Dissertations / Theses on the topic 'Values of thermally stimulated current'

Thermally stimulated current (TSC) spectroscopy has been used, in these reported investigations, as an innovative experimental technique for the detection and characterisation of secondary relaxation phenomena in the crystalline state for a number of low-molecular weight organic materials (LOMs), namely α-alanine, β-alanine, a co-crystal system (salicylic acid and benzamide) and the drug paracetamol (form I). The aim of the research was to significantly improve the contemporary understanding of secondary relaxation phenomena in crystalline LOMs; an area of science that is of significant inter-disciplinary interest throughout academia and industry. In all the materials examined, secondary relaxations were detected and characterised by TSC; in order to achieve the foregoing different experimental methodologies (e.g. thermal windowing) and data processing techniques (e.g. relaxation map analysis) were employed. Of significant prevalence throughout is the phenomenon of compensation behaviour; in both the thermodynamic (i.e. enthalpy and entropy compensation) and in the kinetic (i.e. pre-exponent and activation energy (of the Arrhenius equation)) sense. Derived compensation points were linked to real physical processes; identifying a relationship between secondary relaxations and primary transitions. It is proposed, herein, that secondary relaxations have a pre-requisite relationship with primary transitions. Secondary relaxations are preparatory motions, essential for primary transitions to occur. In addition, throughout this thesis, additional information on topics such as co-crystal synthesis, chirality, degradation, the nature of secondary/primary relaxations, the nature of primary transitions and physical properties of LOMs, are examined in relation to the LOMs under investigation. TSC has proven to be an effective, and perhaps unique, instrumental technique for the detection and characterisation of secondary relaxations in LOMs.

In this thesis, in order to study the structural, optical and electrical transport properties of Tl2In2S3Se, TlInSeS and Tl2In2SSe3 crystals, X-ray diffraction (XRD), energy dispersive spectroscopic analysis (EDSA), transmission, reflection, photoluminescence (PL), thermally stimulated current (TSC) and photoconductivity decay (PC) measurements were carried out. Lattice parameters and atomic composition of these crystals were determined from XRD and EDSA experiments, respectively. By the help of transmission and reflection experiments, the room temperature absorption data were analyzed and it was revealed the coexistence of indirect and direct band gap energies of the studied crystals. Moreover, the refractive index dispersion parameters - oscillator energies, dispersion energies, oscillator strengths, oscillator wavelengths and zero-frequency refractive indexes were determined. Temperature-dependent transmission measurements made it possible to find the rate of change of indirect band gaps with temperature, absolute zero values of the band gap energies and Debye temperatures of these crystals. From the analysis of the transmission and reflection measurements, it was established that, there is a decrease in the values of indirect and direct band gaps energies and an increase in zero-frequency refractive indexes with increasing of selenium content. PL measurements were carried out to obtain the detailed information about recombination levels in crystals studied. The behavior of PL spectra were investigated as a function of laser excitation intensity and temperature. The variation of the spectra with laser excitation intensity and temperature suggested that the observed emission bands in these crystals were due to the donor-acceptor pair recombination. TSC measurements were carried out with various heating rates at different illumination temperatures to obtain information about trap levels in these crystals. The mean activation energies, attempt-to-escape frequencies, concentrations and capture cross sections of the traps were determined as a result of TSC spectra analysis. The analysis of experimental TSC curves registered at different light illumination temperatures revealed the exponential trap distribution in the studied crystals. From the analysis of PC measurements, carrier lifetimes were obtained.

碩士<br>國立交通大學<br>光電系統研究所<br>102<br>Thermally stimulated depolarization current technique is a fundamental method for the investigation of microscopic dielectric characteristics of dielectric materials. In this technique, when a system is applied with a DC voltage source, the energy can be stored in the forms of molecular dipoles, free charges, bonds and reorientation of polar fragments. Normally, as the field being released, the stored energy dissipates, and the system relaxes to its original state. With proper treatment, one can release the energy in the form of electrical current and extract useful information. Based on the measurement of the TSDC, in this study, the mechanism of obtaining phase transition temperatures of liquid crystals is presented. The behavior of the TSDC signal from a typical nematic cell is investigated with pre-applied voltages, polarization time and heating rate as variables.

博士<br>國立交通大學<br>應用化學研究所<br>85<br>Thermally stimulated current and relaxation map analysis (TSC/RMA) are used to study the molecular dynamics of main-chain and side-chain liquid crystalline(LC) polymers. In the frist series of side-chain liquid crystalline copolysil-oxanes, the dual glass transitions as well as effect of spacer length, polymer backbone and lateral substitution on the molecular dynamics, are investigated byTSC/RMA. The results demonstrate that a longer spacers, a more flexible polymerbackbone and a smaller lateral subsitituent will lead easier moving of the sidegroup molecules. In the second series of ferroelectric side-chain LC polyoxetanes , the glass transition, side-chain motions as well as ferroelectric propertiesof the obtained polymers are studied by TSC/RMA. The results demonstrate that ferroelectric mode, i.e. Goldstone mode, can not be observed in the TSC measure-ment. TSC/RMA are used to study the third and forth series of side-chain LC poly-methacrylates and polyacrylates. All of the LC polymethacrylates and polyacrylatesreveal a glass transition and a depolarization peak which is attributed to the motions of the longitudinal components of the dipole moment of the longitudinalcomponents of the dipole moment of the mesogenic side groups on the TSC curves. All obtained LC polymethacrylates show a glass transition temperature lower than that of poly(methyl methacrylate) and all obtained LC polyacrylates presenta glass transition higher than that poly(methyl acrylate). The results indicatethat the side group has prominent effect on the glass transition temperature. The glass transition is generally difficult to be observed in the DSC measure-ments. The glass transition as well as molecular dynamics of some main-chain LCpolymers are investigated by TSC/RMA also. In the fifth series of LC polyesterswhich containing the 4-hydroxy-3,5-dibromobenzoic acid monomer, their glass tran-sition are observed on the TSC curves. The results demonstrate that introducingthe 4-hydroxy-3,5-dibromobenzate unit into the rigid-rod polyesters decreasesthe melting points and glass transition temperatures of the obtained copolyesters. The TSC is also used to study the glass transition of a series of LC polyestersand their block copolymers with polyether sulfone. The LC polymers which displayno glass transition on the DSC scans, however, reveal very clear glass trasition on TSC curves. The block copolymers of LC polyesters and polyether sulfone whichdisplay very broad glass transition DSC curves, show very sharp glass transitionpeaks on the TSC curves. The results indicate that TSC is a very useful techniqueto study the glass transition of the main-chain LC polymers

碩士<br>大葉大學<br>電機工程學系<br>98<br>This study investigates the optical and electrical property of semiconductor at different temperature. We use Close-Cycle system to control temperature and observe thermally stimulated current of semiconductor. We use the measurement of thermally stimulated current (TSC) method to determine the energy trap of silicon semiconductor. According to the TSC measurement of semiconductor, the sample will produce photoinduced carriers due to be exposed by light at low temperature. Then these carriers are captured by defects. Because of these carriers will be released by defect at different temperature, we can observe a maximum current in this time. This impurity center energy is about 70 meV which is agreement with the previous result. We also obtain the electron transport characteristic from the Hall effect at different temperature. This simple is P-type according to the result of our study. The activation energy is about 66 meV in the fitting of our TSC measurement at the feature temperature. We can determine the dopents in the semiconductor by the simple method of TSC measurement.

碩士<br>中原大學<br>應用物理研究所<br>94<br>In this study, by controlling the temperatures of the source powder side and of the substrate side in bath furnace, α-HgI2 polycrystalline films are prepared by physical vapor deposition. The surface morphology and the degree of crystallinity of the as-grown α-HgI2 polycrystalline films are characterized by scanning electron microscopy and X-ray diffraction, respectively. For electrical properties, resistivity and dielectric constant measurements by Precision Component Analyzer. Furthermore, thermally stimulated current measurements are performed to reveal the characters of deep-trapping levels.

碩士<br>中原大學<br>應用物理研究所<br>93<br>In this study, by controlling the temperatures of the source powder side and of the substrate side, α-HgI2 polycrystalline films are prepared by physical vapor deposition (PVD). The surface morphology and the degree of crystallinity of the as-grown α-HgI2 polycrystalline films are characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), Respectively. For electrical properties, thermally stimulated current (TSC) measurements are performed to reveal the characters of deep-trapping levels. Furthermore, the temperature dependence of band gap deduced from photoconductivity (PC) spectra is measured, and the persistent photoconductivity (PPC) decay is studied.

博士<br>國立臺灣科技大學<br>化學工程系<br>87<br>As engineering plastics, epoxy resins possess unique and excellent material properties such as high modulus and high tensile strength. In addition, it has been well known for its dimensional stability, thermal and weatherability. Epoxy resins are particularly known for the adhesion toward metals. It has a very flexible processability. Therefore, epoxy resins are used rather extensively in aerospace, electronic composite, printed circuit, semiconductor molding compounds, adhesives transportation construction among others. However, there is also in need for improvements such as on the highest application temperature and the nonflammibility. The current researches are focused on these aspects. Regarding the problem on the plastic nonflammibility, there are two main research approach: structural modification and the nonflammable additivity. Most research as on plastics are concentrated in the structural modification and little on the inorganic additives. The addition of the brominated epoxy resin as the nonflammabile material has been well known in epoxy application. The addition of DER 542, a brominated epoxy resin which however, emits toxic bromine and brominated volatiles upon ignition in case of the fire. Upon taking these emission in case of fire in consideration, our research interests lie on the phosphonated or phosphated modification. Phosphorus-containing plastics are known as one of the potential candidate as the nonflammable materials. It does not emit toxic volatiles upon the thermal degradation. Instead it forms phosphate film and acts as a heat shield that prevents further degradation. The use of the phosphorus-containing polymeric materials are limited at prevent time, mainly due to its infant stage in the development on the synthesis of the phosphorus-polymers. Our research works involves two stages of phosphorus-containing polymeric materials: epoxy resins and polyurethane elastomers. Characteristics of phosphonated and/or phosphated epoxy resins: 1. Processability: A better and lower Tg (the minimum process temperature) was resulted from the addition of phosphonate and/or tribranched phosphated epoxy resins. The result was indicated in our thermal characterization by DSC and TSC thermal analysis. 2. Curability: Curing kinetic study indicated that the shelf-life has been extended and yet the curing process temperature was narrowed with faster curing time. 3. Nonflammability A thermal tranformation of organsphosphorus to the inorganic salt as heat shield was occurred at the lower temperature that provides the nonflammability. The effect of phosphorus was limited as the heat shield and no toxic volatiles were detected. 4. LOI value Calculated LOI value of phosphorus-containing specimens from the residue of the TGA at 850°C indicated the nonflammability in the values above 26 while the unmodified epoxy resin was found to be 20 below the level indicating the nonflammability in the material. 5. TSC/RMA study on the morphology The recent innovated TSC/RMA possesses a high sensitivity, which can be used to study the morphology in terms of molecular motion/relaxation. The result on the phosphorus-containing epoxy specimens revealed that tribranched phosphated resin has better homogeneity than linear phosphonated resin. 6. Thermally degradative volatiles of the phosphorus-modified materials DP-GC/MS data indicated that phenol was the main degradative volatile regardless the specimens whether or not phosphorus-atom was presented. Phenol was the thermally degradative product from aromatic moiety. Results of this study can be summarized that phosphorus-modified epoxy resins and/or polyurethanes improve nonflammability with the ease of processability. Therefore, phosphonated, particularly phosphated epoxy resins, and/or polyurethanes, may be considered as the suitable candidates for the R&D works in further which may develop as the new engineering plastics.