The preparation and performance of high-temperature adhesives for graphite bonding

Two kinds of high-temperature adhesives (HTAs) were prepared. One was composed of phenol-formaldehyde (PF) resin and boron carbide (PF+B₄C), the other was composed of PF resin, B₄C and fumed silica (PF+B₄C+SiO₂). Graphite materials were bonded by the above adhesives and heat-treated at temperatures ranging from 200 to 1500 °C. The joining strength was tested at room temperature. The results show that the graphite joints exhibit satisfactory bonding strength and that ceramics fillers show a marked property modification effect. The strength of graphite joints bonded by PF+B₄C and PF+B₄C+SiO₂ adhesive and treated at 1500 °C are 9.3 and 17.1 MPa, respectively. The property modification mechanism of ceramics fillers is also discussed in this paper. A strong chemical bonding force is introduced at the bonding interface and the volume shrinkage is restrained, which can be responsible for the good adhesive properties of HTAs for graphite bonding.     

Effect of the evolution of phenol–formaldehyde resin on the high-temperature bonding

The novel high-temperature adhesives (HTAs) were prepared using phenol–formaldehyde (PF) resin as matrix and elemental silicon or boron carbide as modification additives. The bonding properties of the above adhesives were investigated by the bonding experiment on graphite substrate. The graphite joints were heat treated at high temperatures ranging from 200 to 1500 °C. It was shown that the degradation and the content of PF resin had important influences on the bonding properties of the HTAs. The pyrolysis and degradation of the organic resin led to the drastic volume shrinkage and the decrease of mechanical strength of resin matrix. It is the main reason leading to the failure of the joints treated at high temperatures, especially in the range of 400–650 °C. It is concluded that the satisfactory bonding property of the novel organic resin matrix HTAs lies in two aspects: (i) the selection of additives with good modification effect, and (ii) the optimized ratio between resin matrix and modification additives.     

Synthesis of spherical shape borate-based bioactive glass powders prepared by ultrasonic spray pyrolysis

Spherical shape borate-based bioactive glass powders with fine size were directly prepared by high temperature spray pyrolysis. The powders prepared at temperatures between 1200 and 1400 °C had mixed phase with small amounts of fine crystal and an amorphous rich phase. Glass powders with amorphous phase were prepared at a temperature of 1500 °C. The mean size of the glass powders prepared by spray pyrolysis was 0.76 μm. The glass powders prepared at a temperature of 1200 °C had two distinct exothermic peaks (T_c1 and T_c2) at temperatures of 588 and 695 °C indicating crystallization. The glass transition temperature (T_g) of the powders prepared at a temperature of 1200 °C was 480 °C. Phase-separated crystalline phases with spherical shape were observed from the surface of the pellet sintered at a temperature of 550 °C. Crystallization of the pellet was completely occurred at temperatures of 750 and 800 °C. The pellets sintered at temperatures below 700 °C had single crystalline phase of CaNa₃B₅O₁₀. The pellet sintered at a temperature of 800 °C had two crystalline phases of CaNa₃B₅O₁₀ and CaB₂O₄.     

The effect of high temperature heat-treatment on the strength of C/C to C/C-SiC joints

Joining of carbon fiber reinforced carbon matrix composite to a carbon fiber reinforced C-SiC dual matrix composite has been realized through a reaction joining process using boron-modified phenolic resin with micro-size B₄C and nano-size SiO₂ powder additives. The effect of the heat-treatment temperature on the retained strength of the joints, calculated by dividing the strength of the heat-treated joints by the strength of the joints before heat-treatment, was studied. The maximum retained strength of the joints is 91.9% after heat-treatment at 1200 °C for 30 min in vacuum, indicating good heat-resistance of the joints. The interlayer with a thickness of about 25 μm is uniform and densified. There are no obvious cracks or pores at the interfaces. The interlayer is composed of B₄C, SiO₂, glassy carbon, B₂O₃ and borosilicate glass. Si diffuses from the interlayer into the substrates and reacts with carbon to form SiC. Both B and O migrate from the interlayer into the substrates, contributing to the interfacial bonding. The B₄C and the SiO₂ powder additives contribute to the densification of the interlayer, the bonding at the interfaces and the heat-resistance of the joints.     

Bonding ZrB2-SiC-G ceramics using modified organic adhesive for engineering applications at ultra high temperatures in air

It is a challenge to bond ceramics for engineering applications at ultrahigh temperatures in air. In this paper, a high temperature organic adhesive (HTOA) was prepared using methylphenylsilicone resins (MPSR) as the matrix, trisilanolisobutyl-methylsilicone resin/polyhedral oligomeric silsesquioxane (POSS) as the modifier, ZrB₂, SiO₂ and Si powders as the inorganic fillers, and γ-aminopropyltriethoxysilane (KH550) as the curing agent. The synthesized HTOA was used to bond ZrB₂-SiC-G ceramic (ZSG). The ceramic yield of MPSR was increased from 71% to 91% after being modified by trisilanolisobutyl-POSS. The average shear strength of ZSG joints bonded by HTOA was 13.2 MPa at room temperature. After 1500 °C/1 h processing, the bonding strength between HTOA and ZSG ceramic was 53.8 MPa. The inter-diffusion of elements between the HTOA and the ZSG occurred at 1500 °C and ZrSiO₄ compound was formed via the interface reaction. The excellent high-temperature performance of the prepared HTOA makes it one of the convenient and effective organic adhesive for joining ZSG for engineering applications at ultrahigh temperatures in air.     

Effect of Al(H2PO4)3/Zn/B4C doped resin on properties and microstructure of unfired Al2O3–C slide plate materials

The unfired Al₂O₃–C slide plates have the advantages of energy saving, environment friendly, efficiency and relatively low-cost. However, the decomposition and oxidation of the phenol-formaldehyde (PF) resin at elevated temperatures deteriorate the properties and decrease service life of the unfired slide plates. In order to improve the property of resin, the doped PF resin is prepared by incorporating Al(H₂PO₄)₃, Zn and B₄C powders. The effects of the doped resin on medium-high temperature properties and microstructure of the unfired slide plate materials have been investigated. The results show that Zn and B₄C doped resin contributes to notable increasing the density and strength properties at medium temperature, because Zn and B₄C easily oxide and thus protect resin from oxidation, leading to form a dense structure. Zn and B₄C doped resin can significantly improve hot modulus of rupture of the materials at 1400 °C, which is due to Zn and B₄C react with oxidative gases leading to increase in concentration of C(g), CO and N₂, Al and Si would react with C(g), CO(g) and N₂(g) to form AlN and SiC whiskers creating strengthening effect. Specimens with Zn and B₄C doped resin addition have good oxidation resistance at 1500 °C, because Zn and B₄C in the surface of the material react with O₂ to form ZnAl₂O₄ or mullite containing dense glass film, which would retard O₂ diffusion into the inner of the specimens.     

Oxidation resistance of a gradient self-healing coating for carbon/carbon composites

A gradient self-healing coating consisting of three layers, SiC-B₄C/SiC/SiO₂, was examined as a multilayer protection for carbon/carbon composites. The inner layer was made of B₄C and β-SiC, the middle layer was a SiC based layer, and the outer layer was SiO₂ as an airproof layer. Both inner and middle layers were produced to be diphase structure by a pack cementation technique, and the outer airproof layer was prepared by hydrolyzing tetraethylorthosilicate. SEM and EDS investigations showed that the coating had a compositional gradient between B₄C and SiC. The coating showed great self-healing properties from 500 °C to 1500 °C. The weight loss rate of the coated composites was less than 1.3% after 50 h at 1500 °C, and coating represented excellent thermal shock resistance at 1500 °C. The oxidation kinetics of coated carbon/carbon composites showed that the Arrhenius curve consisted of three parts with two broken points at about 700 °C and 1100 °C, and the three parts corresponded to three different self-healing mechanisms in different temperature regions.     

Preparation and high temperature oxidation of SiC compositionally graded graphite coated with HfO2

SiC compositionally graded (SCG) graphite was coated with sol-gel-derived HfO₂ films and oxidized at 1500 °C in air. SCGed graphite was produced by reaction of graphite with molten Si at 1450 °C for 10 h. The sol-gel HfO₂ precursor solution was prepared by dissolving HfCl₄ in ethanol and refluxing with diethanol-amine and HNO₃, and was coated on SCGed graphite by dipping. The HfO₂-coated SCGed graphite was produced by decomposition of the precursor under conditions determined from the results of TG, DTA, and MS analysis. Oxidation of HfO₂-coated SCGed graphite was performed at 1500 °C in air, revealing a small weight loss (0.6 mg cm⁻²) after 15 h. It was found that HfO₂-coated SCGed graphite exhibits extremely high oxidation resistance, which may be due to the formation of HfSiO₄ acting to heal pores or cracks.     

The effects of rare-earth oxide additives on the densification of pressureless sintering B4C ceramics

Boron carbide ceramics used as neutron absorbing materials in fast breeder reactor were fabricated with boron carbide powders and different rare-earth oxide additives by pressureless sintering. The effects of rare-earth oxide as well as phenolic resin on densities and mechanical properties of the composites were studied. The addition of Dy₂O₃, Eu₂O₃, and Sm₂O₃ was found to be beneficial in the densification of B₄C ceramics. B₄C with 4 wt% rare-earth oxide and 18 wt% phenolic resin, exhibiting bulk density of 90–96% T.D., flexural strength of 276–358 MPa, could be prepared by pressureless sintering at 1960–2080 °C, which are capable of meeting the requirement of fast breeder reactor.     

Phase, microstructure evolution and sintering of Sr-doped TiB2 precursors

Precursors for the preparation of bulk Sr-doped TiB₂ composites were synthesized by modified Pechini method. The high temperature behaviour of homogeneous Sr-Ti-B-C-O gels was investigated in the range 1200-1650 °C. DTA-TG analysis of the precursor powder shows two steps of the carbothermal reduction with endothermic peaks at temperatures of 1335 °C and 1500 °C. The influence of strontium content (2, 5, 10, 20 and 50 mol.%) on the phase composition and morphology of powders at 1650 °C was studied.Due to the shift of TiB₂ diffractions and the detection of strontium in TiB₂ grains by EDX analysis the formation of Ti_1 − xSr_xB₂ solid solution is assumed in the Sr-doped powders. Finally, Sr-doped TiB₂ composites were inductive hot-pressed from the as prepared powders at 1900 °C for 7 min. The formation of SrTiO₃ phase in the powders is serving as a sintering aid during the preparation of bulk Sr-Ti-B composites. The exaggerated grain growth (grain size up to ∼60 μm) occurs during the sintering with increasing content of strontium in the precursor.     

The preparation and properties of SiCw/B4C composites infiltrated with molten silicon

Silicon carbide whisker (SiC_w) toughened B₄C composites have been prepared by pressureless infiltration of B₄C–SiC_w–C preforms with molten silicon under vacuum at 1500 °C. The effect of SiC_w addition on bulk density, hardness, bending strength, fracture toughness and microstructure of SiC_w/B₄C composites is discussed. It is revealed that the addition of SiC_w improves the fracture toughness of B₄C ceramic, but reduces its bending strength at the same time. The maximum fracture toughness for SiC_w/B₄C composite with 24 wt% SiC_w addition is 4.88 MPa m^1/2, which is about 9% higher than that of the one without SiC_w, but at the same time, the bending strength reduces to the minimum value 243 MPa, reduced by 25%. XRD analysis shows that the phase composition of reaction bonded SiC_w/B₄C composites is B₄C, SiC, Si, and B₁₂ (C, Si, B)₃, with no residual C. And the main toughening mechanism of SiC_w is whisker pulling up.     

Microstructure of carbon/carbon composites reinforced with pitch-based ribbon-shape carbon fibers

Carbon/carbon composites were prepared with ribbon-shape pitch-based carbon fibers serving as reinforcement and thermosetting PFA resin and thermoplastic pitch as matrix precursors. The composites were heat treated to 1000, 1600 and 2700 °C. Microstructural transformations taking place in the reinforcement, carbon matrix, and the interface were studied using polarized optical and scanning electron microscopy. The fiber/matrix bond and ordering of the carbon matrix in heat-treated composites was found to vary depending on the heat treatment temperature of the fibers. Stabilized fiber cleaved during carbonization of resin-derived composites. In contrast, fibers retain their shape during carbonization of pitch matrix composites. Optical activity was observed in composites made with carbonized fibers; the extent decreases with increased heat treatment of the fibers. Studies at various heat treatment temperatures indicate that ribbon-shape fibers developed ordered structure at 1600 °C when co-carbonized with thermosetting resin or thermoplastic pitches.     

Ceramizable phenolic adhesive modified by different inorganic particles: A comparative study of thermal stability,bonding strength and bonding mechanism

In this work, two ceramizable phenolic adhesives were prepared using ZrSi₂ particles or ZrSi₂/B₄C mix particles as the inorganic fillers. The thermal stability, bonding strength, microstructure and phase composition of the adhesives were investigated by TGA, shear strength of Al₂O₃ joints, SEM, EDS, XRD and XPS. The results show that these two adhesives have different bonding performances and ceramicization evolutions above 600 °C in air due to the addition of the second phase particles B₄C. The bonding strength of ZrSi₂/B₄C modified phenolic adhesive after treatment at 1200 ℃ can be as high as 36.6 MPa, while the bonding strength of ZrSi₂ modified phenolic adhesive under the same conditions is only 17.5 MPa. B₄C undergoes oxidation reaction before ZrSi₂, and the oxidation product B₂O₃ liquid phase not only reacts with ZrSi₂ to form oxidation-resistant ZrB₂, but also can dissolve the high temperatures defects of the adhesive and chemically bond with the Al₂O₃ substrates at the interface.     

Crystallization behaviour of neodymium doped yttrium silicate nanophosphors

Sol-gel technique was used to prepare yttrium silicate powders doped with 0.01Nd³⁺. The crystallite sizes were determined to be 23 ± 0.5 nm from the XRD patterns of the powders annealed at 960 °C. The Y_4.67(SiO₄)₃O, Y₂Si₂O₇ and Y₂SiO₅ crystalline phases were observed upon heat treatment at 960 °C which is much lower than 1500-1650 °C are reported before.     

Preparation and characterization of carbon films prepared from poly(vinyl alcohol) containing metal oxide and nano fibers with iodine pretreatment

This research focused on the combination of catalyst effect of metal oxide, thermal conductive effect of carbon nano fibers and iodine pretreatment during carbonization of polymer precursor in order to prepare tough carbon films. Poly(vinyl alcohol) (PVA) composites containing metal oxide (Fe₃O₄) and vapor-grown carbon fibers (VGCFs) were prepared by gelation/crystallization method with the freezing/thawing technique. The dry gel films as precursor were pretreated in the atmosphere of vapor iodine, and then heat-treated at 600-1200 °C. The combined effect of iodine, Fe₃O₄ and VGCF on the carbonization of PVA was analyzed with thermo-gravimetric analysis, X-ray diffraction, and scanning electron microscope in details. Iodine pretreatment for 24 h significantly promoted the dehydration of PVA, and resulted the carbon film with a high crystallinity. Fe₃O₄ as catalyst facilitated the carbonization of PVA at a low temperature of 800-900 °C. The addition of VGCFs was found to play an important role to prepare tough films by mild carbonization due to its high thermal conductivity. The degree of graphitization in the carbon film depended on the filler contents, pretreatment conditions and carbonization conditions. The graphitization degrees for G- and T-components in the film were investigated on the basis of X-ray diffraction intensity distribution from the (0 0 2) plane.     

Interaction of boron suboxide with compacted graphite cast iron

The chemical interaction of boron suboxide (B₆O) with compacted graphite cast iron (CGI) was investigated using static interaction diffusion couples between B₆O and CGI at 700 °C, 900 °C and 1100 °C for 1 h. This interaction offers the possibility to evaluate the potential of B₆O as a cutting tool. The microstructures and phase compositions of the interaction zones were investigated. At 700 °C and 900 °C the chemical interaction was minimal. However, at 1100 °C, Fe₂B and SiO₂ were formed at the interface. Hence, machining at 1100 °C is likely to result in chemical wear.     

The preparation and performance of short carbon fiber reinforced adhesive for bonding carbon/carbon composites

A short carbon fiber reinforced adhesive for bonding carbon/carbon composites was developed. We found that when the thickness of the bonding layer was 80 μm, the concentration of short carbon fiber was 0.2 wt.%, and the heat-treatment temperature was 1000 °C, the adhesive could operate below 1700 °C and endure 20 times of thermal shock circles at 1500 °C. Finite element and micrograph analysis indicated that the bonding strength was larger than the interlaminar shear strength of carbon/carbon substrate, so that the fracture did not occur in the bonding layer but the carbon/carbon substrate. Weibull distribution analysis results showed that the Weibull modulus was 21.56 and the bonding strength was 11.43 MPa. We investigated that short carbon fiber could advance the tensile strength and thermal shock resistance of the adhesive, release residual stress and inhibit extension of micro-crack in the bonding layer.     

Thermal and ablative properties of low temperature carbon fiber-phenol formaldehyde resin composites

The thermal and ablative properties of phenol formaldehyde resin (PF) composites reinforced with carbon fibers heat-treated at low temperature have been investigated. Low temperature carbon fibers (LTCF) were obtained by a continuous carbonization process from stabilized PAN fibers at 1100 °C. The properties of LTCF reinforced PF (LTCF-PF) composites are compared with those of high temperature carbon fiber (HTCF) reinforced PF (HTCF-PF) composites. The thermal conductivity of the LTCF-PF composite is lower than that of HTCF-PF composite by about 35 and 10% along the directions parallel and perpendicular to the laminar plane, respectively. It was found from the ablation test using an arc plasma touch flame that the erosion rate is higher by about 30% in comparison with HTCF-PF composite. The result suggests that use of LTCFs as reinforcement in a composite may improve the thermal insulation of the composite but decrease the ablative resistance.     

Preparation of microporous sorbents from cedar nutshells and hydrolytic lignin

Carbon nutshells and hydrolytic lignin were used as starting materials for the preparation of microporous active carbons. Optimum parameters for cedar nutshell carbonization have been selected (temperature of carbonization 700-800 °C, rate of heating less than 3 °C/min) for the preparation of microporous carbons (average pore width 0.56 nm). The textural characteristics of microporous carbons made from nutshell are similar to those of a ‘Coconut’ carbon molecular sieve, but the latter has both a higher CO₂ adsorption capacity and a higher coefficient of N₂/O₂ separation. The influence of carbonization and steam-activation parameters on the microtexture and molecular-sieve properties of granular carbons made from hydrolytic lignin was also investigated. A low rate of heating (less 3 °C/min) promotes the formation of micropores with average sizes around 0.56-0.58 nm at carbonization temperature 700 °C. At the same carbonization temperature the average sizes of micropores were 0.7-0.78 nm at rates of heating more than 3 °C/min. The activation of lignin-char with steam at 800 °C resulted in the formation of active carbons with more developed micropore volume (0.3-0.35 cm³ g⁻¹) and with micropores of widths around 0.6-0.66 nm which are able to separate He from a He-CH₄ mixture. The size of the micropores was varied as a function of burn off value.     

Preparation and Performance of the Room‐Temperature‐Cured Heat‐Resistant Phosphate Adhesive for C/C Composites Bonding

Carbon/carbon(C/C) composites were bonded with a phosphate adhesive prepared from H₃PO₄ (orthophosphoric acid) and Al(OH)₃ as the matrix and Si and CuO as the inorganic fillers. The mechanical properties were tested at room temperature after the bonded specimens were treated from RT to 1500°C. The results showed that the addition of CuO could greatly improve the adhesive's mechanical properties due to the formation of Cu_nSi (copper silicides). The shear strength of bonded specimens could be up to 8.2 MPa without heat treatment and remain above 2 MPa within the whole heat‐treatment temperature range.