Abstract

INTRODUCCIÓNEl rendimiento de los materiales termoeléctricos a una temperatura dada, T, suele definirse mediante la figura de mérito (Z), donde Z es función del coeficiente de Seebeck (α), la conductividad eléctrica (σ) y la conductividad térmica (κ):De acuerdo con la ecuación [1], para incrementar la figura de mérito es necesario incrementar el coeficiente de Seebeck y la conductividad eléctrica y disminuir la conductividad térmica. Sin embargo, estas tres magnitudes dependen de la concentración de portadores, por lo tanto, es muy complicado introducir cambios en una de ellas sin alterar las demás.En la figura 1 se muestra la dependencia de la figura de mérito en función de la concentración de portadores. Se observa un máximo para concentraciones en torno a 10 20 cm -3 , que corresponde a semiconductores altamente dopados o degenerados (2). Los materiales basados en Bi 2 Te 3 se encuentran en este intervalo y son muy adecuados para la Actualmente el telururo de bismuto (Bi 2 Te 3 ) es el material termoeléctrico más ampliamente usado en sistemas de refrigeración comerciales o en la conversión de energía en torno a temperatura ambiente. Debido a su estructura laminar altamente anisótropa, el Bi 2 Te 3 es muy frágil y suele agrietarse fácilmente a lo largo de su plano basal. Se espera que el afino del tamaño de grano incremente su tenacidad, con la ventaja de que al mismo tiempo la figura de mérito termoeléctrica se vea incrementada. En este trabajo, polvos del compuesto Bi 2 Te 3 se han compactado mediante dos métodos convencionales y mediante deformación plástica severa bajo alta presión (3 GPa) usando la técnica HPT (torsión a alta presión, 1 giro de deformación). Se ha conseguido una densidad cercana a la teórica. La dureza y tenacidad de los compuestos se han ensayado mediante micro-y nano-indentación. Palabras clave: termoeléctrico, figura de mérito, dureza, conductividad, microestructura, prensado. Mechanical properties of bismuth telluride (Bi 2 Te 3 ) processed by high pressure torsion (HPT)Bismuth telluride, Bi 2 Te 3 , is the main thermoelectric material currently in use for commercial cooling devices or for energy harvesting near room temperature. Because of its highly anisotropic layered structure, Bi 2 Te 3 is very brittle, failing by cleavage along its basal plane. Refining its grain size is expected to increase its toughness with the advantage that, simultaneously, its thermoelectric "figure of merit" results increased. In this work, powders of the compound have been compacted by conventional methods as well as by severe plastic deformation under high pressure (3 GPa) using high pressure torsion (HPT, one turn at room temperature). Near-theoretical density has been achieved. The hardness and toughness of the compacts have been assessed by micro and nano-indentation.

Aluminum-silicon-magnesium alloys are commonly used in the automotive industry to produce structural components. Among usual quality controls of produced castings, microstructure characterization and determination of mechanical properties are the most critical aspects. However, important problems can be found when measuring mechanical properties in those areas of castings with geometrical limitations. In this investigation, a set of A356 alloys have been prepared and then used to manufacture test castings and automotive castings in a laboratory and in industrial conditions, respectively, using Low Pressure Die Casting (LPDC) technology. Test castings were used to predict secondary dendritic arm spacing (SDAS) by using thermal parameters obtained from experimental cooling curves. The results have been then compared to the ones found in the literature and improved methods for estimating SDAS from cooling curves have been developed. In a subsequent step, these methodologies have been checked with different industrial castings by using simulated cooling curves and experimentally measured SDAS values. Finally, the calculated SDAS values together with the Mg contents present in A356 alloys and the temperature and aging time data have been used to develop new models so as to predict the tensile properties in different areas of a given casting prototype. These developed models allow casters and designers predicting tensile properties in selected areas of a given prototype casting even during design and simulation steps and considering the processing variables expected in a given foundry plant. The structures of these new models have been described and experimentally validated using different processing conditions.