Papers by Author: Traian Florin Marinca

Abstract: Nanocrystalline/nanosized magnetite - Fe₃O₄ powder was obtained by mechanical milling of well crystallized magnetite obtained by ceramic method starting from stoichiometric mixture of commercial hematite - Fe₂O₃ and iron - Fe powders. The mean crystallites size of the magnetite is decreasing upon increasing the milling time down to 6 nm after 240 minutes of milling. After 30 minutes of milling an undesired hematite phase is formed in the material. The amount of this phase increases upon increasing the milling time. In the early stage of milling (up to 30 minutes) the existence of nanometric particles (mean size below 100 nm) is noticed. The d₅₀ median diameter decreases first (up to 5 minutes of milling) and after that, an increase follows for milling times up to 120 minutes. Saturation magnetization decreases upon increasing the milling time and is more difficult to saturate. X-ray diffraction, laser particle size analysis and magnetic measurements have been used for powder characterization.

Abstract: Result of research concerning the influence of milling conditions on the amorphisation of the Fe₇₅Si₂₀B₅ (at.%) alloy is presented. Amorphous powder of Fe₇₅Si₂₀B₅ (at.%) was prepared by dry and wet mechanical alloying (MA) route starting from a mixture of Fe, Si and B elemental powders. The mixture was wet/dry milled up to 50 hours. Benzene, oleic acid and ethanol were used as process control agents (PCA) in order to investigate the influence of their chemical composition on the powder amorphisation. The evolution of the powder crystalline structure, thermal stability and magnetic properties were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetry (TG) as well as magnetic measurements versus temperature and field. It is proved that the chemical composition of the PCA (especially the carbon content) plays an important role in the amorphisation process induced by wet MA.

Abstract: Fe/Fe₂O₃ composite powders were obtained by mechanical milling of iron and hematite up to 120 minutes in a high energy planetary ball mill. The particles size decreases by mechanical milling upon the formation of the Fe/Fe₂O₃ composite particles. After 120 minutes of milling the median particles size is at 7.2 μm. The Fe/Fe₃O₄ type composite were obtained by reactive sintering in argon atmosphere at 1100 °C of the Fe/Fe₂O₃ composite powders milled for 60 and 120 minutes. After sintering a FeO-wüstite residual phase is formed and this phase is eliminated by applying a subsequent annealing at a temperature of 550 °C. The sintered compact before and after annealing is composed by a quasi-continuous iron matrix in which are embedded iron oxides clusters (Fe₃O₄ and FeO before annealing and Fe₃O₄ after annealing). The iron oxide clusters are analogous with the Widmanstatten structure observed in steels before and after annealing. The materials have been investigated using laser particle size analysis, optical microscopy, scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction.

Abstract: Amorphous Fe₇₅Si_20-xB₅M_x powders with M= Ti, Ta or Zr and x = 0 and 5 were synthesized by wet mechanical alloying, using benzene as a surfactant. The thermal stability of the Fe-Si-B alloy increases by introducing transition metals. The replacement of 5% Si with Ti, Ta or Zr leads to an increase of the crystallization temperature. It was found that the replacement of 5% Si with Zr increases the crystallization temperature with 115 °C, and also reveals a glass transition temperature around 580 °C.

Abstract: Fe-Si alloy with a Si content of 10 wt. % was obtained in nanocrystalline state by mechanical alloying of elemental iron and silicon powders. The mechanical alloying process was carried out in a high energy ball mill (Fritsch, Pulverisette 4) in argon atmosphere. The X-ray diffraction (XRD) studies indicated that after 4 hours of milling the Fe-Si alloy is formed. The mean crystallites size decreases down to 7 nm after 8 hours of milling. The particles morphology investigated by scanning electron microscopy (SEM) showed an evolution during milling process from two different kinds of particles to a one kind of particles with irregular shape. The magnetisation of powders decreases upon increasing the milling time up to 4 hours as a consequence of the Fe-Si alloy formation.

Abstract: A sum of mixed nickel-manganese ferrites, Ni_xMn_1-xFe₂O₄ (x=0, 0.3, 0.5, 0.7) were synthesized by classical ceramic route starting from stoichiometric mixtures of commercially MnO₂, NiO and Fe₂O₃. The polycrystalline ferrites obtained by ceramic route were subjected to the mechanical milling procedure in order to reduce the particles size and to refine de crystallites size. A planetary high energy ball mill Fritch Pulverisette 4 was used and the milling time was up to 120 minutes. The ceramic and as-milled ferrites samples were investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and laser particles size analysis (LPSA). After 15 minutes of milling the mean crystallites size for each one of the nickel-manganese ferrites is in nanometric range. After 120 minutes of mechanical milling for all ferrites types the mean crystallites size is at 6-8 nm, depending on Ni/Mn ratio. According to the SEM and LPSA investigations the milled ferrites powders consists in nanometric particles alongside of the micrometric ones. The micrometric particles are formed by multiple nanocrystallites.

Abstract: Wet mechanical alloying (MA) were used to prepare amorphous soft magnetic Fe₇₅Si₂₀B₅ (at.%) powders starting from elemental powders of Fe, Si and B. The structural, morphological and magnetic properties of the powders were investigated. It was found that wet MA leads to the amorphisation of the alloy after 40 hours of wet milling using benzene (C₆H₆) as process control agent (PCA). The influence of the wet MA process on the saturation magnetization of the powders was investigated. Amorphous powder of Fe₇₅Si₂₀B₅ (at.%) obtained by wet MA route was used to prepare compacts by spark plasma sintering (SPS). The chosen sintering temperature was 800, 850 and 900 oC. Toroidal samples of Fe₇₅Si₂₀B₅ (at.%) were investigated in DC and AC magnetization regime and their magnetic properties were correlated with sintering parameters, compacts density and phases evolution during sintering.

Abstract: Composite powder of Fe/Fe₂O₃ type was synthesized by mechanical milling using commercially Fe and Fe₂O₃ powders in mass ratio of 35/65. The milling process leads to the powder homogenization, powder activation and formation of some Fe/Fe₂O₃ composite particles. The Fe/Fe₂O₃ composite powder obtained by mechanical milling and the un-milled Fe/Fe₂O₃ mixture were subjected to the reactive sintering procedure in argon atmosphere at 1100 °C for 6 hours. The sintering procedure promotes the reaction of the Fe with the Fe₂O₃ and the result is a sintered composite compact of Fe/Fe₃O₄/FeO type. The microstructure of the Fe/Fe₃O₄/FeO sintered composite compacts presents iron clusters in an oxide matrix. A more homogeneous iron clusters size and distribution in oxide matrix is observed in the case of the sintered compact obtained from mechano-activated powder. The X-ray diffraction (XRD), laser particles size analysis (LPSA), optical (OM) and scanning electron (SEM) microscopies techniques were used for the investigations.

Abstract: The ZnFe₂O₄/α-Fe nanocomposite powders were obtained by ball milling starting from ZnFe₂O₄ powder synthesized by classical ceramic method and commercial iron powder. Two way of milling were used for the synthesis of the ZnFe₂O₄/α-Fe nanocomposite. In both cases after milling process the phases are relatively uniformly distributed in material and zinc ferrite mean crystallite size decreases from micrometric range up to 11 nm for the first milling mode and up 48 nm for second milling mode. The ZnFe₂O₄/α-Fe nanocomposite powders were compacted by Spark Plasma Sintering method (SPS). During sintering a reaction between nanocomposite phases occurs, thus leading to the formation of ZnO and FeO. The evolution of the powders during milling and stability of the nanocomposite phases was investigated by X-ray diffraction. The powders and compacts morphology and local chemical homogeneity were investigated by scanning electron microscopy (SEM) and respectively by energy dispersive x-ray spectrometry (EDX). The influence of the sintering parameters on the stability of nanocomposites phases is studied.

Abstract: The polycrystalline nickel ferrite - NiFe₂O₄ has been obtained by ceramic route starting from a stoichiometric mixture of oxides (NiO and α-Fe₂O₃ powders). The obtained NiFe₂O₄ was subjected to high energy ball milling. The formation of NiFe₂O₄ by ceramic method and also the evolution of the powder during milling were studied by X-ray diffraction. The mean crystallite size of the NiFe₂O₄ continuously decreases with the increasing of the milling time and for all the milling time it is in nanometric range. The particles sizes are drastically reduced by milling process. For the milled samples, the particles size is ranging from tens of microns to few nanometers. The powder morphology and local chemical homogeneity were investigated by scanning electron microscopy (SEM) and respectively by energy dispersive x-ray spectrometry (EDX).