Showing papers in "Journal of Non-crystalline Solids in 1993"

Glass-forming ability of alloys

Abstract: New amorphous alloys exhibiting a wide supercooled liquid region before crystallization were found to form by melt spinning in wide composition ranges of LaAlM, MgYM and ZrAlM (M = Ni or Cu) systems consisting of the constituent elements with significantly different atomic sizes. The temperature span between glass transition temperature, Tg, and crystallization temperature, Tx, ΔTx ( = Tx − Tg) is > 50 K in the compositional ranges around La2AlM, Mg6Ln3M and Zr3AlM and the largest ΔTx reaches 126 K. The critical cooling rate for the glass formation, Rc, is as low as 87–115 K/s and Tg/Tm is > 0.6 in the composition range where ΔTx > 50 K. There is a clear tendency for Rc to decrease with an increase of ΔTx and Tg/Tm. The crystallization of the alloys with large ΔTx occurs through the simultaneous precipitation of several compounds. Based on these results, it is presumed that the large glass-forming ability for these alloys is due to a combined effect of the difficulty of long-range atomic redistribution required for the precipitation of the compounds, the rapid increase of viscosity with decreasing temperature and the large liquidus-solidus interfacial energy which originates from the optimally bonding and packing states resulting from large negative heat of mixing and large atomic size ratios.

Raman spectral studies of borate glasses

Abstract: Raman spectroscopic measurements in borate glasses have been reviewe. The review shows that the technique is useful in identifying the structural groups present in the borate on the basis of the Krogh-Moe hypothesis. Vitreous B2O3 and alkali borates are extensvvely studied and a satisfactory assignment of bands is possible by a careful consideration of the literature. A cation effect on the borate netwoork is observed. Availaable measurements on binary borates other than alkali borates and on ternary borates are limited and more work is required to identify the structural modifications that take place with composition. Mixed alkali effect is reported only lithium-caesium borade and shows the formation of non-bridging oxygens, destroying the six-membered rings when Li2O is replaced by Cs2O. Fast ionic glasses (alkali borates containing alkali halides) yield the same Raman spectra as the alkali borates, except when the alkali is a fluoride.

Spectroscopic properties and mechanisms of excited state absorption and energy transfer upconversion for Er3+-doped glasses

Abstract: The upconversion fluorescences near 0.55 and 0.66 μm and Stokes fluorescences near 0.98 and 2.72 μm were measured for Er3+-doped aluminate, gallate and several other glasses under infrared ation into the 4 I 9 2 level. It was found that the quantum efficiencies of emissions from a given level, except the 4 I 13 2 level, increased in the order of decreasing phonon energies of the glass hosts. The fluorescence originating from the 4 I 13 2 level and its quantum efficiency are not dependent upon the phonon energies of glass hosts, due to influences of the energy transfer upconversion and excited state absorption from the 4 I 13 2 level. Moreover this fluorescence is closely related to the 4 I 11 2 lifetimes, in which the stronger fluorescence can be observed in a glass which has a shorter 4 I 11 2 lifetime. The green upconversion fluorescence originating from the 4 S 3 2 level mainly results from the excited state absorption from the 4 I 13 2 level and the red upconversion fluorescence coming from the 4 I 9 2 level mainly generates from the energy transfer upconversion through the 4 I 13 2 level for Er3+-doped aluminate and gallate glasses.

Activity of substrates in the catalyzed nucleation of glass-forming melts. II. Experimental evidence

Abstract: Methods for determining the nucleating activity of substrates are considered. Experimental data on the activity of nucleating catalysts in various organic and inorganic glass-forming melts are summarized and discussed in terms of the theoretical approach developed in Part I of the present investigation. The nucleation of the same melt induced by different substrates is analysed. In this respect, the data for sodium metaphosphate, lithium metaphosphate, poly(ethylene terephthalate), poly(vinyl chloride), sodium tetraborate nucleated with noble metal cores, for poly(decamethylene terephthalate), lithium disilicate, poly(ethylene) and water initiated by oxide and halide catalysts give a direct proof for the validity of the concepts, developed in Part I.

Activity of substrates in the catalyzed nucleation of glass-forming melts. I. Theory

Abstract: The nucleating activity of different insoluble additives in the heterogeneous nucleation of organic and inorganic glass-forming melts is treated from a generalized thermodynamic standpoint. The activity of a nucleating catalyst is defined through the adhesion energy and calculated from the forces of cohesion in the substrate and in the overgrowing crystal. An estimate is also made of the relative contribution of structure mismatch to the bonding energy by evaluating the total energy of misfit dislocations at the deposit/substrate interface. This approach extends and specifies the limits of application of existing concepts of crystallographic matching and mismatching. By this generalized method, there is a possibility of predicting the nucleating activity of various crystallization cores in the induced crystallization of glass-forming melts by using data on the heat of sublimation (or the temperature of melting) of the respective nucleation catalyst or the value of its thermal expansion coefficient.

Induction time analysis of nucleation and crystal growth in di- and metasilicate glasses

Abstract: The nucleation and crystallization kinetics in three glasses with stoichiometric composition Li2O·2SiO2, Na2O·2CaO·3SiO2 and 2Na2O·CaO·3SiO2 are investigated by determining the nucleation and crystal growth rates and the corresponding induction times. An analysis of the induction times for crystal growth and nucleation indicates that in the two soda-lime metasilicate glasses nucleation and crystallisation of the crystals occur subsequently, i.e., no time lag exists between the two experimentally determined induction times. By contrast with this absence of a time lag, a considerable time lag appears in the case of lithium disilicate, indicating a metastable phase formation and probably compositional changes during crystallisation in this early stage. Transmission electron microscopy shows that a metastable phase (denoted by transient-phase) occurs in the early stage which is totally dissimilar to lithium disilicate and which acts as a precursor to the equilibrium crystallisation of lithium disilicate. This two-stage process leads to the conclusion that the theory of homogeneous nucleation cannot be applied to the Li-disilicate melt.

Non-exponential structural relaxation, anomalous light scattering and nanoscale inhomogeneities in glass-forming liquids

Abstract: Light scattering from glass-forming liquids exhibits an anomalous time dependence in the glass transition region, e.g. maxima in the scattering intensity versus temperature curves during heating. It is shown that this behavior is consistent with the presence of nanoscale inhomogeneities (density fluctuations) which relax at different rates. It is suggested that this could be the source of non-exponential structural relaxation kinetics. An expression relating the size of these regions to structural relaxation kinetic parameters has been developed and predicts sizes in excellent agreement with those determined by other methods.

Mechanical properties of quasicrystalline and crystalline phases in AlCuFe alloys

Abstract: Mechanical properties of icosahedral AlCuFe quasicrystals were investigated and compared with those of related crystalline intermetallic compounds by means of indentation and three-point bending techniques. Quasicrystalline phases possess high hardness (HV ≈ 1000) and high ability for elastic recovery (H/E > 0.08) but exhibit low toughness against unstable fracture (KIC ≈ 1 MPa m12). Icosahedral particles embedded in Al2Cu or in AlFe crystalline matrices lead to increasing resistance against subcritical crack propagation. In contrast to cleavage#1 fracture in crystalline Al13Fe4, the icosahedral quasicrystals exhibit a rough, quasicleavage#1 fracture surface. Evidences for plastic deformation in the quasicrystals were found and its mechanism is discussed.

Zinc phosphate glass surfaces studied by XPS

Abstract: X-ray photoelectron spectroscopy measurements of zinc phosphate glasses with compositions between 30 and 50 mol% P2O5 were used to determine the influence of systematically varied P2O5 content on the oxygen bonding of the glass network Oxygen 1s spectra of zinc phosphate glasses are described in terms of bridging and non-bridging oxygens which are separated by 15 eV The orthophosphate-like oxygen bonding is obtained for binary zinc phosphate glass containing 33 mol% P2O5 The bonding becomes progressively metaphosphate-like with increasing P2O5 content and is correlated with the relative proportions of bridging and non-bridging oxygens

Infrared reflectance investigation of alkali diborate glasses

Abstract: Alkali diborate glasses have been investigated by infrared reflectance spectroscopy to demonstrate the potential of this technique for a quantitative description of the glass structure. Kramers-Kronig transformation was performed on the reflectance data to obtain the optical and dielectric properties of glasses over a broad and continuous spectral range (30–4000 cm−1). Analysis of the mind infrared spectra shows that the normalized absorption area attributed to boron-oxygen tetrahedra decreases upon increasing alkali cation ionic radius, and scales linearly with the fraction of four-coordinated boron atoms determined recently from NMR spectra. The far infrared asymmetric absorption profiles were deconvoluted into two broad bands and attributed to vibrations of alkali cations in two distinct network environments. Cation effective charges were calculated from the integrated absorption of the bands due to cation vibrations, and were found to increase with alkali ionic radius.

Role of photoinduced defects in amorphous GexSe1−x photoconductivity

Abstract: The spectral dependence of photoconductivity of amorphous pure Se and GeSe alloys (4 and 8 at.% Ge) evaporated thin films was measured for a spectral range between 250 and 800 nm. A comparison of the Se and GeSe spectra shows that the magnitude of photocurrent decreases as the Ge content increases. This result indicates a correlation between the magnitude of the photocurrent and the density of photoinduced defects. The contribution of these defects to the enhancement of photoconductivity in a specific region of the spectrum as well as the influence of temperature on their density are also discussed.

Ion implantation- and radiation-induced structural modifications in amorphous SiO2

Abstract: Data on the asymmetric infrared stretching mode of the oxygen in the SiOSi bridge and on the refractive index are examined for crystalline SiO2 polymorphs and compacted, amorphous SiO2. A set of experimentally based relationships is established between the infrared peak position and bridging bond angle and the refractive index and density. Using these relationships, results on radiation- and ion implantation-induced densification in amorphous SiO2 are examined. It is demonstrated that the saturation densification involves reduction in the bridging bond angle by ∼ 10° and a densification ≤3%. This result is inconsistent with the observed bond angle/density relationship established for pressure/temperature compacted amorphous SiO2. It is suggested that the overall densification results from a decrease in the bond angle and a simultaneous generation of both small and large rings.

The comparison of molecular dynamics simulations with diffraction experiments

Abstract: This Commentary discusses the need for a quantitative method of comparing molecular dynamics simulations and other structural models of amorphous solids with diffraction data and hence of assessing the relative merits of the various simulations and models proposed in the literature, most of which claim, erroneously, to be in good agreement with experiment. Using the molecular dynamics simulation of a recent Letter to the Editor [B. Vessal et al., J. Non-Cryst. Solids 159 (1993) 184] as an example, an Rχ, factor is advocated as a quantitative measure of the (dis)agreement with experiment, along the lines of that used in the refinement of diffraction data from polycrystalline powders. The Rχ factor of 9.1% for this simulation is compared with those for other well known structural models proposed in the literature.

DSC and Raman studies of lead borate and lead silicate glasses

Abstract: Several compositions in the xPbO·(1−x)B2O3 (0.20 ≤ x ≤ 0.65) and xPbO·(1−x)SiO2 (0.39 ≤ x ≤ 0.66) vitreous systems were examined by differential scanning calorimetry and by Raman scattering for the former system. Borate glasses with low PbO concentrations are built up in two and three dimensions with three- and four-coordinated boron atoms; Pb2+ acts as a charge balance in the covalent network. Glasses with high PbO content contain ring- and chain-metaborates and PbO4 entities. Differences in Raman scattering of alkali borate glasses are observed; in particular the boroxol rings still exist for x = 0.27. The heat capacities of the glasses at 200°C are an additive function of composition within the accuracy of the differential scanning calorimetry measurements. The heat capacity differences between the vitreous and supercooled liquid states are higher for lead borates than for lead silicates, indicating more profound changes in the lead borates during the glass transition. In the former glasses, the fictive temperature attains a maximum around 0.27PbO corresponding to the highest degree of polymerization. In lead silicates, the fictive temperature decreases continually upon adding PbO, and parallels the depolymerization of the silicate network.

Numerical model for isothermal and non-isothermal crystallization of liquids and glasses☆

Abstract: A new numerical model of isothermal and non-isothermal first order phase transformations, such as the crystallization of liquids and glasses, is presented. This model computes directly the volume fraction transformed, taking into account time-dependent nucleation rates and cluster-size-dependent growth velocities. The model is applied to the crystallization of lithium disilicate glass, using the appropriate kinetic and thermodynamic parameters. The model is used (1) to determine the validity of common methods for computing the volume fraction transformed as a function of time in isothermal experiments when a time-dependent nucleation rate is expected, (2) to simulate non-isothermal differential scanning calorimetry (DSC) studies of glass devitrification as a function of scan rate, and (3) to compute the effect of preannealing on the DSC peak parameters. A novel behavior of the nucleation rate with scan rate is predicted, arising because the relaxation of the cluster distribution cannot be described by a single relaxation time. Comparisons of the calculations with experimental data on this glass demonstrate the validity of the model.

Mechanism of photosurface deposition

Abstract: To understand photosurface deposition (PSD) phenomenon, the deposition mechanism of Ag particles on Ag-rich AgAsS glasses under illumination has been investigated from a view point of electrical effects. The photo-deposited Ag particles are found to be negatively charged under illumination. PSD can effectively be suppressed by coating over the surface with semi-transparent metallic films of work function greater than 4.3 eV, which are positively charged under illumination. The Ag deposition with similar morphology as that observed in PSD can be induced by application of d.c. voltage or by irradiation of an electron beam. These observations suggest that the growth of Ag particles in PSD can be explained by a purely electrical model.

Compositional and temperature effects on five-coordinated silicon in ambient pressure silicate glasses

Abstract: Using 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, silicon coordinated by five oxygens has recently been found in a number of alkali silicate glasses quenched from liquids both at high and ambient pressure. In this report, both the presence of this species in other ambient-pressure samples and the increase in its concentration with fictive temperature are confirmed. However, it has not been possible to detect this species in a hydrated cesium silicate glass sample or in a silica gel, the latter at a detection limit of about 0.02%. Also as previously noted for high-pressure samples, it is found that five-coordinated Si is more abundant in K2Si4O9 than in Na2Si4O9 glass. Given the effects of alkali size on the coordination and bonding to oxygen known from crystalline silicates, this finding supports a mechanism for the formation of the high-coordinate species that involves the consumption of non-bridging oxygens.

Diffuse interface theory of nucleation

Abstract: A nucleation theory for diffuse interfaces is proposed on basis of a parameterization of the cross-interfacial enthalpy and entropy distributions. The predicted size-dependent interface free energy is consistent with experiments on Li2O · 2SiO2.

Non-Debye relaxation and the glass transition

Abstract: A review of some long-standing problems, as well as some new theoretical results regarding non-Debye relaxation and the glass transition is given. The question of whether a phase transition below (or above) the glass transition temperature, Tg, exists is considered. The result that the glass transition temperature and jumps in the dynamic heat capacity and volume expansion coefficient can be calculated if the relevant frequency-dependent linear response functions are known makes the question of the origin of non-Debye relaxation even more important. In effective medium theories, certainly valid at high temperatures, non-Debye relaxation is apparently due to dynamic effects of interactions. At lower temperatures, energy disorder is larger compared with kT, and percolation-based theories may be more appropriate. The question as to the actual magnitude of disorder in energies cannot be conclusively resolved at this time. Certainly, it has been shown that systems exist in which non-Debye relaxation is due exclusively to disorder but, in many systems, an important component of the energy disorder is a manifestation of the influence of topological disorder on interactions. Thus, in these systems, an increase in the magnitude of dynamic interactions relative to kT accompanies the increase in the disorder. The question as to whether universal properties imply universality in the mechanism of non-Debye relaxation is explored in some depth. The present article reaches the tentative conclusion that the relative importance of ‘disorder’ and ‘interactions’ may be dependent on the type of glass considered, and possibly even on specific systems. Certainly, the relative importance of dynamic interaction effects increases as the frequency of an applied ‘force’ is reduced. If their respective influences ‘crossover’ in importance at some finite frequency, ω∗, the relevant question is whether ω∗ is above, at, or below the relaxation peak frequency, ω∗. If universality exists, it relates to the role of disorder, but such a conclusion would require that ω∗ ≤ αc always, the generality of which has not been established.

Dehydration of phosphate glasses

Abstract: For several phosphate glass systems composed of K2O, SrO, (La, Nd)2O3 and P2O5, different dehydration methods have been investigated: variation of raw materials, addition of halides, sintering of raw material mixtures, remelting of glasses, variation of melting time, bubbling of dry oxygen. The methods have been compared with each other concerning their effect on decreasing the OH content. Further, a model of the interactions between glass melt and atmosphere has been established for glasses prepared using fluoride addition or not.

Local structures of rare-earth ions in glasses: the ‘crystal-chemistry’ approach

Abstract: A 'crystal-chemistry' model is proposed to evaluate the rare-earth local structure in glass based on the modified-random-network (MRN) model in glass structure. Three influential factors are summarized, concerning the coordination number, the regularity of the site around the ion. and the spacing between ions. The spectral properties of rare-earth ions in glasses is well described and predicted by the proposed 'crystal-chemistry' model. The concentration-quenching effect of rare-earth ions in glasses in terms of ion-ion separation is analyzed by this model in relation to the role as glass-modifier or glass-intermediate of the rare-earth ion in the glass. The model suggests that glasses in which the rare-earth ions act as intermediates are likely to show less concentration-quenching effect.

The phase diagram and structures of the ternary AlCuFe system in the vicinity of the icosahedral region

Abstract: The phase diagram of the ternary system (Al, Cu, Fe) has been systematically studied around the icosahedral region. At 680°C, the i-region extends approximately over a triangle with vertices of Al, Cu and Fe composition of respectively 62.4-24.4-13.2, 65-23-12 and 61-28.4-10.6. This region splits schematically into 3 fields: (i) the perfect icosahedral phase which is stable down to the lowest possible annealing temperature where atomic diffusion is active in a tiny region of composition close to Al62.3Cu24.9Fe12.8; (ii) a well defined periodic phase with rhombohedral lattice which transforms reversibly into ico near 710°C in the lower part of the triangle; (iii) a complex region characterized by additional diffraction effects (peak broadening, lineshapes, etc.) which may correspond to various approximant structures closely related to the i-phase.

Plastic deformatioon in AlCuFe icosahedral phase

Abstract: Plastic deformation of icosahedral AlCuFe has been studied by compression tests for temperatures ranging from room temperature to 750°C. Whereas at room temperature no ducibility is observed, the samples can be homogeneously deformed up to 130% at high temperatures with no final hardening stage. SEM observations reveal, at the ultimate stage of deformation, a vein pattern similar to the observed in amorphous metals but without slip bands or shear bands. Stress relaxation measurements shows to different regimes at low and high temperatures with a transition at 660°C. It is concluded from these experiments that dislocations are probably not responsible for the plastic deformation of icosahedral quasicrystals.

Surface-induced devitrification of glasses: the influence of elastic strains

Abstract: The influence of elastic strains on devitrification processes in glasses is investigated. It is known that in the vicinity and below the vitrification temperature, Tg, thermodynamic inhibition of crystallization has to be taken into account in addition to kinetic inhibition of crystallization connected with the extremely low values of the mobility (large values of the viscosity). This thermodynamic inhibition results from the evolution of elastic fields due to a difference in the specific volumes of the vitreous and crystalline states of the substance. The inhibiting term is of the same order of magnitude as the thermodynamic driving force of crystallization (and may even exceed it). It is considerably diminished for devitrification at the surface of a glass. In this way, elastic strains can be considered as the origin both for the absence of bulk devitrification and for preferential surface crystallization of glasses in systems with different specific volumes of the vitreous and crystalline states.

Low-frequency vibrations and relaxations in glasses

Abstract: Using molecular dynamics a glass of soft spheres, quenched to zero temperature, is produced and its vibrational behaviour investigated by an analysis of the normal modes. In addition to localized modes with high frequencies, one finds (quasi)localized modes with low frequencies which are centred around structural anomalies of the glass. The glass is then heated in stages to 12.5% of the glass temperature and is allowed to relax. In the relaxed glasses, again (quasi)localized low-frequency modes are found, with effective masses ranging upward from about 10 atomic masses as in the original glasses. The observed relaxations have similar effective masses and are closely related to the (quasi)localized vibrations.

Electric field distribution and near-surface modifications in soda-lime glass submitted to a dc potential

Abstract: The laser-induced pressure pulse method (LIPP) is employed to investigate the electric field distribution in soda-lime glass samples after application of a dc potential (dc poling). The modifications of near-surface composition resulting from the dc polarization procedure were determined by means of three nuclear techniques for near-surface analysis: Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERD) and nuclear reaction analysis. An alkali-depleted layer is formed near the anode. This depletion layer persists for a long time after the dc poling. The electric field in this region is of the order of the intrinsic dielectric breakdown, and its configuration depends on the penetration, or not, of hydrogen. Based on these results, the conduction mechanism in the alkali-depleted layer is discussed. It is suggested that when hydrogen blocking electrodes are used ionized electrons are probably the charge carriers in the alkali-depleted layer.

Liquid state 29Si NMR study on the sol-gel reaction mechanisms of ormosils

Abstract: Sol-gel reaction mechanisms of organically modified silicates (ormosils) were conducted by liquid state 29Si NMR (nuclear magnetic resonance) spectroscopy. Samples were prepared by mixing tetraethoxysilane (TEOS), polydimethylsiloxane (PDMS) and water with isopropanol, tetrahydrofuran (THF) and hydrochloric acid (HCl). Several series of solutions with different reaction times were examined by liquid state 29Si NMR spectroscopy. A new proposed site, D(Q), was assigned from the spectrum of the solution of the dimethyldiethoxysilane (DMDES)/TEOS system. By the reaction between hydrolyzed TEOS and OSi(CH3)2O in the middle of PDMS chains as well as the silanol and groups, HOSi(CH3)2O, of PDMS, bonds between PDMS and TEOS were formed and the PDMS chains were found to exist also as chains (D) and/or cyclic D4 tetramers (D4C) during the reaction. Results of this study also display the expected effects of temperature, acid and water on the reaction rates. The higher the reaction temperature (room temperature to 70°C), the more copolymerization between PDMS and TEOS occurs. The water content (H2O/TEOS = 2 to 4) had little effect on the rate of copolymerization but significantly increased the rate of condensation of TEOS. Increasing acid content (HCl/TEOS = 0.1 to 0.3) increased the rates of both copolymerization and condensation of TEOS. Structural models of the sol-gel reaction mechanisms of ORMOSILs are proposed.

A geometric approach to chemical ordering in icosahedral structures

Abstract: We discuss the chemical ordering of the icosahedral phases by considering the atomic local environments in physical space generated by a set of global atomic surfaces. Each kind of local environment corresponds to a ‘cell’ in the atomic surfaces. From the exploration of the cell decomposition induced by the first neighbours shell in the case of AlCuFe-type icosahedral structures, we conclude that the chemical order in these structures should also depend on the second neighbours shell.

Structural studies in lead germanate glasses: EXAFS and vibrational spectroscopy

Abstract: The Raman, IR absorption and EXAFS spectra at the Ge K-edge and Pb LIII-edge of eight lead germanate glasses, with general formula xPbO(1−x)GeO2 with x = 0.20, 0.25, 0.33, 0.40, 0.50, 0.53, 0.56 and 0.60, have been measured. The occurrence of [GeO6] units besides [GeO4] could not be deduced unambiguously from the data. The vibrational and EXAFS data agree with a progressive depolymerization of the network. Starting from all Ge atoms linked to four bridging oxygens in GeO2 (x = 0), the number of tetrahedral units with one or two non-bridging oxygens increases with x. At low content, Pb2+ ions act as modifiers in the germanate structure, but to a lesser extent than an equivalent number of alkaline ions.

Application of quasicrystalline alloys to surface coating of soft metals

Abstract: Coatings from powdered quasicrystalline alloys may be produced. They show attractive surface mechanical properties that by-pass the brittleness inherent to quasicrystals and they have a good corrosion resistance. Examples of such results are given. An industrial application that follows is mentioned.