Optically anisotropic fragments in a Western Canadian subbituminous coal

Large Isotropic and anisotropic, angular and rounded fragments of inertinite were found in a Canadian subbituminous coal (%R oil = 0.50). The istropic fragments included pyrofusinite, macrinite and pseudovitrinite with pre-oxidized, heat-treated fragments showing oxidation rims. The anisotropic fragments include the heat-treated residue of liptinite, e.g. resinite showing granular anisotropy, pyrolytic carbon and fragments of vitrinite showing basic anisotropy. The morphology of inertinite fragments is described and an attempt is made to correlate these particles with heat-affected coal macerals. The occurrence of pyrolytic carbon with a reflectance of 3.11% and of liptinite showing anisotropy in a coal matrix with a reflectance of 0.50% is unusual. It indicates the detrital nature of the pyrolytic carbon. The formation of isotropic and anisotropic inertinite was possibly due to a thermal event, perhaps a wood or swamp fire. The pseudovitrinite found in these coals may have been formed by heat-treatment of fossil oxidized vitrinite fragments. The anisotropic fragments in the coals (e.g. pyrolytic carbon and anisotropic vitrinite), are detrital and are grouped with inertodetrinite.     

Morphology evolution of TiC-based cermets via different sintering schedules

Solid state sintering, liquid phase and cooling stages play different roles in determining the final morphology and composition of cermets, especially the well-known core-rim structure. In this work, TiC-(5–25 wt%)WC-11Mo₂C-18(Ni-Co) cermets were prepared and sintered by different sintering schedules. Morphology evolution and rim phase composition during sintering from 1250 °C to 1600 °C were investigated. Effects of sintering stages on the final morphology of cermets were also studied. It was shown that submicron (Ti, W, Mo)C grains tend to precipitate in binder during the cooling for cermets with high WC content. After the formation of outer rims during liquid sintering stage, interface reaction began to take effect between the rims and core. Coreless (Ti_0.76, W_0.13, Mo_0.11)C ceramic grains would be formed under high temperature (1600 °C) for TiC cermets with 25% WC. Long time sintering at solid state favored the formation of black core-thick inner rim and bright core-grey rim phases, while cooling near the melting point could result in submicron bright particles. This study provided not only a better view of the formation of rim-core structure but also an easier way to control the final morphology of cermets via reasonable changing the sintering cycle.     

Fabrication and oxidation behavior of Al4SiC4 powders

Al₄SiC₄ powders with high purity were synthesized by heating the powder mixture of aluminum (Al), silicon (Si), and carbon (C) at 1800°C in argon. The microstructure is characterized as platelike single grain. Both the nonisothermal and isothermal oxidation behavior of Al₄SiC₄ was investigated at 800°C‐1500°C in air by means of thermogravimetry method. It is demonstrated that Al₄SiC₄ powder possesses good oxidation resistance up to 1200°C and is almost completely oxidized at 1400°C. At 800°C‐1100°C, the oxide scales consist of an Al₂O₃ outer layer and a transition layer. Al₄SiC₄ remains the main phase. At 1200°C, some spallation resulting from the increment of Al₂O₃ and the mismatch of thermal expansion coefficient between different product layers can be observed. Above 1300°C, the oxide layer is composed of two part, i.e., large‐scale Al₂O₃ crystals (outer layer) and mullite with less amount of SiO₂ (inner layer). The oxidation behavior changes due to the different oxide products. For the reaction kinetics, a new kind of real physical picture model is adopted and obtains a good agreement with the experimental data. The apparent activation energy is calculated to be 176.9 kJ/mol (800°C‐1100°C) and 267.1 kJ/mol (1300°C‐1400°C).     

PdO x /Pd at Work in a Model Three-Way Catalyst for Methane Abatement Monitored by Operando XANES

The oxidation state of palladium in a model Pd/ACZ three-way catalyst was monitored by synchronous XANES and mass spectrometry during two consecutive heating (to 850 °C) and cooling (to 100 °C) cycles under stoichiometric conditions simulating exhaust after treatment of a natural gas engine. During heating in the first cycle, PdO reduction occurred around 500 °C and the initial fully oxidized state of Pd was never recovered upon heating and cooling cycles. A mixed Pd²⁺/Pd oxidation state was at work in the second cycle. Hence, the operando XANES study reveals that the PdO _x /Pd pair exists in a working catalyst but is less active than the catalyst in its initial state of fully oxidized palladium. It is also evident from XANES spectra that ceria–zirconia promotes re-oxidation of metallic Pd, thus reasonably sustaining catalytic activity after exposure to high temperatures.     

An e.s.r. study of the thermal degradation of Kevlar 49 aramid

The thermal degradation of Kevlar 49 aramid in the temperature range 350°–550°C has been investigated by e.s.r. spectroscopy. Accumulation of paramagnetic centres occurs above 470°C in vacuum and above 370°C in air. Above 520°C, the rate of formation of radical centres is approximately the same in air and vacuum. Measurements of g-factors show that the radical centres formed on thermal degradation in air are associated with a greater amount of chemically bound oxygen atoms than those formed in vacuum, indicating that the initial degradative processes in air are associated with oxidation of the fibre.     

Design and optimization of oxidation resistant coating for C/C aircraft brake materials

A SiCN/borosilicate glass anti-oxidation coating with double-layer structure was designed for C/C aircraft brake materials. The SiCN layer was introduced as transition layer to improve the wettability between borosilicate glass and C/C composites, and the microstructure results indicated that the coating with SiCN inner layer was dense and uniform. The oxidation resistance evaluation of the coated samples was conducted at 800 °C in air for 10 h. The weight loss of SiCN/borosilicate glass coated samples valued ~ 5.66% indicated that the oxidation resistant property of the simple SiCN/borosilicate glass coating was not good, which was mainly due to the relative large viscosity of borosilicate glass at 800 °C. B₄C was introduced to add into the outer glass coating to improve the self-healing ability of the coating. After oxidized at 800 °C in air for 10 h, the weight loss of the SiCN/borosilicate glass-B₄C coated samples was ~ 2.48%. B₄C could consume the oxygen diffused into the coating and the reacted product B₂O₃ with a better fluidity at 800 °C could effectively heal cracks and pores in the coating to improve the oxidation resistance property. The reaction of B₄C oxidized to B₂O₃ was accompanied with ~ 1.5 times volume expansion, which was also beneficial for the healing of defects.     

Ethanol oxidation on a high temperature PBI-based DEFC using Pt/C, PtRu/C and Pt3Sn/C as catalysts

A high temperature ethanol-fed polymer electrolyte membrane fuel cell has been implemented by using H₃PO₄-doped m-polybenzimidazole as polymeric electrolyte. Commercial Pt/C, PtRu/C and Pt₃Sn/C catalysts are used in the anode. The performance was assessed in terms of polarization curves at different temperatures, feeding the cell with a high concentration ethanol solution (water/ethanol mass ratio of 2). The product distribution was measured with the support of a gas chromatograph. The use of bimetallic catalysts increased the current density. PtRu/C showed the best performance up to 175 °C, but it is outperformed by Pt₃Sn/C at 200 °C. In terms of oxidation products, higher temperatures and current densities favour the oxidation of ethanol. However, Pt₃Sn/C promoted the generation of more oxidized products compared to PtRu/C (in which most of the ethanol is oxidized to acetaldehyde), especially at high temperature. This accounts for the large current density. In terms of complete oxidation of ethanol to CO₂, Pt/C was by far the most efficient catalyst for C–C scission, achieving percentages of 56 % of CO₂, although operating above 175 °C dramatically boosted an undesirable methanation process that slashed the efficiency. The combination of fuel cell results and product distribution helped to suggest the different oxidation routes on the surface of the different catalysts.     

Effect of air oxidation on coal hydrogenation

Japanese Taiheiyo coal (75.9%C) was oxidized with air at 250 °C for 3–40 h and at 300 °C for 3–10 h. The weight decreased from 16.3 to 46.5% at 250 °C and from 21.7 to 41.3% at 300 °C. Carbon loss and yield of NaOH soluble were obtained. The resultant oxidation products were hydrogenated at 370 °C for 1 h under 10 MPa hydrogen pressure using red mud with sulphur as catalyst and wash oil as solvent. Conversion to pyridine, benzene and n-hexane soluble fraction shows a minimum for the coal oxidized 10 h at 250 °C and for the coal oxidized for 3 h at 300 °C. Initial oxidation makes a network structure which contributes to a reduction in conversion, but in later stages of oxidation, splitting of bridge linkages and/or ring opening of aromatic structures, contribute to increases in conversion.     

Dynamic oxidation and protection of the PAN pre-oxidized fiber C/C composites

Pre-oxidized fibers as reinforcement are candidates for reducing the overall cost of C/C composites with superior properties. This study investigated the dynamic oxidation and protection of the pre-oxidized fiber C/C composites (Pr-Ox-C-C). According to the Arrhenius equation, the oxidation kinetics of the Pr-Ox-C-C consisted of two different oxidation mechanism with the transition point was at about 700 °C. Scanning electron microscopy investigation showed that oxidation initiated from the fiber/matrix interface of composites, whereas the matrix carbon was easily oxidized. To improve the anti-oxidant properties of Pr-Ox-C-C, a ceramic powder-modified organic silicone resin/ZrB₂-SiC coating was prepared by the slurry method. The coated samples were subjected to isothermal oxidation for 320 h at 700 °C, 800 °C, 900 °C, 1000 °C and 1100 °C with incurred weight losses of ? 1.6%, 0.77%, ? 1.28%, 0.68% and 1.19%, respectively. After 110 cycles of thermal shock between 1100 °C and room temperature, a weight loss of 1.30% was obtained. The Arrhenius curve presented four different phases and mechanisms for coating oxidation kinetics. The excellent oxidation resistance properties of the prepared coating could be attributed to the inner layer which was able to form B₂O₃-Cr₂O₃-SiO₂ glass to cure cracks, and the ZrB₂-SiC outer layer that could provide protective oxides to reduce oxygen infiltration and to seal bubbles.     

High-temperature oxidation behavior and oxidation mechanism of C/SiBCN composites in static air

On account of the excellent oxidation resistance of precursor-derived SiBCN ceramics, carbon-fiber-reinforced SiBCN (C/SiBCN) composites are increasingly being used in high-temperature aerospace applications. However, very few studies have investigated the high-temperature oxidation behavior of C/SiBCN composites for their application to high-heat engines. Herein, C/SiBCN composites prepared by precursor infiltration and pyrolysis were tested in static air up to an oxidation temperature of 1700 °C. The composites’ structural evolution after oxidation and their potential oxidation mechanisms were investigated in detail. The carbon fibers were preferentially oxidized at temperatures in the range of 1200–1500 °C and completely oxidized at 1500 °C. The oxidation of the fibers at 1500 °C resulted in the formation of abundant oxygen channels and consequently a high oxide scale growth rate of 5–7 μm² h⁻¹ and a large mass loss of 54.6 wt%. At elevated temperatures in the range of 1600–1700 °C, a dense SiO₂ oxide layer was formed by the sacrificial oxidation of the SiBCN matrix. The oxidation rate of the composites was therefore controlled by the diffusion rate of oxygen through the protective SiO₂ oxide layer and the weight loss of the composites decreased to 28.6% after oxidation at 1600 °C for 60 min. The structural integrity of the composites was maintained after long-term oxidation at 1600 °C.     

Effect of Nano-Size Powders on the Microstructure of Ti(C,N)-xWC-Ni Cermets

Microstructural and compositional analyses of Ti(C,N)– x WC–20Ni cermets were performed to understand the dissolution behavior of nano-size Ti(C,N) and WC, compared with an ultra-fine and a micro-sized system. The WC content was varied from 10 wt% to 70 wt%. The rapid dissolution and reprecipitation of nano-Ti(C,N) powders induced the formation of a coreless microstructure. With an increase in the amount of WC, the concentration of W in the rims increased but was low compared with the ultra-fine or micrometer system. An inversion in W concentration between the inner rim and the outer rim occurred at ∼40 wt% added WC. That is, the outer rim was richer in W than the inner rim. Powder size strongly affected the dissolution and precipitation of particles and was a major determinant of the final microstructure and compositions of the rims. Furthermore, a significant improvement in toughness in the case of simple nano systems was found, compared with that of the corresponding micrometer and ultra-fine systems ( K _{I C} 10–12 vs. 6–10 MPa·m^1/2). The coreless microstructure and the high fraction of rim phase in the nano system are responsible for this difference.     

Strengthened Ti(C,N)-based cermets using high-energy ball-milled NiTiC binders: Microstructure and mechanical properties

High-energy ball-milled NiTiC powders were used for preparing Ti(C, N)-based cermets. Effect of NiTiC content on the morphology, composition, interface structure and mechanical properties of cermets were investigated. NiTiC binders promoted the formation of inner rims on Ti(C, N) cores and hindered their coalescence, leading to well-distributed microstructure. Binder had little effect on the composition of rims, but greatly affected the interface structure of core-rim and rim-binder. Complete inner rim could decrease the lattice mismatch between outer rim and core, forming highly coherent interface. With increasing the Ti-C in Ni, the rim-binder boundaries evolved from semi-coherent to coherent interface, due to the decreased lattice mismatch. Small difference in Vickers hardness of cermets was found, with the values ranging from 1622 to 1684 N/mm². Bending strength of cermets increased from 1330 to 2073?MPa, with the Ti-C content from 0 to 20?wt%. Further increasing the Ti-C could lead to thick rims, resulting in decreased strength and toughness. This work showed us a way to strengthen the Ti(C, N)-based cermets via modifying the interface structure.     

Effect of WC content on the microstructures and corrosion behavior of Ti(C,N)-based cermets

The influence of WC content on the microstructure and corrosion behavior of Ti(C, N)-based cermets in 2 mol/L nitric acid solution was studied in this paper. There exists typical core/rim structure in the cermets. The cores appear black or white, and the rim is divided into white inner rim and grey outer rim. The undissolved Ti(C, N) particles normally appear as black cores, while the white core, inner rim and outer rim are (Ti, W, Mo) (C, N) solid solution formed at different sintering stages. The inner rim and white core appear brighter atomic contrast than the outer rim and black core, which is attributed to their higher W and Mo content. The thickness of the inner rim increases with WC addition, but the grain size of core/rim phase becomes finer. Meanwhile, the amount of white cores increases and that of black cores decreases. WC is more easily oxidized and dissolved in the nitric acid solution, compared with Ti(C, N). Therefore, the degradation of inner rim phase and the white core becomes more considerable with the increase of WC content. Consequently, the corrosion rate of cermets increases and the corrosion resistance of Ti(C, N)-based cermets is deteriorated with the increase of WC content.     

Unmasking the anomalous rapid oxidation of refractory TiB2 at low temperatures

As a refractory transition-metal diboride, monolithic TiB₂ oxidizes at temperatures below 400 °C. Here, we performed detailed microstructural investigations to study the low-temperature oxidation mechanism of monolithic TiB₂. An anomalous rapid oxidation behavior is observed at ～ 500 °C, where the oxidation rate is much higher than that at a higher temperature of 650 °C. The anomalous rapid oxidation behavior originates from an unreported bi-layer oxide scale consisting of outer homogeneous B₂O₃/TiO₂ mixed layer with a supra-nanometre-sized dual-phase amorphous-crystal microstructure and inner unstable Ti-B-O amorphous layer. By contrast, the oxide scale changes from homogeneous mixed microstructure to a laminated configuration at 650 °C, restoring the superior oxidation resistance. Our experimental results indicate that it is the configuration of the oxide scale that determines primarily the oxidation behavior of TiB₂, which has been seldom envisaged in previous studies.     

Fabrication and properties of Ti(C,N) based cermets reinforced by nano-CBN particles

Titanium carbontride (Ti(C,N)) based cermets with and without nano-cubic boron nitride (CBN) particles were prepared by microwave sintering in argon and nitrogen environment, respectively. Two kinds of core–rim microstructure, black core–grey rim and white core–grey rim, are shown in the cermets by scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDX). It is found that, for the cermet with 1.5% nano-CBN particles sintered at 1500 °C for 30 min in argon, its transverse rupture strength (TRS) and hardness are improved to about 25.9% and 1.4%, respectively. The SEM analysis shows that the inhibition effect of nano-CBN particles on the dissolution of Ti(C,N) is weakened with the increase of content of nano-CBN particles. Moreover, for the cermet sintered in argon reinforced by 1.5% nano-CBN particles, more fine black core–grey rims are found in the microstructure compared to the others. For the material sintered in nitrogen, its microstructure accompanied with many white core–grey rims in number and big black core and thin outer rim in size, results in high hardness and low TRS.     

Rim Structure in Ti(C0.7N0.3)–WC–Ni Cermets

The evolution of rim structures during the liquid-phase sintering of Ti(C_0.7N_0.3)· x WC·20Ni ( x = 5–25 wt%) was investigated under a variety of sintering conditions and densification procedures. In addition, the onset of formation of the surrounding structures in the system was examined by analyzing the microstructures and rim compositions. The compositions of the inner and outer rims were found to vary with sintering temperature and holding time. The results indicate that the inner rim forms at sintering temperature and the outer rim forms thereafter during the sintering or cooling stage of sintering. The coalescence of Ti(C,N) appears to occur at the sintering temperature to a great extent before the formation of the rim phases.     

Influence of different coating structures on the oxidation resistance of MoSi2 coatings

Two different structures of MoSi₂ coatings were prepared on Niobium based alloys by using a two step process. The as-deposited type(a) MoSi₂ coating structure consists of a MoSi₂ layer on the surface and a NbSi₂ layer underneath, while the type(b) MoSi₂ coating consists of an outer MoSi₂ layer and an inner unsiliconized Mo layer. The oxidation behaviors of the two different types MoSi₂ coatings were examined at 1200 °C for 100 h in air, and the mass gains of type(a) and type(b) MoSi₂ coated specimens were 0.64 mg/cm² and 0.59 mg/cm² respectively. The excellent oxidation resistance of both type(a) and type(b) MoSi₂ coated samples at 1200 °C was due to the formation of a dense and continuous SiO₂ scale during oxidation. As the CTE mismatch between the outer MoSi₂ coating and the inner layer, cracks distributed within both type(a) and type(b) MoSi₂ coating structures.     

Thermal desorption spectra of the oxidized surfaces of diamond powders

Thermal desorption mass spectrometry was carried out in the range from room temperature to 960°C for the diamond powders oxidized in 10^?2Torr O₂ at temperatures from 25 to 554°C. The spectra show two desorption peaks (α, β) and the major peak α seems to consist of at least four types of adsorption states (α₁?α₄). The first state α₁, may arise from “labile” carbon atoms created during degassing, and disappears with high temperature oxidation changing into the more stable states (α₂-α₄). IR absorption spectroscopy shows that α₂?α₄ states include carbonyl and ether structures. The minor peak β shows little change with oxidation temperature. The amount of oxygen chemisorbed at 420°C (1.22 × 10¹⁵ atom/cm²) is in good agreement with the estimated value of full coverage.     

Influence of Catalyst Pretreatments on Propane Oxidation Over Ru/γ-Al2O3

The influence of different treatments (in H₂ or in O₂ at 250 or 600 °C) of alumina supported Ru catalysts on the total oxidation of propane was investigated. Ruthenium catalysts were prepared using RuCl₃ as metal precursor and characterized by H₂ chemisorption, O₂ uptake, BET, XRD and TEM. The presence of chloride on the catalyst surface was found to exert an inhibiting effect on the activity of Ru. The reduced Ru/γ-Al₂O₃ catalysts after partial removing chlorine ions were more active than the same samples oxidized at 250 °C. The higher activity of the reduced Ru/γ-Al₂O₃ catalysts was attributed to the presence of a large amount of active sites on small Ru _x O _y clusters without well defined stoichiometry or on a poorly ordered layer of a ruthenium oxide on the larger Ru particles. The formation of highly dispersed, but in some extent crystallized RuO₂ phase in catalysts oxidized at 250 °C, leads to slightly lower activity of the Ru phase. Strong decline of the activity was found for catalysts oxidized at 600 °C. At this temperature, the Ru particles were completely oxidized to well-crystallized RuO₂ oxide, and the mean crystallite size of the Ru oxide phase was much higher (9–25 nm) than that of after oxidation at 250 °C (~4 nm). The effect of the regeneration treatment in H₂ on the activity of the Ru/γ-Al₂O₃ catalysts was also studied. The active ruthenium species for propane oxidation were discussed based on the catalytic and characterization data both before and after activity tests.     

Study on the oxygen diffusion in the oxide layers of SiBCN ceramics by SIMS

SiBCN, SiC and SiC-BN ceramics/composites were prepared by mechanical alloyed combined hot-pressing sintering at 1900 °C, and the oxidation kinetics of SiBCN, SiC and SiC-BN were calculated based on the thickness of oxide layers at 1100～1500 °C. The oxide layer can be divided into outer and inner parts under 1300 °C. At 1100 °C, the oxygen molecules diffused in SiC through the gaps in lattice, while diffused in SiBCN by substituting the O in SiO₂. Moreover, BN(C) phase in SiBCN can slow down the generation rate of gases such as CO, N₂, NO₂ and B₂O₃.