Nucleation and Growth of the Supercooled Liquid Phase Control Glass Transition in Bulk Ultrastable Glasses.

We report the anomalous bulk transformation of vapor deposited stable glasses into the liquid state. The transformation proceeds through two competing parallel processes: partial rejuvenation of the stable glass and nucleation and growth of liquid patches within the glass. The kinetics of the transformation extracted from heat capacity curves after isothermal runs is dominated by the heterogeneous nucleation and growth process that initiates at preexisting seeds and propagates radially at a velocity proportional to the alpha relaxation time. Remarkably, the distance between the activation seeds is independent of temperature within experimental uncertainty and amounts to several micrometers, a value in close agreement with the crossover length for TPD glasses. We speculate the initiation sites for the transformation of the glass into the supercooled liquid are localized regions of lower stability (or density).

Multiple glass transitions in vapor-deposited orientational glasses of the most fragile plastic crystal Freon 113.

We investigate by fast-scanning nanocalorimetry the formation of Freon 113 films from the vapor phase at deposition temperatures ranging from 50 to 120 K, that is, spanning above and below the transition temperature of the glassy crystal to the plastic crystal (Tgc = 72 K). Analysis of the heat capacity curves indicates that vapor deposition at T < Tgc of the highly fragile Freon 113 yields structural and orientational glasses in the as-deposited state depending on the temperature range of deposition. Interestingly, growing above Tgc produces plastic crystals with a conformational ratio C1/Cs that changes with Tdep above and below 110-120 K, the temperature at which previous works have identified the arrest of the transformations between the C1 and Cs conformers.

Author Correction: Impact of pore anisotropy on the thermal conductivity of porous Si nanowires.

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Impact of pore anisotropy on the thermal conductivity of porous Si nanowires.

Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87 W·m-1·K-1 for 90 nm diameter wires with 35-40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we find a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifies the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient influences phonon transport along the axis of the NW. Our findings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices.

Relaxation dynamics of glasses along a wide stability and temperature range.

While lots of measurements describe the relaxation dynamics of the liquid state, experimental data of the glass dynamics at high temperatures are much scarcer. We use ultrafast scanning calorimetry to expand the timescales of the glass to much shorter values than previously achieved. Our data show that the relaxation time of glasses follows a super-Arrhenius behaviour in the high-temperature regime above the conventional devitrification temperature heating at 10 K/min. The liquid and glass states can be described by a common VFT-like expression that solely depends on temperature and limiting fictive temperature. We apply this common description to nearly-isotropic glasses of indomethacin, toluene and to recent data on metallic glasses. We also show that the dynamics of indomethacin glasses obey density scaling laws originally derived for the liquid. This work provides a strong connection between the dynamics of the equilibrium supercooled liquid and non-equilibrium glassy states.

Ultrastable glasses portray similar behaviour to ordinary glasses at high pressure.

Pressure experiments provide a unique opportunity to unravel new insights into glass-forming liquids by exploring its effect on the dynamics of viscous liquids and on the evolution of the glass transition temperature. Here we compare the pressure dependence of the onset of devitrification, Ton, between two molecular glasses prepared from the same material but with extremely different ambient-pressure kinetic and thermodynamic stabilities. Our data clearly reveal that, while both glasses exhibit different dTon/dP values at low pressures, they evolve towards closer calorimetric devitrification temperature and pressure dependence as pressure increases. We tentatively interpret these results from the different densities of the starting materials at room temperature and pressure. Our data shows that at the probed pressures, the relaxation time of the glass into the supercooled liquid is determined by temperature and pressure similarly to the behaviour of liquids, but using stability-dependent parameters.

In-plane thermal conductivity of sub-20 nm thick suspended mono-crystalline Si layers.

We measure the thermal conductivity of a 17.5-nm-thick single crystalline Si layer by using a suspended structure developed from a silicon-on-insulator wafer, in which the Si layer bridges the suspended platforms. The obtained value of 19 Wm(-1) K(-1) at room temperature represents a tenfold reduction with respect to bulk Si. This design paves the way for subsequent lateral nanostructuration of the layer with lithographic techniques, to define different geometries such as Si nanowires, nanostrips or phononic grids. As a proof of concept, nanostrips of 0.5 × 10 µm have been defined by focused ion beam (FIB) in the ultrathin Si layer. After the FIB cutting process with Ga ions at 30 kV and 100 pA, the measured thermal conductivity dramatically decreased to 1.7 Wm(-1) K(-1), indicating that the structure became severely damaged (amorphous). Re-crystallization of the structure was promoted by laser annealing while monitoring the Raman spectra. The thermal conductivity of the layer increased again to a value of 9.5 Wm(-1) K(-1) at room temperature, below that of the single crystalline material due to phonon scattering at the grain boundaries.

Review on measurement techniques of transport properties of nanowires.

Physical properties at the nanoscale are novel and different from those in bulk materials. Over the last few decades, there has been an ever growing interest in the fabrication of nanowire structures for a wide variety of applications including energy generation purposes. Nevertheless, the study of their transport properties, such as thermal conductivity, electrical conductivity or Seebeck coefficient, remains an experimental challenge. For instance, in the particular case of nanostructured thermoelectrics, theoretical calculations have shown that nanowires offer a promising way of enhancing the hitherto low efficiency of these materials in the conversion of temperature differences into electricity. Therefore, within the thermoelectrical community there has been a great experimental effort in the measurement of these quantities in actual nanowires. The measurements of these properties at the nanoscale are also of interest in fields other than energy, such as electrical components for microchips, field effect transistors, sensors, and other low scale devices. For all these applications, knowing the transport properties is mandatory. This review deals with the latest techniques developed to perform the measurement of these transport properties in nanowires. A thorough overview of the most important and modern techniques used for the characterization of different kinds of nanowires will be shown.

Acoustic-like dynamics of amorphous drugs in the THz regime.

The high frequency dynamics of Indomethacin and Celecoxib glasses has been investigated by inelastic x-ray scattering, accessing a momentum-energy region still unexplored in amorphous pharmaceuticals. We find evidence of phonon-like acoustic dynamics, and determine the THz behavior of sound velocity and acoustic attenuation. Connections with ordinary sound propagation are discussed, along with the relation between fast and slow degrees of freedom as represented by non-ergodicity factor and kinetic fragility, respectively.

Glass transition in ultrathin films of amorphous solid water.

Nanocalorimetry at ultrafast heating rates is used to investigate the glass transition of nanometer thick films of metastable amorphous solid water grown by vapor deposition in an ultrahigh vacuum environment. Apparent heat capacity curves exhibit characteristic features depending on the deposition temperature. While films grown at T ≥ 155 K are completely crystallized, those deposited at 90 K show a relaxation exotherm prior to crystallization. Films grown between 135 and 140 K and subsequently cooled down to 90 K reveal a clear endothermic feature before crystallization, which is compatible with a glass-to-liquid transition. The onset temperature is located at 174 K at a heating rate of 2.4 × 10(4) K/s and is independent of film thickness in the range of 16-150 nm. Comparison of our data with other calorimetric measurements at various heating rates suggests that water is a strong glass former in the deeply supercooled state.

Anomalous Transformation of Vapor-Deposited Highly Stable Glasses of Toluene into Mixed Glassy States by Annealing Above Tg.

Vapor-deposited glasses have recently emerged as a remarkable new class of materials that can form much denser and stable glasses than those obtained by cooling the liquid. These new amorphous materials reach lower regions of the energy landscape and may impact important technologies that use vapor-deposition. Here, we report on the formation of a glass with two distinct glassy states obtained through the partial annealing of highly stable vapor-deposited glassy films of toluene. The resulting glass exhibits two clear heat capacity overshoots with different onset and fictive temperatures. The transformation times of the ultrastable glass are around 10(5) times slower than the structural relaxation time (τα) of supercooled liquid toluene. We show that the nature of the transformed glass depends on the annealing temperature above Tg. This finding suggests the formation of distinct supercooled liquids at temperatures slightly above Tg during the transformation of the highly stable glass. Our results are compatible with the existence of polyamorphism in toluene.

Accelerated aging in ultrathin films of a molecular glass former.

We report the thermodynamic measurement of the enthalpy released during the aging of supported films of a molecular glass former, toluene, at temperatures well below the glass transition temperature. By using microfabricated devices with very short equilibration times (below 1 s), we evidence a remarkable variation of the relaxation rate on decreasing film thickness from 100 nm down to a 7 nm thick film. Our results demonstrate that surface atoms are more efficient than bulk atoms in attaining low energy configurations within the potential energy landscape.

Crystallisation of amorphous germanium thin films.

By combining cross-sectional transmission and scanning electron microscopy with Raman scattering we have investigated the mechanism of nanocrystal formation in ultrathin amorphous SiO2/Ge/SiO2 trilayers grown by e-beam evaporation as a function of annealing temperature and a-Ge layer thickness. We observe that with decreasing a-Ge thickness the amorphous-to-crystalline (a-to-c) transition occurs at considerably higher temperatures, even avoiding crystallisation for very thin films below 2 nm thickness. Furthermore, we demonstrate that the formation of Ge nanocrystals by annealing at around 900 degrees C takes place driven by a liquid-mediated mechanism. As indicated by the observed microstructure, the metallic liquid film dewets from the surface forming droplets that upon cooling and under the influence of the SiO2 capping layer, solidify into barrel-type nanocrystals.

In situ nanocalorimetry of thin glassy organic films.

In this work, we describe the design and first experimental results of a new setup that combines evaporation of liquids in ultrahigh vacuum conditions with in situ high sensitivity thermal characterization of thin films. Organic compounds are deposited from the vapor directly onto a liquid nitrogen cooled substrate, permitting the preparation and characterization of glassy films. The substrate consists of a microfabricated, membrane-based nanocalorimeter that permits in situ measurements of heat capacity under ultrafast heating rates (up to 10(5) K/s) in the temperature range of 100-300 K. Three glass forming liquids-toluene, methanol, and acetic acid-are characterized. The spikes in heat capacity related to the glass-transition temperature, the fictive temperature and, in some cases, the onset temperature of crystallization are determined for several heating rates.

Zirconia-toughened hydroxyapatite ceramic obtained by wet sintering.

A toughened hydroxyapatite (HA) ceramic has been obtained through the incorporation of magnesia partially stabilized zirconia (Mg-PSZ) under uniaxial pressing and sintering in wet oxygen at 1250 degrees C for 4 h. The powder X-ray diffraction (XRD) patterns and infrared spectra (FT-IR) show that HA is the only calcium phosphate phase present. The composite (MgPSZ-HA) has a density of 94% the theoretical value. The bending strength and the fracture toughness are around 50% higher for Mg-PSZ reinforced than for HA. The grain size and the fracture surface were studied by scanning electron microscopy (SEM). The influence of the Mg-PSZ particles on the fracture mechanism of the HA ceramic is discussed.