Made available in DSpace on 2016-06-02T20:16:54Z (GMT). No. of bitstreams: 1 6611.pdf: 4783832 bytes, checksum: 1dc280ff47cc4df343ea92399b40fdd5 (MD5) Previous issue date: 2015-03-09 / Financiadora de Estudos e Projetos / The search for new lead-free piezoelectric materials has been a major goal of many scientists in recent years. The main cause is the replacement of widely used lead zirconate titanate (PZT) based ceramics due to the highly toxic characteristics of the lead element. Potassium sodium niobate based ceramics have shown high piezoelectric coefficients and a morphotropic phase boundary close to the composition (K0.5Na0.5)NbO3 (KNN), similar to that found in lead zirconate titanate. However, the preparation of highly dense KNN based ceramics is extremely difficult. In this work, the structural, mechanical and electrical properties of lead free ferroelectric ceramics with compositions Li0,03(K0,5Na0,5)0,97Nb0,8Ta0,2O3 + xwt% CuO (x = 0; 2 and 3.5) were studied. All the compositions, sintered at 1050ºC for 2 hours had high density, approximately 95% of the theoretical value. Rietveld refinement of the X ray diffraction patterns showed a mixture of both orthorhombic Bmm2 and tetragonal P4mm phases, for all compositions. Nevertheless, compositions with high CuO contents have mainly the tetragonal phase. Dielectric and dynamic mechanical analysis (DMA) measurements showed two polymorphic phase transitions with increasing temperature. Both phase transitions have diffuse character and they can be related with the transformation of the orthorhombic phase fraction in the tetragonal one, and with the transformation of the tetragonal ferroelectric phase to a cubic paraelectric one. The origin of the difference observed between the temperatures where both techniques, dielectric and mechanical, see the diffuse phase transition is discussed. The ceramic with 2wt% of CuO is electrically softer than the other compositions and it has the highest value of the piezoelectric coefficient d31. Also, in this work we studied the possibility of using high contents of CuO to promote the formation of liquid phase for obtaining and extracting single crystal seeds, which can be used for the texture of KNN-based ceramics. The ceramic Li0,03(Na0,5K0,5)0,97Ta0,2Nb0,8O3 + x wt% CuO with x=16, sintered at 1090ºC for 2 hours, is a perfect candidate for extracting grains which may be used as seeds. Furthermore, ceramics with x=13, sintered at 1110ºC for 2 hours, showed a partial melting of the material, which caused the growth of highly oriented grains. This material can be practically considered as a single crystal and, with a proper cut procedure, the desired single crystal seeds can be obtained. This method to obtain single crystal seeds, as proposed in this work, is very simple and novelty. / Nos últimos anos, o foco principal de muitos cientistas tem sido a procura de novos materiais piezoelétricos livres de chumbo. A causa principal é a substituição dos materiais baseados em titanato zirconato de chumbo (PZT), os quais são amplamente utilizados em aplicações piezoelétricas, devido à alta toxicidade do elemento chumbo. Cerâmicas baseadas em niobato de sódio e potássio têm mostrado altos coeficientes piezoelétricos e um contorno de fases morfotrópico próximo da composição (K0.5Na0.5)NbO3 (KNN), similar ao encontrado no titanato zirconato de chumbo. Porém, a preparação de cerâmicas baseadas em KNN com alta densidade é extremamente dificultosa. Neste trabalho foram estudadas as propriedades estruturais, mecânicas e elétricas de cerâmicas ferroelétricas livres de chumbo com composições Li0,03(K0,5Na0,5)0,97Nb0,8Ta0,2O3 + x %P CuO (x = 0; 2 e 3,5). Todas as cerâmicas sinterizadas a 1050ºC durante 2 horas apresentaram altas densidades, sendo aproximadamente 95% da densidade teórica. O refinamento pelo método de Rietveld dos perfis de difração de raios X mostrou que todas as composições apresentam uma mistura de ambas as fases, ortorrômbica Bmm2 e tetragonal P4mm. Porém, composições com altos teores de CuO apresentam a fase tetragonal como sendo majoritária. As medidas dielétricas e as de análise mecânico dinâmico (DMA) mostraram duas transições de fase polimórficas com o aumento da temperatura. Ambas transições de fase têm caráter difuso e estão relacionadas com a transformação da fração de fase ortorrômbica em tetragonal e com a transformação da fase tetragonal ferroelétrica para cúbica paraelétrica. Foi discutida a origem da diferença observada, nas temperaturas em que ambas as técnicas, dielétrica e mecânica, enxergam a transição de fase difusa. A cerâmica com 2%P de CuO mostrou-se mais mole eletricamente e apresentou um valor maior de coeficiente piezoelétrico d31 do que as outras composições estudadas. Também, neste trabalho foi estudada a possibilidade de usar altos teores de CuO para promover a formação de fase líquida e conseguir a formação e extração de sementes monocristalinas que possam ser utilizadas na textura de cerâmicas baseadas em KNN. A cerâmica de Li0,03(Na0,5K0,5)0,97Ta0,2Nb0,8O3 + x % P CuO com x=16, sinterizada a 1090ºC durante 2 horas, mostrou-se a candidata perfeita para a extração de grãos que possam ser utilizados como sementes. Por outro lado, a cerâmica com x=13, sinterizada a 1110ºC durante 2 horas, apresentou fusão parcial de material, o que promoveu o crescimento dos grãos altamente orientados de forma que esse material já pode ser considerado como sendo praticamente um monocristal e com um procedimento de corte adequado, podem ser obtidas as sementes monocristalinas desejadas. Esse procedimento de obtenção de sementes monocristalinas, proposto neste trabalho, é totalmente simples e inovador.

This dissertation focuses on projects where optical techniques were employed to characterize novel materials, developing concepts toward next generation of devices. The materials that I studied included InAsP, InMnSb and InMnAs, and BT-BCN. I have employed several advanced time resolved and magneto-optical techniques to explore unexplored properties of these structures. The first class of the materials were the ternary alloys InAsP. The electron g-factor of InAsP can be tuned, even allowing for g=0, making InAsP an ideal candidate for quantum communication devices. Furthermore, InAsP shows promises for opto-electronics and spintronics, where the development of devices requires extensive knowledge of carrier and spin dynamics. Thus, I have performed time and polarization resolved pump-probe spectroscopy on InAsP with various compositions. The carrier and spin relaxation time in these structures were observed and demonstrated tunability to the excitation wavelengths, composition and temperature. The sensitivity to these parameters provide several avenues to control carrier and spin dynamics in InAsP alloys. The second project focused on the ferromagnetic narrow gap semiconductors InMnAs and InMnSb. The incorporation of Mn can lead to ferromagnetic behavior of InMnAs and InMnSb, and enhance the g-factors, making them ideal candidates for spintronics devices. When grown using Molecular Beam Epitaxy (MBE), the Curie temperature (textit{$T_c$}) of these structures is textless 100 K, however structures grown using Metalorganic Vapor phase Epitaxy (MOVPE) have textit{$T_c$} textgreater 300 K. Magnetic circular dichroism was performed on MOVPE grown InMnAs and InMnSb. Comparison of the experimental results with the theoretical calculations provides a direct method to map the band structure, including the temperature dependence of the spin-orbit split-off band to conduction band transition and g-factors, as well as the estimated sp-d electron/hole coupling parameters. My final project was on the lead-free ferroelectric BT-BCN. Ferroelectric materials are being investigated for high speed, density, nonvolatile and energy efficient memory devices; however, commercial ferroelectric memories typically contain lead, and use a destructive reading method. Reflectometry measurements were used in order to determine the refractive index of BT-BCN with varying thicknesses, which can provide a means to nondestructively read ferroelectric memory through optical methods. / Ph. D.