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CORDIS - Resultados de investigaciones de la UE
Contenido archivado el 2024-05-29

Nanostructured thermally sprayed magnetic coatings for Microwave absorption applications

Final Report Summary - NAMACO (Nanostructured Thermally Sprayed Magnetic Coatings for Microwave Absorption Applications)

The main objective NAMACO was the development of a new system for absorbing reflected high power microwave radiation in waveguide components used to protect sensitive microwave sources such as magnetrons, klystrons or semiconductor-based power amplifiers (PAs) from damaging reflections. The aim was to replace the current approach, which uses conventional NiZn spinel ferrite tiles. These are currently manufactured by ball milling the raw powder with an organic binder, pressing and firing / sintering to form tiles which are then bonded, for example, onto waveguide walls. This process is problematic in manufacturing and with performance as well as being expensive in labour and materials.

In the project, the types of microwave absorbing ferrite materials currently employed and the processes currently used to manufacture high power microwave absorbing components were critically reviewed. This led to an identification of the materials and powder types to be prepared and evaluated through thermal spraying in the work programme.

In the project, the main interest was in developing high frequency microwave absorbers. This required the measurement of the complex magnetic permeability at frequencies in the range 2- 3 GHz. More specifically, good absorbers are required to possess high values of the imaginary part (µ''). Techniques were developed during the project to carry out low power measurements of µ'' for sprayed deposits and to compare the values with those of conventionally sintered powders including the baseline sintered tiles currently employed in commercial systems. Comparison of the magnetic loss factor, µ'', between pressed and sintered ferrite samples (the baseline material) and sprayed samples (both Top Gun HVOF and APS) showed that in the frequency range of interest (2-3 GHz) the sprayed coatings had µ' values approximately 45 % and 90 % of the baseline materials at 2 and 3 GHz, respectively. These are acceptable for the microwave absorber application envisaged and on this basis scaled-up, industrial, thermal spraying trials were undertaken.

The industrial scale-up spraying trials were undertaken jointly by the appropriate Small and medium-sized enterprises (SMEs) and Research and technological development (RTD) providers. SMEs also conducted particle diagnostic measurements using SprayWatch in support of the scale up trials and to facilitate transfer of spray conditions from RTD providers to the industrial environment. The in-flight particle diagnostic measurements with SprayWatch were also used to establish the repeatability, reliability and limits of the processes. The main emphasis of the spraying trials was to optimise coating characteristics in relation to maximum deposition efficiency (DE), minimum coating porosity, adequate bond strength (~45 MPa), target composition (particularly Zn level), minimum quantity of secondary phases (e.g. FeO) present and maximum magnetic loss factor at 2 and 3 GHz. On the basis of the size of the industrial component to be sprayed, and the coating thickness required, powder was produced for HVOF spraying (6 kg of +5-53 µm) and APS spraying (6 kg of +53 -140 µm) to the previous specifications in terms of morphology, composition, grain size and crystal structure.

The industrial development phase of Top Gun HVOF and APS spray deposition was successfully completed. Procedures for the manufacture of case study demonstrator components were determined and documented. This resulted in a definition of powder characteristics, process parameters (including SprayWatch temperature and velocity measurements) and coating specification in terms of, for example, phases, microstructure and magnetic performance. The magnetic properties of the industrially deposited coatings (specifically µ'' in the 2-3 GHz range) were found to be comparable to the performance data for laboratory processed coatings and baseline material used to manufacture current sintered tiles. Hence, the consortium proceeded with the manufacture and evaluation of full sized demonstrator components.

A cost estimation that has been made suggests that a cost reduction of about 40 % could be obtained compared to the conventional fabrication technology with sintered and bonded ferrite tiles. Even higher economic benefits could be expected for larger production lot quantities. Additionally, the development of this thermal spray approach provides scope for the design of new and improved absorber devices where coatings can be deposited on curved and complex surfaces. This has not hitherto been possible with current technology, involving the use of sintered and adhesively bonded ferrite tiles. The investigation of other ferrite powder compositions for thermal spraying could lead to more broadband devices or Radio frequency (RF) absorbers suited for other microwave frequency bands as well as conformal coatings for EMC-shielding in housings or walls of RF anechoic chambers.