Conductivity and dielectric relaxation in sodium borosulfate glasses

Sodium borosulfate glasses having composition xNa2SO4·(30−x)Na2O·70B2O3 (1.5 ≤ x ≤ 9.0 mol%) were prepared by melt-quenching technique. The influence of Na2SO4 on physical and thermal properties has been interpreted using density, molar volume and DSC data. The decrease in glass transition temperature (Tg) with increase in Na2SO4 content indicates that the average chain length of the host glass matrix decreases. The temperature-dependent conductivity data of the above glass samples have been obtained using impedance measurement in the temperature range 523-593 K and the frequency range 20 Hz to 1 MHz. The dc electrical conductivity (σdc) follows Arrehenius relationship and activation energy is 0.83 ± 0.03 eV for ionic migration. The dc conductivity (at 553 K) of the glass with x = 1.5 mol% is 2.08 × 10−4 Ω−1 m−1 and it is 6.16 × 10−4 Ω−1 m−1 for x = 9 mol%. The ac conductivity obeys Almond-West type power law behaviour: σ(ω) = σdc + σac(ω) = σdc + Aωs. The behaviour of the complex permittivity, dielectric loss and dielectric modulus as a function of frequency was also investigated. An excellent time-temperature superposition in the imaginary part of dielectric modulus (M″) confirms that the dynamical processes are temperature independent and is also an indicative of a common ion transport mechanism.