Reaction flash sintering for producing high quality functional ceramics within seconds

For ceramic materials, it has been recently shown in literature that applying a small electric field and a small DC current through a sample produces sudden sintering (within seconds) at relatively low temperatures. This method is known as Flash Sintering and it has been applied to number of materials. In this work it is shown that both chemical reaction and sintering can be combined into a single flash sintering experiments. This new approach is known as Reaction Flash Sintering. To demonstrate the feasibility of this method, a multiferroic material, BiFeO3, is prepared from a stoichiometric mixture of Bi2O3 and Fe2O3 oxides. Thus, in a single process, dense nanostructured BiFeO3 ceramics are obtained by applying an electric field of 50 V cm-1 and with a current limit of 35 mA mm-2 within seconds at a furnace temperature of about 625 °C. The resulting materials were pure-phase perovskites without any evidence of secondary phases, sillenite or mullite, that are commonly present in materials prepared by conventional procedures. Moreover, samples were electrically insulating, as measured by complex impedance spectroscopy. It is shown here that the synthesis of pure single-phase ceramics of complex oxides from stoichiometric mixtures of single oxides is possible by reaction flash sintering, even for materials difficult to prepare by conventional procedures. This discovery is a breakthrough in materials preparation

Vliv DSC tepelné setrvačnosti na vývoj krystalizační kinetiky

Influence of added thermal resistance on crystallization kinetics, as measured by differential scanning calorimetry (DSC), of the Se70Te30 glass was studied. The increase of thermal resistance was achieved by adding polytetrafluorethylene discs of different thicknesses (up to 0.5 mm) in-between the DSC platform and the pan with sample. Increase of the thermal resistance led to an apparent decrease (by more than 30%) in the crystallization enthalpy. Significant change of model-free kinetics occurred: apparent activation energy E of the crystallization process decreased (by more than 20%) due to the DSC data being progressively shifted to higher temperatures with increasing heating rate. The model-based kinetics was changed only slightly; the DSC peaks retained their asymmetry and the choice of the appropriate model was not influenced by the added thermal resistance. The temperature shift caused by added thermal lag was modeled for the low-to-moderate heating rates.Pomocí diferenční skenovací kalorimetrie byl sledován vliv přidaného tepelného odporu na měřená kinetická data pro krystalizaci sklovitého Se70Te30

Pressure Effect on the Multicycle Activity of Natural Carbonates and a Ca/Zr Composite for Energy Storage of Concentrated Solar Power

This work is focused on the use of the Calcium-Looping process (CaL) in Concentrated Solar Power (CSP) plants for Thermochemical Energy Storage (TCES). Cheap, abundant and nontoxic natural carbonate minerals, such as limestone and dolomite, can be employed in this application to store energy through the cyclic calcination/carbonation of CaCO₃. In a recent work, a closed CO₂ cycle has been proposed for an efficient CaL-CSP integration in which the CO₂ in excess effluent from the carbonator is used to generate electricity by means of a gas turbine. Process simulations show that the thermoelectric efficiency is enhanced as the carbonator pressure and temperature are increased provided that the multicycle CaO conversion is not affected. On the other hand, the use of just one reactor for both calcination and carbonation has been suggested to reduce capital cost. However, the experimental results shown in the present work indicate that sintering is notably enhanced as the pressure in the reactor is increased. Such an adverse effect is mitigated for a ZrO₂/CaCO₃ composite with a low Zr content as compared to natural carbonates. These results are relevant to process simulations for better assessing the global efficiency of the CaL-CSP integration