Scientific report of phase 1
Raport stiintific faza 1
(04.01.2021 – 31.12.2021)
Main objectives/Obiective principale:
(i) Fe-based amorphous fibres coated with a ceramic layer – preparation and characterisation.
(ii) Cold sintered magnetic cores – preparation and characterisation.
Summary of results/Sinteza rezultatelor
The Fe77.5Si7.5B15 at.% amorphous fibres used in this project were prepared by the method of rapid cooling from the melt in a rotating water layer (in rotating water quenching technique). The amorphous structure of the fibres was proved via XRD investigations. The fibres have cylindrical shape, and their diameter is ranging between 100 µm and 105 µm as proved by SEM-EDX analysis. The saturation magnetisation and the coercive field of the amorphous fibres was determined by VSM measurement in longitudinal and transversal direction. The thermal stability of the amorphous fibres, investigated via DSC and HT-XRD, highlighted that the amorphous structure is stable up to the temperature of 500 °C. After this temperature, a mixture of α-Fe(Si) and Fe2B is formed in the fibres.
The Fe77.5Si7.5B15 at.% amorphous fibres were coated with a ZnO ceramic layer via hydrothermal method. The study concerning the influence of the precursor concentration and the deposition duration (SEM-EDX investigations) revealed that a satisfying ZnO layer is obtained by using a precursor of 0.05M concentration and a disposition duration of 24h. A uniform coating of the fibres with ZnO was also obtained via vapor deposition process that represent the second coating technique used in this study.
The thermal stability of the coated fibres was investigated via DTA-TG-MS and highlighted that to transform the layer produced via hydrothermal method in ZnO, the coated fibres must be heated up to 350 °C. The presence of ZnO on the surface of the fibres was proved by XRD and FTIR analysis.
The VSM analysis of the coated and annealed samples revealed that their saturation magnetisation decreases as the concentration of the precursor and the deposition duration increases.
Cold sintering experiments revealed that this technique have a great potential to produce SMCs in general and FSMC in particular. The main shortcut of the prepared CS-FSMC is their low density and further studies are required in order to eliminate this aspect. The DC magnetic measurements revealed that the coercivity of 0.05 M sample is 8.6 times lower as the lowest coercivity reported up to now for a FSMC and its µ rmax is 2.4 times higher as the highest µ rmax reported in the literature for a FSMC. The AC magnetic characterisation highlighted the superior characteristics of the CS-FSMC based on amorphous fibres as compared with cold sintered polycrystalline composites based on Fe fibres or Fe powders. Also, the CS-FSMC prepared form amorphous fibres are superior to the magnetic cores based on Fe-Si laminates in terms of the frequency range in which this compacts can be used.
Characterisation of the Fe77.5Si7.5B15 at.% amorphous fibres
Coating of the amorphous fibres with a ceramic layer
Cold sintered fibres based soft magnetic composites
This work was supported by a grant of the Romanian Ministry of Education and Research, CNCS - UEFISCDI,
project number PN-III-P4-ID-PCE-2020-0175, within PNCDI III”.
Scientific report of phase 2
Raport stiintific faza 2
(01/01/2022 - 31/12/2022)
Main objectives/Obiective principale:
(i) Co-based amorphous fibres coated with a ceramic layer – preparation and characterisation.
(ii) Cold sintered magnetic cores based on amorphous Co-based fibres – preparation and characterisation.
Summary of results/Sinteza rezultatelor
The Co68.18Fe4.32Si12.5B15 at.% amorphous fibres used in this phase of the project were prepared by the method of rapid cooling from the melt in a rotating water layer (in rotating water quenching technique). The fibres have a cylindrical shape, and their diameter is ranging between 110 µm and 115 µm as proved by SEM-EDX analysis. The amorphous structure of the fibres was proved via XRD investigations. VSM measurements were used to determine the magnetic characteristics of the amorphous fibres. The thermal stability of the amorphous fibres was investigated via DSC and HT-XRD techniques.
The Co-based amorphous fibres were coated with two types of ceramic coatings: BaTiO3 and ZnO. The BaTiO3 coating was prepared via hydrothermal technique (using two stabilisers, triethanolamine and glycerol) and vapour deposition. The ZnO coating was prepared via vapour deposition (deposition duration of 10 and 20 minutes). The deposition parameters vs. characteristics of the ceramic coating were investigated and correlated. The best results (thin and uniform coating) were obtained, in the case of the BaTiO3 deposed via hydrothermal technique, when glycerol was used as stabiliser. By vapour deposition of BaTiO3, the best results were obtained for a deposition duration of 10 minutes. The influence of the coating technique on the saturation magnetisation of the fibres was evaluated via VSM measurements.
Cold sintering experiments revealed that this technique has great potential to produce SMCs in general and FSMC in particular. The DC magnetic measurements highlight the following: (i) the coercive field of the compacts prepared in this study falls within the range 1.71 - 1.97 A/m which is 20 times lower than the lowest coercive field reported up to now for CS-FSMCs, which was about 40 A/m; (ii) the maximum relative permeability of the compacts falls within the range of 5000 – 49500. The largest value of the maximum relative permeability reported in this study is about that is about 20 times higher than the highest value reported for a CS-FSMC up to now. The main conclusions resulting from the AC measurements are: (i) the initial relative permeability of the composites is practically constant up to the maximum tested frequency of 10 kHz (except the compact without supplementary addition of ZnO nanoparticles); (ii) the addition of ZnO-MnZn ferrite nanoparticle mixture is recommended since leads to compacts with high µri (about double as the µri of the compacts that contains ZnO nanoparticles); (iii) the vapor deposition of the BTO is the recommended technique for the preparation of the dielectric coating. (iv) Regardless of the coating type (ZnO or BTO), supplementary nanoparticles added or the technique used for coating the fibres, the total losses of the compacts are remarkably low (under 10 W/kg at 10kHz and 0.1 T). The lowest losses, of only 3.3 W/kg at 10 kHz, were obtained for the composite core based on fibres coated with BTO via vapor deposition and containing a supplementary 10 wt.% of BTO nanoparticles
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Characterisation of the Co-based amorphous fibres
Coating of the Co-based amorphous fibres with a ceramic layer
Scientific report of phase 3
Raport stiintific faza 3
(01/01/2023 - 31/12/2023)
Main objectives/Obiective principale:
(i) Hybrid Cold sintered/Spark plasma sintered magnetic cores based on amorphous Fe-based fibres – preparation and characterisation
(ii) Hybrid Cold sintered/Spark plasma sintered magnetic cores based on amorphous Co-based fibres – preparation and characterisation
Summary of results/Sinteza rezultatelor
Fibre-based soft magnetic composites were prepared using a hybrid cold sintering/spark plasma sintering technique. The fibres used in this phase of the project (Fe77.7Si7.5B15 at.% and Co68.18Fe4.32Si12.5B15 at.%) were found to be amorphous, as confirmed by XRD analysis. The thickness of the ZnO coating obtained via vapour deposition ranged from 50 nm to 250 nm, as confirmed by STEM-EDX investigations. To prepare the composite compacts, a novel approach was developed, involving the winding of fibres onto a ceramic support followed by applying of a hybrid cold sintering/spark plasma sintering technique at a temperature in the range of 150 °C - 250 °C for 0, 5 and 10 minutes. A particular aspect of the hybrid CS/SPS technique we have used is that the graphite die was half-filled with fine graphite powder, and the sample was immersed in this powder. The graphite powder will act similarly to a liquid, distributing the compaction pressure uniformly on the entire surface of the sample subjected to sintering (similar to hot isostatic pressing). The graphite powder was used due to its ability of the graphite to conduct the electrical current and its antifriction properties. A set of graphite punches was used to apply a compaction pressure of 25 MPa; this pressure was maintained constant during the entire sintering process.
During the winding of the coated fibres onto the alumina support, an additional 20 wt.% of ZnO nanopowders or a mixture of 10 wt.% of or ZnO nanopowders + 10 wt.% carbonyl Fe, or 10 wt.% of ZnO nanopowders + 10 wt.% Mn-Zn ferrite, dispersed in 1M acetic acid is added. Also, composites with the addition of the following mixtures were prepared: (i) 15 wt.% of or ZnO nanopowders + 5 wt.% carbonyl Fe, (ii) 15 wt.% of ZnO nanopowders + 5 wt.% Mn-Zn ferrite, (iii) 10 wt.% of ZnO nanopowders + 5 wt.% carbonyl Fe + 5 wt.% Mn-Zn ferrite.
The influence of substituting 10 wt.% of ZnO with 10 wt.% of carbonyl Fe or Mn-Zn ferrite on the DC and AC magnetic characteristics of the composite compacts was investigated. The key findings were as follows: (i) Substituting 10 wt.% of ZnO with Fe led to an increase in the saturation induction of the composite from 0.61 T to 0.7 T, while using Mn-Zn ferrite for substitution resulted in a decrease in saturation induction from 0.61 T to 0.5 T. (ii) Regardless of the type of powder used for substitution, there was an increase in coercivity and a decrease in the density of the compacts. (iii) The relative permeability of the compacts, measured at 0.4 T, remained constant in the frequency range of 50 Hz to 10 kHz, but decreased when ZnO was partially substituted with Fe or Mn-Zn ferrite powders. (iv) Partial substitution of ZnO with carbonyl Fe or Mn-Zn ferrite led to an increase in the core losses of the composite compacts. The loss separation revealed a significant increase in hysteresis losses, from 110 W/kg to 140 W/kg and 148 W/kg, respectively, at f = 10 kHz when using Mn-Zn ferrite or carbonyl Fe for substitution. Moreover, a significant increase in core losses was observed when Fe powder was used for substitution, attributed to the formation of electrical contacts between fibres that promoted the excessive development of local and global eddy currents.
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