Study On Thermoelectric Properties Of Phase Change Modified Expanded Graphite/Cement Composites

Thermoelectric cement-based composites can convert thermal energy in the building environment into electrical energy through the thermoelectric effect,and have broad application prospects in energy collection and conversion.They are of great significance in reducing environmental temperature and alleviating urban heat island effects.There is a mismatch between the optimal working temperature and the actual environmental temperature in the application process of thermoelectric cement-based composites,which leads to the inability to continuously and efficiently perform thermoelectric conversion,and restricts the large-scale promotion and application of thermoelectric cement-based composites.In response to this issue,this paper proposes a temperature range for controlling the optimal thermoelectric performance of cement-based composites through phase change materials.Carbon foam（CF）and expanded graphite（EG）were selected to adsorb paraffin wax（PW）to prepare CF/PW phase change composites and EG/PW phase change composites,respectively.Add EG/PW phase change composite materials and metal oxides into cement-based materials to prepare phase change modified cement-based composite materials and metal oxides enhanced phase change modified cement-based composites.Study the influence of phase change composite materials and metal oxide doping on the thermoelectric properties of cement-based materials,and systematically analyze the influence of EG/PW phase change composite materials on the Seebeck coefficient,electrical conductivity,thermal conductivity,power factor（PF）,and thermoelectric value of merit（ZT）of cement-based materials.The influence mechanism of thermoelectric properties of phase change modified cement-based composites and metal oxide reinforced phase change modified cement-based composites was elucidated.The main research content is as follows:Melamine foam（MF）was heat-treated to obtain carbon foam.CF/PW phase change composites and EG/PW phase change composites were obtained by adsorption of paraffin with CF and EG.The effect of heat treatment temperature on the microstructure of CF was studied,and the results showed that CF with a heat treatment temperature of 400℃had a complete network structure.As the adsorption capacity of paraffin increases,the network skeleton structure is gradually filled,and the enthalpy of phase transition increases.The adsorption melting point of paraffin at 38～42℃,and the enthalpy values of CF/PW phase change composite material with 98.0 wt%paraffin content are 118.8 J/g and 121.12 J/g,respectively,with a thermal conductivity of 0.973 W/（m·K）.The endothermic and exothermic enthalpies of the EG/PW phase change composite material with 90.91 wt%paraffin content are 109.92 J/g and 112.37 J/g,respectively,and the thermal conductivity is 4.395 W/（m·K）.Phase change composite materials that undergo100 temperature cycles can maintain their original properties.The capillary force between the pore structures of CF and EG can shape and encapsulate paraffin,and the graphitized thermal conductivity network provides a pathway for heat transfer during paraffin phase change,improving the thermal storage efficiency of phase change composite materials.Mixing EG/PW phase change materials into cement-based materials to prepare phase change modified expanded graphite/cement composite materials.Study the effect of the addition amount of phase change composite materials on the thermoelectric properties of cement-based materials.The increase in the content of phase change composite materials regulates the temperature range for the optimal thermoelectric performance of cement-based composites.The test results showed that the temperature point corresponding to the maximum thermoelectric performance was adjusted from 55℃to 60℃and 65℃.The corresponding Seebeck coefficient is-24.65μV/℃,-30.97μV/℃and-30.90μV/℃.The power factor is 1.39μW·m^-1·K^-2,1.57μW·m^-1·K^-2 and 1.67μW·m^-1·K^-2.The values of ZT is 5.53×10^-5,6.50×10^-5 and 7.07×10^-5.Phase change composite materials absorb heat during the phase change process,reducing the heating rate of cement-based materials,weakening the decrease in Seebeck coefficient caused by an increase in carrier concentration due to temperature rise,adjusting the temperature range corresponding to the peak power factor of cement-based materials,and regulating the temperature range of use of thermoelectric cement-based composite materials.Adding metal oxides into phase change modified cement-based materials to prepare metal oxides enhanced phase change modified cement-based composites.The study investigated the effect of metal oxides addition on the thermoelectric properties of phase change modified cement-based composites.The results showed that with the increase of metal oxides content,the Seebeck coefficient of metal oxide reinforced phase change modified cement-based composites increased,the electrical conductivity first increased and then decreased,and the power factor showed a change pattern of first increasing and then decreasing.The power factor of 10 wt%Zn O,Cu O,and 5 wt%Ti O₂ reinforced phase change modified cement-based composites are 3.15μW·m^-1·K^-2,3.60μW·m^-1·K^-2and2.53μW·m^-1·K^-2.The high Seebeck coefficient of metal oxides and the scattering enhancement of charge carriers caused by a large number of interfaces in the cement matrix enhance the Seebeck coefficient of phase change modified cement-based materials,strengthen the thermoelectric effect of cement-based materials.