Partial correlations in Ni60Nb40 metallic glass

Combined X-ray and neutron diffraction experiments were performed on the Ni₆₀Nb₄₀ metallic glass samples prepared by rapid quenching from the melt with natural Ni^nat and isotope ⁵⁸Ni, respectively. The partial structure factors were separated for the three kinds of atomic pairs: NiNi, NiNb and NbNb. The partial distribution functions were calculated by means of Fourier transformation and the following atomic distances were obtained: $\text{r}{}_{\mathrm{<mtext>NiNi</mtext>}}\text{=2.52}\text{A}\text{?}\text{, r}{}_{\mathrm{<mtext>NiNb</mtext>}}\text{=2.72}\text{A}\text{?}$ and $\text{r}{}_{\mathrm{<mtext>NbNb</mtext>}}\text{=2.70}\text{A}\text{?}$. The values n_NiNi=7.3, n_NiNb=4.5, n_NbNi=6.8 and n_NbNb=5.4 were obtained for the number of nearest neighbours.     

Prediction of glass-forming ability for metallic systems

The relative glass-forming ability (GFA) of metallic alloys is considered in terms of a parameter $\text{ΔT}{}^1\text{= (T}{}_{\mathrm{<mtext>liq</mtext>}}{}^{\mathrm{<mtext>mix</mtext>}}\text{? T}{}_{\mathrm{<mtext>liq</mtext>}}\text{)/T}{}_{\mathrm{<mtext>liq</mtext>}}{}^{\mathrm{mix}}$, which represents the departure of the alloy liquids temperature, T_liq, from that of the simple rule of mixtures liquids temperature, T_liq^mix. For values of $\text{ΔT}{}^1\text{> 0.20}$ a metallic system is likely to form a glass by melt-quenching in useful thicknesses (i.e. > 20 μm) at a cooling rate of 10⁵?10⁷ K s^?1. Hence, a rapid assessment of the GFA of novel compositions may in general be obtained simply from a knowledge of the melting points of the pure components and the liquidust emperatures of the alloys.     

Kinetics of ion exchange in glasses

The kinetics of $\text{K}{}^{\mathrm{+}}\text{?}\text{Na}{}^{\mathrm{+}}$ exchange in two glass systems, 20Na₂O·(60?x)B₂O₃· (20 + x)Si₂ (where x = 0, 15, 30 and 45 mol%) and Na₂O·3SiO₂, were studied as a function of glass composition, salt bath composition, exchange temperature and time The distribution of K in the glass specimens after exchange in molten KNO₃ was determined with an electron probe. Stresses in these speciments were measured photoelastically. The interdiffusion coefficient $\text{D}$ for ion exchange was calculated as a function of local composition in the glass using the Boltzmann-Matano method. The strong variation of $\text{D}$ in any particular glass approximated that predicted by a mixed alkali model (as advanced by Lacharme), where the glass in the ion-exchanged region approximates a composite of stacked layers of mixed alkali glasses with a gradually varying alkali ratio. The small discrepancy between the experiment and the mixed alkali model was partly, but not fully, reconciled by considering the strains in the glasses. The observation which remained unexplained was that the calculated stress profiles did not show perfect agreement, both in magnitude and in shape, with the experimentally measured stress profiles. It appeared that the kinetics of ion exchange in the glasses were also influenced by a network relaxation process which may have occurred well below the glass transition temperature.     

Pressure-volume-temperature relations in liquid and glassy selenium

The PVT properties of amorphous selenium are studied experimentally and theoretically in the temperature range 0–70°C and for pressures up to 200 MPa. PVT surfaces are determined for the metastable liquid and for a glass formed by a pressurization and cooling procedure. Its liquid—glass intersection line $\text{T}{}_{\mathrm{<mtext>g</mtext>}}{}^2\text{(P)}$ is compared with the glass transition line T_g (P), here obtained by pressurizing the liquid isothermally at a rate of 2 MPa/min. Analytical expressions based on the PVT data are compared with the predictions of the Simha-Somcynsky hole theory originally formulated for open chainmolecular fluids. The agreement for the liquid, although satisfactory, is not as good as for amorphous organic polymers thus far studied, possibly because selenium contains both open-chain and Se₈ ring molecules. The theoretical scaling parameters for the best fit to experiment are compared with those obtained for polymers. A very high characteristic pressure and thus cohesive energy density are noted. The theoretical hole fraction is found to be nearly constant along the $\text{T}{}_{\mathrm{<mtext>g</mtext>}}{}^2\text{(P)}$ line for low pressures. For the glass, a theoretical equation of state obtained from that for the liquid by freezing the hole fraction, compares favorably with experiment. The Prigogine-Defray ratio ΔκΔC_p/TV(Δα)² at atmospheric pressure, calculated using literature values of ΔC_p, is found to be about 2.0.     

Effect of thermal history on conductivity and electrical relaxation in alkali silicate glasses

Electrical conductivity σ₀ and electric field relaxation measurements have been carried out as a function of thermal history for two alkali silicate glasses, Na₂O3SiO₂ and K₂O3SiO₂. Specimens of each glass with three different thermal histories, two of the anneal-and-quench type and one of the rate-cool type, were studied. The average structural or fictive temperature T_f of each of the specimens was characterized by measuring their indices of refraction. Effects of thermal history on σ₀ and its activation enthalpy $\text{H}{}_{\mathrm{\sigma }}{}^1$ were in accord with results of previous investigators. That is, for a given type of thermal history σ₀ was lower and $\text{H}{}_{\mathrm{\sigma }}{}^1$ higher the lower T_f. In addition it was found that for two specimens with the same T_f or index of refraction but different thermal histories the rate-cooled specimen exhibited a lower conductivity than the annealed-and-quenched specimen, in accord with the results of Ritland. The distribution of relaxation times τ_σ for decay of the electric field due to ionic migration was found to be due primarily to a distribution in the pre-exponential term ln $\text{τ}{}_{\mathrm{\sigma }}{}^1$ in the equation $\text{ln}\text{τ}{}_{\mathrm{\sigma }}\text{=}\text{ln}\text{τ}{}_{\mathrm{\sigma }}{}^1\text{+ H}{}^1\text{/RT}$; the distribution in $\text{H}{}^1$ was extremely narrow. Differences in thermal history caused small differences in the distribution of τ_σ, but no difference in the average activation enthalpy $\text{〈H}{}^1\text{〉}$ for τ_σ. From this result it appeared that the dependence of the conductivity activation enthalpy $\text{H}{}_{\mathrm{\sigma }}{}^1$ on thermal history was due to the effect of thermal history on the temperature dependence of the distribution in τ_σ.     

Electrical conductivity and photoconductivity of As2S3PbS glasses

Measurements of dc electrical conductivity and photoconductivity of various glassy compositions (x = 0.1?0.625) in (As₂S₃)_1?x(PbS)_x have been made. Experimental results of the temperature dependence of dc conductivity from room temperature to 200°C (which includes the glass transition temperature) are reported. All the compositions exhibit intrinsic conduction in the measured temperature range. Thermal activation energy, glass transition temperature and σ₀ for the compositions studied, were determined from the experimental data. The low value of $\text{σ}{}_0\text{(10?10}{}^{\mathrm{?2}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$) in these semiconducting glasses is attributed to the greater participation of localized states in the conduction process.In the measurements of photoconductivity, the variation of photocurrent with temperature, photon energy, light intensity and electric field is observed. The recombination model has been involved to explain the results of photoconductivity. Both electrical and photoconductivity data support the presence of higher density of localized states in the x = 0.1 composition than in others.     

On mechanisms of structural relaxation in a Pd48Ni32P20 glass

Structural relaxation processes are investigated calorimetrically for a pre-conditioned Pd₄₈Ni₃₂P₂₀ glass over a wide temperature range from well below to just above the glass transition. The low temperature anneals further stabilize the glassy structure. Upon heating, the annealed sample shows an excess endothermic specific heat ΔC_p above the annealing temperature and completely recovers the initial enthalpy before any manifestation of glass transition, T_g. Significantly the ΔC_p peak evolves in a continuous manner with annealing time. A physically reasonable activation energy spectrum $\text{N}{}_0{}^1\text{(Q)}$ is obtained with the proper choice of coupling constants which are dependent on annealing temperature. Results suggest the existence of localized relaxation modes which do not contribute to macroscopic flow. A concept of distribution in glass transition temperatures H(T_g,m) is conceived to account for the reversible relaxation with temperature. A model glass transition based on percolation theory is proposed and is found to reproduce the calorimetric relaxation phenomena well.     

Retarded elasticity in B2O3GeO2 glasses

The retarded elasticity was investigated for B₂O₃GeO₂ glasses having network structure in the glass transition range by using a compressive method. The compliance (J_∞) determined at the final stage of each measurement displayed a maximum for roughly constant viscosity (η ? 10¹⁴ P) in all the B₂O₃GeO₂ glasses and was simulated by the same equation applied for AsS glasses reported in a previous paper [1].

$\text{J}{}_{\mathrm{\infty }}\text{=(1?k}{}_2\text{keta;}{}_{\mathrm{G}}\text{keta;}\text{)[?}{}_1\text{(}\text{k}{}_1\text{keta;}\text{)+?}{}_2\text{(}\text{nk}{}_1\text{keta;}\text{)]}$

, where $\text{K}{}_1\text{, k}{}_2\text{, ?}{}_1\text{, ?}{}_2\text{and}\text{n}$ are parameters and η_G is the viscosity related to the retarded elasticity. The terms (k₁/η) and (nk₁/η) are assumed to be equal to one for all their values exceeding one. For B₂O₃GeO₂ glasses, the deformation due to the retarded elasticity could be alloted to two structural elements: the first element related to the term $\text{?}{}_1\text{(k}{}_1\text{/η)}$ and the second element related to the term $\text{?}{}_2\text{(nk}{}_1\text{/η)}$. The values of $\text{?}{}_1$ showed almost no variance with the glass composition, but $\text{?}{}_2$ had a minimum at the composition of 50 mol% GeO₂. These data suggest that the contribution of the second element is the smallest at B₂O₃/GeO₂ = 1. The values of k₂ were close to that of As₂S₃ glass having the network structure. k₁ and n (or nk₁) were almost constant regardless of the composition, respectively. These data suggest that the inhibition due to the viscosity starts at an approximately constant viscosity in B₂O₃GeO₂ glasses.     

X-ray photoemission study of fluorozirconate glass and related crystals

Silicate glasses with the composition (in per cent by weight) of 74 SiO₂, 20 Na₂O, 6 CaO and different Fe₂O₃ concentrations were subjected to X-ray irradiation and the behaviour of the iron as well as that of the radiation-induced defects was investigated systematically using ESR. The defects which are abundant in iron-free glasses (they produce a strong signal at g = 2.01) and which lead to colouring of the glass, decrease with rising Fe₂O₃ concentration and cease to exist at 2% Fe₂O₃. These glasses are radiation-resistant.Because of the X-rays a valence change of the iron takes place. This process is initiated by a radiation-induced redox interaction between Fe³⁺ and Fe²⁺ according to:

$\text{Fe}{}^{\mathrm{2+}}\text{+}\text{Fe}{}^{\mathrm{3+}}\text{→}\text{hv}\text{Fe}{}^{\mathrm{3+1}}\text{+}\text{Fe}{}^{\mathrm{2+1}}$

Because of the different structures (coordinations) of Fe²⁺ and Fe³⁺ the process leads to a decrease in Fe³⁺ at g = 4.2 in glasses with low Fe₂O₃ concentration. This decrease is reduced with increasing concentration of Fe₂O₃.     

Effects of pressure on the electrical conductivity of chalcogenide glasses

The effects of hydrostatic pressure (to ～2.4 GPa) on the electrical conductivity of AsTe, AsTeI and AsTeGe bulk semiconducting glasses have been determined. The electrical conductivity σ increases nearly exponentially with increasing pressure P. The Δ 1n σ/ΔP values are dependent upon composition and pressure, and vary from about 2 to 6 GPa^?1. This is a narrow range of values considering that the initial conductivies vary over five orders of magnitude for the compositions studied. Many of the glasses exhibited time-dependent conductivity changes both at high pressure and after cycling to high pressure. At high pressure the conductivity drifted to higher values over a period of several hours, initially following a logarithmic time dependence. Generally, the drifts were observed for P ? 0.8 GPa and for $\text{σ ? 10}{}^{\mathrm{t-1}}\text{(Ω-}\text{m}\text{)}{}^{\mathrm{?1}}$. Following the high-pressure experiment, the conductivity (and also the density) of some glasses were above that for the as-prepared material. These same samples had a slightly different conductivity temperature dependence. The conductivity slowly relaxed (over many months) toward the original conductivity state, again initially following a logarithmic time dependence. Much of our data can be interpreted consistently if we assumed that the conductivity changes depend primarily on “expected” volume changes. The kinds of behavior reported here are similar to those observed for a wide variety of glass systems. Any models developed for describing electrical transport under pressure must account for time-dependent as well as pressure-dependent effects.     

Optical and lasing properties of fluorophosphate glass

Neodymium-doped fluorophosphate glass is a laser material newly-developed for use in high power laser fusion systems. The low refractive index (n_d ～ 1.45) and low dispersion (Abbe number ～90) of fluorophosphate glasses give them the properties of low nonlinear refractive indices and long Nd³⁺ fluorescence lifetimes, which are desirable for the high power laser applications. We have measured the intensity gain of 1.052 and 1.064 nm laser light produced by flashlamp-pumped fluorophosphate glass amplifiers, varying in size from 4–34 clear aperture. The measured gains are compared with those measured in other laser glass types and with those predicted from the spectroscopic properties of Nd³⁺. We estimate that the peak cross section for the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>11</mtext><mtext>12</mtext>}}$ transition in commercial fluorophosphate laser glasses is ～2.2 × 10^?20 cm².     

Optical spectra of erbium and thulium in germanate glass

Oscillation strengths, emission and excitation spectra and decay times of Tm³⁺ and Er³⁺ in germanite glasses were measured. The results were compared with the data obtained previously in borate and phosphate glasses. The role of glass phonons on the transition probabilities, half-width of emission bands and the Stokes shift is discussed. Also, the influence of the cavity and the site symmetry in which the rare earth is situated on transition probabilities is discussed. Bands in the emission spectrum of Er³⁺, absent in the borate and phosphate, are detected in the germanite glasses. Probabilities of relaxation from ${}^1\text{D}{}_2\text{→}{}^1\text{G}{}_4$ levels of Tm³⁺ in various glass hosts are calculated.     

Nucleation kinetics of sodium disilicate

The kinetics of crystal nucleation in Na₂O · 2SiO₂ have been determined over the range of undercoolings between 173 and 373°C. The plot of log(I_v?) versus $\text{1}\text{ΔT}{}^2{}_{\mathrm{r}}\text{T}{}^3{}_{\mathrm{r}}$ is a straight line of negative slope over some 13 orders of magnitude in I_v. The slope of this relation indicates a nucleation barrier of about 45 kT at ΔT_r = 0.2, and the intercept at $\text{1}\text{ΔT}{}^2{}_{\mathrm{r}}\text{T}{}^3{}_{\mathrm{r}}\text{= 0}$ is 10²⁶ cm^-3 sec^-1. poise. The results are in good agreement with predictions of the theory of homogeneous nucleation, even in the pre-exponential factor.     

Radiative and non-radiative transition probabilities and quantum yields for excited states of Er3+ in germanate and tellurite glasses

Absorption and fluorescence spectra of Er³⁺ in germanate and tellurite glasses were obtained. Spontaneous transition probabilities of the ${}^4\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and ${}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ to all terminal levels of Er³⁺ were calculated using the Judd-Ofelt theory and intensity parameters obtained from measured intergrated absorption coefficients. Quantum efficiencies of the ${}^4\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and ${}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ fluorescences were measured by the comparative method and by the use of measured decay times. Multiphonon relaxation rates for ${}^4\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ and ${}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ were calculated using the experimental data. The average rate for ${}^4\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ in germanate is 1.16 × 10⁵ sec^?1 and in tellurite is 1.60 × 10⁴ sec^?1, and the rate for ${}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{is}\text{2.85 × 10}{}^5\text{sec}{}^{\mathrm{?1}}$ in germanate and 2.33 × 10⁵ sec^?1 in tellurite. The higher rates in germanate glasses are explained by the stronger interaction of the glass phonons with the electronic states of Er³⁺ in germanate than in tellurite glasses. This also explains the higher quantum yield of the visible fluorescence of Er³⁺ in tellurite glasses compared to other glasses.     

Composition dependence of the glass transition temperatures of PdNiP and PtNiP glasses

Thermal properties of glassy PdNiP and PtNiP alloys have been measured as a function of the concentration of transition metals. The glass transiion temperature, T_g, of these alloys glasses exhibits a negative linear deviation with transition metal content - which is in contrast to the increasing T_g of binary glassy alloys with increasing metalloids.It is suggested that the suppression of the glass transition temperature of these glassy alloys may be attributed to the excess configurational entropy of disorder associated with a mixture of hard spheres differing in radius. In contrast, the increasing T_g of binary glassy alloys with the metalloid content may be associated with the short-range order resulting from strong interactions between metal and metalloid atoms.     

Multiple sites for Er3+ in alkali silicate glasses (II). Evidence of four sites for Er3+

The low-temperature absorption spectrum of a series of binary alkali silicate glasses doped with Er³⁺ shows the presence of four different sites for the Er³⁺. The near-octahedral A site reported in Part I of this series is the principal site and is present in all these glasses. The B site appears in the binary lithium and sodium silicate glasses. The binary potassium, rubidium and cesium glasses show the presence of the C and D sites. The C site absorption spectrum is similar to that found by Gruber et al. for Er³⁺ in Er₂O₃. The D site is probably a variation of the B site. Tentative sixfold-coordinated models are suggested for the B and D sites.     

Small polaron conduction in V2O5P2O5 glasses

Dc conductivity measurements have been made between 90 and 520 K on three bulk samples of V₂O₅P₂O₅ glass. Heat treatment is found to result in a reduction of the activation energy at a given temperature and this is most noticeable at low temperatures. The behaviour at low temperatures can be described using Mott's variable range hopping arguments, and at high temperatures by non-adiabatic small polaron hopping between nearest neighbours. At intermediate temperatures a simple model is used in which excitations by optical and acoustic phonons are considered to make independent contributions to the jump frequency. Mott's theory is extended to the polaron case for $\text{T>}\text{1}\text{4}\text{?}$ and is shown to be in good agreement with results. Values for $\text{r}{}_{\mathrm{<mtext>p</mtext>}}\text{(～2.8}\text{A}\text{?}\text{)}$ the polaron radius and $\text{α(～3.5}\text{A}\text{?}{}^{\mathrm{?1}}\text{)}$ the electron decay constant are shown to be consistent with the model for small polarons. A method is suggested for obtaining α and N(E_F) from the ac conductivity and the slope of 1nσ versus $\text{1}\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ at low temperatures. Values of N(E) are obtained which correlate with those obtained by the previous analysis. This implies that the disorder energy separating adjacent sites Δ₀ is large (～0.4 eV) in these materials.     

Spinodal decomposition during continuous cooling of a PbOB2O3Al2O3 glass

Spinodal decomposition during continuous cooling of the PbOB₂O₃Al₂O₃ quasi-binary glass system was analysed by numerical integration of Cook's differential equation (which includes the contribution of random density fluctuations) for small angle X-ray scattering (SAXS) intensity. The SAXS curves derived from the calculations have a wide range of k-Fourier components (0 < k < k_c) for which a positive amplification factor occurs and they show a “crossover” point at $\text{k}{}_{\mathrm{c}}\text{= 0.155}\text{A}\text{?}{}^{\mathrm{?1}}$. The wavenumber which receives maximal amplification, k_m, increases with the cooling rate, Q, as $\text{Q}{}^{\mathrm{<mtext>1</mtext><mtext>n</mtext>}}$, with n = 10.9. This Q dependence of k_m is similar to that predicted by Huston et al., however our results show a higher value of n. The dependence on Q and k_m of the SAXS intensity I(k_m) was also deduced. The measurements of SAXS curves were performed on glass samples prepared by the splat-cooling technique. Because of the difficulties which arise in the determination of the cooling rate of the samples, the only experimental results that could be compared with the theory are the k_m dependence of I and the value of k_c. These results are satisfactorily understood in terms of the present analysis.     

Energy transfer in Eu3+-Yb3+ and Tb3+-Yb3+ system in glasses

The behavior of the phonon-assisted energy transfer between trivalent rare-earth ions in glasses was investigated. The ions Eu³⁺ and Tb³⁺ as energy donors and Yb³⁺ as acceptor were selected. The energy gap between the levels of the donor and acceptor was estimated on the basis of the energy diagram of each ion determined from absorption and emission spectra. The probability for the transfers of (Eu, ⁵D₀-⁷F₆): (Yb, ${}^2\text{F}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{-}{}^2\text{F}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$) and (Tb, ⁵D₄-⁷F₀): (Yb, ${}^2\text{F}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{-}{}^2\text{F}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$) in silicate, borosilicate, phosphate and germanate glasses was measured in the temperature range of liquid-nitrogen temperature - 650K. The probability of transfer was the smallest in phosphate glass and B₂O₃ had the effect of increasing it. In germanate glass the dependence of the probability of the energy gap was relatively weak. These results were correlated to the difference in the phonon energy and the strength of the electron-lattice coupling in each glass.     

Theory of molten zone shape and stability

The Laplace-Young capillary equation for the shape of an axisymmetric liquid-vapor interface has been solved numerically for boundary conditions relevant to a model of the floating zone process. The stability of these solutions with respect to axisymmetric and asymmetric perturbations which conserve volume has been determined via the conjugate point criterion of the calculus of variations. The liquid zone shape is governed by five dimensionless parameters: $\text{R}{}_{\mathrm{m}}\text{R}{}_{\mathrm{f}}$, $\text{L}\text{Rf}$, $\text{V}\text{φR}{}^2{}_{\mathrm{f}}\text{L}$, $\text{? = ρg}\text{R}{}^2{}_{\mathrm{f}}\text{γ}$, and $\text{?}{}_{\mathrm{R}}\text{= ρΩ}{}^2\text{R}{}^3{}_{\mathrm{f}}\text{γ}$, where R_m and R_f are the radii of the melting and freezing solids, respectively, L is zone length , V is the zone volume, ρ is the density difference between liquid vapor, g is the gravitational acceleration, γ is the liquid-vapor surface tension, and Ω is the constant angular velocity of the uniformly rotating zone. For growth of constant diameter crystals, the angle ø_f, measured between the meniscus and the growth axis at the freezing interface, is constant. For R_m = R_f, ?_R = 0, and ø_f = 0, the maxi mum value of ? for which a stable liquid zone exists has been calculated for various values of L/R_f. For some values of ?, two different stable liquid zones with different volumes (but all other parameters identical) give the same value of ø_f.