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.     

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.     

Mechanical and Corrosion Characteristics of Zr + 2.5% Nb Zirconium Reactor Alloy

The results of structural and phase hardening of pipes made of Zr + 2.5% Nb alloy show that ultrahigh-frequency thermal treatment of pipes (fast heating to the temperature of existence of the -phase followed by sharp cooling and annealing in the high-temperature range of the -phase) destroys the texture and forms a fine-grained structure (the grain diameter is about 1 m) with numerous transitional twins and a high density of precipitations of the secondary -niobium phase ( 10¹⁶cm^–3). In this state, the alloy is rather strong and plastic (at room temperature, _u 650 MPa, _0.2 550 MPa, and 20% both in the longitudinal and transverse directions). The efficiency of hardening by ultrahigh-frequency thermal treatment is not reduced with increase in the temperature of testing up to 500°C. Corrosion tests of channel pipes made of Zr + 2.5% Nb alloy subjected to ultrahigh-frequency thermal treatment in water containing various amounts of oxygen (from 0.1–0.3 to 600 mg/kg) at temperatures of 285–350°C for 700–6600 h under static conditions and in reactor water of the Ignalina Nuclear Power Plant for 5000 h under dynamic conditions showed that the corrosion resistance of this alloy is on a par with the corrosion resistance of the material of assembly channels of high-power channel reactors subjected to a standard treatment.     

Preparation and structure of mesophase pitch-based thin carbon tape

A mesophase pitch of 100 vol % anisotropy prepared from methylnaphthalene using HF/BF₃ was spun through a slit-shaped nozzle, stabilized in air under strain and carbonized at 1300°C into a very thin slit-shaped carbon tape 1.6 m thick and 14 m wide. Better crystalline orientation of the carbon tape always provided a Bacon anisotropic factor higher by 2% than that of the circular carbon fibre prepared from the same pitch. Excellent mechanical properties of the present carbon tape were obtained. Factors influencing the shape and orientation of the carbon tape were examined in terms of properties of mesophase pitches, spinning temperature, and the extent and strain of stabilization.     

Solubility of heat-resisting polymers in mesophase pitches at their spinning temperatures

Solubilities of two thermoresisting polymers (polyethylene telephthalatep-hydroxybenzoate, a liquid crystal polymer and polyethylene naphthalate) in mesophase pitches (MP) derived from coal tar (C1) and petroleum residues (P1 and P2) were examined to prepare blended fibre as the better precursor for the high-performance carbon fibre. MP-C1 and MP-P1 of high aromaticity dissolved 5 wt% of both polymers on mixing at 360 °C for 3 h, maintaining 100 vol% anisotropy which the parent mesophase pitches exhibited, although only MP-C1 did so at 340 °C. In a marked contrast, a number of isotropic droplets from both polymers dispersed in MP-P2 after the blending under the same conditions. The blends of MP-C1 with polymers were spinnable at around 370 °C into fibres of 10 to 20 m diameter although their spinnability was slightly inferior to that of the pitch alone. The blended fibre exhibited slightly higher stabilization reactivity at 270 to 300 °C than that of the parent mesophase pitch fibre. Structural factors of the mesophase pitch which influence their dissolving ability are discussed. Thermal stability of the polymers is also briefly examined.     

Formation of the Structure of a Surface Layer in Multiple Laser Doping

Results of investigations into the influence of multiple laser doping on the structure and properties of the layers formed are given. It is established that repeated treatment enables one to implement the processes of both laser quenching and laser annealing. The possibility of attaining a smooth gradient of the properties in the layer depth is shown.     

Response of a spinning liquid column to pitch excitation

Summary The response of a solidly rotating liquid bridge consisting of inviscid liquid is determined for pitch excitation about its undisturbed center of mass. Free liquid surface displacement and velocity distribution has been determined in the elliptic (>2₀) and hyperbolic (<2₀) excitation frequency range.List of symbols a radius of liquid column - h length of column - I ₁ modified Besselfunction of first kind and first order - J ₁ Besselfunction of first kind and first order - r, ,z cylindrical coordinates - t time - u, v, w velocity distribution in radial-, circumferential-and axial direction resp. - mass density of liquid - free surface displacement - velocity potential - ₀ rotational excitation angle - ₀ velocity of spin - forcing frequency - _1n natural frequency - surface tension - acceleration potential - for elliptic range >2₀ - for hyperbolic range >2₀     

Diels-Alder reaction of mesophase pitch insoluble fractions with maleic anhydride

Pyridine insoluble fractions in the mesophase pitches derived from a decant oil (PMP-PI) and naphthalene (NMP-PI) were found to be rendered almost completely soluble in pyridine by the Diels-Alder reaction with maleic anhydride, maintaining their optical anisotropy and molecular association. Their solubility in pyridine reached up to 95 wt % by the reaction at 175–200 °C. Solubilized fractions were analysed to obtain their average structures, suggesting that PMP-PI consists of aromatic nuclei of peri-condensed rings connected through a small number of methylene and aryl-aryl linkages, having a molecular weight (MW) of 1800, and that NMP-PI consists of oligomeric naphthalene with a large number of naphthenic groups (MW 1000). Such structures are basically much the same as those of their soluble fractions, although the molecules in Pls were much larger. The reaction sites in such structures for the Diels-Alder reaction are discussed.     

Polypropylene nucleation on a glass fibre after melt shearing

Thermoplastic composites may exhibit a wide range of crystalline morphologies in the neighbourhood of fibres. It was found that glass fibre shearing of a molten polypropylene at high temperature modifies the subsequent isothermal crystallization (T _c=122°C) under static conditions. The crystallization results have been analysed as a function of the previous high temperature, shear, , shear rate, , and shear stress, . The mechanical parameters at high temperature, , , , have been calculated for two shearing temperatures (T=170, 210°C) and two fibre displacements. An -phase nucleation process took place at the fibre surface after shearing at the higher temperature (T=210°C). The nucleation increased with shear but did not appear for static conditions. A strong nucleation process in phase appeared on the fibre surface after shearing of the polymer at the lower temperature (T=170°C). These strong morphological modifications with shearing temperature have been analysed as a function of mechanical and thermodynamical parameters.     

Steady state creep of a composite material with short fibres

The shear within a matrix volume is assumed to be an important process during the creep of composite material reinforced with short rigid fibres. The rate of elongation of such a composite with certain fibre distributions can be estimated. The agreement with a few experimental data is reasonably good.List of main symbols V _f volume fraction of fibres in composite - aspect ratio of a fibre - L length of a fibre - h transverse size of a fibre - h interfibre spacing - m, _m , _m constants for creep for a matrix material - n, _f, _f constants of creep for a fibre material _{f m} - v rate of relative motion of two fibres - _* ultimate strength of a fibre - ^* the first critical value of aspect ratio - ^** the second critical value of aspect ratio This work was carried out when the author was a guest worker at the National Physical Laboratory, Teddington, Middlesex, UK.     

Structure and formation of shot coke — a microscopy study

A series of shot cokes and sponge cokes from industrial delayed cokers was examined by scanning electron micrsocopy (SEM) and optical micrscopy (OM) of external, fractured internal and polished surfaces. Calcined cokes were etched with chromic acid solution and etching behaviour related to the optical texture of the cokes. The shot coke spheres have an inner structure of fine-grained mosaic and a smooth external skin, 50m thick, of coarse-grained mosaic and small domains. Sponge cokes can be heterogeneous with inclusions of well-ordered carbon within the matrix mosaics. It is considered that the feedstock leading to shot coke in the delayed coker forms a high viscosity fused pitch/ mesophase system which is subject to significant disturbance by volatile evolution. The paste-like system breaks up into fragments as a result of deep agitation by volatile release and movement through the system. The viscosity of the fragments is too high to permit their coalescence and reform a continuous system. Instead, they remain as fragments >1.0m diameter. Structure of cokes from delayed cokers and the associated manipulation of the system by the volatile evolution within the coker. These two factors are inter-related. Differences between cokes arise from differences between viscosities of the mesophase from the feedstock and control the balance between fragmentation and re-coalescence of the charge, as a result of manipulation by volatile release, leading to the two extremes of shot and needle coke.     

Mechanical properties and structure of a new commercial SiC-type fibre (Tyranno)

An initial evaluation of a new titanium-doped polymer-pyrolysis SiC fibre, Tyranno (Ube Industries, Ltd) showed a narrow distribution of diameters with a mean of 8.5m; average tensile strengths 3 G Pa, nearly independent of gauge length in the 1 to 4cm range; Weibull parameterm 7.3; and mean Young's modulus 170 G Pa. Heat treatment at 1300 to 1350° C in N₂ appreciably reducedS, m andE; and X-ray diffraction showed that significant crystallite growth as well as some Si₃N₄ formation had occurred. Tyranno fibre properties are compared with those of the well-known Nicalon SiC fibre.     

The tensile strength and ductility of continuous fibre composites

The plastic instability approach has been applied to the tensile behaviour of a continuous fibre composite. It is shown that the combination of two components with different strengths and degrees of work-hardening produces a new material with a new degree of work-hardening, which may be determined by the present analysis. Expressions for the elongation at rupture and the strength of a composite have been obtained and the results of the calculation are compared with some experimental data.List of symbols V _f volume fraction of fibres in composite - , , true strain of fibre, matrix and composite - s true stress - , , nominal stress on fibre, matrix and composite - _*, _*, _* critical stress of fibre, matrix and composite (ultimate tensile strength) - _*, _* critical strain of separate fibre and matrix - _* critical strain of composite - Q external load - A cross-sectional area - A ₀ initial value of area     

Interlaminar Fracture Toughness of CF/PEI and GF/PEI Composites at Elevated Temperatures

An experimental study has been conducted to assess temperature effects on mode-I and mode-II interlaminar fracture toughness of carbon fibre/polyetherimide (CF/PEI) and glass fibre/polyetherimide (GF/PEI) thermoplastic composites. Mode-I double cantilever beam (DCB) and mode-II end notched flexure (ENF) tests were carried out in a temperature range from 25 to 130°C. For both composite systems, the initiation toughness, G _IC,ini and G _IIC,ini, of mode-I and mode-II interlaminar fracture decreased with an increase in temperature, while the propagation toughness, G _IC,prop and G _IIC,prop, displayed a reverse trend. Three main mechanisms were identified to contribute to the interlaminar fracture toughness, namely matrix deformation, fibre/matrix interfacial failure and fibre bridging during the delamination process. At delamination initiation, the weakened fibre/matrix interface at elevated temperatures plays an overriding role with the delamination growth initiating at the fibre/matrix interface, rather than from a blunt crack tip introduced by the insert film, leading to low values of G _IC,ini and G _IIC,ini. On the other hand, during delamination propagation, enhanced matrix deformation at elevated temperatures and fibre bridging promoted by weakened fibre/matrix interface result in greater G _IC,prop values. Meanwhile enhanced matrix toughness and ductility at elevated temperatures also increase the stability of mode-II crack growth.     

Viscoelastic and rheological behavior of concentrated colloidal suspensions

Molecular approaches are discussed to the density (), viscoeleastic (), and rheological () behavior of the viscosity(,,) of concentrated colloidal suspensions with 0.3 < < 0.6, where, is the volume fraction, the applied frequency, and ; the shear rate. These theories are based on the calculation of the pair distribution functionP ₂(r,,), wherer is the relative position of a pair of colloidal particles. The linear viscoelastic behavior(,,=0) follows from an equation forP ₂(r,,) derived from the Smoluchowski equation for small, generalized to large by introducing the spatial ordering and (cage) diffusion typical for concentrated suspensions. The rheological behavior(,,=0) follows from an equation forP ₂(r,) of a dense hard-sphere fluid derived from the Liouville equation. This leads to a hard-sphere viscosity^hs(,) which yields the colloidal one(,) by the scaling relation(,) ₀=^hs(,) _B, where ₀ is the solvent viscosity. _B is the dilute hard-sphere (Boltzmann ) viscosity and the's are appropriately scaled,(,) and(,) agree well with experiment. A unified theore for(,,) is clearly needed and pursued.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994. Boulder, Colorado, U.S.A.     

Oxidation behavior of Mo≤5Si3C≤1 and its composites

The oxidation behavior of Mo₅Si₃C₁ and its composites was studied in air over the temperature range of 500°C–1600°C. Experiments revealed poor oxidation resistance of monolithic Mo₅Si₃C₁ at high temperature. The oxidation was quite rapid at 1200°C and above, resulting in complete oxidation of specimens in a short time. The addition of 2.0 wt% boron was found to produce a Mo₅Si₃C₁ composite with three other phases of MoB, MoSi₂, and SiC, and showed remarkable improvement in oxidation resistance. The mechanism for the improvement was attributed to the viscous sintering of the scale to close the pores formed during the initial oxidation period. Oxidation tests were also conducted on SiC-Mo₅Si₃C₁ composite at 800°C, 1300°C and 1600°C for more than 100 hours. The oxidation resistance of the composite was found to be very good. The results demonstrate that, though oxidation resistance of monolithic Mo₅Si₃C₁ is far insufficient for high-temperature applications, boron-modification and/or composites with SiC are viable methods to improve oxidation resistance to a practically acceptable level.     

Precipitate strengthening mechanisms in magnesium zinc alloy single crystals

The mechanical behaviour of Mg-5.1 wt % Zn alloy single crystals was studied in the 4.2 to 300° K temperature range. Quenched crystals have activation energies and volumes best associated with the cutting of small clusters of Zn atoms by dislocations. Fully hardened crystals contain fine ₁ and occasional ₂ precipitates with an average ₁ interparticle spacing of 330 to 660 Å. Strengthening in these crystals is mainly ascribed to the cutting of ₁ particles by dislocations. In the overaged condition ₁, ₂ and equilibrium particles are present and lead to a considerable temperature-dependence unusual for an overaged condition. Analysis of this temperature-dependence suggests that below 77° K the relatively easy cutting of ₁ particles by dislocations takes place in addition to the cutting of ₂ and particles. Above 77° K the difficult cutting of ₂ and particles alone controls the deformation, ₁ being more easily cut with the aid of thermal fluctuations.     

Texture study of two molybdenum shaped charge liners by neutron diffraction

The textures of two different conical shaped liners, fabricated by the same forging processes from arc-cast and powder-sintered ingots, were investigated by using neutron-diffraction measurements and three-dimensional orientation-distribution-function (ODF) analysis. The major textures of both liners could be described by the (1 1 1) uv w and (1 0 0) u v w type. The two liners had essentially identical texture at the 8 cm position (measured from the base of the cone) with strong sheet-type texture components, i.e. (1 1 1) ¯1 0 1, (1 1 1) ¯1 1 0 and (1 0 0) 0 1 1. However, the dominant textures at the 3 cm positions were 1 1 1 and 1 0 0 fibre textures with the fibre axes oriented parallel to the normal direction in both liners. A strong cube texture was observed at the 3 cm position of the arc-cast liner but it was not observed for the powder-sintered liner. The arc-cast liner had a generally higher degree of texture than the powder-sintered liner.     

Diffusion controlled precipitation of austenitic bi-crystals possessing twin related orientation in the ferrite of a duplex stainless steel

The ferritic matrix in the Fe-22Cr-5Ni-3Mo-0.03C ferritic-austenitic duplex stainless steel can undergo a variety of decomposition processes when aged in the temperature range 650–550°C. One of these processes is the formation of austenite in the shape of a spearhead. Unlike the one found at high temperature, this austenite is characterized by a habit plane, which is similar to the midrib of the martensite. This feature suggests that a diffusionless process as referred to in the literature controls its formation. In the present investigation, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) results indicate that the proposed precipitation mechanism must be reviewed. Based on the crystallographic results in conjunction with the clear-cut difference in chemical composition between the matrix and the austenitic second phase, the equilibrium shape is explained and the ambiguity of precipitation mechanism has been brought to light. It has been suggested that the latter one goes through the following steps: (i) enrichment of the (110)-ferritic planes with -forming elements by diffusion, (ii) double shear to transform (110)-ferritic planes into (111)-austenitic planes, i.e., to change from BCC () to FCC (), (iii) twinning of the FCC structure to reduce local strains and formation of bi-crystals and (iv) growth of the twinned towards the ferritic matrix by volume diffusion.     

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.