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.     

Chemistry effects on interface microstructure and reaction in titanium-based composites

The effects of fibre and matrix chemistry on the interface microstructure and stability of Ti-based composites were investigated using Sigma SiC monofilaments as reinforcements and different titanium alloys, namely Ti-6Al-4V, Ti-1100 and Super-α₂, as matrices. Both monomatrix composites and ‘hybrid’ composites were processed by a solid-state diffusion-bonding technique, respectively incorporating either one type of matrix or different matrices into a single sample. Interface microstructures and reaction were analysed with respect to the reactivities of the interfaces and the diffusivities of the relevant elements involved. When the coated fibre was used, TiB needles were found in Ti-6Al-4V and Ti-1100 matrices but not in the Super-α₂ matrix. The reaction rate was slower in a more heavily alloyed matrix. The question of optimizing interface microstructure through matrix chemistry and hybrid composites is addressed.     

Investigation of thermal properties of Al–Si matrix reinforced fine SiCp composites

Abstract

The characterisation of thermal expansion coefficient and thermal conductivity of Al–Si matrix alloy and Al–Si alloy reinforced with fine SiC_p (5 and 20 wt-%) composites fabricated by stir casting process are investigated. The results show that with increasing temperature up to 350°C, thermal expansion of composites increases and slowly reduces when the temperature reaches to 500°C. The values of both thermal expansion and conductivity of composites are less than those for Al–Si matrix. Microstructure and particles/matrix interface properties play an important role in the thermal properties of composites. Thermal properties of composites are strongly dependent on the weight percentage of SiC_p.     

Interface stress relaxation in magnesium matrix composites studied by mechanical spectroscopy

Abstract

Mechanical spectroscopy has been used to study magnesium matrix composites reinforced either by long SiC fibres or randomly distributed Al₂O₃ (Saffil) short fibres. It is well known that, in metal matrix composites, thermal stresses can be built up at the interface due to the mismatch between the thermal expansion coefficients of matrix and reinforcements. In magnesium matrix composites, these thermal stresses are relaxed by dislocation motion in the matrix. This mechanism of thermal stress relaxation yields an extra transient component in the mechanical loss spectrum, which depends on the heating/cooling rate and disappears in isothermal condition in the behaviour of the shear modulus G with temperature has been observed during thermal cycling between 100 K and 500 K. The intensity of this phenomenondepends on the spatial distribution of the reinforcements in the matrix. In particular, composites reinforced with long fibres exhibit a more pronounced anomaly. This is interpreted by the modification of the interface strength when temperature is changed.     

Electrical conductivity and structural properties of proton electrolytes based on CsH2PO4 and silicophosphate matrices with low phosphorus content

We have demonstrated that silicophosphate matrices with various P₂O₅ contents can be used to produce medium-temperature highly conductive CsH₂PO₄-based composites. The low-temperature conductivity of all the composites studied is higher than that of the salt at low humidity by up to three and half to four orders of magnitude. The phase transition disappears with increasing additive content. The proton conductivity, thermal characteristics, and phase composition of the salt are shown to depend significantly not only on the composition of the composite but also on that of the silicophosphate matrix. The composites with Si : P = 1 : 0.14 contain disordered CsH₂PO₄ in the range x = 0.1–0.6. Increasing the percentage of the heterogeneous component leads to CsH₂PO₄ amorphization at x = 0.7 and the formation of the CsH₅(PO₄)₂ compound at x = 0.8–0.9. The thermodynamic properties and thermal stability of the composites vary in accordance with this. We have assessed the thermal stability of the electrolytes of various compositions under isothermal conditions at 200–210°C and water vapor content of ?0.6–1% in air in prolonged tests. The thermal stability of the materials is shown to depend significantly on both the percentage of the salt in the composite and the composition of the matrix. We have determined the optimal thermal and transport characteristics of the composites based on silicophosphate gel with lower phosphorus content. This opens up the possibility of using them as membranes in medium-temperature hydrogen fuel cells.     

一维高导热C/C复合材料的制备研究

以三种沥青作为基体前驱体, 实验室自制的AR中间相沥青基纤维为增强体, 通过500℃热压成型, 随后经炭化和石墨化处理制备出一维炭/炭(C/C)复合材料。研究了前驱体沥青种类和热处理温度对复合材料导热性能的影响, 并采用扫描电子显微镜和偏光显微镜对其石墨化样品的形貌和微观结构进行表征。结果表明; C/C复合材料在沿纤维轴向的室温热扩散系数和导热率均随热处理温度的升高而逐渐增大; 由AR沥青作为基体前驱体所制备的C/C复合材料具有更加明显的沿纤维轴向取向的石墨层状结构以及最好的导热性能, 其3000℃石墨化样品沿纤维轴向的室温热扩散系数和导热率分别达到594.5 mm²/s和734.4 W/(m·K)。     

Tensile properties and thermal expansion behaviors of continuous molybdenum fiber reinforced aluminum matrix composites

The tensile properties and thermal expansion behaviors of continuous molybdenum fiber reinforced aluminum matrix composites (Mo_f/Al) have been studied. The Mo_f/Al composites containing different volume percents of Mo fibers were processed by diffusion bonding. The strengths of unidirectional Mo_f/Al composites were close to the rule-of-mixtures. The strengths of 0°/90° dual-directional composites increased with fiber content, while those of 45°/135° composites remained relatively low. The coefficients of thermal expansion (CTEs) of the composites decreased as the fiber content increased, close to the values of Mo fibers. With increasing temperature, the CTEs of unidirectional composites increased, while those of dual-directional composites decreased due to large accumulated thermal stresses. The CTEs of 45°/135° composites were lower than those of 0°/90° composites because of contraction effect. At temperatures above 250 °C, the CTEs of the dual-directional composites gradually increased due to matrix yielding and interfacial decohesion.     

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.     

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.     

The spheroidization of fibrous,eutectic composites at high temperatures

An analysis is made of experimental data on the high-temperature spheroidization of Fe, Cu₂S and Cu₂O rods in the FeS-Fe, Cu-Cu₂S and Cu-Cu₂O eutectics. The work indicated that breakdown in FeS-Fe took place by diffusion in either the matrix or in the rod-matrix boundary. In contrast, spheroidization in Cu-Cu₂S probably depended on transport in either the rod or the interphase boundary, and in Cu-Cu₂O breakdown took place by diffusion in the eutectic rods only. The latter results confirm that the diffusivities of atomic species in eutectic matrices are not necessarily good guides for predicting the high-temperature stabilities of fibrous, eutectic composites.     

Investigation of thermal conductivity and dielectric properties of LDPE-matrix composites filled with hybrid filler of hollow glass microspheres and nitride particles

The hybrid filler of hollow glass microspheres (HGM) and nitride particles was filled into low-density polyethylene (LDPE) matrix via powder mixing and then hot pressing technology to obtain the composites with higher thermal conductivity as well as lower dielectric constant (D_k) and loss (D_f). The effects of surface modification of nitride particles and HGMs as well as volume ratio between them on the thermal conductivity and dielectric properties at 1 MHz of the composites were first investigated. The results indicate that the surface modification of the filler has a beneficial effect on thermal conductivity and dielectric properties of the composites due to the good interfacial adhesion between the filler and matrix. An optimal volume ratio of nitride particles to HGMs of 1:1 is determined on the basis of overall performance of the composites. The thermal conductivity as well as dielectric properties at 1 MHz and microwave frequency of the composites made from surface-modified fillers with the optimal nitride to HGM volume ratio were investigated as a function of the total volume fraction of hybrid filler. It is found that the thermal conductivity increases with filler volume fraction, and it is mainly related to the type of nitride particle other than HGM. The D_k values at 1 MHz and microwave frequency show an increasing trend with filler volume fraction and depend largely on the types of both nitride particles and HGMs. The D_f values at 1 MHz or quality factor (Q × f) at microwave frequency show an increasing or decreasing trend with filler volume fraction and also depend on the types of both nitride particle and HGM. Finally, optimal type of HGM and nitride particles as well as corresponding thermal conductivity and dielectric properties is obtained. SEM observations show that the hybrid filler particles are agglomerated around the LDPE matrix particles, and within the agglomerates the smaller-sized nitride particles in the hybrid filler can easily form thermally conductive networks to make the composites with high thermal conductivity. At the same time, the increase of the value D_k of the composites is restricted due to the presence of HGMs.     

Evaluation of microstructure and properties deterioration in short fiber reinforced thermoplastics subjected to hydrothermal aging

The problem of quantitative characterization of damage accumulation during hydrothermal aging of polymer matrix composites is addressed. Effective elastic stiffnesses and thermal diffusivities of glass fiber reinforced thermoplastic are measured at several steps of aging. Anisotropic damage accumulation is identified. It is shown that both elastic and thermal properties of the composite degenerate with the accumulation of damage. The extents of degenerations are linked to each other using the methods of micromechanics. The established cross-property connection is in a good agreement with the experimental measurements.     

Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites

Thermal conductivity of SiCp/Cu composites was usually far below the expectation, which is usually attributed to the low real thermal conductivity of matrix. In the present work, highly pure Cu matrix composites reinforced with acid washed SiC particles were prepared by the pressure infiltration method. The interfacial microstructure of SiCp/Cu composites was characterized by layered interfacial products, including un-reacted SiC particles, a Cu–Si layer, a polycrystalline C layer and Cu–Si matrix. However, no Cu₃Si was found in the present work, which is evidence for the hypothesis that the formation of Cu₃Si phase in SiC/Cu system might be related to the alloying elements in Cu matrix and residual Si in SiC particles. The thermal conductivity of SiCp/Cu composites was slightly increased with the particle size from 69.9 to 78.6 W/(m K). Due to high density defects, the real thermal conductivity of Cu matrix calculated by H–J model was only about 70 W/(m K). The significant decrease in thermal conductivity of Cu matrix is an important factor for the low thermal conductivity of SiCp/Cu composites. However, even considered the significant decrease of thermal conductivity of Cu matrix, theoretical values of SiCp/Cu composites calculated by H–J model were still higher than the experimental results. Therefore, an ideal particle was introduced in the present work to evaluate the effect of interfacial thermal resistance. The reverse-deduced effective thermal conductivities of ideal particles according to H–J model was about 80 W/(m K). Therefore, severe interfacial reaction in SiCp/Cu composites also leads to the low thermal conductivity of SiCp/Cu composites.     

废印刷电路板非金属粉负载二氧化硅杂化填料的制备及其在不饱和聚酯中的应用

为增强废印刷电路板非金属粉(WPCBP)与聚合物基体之间的界面结合作用,采用溶胶-凝胶法在WPCBP表面原位负载了一层纳米二氧化硅粒子(SiO_2),制备了一种新型的WPCBP-SiO_2杂化填料。SEM、TGA和FTIR证明SiO_2通过化学键成功负载到了杂化填料的表面。采用含双键的界面改性剂对杂化填料进行改性后,应用于不饱和聚酯树脂基体,探讨了未改性杂化填料及表面改性杂化填料对不饱和聚酯复合材料的力学性能、界面结合作用和热稳定性能的影响。结果表明,新型的杂化填料WPCBP-SiO_2能够与不饱和聚酯基体形成强的界面结合作用,显著提高不饱和聚酯复合材料的力学性能和热稳定性能,且表面改性后复合材料的各项性能得到进一步提高。     

In-plane thermal enhancement behaviors of Al matrix composites with oriented graphite flake alignment

Oriented graphite flakes (G_f)/Si/Al composites were fabricated to study their thermal enhancement behaviors. The in-plane thermal conductivity (TC) of the composites increases with the increase of G_f volume fraction. At a given volume fraction, a larger G_f size can achieve a higher in-plane TC of the composites. Microstructural characterization revealed a clean and Al₄C₃-free interface between the side surface of G_f and the Al matrix. Based on the observed microstructures, an analytical model was presented to predict the in-plane TC of the composites with oriented G_f alignment by incorporating interfacial thermal resistance within the framework of effective medium approach (EMA). Comparisons of the present model predictions with the experimental data of the as-fabricated G_f/Si/Al and previously reported G_f/Al and G_f/polymer (polyvinyl butyral, PVB) composites show good agreement. The results indicate that our model can well predict the in-plane thermal enhancement behaviors of the composites at different effective phase contrasts (i.e. the ratio between effective TC of the G_f and TC of the matrix).     

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.     

Photothermal Study of Two Different Nanofluids Containing SiO2 and TiO2 Semiconductor Nanoparticles

Thermal and optical properties of two different nanofluids containing SiO₂ and TiO₂ semiconductor nanoparticles were studied by thermal lens spectrometry (TLS) and spectrophotometry. In the case of SiO₂ nanofluids the transmission electron microscopy technique was used to obtain the SiO₂ nanoparticle sizes to investigate the size effect of these nanoparticles on the sample thermal diffusivity which is important in some medical applications such as photothermal-modulated drug delivery systems. On the other hand for the case of TiO₂ nanofluids, the photopyroelectric technique, TLS, scanning electron microscopy, and X-ray diffraction were employed to investigate the concentration effect on the thermal properties of these nanofluids. Thermal diffusivities and effusivities as functions of the TiO₂ nanoparticle concentrations were obtained. From the experimental results, an incremental increase in the thermal diffusivities and effusivities was observed when the nanoparticle concentration was increased, indicating that the nanoparticle concentration is an important factor to be considered to obtain nanofluids with more thermal efficiency which are required for some applications, such as degradation of residual water.     

Enhanced dielectric properties of low density polyethylene with bismuth sulfide used as inorganic filler

A series of Bi₂S₃/LDPE composites, with low density polyethylene (LDPE) as matrix and bismuth sulfide as filler, are fabricated by a simple process. The microstructure, dielectric properties and tensile strength of the composites have been studied. The variation of dielectric properties of the Bi₂S₃/LDPE composites with the volume fraction of Bi₂S₃, frequency and temperature is discussed. The composites have significantly high dielectric constants and good thermal stability, with a quite low percolation threshold. The addition of low content of Bi₂S₃ significantly improves the dielectric constant of polymer matrix from 3 to above 60 at 100 Hz.     

Fabrication and thermal conductivity of copper matrix composites reinforced with Mo2C or TiC coated graphite fibers

Graphite fiber reinforced Cu-based composites have good thermal conductivity, low coefficient of thermal expansion for heat sink applications. In these composites, the quality of interfacial bonding between the copper matrix and the graphite fibers has significant influence on the thermal properties of composites. In this study, two different carbide coatings (Mo₂C or TiC) were synthesized on graphite fiber to promote the interfacial bonding in composites. Fibers/Cu composites had been produced by spark plasma sintering process. The results showed that the densification, interfacial bonding and thermal conductivity of coated composites were improved distinctly compared to that of uncoated ones. The enhanced composites present 16–44% increase of thermal conductivity in X–Y plane. An original theoretical model was proposed to estimate the interface thermal resistance. The result showed that the interfacial thermal resistance was largely reduced by one order of magnitude with the introduction of carbide interlayer.     

Thermal Diffusivity and Thermal Conductivity of Dispersed Glass Sphere Composites Over a Range of Volume Fractions

Glass spheres are often used as filler materials for composites. Comparatively few articles in the literature have been devoted to the measurement or modelling of thermal properties of composites containing glass spheres, and there does not appear to be any reported data on the measurement of thermal diffusivities over a range of filler volume fractions. In this study, the thermal diffusivities of guar-gel/glass sphere composites were measured using a transient comparative method. The addition of the glass beads to the gel increased the thermal diffusivity of the composite, more than doubling the thermal diffusivity of the composite relative to the diffusivity of the gel at the maximum glass volume fraction of approximately 0.57. Thermal conductivities of the composites were derived from the thermal diffusivity measurements, measured densities and estimated specific heat capacities of the composites. Two approaches to modelling the effective thermal diffusivity were considered.