Oxidation Behavior of a Fully Dense Polymer-Derived Amorphous Silicon Carbonitride Ceramic

The oxidation behavior of a polymer-derived amorphous silicon carbonitride (SiCN) ceramic was studied at temperature range of 900°–1200°C using fully dense samples, which were obtained using a novel pressure-assisted pyrolysis technique. The oxidation kinetics was investigated by measuring the thickness of oxide layers. The data were found to fit a typical parabolic kinetics. The measured oxidation rate constant and activation energy of the SiCN are close to those of CVD and single-crystal SiC. The results suggest that the oxidation mechanism of the SiCN is the same as that of SiC: oxygen diffusion through a silica layer.     

Nickel oxide/hydroxide nanoplatelets synthesized by chemical precipitation for electrochemical capacitors

Nickel hydroxide powder prepared by directly chemical precipitation method at room temperature has a nanoplatelet-like morphology and could be converted into nickel oxide at annealing temperature higher than 300 °C, confirmed by the thermal gravimetric analysis and X-ray diffraction. Annealing temperature influences significantly both the electrical conductivity and the specific surface area of nickel oxide/hydroxide powder, and consequently determines the capacitor behavior. Electrochemical capacitive behavior of the synthesized nickel hydroxide/oxide film is investigated by cyclic voltammetry and electrochemical impedance spectroscope methods. After 300 °C annealing, the highest specific capacitance of 108 F g⁻¹ is obtained at scan rate of 10 mV s⁻¹. When annealing temperature is lower than 300 °C, the electrical conductivity of nickel hydroxide dominates primarily the capacitive behavior. When annealing temperature is higher than 300 °C, both electrical conductivity and specific surface area of the nickel oxide dominate the capacitive behavior.     

Oxidation Resistance Coating of LSM and LSCF on SOFC Metallic Interconnects by the Aerosol Deposition Process

Conductive La_0.8Sr_0.2MnO₃ (LSM) and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) layers with a thickness of ∼10 μm were deposited on ferritic stainless steel (SS) by the aerosol deposition method, for use as an oxidation resistance-coating layer in the metallic interconnector of a solid oxide fuel cell. The coated layers were fairly dense without pores or cracks, and maintained good adhesion even after oxidation at 800°C for 100 h. The surface of the bare SS after annealing at 800°C for 100 h was covered with Cr₂O₃ and Fe₃O₄ oxide scales, and the electrical conductivity was sharply decreased. However, the LSM- and LSCF-coated SSs showed a surface microstructure with almost no oxidation and maintained good electrical conductivity after annealing at 800°C for 100 h. The area-specific resistance (ASR) of LSM- and LSCF-coated alloys after 100 h of oxidation at 800°C was 20.6 and 11.7 mΩ·cm², respectively.     

Oxidation Behavior of Ferritic Steel Alloy Coated with Highly Dense Conducting Ceramics by Aerosol Deposition

Conducting La_0.8Sr_0.2MnO₃ (LSM) ceramic layers with a thickness of ∼10 μm were deposited on ferritic stainless steel (SS) by aerosol deposition for use as an oxidation resistance coating layer in the metallic interconnects of solid oxide fuel cell. The microstructural evolution and electrical properties of the LSM-coated SS were observed. The coated layers were fairly dense without pores or cracks, and maintained good adhesion even after oxidation at 800°C for 1000 h in air atmosphere. The surface of the bare SS after heat treatment at 800°C for 1000 h was covered with Cr-containing oxide scales, and the electrical conductivity was sharply decreased. However, the LSM-coated SS alloy showed a surface microstructure with almost no chromic oxide formation and maintained good electrical conductivity after the heat treatment. Close observation of the interface between LSM and SS indicated the presence of ∼500-nm-thick Cr₂O₃ and MnCr₂O₄ spinel phase, which may have caused the long-time deterioration of the interconnector performance. The area-specific resistance of the LSM-coated alloy after heat treatment at 800°C for 1000 h was 10.4 mΩ·cm².     

Electronic properties of thermally formed thin iron oxide films

The oxide layer, present between an organic coating and the substrate, guarantees adhesion of the coating and plays a determinating role in the delamination rate of the organic coating. The purpose of this study is to compare the resistive and semiconducting properties of thermal oxides formed on steel in two different atmospheres at 250 °C: an oxygen rich atmosphere, air, and an oxygen deficient atmosphere, N₂. In N₂, a magnetite layer grows while in air a duplex oxide film forms composed by an inner magnetite layer and a thin outer hematite scale. The heat treatment for different amounts of time at high temperature was used as method to sample the thickness variation and change in electronic and semiconducting properties of the thermal oxide layers. Firstly, linear voltammetric measurements were performed to have a first insight in the electrochemical behavior of the thermal oxides in a borate buffer solution. Electrochemical impedance spectroscopy in the same buffer combined with the Mott-Schottky analysis were used to determine the semiconducting properties of the thermal oxides. By spectroscopic ellipsometry (SE) and atomic force microscopy (AFM), respectively, the thickness and roughness of the oxide layers were determined supporting the physical interpretation of the voltammetric and EIS data. These measurements clearly showed that oxide layers with different constitution, oxide resistance, flatband potential and doping concentration can be grown by changing the atmosphere.     

Nernstian behaviour of zirconia oxygen sensors incorporating composite electrodes

The Nernstian behaviour of zirconia oxygen sensors incorporating composite electrodes has been described in the low temperature (<600°C) range. The composite electrodes consisting of a semiconducting metal oxide and platinum lower the operating temperature of oxygen sensors to about 300–350°C, well below that achievable with conventional metal or metal oxide electrodes. Complex impedance measurements made as a function of Pt/metal oxide ratio show that both the electrode resistance and the time constant go through a minimum when plotted against Pt content in the composite.     

HP40合金在H20／H2和空气中氧化行为的研究

在H2O/H2和空气的状态下,HP40合金试样表面氧化出不同的复合氧化层,对比研究在不同的氧化条件下试件表面氧化物的组成。用XRD、EDS和sEM分别对两种环境下的试样表面氧化层的情况进行了对比分析。结果表明,在H20／H2气氛下,试样表面生成了包含尖晶石的氧化层,随温度的升高,由针状变为颗粒状;在空气条件下,随着温度升高,形成了致密颗粒状富铬氧化物,并且逐渐长大,部分高温氧化下,试样表面氧化层出现了剥落。不同氧化对H蝴表面氧化影响较大,不同金属氧化物在氧分压不同时,表现出稳定性不同。     

Studies of interfacial layers between 4H-SiC (0 0 0 1) and graphene

The region between epitaxial graphene and the SiC substrate has been investigated. 4H-SiC (0 0 0 1) samples were annealed in a high temperature molecular beam epitaxy system at temperatures between 1100 and 1700 °C. The interfacial layers between the pristine SiC and the graphene layers were studied by X-ray photoelectron spectroscopy. Graphene was found to grow on the SiC surface at temperatures above 1200 °C. Below this temperature, however, sp³ bonded carbon layers were formed with a constant atomic Si concentration. C1s and Si2p core level spectra of the graphene samples suggest that the interface layer we observe has a high carbon concentration and its thickness increases during the graphitization process. A significant concentration of Si atoms is trapped in the interface layer and their concentration also increases during graphitization.     

Study of Nano-Porous Silicon with Low Thermal Conductivity as Thermal Insulating Material

Recently discovered phenomenon of extremely low thermal conductivity of nano-porous silicon (nano-PS) is discussed in detail. A theoretical model describing specific mechanisms of heat transport in as-prepared and oxidized nano-PS layers is described. The theoretical estimations are in a good agreement with experimental data obtained earlier. The low thermal conductivity values allow to use this promising material as thermal insulator in microsensors and microsystems. To ensure an efficient thermal isolation, a nano-PS layer has to be as thick as possible and mechanically stable. We describe here the procedures to form thick (up to 200 m) and stable nano-PS layers. Distribution of Si oxidized fraction along the layer thickness after thermal oxidation in dry O₂ atmosphere at 300°C during 1 h is studied.     

Oxidation and Crack Healing Behavior of a Fine‐Grained Cr2AlC Ceramic

This work reports the oxidation and crack healing behavior of a fine‐grained (~2 μm) Cr₂AlC MAX phase ceramic. The oxidation behavior was investigated in the temperature range 900°C–1200°C for times up to 100 h. The material showed a good oxidation resistance, owing to the formation of a dense and thin α‐Al₂O₃ layer. The microstructure, composition and thickness of the oxide scale were characterized. Its oxidative crack healing behavior as a function of temperature, healing time, and initial crack size was studied systematically. The material showed excellent healing behavior. The main crack healing mechanism is the filling of the crack by oxides well adhering to the crack faces. The crack geometry before and after healing was characterized by X‐ray tomography. Three‐point bend tests showed the dependence of strength recovery at 1100°C as a function of initial crack length and healing time.     

Effect of Support Modification on Reduction and CO Oxidation Activity of Doped Ceria-Supported Copper Oxide Catalyst

Ceria materials were modified by doping with gadolinia or yttria and by a hold period at 260 °C for 2 h during temperature-programmed calcinations to 650 °C. These doped ceria-supported copper oxide catalysts and the doped ceria material were characterized by temperature-programmed reduction, electron paramagnetic resonance, and CO oxidation activity test. It was observed that, as the doping concentration of gadolinia increases, the reduction temperature of the copper oxide species increases and the CO oxidation activity decreases. This is due to increased formation of the surface spinel species of copper oxide with gadolinia. As the yttria content increases to greater than 10 mol%, surface segregation occurs, which causes the amount of surface oxygen vacancies to decrease. It was also found that maintaining the temperature at 260 °C during calcination may decrease the amount of oxygen vacancies. The surface oxygen vacancies may be the active sites for CO oxidation over the oxygen ion conducting materials in the absence of any metal present. Gd doping leads to the formation of extrinsic oxygen vacancies, which increases the oxygen ionic conductivity of the doped ceria and thus increases the CO oxidation activities of the supported catalysts as well as of the doped ceria.     

The effect of confined crystallization on high-density poly(ethylene) lamellar morphology

High-density polyethylene (HDPE) was co-extruded against high glassy transition temperature (Tg) polycarbonate (PC) to fabricate multilayer films. Melt and recrystallization experiments were conducted on these extruded films to study the effects of isothermal recrystallization temperature and layer thickness on HDPE lamellae orientation. WAXS and AFM were used to demonstrate lamellar morphology of HDPE layers. We report that HDPE lamellae show twisted morphology in 30 nm thin layers after confined crystallization at a high temperature (128 °C). It may be the first time that anyone has created such twisted lamellar morphology with HDPE in such a thin layer. Similar twisted morphology of HDPE was also observed when HDPE was coextruded with another high Tg glassy polymer, polysulfone (PSF). Interestingly, the twisted HDPE lamellar morphology associated with an increased crystallinity improves both the oxygen and water vapor barrier properties of the multilayer films.     

Interfacial polarization and layer thickness effect on electrical insulation in multilayered polysulfone/poly(vinylidene fluoride) films

In this study, we report layer thickness effect on the electrical insulation property of polysulfone (PSF)/poly(vinylidene fluoride) (PVDF) multilayer films having a fixed composition of PSF/PVDF = 30/70 (vol./vol.). Breakdown strength, dielectric lifetime, and electrical conductivity were studied for 32- and 256-layer films having various total film thicknesses. Among these films, those having thinner PVDF and PSF layers exhibited lower breakdown strength, shorter lifetime, and higher electrical conductivity than those having thicker layers. These experimental results were explained by Maxwell–Wagner–Sillars interfacial polarization due to contrasts in dielectric constant and electronic conductivity for PVDF and PSF, respectively. When both PVDF and PSF layers were thick (ca. > 100–200 nm), more space charges were available in PVDF and no electronic conduction was allowed for PSF. These accumulated interfacial charges could serve as effective traps for injected electrons from metal electrodes under high electric fields. As a result, reduced electrical conductivity and enhanced breakdown strength/dielectric lifetime properties were obtained. When both layers were thin (ca. < 100 nm), fewer space charges were available in PVDF and significant electronic conduction through PSF resulted in low interfacial polarization. Consequently, higher electrical conductivity, lower breakdown strength, and shorter lifetime were observed. These results provide us insights into potential physics to enhance electrical insulation property of polymer films using a multilayered structure having large dielectric constant contrast.     

Oxidation behavior of high-entropy carbide (Hf0.2Ta0.2Zr0.2Ti0.2Nb0.2)C at 1400–1600 °C

Oxidation behavior of high-entropy carbide (Hf_0.2Ta_0.2Zr_0.2Ti_0.2Nb_0.2)C (HTZTNC) was investigated over temperature range of 1400–1600 °C. Results showed improved oxidation resistance of high-entropy carbide compared with individual carbide ceramics. In oxide layer, Ta₂O₅ and Nb₂O₅ were found to be dominant phases at 1400 °C, whereas ZrTiO₄ and HfTiO₄ were main phases obtained at 1500 and 1600 °C. Moreover, these complex dense oxide layer structures on the surface of HTZTNC at high temperature led to excellent oxidation resistance. The observation of Ti-depleted layer at 1500 and 1600 °C after 20 min of oxidation indicated that oxidation mechanism involved outward diffusion of titanium oxide, which was further confirmed by reoxidation experiments. In sum, these findings are promising for future development of high-entropy ultrahigh temperature ceramics with good oxidation resistance.     

Effect of Sm content on the properties and electrochemical performance of SmxSr1 − xCoO3 − δ (0.2 ≤ x ≤ 0.8) as an oxygen reduction electrodes on doped ceria electrolytes

Sm_xSr_{1 − x}CoO_{3 − δ} (SSCx) materials are promising cathodes for IT-SOFCs. The influence of Sm content in SSCx (0.2 ≤ x ≤ 0.8) oxides on their oxygen nonstoichiometry, oxygen desorption, thermal expansion behavior, electrical conductivity and electrochemical activity for oxygen reduction is systematically studied by iodometric titration, oxygen-temperature programmed desorption (O₂-TPD), dilatometer, four-probe DC conductivity, electrochemical impedance spectroscopy (EIS) and three-electrode polarization test, respectively. Iodometric titration experiments demonstrate that the electrical charge neutrality compensation in SSCx proceeds preferably through the oxidation of cobalt ion for high Sm³⁺ contents (x ≥ 0.6). However, it proceeds mainly through the creation of oxygen vacancies at x ≤ 0.5. O₂-TPD shows SSC5 possesses the highest oxygen desorption ability among the range of SSCx materials tested. The thermal expansion coefficients (TECs) are high between the transition temperature and 900 °C, showing values typically larger than 20 × 10⁻⁶ K⁻¹. All dense materials show high electrical conductivity with a maximum value of ∼1885 S cm⁻¹ for SSC6 in air, while SSC5 has the highest electrical conductivity in nitrogen. EIS analysis of porous electrodes demonstrates that SSC5 has the lowest area specific resistance (ASR) value (0.42 Ω cm²) at 600 °C. Cathodic overpotential testing demonstrates that SSC5 also has the largest exchange current density of 60 mA cm⁻² at 600 °C in air.     

High-temperature oxidation behavior of the SiC layer of TRISO particles in low-pressure oxygen

Surrogate tristructural-isotropic (TRISO)-coated fuel particles were oxidized in 0.2 kPa O₂ at 1200–1600°C to examine the behavior of the SiC layer and understand the mechanisms. The thickness and microstructure of the resultant SiO₂ layers were analyzed using scanning electron microscopy, focused ion beam, and transmission electron microscopy. The majority of the surface comprised smooth, amorphous SiO₂ with a constant thickness indicative of passive oxidation. The apparent activation energy for oxide growth was 188 ± 8 kJ/mol and consistent across all temperatures in 0.2 kPa O₂. The relationship between activation energy and oxidation mechanism is discussed. Raised nodules of porous, crystalline SiO₂ were dispersed across the surface, suggesting that active oxidation and redeposition occurred in those locations. These nodules were correlated with clusters of nanocrystalline SiC grains, which may facilitate active oxidation. These findings suggest that microstructural inhomogeneities such as irregular grain size influence the oxidation response of the SiC layer of TRISO particles and may influence their accident tolerance.     

Poly(p-phenylenesulfide)-based nano-composite formation: Delamination of organically modified layered filler via solid-state processing

Solid-state processing for the preparation of poly(p-phenylenesulfide) (PPS)-based nano-composites having finely dispersed layered fillers was conducted. The mixture of PPS and organically modified layered filler (OMLF) (95:5 wt./wt.) was subjected to the processing using thermostatted hot-press at ambient temperature and 150 °C, below T_m of PPS (i.e., PPS is still at the solid-state), and applying pressures of 7, 14 and 33 MPa for 30 s. The mixture exhibited disorder and delaminated layer structure with the thickness of 40-80 nm into PPS matrix. On the contrary, nano-composite prepared by melt compounding at 300 °C for 3 min showed the large stacked silicate layers in the PPS matrix. The solid-state processing led to delamination of the silicate layers and attained the discrete dispersion.     

Generation and Evolution of Surface Oxide Layer of Amorphous Boron during Thermal Oxidation: A Micro/nanofabricated Slice Measurement

The generation and evolution of the surface oxide layer of boron were thoroughly investigated due to the key role of the oxide layer in ignition and combustion of amorphous boron (B). Samples in different oxidation degrees were obtained by heating B particles until 600, 650, and 700 °C, using a temperature programmed thermobalance. A dual beam focused ion beam micro/nanofabricator was used to etch and cut the samples into thin slices (ca. 327 nm). The slices were observed under a scanning transmission electron microscope, accompanied with energy dispersive X‐ray analysis. During the thermal oxidation process, B particles initially lost mass through dehydration. Then they began to get oxidized and gain weight markedly. The sample surface became more rough as the final temperature increased. Two different reaction modes took place in sequence during the thermal oxidation of the samples. Below 650 °C, the oxidation reaction occurred only on the surface of the particle (the surface reaction mode). However, when the samples were heated to 700 °C, the particle interior was also involved in the reaction (the global reaction mode), and a large number of pores were formed. The O content of the initial surface oxide layer was fairly high. The thickness distribution was uniform (average thickness 148.1 nm) and the two edges were both smooth. During the heating, the oxygen content of the surface oxide layer increased after an initial decrease. The average oxide layer thickness increased and the thickness distribution became irregular and unequal. The sample heated until 700 °C had an average surface oxide layer thickness of 379.3 nm, and the thickness span reached 354.3 nm. During the global reaction process (700 °C), the oxidation degree within the interior of the particle was lower than that on its surface. In the particle interior, pores near the center were smaller than those close to the edge, whereas the oxidation degree was uniformly distributed. Results in this work provide a deeper understanding of the surface oxide layer, which can potentially help improve the ignition and combustion features of B.     

Fabrication and characterization of ZrB2–SiC ceramic electrodes coated with a proton conducting, SiO2-rich glass layer

3.5 μm thin layers of dense, crack-free, proton conducting, SiO₂-rich glass have been developed on ZrB₂–SiC ceramic composites, by thermal oxidation at 1400 °C for 30 min in air. A conductivity of 2 mS cm⁻¹ at 25 °C was found, as measured by AC impedance and steady-state voltammetry, and was estimated at ca. 2 × 10⁻² S cm⁻¹ at 80 °C. A striking behaviour of the oxidized ZrB₂–SiC composites is also pointed out: underneath the glass layer, there is a porous layer rich in electronic conductive ZrB₂, without well-defined interface between them, i.e., exhibiting a composition gradient in oxygen. In other words, protonic half-fuel cells could be fabricated under such conditions, for future use in hydrogen or direct alcohol fuel cells.     

Mechanism and Kinetics of Oxidation of ZrN Ceramics

Oxidation of ZrN ceramics from 973–1373 K under static conditions reveals parabolic rate behavior, indicative of a diffusion‐controlled process. In‐situ high temperature powder XRD found the oxidation mechanism begins with destabilization of ZrN through formation of a ZrN_1?x phase with oxide peaks initially detected at around 773 K. The zirconium oxide layer was found to be monoclinic by in‐situ XRD with no evidence of tetragonal or cubic polymorphs present to 1023 K. Bulk ceramic samples oxidized at 1173 and 1273 K underwent slower oxidation than those oxidized at 973 and 1073 K. This change in oxidation rate and hence mechanism was due to formation of a denser c‐ZrO₂ polymorph stabilized by nitrogen defects. This N‐doped dense ZrO₂ layer acts as a diffusion barrier to oxygen diffusion. However, at an oxidation temperature of 1373 K this layer is no longer protective due to increased diffusion through it resulting in grain boundary oxidation.