INCREASE IN THE ADHESIVE PROPERTIES OF COMMUTATION LAYERS ON SEMICONDUCTOR p-TYPE BRANCHES OF THERMOELECTRIC GENERATOR BATTERIES
Existing studies demonstrate that the main cause of the degradation of switching contacts during the operation of thermoelements based on bismuth telluride are the low strength properties of this material and the insufficient adhesion of the barrier metallization layer to the surface of the semiconductor branch.
The objective of this paper is to study the phase composition, morphology, hardness and adhesive properties of the surface of the semiconductor thermoelectric branches Bi2Te3-Sb2Te3 (p-type conductivity) obtained by hot pressing, after various types of surface modification.
P-type semiconductor branches based on Bi2Te3–Sb2Te3 solid solution were obtained by cold and subsequent hot isostatic pressing in vacuum (10-2 Pa) of respective powders and annealing semiconductor material blanks at T = 570 K for 24 hours in vacuum (10-1 Pa). Surface modification of semiconductor branches was carried out by mechanical polishing and subsequent electrochemical polishing or pulsed photon treatment with incoherent light. Mechanical polishing was performed on a carbide-silicon abrasive disc with different grain sizes (from Р2000 to Р5000), to obtain a mirror surface.
Pulsed photon treatment was performed by means of the radiation of xenon lamps (wavelength 0.2–1.2 μm) in an Ar atmosphere under the following conditions: double irradiation with a set of pulses with a duration of 10-2 s in 0.8 s, which corresponds to the radiation energy reaching the sample (EP), ~ 80 J/cm2.
Electrochemical polishing was carried out in the following electrolyte composition: KOH – 90 g/l; H2C4H4O6 – 65 g/l, distilled water 845 g/l.
Nickel was used as the switching layer, and molybdenum was used as the barrier layer. The layered deposition of metallic layers (Mo and Ni) onto the prepared surfaces of semiconductor branches was applied by magnetron sputtering of the corresponding targets in Ar.
The phase analyses were performed by X-ray diffractometry (Bruker D2 Phaser). The relief and surface roughness of the samples were studied by atomic force microscopy (NT-MDT Solver P47). The hardness of the samples was measured by nanoindentation method. The adhesion of coatings to the branches was determined by the shear on the tensile testing machine RPM-10MG4.
It has been determined that mechanical polishing of the p-type semiconductor branches based on Bi2Te3-Sb2Te3 solid solution results in surface hardening, and also increases the adhesion of the Mo–Ni switching barrier layers by a factor of 2 compared with the adhesion of Mo–Ni layers on the surface of untreated branches. Subsequent electrochemical polishing increases the adhesion of Mo–Ni layers by 1.3 times, and pulsed photon treatment by 1.4 times compared with mechanically polished branches.
It is shown that the energy effect of an PPT stimulates local recrystallization of the defective layer near the surface of semiconductor branches at a depth of 100-200 nm and increases the hardness of the near-surface layers of the Bi2Te3- Sb2Te3 solid solution by 1.2 times.
The work was performed using the scientific equipment of the Center for collective use of them. prof. Yu.M. Borisov, Voronezh State University, Central Chernozem Collective Center for Analysis of the Structure, Elemental and Chemical Composition of Materials of the Voronezh State Technical University and with financial support from the Ministry of Education and Science of the Russian Federation in the framework of the Russian Federation Government Resolution of April 9, 2010 No. 218 (Contract No. 03.G25.31.0246).
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