In the current thesis, the 4-probe electrical resistance of carbon fiber-reinforced polymer (CFRP) composites is utilized as a metric for sensing low-velocity impact damage. A robust method has been developed for recovering the directionally dependent electrical resistivities using an experimental line-type 4-probe resistance method. Next, the concept of effective conducting thickness was uniquely applied in the development of a brand new point-type 4-probe method for applications with electrically anisotropic materials. An extensive experimental study was completed to characterize the 4-probe electrical resistance of CFRP specimens using both the traditional line-type and new point-type methods. Leveraging the concept of effective conducting thickness, a novel method was developed for building 4-probe electrical finite element (FE) models in COMSOL. The electrical models were validated against experimental resistance measurements and the FE models demonstrated predictive capabilities when applied to CFRP specimens with varying thickness and layup. These new models demonstrated a significant improvement in accuracy compared to previous literature and could provide a framework for future advancements in FE modeling of electrically anisotropic materials. FE models were then developed in ABAQUS for evaluating the influence of prescribed localized damage on the 4-probe resistance. Experimental data was compiled on the impact response of various CFRP laminates, and was used in the development of quasi- static FE models for predicting presence of impact-induced delamination. The simulation-based delamination predictions were then integrated into the electrical FE models for the purpose of studying the influence of realistic damage patterns on electrical resistance. When the size of the delamination damage was moderate compared to the electrode spacing, the electrical resistance increased by less than 1% due to the delamination damage. However, for a specimen with large delamination extending beyond the electrode locations, the oblique resistance increased by 30%. This result suggests that for damage sensing applications, the spacing of electrodes relative to the size of the delamination is important. Finally CT image data was used to model 3-D void distributions and the electrical response of such specimens were compared to models with no voids. As the void content increased, the electrical resistance increased non-linearly. The relationship between void content and electrical resistance was attributed to a combination of three factors: (i) size and shape, (ii) orientation, and (iii) distribution of voids. As a whole, the current thesis provides a comprehensive framework for developing predictive, resistance-based damage sensing models for CFRP laminates of various layup and thickness.

Since their first observation in 1991, carbon nanotubes (CNTs) have attracted a lot of attention owing to their exceptional properties. Their excellent electrical and thermal conducting performances combined with their high toughness and transverse flexibility allow their use in a large range of varied applications. Offering at the same time a high aspect ratio (length-to-diameter) and a low density, carbon nanotubes show strong application potential in reinforced composite materials. Unfortunately, CNTs have the strong tendency to form bundles very difficult to dissociate and disperse in a majority of polymer matrices. Without efficient CNTs dispersion, nanocomposites can not present optimal mechanical, thermal and electrical properties. To overcome this drawback, one solution consists to graft polymer chains on the carbon nanotubes surface in order to disaggregate bundles and, in few cases, to improve interaction between the polymer matrix and nanotubes. The thesis work can be divided into three parts. The first is the one-step amination of multi-walled carbon nanotubes (MWNTs) via an original microwave plasma process. The MWNTs, placed in the post-discharge chamber in presence of H2, are subjected to a reactive flow of atomic nitrogen produced by the plasma. The results give evidence for efficient covalent grafting of primary amine groups along the sidewalls of MWNTs, avoiding any structural damage and alteration of properties. The so-grafted amine groups have been further consider as initiation sites for promoting the ring opening polymerization of lactone monomers yielding polyester-grafted MWNT nanohybrids. Finally, these nanohybrids have been used as highly filled masterbatches to be dispersed in the molten state within several polymer matrices, such as polycaprolactone (PCL) and high density polyethylene (HDPE), to obtain nanocomposites with largely improved properties. For instance, electrical measurements and morphological characterizations showed that the polyester surface-grafting allows for improving the dispersion state of the nanotubes in the different polymer matrices leading to enhanced electrical properties as well as thermal and mechanical performances.

Magnetic resonance imaging (MRI) is an imaging modality that noninvasively measures magnetic fields by selectively exciting the magnetization of protons inside the body. When combined with an understanding of electromagnetic theory, MRI can be used in a novel way to provide a powerful tool for measuring the electromagnetic fields and electrical properties of biological tissues. This thesis presents the analytical, numerical, processing and experimental components of a successful implementation of Low-Frequency Current Density Impedance Imaging (LF-CDII), an impedance imaging method based on MRI measurements. The accuracy, stability and noise tolerance of this technique are examined. The first in-vivo LF-CDII experiment was conducted with a clinical MRI scanner, and the conductivity distribution of the heart of a live piglet was obtained. Both the simulation and experimental results show that LF-CDII can be used as a reliable tool for accurate noninvasive, quantitative imaging of tissue conductivities. This thesis also presents new data processing algorithms, imaging procedures and hardware development for the measurement of electromagnetic fields at radio frequencies, based on Polar Decomposition Radio Frequency Current Density Imaging (PD-RFCDI). The method was tested on both numerical models and experiments on phantoms. The results show that the techniques presented here are able to successfully image current density fields without the strict restrictions on the direction and magnitude of the currents required in previous versions of RFCDI.

In the present thesis, optical and electrical properties of Ga0.75In0.25Se layered single crystals have been studied. The optical properties of the crystals have been investigated by means of visible and infrared reflectivity and transmittance, ellipsometry, Raman spectroscopy, photoluminescence (PL) and thermoluminescence (TL) measurements. The analysis of the absorption data at room temperature revealed the existence of indirect transitions in the crystal. Moreover, the rate of change of the band gap energy with temperature was calculated from the analysis of the temperature dependence of transmission measurements. The spectroscopic ellipsometry measurements on Ga0.75In0.25Se crystals were also performed to get detailed information about the real and imaginary parts of the pseudodielectric function, pseudorefractive index and pseudoextinction coefficient. The critical point analysis of the second derivative spectra of the dielectric function was done to reveal the interband transition energies. The vibrational spectra of Ga0.75In0.25Se crystals were studied by means of infrared reflectivity and transmittance and Raman scattering. The refractive and absorption indices, the frequencies of transverse and longitudinal optical modes, high- and low-frequency dielectric constants were obtained from the analysis of the IR reflectivity spectra. PL experiments were carried out as a function of temperature and excitation laser intensity to get detailed knowledge about the recombination levels in Ga0.75In0.25Se crystals. The observed emission bands in PL spectra were interpreted as the transitions from donor levels to an acceptor level. Electrical characterization of the crystal have been performed using dark electrical conductivity, space charge limited current, photoconductivity and thermally stimulated current (TSC) measurements. The detailed information about the localized levels in the band gap has been obtained from the analysis. The photoconductivity measurements were performed to determine the dominant recombination mechanism in the crystal. Defect centers in the crystal were characterized from TSC and TL measurements accomplished in the low temperature range. The activation energies, attempt-to-escape frequencies, concentrations and capture cross sections of the traps were calculated from the analysis of the experimental data.

Magnetic resonance imaging (MRI) is an imaging modality that noninvasively measures magnetic fields by selectively exciting the magnetization of protons inside the body. When combined with an understanding of electromagnetic theory, MRI can be used in a novel way to provide a powerful tool for measuring the electromagnetic fields and electrical properties of biological tissues. This thesis presents the analytical, numerical, processing and experimental components of a successful implementation of Low-Frequency Current Density Impedance Imaging (LF-CDII), an impedance imaging method based on MRI measurements. The accuracy, stability and noise tolerance of this technique are examined. The first in-vivo LF-CDII experiment was conducted with a clinical MRI scanner, and the conductivity distribution of the heart of a live piglet was obtained. Both the simulation and experimental results show that LF-CDII can be used as a reliable tool for accurate noninvasive, quantitative imaging of tissue conductivities. This thesis also presents new data processing algorithms, imaging procedures and hardware development for the measurement of electromagnetic fields at radio frequencies, based on Polar Decomposition Radio Frequency Current Density Imaging (PD-RFCDI). The method was tested on both numerical models and experiments on phantoms. The results show that the techniques presented here are able to successfully image current density fields without the strict restrictions on the direction and magnitude of the currents required in previous versions of RFCDI.

Cadmium sulfide (CdS) thin films are very attractive materials due to their tunable optical properties and potential applications in not only photovoltaic devices but also in electronics, bio-labeling and fluorescence imaging. Recently, there is a great interest in flexible photovoltaic devices due to their unique properties such as very low weight, mechanical durability and large area applications. Organic semiconductors and their heterojunctions with inorganic materials are the most promising candidates for flexible photovoltaic applications. Preparation of CdS and Polypyrrole (PPy) semiconducting thin films on flexible polyethyleneterephtalate (PET) substrates and investigation of their morphological, structural, optical and electrical properties are the main scopes of this thesis. In the first part of the study, CdS thin films were deposited on PET via electrodeposition method. Taking the advantages of electrodeposition methods, CdS thin films with good optical and electrical properties were produced. CdS thin films were also deposited on soda-lime glass substrates in order to observe substrate effect. Scanning electron microscopy equipped with energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD) and UV-vis spectrometry have been used to determine the structural and optical properties of the films deposited at various temperatures and for different deposition time intervals. For all samples, molecularly homogenous CdS films have been observed with atomic percent ratios of the Cd to S very close to 1:1. Thin films showed (002) hexagonal crystal structure around 26 (2) with average grain size 7.0 nm. CdS films have high transmittance for the wavelength greater than 500nm. Band gap energies of the films, which range between 2.74 and 2.68 eV, decreased with increasing both deposition temperature and time. For further characterization, photoelectrochemical performances and electrochemical impedance spectroscopy (EIS) of both as deposited and CuCl2 treated CdS thin films have been investigated. Later, following to the deposition of individual CdS thin films, polypyrrole thin films were produced and then heterojunctions of polypyrrole with CdS were examined. It has been observed that cadmium sulfide enhanced the photoelectrochemical properties of the polypyrrole film. Influence of the polypyrrole thin film deposition time on the photoelectrochemical properties has been also investigated in this study. Frequency dependent measurements revealed that type of charge carrier changes as a function of polypyrrole deposition time.

Electrowetting is a phenomenon that controls the wettability of liquids on solid surfaces by the application of electric potential. It is an interesting method to handle tiny amounts of liquid on solid surfaces. In recent times, researchers have been investigating this phenomenon and have reported some unexplained behavior and degradation in the Electrowetting system performance. Electrowetting systems include the presence of electric field and different materials from metals to dielectrics and electrolytes that create an environment in which corrosion processes play a very important role. With the small dimensions of the electrodes, corrosion can cause failure quickly when the dielectric fails. In this work, commonly used dielectric films such as silicon dioxide and silicon nitride were deposited using Plasma Enhanced Chemical Vapor Deposition and characterized on the basis of thickness uniformity, etch rate measurements, Dry current – voltage measurements and Wet current – voltage measurements. Sputtered silicon dioxide films were also characterized using the same methods. The correlation between Dry I – V and Wet I – V measurements was studied and a comparison of dielectric quality of films based on these measurements is presented. Also, impact of different liquids on the dielectric quality of films was studied.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第18982号 / 工博第4024号 / 新制||工||1620 / 31933 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 酒井 明, 教授 邑瀬 邦明 / 学位規則第4条第1項該当

Titano dioksidas, pasižymintis unikaliomis fizikinėmis bei cheminėmis savybėmis, yra gerai žinoma, plačiai naudojama bei tyrinėjama medžiaga. Tiriant TiO2 elektrines savybes, galima gauti informacijos, kokios naujos medžiagos fazės formuojasi plonasluoksnėse dangose. Elektrinė titano dioksido varža priklauso nuo bandinio stechiometriškumo, nusakančio vyraujančius defektus bei laidumo tipą. Siekiant gauti norimų savybių plonasluoksnes dangas, būtina atsižvelgti į bandinių paruošimo ir gamybos procesus. Bandinių nusodinimui naudojama vakuuminė užnešimo technika leidžia kontroliuoti procesus, lemiančius dangos savybes. Šiame darbe buvo atliekama literatūros analizė apie elektrines TiO2 savybes bei įvairių faktorių įtaką šio parametro kitimui. Taip pat atkreiptas dėmesys į plonasluoksnių struktūrų nusodinimą, naudojant fizikinį dangų nusodinimą iš garų fazės (PVD). Paviršinės TiO2 varžos tyrimas buvo atliekamas panaudojant 4 zondų metodą. / Titanium dioxide is well known, widely used and investigated material that shows unique physical and chemical properties. Investigations of TiO2 electrical conductivity gives an information about the new phases formed in thin films. Surface resistance of titanium dioxide depends on sample‘s stoichiometry and it shows predominant defects and type of conductivity being in the thin film structure. It is very important to pay an attention into processes of sample‘s preparation and formation technique in order to get thin films with desirable properties. Thin films were formed using vacuum deposition technique. There has been done a scientific literature survey in order to know the main factors that causes changes of TiO2 electrical properties in this work. An attention was paid into deposition of thin film structures using physical vapor deposition (PVD) technique as well. Measurements of TiO2 surface resistance were done using four probe method.

This thesis is about the growth of ZnO thin films for optoelectronic applications. ZnO thin films were grown using plasma assisted molecular beam epitaxy and were studied using various conventional and novel characterisation techniques. The significance of different growth variables on growth efficiency was investigated. The growth rate of ZnO films was found to be linearly dependent on the Zn flux under O-rich growth conditions. Under Zn-rich conditions, the growth rate was dependent on both atomic and molecular oxygen flux. By characterising the oxygen plasma generated using different RF power and aperture plate designs and correlating the results with the growth rates observed, it was found that atomic oxygen was the dominant growth species under all conditions. Molecular oxygen also participated in the growth process, with its importance dependent on the aperture plate design. In addition, an increase in growth temperature was found to monotonically decrease the growth rate. A growth rate of 1.4 Å/s was achieved at a growth temperature of 650 ℃ by using an oxygen flow rate of 1.6 standard cubic centimetres utilising a plasma source with a 276 hole plate operating at 400 W, and a Zn flux 1.4✕10¹⁵ atoms/cm²⋅s. Characterisation of the MBE grown thin films revealed that the qualities of ZnO thin films were dependent on the growth conditions. Experimental evidence suggested that a maximum adatom diffusion rate can be achieved under Zn-rich conditions, giving samples with the best structural quality. O-rich conditions in general led to statistical roughening which resulted in rough and irregular film surfaces. Experimental results also suggested that by increasing the atomic oxygen content and decreasing the ion content of the plasma, the excitonic emission of the ZnO thin films can possibly be improved. It was also found that the conductivity of the films can possibly be reduced by increasing the plasma ion content. By investigating the evolution of the buffer layer surface during the early stages of growth, dislocation nucleation and surface roughening were found to be important strain relief mechanisms in MBE grown ZnO thin films that affected the crystal quality. The usage of LT-buffer layers was found to improve substrate wetting, and was shown to significantly reduce dislocation propagation. Further strain reduction was achieved via the application of a 1 nm MgO buffer layer, and a significant reduction of carrier concentration and improvement in optical quality was subsequently observed. A carrier concentration of <1✕10¹⁶ cm⁻³ and a near band emission full width half maximum of 2 meV was observed for the best sample. The study of electrical characteristics using the variable magnetic field Hall effect confirmed the existence of a degenerate carrier and a bulk carrier in most MBE grown ZnO thin films. The bulk carrier mobility was measured to be ~120 - 150 cm²/Vs for most as-grown samples, comparable to the best reported value. A typical bulk carrier concentration of ~1✕10¹⁶ - 1✕10¹⁸ cm⁻³ was observed for as-grown samples. Annealing was found to increase the mobility of the bulk carrier to ~120 - 225 cm²/Vs and decrease the bulk carrier concentration by two orders of magnitude. Using time resolved photoluminescence, it was found that the radiative recombination in MBE grown ZnO thin films was dominated by excitonic processes, and followed a T³⁄² trend with temperature. A maximum radiative lifetime of 10 ns was observed for as-grown samples. The non-radiative lifetime in ZnO thin films was dominated by the Shockley-Read-Hall recombination processes. The modelling of the temperature dependence of the non-radiative lifetime suggested that an electron trap at ~0.065 eV and a hole trap at ~0.1 eV may be present in these samples. The application of time resolved photoluminescence also allowed the direct observation of carrier freeze-out in these ZnO films at low temperature.