Preparation And Gas-sensing Properties Of Composites Based On Polyaniline

Since 1960,the conventional semiconductor gas-sensing materials(SnO₂,Fe₂O₃, ZnO,and so on,) have been extensively studied by many researchers,and the application of gas sensors has been developed in domestic and industrial fields.But the semiconductor gas sensors have some disadvantages such as poor sensitivity,high power consumption and large dependence of environmental temperature and humidity, thus the development and application of semiconductor gas sensor are limited.In order to upgrade the gas-sensing properties of semiconductor gas sensor,the preparation,gas-sensing properties,and sensing mechanism of gas-sensing materials based on polyaniline(PAn) have been investigated systemically in this thesis.The contents are listed below:Aniline was oxidized by ammonium persulfate(APS) in HC1 for the synthesis of PAn.By means of FTIR spectra and UV-Vis spectra,the structural changes of intrinsic and doped PAn were measured.The influence of the reaction condition on the PAn sensitive properties have been investigated systemically such as the concentration of monomer,APS and HC1,the reaction temperature and reaction time, environmental temperature and humidity et al.The results reveal that gas sensors based on HC1 doped PAn have a high sensitive and selective properties to NH3 under the room temperature,but the gas-sensing properties of materials were influenced by synthesis and environmental conditions.In the same time,the response and recovery process has been analysised,the interaction mechanism of PAn and target gas was also investigated by measure the structural changes of HC1 doped PAn after it contact with NH3,which establish the base to investigate the gas-sensing mechanism of composites based on PAn.In order to upgrade the environmental stability of gas sensors based on PAn, organic sulfonic acid such as TSA,DBSA and SSA were used as doped acids to synthesis PAn.The thermal decomposition process of different acids doped PAn were investigated by means of TGA-DTA.It can be found that the dopants reduced the decomposed temperature of the molecule chain of PAn,organic sulfonic acid doped PAn has better thermal stability than HC1 doped PAn.Sensitive properties of organic sulfonic acid doped PAn have also been investigated,the results reveal that the gas sensors based on organic sulfonic acid doped PAn have better sensitive and response-recovery properties than HC1 doped PAn,especially the sensitivity of PAn-SSA to 1000ppm NH₃ reached 15.47,the response time less than 30S,recovery time less than 3min.Long-term stability of different acid doped PAn were also investigated,combine with the analysis result of TGA-DTA,it can be concluded that organic sulfonic acid doped PAn have better environmental stability.In order to improve the processability of PAn,PAn-SSA/nano-SnO₂ and PAn-SSA/nano-In₂O₃ composites were synthesized respectively by in situ polymerization.Interaction of PAn and inorganic nano-particles were confirmed by FTIR and XRD.The thermal stability of the two composites was investigated by TGA-DTA.The results suggest that the thermal stability of the two composites is higher than that of the PAn.Sensitive properties of the composites were influenced by the contents of inorganic nano-particles,sensitivity of the two composites reached highest when the content of nano-SnO₂ and nano-In₂O₃ reached 50%and 30% respectively.PAn-SSA/nano-In₂O₃ composites are more suitable as practical sensor materials because it can improve the sensitivity and selectivity of composites simultaneousely.The synthesis of its blends with ordinary polymers was the other way to improve the processability of PAn,but the ordinary PAn/polymer composites usually harmed the sensitive properties of PAn.In this thesis,PAn were synthesized in the composite emulsion system of PVA-PVAc,DBSA was used as dopants and emulsifier.Polymer assisted phase inversion(PAPI) process was used first to get PAn/PVA porous composite gas sensitive material.The FTIR and SEM indicates that the extraction has little effect on the structure of PAn in the composite membrane but the porous structure was formed on the composite film after extracting by acetone,which can improve the sensitive and response-recovery properties of the composites.The environmental stability is perfect because its sensitivity varieties in a narrow range during 50 days.The selective property of the composite membrane should also be improved if the pore structure can be controlled.