Virgil C. Solomon

Arizona State University, Phoenix, Arizona, United States

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Publications (9)19.96 Total impact

  • Microscopy and Microanalysis 07/2009; 15. · 2.50 Impact Factor
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    ABSTRACT: By spark-eroding Fe75Si15B10 in water/ethanol mixtures, spherical particles with nanostructured cores consisting of mixed amorphous and crystalline phases were produced. The relative volume fractions of the amorphous and crystalline phases were dependent on the water/ethanol ratio. In the same process, continuous oxide layers were formed on the particle surfaces. The basic mechanisms responsible for the formation of the surface oxide layers and the core nanostructures were modeled. At frequencies ranging from 1 to 100 MHz, the combination of the core nanostructures and the insulating oxide shells yielded exceptionally low-loss magnetic behavior.
    Journal of Materials Research. 05/2008; 23(06):1758 - 1763.
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    ABSTRACT: The microstructural and chemical characteristics of Ni–Mn–Ga particles produced by spark-erosion using liquid argon or liquid nitrogen dielectrics were investigated by electron microscopy techniques. The particles were predominantly micron-sized spheres; those obtained in argon were solid, whereas those obtained in nitrogen were primarily hollow. The investigation of annealed particles revealed twinned microstructure and provided evidence supporting the cryogen solubility model of hollow particle formation, as well as possibly explaining the non-homogeneous structure of particles formed by spark-erosion in liquid nitrogen.
    Scripta Materialia. 01/2007;
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    ABSTRACT: Spherical Fe–Ga particles were prepared by spark erosion in liquid Ar, which directly incorporated the desirable rapid quench from high temperatures. The compositions of the particles investigated were 15.0, 16.3, and 18.9 at. % Ga, respectively, as determined from electron-probe microanalysis, x-ray diffraction, and Mössbauer spectra. Composites for magnetostriction measurements were prepared by mixing particles with epoxy at the volume fraction of 48% and curing in a magnetic field. Magnetostriction values of the composites were comparable to those of polycrystalline chill-cast alloys of the same compositions. Composites with particles having Ga concentrations of 18.9 at. % had the highest magnetostriction, similar to results reported for bulk Fe–Ga alloys.
    Applied Physics Letters 10/2006; 89(14). · 3.52 Impact Factor
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    ABSTRACT: Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005.
    Microscopy and Microanalysis 08/2005; 11(S02):1832-1833. · 2.50 Impact Factor
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    ABSTRACT: The magnetic entropy change of tetragonal and orthorhombic NiMnGa fine particles made by spark erosion was investigated in this paper. It was found that the structure and crystalline phase transformation temperatures can be strongly affected by the compositions of the particles, while Curie temperature is less sensitive to the compositions. Due to the possible distribution of the particle size and compositions in these particles, the magnetic entropy changes observed are much broader and smaller than those of bulk NiMnGa alloys. The maximum absolute value of entropy change ΔS = 2 J Kg−1 K−1 was observed for tetragonal structure NiMnGa particles at 95 °C in a field of 2 T.
    Journal of Applied Physics 05/2005; 97(10):10M309-10M309-3. · 2.21 Impact Factor
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    ABSTRACT: Spherical particles of the ferromagnetic shape memory material Ni51Mn29Ga20 obtained by spark erosion transform during cooling from the high-temperature Heusler L21 cubic phase into tetragonal martensite. Using the Fresnel (i.e., Lorentz) imaging mode, magnetic domains with an average width of 100 nm are observed in the modulated martensitic phase in the absence of a magnetic field. The magnetization distribution within individual particles is determined using electron holography. The magnetic flux lines change direction when crossing boundaries between crystallographic twin variants. These boundaries, where the easy c-axis of the crystallographic variants rotate by 86.5°, coincide with quasi-90° magnetic domain walls, with thickness of approximately 17 nm. The magnetization saturation determined by electron holography is about 0.57 T.
    Applied Physics Letters 05/2005; 86(19). · 3.52 Impact Factor
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    ABSTRACT: Hollow spherical particles of Ni, CoFe, the ferromagnetic shape memory alloy Ni49Mn30Ga21, and the giant magnetostrictive alloy Fe83Ga17, with diameters up to several tens of microns were produced by spark erosion, using liquid nitrogen as the dielectric liquid. In contrast, the particles were primarily solid when the dielectric liquid was argon. The wall thicknesses of the hollow particles depended on the elemental composition. Different models are considered to account for the formation of the spark-eroded hollow spheres, and some of the potential benefits to be derived from their use are described.
    Applied Physics Letters 08/2004; 85(6):940-942. · 3.52 Impact Factor
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    ABSTRACT: Powders of Ni-Mn-Ga ferromagnetic shape memory alloy (FSMA) were produced by spark erosion. Powders quenched in liquid nitrogen are primarily hollow spherical particles, whereas those quenched in liquid argon are mostly solid spheres. Electron microscopy observations of powders as-sparked in nitrogen show highly disordered nanocrystalline grains with an average size of less than 100 nm whereas those sparked in liquid argon display extensive crystallinity. Magnetic measurements indicate that the powders do not become fully ferromagnetic until after annealing at 973 K for 5 h. Investigations of annealed powders using differential scanning calorimetry reveal thermoelastic martensitic transition behavior. The annealed powders have a microtwinned martensite with many large grains showing a five-layer period modulation. These Ni-Mn-Ga powders should be suitable for the development of magnetic-field-driven FSMA powder/polymer microcomposites. © 2004 American Institute of Physics.
    Journal of Applied Physics 05/2004; 95(11):6954-6956. · 2.21 Impact Factor