Preparation and magnetic properties of NiFeMo powdered compacts of powder elements with smoothed surfaces

doi:10.1016/j.jmmm.2019.165770

Journal of Magnetism and Magnetic Materials

Volume 494, 15 January 2020, 165770

https://doi.org/10.1016/j.jmmm.2019.165770 Get rights and content

Highlights

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Innovative method of ferromagnetic powder elements surface smoothing presented.
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Positive impact proved by comparison of structure and magnetic properties of compacts.
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Compaction pressure inhomogeneity and number of obstacles for domain walls reduced.

Abstract

Soft magnetic compacted powder materials represent specific material class with expanding application range. They are intensively studied as there is still large potential for their properties improvement. The paper presents the innovative method of smoothing of surfaces of powder elements suitable for further compaction to create bulk materials exhibiting excellent magnetic properties. The positive impact of described method is documented by comparison of structure of compacts prepared from powders with and without smoothed surfaces. The measured results of magnetic properties – coercivity, total energy losses and complex permeability illustrate the positive effect of mechanical treatment on powder elements applied before pressing procedure.

Graphical abstract

Introduction

Soft magnetic compacted powder materials represent a remarkable kind of materials with high application potential, with isotropic 3D behavior and mechanical stability, as well as with low-cost production and environmentally friendly recycling [1], [2], [3], [4]. They consist of powder magnetic particles with electrical and magnetic contact between each other, which is in contrast to soft magnetic composites [5], [6] containing the insulation coating on particles. That is why they can offer much higher permeability and saturation induction, and lower coercivity, but at the expense of higher eddy current losses hence higher total losses at elevated frequencies. These materials are suitable candidates for applications in electrical devices as electromotors, transformers, sensors, and other types of electrical equipment, particularly those operating at DC or low frequencies up to ~50/60 Hz where high magnetic performance is required. They are usually prepared by uniaxial compaction of powders obtained by several different methods [1], with various particle surface irregularities and size distribution. In case of the powder resulting from milling process, the particles exhibit significant surface irregularities, which leads the resulting compact to contain large fraction of pores. The lower density of the material (decreased volume fraction of magnetic phase) negatively influences the magnetic properties [5], [6].

NiFe alloys exhibit remarkable magnetic properties among soft magnetic materials [2], [3], [4], [5] and those with higher Ni content are sufficiently oxidation/corrosion resistant [7]. The highest permeability characterizes the alloys with the nominal composition Ni81Fe19 (wt. %) (often called permalloys). This high value arises from the simultaneous disappearance of crystal anisotropy and magnetostriction. Supermalloy Ni79Fe16Mo5 has an improved loss characteristic over permalloy materials, targeting higher operating frequencies.

The article offers the description of the method of powder elements surface smoothing, enabling to reduce some negative effects responsible for deterioration of the soft magnetic properties of resulting compacts.

Section snippets

Experimental

Four samples of soft magnetic Ni80Fe15Mo5 (wt. %) powder compacts were prepared by the following way. Small chips (with size from 1 mm to 5 mm), Fig. 1(a), were obtained by grinding from the sheet (0.6 mm thick) by a rotary drill grinder mounted in a lathe. The chips were milled in planetary ball mill Retch PM100 in steel vial with steel balls (the diameter of 10 mm) for 3 h, the BPR was 9:1, the rpm was 300. The resulting powder was sieved to obtain size fraction from 100 μm to 300 μm. The

Structure

The microstructure of the compacted powder samples was investigated by optical microscope Nikon Eclipse LV DIA. The Ar ion polishing technique using Gatan 682 PECS was employed for the preparation of the surface of non-annealed samples for the analysis, while the surface of annealed samples was etched by Marble’s reagent [8].

Fig. 2(a), (b) shows the cross-sectional view of the deformed structure of a ferromagnetic element of the non-annealed Samples A and C, respectively, which is characterized

Magnetic properties

The coercivity of the compacted samples was measured by the Foerster Koerzimat 1.097 HCJ. The coercivity of small ferromagnetic particles (Table 1) forming the compacts is determined by volume structural defects, hindering displacement of domain walls, where the majority of the defects were introduced to Ni80Fe15Mo5 material during grinding of the sheets, milling of the chips in the mill and compacting of the powder by the press at a relatively high pressure. Besides the volume defects, the

Conclusion

By investigating of the powder surface irregularities, and the structure and magnetic properties of compacted Ni80Fe15Mo5 (wt. %) material prepared by innovative technique of powder elements smoothing we found that the compaction of smoothed powder particles, when the surface irregularities of the particles are partially removed, leads to the preparation of compacts with better soft magnetic properties. The smoothed particle surfaces are responsible for:

1.
lowering of the inhomogeneity of the

Acknowledgements

This work was realized within the frame of the project “MACOMA” financed by Slovak Research and Development Agency under the contract APVV-15-0115 and by Scientific Grant Agency of Ministry of Education of Slovak Republic and Slovak Academy of Science – project VEGA 1/0377/16.

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Research articlesPreparation and magnetic properties of NiFeMo powdered compacts of powder elements with smoothed surfaces

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Structure

Magnetic properties

Conclusion

Acknowledgements

Prog. Mater. Sci.

Scr. Mater.

J. Magn. Magn. Mater.

J. Alloy. Compd.

Mater. Sci. Eng., B

Acta Mater.

Powder Technol.

Scr. Mater.

Int. J. Impact Eng.

Research articles
Preparation and magnetic properties of NiFeMo powdered compacts of powder elements with smoothed surfaces