Fiber Thermal Conductivity Research Articles

A study on thermal conductivity enhancement of silicon carbide filler glass fiber epoxy resin hybrid composites

The silicon carbide filler glass fiber reinforced polymer composites have been prepared by using hand- lay- up technique compression molding machine. Thermal conductivity of silicon carbide filler glass fiber reinforced polymer composites at different volume fractions of glass fiber and silicon carbide is determined using Lee’s apparatus experimentally. The experimental results show that the thermal conductivity of the hybrid polymer composite increases with increase in silicon carbide percentage. Experimental results are compared with mathematical models Hashin formula, Maxwell model and Rule of mixture model to find the variation of the thermal conductivity with volume fraction of silicon carbide. All three mathematical models exhibited results close to each other at low percentage silicon content. From mathematical models and experimental results the difference between the two has been found to be small for 20% SiC content; for example, Maxwell model showed a difference 10.40%, Rule of mixtures 16.12% and Hashin formula 3.62% for 20% SiC content.

Experimental study on effect of support fiber on fuel droplet vaporization at high temperatures

The effect of support fiber on fuel droplet vaporization is experimentally investigated in a stagnant high-temperature environment (673–973 K) using high-speed backlit imaging technique. Droplets with nearly identical initial diameter suspended on the different support fibers, thus the effect of support fiber on droplet vaporization due to different initial droplet diameter is removed. The fiber diameters are 0.08, 0.1 and 0.15 mm, and thermal conductivities are 1.4, 80 and 400 W/m·K, respectively. Meanwhile, the experimental results are verified by theoretical analysis. The results show that the droplet vaporization rate constant is linearly related to the squared diameter and thermal conductivity of fiber, which is consistent with the results in the literature. Moreover, the droplet lifetime changes linearly with fiber diameter and thermal conductivity. An increase in the droplet vaporization rate constant with thermal conductivity of fiber, but, a decrease for droplet lifetime. It is proved for the first time that support fiber can cause fuel droplet micro-explosion at high temperatures, however, the previous studies have not been reported. When the fiber diameter is greater than 0.15 mm or the thermal conductivity is higher than 400 W/m·K, the fiber heat transfer induced micro-explosion occurred. This is caused by support fiber induced local boiling or Leidenfrost effect inside the droplet. The optimum choice is that the fiber diameter is less than 0.1 mm and thermal conductivity is less than 80 W/m·K.

Opto-thermal technique for measuring thermal conductivity of polyacrylonitrile based carbon fibers

Thermal conductivity of carbon fibers (CFs) is an important property because CFs are used as heat dissipation fillers in composites for aerospace and electronics applications. However, evaluating thermal conductivity of a single filament of CFs is an arduous task due to dimensional issue of specimens and limitations of conventional measurement system. Therefore, we suggest an opto-thermal technique using Raman spectroscopy to measure thermal conductivity of commercial polyacrylonitrile based CFs (T300, T700SC and T800H). The opto-thermal technique used that G band from Raman spectroscopy of carbon materials is shifted depending on temperature. For verifying an accuracy of the technique, the laser absorbance of CFs were estimated, and the thermal conductivity was measured depending on the length of CF. The measured data were reflected in the thermal conductivity calculation formula. It was demonstrated that the method provides more reasonable thermal conductivity values compare to a conventional Angstrom method. In addition, this simple technique confirmed that graphitic structure of CFs played a critical role in their thermal conductivity.

Investigation on Thermal Conductivity of Steel Fiber Reinforced Concrete Using Mesoscale Modeling

A mesoscale model was developed to investigate the effect of steel fiber on the thermal conductivity of steel fiber-reinforced concrete (SFRC). Delaunay triangulation was employed to generate the unstructured mesh for SFRC materials. The model was validated using the existing experimental data. Then, it was used to study how model thickness affected simulation outcomes of thermal conductivity of models with different fiber lengths, by which an appropriate thickness was determined for the later analyses. The validated and optimized model was applied to the study of relationships between thermal conductivity and factors such as fiber content, fiber aspect ratio and different parts of an SFRC block by conducting steady-state heat analyses with the finite element analysis software ANSYS. The simulation results reveal that adding steel fiber increases thermal conductivity considerably, while fiber aspect ratio only has an insignificant effect. Besides, the presence of steel fibers has an obvious impact on the distribution of temperature and heat flux vector of the SFRC blocks.

Effective Thermal Conductivity of Stainless Steel Fiber Sintered Felt With Honeycombed Channels

Abstract A novel stainless steel fiber sintered felt (SSFSF) with honeycombed channels (SSFSFHC) is a promising support for catalytic combustion of the volatile organic compounds (VOCs). The SSFSFHC consists of stainless steel fiber, three-dimensionally reticulated porous structures, and interconnected honeycombed channels. The equivalent thermal conductivity (ETC) of the SSFSFHC is tested. It is found that the ETC of the SSFSFHC increases with the hot side temperature increasing but decreases with the porosity increasing and channel occupied area ratio increasing. The ETC of the SSFSFHC changes little with channel diameter increasing. The heat transfer model of the SSFSFHC is considered as parallel/series combinations of relevant thermal resistances. In order to estimate the ETC of the SSFSFHC, the correlation of the ETC of the SSFSF is derived. The expressions of the axial temperature under different porosities are deduced when eliminating the radial heat transfer between the channel section and the SSFSF section. The relationships of the transferred heats and the corresponding resistances along the radial direction are obtained by assuming that the radial heat transfer can be simplified as a serial of heat resistances located between the channels and the SSFSF.

Effect of Carbon Fiber Coating on Thermal Conductivity of Carbon Fiber Reinforced Lithium Alumina Silicate Glass-ceramics Composites

Effect of Carbon Fiber Coating on Thermal Conductivity of Carbon Fiber Reinforced Lithium Alumina Silicate Glass-ceramics Composites

Thermal Characterization of Alkali Treated Kenaf Fibers and Kenaf-Epoxy Composites

Chemical treatment of natural fibers is a well-defined means of mechanical property improvement in natural fiberreinforced composites. An understanding of mechanical and thermal properties in these media is essential for evaluating heat transfer, thermal degradation, and overall performance of these composites over their product lifetime. However, very little information is available illustrating the effect of such treatment on the thermal properties of kenaf composites. Also, no study to date has reported the thermal conductivity of individual kenaf fibers. This study reports the effects of fiber treatment (in 6 % NaOH) on thermal transport in unidirectionally oriented kenaf-epoxy composites and individual kenaf fibers. The effective thermal conductivities and thermal diffusivities of chemically treated fiber composites show a general increase over untreated fiber composites (0.210 to 0.232 W/m/K at 28 °C, 0.206 to 0.234 W/m/K at 200 °C). This improvement may be attributed to improved interfacial contact between the fibers and epoxy matrix shown in microstructural images after chemical treatment. The thermal conductivity of individual fibers was evaluated at room temperature using two techniques. Results from both techniques showed slight increases after chemical treatment (0.58±0.53 to 1.0±0.13 W/m/K and 1.2±0.54 to 1.6±0.28 W/m/K) but lacked statistical significance. Any improvement in surface crystallinity after chemical treatment does not appear to affect overall fiber thermal conductivity. A better understanding of thermal transport in kenaf fibers and composites enables better estimation of the performance of these composites in different applications. Moreover, the thermal conductivities of individual fibers are useful in understanding the fiber’s contribution to conduction in different fiber reinforcement configurations.

Prediction of the Effective Thermal Conductivity of Fiber Reinforced Composites Using a Micromechanical Approach

ABSTRACTA novel micromechanical approach is proposed to calculate the effective thermal conductivities of fiber reinforced composite materials. The key advantage of the present formulation is its ability to yield closed form solutions for the effective thermal conductivity of composites in both longitudinal and transverse directions for three dimensional heat transfer problems. The obtained results are in good agreement with the experimental data reported in the literature. When compared with analytical and finite element solutions, the results are seen to be in better agreement with the hexagonal packed array compared to the square packed array which thus represents a more realistic model of the fiber distribution in the matrix medium.

Geometrical modelling and thermal analysis of nonwoven fabrics

Nonwoven fabric can be produced for thermal insulation. It has low fibre volume fraction. Thermal insulation property of fibrous materials depends on not only the thermal conductivity of fibre but also the entrapped static air. If fibre volume fraction is low in fibrous assembly it means that more air in the volume, therefore, the insulation property of the fabric increases, or vice versa. In this research thermal bonded nonwoven fabrics were used to analyse the heat transfer phenomena and predict the effective thermal conductivity and thermal resistance by using finite element method. Finite element models of nonwoven fabrics were created by two techniques: 3D reconstruction and solid modelling. For validation purpose, the effective thermal conductivity results obtained from an in-house developed instrument were compared with predicted results from the developed finite element models. Furthermore, this research work also contains an investigation of the effect of fibre volume fraction and thermal conductivity of fibre on the overall heat transfer of nonwoven structures.

Thermal conductivity of fiber and opacifier loaded silica aerogel composite

Due to pure silica aerogel is fragile and nearly transparent to 3–8μm infrared beam, reinforced fiber and opacifier are composited with silica aerogel to optimize its performance. The suppression of radiative heat transfer is related to opacifier type, size and content. However, the addition of opacifier will enhance solid conduction inevitably. In addition, the influence of doping opacifier will also be affected by temperature since thermal radiation is temperature and wavelength dependent. Therefore thermal conductivities of 10 fiber and opacifier doped silica aerogel composites are studied experimentally and theoretically. Transmittances of silica aerogel doped with opacifier of different type, size and content are measured within 1.4–25μm by FTIR. The spectral extinction coefficient of fiber/opacifier loaded silica aerogel composite is obtained by considering the practical absorption of KBr diluents. The thermal conductivity of silica aerogel composite is measured by transient plane source method at different temperature and gas pressure. Effective model with considering heat conduction via gaseous molecules, matrix skeleton, fiber, opacifier and thermal radiation via skeleton, fiber and opacifier is developed for fast predicting the thermal conductivity of fiber/opacifier doped silica aerogel composite. It is found that the silica aerogels doped with SiC or TiO2 particles has lower radiative thermal conductivity than that doped with ZrO2 particles with volume fraction of 3.75% and diameter of 3.5μm. Silica aerogel doped with 3.75%, 3.5μm SiC particles has the lowest thermal conductivity at temperature less than 1000K.

Thermal conductivity and mechanical properties of pitch-based carbon fibre reinforced aluminium composites fabricated by squeeze casting

Pure aluminium and high-silicon aluminium alloy were reinforced with the discontinuous pitch-based carbon fibres by squeeze casting, then the thermal conductivity and the mechanical properties of the composites were investigated. Optical microscopy revealed that the fibres were in a random planar arrangement, and the transmission electron microscopy revealed that there is no interfacial reaction between the matrices and the fibres. The random planar arrangement of the fibres leads to the anisotropy of the composite. The fibre-reinforcement increased the thermal conductivity in the parallel direction for both pure aluminium and its alloy matrices, while the thermal conductivity decreased in the vertical direction. The increase in the elastic modulus by the reinforcement was not observed for both matrices. The proof stress of the pure aluminium increased by the reinforcement especially in the parallel direction, while that of the high-silicon alloy decreased by the reinforcement.

Nanoporous SiO2 grafted aramid fibers with low thermal conductivity

Silica aerogel is an excellent thermal insulation material for its mesoporous nanostructures In this study, the aramid fibers were grafted with the mesoporous structure of silica aerogel to reduce the fiber thermal conductivity through a serious process. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) results indicate that siloxane groups were successfully grafted onto the fiber surface through nitration, reduction and grafting with 3-glycidoxypropyltrimethoxysilane (GPTMS). After cohydrolysis with tetraethyl orthosilicate (TEOS), condensation, alkylation and ambient drying, SEM images of the as-prepared fibers showed that a mesoporous structure formed around the fiber surface. Si content within the as-prepared fiber was about 10.5 times higher than the fiber grafted with GPTMS. The fiber was covered with a mesoporous and hyperbranched network. As a result, the thermal conductivity of blanket consisting of the as-prepared aramid fibers, measured by transient hot-wire method, was obviously lower than that of the original ones, whereas the thermal stability changes little.

Modeling of heterogeneous heat transfer in fabrics

Thermal comfort, an important factor when designing fabric, is strongly related to heat transfer in the fabric. Fabric is not homogeneous because the constituent yarns are interlaced at a certain weave angle, and the contact area between yarns varies. Therefore, heat transfer along the yarn in the fabric is not yet well understood. In this study, we developed a heterogeneous fabric model in which heat transfer along the longitudinal and transverse directions of the yarn is treated independently, and proposed a method for estimating the contact heat transfer coefficients for yarns. The geometrical fabric structure was constructed based on the cross section of three fabrics observed with a three-dimensional microscope. The parameters used were the mass density, specific heat, and thermal conductivity of the fibers and air. The heat flow calculated using the model was compared with that measured experimentally. Fairly good agreement was observed, verifying the validity of this heterogeneous model. The simulation results indicated that when the anisotropy of the fiber thermal conductivity is high, heat is transferred significantly faster along the longitudinal direction than along the transverse direction of the yarn, and the equilibrium temperature distribution is strongly influenced by heat transfer in the longitudinal direction.

Thermal conductivity of electrospinning chain-aligned polyethylene oxide (PEO)

Polymer fibers' thermal conductivity along its axis direction are usually much better compared to that of the bulk. Many researches indicate that three main factors have significant impact on polymer fiber's thermal conductivity, which are crystallinity, molecular alignment and crystalline grain orientation of the polymer. Most research focus on polyethylene (PE) because of its relatively high crystallinity. Here, we report exceptionally high thermal conductivity on electrospun polyethylene oxide (PEO) nanofibers, which is an important semicrystalline polymer. This indicates that semicrystalline polymers like PEO can also obtain high thermal conductivity, which may be used in areas such as electronic devices. Its thermal conductivity improved about 150 times over bulk polymer. By using infrared spectroscopy, we evidenced that improvement in thermal conductivity was ascribed to increased molecular alignment. And we discussed the relationship between molecular alignment and crystalline grain orientation, as well as their contribution to the thermal conductivity of PEO fibers. We found that for PEO nanofibers, preferred molecular alignment in the amorphous region has significant contributed to high thermal conductivity.

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Continuous Carbon Nanotube-Based Fibers and Films for Applications Requiring Enhanced Heat Dissipation.

The production of continuous carbon nanotube (CNT) fibers and films has paved the way to leverage the superior properties of individual carbon nanotubes for novel macroscale applications such as electronic cables and multifunctional composites. In this manuscript, we synthesize fibers and films from CNT aerogels that are continuously grown by floating catalyst chemical vapor deposition (FCCVD) and measure thermal conductivity and natural convective heat transfer coefficient from the fiber and film. To probe the mechanisms of heat transfer, we develop a new, robust, steady-state thermal characterization technique that enables measurement of the intrinsic fiber thermal conductivity and the convective heat transfer coefficient from the fiber to the surrounding air. The thermal conductivity of the as-prepared fiber ranges from 4.7 ± 0.3 to 28.0 ± 2.4 W m(-1) K(-1) and depends on fiber volume fraction and diameter. A simple nitric acid treatment increases the thermal conductivity by as much as a factor of ∼3 for the fibers and ∼6.7 for the thin films. These acid-treated CNT materials demonstrate specific thermal conductivities significantly higher than common metals with the same absolute thermal conductivity, which means they are comparatively lightweight, thermally conductive fibers and films. Beyond thermal conductivity, the acid treatment enhances electrical conductivity by a factor of ∼2.3. Further, the measured convective heat transfer coefficients range from 25 to 200 W m(-2) K(-1) for all fibers, which is higher than expected for macroscale materials and demonstrates the impact of the nanoscale CNT features on convective heat losses from the fibers. The measured thermal and electrical performance demonstrates the promise for using these fibers and films in macroscale applications requiring effective heat dissipation.

Effect of fiber transverse isotropy on effective thermal conductivity of metal matrix composites reinforced by randomly distributed fibers

The effective thermal conductivity of metal matrix composites (MMCs) reinforced by the randomly distributed transversely isotropic fibers is evaluated using the representative volume element (RVE) based finite element (FE) homogenization method. The modified random sequential absorption (RSA) algorithm is proposed to generate the periodic RVEs of MMCs and thus the periodic boundary conditions are introduced. The transversely isotropic thermal conductivity of fiber is considered and its numerical implementation in the FE analysis is detailed. The RVE based FE homogenization method to predict the effective thermal conductivity of MMCs reinforced by the randomly distributed fibers is verified by comparing against the experiment and the Hashin–Shtrikman bounds. The simulation results show that the effect of the transverse isotropy of fiber thermal conductivity on the effective thermal conductivity of MMCs reinforced by the randomly distributed fibers can be neglected owing to the large matrix/fiber thermal conductivity ratio. The effective thermal conductivity of MMCs reinforced by the randomly distributed fibers decreases with the increase of fiber volume fraction, while the effect of fiber aspect ratio on the effective thermal conductivity of MMCs reinforced by the randomly distributed fibers is not significant.

Thermal Conductivity in the Radial Direction of Deformed Polymer Fibers.

Thermal conductivity of polymer fibers in the axial direction has been extensively studied while thermal conductivity in the radial direction Λ remains unknown. In this work, polymer fibers with different molecular arrangements (crystalline, liquid crystalline, and amorphous) were plastically deformed. Λ was measured at engineering strains ε = 0.2-2.3 using time-domain thermoreflectance. Λ decreases with increasing strains for polyethylene (PE) and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers and is independent of strain for poly(methyl methacrylate) (PMMA) fibers. The extrapolated thermal conductivity at zero strain is Λ0 ≈ 0.27 Wm-1 K-1 for crystalline PE, Λ0 ≈ 0.29 Wm-1 K-1 for liquid crystalline PBO, and Λ0 ≈ 0.18 Wm-1 K-1 for amorphous PMMA. Λ of PE drops to Λ ≈ 0.14 Wm-1 K-1 at ε = 1.9; Λ of PBO drops to Λ ≈ 0.12 Wm-1 K-1 at ε = 2.1. We attribute the decrease of Λ with ε in crystalline and liquid crystalline fibers to structural disorder induced by plastic deformation. The combination of structural disorder and phonon focusing effects produces a thermal conductivity in deformed PE and PBO fibers that is lower than amorphous PMMA.

Thermal conductivity of refractory glass fibres

In the present study, the current international standards and corresponding apparatus for measuring the thermal conductivity of refractory glass fibre products have been reviewed. Refractory glass fibres are normally produced in the form of low-density needled mats. A major issue with thermal conductivity measurements of these materials is lack of reproducibility in the test results due to transformation of the test material during the test. Also needled mats are inherently inhomogeneous, and this poses additional problems. To be able to compare the various methods of thermal conductivity measurement, a refractory reference material was designed which is capable of withstanding maximum test temperatures (1673 K) with minimum transformation. The thermal conductivity of this reference material was then measured using various methods according to the different standards surveyed. In order to compare different materials, samples have been acquired from major refractory glass fibre manufacturers and the results have been compared against the newly introduced reference material. Materials manufactured by melt spinning, melt blowing and sol–gel have been studied, and results compared with literature values.

Thermal Conductivity of Turbostratic Carbon Nanofiber Networks

Composite material systems composed of a matrix of nanomaterials can achieve combinations of mechanical and thermophysical properties outside the range of traditional systems. The microstructure of the system dictates the rate, in which heat moves through the material. In this work, air/carbon nanofiber networks are studied to elucidate the system parameters influencing thermal transport. Thermal properties are measured with varying initial carbon fiber fill fraction, environment pressure, loading pressure, and heat treatment temperature (HTT) through a bidirectional modification of the 3ω technique. The nanostructure of the individual fibers is characterized with small angle X-ray scattering and Raman spectroscopy providing insight to individual fiber thermal conductivity. Measured thermal conductivity of the carbon nanofiber networks varied from 0.010 W/(m K) to 0.070 W/(m K). An understanding of the intrinsic properties of the individual fibers and the interactions of the two-phase composite is used to reconcile low measured thermal conductivities with predictive modeling. Accounting for fiber-to-fiber interactions and the nuanced changes in the composite as pressure is applied is necessary to successfully model thermal transport in system.