Tobias Kjær Bechgaard

Bio: Tobias Kjær Bechgaard is an academic researcher from Aalborg University. The author has contributed to research in topics: Glass transition & Differential scanning calorimetry. The author has an hindex of 6, co-authored 12 publications receiving 209 citations.

Structure and mechanical properties of compressed sodium aluminosilicate glasses: Role of non-bridging oxygens

Abstract: Clarifying the effect of pressure on the structure of aluminosilicate glasses is important for understanding the densification mechanism of these materials under pressure and the corresponding changes in macroscopic properties. In this study, we examine changes in density, network structure, indentation hardness, and crack resistance of sodium aluminosilicate glasses with varying Al/Si ratio and thus non-bridging oxygen (NBO) content before and after 1 GPa isostatic compression at elevated temperature. With increasing NBO content, the silicate network depolymerizes, resulting in higher atomic packing density, lower hardness, and higher crack resistance. The ability of the glasses to densify under isostatic compression is higher in the high-NBO glasses, and these glasses also exhibit more pronounced pressure-induced changes in mechanical properties. The 27Al NMR data show a surprising presence of five-fold aluminum in the as-made high-NBO glasses, with additional formation upon compression. Our study therefore provides new insights into the complicated relationship between Al coordination and NBO content in aluminosilicate glasses and how it affects their densification behavior.

Composition-Structure-Property Relations of Compressed Borosilicate Glasses

Abstract: Glass is ubiquitous, yet our understanding of its structure-function relationships remains far from complete and limits technology. For example, while compression is an important tool in the synthesis of crystalline materials, comparable breakthroughs in preparing bulk glasses are still largely lacking. This work reveals the striking linear dependence of the plastic compressibility of borosilicate glasses on both initial trigonal boron content and relative change in hardness with pressure, with important implications for manufacturing tailored damage-resistant glassy materials.

Structure of MgO/CaO sodium aluminosilicate glasses: Raman spectroscopy study

Abstract: Understanding the composition dependence of the atomic structure of multicomponent aluminosilicate glasses is a challenging problem Aluminum has a crucial influence on the structure-property relations in these systems, but there are still questions regarding its local structural environment Here, we present results detailing the network structure of twenty quaternary MgO/CaO-Na2O-Al2O3-SiO2 glasses upon Al2O3-for-SiO2 and MgO-for-CaO substitutions using room temperature and in situ high-temperature Raman spectroscopy The Raman spectra reveal that the Mg-containing glasses violate the Al-avoidance at lower Al2O3 concentration than the Ca-containing glasses Furthermore, the alkaline earth ions acting as charge-compensators for tetrahedral aluminum are found to have a similar effect on the network structure (Qn speciation and Al/Si mixing), while they affect the network differently when they are in a modifying role Increasing cation field strength allows for stabilization of networks with a larger distribution of bond angles

Fragility and configurational heat capacity of calcium aluminosilicate glass-forming liquids

Abstract: Enabling accurate prediction of the properties of aluminosilicate glasses is important for the development of new glass compositions for high-tech applications. In this study, we use a combined topological and thermodynamic approach to connect the configurational heat capacity (Cp,conf) with the liquid fragility (m) and glass transition temperature (Tg) of calcium aluminosilicate glass-forming liquids. To study structural and dynamical features of these systems, we choose two representative glass composition series: one at the tectosilicate join with varying SiO2 content and one with constant CaO content but varying Al2O3/SiO2 ratio. Cp,conf is determined using differential scanning calorimetry (DSC), while m and Tg are determined through both DSC and viscosity measurements. The Cp,conf model is found to predict the measured data well for most systems, but deviations between the modeled and measured Cp,conf values appear for the "strongest" glasses in the tectosilicate series and for the most peraluminous glasses in the constant CaO series. We discuss the structural origins of these model-from-data deviations based on Raman spectroscopy measurements.

Crucial effect of angular flexibility on the fracture toughness and nano-ductility of aluminosilicate glasses

Abstract: Understanding and controlling materials' resistance to fracture is critical for various applications. However, the structural origin of toughness, brittleness, and ductility remains poorly understood. Here, based on the experimental testing and atomistic simulations of a series of aluminosilicate glasses with varying thermal and pressure histories, we investigate the role of structure in controlling fracture toughness at fixed composition. We show that fracture toughness decreases with density, but strongly depends on the details of the temperature and pressure histories of the glass. This behavior is found to arise from a loss of nano-ductility rather than a loss of cohesion. Finally, we demonstrate that the propensity for nano-ductility is primarily controlled by the extent of angular flexibility between the rigid polytopes of the network. Tuning the extent of nano-ductility in silicate glasses would permit the design of ultra-tough glasses.

Understanding Glass through Differential Scanning Calorimetry

Abstract: Differential scanning calorimetry (DSC) is a powerful tool to address some of the most challenging issues in glass science and technology, such as the nonequilibrium nature of the glassy state and ...

A new transferable interatomic potential for molecular dynamics simulations of borosilicate glasses

Abstract: Borosilicate glasses are traditionally challenging to model using atomic scale simulations due to the composition and thermal history dependence of the coordination state of B atoms. Here, we report a new empirical interatomic potential that shows a good transferability over a wide range of borosilicate glasses—ranging from pure silicate to pure borate end members—while relying on a simple formulation and a constant set of energy parameters. In particular, we show that our new potential accurately predicts the compositional dependence of the average coordination number of boron atoms, glass density, overall short-range and medium-range order structure, and shear viscosity values for several borosilicate glasses and liquids. This suggests that our new potential could be used to gain new insights into the structure of a variety of advanced borosilicate glasses to help elucidate composition-structure-property relationships—including in complex nuclear waste immobilization glasses.

Cooling rate effects in sodium silicate glasses: Bridging the gap between molecular dynamics simulations and experiments

Abstract: Although molecular dynamics (MD) simulations are commonly used to predict the structure and properties of glasses, they are intrinsically limited to short time scales, necessitating the use of fast cooling rates. It is therefore challenging to compare results from MD simulations to experimental results for glasses cooled on typical laboratory time scales. Based on MD simulations of a sodium silicate glass with varying cooling rate (from 0.01 to 100 K/ps), here we show that thermal history primarily affects the medium-range order structure, while the short-range order is largely unaffected over the range of cooling rates simulated. This results in a decoupling between the enthalpy and volume relaxation functions, where the enthalpy quickly plateaus as the cooling rate decreases, whereas density exhibits a slower relaxation. Finally, we show that, using the proper extrapolation method, the outcomes of MD simulations can be meaningfully compared to experimental values when extrapolated to slower cooling rates.

Discovery of Ultra-Crack-Resistant Oxide Glasses with Adaptive Networks

Abstract: Despite their transformative role in our society, oxide glasses remain brittle. Although extrinsic postprocessing techniques can partially mitigate this drawback, they come with undesirable side effects. Alternatively, topological engineering offers an attractive option to enhance the intrinsic strength and damage resistance of glass. On the basis of this approach, we report here the discovery of a novel melt-quenched lithium aluminoborate glass featuring the highest crack resistance ever reported for a bulk oxide glass. Relying on combined mechanical and structural characterizations, we demonstrate that this unusual damage resistance originates from a significant self-adaptivity of the local atomic topology under stress, which, based on a selection of various oxide glasses, is shown to control crack resistance. This renders the lithium aluminoborate glass a promising candidate for engineering applications, such as ultrathin, yet ultrastrong, protective screens.

Structure and mechanical properties of compressed sodium aluminosilicate glasses: Role of non-bridging oxygens

Abstract: Clarifying the effect of pressure on the structure of aluminosilicate glasses is important for understanding the densification mechanism of these materials under pressure and the corresponding changes in macroscopic properties. In this study, we examine changes in density, network structure, indentation hardness, and crack resistance of sodium aluminosilicate glasses with varying Al/Si ratio and thus non-bridging oxygen (NBO) content before and after 1 GPa isostatic compression at elevated temperature. With increasing NBO content, the silicate network depolymerizes, resulting in higher atomic packing density, lower hardness, and higher crack resistance. The ability of the glasses to densify under isostatic compression is higher in the high-NBO glasses, and these glasses also exhibit more pronounced pressure-induced changes in mechanical properties. The 27Al NMR data show a surprising presence of five-fold aluminum in the as-made high-NBO glasses, with additional formation upon compression. Our study therefore provides new insights into the complicated relationship between Al coordination and NBO content in aluminosilicate glasses and how it affects their densification behavior.