Structural characterization of lead metaniobate thin films deposited by pulsed laser ablation

The ferroelectric polymorph of lead metaniobate (PbNb2O6) presents an orthorhombic structure that is metastable at room temperature. This phase is obtained by quenching from high temperature. The fabrication of lead niobate thin films with this orthorhombic form has been reported to be difficult due to the presence of phases with the rhombohedric form or other non-stoichimetric phases. In this work, lead niobate thin films have been prepared by laser ablation, at different oxygen pressures and with different substrate temperatures. Their structure was studied by X-ray diffraction and their surface was examined by scanning electron microscopy (SEM). The results show that for low deposition temperatures the films presented a rhombohedric-PbNb2O6 structural phase. As Tdep increases the films started to develop an orthorhombic- PbNb2O6 structure that appeared at 400ºC and remains up to 600ºC. For lower oxygen pressure during deposition, a mixture of this phase and other orthorhombic lead deficient phases are present in the films. On the other hand, by increasing the oxygen pressure the lead deficient phases are strongly reduced and the films present only the orthorhombic- PbNb2O6 structure.(undefined

Production and characterisation of composites materials based on Ge nanoparticles-doped dielectric layer

Dissertação de mestrado (área de especialização em Processamento e Caracterização de Materiais)The main goal of this project was the processing and structural, chemical and optical characterisation of nanocomposite thin film Germanium (Ge) semiconductor nanoparticles (NPs) embedded in Alumina (Al2O3) layer. Such type of materials structures has several potential applications, mainly in electronics and optoelectronic devices like it is the case of memory or light emitting devices (LED’s). Stand-alone Alumina films were initially produced and studied as the reference starting point. The nanocomposite were produced by RF-magnetron sputtering technique. A significant number of characterisation techniques were used in order to evaluate the nanocomposite properties, namely X-ray diffraction (XRD), Raman scattering, scanning and transmission electron microscopy (SEM and TEM), Rutherford backscattering spectroscopy (RBS), and photoluminescence (PL). The results and discussion, based on the particular findings revealed by the detailed analysis of all data from each characterisation technique, are presented cautiously. The study of the deposition and annealing parameters led to processing parameters optimization. The ability to (re)produce such type of materials structures is discussed. The conclusions are presented in a concise way. Ultimately, some light emission that might be related to excitonic recombination in the Germanium nanocrystals was observed during PL measurements. The temperature dependence of the PL demonstrates the confinement effect.Este trabalho teve como objectivo o processamento e a caracterização estrutural, química e óptica de filmes finos de nanocompósitos de Alumina com nanopartículas de Germânio. Estes materiais possuem várias potenciais aplicações, sobretudo em dispositivos electrónicos e opto-electrónicos como são o caso de dispositivos de memória ou emissores de luz (vulgarmente denominados LED´s). Filmes de Alumina foram inicialmente estudados para servirem como ponto inicial de referência. Os filmes de nanocompósitos foram produzidos por pulverização catódica em magnetrão por rádio frequência (RF-magnetron sputtering). A avaliação das propriedades dos filmes foi efectuada recorrendo a diversas técnicas de carcaterização, nomeadamente difracção de Raios-X (XRD), difusão Raman, espectroscopia electrónica de varrimento e de transmissão (SEM e TEM), espectroscopia de retrodispersão de Rutherford (RBS), e fotoluminescência (PL). Os resultados e a discussão, baseados nas conclusões individuais reveladas pela análise detalhada de todos os dados provenientes de cada técnica de caracterização, são apresentados de forma prudente. Os parâmetros de deposição e recozimento para a produção dos materiais nanocompósitos foram estudados e optimizados. A capacidade de (re)produzir tais estruturas de materiais é discutida. As conclusões são apresentadas de forma concisa. No final, os resultados de PL revelaram uma emissão de luz que poderá estar associada à recombinação excitónica dos nanocristais de Germânio. A dependência da temperatura do PL demonstra o efeito de confinamento quântico

An experimental analysis of the high-cycle fatigue fracture of H13 hot forging tool steels

In this study, the axial fatigue behaviour of hot forging tool steels at room temperature was investigated. Fatigue tests were performed on two steels within the same H13 specification. The fatigue tests were carried out in the high-cycle fatigue domain under normal conditions. These tests were also performed on specimens in contact with a corrosive medium, applying stress values that led to the high-cycle fatigue domain under normal conditions for the sake of comparison. Both materials showed similar fatigue strengths when they were tested under normal conditions. In contrast, corrosion fatigue lives were much lower than in normal tests and differed significantly between the two steels. Crack initiation was triggered by microstructural and surface defects in the normal tests, whereas the formation of corrosion pits caused crack initiation in the corrosion fatigue tests. Moreover, a fracture surface analysis revealed dissimilar crack propagation areas between both steels, which suggested that both steels had different fracture toughness. These results were in line with the differences observed between the carbide and grain sizes of both of the material microstructures.Ministerio de Universidades | Ref. CAS21/00454Centro Español para el Desarrollo Tecnológico Industrial- CDTI | Ref. INCRESTA IDI-2019118