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Final Report Summary - NANOTESULPHIDE (Spark Plasma Sintering Nanostructured Thermoelectric Sulphides)

Based on the progresses made in Queen Mary university of London, my European host university, I set up my research lab and group in Henan Polytechnic University and continued the research topics focusing on thermoelectric sulphides.
First we continued the work of discovering and designing of compounds with intrinsically low thermal conductivity, especially compounds with special bonding nature and stable crystal structure. This project revealed unambiguously the origin of the role of the lone pair electrons on lattice thermal conductivity in Cu-Sb-S compounds by correlating the special bonding on Sb site with the phonon dispersion spectrum and density of state. By substitution of Sb by transition metal Fe, Co and group IIIA element Ga, In without s2 electrons, lone-pair electrons on some of the Sb sites were removed, which created a scenario with opposite influences on lattice thermal conductivity from the loss of lone-pair electrons and gain of alloy scattering. We investigated the competition between the alloy phonon scattering and the extra phonon scattering mechanism linked to lone-pair electrons on trivalent Sb3+ sites in chalcostibite CuSbS2, which is a model system for benchmarking and quantifying the impact of lone-pair on the lattice thermal conductivity of Cu-Sb-S compounds. A significant deviation from the classic alloy model was observed. Along with the impact of the lone-pair electrons on the bonding arrangement and crystal structure, the role of lone-pair electrons in the phonon transport of the TE compound CuSbS2 was well demonstrated and quantified. Two Sb-related quasi-single-frequency vibration modes behaving like localised Einstein harmonic oscillators were discovered and correlated to the bonding circumstance around Sb sites. These results give unequivocal evidence that the trivalent VA atom creates special bonding and vibration modes because of its nonbonding 5s lone-pair electrons.
Thermoelectric materials with phase transitions and strong vibrational anharmonicity have been identified with very promising figure-of-merits, such as SnSe, Cu2-xSe, and provide a new direction in the search for low thermal conductivity and high performance TE materials. However, a phase transition within the working temperature range inevitably deteriorates the joint between the TE legs and the substrates. Therefore it is necessary to stabilise the phase structure to ensure a sustainable connection between the TE materials and substrates while taking advantage of the low lattice thermal conductivity in these materials. Cu3SbS3 is a copper-based sulfide with strong vibrational anharmonicity, a member of Cu-Sb-S system with lone pair electrons on Sb sites and three-coordinated Cu ions exhibiting large vibrational amplitude. It is composed of earth-abundant elements with three well defined temperature dependent polymorphs. Cu3SbS3 may also form an additional high temperature cubic structure similar to that of the well-known tetrahedrite (Cu3SbS3.25), with the S atom at the centre of the unit cell missing. In this study, we tried to stabilize the cubic structure by incorporation of Fe atoms into Cu3SbS3. The phase evolution with increasing Fe concentration was discussed. The thermodynamic stability of the materials was elaborated up to 350 oC in both air and Ar atmospheres. The improved TE performance and the electron band structure were also identified and correlated. Our results firmly demonstrate the possibility of stabilising the cubic structure of Cu3SbS3 material by Fe incorporation and highlight it as a strong candidate for high performance TE materials.
To elaborate the thermodynamic stability of the cubic Cu3SbS3 compound, variable temperature XRD and DSC were performed on the Fe, Ni and Co stabilized samples. The results show the cubic structure is stable in flow Ar atmosphere up to 350oC.
Flash sintering using direct current with a heating rate of 1000 oC/min was performed on materials other than MSS in a SPS furnace. This part of work is in collaboration with QMUL and progresses very well. We are in confidence that flash sintering is capable of maintaining the nanostructure of powders and optimisation the distribution or removal of grain/agglomerate surface oxidation.
Now we are working with Wuhan University of technology and our industrial collaborators to confirm the thermal stability of the materials by evacuating the long-time performance of the demonstration modules and devices. All the derivative results will be published in scientific journals, and intellectual properties be protected by applying patents.

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