Role of Interfacial Carbon layer in the Thermal Diffusivity/Conductivity of Silicon Carbide Fiber-Reinforced Reaction-Bonded Silicon Nitride Matrix Composites

The role of an interfacial carbon coating in the heat conduction behavior of a uniaxial silicon carbide nitride was investigated. For such a composite without an interfacial carbon coating the values for the thermal conductivity transverse to the fiber direction agreed very well with the values calculated from composite theory using experimental data parallel to the fiber direction, regardless of the ambient atmosphere. However, for a composite made with carbon-coated fibers the experimental values for the thermal conductivity transverse to the fiber direction under vacuum at room temperature were about a factor of 2 lower than those calculated from composite theory assuming perfect interfacial thermal contact. This discrepancy was attributed to the formation of an interfacial gap, resulting from the thermal expansion mismatch between the fibers and the matrix in combination with the low adhesive strength of the carbon coating. In nitrogen or helium the thermal conductivity was found to be higher because of the contribution of gaseous conduction across the interfacial gap. On switching from vacuum to nitrogen a transient effect in the thermal diffusivity was observed, attributed to the diffusion-limited entry of the gas phase into the interfacial gap. These effects decreased with increasing temperature, due to gap closure, to be virtually absent at 1000°C.     

Role of the Interfacial Thermal Barrier in the Effective Thermal Diffusivity/Conductivity of SiC-Fiber-Reinforced Reaction-Bonded Silicon Nitride

Experimental thermal diffusivity data transverse to the fiber direction for composites composed of a reaction bonded silicon nitride matrix reinforced with uniaxially aligned carbon-coated silicon carbide fibers indicate the existence of a significant thermal barrier at the matrix-fiber interface. Calculations of the interfacial thermal conductances indicate that at 300°C and 1-atm N₂, more than 90% of the heat conduction across the interface occurs by gaseous conduction. The magnitude of the interfacial conductance is decreased significantly under vacuum or by removal of the carbon surface layer from the fibers by selective oxidation. Good agreement is obtained between thermal conductance values for the oxidized composite at 1 atm calculated from the thermal conductivity of the N₂ gas and those inferred from the data for the effective composite thermal conductivity.     

Thermal Conductivity of a Particulate-Diamond-Reinforced Cordierite Matrix Composite

The effect of 15 vol% particulate diamond reinforcement on the thermal conductivity of a cordierite matrix was studied as a function of diamond particle size from room temperature to 700°C. The thermal conductivity was found to increase with increasing particle size to a maximum increase of about 75% for a mean particle size of 50 μ. The particle size effect was found to be more pronounced at the lower temperatures than at the higher temperatures. The observed effect of particle size and temperature was attributed to the existence of an interfacial thermal barrier, possibly resulting from interfacial phonon scattering, with a positive temperature dependence of the interfacial thermal conductance. The magnitude of this conductance suggested strong adhesion between the diamond and cordierite.     

Interface Contributions to Localized Heating of Dielectric Thin Films

Measurements of the thermal conductivity of thin dielectric films in the last ten years have established that thin film thermal conductivity may be much lower than that of the corresponding bulk solid, by as much as two orders of magnitude, and that significant interfacial thermal resistance may be present along the film/substrate interface. We review such measurements of thin film thermal conductivity and interfacial thermal resistance, and use the heat conduction equation to determine their implications for the localized heating of thermally anisotropic thin films bonded to substrates. It is found that for surface heating an equivalent isotropic film can be established and that the presence of large interfacial thermal resistance leads to a strong dependence of film thermal conductivity on film thickness, especially for thin films. A microscopic model of the film/substrate interface is used to establish the dependence of the interfacial thermal resistance on porosity along the interface.     

A comparison of the interfacial, thermal, and ablative properties between spun and filament yarn type carbon fabric/phenolic composites

In the present paper, the interfacial, thermal, and ablative properties of phenolic composites reinforced with spun yarn type carbon fabrics (spun C/P composite) and filament yarn type carbon fabrics (filament C/P composite) heat-treated at 1100 °C have been extensively compared. The interlaminar shear strength, crack growth rate, and fracture surface were studied to evaluate the interfacial characteristics of the composites using short-beam shear test, double cantilever beam test, and scanning electron microscopy, respectively. The thermal conductivity and the coefficient of thermal expansion were also measured in the longitudinal and transverse directions, respectively. To explore the ablative characteristics of the composites in terms of insulation index, erosion rate, and microscopic pattern of ablation, an arc plasma torch was used. The interfacial properties of the spun C/P composite are significantly greater than those of the filament C/P composite, with qualitative support of fracture surface observations. It has been investigated that the presence of protruded fibers in the phenolic matrix of the spun C/P composite may play an important role in enhancing the properties due to a fiber bridging effect. The longitudinal thermal conductivity of the spun C/P composite is about 7% lower than that of the filament C/P counterpart. It has been found from the ablation test using arc plasma torch flame that the erosion rate is 14% higher than that of the filament C/P counterpart. Consequently, all the experimental results suggest that use of spun yarn type carbon fabrics heat-treated at low carbonization temperature as reinforcement in a phenolic composite may significantly contribute to improving the interfacial, thermal, and ablative properties of C/P composites.     

Thermal Conductivity of Vapor-Liquid-Solid and Vapor-Solid Silicon Carbide Whisker-Reinforced Lithium Aluminosilicate Glass-Ceramic Composites

The thermal conductivities of two lithium aluminosilicate glass-ceramic matrix composites reinforced with 30 vol% of either SiC VS (rice hull) whiskers or SiC VLS (vapor-liquid-solid) whiskers were determined from room temperature to 500°C. Because of the preferred alignment of the whiskers, the thermal conductivity values normal to the hot-pressing direction were found to be significantly higher than those in the parallel direction. The composites with the VLS whiskers exhibited higher thermal conductivity values than those with the VS whiskers. An analysis of the room-temperature data showed that the thermal conductivity values parallel to the hot-pressing direction were higher than those predicted from theory, even for whiskers with infinite thermal conductivity and perfect interfacial thermal contact. This effect was attributed to a significant contribution of percolation to the total heat flow as a result of direct whisker-to-whisker contact. For both types of whiskers, the interfacial thermal conductance and thermal conductivity values (at ∼6.5 × 10⁵ W/(m²-K) and 200 W/(m·K), respectively) inferred from the composite thermal conductivity values perpendicular to the hot-pressing direction were essentially the same. It was concluded that the order of magnitude difference in thickness for the two whisker types was primarily responsible for the differences in thermal conductivity measured for these two composites.     

Effect of Interfacial Reaction on the Thermal Conductivity of Al–SiC Composites with SiC Dispersions

The effect of interfacial reactions between Al and SiC on the thermal conductivity of SiC-particle-dispersed Al-matrix composites was investigated by X-ray diffraction and transmission electron microscopy (TEM), and the thermal barrier conductance ( h _c) of the interface in the Al–SiC composites was quantified using a rule of mixture regarding thermal conductivity. Al–SiC composites with a composition of Al (pure Al or Al–11 vol% Si alloy)–66.3 vol% SiC and a variety of SiC particle sizes were used as specimens. The addition of Si to an Al matrix increased the thermal barrier conductance although it decreased overall thermal conductivity. X-ray diffraction showed the formation of Al₄C₃ and Si as byproducts in addition to Al and SiC in some specimens. TEM observation indicated that whiskerlike products, possibly Al₄C₃, were formed at the interface between the SiC particles and the Al matrix. The thermal barrier conductance and the thermal conductivity of the Al–SiC composites decreased with increasing Al₄C₃ content. The role of Si addition to an Al matrix was concluded to be restraining an excessive progress of the interfacial reaction between Al and SiC.     

The structure and properties of ribbon-shaped carbon fibers with high orientation

Using a naphthalene-derived mesophase pitch as a starting material, highly oriented ribbon-shaped carbon fibers with a smooth and flat surface were prepared by melt-spinning, oxidative stabilization, carbonization, and graphitization. The preferred orientation, morphology, and microstructure, as well as physical properties, of the ribbon-shaped carbon fibers were characterized. The results show that, the ribbon-shaped fibers possessed uniform shrinkage upon heat treatment, thereby avoiding shrinkage cracking commonly observed in round-shaped fibers. As heat treatment progressed, the ribbon-shaped graphite fibers displayed larger crystallite sizes and higher orientation of graphene layers along the main surface of the ribbon-shaped fiber in comparison with corresponding round-shaped fibers. The stability of the ribbon-shaped graphite fibers towards thermal oxidation was significantly higher than that of K-1100 graphite fibers. The longitudinal thermal conductivity of the ribbon fibers increased, and electrical resistivity decreased, with increasing the heat treatment temperatures. The longitudinal electrical resistivity and the calculated thermal conductivity of the ribbon-shaped fibers graphitized at 3000 °C are about 1.1 μΩ m and above 1100 W/m K at room temperature, respectively. The tensile strength and Young’s modulus of these fibers approach 2.53 and 842 GPa, respectively.     

Theoretical analysis of laminar flow heat transfer in a flat-duct wall reactor

Conjugated heat transfer in steady state was studied, assuming the interfacial reaction to be instantaneous and the flow to be a fully-developed laminar condition. The thermal boundary condition at the outer surface of the duct wall is specified as the first kind or the third kind. Influences of the interfacial reaction, the conductance of the wall and the external convection on the heat transfer characteristics were found to be substantial through calculated numerical results of the interfacial temperature and the local Nusselt number. Cases with uniform interfacial heat source and without it were also examined under the third kind boundary condition.     

Thermophysical properties of densified pitch based carbon/carbon materials—I. Unidirectional composites

Unidirectional carbon/carbon composites were developed using high-pressure impregnation/carbonization technique with PAN and pitch based carbon fibers of varying microstructure as reinforcements and different types of pitches as matrix precursors. The composites have been given final heat treatment to 2500-2700 °C. Microstructure of these composites has been evaluated using scanning electron microscope and polarized light optical microscope. Thermophysical properties, i.e., thermal conductivity, coefficient of thermal expansion and specific heat have been evaluated. It is found that the type of fibers and matrix present in the composites influences the absorption (specific heat) and transmission (conductivity) of thermal energy. The temperature dependence of thermal diffusion, specific heat, thermal conductivity and coefficient of thermal expansion has been studied and correlated with microstructure of carbon/carbon composites.     

Covalent functionalization of carbon nanotubes and their use in dielectric epoxy composites to improve heat dissipation

A method to enhance the thermal dissipation of epoxies is described. The method exploits the transport of heat by phonons so that the composite material designed remains dielectric. The improvement in thermal transport is guaranteed by the addition of functionalized carbon nanotube fillers which are covalently bonded to the epoxy matrix. We demonstrate that even if the covalent grafting of functional molecules affects the thermal transport within the nanotubes because of the disruption in periodicity of the structure, it improves the interfacial thermal conductance between the matrix and fillers. The trade-off has a net positive impact on the effective thermal conductivity of the composite material.     

自组装单分子层调控界面热输运的研究进展

从微观角度出发,综述了自组装单分子层（SAM）在调控软?硬材料界面热输运方面的研究进展,介绍了SAM的传热机理,系统总结了SAM的微观结构对界面热导的影响规律,探讨了SAM的密度、长度、末端官能团等因素对界面热阻或界面热导的影响机理。此外还介绍了目前SAM在聚合物基导热复合材料中的研究,并展望了未来SAM在界面热输运的研究方向。     

Effect of the addition of date palm fibers on thermal properties of plaster concrete: experimental study and modeling

Simultaneous measurement of effective thermal conductivity, thermal diffusivity, specific heat, and thermal effusivity of date palm fibers reinforced plaster concrete have been studied by CT meter device using a ring sensor technique. Samples of different weight percentages 1, 1.5, and 2% for different length fibers have been considered. It is found that the thermal conductivity, thermal diffusivity, and effusivity decrease as the fraction of fibers increases, even an increase for the specific heat is recorded. A modeling application reveals good correspondence with the experimental results. Moreover, a scanning microscopy study has shown a good interaction between the gypsum matrix and date palm fiber cells. Finally, it should be noted that date palm fibers and plaster are mutually compatible and many new building components can be made by incorporating these materials.     

Flexible graphite modified by carbon black paste for use as a thermal interface material

Polyol-ester-based carbon black pastes are used to either coat or penetrate flexible graphite, thereby increasing the thermal contact conductance of flexible graphite between copper surfaces. Paste penetration by up to an effective paste thickness (the volume of the penetrated paste divided by the geometric area of the flexible graphite) of 5 μm increases the conductance by up to 350%, 98% and 36% for thicknesses of 50, 130 and 300 μm, respectively. Paste coating up to 10 μm increases the conductance by up to 200%, 120% and 65% for thicknesses of 50, 130 and 300 μm, respectively. The paste penetration is more effective than the paste coating in enhancing the conductance, when the thickness is below 130 μm. At thickness ?130 μm, paste penetration and paste coating are similarly effective. These results stem from the relatively low interfacial thermal resistivity provided by paste penetration and the relatively high through-thickness thermal conductivity provided by paste coating. Paste penetration decreases the thermal conductivity of flexible graphite, but paste coating does not affect the conductivity. Both penetration and coating decrease the interfacial resistivity. The highest thermal contact conductance is 1.4 × 10⁵ W/m² K, as provided by paste-penetrated flexible graphite of thickness 26 μm.     

Thermal conductivity of Al2O3/SiC platelet composites

The thermal conductivity of hot-pressed Al₂O₃/SiC platelet composites is determined as a function of the platelet content, from 0 to 30 vol.% of SiC. Existing heat conduction models are employed to discuss the experimental data. Data agree with the presence of an interfacial thermal resistance at the Al₂O₃/SiC grain boundaries, which precludes the effect of percolation on the thermal conductivity for the higher percentage of SiC platelets. The observed orientation effect on the thermal conductivity due to an alignment of the platelets is also modelled using the Hasselman's approach. The thermal conductivity of the SiC platelets is calculated from the effective thermal conductivity of the composites.     

Thermal properties of carbon fibers at very high temperature

Thermal properties such as specific heat C_p, thermal diffusivity a, and thermal conductivity λ of carbon fibers are important parameters in the behaviour of the carbon/carbon composites. In this study, the specific heat and the thermal diffusivity are measured at very high temperatures (up to 2500 K). The experimental thermal conductivity estimated by the indirect relation λ = aρC_p is presented as a function of the temperature. Validations are carried out on isotropic metallic (tungsten) and ceramic (Al₂O₃) fibers. Measurements were obtained on three carbon fibers (rayon-based, PAN-based and pitch-based). Thermal conductivity results allow us to classify fibers from the most insulated to most conductive. The main result is that insulated carbon fibers have an increasing thermal conductivity when the temperature and the heat treatment temperature rise. Relationships between thermal conductivity and the structural properties (L_c and d₀₀₂) of such carbon fibers are studied. We also describe the influence of heat treatment on the thermal conductivity of carbon fibers.     

Thermal Properties of Copy Paper Sheets

The thermal conductivity and specific heat of paper are important in determining its response to heat pulses encountered in applications such as copying or digital printing. This work reports measurements of the thermal conductivity, contact resistance, and specific heat for a number of commercial copy paper sheets. The experimental setup was designed to measure transient and steady-state temperature distribution in stacks of paper sheets from which the thermal properties were determined. Steady-state measurements of the temperature difference were used to determine the thermal contact resistance and the thermal conductivity of the sheets. The specific heat was determined from the transient temperatures recorded during heat-up and cool-down periods. The thermal conductivity depends upon the sheet density, filler content, and nature of the fibers. It also showed a small increase with temperature of approximately 10^?4 W/(mK)/K. Models of thermal conductivity based on the resistance of the fibers and the fillers were developed. The thermal contact resistance increased with the surface roughness as measured by the Gurley permeability (referring to surface roughness). The specific heat of paper was dependent on its ash content.     

Thermal Diffusivity/Conductivity of Magnesium Oxide/Silicon Carbide Composites

SiC-particle-reinforced MgO composites have been fabricated by hot pressing, and the thermal diffusivities of the composites measured in the temperature range 200–1000°C using a laser flash technique. The thermal conductivity of the composites was calculated by multiplying the diffusivity with density and with heat capacity. The Eshelby inclusion model has been examined, and an equation suitable for particulate composites with porosity has been derived using the multiphase Eshelby model. The model also considers the interfacial thermal condition. Good agreement was obtained between the predictions and the experimental results of the thermal conductivity of the composites, even for various levels of porosity in the composites. Crystal defects, observed in the composites, influenced the thermal conductivity, resulting in a deviation from isothermal interfacial condition. This was reflected in the interfacial thermal parameter,β used in the modeling, and the predicted value of β was in the range of 3–10, depending on the thermal conductivity of SiC used for the calculations.     

Role of Interfacial Debonding and Matrix Cracking in the Effective Thermal Diffusivity of Alumina-Fiber-Reinforced Chemical-Vapor-Infiltrated Silicon Carbide Matrix Composites

The thermal diffusivity of a biaxial weave alumina-fiberreinforced chemical-vapor-deposited (CVD) SiC composite heated to 1500°C, which is above the manufacturing temperature, was found to exhibit an increase for heat flow parallel to the fiber plane, whereas a decrease was observed perpendicular to the fiber plane. The increase parallel to the fiber plane was thought to be due to the annealing of the fibers and matrix. The decrease perpendicular to the fiber plane was found to be the result of interfacial debonding and matrix cracking within the plane of the fibers.     

Electrical and thermal phenomena in low-density polyethylene/carbon black composites near the percolation threshold

Low-density polyethylene (LDPE)/carbon black (CB) composites were fabricated via melt-compounding technique. The percolation threshold was found to be around 20 wt % CB, and an electrical network formed by conductive CB was proven by scanning electron microscopy investigation. Dielectric responses depicted an interfacial relaxation peak at 20 wt % CB content. LDPE/CB composites showed an electric field-dependent conductivity as and a hysteresis behavior around the percolation threshold region. The CB particles with high thermal conductivity increased the heat conductance of the LDPE/CB20 up to 56%. The dynamic mechanical analysis of the LDPE/CB composites exhibited a noticeable contribution of CB throughout the composites, increasing the storage and loss modulus. The physical interactions between CB particles in the filler network enhanced the thermal degradation of the LDPE/CB25 composite for more than 76°C. The maximum breakdown strength of the LDPE/CB composites appeared with an approximately 10% improvement for LDPE/CB5 than pure LDPE. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47043.