Whisker reinforcement of glass-ceramics

Silicon carbide whisker reinforcement of anorthite and cordierite glass ceramics has been studied. At 25 vol% whisker loading the flexural strengths increased from 65–103 MPa to 380–410 MPa, the fracture toughnesses increased from 1.0–1.5 MPa m^1/2 to 5.2–5.5 MPa m^1/2. The strengths decline to 240–276 MPa at 1200 °C. The reasons for the decrease in strength with temperature are discussed. Whiskers from two different sources with differences in diameters and aspect ratios were evaluated and the effect of the whisker morphology on the composite properties was studied. It was found that larger diameter, higher aspect ratio whiskers result in improved composite performance. The composites were also characterized in terms of their thermal properties, i.e. thermal expansions and thermal conductivities. The thermal expansion coefficient from 25–1000 °C for anorthite-based composite was 4.6×10^–6 °C^–1 and that for the cordierite-based composite was 3.62×10^–6 °C^–1. The thermal conductivities at 1000 °C were 3.75 and 4.1 Wm^–1 K^–1 for cordierite and anorthite composites, respectively.     

A two-scale homogenization framework for nonlinear effective thermal conductivity of laminated composites

In this study, we formulate the effective temperature-dependent thermal conductivity of laminated composites. The studied laminated composites consist of laminas (plies) made of unidirectional fiber-reinforced matrix with various fiber orientations. The effective thermal conductivity is obtained through a two-scale homogenization scheme. A simplified micromechanical model of a unidirectional fiber-reinforced lamina is formulated at the lower scale. Thermal conductivities of fiber and matrix constituents are allowed to change with temperature. The upper scale uses a sublaminate model to homogenize temperature-dependent thermal conductivities of only a representative lamina stacking sequence in laminated composites. The effective thermal conductivity of each lamina, in the sublaminate model, is obtained using the simplified micromechanical model. The thermal conductivities from the micromechanical and sublaminate models represent average nonlinear properties of fictitiously homogeneous composite media. Interface conditions between fiber and matrix constituents and within laminas are assumed to be perfect. Experimental data available in the literature are used to verify the proposed multi-scale framework. We then analyze transient heat conduction in the homogenized composites. Temperature profiles, during transient heat conduction, in the homogenized composites are compared to the ones in heterogeneous composites. The heterogeneous composites, having different fiber arrangements and sizes, are modeled using finite element (FE) method.     

A study of the thermal conductivity of alumina/glass dispersed composites

The thermal diffusivity of five groups of alumina/glass composite systems has been measured at room temperature using a laser flash system. These data have been used, in conjunction with specific heat and density measurements, to calculate the effective thermal conductivity of these composites. In each of the five groups a systematic variation in glass concentration was made, and each group represents systematic variations in glass and alumina particle sizes. The thermal conductivities calculated are compared with those predicted by four models. It is apparent from these comparisons that the geometry and orientation of porosity within the sample measured are a key factor in determining which of these models (if any) is appropriate for describing the thermal conductivity of these composites.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Nonsteady transfer and dispersional effects in heterogeneous media

A single transport equation taking account of the dispersion of effective conductivities and interphase exchange due to relaxation effects, as well as the inhomogeneity of the corresponding fields, is obtained in Laplace transforms. The asymptotes of this equation are considered.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 5, pp. 807–816, November, 1990.     

Effective thermal conductivity and electrical conductivity of anisotropic solids of low porosity

Equations are derived to determine the effective thermal and electrical conductivities of anisotropic media of low porosity. The influence of porosity on the thermal and electrical conductivities of anisotropic ternary alloys is established.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 4, pp. 686–692, April, 1976.     

Experimental research to determine the effective thermal conductivities of a layer of furnace charge

This article describes the results of experimental research on the effective thermal conductivities of a layer of furnace charge consisting of steel pellets, small cylinders, and pelletized iron ores at temperatures up to 800°C, obtained by a steady-state method in a plane layer.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 19, No. 1, pp. 42–46, July, 1972.     

Thermodynamic analysis of the reasons for the difference between the values of the thermal conductivities of gases as measured by steady-state and transient methods

It is shown that the effective thermal conductivities of a gas measured by steady-state and transient methods are not equal.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 5, pp. 804–807, November, 1982.     

Viscosity and thermal conductivity of alkali metal vapors at temperatures up to 2000 K

New experimental data were obtained on transport coefficients of alkali metals in gaseous phase at high temperatures and within the pressure range from about 10 to about 100 kPa: lithium—thermal conductivity, T= 1400–1800 K, and viscosity, T=1600–2000 K; sodium-viscosity, T= 1100–1500 K; and cesiumviscosity, T=900–1250 K. Viscosity of the alkali metal vapors has been measured using a stationary-technique viscometer with an annular gap. Thermal conductivity was measured by the method of the nonstationary monotonous heating. Experimental data were used as a basis for computing effective atomatom and atom-molecule collision cross section, the values obtained from data on viscosity being in good agreement with those derived from thermal conductivity data. In the case of lithium, the atom-atom cross sections yielded by experiments are fairly consistent with the results of calculations with exact formulae of kinetic theory on the basis of quantum-mechanical potential curves for atom-atom interactions. This has enabled the authors to compile consistent tables of viscosities and thermal conductivities for lithium in a gaseous phase within the temperature range from 800 to 2500 K and pressures from 0.5 to 800 kPa, including the saturation curve.     

Measurement of the thermal conductivities of neon,krypton, and xenon over a wide range of temperatures

The thermal conductivities of certain monatomic gases have been measured at temperatures from 300 to 1200°K and pressures p1 atm. The parameters for the Lennard-Jones (6–12) potential are obtained from the experimental data and used to calculate the viscosities of these gases.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol.21, No. 3, pp. 491–499, September, 1971.     

Physical properties of hot-pressed tungsten-copper pseudo alloys

The results obtained in investigating the thermal and electrical conductivities and the thermal expansion in the 300–2200°K range of the hot-pressed W + (8–9)%Cu pseudo alloy are described. The effect of copper on the physical properties of the composition is discussed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 1, pp. 96–99, January, 1971.     

Effective conductivity of systems with interpenetrating components

Various methods of calculating effective conductivities of systems with interpenetrating components are compared, and the accuracy of the recommended method is examined.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 2, pp. 271–274, August, 1977.     

A study on selected properties of La1−xSrxCoO3 and its application in sealed CO2 lasers

Samples in the composition series La1–xSrxCoO3 with x=0–0.8 were synthesized, and their crystal structures, conductivities and catalytic activities were studied. When x<0.6, they have a rhombohedrally distorted perovskite structure. When x=0.3–0.5, their conductivities at room temperature are of the order of 103S m-1. When the temperature is higher than 250°C, the catalytic activities of La1–xSrxCoO3 were very high which reaches a maximum for the sample at x=0.3. Cylindrical samples were made from La1–xSrxCoO3 x=0.3 and used as a cathode in sealed CO2 lasers. Compared to the traditional Ag–Cu alloy cathode, the performance of La0.7Sr0.3CoO3 is better. The maximum output of a 1.0 m length laser tube using the new cathode is 53.1 Wm-1.     

Experimental determination and modeling of thermal conductivity tensor of carbon/epoxy composite

The modeling of thermal behavior of composite parts during their forming requires an accurate knowledge of their thermo-physical properties. Because of the heterogeneous nature of composites, the thermal conductivity tensor appears to be the most tricky to determine experimentally but also to model. A wide range of experimental methods can be found in the literature in order to measure either in-plane or transverse conductivity of composite parts, but very few succeed in performing it on dry preform or uncured laminates. In this study, the effective thermal conductivity tensor of carbon/epoxy laminates is investigated experimentally in the three states of a typical LCM-process: dry-reinforcement, raw and cured composite. Samples are made of twill-weave carbon fabric impregnated with epoxy resin. The transverse thermal conductivity is determined using a classical estimation algorithm, whereas a special testing apparatus is designed to estimate in-plane conductivity for different temperatures and different states of the composite. Experimental results are then compared to modified Charles & Wilson and Maxwell models. The fiber crimping of a ply is also taken into account in modeling. The comparison shows clearly that these models can be used to predict the effective thermal conductivities of woven-reinforced composites provided that the material properties are well known.     

The effective conductivity of composites with imperfect thermal contact at constituent interfaces

This paper is an investigation of the effective conductivities of composite media in which a thermal boundary resistance exists at constituent interfaces. The fundamental concepts in the theory of composite media in the context of thermal conduction are reconsidered in the presence of a temperature discontinuity between the constituents. The well-known procedures for defining and computing effective moduli are generalized to include the above mentioned interface effects.The developed procedure is applied to derive the effective conductivities of a composite with embedded spheroidal inclusions at a dilute concentration ratio. The method of solution of the auxiliary problem is based on the use of spheroidal harmonics in prolate and oblate configurations.     

An effective thermal conductivity model of nanofluids with a cubical arrangement of spherical particles

The theoretical investigation of the effective thermal conductivities of nanofluids, a new class of solid-liquid suspensions, is important in both predicting and designing nanofluids with effective thermal conductivities. We have developed a new thermal conductivity model for nanofluids that is based on the assumption that monosized spherical particles are uniformly dispersed in the liquid and are located at the vertexes of a simple cubic lattice, with each particle surrounded by a liquid layer having a thermal conductivity that differs from that of the bulk liquid. This model nanofluid with a cubical arrangement of nanoparticles gives a more practical upper limit of thermal conduction than a model nanofluid with a parallel arrangement of nanoparticles. The new model unexpectedly shows a nonlinear relationship of thermal conductivity with particle concentration, whereas the conductivity-concentration curve changes from convex upward to concave upward with increasing volume concentration. The effects of particle and layer parameters on the effective thermal conductivities are also analyzed. A comparison of predicted thermal conductivity values and experimental data shows that the predicted values are much higher than the experimental data, a finding that indicates that there is a potential to further improve the effective thermal conductivities of nanofluids with more uniformly dispersed particles.     

Thermal conductivity of refrigerants R123, R134a,and R125 at low temperatures

Using a transient coaxial cylinder technique, thermal conductivities were measured for liquid 1,1,1-trifluoro-2,2-dichloroethane (refrigerant R123), 1,1,1,2-tetrafluoroethane (refrigerant R134a). and pentalluoroethane (refrigerant R 125). The uncertainty of the experimental data is estimated to be within 2–3 %. Thermal conductivities of refrigerants were measured at temperatures ranging from –114 to 20°C under pressures up to IOMPa. The apparatus was calibrated with four kinds of liquids and gases. The features of the density dependence of thermal conductivity are indicated. Existing equations for calculating the coefficient are analyzed in cases where development has been sufficient to enable comparisons to be made with experiment. Saturated-liquid thermal conductivities for R134a and R123 are compared with corresponding experimental values.     

Predicting the thermal conductivity of composite materials with imperfect interfaces

This paper compares the predicted values of the thermal conductivity of a composite made using the equivalent inclusion method (EIM) and the finite element method (FEM) using representative volume elements. The effects of inclusion anisotropy, inclusion orientation distribution, thermal interface conductance, h, and inclusion dimensions have been considered. Both methods predict similar overall behaviour, whereby at high h values, the effective thermal conductivity of the composite is limited by the inclusion anisotropy, while at lower h values, the effect of anisotropy is greatly diminished due to the more dominant effect of limited heat flow across the inclusion/matrix interface. The simulation results are then used to understand why in those cases where it has been possible to produce CNF reinforced Cu matrix composites with a large volume fraction of well dispersed CNFs, the measured thermal properties of the composite have failed to meet the expectations in terms of thermal conductivity, with measured conductivities in the range 200–300 W/m K. The simulation results show that, although degradation of the thermal properties of the CNFs and a poor interfacial thermal conductance are very likely the reasons behind the low conductivities reported, great care should be taken when measuring the thermal conductivity of this new class of materials, to avoid misleading results due to anisotropic effects.     

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.     

Effect of the addition of milled carbon fiber as a secondary filler on the electrical conductivity of graphite/epoxy composites for electrical conductive material

In this work, carbon composite bipolar plates consisting of synthetic graphite and milled carbon fibers as a conductive filler and epoxy as a polymer matrix developed using compression molding is described. The highest electrical conductivity obtained from the described material is 69.8 S/cm for the in-plane conductivity and 50.34 S/cm for the through-plane conductivity for the composite containing 2 wt.% carbon fiber (CF) with 80 wt.% filler loading. This value is 30% greater than the electrical conductivity of a typical graphite/epoxy composite with 80 wt.% filler loading, which is 53 S/cm for the in-plane conductivity and 40 S/cm for the through-plane conductivity. The flexural strength is increased to 36.28 MPa compared to a single filler system, which is approximately 25.22 MPa. This study also found that the General Effective Media (GEM) model was able to predict the in-plane and through-plane electrical conductivities for single filler and multiple filler composites.