| The thermoelectric material can realize the direct conversion between heat and electricity and its energy conversion efficiency is determined by the figure of merit(Z7).The research of high-quality thermoelectric materials has never stopped.The thermoelectric properties of the material are determined by the Seebeck coefficient,thermal conductivity,and electrical conductivity.An excellent thermoelectric material generally has high Seebeck coefficient,low thermal conductivity and high electrical conductivity,but these three have a mutual restrictive relationship.Thus,materials with 'phonon glass-electron crystal' properties will become promising thermoelectric materials.In thinking about how to reduce the thermal conductivity of a material without deteriorating or even increasing its conductivity,the author took note of the semiconductor-metal superlattice structure.Sb2Te3-based thin-film materials are well-performing thermoelectric materials and have been extensively studied.By the time-domain thermoreflectance method,thermal conductivity of pure semiconductor films and semiconductor-metal superlattice films were measured before and after annealing and electron irradiation.We summarized the similarities and differences of the thermal conductivity of the two material after the above process.And we further measured the thermal conductivity of the superlattice material with different components.The thermal conductivity of the superlattice material was also measured varying the temperature from 60K to 300K.The effect of electron-phonon coupling and interface scattering on the overall thermal conductivity of the material was described.The details are as follows:(1)Based on the principle of pump-probe techine,we had set up a TDTR system to measure the thermal conductivity of the thin film materials.We have carried out a series of experimental calibrations on several standard samples to ensure the stability and reliability of our TDTR measurement system.Not only that,we also achieved the in-situ measurement of aluminum film thickness and pump spot diameter required for fitting experimental data through the TDTR experimental platform,which ensured the accuracy and consistency of our measurement data.(2)We characterized the morphology and structure of Sb2Te3 thin films and Sb2Te3/Metal superlattice thin film prepared by magnetron sputtering using SEM,AFM,XRD,and EDS to confirm these samples meets our experimental needs.(3)Using TDTR methods,we measured the thermal conductivity of different composition Sb2Te3/Metal superlattice thin film samples.It was found that the superlattice of antimony telluride/silver has a lower thermal conductivity,while the superlattice structure of antimony telluride/platinum has the highest thermal conductivity.We speculated that the interface morphology and electron-phonon coupling in semiconductors and metals lead to this result.At 60K,the thermal conductivity of Sb2Te3/Ag sample with ratio of 15nm:15nm dropped to less than 0.3W/(m·K).Considering the excellent conductivity of metal at low temperature,we believe that Sb2Te3/Ag is expected to have good performance at a certain temperature as thermoelectric material.(4)Using TDTR methods,we measured the thermal conductivity of Sb2Te3 films and Sb2Te3/Metal superlattice films before and after annealing or electron irradiation,then compared the thermal conductivity of pure semiconductor film samples and superlattice samples.It was found that the Sb2Te3 film directly obtained by magnetron sputtering was amorphous,and its crystal structure could be recovered after annealing.After annealing,the thermal conductivity of the sample has a rising trend,even the sample with a superlattice structure has a higher thermal conductivity.We attribute it to the reduction of the thermal resistance of the semiconductor-metal interface in the superlattice.After the electron irradiation process,similarly,the thermal conductivity of all the samples has increased,but the increase of the thermal conductivity of the sample with the superlattice structure was also significantly higher than that of the pure semiconductor film,which was likely to be that metal materials should be responsible for extra tunnels of electron.The above measurement and research results not only laid the foundation for the development of semiconductor/metal superlattice thin film materials in the field of thermoelectricity,but also provided an experimental basis for regulating the properties of functional materials in terms of heat conduction by artificial means,which would facilitate the further search for potential new thermoelectric materials... |
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