DC magnetoelectric sensors for electrical monitoring applications

Abstract

The rapid increase in direct current (dc) applications in recent years has urgently called for advanced materials and technologies for improved sensing of dc magnetic fields and electric currents. Magnetic field and electric current sensors based on the extrinsic magnetoelectric (ME) effect in magnetostrictive-piezoelectric heterostructures have formed an important research and development trend over the past decade due to their passive sensing and high sensitivity (>10 mV/Oe) nature in contrast to the active sensing and low sensitivity (540 μV/Oe) nature in traditional Hall sensors. Today, almost all of the reported ME sensors are "ac" ME sensors, and the greatest weakness is their inability to perform dc sensing because the extrinsic (“ac) ME effect has to be underpinned by the coupled magneto-mechano-electric dynamics of the magnetostrictive and piezoelectric phases in the heterostructures and is weakened inherently by the decay of piezoelectric charges with time in the piezoelectric phase, especially for frequencies below 100 Hz. In this research, a novel class of “dc ME sensors is proposed and realized into two different modes of operation, namely, current and voltage modes, based on specifically designed magnetic-conductive-piezoelectric heterostructures and piezoelectric-magnetostrictive heterostructures, respectively. Original contributions reported in this research are presented as follows: (1) The current-mode ME sensors are developed by driving an ac current of controlled amplitude and frequency into the conductive phase of the heterostructures upon an applied dc magnetic field to be measured with/without an external magnetic biasing by the magnetic phase in order to induce Lorentz forces for stressing the piezoelectric phase and producing an ac voltage response. Physical models for describing the working principles of the sensors are established to predict the dc magnetic field sensitivity (SI=dV/dH) under different driving currents (I) as well as the current-controlled dc magnetic field sensitivity (S=dSI/dI). Two characteristic designs are fabricated using NdFeB bars, Al strips, and 0.7Pb(Mg1/3Nb2/3)O3 0.3PbTiO3 (PMNPT) plate as the magnetic, conductive, and piezoelectric phases, respectively, and their performances are evaluated experimentally with good agreements with the model predictions. For the 1st design without an external magnetic biasing (denoted as the basic version), it can only sense dc magnetic fields and is preferably driven by low-amplitude and low-frequency ac currents to minimize the Joule heating-induced instability. SI and S are measured to be 12 mV/T (at I = 50 mA amplitude and 1 kHz frequency) and 0.23 V/T/A (at I =1 kHz frequency), respectively. For the 2nd design with an external magnetic biasing (denoted as the modified version), it is capable of sensing both dc and ac magnetic fields of up to 15 kHz by operating at resonance to achieve much higher SI and S of 88 mV/T (at 50 mA amplitude) and 1.7 V/T/A, respectively. (2) For the voltage-mode ME sensors, they employ the relatively stable and easily configured ac voltage driving to mitigate the problem of Joule heating associated with the current-mode sensors. In fact, this ac voltage is aimed to induce a natural longitudinal resonance in the heterostructure under zero dc magnetic fields. With reference to this ac voltage-driven natural longitudinal resonance, an applied dc magnetic field to the heterostructure will result in a tuning effect in the magnetic field-dependent compliance and resonance characteristics governed by the negative-ΔE effect intrinsic in the magnetostrictive plate. Physical models are established to describe the working principles of the sensors and to predict the dc magnetic field sensitivity (SV=dV/dH) under different driving voltages (V) as well as the voltage-controlled dc magnetic field sensitivity (S=dSV/dV). Two characteristic designs are fabricated using Pb(Zr,Ti)O3 (PZT) plates and Tb0.3Dy0.7Fe1.92 (Terfenol-D) plates as the piezoelectric and magnetostrictive phases, respectively. Their performances are measured and compared with the model predictions with good agreements. The 1st design (denoted as the basic version) has a plate-type, transversely-transversely polarized PZT piezoelectric transformer bonded on a plate-shaped, longitudinally magnetized Terfenol-D magnetostrictive substrate to exhibit an ac voltage-driven, dc magnetic field-tuned resonance dc ME effect characterized by a high, linear, and negative ac voltage-controlled dc magnetic field sensitivity of 0.63 mV/Oe/V at a low reference ac voltage of ≤2.5 V peak. The 2nd design (denoted as the modified version) has four thickness-polarized PZT piezoelectric plates bonded symmetrically on a length-magnetized Terfenol-D magnetostrictive plate to give an electrically parallel input and an electrically series output. An interestingly high, linear, and negative ac voltage-controlled dc magnetic field sensitivity of 1.3 mV/Oe/V is obtained in a broad range of dc magnetic field of 0400 Oe by referencing an ac voltage of ≤5 V peak amplitude and 116 kHz frequency at the input of the heterostructure. (3) Peripheral electronic circuits are designed, constructed, characterized for interfacing with the developed dc ME sensors to form portable dc magnetometers. They basically consist of a 8-bit segment LCD as the display module, an AD9850 direct digital synthesis (DDS) module as the driving voltage signal generator, and an ARM chip STM32 controlled signal process system as the signal detection, conversion, and analysis module, all powered by batteries. The dc magnetometer prototypes are evaluated and show great promise for electrical condition monitoring applications. 5 research outputs were produced during the 4 years of PhD study, including 3 papers published/accepted by Journal of Applied Physics (JAP) as the 1st author, 1 paper published by JAP as a co-author, and 1 paper presented and published in an international conference as the 1st author. This elucidates the originality, significance, and excellence of the present work.