热喷涂制Ni阻挡层在碲化铋热电器件中的应用

Application of Thermal Spray Ni Barrier Layer in Bismuth Telluride Thermoelectric Devices

在碲化铋热电制冷器件的服役过程中,焊料与热电材料间的元素扩散将严重制约器件的正常使用,目前最常用在两者间加Ni阻挡层的方法来改善这种问题,以往采用电镀、等离子烧结等制备Ni镀层的方法会产生界面镀层厚度不易控制、镀层易氧化的问题,而热喷涂由于其镀速快、镀层厚度易控制和镀后表面较平整、耐氧化、结合强度高等优点,可作为制备Ni阻挡层的更好选择。采用热喷涂技术制备不同厚度的Ni阻挡层,并对其分别进行200℃下24、72和144h的退火试验。首先探究不同Ni层厚度的p型(Bi_(0.4)Sb_(1.6)Te_(3))和n型(Bi_(2)Te_(2.7)Se_(0.3))碲化铋材料退火前后对镀层硬度和防扩散效果的影响,并将不同Ni层厚度的p、n型碲化铋材料制备成热电器件进行服役性能测试。结果表明:退火对p型材料镀Ni层硬度影响较小,其值变化在10%以内,但对n型材料镀Ni层的硬度影响较大,其最大硬度值下降56.36%;Ni是p型碲化铋材料较好的扩散阻挡层,能显著减少Bi_(0.4)Sb_(1.6)Te_(3)中所有元素的扩散,但其对于n型材料的阻挡效果不明显,仅能较弱地阻挡Bi_(2)Te_(2.7)Se_(0.3)中除Te之外的元素扩散;正常工作时,镀Ni器件在热循环2.5万次后,内阻变化小于5%,相较于无镀Ni器件,其服役寿命得到显著提高。

In recent years,some studies have found that thermoelectric materials exhibit ideal characteristics to realize energy efficiency.Among thermoelectric materials,BizTes compounds,discovered in the 1950s,exhibit the highest thermoelectric conversion performance at room temperature,and their alloys have been widely applied in many fields,such as radioisotope thermoelectric generators and polymerase chain reaction amplifiers.The compounds have become the most widely used commercial materials.The connection between the electrode and thermoelectric material is typically achieved by welding in a thermoelectric cooling device.If the thermoelectric element is in direct contact with a solder,they readily diffuse into each other.With a vigorous diffusion reaction at the interface,many dislocations are generated,decreasing the shear stress and interface adhesion strength;and eventually leading to the failure of the thermoelectric device.The addition of a Ni-based alloy barrier layer between the two materials is the most widely used method for solving this problem.However,the preparation of Ni-based alloys has the problem of incompatible properties during the previous preparation process,which may produce defects and uneven coatings.Using a single element as a barrier layer can reduce or even prevent these phenomena.The thermal spraying process has received increasing attention owing to the advantages of simple operation,uniform coating,and large-area spraying,in large-scale projects,the thermal spraying process can significantly facilitate construction progress.However,few studies on applying thermal sprays to synthesizes Ni plating as a barrier layer have been conducted.Other,traditional methods of adding a Ni barrier layer,such as electroplating and plasma sintering,may cause difficulty in controlling the interface coating thickness,and frequent oxidization of the coatings.The effect of Ni-spray coatings of different thicknesses on the interface of bismuth telluride was evaluated by accelerating the aging methods,and some problems arising from thermoelectric devices in the operating process were investigated based on the degree of diffusion of the material interface through annealing and thermal cycling experiments.Ni layers with various thicknesses were prepared by thermal spraying,and annealing experiments were performed for 24,72,and 144 h at 200℃.First,the effects of p-type and n-type bismuth telluride materials with various Ni layer thicknesses on the coating hardness and anti-diffusion effect were investigated,and p-type and n-type samples were prepared in thermoelectric devices for operating performance tests.The results show that the annealing process minimally influences the Ni coating hardness of p-type materials,with the hardness fluctuating within 10%.In contrast,the process can significantly decrease the Ni coating hardness of n-type materials by 56.36%,indicating that Ni is a superior choice as a diffusion barrier in p-type bismuth telluride materials of almost all the elements in Bi^(0.4)Sb_(1.6)Te_(3).However the blocking effect of the n-type material is not evident,and the Ni barrier can only weakly block the diffusion of elements except Te in Bi_(2)Te_(2.7)Se_(0.3).The electrical resistance in the internal resistance of Ni-plated devices can change by less than 5%after 25000 thermal cycles,significantly increasing the service life compared to non-Ni-plated devices.