A systematic study of the properties of ion-exchanged float glass has been carried out by a.c. impedance spectroscopy. The measurement of electrical parameters allows the ionic mobility within the surface region to be investigated separately from the bulk glass. The correlation of a.c. impedance measurements with electron probe micro analysis, infrared reflectance spectroscopy, differential scanning calorimetry and dynamic mechanical thermal analysis has lent some new insights into anomalous behaviour of ionic transport under these circumstances. A highly resistive layer is found to exist as a result of the ion-exchange process in float glass. This is not attributable to compressive stress alone nor does the mixed alkali effect (MAE) operate within this diffusion zone. Glasses of the same composition prepared by homogeneous mixed-melting, however, indicate a strong mixed alkali effect. Infrared reflectance spectroscopic measurements clearly illustrate a more uniform distribution of non-bridging oxygen ions (and therefore some structural relaxation) as a result of ion-exchange. This also holds true for the comparison of ion-exchanged glass and mixed-melted glass. This result clearly shows that a different structure is generated depending on whether alkali cations are mixed homogeneously or via the ion-exchange process. It is proposed that some type of foreign ion repulsion effect (FIRE) operates when the larger cation is substituted into the glass below T_g. The repulsion of such foreign cations, and their search to find their own new sites causes their immobilisation (and a slow ion-exchange process) via the break up of conduction pathways, and thus the conductivity continues to decrease without any recovery, as more K⁺ ions are introduced. In contrast, only a weak mixed alkali effect is apparent in melt-grown lithium-alumino-silicate compositions and no high resistance (or cation immobilisation effect) is found in the ion-exchanged alumino-silicate system.

No description available.

The fabrication and mechanical properties of Ti2AlC composites reinforced with the alumina oxide fibers, such as NextelTM 720 and ALBF1, were described in this thesis. Alumina fibers and Ti2AlC powders were dispersed in the water and slip cast in the molds to form green bodies. Sedimentation test were carried out to optimize pH of the slurry. It was found that suspensions prepared with PAA as a dispersant and has an excellent stability in the pH range of 4 ~ 5. Composite green bodies were densified by pressureless sintering or hot isotatic pressing (HIP) at different temperatures. The microstructure of fabricated samples was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), and porosimetry. It was found out that HIPing at 1300 oC for 4 hrs at 100 MPa results in almost fully dense composites with majority phases being alumina fibers and Ti2AlC. However, fully dense Ti2AlC composites could not be obtained by the pressurless sintering, even at temperature as high as 1400 oC at which reaction between Ti2AlC and NextelTM 720 was observed. The double torsion (DT) tests were carried out at room temperature to measure the fracture toughness of the HIPed pure and 5vol% alumina fiber reinforced Ti2AlC. DT results showed increase in the fracture toughness of Ti2AlC reinforcing with NextelTM 720 alumina fibers. However, fracture toughness of the samples reinforced with ALBF1 was lower than that of pure Ti2AlC because of the low relative densities of those composites. SEM study of the fracture surfaces after DT tests showed that toughening mechanisms by crack bridging and fiber pull outs at the crack tip are operative in all reinforced samples. In addition, elastic moduli of HIPed Ti2AlC measured by Resonant Ultrasound Spectroscopy (RUS) do not show significant change due to reinforcement with alumina fibers, while the Vickers hardness of composites was found to be larger for Ti2AlC reinforced with NextelTM 720 and lower for the samples reinforced with ALBF1.

Doctor of Philosophy / The drastic improvements in stiffness and strength even with the addition of small percentage of clay to a polymer are commonly traded-off with significant reductions in fracture toughness. It is believed that the presence of a stiff nano-filler will restrict the mobility of the surrounding matrix chains, and thus limit its ability to undergo plastic deformation, thereby decreasing their fracture toughness. To understand the role of rigid nano-fillers, like clay and their constraint effect on the surrounding polymer matrix, the effects of preferentially organized polyamide 6 lamellae in the vicinity of organoclay layers on the toughening processes are studied and compared with polyamide 6 filled with an elastomeric additive (POE-g-MA). It is suggested that to impart high toughness to polymer/organoclay nanocomposites, full debonding at the polymer-organoclay interface is necessary so that shear yielding of large volumes of matrix material can be enhanced. However, due to the strong tethering junctions between the individual organoclay layers and the matrix, full-scale debonding at the polymer-organoclay interface is rarely observed under stress conditions indicating that the constraint on the polymer adjacent to the clay is not relieved. Therefore, this has led to the development of ternary nanocomposites by adding a soft elastomeric dispersed phase to polymer/clay systems to obtain well-balanced mechanical properties. Polyamide 66/SEBS-g-MA/organoclay nanocomposites are prepared with four different blending protocols to understand the effect of blending protocol on the microstructure, mechanical properties and fracture mechanisms of the ternary nanocomposites so as to obtain new insights for producing better toughened polymer nanocomposites. In general, it is found that the level of enhancement of fracture toughness of ternary nanocomposites depends on: (i) the location and extent of dispersion of organoclay and (ii) the internal cavitation of rubber particles leading to effective relief of crack-tip tri-axial constraint and thus activating the matrix plastic deformation. Based on the wear/scratch damage studies on different polymer nanocomposite systems, it is suggested that elastic modulus and toughness of polymer nanocomposites are not the predominant factors controlling the material removal or friction coefficient and cannot be the sole indicators to compare and rank candidate materials. It is also found that nano-fillers by themselves, even if uniformly dispersed with good interfacial interaction with the matrix, do not irrevocably improve the wear (and friction) properties. Although it is important to consider these factors, it is necessary to thoroughly understand all microstructural parameters and their response to wear/scratch damage. Other important factors that should be considered are the formation of a uniform and stable transfer film on the counterface slider and the role of excessive organic surfactants or other modifiers added to disperse nanoparticles in a polymer matrix. It is also emphasized that the mechanisms of removal of materials during the wearing/scratching process should be studied meticulously with the use of high resolution microscopic and other analytical tools as this knowledge is critical to understand the surface integrity of polymer nanocomposites.

Metal carbides have attracted much attention over the past several years due to their unique qualities. The purpose of this research is to develop a cermet that demonstrates desired properties of nano-size titanium carbide (TiC) and copper-nickel (Cu-Ni) metals. In this study stoichiometric, nano-size TiC was synthesized through a patented carbothermal synthesis process (U.S. Patent No.: 5,417,952). The resulting TiC was sintered with varying copper (12.5-37.5wt %) and nickel (12.5-25wt %) contents. Hardness, fracture toughness, and microscopy studies were performed. Average hardness ranging between 325-1292 HV were found, with copper content showing a strongly negative correlation with hardness. Fracture toughness values were found to be between 4.85-14.36 Mpa*m^.5; TiC content had a nearly linear, negative correlation with fracture toughness. Samples with high copper to nickel ratios showed poor homogeneity and wetting.

Made available in DSpace on 2014-06-11T19:34:59Z (GMT). No. of bitstreams: 0 Previous issue date: 2002-10Bitstream added on 2014-06-13T18:45:52Z : No. of bitstreams: 1 graca_mla_dr_guara.pdf: 5050152 bytes, checksum: dce8c01ce72d2ebc0e9caea4d6df5d6b (MD5) / Neste trabalho foi feita uma análise detalhada dos micromecanismos de iniciação da fratura em amostras entalhadas para cinco aços e duas ligas de alumínio. Com esse objetivo foram obtidas curvas de transição frágil-dúctil e de tenacidade à fratura em função do raio da raiz do entalhe, e realizados ensaios interrompidos antes da fratura da amostra. Análises fractográficas e micrográficas das regiões de iniciação das fraturas foram realizadas por microscopia eletrônica de varredura. A variação dos micromecanismos de iniciação em função da variação da capacidade plástica local na raiz dos entalhes, como induzida pela variação da temperatura de ensaio e pela variação do raio da raiz, foi analisada. De um modo geral, três tipos de micromecanismos de iniciação foram observados. Um frágil, em que a iniciação envolve a nucleação de uma microtrinca à frente do entalhe e sua subsequente propagação instável. Dois dúcteis, um pela ruptura por cisalhamento localizado ao longo de linhas de cisalhamento máximo formadas na raiz do entalhe, e outro pela formação de microcavidades cuja ligação entre si e a ponta do entalhe envolve um processo misto de cisalhamento localizado e de coalescência de microcavidades. Aspectos de modelos que relacionam tenacidade com a microestrutura foram discutidos com base nos micromecanismos observados. / In this study a detailed analysis of the micromechanisms of the fracture initiation in notched specimens was made, for five steels and two aluminum alloys. With that purpose brittle/ductile transition and fracture toughness x r1/2 curves were obtained, and interrupted tests before the fracture of the sample were used. Fractographic and micrographic analysis of the fracture initiation areas were accomplished by scanning electron microscopy. The variation of the initiation micromechanisms in function of the variation of the local plastic capacity in the notch root, as induced by the variation of the test temperature and by the variation of the notch root radius, was analyzed. In a general way, three types of initiation micromechanisms were observed. A brittle one, where the initiation involves the microcrack nucleation ahead of the notch and its subsequent unstable propagation. Two ductile, one by localized shear rupture along the maximum shear lines formed in the notch root, and other by microvoids nucleation whose link to each other and the notch tip involves a mixed process of localized shear and microcavoid coalescence. Aspects of relating models of fracture toughness with microstructure were discussed, based in the observed micromechanisms.

No description available.

Sweetpotato is an important staple crop, and a supplementary source of nutrients; minerals, carbohydrates, and vitamins, for the food industry. Quality of sweetpotatoes depends on cultivar, preharvest management practices, and harvest equipment causing skinning. Information on morph-physiological characteristics of storage roots is needed for preharvest management decisions, cultivar selection, and application of harvest aids and harvesting procedures for postharvest storage durability of sweetpotatoes. Also, devices to measure skinning properties of storage roots are needed. This research was conducted to measure skin toughness of various sweetpotato cultivars. The number of skin layers was determined using fluorescent microscopy, and lignin content was determined with the Near Infrared System. Preharvest cultural practices, such as devining to enhance skin set and lignin content, were applied 1, 3, 7 days preharvest, and Ethephon at the rate of 1.68 ha-and 0.84 kg ha-1 applied at 1, 3, and 7 days preharvest. In addition, curing to enhance skin healing and lignin content was evaluated. This research was conducted in the field and in the greenhouse environments. The force gauge and the torquometer were the most accurate and precise devices to measure the force needed to break the skin of the various sweetpotato cultivars. The cultivars, “L07-6R”, “L07-146”, and “Beauregard-14” had the toughest skin compared to the other cultivars. However, “Covington” and “Hatteras” had the highest lignin content. Fluorescent microscopy showed that the cultivars “L07-6R” and “L07-146” had 12 and 10 cell layers, respectively, and the treatment of Ethephon at 1.68 Kg∙ha-1 3 days and 7 days before harvest resulted in the highest lignin content in the skin. Divining 3 days preharvest, and applying Ethephon at 0.84 kg∙ha-1 at 1day and 3days preharvest resulted in the highest lignin content. In addition, the treatments with Ethephon at 1.68 Kg∙ha-1 applied at 3 days and 7 days preharvest resulted in the hardest skin as indicated by torquometer and the force gauge. Curing for 7 days resulted in higher lignin content compared to the others pretreatments. When wounded and cured for 7 days, the healing process was enhanced greatly, resulting in rapid skin set of sweetpotato storage roots.

No description available.

Many coaches are becoming aware of the importance of developing mentally tough performers and are designing programs to develop it in their athletes. One of the most significant problems in designing these programs is the inconsistency in the definition and description of mental toughness. If programs are to be designed with the goal of developing mental toughness in athletes, the components of the construct must be identified. Based on the advantages that mental toughness can offer to the performer, this study was designed to develop an understanding of the components of mental toughness in sport. The purpose of this study was to identify the components of mental toughness as perceived by National Collegiate Athletic Association (NCAA) coaches and was guided by two fundamental questions: what are the essential components of mental toughness? and to what degree are these components teachable (trainable). Twenty-two NCAA coaches were selected to participate from a mix of Division I, II, and III male sports teams. The procedures for this study were divided into a two-phase approach. Phase One consisted of each participant completing a questionnaire for the purpose of evaluating the importance and teachability (trainability) of 20 separate components of mental toughness. Phase Two consisted of follow-up, semi-structured interviews that provided further insight into the perspectives of the participants. The results of this study indicated the essential components of mental toughness based on their importance to the construct. The results also indicated the degree of teachability (trainability) of each component. A unique relationship between teachability and trainability was also revealed in this study. These findings provide a better understanding of the components of mental toughness and support the need for its development in sport. / Ph. D.