Investigation On The Preparation And Thermal Conductivity Of Na_1.4Co₂O₄-based Bulks And Fibers

Thermoelectric material is a kind of functional material that can directly convert heat energy into electric energy.It has the characteristics of environmental protection and pollution-free,and has great development potential for solving the increasingly severe energy crisis and pollution problems.Na_1.4Co₂O₄ is an oxide thermoelectric material,which has the advantages of simple preparation process,low cost,and oxidation resistance.However,its thermoelectric performance can not meet the actual application needs at present,and the research on its thermoelectric performance has important scientific significance.Studies have shown that proper amount of doping can increase the power factor of thermoelectric materials and reduce the lattice thermal conductivity.Low-dimensionality can increase the interface of the material,refine the crystal grains,and further reduce the lattice thermal conductivity of the material.Combining doping and low-dimensionality is an effective way to improve the thermoelectric properties of materials.Based on this,this paper uses the sol-gel method combined with the cold pressing method to prepare the Na_1.4Co₂O₄ bulks,and uses the sol-gel method combined with the electrospinning technology to prepare(Na_1-xLi_x)_1.4Co₂O₄(x=0,0.1,0.2,0.3,0.4)and Na_1.4(Co_1-yCu_y)₂O₄(y=0,0.1,0.2,0.3)nanofibers,systematically studied the effect of doping and low-dimensionalization on the thermal conductivity of Na_1.4Co₂O₄.The main research contents and conclusions of this paper are as follows:(1)The Na_1.4Co₂O₄ bulk was prepared and its thermal conductivity were studied.The Na_1.4Co₂O₄ powder was prepared by the sol-gel method,combined with the cold pressing method to form a bulk,and after a series of experimental characterization,the preparation process and phase components of the powder were analyzed,the surface morphology and micro thermal conductivity of the bulk were characterized.The experimental results show that the precursor powder is sintered at 800°C for 2 hours,and the resulting phase is Na_1.4Co₂O₄.The Na_1.4Co₂O₄ powder was pressed into a bulk and its thermal conductivity was characterized by scanning thermal microscope(STh M),and the thermal conductivity of the bulk was 1.78 W·m^-1K^-1.(2)The sol-gel method and electrospinning technology were used to prepare Na_1.4Co₂O₄nanofibers,and their thermal conductivity were studied.Through the analysis of the crystallinity,grain size and morphological characteristics of the sintered nanofibers,the crystallinity of the nanofibers is better than that of the powder,and the grain size is smaller than that of the powder,indicating that low-dimensionalization improves the crystallinity of the material.At the same time,the grain size of the material is reduced.The thermal conductivity of the Na_1.4Co₂O₄ fiber is reduced to 1.43 W·m^-1K^-1,which is 19.66%lower than that of the bulk.(3)A series of(Na_1-xLi_x)_1.4Co₂O₄(x=0.1,0.2,0.3,0.4)and Na_1.4(Co_1-yCu_y)₂O₄(y=0.1,0.2,0.3)nanofibers were prepared by doping,and their thermal conductivity were studied.Through the characterization of the phase,microscopic morphology and thermal conductivity of the doped nanofibers,it is found that the crystal grains of the Li-doped fiber are reduced and more dense,and the Cu-doped fiber has Cu whiskers.The thermal conductivity of(Na_1-xLi_x)_1.4Co₂O₄ fiber presents a trend of first rising,then falling and then rising,which is due to the combined effect of excessive Li doping to produce the second phase Li Co O₂ and grain refinement.When the doping amount of Li is x=0.3,the thermal conductivity reduce to the minimum value,which is 1.38 W·m^-1K^-1.With the increase of Cu content,the thermal conductivity of the fiber decreases first and then rises.When the Cu content y=0.1,the thermal conductivity of the fiber reaches the minimum value,which is 1.42 W·m^-1K^-1.As a result,we have reduced the thermal conductivity of Na_1.4Co₂O₄thermoelectric materials through the combination of doping and low-dimensionalization,thereby providing a new way to improve its thermoelectric performance.