Biography:

In the past M. Stoica has collaborated on articles with B. Bourguignon and Madalina Nicolescu. One of their most recent publications is Surface scienceLaser modifications of Si(100) : Cl surfaces induced by surface melting: etching and cleaning. Which was published in journal Surface Science.

More information about M. Stoica research including statistics on their citations can be found on their Copernicus Academic profile page.

M. Stoica's Articles: (11)

Surface scienceLaser modifications of Si(100) : Cl surfaces induced by surface melting: etching and cleaning

AbstractPulsed laser driven modifications of one Cl monolayer (ML) chemisorbed on Si(100) are studied by time of flight mass spectrometry (TOF) of the desorbed molecules, Auger electron spectroscopy (AES) of the modified surface, and Cl depth profile analysis by secondary ion mass spectrometry (SIMS). The experimental conditions are that of surface melting. Evidence of Cl diffusion to the bulk during surface melting, and of strong segregation of Cl in Si during surface recrystallization, are presented. The following branching ratios for 1 Cl ML initially adsorbed on clean Si are measured independently: (a) (from TOF and depth profiler measurements) 0.58 ± 0.07 ML desorb reactively in the form of SiCl and SiCl2. The SiCl to SiCl2 ratio is 1.3 ± 0.3; (b) (from AES measurements) 0.37 ± 0.13 ML are found after the laser pulse in a thin surface layer of 7 atomic planes; (c) (from SIMS measurements) 0.10 ± 0.10 ML are found in deeper layers, the solubility of Cl in solid Si being lower than 1019 cm−3, the sensitivity of our SIMS for Cl; (d) 0.12 ± 0.07 ML desorb unreactively in the form of Cl and Cl2. By order of increasing importance, unreactive desorption, stoichiometry and diffusion to the bulk during the laser pulse have the practical effect of limiting the maximum etch rate to 0.40 ± 0.03 Si ML per laser pulse under the conditions of surface melting. Although the laser pulse induces diffusion of Cl towards the bulk during melting, the strong segregation of Cl during recrystallization limits Si contamination by Cl at undetectable levels to our SIMS. Cl in Si is a very good case for laser cleaning, as is shown by comparison with C and O.

Influence of the substrate and nitrogen amount on the microstructural and optical properties of thin r.f.-sputtered ZnO films treated by rapid thermal annealing

AbstractN-doped ZnO (ZnO:N) thin films, intended to be used as one of the layers in solar cell applications were deposited by r.f. sputtering, using ZnN target (99.9% purity), on silicon and fused silica substrates. In the gas flow composition, Ar was kept constant (50%) and the O2/N2 ratio was varied as: 40%/10%, 25%/25% and 10%/40%. After deposition, rapid thermal annealing (RTA) at 400 and 550 °C for 1 min in N2 ambient has been performed. The RTA impact on the optical and microstructural properties of ZnO:N thin films have been investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX), UV–vis–NIR spectroscopy, UV–vis–NIR spectroscopic ellipsometry (SE) and infrared ellipsometry (IR-SE). The as-deposited (ad) ZnO:N films are polycrystalline with preferentially oriented columnar crystals. After RTA we found ZnO:N films with improved crystallinity and fewer boundary defects. We report optical constants of ZnO:N from UV to IR spectral range as well as the infrared active phononic modes.

Metallic glass formation in the Cu47Ti33Zr11Ni8Si1 alloy

AbstractPhase formation in the Cu47Ti33Zr11Ni8Si1 alloy synthesized by various solidification techniques has been studied using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) paying, in particular, attention to the TEM sample preparation. While a reduced cooling rate results in a duplex microstructure, consisting of nanocrystals in an amorphous matrix, glass formation is achieved upon employing a faster quenching rate. The difference in composition and quench temperature influence the phase transition upon heating. The essential differences observed in TEM, notwithstanding the ion-milling effects and the different pre-alloy ingots used for metallic glass synthesis, can be linked to differences in the nucleation and growth mechanism which, in turn, is influenced by the chemical short range order (CSRO) obtained upon quenching.

Short communicationThe role of nonmagnetic phases in improving the magnetic properties of devitrified Pr2Fe14B-based nanocomposites

AbstractThe structure–property relationship of devitrified Pr9Fe81.5Ti2.5B7 (low-B) and Pr9Fe73.5Ti2.5B15 (high-B) nanocomposite ribbons with high coercivity has been investigated. The introduction of excessive B leads to an obvious increase of the amount of TiB2 nonmagnetic phases. TEM observation shows that TiB2 is mainly distributed in the Pr2Fe14B hard magnetic phase, which strongly hinders the domain wall movement and keeps a strong intergrain exchange coupling action among the magnetic phases. Thus, the coercivity is significantly increased by 111% from 4.6 kOe for low-B ribbons to 9.7 kOe for high-B ribbons due to increased B addition. The squareness of the demagnetization curves is also improved, simultaneously and the remanence remains almost constant. Hence, an evidently increased energy product is attained for the devitrified high-B ribbons. The distribution of nonmagnetic phases plays a key role in the improvement of the hard magnetic properties.

Fe65.5Cr4Mo4Ga4P12C5B5.5 BMGs: Sample preparation, thermal stability and mechanical properties

AbstractBulk amorphous Fe-based alloys with the nominal composition Fe65.5Cr4Mo4Ga4P12C5B5.5 have been obtained by copper mold casting in different shapes: cylindrical rods with diameters up to 3 mm, rectangular bars of 2 mm × 2 mm and discs of 10 mm diameter and 1 mm thickness. These alloys exhibit good soft magnetic properties, characterized by low coercivity and high saturation magnetization. Besides the magnetic properties, the Fe65.5Cr4Mo4Ga4P12C5B5.5 bulk metallic glass (BMG) shows a high glass transition temperature Tg, as well as a high crystallization temperature Tx, with an extension of the supercooled liquid region of around 65 K. The mechanical behavior was investigated by compression and Vickers hardness tests. The fracture strength for the as-cast samples σf is 2.8 GPa and the fracture strain ɛf is 1.9%. Upon annealing at 715 K for 10 min, i.e. at a temperature below the calorimetric glass transition, the fracture strain drops to 1.6% and no plastic deformation is observed. The Vickers hardness HV for the as-cast samples is about 885, and increases to 902 upon annealing. The fracture behavior of these Fe-based bulk glassy alloys is significantly different in comparison with the well-studied Zr-, Cu- or Ti-based good glass-formers. The fracture is not propagating along a well-defined direction and the fractured surface looks irregular. Instead of veins, the glassy alloy develops a high number of microcracks.

Atomic structure of Zr–Cu–Al and Zr–Ni–Al amorphous alloys

AbstractWe report on the atomic structure via pair distribution function (PDF) analysis of metallic glasses of the Zr–Cu–Al and Zr–Ni–Al families. The PDFs were evaluated from the X-ray diffraction data. Transmission X-ray diffraction spectra of the investigated samples were measured using monochromatic, high-energy synchrotron radiation. In this work we demonstrate the effect of composition on the pair distribution functions. A variation of first PDF peak shape was observed on modification of Cu and Al content. A split of the first PDF peak was found to be more pronounced in Zr–Ni–Al than in Zr–Cu–Al glasses suggesting differences in their atomic structures.

Influence of hot compaction on microstructure and magnetic properties of mechanically alloyed Fe(Co)-based amorphous compositions

Highlights•Bulk samples are produced after hot compacting mechanically alloyed powders.•Amorphous structure is preserved after hot compaction within the supercooled liquid region.•Enhancement in MCE is observed for hot compacted for Co-free alloy.

Glass-forming ability and microstructural evolution of [(Fe0.6Co0.4)0.75Si0.05B0.20]96-xNb4Mx metallic glasses studied by Mössbauer spectroscopy

Highlights•Mössbauer spectroscopy allows the identification of crystallized phases.•Zr and Mo enhance the glass-forming ability in FeCoBSiNb alloys.•The atomic percentage of Fe atoms in each phase has been calculated by TMS.

Liquid ejection temperature dependence of structure and glass transition behavior for rapidly solidified Zr-Al-M (M=Ni, Cu or Co) ternary glassy alloys

Highlights•The GT and SL region disappear for the ribbons prepared from high Te of Tl+700 K.•The GT and SL region appear with Te at Tl+200 K, although first heated to Tl+700 K.•The reversible phenomenon reflects change in medium-range ordered atomic structure.•The samples prepared at high Te exhibit higher values of Tg, Tx and Vickers hardness.

Preparation of bulk Nd2Fe14B/Fe3B nanocomposite magnets with high rare earth content

AbstractNd6Fe72−xCoxB22 (x = 0, 20) rods with 1 mm diameter have been prepared by suction casting. The Nd6Fe72B22 rod is mainly composed of Nd2Fe23B3, Fe3B and a small amount of amorphous phase, whereas the Nd6Fe52Co20B22 sample is amorphous. Co addition considerably improves the glass-forming ability (GFA) of Nd6Fe72B22, which is believed to result from the large decrease of the eutectic temperature due to Co addition. Co addition promotes the formation of the Nd2Fe14B hard magnetic phase in annealed alloys, which strengthens the exchange coupling action among the magnetic phases and thus leads to a significant enhancement of the magnetic properties of the bulk nanocomposite magnets. For example, the remanence, coercivity, and energy product are increased from 3.9 kG, 1.3 kOe, and 0.9 MGOe for the Co-free alloy to 8 kG, 4.5 kOe, and 6 MGOe after Co addition, respectively.

Full length articleInfluence of ejection temperature on structure and glass transition behavior for Zr-based rapidly quenched disordered alloys

AbstractWe examined the influence of ejection liquid temperature (Tel) on the structure, thermal stability and crystallization of ZrAlNiCu ribbons prepared by the melt-spinning technique. The increase in Tel was found to cause the formation of an oxide phase on the ribbon surface, more loose atomic configurations, the absence of glass transition (GT) and supercooled liquid (SL) region, and the rise of crystallization temperature. The changes in the GT and SL region occur reversibly by controlling the Tel. Neither the change in alloy composition except oxygen nor the difference in crystallized phases is seen. Their hardness increases significantly by the disappearance of GT and SL region. The reversible changes in the appearance and disappearance of GT and SL region was found for different Zr-based glassy ribbons, being independent of alloy compositions. The disappearance is presumably due to the change in atomic configurations from high-coordinated to less-coordinated atomic packing in the melt-spun ribbons by freezing high-temperature liquid. The observed phenomenon of the reversible changes provides a novel opportunity for deep understanding of mutual correlations among liquid structure, GT, stability of SL and bulk glass-forming ability for metallic alloys.

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