Made available in DSpace on 2014-10-09T12:38:31Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:05:14Z (GMT). No. of bitstreams: 1 06055.pdf: 2316471 bytes, checksum: 11ba99ee655171f0e9fe8e8052d3fd7c (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:38:31Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:05:14Z (GMT). No. of bitstreams: 1 06055.pdf: 2316471 bytes, checksum: 11ba99ee655171f0e9fe8e8052d3fd7c (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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<p>With the poor room temperature formability of magnesium, rare earth (RE) additions have proven a promising avenue for wrought magnesium products. However, not much is known regarding the effect of these elements on strength and ductility. Stanford et al. (2010b) summarized it best: “Although the addition of rare earth elements offers the possibility of greatly improved mechanical properties, we still lack fairly basic knowledge about the behaviour of these alloying elements”[p.6773]. Through a systematic study across three Mg-RE binary systems, the effect of gadolinium, samarium and scandium on solution strengthening, work hardening behaviour and strain rate sensitivity of Mg-RE binary alloys have been characterized.</p> <p>The results suggest that samarium offers the greatest solid solution strengthening in both tension and compression relative to the other two binary systems. For the binary alloys explored, only gadolinium and samarium in compression followed the Labusch theory of solution strengthening; in which the yield strength scales with c^2/3 . Gadolinium additions provide the largest strength and ductility in tension and compression. Increasing solute content in the binary systems leads to a decrease of the strain rate sensitivity of the alloys. At the highest level of solute, both Mg-Gd and Mg-Sm exhibit negative strain rate sensitivity under tension. In compression, Mg-Gd also exhibits a negative strain rate sensitivity at the highest level solute, whereas Mg-Sm has a low, but positive value. The asymmetry in work hardening behaviour and the form of the flow curves between tension and compression is the result of the difference in the dominating modes of deformation at a given stage of the plastic flow. The extent of the work performed established the hardening levels and thermodynamic deformation parameters which control the flow stress and work hardening behaviour, that can be applied in future work.</p> / Master of Applied Science (MASc)

The addition of rare earth (RE) alloying elements is a promising method for improving the strength, ductility and overall formability of magnesium (Mg) alloys. However, the underlying mechanism for this phenomenon remains unclear. An investigation on the effect of the rare earth element, scandium (Sc), on binary Mg-Sc alloys has been pursued. Tension and compression tests were performed on a series of dilute binary Mg-Sc alloys at temperatures of 298 K, 78 K and 4.2 K. As a reference, pure Mg was also investigated for comparative purposes. Differences in tension and compression stress-strain curves highlighted distinct activated mechanisms, where slip dominated in tension and twinning governed compression. The observed increase in ductility and prolonged necking was attributed to a weaker basal texture, enhanced twinning and non-basal slip. A decreased work hardening rate suggests an improvement in dislocation recovery with Sc addition. In compression, Mg-Sc alloys followed Fleischer’s theory of solution hardening, where stress scales with concentration, c, as c^1/2; however, there was a very weak fit with both Fleischer and Labusch models under tension. The strengthening rate displayed by Mg-Sc was relatively weak compared to previously studied Mg-RE systems. However, considering the estimated misfit parameters, the size and modulus misfit was not enough to account for the strengthening rate. The results suggest that hardening of the twinning mode may influence strength. Constitutive modelling, based on a self-consistent plasticity model, was used to characterize the deformation behaviour. The simulations predicted an increased relative activity of non-basal <c+a> slip with Sc addition, supporting experimental results and proposed mechanisms in literature. The results of Mg-Sc alloys have been connected to theories that identify a decrease in stacking fault energy (SFE) as the determining factor for increased strength and ductility of Mg-RE alloys. A comparison of the SFE of previously studied REEs with Sc, demonstrated strong evidence towards the theory’s validity. Sc has been shown to only moderately reduce the SFE of Mg and hence, the present experimental results have shown a moderate increase in strength and ductility. Additional modelling and detailed dislocation analysis are suggested as future steps to further support this theory. / Thesis / Master of Applied Science (MASc)

No description available.

Luminescence measurements have been applied to three different structures namely, sulphate, fluorides and YAG. In all cases the RE doping suppresses the intrinsic emission and results in intense luminescence characteristic of the RE dopant. Additionally, in double doped samples, or contaminated ones, the TL data show that each dopant defines a glow peak, which is displaced in temperature relative to the others. Examples of this were discussed for CaS04:Ce,Mn; YAG:Nd,Tb,Cr,Mn; BaF2:Ho,Ce and BaF2:Tm,Ce. The data are discussed in terms of an energy transfer model between different parts of extended defect complexes which encompass the RE ion and the lattice defects. Calcium sulphate doped with Dy define a TL peak near 200°C suitable for radiation measurements, but when co-doped with Ag the TL peak move to higher temperatures with minor effects on the peak sensitivity. In Ce,Mn double doped samples, the peak temperatures differ by -7°C between the Ce and Mn sites. The TL glow curves from alkaline earth fluorides are complex and contain several overlapping peaks. Curve fitting show that the peak maxima below room temperature are insensitive to the RE dopant. Additionally the host material has a modest effect on the peak positions. Above room temperature each dopant provides a TL curve specific to the added RE ion and do not show common peaks. Concentration has many effects on the resultant glow curve, and even at the lowest concentration used here (0.01%) there is evidence of cluster formation. Samples with high RE content show low values of the frequency factor consistent with the energy transfer model in that the emission from RE-RE cluster dominates over the emission from direct charge recombination within the defect complex. The effect of concentration and the TL mechanism operating below room temperature are also discussed. Luminescence signals from the near surface of YAG:Nd (via CL) were contrasted with those from the bulk material via RL. Results indicate that the outer few micron layers differ significantly in luminescence response from the bulk crystal. The differences were ascribed to result from solvents that enter the YAG lattice during the growth stage or subsequently from cleaning treatments via the dislocations caused by cutting and polishing. Additionally, the growth stage may include gases from the residual air in the growth furnace trapped into the YAG lattice. In each case there is a discontinuity in luminescence intensity and/or emission wavelengths at temperatures which mach the phase transitions of the contaminants. At the transition temperature there will be a sudden pressure change and this will induce surface expansion or bulk compression. The differences between the two cases were detected by the alternatives of CL and RL excitation, where the Nd or Er lines have moved in opposite directions. The detection of such low concentrations of solvents/trapped gases by luminescence is extremely difficult due to experimental limitations. Hence their role in luminescence generation is normally ignored.

Submitted by Marco Antonio Oliveira da Silva (maosilva@ipen.br) on 2016-12-21T16:28:43Z No. of bitstreams: 0 / Made available in DSpace on 2016-12-21T16:28:43Z (GMT). No. of bitstreams: 0 / O titanato de estrôncio (SrTiO3) possui estrutura cristalina do tipo perovsquita. Materiais com este tipo de estrutura são utilizados para diversas aplicações, tais como, sensores, atuadores, em células a combustível de óxido sólido, entre outros. Devido as suas interessantes propriedades físicas, o SrTiO3 vem sendo intensamente estudado, em especial com a introdução de dopantes. Portanto, neste trabalho foi investigada a influência de diferentes teores de Ca (1; 2,5 e 5% mol) e Pr (0,025; 0,050; 0,075 e 1% mol) na microestrutura e propriedades elétricas e dielétricas do SrTiO3, assim como o material sem aditivos (puro). Os resultados mostram que após a sinterização do SrTiO3 puro, a microestrutura consiste de grãos poligonais com tamanho médio micrométrico, além de texturas lisas e rugosas. A condutividade elétrica das amostras sintetizadas sinterizadas a 1450 e 1500ºC é máxima para 2 horas de patamar. Apenas as amostras de SrTiO3 contendo 1% em mol de Ca apresentam fase única. O tamanho médio de grãos das amostras contendo 1% em mol de Ca é 10,65 &plusmn; 0,28 &micro;m e para teores acima deste valor ocorre crescimento significativo dos grãos. As medidas de condutividade elétrica mostraram que as amostras contendo a adição de 1% em mol de Ca possuem maior condutividade dos grãos em relação ao material puro. Para as amostras contendo teores de até 0,075% mol de Pr, pode-se observar alguns grãos lisos e outros rugosos e não há variação considerável do tamanho médio de grãos. As amostras contendo menor teor de Pr (0,025% mol) apresentam maior condutividade dos grãos e contornos de grãos. As amostras de SrTiO3 sintetizado sinterizadas a 1450ºC/10 h apresentaram permissividade elétrica colossal em temperatura ambiente em altas frequências. / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP