Magnetic anisotropy of the grain oriented and non-oriented silicon iron sheets.

Popovici, Dorina ; Paltanea, Veronica ; Paltanea, Gheorghe 等

1. INTRODUCTION

Grain oriented (GO) and non-oriented (NO) laminations owe their continuing importance as a unique subject of basic research studies and industrial applications to their excellent crystallographic properties (Paltanea & Paltanea, 2006, b). While in many cases the physical modeling has been limited to the properties related to the rolling directions, the use of computational methods in magnetic cores call for the knowledge of magnetization curve and hysteresis loops in directions that are different from rolling directions possibility of defining an intrinsic magnetization behavior in a [001](110) Fe Si (GO) single crystal is limited to the [001] and [0[bar.1]0] directions, because the magnetization process in all the other directions is affected by the sample geometry (Fiorillo et al., 2002).

Magnetic measurements represent a relatively difficult area of the electrical engineering. Classical measurements have been taken for the closed specimens, but the modern AC measurement should be made for open steel strips and sheets. For these measurements, it is necessary to keep the prescribed conditions. The total error of any magnetic measurement depends on the exciting field error, the error of the magnetic to electric variables conversion and the error of integral electric values measurement (Nencib et al., 1995).

The standard magnetic measurements using either the Epstein frame or the single strip tester are limited to the rolling direction measurements for the GO Fe-Si materials. For the NO ones, standard measurements give an average of the magnetic behavior in the rolling and the transverse directions. The hysteresis in the rolling direction [B.sub.L]([H.sub.L]) in the case of Fe-Si GO strips and an average one in the case of Fe-Si NO strips were obtain (Nencib et al., 1995; Soinski, 1987).

2. EXPERIMENTAL DETERMINATIONS

In the case of the GO sample the hard magnetization axis is orientated at 90[degrees] to the easy axis at low level induction and rotates towards 55[degrees] when magnetic flux density increases (Soinski, 1987). Experimentally, in our case, samples measured with a unidirectional single strip tester, led to the same results. At industrial frequency (f = 50 Hz) and peak magnetic flux density B = 1.5 T, the hard axis was obtained for a 60[degrees] angle.

[FIGURE 1 OMITTED]

This happened because a strip cut at 55[degrees] could not be obtained for the uniform distributed angles (between 0[degrees] and 90[degrees]). The easy axis was found at 0[degrees] with the rolling direction for GO strips (see figure 1). For the representation of the polar diagrams at constant magnetic flux density, an original program was developed to interpolate the experimental results, in order to obtain the same vector of the magnetic flux density for all the measuring directions. Using the symmetry a complete diagram from 0[degrees] to 360[degrees] was made.

At low induction (figure 2) the hard axis at 90[degrees] and the easy axis at 0[degrees] were obtained, because in this case the orientation of the grain is not as important as in the interval of high induction. In Goss texture polycrystalline materials, the directions representing different degrees of difficulty in magnetization are 0[degrees], 55[degrees]-60[degrees] (according to the technology of the strip production) and 90[degrees]. Since the plastic working of polycrystalline materials results in the formation of the preferred orientation of crystallographic axes (texture formation), optimum orientation of a single crystal is theoretically possible. Thus, an ideally isotropic sample (of random orientation of crystalline grains with regard to the rolling direction and area) will bring a little change in the perpendicular component of magnetization, which is not the case with crystalline grains oriented in a given way (Paltanea & Paltanea, 2006, a).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

In the case of NO strips (figure 3), a different behavior of the easy and hard axis was observed: the hard axis is located at 90[degrees] and the easy axis is located at 0 for high level induction. It has been noticed that the anisotropy is not so strong as in the case of Fe-Si GO and that observation sustains theory (Fiorillo et al., 2002; Paltanea & Paltanea, 2006, a).

In order to explain the results presented in this paper, a domain structure observation was performed and also to better clarify the relationship between metallurgical structure and the anisotropy effects on the strips (grain-oriented and non-oriented), cut parallel with the rolling direction. The domain observation was made using Kerr microscopy. The magneto-optical techniques have the advantage of being able to follow a rapid magnetic domain wall motion. Two disadvantages of this method are the difficulty of sample preparation and the need of a very powerful light source.

The magnetic domain structures for two Fe-Si GO strips cut at 0[degrees] with the rolling direction (RD) in demagnetized state can be observed in figure 4.

In Fe-Si grain oriented strips, the first magnetocrystalline anisotropy constant [K.sub.1] is high and positive (3.5x[10.sup.4] J/[m.sup.3]) and therefore <100> is an easy direction, since the magnetocrystalline anisotropy energy is equal to zero. The magnetocrystalline anisotropy energy is minimum for domains located parallel to certain crystallographic directions, which are often called "easy directions of magnetization". The high value of [K.sub.1] forces all the domains to be parallel to <100> directions throughout magnetization up to the knee of the magnetization curve. This constraint greatly simplifies the analysis and prediction of domain structure. From figure 1 and figure 2 one can observe that the easy axis is parallel with the rolling directions and this observation is also sustained by the domain structure observation.

The magnetic domain structures for two Fe-Si NO strips cut at 0[degrees] with the rolling direction (RD) in demagnetized state can be observed in figure 5.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

In the case of Fe-Si NO strips, the magnetic domains are barely visible. It can be observed that these materials are less anisotropic that the grain-oriented strips.

3. CONCLUSIONS

The experiments presented in the paper reflect the macroscopic anisotropy of the material, which is deliberately strong for the Fe-Si GO strips and must not be neglected in the case of Fe-Si NO strips.

In the case of Fe-Si GO strips it was observed that in the low induction domain the hard axis is located at 75[degrees] and not at 90[degrees], because not all the grains are oriented in the lamination direction. In the domain of high induction, the hard axis is at 60[degrees] and the easy axis at 0[degrees]. In the case of small magnetic fields, the probable reason of change for the hard axis from 60[degrees] to 90[degrees] is close related to the magnetic domain structure of the material.

The rolling and the transverse direction are the only ones for which properties can be defined independently on the specific sample shape. For a generic direction, the measuring conditions (i.e. sample geometry) must be specified. Single strips are expected to display intermediate behaviors.

The paper refers to a new direction in studying macroscopic anisotropy by making use of simpler measurement methods with strong impact in practical applications linked to a new and more efficient design of electrical machines. Through this research a new perspective in the analysis of the anisotropy for magnetic polycrystalline materials will be open, which will permit to develop new strong tools for magnetic hysteresis analyse and modelling.

4. REFERENCES

Fiorillo, F.; Appino, C. & Beatrice, C. (2002). Magnetization process under generically directed field in GO Fe-(3 wt%)Si laminations. J. Magn. Magn. Mater., pp. 257-260, May, 2002, ISSN 0018-9464

Nencib, N.; Spornic, S., Kedous-Lebouc, A. & Cornut, B. (1995). Macroscopic Anisotropy Characterization of SiFe Using a Rotational Single Sheet Tester. IEEE Transaction of Magnetics, Vo. 31, No. 6, pp. 4047- p.4049, ISSN 00189464

Paltanea, V.; Paltanea G. (2006, a). Magnetic characterization of the anisotropy of the GO and NO silicon iron sheets, Proceedings of the MMDE 2006, pp. 106-109, ISBN: 973-718-503-X, Bucharest, June, 2006

Paltanea, V.; Paltanea G. (2006, b). Utilization of the D8 Advance diffractometer for the determination of the crystalline structure of the grain oriented and nonoriented silicon iron sheets, Proceedings of the MMDE 2006, pp. 102-105, ISBN: 973-718-503-X, Bucharest, June, 2006

Soinski, M. (1987). The Anisotropy of Coercive Force in Cold-Rolled Goss-Texture Electrical Sheets. IEEE Trans. Magn, Vol.23, No. 6, 1987, ISSN 0018-9464