Size-dependent vitrification in metallic glasses

High-sensitivity differential scanning calorimetry using a MEMS thermopile chip for analyzing polymer crystallization

This paper introduces a high-sensitivity differential scanning calorimetry (DSC) technique based on a MEMS single-crystalline silicon thermopile chip and its application for analyzing the crystallization process of polyamide 6 (PA6) under various thermal processing conditions. The chip integrates 54 pairs of single-crystalline silicon thermocouples beneath a SiNx-suspended film, achieving a temperature responsivity of 31.5 mV per °C and a power responsivity of 147 V W-1. Additionally, the chip's cooling time constant is only 2.4 ms. The non-isothermal experimental results of PA6 suggest that melt-crystallization is suppressed at cooling rates exceeding the critical rate of 50 °C s-1, and cold-crystallization is suppressed at heating rates above the critical rate of 300 °C s-1. Thanks to its high sensitivity, this chip can detect subtle exothermic signals associated with the γ-α phase transition in PA6. The critical heating rate for this phase transition is determined to be 25 °C s-1. Isothermal experimental results show that PA6 undergoes crystallization within 70 °C to 170 °C, with the shortest half-crystallization time of ∼1.1 s at 120 °C. The high-sensitivity DSC technique proposed in this work holds great promise for studying the thermal behaviour of various materials at high heating and cooling rates.

Femtosecond x-ray photon correlation spectroscopy enables direct observations of atomic-scale relaxations of glass forming liquids

Glass-forming liquids exhibit structural relaxation behaviors, reflecting underlying atomic rearrangements on a wide range of timescales and playing a crucial role in determining material properties. However, the relaxation processes on the atomic scale are not well-understood due to the experimental difficulties in directly characterizing the evolving correlations of atomic-scale order in disordered systems. Here, we harness the coherence and ultrashort pulse characteristics of an x-ray free electron laser to directly probe atomic-scale ultrafast relaxation dynamics in the model system Ge15Te85. We demonstrate an analysis strategy for determining the intermediate scattering function by extracting the contrast decay of summed scattering patterns from two rapidly successive, nearly identical femtosecond x-ray pulses generated by a split-delay system. The result indicates a full decorrelation of atomic-scale order on the sub-picosecond timescale, supporting the argument for a high-fluidity fragile state of liquid Ge15Te85 above its dynamic crossover temperature. The demonstrated strategy opens an avenue for experimental studies of relaxation dynamics in liquids, glasses, and other highly disordered systems.

Temperature-modulated ellipsometry for the measurement of dynamic expansion of nanoscale thin films

Direct characterization of the chemco-physical properties of nanometer-thick thin films is essential for understanding and optimizing their performance to meet the evolving demands of nanodevice applications. Herein, temperature-modulated ellipsometry was demonstrated as a capable technique for measuring the dynamic expansion of nanoscale thin polymer films, enabling the direct assessment of both reversible and irreversible processes associated with thermal transitions. A sinusoidal plus linear temperature modulation protocol was implemented to induce oscillatory thickness variations in the thin film, and a spectroscopic ellipsometer was employed for the real-time measurement of dynamic expansion. The reversing thermal expansion (αr), which is related to intrinsic molecular dynamics, was determined from the amplitude of the thickness oscillations in response to temperature variations. By contrast, the apparent thermal expansion (αapp), which encompasses contributions from both reversible and irreversible processes (e.g., degradation), was obtained from the response to a linear temperature ramp. Their difference (Θ = αapp - αr) reflects the contribution of irreversible processes to film expansion. This technique provides a multidimensional assessment of the various processes occurring in thin films, making it a valuable tool for a wide range of modern technological applications.

Mechanistic Picture for the Slow Arrhenius Process in Glass Forming Systems: The Collective Small Displacements Model

Recent experiments reveal increasing detail about the so-called slow Arrhenius process (SAP), which leads to molecular relaxation in glassy systems. The SAP operates over a temperature range that spans melt and glassy states and acts alongside the familiar α-relaxation. In this Letter, we propose a mechanistic picture for SAP relaxation wherein local amorphous packing is reshaped by molecular movements that are small in comparison to those in the α-modes. Our approach, the collective small displacements (CSD) model, captures a number of experimentally observed SAP characteristics, including its connection to rheology and shear flow. Using a statistical mechanics-based equation of state to obtain nonbonded segmental interaction energies, the CSD model predicts the SAP's activation energies a priori, based solely on thermodynamic analysis of material properties. The other parameters are all shown to be reasonably material-independent, which means that the model is able to predict SAP relaxation rates and SAP-driven equilibration rates (e.g., adsorption) without using any data on the SAP at all.

Near-Field-Regulated Ultrafast Laser Supra-Wavelength Structuring Directly on Ultrahard Metallic Glasses

The ultrafast-laser-matter interactions enable "top-down" laser surface structuring, especially for materials difficult to process, with "bottom-up" self-organizing features. The subwavelength scenarios of laser-induced structuring are improved in defects and long-range order by applying positive/negative feedbacks. It is still hardly reported for supra-wavelength laser structuring more associated with complicated thermo/hydro-dynamics. For the first time to the knowledge, the near-field-regulated ultrafast-laser lithography of self-arrayed supra-wavelength micro/nano-pores directly on ultra-hard metallic glass is developed here. The plasmonic hot spots on pre-structures, as the positive feedback, clamped the lateral geometries (i.e., position, size). Simultaneously, it drilled and self-organized into micro/nano-pore arrays by photo-dynamic plasma ablation and Marangoni removal confined under specific femtosecond-laser irradiation, as the negative feedback. The mechanisms and finite element modeling of the multi-physical transduction (based on the two-temperature model), the far-field/near-field coupling, and the polarization dependence during laser-matter interactions are studied. Large-area micro/nano-pore arrays (centimeter scale or larger)  are manufactured with tunable periods (1-5 µm) and geometries (e.g., diameters of 500 nm-6 µm using 343, 515, and 1030 lasers, respectively). Consequently, the mid/far-infrared reflectivity at 2.5-6.5 µm iss decreased from ≈80% to ≈5%. The universality of multi-physical coupling and near-field enhancements makes this approach widely applicable, or even irreplaceable, in various applications.

Simple Model to Predict the Adsorption Rate of Polymer Melts

We determine the adsorption rate of polymer melts by means of measurements of molecular mobility. We show that the complex set of molecular rearrangements involved in the adsorption of polymers on flat surfaces can be modeled as an equilibration kinetics driven by the slow Arrhenius process (SAP), a recently discovered molecular mechanism. Our predictive model is based on the single hypothesis that the number of chains adsorbed per unit surface within the timescale of spontaneous fluctuations associated to the SAP is a temperature-invariant constant, not depending on the chemical structure of the polymer. Going beyond the qualitative arguments setting a correlation between equilibrium and nonequilibrium properties, we demonstrate that the rate at which an adsorbed layer grows does not depend on interfacial interactions. By considering simple physical arguments, we demonstrate that this quantity can be straightforwardly determined using the energy barrier of molecular motion as only input.

The slow Arrhenius process in small organic molecules

Equilibration, the complex set of molecular rearrangements leading to more stable states, is usually dominated by density fluctuations, occurring through the structural (α-)relaxation, whose timescale quickly increases upon cooling. Growing evidence shows, however, that equilibration can be reached also through an alternative pathway provided by the Slow Arrhenius process (SAP), a molecular mode slower than the structural processes in the liquid state and faster in glass. The SAP, widely observed in polymers, has not yet been reported in small molecules, probably because of the larger experimental difficulties in handling these systems. Here, we report the presence of the SAP in three different molecular glassformers, by investigating these systems in the thin film geometry via dielectric spectroscopy. These results reinforce the idea that the SAP is a universal feature of liquid and glassy dynamics.

Enthalpy-entropy compensation in the slow Arrhenius process

The Meyer-Neldel compensation law, observed in a wide variety of chemical reactions and other thermally activated processes, provides a proportionality between the entropic and the enthalpic components of an energy barrier. By analyzing 31 different polymer systems, we show that such an intriguing behavior is encountered also in the slow Arrhenius process, a recently discovered microscopic relaxation mode, responsible for several equilibration mechanisms both in the liquid and the glassy state. We interpret this behavior in terms of the multiexcitation entropy model, indicating that overcoming large energy barriers can require a high number of low-energy local excitations, providing a multiphonon relaxation process.