Alkali Borate Glasses Research Articles

Temperature dependence of elastic properties in alkali borate binary glasses

The elastic properties of alkali borate glasses, xM 2O·(100 − x)B 2O 3 (M = Li, Na, K, Rb, Cs, x = 14, 28), have been investigated by Brillouin scattering spectroscopy from room temperature up to 1100 °C. Above the glass transition temperature, T g, the longitudinal sound velocity, V L, decreases markedly on heating. Such significant changes of the elastic properties result from the breakdown of the glass network above T g. Alkali borate family with the same x shows the similar behavior in the temperature variations of V L up to around T g. The absorption coefficient, α L, increases gradually above T g. With the increase of the size of an alkali ion, the slope of V L just above T g decreases. Since the fragility is related to the slope, the present results suggest that the fragility of alkali borate glasses increases as the size of alkali ion decreases. Such an alkali dependence of the fragility is discussed on the basis of the fluctuation of the boron coordination number.

Radiation damage of alkali borate glasses for application in safe nuclear waste disposal

The Electron Paramagnetic Resonance technique has been used to study the time decay of paramagnetic species induced by gamma irradiation and the radiation hardness of different alkali borate glasses for their application in safe nuclear waste disposal. Glasses with different composition have been prepared by conventional melt-quenching. Glass compositions have been chosen to elucidate the role of different alkali cations and of aluminium oxide on the borate glass network. The paramagnetic states detected in these glasses have been attributed, according to the literature, to the formation of hole centers associated with threefold coordinated boron. The results indicate that the time decay trend of the different glasses is slow and that the constant decay does not appear related to the chemical composition. Moreover, the undesired strong fading of the radiation-induced signal during the first 24 h after irradiation, observable in silicate glasses has not been detected. Although no species detectable by a X band spectrometer have been generated, the interaction of lithium borate glasses with air seem to accelerate the system decay rate. Annealing was finally performed and optimized, investigating the correlation between the chemical composition and the radiation damage recovery.

Conductivity and dielectric relaxation in niobium alkali borate glasses

The frequency and temperature dependent conductivity investigations for alkali niobium borate glasses of composition xNb 2O 5·(30− x)M 2O·70B 2O 3 (where M=Li, Na; x=0, 4, 8 mol%) have been carried out using impedance spectroscopy (IS). The complex impedance data have been analyzed by using both the conductivity and the electric modulus formalisms. The conductivity decreases with the decrease in M 2O:Nb 2O 5 ratio. The effect of temperature on the scaling of dielectric modulus indicates that the conductivity relaxation mechanism is temperature independent. The overlapping of the normalized peaks corresponding to impedance and electric modulus and the identical values of thermal activation energy for conduction and relaxation suggest the single mechanism for the dynamic processes occurring in the present glasses. The variation of density, molar volume and glass transition temperature with glass composition has also been reported to supplement the conductivity results.

Optical absorption and EPR spectroscopic studies of (30 − x)Li 2O– xK 2O–10CdO–59B 2O 3–1Fe 2O 3: An evidence for mixed alkali effect

Optical absorption and EPR spectroscopic studies were carried on (30 − x)Li 2O– xK 2O–10CdO–59B 2O 3–1Fe 2O 3 ( x = 0–30) glass system to understand the effect of progressive doping of Li + ion with K + ion. Optical absorption results show typical spectra of Fe 3+ ions and the various optical parameters such as, optical band gap, Urbach energy, oxide ion polarizability, optical basicity and interaction parameter were evaluated from the experimental data. The observed optical band gap and Urbach energy values show large deviation from the linearity where as the other parameters show small deviation from the linearity with the progressive substitution of Li + ions with K + ions. The observed EPR spectra are representative of Fe 3+ ion in octahedral and axial fields in the glass network. The number of paramagnetic centers and paramagnetic susceptibility values were evaluated at different resonance lines for all the specimens and these parameters show non-additive nature with the progressive substitution of Li + ions with K + ions in the glass network. This is first ever observation of mixed alkali effect (MAE) in EPR and optical parameters of mixed alkali borate glasses.

The Thermodynamic Significance of Order Parameters During Glass Relaxation

The thermodynamic state of a glass is often described in terms of a number of order parameters, each of which evolves toward its equilibrium value during glass relaxation. The problem comes in identifying these order parameters and determining their physical significance. The authors argue that the order parameters in a glass can be defined in terms of a set of configurational temperatures, each corresponding to a particular mode of energy storage. The evolution of glass properties can then be calculated using a coupled set of rate equations describing the relaxation of the configurational temperatures toward their equilibrium values, viz., the reservoir temperature. The authors illustrate the concept of configurational temperatures by considering the free energy of mixing in alkali borate and silicate glasses.

Optical characterization of Mn2+: Li2O-K2O-CdO-B2O3 glass system: Absorption edge, optical band gap, optical polarizability and optical basicity

Mixed alkali borate glasses (30-x)Li2O-xK2O-10CdO-59B2O3 (0 ≤ × ≤ 30) implanted with 1mol% manganese ions (LKCBM) were prepared from the melts. These glasses were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and density measurements. Optical absorption studies were carried out as a function of alkali content to look for `Mixed Alkali Effect' (MAE) on the spectral properties of these glasses. The optical absorption spectra of LKCBM exhibit a broad band around 21000 cm-1 which has been assigned to the transition 6A1g(S) → 4T1g(G). From the study of ultraviolet absorption edge, the optical band gap energies and Urbach energies were evaluated. The average electronic polarizability of the oxide ion (αo2-), Optical basicity (∧) and Yamasita-Kurosawas's interaction parameter (A) were also evaluated for all the glasses. In the present investigation many of the physical parameters like glass transition temperature, density, optical band gap and optical basicity vary non-linearly with alkali concentration (x) indicating MAE.

Effect of Doping by Different Transition Metals on the Acoustical Properties of Alkali Borate Glasses

Effect of Doping by Different Transition Metals on the Acoustical Properties of Alkali Borate Glasses

Infrared and Raman Spectroscopic Studies on Alkali Borate Glasses: Evidence of Mixed Alkali Effect

A lithium-potassium-borate glass system containing manganese and iron cations has been thoroughly investigated in order to obtain information about the mixed alkali effect and the structural role of both the manganese and iron in such glass hosts. Mixed alkali borate glasses of the (30 - x)Li(2)O - xK(2)O - 10CdO/ZnO - 59B(2)O(3) (x = 0, 10, 15, 20, and 30) doped with 1MnO(2)/1Fe(2)O(3) system were prepared by a melt quench technique. The amorphous phase of the prepared glass samples was confirmed from their X-ray diffraction. The spectroscopic properties of glass samples were studied using infrared (IR) and Raman spectroscopic techniques. The density of all the prepared glasses was measured using Archimedes principle. Molar volumes were estimated from the density data. IR spectra of these glasses revealed a dramatic variation of three- and four-coordinated boron structures as a function of mixed alkali concentration. The vibrations due to Li-O, K-O, and MnO(4)/FeO(4) arrangements are consistent in all the compositions and show a nonlinear variation in the intensity with alkali content. Raman spectra of different alkali combinations with CdO and ZnO present drastic changes in the intensity of various Raman bands. The observation of disappearance and reappearance of IR and Raman bands as a function of various alkali concentrations is an important result pertaining to the mixed alkali effect in borate glasses. Acting as complementary spectroscopic techniques, both types of measurements, IR and Raman, revealed that the network structure of the studied glasses is mainly based on BO(3) and BO(4) units placed in different structural groups, the BO(3) units being dominant. The measured IR and Raman spectra of different glasses are used to clarify the optical properties of the present glasses correlating them with their structure and composition.

Viscosity properties of sodium borophosphate glasses

The viscosity behavior of (1 − x)NaPO 3− xNa 2B 4O 7 glasses ( x = 0.05–0.20) have been measured as a function of temperature using beam-bending and parallel-plate viscometry. The viscosity was found to shift to higher temperatures with increasing sodium borate content. The kinetic fragility parameter, m, estimated from the viscosity curve, decreases from 52 to 33 when x increases from 0.05 to 0.20 indicating that the glass network transforms from fragile to strong with the addition of Na 2B 4O 7. The decrease in fragility with increasing x is due to the progressive depolymerization of the phosphate network by the preferred four-coordinated boron atoms present in the low alkali borate glasses. As confirmed by Raman spectroscopy increasing alkali borate leads to enhanced B–O–P linkages realized with the accompanying transition from solely four-coordinated boron (in BO 4 units) to mixed BO 4/BO 3 structures. The glass viscosity characteristics of the investigated glasses were compared to those of P-SF67 and N-FK5 commercial glasses from SCHOTT. We showed that the dependence of the viscosity of P-SF67 was similar to the investigated glasses due to similar phosphate network organization confirmed by Raman spectroscopy, whereas N-FK5 exhibited a very different viscosity curve and fragility parameter due to its highly coordinated silicate network.

Thermally activated relaxations and vibrational anharmonicity in alkali-borate glasses: Brillouin scattering study

Measurements of Brillouin light scattering have been performed in ${({M}_{2}\text{O})}_{0.14}{({\text{B}}_{2}{\text{O}}_{3})}_{0.86}$ alkali-borate glasses, where $M=\text{Li}$ and K, as a function of temperature between 15 and 300 K. The temperature behaviors of hypersonic attenuation and velocity have been explained in terms of thermally activated relaxations of intrinsic structural defects and of anharmonic interactions between hypersonic waves and thermal vibrational modes. In the temperature region above 150 K, where the mean free path of thermal modes is shorter than the acoustic wavelength, it has been shown that the sound propagation is mainly regulated by the Akhiezer mechanism of ``phonon viscosity.'' It causes a linear increase in the hypersonic attenuation and a linear decrease in the sound velocity with increasing temperature.

Using pressure, temperature and frequency as variables to study the dynamics of mobile ions in materials with disordered structures

Complementary ways for studying the motion of mobile ions in materials with disordered structures are obtained by varying pressure, tempe- rature and frequency. New results are presented based on a combination of experimental work and modelling. Pressure-dependent measurements on alkali borate glasses show there is a remarkable difference between the activation volumes for conduction and diffusion, with ΔVσ< ΔVD, implying that the Haven ratio decreases with increasing pressure. We propose a mechanism that is characterised by a directionally positive correlation between successive hops of different ions into a moving vacant site. The effect of increasing pressure is to increase the degree of directional correlation and thus to make the conduction pathways increasingly linear in aspect. In sodium borate glasses with much lower sodium content, a maximum has been observed when ionic conductivity is plotted versus temperature at fixed frequency. This feature is identified as being of the nearly constant loss (NCL) type, caused by localised flip-flop movements of interacting charges in the B2O3 network. In crystalline γ-RbAg4I5, a related localised effect has also been found, in this case caused by activated hops of silver ions confined within structural “pockets”. Finally, the frequency dependence of the ionic conductivity is reviewed in fragile ionic melts. Fragility is interpreted here as a consequence of the shape of the local ionic potentials, which unlike in glass do not reflect the pre-existence of empty cation sites for successive ions to hop into. This difference in short-range, short-time behaviour leads directly to the emergence of non-Arrhenius dc conductivity and fluidity behaviours in molten salts. We are thus able to establish a common phenomenological and theoretical approach to ion transport in a wide range of systems, largely based on broadband conductivity spectroscopy.

Diffusion and ionic conduction in oxide glasses

The ion transport properties of soda-lime silicate and alkali borate glasses have been studied with complimentary tracer diffusion and impedance spectroscopy techniques in order to investigate the ion dynamics and mixed-alkali effect (MAE). In soda-lime silicate glasses the tracer diffusivity of 22Na alkali ions is more than six orders of magnitude faster than the diffusivity of earth alkali 45Ca ions. This observation is attributed to a stronger binding of bivalent earth alkali ions to the glass network as compared to that of alkali ions. The conductivity of the investigated standard soda-lime silicate glasses is mostly due to the high mobility of sodium ions and a temperature independent Haven ratio of about 0.45 is obtained. For single alkali sodium-borate glasses, the Haven ratio is also temperature independent, however, it is decreases with decreasing temperature for rubidium-borate glass. The MAE was investigated for Na-Rb borate glasses and it was observed that the tracer diffusivities of 22Na and 86Rb ions cross, when plotted as function of the relative alkali content. This crossover occurs near the Na/(Na+Rb) ratio of the conductivity minimum due to MAE. The authors suggest that this crossover and the trend of diffusion coefficients is the key to an understanding of the MAE.

The structure of borate glass

The existing hypotheses on the structure of alkali borate glass were comprehensively analyzed. Their imperfection in explaining the dependences of the properties of the glasses on their composition was noted. Alternative schemes of structural borate complexes which more convincingly explain the characteristics of the change in the properties of this glass are proposed.

Transition from a single-ion to a collective diffusion mechanism in alkali borate glasses

Two distinct regions of the dc conductivity and its temperature dependence as a function of the mol fraction of alkali oxides, X, are observed in Na- and Li-borate glasses. At low alkali contents the dc conductivity σdc increases only moderately with X. However, at higher alkali contents, log σdc increases linearly and the activation enthalpy ΔH of σdc×T decreases linearly with log X, i.e. the dc conductivity reveals an effective power-law behavior. The transition between low alkali and high alkali behavior takes place at X≈0.08 for Na-borate and at X≈0.09 for Li-borate glasses. This behavior suggests that the diffusion mechanism changes at these alkali contents. The results are discussed in terms of ion separations and the transition from a single-ion jump to a collective diffusion mechanism. The vanishing of the mixed-alkali peak in Na–Rb borate and alumino-germanate glasses at sodium contents similar to that observed for the change in slope of σdc(X) in this work suggests that both phenomena share the same origin.

A molecular dynamics study of the role of the cation in modifying the structure of alkali borate glasses

The structures of the full series of alkali borate glasses (M2O)x(B2O3)1−x (M=Li, Na, K, Rb and Cs) at two different concentrations, x=0.14 and x=0.30, have been investigated by means of molecular dynamics simulations. Additional compositions have also been investigated for the lithium and caesium borate glasses (x=0.10, 0.20, 0.25, and 0.40). The main experimental trends are well reproduced by the simulations, even if the agreement is not quantitative. Our results indicate that lithium atoms can enter into the matrix of pure vitreous B2O3 without inducing large modifications in the B–O network, even at large concentrations. However when the other alkali ions are added to the initial structure, the network opens to accommodate the larger size of the cation. These modifications induce the appearance of a low-Q shoulder or pre-peak, whose intensity increases with increasing alkali concentration as well as with increasing alkali size.

Ionic Conductivity in Mixed Alkali-Borate Glasses

The temperatureand frequency-dependent ionic conductivity in (Li1−xNax)2B4O7 (LNBO), (Li1−xKx)2B4O7 (LKBO) and (Li1−xRbx)2B4O7 (LRBO) glasses have been investigated using Xray diffraction (XRD) and an impedance/gain-phase analyzer. We have specifically focused on the dielectric properties in frequency and temperature domains, and those have been analyzed by using a power law. We have found that the dc conductivity value decreases with heavy elements on mixing the alkali ions in the glass structure. For the LNBO, LKBO and LRBO glasses, the mixed alkali effect is observed to become stronger with decreasing temperature and increasing mismatch of the element size. The concentration dependence of the activation energy of the dc conductivity is characterized in the framework of reverse-Monte-Carlo-produced structural models in combination with the bond-valence technique.

Sodium germanate glasses and crystals: NMR constraints on variation in structure with composition

We present new results from high-resolution 17O and 23Na NMR spectroscopy on sodium germanate glasses ranging from 4 to 36mol% Na2O, and on crystalline sodium digermanate (Na2Ge2O5). Combined with previously published results, these provide a more complete, direct view of changes in oxygen speciation with composition, and the corresponding changes in Ge coordination. Until about 15–20% Na2O, non-bridging oxygens are not detected (<2% of oxygens). At higher Na contents, NBO begin to form in significant quantities and, by 36% Na2O, are at the level expected for a ‘silicate-like’ speciation reaction, with all four-coordinated Ge ([4]Ge). At intermediate compositions, a clear NMR signal is seen for oxygens bridging between one [4]Ge and one [5]Ge or [6]Ge. Estimated mean Ge coordination numbers are consistent with previous X-ray and neutron scattering results. We observe systematic, monotonic changes in the NMR parameters for both Na+ cations and [4]Ge–O–[4]Ge bridging oxygens, suggesting a lack of clustering. This behavior is analogous to that of alkali borate glasses, where network cation coordination is well-known to shift with composition, but is quite distinct from that of alkali silicates, where coordination changes are large only at high pressures.

Probing alkali coordination environments in alkali borate glasses by multinuclear magnetic resonance

Five series of binary alkali borate glasses were prepared to compare the alkali dependence of network and modifier short-range order. 11B magic-angle spinning (MAS) NMR reveals that the fraction of four-coordinate boron depends strongly upon alkali type at high-alkali concentrations: heavier alkalis favour the formation of non-bridging oxygens, whereas lithium borates contain a much higher concentration of tetrahedral boron units. The alkali modifiers were observed directly by MAS NMR to measure the change in chemical shift with composition. All alkali peaks shift to higher frequency with increasing loading, indicative of decreasing average coordination numbers. Relative to their known chemical shift ranges, the heavier alkalis exhibit the greatest shifts, whereas the lithium shifts are subtle. This is interpreted in terms of the availability of charged and partially charged coordinating oxygens in the network. Moreover, the 133Cs chemical shifts plateau at 40 mol%, implying that the Cs + coordination number reaches a lower limit at this composition. This work demonstrates that NMR instrumentation and methodology have reached a level where even challenging nuclei like 39K and 87Rb can be probed to yield structural information in glasses.

Structure of Alkali Borate Glasses at High Pressure: B and LiK-Edge Inelastic X-Ray Scattering Study

We report the first in situ boron K-edge inelastic x-ray scattering (IXS) spectra for alkali borate glasses (Li2B4O7) at high pressure up to 30 GPa where pressure-induced coordination transformation from three-coordinated to four-coordinated boron was directly probed. Coordination transformation (reversible upon decompression) begins around 5 GPa and the fraction of four-coordinated boron increases with pressure from about 50% (at 1 atm) to more than 95% (at 30 GPa) with multiple densification mechanisms, evidenced by three distinct pressure ranges for (d[4]B/dP)T. The lithium K-edge IXS spectrum for Li-borate glasses at 5 GPa shows IXS features similar to that at 1 atm, suggesting that the Li environment does not change much with pressure up to 5 GPa. These results provide improved understanding of the structure of low-z glass at high pressure.

Spectral studies of Sm3+ and Dy3+ doped lithium cesium mixed alkali borate glasses

The effect of host glass composition on the optical absorption and fluorescence spectra of Sm3+ and Dy3+ has been studied in mixed alkali borate glasses of the type 67B2O3·xLi2O·(32−x)Cs2O (x=8, 12, 16, 20 and 24). The Judd–Ofelt intensity parameters (Ω2, Ω4 and Ω6) are calculated. The radiative transition probabilities (A), radiative lifetimes (τR), branching ratios (β) and integrated absorption cross-sections (Σ) are computed for certain excited states of Sm3+ and Dy3+ ions for different x values in the glass matrix. Stimulated emission cross-sections (σp) are obtained for certain emission transitions of two ions in these mixed alkali borate glasses. These parameters are compared for different x values in the glass matrix. Variation of these parameters with x in the glass matrix has been studied.