Research On Thermoelectric Properties Of Boron-doped Carbon Nanotubes Cement-based Composites

In recent years,a large amount of thermal energy generated by fossil fuel emissions and human activities has been released into the environment,making the ambient temperature rise.Cementitious composites with thermoelectric properties can realize the mutual conversion of thermal and electrical energy,and this green technology has broad application prospects in energy harvesting and snow and ice melting.Although the research of thermoelectric cement matrix composites has achieved great results,the low Seebeck coefficient and low electrical conductivity still limit its application.To address the above problems,boron-doped carbon nanotubes（B-doped-CNTs）were prepared in this paper,and B-doped-CNTs cementitious composites and EG-（B-doped-CNTs）/cement composites were prepared.The effects of B-doped-CNTs and EG content,freeze-thaw cycles on the microstructure,Seebeck coefficient,electrical conductivity（σ）,thermal conductivity（κ）,power factor（PF）,and thermoelectric optimum（ZT）of the cement matrix composites and the strengthening mechanism were systematically investigated.The main studies are as follows:（1）Boron-doped carbon nanotubes（B-doped-CNTs）were prepared by the thermal diffusion method,which were incorporated into the cement matrix,and the effect of B-doped-CNTs content on the thermoelectric properties of the cement matrix composites was investigated.It was shown that the electrical conductivity and Seebeck coefficient of the cement matrix composites gradually increased with the increase of B-doped-CNTs content.When the content of 0.76 at.%of 0.25-B-doped-CNTs（H₃BO₃ to CNTs mass ratio of 1:4）was 7.0 wt%,the conductivity of cement matrix composites was 0.44 S/cm and the absolute value of Seebeck coefficient reached 84.8μV/K with the highest thermoelectric optimum value of 1.10×10^-4,which was 3.1 times higher than that of the cement matrix composites with the same content of undoped CNTs.The enhanced thermoelectric properties are attributed to the boron atoms in the doped CNTs acting as host energy levels,the increased carrier concentration in the CNTs,and the lower contact resistance between the CNTs,leading to the increased conductivity.The doping of boron atoms selectively changes the local energy density of the Fermi energy level of CNTs,leading to a“resonance effect”,which increases the effective mass of the energy band and increases the Seebeck coefficient,thus improving the thermoelectric properties of the cement matrix composites.（2）EG-（4-B-doped-CNTs）（H₃BO₃ to CNTs mass ratio of 4:1）/cement composites were prepared by dry compression molding process,and the effect of EG incorporation on the thermoelectric properties of EG-（4-B-doped-CNTs）/cement composites was investigated.It was shown that the electrical conductivity of the cementitious composites increased and the Seebeck coefficient decreased as the EG content increased.When the EG content was 10.0 wt%,the compressive strength of 10.0 wt%EG-5.0 wt%（4-B-doped-CNTs）/cement composite was 65.4 MPa and the porosity was 20.2%.The absolute values of conductivity and Seebeck coefficient of 10.0 wt%EG-5.0 wt%（4-B-doped-CNTs）/cement composite were 3.62 S/cm and 64.1μV/K,respectively,and the thermoelectric power factor was 1.49μW·m^-1·K^-2,which was 11 times higher than that of the cement matrix composite without EG addition,and its output power per unit area and thermoelectric conversion efficiency were 75.1μW/m² and 3.30×10^-5,respectively.The synergistic effect of one-dimensional nanoscale CNTs and micron-scale EG multi-scale hybridization with high crystallinity EG results in increased electrical conductivity of cementitious composites.Although the increase in carrier concentration leads to a decrease in the Seebeck coefficient,the overall enhancement of the thermoelectric properties of the cement matrix composites is achieved.（3）Freeze-thaw cycle experiments were conducted on 10.0 wt%EG-5.0 wt%（4-B-doped-CNTs）/cement composites to study their effects on the mechanical and thermoelectric properties of the cementitious composites.It was shown that the porosity of cementitious composites increased and the compressive strength decreased as the number of freeze-thaw cycles increased,and when the freeze-thaw cycles were 35 times,the compressive strength of cementitious composites was 38.4 MPa and the porosity was28.4%.As the number of freeze-thaw cycles increased,the electrical conductivity of the cementitious composites gradually decreased and the Seebeck coefficient first increased and then decreased.The absolute values of conductivity and Seebeck coefficient of 10.0wt%EG-5.0 wt%（4-B-doped-CNTs）/cement composites were 2.98 S/cm and 71.5μV/K when freeze-thaw cycles were performed 15 times,and the highest thermoelectric figure of merit was 1.37×10^-4,which was improved compared with that of cement matrix composites without freeze-thaw cycles,with the output power per unit area and thermoelectric conversion efficiency were 76.0μW/m² and 3.42×10^-5,respectively.The enhancement of thermoelectric properties is mainly due to the increase in porosity of cement matrix composites caused by freeze-thaw cycles,which increases the carrier scattering inside the cement matrix and decreases the conductivity of cement matrix composites.However,the pores and cracks generated by freeze-thaw cycles introduce an increase in carrier scattering at the interface leading to the formation of an energy barrier in the direction of carrier motion,which enhances the Seebeck effect under the scattering and energy filtering effects,thus improving the thermoelectric properties of cementitious composites.