RESUMEN
We develop a thermoelectric generator based on catalytic combustion and operating in the low power range (up to 10 W). Considering the target of small-scale thermoelectric generators, the additive technique was chosen as an enabling technology to customize the different parts of the presented device. The generator consists of a hexagonal shaped combustion chamber coupled to commercial thermoelectric modules, water-cooled at the cold side. Thanks to the components design, heat transfer across each part of the system is properly driven enhancing the thermal management of the system. Moreover, in order to improve the overall efficiency, exhausts outlet is designed to promote heat recovery. The generator is characterized achieving an electrical power output close to 9 W in continuous regime, with an overall efficiency of 3.55%. The compact size, the light weight, the simple design and the reliability in continuous operating conditions are all promising features of the device described. Furthermore, the materials chosen for the device can suggest a way to fabricate cheaper heat exchangers, actually one of the main costs of the device development.
RESUMEN
The half Heusler TiNiSn compound is a model system for understanding the relationship among structural, electronic, microstructural and thermoelectric properties. However, the role of defects that deviate from the ideal crystal structure is far from being fully described. In this work, TiNi1+xSn alloys (x= 0, 0.03, 0.06, 0.12) were synthesized by arc melting elemental metals and annealed to achieve equilibrium conditions. Experimental values of the Seebeck coefficient and electrical resistivity, obtained from this work and from the literature, scale with the measured carrier concentration, due to different amounts of secondary phases and interstitial nickel. Density functional theory calculations showed that the presence of both interstitial Ni defects and composition conserving defects narrows the band gap with respect to the defect free structure, affecting the transport properties. Accordingly, results of experimental investigations have been explained confirming that interstitial Ni defects, as well as secondary phases, promote a metallic behavior, raising the electrical conductivity and lowering the absolute values of the Seebeck coefficient.
RESUMEN
The thermoelectric behavior and stability of Cu2SnS3 (CTS) has been investigated in relation to different preparations and sintering conditions, leading to different microstructures and porosities. The studied system is CTS in its cubic polymorph, produced in powder form via a bottom-up approach based on high-energy reactive milling. The as-milled powder was sintered in two batches with different synthesis conditions to produce bulk CTS samples: manual cold pressing followed by traditional sintering (TS), or open die pressing (ODP). Despite the significant differences in densities, ~75% and ~90% of the theoretical density for TS and ODP, respectively, we observed no significant difference in electrical transport. The stable, best performing TS samples reached zT ~0.45, above 700 K, whereas zT reached ~0.34 for the best performing ODP in the same conditions. The higher zT of the TS sintered sample is due to the ultra-low thermal conductivity (κ ~0.3-0.2 W/mK), three-fold lower than ODP in the entire measured temperature range. The effect of porosity and production conditions on the transport properties is highlighted, which could pave the way to produce high-performing TE materials.
RESUMEN
We present the first candidate for the realization of a disorder-induced Topological Anderson Insulator in a real material system. High-energy reactive mechanical alloying produces a polymorph of Cu2ZnSnS4 with high cation disorder. Density functional theory calculations show an inverted ordering of bands at the Brillouin zone center for this polymorph, which is in contrast to its ordered phase. Adiabatic continuity arguments establish that this disordered Cu2ZnSnS4 can be connected to the closely related Cu2ZnSnSe4, which was previously predicted to be a 3D topological insulator, while band structure calculations with a slab geometry reveal the presence of robust surface states. This evidence makes a strong case in favor of a novel topological phase. As such, the study opens up a window to understanding and potentially exploiting topological behavior in a rich class of easily-synthesized multinary, disordered compounds.
RESUMEN
The present work focused on the microstructural, thermal, electrical, and damping characterization of NiMnGa samples produced through a powder pressing and a sintering process; the effect of sintering times and of the starting powder size were evaluated. Moreover, an observation of the evolution of martensitic transformation typical of NiMnGa ferromagnetic shape memory alloy was conducted in comparison with the cast material behavior and in correlation with the material densification. The optimum powder size and sintering time for the process, i.e., 50 µm or lower and 72 h, were identified considering the investigated physical properties of the sintered samples in comparison to the cast material. The corresponding sample showed the best compromise between density, thermal and electrical properties, and damping and functional behaviour. In general, the outcomes of this study could be the basis of a useful tool for production processes that include a sintering step as well as being a starting point for the evaluation of an alternative low cost fabrication method of this alloy.
RESUMEN
Bulk samples of kesterite (Cu2ZnSnS4, CZTS) were produced by cold-pressing and sintering of CZTS powders obtained via reactive ball-milling. An increase in the Seebeck coefficient of more than 100 µV/K, almost doubling the expected value, is noticed around a temperature of 260 °C. As pointed out by thermal analyses, this is due to a second order transition of kesterite from an ordered I-4 to a disordered I-42m crystal structure. Conversely to what happens for solar cell materials, where the transition is considered to be detrimental for the performance, it appears to be beneficial for the thermoelectric Seebeck coefficient, suggesting that higher crystal symmetry and cation-disorder due to the transition lead to thermopower enhancement.
RESUMEN
In this work the thermoelectric generator (TEG) based on catalytic combustion already developed in our lab has been further investigated and improved. The system made of two thermoelectric (TE) modules coupled with a catalytic combustor has been used in this work to obtain higher overall efficiency by adding hydrogen to the fuel mixture. Since implementation of hydrogen as a fuel has shown low and stable combustion temperature in literature, it is expected to achieve good overall efficiency of TEG. Moreover, hydrogen can be used to improve the system inducing self-ignition. Focus of the present work is the implementation of different mixture proportions, varying the amount of hydrogen, and the investigation of their effects on the overall efficiency. The overall TEG efficiency, has been evaluated by parallel characterization of thermoelectric modules and exhaust gases composition. The system performances have been characterized using different mixtures: the results indicate that addition of H2 to the fuel contribute to increase the chemical and overall TEG efficiency respect to previous work, producing up to 5.92 W of electrical power. Finally, the effects of H2 for on self-ignition conditions have been investigated finding the minimum H2 amount for different gas flow rates.
RESUMEN
Nowadays, a big challenge in the thermoelectric field is the identification of efficient thermoelectric materials but inexpensive, easy to synthesize, and comprised of Earth-abundant elements. On this basis, tetrahedrite mineral family (Cu(12−x)Tr(x)Sb4S13 where Tr = Cu, Mn, Fe, Co, Ni, Zn) seems to be an attractive p-Type Pb-free thermoelectric material, showing a relatively high conversion efficiency. In this work, a solvothermal synthesis method was developed for undoped tetrahedrite Cu12Sb4S13 and the introduction of Zn and Ni as substituents of copper was also tested. The influence of the stoichiometry and the synthesis conditions on the tetrahedrite phase content and density of the samples were investigated by X-ray diffraction (with profile Rietveld refinements) and scanning electron microscope (equipped with energy dispersive X-ray spectroscopy). Preliminary sintering test were performed by Open Die Pressing.
RESUMEN
A structural study of the filled skutterudite Smy(FexNi1-x)4Sb12 was performed by means of X-ray powder diffraction and µ-Raman spectroscopy with the aim to unveil the correlations between structural and electronic properties of this material and to favor the improvement of its thermoelectric performance. Samples were prepared by direct reaction of the elements at 1223 K, followed by quenching and subsequent sintering at 873 K; microstructure and composition of the obtained products were determined by SEM-EDS. The position of the boundary separating regions that obey hole- and electron-based conduction mechanisms was found by X-ray diffraction at x ≈ 0.63 and y ≈ 0.30, confirmed by measurements of room-temperature Seebeck coefficient, and discussed on the basis of crystallographic data. The presence of a discontinuity is observed in several structural and spectroscopic parameters at the p/n crossover; it is interpreted as associated with the change in the conduction mechanism. The role of the rare earth filling fraction in driving the structural response of the material is investigated too. The advantage of using X-ray diffraction and µ-Raman spectroscopy as aids in the study of electronic properties of this material is highlighted, as well as the complementarity of the two techniques.
