Generalized Maxwell-Wagner response in dispersive silver borophosphate glasses

The dielectric response of a glass-forming system (Ag₂OB₂O₃P₂O₅) has been measured in the frequency range from 10^–3–10⁵ Hz and over temperatures in the range 150–400 K for three different compositions. The dynamic behaviour of the conductance and capacitance in these glasses has been observed to follow fractional power-law dependencies on frequency which obey the generalized Maxwell-Wagner relationships. The power-law dispersions for the bulk and the surface layer of the non-ideal solid electrolyte 0.6Ag₂OxB₂O₃(0.4–x) P₂O₅ have been modelled mathematically using frequency-dependent resistive and capacitive elements in a conventional equivalent network. It is shown that controlled substitution of B₂O₃ in the glassy network influences the response and introduces an imperfect charge transport, the quasi-d.c. process of limited charge transport in place of bulk conduction, at higher frequencies, and affects the diffusion barrier at the electrodes to make them, weakly, more conductive at the lowest frequencies. The magnitudes of the activation energies of conduction indicate thermally activated localized hopping of silver ions between neighbouring sites in a structure that is modified by the addition of boron oxide.     

Viscosity properties of sodium borophosphate glasses

The viscosity behavior of (1 − x)NaPO₃−xNa₂B₄O₇ 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 Na₂B₄O₇. 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 BO₄ units) to mixed BO₄/BO₃ 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.     

Thermomechanical properties and stability of mixed strontium-zinc borophosphate glasses

Borophosphate glasses of the SrO-ZnO-B₂O₃-P₂O₅ system were prepared in 3 compositional series of 25ZnO-25SrO-xB₂O₃-(50 – x)P₂O₅, (58.3 – x) ZnO-xSrO-16.6B₂O₃-33.33P₂O₅ and (50 – x)ZnO-20B₂O₃-30P₂O₅ and the changes in their thermomechanical and thermal properties with changes in their composition were investigated. The thermal expansion coefficient decreases with increasing B₂O₃ content, whereas the values of glass transformation temperature T _g and dilatation softening temperature T _d increase. The replacement of ZnO by SrO increases both T _g and T _d. From the DSC results the value of the the Hruby's criterion K _gl was calculated and changes in the glass-forming tendency with changes in glass composition were discussed.     

Study of potassium-lead borophosphate glasses

Mixed potassium-lead borophosphate glasses were prepared and studied in two compositional series xK₂O-(50-x)PbO-10B₂O₃-40P₂O₅ and xK₂O-(50-x)PbO-20B₂O₃-30P₂O₅ with x = 0, 10, 20, 30, 40 and 50 mol% K₂O. The replacement of lead by potassium decreases the density and increases the molar volume of these glasses. On the other hand both glass transition temperature and chemical durability decrease. The observed changes in the properties of these glasses are explained as due to changes in their structure and ionicity of chemical bonds between cations and the anionic network of the glass. The major role is played by differences in the space occupied by cations, differences in electronegativity and the field strength of the corresponding cations.     

Dielectric properties of some berate glasses

The dielectric constant, epsilon', and the dielectric loss, epsilon ", for some selected lead berate glasses within the frequency band 10(5) to 10(7) Hz and the temperature range (20 similar to 50)degreesC were measured. The dielectric dispersion and the dielectric loss absorption bands were observed. the relaxation time, the activation enthalpy and entropy change of the dielectric relaxation were calculated. The results obtained were discussed and correlated to the internal network structure of the glasses studied.     

Study of lithium–zinc borophosphate glasses

Mixed lithium–zinc borophosphate glasses were prepared and studied in three compositional series xLi₂O–(50−x)ZnO–50P₂O₅, xLi₂O–(50−x)ZnO–10B₂O₃–40P₂O₅ and xLi₂O–(50−x)ZnO–20B₂O₃–30P₂O₅ with x = 0, 10, 20, 30, 40 and 50 mol% Li₂O. The obtained glasses were characterized by the measurements of the density (ρ), molar volume (V _M), glass transition temperature (T _g) and thermal expansion coefficient (α). For the investigation of structural changes ¹¹B and ³¹P MAS NMR and Raman spectroscopy were applied. The replacement of zinc by lithium in borophosphate glasses slightly decreases V _M and T _g, while α increases. In Li–Zn metaphosphate glasses the compositional dependence of T _g reveals a minimum, while at the borophosphate series T _g decreases monotonously with increasing Li₂O content. Chemical stability of Li–Zn borophosphate glasses is very good for glasses with x = 0–30 mol% Li₂O. Spectral studies showed in the glass series with 10 mol% B₂O₃ only the presence of BO₄ sites. In the glasses with 20 mol% B₂O₃ the presence of BO₃ and two BO₄ sites was revealed in ZnO-rich glasses and only one BO₄ site in Li₂O-rich glasses; the number of BO₃ groups decreases with increasing Li₂O content which is ascribed to the formation of P–O–Zn covalent bonds in ZnO-rich glasses.     

Dielectric properties and polarizability of molybdenum tellurite glasses

The dielectric behaviour of the [TeO₂]_1–x [MoO₃] _x , x=0.2, 0.3 and 0.45 mol%, glassy system is reported for the temperature range 300–573 K and the frequency range of 0.1–10 kHz. Both the static and high frequency dielectric constants for these binary tellurite glasses decrease with increasing MoO₃ content. The temperature dependence of the dielectric constants of these glasses are positive. The frequency dependence of the dielectric constant identifies a frequency dependence which does not show a flattening at low frequency. The room temperature static dielectric polarizability is discussed in terms of the MoO₃ concentration. The temperature dependence of the dielectric constant has been analysed in terms of the temperature changes of both volume and polarizability and also a volume change of the polarizability.     

Dielectric and electrical properties of MoO3-doped borophosphate glass: dielectric spectroscopy investigations

A series of (50 ? x) P₂O₅–20B₂O₃–20CaO–10Na₂O (x?=?0–15 mol% MoO₃) glass composition was prepared. Glass structure was analyzed using infrared absorption, UV–visible spectroscopy, electron spin resonance, density, and molar volume calculations. FTIR confirmed that Mo ions are contributed as MoO₆ octahedral units in the glassy matrix, resulting in an increase in the pyrophosphate and BO₃ groups at the expense of metaphosphate and BO₄ units. UV–visible and ESR spectra detected Mo³⁺ and Mo⁵⁺ ions as species in the host glass due to the increase in MoO₃ content. Broadband dielectric spectroscopy investigation on a broad range of frequencies and at different temperatures indicated that the enhancement of electrical conductivity of the prepared glasses due to molybdenum doping was prevented using confinement effect at the wells, causing demobilization of the charge carriers. Hence the dielectric spectra were caused by the mobility of charge carriers rather than the dynamics at the molecular scale. There is a clear correlation between the transport mechanism and dynamics at the interface of the charge carriers. Presently, the challenge is to understand if optimizing the accumulation of charges at the interfaces and electrodes is the origin of electrical storage energy.

     

Dielectric properties of glasses at ultra low temperatures

A temperature dependence in the dielectric constant of vitreous silica has been measured down to a few hundred microkelvin at frequencies between 110Hz and 10kHz. Homosil glass shows a logarithmic increase below 10 mK down to the lowest temperature of 0.61 mK. On the other hand, Suprasil glass exibits a saturation behavior following a logarithmic increase above 3 mK. These phenomena are discussed on the basis of the two level system.     

Behavior of indium oxide in zinc phosphate and borophosphate glasses

The effect of indium oxide on the structure and properties of zinc phosphate and zinc borophosphate glasses has been investigated in the series (50?x)ZnO–xIn₂O₃–50P₂O₅ and 30ZnO–(20?x)B₂O₃–xIn₂O₃–50P₂O₅ with x = 0–20 mol% In₂O₃ in both series. Their physical properties were measured, and their structure was studied by Raman and NMR spectroscopy. Glass transition temperature in the phosphate glass series increases with increasing In₂O₃ content as well as their chemical durability. The observed changes in the ³¹P MAS NMR spectra with increasing In₂O₃ content revealed the depolymerization of phosphate chains due to the replacement of ZnO by In₂O₃ resulting in an increase in the O/P ratio in the glass structure. ¹¹B MAS NMR spectra revealed the presence of BO₄ units in the glass structure in the borophosphate glass series; these spectra give no evidence for the formation of B–O–In bonds. Their glass transition temperature slightly decreases with increasing In₂O₃ content. The values of the molar volume in these glasses do not change substantially when B₂O₃ is replaced by In₂O₃. This can be ascribed not only to differences in boron (BO₄) and indium (InO₆) coordination, but also to a higher ionicity of In···O bonds in comparison with B–O bonds. The obtained data resulted in the conclusion that indium oxide behaves as a modifying oxide in both glasses.     

Dielectric properties of vanadium phosphate glasses in a combined electric field

The dielectric properties of vanadium phosphate glasses, composed of 70 mol% V2O5 and 30 mol% P2O5 with an addition of 1 wt% of CuO, have been measured under combined voltages simultaneously composed of dc voltages of 0, 30, 45 and 65 V, and an ac voltage of 0.5 V. Measurements were taken over a temperature range of 20–110°C and a frequency range of 10–105 Hz. The NiCr alloy electrodes used in this study were prepared by chemical vapour deposition techniques. The results of the measurements show that the conductivity and dielectric properties of the glasses could be described by a mechanism of electron hopping between energy states of the chain of molecules or by a tunneling effect. The transport of the electric charges involves movement across a potential barrier and the movement could be improved if the barrier was deformed by the dc voltage.     

Correlation of large SHG responses with structural characterization in borophosphate niobium glasses

Transparent and homogeneous sodium borophosphate niobium glasses with good optical quality were obtained for Nb₂O₅ proportion up to 50 mol%. Large NLO second-order susceptibilities, up to 5 pm/V for the highest niobium loading, are generated through an efficient thermal poling process which induces a strong space-charge migration of Na⁺ cations. From combined IR, Raman and thermal analyses, we establish that the observed strong increase of χ⁽²⁾ responses with niobium concentration is linked to the extent of the niobium–oxygen 3D structure which enhances the third-order susceptibility.     

Optical properties of aluminophosphate glasses containing silver, tin, and dysprosium

The spectroscopic properties of a Dy³⁺-doped aluminophosphate glass containing silver and tin were reported. Different oxidation and aggregation states of silver were obtained by varying silver concentration and glass thermal history. The addition of silver and tin at the lowest concentration studied results in Dy³⁺ ions emission under nonresonant UV excitation in connection with the appearance of an excitation band around 270 nm, which is associated to isolated Ag⁺ ions and twofold-coordinated Sn centers. The increase in silver and tin concentration leads to a broadening of aforementioned band and to the presence of charged silver dimers as evidenced by the appearance of an excitation band around 330 nm. The data indicated that light absorption might take place at ionic silver species and twofold-coordinated Sn centers, followed by energy transfer to Dy³⁺ ions. After heat treatment, ionic silver species were reduced to atomic Ag by tin with the subsequent formation of Ag nanoparticles (NPs) inside the dielectric host. A quenching effect in Dy³⁺ ions luminescence was shown with the presence of the Ag NPs, most notably for excitation of ⁶H_15/2 → ⁴F_9/2, ⁴I_15/2, ⁴G_11/2 transitions, which were in resonance with the dipole absorption mode of the particles. The silver NPs were believed to provide radiationless pathways for excitation energy loss in Dy³⁺ ions.     

Cerium doped soda-lime-silicate glasses: effects of silver ion-exchange on optical properties

Effects of silver ion-exchange on optical absorption (OA) and photoluminescence (PL) spectra of a cerium doped soda-lime-silicate glass at room temperature are investigated. The optical spectra are described in terms of the characteristic transitions 4f↔5d originated in Ce³⁺ ions placed mainly in two different sites of the glass network. As Ag⁺ ions are introduced into the cerium doped glass, they are reduced to elementary silver (Ag⁰) which are favoured by the reaction Ce³⁺+Ag⁺→Ce⁴⁺+Ag⁰. Then, the number of Ce³⁺ ions decrease inversely with depth from the surface contrarily to Ce⁴⁺ ions does, and elementary silver diffuses and aggregates to form nanoparticles. As a consequence of these changes, the OA spectra of exchanged samples increase substantially in the UV range and the luminescence decreases significantly. The high sensitivity of PL together with deconvolution analysis of spectra, however, allows us to detect changes in the excitation and emission spectra from the earlier stages of ion-exchange. This indicates that during the ion-exchange we deal with fast processes (much shorter than 1 min). In fact, transmission electron microscopy observations of samples from the glass exchanged for a short time as 1 min at 325°C show the presence of a scanty number of silver nanoparticles, which confirms this point. Furthermore, with increasing the length of time of ion-exchange, PL spectra exhibit a progressive red shift indicative in part of a covalence increment in the oxygen–cerium coordinated bonding. We observe no luminescence from Ag⁺ ions and other silver molecular species in contrast with other preliminary PL studies on silver ion-exchange in soda-lime-silicate glasses free of cerium. The effect is discussed on the basis of a supplementary increase in the number of Ce⁴⁺ ions mainly due to the reaction Ce³⁺+Ag⁺→Ce⁴⁺+Ag⁰, which prevents efficiently the luminescence of the silver centers.     

Dielectric properties of R2O−SiO2 glasses prepared via sol-gel

Gels from R₂O−SiO₂ (R=potassium, sodium and lithium) systems with a maximum alkaline content of 5 mol % were prepared. The conductivity and the alkaline ion incorporation degree as a function of temperature were studied by a.c. impedance spectroscopy and dielectric relaxation measurements. At low temperature (20 to 200°C) the major contribution to the total electric conductivity is due to water conducting species (mainly protons) which are adsorbed inside the gels. From 450°C gels start to behave as conventional glasses. The total electric conductivity and activation energy values for conduction at those high temperatures are of the same order as those for glasses prepared by direct melting.     

Dielectric properties of LiF-B2O3 glasses doped with certain rare earth ions

Dielectric constantɛ, loss tanδ and a.c. conductivityσ of LiF-B₂O₃: Ln³⁺ (where Ln=Ce, Pr, Nd and Tb) glasses are studied as functions of frequency (in the range 10²–10⁶ Hz) and temperature (range 30–200°C). The dielectric breakdown strength of these glasses was also determined at room temperature in an air medium. The rate of increase ofɛ and tanδ with temperature decreases with decrease in the ionic radius of RE³⁺ ion whereas the dielectric breakdown strength, the activation energy for a.c. conduction in the high temperature region decreases with increase in the ionic radius of RE³⁺ ion. An attempt has been made to explain the a.c. conduction in these glasses on the basis of quantum mechanical tunnelling (QMT) model.     

Dielectric properties of NaF-B2O3 glasses doped with certain transition metal ions

Dielectric constant ε, loss tan δ, a.c. conductivity Σ and dielectric breakdown strength of NaF-B₂O₃ glasses doped with certain transition metal ions (viz. Cu²⁺, VO²⁺, Ti⁴⁺ and Mn⁴⁺) are studied in the frequency range 10²-10⁷ Hz and in the temperature range 30–250°C. The values of ε, tan δ, Σ_a.c. are found to be the highest for Cu²⁺ doped glasses and the lowest for Mn⁴⁺ doped glasses. Activation energy for a.c. conduction and the value of dielectric breakdown strength are found to be the lowest for Cu²⁺ doped glasses and the highest for Mn⁴⁺ doped glasses. With the help of infrared spectra, increase in the values of ε and tan δ of these glasses with frequency and temperature are identified with space charge polarization. An attempt has been made to explain a.c. conduction phenomenon on the basis of quantum mechanical tunneling model (QMT)/carrier barrier hopping model.