Glass-forming Materials Research Articles

Structure of the network glass-former ZnCl2: From the boiling point to the glass

The structure of the network glass-forming material ZnCl2 was measured using both neutron and high-energy x-ray diffraction for the glass at 298(1) K and for the liquid over the temperature range 601(1)–977(2) K. Intermediate range order, as manifested by the appearance of a first-sharp diffraction peak in the measured diffraction pattern for the glass at a scattering vector kFSDP≃1Å−1, is retained in the liquid state even at temperatures close to the boiling point. The correlation lengths associated with both the intermediate and extended range ordering are found to be inversely proportional to temperature. The reverse Monte Carlo (RMC) method was used to model the material, and the results at two different temperatures are compared to those obtained from RMC models based on the partial structure factors measured by using the method of isotope substitution in neutron diffraction. The models show temperature dependent structural variability in which there is an interplay between the fractions of corner-sharing versus edge-sharing ZnCl4 tetrahedra. Corner-sharing motifs are predominant in the glass, and edge-sharing motifs become more numerous in the liquid as the temperature is increased. The appearance of a first-sharp diffraction peak in the Bhatia–Thornton concentration–concentration partial structure factor is discussed in the context of classifying the different network types for glass-forming materials.

Density-driven defect-mediated network collapse ofGeSe2glass

The evolution in structure of the prototypical network-forming glass ${\mathrm{GeSe}}_{2}$ is investigated at pressures up to $\ensuremath{\sim}16$ GPa by using a combination of neutron diffraction and first-principles molecular dynamics. The neutron diffraction work at pressures $\ensuremath{\le}8.2$ GPa employed the method of isotope substitution, and the molecular dynamics simulations were performed with two different exchange-correlation functionals, the Becke-Lee-Yang-Parr (BLYP) and the hybrid Heyd-Scuseria-Ernzerhof HSE06. The results show density-driven structural transformations that differ substantially from those observed in common oxide glasses such as ${\mathrm{SiO}}_{2}$ and ${\mathrm{GeO}}_{2}$. Edge-sharing tetrahedra persist as important structural motifs until a threshold pressure of $\ensuremath{\sim}8.5$ GPa is attained, whereupon a mediating role is found for homopolar bonds in the appearance of higher coordinated Ge-centered polyhedra. These mechanisms of network transformation are likely to be generic for the class of glass-forming materials where homopolar bonds and fragility-promoting edge-sharing motifs are prevalent in the ambient-pressure network.

Determination of the thermodynamic scaling exponent for relaxation in liquids from static ambient-pressure quantities.

An equation is derived that expresses the thermodynamic scaling exponent, γ, which superposes relaxation times τ and other measures of molecular mobility determined over a range of temperatures and densities, in terms of static physical quantities. The latter are available in the literature or can be measured at ambient pressure. We show for 13 materials, both molecular liquids and polymers, that the calculated γ are equivalent to the scaling exponents obtained directly by superpositioning. The assumptions of the analysis are that the glass transition T(g) is isochronal (i.e., τ(α) is constant at T(g), which is true by definition) and that the pressure derivative of the glass temperature is given by the first Ehrenfest relation. The latter, derived assuming continuity of the entropy at the glass transition, has been corroborated for many glass-forming materials at ambient pressure. However, we find that the Ehrenfest relation breaks down at elevated pressure; this limitation is of no consequence herein, since the appeal of the new equation is its applicability to ambient-pressure data. The ability to determine, from ambient-pressure measurements, the scaling exponent describing the high-pressure dynamics extends the applicability of this approach to a broader range of materials. Since γ is linked to the intermolecular potential, the new equation thus provides ready access to information about the forces between molecules.

Physical aging, the local dynamics of glass-forming polymers under nanoscale confinement.

The glass transition temperature marks a point below which a material's properties change significantly, and it is well-established that confinement to the nanoscale modifies the properties of glass-forming materials. We use molecular dynamics simulations to investigate the dynamics and aging behavior of model glass-forming polymers near and below the glass transition temperature of bulk and confined films. We show that both relaxation times and physical age rates vary similarly throughout a free-standing polymer film at temperatures close to the bulk glass transition temperature, where the surfaces have both lower relaxation times and physical age rates. Moreover, we provide evidence suggesting that string lengths in the bulk control dynamic length scales in the film. This realization, combined with the similarity between aging behavior and dynamic profiles, has implications for design rationale in the microelectronics industry.

Continuous-time random-walk approach to supercooled liquids. I. Different definitions of particle jumps and their consequences.

Single-particle trajectories in supercooled liquids display long periods of localization interrupted by "fast moves." This observation suggests a modeling by a continuous-time random walk (CTRW). We perform molecular dynamics simulations of equilibrated short-chain polymer melts near the critical temperature of mode-coupling theory Tc and extract "moves" from the monomer trajectories. We show that not all moves comply with the conditions of a CTRW. Strong forward-backward correlations are found in the supercooled state. A refinement procedure is suggested to exclude these moves from the analysis. We discuss the repercussions of the refinement on the jump-length and waiting-time distributions as well as on characteristic time scales, such as the average waiting time ("exchange time") and the average time for the first move ("persistence time"). The refinement modifies the temperature (T) dependence of these time scales. For instance, the average waiting time changes from an Arrhenius-type to a Vogel-Fulcher-type T dependence. We discuss this observation in the context of the bifurcation of the α process and (Johari) β process found in many glass-forming materials to occur near Tc. Our analysis lays the foundation for a study of the jump-length and waiting-time distributions, their temperature and chain-length dependencies, and the modeling of the monomer dynamics by a CTRW approach in the companion paper [J. Helfferich et al., Phys. Rev. E 89, 042604 (2014)].

Characteristics of the Structural and Johari–Goldstein Relaxations in Pd-Based Metallic Glass-Forming Liquids

The dynamics of Pd-based metallic glass-forming liquids (Pd(40)Ni(10)Cu(30)P(20), Pd(42.5)Ni(7.5)Cu(30)P(20), Pd(40)Ni(40)P(20), and Pd(30)Ni(50)P(20)) was studied by mechanical spectroscopy and modulated differential scanning calorimetry (MDSC). We found that the change in composition has a significant effect on the α relaxation dynamics; the largest difference corresponds to an increase of the glass transition temperature Tg of ∼ 15 K, for materials in which 30% Ni was substituted by 30% Cu (i.e., from Pd(40)Ni(40)P(20) to Pd(40)Ni(10)Cu(30)P(20)). We also found that all Pd-based metallic glasses have very similar fragilities, 59 < m < 67, and Kohlrausch stretched exponents, 0.59 < βKWW < 0.60. It is interesting that the values of m and βKWW correlate well with the general relation proposed by Böhmer et al. for nonmetallic glass formers (Böhmer, R.; et al. J. Chem. Phys. 1993, 99, 4201-4209), which for the observed βKWW values predicts 58 < m < 61. From a linear deconvolution of the α and β relaxations, we find that the substitution of the Ni with Cu induced a large change in the time constant of the Johari-Goldstein relaxation, τβ. The activation energy, Uβ, of the β relaxation was largely independent of chemical composition. In all cases, 25 < Uβ/RT < 28, a range in agreement with results for other glass formers (Kudlik, A.; et al. Europhys. Lett. 1997, 40, 649-654 and Ngai, K. L.; et al. Phys. Rev. E 2004, 69, 031501). From the heat capacity and mechanical loss, estimates were obtained for the number of dynamically correlated units, Nc; we find significantly larger values for these metallic glass-forming liquids than Nc for other glass-forming materials.

Three-Dimensional Nanomechanical Mapping of Amorphous and Crystalline Phase Transitions in Phase-Change Materials

The nanostructure of micrometer-sized domains (bits) in phase-change materials (PCM) that undergo switching between amorphous and crystalline phases plays a key role in the performance of optical PCM-based memories. Here, we explore the dynamics of such phase transitions by mapping PCM nanostructures in three dimensions with nanoscale resolution by combining precision Ar ion beam cross-sectional polishing and nanomechanical ultrasonic force microscopy (UFM) mapping. Surface and bulk phase changes of laser written submicrometer to micrometer sized amorphous-to-crystalline (SET) and crystalline-to-amorphous (RESET) bits in chalcogenide Ge2Sb2Te5 PCM are observed with 10-20 nm lateral and 4 nm depth resolution. UFM mapping shows that the Young's moduli of crystalline SET bits exceed the moduli of amorphous areas by 11 ± 2%, with crystalline content extending from a few nanometers to 50 nm in depth depending on the energy of the switching pulses. The RESET bits written with 50 ps pulses reveal shallower depth penetration and show 30-50 nm lateral and few nanometer vertical wavelike topography that is anticorrelated with the elastic modulus distribution. Reverse switching of amorphous RESET bits results in the full recovery of subsurface nanomechanical properties accompanied with only partial topography recovery, resulting in surface corrugations attributed to quenching. This precision sectioning and nanomechanical mapping approach could be applicable to a wide range of amorphous, nanocrystalline, and glass-forming materials for 3D nanomechanical mapping of amorphous-crystalline transitions.

Large acoustic thermal hysteresis in relaxor ferroelectric Pb(Zn1/3Nb2/3)O3-PbTiO3

The diffuse phase transition in relaxor-based 0.93Pb(Zn1/3Nb2/3)O3–0.07PbTiO3 ferroelectric single crystals is studied by observing the large thermal hysteresis over 100 K of the longitudinal acoustic (LA) phonon. By observing this hysteresis in the LA phonon frequency with different temperature-cycles, it is concluded that nonequilibrium states are induced by supercooling. Relaxor ferroelectrics easily go supercooling, because they demonstrate structural hierarchy like glass-forming materials. The inhomogeneous structure disrupts sharp phase transitions, revealing diffuse phase transition on cooling. However, annealing at low temperatures reduces the inhomogeneity markedly. These interpretations are consistent with LA phonon behavior under electric fields, which reduce the inhomogeneity.

Space-Dependent Dynamics in 1,4-Polybutadiene Nanocomposite

Understanding the dynamics of the glass transition of polymers and molecular liquids has been a goal of physicists for more than a half-century. The nature of this transition is directly linked to the heterogeneity of the dynamics, reflected in spatial and temporal correlations of molecular motions, as well as a distribution of relaxation times. 1−4 The former can be quantified by a dynamic correlation length that grows on approaching the glassy state and the latter from the breadth of the relaxation function or dynamic susceptibility. The majority of experimental studies of this topic rely on dielectric spectroscopy, which provides relaxation spectra over many decades of frequency. However, the higher order susceptibilities involving both spatial and temporal correlations cannot be obtained from linear relaxation measurements without assumptions. 5 Characterizing the spatial distribution of the dynamics requires other methods. Since the 1990s quasi-elastic neutron scattering (QENS) has been widely employed to obtain space-specific information on the dynamic properties, providing momentum (Q )a nd frequency dependences from the double differential scattering function of protons. 6 The Q-dependent scattering from glassforming materials can be described using the Kohlrausch− Williams−Watts (KWW) equation 7−11 τ Φ∝

Collective THz dynamics in living Escherichia coli cells

We have employed neutron Brillouin spectroscopy to study coherent collective density fluctuations in the biological macromolecular components of living Escherichia coli cells. To highlight the contribution of the macromolecular material alone, a suitably prepared mixture of light and heavy water was exploited to cancel the scattering length of intracellular water. The present results indicate that the cellular biomaterial sustains THz coherent density fluctuations, characterised by a propagating mode travelling at about 3600m/s and by a localised mode at energies between 4 and 7meV. A comparison with both hydration water and simpler biomolecules, such as proteins or DNA, brings further support to the idea that the dynamical coupling between biomolecular structures and biological water provides the delicate dynamical adaptation needed to achieve a full biological functionality. Finally, the behaviour of the damping factors of the observed collective modes strengthens the dynamical similarity of biological systems with glass-forming materials.

The influence of the secondary relaxation processes on the structural relaxation in glass-forming materials

In the frame of fractional-kinetic approach, the model of the structural α-relaxation in the presence of the secondary β-relaxation processes is suggested. The model is based on the rigorous bond between β-processes with α-process and leads to the generalized and justified expression for the complex dielectric permittivity (CDP). It allows to form a new sight on the problem of the fitting of multi-peak structure of the dielectric loss spectra in glass-forming materials. The consistency of the CDP expressions obtained is based on a good fit of experimental data for binary methanol-water mixtures.

Understanding the Vogel–Fulcher–Tammann law in terms of the bond strength–coordination number fluctuation model

It is shown that an expression similar to the Vogel–Fulcher–Tammann (VFT) equation of viscosity can be derived from the bond strength–coordination number fluctuation (BSCNF) model. The parameters BVFT and T0 of the VFT equation, η=η0exp[BVFT/(T−T0)], are related directly to the parameters of the BSCNF model; BVFT≅E0Z0/R and T0≅|ΔE||ΔZ|/R, where E0 and Z0 are the mean values of the bond strength and the coordination number of the structural units, and ΔE and ΔZ are their fluctuations and R is the gas constant. According to the BSCNF model, the viscous flow occurs by breaking or twisting the bonds that connect the structural units of the melt. From the analytical expression obtained in the present study, an expression for the strength parameter D can be also derived. Namely, D is written as D=BVFT/T0≅(E0Z0)/(|ΔE||ΔZ|). This relation indicates that D includes information of the fluctuations that characterize the disorder of glass-forming materials.

Using 20-million-year-old amber to test the super-Arrhenius behaviour of glass-forming systems

Fossil amber offers the opportunity to investigate the dynamics of glass-forming materials far below the nominal glass transition temperature. This is important in the context of classical theory, as well as some new theories that challenge the idea of an 'ideal' glass transition. Here we report results from calorimetric and stress relaxation experiments using a 20-million-year-old Dominican amber. By performing the stress relaxation experiments in a step-wise fashion, we measured the relaxation time at each temperature and, above the fictive temperature of this 20-million-year-old glass, this is an upper bound to the equilibrium relaxation time. The results deviate dramatically from the expectation of classical theory and are consistent with some modern ideas, in which the diverging timescale signature of complex fluids disappears below the glass transition temperature.

Double interface formulation for improved α‐tocopherol stabilisation in dehydration of emulsions

Encapsulation of hydrophobic nutrients can be achieved by freezing and freeze-drying of oil-in-water emulsions containing glass-forming materials. The addition of a polyelectrolyte layer on the protein-stabilised oil droplets may provide better protection to the oil phase against external stresses. Soy protein-trehalose and whey protein-trehalose emulsions with (layer-by-layer, LBL) and without (single-layer, SL) ι-carrageenan were used as the delivery systems for olive oil with dissolved α-tocopherol. Emulsions containing 0.125 g kg(-1) protein, 0.42 g kg(-1) oil and 150 g kg(-1) trehalose with (LBL) or without (SL) 0.25 g kg(-1) ι-carrageenan at pH 3 were frozen and freeze-dried and their state transitions were studied. The stability of α-tocopherol in freeze-dried systems at 0 and 0.33 water activity (aw ) during storage at 55 °C was followed. Loss of α-tocopherol was found in soy protein-stabilised SL systems at 0.33 aw , and this loss coincided with trehalose crystallisation. The stability of α-tocopherol was retained in soy protein-stabilised LBL and whey protein-stabilised LBL and SL systems at all conditions. Trehalose crystallisation-induced loss of structure was confirmed from changes in emulsion properties and visual appearance. Component sugar crystallisation contributed to the loss of sensitive compounds, but the stability of these compounds can be improved by the use of LBL formulations.

Evolution of collective motion in a model glass-forming liquid during physical aging

At temperatures moderately below their glass transition temperature, the properties of many glass-forming materials can evolve slowly with time in a process known as physical aging whereby the thermodynamic, mechanical, and dynamic properties all drift towards their equilibrium values. In this work, we study the evolution of the thermodynamic and dynamic properties during physical aging for a model polymer glass. Specifically, we test the relationship between an estimate of the size of the cooperative rearrangements taking the form of strings and the effective structural relaxation time predicted by the Adam-Gibbs relationship for both an equilibrium supercooled liquid and the same fluid undergoing physical aging towards equilibrium after a series of temperature jumps. We find that there is apparently a close correlation between a structural feature of the fluid, the size of the string-like rearrangements, and the structural relaxation time, although the relationship for the aging fluid appears to be distinct from that of the fluid at equilibrium.

Recent advances in small molecular, non-polymeric organic hole transporting materials for solid-state DSSC

Issue from thin-film technologies, dye-sensitized solar cells have become one of the most promising technologies in the field of renewable energies. Their success is not only due to their low weight, the possibility of making large flexible surfaces, but also to their photovoltaic efficiency which are found to be more and more significant (>12% with a liquid electrolyte, >7% with a solid organic hole conductor). This short review highlights recent advances in the characteristics and use of low-molecular-weight glass-forming organic materials as hole transporters in all solid-state dye-sensitized solar cells. These materials must feature specific physical and chemical properties that will ensure both the operation of a photovoltaic cell and the easy implementation. This review is an english extended version based on our recent article published in Matériaux & Techniques 101, 102 (2013).

Glassy Interfacial Dynamics of Ni Nanoparticles: Part II Discrete Breathers as an Explanation of Two-Level Energy Fluctuations.

Recent studies of the dynamics of diverse condensed amorphous materials have indicated significant heterogeneity in the local mobility and a progressive increase in collective particle motion upon cooling that takes the form of string-like particle rearrangements. In a previous paper (Part I), we examined the possibility that fluctuations in potential energy E and particle mobility μ associated with this 'dynamic heterogeneity' might offer information about the scale of collective motion in glassy materials based on molecular dynamics simulations of the glassy interfacial region of Ni nanoparticles (NPs) at elevated temperatures. We found that the noise exponent associated with fluctuations in the Debye-Waller factor, a mobility related quantity, was directly proportional to the scale of collective motion L under a broad range of conditions, but the noise exponent associated with E(t) fluctuations was seemingly unrelated to L. In the present work, we focus on this unanticipated difference between potential energy and mobility fluctuations by examining these quantities at an atomic scale. We find that the string atoms exhibit a jump-like motion between two well-separated bands of energy states and the rate at which these jumps occur seems to be consistent with the phenomenology of the 'slow-beta' relaxation process of glass-forming liquids. Concurrently with these local E(t) jumps, we also find 'quake-like' particle displacements having a power-law distribution in magnitude so that particle displacement fluctuations within the strings are strikingly different from local E(t) fluctuations. An analysis of these E(t) fluctuations suggests that we are dealing with 'discrete breather' excitations in which large energy fluctuations develop in arrays of non-linear oscillators by virtue of large anharmonicity in the interparticle interactions and discreteness effects associated with particle packing. We quantify string collective motions on a fast caging times scale (picoseconds) and explore the significance of these collective motions for understanding the Boson peak of glass-forming materials.

Fragility and cooperative motion in a glass-forming polymer-nanoparticle composite.

Polymer-nanoparticle composites play a vital role in ongoing materials development. The behavior of the glass transition of these materials is important for their processing and applications, and also represents a problem of fundamental physical interest. Changes of the polymer glass transition temperature Tg due to nanoparticles have been fairly well catalogued, but the breadth of the transition and how rapidly transport properties vary with temperature T - termed the fragility m of glass-formation - is comparatively poorly understood. In the present work, we calculate both Tg and m of a model polymer nanocomposite by molecular dynamics simulations. We systematically consider how Tg and m vary both for the material as a whole, as well as locally, for a range of nanoparticle (NP) concentrations and two polymer-NP interactions. We find large positive and negative changes in Tg and m that can be interpreted in terms of the Adam-Gibbs model of glass-formation, where the scale of the cooperative motion is identified with the scale of string-like cooperative motion. This provides a molecular perpective of fragility changes due to the addition of NPs and for glass formation more generally. We also contrast the behavior along isobaric and isochoric approaches to Tg , since these differing paths can be important to compare experiments (isobaric) and simulations (very often isochoric). Our findings have practical implications for understanding the properties of nanocomposites and fundamental significance for understanding the properties glass-forming materials more broadly.

Glassy Interfacial Dynamics of Ni Nanoparticles: Part I Colored Noise, Dynamic Heterogeneity and Collective Atomic Motion.

Most condensed materials exhibit a significant fraction of atoms, molecules or particles that are strongly interacting with each other, while being configured geometrically at any instant of time in an 'amorphous' state having a relatively uniform density. Recently, both simulations and experiments have revealed that the dynamics of diverse condensed amorphous materials is generally characterized by significant heterogeneity in the local mobility and by progressively increasing collective motion upon cooling that takes the form of string-like collective particle rearrangements. The direct experimental observation of this type of collective motion, which has been directly linked to the growing relaxation times of glass-forming materials, and its quantification under different thermodynamic conditions, has so far been restricted to colloidal and driven granular fluids. The present work addresses the fundamental problem of how to determine the scale of this type of collective motion in materials composed of molecules or atoms. The basic premise of our work is that large scale dynamic particle clustering in amorphous materials must give rise to large fluctuations in particle mobility so that transport properties, especially those related to particle mobility, should naturally exhibit noise related to the cooperative motion scale. In our initial exploratory study seeking a relationship of this kind, we find 1/fα or 'colored noise', in both potential energy and particle displacements fluctuations of the atoms within the glassy interfacial layer of Ni nanoparticles (NPs). A direct relation between the particle displacement (mobility) noise exponent α and the average polymerization index of the string-like collective motion L is observed for a range of NP sizes, temperatures and for surface doping of the NPs with other metal atoms (Ag, Au, Pt) to change of fragility of the glassy interfacial layer at the surface of the Ni NPs. We also introduce a successful analytic model to understand this relationship between α and L.

Identifying and characterising the different structural length scales in liquids and glasses: an experimental approach

The structure of several network-forming liquids and glasses is considered, where a focus is placed on the detailed information that is made available by using the method of neutron diffraction with isotope substitution (NDIS). In the case of binary network glass-forming materials with the MX2 stoichiometry (e.g. GeO2, GeSe2, ZnCl2), two different length scales at distances greater than the nearest-neighbour distance manifest themselves by peaks in the measured diffraction patterns. The network properties are influenced by a competition between the ordering on these "intermediate" and "extended" length scales, which can be manipulated by changing the chemical identity of the atomic constituents or by varying state parameters such as the temperature and pressure. The extended-range ordering, which describes the decay of the pair-correlation functions at large-r, can be represented by making a pole analysis of the Ornstein-Zernike equations, an approach that can also be used to describe the large-r behaviour of the pair-correlation functions for liquid and amorphous metals where packing constraints are important. The first applications are then described of the NDIS method to measure the detailed structure of aerodynamically-levitated laser-heated droplets of "fragile" glass-forming liquid oxides (CaAl2O4 and CaSiO3) at high-temperatures (~2000 K) and the structure of a "strong" network-forming glass (GeO2) under pressures ranging from ambient to ~8 GPa. The high-temperature experiments show structural changes on multiple length scales when the oxides are vitrified. The high-pressure experiment offers insight into the density-driven mechanisms of network collapse in GeO2 glass, and parallels are drawn with the high-pressure behaviour of silica glass. Finally, the hydrogen-bonded network of water is considered, where the first application of the method of oxygen NDIS is used to measure the structures of light versus heavy water and a difference of approximately equal to 0.5% is found between the O-D and O-H intra-molecular bond lengths. The experimental data are best matched by using path integral molecular dynamics simulations with a flexible anharmonic water model, and the results support a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intra-molecular and inter-molecular quantum contributions.