Microstructure and hard magnetic properties in bulk rods of Nd 60Fe 30Al 10 glass forming alloys

The Nd60Fe30Al10 alloy exhibits a large glass forming ability which allows to obtain relatively thick cast rods containing large volume fractions of amorphous phases. In this work the microstructure and the hard magnetic properties of as-cast rods are characterized. The alloy is processed by suction casting into a chilled copper mould to obtain cylinders 5 mm diameter and 50 mm length. This diameter is selected because it is an upper limit for this processing route, beyond which the hard properties largely deteriorate. A room temperature coercivity of 0.34 T is obtained. The sample microstructure is heterogeneous, with very different size scales near the surface and along the central zone. However, in both regions a large fraction of an amorphous ferromagnetic phase is observed; it is found that paramagnetic nanocrystalline phases, mainly Nd or Nd-rich particles embedded in the amorphous matrix, are somewhat coarser in the central zone. These larger nanocrystals, less efficient to pin domain walls, are proposed to be responsible for the lower coercive fields observed, as compared with those found in cylinders 1 to 3 mm diameter where no inhomogeneities are found. This conclusion is supported by microstructure, calorimetric and magnetic observations.Fil: Levingston, J. M.. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; ArgentinaFil: Valente, R.. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Ghilarducci, Ada Albertina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Fabietti, Luis Maria Rodolfo. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física. Sección Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Salva, Horacio Ramon. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Urreta, S. E.. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentin

On the nature of defects in liquid-phase exfoliated graphene

Liquid-phase exfoliation is one of the most promising routes for large-scale production of multilayer graphene dispersions. These dispersions, which may be used in coatings, composites, or paints, are believed to contain disorder-free graphene multilayers. Here, we address the nature of defects in such samples obtained by liquid-phase exfoliation of graphite powder in N-methyl-2-pyrrolidone. Our Raman spectroscopy data challenge the assumption that these multilayers are free of bulk defects, revealing that defect localization strongly depends on the sonication time. For short ultrasound times, defects are located mainly at the layer edges but they turn out to build up in the bulk for ultrasonic times above 2 h. This knowledge may help to devise better strategies to achieve high-quality graphene dispersions.submittedVersionFil: Bracamonte, María Victoria. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Bracamonte, María Victoria. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Lacconi, Gabriela Inés. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Departamento de Fisicoquímica; Argentina.Fil: Lacconi, Gabriela Inés. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones en Fisicoquímica de Córdoba; Argentina.Fil: Urreta, Silvia Elena. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Urreta, Silvia Elena. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Foa Torres, Luis Eduardo Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Foa Torres, Luis Eduardo Francisco. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Física de los Materiales Condensado

Design Characteristics Influence Performance of Clinical Prediction Rules in Validation: A Meta-Epidemiological Study

BACKGROUND: Many new clinical prediction rules are derived and validated. But the design and reporting quality of clinical prediction research has been less than optimal. We aimed to assess whether design characteristics of validation studies were associated with the overestimation of clinical prediction rules' performance. We also aimed to evaluate whether validation studies clearly reported important methodological characteristics. METHODS: Electronic databases were searched for systematic reviews of clinical prediction rule studies published between 2006 and 2010. Data were extracted from the eligible validation studies included in the systematic reviews. A meta-analytic meta-epidemiological approach was used to assess the influence of design characteristics on predictive performance. From each validation study, it was assessed whether 7 design and 7 reporting characteristics were properly described. RESULTS: A total of 287 validation studies of clinical prediction rule were collected from 15 systematic reviews (31 meta-analyses). Validation studies using case-control design produced a summary diagnostic odds ratio (DOR) 2.2 times (95% CI: 1.2-4.3) larger than validation studies using cohort design and unclear design. When differential verification was used, the summary DOR was overestimated by twofold (95% CI: 1.2 -3.1) compared to complete, partial and unclear verification. The summary RDOR of validation studies with inadequate sample size was 1.9 (95% CI: 1.2 -3.1) compared to studies with adequate sample size. Study site, reliability, and clinical prediction rule was adequately described in 10.1%, 9.4%, and 7.0% of validation studies respectively. CONCLUSION: Validation studies with design shortcomings may overestimate the performance of clinical prediction rules. The quality of reporting among studies validating clinical prediction rules needs to be improved

Cluster Ferromagnetism in Partially Amorphous Nd 60 Fe 30 Al 10 Alloys

ABSTRACT The magnetic microstructures and the magnetization mechanisms operating in melt spun Nd 60 Fe 30 Al 10 hard magnetic alloys, are investigated by Mössbauer Effect spectroscopy and by room temperature magnetic relaxation measurements. It is found that small ferromagnetic crystalline clusters, identified as a μ-type metastable phase of the NdFeAl system, develop in the matrix as the quenching rate decreases. For the alloys quenched at lower rates, the magnetic relaxation parameters measured -the mean fluctuation field μ 0 H f and apparent activation volume v ac -are practically constant near the coercivity; they are consistent with a granular magnetization mechanism, involving 'magnetic grains' containing about 25 clusters, further supporting a model of cluster superferromagnetism. For higher cooling rates, the activation volume increases with the applied field suggesting a different magnetic microstructure; even when the experimental data can not be systematized by a mechanism based on strong domain wall pinning, reversion by displacements of domain wall-like spin configurations, controlled by pinning interactions can not be conclusively excluded. A matrix microstructure is briefly discussed. Keywords: Bulk amorphous alloys, Hard magnetic properties, Cluster ferromagnetism, Magnetic viscosity. INTRODUCTION Nd 60 Fe 30 Al 10 alloys processed by rapid cooling techniques exhibit hard magnetic properties for a relatively wide range of quenching rates, as long as they preserve a partially amorphous microstructure These magnetically hard structures are not fully amorphous; the amount and type of crystalline phases present also depend on the cooling rate. A common feature for all the cooling conditions is the presence (revealed by X ray diffraction (XRD)) of Nd crystallites, with sizes varying [3] from 1μm in the arc-melted master alloy to 10-50 nm in suction cast rods (3mm diameter) and melt spun ribbons (30-100μm thick) The residual matrix, nominally amorphous for XRD and responsible for the hard ferromagnetic properties, is found to be highly inhomogeneous. On the basis of AC susceptibility measurements, HRTEM images and thermomagnetic curves, different author

Nd60Fe30Al10 Glass Forming Magnetic Alloys: a Mechanical Spectroscopy Study at the 300- 560 K Temperature Range

The ferromagnetic amorphous phase in rapidly solidified Nd60Fe30Al10 glass forming alloys is investigated in melt spun ribbons (100 µm thick) and in chill cast cylinders (2 mm diameter). The amorphous resulting for these two different quenching rates were characterized by their room temperature hysteresis loops, magnetization and differential calorimetric measurements in the temperature range 300K-900K. The mechanical damping was explored in the 300-560 K temperature range, by measuring the internal friction and the shear modulus in a forced inverted pendulum operating in the frequency range 0.1-10Hz. Simultaneously, the electrical resistance of the samples was measured. The internal friction spectra of both, ribbons and a cylinder, exhibit a local maximum at about 500K, arising in a relaxation mechanism. After some thermal cycles the peak parameters stabilize reaching an apparent activation enthalpy of 1.5 eV and a limit relaxation time τ0 ≈ 0.4-2.5 . 10-17s. In both samples, the electrical resistance largely decreases during the first heating run to 560K and remains unchanged during subsequent thermal cycles. No changes in the elastic modulus or in the damping properties are detected at the Curie temperature of the alloys.Fil: Salva, Horacio Ramon. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Ghilarducci, Ada Albertina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comision Nacional de Energia Atomica. Gerencia del Area de Investigaciones y Aplicaciones no Nucleares. Gerencia de Fisica (CAB); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Urreta, S. E.. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; ArgentinaFil: Fabietti, Luis Maria Rodolfo. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Levingston, Jorge Matías. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Nucleation and Growth Mechanisms in Cu–Co Films

Nucleation and growth mechanisms in Co, Cu, and Co_<i>x</i>Cu_{100–<i>x</i>} single films, and in Co_<i>x</i>Cu_{100–<i>x</i>}/Cu bilayers, electrodeposited on Cu₇₀Zn₃₀ brass substrates, are studied by the constant potential technique. The recorded current–time transients (CTTs) are rather complex, and they extend for long times. Co, Cu, and Co_<i>x</i>Cu_{100–<i>x</i>} alloys electrocrystallize onto brass, undergoing a process with more than a single maximum during the CTTs, as if two or more consecutive nucleation steps were present. The first stage of electrocrystallization in Co and Co_<i>x</i>Cu_{100–<i>x</i>} films involves 3D instantaneous nucleation, but then, at long times, a progressive nucleation regime predominates. CTTs in Cu/brass and in Cu/Co_<i>x</i>Cu_{100–<i>x</i>}/brass bilayers are well fitted by a 2DP progressive nucleation process at the initial stage, while for longer growing times a transition to a 3DP regime is observed, in which film growth becomes controlled by adatoms incorporation to the lattice. Film morphologies observed by SEM are consistent with these growth mechanisms. XRD results indicate that pure Co layers are <i>hcp</i> phase, while Cu and Cu–Co layers have an <i>fcc</i> lattice. Films are soft ferromagnetic, with an “in-plane” magnetization easy axis; there is evidence of crystallographic texture, which should be responsible for the higher coercivity observed in the “out-of-plane” configuration, with the applied field perpendicular to the film plane

On the Nature of Defects in Liquid-Phase Exfoliated Graphene

Liquid-phase exfoliation is one of the most promising routes for large-scale production of multilayer graphene dispersions. These dispersions, which may be used in coatings, composites, or paints, are believed to contain disorder-free graphene multilayers. Here, we address the nature of defects in such samples obtained by liquid-phase exfoliation of graphite powder in <i>N</i>-methyl-2-pyrrolidone. Our Raman spectroscopy data challenge the assumption that these multilayers are free of bulk defects, revealing that defect localization strongly depends on the sonication time. For short ultrasound times, defects are located mainly at the layer edges but they turn out to build up in the bulk for ultrasonic times above 2 h. This knowledge may help to devise better strategies to achieve high-quality graphene dispersions