Virgil C. Solomon

Vitrification of a monatomic metallic liquid

Although the majority of glasses in use in technology are complex mixtures of oxides or chalcogenides, there are numerous examples of pure substances—‘glassformers’—that also fail to crystallize during cooling. Most glassformers are organic molecular systems, but there are important inorganic examples too, such as silicon dioxide and elemental selenium (the latter being polymeric). Bulk metallic glasses can now be made; but, with the exception of Zr50Cu50 (ref. 4), they require multiple components to avoid crystallization during normal liquid cooling. Two-component ‘metglasses’ can often be achieved by hyperquenching, but this has not hitherto been achieved with a single-component system. Glasses form when crystal nucleation rates are slow, although the factors that create the slow nucleation conditions are not well understood. Here we apply the insights gained in a recent molecular dynamics simulation study to create conditions for successful vitrification of metallic liquid germanium. Our results also provide micrographic evidence for a rare polyamorphic transition preceding crystallization of the diamond cubic phase.

Hollow metallic microspheres produced by spark erosion

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.

Magnetic domain configurations in spark-eroded ferromagnetic shape memory Ni-Mn-Ga particles

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 100nm 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 17nm. The magnetization saturation determined by electron holography is about 0.57T.

Microstructural characterization of Ni-Mn-Ga ferromagnetic shape memory alloy powders

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.

One-step production of optimized Fe–Ga particles by spark erosion

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 18.9at.% Ga, respectively, as determined from electron-probe microanalysis, x-ray diffraction, and Mossbauer 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 18.9at.% had the highest magnetostriction, similar to results reported for bulk Fe–Ga alloys.

Magnetocaloric effect in NiMnGa particles produced by spark erosion

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=2JKg−1K−1 was observed for tetragonal structure NiMnGa particles at 95°C in a field of 2T.

Fabrication of spherical particles with mixed amorphous/crystalline nanostructured cores and insulating oxide shells

By spark-eroding Fe 75 Si 15 B 10 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.

Synthesis of Silicon Oxide Nanowires by Chemical Vapor Deposition

Nanowires can be fabricated by numerous methods such as sol-gel processing, thermal oxidation, ion implantation, and chemical vapor deposition (CVD) [1]. The CVD method either utilizes a vapor solid (VS) or vapor liquid solid (VLS) growth mechanism. The VLS growth mechanism, a liquid catalyst (Au, Fe, Ga, etc.), absorbs and precipitates the nanowire material. Among the possible catalysts for silica nanowire growth, gallium has been shown to be a low melting point catalyst which effectively produces large scale growth of highly aligned and closely packed silica nanowire bunches. Additionally, the molten Ga catalysts can be precipitated onto the substrate in the same processing step as the silica nanowire synthesis. Some of the other benefits to using Ga as a catalyst are that Ga is able to solve Si at a wide range of temperatures and it does not react with Si [2].

Formation of silicon oxide nanowires in nanomaterial synthesis experiments based on the usage of tube furnace

In an effort to synthesize doped ZnO nanowires, SiOx nanowires were obtained accidently. In the experiment, mixed powders containing chemicals such as ZnO, graphite, Ga2O3, and In2O3 were placed in the center of a tube furnace, where the temperature was set to 1200 °C and the vacuum was approximately 27 Pa. Silicon wafers were placed around the vicinity of the furnace exit to collect the expected nanomaterials. After prolonged heating, grey layers were found on top of one wafer located inside the furnace. The layer showed no adhesion to the substrate. Characterization by using Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), and Energy Dispersive X-ray Spectroscopy (EDS) revealed that this layer consisted of SiOx nanowires. Formation of Si-containing liquid drop and the subsequent growth of SiOx nanowires out of it are suggested as the growth mechanism.

Electron Microscopy Characterization of Fe-Ga and Fe-Si-B Fine Particles

Fine particles of Fe-Ga and Fe-Si-B alloys have been obtained by spark-erosion using different dielectrics. The particles are spherical in shape with the average diameter ranging from a few nanometers to tenths of micrometers, as a function of the process parameters (sparking energy, apparatus configuration, dielectrics [1]). In the present work Fe-Ga particles prepared in liquid nitrogen or liquid argon, as dielectrics, and Fe-Si-B particles made in water or water/ethanol (50/50 by volume) have been investigated using analytical scanning and transmission electron microscopy methods in a FEI XL30 FEG SEM equipped with an EDAX-TSL Orientation Imaging Microscopy (OIM) system and a JEOL JEM-2010 S/TEM, respectively. The cross-sectioned samples for EM investigation were prepared in a FEI Dual Beam Nova 200 NanoLab focused ion beam machine.