Reoxidation Temperature Research Articles

Influence of the sintering process on the PTCR effect of chip-type Ba1.022–x Sm x TiO3 ceramics prepared by the reduction sintering-reoxidation method

We investigated the effects of sintering temperature and reoxidation annealing on the positive temperature coefficient of resistance (PTCR) effect of Ba1.022–xSmxTiO3 ceramics that were sintered at 1,180–1,260 °C for 30 min in a reducing atmosphere and reoxidized at 800 °C for 1 h. Results indicated that the room-temperature (RT) resistivity and resistance jump of the ceramics decreased with increasing sintering temperature; moreover, the samples exhibited a remarkable PTCR effect with a resistance jump of 3.3 orders of magnitude and achieved a low RT resistivity of 374.4 Ω cm at a lower sintering temperature. Furthermore, the higher grain-boundary resistivity of the ceramics obtained at a high reoxidation temperature after sintering at low temperature was estimated using an impedance analyzer. In addition, the voltage versus current behavior was investigated in present study.

Effect of Reoxidation Temperature on Electrophysical Properties of High-T<sub>C</sub> Barium Titanate-Based PTCR Ceramics

Ceramic samples of (1 x)BaTiO3–xNa0.5Bi0.5TiO3 system were prepared by sintering in reducing atmosphere of N2/H2 and were subsequently reoxidized in air. The influence of reoxidation temperature firing on the PTCR effect of (1 x)BaTiO3–xNa0.5Bi0.5TiO3 ceramics was investigated. The effect of Na0.5Bi0.5TiO3 concentration on resistivity and microstructure of the reoxidized samples was investigated by means of complex impedance spectroscopy and scanning electron microscopy. It has been found that the grain size decreases with the increase in Na0.5Bi0.5TiO3 content. The values of minimum ρmin and maximum ρmax resistivities of the samples were observed to increase with the increase in reoxidation temperature in the 600 – 1000°C temperature range. It was shown that with increasing in reoxidation temperature of (1-x)BaTiO3-xNa0.5Bi0.5TiO3 solid solutions, potential barrier at grain boundaries increases.

적층 PTC 써미스터의 전기적 특성에 대한 재산화의 영향

The alumina substrates that Ni electrode was printed on and the multi-layered PTCR thermistors of which composition is were fabricated by a thick film process, and the effect of re-oxidation temperature on their resistivities and resistance jumps were investigated, respectively. Ni electroded alumina substrate and the multi-layered PTC thermistor were sintered at for 2 h under $PO_2

Effect of reoxidation annealing on the PTCR behaviour of multilayer Nb5+-doped BaTiO3 ceramics with a Ni internal electrode

The effects of reoxidation annealing on the electrical properties and the positive temperature coefficient of resistance (PTCR) effects of multilayer Ba1.005(Ti1−x Nbx)O3 (BTN) ceramics sintered at different temperatures from 1100 to 1160 °C for 2 h in a reducing atmosphere and reoxidized at various temperatures in the range 500–900 °C were investigated. The results indicated that the room-temperature (RT) resistance of the BTN ceramics reoxidized at different temperatures first increased slowly and then quickly with the increase in the reoxidation temperature. Meanwhile, the resistance jump of the samples increased first and then decreased with the increase in the reoxidation temperature. In addition, the PTCR effect could be improved by prolonging the reoxidation time. Finally, the samples reoxidized at a low temperature of 600 °C after sintering at 1160 °C in a reducing atmosphere exhibited a significant PTCR effect, with a resistance jump of 3.5 orders of magnitude, along with a low RT resistance of 0.38 Ω. Additionally, the influences of the reoxidation temperature and time on the grain resistance and grain-boundary resistance were investigated using complex-impedance spectroscopy.

Effect of reoxidation on positive temperature coefficient of resistance behavior for BaTiO3-K0.5Bi0.5TiO3 ceramics

As a positive temperature coefficient of resistivity (PTCR) material, (1-x)BaTiO3-xK0.5Bi0.5TiO3 (BT-KBT, 0.05≦ x ≦0.15) ceramics without any donor doping were prepared by a conventional oxide mixing method. All samples were sintered in an Ar atmosphere at 1280 ~ 1350°C, subsequently, reoxidized at 800 ~ 1100°C in a gas mixture (99 %Ar–1 %O2). The PTCR behavior of BT-KBT ceramics were investigated in terms of KBT content, reoxidation temperature and time. The results showed that the BT-KBT ceramics exhibited an abrupt increase in their resistivity near the Curie temperature (Tc) after annealing in gas mixture, Tc of 0.9BT-0.1KBT ceramic was shifted to a higher temperature (~150°C). Furthermore, the room-temperature resistivity (ρRT) of ceramic samples sintered in Ar and reoxidized in a gas mixture decreased to 102 Ω·cm. The jump in resistivity (maximum resistivity [ρmax]/minimum resistivity [ρmin]) was enhanced by three orders of magnitude through a suitable reduction–reoxidation method without sacrificing the ρRT.

Pt-promoted α-Al2O3-supported Ni catalysts: Effect of preparation conditions on oxi-reduction and catalytic properties for hydrogen production by steam reforming of methane

The effect of Pt addition on the oxi-reduction properties of α-Al2O3-supported Ni catalysts, with different degrees of interaction between NiO and the α-Al2O3 support, was studied using atmospheres of H2, H2/H2O, and CH4/H2O. The effect of Pt promotion on the reduction of NiO with H2 was significant for NiO species that interacted more strongly with the alumina surface, but was much lower when a NiAl2O4-like bulk phase was formed. For samples activated with H2, although metal dispersion decreased with increasing Pt content, the activity was maintained constant by the presence of Pt sites. For samples activated with a CH4/H2O mixture, the activity increased with increasing Pt content, due to the higher reducibility of Ni in the Pt-promoted catalysts. The Pt promotion effect was stable; there was no important decrease in the influence of Pt on NiO reduction, even after high temperature re-oxidation of the catalysts.

Improved reoxidation tolerance of Ni–Fe metal support for LaGaO3 thin film electrolyte cell

Effects of reoxidation and reduction treatment of a Ni–Fe metallic anode substrate were investigated as a function of reoxidation period and temperature. The NiFe2O4 composite anode substrate was highly dense before the reduction treatment. However, after reduction for 2h in H2 atmosphere, the dense oxide composite substrate was changed to a porous metal substrate consisting of a Ni–Fe alloy. The metallic substrate was then reoxidized in 100% O2. When the reoxidation treatment was performed, the porous substrate regained its original density. However, XRD patterns of the reoxidized substrate still exhibited strong Ni base alloy peaks. Moreover, SEM–EDX analysis showed that a large part of the reoxidized substrate was porous and a dense iron base oxide layer formed at the surface of the substrate. In addition, after reduction for 2h with hydrogen gas, the substrate returned to a porous metal substrate again. Therefore, in spite of slightly increased anodic IR loss and a small fuel-leakage, which was observed after the reoxidation and reduction cycles, the Ni–Fe metal supported cell showed an excellent reoxidation tolerance at 973K.

Reoxidation Effects of Ba‐Excessive Barium Titanate Ceramics for Laminated Positive Temperature Coefficient Thermistors

Using tape casting technique Ba‐excessive BaTiO3 ceramics were prepared, and their reoxidation effects for laminated positive temperature coefficient thermistors are investigated. These ceramics exhibit fine grains of size ~1.15 μm. A resistivity of 727 Ω·cm and a PTCR jump about 3.3 orders of magnitude can be observed when the sample was sintered in reducing atmosphere at 1150°C and successively reoxidized at 850°C for 15 min. The influences of reoxidation on ceramic characteristics are discussed. The correlation of room temperature resistivity against reoxidation temperature showed an anomalous trend. This anomaly can be explained by the enhancement of ferroelectricity led by annihilation of oxygen vacancies. The influences of reoxidation time on ceramic characteristics were found to be limited. The impedance analysis revealed that the resistance of grain boundary is more likely to be influenced during reoxidation compared to that of grain bulk. The low acceptor‐state densities of 2 × 1012–6 × 1012 cm−2 at grain boundaries were also estimated.

Effect of Ce<sup>3+</sup>/Ce<sup>4+</sup> Couple on Selective Hydrogenation of Crotonaldehyde over Pt/CeO<sub>2</sub> Catalyst

By pretreatment of Pt/CeO2catalysts with high temperature reduction and subsequent low temperature reoxidation, the concentration of Ce3+species in the Pt/CeO2catalysts could be adjusted, while at the same time the Pt particle size remained constant. Thus the pure effect of Ce3+/Ce4+couples on the selective hydrogenation of crotonaldehdye could be explored. The investigation provided direct proofs supporting that Ce3+species favors the crotyl alcohol selectivity.

Reoxidation behavior of Ni–Fe bimetallic anode substrate in solid oxide fuel cells using a thin LaGaO3 based film electrolyte

The reoxidation behavior of a Ni–Fe metal anode supported cell using a thin LaGaO3 electrolyte film was investigated as a function of the reoxidation temperature. After oxidation and reduction treatments for 2h, the voltage did not return to the initial voltage at higher temperatures (773–973K); however, after reoxidation at 673K, the cell exhibited almost the same OCV as the as-prepared cell. During reoxidation with air at the higher temperatures, the Ni–Fe metal substrate exhibited two different expansion behaviors by the different oxidation rates of Ni and Fe. On the other hand, the volumetric change of the oxidized substrate at 673K was negligible. SEM-EDX results exhibited the reoxidation of Ni–Fe occurred only at the bottom part of the substrate and at the interface between the electrolyte and the substrate. In spite of temperatures as low as 673K, the cell generated a power of 160mWcm−2, which hardly decreased after the redox cycle. The increasing anodic internal resistance accompanied with unreduced Fe.

Effect of the Reoxidation on Positive Temperature Coefficient Behavior of BaTiO3–Bi0.5Na0.5TiO3

As a positive temperature coefficient of resistivity (PTCR) material, (1−x)BaTiO3–x(Bi0.5Na0.5)TiO3 (BT–BNT, 0.01≤x≤0.08) ceramics without any donor doping were prepared by a conventional solid‐state reaction method. All samples were sintered in an N2 atmosphere at 1340°C, followed by reoxidizing at 800°–1100°C in air. The PTCR characteristics of BT–BNT ceramics were investigated in terms of BNT content, reoxidation temperature, and time. Room‐temperature resistivity (ρRT) of simples sintered in N2 decreased to 102Ω·cm, and the jump in resistivity (maximum resistivity [ρmax]/minimum resistivity [ρmin]) was enhanced by three orders of magnitude through a suitable reoxidation process without sacrificing the ρRT. The Curie temperature (Tc) was shifted to a high temperature (>130°C) with an increase in the BNT content. With the addition of 4 mol% BNT, the obtained ceramic exhibited a low ρRT of 2 × 102Ω·cm, a typical PTC effect of ρmax/ρmin>103, and a Tc of 160°C.

Understanding of redox behavior of Ni–YSZ cermets

The oxidation of Ni–YSZ cermet as well the reduction of re-oxidized Ni–YSZ cermet was investigated by using temperature-programmed oxidation (TPO), temperature-programmed reduction (TPR) and scanning electron microscope (SEM). The scanning electron microscope (SEM) photographs and temperature-programmed reduction (TPR) profiles indicated that the sintering of smaller nickel oxide crystallites to larger aggregates occurred concurrently with the formation of smaller nickel oxide crystallites from the oxidation of nickel at 800 °C, and the sintering of smaller nickel oxide crystallites at 600 °C was slower than that at 800 °C. The SEM results showed that each Ni particle was separated into a lot of smaller NiO particles during oxidation. The TPO profiles showed that two kinds of nickel particles exist in the anode reduced at 800 and 600 °C, one with high activity towards oxidation for the nickel crystallites directly from reduction, and another one with low activity towards oxidation for the sintered nickel particles. The Ni–YSZ anodes reduced at higher temperature showed higher re-oxidation temperature than the one reduced at lower temperature because of the accelerated passivating and sintering of the smaller nickel particles at higher temperature. The re-oxidation profiles were almost unchanged during redox cycling at 600 °C, whereas the re-oxidation peak temperature decreased during redox cycling at 800 °C, indicating that the primary nickel grains split to smaller ones upon cyclic reduction at higher temperature.

Transformation of metal-oxide nanostructures in the process of afteroxidation of iron reactively evaporated in oxygen atmosphere

The methods of atomic force microscopy combined with the digital processing of images were used to study the change in the morphology of nanostructured metal-oxide coatings obtained by reactive evaporation at 10−6 torr in the atmosphere of oxygen and further low-temperature iron afteroxidation in an air atmosphere at different temperatures. It is shown that a nanostructured layer with a unimodal distribution according to the size and degree of nonsphericity of the particle is formed in the case of sputtering. Using the sample detection methods, it is shown that a unimodal distribution is transformed into a bimodal distribution at an increase in the reoxidation temperature from 50 to 200°C, after which it becomes a trimodal distribution, which proves that the building blocks of a nanostructured layer are nanoparticles and four- and 16-particle surface nanoparticle conglomerates. This structure can explain the functional properties of these coatings, which are of practical importance.

Redox stability of SOFC: Thermal analysis of Ni–YSZ composites

A re-oxidation of a Ni-based SOFC can seriously damage the cells. Important aspects of this thermomechanical instability are reduction–oxidation kinetics and the dimensional behaviour of the Ni–YSZ composites. These were investigated in the temperature range of 600–1000 °C and different combinations of reduction and oxidation temperatures. Automated temperature and gas change programmes were implemented in thermogravimetry and identically repeated using a high precision dilatometer to show the dimensional behaviour of the cermets simultaneously with the Degree of Oxidation (DoO) as a function of time during redox cycling. The activation energy for reduction was 84.4 kJ/mol and the kinetics was largely linear. Different kinetic models were fitted to the reduction data; the best agreement was found using the Avrami equation. On the re-oxidation, initially linear kinetics was observed, followed by a period of parabolic kinetics slowing down to logarithmic towards full DoO. The shifts in the kinetic shape with time depended on the temperature and DoO. The rate constants for oxidation were fitted to the data. The BET surface area of the cermets after different reduction and oxidation treatments was measured and show decrease of surface area with increasing reduction temperature and no significant differences in the surface area depending on the re-oxidation temperature in the range of 600–1000 °C.

Sm을 첨가한 BaTiO3계의 재산화 온도 및 시간에 따른 PTC 특성 변화

We investigated the effects of the re-oxidation temperature and time on the positive temperature coefficient (PTC) of resistivity characteristics of Sm-doped BaTiO₃ sintered at 1200~1260℃ for 2 h in a reducing atmosphere (3% H₂/N₂), followed by re-oxidization processes in air, in which re-oxidization temperature and time were 600~1000℃ and 1~10 h, respectively. The result reveals that Smdoped (Ba,Ca)TiO₃ ceramics fired in a reducing atmosphere exhibit low PTC characteristics, whereas the sample re-oxidized at 800℃for 1 h in air exhibit pronounced PTC characteristics. The room-temperature resistivity and jumping characteristics of resistivity (ρmax/ρ 25℃ ) decrease with Sm contents. The PTC characteristics with reoxidization time at 800℃ have improved about 2~3 orders of magnitude whereas differed according to the sintering temperature. The 0.7 at% Sm-doped (Ba,Ca)TiO₃ samples reveal the best PTC characteristics in the present range of formula and processes.

Dimensional Behavior of Ni–YSZ Composites during Redox Cycling

The dimensional behavior of Ni–yttria-stabilized zirconia (YSZ) cermets during redox cycling was tested in dilatometry within the temperature range . The effect of humidity on redox stability was investigated at intermediate and low temperatures. We show that both the sintering of nickel depending on temperature of the initial reduction and the operating conditions, and the temperature of reoxidation are very important for the size of the dimensional change. Cumulative redox strain (CRS) is shown to be correlated with temperature. Measured maximum CRS after three redox cycles varies within 0.25–3.2% dL/L in dry gas and respective temperature range of . A high degree of redox reversibility was reached at low temperature, however, reversibility is lost at elevated temperatures. We found that at , 6% steam and a very high ratio is detrimental for redox stability, whereas at no negative effect was observed. Pre-reduction at 1100 instead of more than doubled redox strain on reoxidation at . For samples similarly pre-reduced at , lowering the reoxidation temperature from or below reduces the redox strain to less than half.

Reduction/reoxidation of a multicomponent molybdate catalyst for propylene ammoxidation

The reduction/reoxidation mechanism of a multicomponent molybdate catalyst for propylene ammoxidation was studied by using X-ray powder diffraction (XRD), Laser Raman spectroscopy (LRS) and thermogravimetric analysis methods. XRD and LRS results of the catalysts with various reduced degrees show that catalyst reduction happens in the order: (2) → (3) → (4) → (5), and the lattice oxygen migration range is extended from iron molybdates to cobalt/nickel molybdate. The thermogravimetric analysis results show that the reoxidation of the catalyst is attributed to the replenishment of the bulk lattice oxygen in the lower temperature which is respectively combined with Bi, Fe and Mo, and the replenishment of the surface lattice oxygen in the higher temperature. The favorable reoxidation temperature is found to be around 440 °C.

Oxidation Driven Decomposition of CeNbO4 in Pure Oxygen

CeNbO4 possesses oxygen hyper-stoichiometry at ambient and moderate temperature. At elevated temperature CeNbO4+δ is thermally reduced to near stoichiometric CeNbO4 accompanied by a structural phase transition. Above the phase transition temperature reoxidation of the material occurred in a pure oxygen atmosphere, and in situ X-ray diffraction revealed that CeNbO4 decomposed to CeO2 and CeNb3O9. Upon further heating CeNbO4 reappeared by a solid state reaction between CeO2 and CeNb3O9. The present findings demonstrate that CeNbO4+δ is metastable with respect to CeO2 and CeNb3O9 at low temperature.

Effect of additives on redox behavior of iron oxide for chemical hydrogen storage

The redox behaviors of iron oxides, which were modified with Pd, Pt, Rh, Ru, Al, Ce, Ti and Zr as additives, were investigated using temperature-programmed reaction (TPR) technique. The modified iron oxides were prepared by co-precipitation method using urea precipitant. The role of additives was also examined using XRD and SEM analysis in detail. As a result, Pd, Pt, Rh and Ru additives have an effect on promoting the reduction and lowering the re-oxidation temperature of iron oxide. Especially, it is revealed that the effect of Rh species on lowering the reduction temperature is attributed to decrease of activation energy for H 2 reduction according to Fe 2O 3 → Fe 3O 4 course. Meanwhile, Al, Ce, Ti and Zr additives played an important role in prevention of deactivation of iron oxide by repeated redox cycles. Redox performances of iron oxides were also enhanced due to cooperative effects by co-addition of Rh and another species such as Al, Ce and Zr. Finally, Fe–O/(Rh, Ce, Zr) sample exhibited good performance for H 2 evolution by water-splitting through synergistic effect of component additives.

Reduction of carbon dioxide into carbon by freshly reduced CoFe 2O 4 nanoparticles

The reduction of one of the greenhouse gases, CO 2, has been a current subject in the field of environmental chemistry. Cobalt ferrite nanoparticles with different sizes were prepared by the thermal decomposition of acetate precursors and wet method techniques. CoFe 2O 4 nanoparticles were compacted, then isothermally reduced in H 2 flow at 400–600 °C. The isothermal reduction profiles obtained show that a topochemical mode of reduction is developed as the process proceeds. The metallic phases of iron and cobalt, produced from the complete reduction of CoFe 2O 4 are in nanosize form. The prepared, completely reduced and reoxidized CoFe 2O 4 compacts were characterized by XRD, SEM and reflected light microscope. The reoxidation process is affected by both particle size and reoxidation temperature. At the initial stages, the reaction was controlled by the interfacial chemical reaction mechanism with some contribution of the gaseous diffusion mechanism. On the other hand, at the final stages the mechanism by which the reoxidation process proceeds was found to be the solid-state diffusion. CO 2 decomposes on the surface of metallic phases with 99% conversion at 550 °C to form single- and multi-walled carbon nanotubes.