Preparation of micro-coiled SiC and TiC fibres by vapour phase metallizing of micro-coiled carbon fibres

Micro-coiled fibres of SiC and TiC were prepared by the vapour phase metallizing of micro-coiled carbon fibres with full preservation of the coiling morphology of the source coiled carbon fibres, and their preparation conditions and bulk electrical resistivity were examined. The SiC_1,0 coils were obtained at 1400 °C for 2 h, and TiC_1.0 coils were obtained at 1100–1200 °C for 1.5 h. The bulk resistivity of the coiled TiC fibres sharply decreased with the bulk density and was 10^–2 S^–1 cm at 1.4 g cm^–3.     

Diffusion of tin (IV) in silicate glazes and glasses

The diffusion coefficients of Sn(IV) in an aluminosilicate glass and a commercial glaze have been measured from 809 to 1505° C. Two experimental techniques have been used. In one method, single crystals of SnO₂ were embedded in either the powdered glass or sealed into a bar of the glass. After the diffusion anneal, the Sn(IV) concentration profile was determined by EPMA. In the other method, radioactive ¹¹³Sn was used as a tracer and the profile determined by measuring the X-ray emission. The results gave a good agreement between the two methods. The diffusion coefficients in the glaze ranged from 7×10^–20 m² sec^–1 at 809° C to 1.9×10^–14 m² sec^–1 at 1250° C and in the glass, from 5.6×10^–15 m² sec^–1 at 1307° C to 1.6×10^–11 m² sec^–1 at 1505° C.     

Preparation of cobalt oxide films by plasma-enhanced metalorganic chemical vapour deposition

Co oxide films were prepared on glass substrates at 150–400°C by plasma-enhanced metalorganic chemical vapour deposition using cobalt (II) acetylacetonate as a source material. NaCl-type CoO films were formed at low O₂ flow rate of 7cm³ min^–1 and at a substrate temperature of 150–400°C. The CoO films possessed (100) orientation, independent of substrate temperature. Deposition rates of the CoO films were 40–47 nm min^–1. The CoO film deposited at 400 °C was composed of closely packed columnar grains and average diameter size at film surface was 60 nm. At high O₂ flow rate of 20–50 cm³ min^–1, high crystalline spinel-type Co₃O₄ films were formed at a substrate temperature of 150–400°C. The Co₃O₄ film deposited at 400°C possessed (100) preferred orientation and the film deposited at 150°C possessed (111) preferred orientation. Deposition rates of the Co₃O₄ films were 20–41 nm min^–1. Both Co₃O₄ films with (100) and (111) orientation had columnar structure. The shape and average size of the columnar grains at the film surface were different; a square shape and 35 nm for (100)-oriented Co₃O₄ film and a hexagonal shape and 60 nm for (111)-oriented film, respectively.     

The stability of mother glass for porous glass-ceramics of the TiO2-SiO2 system

The stability of the glasses of TiO₂-SiO₂-Al₂O₃-B₂O₃-CaO-MgO and TiO₂-SiO₂-Al₂O₃-P₂O₅-CaO-MgO systems during formation has been investigated as a basic study on the preparation of porous glass-ceramics. The main factor affecting the stability of these glasses which are used as mother glasses of porous glass-ceramics was CaO content. The stability was remarkably improved by increasing the CaO content.The preparation of porous glass-ceramics with an appropriate amount of pore volume was possible from a glass containing CaO up to 30 mol%. The DTA trace of the glasses showed two distinct exothermic peaks in the ranges 30–130° C and 140–300° C above glass transition temperatures. The porous glass-ceramics of TiO₂-SiO₂ system containing more than 60 mol% TiO₂ and having a surface area larger than 400m²g^–1, a pore volume of 0.3–0.5 ml g^–1, and average pore radius between 1 and 20 nm were fabricated.     

Crystal growth kinetics in BaOAl2O32SiO2 and SrOAl2O32SiO2 glasses

Crystallization kinetics of BaOAl₂O₃2Si0₂ (BAS) and SrOAl₂O₃2SiO₂ (SAS) glasses in bulk and powderforms have been studied by non-isothermal differential scanning calorimetry (DSC). The crystal growth activation energies were evaluated to be 473 and 451 kJ mol^–1 for bulk samples and 560 and 534 kJ mol^–1 for powder specimens in BAS and SAS glasses, respectively. Development of crystalline phases on thermal treatments of glasses at various temperatures has been followed by powder X-ray diffraction. Powder samples crystallized at lower temperatures than the bulk and the crystallization temperature was lower for SAS glass than BAS. Crystallization in both glasses appeared to be surface nucleated. The high temperature phase hexacelsian, MAl₂Si₂O₈ (M = Ba or Sr), crystallized first by nucleating preferentially on the glass surface. Also, monoclinic celsian does not nucleate directly in the glass, but is formed at higher temperatures from the transformation of the metastable hexagonal phase. In SAS the transformation to monoclinic celsian occurred rapidly after 1 h at 1100 °C. In contrast, in BAS this transformation is sluggish and difficult and did not go to completion even after 10 h heat treatment at 1400 °C. The crystal growth morphologies in the glasses have been observed by optical microscopy. Some of the physical properties of the two glasses are also reported.     

Fabrication of cellular structure composite material from recycled soda-lime glass and phlogopite mica powders

A homogeneous composite material with different physical structures has been fabricated from recycled colourless soda-lime glass powders and phlogopite-type mica powders by mixing the two powder components and sintering the mixture at a temperature above 850° C for a period of 30 min or longer. The physical structure of the composite material can be fabricated into either a cellular structure consisting of both closed and open cells or a highly densified ceramic body. The cellular structure composite material is found to have a compressive strength of about 0.877 MN m^–2 and thermal conductivity values in the range of 0.290 to 0.306 W m^–1 °C^–1 when measured at temperatures in the range of 25 to 100° C. The highly densified composite material, on the other hand, is found to have a compressive strength of about 53.0 MN m^–2 and thermal conductivity values in the range of 0.198 to 0.250 W m^–1 °C^–1. The composite material, when compared with other common building materials, is found to be potential material for construction applications because of its superior mechanical and thermal properties.     

Structure and properties of a moulded carbon derived from rice hull

Rice hull was moulded into a tube (outer diameter: 54 mm; inner diameter: 17 mm, length: c. 170 mm) by use of an extruder and then carbonized in nitrogen atmosphere below 1000 °C. Ash content of the hull was 16 wt%, of which c. 94 and 4 wt% were SiO₂ and K₂O, respectively. Carbon yield and shrinkage of the mould after carbonization at 1000 °C were 42 wt%, and 43 vol%, respectively. The bulk density increased with rising of carbonization temperature to reach to 0.93 g ml^–1 at 1000 °C via 0.82 g ml^–1 at 500 °C. The largest compressive strength of 3.6 MPa was obtained after carbonization at 1000 °C. No micropore was developed after carbonization, and the total pore volume measured by a mercury porosimeter was 0.25–0.31 ml g^–1 after carbonization. These data were compared with those of charcoal.     

Carbon fibre-reinforced silicon nitride composite

The processing of silicon nitride reinforced with carbon fibre was studied. The problems of physical and chemical incompatibility between carbon fibre and the silicon nitride matrix were solved by addition of a small amount of zirconia to the matrix and by low-temperature hot-pressing. The composite material possesses a much higher toughness than hot-pressed silicon nitride. Its work of fracture increased from 19.3 J m^–2 for unreinforced Si₃N₄, to 4770 J m^–2; its fracture toughness,K _lc , increased from 3.7 MN m^–3/2 for unreinforced material, to 15.6 MN m^–3/2. The strength remains about the same as unreinforced Si₃N₄ and the thermal expansion coefficient is only 2.51×10^–6 ° C^–1 (RT to 1000° C). It is anticipated that this composite may be promising because of its mechanical and good thermal shock-resistance properties.     

Preparation of cordierite glass by the sol-gel process

Cordierite glass was prepared by the sol-gel process from magnesium, Al(OC₄H₉)₃ and Si(OC₂H₅)₄. The solution having cordierite-like structure was formed by refluxing the raw materials, followed by gelling and drying to 150° C in the saturated water vapour to hydrolyse completely. On heating the gel, AlO₆ groups transformed into AlO₄ groups, in which aluminium ions were incorporated in SiO₄ tetrahedra units to form a Si-O-Al network structure. The gel was converted into the transparent dense glass by heating above 850° C. The glass transition temperature, 820° C, thermal expansion coefficient, 3.1×10^–6° C^–1, Vickers hardness, 6.22 GPa and density, 2.59 g cm^–3 were almost the same as those of the conventional glass.     

Thermal properties of MoSi2 and SiC whisker-reinforced MoSi2

The heat capacity, thermal conductivity and coefficient of thermal expansion of MoSi₂ and 18 vol % SiC whisker-reinforced MoSi₂ were investigated as a function of temperature. The materials were prepared by hot isostatic pressing between 1650 and 1700 °C, the hold time at temperature being 4 h. The heat capacity of MoSi₂ showed an increase from about 0.44 Wsg^–11K^–1 at room temperature to 0.53 at 700 °C. Whisker reinforcement increased heat capacity by about 10%. Thermal conductivity exhibited a decreasing trend from 0.63 Wcm^–1 K^–1 at room temperature to 0.28 Wem^–1 K^–1 at 1400°C. Whiskers reduced conductivity by about 10%. The thermal expansion coefficient increased from 7.42 °C^–1 between room temperature and 200 °C to 9.13 °C^–1 between room temperature and 1200 °C. There was a 10% decrease resulting from the whiskers. The measured data are compared with literature values. The trends in the data and their potential implications for high-temperature aerospace applications of MoSi₂ are discussed.     

Thermal expansion of TiAl + TiB2 alloys and model calculations of stresses and expansion of continuous fiber composites

Thermal expansion values for three TiAl alloys with different additions of TiB₂ can be represented using a third-order equation at temperatures between 20 and 800°C. Expansion values were obtained on both heating and cooling temperature cycles. The total expansion at 800°C is between 0.917 and 0.931% for three different samples. The expansivity increases from about 10×10^–6°C^–1 at 80°C to 14×10^–6°C^–1 at 750°C. A five-coaxial cylinder elastic model for multizone-coated continuous fiber composites is developed for predicting stresses and thermal expansion of composites. Either isotropic or transversely isotropic material properties can be assigned to the various cylinder zones.Paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

Low temperature sintering and crystallisation behaviour of low loss anorthite-based glass-ceramics

Anorthite-based glass-ceramics including TiO₂ as nucleating agent were melted and quenched in this study. The effect of particle size on the sintering behaviour of glass powders was investigated in order to obtain low-temperature sintered glass-ceramics. Anorthite glass-ceramic starts to densify at the transition temperature of glass (T _g = 770°C) and is fully sintered before the crystallisation occurrence (880°C). Therefore, a dense and low-loss glass-ceramic with predominant crystal phase of anorthite is achieved by using fine glass powders (D ₅₀ = 0.45 m) fired at 900°C. The as-sintered density approaches 99% theoretical density and the apparent porosity is as low as 0.05 Vol%. The dense and crystallized anorthite-based glass-ceramic exhibits a fairly low dielectric loss of 4 × 10^–4 at 1 MHz and a thermal expansion coefficient of 4.5 × 10^–6°C^–1. Furthermore, the microwave characteristics were measured at 10 GHz with the results of K = 9.8, Q _f = 2250, and temperature coefficient of resonant frequency _f = –30 ppm/°C.     

Crystallization of leucite as the main phase in glass doped with fluorine anions

The crystallization of a multicomponent glass containing 1.63 wt% of F^– anions was studied. The results show in powder glass with particle sizes less than 0.15 mm, that surface crystallization is dominant, whereby two phases: leucite and dioside are formed. In glass powder or particle size about 0.15 mm, three phases, phlogopite, diopside and leucite, are formed, accompanied by an abrupt decrease in the resistance of the glass to crystallization. If the particle sizes are in the range 0.15 to 0.45 mm, both surface and volume crystallization are significant, while with particle sizes >0.45 mm, volume crystallization is dominant. Two nucleation temperatures, T _n1 = 655°C and T _n2 = 675°C, were determined in the temperature range of 600–710°C. These temperatures satisfy the condition that T _n T _g . Crystallization of bulk glass occurs in the temperature range of T _c = 870–1100°C, the crystal phases appearing in the sequence: phlogopite, followed by diopside, followed by leucite. Kinetical and microstructural studies show that the crystallization process is controlled by volume diffusion.     

Effect of heat treatment on the infrared absorption spectra of Sr-Na borosilicate glass

Internal modes of vibrations are studied here at different temperatures (27–800°C) and in the frequency range 200–4000 cm^–1 through heat treatment. The baseline method was used. The strong bands of SiO₄ tetrahedra in this glass show an increase in absorbance at high temperature (600–800°C). The deformation of SiO₄ tetrahedra is investigated. This is found to depend on the ionic radius of the divalent metal oxide introduced, and the coordination number of the cation. Also from a study of the temperature dependence of the relative integrated intensity of the modes 600–800 and 850–1450 cm^–1, the relaxation time and rotational energy barrier of the glasses selected indicate that the glassy phases are transformed to crystalline phases at 500°C.     

Thermal and chemical properties of TiO2-SiO2 porous glass-ceramics

The thermal and chemical stability of porous glass-ceramics of the TiO₂-SiO₂ system have been investigated. Porous glass-ceramics containing both anatase and rutile had thermal expansion coefficients of 40 to 55 x 10^–7 K^–1 in the range 0 to 700° C. They remained porous up to 1000° C, while the pores of high-silica porous glass of Vycor type collapsed completely at that temperature. The high thermal stability of the porous glass-ceramics may be attributed to a high viscosity due to dispersed crystallites of anatase and rutile in the skeleton. Most of the anatase in the skeleton was transformed to rutile by heat treatment at 900° C, but some of it remained untransformed even after 6 h. Surface -OH groups identified by IR spectroscopy were removed by dehydration polycondensation with heating up to 900° C. The porous glass-ceramics were quite durable to alkali solution compared with high-silica porous glass. The excellent durability of these porous glass-ceramics was attributed to the large amount of TiO₂ contained in the skeletal structure.     

Proton conduction at the surface of Y-doped BaCeO3 and its application to an air/fuel sensor

25 mol% Y³⁺-doped BaCeO₃ (BCY25) showed an extremely low activation energy of 0.3 eV for proton conduction at the surface. The resulting overall conductivity at the surface reached 8.24 × 10^–3 S cm^–1 at 400°C, which was 3, 8, and 28 times higher than those in the bulk of BCY25, 20 mol% Sm³⁺-doped ceria, and 8 mol% yttria-stabilized zirconia, respectively. Such fast proton conduction enabled an air/fuel (A/F ) sensor using BCY25 as the solid electrolyte to work above 150°C for H₂ and above 250°C for C₂H₄.     

Preparation of micro-coiled carbon fibres by metal powder-activated pyrolysis of acetylene containing a small amount of sulphur compounds

Micro-coiled carbon fibres were prepared by the transition metal-activated pyrolysis of acetylene containing a small amount of sulphur compounds, and the preparation conditions were examined in detail. The coiled carbon fibres grew at the reaction temperatures of 700–850 °C and thiophene gas flow rates of 0.14–0.45 standard cm³min^–1 (0.10–0.35 vol % reaction atmosphere). The optimum values depended on the type of metal catalysts used. Among the metal catalysts used, nickel, titanium and tungsten were the most effective for the growth of the coiled carbon fibres and a maximum yield of about 50%–55% was obtained. The bulk resistivity of the coiled carbon fibres decreased with increasing bulk density and was 10⁰ S^–1 cm at a bulk density of 1.     

Effects of ageing treatments on transformation temperatures and precipitation kinetics in a Cu-Zn-Al shape-memory alloy

The effects of ageing treatments on transformation temperatures, hardness, and precipitation kinetics in a Cu-14.2Zn-8.5Al (wt%) shape-memory alloy were investigated. Quench-ageing treatment temperatures varied from 100 to 500° C with times up to 200 h after the solution treatment. The martensitic transformation temperature, M _s, of the hot-rolled material was decreased from 55 to 51 °C by the solution treatment. The temperature hysteresis (A _f-M _f) was 50° C for the hot-rolled condition, but was reduced to 30° C after the solution treatment. The maximum hardness for material aged at 500° C was lower than that for that aged at 300 or 400° C. The apparent activation energy for hardness increase in this alloy was 110 kJ mol^–1, compared with 72 kJ mol^–1 for the similar copper-based shape-memory alloy Cu-21.2 Zn- 6.0 Al. The ordering temperatures for B₂ and DO₃ superlattices were in the neighbourhood of 480 and 260° C, respectively. The tensile ductility and yield strength of this alloy were significantly reduced by the ageing treatment at 400° C.     

Mechanical properties of vanadium beryllide,VBe12

The mechanical properties of VBe₁₂, both at room and elevated temperatures (up to 1200°C), have been measured. Room-temperature properties, including Young's modulus, flexural strength, and fracture toughness are reported. The material behaved elastically at room temperature but became plastic at temperatures above 1000°C. Creep properties of VBe₁₂ were also studied in temperature ranges from 1000–1200°C and applied stress ranges from 33–58 MPa. At low strain rates (approximately < 10^–5s^–1), the stress exponent was about 4, suggesting deformation was controlled by dislocation climb. Microstructural examination indicated that fracture was initiated from grain boundaries subjected to tensile stresses. The creep behaviour of VBe₁₂ is briefly compared with that of other intermetallics.     

Annealing of shear bands in metallic glasses

Shear bands introduced into the as-quenched metallic glass alloy, Metglas^R 2826 (Fe₄₀Ni₄₀P₁₄B₆), were etched by immersion in a solution of CUS04 and HCI. Elimination of the etchability property through isothermal annealing had an apparent activation enthalpy of 250 kJ mol^–1, similar to that of stress relaxation. Direct microscopic observations indicated that the etching of shear bands may be a result of stress corrosion cracking. The electrochemical etchability of shear bands in the nickel-based alloy, BNi₂ (Ni_68.8Cr_6.6Fe_2.6B_14.1Si_7.9), polarized in a solution of perchloric acid and acetic acid at 12° C, could also be eliminated by thermal annealing. The loss of etchability had an apparent activation enthalpy of 580 kJ mol^–1, a value indicative of more complex atomic rearrangements taking place within the deformed material. Finally, shear bands introduced into the as-received BNi₂ material retained the ability to reverse shear after heat treatment at temperatures in the range of 192 to 375° C even if the electrolytic etchability was apparently diminished.