Deformation Processed Al/Al2Ca Nano-filamentary Composite Conductors

A 48% increase in worldwide energy demand is expected by 2040, which will require expansion of electrical power transmission infrastructure. 1 Expanded long-distance transmission grids in China, the United States, and elsewhere are expected to make greater use of high-voltage direct current (HVDC) transmission, the preferred technology for long distances.2 Conventional aluminum- conductor steel-reinforced (ACSR) cables are not well suited for HVDC transmission due to the presence of the heavy, poorly conducting steel core needed for strength and sag-resistance. Al/Ca composite conductors with monolithic construction produced by powder metallurgy and deformation processing have shown promise as a possible next-generation conductor for this application

Deformation processed Al/Ca nano-filamentary composite conductors for HVDC applications

Efficient long-distance power transmission is necessary as the world continues to implement renewable energy sources, often sited in remote areas. Light, strong, high-conductivity materials are desirable for this application to reduce both construction and operational costs. In this study an Al/Ca (11.5% vol.) composite with nano-filamentary reinforcement was produced by powder metallurgy then extruded, swaged, and wire drawn to a maximum true strain of 12.7. The tensile strength increased exponentially as the filament size was reduced to the sub-micron level. In an effort to improve the conductor\u27s ability to operate at elevated temperatures, the deformation-processed wires were heat-treated at 260°C to transform the Ca-reinforcing filaments to Al2Ca. Such a transformation raised the tensile strength by as much as 28%, and caused little change in ductility, while the electrical conductivity was reduced by only 1% to 3%. Al/Al2Ca composites are compared to existing conductor materials to show how implementation could affect installation and performance

In-Situ Gas-Phase Passivation of Molten Calcium Surfaces to Enable Development of Atomization Method for Generating Calcium Powder

Deformation processed aluminum/calcium composites are lighter, stronger, and more conductive than conventional overhead power transmission conductors, which gives them the potential to reduce electrical losses, lower costs, and enhance grid reliability. A principal barrier to full strengthening of Al/Ca composites is availability of fine Ca metal power for co-extrusion with commercial Al powder. Calcium powder does not form a protective oxide layer on its surface and must be passivated to ensure safe production and handling. Experiments using an Induction Melting Passivation (IMPass) apparatus developed earlier and used to successfully passivate Mg powder were used to identify appropriate gas mixture and processing conditions for Ca metal. Auger electron spectroscopy (AES), depth profiling, and scanning electron microscopy (SEM) were used to evaluate the passivation characteristics. These findings will allow for lab-scale atomization trials and eventual scale-up allowing safe commercial production of Ca powder

A deformation-processed Al-matrix/Ca-nanofilamentary composite with low density, high strength, and high conductivity

Light, strong materials with high conductivity are desired for many applications such as power transmission conductors, fly-by-wire systems, and downhole power feeds. However, it is difficult to obtain both high strength and high conductivity simultaneously in a material. In this study, an Al/Ca (20 vol%) nanofilamentary metal-metal composite was produced by powder metallurgy and severe plastic deformation. Fine Ca metal powders (~200 µm) were produced by centrifugal atomization, mixed with pure Al powder, and deformed by warm extrusion, swaging, and wire drawing to a true strain of 12.9. The Ca powder particles became fine Ca nanofilaments that reinforce the composite substantially by interface strengthening. The conductivity of the composite is slightly lower than the rule-of-mixtures prediction due to minor quantities of impurity inclusions. The elevated temperature performance of this composite was also evaluated by differential scanning calorimetry and resistivity measurements

Synthesis and magnetic performance of gadolinium powder produced with rotating disk atomization

Spherical powder is key for achieving good heat transfer and maintaining low pressure drop for heat transfer fluid in magnetocaloric regenerator. Obtaining relatively large spherical powder of rare earth metals that are extremely sensitive to oxygen is challenging. Comparing to gas atomization and plasma rotating electrode methods, rotating disk atomization (RDA) method is more robust for obtaining a small amount of powder with a short turn-around time, thereby, more suitable for research and development of new magnetocaloric materials. In this work, commercially bulk gadolinium was successfully atomized to produce spherical powder in the targeted 150–250 μm range. The obtained powder exhibited the expected magnetocaloric performance with a maximum change of magnetic entropy of 12.1 J/kg K and a maximum adiabatic temperature change of 12.8 K with an applied magnetic field of 7 T

125Te NMR and Seebeck Effect in Bi2Te3 Synthesized from Stoichiometric and Te-Rich Melts

Bi2Te3 is a well-known thermoelectric material and, as a new form of quantum matter, a topological insulator. Variation of local chemical composition in Bi2Te3 results in formation of several types of atomic defects, including Bi and Te vacancies and Bi and Te antisite defects; these defects can strongly affect material functionality via generation of free electrons and/or holes. Nonuniform distribution of atomic defects produces electronic inhomogeneity, which can be detected by 125Te nuclear magnetic resonance (NMR). Here we report on 125Te NMR and Seebeck effect (heat to electrical energy conversion) for two single crystalline samples: (#1) grown from stoichiometric composition by Bridgman technique and (#2) grown out of Te-rich, high temperature flux. The Seebeck coefficients of these samples show p- and n-type conductivity, respectively, arising from different atomic defects. 125Te NMR spectra and spin–lattice relaxation measurements demonstrate that both Bi2Te3 samples are electronically inhomogeneous at the atomic scale, which can be attributed to a different Te environment due to spatial variation of the Bi/Te ratio and formation of atomic defects. Correlations between 125Te NMR spectra, spin–lattice relaxation times, the Seebeck coefficients, carrier concentrations, and atomic defects are discussed. Our data demonstrate that 125Te NMR is an effective probe to study antisite defects in Bi2Te3

Laser angle-resolved photoemission as a probe of initial state k(z) dispersion, final-state band gaps, and spin texture of Dirac states in the Bi2Te3 topological insulator

We have obtained angle-resolved photoemission spectroscopy (ARPES) spectra from single crystals of the topological insulator material Bi2Te3 using a tunable laser spectrometer. The spectra were collected for 11 different photon energies ranging from 5.57 to 6.70 eV for incident light polarized linearly along two different in-plane directions. Parallel first-principles, fully relativistic computations of photointensities were carried out using the experimental geometry within the framework of the one-step model of photoemission. A reasonable overall accord between theory and experiment is used to gain insight into how properties of the initial- and final-state band structures as well as those of the topological surface states and their spin textures are reflected in the laser-ARPES spectra. Our analysis reveals that laser-ARPES is sensitive to both the initial-state kz dispersion and the presence of delicate gaps in the final-state electronic spectrum

Production of fine calcium powders by centrifugal atomization with rotating quench bath

Recently, a novel Al/Ca composite was produced by severe plastic deformation of Al powders and Ca granules for possible use as a high-voltage power transmission conductor. Since the strength of such composites is inversely proportional to the Ca filament size, fine Ca powders (less than ~ 250 μm) are needed to achieve the desired high strength for the powder metallurgy production of an Al-matrix composite reinforced by nano-scale Ca filaments. However, fine Ca powders are not commercially available. Therefore, we have developed a method to produce fine Ca powders via centrifugal atomization to supply Ca powder for prototype development of Al/Ca composite conductor. A secondary goal of the project was to demonstrate that Ca powder can be safely prepared, stored, and handled and could potentially be scaled for commercial production. Our results showed that centrifugal atomization can yield as much as 83 vol.% Ca powder particles smaller than 250 μm. The mean particle size sometimes matches, sometimes deviates substantially from the predictions of the Champagne & Anger equation likely due to unexpected secondary atomization. The particle size distribution is typical for a ligament-disintegration atomization mode. Scanning electron micrographs showed that the morphology of these Ca powders varied with powder size. Spark testing and auto-ignition tests indicated that the atomized powders were difficult to ignite, providing confidence that this material can be handled safely in air

Deformation processed Al/Ca nano-filamentary composite conductors for HVDC applications

Efficient long-distance power transmission is necessary as the world continues to implement renewable energy sources, often sited in remote areas. Light, strong, high-conductivity materials are desirable for this application to reduce both construction and operational costs. In this study an Al/Ca (11.5% vol.) composite with nano-filamentary reinforcement was produced by powder metallurgy then extruded, swaged, and wire drawn to a maximum true strain of 12.7. The tensile strength increased exponentially as the filament size was reduced to the sub-micron level. In an effort to improve the conductor's ability to operate at elevated temperatures, the deformation-processed wires were heat-treated at 260°C to transform the Ca-reinforcing filaments to Al2Ca. Such a transformation raised the tensile strength by as much as 28%, and caused little change in ductility, while the electrical conductivity was reduced by only 1% to 3%. Al/Al2Ca composites are compared to existing conductor materials to show how implementation could affect installation and performance.</p

In-Situ Gas-Phase Passivation of Molten Calcium Surfaces to Enable Development of Atomization Method for Generating Calcium Powder

Deformation processed aluminum/calcium composites are lighter, stronger, and more conductive than conventional overhead power transmission conductors, which gives them the potential to reduce electrical losses, lower costs, and enhance grid reliability. A principal barrier to full strengthening of Al/Ca composites is availability of fine Ca metal power for co-extrusion with commercial Al powder. Calcium powder does not form a protective oxide layer on its surface and must be passivated to ensure safe production and handling. Experiments using an Induction Melting Passivation (IMPass) apparatus developed earlier and used to successfully passivate Mg powder were used to identify appropriate gas mixture and processing conditions for Ca metal. Auger electron spectroscopy (AES), depth profiling, and scanning electron microscopy (SEM) were used to evaluate the passivation characteristics. These findings will allow for lab-scale atomization trials and eventual scale-up allowing safe commercial production of Ca powder.This chapter is published as Czahor, C., Riedemann, T.M., Russell, A.M., Anderson, I.E., “In-Situ Gas-Phase Passivation of Molten Calcium Surfaces to Enable Development of Atomization Method for Generating Calcium Powder,” in Advances in Powder Metallurgy & Particulate Materials—2018, compiled by Animesh Bose and Scott Davis, Metal Powder Industries Federation, 105 College Road East Princeton, NJ, ISBN No. 978-1-943694-18-1, pp. 151-163 (2018). Posted with permission.</p