Abstract
Ni- and Mn-doped tetrahedrites synthesized by a solid-state reaction and densified by hot-pressing were coated with boron nitride and two refractory silicone pastes. The materials were exposed to air for 1000 h and 500 h, while being heated at 648 K to test the protection efficiency of the coatings. Through x-ray diffraction analysis and scanning electron microscopy conjugated with energy dispersive spectroscopy, phases of antimony oxide and dicopper sulfide were detected in the materials coated with boron nitride. Similar secondary phases were also detected on the aged uncoated samples and Cu10.5Ni1.5Sb4S13 tetrahedrites coated with silicone pastes, whereas on silicone-coated Cu11Mn1Sb4S13, no traces of oxidation or degradation were observed. However, measurements of the Seebeck coefficient at 300 K on the manganese tetrahedrites indicated a reduction from 119(6) μV/K to ∼ 77(4) μV/K for the initial and aged coated samples (500 h), respectively, while the same material treated for 1000 h under argon atmosphere showed a much smaller decrease (114(6) μV/K), similar to the initial value within the experimental error. A deeper analysis by optical and electronic microscopy revealed the presence of a MnSO4 interlayer which affects the stoichiometry of the materials and, consequently, changes the Seebeck coefficient.
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