Thermal and dielectric properties of zirconyl phosphate compact

Experimental results are presented on the measurements of thermal expansion (up to 1500°C), thermal conductivity (up to 1000°C), dielectric constant (up to 450 °C) and tan (up to 800 °C) of zirconyl phosphate compacts obtained by sintering at 1600°C. The thermal expansion coefficient of the samples at the temperature below 1100°C was less than 1.7 × 10^–6°C^–1. The samples showed a definite shrinkage at temperatures of 1110 and 1470°C due to the phase transformations. The expansion at 1500°C was less than that at 1100°C probably because of the phase transformation. The thermal conductivity at room temperature was a very small value (0.0046 to 0.0065 cal s^–1 cm°C^–1 cm^–2). The dielectric constant was close to 9. The value of tan° (–0.0001) measured is one of the lowest values for ceramic materials.     

Nickel nanoparticles inserted in tBuONa matrix deposited on alumina Part II Thermal treatment and nickel content effects on their stabilty and catalytitic activity

Nanoparticles (1 nm–3 nm) of metallic nickel supported on alumina (4.3% Ni–17.9% Ni w/w) were prepared from a colloidal precursor inserted in an organic matrix. Their structural and stability properties have been studied by X-Ray Diffraction (XRD), Electron Paramagnetic Resonance (EPR) and Thermal Gravimetric Analysis (TGA). Benzene hydrogenation at atmospheric pressure in the temperature range of 75 °C–200 °C was used as a test reaction of their catalytic capability. The thermal stability of the particles depended on the nature of the reactive atmosphere. Thus, a growth in size (up to around 20 nm) is observed under H₂ flow at 350 °C or during benzene hydrogenation but not under air flow at 300 °C. The growth may be due to the coalescence of the metal particles during the heating and decomposition of the stabilizing organic matrix. Under oxidative atmosphere, stable nickel oxide particles, firmly attached to the support, are formed. The catalysts pre-treated under H₂/350 °C were active and stable in benzene hydrogenation. The observed activities depended on the reaction conditions and nickel composition.     

Determination of thermal conductivity from specific heat and thermal diffusivity measurements of plasma-sprayed cermets

The thermal conductivities of three plasma-sprayed cermets have been determined over the temperature range 23–630°C from the measurement of the specific heat, thermal diffusivity, and density. These cermets are mixtures of Al and SiC prepared by plasma spray deposition and are being considered for various applications in magnetic confinement fusion devices. The samples consisted of three compositions: 61 vol% Al/39 vol% SiC, 74vol% Al/26vol% SiC, and 83 vol% Al/17 vol% SiC. The specific heat was determined by differential scanning calorimetry through the Al melt transition up to 720°C, while the thermal diffusivity was determined using the laser flash technique up to 630°C. The linear thermal expansion was measured and used to correct the diffusivity and density values. The thermal diffusivity showed a significant increase after thermal cycling due to a reduction in the intergrain contact resistance, increasing from 0.4 to 0.6 cm²·^–1 at 160°C. However, effective medium theory calculations indicated that the thermal conductivities of both the Al and the SiC were below the ideal defect-free limit even after high-temperature cycling. The specific heat measurements showed suppressed melting points in the plasmasprayed cermets. The 39 vol% SiC began a melt endotherm at 577°C, which peaked in the 640–650°C range depending on the sample thermal history. Chemical and X-ray diffraction analysis indicated the presence of free silicon in the cermet and in the SiC powder, which resulted in a eutectic Al/Si alloy.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Thermophysical properties of ceramic matrix composites produced by polymer pyrolysis

A unidirectional composite and a series of bidirectionally reinforced composites were fabricated using carbon fibre reinforcement in a silicon carbide matrix, which was produced by the pyrolysis of a polymer precursor. The thermal expansion over the temperature range 20–1000 °C has been measured and the thermal diffusivity measured over the temperature range 200–1200 °C. Thermal diffusivity data was converted to conductivity data using measured density and literature specific heat data. Metallographic examination has been carried out on the composites and the results are discussed in terms of the observed microstructural features.     

Negative thermal expansion of laminates

Measurements have been carried out on the in-plane and through the thickness thermal expansion coefficients of glass polypropylene fibre composites of 50% volume fraction between room temperature and 120°C. Only in the temperature range 20° to 75°C are reproducible results obtained. It is confirmed that in-plane negative values may be obtained in specific directions. The physical reason for this and its connection with the necessary appearance of a large Poisson ratio is pointed out. The expansivity of the matrix material depends strongly on temperature. Very good agreement between the experimental values and those predicted from the properties of the two constituents is found provided that the value for the expansivity of the polyoropylene is that within the temperature range considered. It is shown that the polypropylene matrix does not provide a matrix which is stable enough in its properties to enable the system to yield consistent negative values of expansivity. An alternative system is proposed and a single experiment confirms that a negative value of the thermal expansivity of as large as –50 × 10^–6 K^–1 may be obtained.     

Precise evaluation of fast fracture velocities in acrylic polymers at the slow-to-fast transition

Transient fast fracture velocities at the onset of the slow-to-fast transition in polymethylmethacrylate (PMMA) have been measured precisely in single-edge-notched specimens of various geometries by using ultrasonic fractography. Little sign of the fracture velocity of the order of 10° m sec^–1 have been detected on the instability onset. The fast fracture starts from a point source almost instantaneously. The initial velocity has been shown to fall in a small range, 90 to 150 m sec^–1, almost independent of the loading speeds from 0.1 to 100 mm min^–1 and specimen temperature from –50 to 40° C, with exceptional cases for specimens loaded slowly (0.1 mm min^–1) at a low temperature (–50° C). As the final minimum velocity of an arresting crack, a value of 42 m sec^–1 has been obtained under room temperature. Crack propagation in low molecular weight PMMA has been shown to be more temperature, as well as loading rate, dependent than in higher molecular weight PMMA.     

Thermal degradation of foamed polystyrene

The thermal degradation of foamed polystyrene patterns in the expendable pattern casting (EPC) process has been studied. Various physical transitions that may occur during the degradation of the polymer have been determined with scanning electron microscopy, differential scanning calorimetry and thermogravimetric analysis. The effects of polymer density and bead structure on the degradation characteristics have been investigated. The results indicate that, when exposed to elevated temperatures, the polymer beads collapse at about 110–120 °C. The collapsed beads melt at 160 °C, and start to vaporize at temperatures greater than about 275 °C. Complete volatilization occurs in the temperature range 460–500 °C. The heat of degradation of expanded polystyrene has been estimated to be at the order of 912 Jg^–1.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

Microwave drying of boehmite sol intercalated smectites

Monohydroxy aluminium oxide (boehmite) intercalated (cross-linked) smectites (SCLS) have been prepared from an aqueous suspension containing sodium mono-ion-exchanged bentonite (2 wt%) and boehmite (AlOOH) sol at pH 3.5 and at 32 °C. The SCLS has been separated by centrifugation and repeated washing. The intercalated smectite was dried in an oven at 60 °C over a period of 24 h and also in a microwave oven of 2.45 GHz frequency and 600 W power over a range of 3–15 min. Both samples have identical thermal and electrical properties. However, the microwave-dried samples have a distinctly higher surface area of 120 m² g^–1, stable up to 650 °C with a marginal reduction to 116 m² g^–1 compared with 94 m²g^–1 for the oven-dried sample. Similarly there was clear difference in the morphological features of the two samples, the air-dried sample having a close packed structure while the microwave one is delaminated and porous.     

Rationalizing the calibration method of reference telescopic radiation pyrometers

Conclusions An equation which was derived in this work provides with sufficient accuracy the relationship between the thermal emf and the measured radiation temperature for two types of TERA-50 radiation pyrometer telescopes.Optimum temperatures have been found for plotting a curve (2) through three points, at 600, 900 and 1200°C for telescopes type TERA-50/(600–1400°C), and at 1300, 1600 and 1900°C for telescopes type TERA-50/(1200–2500°C).Thus, it becomes possible to recommend a new and more efficient, three-point calibration technique for 2nd grade telescopes type TERA-50/(600–1400°C) and TERA-50/(1200–2500°C).     

Thermal conductivity of steam from 250 to 510°C at pressures up to 95 MPa including the critical region

New measurements of the thermal conductivity of steam have been performed in the temperature range 250–510°C and in the pressure range from 1 up to 95 MPa. Most of the measurements were taken at temperatures greater than the critical temperature, where the enhancement of the thermal conductivity is observed. The experimental values are compared to the IAPS formulation for the thermal conductivity of water.     

Studies of microstructural and mechanical properties of Nylon/Glass composite Part II The effect of microstructures on mechanical and interfacial properties

This is a part of a series of studies on the influence of thermal processing on microstructures and mechanical properties of thermoplastic composites. In this paper, the effect of cooling rate during thermal moulding processes on the mechanical properties of bulk unidirectional commingled yarn GF/PA6 composites (Iosipescu shear strength, transverse flexural tensile strength and elastic modulus) has been investigated. Cooling rate from fast to slow, –60°C/min, –3°C/min and –1°C/min, were achieved at 1.5 MPa pressure. Scanning electron microscopy (SEM) was used to analyse the damaging mechanisms of the fracture surfaces of the tested samples. The different dynamic responses of the samples were observed by polarised optical microscopy (POM) during the mechanical tests. The results indicated that when the cooling rate was varied from fast to slow, the interfacial tensile and shear strength were improved associated with enhanced elastic modulus. These results may be attributed to the slow cooling achieved a high transcrystallinity between the glass fibres and PA6 matrix, and high crystallinity of phase in the PA6 matrix.     

The environmental response of hybrid composites

Hybrid composite specimens containing a total of 60 or 75 vol % of unidirectional fibre were prepared from HT S-carbon fibre and E-glass fibre, HT S-carbon fibre and Kevlar 49 fibre, and E-glass fibre and Kevlar 49 fibre with a standard anhydride cured epoxide resin. The specimens were divided into four groups and subjected to the following environments: (A) room temperature and humidity; (B) soaked in water for 300 h at 95° C and then oven dried at 60° C to a constant weight; (C) thermally cycled 100 times between –196 and 95° C; (D) cycled 35 times between –196 and water at 95° C. The flexural properties of the samples were measured at room temperature after exposure. The modulus of the hybrid materials was not significantly affected by any of the treatments, although thermal cycling with or without water caused a large decrease in the modulus of all Kevlar fibre/resin and to a lesser extent all glass fibre specimens. The flexural strength of the unexposed carbon fibre/glass fibre and glass fibre/Kevlar fibre hybrids showed a positive deviation from the rule of mixtures behaviour at low volume loadings of the lower extension fibre. Wet thermal cycling or soaking in water caused a substantial reduction in the flexural strength of glass fibre/Kevlar fibre specimens. The interlaminar shear strength of all three fibre combinations was not affected by dry thermal cycling, but the effects of soaking in water and especially thermal cycling with water exposure were significant and irreversible.     

Thermal shock resistance of SiC fibrerein-forced borosilicate glass and lithium aluminosilicate matrix composites

Thermal shock resistance of an SiC fibre-(Nicalon®) reinforced borosilicate glass (Pyrex) and lithium aluminosilicate (LAS) matrix composite has been investigated experimentally in the temperature range 0–1000 K. Longitudinal Young's modulus and flexure strength of the composites after thermal shock were obtained as a function of thermal shock temperature. The results are discussed with the observed damage of the composite. The borosilicate glass matrix composite showed multiple cracking of the glass matrix perpendicular to the fibre axis when the thermal shock temperature was above 600 K. Decreases in Young's modulus and flexure strength were also recognized after multiple cracking of the matrix was initiated. On the other hand, the LAS matrix composite showed no damage at thermal shock temperatures below 800 K. However, at 800 K and above, microcracking of the matrix along the fibre axis was observed. After thermal shock, no decrease in the flexure strength was recognized, while the Young's modulus decreased due to microcracking of the matrix when the thermal shock temperatures were 800 K and above. It was found that the major advantage of the composite against thermal shock was to retain non-catastrophic failure properties even after the development of thermally induced damage in the composite.     

Crystallization kinetics of J-1 polymer under different thermal treatments

A bis-para-amino cyclohexylmethane (PACM)-based polyamide homopolymer (J-1 polymer produced by Du Pont), utilized as a matrix for composites, was subjected to different thermal treatments in order to investigate its crystallization thermodynamics and crystallization kinetics. Various J-1 samples, quenched, annealed from the glassy state, isothermally crystallized from the melt and slowly cooled, were studied by differential scanning calorimetry (DSC). A thermodynamic melting temperature of 352.6 °C was determined from a Hoffman-Weeks diagram of polymer samples annealed at different temperatures between the glass transition and melting temperature. By using DSC isothermal crystallization data from the melt, the existence of two crystallization regimes, already found in a previous investigation, was confirmed, and a transition temperature between the two regimes, equal to 262.2 °C was determined, in good agreement with 260.5 °C, obtained by depolarized light measurements, reported elsewhere. Moreover, the ratio between the crystallization kinetics factor of two crystallization regimes is 1.87, very close to the value of 2 predicted by the Huffman theory. Crystallization of samples from the melt, at different cooling rates, was also performed. The Arrhenius plot of data indicated that the crystallization process proceeds with two distinct activation energies (589 and 244 kJ mol^–1), below or above a cooling rate of 2.67 °C min^–1, corresponding to a temperature of 253.9 °C. This result is in good agreement with the two crystallization regimes reported above.     

Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon)

The oxygen free Si–C fibre (Hi-Nicalon) consists of -SiC nanocrystals (5nm) and stacked carbon layers of 2–3nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si–C–O fibre (Nicalon NL202), the Si–C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200–1400°C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400°C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600°C could be caused by SiC coarsening or superficial degradation. Si–C fibres have a good oxidation resistance up to 1400°C, due to the formation of a protective silica layer.     

Ultralow expansion ceramics in the HfO2-TiO2 system synthesized by an hydrolysis and polycondensation technique

Ultra-refractory ceramics from the HfO₂-TiO₂ system in the range 30–40 mol% TiO₂, with a near-zero thermal expansion, have been synthesized by hydrolysis and polycondensation of titanium alkoxide and hafnium dichloride alcoholic solutions and sintered at moderate temperature. Thermal stability, crystallization, density and microstructure of these materials have been examined. The as-prepared powder, amorphous at room temperature, crystallized quickly when heated at 500 ° C. Entire crystallization occurred after treatment at 1000 °C. Sintering at 1500 °C on cold-pressed samples led to ceramics with weak porosity (7%), low expansion coefficient <1×10^–6 °C^– with a minimum for 30 mol% TiO₂ content. SEM examination on sintered materials at 1500 °C reveals a grain size from 2–6 m, increasing with TiO₂ content.     

Thermal stability of carbon-MoSi2-carbon composites by thermogravimetric analysis

A thermal stability of carbon-carbon composites with increase of oxidation resistant filler, MoSi₂, have been studied by thermogravimetric analysis during the graphitization process. In this work, the initial decomposition temperature, temperature of maximum rate of weight loss, integral procedure decomposition temperature, and decomposition temperature range for the degradation temperatures, and the activation energy based on Horowitz-Metzger calculating method were characterized in a thermal stability study. It has been found that 12–20 wt% filler on the basis of the resin matrix leads to an improvement of degradation temperature and to an effectively increase of activation energy of the composites. This is probably due to the effect of the inherent MoSi₂ properties, resulted from a brittle-to-ductile transition for increasing the interfacial adhesion between fiber and matrix, and a mobile diffusion barrier formation against oxygen attack, in the vicinity of 900–1000°C.     

Hardening and phase stability in rapidly solidified Al–Fe–Ce alloys

Rapidly solidified (RS) Al–Fe–Ce alloys were prepared by melt spinning. The phases present and the thermal stability, at temperatures up to 500 °C, were then followed by X-ray analysis, chemistry, hardness and thermal analysis techniques. The results obtained indicated that the alloys studied have enhanced mechanical properties compared to commercial aluminium alloys and castings of the same alloy compositions, and the RS alloy also exhibit good stability up to about 300 °C; a result of stable second phase particles. It is suggested that these results indicate that there are two mechanisms responsible for the hardening and stability of the RS alloys: solid solution strengthening at lower temperatures, and semicoherent particles formed from supersaturated solid solution at higher temperature. The maximum hardness, after 2 h ageing occurred at about 300 °C. At higher temperatures the dispersed phase became incoherent with a dramatic loss in hardness.     

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.