Holographic techniques were used to study the optical properties and bistable behavior of Ga-doped cadmium fluoride. Ga impurities form bistable centers in CdF���. Illumination causes a phototransformation from a deep to shallow center, which causes a change in the index of refraction. This change is caused by a redistribution of electrons in the centers, and is known as the photorefractive effect. The photorefractive effect makes it possible to write holographic patterns into the material. Using intersecting laser beams, a holographic grating was written into the sample, and the behavior of this grating was studied. The decay and efficiency of the grating are temperature dependent. The thermal decay that is responsible for erasing the grating is a two-center process. The thermal activation energy between the centers is about 780 meV. / Graduation date: 2000

The Purpose of this thesis is to experimentally demonstrate an inversion analysis technique, developed by Dr. Jay Frankel (UTK), that utilizes transient temperature data from probes embedded at known locations in a material. This allows one to determine thermal properties (thermal diffusivity and thermal conductivity) of the material, surface temperature, and the surface heat flux as they change with time. Dr. Frankel’s inversion method can be used to determine surface temperature and heat flux of a one-dimensional semi-infinite slab based on the transient data from one or two embedded probes, if the thermal conductivity and thermal diffusivity of the material are known. Frankel’s theory suggests that the thermal properties of the material can be determined if transient data from two thermocouple (TC) probes at known locations and the heat flux at the surface are known. This thesis investigates finding the thermal properties and surface temperature of materials using a two embedded thermocouple approach. As an initial check to the inversion analysis, the theoretical temperature solution for a one-dimensional semi-infinite slab was used. This validated that the analysis could converge to the constant thermal properties for the theoretical material. An experiment was run again to provide data for the materials copper and aluminum. Using a real material is fundamentally different from using theoretical determined (analytical) data, because the thermal properties for a real material vary with temperature. Since the inversion analysis converged to a constant solution for the theoretical temperatures, it was believed that the real material will converge to a solution. However, it was seen that the thermal diffusivity for the real materials never converged to the expected value. Although, when a constant handbook value for the thermal diffusivity is used to calculate the thermal conductivities from the experimental temperature data collected from the internal probes, the inversion analysis resulted in good agreement with experiment.

M.S. / Materials Science and Engineering / The most fundamental step in the development of a predictive model for microstructure and residual stress distribution in steels is the accurate representation of the transient temperature field. Three constituents of a database of thermophysical properties, namely the thermal conductivity, volumetric specific heat capacity and convective heat transfer coefficient, were isolated and their effects quantified on the accuracy of temperature field predictions using finite element analysis (FEA). The most critical parameter in the heat transfer process was ultimately identified to be the temperature dependent convective heat transfer coefficient. It was determined using an inverse heat transfer method, which was successfully applied to accurately establish the thermal boundary conditions for an arbitrary 3D steel geometry. The temperature dependency of the volumetric specific heat capacity in the transformation range of temperatures has to be known a priori, for which a reliable model describing alloy dependent reaction kinetics has to be developed first. Thermal conductivity and its dependency on temperature have secondary effects on the accuracy of FEA predictions. The impact of the outcome of this study lies in its relevance to the heat treatment industry.

Sweetpotato (<i>Ipomoea batatas</i>) is a popular vegetable across the world. It is a staple food item of many countries in South America, Africa and Asia where the population depends on the crop as an important source of energy and essential nutrients like vitamins A and C, calcium, iron and copper. It is also a very popular crop in North America. Deep fat frying is one of the favourite processing methods for sweetpotato. The method is fast and the finished product is desired for its unique flavour and taste. <p>The main objective of this study was to establish analogy between convective heat and mass transfer during frying. The accurate estimation of the coefficients for both phenomena is challenging. During frying, the rate of heat transfer from the oil to the food surface is largely controlled by the convective heat transfer coefficient. This heat transfer coefficient is dependent on the interaction between the temperature gradient and the drying rate in a frying process. The temperature gradient and the drying rate in turn partly depend on the thermophysical properties of the product. In this study, thermophysical properties of sweetpotato were studied and modeled as a function of moisture content and temperature. The properties of interest are specific heat capacity, thermal conductivity, thermal diffusivity and density. A designed deep fat frying experiment of sweetpotato was carried out under four different oil temperatures (150, 160, 170 and 180°C) and using three different sample sizes (defined as ratio of diameter to thickness (D/L: 2.5, 3.5 and 4.0). Convective heat transfer coefficients under these frying conditions were estimated and computer simulation based on finite element modeling technique was used to determine convective mass transfer coefficients. Correlation between heat transfer coefficient and mass transfer coefficient were investigated with reliable statistical tool. Effects of sample size, oil temperature and frying time on heat and mass transfer were also studied. <p>Specific heat, thermal conductivity and thermal diffusivity of sweetpotato were all found to increase with increase in temperature and moisture content. Density decreased with increase in moisture content. Maximum heat transfer coefficient reached during sweetpotato frying was in the range of 700-850 W/m2.°C. Heat transfer coefficient of sample during frying increased with increase in frying oil temperature but decreased with increase in sample size. Same trend for heat transfer coefficient was observed for effects of oil temperature and sample size on mass transfer coefficient. Maximum mass transfer coefficient reached during sweetpotato frying was in the range of 4×10-6 to 7.2×10-6 kg/m2.s. No general relationship was established between heat transfer coefficient and mass transfer coefficient during frying but a relationship was established between maximum heat transfer coefficient and maximum mass transfer coefficient. A trend was also observed between maximum heat transfer coefficient and the corresponding mass transfer coefficient at that point.

Graduation date: 1995

Application of carbon nanotubes (CNTs) in various fields demands a thorough investigation of their stability under irradiation. Open structure, ability to reorganize and heal defects, and large surface-to-volume ratio of carbon nanotubes affect materials' radiation response so that it differs from their bulk counterparts. Despite the work conducted to this date, radiation damage and mechanisms governing the evolution of CNTs under irradiation are still deficient in fundamental understanding. This dissertation is aimed to comprehend and characterize radiation response and crystalline-to-amorphous transition in ion and electron irradiated carbon nanotubes using various techniques, including but not limited to, transmission electron microscopy (TEM) and Raman spectroscopy. It shows that ion irradiation can be used to engineer properties of nanotubes in a controllable manner and significantly improve thermal diffusivity and conductivity of the material. This work also establishes the role of nuclear and electronic stopping powers in thermal diffusivity enhancement: thermal properties of irradiated CNTs are governed by nuclear stopping power of bombarding species. The change of thermal properties with irradiation is driven by two competing mechanisms: inter-tube displacement-mediated phonon transport and defect-induced phonon scattering. In addition to experiments, molecular dynamic simulations are used to confirm validity of the obtained results. Radiation damage in CNTs at various temperatures as a function of ion energy, flux and fluence is examined. Mechanisms governing crystalline-to-amorphous transition under electron and ion irradiations are explored, applicability of previously suggested models discussed, and new models introduced. The results show enhanced defect annealing at elevated irradiation temperatures, which delays the formation of amorphous regions. Investigation of nanotube stability after various processing techniques and irradiation indicated that radiation response of CNTs in a composite is similar to that of individual nanotubes.

Sweetpotato (<i>Ipomoea batatas</i>) is a popular vegetable across the world. It is a staple food item of many countries in South America, Africa and Asia where the population depends on the crop as an important source of energy and essential nutrients like vitamins A and C, calcium, iron and copper. It is also a very popular crop in North America. Deep fat frying is one of the favourite processing methods for sweetpotato. The method is fast and the finished product is desired for its unique flavour and taste. <p>The main objective of this study was to establish analogy between convective heat and mass transfer during frying. The accurate estimation of the coefficients for both phenomena is challenging. During frying, the rate of heat transfer from the oil to the food surface is largely controlled by the convective heat transfer coefficient. This heat transfer coefficient is dependent on the interaction between the temperature gradient and the drying rate in a frying process. The temperature gradient and the drying rate in turn partly depend on the thermophysical properties of the product. In this study, thermophysical properties of sweetpotato were studied and modeled as a function of moisture content and temperature. The properties of interest are specific heat capacity, thermal conductivity, thermal diffusivity and density. A designed deep fat frying experiment of sweetpotato was carried out under four different oil temperatures (150, 160, 170 and 180°C) and using three different sample sizes (defined as ratio of diameter to thickness (D/L: 2.5, 3.5 and 4.0). Convective heat transfer coefficients under these frying conditions were estimated and computer simulation based on finite element modeling technique was used to determine convective mass transfer coefficients. Correlation between heat transfer coefficient and mass transfer coefficient were investigated with reliable statistical tool. Effects of sample size, oil temperature and frying time on heat and mass transfer were also studied. <p>Specific heat, thermal conductivity and thermal diffusivity of sweetpotato were all found to increase with increase in temperature and moisture content. Density decreased with increase in moisture content. Maximum heat transfer coefficient reached during sweetpotato frying was in the range of 700-850 W/m2.°C. Heat transfer coefficient of sample during frying increased with increase in frying oil temperature but decreased with increase in sample size. Same trend for heat transfer coefficient was observed for effects of oil temperature and sample size on mass transfer coefficient. Maximum mass transfer coefficient reached during sweetpotato frying was in the range of 4×10-6 to 7.2×10-6 kg/m2.s. No general relationship was established between heat transfer coefficient and mass transfer coefficient during frying but a relationship was established between maximum heat transfer coefficient and maximum mass transfer coefficient. A trend was also observed between maximum heat transfer coefficient and the corresponding mass transfer coefficient at that point.

The electronic material graphene has attracted much attention for its unique physical properties such as, linear band structure, high electron mobility, and room temperature ballistic conduction. The possibilities for device applications utilizing graphene show great variety, from transistors for computing to chemical sensors. Yet, there are still several basic physical properties such as thermal conductivity that need to be determined accurately. This work examines the thermal properties of graphene grown by the chemical vapor deposition technique. The thermoelectric power of graphene is studied in ambient and vacuum environments and is shown to be highly sensitive to surface charge doping. Exploiting this effect, we study the change in thermoelectric power due to introduction of gaseous species. The temperature dependent thermal conductivity of graphene is measured using a comparison method. We show that the major contribution to the thermal conductivity is the scattering of in-plane phonons. Graphene also shows promise as an optoelectronic material. We probe the Landau level structure of graphene in high magnetic fields using a differential photoconductivity technique. Using this method we observed the lifting of spin and valley degeneracies of the lowest Landau level in graphene.

The Purpose of this thesis is to experimentally demonstrate an inversion analysis technique, developed by Dr. Jay Frankel (UTK), that utilizes transient temperature data from probes embedded at known locations in a material. This allows one to determine thermal properties (thermal diffusivity and thermal conductivity) of the material, surface temperature, and the surface heat flux as they change with time. Dr. Frankel’s inversion method can be used to determine surface temperature and heat flux of a one-dimensional semi-infinite slab based on the transient data from one or two embedded probes, if the thermal conductivity and thermal diffusivity of the material are known. Frankel’s theory suggests that the thermal properties of the material can be determined if transient data from two thermocouple (TC) probes at known locations and the heat flux at the surface are known. This thesis investigates finding the thermal properties and surface temperature of materials using a two embedded thermocouple approach. As an initial check to the inversion analysis, the theoretical temperature solution for a one-dimensional semi-infinite slab was used. This validated that the analysis could converge to the constant thermal properties for the theoretical material. An experiment was run again to provide data for the materials copper and aluminum. Using a real material is fundamentally different from using theoretical determined (analytical) data, because the thermal properties for a real material vary with temperature. Since the inversion analysis converged to a constant solution for the theoretical temperatures, it was believed that the real material will converge to a solution. However, it was seen that the thermal diffusivity for the real materials never converged to the expected value. Although, when a constant handbook value for the thermal diffusivity is used to calculate the thermal conductivities from the experimental temperature data collected from the internal probes, the inversion analysis resulted in good agreement with experiment.

The structural behaviour of single shear bolted connections and double shear bolted connections of cold-formed stainless steel at elevated temperatures and post-fire condition has been investigated in this study. The current design rules on bolted connections of cold-formed stainless steel are mainly based on those of carbon steel, and are applicable for room (ambient) temperature condition only. These design rules may not be applicable for elevated temperatures. Therefore, design guidelines should be prepared for bolted connections of cold-formed stainless steel structures at elevated temperatures. The key findings of the investigation are described in the following paragraphs. A total of 25 tensile coupon tests were conducted to investigate the material deterioration of three different grades of stainless steel at elevated temperatures. The stainless steels are austenitic stainless steel EN 1.4301 (AISI 304) and EN 1.4571 (AISI 316Ti having small amount of titanium) as well as lean duplex stainless steel EN 1.4162 (AISI S32101). Totally 434 tests on bolted connections of stainless steel were performed in the temperature ranged from 22 to 950 ºC using both steady state and transient state test methods. The test results were compared with the nominal strengths calculated from the American Specification, Australian/New Zealand Standard and European codes for stainless steel structures. In calculating the nominal strengths of the connections, the material properties at elevated temperatures were used in the design equations for room temperature. It is shown that the nominal strengths predicted by these specifications are generally conservative at elevated temperatures. A total of 78 cold-formed stainless steel single shear and double shear bolted connections were tested in post-fire condition. The test results were compared with those tested at room temperature. Generally, it is found that the bolted connection strengths in post-fire condition cooling down from 350 and 650 ºC are higher than those tested at room temperature for all three grades of stainless steel. Finite element models for single shear and double shear bolted connections were developed and verified against the experimental results. Static analysis technique was used in the numerical analyses. Extensive parametric studies that included 450 specimens were performed using the verified finite element models to evaluate the bearing resistances of bolted connections of stainless steel at elevated temperatures. Design equations for bearing resistances of cold-formed stainless steel single shear and double shear bolted connections were proposed based on both the experimental and numerical results in the temperature ranged from 22 to 950 ºC. The bearing resistances of bolted connections obtained from the tests and the finite element analyses were compared with the nominal strengths calculated using the current design rules and also compared with the predicted strengths calculated using the proposed design equations. It is shown that the proposed design equations are generally more accurate and reliable in predicting the bearing resistances of bolted connections at elevated temperatures than the current design rules. The reliability of the current and proposed design rules was evaluated using reliability analysis. The proposed design equations are recommended for bolted connections assembled using cold-formed stainless steels. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy