Browsing by Author "Li, Jie-Fang"

Now showing 1 - 20 of 25
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    Advanced Synthesis of Ultra-High Temperature Ceramics (UHTCs) and High Temperature Electron Emitting Materials
    Mondal, Santanu (Virginia Tech, 2024-02-06)
    From space exploration and advanced aircraft to next generation weapons, achieving hypersonic speed is becoming increasingly important across a range of research domains. The immense challenge associated with this goal involves the development of suitable materials and systems for the different components of a hypersonic vehicle, each of which must have the inherent capability to resist extreme temperatures, high thermal shock due to high heat flux, and high oxidation and ablation. First, the ultra-high temperature ceramic (UHTC) zirconium diboride or ZrB2 was sintered by ultra-fast high temperature sintering (UHS). The UHS process was optimized and the sintering parameters for ZrB2 and other UHTCs were studied. ZrB2 is an ultra-high temperature ceramic (UHTC) with a very high melting point; thus, its densification is difficult, energy intensive, and time-consuming. Commercial ZrB2 powders were rapidly densified via UHS to >90% relative density within 60 second in vacuum without pressure. The effect of sintering time on densification and final grain size were studied. An innovative process for manufacturing bulk UHTC materials was studied and is detailed herein. Second, the work function (W_f) of electron emitting materials was reduced to improved performance. A reduction of W_f in multicomponent hexaborides was achieved by doping with highly electropositive Ba, which enhances electron emission. Single-phase bulk multicomponent polycrystalline hexaborides of La0.5Ba0.5B6, Ce0.5Ba0.5B6, and BaB6 powders were first synthesized and then densified by UHS sintering. W_f measurements were obtained by Kelvin probe force microscopy. Ba-substitution was found to lower W_f (~25%) in synthesized multicomponent hexaborides. The specific techniques required to engineer the W_f of these materials are also provided herein. Finally, combining low W_f materials with UHTCs was explored for thin film systems for the exterior surface of hypersonic vehicles. The thin films of CeB6, a low W_f material, was deposited on sintered ZrB2 by RF-sputtering and single crystalline SrTiO3 (STO) substrates. Epitaxial thin films of SrHfO3 (SHO) were also deposited on (100), (110) and (111) STO substrates at 600°C. X-ray diffraction (XRD) results confirmed the formation of epitaxial layer, and reciprocal space mapping (RSM) was used to characterize film's mosaicity / texture on different substrates. XRD and RSM data demonstrated that the most favorable film growth direction was (110). As detailed herein, an inexpensive thin film production process, RF-sputtering, was exploited to manufacture various epitaxial and non-epitaxial layers of low W_f materials on UHTC and single-crystal substrates for hypersonic vehicles. To summarize, a range of bulk UHTCs and low W_f materials were prepared by UHS, and various thin films of low W_f material were produced on UHTC. Thereafter, the properties of synthesized materials were studied to develop new material systems for hypersonic applications. The findings from this research shed light on the development of suitable materials for implementation of electron transpiration cooling for hypersonic vehicle development.
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    Applications of Magnetoelectric Sensors
    Shen, Ying (Virginia Tech, 2014-02-11)
    The magnetoelectric (ME) effect is an electric output in response to an applied magnetic field. In a heterostructure configuration where the two-phases are engineered with close interface contact, a giant electric response to a magnetic field has been found, which is designated as the ME voltage (or charge) coefficient α^ME. This effect is mediated by a mechanical-coupling between magnetostrictive and piezoelectric phases. In this thesis, I concentrate on application study for ME sensors with respect to noise control and rejection, thermal stability, triple-axis sensor design, array imaging, DC and AC magnetic sources detection and active mode ME sensor development, which is important for future ME sensor device applications.
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    Barium Titanate-Based Magnetoelectric Nanocomposites
    Yang, Yaodong (Virginia Tech, 2011-06-21)
    Barium Titanate (BaTiO3 or BTO) has attracted an ever increasing research interest because of its wide range of potential applications. Nano-sized or nanostructured BTO has found applications in new, useful smart devices, such as sensors and piezoelectric devices. Not only limited to one material, multi-layers or multi-phases can lead to multifunctional applications; for example, nanocomposites can be fabricated with ferrite or metal phase with BTO. In this study, I synthesized various BTO-ferrites, ranging from nanoparticles, nanowires to thin films. BTO-ferrite coaxial nanotubes, BTO-ferrite self-assemble thin films, and BTO single phase films were prepared by pulsed laser deposition (PLD) and sol-gel process. BTO-ferrite nanocomposites were grown by solid state reaction. Furthermore, BTO-metal nanostructures were also synthesized by solid state reaction under hydrogen gas which gave us a great inspiration to fabricate metal-ceramic composites. To understand the relationship between metal and BTO ceramic phase, I also deposited BTO film on Au buffered substrates. A metal layer can affect the grain size and orientation in BTO film which can further help us to control the distribution of dielectric properties of BTO films. After obtaining different nanomaterials, I am interested in the applications of these materials. Recently, many interesting electric devices are developed based on nanotechnology, e.g.: memristor. Memristor is a resistor with memory, which is very important in the computer memory. I believe these newly-synthesized BTO based nanostructures are useful for development of memristor, sensors and other devices to fit increasing needs.
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    Fabrication of reliable, self-biased and nonlinear magnetoelectric composites and their applications
    Li, Menghui (Virginia Tech, 2014-10-31)
    The magnetoelectric (ME) effect, i.e., the induction of magnetization by an applied electric field (E) or a polarization by an applied magnetic field (H), is of great interest to researchers due to its potential applications in magnetic sensors. Moreover, the ME effect in laminate composites is known to be much higher than in single phase and particulate composites due to combination of the magnetostrictive and piezoelectric effects in the individual layers. Given that the highest ME coefficient have been found in Metglas/piezo-fiber laminate composites, this study was designed to investigate and enhance the magnetoelectric (ME) effect in Metglas/piezo-fiber laminate composites, as well as develop their potential for magnetic sensor applications. To initiate this investigation, a theoretical model was derived to analyze the thickness effect of the magnetostrictive, piezoelectric, epoxy and Kapton layers on the ME coefficient. As a result, the importance of the coupling effect by epoxy layers was revealed. I used spin-coating, vacuum bagging, hot pressing, and screen printing techniques to decrease the thickness of the epoxy layer in order to maintain homogeneity, and to obtain good repeatability of the 16 ME laminates fabricated at one time. This protocol resulted in a more efficient way to induce self-stress to Metglas/PZT laminates, which is essential for increasing the ME coefficient. With an enhanced ME effect in the Metglas/piezo-fiber laminates, magnetic field sensitivity could then be increased. An ME sensor unit, which consisted of a Metglas/PMN-PT laminate and a low noise charge amplifier, had a magnetic field sensitivity of 10 pT/Hz0.5 in a well-shielded environment. Stacking four of these ME laminates could further increase the signal-to-noise (SNR) ratio. I studied the optimized distance between a pair of Metglas/PZT ME laminates. A stack of up to four ME sensors was constructed to decrease the equivalent magnetic noise. The magnetic field sensitivity was effectively enhanced compared to a single laminate. Finally, a number of four Metglas/PZT sensor units array was constructed to further increase the sensitivity. ME laminate composites operated in passive mode have typically required an external magnetic bias field in order to maximize the value of the piezomagnetic coefficient, which has many drawbacks. I studied the ME effect in an Ni/Metglas/PZT laminate at zero bias field by utilizing the remnant magnetization between the Ni and Metglas layers. To further enhance this effect, annealed Metglas was bonded on the Metglas/PZT laminate since it is known that hard-soft ferromagnetic bilayers generate built-in magnetic field in these Metglas layers. As a result, giant αME values could be achieved at a zero bias field at low frequency range or at electromechanical resonance (EMR). The sensor unit consisting of self-biased ME laminate arrays is considerably smaller compared to a unit that uses magnet-biased ME laminates. Introducing the converse ME effect and nonlinear ME effect in Metglas/piezo-fiber laminates affords a variety of potential applications. Therefore, I theoretically and experimentally studied converse ME effects in laminates with longitudinally magnetized and longitudinally poled, or (L-L) mode. The optimum structure for producing the maximum effect was obtained for Metglas/PZT laminates. Additionally, the optimum structure and materials for enhancing the nonlinear ME effect in Metglas/PZT laminates are reviewed herein. In particular, this study revealed that modulating the EMR in laminates with high-Q piezo-fibers could enhance the SNR. The stress effect on nonlinear ME effect is also discussed—namely that magnetic field sensitivities can be enhanced by this modulation-demodulation technique.
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    Induced Phase Transition in Magnetoelectric BiFeO3 Crystals, Thin-layers and Ceramics
    Ruette, Benjamin Thibault (Virginia Tech, 2003-05-22)
    Bismuth ferrite (BiFeO₃) is a magneto-electric material which exhibits simultaneously ferroelectric and antiferromagnetic properties. We have used high-field electron spin resonance (ESR) as a local probe of the magnetic order in the magnetic range of 0-25 Tesla. With increasing magnetic field, an induced transition has been found between incommensurately modulated cycloidal antiferromagnetic and homogeneous magnetized spin state. The data reveal a number of interesting changes with increasing field, including: (i) significant changes in the ESR spectra; (ii) hysteresis in the spectra near the critical field. We have analyzed the changes in the ESR spectra by taking into account the magnetic anisotropy of the crystal and the homogeneous anti-symmetric Dzyaloshinsky-Moria exchange. We have also investigated phase induced transition by epitaxial constraint, and substituent and cystalline solution effects. Variously oriented BiFeO₃ epitaxial thin films have been deposited by pulsed laser deposition. Dramatically enhanced polarization has been found for (001)c, (110)c, and (111)c films, relative to that of BiFeO₃ crystals. The easy axis of spontaneous polarization lies close to (111)c for the variously oriented films. BiFeO₃ films grown on (111)c have a rhombohedral structure, identical to that of single crystals. Whereas, films grown on (110)c or (001)c are explained in terms of an epitaxially-induced transition between cycloidal and homogeneous spin states, via magneto-electric interactions. Finally, lanthanum modified BiFeO₃-xPbTiO₃ crystalline solutions have been found to have a large linear magneto-electric coefficient, ∝p. The value of ∝p (2.5x10⁻⁹ s/m or C/m²-Oe) is ∼10x greater than that of any other material (cg., Cr₂O₃ ∼2.5x10⁻¹⁰ s/m), and many order(s) of magnitude higher than unmodified BiFeO₃ crystals. The data also reveal: (i) that ∝p is due to a linear coupling between polarization and magnetization; and (ii) that ∝p is independent of dc magnetic bias and ac magnetic field. We show that the ME effect is significantly enhanced due to the breaking of the transitional invariance of a long-period spiral spin structure, via randomly distributed charged imperfections.
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    Investigation of polarization switching over broad time and field domains in various ferroelectrics
    Jullian, Christelle Francoise (Virginia Tech, 2003-12-10)
    Investigations of polarization switching over broad time and electric field domains, in various modified Pb-based perovskite ferroelectrics, were systematically performed by ferroelectric switching current transient and bipolar drive P-E responses. Studies were performed from E«Ec to E»Ec, where Ec is the coercive field These investigations have shown the presence of broad relaxation time distributions for the switching process, which can extend over several decades in order of magnitude in time, and where the distribution is strongly dependent on the applied electric field. By performing the study of domain dynamics and polarization switching over extremely broad time domains (10⁻⁸ t < 10² sec), more complete information has been obtained that allows for development of a better mechanistic understanding. Prior polarization kinetics studies have focused on relatively narrow time ranges, and were fit to the Avarami equation, which contains a single relaxation time. However, our broad band width polarization dynamics and frequency relaxation studies have been fit to multiple stretched exponential functions extending over decades of order of magnitude in the time domain. Stretched exponential functions for domain nuclei formation, and for domain variant growth have been found. For example, [001]c, [110]c, and [111]c oriented PZN-4.5%PT crystals, nucleation was found to be a volume process (n=3) rather than just a domain wall restricted process. Consequently, nucleation is heterogeneous. And, growth of a domain variant with reversed polarization was found to be a boundary process (n=2), involving diffuse or rough domain walls. We have extended these studies to various types of ferroelectrics including hard, soft and relaxor types.
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    Investigation of the Magnetic Properties of Non-Thiolated Au Nano-Structures Grown by Laser Ablation
    Zhao, Chenlin (Virginia Tech, 2014-09-09)
    Although it is known that gold (Au) is diamagnetic in bulk form, it has been reported that Au displays magnetic properties when reduced to the nano-scale. Researchers found magnetism in Au nanoparticles (NPs) in a size range from 2 to 10 nanometers. Moreover, the Au nanoparticles are usually coated by thiol-containing organic caps, which are believed to be responsible for the magnetism. However, others suggest that organic capping is not necessary to observe magnetism in Au NPs, and magnetism may be an intrinsic property for nano-structured gold. For this investigation, we used pulsed laser deposition to prepare nano-structured gold of different sizes and concentrations to investigate the magnetic properties. Our experiment results confirmed that for the samples in which Au is in the metallic state as nanoparticles with ~5 nm diameter, as well as inthe alloy form, bonded with indium, the samples show ferromagnetism when embedded in an Al2O3 matrix without any thiol-containing organic capping. Our results suggest that ferromagnetism is an intrinsic property of Au nano-structures, which means that it is not necessary to incorporate Au-S bonds with organic coatings in order to observe this phenomenon. We believe due to the significant broken symmetry at the surface of the nanoparticles, holes are generated in d bands of the surface Au atoms. These holes are most possibly responsible for ferromagnetism in Au nanoparticles. The realization of magnetism in Au coupled with the lack of clear understanding of its origin makes the investigation of magnetism of diamagnetic metals ripe for further inquiry.
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    Magnetoelectric (ME) composites and functional devices based on ME effect
    Gao, Junqi (Virginia Tech, 2013-06-03)
    Magnetoelectric (ME) effect, a cross-coupling effect between magnetic and electric orders, has stimulated lots of investigations due to the potential for applications as multifunctional devices. In this thesis, I have investigated and optimized the ME effect in Metglas/piezo-fibers ME composites with a multi-push pull configuration. Moreover, I have also proposed several devices based on such composites. In this thesis, several methods for ME composites optimization have been investigated. (i)  the ME coefficients can be enhanced greatly by using single crystal fibers with high piezoelectric properties; (ii) the influence of volume ratio between Metglas and piezo-fibers on ME coefficients has been studied both experimentally and theoretically. Modulating the volume ratio can increase the ME coefficient greatly; and (iii) the annealing process can change the properties of Metglas, which can enhance the ME response as well. Moreover, one differential structure for ME composites has been proposed, which can reject the external vibration noise by a factor of 10 to 20 dB. This differential structure may allow for practical applications of such sensors in real-world environments. Based on optimized ME composites, two types of AC magnetic sensor have been developed. The objective is to develop one alternative type of magnetic sensor with low noise, low cost and room-temperature operation; that makes the sensor competitive with the commercially available magnetic sensor, such as Fluxgate, GMR, SQUID, etc. Conventional passive sensors have been fully investigated, including the design of sensor working at specific frequency range, sensitivity, noise density characterization, etc. Furthermore, the extremely low frequency (< 10-3 Hz) magnetic sensor has undergone a redesign of the charge amplifier circuit. Additionally, the noise model has been established to simulate the noise density for this device which can predict the noise floor precisely. Based on theoretical noise analysis, the noise floor can be eliminated greatly. Moreover, another active magnetic senor based on nonlinear ME voltage coefficient is also developed. Such sensor is not required for external DC bias that can help the sensor for sensor arrays application. Inspired by the bio-behaviors in nature, the geomagnetic sensor is designed for sensing geomagnetic fields; it is also potentially used for positioning systems based on the geomagnetic field. In this section, some works for DC sensor optimization have been performed, including the different piezo-fibers, driving frequency and magnetic flux concentration. Meanwhile, the lock-in circuit is designed for the magnetic sensor to replace of the commercial instruments. Finally, the man-portable multi-axial geomagnetic sensor has been developed which has the highest resolution of 10 nT for DC magnetic field. Based on the geomagnetic sensor, some demonstrations have been finished, such as orientation monitor, magnetic field mapping, and geomagnetic sensing. Other devices have been also developed besides the magnetic sensor: (i) magnetic energy harvesters are developed under the resonant frequency condition. Especially, one 60 Hz magnetic harvester is designed which can harvester the magnetic energy source generated by instruments; and (ii) frequency multiplication tuned by geomagnetic field is investigated which potentially can be used for frequency multiplier or geomagnetic guidance devices.
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    Magnetoelectric Device and the Measurement Unit
    Xing, Zengping (Virginia Tech, 2009-04-10)
    Magnetic sensors are widely used in the field of mineral, navigational, automotive, medical, industrial, military, and consumer electronics. Many magnetic sensors have been developed that are generated by specific laws or phenomena: such as search-coil, fluxgate, Hall Effect, anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), magnetoelectric (ME), magnetodiode, magnetotransictor, fiber-optic, optical pump, superconducting quantum interference device (SQUID), etc. Each of these magnetic field sensors has their merits and application areas. For low power consumption (<10uW), quasi-static frequency (<10Hz) and high sensitivity (ME is the most important parameter. To enhance resonant gain in αME, I have developed a three phase laminate concept, which is based on increasing the effective mechanical factor Q while reducing the resonant frequency. A ME voltage coefficient of αME ~40V/cm.Oe has been achieved at resonance, which is about 2x higher than that of a conventional bending mode. Investigations of detection circuit optimization were also performed. Component selection strategies and a new charge topology were considered. Proper component values were required to optimize the charge detection scheme. It was also found, under some specific conditions to satisfy the circuit stability, that if the lowest required measurement frequency of the charge source was f1, then that it was not necessary to make the high corner frequency fp of the charge amplifier lower than f₁: as doing so would decrease the system's signal-to-noise ratio (SNR). A high pass, high order filter placed behind the charge amplifier was found to increase the charge sensitivity, as it narrows the intrinsic noise bandwidth and decreases the output noise contribution, while only slightly affecting the signal's output amplitude. Prototype ME unit were also constructed, and their noise level simulated by Pspice. Experimental results showed that prototypes ME unit can reach their detection limit. In addition, a new magneto-electric coupling mechanism was also found, which had a giant ME effect.
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    Magnetoelectric Effect in Ferroelectric-Ferromagnetic Heterostructures
    Wang, Zhiguang (Virginia Tech, 2014-05-28)
    The magnetoelectric (ME) effect, a coupling effect between magnetic and electric orders, has been widely investigated, both from a fundamental science perspective and an applications point of view. Magnetoelectric composites with one piezoelectric phase and one magnetostrictive phase can be magneto-electrically coupled via elastic strain mediation. Bulk magnetoelectric composites have been intensively studied as magnetic sensors due their significant magnetic-to-electric signal transforming efficiency, which promises high magnetic field sensitivity. In contrast, electric field-controlled magnetization in magnetoelectric thin films is more attractive for information recording and novel electrically-tunable microwave magnetic devices. For the present work, we prepared a series of magnetoelectric structures capable of modulating the magnetization with an electric field -- all of which display unprecedented magnetic coercive field tunability. These structures show promise for a number of applications, including magnetic memory and spintronics. First, we generated self-assembled BiFeO3-CoFe2O4 (BFO-CFO) nanostructures of varying architectural structures on differently-oriented perovskite substrates. We were able to control aspect ratio through both thickness control and by manipulating growth thermodynamics. The relationship between magnetic shape and strain anisotropy was systematically analyzed using both in-plane and out-of-plane magnetic easy axis data. The BFO-CFO self-assembled structures may be useful for applications, including longitudinal and perpendicular magnetic memory; additionally they can serve as a prototype for analyzing the magnetoelectric effect-based magnetoresistive random-access memory. BFO-CFO grown on piezoelectric Pb(Mg,Nb)O3-PbTiO3 (PMN-PT) shows a large magnetoelectric coupling coeffcient. Second, we sought to clarify the relationship between ferroelectric/ferroelastic phase transformation and the magnetoelectric effect in CFO films on PMN-PT heterostructures. Elastic strain is an essential component of electro-mechanical-magnetic coupling. Most prior studies that used piezoelectric materials as a strain source assumed that these materials shared a linear relationship (d31 or d33) with the electric field, which is true only with small electric field signals. In contrast, the largest strain is produced during phase transformation in piezoelectric single crystals. In this work, we systematically investigated electric field induced phase transformation in PMN-PT single crystals with different compositions. A signficant finding that emerged from this study is that a large in-plane uniaxial strain can be controlled by an electric field, and this strain can be used to control the magnetic easy axis distribution in the in-plane. The electric field is along the out-of-plane direction, which is perpendicular to the uniaxial strain and the surface of the sample, and thus can be easily incorporated into real device design. Finally, we identified very large magnetic coercive field tunability in the CFO/PMN-PT monolithic structures -- in fact, more than ten times larger than previously reported magnetoelectric heterostructures. We used a <011> oriented PMN-PT substrate, where a large uniaxial strain can be induced by an electric field. Importantly, since the two in-plane directions have the same dimensions, the uniaxial strain can induce a significant magnetic anisotropy distribution change in the two in-plane directions. A unprecedented magnetic coercive field change of up to 580 Oe has been observed, which shows great potential for applications in both magnetic memory and microwave magnetic devices.
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    Magnetoelectric laminated composites and devices
    Zhai, Junyi (Virginia Tech, 2009-02-10)
    Since the turn of the millennium, giant magnetoelectric (ME) effects have been found in laminated composites of piezoelectric and magnetostrictive layers. Compared to ME single phase and two phase particulate composites, laminated composites have much higher ME coefficients and are also readily fabricated. In this thesis, I have investigated ME effect in laminated composites including materials, structures, fundamental properties and devices. Giant permeability Metglas was incorporated in ME laminates. The piezomagnetic coefficient of the Metglas is larger than that of widely used magnetostrictive materials, such as Terfenol-D or nickel ferrite. The experimental results show that Metglas based ME laminates have giant ME voltage coefficients and small required DC magnetic biases. Besides, the laminates have a good directional dependence of the magnetic field: it can only sense the magnetic field along its longitudinal direction. Symmetric bimorph and differential mode magnetoelectric laminates have been designed to reject (decrease) thermal and vibration noise sources, respectively. The mechanism for the noise cancellation capability is that the laminate operates in a bending (or longitudinal) mode, whereas the noise is contained in the other mode. The ME susceptibility (αme) is the fundamental property that describes the coupling between the polarization and magnetization of a ME media. It is a complex quantity ( ). I discuss the relationship of the ME susceptibility between the magnetic permeability, dielectric permittivity of the materials, and the widely used ME voltage coefficient. The shape of the magnetic layer has a large impact on the giant permeability due to shape demagnetization effects. A long, thin and narrow shape increases the ME voltage coefficient and decreases the required optimum DC bias. The resonance frequency of Terfenol-D/PZT laminates can be continuously tuned by magnetic field over a wide range. This large tunability is due to the large magnetostriction of Terfenol-D. It results in a dramatic increase in the bandwidth over which devices might take advantage of the resonance enhanced ME coefficient. Four device applications have also been studied based on the giant ME effect of laminate composites. (i) ME laminates offer much potential for low-frequency (10⁻² to 10³ Hz) detection of minute magnetic fields (10-12Tesla or below) in a passive mode of operation. With a wrapped active coil, the Metglas/PZT laminates are also capable of detecting changes of 0.8 nano-Tesla in DC magnetic fields without an applied DC bias. (ii) A geomagnetic field sensor is shown to have high sensitivity to variations in Earth's field of HDC=0.8nano-Tesla. It could offer potential applications in global positioning. (iii) Under electro-mechanical resonance drive conditions, ME laminates have been shown to have a high gyration effect. These findings indicate the potential existence of a fifth fundamental network element. (iv) A multimodal system has been developed for simultaneously harvesting mechanical vibration and magnetic energies.
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    Magnetoelectric Laminates with Novel Properties for Sensor, Transmitter, and Gyrator Applications
    Xu, Junran (Virginia Tech, 2020-05-20)
    The magnetoelectric (ME) effect is a property that results in power/energy conversion between magnetic and electric forms. Two-phase composites consisting of magnetostrictive and piezoelectric materials have been developed that show remarkable ME voltage/charge coefficients. This extrinsic ME effect is achieved by using mechanical coupling as a medium between the magnetostrictive and piezoelectric phases. As described in this thesis, I investigated the optimization of the material properties of sensors/gradiometers, transmitters, and gyrator applications using ME heterostructures with a multi-push-pull structure. In applications, ME sensors will need to work in an open environment where there will be a mix of magnetic signals and microphonic noises. Prior research has determined that both passive and active mode ME sensors are affected by vibrational noise in the open environment. Therefore, as described herein, an ME gradiometer consisting of a pair of ME sensors working under H-field modulation (active mode) was developed to address the issue of microphonic noise. The common mode rejection ratio of my ME gradiometer was determined to be 74. Gradiometer curves were also measured, which presented the gradiometer outputs as a function of the normalized distance between the magnetic source and the ME gradiometer. Based on resulting data, the proposed ME gradiometer was confirmed to be capable of significant vibration noise rejection. However, this method is not appropriate for rejecting longitudinal vibrations due to the propagation direction being the same as the magnetic field. To resolve this dilemma, a new ME laminate structure was designed that could better reject vibrational noise. Additionally, two different configurations were developed to measure the gradiometer curve. Second, in order to understand how much energy can be wirelessly transmitted by ME laminates within a local area, a portable (area ~ 16 cm^2), a very low-frequency transmitter was developed using ME laminate with Metglas/PZT structure. The proposed strain-driven ME laminate transmitter functions as follows: (a) a piezoelectric layer is first driven by alternating current electric voltage at its electromechanical resonance (EMR) frequency; (b) subsequently, this EMR excites the magnetostrictive layers, giving rise to a magnetization change; (c) in turn, the magnetization oscillations result in oscillating magnetic fluxes, which can be detected through the use of a search coil as a receiver. The prototype measurements revealed an induction transmission capabilities in the near field. Furthermore, the developed prototype evidenced a 10^4 times higher efficiency in the near field over a small-circular loop of the same area, exhibiting its superiority over the class of traditional small antennas. Next, recent efforts in our group resulted in the development of an ME gyrator based on ME heterostructures. Such gyrators facilitate current-to-voltage conversion with high power efficiency. ME gyrators working at their resonance frequency are capable of converting power with an efficiency of > 90 %, which show potential for use in power convertors. Here, we found that the resonance frequency could be tuned through the use of a frequency-modulation technique. Accordingly, this method can be utilized to match the frequency difference between the power supply and the piezoelectric transducer in actual applications, which will increase the power efficiency. Another problematic issue is that the electromechanical coupling factor of piezoelectric transducers is limited by bandwidth. Typically, transducers cannot be impedance matched to a power supply, which significantly reduces power efficiency. Our initial studies have shown that an improved impedance match can be realized by using an ME gyrator to geometrically tune a transducer, which will substantially enhance power efficiency. The last chapter will mainly focus on ME gyrator applications. Designing linear power amplifiers that operate reliably at high frequency is quite challenging, which is mainly due to the fact that the parasitic impedances of their electronic components tend to dominate at higher frequencies, thereby leading to significant power-efficiency loss. Therefore, ME gyrator may play an important role between the power amplifier and the acoustic transducer to reduce the power loss. In this chapter, we achieved the impedance matching between a piezoelectric transducer and a power supply by implementing geometric changes to the gyrator. Both the power efficiency of an individual ME gyrator and a piezoelectric transducer are > 90%. Therefore, the total power efficiency of the ME gyrator and the piezoelectric transducer also approach > 80% when they got connected together. The second aspect of this chapter pertains to resonance-frequency tuning using three method. Since an ME gyrator will be used to achieve impedance matching, the resonance frequency of the ME gyrator and a piezoelectric transducer may not exactly match. This limitation will be overcome through capacitance tuning of the piezoelectric transducer in order to achieve frequency matching. Finally, an equivalent circuit will be developed that connects a piezoelectric transducer with a gyrator, thereby enabling the impedance of the output port of the transducer and the shifted EMR frequency of the transducer to be modified.
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    Magnetoelectric magnetic field sensor with longitudinally biased magnetostrictive layer
    (United States Patent and Trademark Office, 2006-04-04)
    A magnetoelectric magnetic field sensor has one or more laminated magnetostrictive layers and piezoelectric layers. The magnetostrictive layers are magnetized by a bias magnetic field in a longitudinal, in-plane direction. The piezoelectric layers can be poled in the longitudinal direction or perpendicular direction. The longitudinal magnetization of the magnetostrictive layers provides greatly increased sensitivity at lower bias fields compared to other magnetoelectric sensors. Perpendicular poling of the piezoelectric layers tends to provide higher sensitivity at lower detection frequency (e.g. less than 1 Hz). Longitudinal poling tends to provide higher sensitivity at high detection frequency (e.g. above 10 Hz). Also included are embodiments having relative thicknesses for the magnetostrictive layers that are optimized for sensitivity. Equations are provided for calculating the best relative thickness for the magnetostrictive layer for maximum sensitivity.
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    Magnetoelectric Oxide Nanocomposite Heterostructures
    Li, Yanxi (Virginia Tech, 2017-02-28)
    Multiferroics have attracted lots of research interest due to their potential in numerous multifunctional applications. The multiferroic materials could simultaneously exhibit two or more ferroic order parameters, and the coupling effects between ferroelectricity and ferromagnetism are named as magnetoelectric (ME) effect. Recently, with the development of thin film growth techniques, the multiferroics magnetoelectric composite heterostructures exhibit a very promising future prospects. This dissertation focused on the design, fabrication and characterization of new multiferroics magnetoelectric composite heterostructures. First, based on the specific phase architectures in BFO-CFO self-assembled thin films grown on variously oriented STO substrates and the epitaxial film growth knowledge, I designed two kinds of new film heterostructures: (i) I utilized self-assembled BFO nanopillars in a BFO-CFO two phase layer on (111) STO as a seed layer on which to deposit a secondary top BiFeO3 layer. The growth mechanism and multiferroic properties of these new heterostructures were investigated. (ii) I demonstrated the formation of a new quasi-(0-3) heterostructure by alternately growing (2-2) and (1-3) layers within the film. I proposed a new concept to overcome limitations of both the (2-2) and (1-3) phase connectivities and identified an indirect ME effect by the switching the characteristics of the piezoresponse for the new heterostructure. Second, for the option for candidates thin film materials with a high piezoelectric coefficient, which is a critical factor for ME composite films, I utilized the simple compositional BaSn0.11Ti0.89O3 bulk ceramic material as a target to grow films with the large piezoelectric properties. The grown high qualify lead-free epitaxial thin films had a chemical constituent similar to the reported giant piezoelectric ceramics near the MPB and with the QP. Both coherent and incoherent regions were observed in the interface and a larger piezoelectric coefficient d33 was achieved in this film. Finally, with respect to their characteristics and potential, I redirected from two-dimensional thin film materials to one-dimensional nanowire materials. By utilizing vertically aligned templates, I fabricated a new type of coaxial two-phase composite nanowires. Multiferroic properties of these new one-dimensional materials have been investigated. All these multiferroics magnetoelectric composite herterostructures would provide lots of potential in applications.
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    Magnetoelectric Thin Film Heterostructures and Electric Field Manipulation of Magnetization
    Zhang, Yue (Virginia Tech, 2015-06-21)
    The coupling of magnetic and electric order parameters, i.e., the magnetoelectric effect, has been widely studied for its intriguing physical principles and potentially broad industrial applications. The important interactions between ferroic orderings -- ferromagnetism, ferroelectricity and ferroelasticity -- will enable the manipulation of one order through the other in miniaturized materials, and in so doing stimulate emerging technologies such as spintronics, magnetic sensors, quantum electromagnets and information storage. By growing ferromagnetic-ferroelectric heterostructures that are able to magneto-electrically couple via interface elastic strain, the various challenges associated with the lack of single-phase multiferroic materials can be overcome and the magnetoelectric (ME) coupling effect can be substantially enhanced. Compared with magnetic field-controlled electric phenomena (i.e., the direct magnetoelectric coupling effect), the converse magnetoelectric effect (CME), whereby an electric field manipulates magnetization, is more exciting due to easier implementation and handling of electric fields or voltages. CME also affords the possibility of fabricating highly-efficient electric-write/magnetic-read memories. This study involved two avenues of inquiry: (a) exploring the strain-mediated electric field manipulation of magnetization in ferroelectric-ferromagnetic heterostructures, and (b) investigating coupling and switching behaviors at the nanoscale. Accordingly, a series of magnetoelectric heterostructures were prepared and characterized, and their electric field tunability of magnetic properties was explored by various techniques and custom-designed experiments. Firstly, the relevant properties of the individual components in the heterostructures were systematically investigated, including the piezoelectricity and ferroelectric/ferroelastic phase transformations of the ferroelectric substrates, lead magnesium niobate-lead titanate, or Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT). This investigation revealed significant information on the structure-property relationships in crystals oriented at <110>, as well as shed light on the effect of ferroelectric phase transformation on magnetoelectric coupling. This investigation of electric field controlled strain, in contrast to many prior studies, enables a more rational and detailed understanding of the magnetoelectric effect in complex ferroelectric-ferromagnetic heterostructures. The magnetoelectric thin film heterostructures were fabricated by depositing ferromagnetic iron-gallium (Fe-Ga) or cobalt ferrite (CoFe2o4 or CFO) films on top of differently-oriented ferroelectric PMN-PT substrates. Through significant electric field-induced strain in the piezoelectric substrate, the magnetic remanence and coercive field, as well as the magnetization direction of the ferromagnetic overlayer, can be substantially tuned. These goals were achieved by the interfacial strain modification of the magnetic anisotropy energy profile. The observation and analysis of the electric field tunability of magnetization and the establishment of novel controlling schemes provide valuable directions for both theoretical development and future application endeavors.
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    Method and apparatus for high voltage gain using a magnetostrictive-piezoelectric composite
    (United States Patent and Trademark Office, 2007-08-14)
    A method and apparatus attains high voltage gain by using a composite structure of an elastic section of piezoelectric layers bonded between magnetic and electric sections of magnetostrictive layers, with a harmonic magnetic field being applied along the layers at a mechanical resonance frequency of the composite structure, through coils around the laminate carrying current, such as to produce a continuity of both magnetic and electric flux lines, and achieving a high voltage output.
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    Method and Apparatus for High-Permeability Magnetostrictive/Piezo-Fiber Laminates Having Colossal, Near-Ideal Magnetoelectricity
    (United States Patent and Trademark Office, 2010-08-10)
    An ME composite laminate of at least one (1-3) piezo-fiber layer coupled with high-permeability alloy magnetostrictive layers, optionally formed of FeBSiC or equivalent. The composite laminate alternates the (1-3) piezo-fiber and high-permeability alloy magnetostrictive layers in a stacked manner. Optionally, the magnetization direction of the high-permeability alloy magnetostrictive layers and polarization direction of the piezo-fiber layer are an (L-L) arrangement. Optionally, thin film polymer layers are between the (1-3) piezo-fiber layer and high-permeability alloy magnetostrictive layers. Optionally, piezo-electric fibers within the (1-3) piezo-fiber layer are poled by inter-digitated electrodes supported by the thin film polymer, arranged as alternating symmetric longitudinally-poled “push-pull” units.
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    Multiferroic Bismuth Ferrite-Lead Titanate and Iron-Gallium Crystalline Solutions: Structure-Property Investigations
    Wang, Naigang (Virginia Tech, 2005-05-17)
    Recently, multiferroics-defined as materials with coexistence of at least two of the ferroelectric, ferroelastic and ferromagnetic effects-have attracted enormous research activities. In this thesis, the structure and properties of multiferrioic BiFeO3-x%PbTiO3 and Fe-x%Ga crystalline solutions were investigated. First, the results show that modified BiFeO3-PbTiO3 based ceramics have significantly enhanced multiferroic properties, relative to BiFeO3 single crystals. The data reveal: (i) a dramatic increase in the induced polarization; and (ii) the establishment of a remnant magnetization by a breaking of the translational invariance of a long-period cycloidal spin structure, via substituent effects. In addition, temperature dependent magnetic permeability investigations of BiFeO3-xPbTiO3 crystalline solutions have shown that aliovalent La substitution results in a significant increase in the permeability. Second, room temperature high-resolution neutron and x-ray diffraction studies have been performed on Fe-x%Ga crystals for 12
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    Phase transformations in highly electrostrictive and magnetostrictive crystals: structural heterogeneity and history dependent phase stability
    Cao, Hu (Virginia Tech, 2008-05-08)
    Ferroelectric and ferromagnetic materials have been extensively studied for potential applications in sensors, actuators and transducers. Highly electrostrictive (1-x)Pb(Mg1/3Nb2/3)-xPbTiO₃ (PMN-xPT) and highly magnetostrictive Fe-xat.%Ga are two such novel materials. Both materials systems have chemical disorders and structural inhomogeneity on a microscale, giving rise to an interesting diversity of crystal structures and novel macroscopic physical properties, which are dependent on thermal and electrical histories of the crystals. In this thesis, I have to investigated phase transformations in these two systems under thermal and field (electric/magnetic) histories, using x-ray and neutron scattering techniques. In PMN-xPT crystals, x-ray and neutron diffractions were performed along the different crystallographic orientations and for different thermal and electrical histories. Various intermediate monoclinic (M) phases that structurally “bridge” the rhombohedral (R) and tetragonal (T) ones across a morphtropic phase boundary (MPB) have been observed. Systematic investigations of (001) and (110) electric (E) field-temperature phase diagrams of PMN-xPT crystals have demonstrated that the phase stability of PMN-xPT crystals is quite fragile: depending not only on modest changes in E (≤ 0.5kV/cm), but also on the direction along which E is applied. Structurally bridging monoclinic Mc or orthorhombic (O) phases were found to be associated with the T phase, whereas the monoclinic Ma or Mb phases bridged the Cubic (C) and R ones. In addition, neutron inelastic scattering was performed on PMN-0.32PT to study the dynamic origin of the MPB. Data were obtained between 100 and 600 K under various E applied along the cubic [001] direction. The lowest frequency zone-center, transverse optic phonon was strongly damped and softened over a wide temperature range, but started to recover on cooling into the T phase at the Curie temperature (TC). Comparisons of my findings with prior ones for PMN and PMN-0.60PT suggest that the temperature dependence and energy scales of the soft mode dynamics in PMN-xPT are independent of PT concentration below the MPB, and that the MPB may be defined in composition space x when TC matches the temperature at which the soft mode frequency begins to recover. High-resolution x-ray studies then showed that the C–T phase boundary shifted to higher temperatures under E by an expected amount within the MPB region: suggesting an unusual instability within the apparently cubic phase at the MPB. In Fe-xat.%Ga alloys, the addition of Ga atoms into the b.c.c. α-Fe phase also results in diversity of crystal structures and structural inhomogeneity, which are likely the source of its unusual magneto-elastic properties. I have carefully investigated decomposition of Fe-xat.%Ga alloys subjected to different thermal treatments by x-ray and neutron diffraction for 12 < x < 25. Quenching was found to suppress the formation of a DO₃ structure in favor of a high-temperature disordered bcc (A2) one. By contrast, annealing produced a two-phase mixture of A2 + DO₃ for 14 < x < 20 and a fully DO₃ phase for x = 25. A splitting of the (2 0 0) and (0 0 2) Bragg peaks observed along the respective transverse directions indicated that Fe-xat.%Ga –crystals' are composed of several crystal grain orientations (or texture structures), which are slightly tilted with respect to each other. In order to investigate the local structural distortions and heterogeneities, neutron diffuse scattering was performed on Fe-x%Ga alloys for different thermal conditions. Diffuse scattering around a (100) superlattice reflection was found for 14 < x < 22 in the furnace-cooled condition, indicative of short-range ordered DO₃ nanoprecipitates in an A2 matrix. This diffuse intensity had an asymmetric radial contour and an off-centering. Analysis (x=19) revealed two broad peaks with c/a–1.2: indicating that the DO₃-like nanoprecipitates are not cubic, but rather of lower symmetry with a large elastic strain. The strongest diffuse scattering was observed for x=19, which correspondingly had maximum magnetostriction: indicating a structural origin for enhanced magnetostriction.
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    Structural Investigations of Highly Strictive Materials
    Yao, Jianjun (Virginia Tech, 2012-04-25)
    Ferroelectric (piezoelectric) and ferromagnetic materials have extensively permeated in modern industry. (Na1/2Bi1/2)TiO3-BaTiO3 (NBT-x%BT) single crystals and K1/2Na1/2NbO3 (KNN) textured ceramics are top environment-friendly candidates which have potential to replace the commercial lead zirconate titanate or PZT. High magnetostrictive strain (up to 400 ppm) of Fe-xat.%Ga makes this alloys promising alternatives to existing magnetostrictive materials, which commonly either contain costly rare-earth elements or have undesirable mechanical properties for device applications. These systems have common characteristics: compositional/thermal/ electrical dependent structural heterogeneity and chemical disorder on sub-micron or nano scale, resulting in diverse local structures and different physical properties. In this work, I have investigated domain and local structures of NBT-x%BT crystals, KNN ceramics and Fe-xat.%Ga alloys under various conditions, mainly by scanning probe and electron transmission techniques. In NBT-x%BT single crystals, polarized light, piezo-response force (PFM) and transmission electron (TEM) microscopies were used to study domain structures and oxygen octahedral tiltings. Hierarchical domain structures were found in NBT: a high-temperature tetragonal ferroelastic domain structure is elastically inherited into a lower temperature rhombohedral ferroelectric phase. Nanoscale domain engineering mechanism was found to still work in NBT-x%BT system and a modified phase diagram was proposed based on domain observations. An increased intensity of octahedral in-phase tilted reflections and a decrease in the anti-phase ones was observed, with increasing x as the morphotropic phase boundary (MPB) is approached. It was also found that Mn substituents favor the formation of long range ordered micro-sized ferroelectric domains and octahedral in-phase tilted regions near the MPB. Nano-size heterogeneous regions were observed within submicron domain structure, indicating that the nanoscale polarization dynamics are not confined by domain boundaries, and the high piezoelectricity of NBT-x%BT is due to a polarization dynamics with high sensitivity to electric field and a broadened relaxation time distribution. In KNN textured ceramics, an aging effect was found to exist in the orthorhombic single phase field, not only in the orthorhombic and tetragonal two-phase field as previously reported. No variation of phase structure was revealed between before and after aging states. However, pronounced changes in domain morphology were observed by both PFM and TEM: more uniform and finer domain structures were then found with aging. These changes were even more pronounced after poling the aged state. A large number of sub-micron lamellar domains within micron-domains were observed: suggesting a domain origin for improved piezoelectric properties. In Fe-xat.%Ga alloys, an underlying inhomogeneity from Ga atoms embedded into the α-Fe matrix was believed to be the origin of giant magneostrictive properties. I have systematically investigated the phase structure and nano-size heterogeneity of Fe-xat.%Ga alloys subjected to different thermal treatments using standard TEM and high resolution TEM for 10