Dielectric response of alkali aluminosilicate hydrate and silico alumino phosphate network present in metakaolin–graphene oxide geopolymer: A comparative study

Abstract

Metakaolin-based geopolymeric composites are regarded as potentially useful materials for a number of applications because of the strength of networks present in them. In the present study, the binding network of aluminosilicate hydrate or silico alumino phosphate network of metakaolin is activated using sodium silicate (MA) and phosphoric acid (MP), respectively, and reinforced with an optimised dose of graphene oxide (GO) for dielectric applications. The sheet-like structure of graphene oxide present in the network was confirmed by transmission electron microscopic (TEM) analysis and the selected area diffraction (SAD) pattern showed relatively diffuse scattering spots. The alignment in the growth of N-A-S-H and S-A-P gels with fibrillar products at various points on the GO surface was observed in scanning electron microscopic (SEM) images. The reinforcement with GO improves the compressive strength of both types of geopolymers (G-MA, G-MP) by 20 % compared to that of control specimens (MA, MP). This could be due to the acceleration of the reaction with a higher amount of N-A-S-H gel in MA and a synergistic formation of silicoalumino phosphate and aluminium phosphate in MP. GO intercalation was confirmed from the intense peaks of D band of GO at 1300–1450 cm−1 in the Raman spectrum in MA and MP as well as different Q1–4Si environments pertaining to the geopolymeric network. The impedance spectroscopy was used to investigate electrochemical properties like static high-frequency dielectric permittivity, suppressed electrical conductivity, reduced loss tangent, increased resistivity and modulus. Dielectric constant (K) calculated for was found to be in the order G-MP < G-MA < MP < MA with the corresponding values of 1.17 < 1.26 < 1.78 < 2.68 × 1013. Free cation (Na+) transference under the influence of an electric field in alkali-based geopolymer is more pronounced than in phosphate-based composites. This study provides a new insight into the design of metakaolin nanocomposites as an alternative to solid oxide fuel cell/fast inorganic ionic conductors as well for insulation packaging.