Estimating ionic conductivity during flash sintering of 8ysz

We provide evidence for temperature-driven flash sintering process, accounting for otherwise interpreted as abnormal behavior. Using 8%-Yttria-doped Zirconia as reference material, we selected moderate flash conditions to avoid external current-limiting control. Samples were dry-pressed and pre-sintered to relative densities between 50 and 85 %TD by interrupting sintering at pre-determined shrinkage values, in conventional and flash sintering processes. SEM characterization shows similar trends for average grain size, porosity levels and distribution between flash and conventional samples. Bulk and grain boundary conductivity and characteristic frequencies, measured by impedance spectroscopy, show similar behavior regardless of the processing route. A third series of samples were pre-sintered and then flash sintered. The conductivity was measured as a function of relative density for different temperatures. These data were used to derive a scaling-law for flash sintering, which expresses sample conductivity versus temperature, corrected for instantaneous density. It replicates the signature current runaway during flash sintering which has been associated with unconventional sintering mechanisms. We conclude instead that it derives simply from the interdependence of conductivity with densification and sample self-heating through joule effect. A rough estimate of sample temperature can convert this non-linearity to a well-known Arrhenius behavior. Results apply equally well to previously published data, both in isothermal or constant heating conditions

Preparação e caracterização do supercondutor MgB2

Mestrado em Ciência e Engenharia de MateriaisAmostras supercondutoras de MgB2 foram produzidas seguindo os métodos ex situ e in situ e posteriormente caracterizadas. As amostras ex situ foram obtidas recorrendo a sinterizações a 950ºC numa prensa isostática a quente (HIP) sob pressões de 30, 50 e 190MPa. Nestas condições obtiveram-se valores de densidade até 98% da densidade teórica do material. Foram usadas técnicas de XRD, SEM, TEM e EDS para caracterizar as amostras que revelaram uma melhoria do controlo sobre a formação de fases secundárias. Recorrendo a uma técnica de encapsulamento em vidro as quantidades finais de MgO foram limitadas a ~10% (em massa) e a formação de MgB4 foi impedida. As amostras in situ foram obtidas através do encapsulamento de misturas de pós de Mg e B nas proporções molares de 1:2, 1.2:2 e 1.5:2 e posterior sinterização em vácuo num forno de grafite. Deste processo resultaram amostras porosas e com fraca resistência mecânica. Técnicas de SEM, EDS e XRD revelaram grandes quantidades finais de MgO enquanto que a formação de MgB4 foi evitada. As amostras ex situ são supercondutoras com valores de Tc~37K e respectivas larguras de transição na ordem dos 0.5 K. Para além destes resultados, as medidas eléctricas e magnéticas permitiram também determinar os valores de densidade de corrente crítica, Jc, do material. Contrariamente ao Tc, o Jc parece ser sensível às condições de processamento e foram obtidos valores que variam entre 0.37 e 3.89 x106 A/cm2 a 10K. ABSTRACT: Superconducting bulk samples of MgB2 were produced, using ex situ and in situ processing routes, and characterized. Ex situ samples were obtained by hot isostatic pressing (HIP) under pressures up to ~200MPa at 950ºC. In these conditions, full densification of samples was obtained (~98% of theoretical density). SEM, TEM, EDS and XRD analysis on final dense bodies were used to evaluate samples. These show increasing improvement in controling the amounts of secondary phases. MgO was limited down to ~10% wt. and complete prevention of formation of MgB4 by using simple glass encapsulation techniques and addition of Mg(s) to the capsule. In situ samples were obtained by encapsulating and sintering a mixture of Mg and B powders with Mg:B molar ratios of 1:2, 1.2:2 and 1.5:2 under vacuum in a graphite furnace. This process resulted in porous samples with poor mechanical resistance. SEM, EDS and XRD analysis have shown large amounts of MgO while MgB4 formation was successfully avoided. Ex situ samples display superconducting properties (Tc~37 K), including narrow critical transition in electrical properties (ΔTc~0.5 K). Magnetic and electric measurements were performed allowing the determination of critical current density, Jc, and critical transition temperature, Tc, of the material. Contrary to Tc, Jc is quite sensitive to the processing conditions and values from 0.37 and 3.89x106 A/cm2 are obtained at 10K

Mixed-ionic and electronic conduction and stability of YSZ-graphene composites

Composites of 8 mol% Yttria-stabilized Zirconia (YSZ) containing 0, 7, 10 and 14 vol.% of graphene nano-platelets (GNP) were fully densified by Spark Plasma Sintering. The effect of GNP on the electrical performance of the composites was analyzed by impedance spectroscopy as a function of temperature (150–800 °C) and oxygen partial pressure (0.21–10 atm). Results show that below GNP percolation threshold (7.1 vol.%), the electrical behavior is dominated by the matrix oxygen-ion conductivity. Above the threshold, the conductivity is predominantly electronic provided by the GNP network. The total conductivity of composites was used as an indicator of GNP stability in different atmospheres. YSZ/GNP composites remain stable in inert conditions up to 600 °C, and in reducing conditions up to 800 °C, making them good alternatives to perovskite-based materials used for electrochemical applications.This work was supported by project MAT2015-67437-R (MINECO, FEDER, UE).Peer Reviewe

Influence of electrospraying parameters on the microstructure of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ films for SOFCs

International audienceCeramics can play a remarkable role in the engineering of intermediate temperature solid oxide fuel cells (IT-SOFCs) capable of meeting the ambitious targets of reduced cost and improved lifetime. While mixed ionic-electronic conductors such as LaxSr1-xCoyFe1-yO3–δ are being used as volumic cathodes to increase the catalytic performance of these components, adequate microstructures are also an important requirement for optimal performance, particularly at lower operating temperatures. This work is devoted to the fabrication of La0.6Sr0.4Co0.2Fe0.8O3–δ films on Ce0.9Gd0.1O2-δ substrates by electrostatic spray deposition (ESD) and to the characterization of the microstructural dependence on the deposition conditions. A wide variety of microstructures ranging from dense to porous, with particular features such as reticulation and micro-porosity, were obtained by varying the ESD deposition parameters: nozzle-to-substrate distance (15, 30, 43, 45, and 58 mm), solution flow rate (0.34 and 1.5 mL/h), and substrate temperature (300, 350, 400 and 450 ºC). The correlation between deposition parameters and resulting microstructures was systematically studied and put into evidence

Long-term stability of iron-doped calcium titanate CaTi0.9Fe0.1O3−δ oxygen transport membranes under non-reactive and reactive atmospheres

Oxygen transport membranes (OTM) are widely considered as a possible solution to limit the carbon footprint, but are notoriously afflicted by performance issues due to chemical instability observed during long-term operation. This paper reports on the stability of an OTM made of CaTi0.9Fe0.1O3−δ (CTF), and addresses its applicability. The redox stability of CTF was investigated using thermal gravimetry up to 1000 °C under air and H2, coupled with XRD and Mössbauer analyses. The redox potential of iron was measured using an electrochemical potential relaxation as a function of temperature. The baseline oxygen semi-permeability flux of dense CTF membranes was measured in inert atmospheres (air/argon or air/helium), and the long-term stability established for up to 1600 h under simulated operation atmospheres containing CO, CO2, H2 and CH4. CTF shows a remarkable performance stability and post mortem XRD, SEM-EDS and Raman analyses show no evidence of decomposition or reaction byproducts

Sintering and conductivity of nano-sized yttria-doped ZrO2synthesized by a supercritical CO2-assisted sol-gel process

International audienceThis study reports on the electrical characterization of nano-sized zirconia powders doped with 3 and8 mol% Yttria, referred to as 3Y-ZrO2and 8Y-ZrO2, respectively. Powders were synthesized using a super-critical CO2assisted sol-gel method to control the size and morphology of the powder particles. Sparkplasma sintering (SPS) was used to sinter the powders and maintain a fine microstructure with small grainsize. The microstructure and chemical composition of the resulting ceramics was evaluated by electronmicroscopy techniques, showing grain boundaries depleted of amorphous phase. Electric conductivity ofthe 8Y-ZrO2pellets was determined by electrochemical impedance spectroscopy techniques, between200 and 800◦C, and correlated with the microstructure. The bulk electric response at 300◦C is compara-ble to commercially available Y-ZrO2compositions. Compared to conventionally prepared samples weobtain lower conductivity in the high temperature range, still values in this study are twice superior to3Y-ZrO2samples similarly prepared by SPS

Catalytic and Electrochemical Properties of Ag Infiltrated Perovskite Coatings for Propene Deep Oxidation

International audienceThis study reports the catalytic properties of Ag nanoparticles dispersed on mixed ionic and electronic conducting layers of LSCF (La0.6Sr0.4Co0.2Fe0.8O3 for propene combustion. A commercial and a synthesized LSCF powder were deposited by screen-printing or spin-coating on dense yttria-stabilized zirconia (YSZ) substrates, an oxygen ion conductor. Equal loadings (50 µg) of Ag nanoparticles were dispersed via drop-casting on the LSCF layers. Electrochemical and catalytic properties have been investigated up to 300 °C with and without Ag in a propene/oxygen feed. The Ag nanoparticles do not influence the electrochemical reduction of oxygen, suggesting that the rate-determining step is the charge transfer at the triple phase boundaries YSZ/LSCF/gas. The anodic electrochemical performances correlate well with the catalytic activity for propene oxidation. This suggests that the diffusion of promoting oxygen ions from YSZ via LSCF grains can take place toward Ag nanoparticles and promote their catalytic activity. The best specific catalytic activity, achieved for a LSCF catalytic layer prepared by screen-printing from the commercial powder, is 800 times higher than that of a pure Ag screen-printed film