Thomaz Augusto Guisard Restivo

Cu-Ni-YSZ anodes for solid oxide fuel cell by mechanical alloying processing

Abstract The work shows some results concerning a new cermet material 40 vol.% [(Cu)–Ni]–YSZ processed by mechanical alloying followed by Sintering by Activated Surface method. The projected cermet microstructure for this application is expected to possess microstructural characteristics that lead to better electric and ionic percolating, higher electrocatalytic activity and fuel reforming. The powder samples prepared by mechanical alloying optimized conditions show a homogeneous mixture. Transmission and scanning electron microscope analysis have demonstrated the powder particles are nanosized after 2 h of milling, showing lamellar internal structure aggregates. Suitable sintered pellets are obtained from these powders, within the required porosity and microstructure. Sintering kinetics studies for pellets of Ni–YSZ and Ni–Cu–YSZ indicate 2-step sintering processes. Copper additive promotes sintering and refines the microstructure.

TG/DTA-MS evaluation of methane cracking and coking on doped nickel-zirconia based cermets

Cermet materials based on metallic nickel and cubic zirconia are the key material for applications on solid oxide fuel cells and high temperature water electrolysis. The main advantage is the possibility of direct feeding a hydrocarbon fuel, like methane, or even an alcohol as a source of hydrogen. The reforming reaction on the Ni catalyst surface can produce hydrogen continuously. However, the resulting catalyst poisoning by carbon deposition (coking) imparts their broad application. The work shows the evaluation of coking tolerance of some cermets prepared by mechanical alloying techniques and compares new additives specially chosen in order to avoid coking and increase the catalytic activity. Refractory metal additives besides copper were added to the basic cermet. While copper is a known doping agent that avoids coking, the refractory metals (Mo and W) have a twofold effect: promote sintering at lower temperatures and increase Cu activity due to their mutual immiscibility. Results of TG/DTA-MS analysis demonstrate both refractory metals have increased the coking tolerance as well as the catalytic activity during diluted methane cracking. Molybdenum and tungsten additives are promised regarding the improvement of these cermet materials for high temperature electrochemical devices.

Advanced Multi-Metallic SOFC Anode Development by Mechanical Alloying Route

Anodes composed of Ni-YSZ (yttria-stabilised zirconia) cermets are the key material to allow direct biofuel feeding to Solid Oxide Fuel Cell (SOFC) devices due to its internal reforming capability. The main challenge among these materials is related to carbon deposition poisoning effect when C-bearing fuels are feed. The work deals with these issues by alloying Ni with some metals like Cu to conform a multi-metallic anode material. Mechanical alloying (MA) at shaker mills is chosen as the route to incorporate the metal and ceramic powders in the anode material, also leading to better sintering behaviour. A projected cermet material is conceived where a third metal can be added based on two criteria: low Cu solubility and similar formation enthalpy of hydrides regarding Ni. Refractory metals like Nb, W and Mo, seems to fulfil these characteristics, as well as Ag. The MA resulted powder morphology is highly homogeneous showing nanometric interpolated metal lamellae. The sintering behaviour is investigated by conventional dilatometry as well as by stepwise isothermal dilatometry (SID) quasi-isothermal method to determine the sintering kinetic parameters. Based on these tools, it is found the Cu additive promotes sintering to obtain a denser anode and therefore allowing lower process temperatures. The consolidation is achieved through the sintering by activated surface (SAS) method allied to liquid phase sintering process, where the third metal additive also has influenced. The final cermet can be obtained at one sole process step, dispensing pore-forming additives and reduction treatments. The sintered microstructure demonstrates the material is homogeneous and possesses suitable percolation networks and pore structure for SOFC anode applications.

New Integrated Cermet Powder Preparation and Consolidation Method–Ni-ZrO2 Case

A new integrated method for direct preparation of cermet materials is proposed consisting of a powder processing method allied to a special sintering step. The powder is obtained by mechanical alloying route where a specific morphologic design is searched to yield thin metal plated ceramic particles. These have the proper characteristics to engage the sintering by activated surface (SAS) consolidation method. The last is triggered by partial evaporation and reactive sintering of thin metal layers, therefore exposing high active surfaces with superior sinterability. Refractory sacrifice metal components are found to play an important role. The application of the integrated method to Ni-ZrO2 cermet with selected metal additives is investigated. Sintering temperatures can be reduced by more than 300°C for the same final density range. The resulted powders and pellets microstructures are analysed accordingly to the projected expected ones. The thermophysical and electrical properties measurements are performed for evaluate phases percolation.

ESTUDO CINÉTICOS DA REDUÇÃO DE PELOTAS DE POEIRA DE ACIARIA LD POR HIDROGÊNIO

The generation of basic oxygen furnace dust is a problem that companies found in the steel industry. In this way, studies are important to obtain, or at least indicate a way to recycle this waste. The direct reduction can be an alternative, once this method produces rich in iron. Thus, the goal of this paper is study the kinetic of reduction of basic oxygen furnace dust pellet using a mixture containing hydrogen and argon. Thereby, the mechanism and apparent activation energy that involve the reduction can be found out. The kinetic analysis was carried out applying the Forced Stepwise Isothermal Analysis method. The temperatures used in the test were between 500°C to 1,100°C. The results showed that reduction occurred in three steps. In the first step, nucleation was the control mechanism. It was found an apparent activation energy about 24kJ/mol in this step. In the second and third stage, the control mechanism was diffusion. In the second step, the apparent activation energy was 54.9 kJ/mol, while in the third step the apparent activation energy was 80.7 kJ/mol

Competitive Precipitation and Recrystallization in U-7.5Nb-2.5Zr Alloy

Abstract. Metallic nuclear fuel plates are nowadays an alternative to the ceramic ones in the sense that the uranium density can be increased at lower enrichment. Higher thermal conductivity is also a key factor favouring such fuels for power reactors. Uranium reacts promptly with oxygen and nitrogen at high temperatures to catastrophic corrosion due to non-protective oxide layers, which imparts hot forming processes. The gamma phase body centred cubic structure can be retained at room temperature by annealing the U-7.5Nb-2.5Zr (wt.%) alloy followed by quenching, where the deformation can be extensive. The resulted highly deformed gamma supersaturated structure is subjected further to competitive recovery/recrystallization and phase precipitation phenomena whose are studied in the work. The U-7.5Nb-2.5Zr alloy was melted into plasma and induction furnaces and afterwards annealed to gamma phase. The normalized alloy was cold rolled and underwent isochronal and isothermal treatments. The microstructure evolution was monitored by optical microscopy, X-ray diffraction analysis and hardness measurements. The results show the precipitation events of α” and α+γ3 phases are dominant over recovery in the range 200oC < T < 500oC. Above 500oC the recrystallization is the main process leading to softening and initial Vickers hardness recovery. One refined gamma phase grain structure was obtained (~8.0 μm) after annealing at 700oC for 2.5 hours.