Study On Thermoelectric Properties Of Carbon Materials Reinforced Cement-based Composites

Thermoelectric cement-based composites is a new type of intelligent structural material,which has widely application prospect in the field of reducing urban heat island effect or deicing in winter.At present,the application of thermoelectric cement-based composites had been limited owing to the low thermoelectric conversion efficiency and the poor stability of Seebeck coefficient.Therefore,in order to promote the application of cement-based composites.In this paper,the expanded graphite,carbon nanotubes and acid treatment carbon fibers were used as reinforcing phase,the influence of the formation pressure,the specific surface area(different pretreatment methods)and the content of conductive phase on the thermoelectric properties of cement-based composites were studied.Thus,the optimization approaches of preparing high thermoelectric properties of cement-based composites were obtained,and the mechanism of thermoelectric performance enhancement was revealed.(1)The effect of the forming pressure(10 MPa,20 MPa,30 MPa and 40 MPa),the pretreatment temperature(700oC,800 oC and 900oC)and the content(5.0 wt%,10.0 wt% and 15.0 wt%)of expanded graphite on the thermoelectric properties of cement-based composites were investigated.The results shown that the thermoelectric properties of expanded graphite reinforced cement-based composites(EGCC)increased with the increasing of molding pressure,and in the whole measurement temperature range(35-75oC),the thermoelectric properties of EGCC increased with the increasing of temperature.The BET test results of expanded graphite at different pretreatment temperature(700oC,800 oC and 900oC)shown that the expanded graphite had higher N2 absorption isothermand,cumulative pore volume and specific surface area at 800 oC.As a result,the thermoelectric properties of cement-based composites were improved.With the increase of the content of expanded graphite,the thermal conductivity and thermoelectric properties of cement-based composites were increased.In summary,the highest thermoelectric properties of EGCC had been obtained under the conditions of the pretreatment temperature at 800 oC,the addition of 15.0 wt% and the forming pressure of 40 MPa,and the highest ZT value of EGCC was about 6.82×10-4.(2)The Hall effect of EGCC test results shown that the Hall coefficient of cement-based composites were negative in addition of expanded graphite,that the main types of conductive carriers in EGCC was electronic carriers,and its belonged to n-type semiconductor.With increasing expanded graphite content,the carrier concentration and mobility of the cement-based composites were increased,so that the higher conductivity and the stable Seebeck coefficient of EGCC havd been obtained.(3)The effect of carbon nanotubes content on cement-based composites was studied.The carbon nanotubes contents were 5.0 wt%,10.0 wt% and 15.0 wt%.The results shown that the thermoelectric properties of carbon nanotubes reinforced cement-based composites(CNTs CC)increased with the increase of carbon nanotubes contents.When the content of carbon nanotubes was 15.0 wt%,the higher ZT value of CNTs CC had been obtained,which was about 1.02×10-4.The high content of carbon nanotubes could improve the thermoelectric properties of cement-based composites,however,the mechanical properties and structural density of CNTs CC had been decreased owing to the high content and high specific surface area of carbon nanotubes.Therefore,in order to enhance the thermoelectric properties of CNTs CC,the carbon nanotubes should be dispersed in the cement matrix with single filament.(4)The thermoelectric properties of acid treated carbon fibers reinforced cement-based composites(CFRC)were studied when the carbon fibers content was 1.2 wt%.The results shown that the higher content of carbon fibers could be uniformly dispersed in the cement matrix using the sand mixer,and the carbon fibers interacted with each other.With the increase of carbon fibers acid treatment time,the electrical conductivity of cement-based composites were decreased,but the Seebeck coefficient,power factor and thermoelectric value were significantly increased,so when the acid treatment time of carbon fibers was 96 h,the cement-based composites had the highest ZT value(2.98×10-2),which was greater than that of the untreated carbon fibers about 5 orders of magnitude.Acid treated carbon fibers had significantly enhance the thermoelectric properties of cement-based composites,because the acid treatment destroyed the graphite microcrystalline layer structure of carbon fibers surface,which reduced the carbon fibers crystallinity and increased the carbon fibers surface roughness,aperture and the specific surface area.The thermoelectric properties of cement-based composites were improved by the order of magnitude,which due to the interface between carbon fibers and cement matrix increased and the carrier scattering enhancement.