Magnetoelectric Particulate Composite Transformers : Design and Characterization

We report the resonance voltage transformation effect of magnetoelectric particulate composite based transformers. The effect of transformer geometry and magnetoelectric composition was investigated to identify the range of transformation ratios. Pb(Zr_0.52Ti_0.48)O₃ (PZT) was selected as the piezoelectric phase and NiFe_1.9Mn_0.1O₄ (NFM), Ni_0.8Zn_0.2Fe₂O₄ (NZF), CoFe₂O₄ (CF) and Co_0.6Zn_0.4Fe₂O₄ (CZF) were selected as the magnetostrictive phase for fabricating the transformer. It was found that the composition PZT – 20 NZF with cylindrical geometry exhibits a high voltage gain of 11. The magnitude of voltage gain is directly correlated with the mechanical quality factor (Q_m). The results reported here present the possibility of developing magnetoelectric transformer for both step-up and step-down applications.

Keywords: Magnetoelectric; transformer; ferroelectric; ferromagnetic; ferrite; PZT

Magnetoelectric composite materials consisting of piezoelectric and magnetostrictive phases respond to both electric and magnetic field. The composites exploit the product property of the materials and various synthesis techniques can be adopted to combine the materials with two different phases depending upon the crystal symmetry, lattice parameters and physical state in the films and bulk. The presence of the piezoelectric and magnetostrictive phases in the same material provides the opportunity to develop a voltage gain device operating on the following principle. An applied AC magnetic field induces strain in the magnetostrictive phase which is transferred onto the piezoelectric phase in an elastically coupled system. The piezoelectric phase produces the electric charge in proportion to the applied strain. Dong et al. have reported such a device utilizing the three layer composite structure as Terfernol-D/epoxy/PZT/epoxy/Terfenol-D and obtained high voltage gain [[1]].

Recently, we have made significant progress in the synthesis of the particulate sintered composites [[3]]. In the sintered ceramic material, the applied magnetic field generates stress through the magnetostrictive grains which is transferred on to the piezoelectric grains where it is converted in to electric charge. The ability to induce large magnetoelectric coupling in the same material opens the possibility to fabricate miniature transformer devices with complex geometries having voltage gain ratios in wide range. In this manuscript we present the data on the magnetoelectric transformer using the sintered composites which can be utilized in both step-up and step-down applications.

Reagent-grade powders of PbO, ZrO2, TiO2, NiO ZnO, CoCO3 Fe2O3, and MnCO3 were mixed according to compositions Pb(Zr0.52Ti0.48)O3 (PZT), NiFe1.9Mn0.1O4 (NFM), Ni0.8Zn0.2Fe2O4 (NZF), CoFe2O4 (CF) and Co0.6Zn0.4Fe2O4 (CZF) and calcined in air. PZT powders were then mixed with 20 mole% ferrite powders to get the final compositions of PZT – 20 NFM, PZT – 20 NZF, PZT – 20 CF and PZT – 20 CZF. The effect of Zn concentration on the magnetostrictive properties in ferrites has been reported by Srinivasan [[6]]. The powders were pressed in cylindrical shape of size 12.7 mm diameter and 6.5 mm height, square shape of area 12.5 mm2, and ring shape with outside diameter 25.4 mm, inside diameter 11.5 mm and height 2 mm. Pressure less sintering of composites was performed in air at 1150°C for 2 hours. In order to perform magnetoelectric measurements, an Ag/Pd electrode was applied on the samples and fired at 825°C for 1 hr. The specimens were poled under a D.C. field of 2.5 kV/mm for 20 minutes in a silicone oil bath at 120°C. The magnetoelectric coefficient was calculated from the measured piezoelectric charge using lock-in amplifier in conjunction with the charge amplifier (5010B Dual Mode Amplifier, Kistler Instrument Co. NY) under an applied magnetic field using Helmholtz coil. Table I shows the magnitude of the magnetoelectric coefficient for the four compositions at the frequency of 1 kHz under DC bias of 100 Oe. The table also lists the piezoelectric coefficients measured using the impedance analyzer and d33 meter.

TABLE I Magnetoelectrical and electromechanical properties of PZT-ferrite magnetoelectric composites

In order to fabricate the transformer as shown in Fig. 1(a)–(c), all the poled samples of various shapes were wrapped by insulated Cu coils (20 turns each). Output wires were soldered to the electroded region of the sample. Figure 2(a)–(c) shows the picture of the fabricated samples. Clearly the advantage of this transformer design is the simple structure and driving as the fabrication process consists of just wrapping the coils around the poled samples and applying AC voltage through the coils. We did not study the effect of the winding density and number of turns. Compared to the design presented here the electromagnetic transformers require primary and secondary windings in a specific ratio. PZT – 20 NFM samples were initially used to conduct the comparative study of the shape effect on the transformer action. The voltage gain measurement was done using the following procedure. The coil was powered by the network analyzer (HP 4194A) and voltage gain (Vout/Vin) as a function of the frequency was measured by using the wires soldered on top and bottom surface of the sample. A 100 Oe magnetic DC bias field was applied on the sample using permanent magnets during the measurement.

Graph: Figure 1 Schematic diagram of different types of magnetoelectric transformers (a) Model 1 with cylindrical geometry, (b) Model 2 with ring geometry, and (c) Model 3 with square geometry.

Graph: Figure 2 Pictures of the fabricated transformer and voltage gain as a function of frequency for the composition PZT – 20 NFM (a) Model 1, (b) Model 2, (c) Model 3, (d) Voltage gain for Model 1, (e) Voltage gain for Model 2, and (f) Voltage gain for Model 3. (See Color Plate I)

Graph: Color Plate I See Figure 2 on page 4.—Pictures of the fabricated transformer and voltage gain as a function of frequency for the composition PZT – 20 NFM (a) Model 1, (b) Model 2, (c) Model 3, (d) Voltage gain for Model 1, (e) Voltage gain for Model 2, and (f) Voltage gain for Model 3.

Figure 2(d)–(f) shows the variation of voltage gain as a function of the frequency for the PZT – 20 NFM transformers. The input current (Iin) from the network analyzer creates AC magnetic field with the same frequency as that of the driving signal through the Cu coils. This magnetic field creates strain in the matrix through the magnetostrictive grains which is converted in to voltage through the piezoelectric grains and measured across the output terminal. The voltage is highest at the resonance frequency because the magnetoelectric coefficient exhibits maximum magnitude at this frequency providing high coupling factor. The cylindrical shape transformer (Model I) exhibited the voltage gain of ∼ 3.6 at the resonance frequency of 238 kHz, ring shape transformer (Model II) a voltage gain of ∼ 0.21 at the resonance frequency of 83 kHz, and square shape transformer (Model III) a voltage gain of ∼ 5.98 at the resonance frequency of 208 kHz. These results clearly imply that square geometry can be used for the step-up application while ring geometry for the step-down applications.

Figure 3 compares the voltage gain of three compositions PZT – 20 CF, PZT – 20 CZF, and PZT – 20 NZF for Model I. The composition PZT – 20 CF exhibited the voltage gain of 0.45 at resonance frequency of 237 kHz, PZT – 20 CZF exhibited voltage gain of 1.5 at resonance frequency of 236 kHz, and PZT – 20 NZF exhibited the voltage gain of 10.9 at resonance frequency of 233 kHz. Comparing the results of Fig. 3 with that in Fig. 2(d) it can be seen that composition PZT – 20 NZF provides the highest voltage gain. These results can be explained using the relation derived by Dong et al. as following [[2]]:

Graph

where Qm is the mechanical quality factor, d33 is longitudinal piezoelectric constant, and ϵ33 is the dielectric constant. The results in the Table I show that the ratio, d33/ϵo.ϵ33, is in the same range for all the compositions, 15.3 × 10− 3 – 13.2 × 10− 3 V-m/N, however the Qm varies significantly. The magnitude of Qm is highest for the PZT – 20 NZF composition of the order of 613 which could be reason for the highest voltage gain effect. Further, the magnitude of the magnetoelectric coefficient for PZT – 20 NZF is about 2 times higher than that of PZT – 20 CF implying that the elastic coupling will better in this system. Combined the results of Fig. 2 and Fig. 3 present magnetoelectric voltage gain devices with the gain ratio varying at the resonance frequency from 0.4–10.90.

Graph: Figure 3 Voltage gain as a function of frequency for Model 1 transformers with different compositions. (See Color Plate II)

Graph: Color Plate II See Figure 3 on page 5.—Voltage gain as a function of frequency for Model 1 transformers with different compositions.

The results reported in this manuscript open the possibility of developing analog transformer which is simpler in fabrication, cost effective, and easy to drive. Compared to electromagnetic transformers these devices do not require primary and secondary coils, have higher efficiencies (resonant operation), and wider bandwidth. Compared to piezoelectric transformer these devices offer the advantage of being a one-port device.

The authors gratefully acknowledge the support from Army Research Office.

Communicated by Prof. Amar M. Bhalla