Periodic Reporting for period 1 - RELAX (Revealing the Hidden Mechanism of Room Temperature Relaxation in Glasses)
Reporting period: 2023-01-01 to 2024-12-31
Does the church glass flow like a liquid at room temperature? This is not just an urban legend but a serious scientific question. Modern glass science believes that although the structure of glass is very similar to that of a supercooled liquid, it is in a deeply dynamics-arrested state at low temperatures (such as room temperature), which makes its properties more like those of a solid than a liquid. However, the latest observation results, showing abnormally occurred relaxation behavior of both oxide glasses and metallic glasses at very low temperatures, have cast doubles on the well-established consensus.
In the RELAX project, we aimed at investigating this irregular relaxation process in glasses and to decipher the underlying atomic-scale structural mechanisms. Specifically, the relaxation parameters, including relaxation time τ and Kohlrausch exponent β, were firstly be derived from fitting of the enthalpy or volume changes within low-temperature relaxation. Then, the relaxation behaviors depicted by different macroscopic state parameters (excess enthalpy vs. volume) or measured at different temperatures were compared to explore underlying decorrelation or crossover features of these relaxation modes. Finally, leveraging advanced experimental and modelling techniques, we strived to decode the atomic-scale details of structure reorganization behind such low-temperature relaxation.
We focused on uncovering these mechanisms in mixed-alkali bioactive (MAB) glasses and metal-organic framework (MOF) glasses, two families of materials with distinct structural and compositional features. MAB glasses, with their bioactive properties, are critical for applications in bone regeneration and drug delivery. MOF glasses, which combine inorganic nodes with organic linkers, have potential in energy storage and catalysis. By studying how these glasses undergo relaxation at low-temperature, the project sought to correlate their structural rearrangement with relaxation behaviors. This understanding is key to improving the stability, reliability, and performance of these glasses in practical applications, particularly in medical devices, energy storage systems, and other advanced technologies.
In the RELAX project, we aimed at investigating this irregular relaxation process in glasses and to decipher the underlying atomic-scale structural mechanisms. Specifically, the relaxation parameters, including relaxation time τ and Kohlrausch exponent β, were firstly be derived from fitting of the enthalpy or volume changes within low-temperature relaxation. Then, the relaxation behaviors depicted by different macroscopic state parameters (excess enthalpy vs. volume) or measured at different temperatures were compared to explore underlying decorrelation or crossover features of these relaxation modes. Finally, leveraging advanced experimental and modelling techniques, we strived to decode the atomic-scale details of structure reorganization behind such low-temperature relaxation.
We focused on uncovering these mechanisms in mixed-alkali bioactive (MAB) glasses and metal-organic framework (MOF) glasses, two families of materials with distinct structural and compositional features. MAB glasses, with their bioactive properties, are critical for applications in bone regeneration and drug delivery. MOF glasses, which combine inorganic nodes with organic linkers, have potential in energy storage and catalysis. By studying how these glasses undergo relaxation at low-temperature, the project sought to correlate their structural rearrangement with relaxation behaviors. This understanding is key to improving the stability, reliability, and performance of these glasses in practical applications, particularly in medical devices, energy storage systems, and other advanced technologies.
We synthesized various MAB and MOF glass compositions using different preparation methods (such as conventional melt-quenching vs. levitation fast cooling) and characterized their relaxation behaviors and structural rearrangements using techniques such as differential scanning calorimetry (DSC), high-energy synchrotron X-ray scattering (HEXRD), and nuclear magnetic resonance (NMR) spectroscopy. We revealed the effects of mixed-alkali, thermal history, and mixed metallic node on τ and β. Additionally, the structural evolution behind the relaxation behavior of MAB and MOF glasses was systematically investigated—with a focus on elucidating structural understanding on distinct relaxation modes. Key findings are synthesized below:
1. Effect of Composition and Thermal History on Relaxation Propensity in MAB Glasses. MAB glasses with different Na/Li ratios were synthesized using two different cooling methods: conventional melt-quenching (slow-cooling glasses) and aerodynamic levitation quenching (fast-cooling glasses). The relaxation propensity towards room temperature, depicted by glass transition temperature Tg, shows a nonlinear deviation with mixed-alkali (Na/Li), with the greatest relaxation propensity around the equimolar ratio (MAB-15Li) composition. The fast-cooled MAB glasses presented an overall depressed relaxation propensity compared the slow-cooled glasses, that is, overall increased Tg values. Structural characterization revealed that the change in relaxation propensity was governed by the network reorganization across SRO and MRO, where the spatially heterogeneous distribution of topological rigidity around network nodes (Si and P) strongly correlate with Tg values. The heterogeneity of topological rigidity in the networks serves as a structural fingerprint for relaxation propensity, which links the variations in external factors (composition, cooling) with modulation of relaxation behavior.
2. Correlating Hierarchical Relaxation Modes with Network Stabilization. We explored hierarchical relaxation mechanisms in MAB glasses through isothermal relaxation experiments at various temperatures (0.9~0.97Tg) for 0Li and 15Li compositions. The 15Li glass showed smaller fluctuations in the Kohlrausch exponent β compared to 0Li composition, indicating a less dynamical heterogeneity. In situ HEXRD experiments and high-temperature Raman spectroscopy confirmed that the 15Li glass had fewer structural changes with temperature, reflecting enhanced network stability. This was further supported by conductivity measurements, which showed significantly reduced ionic mobility in the 15Li glass. Our results demonstrated that the transport properties of modifiers in oxide glass would regulate the stability of surrounding network, both of which play a critical role in the crossover of hierarchical relaxation modes.
3. Coupling Radial and Angular Binding Constraints to Predict Tg in ZIF Glasses.
In MOF glasses, particularly Zn/Co-HDA phases, the relaxation time markedly decreased with increasing Co doping, which was correlated with a monotonical decrease in Tg. We quantified the topological constraints of chemical binding in the ZIF network using peak deconvolution analysis regarding the experimental total correlation functions. The radial freedom (RF) and angular freedom (AF) metrics were determined based on the peak parameters. The results showed that both high and low RF/AF values led to reduced Tg, suggesting that an optimized ZIF network requires balanced radial and angular constraints. This finding provides an approach to depict the complex topological constraints in ZIF glasses and a framework for predicting Tg and relaxation propensity in ZIF glasses.
1. Effect of Composition and Thermal History on Relaxation Propensity in MAB Glasses. MAB glasses with different Na/Li ratios were synthesized using two different cooling methods: conventional melt-quenching (slow-cooling glasses) and aerodynamic levitation quenching (fast-cooling glasses). The relaxation propensity towards room temperature, depicted by glass transition temperature Tg, shows a nonlinear deviation with mixed-alkali (Na/Li), with the greatest relaxation propensity around the equimolar ratio (MAB-15Li) composition. The fast-cooled MAB glasses presented an overall depressed relaxation propensity compared the slow-cooled glasses, that is, overall increased Tg values. Structural characterization revealed that the change in relaxation propensity was governed by the network reorganization across SRO and MRO, where the spatially heterogeneous distribution of topological rigidity around network nodes (Si and P) strongly correlate with Tg values. The heterogeneity of topological rigidity in the networks serves as a structural fingerprint for relaxation propensity, which links the variations in external factors (composition, cooling) with modulation of relaxation behavior.
2. Correlating Hierarchical Relaxation Modes with Network Stabilization. We explored hierarchical relaxation mechanisms in MAB glasses through isothermal relaxation experiments at various temperatures (0.9~0.97Tg) for 0Li and 15Li compositions. The 15Li glass showed smaller fluctuations in the Kohlrausch exponent β compared to 0Li composition, indicating a less dynamical heterogeneity. In situ HEXRD experiments and high-temperature Raman spectroscopy confirmed that the 15Li glass had fewer structural changes with temperature, reflecting enhanced network stability. This was further supported by conductivity measurements, which showed significantly reduced ionic mobility in the 15Li glass. Our results demonstrated that the transport properties of modifiers in oxide glass would regulate the stability of surrounding network, both of which play a critical role in the crossover of hierarchical relaxation modes.
3. Coupling Radial and Angular Binding Constraints to Predict Tg in ZIF Glasses.
In MOF glasses, particularly Zn/Co-HDA phases, the relaxation time markedly decreased with increasing Co doping, which was correlated with a monotonical decrease in Tg. We quantified the topological constraints of chemical binding in the ZIF network using peak deconvolution analysis regarding the experimental total correlation functions. The radial freedom (RF) and angular freedom (AF) metrics were determined based on the peak parameters. The results showed that both high and low RF/AF values led to reduced Tg, suggesting that an optimized ZIF network requires balanced radial and angular constraints. This finding provides an approach to depict the complex topological constraints in ZIF glasses and a framework for predicting Tg and relaxation propensity in ZIF glasses.
The RELAX project extended the current understanding of glass relaxation by providing a detailed atomic-scale insights of the structural mechanisms. Through combining the real-time experimental techniques with effective simulation approaches, the relationship between atomic structure and relaxation dynamics was systemically investigated in the two different glasses, MAB and MOF. Our work represents a significant advancement over previous decoupling between the characterization of relaxation behavior and the tracking of structural evolution on the timescale of real relaxation processes.
A key finding of our project is the identification of the role of topological rigidity in glass relaxation. We demonstrated that the stability of the MAB glass network, influenced by factors like network connectivity and the cationic transport, plays a crucial role in determining the relaxation behavior. Our research also revealed that the relaxation dynamics in the complex network, e.g. MOF glass, are influenced by both radial and angular constraints in the network, with implications for understanding and controlling relaxation in more complex glass systems.
These findings offer new insights into the design of glass materials with tailored relaxation properties, which is crucial for applications where long-term stability is essential, such as in biomedical devices and energy storage systems.
A key finding of our project is the identification of the role of topological rigidity in glass relaxation. We demonstrated that the stability of the MAB glass network, influenced by factors like network connectivity and the cationic transport, plays a crucial role in determining the relaxation behavior. Our research also revealed that the relaxation dynamics in the complex network, e.g. MOF glass, are influenced by both radial and angular constraints in the network, with implications for understanding and controlling relaxation in more complex glass systems.
These findings offer new insights into the design of glass materials with tailored relaxation properties, which is crucial for applications where long-term stability is essential, such as in biomedical devices and energy storage systems.