which is sometimes not well resolved being part of the second peak at 740 degrees C, occurs at similar to 820 degrees C. The critical temperature at the midpoint of the transition, T-C, shows only a relatively small variation between 37.4 and 38 K, and the irreversibility field at a low temperature of 5 K takes values between 8 and 10 T. The pinning force and pinning force related parameters do not correlate with the carbon substituting for boron in MgB2 and suggest a synergetic influence of the microstructural details and carbon. Overall, despite the superconducting properties scattering, the samples are of high quality. Stacked into a column of six samples, they can trap at the center and on the surface of the column a magnetic field of 6.78 and 5.19 Tat 12 K, 5.20 and 3.98 Tat 20 K and 2.39, and 1.96 Tat 30 K. These promising values, combined with facile fabrication of the samples with relatively high quality and reproducibility, show the feasibility of their use in building complex and large compound arrangements for bulk magnets and other applications.

24. Microwave investigation of pinning in Te- and cubic-BN- added MgB2

Authors: Alimenti, A; Torokhtii, K; Grigoroscuta, M; Badica, P; Crisan, A; Silva, E; Pompeo, N

Published: 2020, 14TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS2019), 1559, DOI: 10.1088/1742-6596/1559/1/012039

MgB2 has great potential for many applications, thanks to its relatively high critical temperature and low fabrication cost. Large efforts are done to improve the current carrying capabilities of bulks and tapes in view of different application fields, e.g. with the addition of Te and cubic-BN to MgB2. To elucidate the vortex pinning physics exploiting a different dynamic regime, we present here a microwave study of the pinning properties of spark plasma sintered bulk MgB2 with and without the addition of 0.01 % at. Te or cubic-BN. We show the surface resistance R-s of the MgB2 samples measured with a dielectric-loaded resonator at similar to 16.5 GHz and similar to 26.7 GHz in the 10 K-Tc temperature range at fields up to 1.0 T. Then, the MgB2 R., is studied with high frequency vortex motion models in order to obtain the pinning constant (Labusch parameter) and the depinning frequency. Finally, the microwave behavior of MgB2 in the mixed state is compared with the recent results obtained on Nb3Sn.

25. Extrinsic absorption in spark plasma sintered Zr0.8Sn0.2TiO4 ceramics investigated by terahertz time-domain spectroscopy

Authors: Nedelcu, L; Burdusel, M; Grigoroscuta, MA; Geambasu, CD; Banciu, MG; Badica, P

Published: 2019, 2019 44TH INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER, AND TERAHERTZ WAVES (IRMMW-THZ), DOI:

Solid-state reaction synthesized Zr0.8Sn0.2TiO3 powders have been compacted by spark plasma sintering. In order to reduce the oxygen vacancies, the sintered samples were annealed ex-situ in air. Single-phase ceramics with different amount of oxygen vacancies and, consequently, different extrinsic absorption were investigated by terahertz time-domain spectroscopy. The results showed that the terahertz spectroscopy could be a suitable technique for tailoring the absorption properties of the spark plasma sintered materials.

26. Passive magnetic shielding by machinable MgB2 bulks: measurements and numerical simulations

Authors: Gozzelino, L; Gerbaldo, R; Ghigo, G; Laviano, F; Torsello, D; Bonino, V; Truccato, M; Batalu, D; Grigoroscuta, MA; Burdusel, M; Aldica, GV; Badica, P

Published: MAR 2019, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 32, DOI: 10.1088/1361-6668/aaf99e

We report on a combined experimental and modelling approach towards the design and fabrication of efficient bulk shields for low-frequency magnetic fields. To this aim, MgB2 is a promising material when its growing technique allows the fabrication of suitably shaped products and a realistic numerical modelling can be exploited to guide the shield design. Here, we report the shielding properties of an MgB2 tube grown by a novel technique that produces fully machinable bulks, which can match specific shape requirements. Despite a height/radius aspect ratio of only 1.75, shielding factors higher than 175 and 55 were measured at temperature T = 20 K and in axially-applied magnetic fields mu H-0(appl) = 0.1 and 1.0 T, respectively, by means of cryogenic Hall probes placed on the tube's axis. The magnetic behaviour of the superconductor was then modelled as follows: first we used a two-step procedure to reconstruct the macroscopic critical current density dependence on magnetic field, J(c)(B), at different temperatures from the local magnetic induction cycles measured by the Hall probes. Next, using these J(c)(B) characteristics, by means of finite-element calculations we reproduced the experimental cycles remarkably well at all the investigated temperatures and positions along the tube's axis. Finally, this validated model was exploited to study the influence both of the tube's wall thickness and of a cap addition on the shield performance. In the latter case, assuming the working temperature of 25 K, shielding factors of 10(5) and 10(4) are predicted in axial applied fields it mu H-0(appl) = 0.1 and 1.0 T, respectively.

27. Bulks of Al-B-C obtained by reactively spark plasma sintering and impact properties by Split Hopkinson Pressure Bar

Authors: Vasylkiv, O; Borodianska, H; Demirskyi, D; Li, P; Suzuki, TS; Grigoroscuta, MA; Pasuk, I; Kuncser, A; Badica, P

Published: DEC 20 2019, SCIENTIFIC REPORTS, 9, DOI: 10.1038/s41598-019-55888-z

Mixtures of B4C, alpha-AlB12 and B powders were reactively spark plasma sintered at 1800 degrees C. Crystalline and amorphous boron powders were used. Samples were tested for their impact behavior by the Split Hopkinson Pressure Bar method. When the ratio R = B4C/alpha-AlB12 >= 1.3 for a constant B-amount, the major phase in the samples was the orthorhombic AlB24C4, and when R < 1 the amount of AlB24C4 significantly decreased. Predictions that AlB24C4 has the best mechanical impact properties since it is the most compact and close to the ideal cubic packing among the Al-B-C phases containing B-12-type icosahedra were partially confirmed. Namely, the highest values of the Vickers hardness (32.4 GPa), dynamic strength (1323 MPa), strain and toughness were determined for the samples with R = 1.3, i.e., for the samples with a high amount of AlB24C4. However, the existence of a maximum, detectable especially in the dynamic strength vs. R, indicated the additional influence of the phases and the composite's microstructure in the samples. The type of boron does not influence the dependencies of the indicated mechanical parameters with R, but the curves are shifted to slightly higher values for the samples in which amorphous boron was used.

28. Superconducting MgB2 textured bulk obtained by ex situ spark plasma sintering from green compacts processed by slip casting under a 12T magnetic field

Authors: Grigoroscuta, MA; Sandu, V; Kuncser, A; Pasuk, I; Aldica, G; Suzuki, TS; Vasylkiv, O; Badica, P

Published: DEC 2019, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 32, DOI: 10.1088/1361-6668/ab4620

MgB2 green bodies were prepared by magnetic field slip casting in ethyl alcohol with added polyethyleneimine dispersing agent under a high magnetic field, mu H-0(0) = 12 T. Samples were further processed by spark plasma sintering (SPS) and characterized for superconducting properties. Slip casting provides texturing of MgB2 (the degree of c-axis orientation is approximately 3.5%), which is further increased significantly (to about 21%) in the SPSed sample. The critical current density (J(c)) displays anisotropy relative to the orientation of the measuring magnetic field. Specific features of J(c)(H, T) and of the pinning force extracted from magnetic measurements with the field parallel and perpendicular to H-0 are discussed.

29. Enhanced near-infrared response of a silicon solar cell by using an up-conversion phosphor film of Yb/Er - co-doped CeO2

Authors: Grigoroscuta, M; Secu, M; Trupina, L; Enculescu, M; Besleaga, C; Pintilie, I; Badica, P

Published: SEP 1 2018, SOLAR ENERGY, 171, 46, DOI: 10.1016/j.solener.2018.06.057

Co-doped CeO2 thin films were grown from a bulk target with starting composition Ce0.95Yb0.04Er0.01O2 by pulsed laser deposition (PLD) on a p(+)-n-n(+) single crystal silicon diode. The PLD laser fluence was varied between 1.7 J/cm(2) and 3.7 J/cm(2). The device with the film grown for a laser fluence of 2.3 J/cm(2) delivers the highest performance taking advantage of the up conversion (UC) effect provided by this film. Namely, the increase in the relative power conversion efficiency of the device is 12.1% and 39.2% for illumination under 1 and 2.1 sun, respectively, and its relative external quantum efficiency is 8.2% when illuminated with 980 nm light. The film grown for the optimum 2.3 J/cm(2) fluence shows good target-film composition transfer and a granular morphology with a low roughness. The UC mechanism consists of efficient energy transfer between spatially separated Yb3+ and Er3+ ions, i.e. the absorption of infrared light photons by the Yb3+ ions (F-2(7/2) -> F-2(5/2) transition) is followed by a two-step energy transfer process to neighboring Er3+ ions and by their characteristic luminescent emissions ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and (F-4(9/2) -> I-4(15/2)).

30. Compressive properties of pristine and SiC-Te-added MgB2 powders, green compacts and spark-plasma-sintered bulks

Authors: Badica, P; Batalu, D; Burdusel, M; Grigoroscuta, MA; Aldica, GV; Enculescu, M; Gabor, RA; Wang, ZY; Huang, RX; Li, PF

Published: JUN 15 2018, CERAMICS INTERNATIONAL, 44, 10191, DOI: 10.1016/j.ceramint.2018.03.008

Pristine and (SiC + Te)-added MgB2 powders, green and spark plasma sintered (SPS) compacts were investigated from the viewpoint of quasi-static and dynamic (Split-Hopkinson Pressure Bar, SHPB) compressive mechanical properties The amount of the additive (SiC + Te) was selected to be the optimum one for maximization of the superconducting functional parameters. Pristine and added MgB2 show very similar compressive parameters (tan delta, fracture strength, Vickers hardness, others) and fragment size in the SHPB test. However, for the bulk SPSed samples the ratio of intergranular to transgranular fracturing changes, the first one being stronger in the added sample. This is reflected in the quasi-static K-IC that is higher for the added sample. Despite this result, sintered samples are brittle and have roughly similar fragmentation behavior as for brittle engineering ceramics. In the fragmentation process, the composite nature of our samples should be considered with a special focus on MgB2 blocks (colonies) that show the major contribution to fracturing. The Glenn-Chudnovsky model of fracturing under dynamic load provides the closest values to our experimental fragment size data.

31. POWDER-IN-TUBE TAPES OF MgB2 IN Fe-SHEATH PROCESSED BY EX-SITU SPARK PLASMA SINTERING

Authors: Burdusel, M; Ionescu, AM; Grigoroscuta, M; Batalu, D; Enculescu, M; Popa, S; Mihalache, V; Aldica, G; Badica, P

Published: 2017, UNIVERSITY POLITEHNICA OF BUCHAREST SCIENTIFIC BULLETIN SERIES B-CHEMISTRY AND MATERIALS SCIENCE, 79, 172, DOI:

Commercial MgB2 powder was loaded into a Fe-tube, by plastic deformation a tape of similar to 0.5 mm in thickness and 6.9 mm in width was obtained. Short pieces were processed by Spark Plasma Sintering (SPS) at 950, 1050 and 1150 degrees C for 3 min. The optimum sintering temperature is 1050 degrees C. From magnetic/electrical measurements, the onset critical temperature and the irreversibility field at 5 K were 38.7 / 38.9 K and 6.2 / 13.5 T, respectively. The pinning-force-related parameters indicate that the dominant flux pinning mechanism is of point pinning type. Contribution of grain boundary pinning is stronger at lower temperatures.

32. From an Anomalous Peak Effect to a Second Magnetization Peak in Nb-rich Nb-Ti Alloys

Authors: Ionescu, AM; Ivan, I; Enculescu, M; Grigoroscuta, M; Miu, D; Valeanu, M; Badica, P; Miu, L

Published: APR 2017, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 30, 1108, DOI: 10.1007/s10948-016-3670-4

We investigated the evolution of the DC magnetic hysteresis curves of Nb0.89Ti0.11 alloys thermo-mechanically processed by intermediate heat treatments at 900 degrees C in vacuum and cold rolling (similar to 50% thickness reduction). Starting with a rectangular piece (similar to 0.4 mm thick) cut from the as-grown alloy, after the first thermo-mechanical treatment, the specimen exhibits over a wide temperature T interval a peak effect not far from the DC irreversibility line. With a supplemental thermo-mechanical treatment, the peak effect disappears and is substituted by a second magnetization peak ( where the characteristic fields are significantly lower) induced by pinning enhancement. The second magnetization peak was clearly seen at high temperatures only, due to the occurrence of thermo-magnetic instabilities in the low-T domain. In both cases, analysis of magnetic relaxation evidences a crossover towards plastic vortex creep accompanying the maximum in the effective critical current density. These results suggest a common nature of the observed effects, related to the disordering of the vortex system induced by pinning.

Dr. Eng. Mihai-Alexandru GRIGOROSCUTA

Scientific Researcher