Fe-based amorphous and nanocrystalline soft magnetic alloys have been intensively researched for many applications, including high efficiency electrical distribution and power conditioning applications. New amorphous foils with a saturation magnetization of 1.63T and new nanocrystalline foils with saturation up to 1.9T have been developed. Often these new materials are not drop-in replacements for the conventional competing alloys and require significant changes in processing to be able to fabricate commercial devices.

Vertically aligned BaTiO3 nanowire (NW) arrays on a Ti substrate were adopted for use in piezoelectric energy harvesting device that scavenges electricity from mechanical energy. BaTiO3 NWs were simultaneously grown at the top and bottom surfaces of a Ti substrate by two-step hydrothermal process. To characterized the piezoelectric output performance of the individual NW, we transferred a BaTiO3 single NW that was selected from well-aligned NW arrays onto a flexible substrate and measured the electric signals during the bending/unbending motions. For fabricating a piezoelectric energy harvester (PEH), both NW arrays were sandwiched between two transparent indium tin oxide (ITO)-coated polyethylene terephthalate (PET) plastic films and then packaged with polydimethylsiloxane (PDMS) elastomer. A lead-free BaTiO3 NW array-based PEH produced an output voltage of about 90 V and a maximum current of 1.2 μA under periodically bending motions.

Piezoelectric semiconductors (PS) nanofibers, which simultaneously exhibit piezoelectricity and unique electric conductive behavior, have huge applications in sensors, energy harvesters, and piezoelectric field effect transistors. Electromechanical fields and charge carrier in PS nanofibers can be effectively controlled by a mechanical force. One-dimensional linear equations for PS nanofibers, which are suitable for small axial force and small electron concentration perturbation, are presented. Analytical expressions for the electromechanical fields and electron concentration in the fiber are obtained. Numerical results show that the electromechanical fields near the two ends are sensitive to the initial electron concentration and the applied axial force.

The ordering of polarization field of inhomogeneous ferroelectric systems were investigated. We found that these systems exhibit rather complex polarization ordering behaviors with the coexistence of polar and toroidal ordering, and particularly, a novel and tunable polar-toroidal phase transformation under external mechanical, electrical or thermal fields. Accompanying with this polar-toroidal phase transformation, there is a large change of polarization and strain. As a result, large eletromechanical and thermomechanical performance can be achieved in these systems. The polar/toroidal phase boundaries can be regarded a new kind of morphotropic phase boundary (MPB). The polar-toroidal phase transformation in nanoscale ferroelectric systems should provide us a novel strategy to develop energy conversion nanodevices.

Molecular dynamics (MD) simulations are carried out to investigate the interfacial sliding dynamics of a ZnO piezoelectric nanogenerator in an atomic force microscope (AFM) setting. The molecular system includes a vertically aligned ZnO nanowire along the [0001] direction and a Pt (111) metal tip sliding over it. We calculate the piezoelectric potential distributions based on the equilibrium molecular configurations using classical ionic charges. Simulation results reveal the very detailed evolution changes of the piezopotential within the nanowire, which are largely contributed from the different internal tensile or compressive strains induced by mechanical bending of the ZnO nanowire. Variations of the normal contact and lateral frictional forces versus the sliding distance of the Pt metal tip are also presented.

Research of electrostrictive polymers has generated new opportunities for harvesting energy from the surrounding environment and converting it into usable electrical energy. Electroactive polymer (EAP) research is one of the new opportunities for harvesting energy from the natural environment and converting it into usable electrical energy. Piezoelectric ceramic based energy harvesting devices tend to be unsuitable for low-frequency mechanical excitations such as human movement. Organic polymers are typically softer and more flexible therefore translated electrical energy output is considerably higher under the same mechanical force. In addition, cantilever geometry is one of the most used structures in piezoelectric energy harvesters, especially for mechanical energy harvesting from vibrations. In order to further lower the resonance frequency of the cantilever microstructure, a proof mass can be attached to the free end of the cantilever. Mechanical analysis of an experimental bimorph structure was provided and led to key design rules for post-processing steps to control the performance of the energy harvester. In this work, methods of materials processing and the mechanical to electrical conversion of vibrational energy into usable energy were investigated. Materials such as polyvinyledenedifluoridetetra-fluoroethylene P(VDF-TrFE) copolymer films (1um thick or less) were evaluated and presented a large relative permittivity and greater piezoelectric β-phase without stretching. Further investigations will be used to identify suitable micro-electromechanical systems (MEMs) structures given specific types of low-frequency mechanical excitations (10-100Hz).

LaFe13−xSix compounds display a giant magnetocaloric effect near 200 K. The insertion of light elements (H, C) is used to improve the Curie temperature near ambient temperature for magnetic refrigeration applications. We have developed a synthesis method with a short annealing treatment compared to classical melting techniques. The parent intermetallic alloys were synthesized by high energy ball milling. The insertion of H atoms was carried out using a Sievert apparatus and the carbon atom was inserted by solid/solid reaction. Moreover, structural and magnetic results were carried out by neutron diffraction and Mössbauer spectrometry for H content (y = 0.7,1.5) and C content (y = 0.7). The cell parameter and the Fe magnetic moments versus temperature are determined. The misunderstanding on interstitial site is clarified. The magnetovolume effect on the Curie temperature is explained by combination of the structural and magnetic properties. The advantages and drawbacks of each type of element insertion are discussed.

Magnetocaloric materials have gained significant interest as an environment friendly and efficient refrigeration technology. We have been working on Heusler alloys, primarily the Ni-Mn-Ga system for improved magnetocaloric effect (MCE). We have observed significant MCE increase in stress assisted thermally cycled samples and demonstrated that one primary mechanism is texture change. More recently, we have observed volumetric decrease as well as anisotropy changes due to stressed cycling. The influence of these parameters on magnetic anisotropy and MCE are discussed. We have also utilized isoelectronic Al substitution at the Ga lattice sites for optimizing atomic distances and exchange interactions in an effort to bring about magnetostructural transformation closer to room temperature (RT) while retaining high MCE. The results show that Al substitutions can bring about large decreases (50 – 70K) in both the Curie (TC) and martensite start (Ms) temperatures, and permit fine tuning the magnetostructural transformation temperature.

A Ga0.8Fe1.2O3 epitaxial thin film was fabricated on a SrTiO3(111) substrate using pulsed laser deposition. The film is c-axis-oriented and has multiple in-plane domains. In-plane magnetization measurements show that it exhibits ferrimagnetic behavior with a Curie temperature (TC) of 290 K. The insulating film exhibits hopping conduction with a resistivity (ρ) of 4 × 105 Ωcm at 300 K. The ρ value is four orders lower than that of a BiFeO3 film, probably owing to the formation of multiple in-plane domains in the Ga0.8Fe1.2O3 film. Positive magnetoresistance with a maximum value of 3.5% near TC was observed, suggesting that antiferromagnetic interaction between Fe3+ ions decreases carrier transfer between the ions.

Magnetic core-shell nanoparticles have the potential for numerous applications, such as in magnetic recording media, magnetic resonance imaging, drug delivery or hyperthermia, and spin valves. Inverse core-shell nanoparticles, comprised of an antiferromagnetic (AFM) core covered by a ferromagnetic (FM) or ferrimagnetic (FiM) shell, are of current interest due to the tunability of their magnetic properties. NiO is typically antiferromagnetic in nature and has a Néel temperature of 523 K. Our primary objective in this project is to synthesize and characterize inverted core-shell nanoparticles (CSNs) comprised of a NiO (AFM) core and a shell consisting of a NixMn1-xO (FM/FiM) compound. The synthesis of the CSNs was made using a two-step process. The NiO nanoparticles were synthesized using a chemical reaction method. Subsequently, the NiO nanoparticles were used to grow the NiO@NixMn1-xO CSNs using our hydrothermal nano-phase epitaxy method. XRD structural characterization shows that the NiO@NixMn1-xO CSNs have the rock salt cubic crystal structure throughout. SEM-EDS data indicates the presence of Mn in the CSNs. SQUID magnetic measurements show that the CSNs exhibit AFM/FM or AFM/FiM characteristics with a coercivity field of 425 Oe at 5 K. The field cooled vs zero field cooled hysteresis loop measurements show a significant exchange bias effect between the AFM NiO core and FM/FiM NixMn1-xO shell of the CSNs. The results of additional TEM and magnetic characterization are discussed.