Glass softening in the limit of high heating rates: Heterogeneous devitrification kinetics on nano, meso, and micrometer scale.

When heated rapidly, glasses often devitrify heterogeneously, i.e., by a softening front that originates at the surface of an amorphous film. Yet the fundamentals of this devitrification regime are not completely understood; depending on experimental conditions, the reported front propagation distances differ by an order of magnitude. Using a high-resolution fast scanning calorimetry technique, we have investigated the softening of glassy methylbenzene films with thicknesses between 30 and 1400 nm. We confirm first that, in all films, the devitrification process begins with the formation of a softening front that propagates through the films over distances of ∼50 nm and that the front propagation kinetics at this stage follow an Arrhenius law. However, we also show that, in films with thicknesses above 165 nm, the front propagation does not terminate with the onset of bulk softening. Specifically, increasing the films' thicknesses above 165 nm yields sharp, clearly discernible endotherms that precede the bulk softening endotherms and that are consistent with a two-fold increase in the enthalpic barrier to front propagation at a well-defined critical temperature. We term this phenomenon "Arrhenius discontinuity" and use reaction rate and continuum front dynamics theories to explain its origins and the physical nature of the resulting distinct heterogeneous devitrification processes. Finally, we discuss the findings in the context of recent theoretical, computational, and experimental studies of heterogeneous devitrification by other research groups.

Mass Accommodation of Water on Ice at Environmentally Relevant Temperatures: Insights from Fast Scanning Calorimetry.

Using a conceptually simple, quasi-adiabatic, fast scanning calorimetry technique, we have investigated the sublimation kinetics of ice films with thicknesses ranging from 14 to 400 nm at environmentally relevant temperatures, between 223 and 268 K. The technique enables accurate determination of ice sublimation rates into vacuum under the conditions of free molecular flow during rapid yet quasistatic heating. The measured sublimation fluxes yield the vapor pressure of the ice samples, which is indistinguishable from that derived from experiments under near-equilibrium conditions. Thus, in agreement with the microscopic reversibility principle, we conclude that the mass accommodation coefficient of water by ice is unity and temperature-independent in the temperature range of the studies. We discuss these findings in the context of current computational and theoretical research into the chemistry and physics of aqueous interfaces.

Glass softening kinetics in the limit of high heating rates.

Surface-facilitated, front-propagated softening of glassy materials is now a well-known phenomenon, which is common to stable vapor deposited glasses. As we demonstrate in our recent communication, this softening pathway is not unique to vapor-deposited vitreous phases and can be observed in ordinary melt-cooled glasses in the limit of high heating rates [Cubeta et al., J. Chem. Phys. 147(7), 071101 (2017)]. Expanding on this preliminary report, we use our thin-wire, quasi-adiabatic fast scanning calorimetry technique to investigate softening kinetics of micrometer scale, viscous liquid methylbenzene, and 2-propanol films, which are fully equilibrated at distinct temperatures near the compounds' standard glass hardening transition ranges. Heating of each sample with rates in excess of 105 K·s-1 results in softening kinetics that are well approximated by an Arrhenius temperature function. Remarkably, the apparent activation energy barriers to non-equilibrium, front-propagated softening matches the barriers to near-equilibrium self-diffusivity at the samples' initial temperatures. Furthermore, our analysis also shows an exceptionally strong correlation between the high temperature softening rate and the self-diffusion coefficients at low initial temperatures. Finally, our front softening velocities are also strongly dependent on the samples' initial states, much more so than previously observed. Based on these results, we propose an extended Wilson-Frenkel model of non-equilibrium phase transformations as a general theoretical framework to describe front propagated softening in glassy materials.

Communication: Surface-facilitated softening of ordinary and vapor-deposited glasses.

A common distinction between the ordinary glasses formed by melt cooling and the stable amorphous films formed by vapor deposition is the apparent mechanism of their devitrification. Using quasi-adiabatic, fast scanning calorimetry that is capable of heating rates in excess of 105 K s-1, we have investigated the softening kinetics of micrometer-scale, ordinary glass films of methylbenzene and 2-propanol. At the limit of high heating rates, the transformation mechanism of ordinary glasses is identical to that of their stable vapor-deposited counterparts. In both cases, softening is likely to begin at the sample surface and progress into its bulk via a transformation front. Furthermore, such a surface-facilitated mechanism complies with zero-order, Arrhenius rate law. The activation energy barriers for the softening transformation imply that the kinetics must be defined, at least in part, by the initial thermodynamic and structural state of the samples.

Melting of superheated molecular crystals.

Melting dynamics of micrometer scale, polycrystalline samples of isobutane, dimethyl ether, methyl benzene, and 2-propanol were investigated by fast scanning calorimetry. When films are superheated with rates in excess of 105 K s-1, the melting process follows zero-order, Arrhenius-like kinetics until approximately half of the sample has transformed. Such kinetics strongly imply that melting progresses into the bulk via a rapidly moving solid-liquid interface that is likely to originate at the sample's surface. Remarkably, the apparent activation energies for the phase transformation are large; all exceed the enthalpy of vaporization of each compound and some exceed it by an order of magnitude. In fact, we find that the crystalline melting kinetics are comparable to the kinetics of dielectric α-relaxation in deeply supercooled liquids. Based on these observations, we conclude that the rate of non-isothermal melting for superheated, low-molecular-weight crystals is limited by constituent diffusion into an abnormally dense, glass-like, non-crystalline phase.

Vapor-deposited non-crystalline phase vs ordinary glasses and supercooled liquids: Subtle thermodynamic and kinetic differences.

Vapor deposition of molecules on a substrate often results in glassy materials of high kinetic stability and low enthalpy. The extraordinary properties of such glasses are attributed to high rates of surface diffusion during sample deposition, which makes it possible for constituents to find a configuration of much lower energy on a typical laboratory time scale. However, the exact nature of the resulting phase and the mechanism of its formation are not completely understood. Using fast scanning calorimetry technique, we show that out-of-equilibrium relaxation kinetics and possibly the enthalpy of vapor-deposited films of toluene and ethylbenzene, archetypical fragile glass formers, are distinct from those of ordinary supercooled phase even when the deposition takes place at temperatures above the ordinary glass softening transition temperatures. These observations along with the absolute enthalpy dependences on deposition temperatures support the conjecture that the vapor-deposition may result in formation of non-crystalline phase of unique structural, thermodynamic, and kinetic properties.

Enthalpy and high temperature relaxation kinetics of stable vapor-deposited glasses of toluene.

Stable non-crystalline toluene films of micrometer and nanometer thicknesses were grown by vapor deposition at distinct rates and probed by fast scanning calorimetry. Fast scanning calorimetry is shown to be extremely sensitive to the structure of the vapor-deposited phase and was used to characterize simultaneously its kinetic stability and its thermodynamic properties. According to our analysis, transformation of vapor-deposited samples of toluene during heating with rates in excess 10(5) K s(-1) follows the zero-order kinetics. The transformation rate correlates strongly with the initial enthalpy of the sample, which increases with the deposition rate according to sub-linear law. Analysis of the transformation kinetics of vapor-deposited toluene films of various thicknesses reveal a sudden increase in the transformation rate for films thinner than 250 nm. The change in kinetics seems to correlate with the surface roughness scale of the substrate. The implications of these findings for the formation mechanism and structure of vapor-deposited stable glasses are discussed.

Fast scanning calorimetry studies of the glass transition in doped amorphous solid water: evidence for the existence of a unique vicinal phase.

The fast scanning calorimetry (FSC) was employed to investigate glass transition phenomena in vapor deposited amorphous solid water (ASW) films doped with acetic acid, pentanol, and carbon tetrachloride. In all three cases, FSC thermograms of doped ASW films show well pronounced glass transitions at temperatures near 180 K. Systematic FSC studies of the glass transition temperature and the excess heat capacity dependence on the concentration of impurities indicate the possible existence of two distinct non-crystalline phases of H2O in binary aqueous solutions. According to our conjecture, bulk pure ASW is a glass at temperatures up to its crystallization near 205 K. However, guest molecules in the ASW matrix may be enveloped in an H2O phase which undergoes a glass transition prior to crystallization. In the case of CH3COOH, we estimate that such a viscous liquid shell contains approximately 25 H2O molecules. We discuss the implications of these findings for past studies of molecular kinetics in pure vitreous water and in binary aqueous solutions.

Bulk and interfacial glass transitions of water.

Fast scanning calorimetry (FSC) was employed to investigate glass softening dynamics in bulk-like and ultrathin glassy water films. Bulk-like water samples were prepared by vapor-deposition on the surface of a tungsten filament near 140 K where vapor-deposition results in low enthalpy glassy water films. The vapor-deposition approach was also used to grow multiple nanoscale (approximately 50 nm thick) water films alternated with benzene and methanoic films of similar dimensions. When heated from cryogenic temperatures, the ultrathin water films underwent a well manifested glass softening transition at temperatures 20 K below the onset of crystallization. However, no such transition was observed in bulk-like samples prior to their crystallization. These results indicate that thin-film water demonstrates glass softening dynamics that are dramatically distinct from those of the bulk phase. We attribute these differences to water's interfacial glass transition, which occurs at temperatures tens of degrees lower than that in the bulk. Implications of these findings for past studies of glass softening dynamics in various glassy water samples are discussed.

H/D exchange kinetics in pure and HCl doped polycrystalline ice at temperatures near its melting point: structure, chemical transport, and phase transitions at grain boundaries.

We report the results of a fast thermal desorption spectroscopy study of the H/D isotopic exchange kinetics in a few micrometer thick, pure polycrystalline ice film and in ice films doped with HCl. Using the isotopic exchange reaction as a probe of transport processes in ice, we determined the effective H/D interdiffusion coefficients, D(eff), in pure and doped polycrystalline ice at temperatures ranging from -18 to -1 degree C. In the case of pure polycrystalline ice, D(eff) demonstrates an Arrhenius dependence on temperature with an effective activation energy of 69+/-3 kJ mol(-1) and a pre-exponential of 10(9+/-0.5) microm(2) ms(-1) up to -2 degrees C. According to our analysis, H/D interdiffusion coefficient at the grain boundaries also shows an Arrhenius dependence on temperature with an activation energy of 69+/-3 kJ mol(-1) and a pre-exponential of 10(11+/-1) microm(2) ms(-1). However, the addition of 0.04% of HCl results in a marked deviation of D(eff) from Arrhenius law at -8 degrees C, which is attributed to premelting at intersections of grain boundaries. We discuss the structure and transport properties of condensed aqueous phase at grain boundaries in polycrystalline ice at various temperatures.

Fast thermal desorption spectroscopy study of H/D isotopic exchange reaction in polycrystalline ice near its melting point.

Using fast thermal desorption spectroscopy, a novel technique developed in our laboratory, we investigated the kinetics of HD isotopic exchange in 3 microm thick polycrystalline H2O ice films containing D2O layers at thicknesses ranging from 10 to 300 nm at a temperature of -2.0+/-1.5 degrees C. According to our results over the duration of a typical fast thermal desorption experiment (3-4 ms), the isotopic exchange is confined to a 50+/-10 nm wide reaction zone located at the boundary between polycrystalline H2O and D2O ice. Combining these data with a theoretical analysis of the diffusion in polycrystalline medium, we establish the range of possible values for water self-diffusion coefficients and the grain boundary widths characteristic of our ice samples. Our analysis shows that for the grain boundary width on the order of a few nanometers, the diffusivity of D2O along the grain boundaries must be at least two orders of magnitude lower than that in bulk water at the same temperature. Based on these results, we argue that, in the limit of low concentrations of impurities, polycrystalline ice does not undergo grain boundary premelting at temperatures up to -2 degrees C.

Fast thermal desorption spectroscopy study of morphology and vaporization kinetics of polycrystalline ice films.

Fast thermal desorption spectroscopy was used to investigate the vaporization kinetics of thin (50-100 nm) H(2)O(18) and HDO tracer layers from 2-5 microm thick polycrystalline H(2)O(16) ice films at temperatures ranging from -15 to -2 degrees C. The isothermal desorption spectra of tracer species demonstrate two distinct peaks, alpha and beta, which we attribute to the vaporization of H(2)O(18) initially trapped at or near the grain boundaries and in the crystallites of the polycrystalline ice, respectively. We show that the diffusive transport of the H(2)O(18) and HDO tracer molecules in the bulk of the H(2)O(16) film is slow as compared to the film vaporization. Thus, the two peaks in the isothermal spectra are due to unequal vaporization rates of H(2)O(18) from grain boundary grooves and from the crystallites and, therefore, can be used to determine independently the vaporization rate of the single crystal part of the film and rate of thermal etching of the film. Our analysis of the tracer vaporization kinetics demonstrates that the vaporization coefficient of single crystal ice is significantly greater than those predicted by the classical vaporization mechanism at temperatures near ice melting point. We discuss surface morphological dynamics and the bulk transport phenomena in single crystal and polycrystalline ice near 0 degrees C.

Glass transition in pure and doped amorphous solid water: an ultrafast microcalorimetry study.

Using an ultrafast scanning microcalorimetry apparatus capable of heating rates in excess of 10(5) Ks, we have conducted the first direct measurements of thermodynamic properties of pure and doped amorphous solid water (also referred to as low density amorphous ice) in the temperature range from 120 to 230 K. Ultrafast microcalorimetry experiments show that the heat capacity of pure amorphous solid water (ASW) remains indistinguishable from that of crystalline ice during rapid heating up to a temperature of 205+/-5 K where the ASW undergoes rapid crystallization. Based on these observations, we conclude that the enthalpy relaxation time in pure ASW must be greater than 10(-5) s at 205 K. We argue that this result contradicts the assignment of glass transition temperature to 135 K and that ASW may undergo fragile to strong transition at temperatures greater than 205 K. Unlike pure ASW, we observe an approximately twofold rise in heat capacity of CH3COOH doped ASW at 177+/-5 K. We discuss results of past studies taking into account possible influence of impurities and confinement on physical properties of ASW.

Ice nucleation on BaF2(111).

The mechanism of heterogeneous ice nucleation on inorganic substrates is not well understood despite work on AgI and other materials over the past 50 years. We have selected BaF(2) as a model substrate for study since its (111) surface makes a near perfect match with the lattice of the basal face of I(h) ice and would appear to be an ideal nucleating agent. Two series of experiments were undertaken. In one, nucleation of thin film water formed from deposition of vapor on BaF(2)(111) faces was explored with the finding that supercooling to -30 degrees C was required before freezing occurred. In the other series, nucleation of liquid water on submerged BaF(2) crystals was studied. Here supercooling to -15 degrees C was needed before ice formed. The reason why BaF(2) is such a poor nucleating agent contains clues to realistic mechanisms of heterogeneous nucleation. Our explanation of these results follows the model of Fletcher [J. Chem. Phys. 29, 572 (1958)] who showed that heterogeneous ice nucleating ability depends on how well ice wets a substrate. In this view, a smooth BaF(2)(111) face is poor at nucleation because ice only partially wets its surface. In an extension of Fletcher's model, our calculations, consistent with the experimental results demonstrate that the pitting of a submerged BaF(2) crystal dramatically improves its ice nucleating ability.

The vaporization rate of ice at temperatures near its melting point.

The first study of free vaporization kinetics of ice at temperatures near its melting point is reported. The experimental approach employed is based on a unique combination of thermal desorption spectroscopy, microcalorimetry, and time-of-flight mass spectrometry, making it possible to overcome challenges associated with the introduction of volatile solids into a high vacuum environment. Measurements of the vaporization rate of polycrystalline ice demonstrate that the vaporization kinetics deviate dramatically from those predicted by a simple mobile precursor mechanism. The vaporization rate follows Arrhenius behavior from -40 to 0 degrees C with an effective activation energy of 50+/-4 kJ/mol, which is significantly higher than the value predicted by the simple mobile precursor mechanism. Extrapolation of earlier measurements conducted below -40 degrees C yields a value of approximately 0.02 at 0 degrees C for the vaporization coefficient alphav. In contrast, experimentally determined vaporization coefficient is found to be 0.7+/-0.3 and shows a weak dependence on temperature up to the bulk melting point. The role of possible surface phase transitions in the mechanisms of release and uptake of H2O and other chemical species by ice surfaces is discussed.