National Institute Of Materials Physics - Romania

Publications

860 articles found

331. Effect of green body annealing on laser performance of YAG:Nd3+ ceramics

Authors: Yavetskiy, RP; Parkhomenko, SV; Vorona, IO; Tolmachev, AV; Kosyanov, DY; Kuryavyi, VG; Mayorov, VY; Gheorghe, L; Croitoru, G; Enculescu, M

Published: MAR 2018, CERAMICS INTERNATIONAL, 44, 4490, DOI: 10.1016/j.ceramint.2017.11.192

Annealing temperature of green bodies has been shown to influence greatly the optical properties and laser characteristics of Y3Al5O12:Nd3+ (1 at%) ceramics. Increase the annealing temperature above some critical one (800 degrees C) results in appearance of submicron pores due to Y4Al2O9 phase formation accompanied by specific volume expansion. The energy changes associated with the chemical reaction can lead to the development of microstructures that possess lower sinterability. As a result, Y3Al5O12:Nd3+ ceramics prepared from unannealed green bodies and those annealed at 600 and 800 degrees C possess higher optical transmittance and enhanced slope efficiency (63-67%) compared with that obtained using green bodies annealed at 1000 degrees C (41%). Further investigations are necessary in order to explain this behavior.

332. Wet chemical synthesis of ZnO-CdS composites and their photocatalytic activity

Authors: Zgura, I; Preda, N; Socol, G; Ghica, C; Ghica, D; Enculescu, M; Negut, I; Nedelcu, L; Frunza, L; Ganea, CP; Frunza, S

Published: MAR 2018, MATERIALS RESEARCH BULLETIN, 99, 181, DOI: 10.1016/j.materresbull.2017.11.013

The present study is focused on the wet chemical synthesis and the characterization of ZnO-CdS composites. The X-ray diffraction shows that the composites contain ZnO in hexagonal wurtzite structure and CdS in cubic phase. The scanning/transmission electron microscopy images reveal flower-like structures with different sizes depending on the CdS content. The optical investigations on composites reveal that the reflectance spectra disclose two thresholds of similar to 370 nm and similar to 460 nm associated with the ZnO and CdS, respectively. The photocatalytic activity measurements evidenced that the degradation efficiency of RhB in the presence of composites is higher comparatively with pristine ZnO, depending on the catalyst morphology, which varies with CdS content and the pH value of RhB solution. The electron paramagnetic resonance revealed the presence of the paramagnetic point defects in the samples. Thus, the wet chemical approaches are suitable for a large scale production of such ZnO-CdS composites having enhanced photocatalytic activity.

333. 1532 nm sensitized luminescence and up-conversion in Yb,Er:YAG transparent ceramics

Authors: Vorona, I; Yavetskiy, RP; Dobrotvorskaya, MV; Doroshenko, AG; Parkhomenko, SV; Tolmachev, AV; Kosyanov, DY; Gheorghe, L; Gheorghe, C; Hau, S; Enculescu, M

Published: MAR 2018, OPTICAL MATERIALS, 77, 225, DOI: 10.1016/j.optmat.2018.01.038

Luminescent properties of Yb,Er:YAG transparent ceramics containing 5 at% Yb3+ and 0.5, 1 and 1.5 at%, respectively, Er3+ ions have been studied. It has been found that increasing of erbium ions concentration increases both efficiency of nonradiative energy transfer Yb3+ -> Er3+ (which reaches 72% for 1.5 at% Er3+) and the luminescence in the range of 650-700 nm associated with F-4(9/2), (11/2) -> I-4(15/2) transitions of Er3+ ions. It was also determined that the concentration quenching of sensitized luminescence of Er3+ ions at 1532 nm is associated with secondary excitation of metastable energy level of Er3+ followed by up conversion emission. (C) 2018 Elsevier B.V. All rights reserved.

334. Synthesis and characterization of dextran-coated iron oxide nanoparticles

Authors: Predescu, AM; Matei, E; Berbecaru, AC; Pantilimon, C; Dragan, C; Vidu, R; Predescu, C; Kuncser, V

Published: MAR 2018, ROYAL SOCIETY OPEN SCIENCE, 5, DOI: 10.1098/rsos.171525

Synthesis and characterization of iron oxide nanoparticles coated with a large molar weight dextran for environmental applications are reported. The first experiments involved the synthesis of iron oxide nanoparticles which were coated with dextran at different concentrations. The synthesis was performed by a co-precipitation technique, while the coating of iron oxide nanoparticles was carried out in solution. The obtained nanoparticles were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction spectrometry, Fourier transform infrared spectroscopy and superconducting quantum interference device magnetometry. The results demonstrated a successful coating of iron oxide nanoparticles with large molar weight dextran, of which agglomeration tendency depended on the amount of dextran in the coating solution. SEM and TEM observations have shown that the iron oxide nanoparticles are of about 7 nm in size.

335. High temperature thermo-physical properties of SPS-edW-Cu functional gradient materials

Authors: Galatanu, M; Enculescu, M; Galatanu, A

Published: FEB 2018, MATERIALS RESEARCH EXPRESS, 5, DOI: 10.1088/2053-1591/aaa860

The divertor of a fusion reactor like DEMO requires materials able to withstand high heat fluxes and neutron irradiation for several years. For the water cooling concept of this essential part of the reactor, the most likely plasma facing material will be W, while the heatsink material considered is CuCrZr or an improved version of such a Cu-based alloy. To realize W-Cu alloy joints able to withstand thousands of thermal cycles can be difficult due to the difference between the thermal expansion coefficients of these materials. In this work we investigate the possibility to realize such joints by using W-Cu functional gradient materials (FGMs) produced from nanometric and micrometric metallic powders mixtures and consolidated by spark plasma sintering at about 900 degrees C. Morphological and thermal properties investigations, performed for typical compositions, shows that the best results are obtained using powders with micrometric dimensions. A resulting 1 mm thick, 3 layers W-Cu FGM produced by this simple method shows a remarkable almost constant thermal conductivity value of 200 W m(-1) K-1, from room temperature up to 1000 degrees C.

336. Charge carrier traps in tris-(8-hydroxyquinoline) aluminum

Authors: Secu, M; Polosan, S

Published: FEB 2018, JOURNAL OF LUMINESCENCE, 194, 95, DOI: 10.1016/j.jlumin.2017.10.003

Thermoluminescent measurements between 40 and 300 C revealed two traps in the irradiated Alq(3) powder synthetized from Al(OH)(3), at 0.8 and 1.1 eV below the conduction band. These two traps have been assigned with the Alq(3) facial and meridional isomers and the trapping process stand for a long time after the switching off the irradiation source. The spectra recorded an hour after switching recover 60% from the photoluminescence and also decreasing the TL signals. A theoretical model was proposed in order to explain the trapping and detrapping processes in both meridional and facial Alq(3) isomers based on the XPS measurements for the HOMO, LUMO, ionization potential and optical bandgap, completed with the DFT calculated parameters and similar experimental results concerning the evidences of the triplet states in Alq(3) compound.

337. Electroluminescence Properties of IrQ(ppy)(2) Dual-Emitter Organometallic Compound in Organic Light-Emitting Devices

Authors: Ciobotaru, CC; Polosan, S; Ciobotaru, IC

Published: FEB 2018, JOURNAL OF ELECTRONIC MATERIALS, 47, 1496, DOI: 10.1007/s11664-017-5945-3

This paper reports the influence of the charge carrier mobility on the electroluminescent properties of a dual-emitter organometallic compound dispersed in two conjugated organic small-molecule host materials and embedded in organic light-emitting devices (OLEDs). The electroluminescent processes in OLEDs are strongly influenced by the host-guest interaction. The charge carrier mobility in the host material plays an important role in the electroluminescent processes but also depends on the triplet-triplet interaction with the organometallic compound. The low charge carrier mobility in 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) host material reduces the electroluminescent processes, but they are slightly enhanced by the triplet-triplet exothermic charge transfer. The higher charge carrier mobility in the case of N, N'-bis(3-methylphenyl)-N, N'-diphenylbenzidine (TPD) host material influences the electroluminescent processes by the endothermic energy transfer at room temperature, which facilitates the triplet-triplet harvesting in the host-guest system. The excitation is transferred to the guest molecules by triplet-triplet interaction as a Dexter transfer, which occurs by endothermic transfer from the triplet exciton in the host to the triplet exciton in the guest.

338. Thermophysical properties of Cu-ZrO2 composites as potential thermal barrier materials for a DEMO W-monoblock divertor

Authors: Galatanu, M; Enculescu, M; Galatanu, A

Published: FEB 2018, FUSION ENGINEERING AND DESIGN, 127, 184, DOI: 10.1016/j.fusengdes.2018.01.011

DEMO fusion reactor divertor is expected to extract a heat flux of about 10 MW/m(2). One of the most promising concept design for it is the W-monoblock, which should be connected to a CuCrZr or an advanced Cu ODS alloy pipe passing through the W component. Since the optimum operating temperature windows for W and existing Cu alloys are far away from overlapping, a suited interface is needed to keep the adjacent materials as close as Possible to their respective temperature operating windows. The interface material should therefore have a low enough thermal conductivity to act as a thermal barrier and a thermal expansion coefficient suited to protect the W-pipe joint from stresses induced by the different thermo-mechanical properties of W and Cu-alloys. As interface materials we have considered Cu-ZrO2 composites produced by powder metallurgy route. Such materials can be realized in an unexpected large compositional range (up to at least 90% ZrO2 volume concentration) and be easily further joined to both W and Cu-alloys by an electrical field assisted technology. We analyse their microstructure and thermo-physical properties both as single materials and included in W-thermal barrier-CuCrZr 3-layers systems in comparison to those of previously produced Cu-ased composites and commercially available Cu foams.

339. Dwell Time Influence on Spark Plasma-Sintered MgB2

Authors: Aldica, G; Popa, S; Enculescu, M; Pasuk, I; Ionescu, AM; Badica, P

Published: FEB 2018, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 31, 325, DOI: 10.1007/s10948-017-4236-9

Samples of MgB2 with relative density above 95% were obtained by spark plasma sintering (SPS) at 1150 A degrees C for a heating time of 100 A degrees C/min and a maximum pressure of 95 MPa. Dwell time was of 1, 4, 7, 10, and 20 min. Samples show that dwell time has a low influence on superconducting characteristics. However, small differences were observed and they are discussed from the viewpoint of density, structural, microstructural, critical current density, irreversibility field, and pinning type. The dominant contribution for all investigated samples is given by point and delta T (c) pinning.

340. Commercial Hydroxyapatite Powders for Lead Removal from Aqueous Solution

Authors: Negrila, CC; Iconaru, SL; Motelica-Heino, M; Guegan, R; Predoi, G; Barbuceanu, F; Ghita, RV; Petre, CC; Jiga, G; Badea, ML; Prodan, AM; Predoi, D

Published: 2018, 9TH INTERNATIONAL CONFERENCE ON TIMES OF POLYMERS AND COMPOSITES: FROM AEROSPACE TO NANOTECHNOLOGY, 1981, DOI: 10.1063/1.5045983

Nowadays, the advances materials science have created the possibility of using nanomaterials as adsorbents in environmental technologies. This study is focused on the evaluation of C-HAp powders capacity in lead ions removal from contaminated solutions. The C-HAp powders after removal of lead (Pb2+) ions from contaminated water were characterized by X-Ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-ray analysis (EDX). The presence of lead in the hydroxyapatite powders after removal experiments was observed in the XPS general spectrum and also in the EDX spectra. In addition, the AAS studies revealed that the C-HAp powders possess a high affinity towards lead ions with an effective removal rate of 80% for an initial lead concentration of 20 mg.L-1.