Effect of Mechanical Stresses in Rapidly Heated Fe73Cu1Nb3Si16B7 Ribbon Arising During the Ring Core Formation on Their Magnetic Properties
© The Author(s). 2017
Received: 30 December 2016
Accepted: 31 March 2017
Published: 26 April 2017
The influence of winding-induced mechanical stresses on the magnetic anisotropy and core loss in toroidal cores made of Fe73Cu1Nb3Si16B7 ribbon is studied. The ribbon for the cores was rapidly pre-heated under tensile stress up to 120 MPa. It was found that magnetic characteristics of the material (magnetic anisotropy energy and the core loss) can be controlled by varying the tensile stress during the preliminary rapid heating of the ribbon. It was shown that with reducing core diameter, the magnetic anisotropy energy and core loss significantly increase. However, relatively high winding-induced core loss in small cores can be significantly reduced by increasing tensile stresses applied to the ribbon during pre-heating.
Nowadays, soft magnetic nanocrystalline alloys of Fe–Nb–Cu–B–Si system  are widely used in magnetic cores of various inductive components (transformers or chokes). It is known that formation of α–Fe(Si) nanocrystals in these alloys during heat treatment improves their soft magnetic properties. Volume fraction of nanocrystals in these materials is 75–80% and their size is about 10 nm . The so-called linear hysteresis loop can be obtained in these types of alloys by inducing uniaxial transverse magnetic anisotropy during annealing under tensile stress [2, 3]. Magnetic structure of such type alloys was studied in details by the authors of . In , huge magnetic anisotropy was reported for the ribbons of this type.
Magnetic cores made of the ribbons with induced magnetic anisotropy have a number of advantages, namely, high field stability of magnetic permeability , low core loss in the important frequency range (1–100 kHz) , and DC bias immunity .
The disadvantage of these cores is the sensitivity of the magnetic properties of the ribbon to mechanical stresses arising during core formation [8, 9]. Difficulties arise during production of miniaturized magnetic nanocrystalline cores with transverse anisotropy, particularly for pulse transformers in telecommunication systems. That is why the purpose of the current investigation was to study the influence of mechanical stresses appearing during core fabrication on the transverse magnetic anisotropy that was induced by tensile stress during nanocrystallization process and core loss.
Fe73Nb3Cu1B7Si16 amorphous ribbon (the thickness 20 μm, width 10 mm) was obtained by planar flow casting process (PFC) in the air atmosphere using the equipment for rapid quenching of the melt .
Straight pieces of ribbons were heated in order to obtain nanocrystalline structure of the material. Fast heating was realized by conducting of electric current with the density j h = 42 A/mm2 and frequency 50 Hz through straight piece of ribbon for t h = 3.7 s that provided its heating above 600 °C. To induce a uniaxial transverse magnetic anisotropy in the ribbons, the rapid heating is done under tensile stress σ t = 0, 35, 80, and 120 MPa.
Mechanical testing of ribbons had been done using the universal servohydraulic machine Instron 8802. Loading curves of ribbons during tests on uniaxial tensile stretch were recorded in coordinates “stress-strain” that allowed to determine Young’s modulus.
Magnetic studies were done using toroidal (ring) cores with primary and secondary windings like in a usual transformer (i.e., the magnetic field was applied along the ribbon axis (Fig. 1b)). Dynamic B-H curves and core loss at different frequencies were measured using the measuring complex MS-02 B-H ANALYZER (MSTATOR, Novgorodskaya oblast, Russia). The detailed description of this complex can be found in .
Results and Discussion
The technological processes used for the fabrication of the cores studied in the present paper imply application of two types of mechanical stresses into the material. One type is the tensile stress which is applied during the pre-heating of amorphous ribbons. As it was shown in , this procedure causes irreversible changes in the inner structure of amorphous ribbons on the nanoscale level. Another type of stress is the winding-induced bending of the ribbons. This stress is elastic and permits multiple winding-unwinding cycles. The interplay of these stress influences provides the possibilities to tune the magnetic properties of the material and achieves the targeted parameters of the cores.
where λ s is the saturation magnetostriction.
The origin of the additional magnetic anisotropy K is , according to , is the influence of the environment, i.e., the interaction of ribbon’s surfaces with oxygen, hydrogen, and water vapor which leads to anisotropic crystallization of surfaces.
Using (4) for the fitting of the experimental results for the cores made of the ribbon pre-heated at zero tensile stress (Fig. 4, curve 0 MPa), we obtained the values λ s = 0.31 × 10−6 and K is = 0.042 kJ/m−3. The λ s value correlates with the one obtained in .
Only the cores made of the ribbons pre-heated under large tensile stress (120 MPa) show excellent stability of core loss (independence of winding-induced mechanical stress). The resulting effect can be explained by decreasing of saturation magnetostriction with increasing value of tensile stress .
It is shown that the magnetic anisotropy of nanocrystalline ribbon of Fe73Cu1Nb3Si16B7 alloy in the toroidal core can be controlled by increasing of tensile stress applied during its preliminary rapid heating.
It is demonstrated that increasing of winding-induced mechanical bending stress (with decreasing of magnetic path diameter) above a certain value leads to higher magnetic anisotropy energy as well as higher core loss.
It is shown that the increase of core loss (due to winding-induced stress) can be significantly reduced by increasing the tensile stress applied during pre-heating of the ribbon.
Based on the results on the influence of tensile stresses during rapid heating of ribbons, the values of stresses necessary to minimize core remagnetization loss were obtained; they are important for manufacturing technology of small nanocrystalline magnetic cores with linear DC bias immune remagnetization loop, in particular for the production of pulse transformers broadband telecommunication systems.
The authors are very much obliged to MELTA Ltd. scientific production company for the provided quenching facility  and devices for measurement of magnetic properties.
VN analyzed and discussed the results and formulated conclusions of the paper. AN organized and carried out the experiment, analyzed and discussed the results, and wrote the main part of the paper. TM analyzed and discussed the results, prepared the illustrations, and translated in English. OS analyzed and discussed the results. All authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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