Detachable Contacts with Low Melting Solders for High-Temperature Thermoelectric Characterization of Cobalt-Doped Iron Disilicide

Materials characterization is a prerequisite for the research and development on thermoelectric (TE) materials. The key properties to determine a TE material´s performance are the electrical and thermal conductivities, and the Seebeck coefficient. A measurement facility for combined thermoelectric measurements (CTEM) is under development that will allow the three properties to be measured simultaneously on one sample and in one temperature sweep. However, the contact between the thermoelectric sample and the facility´s sample holder is crucial for an accurate measurement. Besides low thermal and electrical contact resistances, the contact scheme should remain chemically stable during several measurement cycles in the CTEM and should be detachable. To fulfill the CTEM contact requirements, solders with melting points below 200 °C based on bismuth, indium and/or tin were selected as a joint between the metal blocks of the sample holder and the sample under investigation. Especially for the measurement of high-temperature TE materials, the contacting method in the CTEM should be adapted for the required temperature range, i.e. up to 600 °C. This study specifically focuses on the contact development for FeSi2 as a TE material which is chemically and functionally stable up to 800 °C and which has been certified as a reference material for Seebeck measurement up to high temperatures.

The quality of the soldered joints was investigated by measurements of the temperature dependent electrical contact resistance up to 600 °C which was determined under vacuum with an in-house built facility. This was done by a four-point measurement of the resistance between metal blocks attached from both sides to the base faces of the prismatic sample and subsequent subtraction of the sample´s net resistance contribution. Transient changes of the contact resistance were traced as an efficient indicator of changes at the contact such as chemical reactions but also delamination or cracking, dissolution or diffusion.  After temperature cycling, the microstructure of the contact cross section was analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX).

Selected contacting schemes are presented within this study. Copper and molybdenum have been tested as materials of the sample holder blocks. Due to its predisposition to diffuse and the lack of an efficient diffusion barrier, copper was proved unsuitable as a block material for high temperature operation. On the other hand, molybdenum revealed suitable stability with the contact’s constituents. Among other indium-based solders, Field’s metal (FM) alloy (Bi32.5In51Sn16.5) was identified as the best candidate to solder the sample to the blocks due to its chemical stability with the contact constituents and its low melting point (Tm = 62 °C) making the sample dismounting after measurement easy. Nevertheless, experiments proved that molybdenum soldered to FeSi2 with FM was not electrically stable for the CTEM operation due to the formation of an insulating SiO2 nanolayer at the FM/FeSi2 interface above 370 °C. To inhibit the insulating effect of the SiO2, chromium or titanium were introduced into the assembly either by coating on the contact surfaces or by dissolution in the FM. Although the reaction mechanisms caused by each of chromium and titanium were different, the consequences were significantly the same with the inhibition of the formation of SiO2 allowing a direct contact between the solder and the FeSi2. Above 390 °C, the liquid solder superficially corroded the FeSi2 by a silicon leaching leading to the formation of a porous FeSi layer on the sample contact surface. The solubility of silicon in indium is low, inducing a rapid saturation and thus an inhibition of the corrosion. Above 550 °C, silicon dissolved in the solder starts to react with the additive, i.e. with chromium or titanium, and the molybdenum blocks. The consumption of silicon reduced the silicon content in the solder which allowed further silicon dissolution from the FeSi2. Nevertheless, all processes go on in the interface zone between block and sample, i.e. close to the sample surface, since the thickness of the FeSi porous layer stabilized around 30 µm after repeated temperature cycling up to 600 °C; in this way, silicides formation tended to a limit which stabilized the FeSi2 corrosion. The TE properties of the FeSi2 sample were measured before and after exposure at 600 °C. The comparison revealed that the sample properties did not significantly change.

The contacting scheme formed with the molybdenum blocks soldered to the FeSi2 sample with titanium activated FM is considered as the most appropriate for the CTEM operation, among the four operational schemes explored in this study. Due to the titanium’s high reactivity with oxygen, the formation of an SiO2 layer at the contact interface is prevented avoiding a strong increase of the contact resistance during the first heating cycle and following. From a practical point of view, the use of an activated solder is economically and technically more efficient compared to the application of coating processes.


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