Characterization of the glass‐coated CoSb3 thermoelectric material by electron microscopy

Abstract

Over the last years alternative sources of energy took on special significance. Durability, reliability and maintenance‐free operation make the devices for power generation based on thermoelectric materials very attractive. Beside high efficiency, one of the main requirements for thermoelectric materials is the stability of the properties during the long‐time operating at elevated temperatures. Doped cobalt triantimonides (CoSb 3 ) are used as components of thermoelectric devices at temperature range about 400–600 °C. The main difficulty of the CoSb 3 application is the degradation of its thermoelectric properties as a result of antimony sublimation and material oxidation at elevated temperatures. To prevent these processes protective coatings are foreseen. The objective of this work was to characterize the glass coating/CoSb 3 interface and to determine its influence on the CoSb 3 stability at elevated temperatures. The borosilicate glass coatings with different chemical compositions were applied on the CoSb 3 substrates by dipping and then fired in air at temperature up to 700 °C. To determine oxidation resistance, coated samples were oxidized at 600 °C in air. Then samples were examined by SEM/EDS and TEM/EDS. Merlin Gemini II of ZEISS (SEM) as well as a probe Cs‐corrected Titan 3 G2 60‐300 equipped with ChemiSTEM™ system were used to investigate the oxidized samples. TEM lamellae were prepared by FIB facility. Phase identification was performed by STEM‐EDS and SAED electron diffraction supported with the JEMS software. Surfaces, fractures facets and cross‐sections were analyzed to assess the quality of the coatings, adherence to the underlying substrate and glass coating/CoSb 3 interface structure. Depending on the chemical composition of the glass (the content of the network modifiers), different effects were observed at the glass coating/CoSb 3 interface. The results of the study showed that effective protection for CoSb 3 against oxidation at 600 °C in air was possible only if: No crystallization near glass/substrate interface occurred. During coating firing antimony oxides were formed and reacted with the glass. However, glasses with too high Sb 2 O 3 content (more than 50%) tends to crystallize. Air trapped in the voids in the crystallization zone caused degradation of the substrate by oxidation during annealing at elevated temperatures. Porosity at the glass coating/CoSb 3 interface caused by the antimony sublimation during firing was as small as possible (Fig. 1). Bonding mechanism due to mutual solubility of the glass to the substrate was involved. The Sb 2 O 3 and SiO 2 form an eutectic type phase diagram with low eutectic temperature. Increased antimony concentration in the inner part of the coating (Fig. 1) denotes the scale dissolving in the liquid phase. Mechanical bonding was developed (Fig. 2). Chemical bonding was of less importance in studied case. The Cu precipitates observed on the SEM/EDS maps near the interface indicated that following reaction could be involved: 3 CuO (glass) + 2 Sb (substrate) = Sb 2 O 3(glass) + 3 Cu. However the glass coating/CoSb 3 interface as shown in the TEM image (Fig. 3) is sharp and shows no transition zone, characteristic for the chemical bonding. Summarizing, the borosilicate glass with high titania content was found to be an effective protection for CoSb 3 during the exposure to air at 600 °C. The glass coating was an effective barrier for oxygen diffusion into the material and for antimony sublimation, therefore chemical and phase composition of the substrate was not affected by the oxidation. Good coating/substrate adhesion was ensured mainly due to mutual solubility and mechanical bonding mechanisms.