Control of molecular orientations in mesophase pitch-based carbon fibre by blending PVC pitch

Coal tar-derived mesophase pitch and its blends with PVC pitch in 5 or 10 wt% were spun at temperatures from 340 to 390° C by applying pressurized nitrogen. The parent mesophase pitch and the blended pitch showed an excellent spinnability at temperatures from 360 to 380° C and from 350 to 380° C, respectively, to give a thin pitch fibre of 10m diameter. The transverse texture of the fibres from the parent mesophase pitch showed the radial orientation regardless of a higher spinning temperature of 390° C. In contrast, those from the blended pitches showed random orientation even at the lower spinning temperature of 350° C. The amounts of the blend extruded by spinning at each temperature under 0.2 kg cm^–2 G^–1 were always larger than those of the mesophase pitch. It is clarified in the present study that blending PVC pitch can realize stable spinning at lower temperatures, where the molecular orientation in the transverse section of the resultant carbon fibre was controlled through decreasing the viscosity of the whole mesophase pitch.     

Tensile creep behaviour of a silicon carbide-based fibre with a low oxygen content

The high-temperature mechanical behaviour and microstructural evolution of experimental SiC fibres (Hi-Nicalon) with a low oxygen content (<0.5 wt%) have been examined up to 1600 °C. Comparisons have been made with a commercial Si-C-O fibre (Nicalon Ceramic Grade). Their initial microstructure consists of -SiC crystallites averaging 5–10 nm in diameter, with important amounts of graphitic carbon into wrinkled sheet structures of very small sizes between the SiC grains. The fall in strength above 800 °C in air is related to fibre surface degradation involving free carbon. Crystallization of SiC and carbon further develops in both fibres subject to either creep or heat treatment at 1300 °C and above for long periods. The fibres are characterized by steady state creep and greater creep resistance (one order of magnitude) compared to the commercial Nicalon fibre. The experimental fibre has been found to creep above 1280 °C under low applied stresses (0.15 GPa) in air. Significant deformations (up to 14%) have been observed, both in air and argon above 1400 °C. The stress exponents and the apparent activation energies for creep have been found to fall in the range 2–3, both in air and argon, and in the range 200–300 kJ mol^–1 in argon and 340–420 kJ mol^–1 in air. The dewrinkling of carbon layer packets into a position more nearly aligned with the tensile axis, their sliding, and the collapse of pores have been proposed as the mechanisms which control the fibre creep behaviour.     

Characteristics of a continuous Si-Ti-C-O fibre with low oxygen content using an organometallic polymer precursor

A continuous Si-Ti-C-O fibre with 12 wt% oxygen content, which is lower than the usual 18 wt% found in the normal fibres, was synthesized by using polytitanocarbosilane which has fewer Si-Si bonds than the usual precursor polymer. The density, tensile strength, tensile modulus and thermal conductivity were found to be 2.37 g cm^–3, 3.4±0.3 GPa, 190±10 GPa and 1.40 W m^–1 K^–1, respectively. Amongst these properties, the tensile modulus was improved by 20 GPa and the thermal conductivity had a higher value in comparison with that of the ordinary Si-Ti-C-O fibre with 18 wt% oxygen content. The Si-Ti-C-O fibre with a 12 wt% oxygen content has a better heat resistance above 1400 °C in an argon atmosphere and 1300 °C in air, than the usual fibre. About 60 and 40% of its tensile strength at room temperature were retained in air at respectively, 1500 and 1600 °C. This improved ceramic fibre is considered to be useful as a reinforcing material for advanced composites such as high-temperature ceramic matrix composites and metal matrix composites.     

Pulsed chemical vapour infiltration of SiC to three-dimensional carbon fibre preforms

Three-dimensional carbon fibre preforms were infiltrated with silicon carbide from a gas system of CH₃SiCl₃-H₂ using a process of pressure pulsed chemical vapour infiltration. To infiltrate to a deep level, the temperature had to be lowered to 870–900°C, and the hold time per pulse below 1.0 s. Three-dimensional carbon fibre preforms partly filled with SiC fine powder were compared with those without filler. The weight of the preforms increased linearly with increasing number of pulses up to 10⁵ when no filler was present. However, the weight increase slowed down above 8×10⁴ pulses when the filler was used. Preforms with and without SiC filler showed three-point flexural strengths of 160 and 80 MPa after CVI of 10⁵ pulses, respectively. In order to improve the strength, a denser filling of SiC powder is necessary.     

Oxygen distribution in the mesophase pitch fibre after oxidative stabilization

The oxygen distribution in the transverse section of 30m diameter mesophase pitch fibres after oxidative stabilization was measured by using EPMA (electron probe X-ray microanalyser) to clarify the progress of the oxidative reaction and diffusion of the oxidant during the stabilization. Oxygen was distributed in shallow gradients regardless of the stabilization time from the surface to the centre of the mesophase pitch (MP) fibres stabilized at 230° C, suggesting sufficient diffusion of the oxidant to the centre of the fibre at this temperature. In contrast, steeper gradients of distribution were observed in the MP fibres stabilized at 270° C although oxygen up-take of the centre increased steadily with the longer stabilization time to decrease the gradient. Much steeper gradients of the oxygen distribution were observed in the cross-sectioned surface of the fibres stabilized at 300° C for 15 and 30min. The gradient became much steeper with longer stabilization, suggesting some barriers in the deeply oxidized zone which may block the oxygen diffusion. The PVC-10 fibres, whose reactivity was enhanced by blending PVC pitch of 10wt%, showed steeper distributions of oxygen after the stabilization at 270° C comparing to those of the MP fibres stabilized under the same conditions. It showed steeper gradient with the longer stabilization time. In conclusion, stabilization at a lower temperature (230° C) allows relatively rapid diffusion of the oxidant into the centre of the MP fibre during rather slow stabilization but, a higher temperature of stabilization (at 300° C) and/or higher reactivity of the mesophase pitch accelerates the oxidation much more rapidly than the diffusion, providing a blockade zone for the oxygen diffusion near the fibre surface. The extensive oxidation may cross-link three dimensionally the mesophase molecules thus allowing no diffusion of oxygen among the molecules. Such diffusion control tends to provide skin-core structure in the carbonized fibre.It should be noted that fibre thinner than 10m showed no skin-core structure. Diffusion within 5m from the surface may be rapid under any conditions.     

Crack growth resistance of fluoroelastomer vulcanizates filled with particulate and fibre filler

Crack growth characteristics of fluoroelastomer vulcanizates filled with carbon black (size: 30 nm) and short Kevlar fibre (6 mm length) under static and dynamic conditions have been evaluated. The fibre reinforced vulcanizate shows higher J _c values (80 kJ m^–2) than both the carbon black filled sample (44 kJ m^–2) and the control (15 kJ m^–2). However, under dynamic fatigue conditions, the black filled sample is stronger.     

A new glassy carbon fibre

It is shown that phenol-hexamine polymers may be extruded from the melt to produce fibres which may be carbonised to form fine high-strength glassy carbon fibres with a tensile strength of up to 2 GNm^–2 (300 000 Ib in^–2) after 900° C heat-treatment. The fibres have a specific modulus of 5 Mm compared with 14 Mm for carbonised polyacrylonitrile fibres and 3 Mm for silica glass fibres. Both strength and modulus increase rapidly with decrease in diameter. The fibres are subjected to no special surface treatment after extrusion but electron microscopy indicates the presence of a thin textured sheath surrounding a true glassy carbon core in the final fibre. The fibres have the advantages of glassy carbon (inertness to chemical attack, resistance to abrasion) and give promise of a new range of cheap high-strength carbon fibres derived from coal tar fractions.     

Synthesis of polycrystalline zirconia fibre with organozirconium precursor

Polycrystalline zirconia fibre was successfully synthesized by pyrolysis of preceramic fibre formed from an organozirconium compound. Dibutoxybis(2, 4-pentadionato)zirconium (BPZ) was polymerized at 150° C and 10² Pa, yielding a viscous polymeric product. The infrared absorption bands of the Zr-O bond changed from separate to coalesced bands after polymerization. The signals of the¹³C NMR spectrum of BPZ changed from sharp singlets to multiplets after polymerization. The molecular weight of the polymer was between 400 and 1000. The viscosity of polymer was 580 Pa sec at 30° C and a shear rate of 1.0 sec^–1. The polymer viscosity decreased with increased temperature from 30 to 60° C. The precursor polymer pyrolysed at 400° C in air was amorphous to X-rays, and crystallized in a mixture of monoclinic and tetragonal phases at 450° C. Tetragonal zirconia was synthesized from the polymer including 4.3 mol % yttrium compound (2.2 mol % yttria) after heat treatment at 1200° C for 1 h. The precursor fibres were pyrolysed to yield fine-grained fibres of tetragonal zirconia at 1200° C for 1 h.     

Carbon fibre/nickel compatibility

The effects of evaporated pure nickel coatings on the room-temperature fracture strength and microstructure of individual carbon fibres have been investigated. The fracture strength of a nickel-coated fibre was not affected by anneals in a 10^–6 torr vacuum below 1000°C. However, after higher temperature anneals a reduction in strength was noted, the magnitude of which was time-dependent, but almost independent of the thickness of the coating. The reduction in strength was not related to fibre recrystallisation, but appeared to be controlled by the formation of a carbon-nickel interface, with an energy for adhesion of approximately 110 kcal mole^–1.     

Flux growth and characterization of TiC crystals

Single crystals of TiC were grown using nickel and cobalt metal as a flux at soaking temperatures of 1500 to 1800° C and at compositions of 12.5 to 30 mol% TiC. The use of cobalt flux produced crystal sizes less than 0.5 mm under all conditions. With a nickel flux, a maximum crystal size of 1.5 mm was obtained at 1700 and 1800°C and at 20 and 25 mol% TiC composition, using a cooling rate of 3° C min^–1. A slower cooling rate of 0.2° C min^–1 also gave crystals of 1.5 mm at 1600° C. The crystals were cubic and metallic-lustre silver grey in colour. The lattice parameter of the crystal was measured to bea ₀ = 0.432 75 ± 0.000 05 nm, with nearly stoichiometric composition. The grown faces were of the {1 0 0} family with a dislocation density around 10⁷ cm^-2. The Vickers' microhardness on these faces was in the range 2600 to 2800 kg mm^–2. The nickel impurity and free carbon contents in the crystals were 700 to 1000 p.p.m. and 0.8 to 4 wt%, respectively.     

Carbon dioxide decomposition into carbon with the rhodium-bearing magnetite activated by H2-reduction

The CO₂ decomposition into carbon with the rhodium-bearing activated magnetite (Rh-AM) was studied in comparison with the activated magnetite (AM). The Rh-AM and the AM were prepared by flowing hydrogen gas through the rhodium-bearing magnetite (Rh-M) and the magnetite (M), respectively. The rate of activation of the Rh-M to the Rh-AM was about three times higher than that of the M to the AM at 300 °C. The reactivity for the CO₂ decomposition into carbon with the Rh-AM (70% CO₂ was decomposed in 40 min) was higher than that with the AM (30% in 40 min) at 300 °C. The Rh-M was activated to the Rh-AM at a lower temperature of 250 °C, and the Rh-AM decomposed CO₂ into carbon at 250 °C. On the other hand, the M was little activated at 250 °C.     

Microstructure-related fracture behaviour of injection moulded short fibre reinforced polyarylamide in dry and wet states

The fracture behaviour of injection moulded polyarylamide (PAR) composites containing 30, 50 and 60 wt% glass and 30 wt% carbon fibres has been investigated in both dry and wet states. Kinetics of moisture absorption study revealed that PAR and its composites exhibit Fickian behaviour. The incorporation of short fibres into a PAR matrix has resulted in the reduction of both maximum moisture content (M _m ) and diffusion coefficient (D). The fracture mechanical characterization of the various materials was evaluated by using notched compact tension (CT) specimens. Testing was performed as a function of temperature (T = –40, 20 and 80°C) and crosshead speeds (v = 1 and 1000 mm min^–1) on as received (AR) specimens. The influence of water uptake due to the hygrothermal ageing (HA) process on residual fracture performance was also studied. The combined action of moisture-induced plasticization of the PAR matrix and interfacial degradation has been concluded to play a significant role in controlling the fracture behaviour of the (HA) composites. The residual fracture properties of both neat PAR and its composites are almost fully recovered in the case of redrying (RD). Failure mechanisms of both the matrix and the composites, assessed by fractographic studies in a scanning electron microscope (SEM) are discussed.     

Permissible crack widths in steel fibre reinforced marine concrete

The paper presents some results from a continuing study of the marine durability of steel fibre reinforced concrete. The overall aim of the investigation is to develop the material for marine applications. The results reported here pertain to pre-cracked specimens of steel fibre reinforced concrete which were exposed to wet-dry cycles of marine spray in the laboratory simulating tidal zone conditions of exposure. Two types of concrete mixes were used in the investigation—one with standard concrete constituents and OPC and the second replacing about 26% of cement with pfa. The cement content of the mixes was 590 and 435 kg m⁻³, respectively. Fibre reinforcement was provided by means of low carbon steel fibres and melt extract steel fibres at a v _f ℓ/d ratio of 100 and 147. Prism specimens were manufactured and these were precracked to induce cracks of width ranging between 0.03 and 1.73 mm. After cracking, both sealed and unsealed specimens were exposed to laboratory marine spray cycles using sea water. Some control specimens were cured in the laboratory air throughout. Tests were carried out after 650 marine cycles (450 days) and 1450 marine cycles (900 days). Based on data on flexural strength, energy absorption capacity, stiffness and state of corrosion of the fibres, recommendations are made regarding suitable permissible crack widths for the design of steel fibre reinforced concrete for marine applications. The results indicate that a permissible crack width of 0.2 mm is satisfactory for concrete reinforced with melt extract fibres. A smaller value is recommended for concrete reinforced with low carbon steel fibres. Complete healing of open cracks of small widths is observed under exposure to marine cycles.     

Reaction control of TiB2 formation from titanium metal and amorphous boron

TiB₂ powder was synthesized by a controlled formation reaction from titanium metal and amorphous boron. Precursory TiB₂ formed by the pretreatment of the mixed powder (mole ratio: B/Ti=2.0) at 600° C for 60 min in an argon stream. Hollow TiB₂ powder with an average grain size of 15m was obtained by subsequent heat treatment above 900° C for more than 60 min in an argon stream. The formation reaction of TiB₂ powder was further controlled by pretreatment of the mixed powder at 600° C for 60 min in a hydrogen and argon stream and subsequent heat treatment at 1000° C for 360 min in an argon stream, when hollow-free TiB₂ powder was formed by a milder formation reaction between amorphous boron and the reformed titanium metal with hydrogen diffused lattice.     

Synthesis of polycrystalline alumina fibre with aluminium chelate precursor

Polycrystalline alumina fibre was successfully synthesized by pyrolysis of preceramic fibre formed from aluminium chelate compound. Ethyl 3-oxobutanoatodiisopropoxyaluminium (EOPA) was reacted with glacial acetic acid yielding a polymeric product. The absorption bands ascribed to Al-O from 630–705 cm^–1 changed from a sharp to a broad band on treatment with acetic acid. The¹³CNMR spectrum of EOPA changed from sharp singlets to multiplets after the reaction with acetic acid. The viscosity of the polymeric product increased in intensity with increasing amount of acetic acid. The viscosity of the polymeric product formed from EOPA-30 mol % acetic acid was 450 Pa s at 30 °C, and decreased to 5.4 Pa s with increasing measurement temperature from 30–70 °C. The²⁷Al resonance at 35 p.p.m. increased in intensity with increasing viscosity of the polymeric precursor. The molecular weight of the precursor was distributed from 400–800. The polymeric precursor pyrolysed at 500 °C in air was amorphous to X-rays, and crystallized in -alumina at 840 °C. The precursor fibres were pyrolysed, to yield fine-grained fibres of -alumina, at 1300 °C for 1 h.     

Modification of precursor pitch for general performance carbon fibre by blending naphthalene-derived isotropic and mesophase pitches

Modification of an isotropic precursor pitch (IMP) for general performance carbon fibre (GPCF) was carried out by blending isotropic and mesophase pitches (IP and MP, respectively), both of which were synthesized from naphthalene using HF/BF₃ as catalysts because they carried a large amount of naphthenic hydrogens. Both IP and MP were miscible with IMP at temperatures higher than their softening points. The softening point of the blended pitch dropped remarkably, being lower than 200° C with 40% IP. Such lower softening points made spinning much smoother. Modification also enhanced the stabilization reactivities of pitch fibre, the time required for the sufficient stabilization at 270° C decreasing by 20 and 30 min by blending with 20% IP and 20% MP, respectively. The mechanical properties of the resultant carbon fibres were also improved.     

Modification of jute fibre through vinyl grafting aimed at improved rot resistance and dyeability

Raw, dewaxed and oxidized jute fibres and those chemically modified with phenol and formaldehyde (treatment done for 3 h at 95° C and pH 8) before or subsequent to oxidation, were subjected to graft copolymerization with methyl methacrylate (MMA) in the presence or absence of some other monomers such as maleic anhydride (MA) or methacrylic acid (MAA) in limited aqueous system using K₂S₂O₈ as the initiator under photoconditions with the objective of inducing improved rot resistance and dyeability without loss in tensile strength of the fibre. For preparing oxy-jute, dewaxed and preswollen (dewaxed jute fibre swollen with 10% aqueous urea at 90° C for 2h) jute fibre were separately subjected to mild oxidation or bleaching using aqueous H₂O₂ and NalO₄ and non-aqueous chlorine (Cl₂ in CCl₄) under specified conditions. Optimum conditions for graft copolymerization have been established by examination of the effect of such variables as monomer concentration, time of polymerization and nature of chemical modification of jute fibre prior to vinyl grafting. Percentage grafting, tenacity (g denier^–1), dye fixation (%) and rot resistance (expressed as percentage retention of tensile strength of the fibre after a standard soil burial test) were evaluated and analysed. High rot resistance (80–90% retention of tensile strength after soil burial test) and dye fixation (%) of as high as 86% were readily obtained for grafted jute fibres. Washing fastness and light fastness properties of the dyed fibres (grafted and ungrafted) were also examined and compared.     

One-pot synthetic method to prepare highly N-doped nanoporous carbons for CO2 adsorption

A one-pot synthetic method was used for the preparation of nanoporous carbon containing nitrogen from polypyrrole (PPY) using NaOH as the activated agent. The activation process was carried out under set conditions (NaOH/PPY = 2 and NaOH/PPY = 4) at different temperatures in 600–900 °C for 2 h. The effect of the activation conditions on the pore structure, surface functional groups and CO₂ adsorption capacities of the prepared N-doped activated carbons was examined. The carbon was analyzed by X-ray photoelectron spectroscopy (XPS), N2/77 K full isotherms, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The CO₂ adsorption capacity of the N-doped activated carbon was measured at 298 K and 1 bar. By dissolving the activation agents, the N-doped activated carbon exhibited high specific surface areas (755–2169 m² g⁻¹) and high pore volumes (0.394–1.591 cm³ g⁻¹). In addition, the N-doped activated carbons contained a high N content at lower activation temperatures (7.05 wt.%). The N-doped activated carbons showed a very high CO₂ adsorption capacity of 177 mg g⁻¹ at 298 K and 1 bar. The CO₂ adsorption capacity was found to be dependent on the microporosity and N contents.     

Synthesis of thermosetting copolymer of polycarbosilane and perhydropolysilazane

A copolymer of polycarbosilane and perhydropolysilazane was obtained by reacting polycarbosilane with titanium n-butoxide and perhydropolysilazane. Titanium n-butoxide and perhydropolysilazane were essential for the polymer to show a thermosetting property. The thermosetting copolymers were converted into silicon carbide-based ceramics by pyrolysis in a stream of nitrogen to 1000 °C with about 80 wt% ceramic yield. The main phase of the pyrolysis product at 1500 °C in nitrogen was small crystallite -SiC. Elemental carbon, based on rule-of-mixtures composition, in the final ceramics could be reduced by varying the ratio of polycarbosilane/perhydropolysilazane. The copolymer was dry spun and pyrolysed to produce ceramic fibre. Pyrolysis in nitrogen to 1500 °C yielded a silicon carbide-based fibre with low oxygen and low elemental carbon content. A tensile strength of 1.8 GPa and an elastic modulus of 220 GPa were obtained for the fibre which ranged from 10–12 m in diameter. Crystallization to -Si₃N₄, -SiC, and -Si₃N₄ proceeded on annealing in nitrogen at 1700 °C for 1 h.     

TiO2 coatings on silicon carbide and carbon fibre substrates by electrophoretic deposition

Electrophoretic deposition (EPD) has been used to obtain TiO₂ coatings on three dimensional (3-D) SiC fibre (Nicalon ®-type) and carbon fibre substrates. Colloidal suspensions of commercially available TiO₂ nanoparticles in acetylaceton with addition of iodine were used. The EPD parameters, i.e., deposition time and voltage, were optimised for each fibre type. Strongly adhered TiO₂ deposits with high particle packing density were obtained. Scanning electron microscopy observations revealed high penetration of the titania nanoparticles into the fibre preforms. The TiO₂ deposits were sintered at 800°C for 1 h in order to produce relatively dense and uniform TiO₂ coatings covering completely the SiC or carbon fibres. For the carbon fibre/TiO₂ system, an effort was made to produce a 3-D titania matrix composite by further infiltration of the porous fibrous preform with TiO₂ by slurry dipping and subsequent pressureless sintering. The 3-D carbon fibre reinforced TiO₂ matrix composites fabricated contained residual porosity, indicating further infiltration and densification steps are required to produce dense composites of adequate structural integrity. For SiC fibre fabrics, oxidation tests in air established the effectiveness of the TiO₂ coating as oxidation protective barrier at 1000°C. After 120 h the increase of weight due to oxidation of coated fibres was more than twice lower than that of the uncoated fibres. TiO₂ coated SiC fibre preforms are attractive materials for manufacturing hot gas filters and as reinforcing elements for ceramic matrix composites.