Titanium carbide derived nanoporous carbon for energy-related applications

High surface area nanoporous carbon has been prepared by thermo-chemical etching of titanium carbide TiC in chlorine in the temperature range 200-1200 °C. Structural analysis showed that this carbide-derived carbon (CDC) was highly disordered at all synthesis temperatures. Higher temperature resulted in increasing ordering and formation of bent graphene sheets or thin graphitic ribbons. Soft X-ray absorption near-edge structure spectroscopy demonstrated that CDC consisted mostly of sp² bonded carbon. Small-angle X-ray scattering and argon sorption measurements showed that the uniform carbon-carbon distance in cubic TiC resulted in the formation of small pores with a narrow size distribution at low synthesis temperatures; synthesis temperatures above 800 °C resulted in larger pores. CDC produced at 600-800 °C show great potential for energy-related applications. Hydrogen sorption experiments at −195.8 °C and atmospheric pressure showed a maximum gravimetric capacity of ∼330 cm³/g (3.0 wt.%). Methane sorption at 25 °C demonstrated a maximum capacity above 46 cm³/g (45 vol/vol or 3.1 wt.%) at atmospheric pressure. When tested as electrodes for supercapacitors with an organic electrolyte, the hydrogen-treated CDC showed specific capacitance up to 130 F/g with no degradation after 10 000 cycles.     

Joining of Cf-SiC composites and ZrB2-SiC based composites for ultra high temperature applications

Gas tungsten arc welding (GTAW) of Cf–SiC composites to themselves and to ZrB₂-SiC based composites have been carried out with a filler material of (ZrB₂-SiC-B₄C-Y₂O₃-Al₂O₃) composite. The weld interfaces of joints of composites were clean and free from porosity and cracks. Penetration of filler material into voids and pores existing in the Cf-SiC composites was observed. An average shear strength of 25.7?MPa was achieved. The ZrB₂-SiC based composite joined to Cf-SiC (CVD) composite was exposed for 300?s to the oxy-propane flame at 2300?°C. The joint and interfaces between the filler material and parent composites were found to be unaffected by thermal cycling and oxidation during the exposure to the oxy-propane flame.     

Microstructural and mechanical evaluation of porous biomorphic silicon carbide for high temperature filtering applications

Biomorphic SiC (bioSiC) is a low cost SiC/Si composite obtained by melt infiltration of carbon preforms obtained from the pyrolysis of cellulose precursors. The porosity and pore size distribution of bioSiC can be tailored for specific applications by adequate selection of the wood precursor. Natural and artificial industrial woods were explored as possible bioSiC precursors. Silicon was removed by chemical etching. Relevant microstructural parameters such as pore size distribution, total porosity, and permeability were characterized. Since the filtration process involves large pressure gradients along the material at high temperatures, mechanical properties of porous bioSiC from the different precursors were evaluated at room temperature and 800 °C. The feasibility of porous bioSiC as a filtration material for high temperature gasification processes is discussed in terms of these properties. MDF-bioSiC is shown to be a promising material for such applications because of its good mechanical properties, interconnected porosity, pore sizes, and permeability.     

Oxidation behavior of graphene nanoplatelet reinforced tantalum carbide composites in high temperature plasma flow

Graphene nanoplatelets (GNP) reinforced tantalum carbide (TaC) composites are exposed to a high temperature plasma flow in order to evaluate the effects of GNP on the oxidation behavior of TaC at conditions approaching those of hypersonic flight environments. The addition of GNP is found to suppress the formation of the oxide layer by up to 60%. The high thermal conductivity of GNPs dissipates heat throughout the sample thereby reducing thermal gradients and reducing the intensity of heating at the surface exposed to plasma. In addition, GNPs enhance oxidation resistance by providing toughening which suppresses crack formation and bursting that accelerates oxidation. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) reveal that GNPs have the ability to survive the intense high temperature of the plasma. GNPs are believed to seal oxide grain boundaries and hinder the further influx of oxygen. GNPs also provide nano sized carbon needed to induce the localized reduction of Ta₂O₅ to TaC. Micro computed X-ray tomography (MicroCT) validates that the above mechanisms protect the underlying unoxidized material from the structural damage caused by thermal shocks and high shear forces, by reducing thermal gradients and providing toughness.     

Biopolyamide hybrid composites for high performance applications

This study focuses on the performance characteristics of wood/short carbon fiber hybrid biopolyamide11 (PA11) composites. The composites were produced by melt‐compounding of the fibers with the polyamide via extrusion and injection molding. The results showed that mechanical properties, such as tensile and flexural strength and modulus of the wood fiber composites were significantly higher than the PA11 and hybridization with carbon fiber further enhanced the performance properties, as well as the thermal resistance of the composites. Compared to wood fiber composites (30% wood fiber), hybridization with carbon fiber (10% wood fiber and 20% carbon fiber) increased the tensile and flexural modulus by 168% and 142%, respectively. Izod impact strength of the hybrid composites exhibited a good improvement compared to wood fiber composites. Thermal properties and resistance to water absorption of the composites were improved by hybridization with carbon fiber. In overall, the study indicated that the developed hybrid composites are promising candidates for high performance applications, where high stiffness and thermal resistance are required. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43595.     

Tensile creep and rupture of 2D-woven SiC/SiC composites for high temperature applications

The tensile creep and rupture behavior of 2D-woven SiC fiber-reinforced SiC matrix composites with potential for advanced high temperature structural applications was determined in air at 1315 °C. The results are compared to similar SiC/SiC data in the literature in order to understand the underlying creep and rupture mechanisms. Focus was placed on three different near-stoichiometric SiC fiber-types and three SiC-based matrix systems produced by different process routes. In general, the creep and rupture properties of the tested composites were primarily dictated by the creep resistance of the fiber-type, with the Sylramic-iBN fiber typically showing the best behavior. However, the type of matrix did have an effect on the composite creep and rupture lives due to load-sharing differences for the different matrix types and due to stoichiometry in the case of chemical vapor infiltration SiC matrices.     

高温热结构复合材料的连接技术

综述了高温热结构复合材料之间的连接技术.主要包括C/C复合材料之间采用热压炭素、反应生成SiC的连接;C/SiC复合材料的液相硅渗透连接;SiC基复合材料的钎焊连接;SiC/SiC复合材料与钨板的扩散连接等.并简介了高温热结构复合材料连接在航天及核工业上的应用.     

Suitability of mullite for high temperature applications

High temperature mechanical behaviour of mullite has been studied. Our study include tensile, flexural and compressive creep behaviour and fracture up to 1400 °C. The results obtained in creep are analysed and compared with previous work in the literature. Two regions with different behaviour can be distinguished. The creep rates in bending, tension and compression are very similar in the first region at low stresses and temperatures. It is shown that in this region creep takes place by accommodated grain boundary sliding assisted by diffusion. At higher stresses slow crack growth from defects present in the sample occurs. The stress at which this transition in the deformation mechanism happens is dependent on several factors, the loading system during testing, the grain size, the amount and distribution of glassy phase and the environment. It is claimed the existence of a network of mullite–mullite grain boundaries free of glassy phase associated to the low surface energy of [001] planes. The diffusion rate through these boundaries controls the creep rate, and explains the high creep resistance of mullite. The results presented in this work lead to the conclusion that the mechanism controlling high temperature deformation resistance of mullite materials in a wide range of stress–temperature working conditions is independent of the glassy phase content. Slow crack growth limit the use of mullite at high stresses and temperatures.     

Porous alumina-spinel ceramics for high temperature applications

In order to reduce energy costs, high-temperature insulation porous refractory ceramics have been subjected to increasing demands. Among the techniques used to produce these materials (such as the addition of foaming agents and organic compounds), the pore generation via phase transformation presents key aspects, such as easy processing and the absence of toxic volatiles. In this study, this technique was applied to produce porous ceramics by decomposing an aluminum-magnesium hydro-carbonate known as hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆·4H₂O). It was found out that by using this complex compound, a large fraction of pores can be generated and kept at high temperatures (above 1300 °C) due to the in situ formation of spinel-like phases (MgAl₂O₄).     

A monomeric plasticizer for high temperature applications

The results of this study demonstrate that high molecular weight and predominantly linear diundecyl phthalate (DUP) provide improved performance properties and cost savings when used as a partial or total replacement for trimellitate plasticizers in critical applications such as vinyl electrical insulations and crashpad covers. DUP is shown to be a permanent and stable plasticizer with a high efficiency in improving low-temperature flexibility of formulated vinyls.     

Cold sintered LiMgPO4 based composites for low temperature co-fired ceramic (LTCC) applications

Cold sintered, Li₂MoO₄-based ceramics have recently been touted as candidates for electronic packaging and low temperature co-fired ceramic (LTCC) technology but MoO₃ is an expensive and endangered raw material, not suited for large scale commercialization. Here, we present cold sintered temperature-stable composites based on LiMgPO₄ (LMP) in which the Mo (and Li) concentration has been reduced, thereby significantly decreasing raw material costs. Optimum compositions, 0.5LMP-0.1CaTiO₃-0.4K₂MoO₄ (LMP-CTO-KMO), achieved 97% density at <300°C and 600 MPa for 60 minutes. Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray mapping confirmed the coexistence of end-members, LMP, CTO, and KMO, with no interdiffusion and parasitic phases. Composites exhibited temperature coefficient of resonant frequency ~ –6 ppm/°C, relative permittivity ~9.1, and Q × f values ~8500 GHz, properties suitable for LTCC technology and competitive with commercial incumbents.     

Porous silicon carbide structures with anisotropic open porosity for high- temperature cycling applications

We have prepared porous silicon carbide by a novel two-step template method. Graphite/SiC composites of required size and shape are first fabricated by hot pressing at 2125 °C, followed by the removal of the graphite template by controlled heat treatment. The anisotropy in the composite structure is restored after the removal of the template and porous SiC with anisotropic properties is obtained. The composite can be easily machined by electrical discharge machining because of the presence of graphite, and porous SiC can be obtained by heat treatment, solving the inherent difficulty in the machining of SiC. The mechanical properties, thermal conductivity, and thermal shock resistance of porous SiC have been studied in both directions. The material shows good thermal shock resistance in the perpendicular to pressing direction even at 1400 °C. Hence porous SiC suitably machined preserving the proper direction can be a potential candidate for thermal cycling applications.     

Electrical resistivity of silicon nitride-silicon carbide based ternary composites

New electrically conductive ternary composites were developed by adding 8 vol.% of ZrN or ZrB₂ to a Si₃N₄-SiC matrix. During hot pressing, ZrB₂ reacted with Si₃N₄ to form ZrSi₂, ZrN, Si and BN whereas added ZrN did not undergo any reactions in the Si₃N₄-SiC-ZrN composite. The composites modified by ZrN or ZrB₂ addition showed a lower resistivity (7 × 10³ Ω cm and 3 × 10⁻¹ Ω cm) compared to the matrix (3 × 10⁴ Ω cm). Further studies on the grain size distribution and the volume ratio of conducting and non-conducting phases excluded a percolation network of ZrN and ZrSi₂ grains. In fact, doping of SiC grains and modified grain boundaries as a consequence of the formation of liquid phases during sintering are suggested to be the reason for the significantly lower resistivity of materials containing ZrSi₂.A decrease in the composite resistivity due to a subsequent heat treatment was obtained for all hot-pressed composites.     

Studies on unsaturated polyester composites for high‐temperature applications

Unsaturated polyesters are widely used in a number of applications. However, they fall short in areas where high thermal stability and performance at higher temperatures are required. Previous investigations have studied the kinetics and degradation behavior of bismaleimide‐based unsaturated polyester composites. The current study aimed to investigate the effects of bismaleimide on the mechanical properties and thermal class of a bismaleimide unsaturated polyester composition. Addition of bismaleimide to the composition resulted in an increase in the thermal index of the material, thus making it useful in applications where high temperature stability is required. However, once degradation was initiated, the addition of bismaleimide had a detrimental effect on the stability of the composites. J. VINYL ADDIT. TECHNOL., 2012. © 2012 Society of Plastics Engineers     

Growth and high temperature performance of semi-insulating silicon carbide

The problems of obtaining insulating properties in bulk single-crystal silicon carbide by vanadium doping under the LETI method growth process are considered. The prime novelty of this work is the growth of a semi-insulating bulk single-crystal n-4H-SiC:V by the LETI method in vacuum from a vanadium and aluminium-containing source. The obtained 4H-SiC:V material possesses resistivity >10⁷ Ω cm at 20°C and activation energy ∼1.6 eV at 20–800°C and can be applied as a semi-insulating substrate material for extreme electronics based on silicon carbide or nitrides.     

Novel isocyanate-based matrix resins for high temperature composite applications

Polyoxazolidone composites were prepared from polymeric isocyanate (PAPI 901) and epoxides (Epon 828 and DEN 431) in the presence of an oxazolidone-forming catalyst, triphenylantimony iodide. The effects of isocyanate to epoxide equivalent ratio, type of epoxide, and amount of fiberglass reinforcement on the composite properties were studied as well as the effects of post-curing temperature and time. Increasing the fiberglass content of the polyoxazolidone composites resulted in an improvement of the thermal and mechanical strength properties. The heat deflection temperature of all polyoxazolidones was > 250°C. The retention of the tensile strength at 150°C was excellent, ∼90% or higher. Polyoxazolidone composites based on DEN 431 at 1.2 isocyanate to epoxide equivalent ratio with 70 wt.% of fiberglass and post-cured at 150°C for 48 h exhibited the best properties. According to the results of DMA, TMA and DSC, the maximum operating temperature for polyoxazolidone composites is around 200°C. The TGA data showed that the decomposition temperature was ∼330°C.     

Low temperature sintering of barium titanate based ceramics with high dielectric constant for LTCC applications

The sintering temperature of BaTiO₃ powder was reduced to 900 °C due to the ZnO-B₂O₃-Li₂O-Nb₂O₅-Co₂O₃ addition. Excellent densification was achieved after sintering at 900 °C for 2 h. The low sintering temperature of newly developed capacitor materials allows a co-firing with pure silver electrodes. The dielectric constant and the temperature stability of the dielectric constant are strongly correlated with the composition of the ZnO-B₂O₃-Li₂O additives. A high dielectric constant up to 3000 and a dielectric loss less than 0.024 were measured on multilayer capacitors sintered at 900 °C with silver inner electrodes.     

Comparison of two high temperature treatment methods on preparing electrically conductive polysulfide/Ag composites for aerospace sealant applications

High electrical conductivity of aircraft fuel tank sealant is vital to prevent charge accumulation during operation. In this work, we prepare highly conductive polysulfide (PS)/Ag composite (10⁶ S m⁻¹) via two thermal treatment methods-namely post cure annealing and high temperature (HT) curing, where the room temperature (RT) cured sample was previously found insulative. The HT cure samples resulted in a milder hardness increase compared to the RT cure-annealed ones. Through various chemical and thermal analyses, a strong coordination of the sulfide components on Ag surfaces is found blocking the electron tunneling pathways at RT, which turns out to be weakened at HT. Post cure annealing causes the decomposition of the coordinated segments and provides intimate contact of Ag particles to support conductivity. HT cure on the other hand prevents the Ag/S coordination in the first place, therefore producing highly conductive PS/Ag composite with softness well maintained which makes it the preferred method for conductive sealant applications.