Study On Thermoelectric Properties Of Defect-modulated Metal Oxide Reinforced Cement-based Composites

A large number of artificial structures（e.g.,pavements and various types of building walls）that have sprung up in cities are in a temperature differential environment.When cement-based composites with thermoelectric effect are applied to artificial structures,the thermoelectric effect is used to absorb heat from the environment and convert it into electricity,while reducing the surface temperature and effectively mitigating the urban heat island effect.This green technology,which can alleviate both energy and environmental crises,is of great significance for future urban development.However,to achieve large-scale and low-cost conversion of heat energy into electricity,cement-based composites with good thermoelectric properties are required.Current thermoelectric cement-based composites all suffer from low Seebeck coefficient and electrical conductivity,making the improvement of power factor a bottleneck.In this paper,the n-type metal oxides zinc oxide（Zn O）,tin oxide（Sn O₂）,indium tin oxide（ITO）,and titanium oxide（Ti O₂）are treated with reducing atmosphere,and the p-type metal oxides nickel oxide（Ni O）,copper oxide（Cu O）,and cobalt oxide（Co₃O₄）are treated with oxidizing atmosphere to achieve defect engineering to improve the metal oxides thereby improving the thermoelectric properties of cement-based composites,combined with highly conductive expanded graphite（EG）to achieve a substantial increase in power factor.Further,carbon nanotubes are used to construct interfacial micropores to effectively improve the thermoelectric optimum（ZT value）of cement-based composites.By studying the defect content,electrical conductivity,microscopic morphology and structure of metal oxides,we analyze and compare the effect law of defect engineering on the thermoelectric properties of cement-based composites.The main studies are as follows.Defect engineering was used to modulate the defects of Zn O and increase its sender defect concentration to study the effect of the thermoelectric properties of Zn O and expanded graphite bonded cement-based composites with different defect contents.The results show that the treatment of Zn O at 1000^oC with CO atmosphere decreases the host defect concentration by 37.94%,the sender defect concentration increases naturally,the Schottky barrier decreases,the carrier mobility increases to 1.094 cm²V^-1s^-1,the Seebeck coefficient reaches the highest value of-419μV·^oC^-1 at 70^oC,and the conductivity reaches12.78 S·cm^-1,obtaining the reported The highest value of power factor of 224μWm^-1K^-2was obtained for the cement-based composite,but the high carrier concentration resulted in a high thermal conductivity of 8.797 W·m^-1K^-1.Three n-type metal oxides（Sn O₂,ITO,Ti O₂）were treated with reducing atmosphere CO to achieve modulation of oxygen vacancy defects and to investigate the effect of oxygen vacancy defect modulation on the thermoelectric properties of n-type metal oxide/expanded graphite cement-based composites.The results show that the oxygen defects of all three n-type metal oxides were changed equally by CO treatment.The oxygen defect content of the n-type metal oxides treated at 800^oC with CO atmosphere increased the most,and the higher the oxygen vacancy content,the better the thermoelectric properties,and the maximum power factor of the cement-based composites with 5.0 wt%pretreated Ti O₂ reached 0.46μWm^-1K^-2.Therefore,the oxygen vacancy defect control of n-type cement matrix composites using reducing atmosphere is a universal and good way to improve the thermoelectric properties of cement-based composites.The effect of metal vacancy defect modulation on the thermoelectric properties of p-type metal oxide/expanded graphite cement-based composites was investigated by treating three p-type metal oxides（Ni O,Cu O,Co₃O₄）with oxidizing atmosphere oxygen（O₂）.It was found that the three p-type metal oxides were treated by O₂ with similar patterns as n-type metal oxides treated by CO,but the optimal treatment temperature for p-type metal oxides was lower than that for n-type metal oxides.the power factor of Ni O reached a maximum of 0.96μWm^-1K^-2 at 500^oC,and the power factor of Cu O treated at700 ^oC increased from 0.45μWm^-1K^-2 to 0.58μWm^-1K^-2.The power factor of Co₃O₄treated at 400 ^oC was most significantly improved with a maximum value of 1.05μWm^-1K^-2.Therefore,the use of oxidizing atmosphere is effective for the control of metal vacancy defects in p-type cement-based composites to improve their thermoelectric properties.The effect of interfacial micropores on the thermoelectric properties of cement-based composites was investigated.It was found that interfacial micropores constructed through Carbon Nanotubes（CNTs）were beneficial for both n-type and p-type cement-based composites,with significantly higher ZT values.The addition of CNTs had no effect on the semiconductor type and the degree of reinforcing phase dispersion of the metal oxide/expanded graphite cement-based composites themselves.With increasing CNTs content,the average Seebeck coefficient at 2.0 wt%reaches the highest value,but the electrical conductivity decreases.The interfacial micropores led to a significant decrease in thermal conductivity,so that all ZT values increased.However,the content of interfacial micropores needs to be properly adjusted and designed to obtain cement-based composites with good thermoelectric properties.