Showing papers in "Journal of Non-crystalline Solids in 2015"

Active colloidal suspensions: Clustering and phase behavior

Abstract: We review recent experimental, numerical, and analytical results on active suspensions of self-propelled colloidal beads moving in (quasi-)two dimensions. Active colloids form part of the larger theme of active matter , which is noted for the emergence of collective dynamic phenomena away from thermal equilibrium. Both in experiments and computer simulations, a separation into dense aggregates, i.e., clusters, and a dilute gas phase has been reported even when attractive interactions and an alignment mechanism are absent. Here, we describe three experimental setups, discuss the different propelling mechanisms, and summarize the evidence for phase separation. We then compare experimental observations with numerical studies based on a minimal model of colloidal swimmers. Finally, we review a mean-field approach derived from first principles, which provides a theoretical framework for the density instability causing the phase separation in active colloids.

Fracture toughness of calcium-silicate-hydrate from molecular dynamics simulations

Abstract: Concrete is the most widely manufactured material in the world. Its binding phase, calcium–silicate–hydrate (C–S–H), is responsible for its mechanical properties and has an atomic structure fairly similar to that of usual calcium silicate glasses, which makes it appealing to study this material with tools and theories traditionally used for non-crystalline solids. Here, following this idea, we use molecular dynamics simulations to evaluate the fracture toughness of C–S–H, inaccessible experimentally. This allows us to discuss the brittleness of the material at the atomic scale. We show that, at this scale, C–S–H breaks in a ductile way, which prevents one from using methods based on linear elastic fracture mechanics. Knowledge of the fracture properties of C–S–H at the atomic scale opens the way for an upscaling approach to the design of tougher cement paste, which would allow for the design of slender environment-friendly infrastructures, requiring less material.

Design and control of crystallization in oxide glasses

Abstract: Crystallization of oxide glasses has a long history since the first proposal by Stookey in the 1950s, and nowadays various glass-ceramics (crystallized glasses) having potential for practical applications have been reported There has been great progress in the field of the crystallization of glasses such as the relationship between glass composition and crystalline phase Because crystal nucleation and growth start from atomic rearrangements in the nanometer scale (ie, nano-scale) level, information on the nano-scale glass structure is very important for an understanding of the crystallization mechanism and thus for the design and control of crystalline phase and morphology In this article, we review the basic concept of the oxide glass structure related to crystallization, the basic scenario and typical types of glass crystallization, and features of laser-induced crystallization The design and control of crystal morphologies and glass-ceramic properties are also reviewed with some examples The crystallization processing of glasses is becoming more and more important as a novel technique for the development of new functional glass/crystal hybrid materials and also as an approach for a deep understanding of the nano-scale heterogeneous structure of glasses

On the equivalence between the thermodynamic and dynamic measurements of the glass transition in confined polymers

Abstract: Understanding why the glass transition temperature (Tg) of polymers deviates substantially from the bulk with nanoscale confinement has been a 20-year mystery. Ever since the observation in the mid-1990s that the Tg values of amorphous polymer thin films are different from their bulk values, efforts to understand this behavior have intensified, and the topic remains the subject of intense research and debate. This is due to the combined scientific and technological implications of size-dependent glassy properties. Here, we discuss an intriguing aspect of the glassy behavior of confined amorphous polymers. As experimentally assessed, the glass transition is a dynamic event mediated by segmental dynamics. Thus, it seems intuitive to expect that a change in Tg due to confinement necessitates a corresponding change in molecular dynamics, and that such change in dynamics may be predicted based on our understanding of the glass transition. The aim of this perspectives article is to examine whether or not segmental dynamics change in accordance with the value of Tg for confined polymers based on bulk rules. We highlight past and recent findings that have examined the relationship between Tg and segmental dynamics of confined polymers. Within this context, the decoupling between these two aspects of the glass transition in confinement is emphasized. We discuss these results within the framework of our current understanding of the glass transition as well as efforts to resolve this decoupling. Finally, the anomalous decoupling between translational (diffusion) and rotational (segmental) motion taking place in the proximity of attractive interfaces in polymer thin films is discussed.

Effect of the reactivity of alkaline solution and metakaolin on geopolymer formation

Abstract: This paper focuses on the effects of alkaline solution reactivity and metakaolin properties on geopolymer formation. To examine these effects, several geopolymer samples were synthesized from two alkaline solutions and four metakaolins. The structural evolution of the formed geopolymers was investigated using FTIR spectroscopy and pH values during the material formation. The mechanical properties were measured using compression tests. The results show that the type and amount of siliceous species and non-bridging oxygen atoms control the alkaline solution reactivity. The effect of the metakaolin reactivity is more significant when it is activated with a poorly reactive alkaline solution. However, the alkaline solution governs the reaction when it is highly reactive. Therefore, the extent of depolymerization of the alkaline solution and the reactivity of metakaolin are crucial parameters that control the rate of polycondensation and the compressive strengths of geopolymer materials.

Plasticity, crack initiation and defect resistance in alkali-borosilicate glasses: From normal to anomalous behavior

Abstract: We provide a comprehensive description of the defect tolerance of sodium-borosilicate glasses upon sharp contact loading. This is motivated by the key role which is taken by this particular glass system in a wide variety of applications, ranging from electronic substrates, display covers and substrates for biomedical imaging and sensing to, e.g., radioactive waste vitrification. The present report covers the mechanical properties of glasses in the Na 2 O–B 2 O 3 –SiO 2 ternary over the broad range of compositions from pure SiO 2 to binary sodium-borates, and crossing the regions of various commercially relevant specialty borosilicate glasses, such as the multi-component Duran-, Pyrex- and BK7-type compositions and typical soda-lime silicate glasses, which are also included in this study. In terms of structure, the considered glasses may be separated into two groups, that is, one series which contains only bridging oxygen atoms, and another series which is designed with an increasing number of non-bridging oxygen ions. Elastic moduli, Poisson ratio, hardness as well as creep and crack resistance were evaluated, as well as the contribution of densification to the overall amount of indentation deformation. Correlations between the mechanical properties and structural characteristics of near- and mid-range order are discussed, from which we obtain a mechanistic view at the molecular reactions which govern the overall deformation reaction and, ultimately, contact cracking.

Hardness and toughness of sodium borosilicate glasses via Vickers's indentations

Abstract: This study investigates the mechanical response of sodium borosilicate (SBN) glasses as a function of their chemical composition Vickers's indentation tests provide an estimate of the material hardness ( H V ) and indentation fracture toughness ( K C VIF ) plus the amount of densification/shear flow processes Sodium content significantly impacts the glass behavior under a sharp indenter Low sodium glasses maintain high connected networks and low Poisson's ratios ( ν ) This entails significant densification processes during deformation Conversely, glasses with high sodium content, ie large ν , partake in a more depolymerized network favoring deformation by shear flow As a consequence, indentation patterns differ depending on the processes occurring Densification processes appear to hinder the formation of half-penny median–radial cracks Increasing ν favors shear flow and residual stresses enhance the development of half-penny median–radial cracks Hence, K C VIF decreases linearly with ν

FTIR, Raman and NMR investigation of CaO–SiO2–P2O5 and CaO–SiO2–TiO2–P2O5 glasses

Abstract: The role of P2O5 on the structure of CaO–SiO2–P2O5 and CaO–SiO2–TiO2–P2O5 glasses was identified using Fourier transformation infrared (FTIR), Raman and Magic Angular Spinning Nuclear Magnetic Resonance (MAS NMR) spectra in this study to provide some fundamental clues for titanium and phosphorus extraction. In both systems, the vibration signals of Q0(Si), Q1(Si), Q2(Si), and Q3(Si) were detected and the dominant structural units associated with P–O groups were isolated Q0(P) and terminal Q1(P) (Qi(Si,P), i represents the number of bridging oxygen per Si or P). The added P2O5 resulted in an increase of Q3(Si) at the cost of Q0(Si), Q1(Si), and Q2(Si) and the degree of polymerization (DOP) of the glasses was therefore increased; additionally, the mole ratio of Q0(P) to Q1(P) decreased with increasing P2O5 content, indicating an equilibrium reaction between Q0(P) and Q1(P). Furthermore, the presence of TiO2 resulted in a more complicated structure in the CaO–SiO2–TiO2–P2O5 glasses and the structural units related to Q0(P), O–Ti–O deformation and TiO44 − monomers were clearly clarified.

Escaping the no man's land: Recent experiments on metastable liquid water

Abstract: The properties of supercooled water have been the subject of intense studies for decades. One of the main goals was to follow the evolution of water anomalies, already present in the stable liquid, as far as possible in the metastable phase. All anomalies were found to become more pronounced, but their origin has hitherto remained hidden because of crystallization into ice. We review the recent experimental developments in the field, with a focus on the techniques used to reach a larger metastability, or to extend the investigations to the negative pressure region of the phase diagram, where the liquid is also metastable with respect to its vapor.

Crystallization of glass-forming liquids: Maxima of nucleation, growth, and overall crystallization rates

Abstract: A set of equations for determining the temperatures and magnitudes of the maximum nucleation, growth, and overall crystallization rates of glass-forming liquids is derived and analyzed The analysis is performed based on the classical theories of nucleation and growth, without introducing additional assumptions such as the Stokes–Einstein–Eyring (SEE) equation, models for specific kinetic mechanisms of aggregation, a specification of the type of temperature dependence of the diffusion coefficient, or specific models for the computation of the driving force of crystallization and the work of critical cluster formation Such approximations are employed only for analytical estimates and to illustrate the general results In particular, it is shown that the magnitude of the maximum of the steady-state nucleation rate Jmax decreases upon increasing the ratio Tmax(nucl)/Tm (Tmax(nucl): temperature of maximum of the steady-state nucleation rate, Tm: melting or liquidus temperature) Similarly, the maximum growth rate, umax, decreases with increasing values of the ratio Tmax(growth)/Tm (Tmax(growth): temperature of maximum of the growth rate) Several experimental results on the crystallization kinetics of glass-forming liquids are interpreted theoretically for the first time employing the concepts developed here

Glassy dynamics and glass transition in nanometric layers and films: A silver lining on the horizon

Abstract: The long lasting highly controversial discussion on glassy dynamics and the glass transition temperature of polymers in nanometric layers and films seems to be converging based on a multitude of recent experiments: (i) Linear response spectroscopies (e.g. alternating current calorimetry (ACC), broadband dielectric spectroscopy (BDS)) measuring in the liquid state do not observe shifts of the mean relaxation rate in dependence on the 1-dimensional confinement down to layer thicknesses of 8 nm (ACC) and 4 nm (BDS); (ii) Frequency dependent photobleaching techniques working essentially below the bulk glass transition temperature find as well a glassy dynamics in thin (freestanding or supported) films of polystyrene (PS) which is primarily bulk-like and does not depend on the thickness of the layer as demonstrated down to 14 nm. (iii) Evidence exists, that close to the bulk T g , a layer of strongly enhanced mobility—and having a temperature dependent thickness—is formed on a free polymer surface. This enormously complicates the interpretation of ellipsometric and fluorescent based experiments, and might be the reason for the widely diverging results. In summary the dynamic glass transition does not show a confinement effect above and below the glass transition temperature in pronounced contrast to the non-equilibrium dynamics in the glassy state, which depend strongly on a variety of parameters including the layer thickness. This decoupling is well described by the free volume hole diffusion (FVHD) model as developed by Cangialosi et al.

Synthesis and characterization of silica aerogels dried under ambient pressure bed on water glass

Abstract: In this paper, we report the experimental results on the synthesis of water glass based silica aerogels, which were dried under ambient pressure. Water glass was hydrolyzed and condensed in water using HCl as the catalyst. To minimize shrinkage during drying process, N,N-dimethylformamide (DMF), acting as drying control chemical additive (DCCA), was introduced. Before the ambient pressure drying, solvent exchange and surface modification were completed. In order to get hydrophobic aerogel, trimethylchlorosilane (TMCS) was used to modify the hydrophilic hydrogel surface. Here a large amount of TMCS can be saved compared with that in single step solvent exchange/surface modification method. The effects of DMF on the physical and textural properties of the resulting aerogels were investigated. When the molar ratio of Si in water glass to DMF is 2.23, the synthesized silica aerogels have better properties. Characterized by FT-IR, SEM, BET, etc., the resulting aerogels have well-developed mesoporous structure (mean pore size of ~ 15 nm) with super hydrophobicity (contact angle of 161°) and excellent absorption capacity of organic liquids.

Influence of Al3 + ions on luminescence efficiency of Eu3 + ions in barium boro-phosphate glasses

Abstract: The emission and life time decay characteristics of Eu 3 + ions in barium boro-phosphate glasses mixed with different concentrations of Al 2 O 3 are studied. The emission spectra exhibited bands due to 5 D 0 → 7 F 0 , 7 F 1 , 7 F 2 , 7 F 3 , 7 F 4 transitions of Eu 3 + ions. The highest intensity of these transitions is observed when the glasses are mixed with 3.0 mol% of Al 2 O 3 . Using the emission spectra, the Judd–Ofelt (J–O) parameters and the radiative parameters viz., emission probability, A, the radiative lifetime, τ, and the fluorescent branching ratio, β of different transitions originated from 5 D 0 level of Eu 3 + ions are evaluated. The results have been analyzed as a function of Al 2 O 3 concentration with the aid of IR spectral data. The IR spectral data have indicated that Al 3 + ions participate in larger proportions in octahedral positions in the glasses mixed with 3.0 mol% of Al 2 O 3 , which induce bonding defects, de-cluster the Eu 3 + ions and facilitate for minimizing the emission losses due to quenching. The quantitative analysis of these results further indicated that 3.0 mol% of Al 2 O 3 is optimal for achieving the highest luminescence efficiency in these glasses.

Characterization and biological properties of TiO2/PCL hybrid layers prepared via sol–gel dip coating for surface modification of titanium implants

Abstract: Surface modification of implants to facilitate the integration process of the biomaterials is a strategy which has been attracting increasing attention. In this work, the sol–gel method has been used to synthesize organic–inorganic nanocomposite materials consisting of an inorganic titania matrix in which different percentages of a biocompatible polymer, the poly-e-caprolactone (PCL), have been incorporated. The synthesized materials, still in sol phase, have been used to dip-coat titanium grade 4 substrates to improve the biological properties of the surface. Fourier transform infrared spectroscopy detected the formation of H-bonds between the C O of PCL chains and the –OH groups of the sol–gel matrix. The morphological analysis of films has been performed via scanning electron microscopy (SEM) and has shown that the PCL addition allows the preparation of crack-free and porous coatings; however, uncoated areas develop if PCL is present in high percentages. Bioactivity properties of the coatings have been investigated by soaking coated substrates in a fluid simulating the human blood plasma and evaluating at a later stage the formation of a hydroxyapatite layer on the surface by means of SEM/EDX (energy dispersive X-ray). Coating biocompatibility has been studied by WST-8 assay using 3T3 cells seeded on coated and uncoated substrates.

The mechanism of foaming and thermal conductivity of glasses foamed with MnO2

Abstract: We prepare foam glass from cathode ray tube (CRT) panels using MnO2 as foaming agent at different temperatures for various durations. The reduction of MnO2 to Mn2O3 leads to formation of O2 gas, and hence, causes initial foaming. The Mn2O3 particles dissolve into the glass melt and subsequently reduce, causing further formation of O2 gas and foaming of the glass melt. Increasing the treatment temperature and time enhances foam expansion, Mn2O3 dissolution, and lowers the closed porosity. Once the foam reaches a percolated stage, the foam continues to grow. This is caused by nucleation of new bubbles and subsequent growth. We discuss evolution of pore morphology in terms of pore number density, pore size and closed porosity. The thermal conductivity of the foam glasses is linearly dependent on density. The heat transfer mechanism is revealed by comparing the experimental data with structural data and analytical models. We show that the effect of pore size, presence of crystal inclusions and degree of closed porosity do not affect the overall thermal conductivity.

Thermal properties, density and structure of percalcic and peraluminus CaO–Al2O3–SiO2 glasses

Abstract: Calcium silicate and aluminosilicate glasses with various SiO 2 contents (10–76 mol%) were prepared in the wide range of [Al 2 O 3 ]/[CaO] molar ratio from 0.10 to 4.00 by conventional melt-quenching method and an aerodynamic levitation device for a part of peraluminus glasses. Variations of glass transition temperature, the coefficient of linear expansion, and ionic packing factor derived from density measurements with [Al 2 O 3 ]/[CaO] molar ratio were studied. The glass structures of SiO 4 three dimensional network, mixed anions of (Si, Al)O 4 tetrahedra, isolated AlO 4 and Al–O–Al linkages were evaluated by FT-IR measurements. Compositional dependences of the properties are qualitatively related to the glass structure examined by IR spectroscopy and the proportions of AlO x (x = 4, 5, 6) species evaluated in a previous study by using 27 Al MQ NMR spectroscopy.

Strong geometric frustration in model glassformers

Abstract: We consider three popular model glassformers, the Kob–Andersen and Wahnstrom binary Lennard–Jones models and weakly polydisperse hard spheres. Although these systems exhibit a range of fragilities, all feature a rather similar behaviour in their local structure approaching dynamic arrest. In particular we use the dynamic topological cluster classification to extract a locally favoured structure which is particular to each system. These structures form percolating networks, however in all cases there is a strong decoupling between structural and dynamic lengthscales. We suggest that the lack of growth of the structural lengthscale may be related to strong geometric frustration.

Effect of alkaline earths on spectroscopic and structural properties of Cu2 + ions-doped lithium borate glasses

Abstract: Alkaline earth lithium borate glasses with composition 15RO–25Li2O–(60-x)B2O3: xCuO (where R = Ca, Ba and x = 0, 0.4 and 0.8) were prepared by the conventional melt quenching technique. X-ray diffractograms confirm the amorphous nature of the glass samples. Spectroscopic studies such as optical absorption, EPR and FTIR were used to understand the effect of modifier oxide and CuO dopant in the glass matrix. From the optical absorption spectra the absorption band which is attributed to the 2B1g → 2B2g characteristic transition of Cu2 + ions in the distorted octahedral sites. The cut-off wavelength (λc), optical band gap (Eopt), and Urbach energy (ΔE) were also determined. The average electronic polarizability of the oxide ion αo2 −(Eopt), optical basicity Λ(Eopt) and Yamashita–Kurosawa's interaction parameter A(Eopt) were also examined. From the EPR spectra the spin-Hamiltonian parameters are evaluated. The values of spin-Hamiltonian parameters indicate that the site symmetry around Cu2 + ions is tetragonally distorted octahedral. Using electron paramagnetic resonance (EPR) and optical absorption data, the molecular orbital coefficients α2 and β21 are calculated. The FTIR studies show that these glasses are made up of BO3 and BO4 units. The variation of optical and spectroscopic parameters was discussed in terms of the ionic radius of alkaline earth metal modifier.

The white light emission properties of Tm3 +/Tb3 +/Sm3 + triply doped SrO–ZnO–P2O5 glass

Abstract: The phosphate parent glasses, the rare earth ion single and tri-doped phosphate glasses in the SrO–ZnO–P 2 O 5 ternary system were prepared by conventional melting–quenching method. The structure and thermal stability of the parent glasses were investigated by Fourier transform infrared spectroscopy (FT-IR) and differential thermal analyzer (DTA). The results show that with the P/O molar ratio increasing, the relative intensity of 530 cm − 1 bond becomes weaker and the relative intensity of 1284 cm − 1 bond gets strengthened, and the values of the glass stability parameters ( K 2 , K SP ) decrease after a previous increase, and reach the maximum when the mole ratio of P/O is 0.333 (P 2 O 5 = 50 mol%). Blue, green and red light emissions are separately observed in the photoluminescence spectra of Tm 3 + , Tb 3 + or Sm 3 + single-doped phosphate glasses excited by 357 nm. The combination of these three emissions allows white light emission via adjustment of dopants' concentration. It is confirmed that energy transfer from Tm 3 + (Tb 3 + ) to Sm 3 + occurs, which is reasonably interpreted based on energy level diagrams and the analysis of the fluorescence lifetimes.

Specular reflectance (SR) and attenuated total reflectance (ATR) infrared (IR) spectroscopy of transparent flat glass surfaces: A case study for soda lime float glass

Abstract: This paper describes the optical principles of specular reflection (SR) and attenuated total reflection (ATR)-infrared (IR) spectroscopy, both of which are useful methods for glass surface analyses. It should be noted that the shape, position, and relative intensity of peaks in reflectance spectra vary drastically depending on the IR incidence angle as well as the probe method (SR vs. ATR). For example, in SR-IR analyses of soda lime glass, the Si–O–Si stretch band shows a blue-shift from its original position and a new peak grows at ~ 1200 cm− 1 as the IR incidence angle increases. In contrast, the Si–O–Si stretch band appears significantly red-shifted from its original position in the ATR-IR spectra of soda lime glass. SR-IR spectra of thin and flat samples can contain transmission spectral features, due to the reflection from the backside, in the region where the bulk absorption coefficient is low. These artifacts are due to the complex nature of refractive index and should not be interpreted as new peaks or new chemical states of soda lime glass surfaces. An ATR-IR analysis to probe hydrous species in glass surface is demonstrated using two ATR crystals with different refractive indices.

Protein dynamics in a broad frequency range: Dielectric spectroscopy studies

Abstract: We present detailed dielectric spectroscopy studies of dynamics in two hydrated proteins, lysozyme and myoglobin. We emphasize the importance of explicit account for possible Maxwell–Wagner (MW) polarization effects in protein powder samples. Combining our data with earlier literature results, we demonstrate the existence of three major relaxation processes in globular proteins. To understand the mechanisms of these relaxations we involve literature data on neutron scattering, simulations and NMR studies. The faster process is ascribed to coupled protein–hydration water motions and has relaxation time ~ 10–50 ps at room temperature. The intermediate process is ~ 10 2 –10 3 times slower than the faster process and might be strongly affected by MW polarizations. Based on the analysis of data obtained by different experimental techniques and simulations, we ascribe this process to large scale domain-like motions of proteins. The slowest observed process is ~ 10 6 –10 7 times slower than the faster process and has anomalously large dielectric amplitude Δ e ~ 10 2 –10 4 . The microscopic nature of this process is not clear, but it seems to be related to the glass transition of hydrated proteins. The presented results suggest a general classification of the relaxation processes in hydrated proteins.

Investigation on bioactivity, biocompatibility, thermal behavior and antibacterial properties of calcium silicate glass coatings containing Ag

Abstract: The aim of this study has been the preparation of dental implants with potential antibacterial properties. Bioactive glasses containing different percentages of silver have been synthesized via the sol–gel method and used to coat titanium implants by means of a dip coating technique. The glasses obtained have the following general formula, 70S30CxA, which is related to its composition (in mol%): 70% of SiO 2 (S), 30% of CaO (C) and x% of Ag 2 O (A), with 0.08 ≤ x ≤ 0.27. Fourier Transform Infrared (FTIR) spectroscopy and simultaneous thermogravimetry/differential thermal analysis (TG/DTA) were used to characterize the materials. Scanning electron microscopy (SEM) has been used to investigate the coating morphology. Moreover, the films obtained have been characterized in order to verify their antibacterial activities as well as their bioactivity and biocompatibility as a function of Ag content. SEM/EDX analysis has shown that the films are bioactive because they are able to stimulate the hydroxyapatite nucleation on their surface when soaked in a simulated body fluid (SBF). WST-8 assay on 3T3 cells seeded on coated titanium substrates has proved that the coatings don't induce cytotoxicity. However, the results have shown that both the bioactivity and biocompatibility of coatings decrease slightly at high Ag contents. In contrast, antibacterial activity of the films against the Staphylococcus aureus increases with an increase of the silver amount.

The structure of water; from ambient to deeply supercooled

Abstract: Here we discuss the structure of water in terms of a temperature-dependent balance between two classes of hydrogen-bonded structures. At high and down to mildly supercooled temperatures most molecules favor a closer packing than tetrahedral, with strongly distorted hydrogen bonds. This allows the quantized librational modes to be excited and contribute to the entropy while the loss of enthalpy due to breaking hydrogen bonds is compensated by enhanced van der Waals interactions. Tetrahedral hydrogen bonding is of lower enthalpy resulting in tetrahedrally bonded water patches appearing, but only as fluctuations with size and life-time increasing at lower temperatures. Measurements of the structure at deeply supercooled conditions show a continuous increase in tetrahedrality which becomes accelerated below the temperature of homogeneous ice nucleation. The two local structures are connected to the liquid–liquid critical point (LLCP) hypothesis in supercooled water and correspond to high density liquid (HDL) and low density liquid (LDL). We propose that both HDL and LDL behave as normal liquids and that the anomalous properties of water result from the transition between them, which occurs over a wide temperature range at ambient pressure. The key issue is the competition between incompatible conditions for maximizing the entropy, favored in HDL, and minimizing the enthalpy, favored in LDL, which leads to the instability in the liquid and is the fundamental origin of the proposed LLCP.

Molecular dynamics simulation of the structure and properties for the CaO–SiO2 and CaO–Al2O3 systems

Abstract: The microstructure and transport properties of CaO–SiO 2 and CaO–Al 2 O 3 systems were studied by molecular dynamics simulations and the relations between the structure and transport properties were also discussed. The results show that the simulation results of both structure and transport properties of CaO–SiO 2 and CaO–Al 2 O 3 systems are well consistent with the experimental values. With the addition of CaO content, the melt structure would be depolymerized resulting in decrease of viscosity and enhancement of electrical conductivity in both systems. The relations between the structure and transport properties were established by fitting the calculated results and those results show that the decrease of melt polymerization degree leads to decrease of viscosity and increase of electrical conductivity in both systems. Relation between the logarithm of viscosity and the logarithm of conductivity of CaO–SiO 2 system shows good agreement with the published literature. However, it needs further investigation for CaO–Al 2 O 3 system.

Mg-based bulk metallic glasses for biodegradable implant materials: A review on glass forming ability, mechanical properties, and biocompatibility

Abstract: Amorphous Mg-based bulk metallic glasses (BMGs) are relatively new materials for various engineering applications because of their superior mechanical properties and corrosion resistance. Recently, BMGs attract significant attention as a new class of biodegradable materials. Some of their properties have been observed to be superior as compared to their crystalline counterparts. The current state of the art of Mg-based BMGs development for biomedical implant applications is still focused on the glass forming ability and their formation mechanism. Some types of Mg-based BMGs demonstrate very encouraging results in terms of biodegradability and biocompatibility performances. However, there are still many Mg-based BMGs in development stage where toxic alloying elements are used. This study reviews the characteristics and role of elements with good glass forming ability toward development of Mg-based bulk metallic glasses. It discusses the glass forming ability, mechanical properties, corrosion behavior, and biocompatibility of previously reported Mg-based BMGs. It ends with the proposed strategy for future development of Mg-based BMGs for biodegradable implant utilization.

Optimizing Nd/Er ratio for enhancement of broadband near-infrared emission and energy transfer in the Er3 +–Nd3 + co-doped transparent silicate glass-ceramics

Abstract: The optimizing Nd/Er ratio for enhancement of broadband near-infrared (NIR) emissions in the Er 3 + –Nd 3 + co-doped SiO 2 –AlF 3 –BaF 2 –TiO 2 –CaCO 3 (SABTC) transparent silicate glass-ceramics containing BaF 2 nanocrystals under 808 nm excitation was investigated. The broadband NIR emission bands from 850 to 1700 nm, which covered the whole O, E, S, C, L and U bands were observed in the Er 3 + –Nd 3 + co-doped glass-ceramics samples, as a result of the overlap of Nd 3 + -doped emission band centered at 1342 nm and the emission from 4 I 13/2 → 4 I 15/2 transition of Er 3 + band centered at 1546 nm. The NIR emission intensity of Er 3 + –Nd 3 + co-doped bands centered at 895, 1065, 1342 and 1546 nm, becomes stronger with varied depending on the mixing ratio of Nd 3 + and Er 3 + ions, and the full width at half-maximum (FWHM) extending from 1250 to 1700 nm could be 340 nm. Most NIR emission bands at 1065 and 1546 nm are observed for the SABTC-1.0N0.4E glass-ceramics sample, corresponding to Nd/Er ratio of 2.5. At the same time, the energy transfer processes between the Nd 3 + and Er 3 + ions were also discussed.

Structure and properties of alkali and silver sulfophosphate glasses

Abstract: Glasses of the composition 24 M2SO4-1 M2O-52 ZnO-23 P2O5 with M Li, Na, K, Rb, Cs, and Ag were prepared by conventional melt-quenching. Variation of the monovalent cation results in distinct changes in the thermo-physical, mechanical, and chemical properties. From Li to Cs, the calorimetric glass transition temperature and the stability against crystallization increase significantly. This trend is contrary to expectations in covalent glasses, but consistent with the behavior of ionic fluoride glasses. However, chemical and mechanical properties are consistent with the trend observed for covalent glasses as hardness and elastic modulus are drastically lowered from Li to Cs glasses. Modifier substitution with the heavier Ag ion has often similar effects as lithium substitution due to their comparably small coordination number (i.e., four) in the studied glasses. Raman- and IR-spectroscopic studies were carried out in order to correlate the variations in the glass properties with variations of the glass structure. Here, sulfate anions show preferential bonding to the monovalent cations while phosphate groups are preferentially linked to zinc-cations. For larger alkali cations the tetrahedral phosphate and sulfate groups show a reduction of symmetry, i.e., deviations from tetrahedral symmetry when acting as bi- or multi-dentate ligands.

Theory of vibrational anomalies in glasses

Abstract: The theory of elasticity with spatially fluctuating elastic constants (heterogeneous-elasticity theory) is reviewed. It is shown that the vibrational anomalies associated with the boson peak can be qualitatively and quantitatively explained in terms of this theory. Two versions of a mean-field theory for solving the stochastic equation of motion are presented: the coherent-potential approximation (CPA) and the self-consistent Born approximation (SCBA). It is shown that the latter is included in the former in the Gaussian and weak-disorder limit. We are able to discuss and explain cases in which the change of the vibrational spectrum by varying an external parameter can be accounted for by changing the Debye frequency (elastic transformation) and cases in which this is not possible. In the latter case a change in the distribution of the elastic moduli has occurred.

A review of experiments testing the shoving model

Abstract: According to the shoving model the non-Arrhenius temperature dependence of supercooled liquids' relaxation time (or viscosity) derives from the fact that the high-frequency shear modulus is temperature dependent in the supercooled phase, often increasing a factor of three or four in the temperature interval over which the relaxation time increases by ten to fifteen decades. In this paper we have compiled all tests of the shoving model known to us. These involve rheological data obtained by different techniques, high-frequency sound-wave data, neutron scattering data for the vibrational mean-square displacement, data obtained at or below the glass transition, as well as data testing the model under out-of-equilibrium conditions, i.e., during aging. Most data confirm the model, some do not. We conclude that more work is needed to precisely characterize the model's range of applicability.

Variation of the physicochemical and morphological characteristics of solvent casted poly(vinylidene fluoride) along its binary phase diagram with dimethylformamide

Abstract: Poly(vinylidene fluoride), PVDF, films and membranes were prepared by solvent casting from dimethylformamide, DMF, by systematically varying polymer/solvent ratio and solvent evaporation temperature. The effect of the processing conditions on the morphology, degree of porosity, mechanical and thermal properties and crystalline phase of the polymer was evaluated. The obtained microstructure is explained by the Flory–Huggins theory. For the binary system, the porous membrane formation is attributed to a spinodal decomposition of the liquid–liquid phase separation. The morphological features were simulated through the correlation between the Gibbs total free energy and the Flory–Huggins theory. This correlation allowed the calculation of the PVDF/DMF phase diagram and the evolution of the microstructure in different regions of the phase diagram. Varying preparation conditions allow tailoring polymer microstructure while maintaining a high degree of crystallinity and a large β crystalline phase content. Further, the membranes show adequate mechanical properties for applications in filtration or battery separator membranes.