Effect of doping of organic and inorganic acids on polyaniline/Mn₃O₄ composite for NTC and conductivity behaviour

Abstract

The paper deals with the study of TCR and conductivity behaviour of the PANI/Mn₃O₄ composite in its emeraldine salt state doped with different acids like H₃PO₄, H₂SO₄, HClO₄, CH₃COOH and CH₂=CHCOOH. The conductivity results have also been compared with pure emeraldine salt doped with same acid. The characterisation of each composite sample and emeraldine salt was done using XRD, FT-IR, and TGA. From the TCR studies it has been observed that PANI/Mn₃O₄ composite doped with acetic acid and acrylic acid have negative temperature co-efficient (NTC) behaviour in the temperature range 20-110 and 20-160° C, respectively. Thermistors constant (B) values calculated from the data are 3625 K^-1 for composite doped with acetic acid and 3745 K^-1 for acrylic acid. Composite samples doped with H₃PO₄, H₂SO₄ and HClO₄ acid did not show NTC behaviour, as the thermistor constant values in these composites are found to be on lower side. The results of the conductivity behaviour for the composite doped with CH₃COOH and HClO₄ are of the same order as that of doped emeraldine salt, whereas in other three composite samples the conductivity is much lower in comparison to emeraldine salt samples. XRD studies reveal that crystalline PANI/Mn₃O₄ composite has been transformed into amorphous phase during doping. From FT-IR spectra, it can be concluded that adsorbed acids anion on the oxide surface work as the charge compensator for positive charge on PANI. TG curves of composite suggest that the thermal degradation process of PANI/Mn₃O₄ composite proceeds in three steps and thermal stability of the composite depend on the dopant acid. The improvement in the thermal stability of composite is attributed to the interaction between a specific acid and PANI/Mn₃O₄ composite which may restrict the thermal motion of the PANI chain and shield the degradation of PANI in the composite. From the above study it can be concluded that the PANI/Mn₃O₄ composite doped with organic acids has the potential to act as NTC material for thermistor applications. Due to variation in the conductivity behaviour of doped composites and PANI, it may find application as sensing material.