Evidence for a simple monatomic ideal glass former: the thermodynamic glass transition from a stable liquid phase

Under cooling, a liquid can undergo a transition to the glassy state either as a result of a continuous slowing down or by a first-order polyamorphous phase transition. The second scenario has so far always been observed in a metastable liquid domain below the melting point where crystalline nucleation interfered with the glass formation. We report the first observation of the liquid-glass transition by a first-order polyamorphous phase transition from the equilibrium stable liquid phase. The observation was made in a molecular dynamics simulation of a one-component system with a model metallic pair potential. In this way, the model, demonstrating the thermodynamic glass transition from a stable liquid phase, may be regarded as a candidate for a simple monatomic ideal glass former. This observation is of conceptual importance in the context of continuing attempts to resolve the long-standing Kauzmann paradox. The possibility of a thermodynamic glass transition from an equilibrium melt in a metallic system also indicates a new strategy for the development of bulk metallic glass-forming alloys.     

聚(乙烯基对苯二甲酸二烷基酯)的非寻常相转变

以聚(乙烯基对苯二甲酸二烷基酯)(PDAVT)为对象,用热分析、X射线衍射、流变及固体核磁共振等方法考察了甲壳型液晶高分子中存在的"低温无序、高温有序"的非寻常相行为.实验证明,在升温过程中,烷基基团为丁基、己基和辛基的PDAVT(P4、P6和P8)可从无定形态发育出柱状(Col)液晶相,导致在高温区体系模量升高.降温时P6和P8可完全回到无定形态,表现出典型的"各向同性相重入"行为.PDAVT液晶相畴的生长具有一维生长的特点,是成核控制的,升高温度可加快Col相的形成.固体核磁共振实验表明,样品从无定形态转变为Col相的过程与侧链运动性的不断加强有密切关系.我们也初步探索了剪切场或拉伸场对液晶转变的影响,发现当温度不高时,剪切或拉伸既不改变液晶转变温度,也没有提高液晶态样品的液晶化程度.综合分析多方面的实验结果可以看出,PDAVT的非寻常相行为是熵主导的.侧链的加速运动会增强"甲壳效应",Col相是侧链熵最大化的结果.     

Tunable red-green upconversion luminescence in novel transparent glass ceramics containing Er: NaYF4 nanocrystals

To develop NaYF(4) as bulk luminescence material, transparent glass ceramics containing Er(3+): NaYF(4) nanocrystals were fabricated for the first time, and the influences of heat-treatment temperature and Er(3+) doping level on their upconversion luminescence were investigated. With increasing heating temperature, the upconversion intensity enhanced accordingly, attributing to the incorporation of more Er(3+) into the grown NaYF(4). Notably, when the heating temperature reached 650 degrees C, the upconversion intensity augmented drastically due to the occurrence of phase transition from the cubic NaYF(4) to the hexagonal one. Interestingly, for the samples heat-treated at 620 degrees C, when the Er(3+) doping level was increased from 0.05 to 2.0 mol %, the upconversion emission was whole-range tunable from monochromatic green to approximately monochromatic red, which could be mainly attributed to the cross-relaxation between Er(3+) ions. The excellent optical properties and its convenient, low-cost synthesis of the present glass ceramic imply that it is an excellent substitution material for the unobtainable bulk NaYF(4) crystal, potentially applicable in many fields.     

Surface and bulk morphology of cold-crystallized poly(ethylene terephthalate)

The morphology/habit of crystals of cold-crystallized poly(ethylene terephthalate) (PET) has been evaluated using scanning and transmission electron microscopy and using atomic force microscopy. The combination of different preparation and analysis techniques allowed assessing the structure at the nanometer scale of films of PET at both the surface and the bulk. It is found that crystals formed on heating the amorphous glass to a temperature higher than the glass transition temperature are of lamellar shape in the bulk and almost isometric habit at the surface. This finding is explained by different rates of nucleation/crystallization in the bulk and at the surface, being supported by the observation of nanometer-scale surface heterogeneities after quenching PET to ambient temperature before crystallization was initiated by heating.     

Chemical heterogeneity in emulsion copolymers of carboxylic monomers

Copolymer latices of butylacrylate (BA) with acrylic and methacrylic acid (AA and MAA) were prepared by batch type emulsion polymerization, and, for comparison, copolymers with identical monomer composition were prepared by batch type solution polymerization.The distribution of the carboxylic monomers in the latex particles and the serum was studied by density gradient and sedimentation experiments with the analytical ultracentrifuge. Dynamic mechanical measurements of films of these copolymers were used to determine the storage and loss moduli as a function of temperature. From these measurements the position and extension of the glass transition range on the temperature scale is obtained. For heterogeneous emulsion copolymers with two glass transition temperatures the distribution of the carboxylic monomer units in the different copolymer phases can be determined. Electron microscopy of ultra thin cross-sections of stained films gave further insight into the film morphology.The combination of the results obtained with the different methods gives rise to the following clues: In the BA/AA latices about 40% (by weight) of the total AA used in the recipe are found in the serum as a water soluble polymer, about 50% are found to increase the glass transition temperatureT _g of the bulk of the BA copolymer and, therefore, are thought to be incorporated into the interior of the latex particles, and the remaining 10% are, conclusively, located on the particle surface.In the BA/MAA latices no water soluble copolymer could be detected in the serum, about 90% of the MAA used is found in the bulk of the copolymer, and about 10% form a second hard phase on the surface of the latex particles.Dynamic mechanical measurements on the copolymer latex films show at least two phases with different glass transition temperatures: the bulk of the copolymer with a relatively low content of (M)AA units and a glass transition range at low temperatures, and a second (M)AA rich phase with a highT _g.The latter phase forms a honeycomb-like structure surrounding the packed latex particles. That results in a three-dimensional network of polymer with a highT _g extending throughout the latex film. In spite of the fact that this phase is built from a small fraction of the total copolymer only, it has a very pronounced influence on the performance behaviour of latex films.Dedicated to Professor Dr. R. Manecke on the occasion of his 70th birthday.     

Heterogeneous critical nucleation on a completely wettable substrate

Heterogeneous nucleation of a new bulk phase on a flat substrate can be associated with the surface phase transition called wetting transition. When this bulk heterogeneous nucleation occurs on a completely wettable flat substrate with a zero contact angle, the classical nucleation theory predicts that the free-energy barrier of nucleation vanishes. In fact, there always exists a critical nucleus and a free-energy barrier as the first-order prewetting transition will occur even when the contact angle is zero. Furthermore, the critical nucleus changes its character from the critical nucleus of surface phase transition below bulk coexistence (undersaturation) to the critical nucleus of bulk heterogeneous nucleation above the coexistence (oversaturation) when it crosses the coexistence. Recently, Sear [J. Chem. Phys. 129, 164510 (2008)] has shown, by a direct numerical calculation of nucleation rate, that the nucleus does not notice this change when it crosses the coexistence. In our work, the morphology and the work of formation of critical nucleus on a completely wettable substrate are re-examined across the coexistence using the interface-displacement model. Indeed, the morphology and the work of formation changes continuously at the coexistence. Our results support the prediction of Sear and will rekindle the interest on heterogeneous nucleation on a completely wettable substrate.     

Nanostructures boost the thermoelectric performance of PbS

In situ nanostructuring in bulk thermoelectric materials through thermo-dynamic phase segregation has established itself as an effective paradigm for optimizing the performance of thermoelectric materials. In bulk PbTe small compositional variations create coherent and semicoherent nanometer sized precipitates embedded in a PbTe matrix, where they can impede phonon propagation at little or no expense to the electronic properties. In this paper the nanostructuring paradigm is for the first time extended to a bulk PbS based system, which despite obvious advantages of price and abundancy, so far has been largely disregarded in thermoelectric research due to inferior room temperature thermoelectric properties relative to the pristine fellow chalcogenides, PbSe and PbTe. Herein we report on the synthesis, microstructural morphology and thermoelectric properties of two phase (PbS)(1-x)(PbTe)(x)x = 0-0.16 samples. We have found that the addition of only a few percent PbTe to PbS results in a highly nanostructured material, where PbTe precipitates are coherently and semicoherently embedded in a PbS matrix. The present (PbS)(1-x)(PbTe)(x) nanostructured samples show substantial decreases in lattice thermal conductivity relative to pristine PbS, while the electronic properties are left largely unaltered. This in turn leads to a marked increase in the thermoelectric figure of merit. This study underlines the efficiency of the nanostructuring approach and strongly supports its generality and applicability to other material systems. We demonstrate that these PbS-based materials, which are made primarily from abundant Pb and S, outperform optimally n-type doped pristine PbTe above 770 K.     

Microfluorescence analysis of nanostructuring inhomogeneity in optical fibers with embedded gallium oxide nanocrystals

A spectroscopic protocol is proposed to implement confocal microfluorescence imaging to the analysis of microinhomogeneity in the nanocrystallization of the core of fibers belonging to a new kind of broadband fiber amplifier based on glass with embedded nanocrystals. Nanocrystallization, crucial for achieving an adequate light emission efficiency of transition metal ions in these materials, has to be as homogeneous as possible in the fiber to assure optical amplification. This requirement calls for a sensitive method for monitoring nanostructuring in oxide glasses. Here we show that mapping microfluorescence excited at 633 nm by a He-Ne laser may give a useful tool in this regard, thanks to quasi-resonant excitation of coordination defects typical of germanosilicate materials, such as nonbridging oxygens and charged Ge-O-Ge sites, whose fluorescence are shown to undergo spectral modifications when nanocrystals form into the glass. The method has been positively checked on prototypes of optical fibers--preventively characterized by means of scanning electron microscopy and energy dispersive spectroscopy--fabricated from preforms of Ni-doped Li?O-Na?O-Sb?O?-Ga?O?-GeO?-SiO? glass in silica cladding and subjected to heat treatment to activate gallium oxide nanocrystal growth. The method indeed enables not only the mapping of the crystallization degree but also the identification of drawing-induced defects in the fiber cladding.     

Size dependence of the solid-solid phase transition in CdSe nanocrystals

High pressure optical absorption spectra are presented for CdSe nanocrystals as a function of size. The spectra show a transition to a high pressure Rock Salt type phase at pressure greatly elevated from the bulk. The size dependence of the transition pressure can be explained in part by an increased surface tension in the Rock Salt phase relative to the low pressure tetrahedral phase.     

Diffusion coefficients for crystal nucleation and growth in deeply undercooled glass-forming liquids

We calculate, employing the classical theory of nucleation and growth, the effective diffusion coefficients controlling crystal nucleation of nanosize clusters and the subsequent growth of micron-size crystals at very deep undercoolings, below and above Tg, using experimental nucleation and growth data obtained for stoichiometric Li2O.2SiO2 and Na2O.2CaO.3SiO2 glasses. The results show significant differences in the magnitude and temperature dependence of these kinetic coefficients. We explain this difference showing that the composition and/or structure of the nucleating critical clusters deviate from those of the stable crystalline phase. These results for diffusion coefficients corroborate our previous conclusion for the same glasses, based on different experiments, and support the view that, even for the so-called case of stoichiometric (polymorphic) crystallization, the nucleating phase may have a different composition and/or structure as compared to the parent glass and the evolving macroscopic crystalline phase. This finding gives a key to explain the discrepancies between calculated (by classical nucleation theory) and experimentally observed nucleation rates in these systems, in particular, and in deeply undercooled glass-forming liquids, in general.     

Effect of Nanoscale Confinement on Freezing of Modified Water at Room Temperature and Ambient Pressure

Understanding the phase behavior of confined water is central to fields as diverse as heterogeneous catalysis, corrosion, nanofluidics, and to emerging energy technologies. Altering the state points (temperature, pressure, etc.) or introduction of a foreign surface can result in the phase transformation of water. At room temperature, ice nucleation is a very rare event and extremely high pressures in the GPa–TPa range are required to freeze water. Here, we perform computer experiments to artificially alter the balance between electrostatic and dispersion interactions between water molecules, and demonstrate nucleation and growth of ice at room temperature in a nanoconfined environment. Local perturbations in dispersive and electrostatic interactions near the surface are shown to provide the seed for nucleation (nucleation sites), which lead to room temperature liquid–solid phase transition of confined water. Crystallization of water occurs over several tens of nanometers and is shown to be independent of the nature of the substrate (hydrophilic oxide vs. hydrophobic graphene and crystalline oxide vs. amorphous diamond‐like carbon). Our results lead us to hypothesize that the freezing transition of confined water can be controlled by tuning the relative dispersive and electrostatic interaction.     

Syringe pump-assisted synthesis of water-soluble cubic structure Ag2Se nanocrystals by a cation-exchange reaction

Water-soluble cubic structure Ag(2)Se (alpha-Ag(2)Se) nanocrystals smaller than 5 nm can be obtained by cation-exchange reaction at room temperature, using water-dispersed ZnSe nanocrystals as precursors, which is achieved by controlling the injection speed of AgNO(3) solutions via a syringe pump in the presence of the stabilizer of trisodium citrate. Meanwhile, the thermal stability of the product Ag(2)Se nanocrystals is studied. The results show that the mean sizes and shapes of the precursor ZnSe and product Ag(2)Se nanocrystals are similar, and Se anion sublattices between them are topotaxial. In addition, no phase transition is observed for the product Ag(2)Se (cubic structure) nanocrystals below 180 degrees C. The present synthetic method based on cation-exchange reactions can also be applied to the syntheses of PbSe and CuSe nanocrystals.     

不同结晶度的乙二醇及其水溶液玻璃化转变与焓松弛

为了考察晶体成分对无定形成分玻璃化转变和结构松弛行为的影响,利用差示扫描量热法(DSC),结合低温显微技术,研究了乙二醇(EG)及其50％水溶液在不同结晶度时的玻璃化转变和焓松弛行为.采用等温结晶方法控制骤冷的部分结晶玻璃体中的晶体份额.DSC结果表明,对于部分结晶的EG,只有单一的玻璃化转变过程,而对于50％EG,当结晶度不同时,不同程度地表现出两次玻璃化转变（无定形相Ⅰ和无定形相Ⅱ）.相Ⅰ的玻璃化转变温度和完全无定形态的含水EG的玻璃化转变温度相一致;相Ⅱ的玻璃化转变温度要比此温度约高6 ℃.低温显微观察结果印证了DSC实验结果.DSC等温退火的实验和KWW(Kohlrausch-Williams-Watts)衰变函数分析结果表明,EG无定形和50％EG中的两种无定形有不同的焓松弛行为.     

Superionic phase transition in silver chalcogenide nanocrystals realizing optimized thermoelectric performance

Thermoelectric has long been recognized as a potentially transformative energy conversion technology due to its ability to convert heat directly into electricity. However, how to optimize the three interdependent thermoelectric parameters (i.e., electrical conductivity σ, Seebeck coefficient S, and thermal conductivity κ) for improving thermoelectric properties is still challenging. Here, we put forward for the first time the semiconductor-superionic conductor phase transition as a new and effective way to selectively optimize the thermoelectric power factor based on the modulation of the electric transport property across the phase transition. Ultra low value of thermal conductivity was successfully retained over the whole investigated temperature range through the reduction of grain size. As a result, taking monodisperse Ag(2)Se nanocrystals for an example, the maximized ZT value can be achieved around the temperature of phase transition. Furthermore, along with the effective scattering of short-wavelength phonons by atomic defects created by alloying, the alloyed ternary silver chalcogenide compounds, monodisperse Ag(4)SeS nanocrystals, show better ZT value around phase transition temperature, which is cooperatively contributed by superionic phase transition and alloying at nanoscale.     

Theory of bulk, surface and interface phase transition kinetics in thin films

We report a theoretical study of phase transition kinetics in confined two-dimensional systems, motivated by recent experimental results on the amorphous-to-crystalline transition in supported, thin amorphous water films [E.H.G. Backus, M.L. Grecea, A.W. Kleyn, and M. Bonn, Phys. Rev. Lett. (to be published)]. We generalize and extend existing theories to simultaneously describe the converted (crystalline) fractions in the bulk, at the sample-vacuum surface, and at the sample-support interface as a function of time. The general approach presented here results in expressions for the time-dependent converted bulk, surface, and interface fractions, for arbitrary desorption rate from the thin film, nucleation and growth rates and also includes finite nucleation grain size. The converted bulk, surface, and interface fractions are calculated for nucleation of the new phase occurring (i) in the bulk, (ii) at the support-sample interface, and (iii) at the sample surface (sample-vacuum interface), resulting in nine expressions. The results demonstrate the advantage of monitoring bulk, surface and interface fractions simultaneously to make definite statements regarding the location of the nucleation, and to reliably determine the values of the relevant crystallization parameters.     

Simulation of the pressure-driven wurtzite to rock salt phase transition in nanocrystals

Nanocrystals in the size range 12-21 nm of a model binary ionic material in the wurtzite (B4) structure were constructed with morphologies which minimize the surface energy. These were then embedded in a pressurization medium, consisting of a binary Lennard-Jones-type fluid and progressively pressurized in "constant pressure" molecular dynamics simulation runs. Phase transitions to the rocksalt (B1) phase were confirmed by examining calculated powder diffraction patterns, which show the same changes in features as seen for experimental systems. By directly observing the atomic trajectories throughout the duration of the transition the local mechanism has been identified. The transition proceeds via a trigonal bipyramidal intermediate, denoted as the h-MgO structure. It is initiated by a single nucleation event at a [1120]B4 surface with subsequent growth of the B1 region throughout the remainder of the nanocrystal. The consequences of this mechanism for the particle shape of the product phase are detailed and contrasted with those previously found for initially zincblende (B3) structured nanoparticles, using the same interaction potential. The observed transition pressures are elevated relative to the thermodynamically predicted pressure for the bulk, but there is no observable system size effect on the transition pressure across the size range of nanocrystals investigated.     

Nucleation of bulk phases in the HCl/H2O system

We report experimental results on the low-temperature uptake of HCl on H(2)O ice (ice). HCl was deposited on the surface at greater than monolayer amounts at 85 K, and the ice substrate was heated. The temperature dependence of the HCl vapor pressure from this phase was measured from 110 to 150 K, with the nucleation of a bulk hydrate phase observed at 150 K. Measurements were conducted in a closed system by simultaneous application of gas phase mass spectrometry and surface spectroscopy to characterize vapor/solid equilibrium and the nucleation of bulk hydrate phases. Combining the nucleation data reported here with data we reported previously (180 to 200 K) and data from two other laboratories (165 and 170 K), the thermodynamic boundaries for the nucleation of both the metastable bulk solution and bulk hydrate phases subsequent to monolayer adsorption of HCl have been determined. The nucleation of the metastable bulk solution phase occurs promptly at monolayer coverage at the ice/liquid coexistence boundary on the binary bulk phase diagram. The nucleation of the bulk hexahydrate occurs from this metastable solution along a locus of points defining a state of constant solution free energy. This measured free energy is -51.2 +/- 0.9 kJ/mol. Finally, the temperature dependence of the HCl vapor pressure from the low-temperature phase is reported here for the first time and is consistent with that of the metastable solution predicted by this thermodynamic model of uptake, extending the range of validity of this model of adsorption followed by bulk solution and hydrate nucleation to a lower bound in temperature of 110 K.     

Phase separation dynamics of a poly(vinyl methyl ether)/polystyrene (PVME/PS) blend studied by ultrafast differential scanning calorimetry

In this work, ultrafast differential scanning calorimetry (UFDSC) is used to study the dynamics of phase separation. Taking poly(vinyl methyl ether)/polystyrene (PVME/PS) blend as the example, we firstly obtained the phase diagram that has lower critical solution temperature (LCST), together with the glass transition temperature (T_g) of the homogeneous blend with different composition. Then, the dynamics of the phase separation of the PVME/PS blend with a mass ratio of 7:3 was studied in the time range from milliseconds to hours, by the virtue of small time and spatial resolution that UFDSC offers. The time dependence of the glass transition temperature (T_g) of PVME‐rich phase, shows a distinct change when the annealing temperature (T_a) changes from below to above 385 K. This corresponds to the transition from the nucleation and growth (NG) mechanism to the spinodal decomposition (SD) mechanism, as was verified by morphological and rheometric investigations. For the SD mechanism, the temperature‐dependent composition evolution in PVME‐rich domain was found to follow the Williams–Landel–Ferry (WLF) laws. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1357–1364     

Pressure-volume-temperature properties of blends of poly(2,6-dimethyl-1,4-phenylene ether) with polystyrene

The pressure-volume-temperature (PVT) properties of blends of poly(2,6-dimethyl-1,4-phenylene ether) (PPO) with polystyrene (PS) have been studied experimentally in both the glassy and melt states at 0, 20, 40, 50, 60, 80, and 100% PPO content. In all compositions a strong glass transition was observed varying linearly with composition. For all but the 40% PPO composition this was the only transition, indicating molecular compatibility of the components in these blends. The 40% PPO composition showed a very weak second transition near the glass transition of pure PS. A small amount of phase separation may have occurred in this blend. The data for the glassy and melt states were fitted to an empirical equation of state based on the Tait equation. The volume of the melts at constant pressure and temperature showed a virtually linear dependence on composition. Any negative excess volume of mixing compatible with the data would have to be very small, smaller than expected from previous measurements in the glassy state. Various properties relating to the glassy and melt states and to the glass transition were evaluated and are discussed as a function of composition. It was found that most properties of the glasses could not be modeled by simple functions of composition.     

Phase formation and controlled nanostructuring in amorphous Fe80B20 alloy

The temperature dependence of the volume fraction of the crystalline phase in Fe₈₀B₂₀ amorphous alloy is calculated using equations from the homogeneous nucleation theory of binary systems. It is shown that the crystallization of Fe₈₀B₂₀ alloy is two-stage, as is confirmed by the experimental data obtained by means of highly sensitive dilatometry and X-ray diffractometry. On the basis of results of calculations performed within the theory of the high-temperature stability of amorphous alloys, two areas of its practical application are proposed: (i) enhancing the thermal stability of amorphous alloys by isothermal annealing in the range of temperatures where crystalline nuclei can transition to the amorphous phase; (ii) controlled nanostructuring of the amorphous state with different modes of treatment. Methods are proposed for obtaining the nanostructured state from the initial amorphous state. Alloys in the nanocrystalline state are obtained, as is confirmed by the results from electron microscope investigations.