Protective Coatings for Cu 11 Mn 1 Sb 4 S 13 and Cu 10.5 Ni 1.5 Sb 4 S 13 Tetrahedrites
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https://doi.org/10.1007/s11664-020-08575-z Ó 2020 The Minerals, Metals & Materials Society
Protective Coatings for Cu11Mn1Sb4S13 and Cu10.5Ni1.5Sb4S13 Tetrahedrites RODRIGO COELHO,1 ELSA BRANCO LOPES,1 ´ NIO PEREIRA GONC and ANTO ¸ ALVES 1,2 1.—C2TN, Departamento de Engenharia e Cieˆncias Nucleares, Instituto Superior Te´cnico, Universidade de Lisboa, Estrada Nacional 10, 2695-066 Bobadela LRS, Portugal. 2.—e-mail: [email protected]
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) lV/K to 77(4) lV/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) lV/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. Key words: Thermoelectric materials, tetrahedrites, protective coatings, aging tests, Seebeck coefficient
INTRODUCTION Thermoelectric generators (TEGs) are devices with the capacity to generate electricity directly from a heat source using the Seebeck effect.1 These generators are constituted of multiple pairs, each pair containing two legs connected between them, one leg made of a p-type semiconductor and the other one of an n-type semiconductor. For power generation, the multiple pairs need to be connected electrically in series and thermally in parallel constituting a module.2 When one side of the module is placed close to a heat source, a thermal
(Received March 24, 2020; accepted September 23, 2020)
gradient is created between the two sides (hot and cold)3 and an electrical potential difference is generated. These devices have the advantages of being light, easy to transport, and able to be coupled to flat surfaces. They have no moving parts, can be connected in series for scalability and increased efficiency, and can generate energy from waste heat sources, which is very important for the development of a greener and more sustainable world.4 Many materials are used for the fabrication of TEGs, but most of them contain toxic or rare elements (e.g., Bi, Te, Pb,
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