Fiber Thermal Conductivity Research Articles

High-Temperature Thermal Conductivity of Ceramic Fibers

The ceramic fibers VK-60, ABK, and VK-80 produced by steam blowing and nozzle dissemination methods have been investigated for the effect of nonfibrous material content, pressure, and temperature on the thermal conductivity at ambient and higher temperatures. It was noticed that with an increase in the aluminum content of the ceramic fibers the thermal conductivity of the material decreased while the insulation properties improved. The VK-80 fibers have the lowest and the VK-60 fibers the highest value of thermal conductivity at ambient temperature. At ambient temperature, the value of thermal conductivity increased with an increase in pressure for all analyzed fibers. ABK fibers showed the least increase and VK-80 registered an increase of about 10% in the values of thermal conductivity for pressures ranging from 0.6 to 6.6 kN/2. However, beyond a pressure of 6.6 kN/m2, the thermal conductivity of all samples increased. To assess the insulation properties of investigated fibers, the thermal conductivity was measured at different temperatures up to 800 °C. From the obtained results, it was concluded that all three types of fibers have a good potential for future applications, showing good performance in the investigated temperature range.

Thermal conductivity of ceramic fibres as a function of temperature andpress load

Three ceramic fibres VK-60, ABK and Nextel/VK-80 produced by the steamblowing and nozzle dissemination methods have been investigated for the effectof press load per unit area and temperatures using the transient plane source(TPS) method in air. It was noticed that, with the increase of the aluminiumcontent in the composition of the ceramic fibres, the thermal conductivity of thematerial decreases and the isolation properties improved. TheNextel/VK-80 fibre has the lowest and VK-60 the highest value of thermalconductivity at room temperature.The application of a press load results in an increase in the value of the thermalconductivity for all the fibres analysed. ABK fibres showed the least increase andNextel/VK-80 registered an increase of about 10% in the value of thermalconductivity within the load increase from 0.6 to 6.6 kN m-2. However, above6.6 kN m-2 the thermal conductivity of all the samples increased almostlinearly. The thermal conductivity measurements as a function of temperatureindicated the same trend for an increase in thermal conductivity for all thesamples.

Noncontact determination of the thermal conductivity of fibres

Because of the small size and the fragility of ceramic fibres, noncontact methods are highly advantageous for their thermal characterisation. The development of such a noncontact method for the direct measurement of the thermal conductivity of fibres and wires is described. The presented measurement method uses a high-power laser diode emitting in the visible spectrum as a line-shaped heat source and a high resolution infrared camera as a spatial temperature sensor. The latter detects the stationary temperature profile adjacent to the locally heated fibres as a result of heat losses by radiation and convection. An evaluation of this temperature allows the derivation of the thermal conductivity in the direction of the fibre axis. Variation of the heating power and of the sample geometry by investigating single fibres or bundles of fibres can be used to check the consistency of the results and to improve accuracy. The noncontact method was applied to fibre samples with diameters in the range of tens of micrometres at ambient temperatures. Tests were performed with glass fibres of known thermal conductivity. Hereby a light-absorbing layer was deposited on the samples by spraying. The method was also applied to ceramic fibres of the Si/B/N/C system. In this case there was no need for any coatings because of the small reflectivity in the infrared spectrum. A thermal conductivity of (1.1 ± 0.3) W m -1 K -1 was obtained along the axis of the Si/B/N/C fibres. The presented measurement method, which is also applicable to foils and plates, stresses simplicity of preparation and wide applicability rather than accuracy. However, it offers the possibility of being easily extended to dynamic measurement procedures and different heater and sample geometries, which leads to a reduction of the uncertainties in the determined thermal conductivity.

Thermal Conductivity of Fiber Reinforced Composites by the FEM

Applicability of the finite element method (FEM) in predicting the effective transverse thermal conductivity of fiber reinforced composites is systematically studied. Four different boundary condition combinations representing the periodicity of the temperature field are employed for ideal composites having perfect bond between fiber and matrix. Both circular and square cross-section fibers are studied. Comparisons of present FEM results with available analytical and experimental results reveal that periodicity realized by prescribed temperatures yields most accurate results up to high fiber volume fractions. In composites with interfacial thermal barrier resistance the effective conductivity varies in a wide range depending on the interfacial conductance between fiber and matrix. Best fit with available experimental results is obtained for both circular and square fibers when the dimensionless interfacial conductance is about 30. By employing the modeling practice found successful in the cases for which analytical andlor experimental results exist, some typical combined effects of partial debonding and matrix cracking, for which no such results exist, are finally considered.

Measured Through-the-Thickness Thermal Diffusivity of Carbon Fiber Reinforced Composite Materials

The thermal diffusivity of carbon fiber reinforced composite materials is dependent on fiber, matrix, and porosity volume fractions. Since the carbon fiber thermal conductivity is much greater than the epoxy resin matrix thermal conductivity, thermal diffusivity is a good indicator of the fiber volume fraction. In this study, through-the-thickness thermal diffusivity images were obtained nondestructively on carbon fiber reinforced composite plates, and the values varied from 0.0035 to 0.0062 cm2/s. The fiber and porosity volume fractions were measured destructively, and their values varied from 56.0 to 70.5% and from 0.9 to 7.2%, respectively. The destructive test data were used as a standard for comparison of three commonly used transverse equivalent thermal conductivity models: the Stacked Plate, the mixed flow Cylindrical Fiber model, and the Composite Circular Assemblage model. In these models, a porosity correction is also studied. There was very good agreement between the Composite Circular Assemblage and Cylindrical Fiber model when porosity levels were less than 2%. The porosity correction helped to reduce the chi-squared value by 24% in the Cylindrical Filament model. Finally, the potential is discussed to image fiber volume fraction nondestructively using the described thermal measurement system.

Thermal diffusivity measurement of single fibers by an ac calorimetric method

An ac calorimetric method is applied to the direct measurement of thermal diffusivity of single fibers. Theoretical analysis is presented, taking into account the two-dimensional ac thermal wave for cylindrical systems and the effect of time delay at the temperature detecting system. Experimental tests are performed using nickel and stainless steel wires about 20 μm in diameter. The thermal conductivity of fibers about 10 μm in diameter of the advanced materials, polyacrylonitrile-based carbon, TORAYCA, and aramide, Kevlar, is determined. The relationship between the thermal conductivity and electric conductivity and the crystalline size of these fibers is discussed. For composite materials made of carbon fibers, the observed thermal conductivity agrees with the thermal conductivity estimated using the values determined by the present method.

Thermal Conductivity of Uniaxial Coated Cylindrically Orthotropic Fiber-Reinforced Composite with Thermal Barriers

The theories of Hasselman et al. and Benveniste et al. were extended to derive expressions for the thermal conductivity of continuous matrix composites containing dilute volume fractions of uniaxial coated cylindrically orthotropic fibers with interfacial thermal barriers at the fiber-coating and coating-matrix interfaces. The transverse thermal conductivity of this type of composite was found to depend on ten variables: the thermal conductivities of the matrix and coating, the radial and tangential components of the fiber thermal conductivity, the volume fractions of fibers and coating, the absolute dimensions of the fiber and coating, and the interracial thermal conductances at both interfaces.

Thermal conductivity of glass fiber reinforced polypropylene under high pressure

AbstractThe thermal conductivities of molten polypropylene and its glass fiber composites were measured by the compensating hot wire method. The testing apparatus empolyed was designed and tested in our laboratory. The measurements were carried out with temperatures ranging from 170 to 230°C and pressures from 1 to 2000 kg/cm2. The results show that the thermal conductivity increases with increasing pressure and glass fiber content, but is almost independent of temperature. The thermal conductivity data were fitted satisfactorily with a proposed empirical equation for polypropylene and Lewies‐Nielsen equation for the composites, respectively.

Thermal conductivity and Raman spectra of carbon fibers.

The thermal conductivity between 12 and 300 K is reported for graphite fibers carbonized from a variety of precursors, including spun polyacrylonitrile, petroleum pitch, or grown by chemical vapor deposition of methane. The fibers were studied as grown as well as heat treated to temperatures up to 3000 \ifmmode^\circ\else\textdegree\fi{}C. First-order Raman spectra of the same fibers were taken with use of a microprobe, and the intensity ratio ${I}_{1360}$/${I}_{1580}$ between the disorder-induced line at 1360 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the Raman-allowed line at 1580 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ correlates well with the amplitude of the low-temperature thermal-conductivity data. Using the Kelly model for the phonon-dominated thermal conductivity, we deduce the defect-limited phonon mean free path ${L}_{\ensuremath{\varphi}}$ from our data below 100 K. We find ${L}_{\ensuremath{\varphi}}$ approximately equal to ${L}_{a}$, the x-ray in-plane correlation length which had been related by Tuinstra and Koenig to the Raman intensity ratio ${I}_{1360}$/${I}_{1580}$. We show further correlations between ${L}_{\ensuremath{\varphi}}$ and the electrical resistivity \ensuremath{\rho}, the interplane distance (1/2)${c}_{0}$, and the c-axis correlation length ${L}_{c}$.

Thermal conductivity and thermopower of vapor-grown graphite fibers.

Values for the thermal conductivity and thermopower of graphite fibers grown by pyrolysis of natural gas are reported between 10 and 370 K. The measurements were made along the fiber axis on both as-grown fibers and fibers heat treated to 3000 \ifmmode^\circ\else\textdegree\fi{}C. It is shown that heat treatment improves the thermal conductivity by a factor of 50: at room temperature highly heat-treated fibers rank among the very best thermal conductors available. We quantitatively interpret our low-temperature thermal conductivity data using the phonon dispersion relation for graphite. Remarkable agreement is achieved between the phonon mean free paths we deduce from our measurements and defect structures we identify in electron micrographs. The thermopower data we report on the highly heat-treated fibers are very similar to those obtained on single-crystal graphite.

Thermal conductivity of pitch-derived fibres

The temperature variation of the thermal conductivity of pitch-derived carbon fibres measured from 3 to 300K is reported and discussed. The results are compared with those previously obtained on benzene-derived fibres (BDF) and with the room-temperature thermal conductivity of PAN-based carbon fibres, the GY70. The data confirm that the structure of pitch fibres is less perfect than that of BDF. At room temperature a thermal conductivity comparable to that of copper is found.

Thermal conductivity of high-modulus carbon fibers : Kalnin, I.L. Conference on Composite Reliability, ASTM STP 580 (American Society for Testing and Materials, 1975) pp 560–573

Thermal conductivity of high-modulus carbon fibers : Kalnin, I.L. Conference on Composite Reliability, ASTM STP 580 (American Society for Testing and Materials, 1975) pp 560–573