J. Kováč

AC loss in ReBCO pancake coils and stacks of them: modelling and measurement

Many applications of ReBCO-coated conductors contain stacks of pancake coils. In order to reduce their high AC loss, it is necessary to understand the loss mechanisms. In this paper, we measure and simulate the AC loss and the critical current, Ic, in stacks of pancake coils (‘pancakes’). We construct stacks of up to four pancakes and we measure them by electrical means. We also obtain the anisotropic field dependence of Jc from Ic measurements of the tape. This Jc is the only input to the simulations, together with the coil dimensions. After validating our computations with the measurements, we simulate stacks of many pancakes, up to 32. We found that the AC loss in a stack of (four) pancakes is very high, two orders of magnitude larger than for a single tape. A double pancake behaves as a single one with double the width but a stack of more pancakes is very different. Finally, we found that a two-strand Roebel cable reduces the AC loss in a stack of pancakes but not in a single pancake. In conclusion, our simulations are useful to predict the AC loss of stacks of coated conductor pancake coils and to reduce the AC loss by optimizing the coil design.

Structural and magnetic properties of metastable Fe1-xSix (0.15<x<0.34) alloys prepared by a rapid-quenching technique

Fe1-xSix (0.15<x<0.375) alloys have been prepared by a rapid-quenching technique. X-ray diffraction patterns show that these alloys consist of submicron grains between 100 and 300 nm. The DO3 cubic symmetry could be obtained for Si content up to 34 at.% Si. This feature is particularly interesting since alloys containing more than 26 at.% Si and cast with usual techniques consist of a mixture of Fe3Si and Fe5Si3. The magnetization, Curie temperature and resistivity data for these fine-grained alloys are consistent with those for bulk alloys for Si content below 25 at.% Si. In contrast, the structural and physical data for single-phase alloys with 28-34 silicon content are reported for the first time. Within this composition range, the magnetization and Curie point values roughly correspond to those suggested by the extrapolation of existing data. However, the resistivity shows an unexpected marked increase above 25 at.% Si. The values of resistivity, magnetization and effective anisotropy of soft-magnetic nanocrystalline Fe2Si are found to be 200 µΩ cm, 0.6 T and 15 kJ m-3 respectively, suggesting that this alloy has potential for high-frequency applications.

The influence of microstructure on magnetic properties of nanocrystalline Fe-Pt-Nb-B permanent magnet ribbons

A FePt-based hard-magnetic nanocomposite of exchange spring type was prepared by isothermal annealing of melt-spun Fe52Pt28Nb2B18 (atomic percent) ribbons. The relationship between microstructure and magnetic properties was investigated by qualitative and quantitative structural analysis based on the x-ray diffraction, transmission electron microscopy, and F57e Mossbauer spectrometry on one hand and the superconducting quantum interference device magnetometry on the other hand. The microstructure consists of L10-FePt hard-magnetic grains (15–45 nm in diameter) dispersed in a soft magnetic medium composed by A1 FePt, Fe2B, and boron-rich (FeB)PtNb remainder phase. The ribbons annealed at 700 °C for 1 h exhibit promising hard-magnetic properties at room temperature: Mr/Ms=0.69; Hc=820 kA/m and (BH)max=70 kJ/m3. Strong exchange coupling between hard and soft magnetic phases was demonstrated by a smooth demagnetizing curve and positive δM-peak in the Henkel plot. The magnetic properties measured from 5 to 750...

Low AC loss cable produced from transposed striated CC tapes

In this work we demonstrate that the use of striated tapes from coated conductors (CCs) significantly reduces the dissipation of a cable made of tapes wound helically on a round core when it is exposed to AC magnetic field. The coupling loss can vanish provided that the striations ensure electrical insulation between filaments and the cable length corresponds to an entire number of lay pitches. In our study we compare the magnetization loss in two cable models exposed to magnetic field perpendicular to their longitudinal axis. The overall geometry of the models was identical: each consisted of three tapes 4?mm wide that were placed with a pitch of 50?mm in a single layer on the 8?mm diameter round core. The cable length was designed to reach two complete tape pitches. In the first cable (the reference cable) tapes without striation were used; the second cable was prepared using similar tapes but striated to five filaments by laser processing. The AC loss was measured for cables without terminations as well as with low resistance terminations; this latter configuration simulates the conditions in a magnet winding. Our experiments have clearly shown the loss behavior expected in the regime of uncoupled filaments. In particular, at AC fields of 0.1?T amplitude the loss in the cable from striated tapes is five times lower than in the reference cable. Numerical models have explained the experimentally observed cable behavior in the whole range of AC fields.

Magnetic characteristics of RNi4B compounds (R=Y, Pr, Sm, Tb, Ho and Er)

The magnetic properties of RNi4B compounds with R=Y, Pr, Sm, Tb, Ho and Er have been studied. These compounds crystallize in a hexagonal structure with the P6/mmm space group. YNi4B shows superconducting behavior below ∼12 K, while PrNi4B is paramagnetic but contains a contribution of a ferromagnetic impurity phase. For compounds with R=Sm, Tb, Ho and Er transitions to magnetically ordered state are visible at 38, 21, 6 and 12 K, respectively. A large hysteresis loop is observed for the SmNi4B compound with a coercive field HC>7.0 T, whereas the RNi4B compounds of the other rare earths are characterized by an HC lower than 0.06 T at 5 K.

Dielectric response of transformer oil based ferrofluid in low frequency range

In this article, our experimental study of the dynamic dielectric behaviour of transformer oil-based ferrofluid with magnetite nanoparticles is presented. Frequency-dependent dielectric permittivity and dissipation factor were measured within the frequency range from 20 Hz to 2 MHz by a capacitance method. The ferrofluid samples were placed in a liquid crystal cell, and experiments were carried out in an electromagnetically anechoic chamber. Two polarization processes and corresponding relaxations were revealed within the applied frequency range. Schwarz theory of electric double layer polarization is used to explain the low frequency relaxation maximum. Moreover, the shift of the maximum position towards higher frequencies is observed as the magnetic volume fraction in the ferrofluid increases. The related decrease in relaxation time due to higher counterion mobility is analysed. Reduced electric field intensity due to depolarization field, which is dependent on the particle concentration, is proposed as...

Magnetic properties of Fe2O3 nanoparticles embedded in hollows of periodic nanoporous silica

The magnetic properties of the nanocomposite consisting of hematite (α-Fe2O3) nanoparticles embedded in the hollows of the periodic nanoporous silica matrix were investigated. The magnetic measurements showed on the superparamagnetic behavior of the nanoparticles. This behavior was evidenced by the irreversibility of magnetization curves measured in zero-field-cooled and field cooled regimes, by the presence of a maximum in zero-field-cooled magnetization curve related to blocking temperature TB∼37 K and also by revealing of the coercivity HC below TB. The value of the magnetic moment mP∼300 μB of each iron-oxide particle was estimated. The interparticle interactions were investigated from dynamic properties using ac susceptibility. The susceptibility study shows, that the in-phase ac susceptibility χ′ is frequency dependent and the peak position increases with the increase in the frequency. The quantitative analysis of χ′ using theoretical models shows on the existence of weak dipolar interactions betwee...

Influence of microstructure on the magnetic and mechanical behaviour of amorphous and nanocrystalline FeNbB alloy

The formation of a nanocrystalline structure and its influence on the magnetic and mechanical properties in a ternary Fe80.5Nb7B12.5 alloy has been investigated using a variety of complementary methods. The crystallization studies performed by DSC calorimetry, magnetization and electrical resistivity measurements have confirmed a two-stage nature of the primary crystallization process. The microstructure in the series of heat-treated amorphous and nanocrystalline specimens with different volume fractions of crystalline phase was examined by transmission electron microscopy, x-ray diffraction and 57Fe Mossbauer spectrometry. The results obtained by a combination of static magnetic measurements and Mossbauer spectrometry have indicated a higher degree of structural and magnetic inhomogeneity of the residual amorphous phase after nanocrystallization. Striking differences in the magnetic hardening regime at elevated temperatures have been observed for the samples with different volume fractions of nanocrystalline particles. The strongest magnetic hardening effects are visible for the samples exhibiting a medium degree of crystallinity, while the best soft-magnetic properties are obtained for the samples where the primary crystallization process is nearly finished. The ductility tests have revealed that the transition from ductile to brittle behaviour develops predominantly in an amorphous phase just before crystallization and the subsequent crystallization causes only slight changes in the embrittlement level. On the other hand, the hardness is rather insensitive to the structural relaxation processes before crystallization and its value increases proportionally to the volume fraction of precipitated nanocrystalline grains.

Annealing effects on the magnetic properties of nanocrystalline FeNbB alloys

Abstract The influence of microstructure on magnetic behavior in nanocrystalline Fe 80.5 Nb 7 B 12.5 alloys is investigated in a series of specimens with different volume fractions of crystalline phase. The magnetic behavior is studied by VSM magnetometry and by hysteresis loop measurements. A marked increase of Curie temperature of the residual amorphous matrix is observed with increasing volume fraction of BCC Fe nanocrystals. An abrupt deterioration of the soft magnetic behavior takes place at the beginning of the crystallization process. The minimum value of coercivity is found after annealing of specimens in the temperature range 610–630°C, i.e. just before the onset of the crystallization of the residual amorphous matrix.

The influence of partial substitution of Co by Al atoms on the magnetic properties of DyCo2 compound

Abstract New results of experimental investigations of the physical properties of Dy(Co 1− x Al x ) 2 compounds, where x = 0.0, 0.1 and 0.2, are presented. These systems crystallize in the whole range of the concentration x in the Laves-phase MgCu 2 crystal structure. The zero field cooled (ZFC) and field cooled (FC) magnetization measurements were performed on the pseudobinary Dy(Co 1− x Al x ) 2 cubic compounds. The composition dependence of the Curie temperature is discussed on the basis of electronic structure calculations.