Estrutura e cristalização de vidros de silicato de lítio multicomponents

Doutoramento em Ciência e Engenharia de MateriaisA presente tese tem como objetivo adquirir uma compreensão aprofundada acerca do processo de cristalização de vidros à base de silicato de lítio com a adição de pequenas quantidades de outros componentes. Os principais componentes investigados neste estudo são os óxidos de Mn, Al, B e P. Estudaram-se os efeitos de cada um destes componentes na estrutura do vidro, na separação de fases líquido-líquido, nos processos de nucleação e crescimento de cristais, na microestrutura e no conjunto das fases cristalinas formadas. Os vitro-cerâmicos utilizados neste estudo são produzidos a partir de amostras tridimensionais de vidro fundido e vertido em moldes, ou a partir de pós de frita obtida por arrefecimento dos fundidos em água. A adição de óxidos de Mn aos vidros de silicato de lítio resulta na criação de entidades moleculares individuais de Mn. Por conseguinte, estas entidades moleculares dificultam o todo o processo de cristalização do vidro. Óxidos de Al e B são incorporados na rede de vidro como formadores de rede. Estes componentes, por conseguinte, também diminuem a tendência do vidro para a cristalização. O P2O5 também desempenha um papel de formador de rede do vidro. No entanto, ele aumenta a tendência do vidro para a cristalização. Dá-se uma ênfase especial ao estabelecimento de correlações entre a estrutura do vidro e seu comportamento na cristalização. Estes esforços levaram à introdução de um novo modelo matemático baseado na mecânica estatística para descrever a estrutura de vidro. O modelo foi desenvolvido principalmente para silicatos binários e mais tarde estendido para composições de silicatos multicomponentes.The present thesis is aimed at gaining an in-depth understanding of the crystallization process in multicomponent lithium silicate based glasses when other components are added in small amounts. The added components investigated in this study are oxides of Mn, Al, B and P. The effects of each of these components on glass structure, liquid-liquid phase separation, crystal nucleation, crystal growth, microstructure and phase assemblage are studied. The glass ceramics used in this study are produced by both bulk glasses obtained by melt quenching as well as by powder methods from glass frits. Oxides of Mn when added to lithium silicate glasses result in creating individual Mn molecular entities. Consequently, these molecular entities hinder the overall crystallization ability of the glass. Oxides of Al and B are incorporated into glass network as network formers. These components consequently decrease the overall crystallization ability of the glass. P2O5 is also incorporated into glass network as network former. However, it increases the overall crystallization ability of the glass. Particular emphasis is given to establishing correlations between glass structure and its corresponding crystallization behaviour. These efforts led to introducing a new mathematical model based on statistical mechanics for describing the glass structure. The model was primarily developed for binary silicates and later on extended to multicomponent silicates

Statistics of silicate units in binary glasses

In this paper, we derive a new model to determine the distribution of silicate units in binary glasses (or liquids). The model is based on statistical mechanics and assumes grand canonical ensemble of silicate units which exchange energy and network modifiers from the reservoir. This model complements experimental techniques, which measure short range order in glasses such as nuclear magnetic resonance (NMR) spectroscopy. The model has potential in calculating the amounts of liquid-liquid phase segregation and crystal nucleation, and it can be easily extended to more complicated compositions. The structural relaxation of the glass as probed by NMR spectroscopy is also reported, where the model could find its usefulness. Published by AIP Publishing

Structure and thermal relaxation of network units and crystallization of lithium silicate based glasses doped with oxides of Al and B

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Glass structure and crystallization of Al and B containing glasses belonging to the Li2O-SiO2 system

The aim of the present work is to investigate the effect of substituting B2O3 for Al2O3 in a nonstoichiometric lithium disilicate (Li2Si2O5, LS2) glass composition belonging to the system Li2O-K2O-Al2O3-SiO2. Addition of equimolar amounts of K2O and Al2O3 to binary lithium silicate glass compositions improves chemical resistance, sintering behaviour and mechanical properties of the glassceramics produced from sintered glass powder compacts. However, in bulk (monolithic) glasses Al2O3 addition hinders bulk nucleation. It also suppresses crystallization of LS2 and promotes formation of a meta-stable crystalline phase called lithium metasilicate (Li2SiO3, LS). The results showed that B substitution resulted in the depolymerisation of the glass network increasing the percentage of NBOs leading to decreasing viscosity, molar volumes, oxygen densities and glass transition temperatures. The simultaneous addition of Al and B into the glass composition resulted in decreased liquid-liquid phase segregation (LLPS) and lower crystal nucleation tendency when compared to Al pure or B pure compositions. Further, Al rich glasses featured lithium metasilicate crystallization at initial stages and then transformed into LS2 at higher temperatures, while with B addition glasses crystallize directly into LS2

Structural relaxation of Qn units by statistics mechanical modelling

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Structure, properties and crystallization of non-stoichiometric lithium disilicate glasses containing CaF2

The role of CaF2 on the structure, crystallization behaviour and properties of a relatively simple non-stoichiometric lithium disilicate (Li2Si2O5) based glass composition was studied. Different x amounts (x = 0, 1, 3 and 5 mol%) of CaF2 were added to (100 - x) of a parent glass (22.96Li(2)O-2.63K(2)O-2.63Al(2)O(3)-71.78SiO(2)) composition. The glasses were produced by conventional melt-quenching technique, whilst glass-ceramics were produced via crystallization of monolithic bulk glasses. A scanning electron microscopy (SEM) examination of as cast non-annealed monolithic glasses revealed precipitation of nanosize droplet phase in glassy matrices suggesting the occurrence of liquid phase separation in all investigated compositions. The extent of phase segregation, as judged from the mean droplet diameter and the packing density of droplets, decreased with increasing CaF2 content in the glasses. A slight depolymerisation of the glass network was observed according to magic angle spinning nuclear magnetic resonance (MAS-NMR) and Fourier transform infrared (FTIR) spectroscopic studies, suggesting a network modifier role for CaF2. The presence of CaF2 enhanced the crystallization at lower temperatures in comparison to CaF2-free glass. (C) 2014 Elsevier B.V. All rights reserved

Bioactive Glasses and Glass-Ceramics for Healthcare Applications in Bone Regeneration and Tissue Engineering

The discovery of bioactive glasses (BGs) in the late 1960s by Larry Hench et al. was driven by the need for implant materials with an ability to bond to living tissues, which were intended to replace inert metal and plastic implants that were not well tolerated by the body. Among a number of tested compositions, the one that later became designated by the well-known trademark of 45S5 Bioglass® excelled in its ability to bond to bone and soft tissues. Bonding to living tissues was mediated through the formation of an interfacial bone-like hydroxyapatite layer when the bioglass was put in contact with biological fluids in vivo. This feature represented a remarkable milestone, and has inspired many other investigations aiming at further exploring the in vitro and in vivo performances of this and other related BG compositions. This paradigmatic example of a target-oriented research is certainly one of the most valuable contributions that one can learn from Larry Hench. Such a goal-oriented approach needs to be continuously stimulated, aiming at finding out better performing materials to overcome the limitations of the existing ones, including the 45S5 Bioglass®. Its well-known that its main limitations include: (i) the high pH environment that is created by its high sodium content could turn it cytotoxic; (ii) and the poor sintering ability makes the fabrication of porous three-dimensional (3D) scaffolds difficult. All of these relevant features strongly depend on a number of interrelated factors that need to be well compromised. The selected chemical composition strongly determines the glass structure, the biocompatibility, the degradation rate, and the ease of processing (scaffolds fabrication and sintering). This manuscript presents a first general appraisal of the scientific output in the interrelated areas of bioactive glasses and glass-ceramics, scaffolds, implant coatings, and tissue engineering. Then, it gives an overview of the critical issues that need to be considered when developing bioactive glasses for healthcare applications. The aim is to provide knowledge-based tools towards guiding young researchers in the design of new bioactive glass compositions, taking into account the desired functional properties

Role of vanadium oxide on the lithium silicate glass structure and properties

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Structure and thermal relaxation of network units and crystallization of lithium silicate based glasses doped with oxides of Al and B

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Structure and Stability of High CaO- and P2O5-Containing Silicate and Borosilicate Bioactive Glasses

The present work elucidates about the structure of bioactive glasses having chemical compositions expressed as (mol %) (50.0 - x)SiO2-xB(2)O(3)-9.3Na(2)O-37CaO-3.7P(2)O(5), where x = 0.0, 12.5, 25, and 37.5, and establishes a correlation between the structure and thermal stability. The structural modifications in the parent boron-free glass (BO) with the gradual substitutions of B2O3 for SiO2 are assessed by Raman and Si-29, P-31, B-11, and Na-23 magic angle spinning (MAS)nuclear magnetic resonance (NMR) spectroscopies. The structural studies reveal the presence of Q(Si)(2) and Q(Si)(3) structural units in both silicate and borosilicate glasses. However, Q(Si)(4)(3B) units additionally form upon incorporating B2O3 in BO glass. B-containing silicate glasses exhibit both three-coordinated boron (B-III) and fourcoordinated boron (B-IV) units. The P-31 MAS-NMR studies reveal that the majority of phosphate species exist as isolated orthophosphate (Q(p)(0)) units. The incorporation of B2O3 in B0 glass increases the cross-linking between the SiO4 and BO4 structural units. However, incorporation of B2O3 lowers the glass thermal stability (Delta T), as shown by differential scanning calorimetry. Although both silicate and borosilicate glasses exhibit good in vitro apatite-forming ability and cell compatibility, the bactericidal action against Escherichia coli bacteria is more evident in borosilicate glass in comparison to silicate base glass. The controlled release of (BO3)(3-) ions from boron-modified bioactive glasses improves both the cell proliferation and the antibacterial properties, making them promising for hard tissue engineering applications