Conductivity and morphological studies on iron borosilicate glasses

Abstract

A set of glass samples of nominal composition xFe₂O₃.(43-x)B₂O₃.25SiO₂.30Na₂O. 2Al₂O₃, (x ≤ 40 mol%) was prepared via an ordinary melt quenching route. The variation of structural and electrical characteristics of the prepared samples have been studied. Electrical conduction in glasses that contain Fe₂O₃ (≤ 20 mol%) is considered to be ionic. Otherwise, the polaronic conduction is dominant in glasses of higher Fe₂O₃ concentrations (>20 mol%). Energy dispersive x-ray analysis of glass containing 5 mol% Fe₂O₃ showed that only one type of modifying iron ions Fe³⁺ present in their octahedral coordination. Besides, glasses containing higher Fe₂O₃ content reveal the presence of a mixture of both ferrous and ferric ions (Fe²⁺ & Fe³⁺). A fraction of both tetrahedral (BO₄ and FeO₄) units and BO₃ groups are determined from Fourier transform infra-red spectroscopic analysis (FTIR). Increasing the fraction of tetrahedral units and decreasing BO₃ are considered as the main reasons in conductivity enhancements.

Introduction

For most commercial and scientific uses, glasses containing transition metal oxides (TMO) and their glass-ceramic derivatives have a broad diversity of applications [1], [2], [3]. Nevertheless, increasing technology advancement needs different glass types. For instance, different compositions from borosilicate glasses [4,5] have been devoted to Molten Carbonate Fuel Cells (MCFC) as special sealants.

Both the academic and technological interest of borosilicate glasses were associated with their composition and structural arrangement of constituents. FTIR vibrational spectroscopy may represent a powerful tool used for the investigation of glass structural building units and their arrangement [6,7]. Glasses containing Fe₂O₃ could have spin-allowed electronic transitions corresponding to the electron transfer from one oxidation state iron ion (Fe³⁺) to another Fe²⁺ [8,9] combined with the change in boron coordination's. This property enables the glass as a possible martial in a broad variety of applications such as diodes producing white light, ground emission screens, and spin glasses. Silicate [6], [7], [8], phosphate [9] and borate [10,11] glassy matrix observed to be unique hosts for iron ions. On the other hand, studies of large Fe₂O₃ content in borosilicate glasses and excellent spin-allowed transitions are limited.

Cochain et al [11] studied Raman, Mössbuer, and X-ray absorption near edge structure (XANES) spectroscopy to investigate the role of iron content up to 12 mol% and redox state on the structure of borosilicate glasses. They concluded that oxygen coordination of Fe²⁺ changes between fourfold and sixfold whereas Fe³⁺ is mainly network-former in tetrahedral coordination. They also observed that the fraction of tetrahedral boron is independent of both iron content and redox state.

The dielectric characteristics and the microstructure of sodium borosilicate glasses containing iron oxide up to 6 mol% are previously studied and the results were interpreted in terms of the double channel conduction mechanism while FTIR spectral bands were attributed for different borate and silicate structural units [12].

Presented work aims to shed more light on the structural role of iron oxide addition both in lower dopant concentrations < 20 mol% Fe₂O₃, and in higher dopant concentrations from 25mol% and to 40 mol%. In addition, a correlation between the structural changes, different valences of iron ions and electrical characteristics were studied and interpreted in terms of structural variations.