Thesis (Ph.D.)--Materials Science and Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Speyer, Robert; Committee Member: Gerhardt, Rosario; Committee Member: Liu, Meilin; Committee Member: Lynch, Christopher; Committee Member: Wilkinson, Angus

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he extent of BiInO₃ substitution in the perovskite system xBiInO(₃)-(1 - x)PbTiO₃ and the corresponding raise in the transition temperature were investigated using thermal analysis, dielectric measurements, x-ray diffraction, and electron microscopy. Maximum tetragonal perovskite distortion (c/a = 1.082) was obtained for x = 0.20, with a corresponding Curie temperature of 582°C. Phase-pure tetragonal perovskite was obtained for x less than or equal to 0.25. Compound formation after calcining mixed oxide powders resulted in agglomerated cube-shaped tetragonal perovskite particles, which could be fired to 94.7% of theoretical density (TD). Niobium-modified BIPT ceramics with PT contents of 80% and 85% were found to possess significantly lower dielectric loss at elevated temperatures, making it possible to polarize the materials. Piezoelectric properties were measured for a 1.5 mol% Nb -0.15BI-0.85PT composition with a transition temperature of 542°C; the longitudinal piezoelectric coefficient and coercive field were found to be 60 pC/N and 125 kV/cm, respectively. Compositions of xBiLaO₃-(1 − x)PbTiO₃ over the range 0 < x < 0.225 were calcined and sintered. Dielectric constant with temperature and differential scanning calorimetry measurements were in excellent agreement with respect to a Curie-like tetragonal to cubic transformations starting at 495°C for pure PbTiO₃, shifting to lower temperatures with increasing x. For compositions of x > 0.05, a second higher-temperature (∼600°C) endotherm, and matching dielectric anomaly, were consistently observed, for which there were no structural changes indicated by hot-stage x-ray diffraction. This transformation was interpreted to be similar to a Curie transformation in relaxor ferroelectrics in which localized segregation of B-site cations (below the resolution limit of x-ray diffraction) facilitated ferroelectric behavior.

For heat transfer through building insulations such as fiberglass, radiation and conduction are important modes of heat transfer. Moreover, materials like fiberglass scatter radiation in a highly anisotropic manner. The equations for heat transfer by simultaneous conduction and radiation are a coupled pair, one of which is of the nonlinear integrodifferential type. Exact solution for transient heat transfer in this case is not available, and the approximate solution available is the two-flux model. The two-flux model does not give good results for transient, combined mode heat transfer, through an absorbing, emitting, and anisotropically scattering medium. In this thesis a higher order approximate solution has been developed. It is found that this model gives appreciably better results than the two-flux model. / Master of Science

Les agro-composites font l'objet de nombreuses études et applications industrielles en raison des multiples propriétés mécaniques qu'ils présentent. Ces propriétés présentent de grandes perspectives comparées à celles des composites traditionnels. Cependant les connaissances sur le comportement mécanique de l'interface fibre/matrice restent limitées. De plus, la différence de propriété entre la fibre hydrophile et la matrice hydrophobe peut causer des défauts au niveau de l’interface. Il est donc important de caractériser finement la décohésion à l'interface au cours d'une sollicitation. Plusieurs méthodes ont été proposées dans la littérature, elles sont généralement très complexes à mettre en œuvre et sont coûteuses. Dans cette étude, nous avons développé une méthode de caractérisation en se basant sur l'essai de « pull-out ». La géométrie de la fibre a été prise en compte dans le calcul des propriétés mécaniques de l'interface par la mise à profit d'une approche inspirée de la tomographie. L'influence de la température d'élaboration sur les propriétés mécaniques de l'interface a été étudiée de manière à définir la température optimale. L'évolution des propriétés interfaciales a été suivie au cours d'un vieillissement en humidité relative. Après quatre semaines, la résistance au cisaillement et la rigidité au cisaillement de l'interface sont diminuées fortement alors que la déformation à la rupture est augmentée / Agro-composites are increasingly studied and applied to various industries over recent years due to good mechanical properties compared to conventional composites especially in terms of specific values. However, since low adhesion between the hydrophilic fiber and hydrophobic matrix, which occurs one of the main breaks modes in this kind of material, the characterization of the interface becomes a key problem. For investigation of this issue, existing methods show limitation for reasons of complexity (in preparation, in principle) and of cost. In this study, we developed a « pull-out ». In particular, the real fiber geometry of the plant fiber was taken into the calculation of mechanical properties of interface using a tomography inspired method. By checking the effective temperature of the molding then varying it, we studied the effect of this processing parameter to mechanical properties of fibre/matrix interface and determined the optimal conditions. The developed experimental protocol is applied to aged interfaces in order to clarifying the evolution of interfacial properties during the aging time to relative humidity. After four weeks, the interfacial shear strength and the shear modulus of the interface were greatly reduced while the shear deformation at the rupture was greatly increased

Thesis (M. Tech. - Engineering: Electrical, Department Electronic Engineering, Faculty of Engineering and Technology)--Vaal University of Technology. / Curing of materials in order to obtain different properties has been a practice for many years. New developments in composite materials increase the need to control certain variables during the curing process. One very significant variable is temperature. Temperature control by itself is an old practice, however when the need for repeatedly controlling the process accurately over long periods of time arises, a system is required that outperforms normal manual control. One of the aspects within such a system that needs to be considered is the ability to replicate the temperatures within an oven which were originally used for a specific material’s curing profile. This means that a curing profile would need to be defined, saved for later and finally be interpreted correctly by the controlling system. Different control methods were simulated to enable the system to control the temperature which has been defined by literature. This dissertation introduces a variation on the standard control methods and shows improved results. Switching the oven on and off in order to increase or decrease internal oven temperature seems simple, but can cause switching devices to decrease their operational life span, if not designed carefully. A combination switch was introduced which harnesses the advantages of two very common switching devices to form an improved combination switch. Software for the personal computer environment, as well as software for the embedded environment were developed and formed a control system that produced acceptable results for temperature control. Accuracies of 98% and more were achieved and found to be acceptable according to standard engineering control practices. An accurate temperature profile controller was designed, simulated and built in order to control the temperature inside a specific curing oven which, in turn, determined the curing properties of specific materials. The overall results were satisfactory which lead to achieving the objectives outlined in this dissertation.