Yttrium Chromite Research Articles

Influence of Sm and Fe Co-doping on Structural and Electrical Features of Yttrium Chromite Nanoparticles

Influence of Sm and Fe Co-doping on Structural and Electrical Features of Yttrium Chromite Nanoparticles

Influence of Sm and Fe Codoping on Structural and Electrical Features of Yttrium Chromite Nanoparticles

Influence of Sm and Fe Codoping on Structural and Electrical Features of Yttrium Chromite Nanoparticles

Dielectric relaxation and electrical conduction mechanism of Neodymium doped Yttrium Chromite

This research article represents the fabrication and characterization of Neodymium doped YCrO3. Different optical parameters like optical bandgap energy and Urbach energy are evaluated. The gradual decrement of activation energy is observed when Neodymium doping content gradually increases. From the capacitance-voltage characteristics, free carrier concentration and trap carrier concentration are calculated, which increase with Nd doping content. Current-Voltage Characteristics of the different samples are measured at different temperatures. It follows the diode equation for the Schottky Barrier Diode (SBD) following thermionic emission. The graphical representation of current and voltage shows non-linearity, which cites the concept of Schottky diode. Another observation is that the barrier height is reduced, and the ideality factor is increased with the increase of doping concentration of Neodymium. The obtained value of the ideality factor is large, and it may be due to generation-recombination of charges inside the depletion layer and also tunneling through the barrier.

On the optical and magnetic studies of YCrO3 perovskites

In this paper we studied the Electron Paramagnetic Resonance (EPR) and Optical spectroscopies of mono disperse yttrium chromite (YCrO3) nanoparticles prepared by a modified polymeric precursor method. The particles crystallized in roughly spherical shape with size ranging from 100 to 200 nm. Diffuse reflectance spectrum exhibited very well defined bands related to the 4A2 → 4T2, 4A2 → 4T1, 4A2 → 2T1, 4A2 → 2E transitions, characteristic of Cr3+ (3d3, 2D5/2) ions in nearly octahedral symmetry. The crystalline field Dq, Racah parameters (B and C) and Dq/B, could be determined and the results indicated that Cr3+ ions are in a strong ligand field site. The EPR spectra of YCrO3 shows a single broad resonance in the whole range of temperature studied, 100–300 K. Combining optical and EPR results the effective g-values of the paramagnetic Cr3+ were estimated to be 1.97 and 1.98, respectively, typical of Cr3+ in orthorhombic crystal field environment. The onset of the antiferro-paramagnetic transition, TÑ140 K, attributed to Cr3+-Cr3+ exchange interaction (J = −41.5 cm−1) was determinate by means of Magnetization and EPR measurements. The results were discussed in terms of the magnetic short-rang spin-spin exchange interaction between pairs of Cr3+.

Effects of sintering atmosphere on the interaction between doped yttrium chromite anode and yttria stabilized zirconia electrolyte

The perovskite chromite-based oxide materials are potential anodes for solid oxide fuel cells (SOFCs). However, the chemical reactivity between chromite-based anodes and yttria-stabilized zirconia (YSZ) electrolyte is one of the important factors that its limits application in SOFCs. Here, we report the effect of sintering atmosphere on the interaction between Ca and Co co-doped YCrO3 (Y0.8Ca0.2Cr0.8Co0.2O3, YCCC) chromite anode and the YSZ electrolyte. The results of SEM, EDX, and XRD analyses show that the reducing atmosphere of 1% H2-Ar gas can efficiently prevent the interfacial reaction between YCCC and YSZ. However, sintering in air results in the formation of CaZrO3 intermediate reaction layer at the YCCC/YSZ interface. The electrochemical impedance measurements also revealed that the polarization and ohmic resistances of cells sintered in reducing atmosphere are much smaller than the ones sintered in air due to inhibited formation of CaZrO3 at the YCCC/YSZ interface. The reduction of Cr4+ concentration and Ca2+ segregation in the YCCC solid solution in reducing atmosphere may have led to the reduced interaction between YCCC and YSZ.

Structural and electrical transport studies in Bi-substituted Yttrium Chromite

Perovskite type chromites are studied recently for their fascinating structure and inter-related functional properties. YCrO3 (YCO) is one among the chromites which possess structural heterogeneity. The structural studies on YCrO3 (YCO) and Y0.9Bi0.1CrO3 (YBiCO-10) reveal that the powder diffraction patterns exhibits a single phase with orthorhombic Pnma space group. Compositional studies on YCO and YBiCO-10 indicates the cation ratios of (Y, Bi):Cr (1:1) is preserved. Microstructural studies reveal that there is an increment in grain growth in YBiCO-10 ceramics. The microstructural studies also evidently show that YBiCO-10 has more of faceted orthorhombic type of grains, whereas, YCO has a spherical grain type of features. Impedance spectroscopy studies were performed on both YCO and YBiCO ceramics and the studies reveal a significant change in grain resistance. The decrease of resistance could be due to increase of grain size which offers the conductive path of hopping of charge carriers or inherent electrical transitions of Cr3+ in YBiCO-10 sample. These chromites can be used as potential candidates for the thermistor applications whose thermistor parameter (β) and sensitivity (α) values are 2873 K, −5 × 10− 3 K−1 (YCO) and 3102 K,−5.44 × 10− 3K−1 (YBiCO-10) at 755 K.

Local structural distortion and interrelated phonon mode studies in yttrium chromite

Abstract

Highly Stable Dual-Phase Y0.8Ca0.2Cr0.8Co0.2O3 - Sm0.2Ce0.8O1.9 ceramic Composite Membrane for Oxygen Separation

High-temperature oxygen transport membranes based on mixed ionic- and electronic-conducting (MIEC) ceramics are receiving increasing attention due to their potential applications for high-purity oxygen production, oxyfuel combustion, hydrogen/syngas production, coal gasification, and waste recovery. Dense MIEC membranes exclusively allow the permeation of oxygen ions in the presence of an oxygen partial pressure gradient because the oxygen ionic flux is counterbalanced by the flux of electrons to form an internal short circuit. In this work, a highly stable ceramic composite membrane composed of Ca- and Co-doped yttrium chromite, Y0.8Ca0.2Cr0.8Co0.2O3 (YCCC), and samaria-doped ceria, Sm0.2Ce0.8O1.9 (SDC), is demonstrated for oxygen separation. Homogeneously dispersed nano-scale composite powders were synthesized by a single-step combustion process based on the glycine-nitrate method. Dense composite membranes were achieved having submicron grain sizes and well-percolated electronic and ionic conduction pathways. Densification of the composite membrane was assisted by liquid phase sintering caused by cobalt-doping in yttrium chromite, and gas-tight membranes are fabricated at 1400oC. The YCCC and SDC phases were chemically and thermo-mechanically compatible at both processing and operating temperatures. The composite membrane exhibited an oxygen permeation flux comparable to those of the state-of-the-art single-phase membrane materials and excellent stability in harsh operating conditions under a H2-CO2 environment for long-term operation, which suggests potential application in various combustion and fuel production processes.

Hodrogen Oxidation Reaction Mechanisms in Mixed Ionic and Electronic Conducting Oxide Anode for Solid Oxide Fuel Cells

Ceramics anode offers certain advantages in terms of impurity tolerance and other properties, as compared to conventional Ni/YSZ anode for solid oxide fuel cells (SOFC). However, ceramic anodes typically have much lower electrochemical performance, and there is very few studies on the hydrogen oxidation reaction (HOR) mechanisms and kinetics in ceramic anodes. In this research, using Doped YCrO3 as baseline material, we systematically investigated the HOR mechanisms in mixed ionic and electronic conducting (MIEC) ceramic anodes, with the focus on the effect of electronic conductivity, electro-catalytic activity, triple-phase-boundary (3PB), and electrolyte, on the HOR kinetics. It is found that both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Co and Ni doping. Electrochemical impedance spectroscopy (EIS) results indicate that charge transfer process at high frequency and surface adsorption/diffusion processes at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes by reducing the related activation energy from 1.2 to 0.5 eV. It also substantially improves the charge transfer process probably by increasing the amount of adsorbed H on electrode surface. The resistance of high frequency is found to be dependent on H2 content. The observed reaction order is 1/4 for Co doped and 1/3 ~ 1/2 for Ni doped yttrium chromites. A model of H2oxidation reaction is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen. The influence of electrolyte to electrode performance was investigated by replacing YSZ electrolyte with scandium stabilized zirconium (SSZ). Smaller polarization resistances are observed on each electrode in both wet H2 and air atmospheres. Replacement of electrolyte can alter not only the rate of charge transfer process but also in some cases other surface processes not related to the electrolyte directly. It is proposed that the impact of the electrolyte on each electrode process is passed down as in a chain and the charge transfer step functions as the first ring in the chain.

Performance and sulfur resistance of doped yttrium chromite-ceria composite as anode material for the SOFCs operating on H2S-containing fuel

The sulfur resistance and performance of Sr- and Mn-doped yttrium chromite (YSCM)-samaria-doped ceria (SDC) composite material were investigated for potential use as anodes in solid oxide fuel cells (SOFCs). The anode was well adhered to the electrolyte, which was ascribed to their similar coefficient of thermal expansion (TEC). The electro-catalytic activity of YSCM-SDC anodes in yttria-stabilized zirconia (YSZ) electrolyte-supported cells toward hydrogen oxidation was superior to that of the La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM) anode. Sr depletion in the YSCM structure and the formation of SrSO4 in the presence of sulfur led to performance degradation of the anode. Irreversible and reversible performance degradation suggested that YSCM and SDC played a respective role during the anode deactivation process. The voltage decreased at a rate of 31 mV/h and stabilized at 0.49 V under a 3000 ppm H2S atmosphere. In addition, the sulfur tolerance of YSCM-SDC anode was better than SDC under strictly identical conditions.

Studies on Local Structural Inhomogeneity and Origin of Ferroelectricity in Yttrium chromite Ceramics

ABSTRACTYCrO3 (YCO) is known to be a multiferroic with orthorhombic, Pnma, structure with center of inversion. However, the local structural inhomogeneity in this compound is believed to give rise to ferroelectric behavior. In this study we explore high temperature Raman investigations of YCO and observed that one of its Raman mode B3g (3) (CrO6 Octahedral tilt mode) softens around the structural phase transition which could be the origin of ferroelectric nature in YCO. In addition, we substitute bismuth (Bi) in YCO to understand the structural distortions that lead to local structural inhomogeneity. Besides B3g (3) mode softening with composition and high temperature studies in Y1-xBixCrO3 the study reveals the structural distortions and the structural tunability Bi offers in such systems.

Chemical compatibility of doped yttrium chromite and ceria composite materials with YSZ electrolyte

The interfacial reaction between yttria-stabilized zirconia (YSZ) electrolyte and different anode materials of single-phase Y0.8Ca0.2Cr0.8Co0.2O3 (YCCC) and YCCC- Ce0.9Gd0.1O1.95 (YCCC-GDC) composite is investigated. The single-phase YCCC anode is found to react with YSZ and forms a CaZrO3 interlayer at the anode/electrolyte interface during the cell fabrication at 1100°C in air. On the other hand, the YCCC-GDC composite anode shows no interfacial reaction at the anode/electrolyte interface suggesting good chemical compatibility with YSZ electrolyte. The Gd and Ce in GDC diffuse to YCCC particle surface and grain boundary in the YCCC-GDC composite anode. The electrochemical properties characterizations reveal that the interfacial reaction of YCCC/YSZ leads to low performance of single-phase YCCC anode. The GDC and YCCC mixed particles without reaction with electrolyte form anode frameworks with a large amount of triple phase boundary (TPB) in the YCCC-GDC composite anode. This leads to reduction of the polarization resistance and results in large performance enhancement.

Effect of heterovalent substitutions in yttrium chromite on the hyperfine interactions of 119Sn4+ studied by Mössbauer spectroscopy

In order to develop the 119Sn Mössbauer spectroscopic probe technique to study magnetically ordered materials, three Ca-substituted yttrium chromites, i.e. Y0.9Ca0.1CrO3, Y0.9Ca0.1Cr0.9Ti0.1O3 and Y0.8Ca0.2Cr0.8Ti0.2O3, doped with 0.3 atom-% Sn4+, were for the first time investigated. 119Sn Mössbauer spectra, recorded at 4.2K, have allowed, through analysis of the magnetic hyperfine field values, probed by 119Sn nuclei, to gain insight into the local magnetically active surrounding of different Sn4+ ions. In all of these compounds, partial segregation of Sn4+ ions is revealed. In the case of Y0.9Ca0.1CrO3, neither highly oxidized Cr4+ nor Cr6+ species, expected to compensate for the Ca2+ positive charge deficit, is found in the vicinity of the 119Sn4+ probe. In the case of both studied Ti-containing chromites, 119Sn Mössbauer spectra have provided the original indirect evidence for the statistical distribution of Cr3+ and Ti4+ ions over the octahedral sites and permitted characterization of the occurring associates of Sn4+.

Highly stable dual-phase Y0.8Ca0.2Cr0.8Co0.2O3–Sm0.2Ce0.8O1.9 ceramic composite membrane for oxygen separation

A highly stable ceramic composite membrane composed of Ca- and Co-doped yttrium chromite, Y0.8Ca0.2Cr0.8Co0.2O3 (YCCC), and samaria-doped ceria, Sm0.2Ce0.8O1.9 (SDC), was demonstrated for oxygen separation. Homogeneously dispersed nano-scale composite powders were synthesized by a single-step combustion process based on the glycine–nitrate method. Dense composite membranes were achieved having submicron grain sizes and well-percolated electronic and ionic conduction pathways. Densification of the composite membrane was assisted by liquid phase sintering caused by cobalt-doping in yttrium chromite, and gas-tight membranes are fabricated at 1400°C. The YCCC and SDC phases were chemically and thermo-mechanically compatible at both processing and operating temperatures. The composite membrane exhibited an oxygen permeation flux comparable to those of the state-of-the-art single-phase membrane materials and excellent stability in harsh operating conditions under a H2–CO2 environment for long-term operation, which suggests potential application in various combustion and fuel production processes.

Characterization of Doped Yttrium Chromites as Electrodes for Solid Oxide Fuel Cell by Impedance Method

Redox-stable ceramic electrodes, Co and Ni doped yttrium chromites, for solid oxide fuel cell (SOFC) are characterized by electrochemical impedance spectroscopy (EIS) in various PO2 and PH2 atmosphere. The polarization resistances for Y0.8Ca0.2Cr0.8Co0.2O3-δ (YCCC) and Y0.8Ca0.2Cr0.9Ni0.1O3-δ (YCCN) on yttrium stabilized zirconia (YSZ) electrolyte are 0.96 and 8.4 Ωcm2 in wet H2, 1.2 and 36.7 Ωcm2 in air at 850°C, respectively. For the anode application, the rate-limiting steps (RDS) are identified as charge transfer and surface diffusion for both YCCC and YCCN. The primary active zone in YCCN is three-phase boundary (3PB) but extends to a small portion of the electrode bulk in the YCCC anode. For the cathode application, O2 dissociative adsorption or diffusion is one of the RDS for both electrodes, and the active zone is limited to the 3PB area. The influence of electrolyte to electrode performance is investigated by replacing YSZ electrolyte with scandium stabilized zirconium (SSZ). Smaller polarization resistances are observed on each electrode in both wet H2 and air atmospheres. Replacement of electrolyte can alter not only the rate of charge transfer process but also in some cases other surface processes not related to the electrolyte directly. It is proposed that the impact of the electrolyte on each electrode process is passed down as in a chain and the charge transfer step functions as the first ring in the chain. The best performance is obtained with the YCCC/SSZ combination, 0.49 and 1 Ωcm2 in wet H2 and air at 850°C, respectively, making YCCC a promising electrode.

H2 Oxidation on Doped Yttrium Chromites Anode of Solid Oxide Fuel Cell

Doped yttrium chromites as potential anodes for SOFC are studied with respect to the electrode performance and anode reaction mechanisms. Both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Ni doping. Electrochemical impedance spectra results indicate that charge transfer process at high frequency and surface adsorption/diffusion processes at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes and improves the charge transfer process probably by increasing the amount of adsorbed H on the electrode surface. A model of H2 oxidation reaction is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen.

H2 oxidation on doped yttrium chromites/yttrium stabilized zirconia anode of solid oxide fuel cell

Co and Ni doped yttrium chromites as potential anodes for solid oxide fuel cell (SOFC) are studied with respect to the electrode performance and anode reaction mechanisms. Both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Co and Ni doping. Electrochemical impedance spectroscopy (EIS) results indicate that charge transfer process at high frequency and surface adsorption/diffusion processes at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes by reducing the related activation energy from 1.2 to 0.5eV. It also substantially improves the charge transfer process probably by increasing the amount of adsorbed H on electrode surface. The resistance of high frequency is found to be dependent on H2 content. The observed reaction order is 1/4 for Co doped and 1/3–1/2 for Ni doped yttrium chromites. A model of H2 oxidation reaction is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen.

Doped yttrium chromite–ceria composite as a redox-stable and sulfur-tolerant anode for solid oxide fuel cells

A new high performance ceramic solid oxide fuel cell (SOFC) anode, Ca- and Co-doped yttrium chromite (YCCC)–samaria-doped ceria (SDC) composite, that resists deactivation by sulfur and does not show degradation during multiple reduction–oxidation cycles is developed. The electrocatalytic activity of the YCCC–SDC anodes in yttria-stabilized zirconia (YSZ) electrolyte-supported cells toward hydrogen oxidation is comparable to that of the Ni/YSZ anode. YCCC–SDC exhibits superior sulfur tolerance showing less than 10% increase in electrode resistance, fully reversible, upon exposure to 20 ppm H 2S. The excellent redox tolerance is attributed to the dimensional and chemical stability of the YCCC exhibiting minimal isothermal “chemical” expansion.

High performance ceramic interconnect material for solid oxide fuel cells (SOFCs): Ca- and transition metal-doped yttrium chromite

The effect of transition metal substitution on thermal and electrical properties of Ca-doped yttrium chromite was investigated in relation to use as a ceramic interconnect in high temperature solid oxide fuel cells (SOFCs). 10at.% Co, 4at.% Ni, and 1at.% Cu substitution on B-site of 20at.% Ca-doped yttrium chromite led to a close match of thermal expansion coefficient (TEC) with that of 8mol% yttria-stabilized zirconia (YSZ), and a single phase Y0.8Ca0.2Cr0.85Co0.1Ni0.04Cu0.01O3 remained stable between 25 and 1100°C over a wide oxygen partial pressure range. Doping with Cu significantly facilitated densification of yttrium chromite. Ni dopant improved both electrical conductivity and dimensional stability in reducing environments, likely through diminishing the oxygen vacancy formation. Substitution with Co substantially enhanced electrical conductivity in oxidizing atmosphere, which was attributed to an increase in charge carrier density and hopping mobility. Electrical conductivity of Y0.8Ca0.2Cr0.85Co0.1Ni0.04Cu0.01O3 at 900°C is 57Scm−1 in air and 11Scm−1 in fuel (pO2=5×10−17atm) environments. Chemical compatibility of doped yttrium chromite with other cell components was verified at the processing temperatures. Based on the chemical and dimensional stability, sinterability, and thermal and electrical properties, Y0.8Ca0.2Cr0.85Co0.1Ni0.04Cu0.01O3 is suggested as a promising SOFC ceramic interconnect to potentially overcome technical limitations of conventional acceptor-doped lanthanum chromites.

Effect of nickel substitution on defect chemistry, electrical properties, and dimensional stability of calcium-doped yttrium chromite

The effect of nickel substitution on defect chemistry, electrical properties, and dimensional stability of calcium-doped yttrium chromite was studied for use as an interconnect material in high temperature solid oxide fuel cells (SOFCs). The compositions of Y 0.8Ca 0.2Cr 1 − x Ni xO 3 ± δ (x = 0–0.15), prepared using the glycine nitrate process, showed single phase orthorhombic perovskite structures over a wide range of oxygen partial pressures (4.6 × 10 − 20 atm ≤ pO 2 ≤ 0.21 atm at 900 °C). X-ray diffraction (XRD) analysis indicated that most of the nickel ions replacing chromium ions are divalent and act as acceptor dopants, leading to a substantial increase in conductivity. In particular, the conductivity at 900 °C in air increased from 10 S/cm to 34 S/cm with 15% nickel substitution, and an increase in charge carrier density was confirmed by Seebeck measurements, which validated the predominant Ni 2+ oxidation state. A point defect model was derived, and the relationship between electrical conductivity and oxygen partial pressure was successfully fitted into the proposed model. The defect modeling results indicated that nickel substitution improves the stability of calcium-doped yttrium chromite toward reduction and suppresses the oxygen vacancy formation, which results in significantly increased electrical conductivity in reducing environment. The electrical conductivity of Y 0.8Ca 0.2Cr 0.85Ni 0.15O 3 ± δ at 900 °C in reducing atmosphere ( pO 2 = 10 − 17 atm) was 5.8 S/cm, which was more than an order of magnitude higher than that of Y 0.8Ca 0.2CrO 3 ± δ (0.2 S/cm). Improved stability in reducing atmosphere was further confirmed by dilatometry measurements showing reduced isothermal “chemical” expansion, and the isothermal expansion in reducing atmosphere ( pO 2 = 10 − 17 atm) at 900 °C decreased from 0.07% for Y 0.8Ca 0.2CrO 3 ± δ to 0.03% for Y 0.8Ca 0.2Cr 0.85Ni 0.15O 3 ± δ . Based on these results, enhanced electrical performance and mechanical integrity is expected with nickel substitution on calcium-doped yttrium chromite in SOFC operating conditions.