Jung-Pyung Choi

Thermal stability of MnBi magnetic materials

MnBi has attracted much attention in recent years due to its potential as a rare-earth-free permanent magnet material. It is unique because its coercivity increases with increasing temperature, which makes it a good hard phase material for exchange coupling nanocomposite magnets. MnBi phase is difficult to obtain, partly because the reaction between Mn and Bi is peritectic, and partly because Mn reacts readily with oxygen. MnO formation is irreversible and harmful to magnet performance. In this paper, we report our efforts toward developing MnBi permanent magnets. To date, high purity MnBi (>90%) can be routinely produced in large quantities. The produced powder exhibits 74.6 emu g(-1) saturation magnetization at room temperature with 9 T applied field. After proper alignment, the maximum energy product (BH)max of the powder reached 11.9 MGOe, and that of the sintered bulk magnet reached 7.8 MGOe at room temperature. A comprehensive study of thermal stability shows that MnBi powder is stable up to 473 K in air.

Alkali Effect on the Electrical Stability of a Solid Oxide Fuel Cell Sealing Glass

An alkaline-earth silicate (Sr-Ca-Y-B-Si) sealing glass with varying K 2 O content (2-5 mol % ) was developed and evaluated for solid oxide fuel cell (SOFC) sealing applications. Two metallic interconnect plates were sealed with the glass and tested for electrical stability in a dual environment at elevated temperatures under a dc loading of 0.7 V. The metallic interconnect material, a ferritic stainless steel, was tested in the as-received state and after surface aluminization. The isothermal aging results showed stable electrical resistivity at 850°C for ~600 to 800 h for all the sealing glasses, with or without K 2 O. The electrical resistivities at 850°C were several orders of magnitude higher than the typical SOFC stack components. However, the glass with high alkali content (5%) showed excessive interfacial reaction, which resulted in debonding from both the as-received and the aluminized steel. Interfacial microstructures were characterized and possible reactions were discussed.

Development of MnBi permanent magnet: Neutron diffraction of MnBi powder

MnBi attracts great attention in recent years for its great potential as permanent magnet materials. MnBi phase is difficult to obtain because of the rather drastic peritectic reaction between Mn and Bi. In this paper, we report our effort on synthesizing high purity MnBi compound using conventional powder metallurgical approaches. Neutron diffraction was carried out to investigate the crystal and nuclear structure of the obtained powder. The result shows that the purity of the obtained powder is about 91 wt. % at 300 K, and the magnetic moment of the Mn atom in MnBi lattice is 4.424 and 4.013 μB at 50 K and 300 K, respectively.

Reactive Air Aluminizing of Nicrofer-6025HT for Use in Advanced Coal-Based Power Plants

Nicrofer-6025HT (Nicrofer) has been selected as a potential manifold material for advanced air separation units that use mixed ionic and electronic conductors (MIECs). Reactive air brazing (RAB) is a recently developed joining technique that has the potential to obtain high-quality joints with the required hermeticity between Nicrofer and the MIEC. Successful RAB joining of these two distinct materials requires an alumina surface layer on the Nicrofer. To aluminize the surface of Nicrofer, a recently developed reactive air aluminizing (RAA) technique was used. The current work demonstrated the feasibility of preparing RAA coatings on Nicrofer and compared the effect of aluminum powder size on the RAA process.

Reactive Air Aluminization

Ferritic stainless steels and other alloys are of great interest to SOFC developers for applications such as interconnects, cell frames, and balance of plant components. While these alloys offer significant advantages (e.g., low material and manufacturing cost, high thermal conductivity, and high temperature oxidation resistance), there are challenges which can hinder their utilization in SOFC systems; these challenges include Cr volatility and reactivity with glass seals. To overcome these challenges, protective coatings and surface treatments for the alloys are under development. In particular, aluminization of alloy surfaces offers the potential for mitigating both evaporation of Cr from the alloy surface and reaction of alloy constituents with glass seals. Commercial aluminization processes are available to SOFC developers, but they tend to be costly due to their use of exotic raw materials and/or processing conditions. As an alternative, PNNL has developed Reactive Air Aluminization (RAA), which offers a low-cost, simpler alternative to conventional aluminization methods.

Mechanical and electrical properties of low temperature phase MnBi

Low temperature phase (LTP) manganese bismuth (MnBi) is a promising rare-earth-free permanent magnet material due to its high intrinsic coercivity and large positive temperature coefficient. While scientists are making progress on fabricating bulk MnBi magnets, engineers have begun considering MnBi magnets for motor applications. Physical properties other than magnetic ones could significantly affect motor design. Here, we report results of our investigation on the mechanical and electrical properties of bulk LTP MnBi and their temperature dependence. A MnBi ingot was prepared using an arc melting technique and subsequently underwent grinding, sieving, heat treatment, and cryomilling. The resultant powders with a particle size of ∼5 μm were magnetically aligned, cold pressed, and sintered at a predefined temperature. Micro-hardness testing was performed on a part of original ingot and we found that the hardness of MnBi was 109 ± 15 HV. The sintered magnets were subjected to compressive testing at differen...

Mechanical Characterization and Corrosion Testing of X608 Al Alloy

This paper describes the mechanical characterization and corrosion testing of X608 Al alloy that is being considered for A-pillar covers for heavy-duty truck applications. Recently, PNNL developed a thermo-mechanical process to stamp A-pillar covers at room temperature using this alloy, and the full-size prototype was successfully stamped by a tier-1 supplier. This study was conducted to obtain additional important information related to the newly developed forming process, and to further improve its mechanical properties. The solutionization temperature, pre-strain and paint-bake heat-treatment were found to influence the alloy’s fabricability and mechanical properties. Natural aging effect on the formability was investigated by limiting dome height (LDH) tests. Preliminary corrosion experiments showed that the employed thermo-mechanical treatments did not significantly affect the corrosion behavior of Al X608.