Sodium Borate Glasses Research Articles

Determination of complex-specific heat and fragility of sodium borate glasses by temperature-modulated DSC

The frequency dependences of the complex-specific heat of the sodium borate glasses, xNa2O·(100 − x)B2O3, where x denotes molar concentration of Na2O, have been investigated by temperature-modulated DSC. The temperature dependences of α-relaxation time have been analyzed in Angell plot, and the fragility index has been determined. The composition dependence of the fragility index has been discussed on the basis of the variations of the structural units of the borate network. The origin of the fragility of the borate system relates to the distribution of the coordination number of boron atom.

Effect of alkali content on AC conductivity of borate glasses containing two transition metals

Abstract Sodium borate glasses containing iron and molybdenum ions with the total concentration of transition ions constant and gradual substitution of sodium oxide (network modifier) by borate oxide (network former) was prepared. Densities, molar volume, DC and AC conductivities are measured. The trends of these properties are attributed to changes in the glass network structure. Their DC and AC conductivity increased with increasing NaO concentration. The increase of AC conductivity of sodium borate glasses is attributed to the chemical composition and the hopping mechanism of conduction. Measurements of the dielectric constant (e) and dielectric loss (tan δ) as a function of frequency (50 Hz–100 kHz) and temperature (RT—600 K) indicate that the increase in dielectric constant and loss (e and tan δ) values with increasing sodium ion content could be attributed to the assumption that Fe and Mo ions tend to assume network-forming position in the glass compositions studied. The variation of the value of frequency exponent s for all glass samples as the function of temperature at a definite frequency indicates that the value of s decreases with increasing the temperature which agrees with the correlated barrier-hopping (CBH) model.

Nearly constant loss effects in borate glasses

Different nearly constant loss phenomena are investigated in borate glasses with compositions xNa(2)O.(1-x)B(2)O(3), for 0 < or =x< or = 0.3. The ionic conductivities caused by these effects are studied in wide ranges of temperature and frequency, spanning 4.3 K to 573 K and 100 mHz to 1 MHz, respectively. In a first step, we show how to identify the nearly constant loss (NCL) in 0.3Na(2)O.0.7B(2)O(3) glass. In the procedure, the scaling property of the conductivity caused by ordinary hopping is used to remove this component from the total conductivity as measured as a function of temperature at fixed frequency. The resulting NCL component is seen to be proportional to frequency and to display no temperature dependence. In a second step, a broad-band relaxation process is shown to exist in amorphous boron oxide and in sodium borate glasses with x< or = 0.1. It is most probably due to the presence of traces of water, with hydrogen ions behaving as reorienting and interacting local dipoles. In a third step, we propose a simple formal treatment of the NCL phenomenon, tracing it back to a large number of interacting ions, each of them moving locally. The key aspect is a "see-saw-type" time dependence of their individual single-particle double-well potentials, which is due to their Coulomb interactions. The individual ion does, therefore, not require thermal activation and is thus kept in motion even at cryogenic temperatures.

Growth and melting of metallic nanoclusters in glass: A review of recent investigations

The mechanisms of nucleation and growth and the solid-to-liquid transition of metallic nanoclusters embedded in sodium borate glass were recently studied in situ via small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). SAXS results indicate that, under isothermal annealing conditions, the formation and growth of Bi or Ag nanoclusters embedded in sodium borate glass occurs through two successive stages after a short incubation period. The first stage is characterized by the nucleation and growth of spherical metal clusters promoted by the diffusion of Bi or Ag atoms through the initially supersaturated glass phase. The second stage is named the coarsening stage and occurs when the (Bi-or Ag-) doping level of the vitreous matrix is close to the equilibrium value. The experimental results demonstrated that, at advanced stages of the growth process, the time dependence of the average radius and density number of the clusters is in agreement with the classical Lifshitz-Slyozov-Wagner (LSW) theory. However, the radius distribution function is better described by a lognormal function than by the function derived from the theoretical LSW model. From the results of SAXS measurements at different temperatures, the activation energies for the diffusion of Ag and Bi through sodium borate glass were determined. In addition, via combination of the results of simultaneous WAXS and SAXS measurements at different temperatures, the crystallographic structure and the dependence of melting temperature T m on crystal radius R of Bi nanocrystals were established. The experimental results indicate that T m is a linear and decreasing function of nanocrystal reciprocal radius 1/R, in agreement with the Couchman and Jesser theoretical model. Finally, a weak contraction in the lattice parameters of Bi nanocrystals with respect to bulk crystals was established.

Temperature dependences of the density of borate glasses in equilibrium states at temperatures below the glass transition point

The kinetics of density relaxation during isothermal treatment of samples of vitreous boron oxide and sodium borate glasses (containing 15, 20, and 30 mol % Na2O) cooled from high temperatures and after heating of the samples stabilized at low temperatures is investigated over a wide range of temperatures below the glass transition point Tg. It is established that there exists a temperature Ts below which the density in the equilibrium state does not depend on the temperature, i.e., the temperature at which the supercooled liquid transforms into a new noncrystalline solid state. The distinguishing feature of this state is the absence of structural transformations after a change in the temperature. The temperatures of the transition of the supercooled liquid to the noncrystalline solid state and the ranges of lower temperatures at which the supercooled liquid can reach an equilibrium state within the limits of experimental error coincide with those previously obtained in the study of relaxation processes in these glasses by the small-angle X-ray scattering technique.

UV-Visible and Infrared Spectroscopic Studies of Gamma-Irradiated Transition Metals-Doped Sodium Borate Glasses

UV-visible and infrared absorption spectra of 3d transition metals-doped sodium borate host have been measured before and after successive gamma-irradiation. Transition metals are known to exhibit characteristic absorption bands. Infrared absorption spectra of the prepared samples exhibit characteristic bands indicating the coexistence of 3- and 4-coordinated boron groups. Transition metals reveal varying response in the UV-visible spectra to the effect of successive gamma irradiation while minor effect is obvious in the infrared spectra. The traditional and induced spectral data are analyzed and interpreted on the basis of current views on the effect of ionizing irradiation on glass.

Sputter-deposited network glasses

Thin films of lithium borate, sodium borate, rubidium borate, and lithium silicate glasses are prepared by ion beam sputter deposition in a thickness range between 70 and 1,400 nm. The chemical, structural, and electrical properties of the films are determined by transmission electron microscopy and impedance spectroscopy. A comparison between the thin glass layers and the corresponding bulk glasses, prepared from the melt, shows that both have similar chemical compositions and atomic short range orders. In contrast, the ionic conductivities of the borate glass films are found to be significantly increased compared to the corresponding bulk materials, while the increase depends on the chemical composition of the glasses and is not observed in the case of the silicate system. We propose a modified network structure of the sputter-deposited glass layers, namely, an increased nonbridging oxygen concentration, to be responsible for the elevated ionic conductivity.

Melting and freezing of spherical bismuth nanoparticles confined in a homogeneous sodium borate glass

The melting temperature and the crystallization temperature of Bi nanoclusters confined in a sodium borate glass were experimentally determined as functions of the cluster radius. The results indicate that, on cooling, liquid Bi nanodroplets exhibit a strong undercooling effect for a wide range of radii. The difference between the melting temperature and the freezing temperature decreases for decreasing radius and vanishes for Bi nanoparticles with a critical radius $R=1.9\text{ }\text{nm}$. The magnitude of the variation in density across the melting and freezing transitions for Bi nanoparticles with $R=2\text{ }\text{nm}$ is 40% smaller than for bulk Bi. These experimental results support a basic core-shell model for the structure of Bi nanocrystals consisting of a central crystalline volume surrounded by a structurally disordered shell. The volume fraction of the crystalline core decreases for decreasing nanoparticle radius and vanishes for $R=1.9\text{ }\text{nm}$. Thus, on cooling, the liquid nanodroplets with $R&lt;1.9\text{ }\text{nm}$ preserve, across the liquid-to-solid transformation, their homogeneous and disordered structure without crystalline core.

Fragility and anharmonicity of lattice vibrations of glass-forming systems

The dependence of the fragility on the Gruneisen lattice parameter, which is a measure of the anharmonicity of lattice vibrations, is established for lead silicate and sodium borate glasses. An expression relating the fragility of glasses to the parameters of the Williams-Landell-Ferry equation is obtained. The calculation performed using this expression is in agreement with the results of the conventional determination of the fragility.

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.

Multilayer optical memory using femtosecond-laser induced fluorescence in rare-earth ion doped glass

We report three-dimensional fluorescent memory by recording optical bits with irradiation of femtosecond laser pulses at 800 nm and by reading photoluminescence change in Eu3+ doped glass. We produced multi-layered micro-bit patterns and read the blue emission from the 405 and 325 nm excitations due to permanent reduction of Eu3+ to Eu2+ in sodium borate glass by scanning the irradiated region in multilayers.

Optical properties and ultrafast optical nonlinearity of Yb3+ doped sodium borate and bismuthate glasses

In this paper, we report the optical and ultrafast nonlinear optical properties of Yb3+ doped sodium borate and bismuthate glasses. The glasses have been prepared through the melt quench technique. Optical absorption measurements show compositional dependent absorption spectrum of Yb3+, which is due to the higher crystal field induced by Bi3+ ions. Local structure of the glasses has been identified by using Fourier transform infrared and Raman studies. From open aperture z-scan measurements done by using 100 fs laser pulses, the ultrafast optical nonlinearity in these materials is calculated at the nonresonant excitation wavelength of 800 nm. The measured three-photon absorption originates from the glass host, with contributions from the nonbridging oxygens and the nonlinear electronic polarization of the Bi3+ ions.

Spectral Change in Silver-Doped Sodium-Borate Glass by Using Femtosecond Laser Irradiation

Spectral Change in Silver-Doped Sodium-Borate Glass by Using Femtosecond Laser Irradiation

Influence of Montmorillonite Clay on Structure and Properties of Sodium Borate Glasses

In the light of environmental perspective, clay minerals attract special interest because of their capability to absorb and chemically stabilize heavy metals in their structure [1, 2]. The absolute suppression of heavy element leakage from clay structures to the environment, which is obviously a strict demand, can be achieved with clay vitrification. This work is related to the influence of the addition of (0-50 wt%) of montmorillonite to a borate glass with composition of 0.33Na2O-0.67B2O3 on its structure. This influence has been studied with Raman and FT-IR spectroscopies, Archimedean density measurements, chemical durability in 90°C water and finally by studying of the ultrasonically measured elastic properties, such as Young’s and shear modulus and Poisson’s ratio, of the resultant glasses. The experimental results showed that the glass structure is predominantly comprised from tetrahedral and trigonal borate units and silicon and/or aluminium tetrahedral units. The results of density and chemical durability are fairly well correlated with structure. Higher amounts of montmorillonite lead to glasses of higher mechanical strength and chemical durability.

Femtosecond laser induced coordination transformation and migration of ions in sodium borate glasses

We report on the coordination transformation of B3+ ions and migration of Na+ and O2− ions in sodium borate glasses, induced by 250kHz, 800nm femtosecond laser irradiation. Micro-Raman spectra show that the ratio of the integrated intensity of the two peaks at 806 and 774cm−1 decreases at first and then increases with increasing distance from the center of the laser modified zone. Electron dispersive x-ray spectra show that a portion of Na+ and O2− ions migrate from the vicinity of focal point after the femtosecond laser irradiation. A possible mechanism is proposed to explain the observed phenomena.

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.

Structural and acoustical studies of lead sodium borate glasses

Abstract Glasses in the system Na2−2xB4−4xPbxO7−6x, 0 ≤ x ≤ 0.8, have been prepared by the melt quenching technique. Elastic properties and Debye temperature have been investigated using sound velocity measurements at 4 MHz. The ultrasonic parameters along with the IR spectroscopic studies have been employed to study the role of PbO on the structure of Na2B4O7 glass. The density, the molar volume, and the ultrasonic parameters of these samples have been found to be compositional dependent. The results indicate that PbO acts as a network modifier in the range 0 ≤ x ≤ 0.4, while beyound x = 0.4, PbO acts as a network former which affects the diborate units that mainly consist the strong borate network. These results are interpreted in terms of the IR analysis that indicates the transformation of the structural units BO3 into BO4, the increase in the number of non-bridging oxygen atoms, and the substitution of longer Pb O bond length, in place of shorter B O bond. The observed compositional dependence of the elastic moduli is interpreted in terms of the effect of PbO on the boron-coordination number of the glass structure and to the relatively large electron–phonon anharmonic interactions. A good correlation was observed between the experimentally determined elastic moduli and those computed according to Makishima–Mackenzie model.

Crystalline patterning in Sm-doped sodium borate glass by CW Nd:YAG laser irradiation

Laser irradiation of glass materials has drawn much attention because this technique can offer a new processing method for spatially selected structural modification and/or crystallization in glass. Crystallized line and dot patterns at the micrometer scale were fabricated on the surface of Sm-doped sodium borate (Na 2O–B 2O 3) glass by irradiation of a continuous-wave Nd:YAG laser at λ = 1064 nm. The pattern sizes could be controlled by adjusting such parameters as scan rate, exposure time, and laser power. Analyses by Raman spectroscopy and X-ray diffraction revealed that the crystalline phase is Na 3Sm 2(BO 3) 3.

Pressure-Dependent Diffusion Coefficients and Haven Ratios in Cation-Conducting Glasses

The influence of hydrostatic pressure on diffusion and ionic conduction is providing deeper insights into the atomistic mechanisms of ionic motion in glasses. We have studied the tracer diffusion of 22Na in a sodium borate glass and of 86Rb in a rubidium borate glass as functions of hydrostatic pressures. The activation volumes of tracer diffusion are DeltaVD(Rb) = 33.5 cm3 mol-1 and DeltaVD(Na) = 6.1 cm3 mol-1. In comparison, the activation volumes of charge diffusion obtained recently from the pressure dependence of conductivity are smaller: DeltaVsigma(Rb) = 7.2 cm3 mol(-1) and DeltaVsigma(Na) = 2.8 cm3 mol(-1). These differences, where (DeltaVD - DeltaVsigma) > 0, imply that the Haven ratios decrease with pressure. This effect is particularly significant for the rubidium borate glass. Starting from basic equations of linear response theory for mass and charge transport, we develop a model that accounts for these experimental findings. The difference between the activation volumes, DeltaVD and DeltaVsigma, and the pressure-dependent Haven ratios are consequences of collective movements of ions in glass, implying a concerted motion of ions in a chain- or caterpillar-like fashion. In our treatment, it is a vacant site (with ions jumping into it successively) that moves along an extended pathway. Hence, we regard vacant sites as the carriers of charge and ions as the carriers of diffusing matter. The decrease of the Haven ratio with pressure is attributed to the influence of pressure on the topology of the conduction pathways, which are progressively straightened out with increasing pressure.

Structural investigations of sodium diborate glasses containing PbO, Bi2O3 and TeO2: Elastic property measurements and spectroscopic studies

Pseudo-binary sodium borate glasses containing (1−y)Na2B4O7–yMaOb (where MaOb=PbO, Bi2O3 and TeO2) (y=0.25, 0.5, 0.67 and 0.79) have been investigated. Sound velocities (longitudinal and shear) have been measured at 10MHz frequency using quartz transducers. Density increases with increase of y and the molar volume decreases. Sound velocities also decrease with increasing y till y≈0.66 above which it increases slightly. Steeper decrease in velocities has been observed in TeO2 containing glasses. Elastic moduli, Poisson’s ratio and Debye temperature have been calculated. Glass transition temperatures have also been determined and it decreases with increase of y. Tg also exhibits a dependence on the cationic charge in MaOb. Infrared spectra of the glasses reveal that the strong network consisting of diborate units is affected only by PbO and only very marginally by Bi2O3 and TeO2. Only glasses with high concentrations of Bi2O3 and TeO2 reveal the presence of mixed bridges such as Bi–O–B and Te–O–B. Consistent with the IR spectral observations, the N4 values of 11B MAS-NMR remain close to the ideal value of 0.5 of the diborate composition in most of the glasses. A structural model based on the observation that the diborate network is unaffected by Bi2O3 and TeO2 where as PbO opens up and breaks the diborate units is shown to be consistent with all of the experimental observations including mechanical properties.