Bond constraint theory applied to complex phosphate glasses

While the mystery of the glassy state and its fundamental relation to the glass transition temperature (Tg) is often touted as the main driving force behind research on how it depends on the glass’ chemical composition, this also elicits a great deal of interest from the glass industry since the Tg is a very important parameter in virtually every modern manufacturing process. This is the background the brought about the Temperature Dependent Bond Constraint Theory (TBCT) by Gupta and Mauro in 2009. The TDBCT is based on the Bond Constraint Theory originally developed by Phillips and Thorpe to help elucidate the composition dependency of the glass forming ability of chalcogenide glasses. By abstracting the glass network as a static mechanical scaffold, they found that the glass compositions with greater glass forming ability generally are “isostatic”, where the network has no excess of dangling bonds (or floppy modes – a “floppy network”) and no redundant bonds (a “stressed-rigid network”), corresponding with an average coordination number of 2.4. Gupta and Mauro extended the theory by introducing the concept of temperature dependency to the constraints, which are organized in hierarchical order and become broken at certain temperatures. This allows the theory to treat the problem of the compositional dependency of the glass transition by linking the appearance of the floppy modes (or broken constraints) to the system’s configurational entropy. The greatest appeal of the TDBCT is its simplicity: with just the knowledge of how the glass structure evolves with changing chemical composition one could easily model the glass transition temperature. But it also depends on several assumptions in order to be applied, some of which are stronger than others. In order to evaluate how the TDBCT holds against closer scrutiny we based our analysis on phosphate glasses, which not only have very precise and easy to calculate evolution of the phosphate network with increasing modifier concentration, but also a plethora of reliable experimental data available in literature. This allows us to subtract the influence of the glass network from the experimental number of constraints and focus on the effect of other variables. We find that for binary phosphate glasses up until the metaphosphate composition the influence of the constraints added by the modifiers are of paramount importance to the overall behaviour of the glass. These constraints are not tied to the coordination number of the first coordination shell around the modifier, but are instead determined by the strength of the electrostatic interactions between the modifier and the surrounding non-bridging oxygens. Coupled with that, we also found that the modifier contribution depends on whether it is located in an “isolated” site (meaning that the majority of the surrounding oxygens are double-bonded to the phosphorus) or a “crosslinking” site (where the majority of the oxygens are non-bridging), and, in the case of mixed alkali ultra- and metaphosphates, whether or not one can find different modifiers in the immediate vicinity. In addition to that, experimental measurements of the glass transition temperature of silver metaphosphate – silver halide glasses are much higher than expected from theoretical estimations; this effect is attributed to the conformation change the phosphate network goes through, transitioning from primarily chains to a mixture of chains and rings. When analyzing the viscosity and glass transition temperature of binary alkali borate glasses there are some inconsistencies that can be attributed to the glass system not complying to one of the base assumptions of the TDBCT: the average energy barrier associated with cooperative motion, represented by B(x) in the Adam-Gibbs viscosity equation, is not held constant throughout the whole compositional range. The same behaviour could also be discerned in binary alkali silicate glasses, accounting for the observed severe drop on the glass transition temperature with the addition of relative small amounts of modifiers. Finally, the outcome of the current development of the Temperature Dependent Bond Constraint Theory emphasizes its ambivalent character. On one hand, the TBCT has been shown to be a powerful model that is easy to apply and to expand, allowing it to model more complex glass compositions; on the other hand, reasonable results are only guaranteed through judiciously selecting a glass system that complies with the underlying theoretical assumptions, and the expansions to the theory highlight its empirical nature, since the additional parameters can’t be calculated from first principles nor have any clear physical meaning

Interfacial Bonding between a Crystalline Metal-Organic Framework and an Inorganic Glass.

The interface within a composite is critically important for the chemical and physical properties of these materials. However, experimental structural studies of the interfacial regions within metal-organic framework (MOF) composites are extremely challenging. Here, we provide the first example of a new MOF composite family, i.e., using an inorganic glass matrix host in place of the commonly used organic polymers. Crucially, we also decipher atom-atom interactions at the interface. In particular, we dispersed a zeolitic imidazolate framework (ZIF-8) within a phosphate glass matrix and identified interactions at the interface using several different analysis methods of pair distribution function and multinuclear multidimensional magic angle spinning nuclear magnetic resonance spectroscopy. These demonstrated glass-ZIF atom-atom correlations. Additionally, carbon dioxide uptake and stability tests were also performed to check the increment of the surface area and the stability and durability of the material in different media. This opens up possibilities for creating new composites that include the intrinsic chemical properties of the constituent MOFs and inorganic glasses

Super and subcr´ıtical crack growth in bioglasses and glass-ceramics analized via indentation

In this dissertation we quantified properties that may better describe the mechanical behaviour of different glasses and glass-ceramics of the Na2O-CaO-SiO2-P2O5 system, based on the Biosilicate®chemical composition developed by LaMaV-UFSCar, after implantation in the human body. We tried to analyse variables such as n (the stress-corrosion susceptibility coefficient, derived from subcritical crack growth experiments), K0 (KI0, KSCC or Kth, the threshold stress intensity factor below which no subcritical crack growth occ-curs), and to characterize the R-Curve (the increase in fracture toughness as the cracks increase in length). The experimental methods utilized are beased on the materials response to Vickers indentation, whether by following the crack length, statistically analyzing the number of nucleated cracks or breaking indented samples on four point bending testing. These techniques pose the great advantage of being very economic in respect of time needed to prepare the samples and in respect of the number of samples used. The experimental results were, whenever possible, compared with literature data and their validity broadly discussed. From the experimental data its possible to conclude that the tested methods do not generate valid and coherent results concerning the materials and conditions tested, contributing to the con side of a discussion very present on the literature.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Nesta dissertação quantificamos propriedades que possam descrever melhor o comportamento mecanico de diferentes vidros e vitrocerâmicos do sistema Na2O-CaO-SiO2-P2O5 após a implantacao no corpo humano, baseados na composicao do Biosilicato®anteriormente desenvolvido no LaMaV-UFSCar. Tentamos analisar variáveis como o n (parâmetro de susceptibilidade do material ao meio, derivado de experimentos de crescimento subcrítico de trincas), K0 (K/o, Kscc ou Kth, fator de concentracao de tensao abaixo do qual nao ocorre o crescimento subcrítico de trincas) e levantar a Curva-R (aumento da tenacidade a fratura do material com o aumento do comprimento da trinca). Os metodos experimentais utilizados sao baseados na resposta do material a indentacao Vickers, seja acompanhando o tamanho das trincas, analisando estatisticamente o numero de trincas nucleadas ou rompendo corpos de prova indentados em ensaios de flexao em quatro pontos. Essas tecnicas tem a grande vantagem de serem muito econômicas tanto em relacao ao tempo de preparacao das amostras quanto em relacao a quantidade de amostras necessarias. Os resultados experimentais foram, sempre que possível, comparados com dados da literatura e sua validade foi amplamente discutida. A partir dos resultados experimentais obtidos e possível concluir que os metodos testados nao geram resultados validos e coerentes para o conjunto de materiais e condicoes testados, contribuindo para o lado negativo da discussao que existe na literatura.FAPESP: 2010/04595-

Decoupling Mobility and Charge Carrier Concentration in Agr-agpo3 Glasses (R = Cl, Br, I)

Halide-containing silver phosphate glasses have been used as model systems for the study of the ionic conductivity of oxide glasses for the past 50 years, mostly due to the massive increase in conductivity observed as the halide concentration is increased, even if the molar ratio of silver is kept constant. Furthermore there is still no consensus if the increased conductivity is due to an enhancement of the number of effective charge carriers (glass as a weak electrolyte) or a higher charge carrier mobility (glass as a strong electrolyte). In this work we investigate the electrical properties of silver-halide-containing silver metaphosphate glasses through Impedance Spectroscopy. We find the glasses to follow the “canonical scaling”, which coupled with the diffusional nature of the conductivity allows us to show that the number of effective charge carriers remains constant with increasing halide concentration, and that the conductivity follows the same scaling as the ionic mobility

Parametrization in Models of Subcritical Glass Fracture: Activation Offset and Concerted Activation

There are two established but fundamentally different empirical approaches to parametrize the rate of subcritical fracture in brittle materials. While both are relying on a thermally activated reaction of bond rupture, the difference lies in the way as to how the externally applied stresses affect the local energy landscape. In the consideration of inorganic glasses, the strain energy is typically taken as an offset on the activation barrier. As an alternative interpretation, the system’s volumetric strain energy is added to its thermal energy. Such an interpretation is consistent with the democratic fiber bundle model. Here, we test this approach of concerted activation against macroscopic data of bond cleavage activation energy, and also against ab initio quantum chemical simulation of the energy barrier for cracking in silica. The fact that both models are able to reproduce experimental observation to a remarkable degree highlights the importance of a holistic consideration toward non-empirical understanding

Hybrid Polyethylene Glycol/Sodium Metaphosphate Composites Prepared via Coacervation

We report on the fabrication and characterization of homogeneous, monophasic sodium metaphosphate and polyethylene glycol hybrid composites achieved via coacervation in aqueous solution. After separation and drying, an amorphous plastic solid is formed, composed mostly of hydrated sodium phosphate moieties amalgamated with polyethylene glycol chains. These composites are largely X-ray amorphous and can contain up to 8 weight percent of polymer. Impedance spectroscopic measurements reveal DC conductivity values of 12 μS/m at room temperature, an enhancement of three orders of magnitude when compared to glassy sodium metaphosphate, and the presence of the polyethylene glycol is reflected in the equivalent circuit and ionic hopping analyses

Características de carcaças de bovinos Nelore comercial inteiros e castrados, criados em pastagens e abatidos em duas idades diferentes

As características de carcaça de bovinos Nelore comercial, inteiros e castrados, com maturidade fisiológica de quatro e seis dentes, aproximadamente 30 e 36 meses de idade, respectivamente, foram estudadas. Dados de trinta e dois animais, criados a pasto, foram utilizados. Não houve interação entre condição sexual e idade para todas as variáveis estudadas. Para o comprimento da perna, os animais castrados foram superiores aos inteiros (93,8 vs 89,6cm), enquanto para espessura do coxão, os animais que apresentavam quatro dentes, aproximadamente 30 meses de idade, foram superiores aos animais com seis dentes que possuíam em torno de 36 meses de idade (24,6 vs 21,6cm). Não foram observadas diferenças para as outras medidas de desenvolvimento da carcaça. Os animais Nelore comercial inteiros avaliados, não são identificados como animais de melhor musculosidade e animais mais jovens são mais eficientes no depósito de musculatura no traseiro, observada pela espessura do coxão

Características de carcaças de bovinos Nelore comercial inteiros e castrados, criados em pastagens e abatidos em duas idades diferentes

As características de carcaça de bovinos Nelore comercial, inteiros e castrados, com maturidade fisiológica de quatro e seis dentes, aproximadamente 30 e 36 meses de idade, respectivamente, foram estudadas. Dados de trinta e dois animais, criados a pasto, foram utilizados. Não houve interação entre condição sexual e idade para todas as variáveis estudadas. Para o comprimento da perna, os animais castrados foram superiores aos inteiros (93,8 vs 89,6cm), enquanto para espessura do coxão, os animais que apresentavam quatro dentes, aproximadamente 30 meses de idade, foram superiores aos animais com seis dentes que possuíam em torno de 36 meses de idade (24,6 vs 21,6cm). Não foram observadas diferenças para as outras medidas de desenvolvimento da carcaça. Os animais Nelore comercial inteiros avaliados, não são identificados como animais de melhor musculosidade e animais mais jovens são mais eficientes no depósito de musculatura no traseiro, observada pela espessura do coxão

Interfacial bonding between a crystalline metal-organic framework and an inorganic glass

The interface within a composite is critically important for the chemical and physical properties of these materials. However, experimental structural studies of the interfacial regions within metal-organic framework (MOF) composites are extremely challenging. Here, we provide the first example of a new MOF composite family, i.e. using an inorganic glass matrix host in place of the commonly used organic polymers. Crucially, we also decipher atom-atom interactions at the interface. Specifically, we dispersed ZIF-8 within a phosphate glass matrix, and identified interactions at the interface using several different analysis methods of pair distribution function (PDF) and multinuclear multidimensional magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. These demonstrated glass-ZIF atom-atom correlations. Additionally, carbon dioxide uptake and stability tests were also performed to check the increment of the surface area and the stability and durability of the material in different media. This opens up the possibility to create new composites including the intrinsic chemical properties of the constituent MOFs and inorganic glasses