OSSIDI AD ALTA ENTROPIA (HEO) COME MATERIALI CERAMICI AVANZATI CON ENORMI POTENZIALITA'

Heating an equimolar mixture of Mg, Co, Ni, Cu and Zn binary oxides at sufficiently high temperature followed by quenching, a single phase compound (HEO) with the cations randomly distributed on one of the face centered cubic (FCC) sublattices of a rocksalt structure, the other one being occupied by oxygen can be produced. So, High entropy oxides HEO are based on a new, quite revolutionary concept of entropy stabilization (already applied to alloys): to stabilize a crystal structure that can differ from those of the constituent elements, thereby increasing the congurational entropy of the resulting compounds. This can allow to discover new intriguing functional properties, such as a colossal dielectric constant, which could make it very promising for applications as largek dielectric materials and a high initial discharge specific capacity of about 1585 mAhg1 and superior cycling stability that allow the application of HEO as anode in lithium ion batteries. It has been also reported that these materials possess big elastic modulus (108 GPa) and a high yield strength (323 MPa) that are similar to those measured for steels. In this work together with the five-elements sample, (All), the corresponding four-element oxides were also prepared and tested. These materials have been synthesized both via solid state synthesis, microwave assisted synthesis and through spark plasma sintering (SPS): the transition to a single phase occur at a different temperature, depending on how many and which cations are present in the composition. The progress of solid-state synthesis has been studied through XRD analysis, obtaining diffractograms every 100°C, until the achieving of a product as pure as possible. As regards syntheses via microwave and SPS, we have tried to find the best possible conditions to obtain a product with an acceptable degree of purity. The study of the reaction dynamics was also completed by recording DSC and TGA curves (with O2 and N2 flow), which showed an interesting behavior particularly towards cobalt oxide. For each synthesis we considered successful, a Rietveld analysis was performed, also investigating possible tetragonal distortions of the materials. SEM analysis conducted on the samples finds out micrometer rounded particles with a diameter between 0.9um and 7um. Together with the SEM measurement an EDS analysis was also conducted, detecting a good cation distribution and almost no element loss. Only for the SPS samples we observed the presence of segregations of single elements. Raman analyses were also conducted to determine purity and homogeneity of the products, determining any segregations, or unreacted oxides. Through the SPS, we obtained half-centimeter pellets that were used for electrical analysis, such as impedance spectroscopy to determine the conductivity and a further analysis to determine the Seebeck coefficient. A semiconductor behavior and very high Seebeck coefficient was determined. We can also calculate a small power factor of about 2*10^-2 uW/m*K^2 at 700°C. These measurements were performed at various Temperatures both at Trento University and also through a homemade apparatus in Pavia university. In conclusion, we can say that this new class of materials has very interesting electrochemical properties, which combined with relative ease of synthesis could make these materials at the cutting edge in multiple fields of application.