Modulus and internal friction in phosphate-silicate bioactive glass

The real and imaginary parts of the dynamic shear modulus, G′ and G″, respectively, of a “bioactive glass” Na₂O, CaO, SiO₂, P₂O₅ have been measured from 10^?4 Hz to 1 Hz at temperatures from 160 K to 610 K. The mechanical loss tangent of the glass (T_g = 816 K) at 1 Hz shows two relaxation regions: one centred at 312 K and the second at 585 K. The low-temperature peak appears in the same region as the peak attributed to Na ion motion in silicate glasses. The modulus of the glass is 32 GPa at 295 K. The spectrum of the high temperature peak has a broad distribution of relaxation times with a half width of five decades of frequency, and Arrhenius “activation energy” of 164 kJ mol.^?1, and the amplitude of this relaxation decreases on cooling. The similarity of these features with those observed in rigid molecular and other types of glasses suggests that the relaxations observed in the bioglass may be more appropriately considered as an intrinsic property of the non-periodic arrangement of atoms or ions in a solid than as being due to the diffusion of specific types of ions in it. It is pointed out that isothermal and isochronal measurements of internal friction in glasses do not give the same relaxation rate at the same temperature, due mainly to a rapid decrease in the amplitude of relaxation with temperature. The shear, Young's and bulk moduli of the glass are, 31.9, 76.6 and 43.0 GPa, respectively, and the Poisson's ratio is 0.20.A sample of the same bioglass containing 0.05% more water showed a shoulder in its tan φ at ～ 400 K and a peak at ～ 560 K, with little change in its modulus or T_g. Thus the presence of water above a certain amount had less significant effect on the internal friction of the bioactive glass.     

In situ high temperature 27Al NMR study of structure and dynamics in a calcium aluminosilicate glass and melt

The temperature dependence (25–1400 °C) of ²⁷Al NMR spectra and spin–lattice relaxation time constants T₁ have been studied for a calcium aluminosilicate (43.1CaO–12.5Al₂O₃–44.4SiO₂) glass and melt using an in situ high temperature probe, and the glass has been characterized by ambient temperature, high field MAS NMR. The peak positions and the line widths show a consistent behavior as motional averaging of the quadrupolar satellites increases with increasing temperature. The rate of decrease with temperature of T₁ drastically increases near the glass transition temperature T_g, which suggests a change in NMR relaxation process from vibrational to translational motions. Above the T₁ minimum (≈1200 °C), NMR correlation times obtained from T₁ are in good agreement with shear relaxation times estimated from viscosity, suggesting that microscopic nuclear spin relaxation is controlled by the same dynamics as macroscopic structural relaxation, and thus that atomic-scale motion is closely related to macroscopic viscous flow.     

Influence of structural relaxation on atomic mobility in a Zr41.2Ti13.8Cu125Ni10.0Be22.5 (Vit1) bulk metallic glass

The effect of an annealing at a temperature above or below the glass transition temperature in a Zr_41.2Ti_13.8Cu₁₂₅Ni_10.0Be_22.5 bulk metallic glass was investigated using dynamic mechanical analysis. Structural relaxation influences both the storage modulus (elastic component) and the loss modulus (viscoelastic component). Kinetics can be captured by a stretched exponential relaxation function. Experimental results are correctly described using a physical model based on the concept of defects for the mechanical response of amorphous materials and especially for the characteristic time relative to atomic mobility.     

Distribution of electrical relaxation times,determined from current decay curves on alkali-còntaining silicate glasses,and an attempt at its explanation

A theory is formulated which offeres the possibility of explaining the occurrence of the very broad distribution of relaxation times with a single activation energy for all relaxation times. It is derived from the potential model of Fröhlich, but with the acceptance of the existence of different long paths through the glass structure for the alkali ions. The theory relates the relaxation spectrum to a distribution of path-lengths of the alkali ions.Electrical decay curves on three silicate glasses, one with Na₂O, one with Na₂O + K₂O, and the third with K₂O only as an alkali, were determined having a time range of almost seven decades (beginning at less than 10^?5 sec). The relaxation spectra could be calculated over 15 decades by superposition of measurements obtained at various temperatures. The disturbing influence of electrode polarization could be minimized by the application of Na-amalgam electrodes. Special distribution functions for the relaxation times fit the measured data according to the theory presented here.     

NMR and infrared study of cation motion in vitreous and polycrystalline TlPO3

The activation energy for Tl⁺ conduction in TlPO₃ glass is obtained from analysis of temperature-dependent motional narrowing for Tl²⁰⁵ NMR spectra and determinations of the localized far infrared (FIR) vibrational frequency for Tl⁺. Use is made of the phenomenological equation of Hendrickson and Bray to analyze the NMR data, yielding E_a = 1.19 eV; the measured FIR Tl⁺ vibrational frequency of 80 cm^?1 yields E_a = 1.09 eV. No significant ionic conduction is observed in polycrystalline TlPO₃. Differential scanning calorimeter measurements yield a glass transition temperature T_g of 96°C and the onset of crystallization temperature of 132°C. Measurements of the Tl²⁰⁵ chemical shift interaction as a function of frequency indicate that (1) the Tl⁺ sites in both polycrystalline and glassy TlPO₃ are ionic, the sites in the polycrystal being slightly more ionic than in the glass; (2) the chemical shift interaction is anisotropic in the glass and isotropic in the polycrystal; and (3) distributions in the values of the principal components of the chemical shift tensor exist in the glass, corresponding to a variety of TlO bond lengths and bond strengths.     

Load relaxation behavior of a Zr41.2Ti13.8Cu12.5Ni10 Be22.5 bulk metallic glass

The load relaxation behavior within the supercooled liquid region of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass has been investigated. To explain the relationship between normalized stress and relaxation time, two different stress relaxation modes such as a Kohlrausch-Williams-Watts (KWW) behavior and a simple power law were applied to the short and long relaxation time regimes, respectively. The apparent activation energy for stress relaxation is 126 ± 10 kJ/mol. Flow curves were obtained by converting load-displacement data into a flow stress-strain rate relation, resulting in three different deformation characteristics through a wide strain rate region interpreted in terms of strain rate sensitivity. A prediction of hot workability has also been attempted by constructing a power dissipation map based on a dynamic materials model.     

High-temperature relaxation mechanisms in inorganic glasses

At temperatures below T_g two relaxation processes are observed in sheet glass (200–500°C) and low-alkali glass (300–600°C): the fast R₁ and the slow R₂ processes which are not connected with the viscous flow, and the structural relaxation occurring R₃ above T_g. The processes R₁ and R₂ proceed at an invariable structure and are characterized by activation energies as high as 5 kcal mol^?1 and 13–15 kcal mol^?1, respectively. The contribution of R₂ amounts to 70–80%. The process R₃, observed near and above T_g, is accompanied with structural variations and, therefore, its activation energy depends on temperature; at T_g it is equal to 60 kcal mol^?1.The processes R₁ and R₂ are due to the mobility and rearrangement of large kinetic units. On the contrary, R₃ is characterized by a low volume of kinetic units. This shows that the ions of silicon and oxygen are involved in this process. The relaxation process R₁ is assumed to be connected with the local fluctuation deformationsof the glass network as in the case of reverse glass deformation under high pressures, and the process R₂ with the mobility of microscopic areas of the glass micro-inhomogeneous structure (structural complexes, microblocks). The continuous spectra corroborate the existence of several high-temperature relaxation processes in silicate glasses.Thus, three relaxation processes are observed in alkali-silicate glasses in the temperature range 200–600°C: the processes R₁ and R₂ are mechanical relaxations, whereas the process R₃ is a structural relaxation determining the viscous flow of glass. The contribution of R₃ to stress relaxation amounts to 5%.There exists a temperature T_k (20–30° below T_g) which is the upper limit of the process R₂. At higher temperatures beginning from T_k the stress relaxation is first determined by the two processes R₁ and R₃, and then by one process R₃. At temperatures below T_k all three processes determine the stress relaxation, but with the decreasing temperature the rate of R₃ becomes negligible and, therefore, in the glass annealing range (below T_k) the mechanical relaxation R₂ and R₁ are mainly responsible; their contribution to the whole relaxation process is as high as 95%.     

Fragility in mixed alkali tellurites

Linear viscoelastic stress relaxation and calorimetric measurements were performed on a series of mixed alkali tellurite glasses of composition 0.3([xNa₂O+(1−x)Li₂O])+0.7TeO₂ at temperatures near and above the glass transition temperature, T_g. The stress relaxation data were well described by the stretched exponential function, G(t)=G₀exp[−(t/τ)^β], where τ is the relaxation time, β is the distribution of relaxation times and G₀ is the high frequency modulus. The fragility, determined from the temperature dependence of τ, exhibited a minimum in the middle of the mixed alkali composition. A possible connection between the kinetic and the thermodynamic dimensions of this system was established, wherein the heat capacity change at the T_g, ΔC_p(T_g), and the fragility are correlated.     

Superplastic flow of a Mg-based bulk metallic glass in the supercooled liquid region

The tensile flow behavior of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass was investigated over a range of strain rates (10⁻³–10⁻¹/s) and deformation temperatures (150–170 °C) in the supercooled liquid region. In this region, the relationship between peak flow stress, strain rate and absolute deformation temperature was described adequately by the classic Sellars–Tegart constitutive relationship. There was also a good correlation between the Zener–Hollomon parameter, Z, and the flow characteristics of the material such as the transition from Newtonian to non-Newtonian flow and maximum achievable tensile elongation; the latter was used for determining the optimum conditions for superplastic flow in the material.     

Defect model for the mixed mobile ion effect revisited: An importance of deformation rates

The progress in understanding the behavior of glassy mixed ionic conductors within the concept of the defect model for the mixed mobile ion effect [V. Belostotsky, J. Non-Cryst. Solids 353 (2007) 1078] is reported. It is shown that in a mixed ionic conductor (e.g., mixed alkali glass) containing two or more types of dissimilar mobile ions of unequal size sufficient local strain arising from the size mismatch of a mobile ion entering a foreign site cannot be, in principle, absorbed by the surrounding network-forming matrix without its damage. Primary site rearrangement occurs immediately, on the time scale close to that of the ion migration process, through the formation of intrinsic defects in the nearest glass network. Neither anelastic relaxation below glass transition temperature, T_g, nor viscoelastic or viscous behavior at or above T_g can be expected being observed in this case because the character of the stress relaxation in a wide temperature range is dictated above all by the deformation rates employed locally to the adjacent network-forming matrix. Since the ion migration occurs on the picosecond time scale, the primary rearrangement of the glass network adjacent to an ionic site occurs at rates orders of magnitude higher than those of the critical minimum values, so the matrix demonstrates brittle-elastic response to the arising strain even at temperatures well above T_g, which explains, among other things, why mixed alkali effect is observable in glass melts.     

High-temperature strain relaxation in silica by optical correlation spectroscopy

Strain relaxation has been studied in silica glass from the glass transition up to 1750°C by means of optical correlation spectroscopy. The decay function shape for longitudinal strain is found to be independent of temperature so that silica can be considered thermorheologically simple over a large part of its transformation region. Measurements on high silica glass (Corning Code 7900) yield the same decay function apart from a viscosity-dependent shift of the average relaxation time.     

Dielectric behaviour in a vanadium phosphate glass

The dielectric constant and conductivity of 80% V₂O₅: 20% P₂O₅ glass has been measured in the frequency range 10² to 10⁹Hz and in the temperature range 80 to 350°K. It is shown that the dielectric behaviour over these ranges is described by a Debye type relaxation process with distribution of relaxation times. A method is proposed to determine the width of distribution from the data at fixed frequencies and different temperatures. The width of distribution increases at frequencies ω > 10/τ, which leads to an a.c. conductivity at these frequencies almost linearly proportional to frequency and independent of temperature. The estimated value of the static dielectric constant of about 30 was found to decrease with temperature while the infinite frequency dielectric constant of 10 was independent of temperature. The carrier concentration calculated from the dielectric relaxation time and the d.c. conductivity through a thermal diffusion model shows reasonable agreement with direct measurement using electron paramagnetic resonance.     

Dynamic deformation of silicon-rich V3Si single crystals at elevated temperatures

The plastic deformation behaviour of the intermetallic phase V₃Si (Cr₃Si type) was investigated under dynamic conditions. The experiments revealed that V₃Si deforms plastically at a strain rate of about 4.7 × 10⁻⁵s⁻¹ above 1200°. Flow stress depends strongly on deformation temperature and strain rate. An alloy hardening by deviation from stoichiometric composition within the range of homogeneity is observed.     

Temperature and Strain Rate Effects on Work-Hardening of KCl Single Crystals

The influence of temperature and strain rate on work hardening of a KCl crystal has been investigated. Using the experimental data on flow stress relaxation, the activation “volume” for dislocation motion and internal counteracting stresses have been calculated in the temperature range of 300 to 40 K. Estimation has been carried out for the variation of a number of mobile dislocations ϱ₂/ϱ₁, when the strain rate is changed by a factor of four.     

The internal friction and elastic modulus of amorphous Pd–Si and Fe–P–C alloys

The internal friction and the shear modulus of the amorphous Pd₈₀–Si₂₀–, Fe₈₀–P₁₃–C₇ and Fe₅₀–Ni₃₃–P₁₂–C₅ alloys have been measured at about 0.5 Hz over a temperature range from room temperature to about 550°C. The internal friction rises steeply with temperature and shows maxima around the crystallization temperature. The activation energy for the steeply rising internal friction has been estimated by the two different procedures; from the shift of the curve by the frequency change and from the slope of the In Q^?1 versus 1/T plot. The two values are very different, which indicates the existence of a wide distribution of activation energies. Crystallization of the amorphous alloys resulted in an increase of about 30% in the shear modulus.     

Low frequency storage and loss moduli of soda-silica glasses in the transformation range

The storage and loss moduli of sodium silicate glasses in the transformation range are presented. The data cover the frequency range from 10^?3 to 1 rad/sec. Using linear viscoelastic theory, the frequency-temperature dependence is separated into master curves for the storage and loss moduli at the glass transition temperature and shift factor data describing the shift of the curves with temperature along the frequency scale. These binary glasses are concluded to be thermorheologically simple. The relationship between the shape of the master curves and the configurational entropy is shown, and various available theories for the shift factor data are discussed.     

Structural relaxation of Ti40Zr25Ni8Cu9Be18 bulk metallic glass

Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ bulk metallic glass has a unique quenched-in nuclei/amorphous matrix structure. The crystallization of quenched-in nuclei, when the experimental isothermal annealing time is within its incubation time, may not disturb the enthalpy relaxation, which makes it have the accordingly common enthalpy relaxation behavior with amorphous materials. The alloy's annealing time dependence of recovery enthalpy follows a stretched exponential function with the mean relaxation time obeying an Arrhenius law. The equilibrium recovery enthalpy ΔH_T^eq, mean relaxation time τ and stretching exponent β are all dependent on the annealing temperature, and generally, a higher annealing temperature comes with a lower value of ΔH_T^eq, τ and a higher value of β. Two parameters, β_g and τ_g, representing the stretching exponent and the mean structural relaxation time at the calorimetric glass transition temperature, respectively, are correlated with glass forming ability and thermal stability, respectively. For Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ BMG, the high value of β_g, which is much higher than 0.84 and approaches unity, reveals its good glass forming ability, while, on the other hand, the low value of τ_g indicates a worse thermal stability compared with typical BMGs.     

Luminescence properties of Tb3+/Sm3+ co-doped 38B2O3―31Al2O3―31SrO glasses and glass ceramics

In this paper, Tb³⁺/Sm³⁺ co-doped 38B₂O₃―31Al₂O₃―31SrO glass was successfully prepared. After heat treatment, single crystal phase SrAl₂B₂O₇ was precipitated from the parent glass. DTA data showed the glass transition temperature at 625 °C and a sharp exothermic peak at 860 °C. XRD patterns demonstrated a regular evolution from glass to glass ceramics with higher treatment temperature and longer treatment time. From the XRD patterns, we supposed that Tb³⁺/Sm³⁺ ions can be most likely contained in the crystal phase. The photoluminescence spectra showed that the crystallization can enhance the emission intensity significantly and there could be an optimum crystallization degree to get the strongest luminescence in glass ceramics. The light scattering of devitrification sample can vary the intensity ratio of Sm³⁺ and Tb³⁺ emission. Therefore, as a potential route, rare earth ions doped glass ceramics could be a further research direction of luminescence glasses for white light emitting diodes application.     

The effect of cooling rate on the glass transition of an amorphous polymer

Cooling down from the equilibrium state at different rates reveals the dynamic behavior of glass forming materials. In particular, the dependence of the glass transition region on the cooling rate, q is commonly agreed to contain information regarding the activation energy of the relaxation time, τ. In this work experimental and theoretical aspects of such a relationship have been highlighted. Experimentally, the glass transition zone of amorphous polystyrene films has been investigated over two decades of cooling rate (0.5–50 K/min) by using refractive index measurements. The shift of the glass transition temperature and the broadening of the transition zone at increased cooling rate have been characterized. Theoretically, the cooling experiments have been simulated within the integral formulation of the Kovacs–Aklonis–Hutchinson–Ramos (KAHR) model using the Vogel temperature dependence for the relaxation time. The Frenkel–Kobeko–Reiner equation, τq = constant, provided the needed relationship between the experiments and the theory, enabling the evaluation of the relevant parameter of the kinetic model, i.e. the Vogel activation energy and the zero configurational entropy temperature, from the shift of the glass transition temperature with cooling rate.     

NMR investigation of rapid structural changes and non-metal-to-metal transition in the molten eutectic Ge–Te alloy

Resonance shift, K¹²⁵, spin-lattice relaxation time, T₁ and spin–spin relaxation time, T₂ for ¹²⁵Te nuclei in molten Ge₁₅Te₈₅ and Te in deeply under-cooled state have been measured as a function of temperature. The temperature dependence of K¹²⁵ is compared with that of the extent of structural change, C, and it is shown that the rapid increase of K¹²⁵ with increasing temperature can be attributed to an increase of the density of states at the Fermi level proportional to the extent of structural change. At high temperatures, the dependences on the temperature of K¹²⁵ and T₁ conform to the Korringa relation with an enhancement factor or the relaxation mechanism expected metallic nature. At low temperatures, in contrast, the Redfield theory assuming localized spins of which the density is proportional to C can explain the overall temperature dependences of T₁ and T₂. These results provide the first clear evidence for a crossover transition from extended to localized states in the electron system accompanying the rapid structural changes.