The overall goals of the present PhD research are to explore the giant magnetoimpedance (GMI) and giant magnetocaloric (GMC) effects in functional magnetic materials and provide guidance on the optimization of the material properties for use in advanced magnetic sensor and refrigeration applications. GMI has attracted growing interest due to its promising applications in high-performance magnetic sensors. Research in this field is focused on the development of new materials with properties appropriate for practical GMI sensor applications. In this project, we have successfully set up a new magneto-impedance measurement system in the Functional Materials Laboratory at USF. We have established, for the first time, the correlation between sample surface, magnetic softness, critical length, and GMI in Co-based amorphous ribbon materials, which provide a good handle on selecting the suitable operating frequency range of magnetic materials for GMI-based field sensor applications. The impact of field-induced magnetic anisotropy on the GMI effect in Co-based nanocrystalline ribbon materials has also been investigated, providing an important understanding of the correlation between the microstructure, magnetic anisotropy, and GMI in these materials. We have shown that coating a thin layer of magnetic metal on the surface of a magnetic ribbon can reduce stray fields due to surface irregularities and enhance the magnetic flux paths closure of the bilayer structure, both of which, in effect, increase the GMI and its field sensitivity. This finding provides a new way for tailoring GMI in surface-modified soft ferromagnetic ribbons for use in highly sensitive magnetic sensors. We have also introduced the new concepts of incorporating GMI technology with superparamagnetic nanopthesiss for biosensing applications and with carbon nanotubes for gas and chemical sensing applications. GMC forms the basis for developing advanced magnetic refrigeration technology and research in this field is of topical interest. In this project, we have systematically studied the ferromagnetism and magnetocaloric effect in Eu8Ga16Ge30 clathrate materials, which are better known for their thermoelectric applications. We have discovered the GMC effect in the type-VIII clathrate and enhanced refrigerant capacity in the type-I clathrate. We have successfully used the clathrates as excellent host matrices to produce novel Eu8Ga16Ge30-EuO composite materials with desirable properties for active magnetic refrigeration technologies. A large refrigerant capacity of 794 J/kg for a field change of 5 T over a temperature interval of 70 K has been achieved in the Eu8Ga16Ge30-EuO composite with a 40%-60% weight ratio. This is the largest value ever achieved among existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K - 100 K. The excellent magnetocaloric properties of the Eu8Ga16Ge30-EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range.

Clathrate materials comprise compounds in which guest atoms or molecules can be encapsulated inside atomic cages formed by host framework polyhedra. The unique relationship that exists between the guest species and its host results in a wide range of physical phenomena, and offers the ability to study the physics of structure-property relationships in crystalline solids. Clathrates are actively being investigated in fields such as thermoelectrics, superconductivity, optoelectronics, and photovoltaics among others. The structural subset known as type II clathrates have been studied far less than other clathrates, and this forms the impetus for the present work. In particular, the known “composition space” of type II clathrates is small, thus the need for a better understanding of possible compositions is evident. A basic research investigation into the synthesis and characterization of silicon and germanium type II clathrates was performed using a range of synthetic, crystallographic, chemical, calorimetric, and transport measurement techniques. A series of framework substituted type II germanium clathrates has been synthesized for the first time, and transport measurements indicate that these compounds show metallic behavior. In the course of the investigation into type II germanium clathrates, a new zeolite-like framework compound with its corresponding novel crystal structure has been discovered and characterized. This compound can be described by the composition Na 1-xGe3 (0 < x < 1), and corresponds to a new binary phase in the Na-Ge system. One of the most interesting aspects of type II clathrates is the ability to create compounds in which the framework cages are partially occupied, as this offers the unique opportunity to study the material properties as a function of guest content. A series of type II sodium-silicon clathrates Na xSi136 (0 < x < 24) has been synthesized in higher purity than previously reported for as-synthesized products. The transport properties of the Na xSi136 clathrates exhibit a clear dependence on the guest content x. In particular, we present for the first time thermal conductivity measurements on Na xSi136 clathrates, and observe evidence that the guest atoms in type II clathrates affect the thermal transport in these materials. Some of the crystalline Na xSi136 compounds studied exhibit very low thermal conductivities, comparable in magnitude to amorphous materials. In addition, for the first time clear evidence from transport measurements was found that resonance phonon scattering may be present in type II clathrates, as is also the case in the type I subset.

Previously we have suggested the concept of tuning hydrate compositions which makes it possible to increase the gas storage capacity of binary hydrates. Herein, we report for the first time the existence of a critical guest concentration (CGC) and establish the complete tuning pattern that appears to exist in binary hydrates, including the water-soluble hydrate formers (promoters) and water insoluble guests,. The first attempt to verify the new features of clathrate hydrate compositions is executed on the binary hydrate of CH4 + THF and involves a detailed examination of the guest distribution by spectroscopic methods. THF molecules by themselves form sII hydrate from a completely miscible aqueous solution, and in this structure, because of their size, THF molecules occupy only the large 51264 cages. The CGC value appears to depend largely on the chemical nature of the liquid guest component participating in the binary hydrate formation. The present experimental findings on the existence of critical guest concentration and the complete tuning phenomenon can be expected to make a meaningful contribution to both inclusion chemistry and a variety of hydrate-based fields.

Clathrate hydrates can be reasonable choices for high-density hydrogen storage into compact host media, which is an essential task for hydrogen-based future society. However, conventional storage scheme where aqueous solution is frozen with hydrogen gas was impractically slow for practical use. Here we propose a much faster scheme where hydrogen gas was directly charged into hydrogen-free, crystalline hydrate powders. The storage kinetics was observed in situ by nuclear magnetic resonance (NMR) spectroscopy in a pressurized tube cell. At pressures up to 20 MPa the storage was complete within 80 minutes, as observed by growth of stored-hydrogen peak into the hydrate. Since the rate-determining step of current storage scheme is body diffusion of hydrogen within the crystalline hydrate media, we have measured the diffusion coefficient of hydrogen molecules using the pulsed field gradient NMR method. The results show that at temperatures down to 250 K the stored hydrogen is highly mobile, so that the powdered hydrate media should work well even in cold environments. Compared with more prevailing hydrogen storage media such as metal hydrides, the clathrate hydrate could offer even more advantages: It is free from hydrogen embrittlement, more chemically durable, more environmentally benign, as well as economically quite affordable.

Recently, clathrate hydrate has attracted much attention because of its energy gas enclathration phenomenon. Since energy gas such as methane, ethane, and hydrogen could be stored in solid hydrate form, clathrate hydrate research has been considerably focused on energy gas storage and transportation medium. Especially, methane hydrate, which is crystalline compound that are formed by physical interaction between water and relatively small sized guest molecules, can contain about as much as 180 volumes of gas at standard pressure and temperature condition. To utilize gas hydrate as energy storage and transportation medium, two important key features: storage capacity and storage condition must be considered. Herein, we report the inclusion phenomena of methane occurred on tert-butyl alcohol hydrate through thermodynamic measurement and spectroscopic analysis by using powder X-ray diffractometer, and 13C solidstate NMR. From spectroscopic analysis, we found the formation of sII type (cubic, Fd3m) clathrate hydrate by introducing methane gas into tert-butyl alcohol hydrate whereas tert-butyl alcohol hydrate alone does not form clathrate hydrate structure. Under equilibrium condition, pressure-lowering effect of methane + tert-butyl alcohol double hydrate was also observed. The present results give us several key features for better understanding of inclusion phenomena occurring in the complex hydrate systems and further developing methane or other gas storage and transportation technique.

In the current work we present a significant advancement in the area of hydrogen storage in clathrates: hydrogen storage from both enclathrated molecular hydrogen as well as storage from the clathrate host lattice. We have investigated the hydrogen storage potential in all of the common clathrate hydrate structures with techniques such as gas evolution, X-ray / neutron diffraction, and NMR / Raman spectroscopy. We have determined that the common clathrate structures may not suffice as H2 storage materials, although these findings will aid in the design and production of enhanced hydrogen storage materials and in the understanding of structure-stability relations of guest-host systems. In view of current storage limitations, we propose a novel chemical – clathrate hybrid hydrogen storage concept that holds great promise for future materials.

Hydrate film growth has been examined at the hydrocarbon/water interface for cyclopentane and methane hydrate. Video microscopy was used to measure hydrate film thickness, propagation rate across the hydrocarbon/water interface and gas consumption measurements characterized the hydrate formation mechanism. Cyclopentane and methane hydrate film formation were measured over the temperature range of 260–279K and pressure range of atmospheric to 8.3MPa. Hydrate formation was initiated by the propagation of a thin, porous film across the hydrocarbon/water interface. The propagation rate and thickening of the hydrate film was strongly dependent on the hydrate former solubility in the aqueous phase, in the absence and presence of hydrate. Cyclopentane hydrate film thickness began at ~12 μm and grew to a final thickness (15–40 μm) which increased with subcooling. Methane hydrate film thickness began at ~ 5 μm and grew to a final thickness (20–100 μm) which also increased with subcooling. The hydrate film grew into the water phase. Gas consumption measurements indicated that the aqueous phase supplied hydrate former during the initial hydrate growth, and the free gas supplied the hydrate former for film thickening and development. Hydrate film formation at the hydrocarbon/water interface was proposed to consist of three consecutive stages: propagation, development and bulk conversion.

Large amounts of CH4 in the form of solid hydrates are stored on continental margins and in permafrost regions. If these CH4 hydrates could be converted into CO2 hydrates, they would serve double duty as CH4 sources and CO2 storage sites. Herein, we report the swapping phenomena between global warming gas and various structures of natural gas hydrate including sI, sII, and sH through 13C solid-state nuclear magnetic resonance, and FT-Raman spectrometer. The present outcome of 85% CH4 recovery rate in sI CH4 hydrate achieved by the direct use of binary N2 + CO2 guests is quite surprising when compared with the rate of 64 % for a pure CO2 guest attained in the previous approach. The direct use of a mixture of N2 + CO2 eliminates the requirement of a CO2 separation/purification process. In addition, the simultaneously-occurring dual mechanism of CO2 sequestration and CH4 recovery is expected to provide the physicochemical background required for developing a promising large-scale approach with economic feasibility. In the case of sII and sH CH4 hydrates, we observe a spontaneous structure transition to sI during the replacement and a cage-specific distribution of guest molecules. A significant change of the lattice dimension due to structure transformation induces a relative number of small cage sites to reduce, resulting in the considerable increase of CH4 recovery rate. The mutually interactive pattern of targeted guest-cage conjugates possesses important implications on the diverse hydratebased inclusion phenomena as clearly illustrated in the swapping process between CO2 stream and complex CH4 hydrate structure.

Nonspherical large cages in structure I (sI) clathrates impose non-uniform motion of nonspherical guest molecules and anisotropic lineshapes in NMR spectra of the guest. In this work, we calculate the lineshape anisotropy of the linear CO2 molecule in large sI clathrate cages based on molecular dynamics simulations of this inclusion compound. The methodology is general and does not depend on the temperature and type of inclusion compound or guest species studied. The nonspherical shape of the sI clathrate hydrate large cages leads to preferential alignment of linear CO2 molecules in directions parallel to the two hexagonal faces of the cages. The angular distribution of the CO2 guests in terms of a polar angle θ and azimuth angle  and small amplitude vibrational motions in the large cage are characterized by molecular dynamics simulations at different temperatures in the stability range of the CO2 sI clathrate. These distributions are used to calculate the NMR powder spectrum of CO2 at different temperatures. The experimental 13C NMR lineshapes of CO2 guests in the large cages show a reversal of the skew between the low temperature (77 K) and the high temperature (238 K) limits of the stability of the clathrate. Good agreement between experimental lineshapes and calculated lineshapes is obtained. No assumptions regarding the nature of the guest motions in the cages are required.

The hydrogen absorption behavior of organic-compound clathrate hydrates was investigated using five kinds of organic compounds as well as tetrahydrofuran (THF). These hydrates were pressurized by hydrogen, and Raman analysis, the determination of the amount of hydrogen and calorimetric measurement were carried out. The Raman results show that the samples investigated in this work formed binary clathrate hydrate of hydrogen and each organic compound. The organic-compound clathrate hydrates presented similar performances to that of THF clathrate hydrate regarding hydrogen absorption and heat of dissociation. These results suggested that the organic compounds investigated in this work may become alternatives to THF.