Microstructure and Mechanical Strength of Sn-57Bi-x（Sb/Ag/Ni） Alloy AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY23120019

Chinese Library Classification Number:TG425.1

Citation Information:

Heterojunction （HJT） solar cells have advantages such as high conversion efficiency，short process，and low temperature process （below 200 ℃）. However，Sn60-Pb40 alloy currently used in the HJT manufacture process has a soldering temperature of up to 230~240 ℃. Such high connection temperatures will not only damage the low temperature made silver surface finish，but also increase the solar cell breakage rate. Therefore，the assembly of heterojunction solar cells in the photovoltaic industry has put forward a low-temperature joining demand. This has once again attracted attention to the cost-effective Sn-Bi low-temperature solder. Based on reducing Bi to 57%，this study added 0.1% and 0.9% Sb/Ag/Ni trace elements respectively to observe the effects of different elements on the melting characteristics，microstructure，and mechanical properties of eutectic （nearby） alloys. Among them，trace elements Sb，Ag，and Ni were all prepared by vacuum sealing and muffle furnace melting during alloy melting. The thermal effects of physical or chemical changes in the sample during heating were characterized using differential scanning calorimetry （DSC，Pyris Diamond） analysis. By observing the changes in the endothermic and exothermic peaks of the alloy on the differential thermal curve，it was possible to determine whether there were new phases generated within the microstructure after microalloying and their effects on melting point and undercooling. Subsequently，scanning electron microscope （SEM，JSM-6480） equipped with energy dispersive spectroscope （EDS） was used for microstructure and composition characterization. And the phase analysis was conducted on an X-ray diffraction analyzer （XRD-6000） . Tensile tests of Sn-57Bi-x（0.1，0.9）Sb/Ag/Ni were implemented at a strain rate of 0.01 s⁻¹ at room temperature to investigate the influence of microelements on the mechanical behavior. The results showed that a small amount of Sb （Sn-57Bi-0.1Sb） mostly existed in solid solution form. When the Sb content reached 0.9% Sb，SbSn phase was found in XRD peak，no SbSn phase was observed under SEM though，indicating that Sb mainly existed as SbSn phase. In order to better understand the formation of SbSn phase，a vertical cross-sectional phase diagram of Sn-57Bi-（0~1）Sb was drawn. From the phase diagram，it was observed that when the Sb content was greater than 0.79%，the primary phase of SbSn would be formed. When the content was lower than this，it should be the precipitated phase of SbSn. Due to the low solid solubility of Ag element in Sn and Bi phases，adding 0.1% Ag to Sn-57Bi alloy revealed Ag₃Sn intermetallic compounds in the microstructure of the alloy. This compound was between the two phases of Sn and Bi，and was in the form of granules and short rods，small in size，generally not exceeding 5 μm. Diffuse distribution in β-Sn phase was quite moderate between the Sn and Bi. When the Ag content was 0.9%，the alloy β-Sn，the size of Sn dendrites significantly decreased，and at this time，Ag₃Sn compounds，as nucleation particles，were mainly distributed inside the Sn matrix in a long strip like aggregation，with a significant increase in size. Like Ag element，Ni added to Sn-57Bi alloy formed Ni₃Sn₄ intermetallic compound phase. When the content of Ni was 0.9%，the size of Ni₃Sn₄ intermetallic compound phase was coarse，even reaching above 200 μm. Generally，these three elements doping reduced the eutectic interlayer spacing and refined the structure，with Ag having the most significant refining effect. And the three elements doping all had strong strengthening effects，and their UTS was higher than that of Sn-58Bi alloy. The maximum tensile strength of Sn-57Bi-0.9Ag alloy was 75.08 MPa，which was 24.0% higher than that of Sn-58Bi eutectic. The minimum elongation of Sn-57Bi-0.9Ni alloy was 13.2%，which was 54% lower than that of Sn-58Bi eutectic. So both Ni and Ag elements strengthened the alloy while damaging the elongation. Although Sb element had a relatively small effect on the tensile strength and elongation of the alloy，it could balance the strength and plasticity indicators. While improving the tensile strength，it also enhanced the elongation of the alloy，Sn-57Bi-0.1Sb alloy had more advantages as a photovoltaic solder alloy.