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Thermal transport properties are critical for applications ranging from thermal management to energy conversion. Passive thermal management has been an area of study for over a century and has only grown as technology has advanced because it requires no additional energy to remove heat. Changing the nanostructure of the materials involved in passive heat transfer methods, either by geometric changes or stoichiometric changes, can greatly improve the effectiveness of this heat transfer method. In order to explore this further, this work employs LAMMPS molecular dynamics (MD) simulation software to calculate the lattice thermal conductivity (��p) of a nanoparticle (NP) and material used indifferent passive heat transfer methods after either modifying their geometry or stoichiometry. The NPs this work will simulate are single-wall carbon nanotubes (SWCNTs), which have been well known for high ��p, and their applications in improving thermal conductivity in matrix materials. The material this work will simulate is magnesium silicide (Mg2Si), a thermoelectric material. Thermoelectric materials, in general, become more efficient in converting heat into electrical power as their ��p decreases. ��p will be calculated for SWC-NTs of varying lengths, diameters, and at varying equilibration temperatures (Teq). ��p will be calculated for samples of pure Mg2Si and Mg2Si with off-stoichiometry over a range of Teq values. Two methods will be used to induce the off-stoichiometry: atomic silicon (Si) substitutionals, and Si NPs. A range of stoichiometric ratios will be applied to the material by both methods, and then ��p will be calculated for each of these cases. This is done so as to observe which method of stoichiometric change, given the same stoichiometric ratio, decreases ��p greater, and, therefore, causes Mg2Si to be a better thermoelectric material. It is expected that increases in length will increase the ��p of the SWCNT, while increases in diameter and Teq will decrease ��p. It is expected that increases in atomic percent (a/o) Si and Teq will decrease ��p regardless of the method of stoichiometric change, and that the Si NP method will decrease ��p more than the atomic Si substitutional method.

The interest in thermoelectric materials raised in the recent years due to the possibility to design thermoelectric generators (TEGs), reliable solid-state devices for waste heat recovery valuable in particular contexts where the employment of other technologies results inconvenient. Nowadays, the challenge in the thermoelectric field seems to be the identification of efficient thermoelectric materials which should be also inexpensive, easy to synthesize, and comprised of earth-abundant elements. On this basis, tetrahedrite mineral family (Cu12-xTrxSb4S13 where, for instance, Tr could be Mn, Fe, Co, Ni, or Zn), one of the most widespread sulfosalts on Earth's crust, seems to meet the right features for an attractive sustainable p-Type Pb-free thermoelectric material showing relatively high conversion efficiency [1,2]. The sulphide precursor powders were ball milled and then one-step simultaneous synthesis and sintering process was performed by open die pressing (ODP) [3]. By this simple, fast, and easy scalable route, thermoelectric pellets with a whole process lasting less than 6 hours. Both Zn and Ni cations were added as partial substituents of Cu in Cu12Sb4S13 permitting to considerably improve the thermoelectric performances of the undoped material prepared by ODP. The influence of precursor ratio on the tetrahedrite phase, secondary phase content and stoichiometry were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The density and mechanical stability of samples were evaluated as a function of the chemical composition and processing parameters. A complete thermoelectric characterization was carried out for samples with different Zn and Ni substitution up to 400 °C.

Tetrahedrite mineral family, one of the most widespread sulfosalts on Earth's crust, seems to meet the right features for an attractive sustainable p-Type Pb-free thermoelectric material, showing relatively high conversion efficiency, achieving a figure of merit value of almost 1 at 450°C [1,2]. In this work, Ni-Zn co-doped tetrahedrite samples were prepared by Open Die Pressing one step reactive consolidation method [3], employing previously ball milled sulphide powder precursors, in order to identify a simple cheap scalable fast production route for thermoelectric pellets. The influence of dopant presence and reactive consolidation process parameters on the tetrahedrite phase content, stoichiometry and density of the samples were investigated by X-ray diffraction, scanning electron microscope (equipped with energy dispersive X-ray spectroscopy), and thermogravimetric analysis. Laser Flash Analysis, electrical conductivity, and Seebeck coefficient measurements were carried out for obtaining a complete thermoelectric characterization. Acknowledgments The authors are grateful to Stefano Boldrini (CNR-ICMATE) for the valuable discussion about the results, to Enrico Bassani (CNR-ICMATE) for his essential technical supports, and Raul Carampin (CNR-IGG) for the inestimable help, advices and data discussion about the quantitative EDS analyses. This work has been funded by the Italian National Research Council - Italian Ministry of Economic Development Agreement "Ricerca di sistema elettrico nazionale". References: [1]X. Lu, D.T. Morelli, Physical Chemistry Chemical Physics 15 (2013) 5762. [2]X. Lu, D.T. Morelli, Y. Xia, V. Ozolins, Chemistry of Materials 27 (2015) 408. [3]S. Ceresara, C. Fanciulli, F. Passaretti, D. Vasilevskiy, Journal of Electronic Materials 42 (2013) 1529.

Nowadays, the real challenge in the thermoelectric field seems to be the identification of efficient thermoelectric materials which should be inexpensive, easy to synthesize, and comprised of Earth-abundant elements. On this basis, tetrahedrite mineral family (Cu12-xTrxSb4S13 where Tr= Cu, Mn, Fe, Co, Ni, Zn), one of the most widespread sulfosalts on Earth's crust, seems to meet the right features for an attractive p-Type Pb-free thermoelectric material, showing a relatively high conversion efficiency [1, 2]. Working on the synthetic geomimetic tetrahedrites is useful to improve the knowledge about this peculiar material, leading to understand how enhance it and how to exploit the natural tetrahedrite mineral more effectively. In this work, both Zn and Ni cations were employed as substituents, according to the stoichiometry Cu10Zn2-xNixSb4S13, by means of ball milling treatment of sulphide precursor powders and one step simultaneous synthesis and sintering by Open Die Pressing [3]. This process is a scalable simple and fast route that lasted few hours, a very short time if compared with those reported in literature so far. The influence of the stoichiometry, the synthesis conditions and the reactive consolidation process parameters on the tetrahedrite phase content and density of the samples were investigated by X-ray diffraction (with a profile Rietveld refinements) and scanning electron microscope (equipped with energy dispersive X-ray spectroscopy). Furthermore, the thermoelectric functional properties of the samples were investigated by electrical conductivity and Seebeck oefficient measurements and Laser Flash Analysis for the thermal conductivity.