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.     

Thermal diffusivity of antifriction polymer composites

We have studied the thermal diffusivity of a series of polymer composites based on poly(2,6-dimethyl-1,4-phenyleneoxide) (PDPO) and filled with copper and aluminum powders, SiC and TiO₂ whiskers, and comminuted carbon, poly(amidobenzimidazole) (PABI), and glass fibers. An increase in the thermal diffusivity of PDPO upon the introduction of a filler is explained by the formation of boundary layers in the polymer matrix near the filler particles, where the polymer molecules are oriented parallel to the particle surface. The boundary layer thickness varies from 1 to 60 μm, depending on the filler type. It is established that the thermal diffusivity of the composite exponentially decreases with increasing mean distance between particles of the filler.     

Thermal and mechanical properties of waste grass broom fiber-reinforced polyester composites

The main focus of this study is to utilize waste grass broom natural fibers as reinforcement and polyester resin as matrix for making partially biodegradable green composites. Thermal conductivity, specific heat capacity and thermal diffusivity of composites were investigated as a function of fiber content and temperature. The waste grass broom fiber has a tensile strength of 297.58 MPa, modulus of 18.28 GPa, and an effective density of 864 kg/m³. The volume fraction of fibers in the composites was varied from 0.163 to 0.358. Thermal conductivity of unidirectional composites was investigated experimentally by a guarded heat flow meter method. The results show that the thermal conductivity of composite decreased with increase in fiber content and the quite opposite trend was observed with respect to temperature. Moreover, the experimental results of thermal conductivity at different volume fractions were compared with two theoretical models. The specific heat capacity of the composite as measured by differential scanning calorimeter showed similar trend as that of the thermal conductivity. The variation in thermal diffusivity with respect to volume fraction of fiber and temperature was not so significant.The tensile strength and tensile modulus of the composites showed a maximum improvement of 222% and 173%, respectively over pure matrix. The work of fracture of the composites with maximum volume fraction of fibers was found to be 296 Jm⁻¹.     

Correction of Edge Effect in AC Calorimetric Method for Measuring Thermal Diffusivity of CVD Diamond Films1

In the thermal diffusivity measurement of a CVD diamond film using an ac calorimetric method, the reflection of an ac temperature wave at the edge of the film sample should be considered due to the limited length of the sample and its high thermal diffusivity, i.e., the edge effect. In this case, the measured thermal diffusivity is given as a function of frequency. The relation between the measured thermal diffusivity and the frequency is represented as an analytical expression. The real thermal diffusivity is obtained by correcting the edge effect by two means. One is an iterative method using the directly measured edge length of the sample to fit the analytical expression. The other is a parameter estimation method by which a simplex method is used to estimate the edge length and the real thermal diffusivity. Thermal diffusivities of two diamond films were measured, and data were analyzed using the above methods. The result shows that the parameter estimation method is relatively accurate and convenient in processing test data.     

Thermally conductive polyamide 6/carbon filler composites based on a hybrid filler system

We explored the use of a hybrid filler consisting of graphite nanoplatelets (GNPs) and single walled carbon nanotubes (SWCNTs) in a polyamide 6 (PA 6) matrix. The composites containing PA 6, powdered GNP, and SWCNT were melt-processed and the effect of filler content in the single filler and hybrid filler systems on the thermal conductivity of the composites was examined. The thermal diffusivities of the composites were measured by the standard laser flash method. Composites containing the hybrid filler system showed enhanced thermal conductivity with values as high as 8.8 W (m · K)⁻¹, which is a 35-fold increase compared to the thermal conductivity of pure PA 6. Thermographic images of heat conduction and heat release behaviors were consistent with the thermal conductivity results, and showed rapid temperature jumps and drops, respectively, for the composites. A composite model based on the Lewis–Nielsen theory was developed to treat GNP and SWCNT as two separate types of fillers. Two approaches, the additive and multiplicative approaches, give rather good quantitative agreement between the predicted values of thermal conductivity and those measured experimentally.     

Thermally conductive polyamide 6/carbon filler composites based on a hybrid filler system

Abstract

We explored the use of a hybrid filler consisting of graphite nanoplatelets (GNPs) and single walled carbon nanotubes (SWCNTs) in a polyamide 6 (PA 6) matrix. The composites containing PA 6, powdered GNP, and SWCNT were melt-processed and the effect of filler content in the single filler and hybrid filler systems on the thermal conductivity of the composites was examined. The thermal diffusivities of the composites were measured by the standard laser flash method. Composites containing the hybrid filler system showed enhanced thermal conductivity with values as high as 8.8 W (m · K)^?1, which is a 35-fold increase compared to the thermal conductivity of pure PA 6. Thermographic images of heat conduction and heat release behaviors were consistent with the thermal conductivity results, and showed rapid temperature jumps and drops, respectively, for the composites. A composite model based on the Lewis–Nielsen theory was developed to treat GNP and SWCNT as two separate types of fillers. Two approaches, the additive and multiplicative approaches, give rather good quantitative agreement between the predicted values of thermal conductivity and those measured experimentally.     

Experimental Values for the Elastic Constants of a Particulate-Filled Glassy Polymer

Young’s modulus and Poisson’s ratio have been measured simultaneously on a series of particulate composites containing volume fractions of filler up to 0.50. The composites consisted of small glass spheres imbedded in a rigid epoxy polymer matrix. The measured values were compared with theoretical values calculated from current theories. A recently generalized and simplified version of van der Poel’s theory provided the best agreement. It predicted values of Young’s modulus for composites with filler volume fractions up to 0.35. Measured values of Poisson’s ratio exhibited scattering, but were consistent with values calculated from van der Poel’s theory.     

Investigation of Thermal Properties of High-Density Polyethylene/Aluminum Nanocomposites by Photothermal Infrared Radiometry

In this study, thermal properties of high-density polyethylene (HDPE) filled with nanosized Al particles (80 nm) were investigated. Samples were prepared using melt mixing method up to filler volume fraction of 29 %, followed by compression molding. By using modulated photothermal radiometry (PTR) technique, thermal diffusivity and thermal effusivity were obtained. The effective thermal conductivity of nanocomposites was calculated directly from PTR measurements and from the measurements of density, specific heat capacity (by differential scanning calorimetry) and thermal diffusivity (obtained from PTR signal amplitude and phase). It is concluded that the thermal conductivity of HDPE composites increases with increasing Al fraction and the highest effective thermal conductivity enhancement of 205 % is achieved at a filler volume fraction of 29 %. The obtained results were compared with the theoretical models and experimental data given in the literature. The results demonstrate that Agari and Uno, and Cheng and Vachon models can predict well the thermal conductivity of HDPE/Al nanocomposites in the whole range of Al fractions.     

石墨和炭纤维分别改性热塑性聚酰亚胺复合材料的导热行为

采用石墨、 炭纤维填充改善热塑性聚酰亚胺(TPI)材料的导热性能, 研究了填料物性对材料力学性能和导热行为的影响。在此基础上, 用Nielsen理论模型和有限元方法模拟了复合材料的导热行为, 进一步探讨了填料形状对材料导热系数的影响。研究表明: 炭纤维、 石墨填充TPI均能提高复合材料的导热性能; 用Nielsen理论模型预测石墨、 炭纤维填充TPI材料导热系数与实验值存在一定偏差; 采用有限元法模拟二维复合材料稳态导热行为, 能有效地预测复合材料的导热系数。基于材料内部热流分布模拟分析发现, 填料自身导热性能对复合材料导热行为的影响不明显; 与圆形填料相比, 方形填料改善材料导热性能效果显著。      

Linking Interfacial Bonding and Thermal Conductivity in Molecularly-Confined Polymer-Glass Nanocomposites with Ultra-High Interfacial Density

Thermal transport in polymer nanocomposites becomes dependent on the interfacial thermal conductance due to the ultra-high density of the internal interfaces when the polymer and filler domains are intimately mixed at the nanoscale. However, there is a lack of experimental measurements that can link the thermal conductance across the interfaces to the chemistry and bonding between the polymer molecules and the glass surface. Characterizing the thermal properties of amorphous composites are a particular challenge as their low intrinsic thermal conductivity leads to poor measurement sensitivity of the interfacial thermal conductance. To address this issue here, polymers are confined in porous organosilicates with high interfacial densities, stable composite structure, and varying surface chemistries. The thermal conductivities and fracture energies of the composites are measured with frequency dependent time-domain thermoreflectance (TDTR) and thin-film fracture testing, respectively. Effective medium theory (EMT) along with finite element analysis (FEA) is then used to uniquely extract the thermal boundary conductance (TBC) from the measured thermal conductivity of the composites. Changes in TBC are then linked to the hydrogen bonding between the polymer and organosilicate as quantified by Fourier-transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopy. This platform for analysis is a new paradigm in the experimental investigation of heat flow across constituent domains.     

Numerical Investigation of Heat Transfer of Silver-Coated Glass Particles Dispersed in Ethylene Vinyl Acetate Matrix

The effective thermal conductivity of silver-coated glass spheres dispersed in an ethylene vinyl acetate matrix was investigated numerically as a function of filler concentration. The finite-element method was carried out for modeling the thermal heat transport and to calculate the effective thermal conductivity of the composite for three elementary cells; simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC). The effect of the inclusion/matrix thermal contact resistance and the ratio of thermal conductivities of the filler-to-matrix material are also taken into account. The numerical results are compared with previously published experimental data and some theoretical models. The calculated values of the thermal conductivity of the SC model are in good agreement with the measured results for all the filler volume fractions. Numerical results for FCC and BCC models were found to be in good agreement with analytical models. The results show that the filler/matrix contact resistance has an important effect on the effective thermal conductivity.     

Measurement of the Thermal Diffusivity of an Anisotropic Graphite Sheet Using a Laser-Heating AC Calorimetric Method

The thermal diffusivity of a graphite sheet having an extremely high anisotropy has been measured by a laser heating AC calorimetric method in the temperature range from 30 to 350 K. This graphite sheet has characteristics of high thermal diffusivity and high anisotropy, and it is only 100 m thick. Thus, it is difficult to apply the conventional AC technique. Therefore, we propose a simultaneous measurement method for the in-plane and out-of-plane thermal diffusivities, by analyzing the three-dimensional heat conduction process, which contains the effects of anisotropy and thermal wave reflections. This method was verified by checking with thermal diffusivity measurements of isotropic materials such as stainless steel and pure copper and was then applied to the anisotropic thermal diffusivity measurement of the graphite sheet.     

Thermal conductivity of polymer composites with close-packed structure of nano and micro fillers

Thermal conductivity of polymer composites with nano and micro fillers has been investigated numerically and experimentally. The nano fillers used were multi-walled carbon nanotubes (MWNTs) and alumina nanoparticles, and the spherical alumina particles were selected as the micro fillers. A periodic unit cell with a random close-packed structure was created using a packing algorithm that treat the micro filler as spheres. Finite element analyses were also performed to predict the potential of nano fillers to enhance thermal conductivity of the composites and to analyze the effect of microstructure of micro fillers. Additionally, the polymer composites with nano and micro fillers were made and the thermal conductivity of the composites were measured. The results showed that the addition of MWNTs to the matrix lead to a large increase in thermal conductivity of the composites. The proposed thermal model predicted a thermal conductivity in good agreement with experiment.     

Experimental Verification to Obtain Intrinsic Thermal Diffusivity by Laser-Flash Method

There is a need to obtain highly reliable values of thermophysical properties. The thermal conductivity of solids is often calculated from the thermal diffusivity, specific heat, and density, respectively, measured by the laser-flash method, differential scanning calorimetry, and Archimedes’ method. The laser-flash method is one of the most well-known methods for measuring the thermal diffusivity of solids above room temperature. This method is very convenient to measure the thermal diffusivity without contact in a short time. On the other hand, it is considered as an absolute reference measurement method, in particular, because only measurements of basic quantities such as time, temperature, length, and electrical quantities are required, and because the uncertainty of measurement can be analytically evaluated. However, it could be difficult in some cases to obtain reliable thermal-diffusivity values. The measurement results can indeed depend on experimental conditions; in particular, the pulse heating energy. A procedure to obtain the intrinsic thermal-diffusivity value was proposed by National Metrology Institute of Japan (NMIJ). Here, “intrinsic” means unique for the material, independent of measurement conditions. In this method, apparent thermal-diffusivity values are first measured by changing the pulse heating energy at the same test temperature. Then, the intrinsic thermal diffusivity is determined by extrapolating these apparent thermal diffusivities to a zero energy pulse. In order to verify and examine the applicability of the procedure for intrinsic thermal-diffusivity measurements, we have measured the thermal diffusivity of some materials (metals, ceramics) using the laser-flash method with this extrapolation procedure. NMIJ and Laboratoire National de Metrologie et d’essais (LNE) have laser-flash thermal-diffusivity measurement systems that are traceable to SI units. The thermal diffusivity measured by NMIJ and LNE on four materials shows good agreement, although they used different measurement systems and different analysis methods of the temperature-rise curve. Experimental verification on the procedure was carried out using the measured results. Some problems and considered solutions for laser-flash thermal-diffusivity measurements are discussed.     

Anisotropic thermal diffusivity characterization of aligned carbon nanotube-polymer composites

The anisotropic thermal diffusivity of aligned carbon nanotube-polymer composites was determined using a photothermoelectric technique. The composites were obtained by infiltrating poly-dimethyl siloxane (PDMS) in aligned multiwall CNT arrays grown by chemical vapor deposition on silicon substrates. The thermal diffusivities are insensitive to temperature in the range of 180 K-300 K. The thermal diffusivity values across the alignment direction are approximately 2-4 times smaller than along the alignment direction and larger than effective media theory predictions using reported values for the thermal diffusivity of millimeter thick aligned multiwall carbon nanotube arrays. The effective room temperature thermal conductivity of the composite along the carbon nanotube alignment direction is at least 6X larger than the thermal conductivity of the polymer matrix and is in good agreement with the effective media predictions. This work indicates that infiltration of long and aligned carbon nanotube arrays is currently the most efficient method to obtain high thermal conductivity polymer composites.     

Thermal conductivities of Ti-SiC and Ti-TiB2 particulate composites

Composites of commercial-purity titanium reinforced with 10 and 20 vol % of SiC and TiB₂ particulates were produced by powder blending and extrusion. Heat treatments were conducted on each of these composites. The thermal diffusivities of the composites were measured as a function of temperature using the laser flash technique. Thermal conductivities were inferred from these measurements, using a rule-of-mixtures assumption for the specific heats. It has been shown that, while an enhancement of the thermal conductivity is expected to arise from the presence of both types of reinforcement, this behaviour is in fact observed only with the Ti-TiB₂ composites. The thermal conductivity of Ti-TiB₂ composites is significantly greater than that of the unreinforced matrix and rises with increasing volume fraction of reinforcement. In contrast, the conductivities of the Ti-SiC composites were considerably lower than that of the unreinforced titanium and decreased with increasing volume fraction of SiC reinforcement. These results have been interpreted in terms of the thermal resistance of the reaction layers which exist between the matrix and two types of particulate reinforcements. The faster reaction kinetics between SiC and Ti gives rise to a thicker reaction layer for a given heat treatment than that between Ti and TiB₂ and is also accompanied by a much larger volume change (– 4.6%). It is proposed that this volume decrease, giving rise to interfacial damage and a network of microcracks, is at least partly responsible for a high interfacial thermal resistance, reducing the conductivity of the Ti-SiC composite. These results indicate that TiB₂ would be preferable to SiC as a reinforcement in Ti for situations where a high thermal conductivity would be beneficial.     

Thermal conductivity of epoxy composites with a binary-particle system of aluminum oxide and aluminum nitride fillers

Aluminum oxide and aluminum nitride with different sizes were used alone or in combination to prepare thermally conductive polymer composites. The composites were categorized into two systems, one including composites filled with large-sized aluminum nitride and small-sized aluminum oxide particles, and the other including composites filled with large-sized aluminum oxide and small-sized aluminum nitride. The use of these hybrid fillers was found to be effective for increasing the thermal conductivity of the composite, which was probably due to the enhanced connectivity offered by the structuring filler. At a total filler content of 58.4 vol.%, the maximum values of both thermal conductivities in the two systems were 3.402 W/mK and 2.842 W/mK, respectively, when the volume ratio of large particles to small particles was 7:3. This result was represented when the composite was filled with the maximum packing density and the minimum surface area at the same volume content. As such, the proposed thermal model predicted thermal conductivity in good agreement with experimental values.     

In Situ Measurement of Thermal Diffusivity in Anisotropic Media

A knowledge of thermal conductivity/diffusivity is essential in several situations in engineering. This material property serves also as a measure of the quality of the manufactured materials. The thermal conductivity and diffusivity are measured by specialized labs using commercially available equipment. Even though both the number of such sites and the available measurement techniques are quite large, non-destructive, fast, and reliable techniques are still demanded. The developed technique, due to its rapidity and nondestructive character, can be embedded in a manufacturing process. As opposed to most methods, it does not require preparation of samples of a special shape (e.g., a small cylinder, a thin foil, cuboid). Moreover, one measurement cycle of the proposed technique yields two principal components of the diffusivities of orthotropic materials.     

Thermal conductivity of ceramic particle filled polymer composites and theoretical predictions

Models and theories for predicting the thermal conductivity of polymer composites were discussed. Effective Medium Theory (EMT), Agari model and Nielsen model respectively are introduced and are applied as predictions for the thermal conductivity of ceramic particle filled polymer composites. Thermal conductivity of experimentally prepared Si₃N₄/epoxy composite and some data cited from the literature are discussed using the above theories. Feasibility of the three methods as a prediction in the whole volume fraction region of the filler from 0 to 1 was evaluated for a comparison. As a conclusion: both EMT and Nielsen model can give a well prediction for the thermal conductivity at a low volume fraction of the filler; Agari model give a better prediction in the whole range, but with larger error percentage.     

A Transient Heating Technique for Measuring the Thermal Diffusivity of Metals

A transient heating technique, improving the constant-rate-heating technique for the measurements of thermal diffusivities of metals, is proposed. For a physical model of a specimen to be measured, the transient heat-conduction equation was solved with some boundary conditions, and the solution obtained was used as the principle of the present transient heating technique for determining the thermal diffusivity of the specimen. Additionally, a thermal analysis was made to satisfy a boundary condition involved in the principle, that is, the condition of radiative thermal insulation at the two end surfaces of the specimen. To verify the validity of the present technique, the thermal diffusivity of iron, whose thermophysical properties are well-known, was measured with the same apparatus as used in our previous work, and the experimental results are discussed. Moreover, thermal diffusivities of thermocouple materials, namely, constantan, chromel, and alumel, were measured by the technique in the temperature range of 360 to 680 K.