Research On The Thermoelectric Properties Of Carbon Nanotube Cement Based Composites

Thermoelectric cement matrix composite is a composite material that achieves mutual conversion of thermal and electrical energy by adding carbon or metal materials and other functional additives.It has broad application prospects in energy harvesting,low-energy snow melting and deicing,and alleviating urban heat island effect.Although great progress has been made in thermoelectric properties of carbon-reinforced cement-based composites,the high carbon content and low Seebeck coefficient in carbon-reinforced cement-based composites have become the main factor to improve the thermoelectric properties of carbon-reinforced cement-based composites.In this work,carbon nanotube（CNTs）cement-based composites were prepared through the pretreatment of CNTs.Combining CNTs and metal oxides effectively improved the thermoelectric properties of CNTs cementitious composites.The changes of surface defects,functional groups and electrical conductivity after pretreatment of CNTs were systematically investigated.Meanwhile,Variation of the microstructure,electrical conductivity,Seebeck coefficient,thermal conductivity,power factor（PF）and thermoelectricity figure of merit（ZT）of CNTs cement-based composites were analyzed.The main research contents are as follows:The effect of CNTs pretreatment by HNO₃:H₂SO₄ on the thermoelectric properties of cementitious composites was investigated.It was shown that the defect density of CNTs increased after pretreatment with HNO₃:H₂SO₄ at 80°C for 4 h.At the same time,oxidation occurred on the surface and C=O bonds were formed,leading to an increase in carrier scattering and a decrease in carrier mobility.The decrease in carrier mobility contributes to the increase in effective carrier mass,which further enhances the thermoelectric properties of the cement matrix composites.When the content of CNTs pretreated with HNO₃:H₂SO₄ in cement matrix composites was 5.0 wt%,the electrical conductivity of cement matrix composites was 0.23 S/cm,the Seebeck coefficient was 59μV/K,and the thermoelectric figure of merit was up to 0.25x10^-4,which was 2.2 times higher than that of cement matrix composites prepared without pretreated CNTs.Therefore,the CNTs pretreatment with H HNO₃:H₂SO₄ is beneficial to enhance the thermoelectric properties of the CNTs cement matrix composites.The effect of CNTs pretreatment time by HCl gas on the thermoelectric properties of cement matrix composites was investigated.It is shown that the pretreatment of CNTs in HCl atmosphere increases the defect density of the resulting material,increases the carrier scattering and decreases the carrier mobility.The decrease of carrier mobility contributes to the increase of carrier effective mass,which further enhances the thermoelectric properties of cement matrix composites.Meanwhile,CNTs formed C-Cl covalent bonds on the surface after pretreatment with HCl gas,and the bonding action destroyed the CNTs covalency,which decreased the CNTs conductivity.The chemisorption of Cl shifted theπ-electron cloud in CNTs,which enhanced theπ-electron leap and increased the conductivity of CNTs.When the content of CNTs in cement matrix composites pretreated with HCl gas for 48 h was 5.0 wt%,the conductivity of cement matrix composites was 0.22 S/cm,and the absolute value of Seebeck coefficient was 66μV/K with the highest thermoelectric figure of 0.38x10^-4,which was three times higher than that of cement matrix composites prepared without pretreated CNTs.Therefore,the pretreatment of CNTs with HCl gas for 48 h is beneficial to enhance the thermoelectric properties of CNTs cement matrix composites.The effects of micro/nano Cu O and Zn O contents（5.0 wt%,10.0 wt%and 15.0wt%）on the thermoelectric properties of CNTs cement matrix composites was investigated.Incorporation of metal oxides into CNTs cement matrix composites has a positive effect on enhancing thermoelectric properties of CNTs cement-based composites.When the contents of Cu O and Zn O in the CNTs cement-based composite are equivalent,the power factor from both nano-scale Cu O and Zn O is higher than that of micro-scale Cu O and Zn O.Therefore,for the same metal oxides（Cu O,Zn O）,nanoscale oxides are more likely to enhance the thermoelectric properties of cement-based composites than micrometer oxides.