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.     

Measurement of thermal properties of liquids with an AC heated-wire technique

An apparatus for the simultaneous absolute measurement of the thermal activity, thermal diffusivity, thermal conductivity, and heat capacity of nonconducting liquids with the AC heated-wire (strip) technique is described. The main advantage of this technique is that the temperature oscillations field can be confined around the sensor in a liquid layer thin enough to suppress the hydrodynamic currents. This leads to the elimination of the convective heat transport. Carrying measurements at different frequencies, the inertia of the sensor can be considered, and the radiative heat transport can be estimated for liquids with known optical properties. The apparatus was constructed and tested using six different liquids in a limited temperature range. The thermal properties of these liquids at 20°C are reported. The thermal conductivity data of toluene and n-heptane (recommended as proposed thermal conductivity standards) are given in the temperature range 10–40°C. Good agreement was found with data reported by other investigators at 20°C, but there is still a considerable discrepancy in the temperature coefficient of thermal conductivity.     

Thermal diffusivity and conductivity of a carbon fibre-reinforced borosilicate glass

A study was conducted of the thermal diffusivity, thermal conductivity and specific heat of hot-pressed Pyrex borosilicate glass reinforced with randomly oriented carbon fibres perpendicular to the hot-pressing direction. The thermal diffusivity and conductivity of the composite perpendicular to the plane of the fibres was only slightly higher than the corresponding value for the matrix, attributed to the low thermal conductivity of the carbon fibres perpendicular to their axis. Within the plane of the fibres the thermal diffusivity and conductivity were approximately twice those of the matrix. The specific heats of the matrix and composite were nearly identical. The temperature dependence of the thermal diffusivity of the matrix and composite was minor only. The corresponding positive temperature dependence of the thermal conductivity of the matrix and composite was attributable primarily to the positive temperature dependence of the specific heat. Heating the composite to temperatures above 600° C resulted in a permanent decrease in thermal diffusivity at a rate which depended strongly on temperature. This change was attributed to the relaxation of the elastically bent fibres by the thermally activated viscous flow of the surrounding matrix, which as shown by electron microscopy resulted in interfacial separation and crack formation within the composite. These effects, in turn, caused the observed decrease in thermal diffusivity.     

A new polymeric route to silicon carbide and silicon nitride using elementary silicon as starting material

Elementary silicon activated with copper was heated up in tetraethylene pentamine to 300°C. After drying in vacuum, an amorphous solid was obtained. Calcining this solid under argon at temperatures of up to 1200°C led to amorphous products, while at higher temperatures silicon carbide was obtained. Calcining in ammonia at temperatures of up to 1000°C also led to amorphous products. At calcination temperatures of 1200 and 1400°C crystalline silicon nitride and silicon nitride fibres respectively were obtained. The dependence of the fibre growth on the ammonia flow rate, as well as the occurrence of spherical iron-enriched particles terminating these fibres gave evidence for a vapour-liquid-solid mechanism being responsible for the fibre growth.     

Effect of HIPing on the effective thermal conductivity/diffusivity and the interfacial thermal conductance of uniaxial SiC fibrereinforced RBSN

Hot isostatic pressing (HIPing) was found to increase the thermal diffusivity/conductivity of uniaxial silicon carbide fibre-reinforced reaction-bonded silicon nitride (RBSN) matrix composites, as the result of the densification of the matrix, the increase in the grain size of the silicon carbide and the improved thermal contact between the fibres and the matrix. Transverse to the fibre direction the thermal diffusivity/conductivity was found to be a function of the surrounding gaseous atmosphere due to the access of the gas phase to the fibre-matrix interface, which was facilitated by the existence of an interfacial gap due to the thermal expansion mismatch between the fibres and the matrix. The interfacial conductance was found to exhibit a strong positive temperature dependence as the result of the closure of the interfacial gap with increasing temperature.     

Effect of heat treatment in air on the thermal properties of SiC fibre reinforced composite: Part 2 A magnesium aluminium silicate (MAS) matrix glass ceramic composite

The thermal properties of a magnesium aluminium silicate (MAS) glass ceramic matrix composite reinforced by SiC (Nicalon) fibres have been investigated before and after heat treatment in the temperature range 600–1,200 °C. Within this temperature range, during the heat treatment at lower temperatures such as 600 and 700 °C, the oxidation of the carbon layer occurred and mixture of silicon and carbon was formed in the interface. This results in a decrease in thermal diffusivity values. After heat treatment at the temperatures higher than 1,000 °C, the carbon layer was thickened and resulted in the higher thermal diffusivity values.     

The investigation of thermophysical properties of fluids by an alternating current hot-wire method

A method and an instrument for the measurement of a number of the thermophysical properties (thermal conductivity, thermal activity, volumetric specific heat, thermal diffusivity) are described. The results obtained for thermal conductivity and specific heat of toluene, over a temperature range 30–350°C and pressures of up to 30 MPa, are presented.     

Heat conduction characteristics of a carbon-fibre-reinforced lithia-alumino-silicate glass-ceramic

A study was conducted of the thermal diffusivity' specific heat and thermal conductivity of a uniaxially carbon-fibre-reinforced lithia-alumino-silicate glass-ceramic. The thermal diffusivity and conductivity parallel to the fibre direction was found to be independent of thermal history and more than an order of magnitude higher than in the transverse directions. During the first thermal cycle, the thermal diffusivi1y transverse to the fibre direction was found to exhibit a decrease attributed to crack formation under the influence of internal stresses. The transverse thermal diffusively on thermal cycling to 1000° C exhibited lower values during heating than during subsequent cooling. This hysteresis was attributed to a thermal history-dependent barrier to heat flow at the matrix-fibre interface. The thermal conductivity of the fibres along their length inferred from composite theory was found to be much lower than the corresponding value for pyrolytic graphite, attributed to less than complete graphitization and associated high density of lattice defects which act as phonon scatterers.     

Thermal conductivity of fibre-phenolic resin composites. Part I: Thermal diffusivity measurements

Measurements have been made on the thermal diffusivity of fibre-phenolic resin composites between 20 and 400°C. The matrix material was the phenolic resin SC-1008 manufactured by Monsanto. Two composite systems were considered: a two-directional composite reinforced with carbon fibres woven into an eight-harness satin weave, and a silica fibre composite orthogonally reinforced in three mutually perpendicular directions. One-directional composites were also prepared using the same fibres.To assist mathematical modelling of the thermal conductivity ( Part II of this report ) heat capacity, density, and volume fraction of constitutents were also determined, together with weight and length change during the diffusivity measurement. The problems and uncertainties in obtaining component data are discussed.     

Calorimetric and transport properties of Zircalloy 2, Zircalloy 4, and Inconel 625

This paper presents the measurements and the results on thermal and electrical transport properties of three nuclear reactor cladding materials: Zircalloy 2, Zircalloy 4, and Inconel 625. Study of these materials constituted a part of the IAEA coordinated research program aimed at the generation and establishment of a reliable and complete database of the thermal properties of reactor materials. Measured properties include thermal diffusivity, specific heat, and electrical resistivity. Thermal diffusivity was measured by the laser pulse technique. Specific heat and electrical resistivity were measured using a millisecond-resolution direct electrical pulse heating technique. Thermal conductivity was computed from the experimentally determined thermal difusivity and specific heat functions and the room temperature density values. Measurements were performed in the 20 to 1500°C temperature range, depending on the material and property concerned.     

Thermal diffusivity of chemically vapour deposited silicon carbide reinforced with silicon carbide or carbon fibres

The thermal diffusivity of chemically vapour deposited silicon carbide reinforced with either Nicalon SiC yarn or PAN-precursor carbon fibres was measured by the laser-flash method during various time-temperature treatments. The diffusivity was found to depend on the degree of densification, the direction of heat flow with respect to the fibre orientation, and the thermal history. Structural modifications, confirmed by X-ray diffraction, produced large permanent changes in the thermal properties of the SiC-SiC composites when heated above 1200° C, while only minor changes were seen in C-SiC composites heated above 1500° C. On sabbatical leave of absence from the Société Européenne de Propulsion, Bordeaux, France.     

Effect of boron-doping on structure and some properties of carbon-carbon composite

Boron-doped carbon-carbon composites with boron concentration around 11–15 mass % were prepared from a carbon fibre felt with dispersed boron carbide powder by infiltration of pyrolytic carbon. The composite was heat treated at several different temperatures from 2000–2800 °C. The highest bending strength was obtained for the composite at a heat treatment temperature (HTT) of 2200 °C. Carbon fibre began to be destroyed after heat treatment at 2400 °C and the structure of the composite was drastically changed above 2600 °C where the anisotropy of the composite originally existing in the thermal expansion coefficient and the thermal conductivity has been faded away. X-ray diffraction measurement indicated that graphitization of the composite was enhanced by boron doping. At HTTs above 2400 °C, the composite became graphitic, the crystallite sizes of which were more than 100 nm in L_c (004) and L_a (110). It was shown that boron was uniformly distributed in the composite at an HTT of 2400 °C and also that heat treatment at higher temperatures, such as 2600 °C, incurred condensation of boron. Air-oxidation loss at 800 °C appeared to be the lowest for the composite with an HTT of 2400 °C and the rate of oxidation loss was 22 times lower than that of the non-boron-doped composite.     

Influence of high-temperature exposure on mechanical properties of zircon-silicon carbide composites

Thermal stability of zircon matrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide monofilaments was determined by measuring mechanical properties and fibre-matrix interfacial characteristics in the as-fabricated state and after annealing treatments between 25 and 1430 °C for times up to 100 h. Composites reinforced with uncoated silicon carbide filaments retained their mechanical properties and fibre-matrix interfacial characteristics up to 1350 °C for 100 h. In contrast, composites reinforced with BN-coated silicon carbide filaments displayed changes in mechanical properties and fibre-matrix interfacial characteristics when annealed beyond 1300 °C for 100 h. Both types of composite displayed a significant reduction in strength and toughness after annealing at 1430 °C for 20 h. These results are consistent with changes in fibre-matrix interfacial properties, and with changes in mechanical characteristics of zircon matrix and silicon carbide filament as a result of the high-temperature annealing treatments.     

Suggestions regarding thermal diffusivity measurements on pyrolytic graphite and pyrolytic boron nitride by the laser pulse method

The laser pulse method can be successfully applied to the measurement of thermal diffusivity of isotropic materials subject to some assumptions. For anisotropic materials, this method is applicable to the measurement of principal thermal diffusivity only on the condition that there is no difference in direction between the principal axis and that of the temperature gradient. After analyzing the heat conduction process in an anisotropic solid, it has been shown that large errors in the measurement of thermal diffusivity would exist if the direction of the principal axis deviates inconspicuously from that of the temperature gradient. The experimental results of thermal diffusivity of highly oriented pyrolytic graphite (HOPG) samples with various deviation angles have been compared with the analytical results. The laser pulse method is not applicable to measurements on semitransparent pyrolytic boron nitride (PBN). We adopted a two-layer composite sample to measure the thermal diffusivity of PBN in the c direction and a particular graphite-PBN composite sample has been prepared which has a very low thermal resistance at the interface. The thermal diffusivity and thermal conductivity of PG (below 2300°C) and PBN (below 1000°C) are given.Invited paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Photopyroelectric Determination of the Order of Liquid Crystal Phase Transitions

The photopyroelectric technique has been used to measure simultaneously the specific heat, the thermal conductivity, and the thermal diffusivity of 9CB liquid crystal in the temperature range 35 to 60°C, where the sample undergoes a weakly first-order phase transition and a second-order one. Measurements of the anisotropy of the thermal conductivity have also been performed, and the data have been used to establish the order of the above-mentioned phase transitions. Pretransitional effects in the isotropic phase in the thermal diffusivity have been found, and they have been associated with similar effects reported for the specific heat.     

Intercomparison of Measurements of the Thermophysical Properties of Polymethyl Methacrylate

Results of an intercomparison of measurements of thermal conductivity, thermal diffusivity, specific heat capacity, and density of polymethyl methacrylate (PMMA) in the temperature range between –70°C and +80°C are presented. The purpose of this comparison is to investigate the variability of the results among guarded hot-plate (GHP) and guarded heat-flow meter (GHF) techniques on the one hand and among GHP/GHF and other measuring instruments on the other. The primary objectives are to characterize the material properties mentioned and to quantify the effects of thermal contact resistances and temperature measurements. With regard to future use of PMMA as a reference material, reference data for the thermal conductivity are derived.     

Thermophysical Analysis of Gioia Marble in Dry and Water-Saturated States by the Pulse Transient Method

Measurements of the thermophysical properties of Gioia marble in the temperature range from −20 to 60°C are presented. Thermophysical properties, namely, thermal diffusivity, thermal conductivity, and specific heat, were measured by the pulse transient technique. The data were compared for dry and water-saturated states. Despite the very low porosity of marble of about 0.6 vol%, an increase of the transport property parameters (thermal diffusivity and thermal conductivity) up to 20% after water saturation was found. To verify the differences in the transport parameters, the ultrasonic pulse velocity method was employed. A detailed analysis of thermophysical property data during the freeze/thaw process for dry and water-saturated marble was carried out in the temperature range from −8 to 1°C, where an anomaly in the water freezing process was observed. In order to study artificial aging of Gioia marble, up to 60 freeze/thaw cycles were performed. No significant changes in the thermophysical properties of Gioia marble were observed during the artificial aging process. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Thermal conductivity of Inconel 718 and 304 stainless steel

The results of thermal conductivity measurements on Inconel 718 and 304 stainless steel by the comparative and flash diffusivity techniques are reported for the temperature range 0–700°C. For 304 stainless steel, excellent agreement with published data is found for the specific heat, thermal diffusivity, and thermal conductivity. In the case of Inconel 718, the measurements show that the conductivity depends critically on the sample thermal history and the metallurgical condition of the alloy. Measurements on a solution-treated sample indicated a conductivity function close to that reported previously, while precipitated samples showed a higher conductivity, similar to the conductivityvs-temperature function used for reduction of comparative thermal conductivity data with Inconel 718 references. These results indicate that Inconel 718 is not a suitable reference for high-accuracy comparative thermal conductivity measurements unless its thermal history and associated conductivity function are known.     

Thermal conductivity,heat capacity,and thermal diffusivity of selected commercial AlN substrates

The thermal transport properties of four commercially available AlN substrates have been investigated using a combination of steady-state and transient techniques. Measurements of thermal conductivity using a guarded longitudinal heat flow apparatus are in good agreement with published room temperature data (in the range 130–170 W · m^–1 · K^–1). Laser flash diffusivity measurements combined with heat capacity data yielded anomalously low results. This was determined to be an experimental effect for which a method of correction is presented. Low-temperature measurements of thermal conductivity and heat capacity are used to probe the mechanisms that limit the thermal conductivity in AlN.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Fabrication of CuSiC metal matrix composites

A CuSiC MMC heatspreader will offer high thermal conductivity between 250 and 325 W/mK and corresponding adjustable thermal expansion coefficient between 8.0 and 12.5 ppm/°C. The primary challenge of CuSiC manufacture was to prevent reaction between copper and silicon carbide during high temperature densification, which dramatically degraded the thermal conductivity. In this study, the key issue addressed was the Si attack of Cu at the temperatures necessary for CuSiC fabrication (850 to 1200°C). Decomposition of SiC in contact with copper will dissolve Si in Cu causing a dramatic decrease of Cu thermal conductivity. This diffusion of Si into Cu can be prevented by the application of reliable barrier layers to diminish mass transport through the diffusion path and thereby minimizing the chemical interaction. A reliable barrier coating was identified and used to fabricate the CuSiC composites. The CuSiC composites were then characterized by SEM, TEM, XRD and XPS. Chemical analysis and thermal conductivity by laser flash diffusivity measurement illustrated the effectiveness of the barriers. A CuSiC composite having thermal conductivity of 322.9 W/m-K was successfully fabricated.