Showing papers in "Journal of Non-crystalline Solids in 2002"

Analysis of complex dielectric spectra. I. One-dimensional derivative techniques and three-dimensional modelling

Abstract: Several strategies for the analysis of isochronal and isothermal spectra as well as complete three-dimensional (3D) spectra are outlined and applied to data of (glass forming) liquids and polymers Conduction-free loss spectra are calculated using a compact solution of the Kramers–Kronig transformation and an approximation based on oe 0 =o ln x The usefulness of the dielectric modulus for data analysis is also evaluated Apart from oe 0 =o ln x, other useful derivatives in the frequency and temperature domain are treated as well Finally, we describe the evaluation of 3Drelaxation maps by means of 3D fitting 2002 Elsevier Science BV All rights reserved

Characterization of biological cells by dielectric spectroscopy

Abstract: This review describes theoretical analysis and measurement techniques for dielectric spectroscopy of biological cells in the radio frequency range. The main focus is the β-dispersion due to interfacial polarization. The β-dispersion has been analyzed using various electrical models proposed for cells of diverse morphology. Recent technical developments in dielectric spectroscopy allow real-time monitoring of dynamic behavior of cells under physiological conditions and characterizing single cells.

Structural investigations of copper doped B2O3–Bi2O3 glasses with high bismuth oxide content

Abstract: Raman and infrared spectroscopy have been employed to investigate the 99.5%[xB2O3(1−x)Bi2O3]0.5%CuO glasses with different Bi/B nominal ratios (0.07⩽x⩽0.625) in order to obtain information about the competitive role of B2O3 and Bi2O3 in the formation of the glass network. The glass samples have been prepared by melting at 1100 °C and rapidly cooling at room temperature. In order to relax the structure, to improve the local order and to develop crystalline phases the glass samples were kept at 575 °C for 10 h. The influence of both Bi2O3 and CuO on the vitreous B2O3 network as well as the local order changes around bismuth and boron atoms in as prepared and heat treated samples was studied. Structural modifications occurring in heat treated samples compared to the untreated glasses have been observed.

Silver incorporation in Ge-Se glasses used in programmable metallization cell devices

Abstract: We investigate the nature of thin films formed by the photodissolution of Ag into Se-rich Ge–Se glasses for use in programmable metallization cell devices. These devices rely on ion transport in the film so produced to create electrically programmable resistance states. The way in which Ag incorporates into the chalcogenide film during photodiffusion is examined using Rutherford backscattering spectroscopy analysis and Raman spectroscopy. The results suggest that an Ag-rich phase separates due to the reaction of Ag with free Se from the chalcogenide glass leaving a Ge-rich chalcogenide matrix.

Determination of mesopore size of aerogels from thermal conductivity measurements

Abstract: A novel method to determine the average mesopore size of aerogels was developed. This method is based on the findings that the heat transfer through the gas phase in porous materials is strongly affected by the size of mesopores. Polyisocyanurate aerogels were synthesized and the thermal conductivity of these materials was investigated from vacuum to ambient pressure using the transient hot-wire method. Thermal conductivity of porous materials can be recognized as the sum of gaseous, solid, radiative, and convective thermal conductivity. Extraction of gas phase thermal conductivity from total thermal conductivity was performed by considering different heat transfer mechanisms. Gas phase thermal conductivity increases with pressure in `S' shape due to collisions of gas molecules with pore walls. Average mesopore size could be obtained by fitting gas phase thermal conductivity data with existing equations. Investigation of polyisocyanurate aerogels by SEM revealed that this method gives a reasonable estimate for the average mesopore size. It is expected that pore structure of fragile mesoporous materials can be analyzed by further refinements of this novel method.

Localized molecular motions of β-relaxation and its energy landscape

Abstract: The β-relaxation process in a highly viscous liquid and mechanically rigid glass may be attributed to reorientational motions of (i) essentially all molecules with a nearly temperature-independent small angle, or (ii) a small number of molecules or molecular groups confined to certain sites of loose packing resulting from frozen-in density fluctuations. Relative merits of these mechanisms are examined by considering the changes in the orientation polarization and entropy observed on changing the temperature, and observed on spontaneous structural relaxation at a fixed temperature. The effects of isothermal densification by pressure and by structural relaxation are also considered. It is found that when several properties are examined, the first mechanism for the β-relaxation is not supported. The β-relaxation process also involves translational diffusion, which has been overlooked. Implications of these finding for the β-relaxation landscape in the currently used potential energy surface are discussed. Experiments have been suggested for resolving the mechanism of this important process.

Dependence of nanocrystalline SnO2 particle size on synthesis route

Abstract: Very fine SnO2 powders were produced by (a) slow and (b) forced hydrolysis of aqueous SnCl4 solutions and (c) hydrolysis of tin(IV)-isopropoxide dissolved in isopropanol (sol–gel route) and then characterized by X-ray powder diffraction, Fourier transform infrared and laser Raman spectroscopies, TEM and BET. The XRD patterns showed the presence of the cassiterite structure. As found from XRD line broadening the crystallite sizes of all powders were in the nanometric range. TEM results also showed that the sizes of SnO2 particles in all powders are in nanometric range. Very fine SnO2 powders showed different features in the FT-IR spectra, depending on the route of their synthesis. The reference Raman spectrum of SnO2 showed four bands at 773, 630, 472 and 86 (shoulder) cm−1, as predicted by group theory. Very fine SnO2 powders showed additional Raman bands, in dependence on their synthesis. The broad Raman band at 571 cm−1 was ascribed to amorphous tin(IV)-hydrous oxide. The additional Raman bands at 500, 435 and 327 cm−1 were recorded for nanosized SnO2 particles produced by forced hydrolysis of SnCl4 solutions. However, these additional Raman bands were not observed for nanosized SnO2 particles produced by slow hydrolysis of SnCl4 solution or the sol–gel route. The aggregation effects of nanosized particles were considered in the interpretation of the Raman band at 327 cm−1. The method of low frequency Raman scattering was applied for SnO2 particle size determination. On the basis of these measurements it was concluded that the size of SnO2 particles was also in the nanometric range and that, the sol–gel particles heated to 400 °C consisted of several SnO2 crystallites.

Modelling glass transition temperatures of chalcogenide glasses. Applied to phase-change optical recording materials

Abstract: A model is presented for estimating the glass transition temperature of covalent amorphous materials from their enthalpy of atomisation. The enthalpy of atomisation is calculated by summing all individual bond enthalpies taking account of the coordination numbers of the constituting elements. A consistent set of bond enthalpies has been derived from the enthalpies of atomisation of stoichiometric compounds. The model is applicable to covalent amorphous materials consisting of the elements from group IB–VIB and for which the average number of valence electrons is no less than 4. It is shown that the model can be used for the development of phase-change materials for rewritable optical recording. Since the glass transition temperature is a lower limit for the crystallisation temperature, the model can predict the stability against spontaneous crystallisation of amorphous phase-change materials.

Dielectric spectroscopy – yesterday, today and tomorrow

Abstract: Dielectric spectroscopy is an old experimental tool which has dramatically developed in the last two decades. It covers nowadays the extraordinary spectral range from 10−6 to 1012 Hz. This enables researchers to make sound contributions to contemporary problems in modern physics. Two examples will be briefly presented: the scaling behavior of the glass transition and the dynamics of molecules in confined space. In a brief outlook novel perspectives of dielectric techniques will be discussed, e.g. non-resonant dielectric hole-burning spectroscopy, dielectric thermal expansion spectroscopy or dielectric measurements using an atomic force microscope.

Filler–elastomer interaction in model filled rubbers, a 1H NMR study

Abstract: 1 H NMR experiments on filled rubbers allow the observation of two features: the introduction of supplementary topological constraints by the fillers, and a layer of immobilised segments at the particle surface. In order to assess the role of the filler–elastomer interaction on the local chain dynamics, silica filled elastomer model systems were prepared. The synthesis methods used allowed us to test vastly different interaction strengths. It is shown that in the case of a weak interaction, the influence of the fillers is small, whereas a strong anchoring at the interface leads to an increase in the density of the topological constraints. The latter is correlated to a layer of highly immobilised segments whose thickness decreases with temperature. This layer is seen as a glassy shell.

Indentation creep of Ge–Se chalcogenide glasses below Tg: elastic recovery and non-Newtonian flow

Abstract: Chalcogenide glasses from the Ge–Se system behave viscoelastically at room temperature. It follows that indentation measurements are time- or rate-dependent. The study of the dependence of hardness ( H ) on the loading duration for Ge x Se 1− x glasses with x between 0 and 0.4 shows that the penetration displacement is the sum of an elastic component which reaches values as high as 60% of the total displacement, and a creep one, which is strongly non-Newtonian (shear thinning), and leads to a significant decrease of H with an increase of the loading time. The apparent viscosity and activation energy for flow were derived from the H ( t ) data on the basis of a theoretical analysis of the indentation process, and the results are in good agreement with those obtained from conventional viscosity measurements.

Study of CaO–WO3–P2O5 glass system by dielectric properties, IR spectra and differential thermal analysis

Abstract: Dielectric constant e ′ , loss tan δ and ac conductivity σ ac of 40CaO– x WO 3 –(60− x )P 2 O 5 (with 0⩽ x ⩽15) glasses are studied over a range of frequencies and temperature. The dielectric breakdown strength of these glasses is also determined at room temperature. The values of dielectric parameters, viz., e ′ , tan δ and σ ac of CaO–P 2 O 5 glasses are found to decrease with the introduction of WO 3 up to 3 mol% and increase with further increase in the concentration of WO 3 ; the probable reasons for such an increase are identified and explained with the aid of IR spectra and differential thermal analysis of these glasses. The variation of tan δ with temperature at different frequencies of CaO–P 2 O 5 glasses has exhibited dielectric relaxation effects with decreasing relaxation intensity with increase in the concentration of WO 3 from 0 to 3 mol%; such relaxation effects seem to have been absent in glasses containing WO 3 beyond 3 mol%. The relaxation phenomenon has been analysed by a pseudo-Cole–Cole plot method and the possible mechanism responsible for such relaxation effects has been suggested.

Hydrogen network fluctuations and dielectric spectrometry of liquids

Abstract: Complex dielectric spectra of associating liquids are presented and are discussed considering their relaxation properties Particular emphasis is given to water and aqueous solutions and to alcohols and mixtures of alcohols with dipolar and non-polar solvents The relaxation properties of the associating systems are examined in the light of a waitand-switch model of dielectric relaxation in which the relaxation time is governed by the period for which a given ensemble of hydrogen bond partners within the hydrogen network has to wait until favorable conditions for a reorientation of a molecular permanent dipole exist These conditions are provided by an additional molecule or group which firstly tends to lower the potential energy barrier for reorientation and which secondly, at the same time, offers a site for the formation of a new hydrogen bond Some experimental findings are discussed in terms of the wait-and-switch model, among them the reduced dielectric relaxation time of water under hydrostatic pressure, effects of negative and hydrophobic hydration of ions, and the influence that water, on the one hand, and n-alkanes, on the other hand, act on the relaxation of alcohols © 2002 Elsevier Science BV All rights reserved

Theory of non-linear susceptibility and correlation length in glasses and liquids

Abstract: Within the framework of the effective potential theory of the structural glass transition, we calculate for the p-spin model and for a hard sphere liquid in the hypernetted chain approximation a static non-linear susceptibility related to a four-point density correlation function, and show that it diverges in mean field with exponent γ=1/2 as the critical temperature Tc is approached from below. When Tc is approached from above, we calculate for the p-spin model a time dependent non-linear susceptibility and show that there is a characteristic time where this susceptibility has a maximum, and that this time grows with decreasing T. This susceptibility diverges as Tc is approached from above, and has key features in common with a generalized susceptibility related to particle displacements, previously introduced to measure correlated particle motion in simulations of glass-forming liquids.

Band offsets of high dielectric constant gate oxides on silicon

Abstract: High dielectric constant oxides will soon be needed to replace silicon dioxide as the gate dielectric material in complementary metal oxide semiconductor technology. The oxides must have band offsets with silicon of over 1 eV for both electrons and holes in order to have low leakage currents. We have calculated the band offsets for many candidate oxides using the method of charge neutrality levels. Ta2O5 and SrTiO3 have small or vanishing conduction band offsets on Si. La2O3, Y2O3, Gd2O3, ZrO2, HfO2, Al2O3 and silicates like ZrSiO4 have offsets over 1.4 eV for both electrons and holes, making them better gate dielectrics. Zirconates are better than titanates as they have wider gaps.

Nanocrystalline TiO2 films studied by optical, XRD and FTIR spectroscopy

Abstract: We report the deposition of thin titanium dioxide films on Si(1 0 0) and silica glass at low temperatures between 200 and 350 °C by a technique of ultraviolet-assisted injection liquid source chemical vapor deposition (UVILS-CVD) with 222 nm radiation. The composition and optical properties of the films deposited have been studied using a variety of standard characterisation methods. A strong absorption peak around 438 cm −1 , corresponding to Ti–O stretching vibration, was observed by Fourier transform infrared spectroscopy for different deposition temperatures. Nanostructured films on Si wafers were observed by atomic force microscopy while X-ray diffraction results showed that crystalline TiO 2 layers could be formed at deposition temperatures as low as 210 °C. The deposition kinetics and influence of the substrate temperature on the film are discussed. The activation energy for this photo-CVD process at temperatures between 200 and 350 °C was found to be 0.435 eV. This is much lower than the value ( E a =5.64 eV) obtained by conventional thermal CVD. The thicknesses of the films grown, from several nanometers to micrometers can be accurately controlled by changing the number of drops introduced by the injection liquid source. Under optimum deposition conditions, refractive index values as high as 2.5 and optical transmittance of between 85% and 90% in the visible region of the spectrum can be obtained.

Stable sealing glass for planar solid oxide fuel cell

Abstract: The thermal and chemical stability of glasses in the BaO–Al2O3–La2O3–B2O3–SiO2 system were investigated as well as bonding characteristics and wetting behavior to yttria stabilized zirconia (YSZ) electrolyte, to develop a suitable sealing glass for planar solid oxide fuel cell operating at 800–850 °C. The thermal properties such as glass transition temperature, softening temperature and thermal expansion coefficient were found to depend on the B2O3:SiO2 ratio in glass composition; thus the bonding characteristics of the glass to YSZ were also influenced by this ratio. The glass having a minimum thermal expansion mismatch with YSZ showed an excellent endurance during thermal cycling. No interface reaction was observed for all the glass/YSZ specimens heat-treated at 800–850 °C up to 100 h.

Characterization of ALCVD-Al2O3 and ZrO2 layer using X-ray photoelectron spectroscopy

Abstract: The atomic layer chemical vapor deposition (ALCVD) deposited Al 2 O 3 and ZrO 2 films were investigated by ex situ X-ray photoelectron spectroscopy. The thickness dependence of band gap and valence band alignment was determined for these two dielectric layers. For layers thicker than 0.9 nm (Al 2 O 3 ) or 0.6 nm (ZrO 2 ), the band gaps of the Al 2 O 3 and ZrO 2 films deposited by ALCVD are 6.7 ± 0.2 and 5.6 ± 0.2 eV, respectively. The valence band offsets at the Al 2 O 3 /Si and ZrO 2 /Si interface are determined to be 2.9 ± 0.2 and 2.5 ± 0.2 eV, respectively. Finally, the escape depths of Al2p in Al 2 O 3 and Zr 3p3 in ZrO 2 are 2.7 and 2.0 nm, respectively.

The nature of phase separation in binary oxide melts and glasses. I. Silicate systems

Abstract: An exhaustive review of compositional and thermal extents of miscibility gaps in 41 binary silicate systems permits identification of three groups of cations exhibiting different immiscibility behaviours. The first group comprises network-modifier cations with an ionic radius larger than about 87.2 pm. They have coordination numbers equal to, or higher than, 5 and their miscibility gap size increases linearly with increasing ionic potential. The second group involves cations with an ionic radius larger than 26 pm and smaller than about 87.2 pm (in octahedral coordination). They have at least two coordination numbers: the first one is always 4 and the other 5 (or more). For this reason they are called amphoteric. Their miscibility gap sizes do not increase linearly with an increase of the ionic potential, but follow curves. The third group includes cations with variable crystal field stabilization energies. They are characterized by larger miscibility gap sizes than expected when they are compared with cations with similar ionic radii despite the fact that some of them (e.g. Cr3+) may behave as an amphoteric element because their ionic radii in octahedral coordination are smaller than about 87.2 pm. The origin of phase separation in binary silicate systems is due to coulombic repulsions between poorly screened cations bounded by bridging oxygen strongly polarized towards the silicon, and by non-bridging oxygen.

Characterisation of porosity and sensor response times of sol–gel-derived thin films for oxygen sensor applications

Abstract: In recent years, sol–gel-derived films have played a significant role in optical sensor development. This paper focuses on the characterisation of oxygen-sensitive tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS)-based films. Film porosity and sensor response times are reported for a range of films fabricated under different conditions. Porosity data are correlated with predicted film behaviour and also with previous characterisation studies. Oxygen diffusion coefficients are estimated from the response times. The enhanced diffusion coefficient of MTEOS films compared to TEOS films is discussed in terms of the relative oxygen solubility of the films. Comparisons are made with data for oxygen-sensitive polymer films, indicating an enhanced solubility for sol–gel films compared with typical polymer films.

From stretched exponential to inverse power-law: fractional dynamics, Cole-Cole relaxation processes, and beyond

Abstract: We present a generalisation of the classical exponential relaxation based on the fractional Fokker–Planck equation framework. We show how fractional dynamics modifies the Brownian dynamics underlying standard relaxation processes, and gives rise to the Mittag–Leffler relaxation of modes and moments. The latter is characterised through a turnover from an initial stretched exponential to a final inverse power-law pattern and the associated complex susceptibility corresponds to the Cole–Cole pattern.

Cu-based bulk glassy alloys with high tensile strength of over 2000 MPa

Abstract: Cu-based glassy alloys with tensile strength of over 2000 MPa were synthesized in Cu–Zr–Ti and Cu–Hf–Ti ternary and Cu–Zr–Hf–Ti and Cu–Zr–Ti–Y quaternary systems by copper mold casting. The critical diameter was 4 mm for the Cu 60 Zr 30 Ti 10 and Cu 60 Hf 25 Ti 15 ternary alloys and the Cu 60 Zr 20 Hf 10 Ti 10 and Cu 60 Zr 10 Hf 15 Ti 15 quaternary alloys and 5 mm for the (Cu 0.6 Zr 0.3 Ti 0.1 ) 98 Y 2 alloy. As the Ti content increases, the glass transition temperature ( T g ), crystallization temperature ( T x ) and temperature interval of supercooled liquid region, ΔT x (= T x − T g ), decrease, while the liquidus temperature ( T l ) has a minimum of 1127 K around 20% Ti, resulting in a maximum T g / T l of 0.63 in the vicinity of 20% Ti. The bulk glass-forming ability was obtained at the compositions with T g / T l >0.60. The glassy alloys have a fracture strengths between 2050 and 2160 MPa and plastic elongations of 0.8–1.7%. The finding of the Cu-based bulk glassy alloys with T g / T l >0.60, fracture strength >2000 MPa and plastic elongation indicates a possibility for subsequent development of glassy alloys with new compositions which can be used for structural materials.

Aluminum-induced crystallization of amorphous silicon

Abstract: We investigated the aluminum-induced crystallization of amorphous silicon (a-Si) during the aluminum-induced layer exchange (ALILE) process, in which a stack of glass/Al/a-Si is transformed into a glass/polycrystalline silicon (poly-Si)/Al(Si) structure by an annealing step well below the eutectic temperature of the Al/Si system. Our experiments resulted in continuous large-grained poly-Si films on glass substrates. The nucleation and the growth of the crystalline phase during the ALILE process was observed using an optical microscope. We found an activation energy of 1.8 eV for the nucleation process and we related this energy to a large barrier at the a-Si/Al interface.

Interface and material characterization of thin Al2O3 layers deposited by ALD using TMA/H2O

Abstract: Thin Al 2 O 3 layers were grown by atomic layer deposition using trimethylaluminum (TMA) and water as precursors on 1.2 nm thermal SiO 2 and HF cleaned Si surfaces. The stoichiometry and the contamination (H, OH and C) of as-deposited and N 2 annealed thick Al 2 O 3 layers were characterized by secondary ion mass spectrometry (SIMS), elastic recoil detection analysis (ERDA) and X-ray photoelectron spectroscopy (XPS). We show a perturbed region (5 nm thick) at the Al 2 O 3 /Si interface by XPS and Auger electron spectrometry (AES). Post-deposition annealings induced important interface oxidation, Si atoms injection and SiO 2 /Al 2 O 3 mixture whereas the initial interface was abrupt. Silicon oxidation before Al 2 O 3 growth highly limits interfacial oxidation and improves interfacial quality. We proposed that OH groups may play a key role to explain silicon oxidation during post-deposition annealings in inert ambience with low oxygen contamination levels.

The effect of Sn addition on the glass-forming ability of Cu–Ti–Zr–Ni–Si metallic glass alloys

Abstract: The effect of Sn substitution for Ni on the glass-forming ability was studied in Cu 47 Ti 33 Zr 11 Ni 8− x Sn x Si 1 ( x =0,2,4,6,8) alloys by using thermal analysis and X-ray diffractometry. With increasing x from 0 to 8, the glass transition temperature, T g , of melt-spun Cu 47 Ti 33 Zr 11 Ni 8− x Sn x Si 1 alloys increased gradually from 720 to 737 K. On the other hand, the crystallization temperature, T x , increased from 757 K at x =0 to 765 K at x =2, being nearly same with further increase of x . Partial substitution of Ni by Sn in Cu 47 Ti 33 Zr 11 Ni 8 Si 1 promotes the glass formation. Both amorphous Cu 47 Ti 33 Zr 11 Ni 8− x Sn x Si 1 alloys prepared by melt spinning and injection casting showed similar crystallization process during continuous heating in DSC. Temperature range of undercooled liquid region exhibits good correlation with the critical diameter for the formation of an amorphous phase in injection casting.

Segmental and chain dynamics of polymers: from the bulk to the confined state

Abstract: Dielectric spectroscopy and temperature modulated DSC are employed to study the molecular dynamics of oligomeric poly(propylene glycol) (PPG) melts of different molecular weights confined to nanoporous glasses (pore sizes 2.5, 5.0, 7.5 and 20 nm). Moreover the results obtained for the polymer are compared with the corresponding monomer. For large pore sizes an acceleration of the segmental dynamics compared to the bulk state is observed which is already known for low molecular-weight glass forming liquids. For smaller pore sizes the molecular dynamics is slower than in the bulk. The observed behavior is nearly independent of the molar mass of the polymer and of the treatment of internal glass surfaces. The experimental results are discussed in the frame of an interplay of confinement and adsorption effects. Moreover a length scale of about 1.6 nm is estimated as a minimal length scale for the cooperativity for the glass transition. In addition to the α-relaxation the whole chain dynamics (normal mode relaxation) can be measured by dielectric spectroscopy because PPG has a dipole component parallel to the chain. For virgin internal surfaces the relaxation rate of the normal mode relaxation is shifted dramatically to lower relaxation rates. That can be explained by adsorption effects. For treated surfaces this effect is strongly reduced and it is concluded that also in this case the chain dynamics are influenced by geometric (confinement) effects.

Investigation on kinetics of thermal decomposition in polysiloxane networks used as precursors of silicon oxycarbide glasses

Abstract: In this study, polysiloxane networks prepared by hydrosilylation or hydrolysis/condensation reactions were considered to be potential precursors for Si–C–O systems. Different precursors had different pyrolytic properties, which was essentially due to their molecular architecture. The kinetics parameters, such as the activation energy, E (kJ/mol) involved in the polymer-to-ceramic conversion, were investigated by thermogravimetry using a multiple heating rate kinetic method. The relationships between the molecular architecture and the precursor composition were compared to that of a linear poly(dimethylsiloxane) precursor. Solid-state 29 Si nuclear magnetic resonance, infrared spectroscopies, density measurements, and X-ray diffraction measurements were made on the final samples. These products were typically amorphous, with a molecular structure formed by a random distribution of different silicon sites and variable amounts of free carbon residue.

Microstructure-property relationship in high-strength MgO–Al2O3–SiO2–TiO2 glass-ceramics

Abstract: An MgO–Al 2 O 3 –TiO 2 –SiO 2 glass was exposed to various two-step heat treatments in order to produce high-strength glass-ceramics. The maximum fracture toughness of 4.3 MPa m 1/2 (accompanied by an elastic modulus of 139 GPa and a hardness of 9.5 GPa) was obtained after a two-hour pre-treatment at 800 °C followed by annealing at 1080 °C. Based on TEM imaging, energy-dispersive X-ray spectrometry, X-ray diffraction and dilatometry, an understanding of the microstructure-property relationship is developed. The high-quartz low-quartz phase transition is shown to play the key role for achieving high-strength glass-ceramics.

The electronic polarisability of oxygen in glass and the effect of composition

Abstract: The electronic polarisability of the oxide species, αoxide(-II), extends over a wide range of values depending on chemical bonding. Several hundred silicate glasses containing alkali and/or alkaline earth metal ions (also Mg2+) have been investigated, and the value of αoxide(-II) for each of these was calculated from refractive index and density. The relationship between αoxide(-II) and the acid–base nature of the glass, expressed as optical basicity, Λ, was investigated, and the results were used to calculate basicity moderating parameters of the constituent sodium, calcium, silicon, etc. These parameters allow calculation of Λ from glass composition, and hence it is possible to predict how oxide polarisability in a glass system will be affected by compositional changes. This facility is also useful for exposing deviations in αoxide(-II) arising from abnormal chemical or physical behaviour. Such examples include (i) the identification of the `mixed alkali effect' in Li2O–K2O–SiO2 glasses, (ii) abnormal refractivity behaviour in glasses based on BeO (in an assessment of the basicity moderating parameter for beryllium in these glasses), and (iii) the effect of stereochemistry on glass basicity, as found for threefold and fourfold co-ordinated boron in borate glasses (such data providing a guide for identifying the co-ordination number of boron in borosilicate glasses). The relevance of the basicity moderating parameter to the polarising power of cations and to electrovalency, covalency and metallic bonding in oxidic systems generally is also discussed.

Variable range hopping revisited: the case of an exponential distribution of localized states

Abstract: Numerical modeling of single-phonon hopping transport within distributions of localized electronic states shows that many transport data for amorphous semiconductors and polymers are consistent with exponential tail state distributions rather than with Mott's hypothesis of an energy-independent distribution near the Fermi level. Although both cases lead to a T −1/4 temperature dependence of the 3D hopping conductivity, they can be discriminated by their different correlations between the slope and the conductivity prefactor. This numerical approach allows a determination of the localization parameter N ( E F ) γ −3 , where (1/ γ ) is the decay length of the electronic wave function, and the assessment of the validity conditions for the single-phonon approximation.