Study Of Higher Manganese Silicides Coating Fabricated By Plasma Spraying And Its Thermoelectric Properities

Thermoelectric device is a solid-state device that converts heat flux directly into electrical energy based on thermoelectric effect.Making full use of thermoelectric conversion technology to harvest of waste heat is significant to improving energy utilization efficiency or solving practical problems such as passive sensing in special situation.In addition to being limited by the development of thermoelectric materials which govern the conversion efficiency thermoelectric devices,the restriction on large-scale application of thermoelectric devices also includes the cost-performance ratio in device manufacturing,facing with huge amount of low-cost or even zero-cost waste heat resources.As an important surface engineering technology,plasma spraying technology is maturely used in aerospace and other fields.It has the advantages of low process cost,high production efficiency,and flexible surface adaptability.Therefore,considering the abundant element reserve and environment-friendly feature of higher manganese silicides(HMS),this paper proposes to prepare HMS thermoelectric functional coatings by plasma spraying,aiming to realize the large-scale and low-cost preparation of thermoelectric coatings and to explore layered TEG by spraying.Focusing on the problems discussed above,this paper carried out the research on the structure and performance of the plasma sprayed HMS coating.Combined with the flattening behavior of the HMS particles,the influence of the process parameters on the microstructure,mechanical properties,electrical conductivity and Seebeck coefficient of the HMS coating is determined.By analyzing the phase composition and microstructure of the HMS coating corresponding to different melting states,the mechanism of the interaction between the HMS particles and the plasma jet on the thermoelectric properties of the HMS coating was revealed and the relationship between spraying process-coating structure-coating performance was constructed.Firstly,in order to facilitate the characterization of the thermoelectric properties of the HMS coating,a testing setup for Seebeck coefficient and electrical conductivity of the coating sample was designed and developed.The basic principles of Seebeck coefficient and electrical conductivity measurement were explained in detail.Hardware constitution and software system were demonstrated.The measurement setup has a simple structure and can achieve rapid measurement in the temperature range of 50?to500?.Measurement examples were given to verify the effectiveness of the test method and equipment.Based on the study of the flattening behavior of HMS particles,the process window of plasma spraying power,spraying distance,and preheating temperature was determined.The influence of process parameters on the flattening process of particles was explained with the solidification-induced splash model.The morphology of HMS splat is significantly affected by spraying distance and spraying power.Preheating substrate can increase the number of deposited splat and depress particle splashing notably.The orthogonal experimental design was designed to study the effect of plasma spraying process on the performance of HMS coating:the main influencing factors on the porosity,conductivity,Seebeck coefficient and tensile strength of the coating are spraying distance,spraying current,H₂ gas flow and spraying distance in order.Spraying current and H₂ gas flow have the most significant influence on the thermoelectric properties of HMS coating,while the influence of HMS porosity on the conductivity of the coating is not apparent.In order to further explain the influence of spraying process on the thermoelectric properties of the HMS coating,the mechanism of the interaction between the HMS particles and the plasma jet on the thermoelectric properties of the HMS was analyzed by controlling the melting state of the particles.The total amount of Mn evaporated in the HMS particles depends on the mass transfer process of liquid Mn atoms to the liquid/gas interface:in the semi-molten state,Mn quickly evaporates to form a Mn shortage shell to prevent further Mn evaporation in the internal liquid phase;When the particles are completely melted,the Mn-poor layer will be destroyed under the action of continuous convection,and at the same time,a new uniform Mn distribution will be continuously formed in the entire droplet,resulting in a significant decrease in the internal Mn content of the particles and a stepwise change in the O and Mn content on the surface of the particles.Mn evaporation drives the phase transition,resulting in an increase of Si phase,and the HMS phase in the higher power sprayed coating is(Mn₁₅Si₂₆+Mn₂₇Si₄₇).The electrical conductivity of the HMS coating exhibits a degenerate semiconductor metal conduction characteristic,which decreases with the increase of the spraying power in the range of 19-28 k W,and shows a strong consistency with the decreasing trend of the Mn content.The Seebeck coefficient of the coating exhibits a p-type conduction mechanism,which is opposite to the effect of spraying power on the conductivity of the coating.Based on the process research,a tubular layered thermoelectric device was designed.Numerical model of the thermoelectric device was constructed for optimizing geometry size of following the maximum output power optimization strategy.According to the numerical calculation,the tubular thermoelectric device was plasma sprayed.The tubular TEG prepared by plasma spraying has a voltage output of 24.69 m V at 450?under natural heat elimination condition,which meets the input requirements of the ultra-low energy harvest device,showing practical application potential for passive wireless sensing through waste heat recovery.