Preparation And Performance Optimization Of Ni Coating On Bi₂Te₃ Based Thermoelectric Materials

Bi₂Te₃ based thermoelectric?TE?materials are one of the most widely used materials in commercial TE devices at room temperature,and mainly used for solid-state cooling and power generation.However,there are some problems of connection and diffusion between the electrodes and Bi₂Te₃ in the process of preparing TE devices.Among them,the Ni transition layer is the key to determine the service life of the device.In this paper,by preparing a Ni transition layer and discussing its structural control methods to improve its performance,the following conclusions were obtained:The effects of the main process parameters on the structure and performance of the Ni coatings obtained by sulphate nickel plating and sulfamic acid nickel plating of Bi₂Te₃ based thermoelectric materials were analyzed by orthogonal experiments.For the sulfate electroplating and sulfamate plating nickel,the sample with the saccharin concentration was 1 g/L,the current density was 1 A/dm²,the temperature was 40°C,and the plating time of 5 min achieved the highest bonding strength of 3.58 MPa and4.66 MPa respectively.The influences of the selected process parameters on the bonding strength are ranked as following:saccharin concentration>plating time>current density>temperature.In addition,for the sulfate nickel plating and sulfamate nickel plating,the differences in the microhardness of the samples of each group are relatively small;the samples with saccharin added to the porosity of the coating are significantly lower than the samples without saccharin added.For the sulfate plating nickel,the bonding strength of the coating with the saccharin concentration of 1.4 g/L is the highest,reached 5.31 MPa.For the sulfamate plating nickel,the bond strength of the coating with the saccharin concentration of 1.2 g/L was the highest,reached6.27 MPa.In addition,the bond strength and hardness of the sulfamate-plated nickel samples were higher than those of the sulfate-plated nickel samples when other conditions are the same.The effects of reaction temperature and reaction time on the interface microstructure and surface hardness of Ni/Bi_0.4Sb_1.6Te₃ samples were studied by Bi_0.4Sb_1.6Te₃ powder/Ni foil diffusion welding experiments.It shows that Ni?Sb,Te?intermetallic compound layer is formed between Ni foil and Bi_0.4Sb_1.6Te₃ powder.As the reaction temperature increases,the thickness of the nickel layer decreases,while the thickness of the intermetallic compound layer increases,and the surface hardness of Ni/Bi_0.4Sb_1.6Te₃ increases rapidly.As the reaction time increases,the thickness of the nickel layer changes little,but the thickness of the intermetallic compound layer tends to increase,the surface hardness of the sample increases slowly.The effects of reaction temperature and reaction time on the interface microstructure and surface of Ni/Bi_0.4Sb_1.6Te₃ samples were studied by Bi_0.4Sb_1.6Te₃powder/Ni foil diffusion welding experiments.Ni?Sb,Te?intermetallic compound layer was formed between the Ni foil and the Bi_0.4Sb_1.6Te₃ wafer.As the reaction temperature increases,the thickness of the nickel layer becomes thinner,and the intermetallic compound layer becomes thicker;prolong the reaction time,the thickness of nickel layer and intermetallic compound layer has no obvious change.