The Study Of Interfacial And Surface Behaviors Of CoSb₃-based Thermoelectric Joint

Thermoelectric?TE?technology can employ the Seebeck and Peltier effect to realize the conversation between electricity and temperature difference.Because of its advantages including no mechanical structure and produce no pollution,TE technology has wide potential applications in space power and waste heat harvest.In the recent two decades,amount of impressive achievements has earned in TE materials,leading z T values to be over 1.5 and even thrilling 2.0,which build the cornerstone for TE application.However,the unsatisfied stability of the TE device drags the step of TE technology from lab to industry.The stability problems of the TE device majorly include the hot-side interfacial stability and the sublimation.The TE joint is the fundamental unit of the TE device and includes TE material and the interface layer?barrier layer and electrode connecting layer?.The core issue of the TE joint is the hot-side interface problems,of which the element diffusion is the top,which can also be suppressed by the barrier layer.However,the traditional trial-and-error method for interface design is not only born to be time-and-cost-consuming,but also hard to achieve optimized bonding.In order to realize the application of novel high-performance TE material,a new interface design model,which can connect the experimental interface behaviors and theoretical calculations,is urgingly required to accelerate the study of TE device.Therefore,this paper focuses on the CoSb₃-based skutterudite?SKD?material,which has a promising application in middle temperature range,and takes the advantage of lots of previous studies on CoSb₃-based joint interface to create an interface design model with reaction-diffusion theory describing the growth of interfacial reaction layer?IRL?.The validity and accuracy of the interface design model are verified by the experimental stability of the TE joint and TE module.This paper finishes the full research path of“interface design-joint verifying-module evaluation”.In addition,most TE materials suffer the sublimation problem.The element atom sublimates from the surface of the TE material and leaves the sublimation layer in the surface.The sublimation layer reduces the effective area of the TE devices and leads to the increase of the inner resistance and decline of the output performance.In this paper the sublimation layer for SKD is studied systematically and kinetically and the effect of the sublimation layer on the performance of TE device is simulated.The major achievements are listed as follows:1.Based on the reaction-diffusion theory,the growth mechanism of the IRL in CoSb₃-based joint has been revealed,which contains two steps,the chemical reaction and element diffusion.According to the two steps,the interface design model has been built,in which the interfacial reaction energy for chemical reaction should be negative to obtain the strong chemical bonding and the activation energy for Sb diffusion should be larger than that of Ti barrier layer to suppress the IRL growth.By combining the interfacial reaction energy and activation energy,the“sweet spot”is produced to guide the interfacial design.N-type Yb_0.3Co₄Sb₁₂/Nb joint and Yb_0.3Co₄Sb₁₂/Zr joint have been fabricated and their interfacial behaviors under the accelerating experiment have been investigated.The decomposition layer?CoSb₂?and IRL?Nb Sb₂ and Zr Sb₂?are formed.Based on the Deal and Grove`s relation,the kinetic analysis for the IRL growth has been carried out.The activation energy for Nb joint?311 k J/mol?is much larger than Zr joint?242 k J/mol?.The experimental results agree with the interface design model.Based on the interfacial structure,the interfacial resistivity analysis model has been built and the analysis results demonstrate that the decomposition layer is the major cause of the interfacial resistivity.The prediction of the interfacial resistivity is achieved for the first time.The interfacial resistivity for Nb joint will reach 10??cm<sup>2</sup> after 87years at 773 K.2.P-type Ce<sub>y</sub>Fe<sub>x</sub>Co<sub>4-x</sub>Sb<sub>12</sub>/Nb joint is fabricated to study the interfacial behaviors in p-type joints and the influence of TE material composition on the interfacial behaviors.In p-type joints,the IRL?Nb Sb<sub>2</sub>?is formed but decomposition layer isn`t formed.The kinetic analysis demonstrates that the TE material composition only has influence on the Sb diffusion.The activation energy for Sb atoms in p-type joint?185-204 k J/mol?is much lower than n-type?311 k J/mol?.Therefore,Sb atoms in the p-type CoSb₃-based SKD can diffuse to the interface to replenish the loss and prevent the decomposition layer.The interfacial resistivity for p-type joints is related with the Fe content in TE material and the IRL thickness.3.The sublimation layer of the CoSb₃-based SKD has been studied.The micro-XRD results demonstrate that the composition of the sublimation layer for p-type material is the combination of FeSb and FeSb₂ and n-type is CoSb₂.The sublimation layer growth is based on the interfacial reaction and Sb diffusion.The kinetic analysis reveals that n-type sublimation layer grows much more slowly than p-type.The increase of Fe content in p-type material results in the decrease of activation energy for chemical reaction?84-176 k J/mol?but does not affect the activation energy for Sb diffusion?119-146 k J/mol?.The simulation results demonstrate that after 6 years when the hot-side temperature is 873 K,the internal resistance of p-type Ce Fe₄Sb₁₂ device will increase to 223%and its output power will decrease 58%and the internal resistance of n-type Yb_0.3Co₄Sb₁₂ device will increase 104%and its output power will only decline 5%.4.The stability of the single-leg device and 8-pair module using Nb as barrier layer is investigated.After 1000 hours service test with the hot-side of 869 K,the internal resistance of the single-leg device increases 1%,the maximum output power reduces1.8%.The performance of the module before and after a 846-hour long-term service test demonstrates no difference.The maximum conversion efficiency is 10.4%,which is the highest record.After more than 300 thermal shocks,the internal resistance of the module increases 0.5%.Under the maximum current of 5.2 A,the internal resistance of the module shows no change.The module using Nb as barrier layer has not only a great stability,but also a impressive output performance.