Design,Preparation And Transport Properties Of Materials And Electrodes For N-type Bi₂Te₃ Based Artificially Tilted Multilayer Thermoelectric Devices

In the past two decades,thermoelectric?TE?materials have been developed rapidly,and the figure of merit value?ZT?of TE materials has been increased from 1.0 to 2.0.However,it is still difficult to individually optimize electrical and thermal transport properties in a TE material,and the fundamental challenge of TE properties stems from the same direction of electric current and heat flux.Actually,for artificially tilted multilayer thermoelectric devices?ATMTDs?,the electric field and thermal gradient are no longer parallel or anti-parallel,which provides a novel approach for solving this problem.As a kind of commercialized n-type TE materials,Bi₂Te_2.7Se_0.3?BTS?has excellent TE properties near the room temperature.However,there is few research achievements in BTS-based ATMTDs and the related electrodes because many bottleneck problems exist in BTS,such as the complex relationship among components,structure and properties in BTS-based ATMTDs,and the difficulty in preparing high-quality electrodes for BTS-based devices.In this dissertation,a genome database of 230 BTS-based ATMTDs is established.The preliminary screening and quick screening methods for predicting the maximum transverse figure of merit(ZT_zx,max)values are proposed to facilitate the screening of high-performance ATMTDs.The effects of geometrical configurations and working conditions on the conversion efficiency???are studied.A p-Bi₂Te₃/BTS ATMTD with high performance is fabricated and characterized.The use of multilayer thin-film electrode is demonstrated as a tool to lower interface resistance and improve anti-oxidation for BTS-based devices.The main contributions are as follows:A genome database of 230 B/BTS?B stands matching materials?ATMTDs is established by high throughput screening,and the relationship among the transport properties of material B,the geometric configurations of B/BTS ATMTDs and the TE properties of B/BTS ATMTDs are investigated.The results indicate that the ZTzx,max values of a given ATMTD can be significantly improved by optimizing tilted angle???and thickness ratio?a?ATMTDs.For different ATMTDs,the transport properties of matching material B not only determine the ZTzx,max values,but also the optimal geometrical configurations including a and?of the ATMTDs.The key parameters in high ZT_zx,max B/BTS ATMTDs are the electrical conductivity of matching material B and?of ATMTDs.Bi_0.1Sb_1.9Te₃/BTS is the promising BTS-based ATMTDs,which has high ZTzx,max of 0.43 and large tilted angle around 32°.To facilitate the screening of high-performance B/BTS ATMTDs,the preliminary screening and quick screening methods are developed.The error rate of those methods is less than 10%.The optimal electrical conductivity ratio p??B/?BST?of the components is in the range from 0.0025 to 400,and p?1.The matching materials B with high conductivity and large Seebeck coefficient combined with BTS shows high ZT_zx,max.However,the ZT_zx,max value of all the B/BTS ATMTDs are less than 0.7.An approach to break through the bottleneck of low ZT_zx,max values in ATMTDs is to reduce the electrical conductivity of BTS without deteriorating the Seebeck coefficient,such as,preparation of organics-BTS composites.A promising ATMTD,composed of Bi_0.1Sb_1.9Te₃ and BTS,is constructed by finite analysis model to understand the relationship among the length?l?,width?w?,height?h?,thickness of the components?d?,contact resistance,working conditions and?.The results reveal that?was increased with increasing l/w and temperature difference;?was decreased with increasing l/d,d and contact resistance.The transport properties of Bi_0.1Sb_1.9Te₃/BTS,Ag/BTS and Co/BTS ATMTDs demonstrate that the direction of current is governed by the conductivity of components.The carriers in components with high electrical conductivity will move parallel to the alternating layers and form a net heat flux in longitudinal,while carrier in components with low electrical conductivity will move perpendicular to the alternating layers and form a net current in horizontal.As a result,a lateral electric field is generated in ATMTDs under a vertical thermal gradient.The open circuit voltage is governed by the transverse Seebeck coefficient(S_zx).The open circuit voltage increases with increasing S_zx.Bi_0.5Sb_1.5Te₃?BST?single crystal with 90°cutting angle and Bi₂Te_2.7Se_0.3?BTS?are selected as the optimal components for high-performance p-Bi2Te3/BTS ATMTD.The BTS based ATMTD is prepared by spark plasma sintering and wire cutting.The performances of the BTS-based ATMTD are measured with a self-made test platform.The microstructures indicate that the interface of the BTS-based ATMTD is compact.The diffusion of elements is not obvious and the thickness of the diffusion layer is about9?m.The performance results show that the S_zx of the BTS-based ATMTD is-67?V/K,which is in good agreement with the theoretical value of-72?V/K.A maximum?of 1.2%is obtained when the hot side temperature is 100°C and cold side temperature is 20°C.Based on the principle of low lattice mismatches,a Ni transition layer?TL?is designed between Cu thin-film electrode and BTS to reduce interfacial electrical resistances.An Al protective layer?PL?is introduced on the surface of Cu thin-film electrode to improve the thermal stability of Cu.It shows that the Ni TL can significantly lower the interface resistance through improving the interfacial contact between Cu and BTS.As a result,a decrease of 20%in electric resistance is achieved by introducing the Ni TL between Cu and BTS.The anti-oxidation of Al/Cu/Ni multilayer thin-film electrode is improved significantly,as compared to that of Cu monolayer and Cu/Ni bilayer thin-film electrodes without Al protective layer.After annealing for 72 h at 200°C in air,the electric resistance of Al/Cu/Ni/BTS multilayer materials was only 22m?.The microstructure and XPS results reveal that the robust anti-oxidation of Al/Cu/Ni multilayer thin film electrode is attributed to the dense Al₂O₃ layer with100-200 nm in diameter covering on the surface of Al PL,which can effectively prevent the inner Cu and Ni layers from further oxidation.Based on the above results,an Al/Cu/Ni multilayer thin-film electrode with low interface resistance and excellent anti-oxidation for BTS-based TE devices has been developed.