Preparation And Thermoelectric Properties Of Polyaniline/Carbon Nanotubes Composites

Polyaniline (PANI), as one kind of conducting polymers, is attracting more and moreattention owing to its easy synthesis, low cost, facile processability and environmentallybenign characteristics. However, the thermoelectric performance of PANI has yet to beimproved. In this paper, doped PANI was synthesized by in situ polymerization withdifferent ratio of ammonium persulfate (APS) and potassium dichromate (PDS) inaqueous solution and water-acetone solvent, respectively. The structure and morphologyof the obtained PANI were characterized, and their thermoelectric properties were alsoevaluated. The influences of carbon nanotubes (CNT) with different length and diameterand polyaniline-coated carbon nanotubes (PCNT) with different contents on the structureand thermoelectric performance of PANI-based composites were also investigated.The electrical conductivity of PANI synthesized in water-acetone solvent reaches2190S/m at60℃, while the ratio of APS and PDS in mixed oxidizer was7.5:1. Ascompared to the great enhancement in the aspect of electrical conductivity in PANI, theSeebeck coefficient shows a disappointing opposite trend which is not benefit for theimprovement of thermoelectric properties. The thermal conductivity of PANI with mixedoxidizer obtained from aqueous solution is a little higher than that of PANI with APS, butlower than that of PANI obtained from water-acetone solvent. The ZT value of PANIobtained from aqueous solution reaches the maximum10.910-4at153℃, which is11%higher than that of PANI with APS, while the ratio of APS and PDS in mixed oxidizerwas7.5:1.Polyaniline composited with carbon nanotubes (CNT)(recorded as PANI/CNT)show a great enhancement in thermoelectric properties due to excellent mechanical andelectrical performance of CNT. PANI/CNT composites prepared by in situ polymerizationwith five different kinds of CNT were recorded as PANI/CNT1,PANI/CNT2,···PANI/CNT5, respectively. The structure and morphology of PANI/CNTcomposites were characterized and their thermoelectric performances were evaluated.The analysis of SEM indicated that the microstructure of polyaniline composited withcarboxyl CNT (CNT4) was more uniform and ordering than that of other samples. Theelectrical conductivity of polyaniline composited with CNT5(length=10-30μm,diameter=10-20nm) reached2650S/m at60℃, which was210%higher than that of PANI (APS:PDS=7.5:1). The ZT value of PANI/CNT4composite reached2.0510-3at180℃, which was89.8%higher than that of PANI (APS:PDS=7.5:1).Generally, the highly cohesive van der Waals energy of CNT makes it difficult toform well-dispersed PANI/CNT composites. One of the best ways to uniformly dispersethe CNT into PANI matrix is to attach a PANI coating on the surface of CNT forimproving the dispersion and overall properties of the composites. In this thesis,Polyaniline-coated carbon nanotubes (PCNT) were firstly synthesized by in situpolymerization and then incorporated into the PANI matrix through mechanical ballmilling method to prepare PANI/PCNT composites. PCNT, PCNT-SA and PCNT-MAwere synthesized by in situ polymerization with Hydrochloric acid, sulfuric acid (SA) andmixed acid (MA)(sulfuric acid and sulfosalicylic acid at the molar ratio of4:1),respectively. The analysis of SEM indicated that PCNT had better dispersivity than thatof CNT.PANI/PCNT composites with different contents of PCNT were prepared throughmechanical ball milling method. It was observed that both the electrical conductivity andSeebeck coefficient of composites were greatly enhanced by incorporation of PCNT. TheSeebeck coefficient of PANI/PCNT-30%(the PCNT content is30wt%) reaches themaximum34.9μV/K at180℃. The ZT value of PANI/PCNT-20%(the PCNT content is20wt%) reached2.2110-3at180℃, which is104.6%higher than that of PANI(APS:PDS=7.5:1). There are three possible reasons for the improvement in thethermoelectric performance of PANI/PCNT composites.(1) A PANI coating on thesurface of CNT can uniformly disperse the CNT into PANI matrix and improve the overallproperties of the composites;(2) Low energy carriers could be filtered by the interfaceenergy filtering effect attributed to existence of the interface between PANI moleculesand CNT. As a result, the Seebeck coefficient of composites which is in inverse proportionto the carrier concentration in composites is improved.(3) Plenty of the interfacesbetween PANI molecules and CNT result in the formation of scattering centers forphonons, which may lead to the decrease of thermal conductivity.