Thermal spraying of Co,Ti-substituted Ba-hexaferrite coatings for electromagnetic wave absorption applications

Characterisation of plasma-sprayed SrFe12O19 coatings for electromagnetic wave absorption

“…Very large quenching stresses arising in those stackings of well-bonded lamellae 27,28 also cause extensive transverse microcracking across the coating. Previous studies on plasma-and HVOF-sprayed Bahexaferrites clearly showed that, when such extensive melting of the sprayed agglomerates occurred, no hexaferrite phase could ever be retained in the coatings, [20][21][22] because the time required to complete the crystallisation process of Ba-hexaferrite, which involves various peritectic reactions, 29 is definitely too long, compared to the very short solidification times (of the order of few microseconds, with cooling rates of ∼10 6 K/s) of impactquenched molten lamellae. 30,31 A similar phenomenon can be expected for Sr-hexaferrite as well; indeed, according to the most recent phase diagram published on the SrO-Fe 2 O 3 system, 32 the Sr-hexaferrite compound exhibits incongruent melting: as temperature increases, the stoichiometric hexagonal ferrite compound is progressively altered by three peritectic reactions, each involving loss of oxygen and partial reduction of some Fe 3+ to Fe 2+ .…”

“…On the one hand, the spray parameters and the size distribution of the feedstock powder have to be optimised in order to embed a controlled fraction of unmelted material in the sprayed layer while retaining sufficient deposition efficiency 22,23 ; specifically, the best results were obtained using the APS technique. 22 On the other hand, the production route of the feedstock powder also plays a crucial role.…”

“…22 On the other hand, the production route of the feedstock powder also plays a crucial role. APS-processing of spraydried agglomerates of fine Ba-hexaferrite particles gave rise to coatings with "bimodal" microstructure, where dense regions, produced by molten agglomerates and containing no crystalline hexaferrite, surround unmelted regions containing the original micron-sized hexaferrite particles.…”

“…APS-processing of spraydried agglomerates of fine Ba-hexaferrite particles gave rise to coatings with "bimodal" microstructure, where dense regions, produced by molten agglomerates and containing no crystalline hexaferrite, surround unmelted regions containing the original micron-sized hexaferrite particles. 22 On the contrary, reactively sintered hexaferrite agglomerates gave different results. Reactively sintered agglomerates were produced by spraydrying a stoichiometric mixture of micrometric reagent particles (BaCO 3 + Fe 2 O 3 in the case of Ba-hexaferrite) and by heattreating the resulting agglomerates in order to promote reaction and develop crystalline hexaferrite.…”

Low material density and high microwave-absorption performance of hollow strontium ferrite nanofibers prepared via coaxial electrospinning

“…Because of their high crystal anisotropy, magnetic permeability, saturation magnetization, coercivity, and excellent chemical stability, hexagonal ferrites are used widely to make permanent magnets for a number of high-technology fields. They have also been used to produce microwave-absorbing nanocomposites [10,11] and highdensity magnetic recording media [12e14]. Hexagonal ferrites with various structures and morphologies have been studied.…”