Preparation And Properties Of Gradient Composite Electrodes For Mg₂Si Thermoelectric Devices

Mg₂Si based thermoelectric device is a kind of Power generating element worked at intermediate temperature?300-500??,which can realize the direct conversion between thermal energy and electric energy.It is a combination of electrode materials and Mg₂Si based thermoelectric materials with the advantage of environment-friendly and excellent thermoelectric properties?ZT¹.4?.Under the high temperature environment?T?400??thermal contact resistance and resistance of the interface will affect the conversion efficiency of the device heavily,Thermal match and temperature stability of the interface determines the reliability and service life of the device.but for now,there is a lack of the research about the designing,fabrication and high temperature stability of the electrode interface of Mg₂Si based thermoelectric devices,The electrode structure is also confined to a single-layer structure,which makes the practical application of Mg₂Si based thermoelectric devices hindered.In this paper,we selected a suitable electrode material for Mg₂Si based thermoelectric devices,explored a connection technology of electrode materials and Mg₂Si material,and a kind of gradient composite electrodes was successfully designed and fabricated.The composition,physical properties and high temperature stability of the gradient buffer materials were studied systematically.The gradient composite electrode structure and the traditional electrode structure are compared in the aspect of thermal conductivity and contact resistance of the electrode,.Through the analysis of the data,the following conclusions was listed below:?1?Ni has a high thermal conductivity(60W·m^-1?^-1)and electrical conductivity?0.87×10⁷S/m?.connection technology is better than Cu and Al,which can be sintered under 780?by whole SPS,and achieved a connection with Mg₂Si tightly and flatly.Furthermore Ni/Mg₂Si interface has not found a clear diffusion phenomenon after annealing for 3 weeks.?2?The Ni/Mg₂Si interface is designed to introduce one or more layers of gradient buffer materials,and then formed a new Segment structure:Ni/gradient buffer layer?GBLn:1?/Mg2Si,then,this new electrode structure was fabricated successfully by optimizing the parameters of ball milling process,cold press process and whole sintering process.The interface structure of the electrode is clear and without holes and cracks,The results of XRD showed:Gradient buffer layer are the composite materials which consist of Mg₂Ni₃Si,MgNi₂,Ni-Si compounds and a small amount of Ni or Mg₂Si,Component analysis of Ni/GBL_4:1/Mg₂Si,a middle layer of about 20-25?m is formed between Ni and the GBL layer,The component ratio of this layer is Mg:Si:Ni=50:50:120 approximately and No obvious interlayer diffusion was found between GBL and Mg₂Si layers.?3?The gradient buffer layer?GBLn:1?have a high thermal conductivity(19-22w·m^-1?^-1)and electrical conductivity?45⁶0×10⁵S/m?,The thermal expansion of the Ni/Mg₂Si interface can be reduced to the original 1/7th due to the introduction of GBL,This material is annealed at 500?for 1-3 weeks and still has a high electrical conductivity of?50⁵5×10⁵S/m?and thermal conductivity(17²3w·m^-1?^-1)The average heat transfer coefficient of Ni/GBL_n:1/Mg₂Si is about 4.25-4.47mm²/s which greater than Ni/Mg₂Si thirty percent,Ni/GBL_n:1/Mg₂Si heat-conduction capacity at 500?is 1.32-1.48 times of Ni/Mg₂Si,The thermal contact resistance for Ni/GBL_n:1/Mg₂Si decreased about 3-12%,The contact resistance of Ni/GBL_2:1/Mg₂Si and Ni/GBL_4:1/Mg₂Si is about 0.01?,Only 1/20th of Ni/Mg₂Si.