G. Bolelli, L. Lusvarghi, University of Modena and Reggio Emilia, Modena, Italy; M. Begard, K. Richardt, T. Schläfer, Surface Engineering Institute, RWTH Aachen University, Aachen, Germany; K. Bobzin, Surface Engineering Institute (IOT), RWTH Aachen University, Aachen, Germany; A. Hujanen, P. Lintunen, U. Kanerva, T. Varis, VTT Technical Research Centre of Finland, Espoo, Finland; D. Lisjak, Jozef Stefan Institute, Ljubljana, Slovenia; M. Pasquale, National Institute of Metrological Research (INRIM), Torino, Italy
The increasing exploitation of millimeter-waves and microwaves in electronics and telecommunications has aroused the need to protect electronic parts from interferences with electromagnetic wave-absorbers, which can be based on the Ba-hexaferrites. These absorbers can be ceramics or composites and have some limitations. Pure Ba-hexaferrites-based coatings could instead be produced by thermal spraying. In this research, Co,Ti-substituted Ba-hexaferrite (BaCoTiFe10O19) coatings were prepared by APS and HVOF, using BaCoTiFe10O19 powders manufactured with solid-state reaction at 1100°C followed by spray-drying.The SEM+EDX, XRD and micro-Raman analyses indicated that the melting and quenching of the agglomerates during spraying hindered the crystallisation of the hexaferrite structure and consequently delivers poor magnetic properties of the coatings. Adjusted processing conditions enabled the deposition of a coating retaining enough hexaferrite phase. Its magnetic properties, close to bulk BaCoTiFe10O19, show that thermally-sprayed hexaferrites hold promise as high-quality electromagnetic wave-absorbing layers.
Summary: With the increasing exploitation of millimeter-waves and microwaves in electronic devices and telecommunications, the development of electromagnetic wave absorbers has been acquiring a large importance, in order to protect critical electronic parts from electromagnetic interference.
Ba-hexaferrites-based compounds, well-known ferrimagnetic materials, are particularly suitable for the production of electromagnetic wave absorbers. In numerous applications, absorbing films must be deposited onto the surface of the equipment to be protected: accordingly, ferrite-containing paints and polymeric composites are generally employed, but they have several limitations. Thermal spraying appears a suitable technique for the deposition of pure Ba-hexaferrite coatings whose thickness is in the range of a few hundred microns onto objects with various shapes. In this research, Co,Ti-substituted Ba-hexaferrite (BaCoTiFe10O19) coatings were prepared by APS and HVOF techniques, using BaCoTiFe10O19 pre-reacted powders prepared by ball-milling and spray-drying. Various parameter sets were tested, in order to limit the loss of crystallinity in the as-deposited coating, thus preserving the magnetic properties of the starting material.
The phase composition and the microstructure of the coatings were analysed by SEM, X-ray diffractometry and micro-Raman spectroscopy. Their magnetic properties were characterised by the vibrating sample magnetometer (VSM) technique, in order to assess their saturation magnetization and coercivity, and by free space measurements, in order to measure the complex permeability and permittivity, thus determining their ferromagnetic resonance frequency.
It was noted that, in many cases, the hexaferrite structure was largely lost in the as-deposited coatings, because full melting of the sprayed agglomerates and subsequent impact quenching did not permit the completion of the slow and complicated crystallisation process leading to the hexaferrite structure; consequently, the coatings showed poor magnetic properties. Proper adjustment of the spray-dried agglomerate size and of the deposition parameters, however, eventually allowed the production of a coating retaining an adequate amount of hexaferrite phase, showing magnetic properties close to bulk BaCoTiFe10O19. Thermally-sprayed hexaferrites therefore hold promise as high-quality electromagnetic wave-absorbing layers; the results obtained so far indicate the way for future improvements.
