A very low frequency (VLF) antenna based on clamped bending-mode structure magnetoelectric laminates

A VLF ME mechanical transmitting antenna consisting of magnetostrictive Metglas 2605 amorphous ribbons and piezoelectric PMN–PT single crystal plates has been proposed, as shown in figure 1(a-i). As shown in figure 1(b), the piezoelectric phase deformed due to the converse piezoelectric effect and conducted the deformation to the magnetostrictive layer under the external electric field. Then the magnetostrictive phase induces magnetic flux, and the ME antenna can be fabricated based on converse ME effect.

For strip-shaped ME laminates, the electromechanical resonance (EMR) frequency of the length vibration mode is generally higher to about 30 kHz. For ME laminates with asymmetric structure, there is a resonant peak, which is much lower than the resonant peak of the length mode. The vibration mode has been changed from a simple length vibration mode to a complex vibration mode. The drop in working frequency is mainly contributed by the bending vibration modes. It is a generalized bending vibration mode of a pure cantilever beam. In order to enhance the value of the resonant peak, we clamped one single end of the ME laminates to form a cantilever beam structure as shown in figure 1(a-ii).

Solenoid coils were used to measure the near-field radiation ability of the ME transmitting antenna, as shown in figure 1(a-iii), and another magnetic detector was used as the receiver to measure the far-field radiation ability of the ME transmitting antenna, as shown in figure 1(a-iv). The optimal direct current (DC) bias magnetic intensity was provided by Hall coil in order to maximize the converse ME coefficient of the ME transmitting antenna.

The ME transmitting antenna shown in figure 1(b) (size of 120 mm × 5 mm × 0.75 mm) constituted by Metglas/PMN–PT bilayer. Six Metglas ribbons (of dimension 120 mm × 5 mm × 0.25 mm) were bonded together to be the magnetostrictive phase by epoxy resin (West system 105/206). Then, a piece of PMN–PT single crystal plate (20 mm × 5 mm × 0.5 mm) was bonded to the middle of the upper surface of the magnetostrictive phase as shown in figure 1(c). The laminate was treated at 50 °C under certain pressure for 6 h in order to obtain a higher ME coupling. The PMN–PT layer was polarized along the thickness direction and the Metglas layer was magnetized along the length direction. Polarization and magnetized are necessary in order to induce the desired piezoelectric effect and magnetostrictive effect in the PMN–PT layer and Metglas ribbons. Figure 1(a-iii) shows a near-field receiver which made up of copper coil with a diameter of 30 mm, a length of 120 mm and three hundred turns to measure the near-field radiation of the ME transmitting antenna. Figure 1(a-iv) shows a magnetic detector based on ME laminates to measure the far-field radiation of the ME transmitting antenna.

First of all, COMSOL Multiphysics software was used to find a structure which can reduce the resonance frequency of the ME transmitting antenna. The vibration mode of the ME transmitting antenna was studied by using the finite element method.

Based on previous research, the finite element simulation of the magnetic flux density and strain of ME materials was carried out. We modeled and simulated the unclamped and clamped ME transmitting antennas, respectively. The ME sample model was constructed, and a DC magnetic field and an alternating (AC) magnetic field were applied along the longitudinal axis. As shown in figure 2(a), for no-clamped ME laminates, the boundary conditions of solid mechanics were set as free. It clearly indicates that the magnetic flux density and strain exhibit maximum value at the center of the piezomagnetic. As shown in figure 2(b), for the ME antenna sample clamped on one side, the solid mechanical boundary condition on the side was set to be fixed, and obviously its magnetic flux density distribution is different from that of a no-clamped ME transmitting antenna. Figures 2(c) and (d) show the modal analysis of the no-clamped and clamped ME transmitting antenna, respectively. It is observed that the ME transmitting antenna without clamping had a single vibration mode, which referred to a length vibration mode at the resonance frequency of 17.5 kHz; while the vibration mode of the ME antenna single-sided clamped was more complicated, and it mainly included the lower frequency in bending vibration mode and a higher resonant frequency in length vibration mode. It can be predicted that the resonant frequency can be effectively reduced by clamping the unilateral side of the asymmetric geomagnetic ME transmitting antenna. In the following section, sample preparation and performance testing will be carried out.

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After the fabrication of the ME antennas, we characterized the properties of ME transmitting antennas. As shown in figure 3(a), the impedance and phase degree of the no-clamped ME antenna have been measured through impedance analyzer. For the ME transmitting antenna not clamped, there is a EMR frequency of 17.5 kHz. For the ME transmitting antenna of the clamped state, two resonance frequency can be clearly observed, respectively at 6.3 kHz and 22 kHz as shown in figure 3(c).

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We used the solenoid coil (shown in figure 1(a)) wrapping the ME transmitting antenna to measure the near- field converse ME coefficient, as shown in figure 3(c). A signal analyzer was used to detect the electromotive force of the solenoid coil. From the electromagnetic coefficient of the solenoid coil, the radiated magnetic flux of the ME antenna can be calculated. Then the converse ME coefficients at the resonance frequency have been characterized. As shown in figure 3(d), the ME transmitting antenna in the clamped state exhibits a converse ME coefficient of 6 Oe cm V⁻¹ and shows an obvious resonant frequency at 6.3 kHz, which is nearly four times lower than the 25 kHz (made by Dong et al) [9] and the 24 kHz (made by the team of Virginia Tech) [6]. We used a more advanced PMN–PT as the piezoelectric phase (with higher piezoelectric properties than PZT), such a high converse ME coefficient is more than twice that of the samples made by the Northeastern University team [9]. The experimental results agree well with the simulation predictions.

We tested the optimal DC bias magnetic field of the ME transmitting antenna, which directly had impact on the performance of radiation ability, as can be seen in figure 4. We placed the ME laminate in the Hall coil, change the DC current of the Hall coil, and measure the maximum value of the CME coefficient at the resonant frequency, so as to measure the optimal DC bias magnetic field of the ME material. The responsivity of the ME laminates at the resonance frequency is affected by the magnitude of the bias magnetic field (H_DC). The resonance frequencies show slight dispersion with the biased DC magnetic field changing varies in the range of 0–10 Oe to the no-clamped ME antenna and the optimal H_DC is about 5 Oe, as shown in figure 4(a). Figure 4(b) reveals the resonant state converse ME coefficients of the ME antenna under different H_DC, where the optimal H_DC is about 3 Oe. More importantly, the converse ME coefficients of the clamped Metglas/PMN–PT laminates can reach as high as 6 V cm Oe⁻¹ at a much lower resonant frequency of ∼6.2 kHz, which guarantee the antenna sufficient radiation capacity. Different to the non-clamped ME transmitting antenna, the resonance frequencies show almost imperceptible dispersion with the biased DC magnetic field changing varies in the range of 0–9 Oe.

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