Studies on dielectric dispersion, relaxation kinetics and a.c. conductivity of Na2OCuOSiO2 glasses mixed with Bi2O3-Influence of redox behavior of copper ions
Graphical abstract
Dispersion of electric moduli, M′(ω) and M″(ω) with the frequency evaluated in the temperature range 300–400 K for the glass B4.
Introduction
Studies on dielectric properties viz., dielectric constant, ε′, loss tan δ, ac conductivity, σ ac(ω), electric moduli, M, and electrical impedance, Z, of amorphous solid electrolyte materials like alkali silicate glasses are helpful in assessing the insulating character and in optimizing electrical conductivity [1], [2], [3], [4]. Such studies are also being extensively used to obtain the information on structural aspects of the glass materials [5], [6], [7], [8]. Addition of heavy metal oxide like Bi2O3 to conventional alkali silicate glasses like Na2OSiO2, endows them with outstanding electrical characteristics like wide band-gap, high dielectric permittivity because of their larger electrical polarizability. In view of these qualities, Bi2O3 mixed amorphous materials are being extensively used in developing microwave-integrated circuits, semiconductor capacitors, transparent ceramic glasses etc., [9], [10], [11], [12], [13]. Bi2O3 is an incipient/conditional network former and as such does not readily form the glass; however in the presence of other glass forming oxides like SiO2 and the modifiers like Na2O, it participates in the glass network forming with BiO3 structural units and also as network modifier with BiO6 structural units. The concentration of these units, however, depends upon the percentage of Bi2O3 and the other constituents of the glass matrix [14], [15].
The studies on electrical properties of transition metal oxides doped glasses have been the subject of extensive research for the last several years. The amorphous materials containing mixed valence transition metal ions are appealing systems for several applications such as electrodes or electrolyte in solid state batteries owing due to their very high energy density and high capacitance and also in memory and photoconducting devices [16], [17], [18], [19]. Among different transition metal oxides, CuO is an interesting p-type material with narrow band gap (∼1.2 eV) [20], [21]. Further, it is well known that in the silicate glass hosts copper ions do exist in Cu+ and Cu2+ states and are expected to influence electrical characteristics of the material. By appropriate monitoring of the proportions of Cu+ and Cu2+ ion concentrations in the glass matrix, the material can be made more useful for the applications as electrolytes as mentioned above [22].
In the earlier studies we have reported spectroscopic properties mainly the photoluminescence, thermoluminescence and also d.c. conductivity studies of CuO doped sodium silicate glasses mixed with variable concentrations of Bi2O3 [23], [24]. In such studies, it was understood that redox ratio of copper ions that influences the physical properties, depends on the concentration of bismuth ions. To be more specific, those studies have indicated that the copper ions gradually reduced to monovalent state with increase of Bi2O3 concentration. These Cu+ ions increased the degree of polymerization of the glass network and influenced above mentioned properties to a large extent. In continuation of the earlier studies, to have deeper understanding on the influence of redox behavior of copper ion on structural and physical properties, in this investigation we have reported the variations of dielectric constant ε′, loss tan δ, a.c conductivity σac and other related properties over larger ranges of frequency (102–106 Hz) and temperature (200–400 K) as a function of Bi2O3 concentration. The obtained results are analyzed in the light of changes in the oxidation states and environment of copper ions in the scenario of varying concentrations of Bi2O3 in Na2OSiO2Bi2O3 glass matrix.
Section snippets
Experimental
The chemical composition of the glasses chosen for the present study is (49‒x) Na2O‒x Bi2O3‒50 SiO2:1.0 CuO (with x = 0, 4, 8,12,16, and 20 mol% and the samples are labeled as B0, B4, B8, B12, B16 and B20, respectively).
The glasses were prepared by usual melt quenching technique. AR grade reagents of Na2CO3, Bi2O3, SiO2 and CuO (99.9% purity, Sigma Aldrich) powders in required proportions were taken and thoroughly mixed in an agate mortar and melted in a silica crucible in the temperature range
Results and discussion
The titled sodium bismuth silicate glass doped with CuO is an admixture of network former, modifier and intermediate. SiO2 is well-known glass former; it participates in the glass network with tetrahedral silicate units. The Na2O is a traditional alkali modifier and participates in the glass network by depolymerizing the silicate network with the creation of meta, pyro and ortho−silicates structural units. Bi2O3 is an intermediate glass former; it participates in the glass network with BiO3
Conclusions
Na2OCuOSiO2 glasses mixed with different concentrations Bi2O3 are synthesized. Dielectric properties and a.c. conductivity are investigated over broad ranges of frequency and temperature. The values of dielectric parameters and the conductivity exhibited a decreasing trend with increase of Bi2O3 concentration. The decrease is attributed to the increasing degree of polymerization of the glass network. The dielectric loss and also the electric moduli exhibited relaxation effects. These effects
Acknowledgement
Two of the authors, J. Ashok and B. Suresh wish to thank UGC, New Delhi, for sanctioning RGNF fellowship to carry out this work. M.S. Reddy is grateful to CSIR, New Delhi for the financial support in the form of Major Research Project to carry out this work (Grant No: 03 (1234)/12/EMR-II). N. Veeraiah wishes to thank DST, New Delhi, for the financial support through FIST programme and also University Grants Commission, New Delhi, for support through DSA-I programme to carry out this work.
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