Thermal plasma synthesis of coated iron cobalt-iron cobalt vanadium nanoparticles as precursors for compacted nanocrystalline bulk magnets.

摘要

High temperature power applications such as starter and generator components of the aircraft engines require soft magnetic materials with optimum magnetic properties. Thus creep resistance and yield strength become important material properties due to the high temperatures and high rotational forces. FeCo based alloys are the only material that can meet desired magnetic properties but they exhibit poor creep resistance at temperatures up to 775 K. Eddy current losses, which are strong dependent on the materials' volume resistivity, are also one of the main concerns designing the aforementioned devices. Current technology utilizes stacks of ∼150 μm thick FeCo alloy laminates and limitations on dimensions arising from the eddy currents and skin depth issues.; It is a well known fact that any improvement in mechanical properties through a secondary phase hardening will result in poor magnetic properties due to the domain wall pinning effect of the secondary phase. Engineering of fiber re-enforced structures to improve the mechanical properties also is not feasible due to the dimensions of the material. This indicates that any improvement on mechanical properties will interfere with the magnetic performance of the system.; Coated nanoparticles eventually compacted in a bulk form, may offer a solution to poor mechanical properties thus magnetic properties can be further improved, i.e. lower coercivities and higher permeabilities, by tailoring the grain sizes to be smaller than the magnetic exchange length, L_ex . Presence of a highly resistive coating phase can also reduce the eddy current losses and ease the limitations on the materials thickness.; Oxide and carbon coated FeCo and FeCoV nanoparticles were synthesized through thermal plasma processing as precursors for the compacted bulk magnets. Their densification characteristics as well as the magnetic, structural and microstructural properties were studied before and after compaction. A hot isostatic pressing (HIP) and dynamic magnetic compaction (DMC) methods were employed for densification studies. Order - disorder (α→α ′), magnetic-nonmagnetic (α→γ) phase transformations and a 550°C anomaly in nanoparticles were also studied and their deviation from the bulk properties were explored.; Combining the capabilities of thermal plasma synthesis and HIP processing, exchange coupled composite hard magnets consisting of Sm(FeCo)CuZrB and plasma synthesized FeCo[C] and FeCo[O] were synthesized in order to fabricate bulk magnets with an enhanced energy product, BH_max.; Thermal plasma synthesis was also employed in producing nitrogen martensite (γ-FeN_x) nanoparticles as precursors for the annealing studies to synthesis α″-FeN_x giant moment phase, which is still a controversial subject. FeN_x nanoparticles containing up to 50% γ-phase were synthesized by using nitrogen as a nitrogenization agent and the α″-phase was precipitated by low temperature annealing. No evidence was found to assure that α ″-FeN_x phase possesses a giant moment. It was also experimentally demonstrated that thermal plasma synthesis could be used as a deposition tool to fabricate bulk dense structures.