Percolation based enhancement in effective thermal conductivity of HDPE/LBSMO composites

Thermal conductivity of composites with electrically conducting La_0·7Ba_0·15Sr_0·15MnO₃ (LBSMO) filler of nanometric grain size in HDPE matrix is investigated. Volume fraction of LBSMO fillers was varied between 0 and 0·30. SEM photographs of the composites show the presence of clusters and percolative paths, particularly for composites prepared with higher filler volume fractions. The effective thermal conductivity of the composites displays significant enhancement with increasing filler content in HDPE. A maximum enhancement of ~65% compared to that for pure HDPE has been observed for composite with 0·30 volume fraction of LBSMO filler. Most of the models those are generally used to predict the properties of two phase mixtures, has been found either to under/overestimate the measured effective thermal conductivity of the composites. We confirm that the observed rapid increase in the effective thermal conductivity of HDPE/LBSMO composite over the studied range of filler volume fraction (viz. 0?C0·30), is predicted very well, considering the effect of percolation as proposed by Zhang et al (2009).     

Effect of aspect ratio on thermal conductivity of high density polyethylene/multi-walled carbon nanotubes nanocomposites

This study aims to investigate experimentally the effects of aspect ratio (length/diameter ratio) and concentration of multiwalled carbon nanotubes (MWCNTs) on thermal properties of high density polyethylene (HDPE) based composites. The aspect ratios of two types of MWCNT fillers are in the range of 200–400 and 500–3000. Composite samples were prepared by melt mixing up to weight fraction of 19% filler content, followed by a compression molding. Measurements of density, specific heat and thermal diffusivity (by modulated photothermal radiometry, PTR) were performed and effective thermal conductivities k_e of nanocomposites were calculated using these values. The results show that the composites containing MWCNTs with higher aspect ratio have higher thermal conductivities than the ones with lower aspect ratio. In terms of conductivity enhancement k_e/k_m − 1, the results indicate that MWCNTs with higher aspect ratio provide three to fourfold larger enhancement than the ones with lower aspect ratio, at low filler concentrations.     

Thermal-Diffusivity Measurements of Conductive Composites Based on EVA Copolymer Filled With Expanded and Unexpanded Graphite

In this research, the thermal diffusivity of composites based on ethylene- vinyl acetate (EVA) copolymer filled with two kinds of reinforcement graphite materials was investigated. The reinforcement graphite fillers were untreated natural graphite (UG) and expanded graphite (EG). Composite samples up to 29.3 % graphite particle volumetric concentrations (50 % mass concentration) were prepared by the melt- mixing process in a Brabender Plasticorder. Upon mixing, the EG exfoliates in these films having nanosized thicknesses as evidenced by TEM micrographs. Thus, the thermal diffusivity and electrical conductivity of composites based on the ethylene-vinyl acetate matrix filled with nanostructuralized expanded graphite and standard, micro-sized graphite were investigated. From the experimental results it was deduced that the electrical conductivity was not only a function of filler concentration, but also strongly dependent on the graphite structure. The percolation concentration of the filler was found to be (15 to 17) vol% for micro-sized natural graphite, whereas the percolation concentration of the filler in nanocomposites filled with expanded graphite was much lower, about (5 to 6) vol%. The electrical conductivity of nanocomposites was also much higher than the electrical conductivity of composites filled with micro-sized filler at similar concentrations. Similarly, the values of the thermal diffusivity for the nanocomposites, EG-filled EVA, were significantly higher than the thermal diffusivity of the composites filled with micro-sized filler, UG-filled EVA, at similar concentrations. For 29.3 % graphite particle volumetric concentrations, the thermal diffusivity was 8.23 × 10^?7 m² · s^?1 for EG-filled EVA and 6.14 × 10^?7 m² · s^?1 for UG-filled EVA. The thermal diffusivity was measured by the flash method.     

BN/HDPE导热塑料的热导率

用粉末混合法制备了氮化硼增强高密聚乙烯塑料,研究了材料内部填料分散状态,填料含量,基体粒径和温度对热导率的影响。结果表明,材料中填料粒子围绕在聚乙烯粒子周围,形成了特殊的网状导热通路;增大填料用量和基体粒径,热导率升高;填料体积用量为30%时体系热导率达0.96 W/m.K,是基体热导率的3倍多。用Y.Agari模型分析了基体粒径对形成导热通路的影响。此外,使用氧化铝短纤维和氮化硼混杂填料能获得更高的热导率。     

Fabrication, thermal, and dielectric properties of self-passivated Al/epoxy nanocomposites

The current paper reports the effects of an epoxide-functionalized, silane surface-treated, self-passivated aluminum (Al) nanoparticles on the glass transition, morphology, thermal conductivity, dielectric properties of an epoxy composite. The surface modification of the Al nanoparticles improved the dispersion of the filler, as well as the glass transition temperature, thermal conductivity, and dielectric properties of the epoxy composites. The epoxy/Al nanocomposites showed a dielectric constant transition concentration. The dielectric constant and dissipation factor increased when the Al particle loading exceeded the critical content but gradually decreased with the frequency. The epoxy nanocomposites containing 15 % by weight Al nanoparticles have a high thermal conductivity and a high dielectric constant but a low dissipation factor. The enhancements in the thermal and dielectric properties of the epoxy nanocomposites show potential for future engineering applications.     

Ba(Zn1/3Ta2/3)O3 ceramics reinforced high density polyethylene for microwave applications

The effect of Ba(Zn_1/3Ta_2/3)O₃ (BZT) ceramic filler on the dielectric, mechanical and thermal properties of high density polyethylene (HDPE) matrix have been investigated. The dispersion of BZT particles in the matrix was varied up to 0.45 volume fraction (V_f)_. The SEM images confirmed the increase in connectivity between the filler particles with the increase in filler loading. All the composites showed excellent densification (>99 %) with relatively low moisture absorption (<0.04 wt%). The dielectric properties of the composites were investigated at 1 MHz, 5 GHz and at 10 GHz. The relative permittivity and the dielectric loss were found to increase as a function of BZT loading. Different theoretical models were used to predict the relative permittivity at 10 GHz. Effective medium theory gave the best correlation with the experimental results. An enhancement in the thermal conductivity (TC) and a reduction in the coefficient of linear thermal expansion (CTE) were achieved with filler loading. A slight decrease in the tensile strength was also observed with BZT loading. At 10 GHz, 0.45 V_f BZT reinforced HDPE showed a low relative permittivity (ε_r = 8.2) and a low dielectric loss (tanδ = 1.6 × 10^?3) with good thermal (TC = 1.4 W m^?1 K^?1, CTE = 92 ppm/°C) and mechanical (tensile strength = 18 MPa) properties.     

Enhancement of thermal and electrical conductivities of cyanoacrylate by addition of carbon based nanofillers

Nanocomposites with addition of graphite nanoparticles, multi-walled carbon nanotubes (MWCNTs), and graphene in cyanoacrylate from 0.1 to 0.5 or 0.6 vol% were fabricated. The influences of morphology towards thermal and electrical conductivities of cyanoacrylate nanocomposites were studied. Microstructure based on field emission scanning electron microscopy and transmission electron microscopy images indicated that nanofillers have unique morphologies which affect the thermal and electrical conductivities of nanocomposites. The maximum thermal conductivity values were measured at 0.3195 and 0.3500 W/mK for 0.4 vol% of MWCNTs/cyanoacrylate and 0.5 vol% of graphene/cyanoacrylate nanocomposite, respectively. These values were improved as high as 204 and 233% as compared with the thermal conductivity of neat cyanoacrylate. Nanocomposites with 0.2 vol% MWCNTs/cyanoacrylate fulfilled the requirement for ESD protection material with surface resistivity of 6.52?×?10⁶ Ω/sq and volume resistivity of 6.97?×?10⁹ Ω m. On the other hand, 0.5 vol% MWCNTs/cyanoacrylate nanocomposite can be used as electrical conductive adhesive. Compared with graphene and graphite nanofillers, MWCNTs is the best filler to be used in cyanoacrylate for improvement in thermal and electrical conductivity enhancement at low filler loading.     

填充型聚合物基复合材料的导电和导热性能

研究了高密度聚乙烯为基体、炭黑和炭纤维为填料复合体系的导电和导热性能。发现当导电填料的含量达到渗流阈值时,复合材料的电导率急剧升高;而在渗流阈值附近,其热导率未出现突变。这表明电导渗流现象不完全是由导电粒子通过物理接触生成导电链所致。其导电机制是相当数量的导电粒子相互发生隧道效应。     

无机填料对环氧模塑料导热和阻燃性能的影响

采用两种无机填料Si₃N₄和Al(OH)₃ 复合填充环氧树脂制备了环氧模塑料(EMCs), 研究了两种填料用量及单独添加和复合添加对环氧模塑料导热性能和阻燃性能的影响。研究结果表明, 单独添加Si₃N₄或Al(OH)₃对环氧模塑料导热性能和阻燃性能的影响规律基本一致, 即随着填料含量的增加, 环氧模塑料的导热性能和阻燃性能均有不同程度的提高; 复合添加Si₃N₄和Al(OH)₃对环氧模塑料的导热性能和阻燃性能均起到积极作用, 但是随着填料中Si₃N₄与Al(OH)₃体积比的变化, 材料导热性能与阻燃性能会产生交叉耦合作用。当 填料中Si₃N₄与Al(OH)₃体积比为3∶2, 总体积分数为60%时, 环氧模塑料的导热率可以达到2.15 W/(m·K), 氧指数为53.5%, 垂直燃烧达到UL-94 V-0级。      

Microstructure and thermo-mechanical properties of SiCp/Al composites prepared by pressureless infiltration

In this paper, SiCp/Al composites with high reinforcement content are fabricated by pressureless infiltration with aluminum alloy into porous SiC preforms obtained by cold press forming. Microstructures and particulate distributions are analyzed with scanning electron microscope, X-ray diffraction and energy dispersive spectrometer. The reinforcement volume fraction reaches 65 % by using bimodal particle distributions. The bending strength ranges from 320 to 342 MPa, depending on particle sizes. Due to the intrinsically low thermal conductivity of the matrix, the thermal conductivity of SiCp/Al composites are in the range of 121–143 W m^?1 K^?1.     

Thermophysical Properties of U–Mo/Al Alloy Dispersion Fuel Meats

Uranium–molybdenum alloy dispersion fuel meats are being studied for utilization as a research reactor fuel. Thermophysical properties of U–Mo/Al dispersion fuel, where U–Mo was dispersed in aluminum in research reactor fuel for the study, were determined by computing the thermal conductivity through measurements of the specific heat capacity and thermal diffusivity. Uranium molybdenum powder was first fabricated and utilized as U–Mo/Al dispersion fuel; the molybdenum-to-uranium ratios were 6, 8, and 10 mass% to produce the initial powder, which was then combined with aluminum (Al 1060). The volume fractions of U–Mo powder to aluminum were 10, 30, 40, and 50 vol.% to fabricate the dispersion fuel. The thermal diffusivity and specific heat capacity were measured by the laser-flash and differential scanning calorimetry (DSC) methods, respectively. Although the thermal diffusivity showed a decreasing trend with the U–Mo volume fraction when the dispersion quantity was insignificant, the trend reversed with a higher dispersion level. The specific heat capacity increases monotonically with temperature; its value is larger for a smaller dispersion level. Additionally, the overall thermal conductivity increases with temperature. Finally, the thermal conductivity decreases with an increase in the amount of U–Mo powder in the dispersion fuel. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Electrical and mechanical properties of expanded graphite/high density polyethylene nanocomposites

High density polyethylene (HDPE) were filled with expanded graphite particles that have different particle sizes, 5–7 μm (EG5) and 40–55 μm (EG50) in diameter. Nanocomposites were prepared by the melt-mixing technique using EG5 and EG50 at different weight ratios. Transmission Electron Microscopy (TEM) was used to observe the morphology of the nanocomposites. X-ray diffraction patterns of EG5-HDPE and EG50-HDPE nanocomposites were investigated. Tensile tests were carried out to determine tensile strength, Young’s modulus and elongation at break values. The storage modulus and loss modulus were evaluated by Dynamic Mechanical Analysis (DMA). The effect of EG5 and EG50 on electrical conductivity of HDPE was also determined. The tensile strength of HDPE increased 18.7% and 8.5% when 40 wt% EG5 and EG50 was added into HDPE, respectively. The storage modulus of EG5-HDPE and EG50-HDPE is higher compared to that of HDPE. Incorporation of EG5 and EG10 into HDPE also increased the relaxation transition peak of HDPE. The values of electrical conductivity for EG50-HDPE nanocomposites under the same filler content obtained higher in comparison with those for EG5-HDPE nanocomposites.     

Thermophysical Properties of Thermal Sprayed Coatings on Carbon Steel Substrates by Photothermal Radiometry

Laser infrared photothermal radiometry (PTR) was used to measure the thermophysical properties (thermal diffusivity and conductivity) of various thermal sprayed coatings on carbon steel. A one-dimensional photothermal model of a three-layered system in the backscattered mode was introduced and compared with experimental measurements. The uppermost layer was used to represent a roughness-equivalent layer, a second layer represented the thermal sprayed coating, and the third layer represented the substrate. The thermophysical parameters of thermal sprayed coatings examined in this work were obtained when a multiparameter-fit optimization algorithm was used with the backscattered PTR experimental results. The results also suggested a good method to determine the thickness of tungsten carbide and stainless-steel thermal spray coatings once the thermophysical properties are known. The ability of PTR to measure the thermophysical properties and the coating thickness has a strong potential as a method for in situ characterization of thermal spray coatings.     

Composition dependence of some thermo-physical properties of multi-component Se78−x Te20Sn2Bi x (0 ≤ x ≤ 6) chalcogenide glasses

New multi-component glassy materials with composition of Se_78?x Te₂₀Sn₂Bi _x (0 ≤ x ≤ 6) have been synthesized by well-known melt quenching technique. The as-prepared glasses have been characterized by applying an advanced transient plane source technique to study their thermal transport properties (effective thermal conductivity, diffusivity, specific heat per unit volume) at room temperature. Density measurements have been done to correlate the obtained results. Using the experimental data of density measurements, the basic physical parameters, such as mean atomic volume, compactness, average coordination number etc., are evaluated for the synthesized glasses and the results are discussed as a function of glass composition. We have also determined the phonon mean free path (τ) using the experimental value of effective thermal diffusivity. The composition dependence of the thermal transport properties of aforesaid glassy system has also been discussed.     

Thermal conductivity of boron nitride reinforced polyethylene composites

The thermal conductivity of boron nitride (BN) particulates reinforced high density polyethylene (HDPE) composites was investigated under a special dispersion state of BN particles in HDPE, i.e., BN particles surrounding HDPE particles. The effects of BN content, particle size of HDPE and temperature on the thermal conductivity of the composites were discussed. The results indicate that the special dispersion of BN in matrix provides the composites with high thermal conductivity; moreover, the thermal conductivity of composites is higher for the larger size HDPE than for the smaller size one. The thermal conductivity increases with increasing filler content, and significantly deviates the predictions from the theoretic models. It is found also that the combined use of BN particles and alumina short fiber obtains higher thermal conductivity of composites compared to the BN particles used alone.     

Thermal conductivity and dynamic mechanical analysis of NiZn ferrite nanoparticles filled thermoplastic natural rubber nanocomposite

The effect of NiZn ferrite nanoparticles on the thermal behaviour of thermoplastic natural rubber (TPNR) composite is investigated. Melt blending technique was employed to prepare TPNR matrix, which comprised of natural rubber (NR), liquid natural rubber (LNR) and high-density polyethylene (HDPE) in a ratio of 20:10:70. Dynamic mechanical analysis results show that the thermal stability of the nanocomposites enhanced with increasing filler loading. Moreover, thermal conductivity of the nanocomposites increased with filler content until 8 wt%, which is believed to be the optimum loading that formed the suitable percolated network for phonon conduction facilitation.     

Viscosity,density, and thermal conductivity of aluminum oxide and zinc oxide nanolubricants

This paper presents liquid kinematic viscosity, density, and thermal conductivity measurements of eleven different synthetic polyolester-based nanoparticle nanolubricants (dispersions) at atmospheric pressure over the temperature range 288 K to 318 K. Aluminum oxide (Al₂O₃) and zinc oxide (ZnO) nanoparticles with nominal diameters of 127 nm and 135 nm, respectively, were investigated. A good dispersion of the spherical and non-spherical nanoparticles in the lubricant was maintained with a surfactant. Viscosity, density, and thermal conductivity measurements were made for the neat lubricant along with eleven nanolubricants with differing nanoparticle and surfactant mass fractions. Existing models were used to predict kinematic viscosity (±20%), thermal conductivity (±1%), and specific volume (±6%) of the nanolubricant as a function of temperature, nanoparticle mass fraction, surfactant mass fraction, and nanoparticle diameter. The liquid viscosity, density and thermal conductivity were shown to increase with respect to increasing nanoparticle mass fraction.     

Thermal conductivity and mechanical properties of high density polyethylene composites filled with silicon carbide whiskers modified by cross-linked poly (vinyl alcohol)

A thin layer of poly (vinyl alcohol) (PVA) was coated on the surface of silicon carbide whiskers (SCWs) and crosslinked by glutaraldehyde, and then these modified whiskers (mSCWs) were incorporated into high density polyethylene (HDPE) to prepare HDPE/mSCW composites with a high thermal conductivity. The thermal conductivity, mechanical properties, heat resistance, thermal stability and morphology of HDPE/mSCW and HDPE/SCW composites were characterized and compared. The results reveal that the thermal conductivity of both HDPE/SCW and HDPE/mSCW composites increases with the increase of filler loading, and reaches a maximum of 1.48 and 1.69?W/(m?K) at 40?wt% filler loading, which is 251.2% and 300.75% higher than that of HDPE, respectively. Significantly, HDPE/mSCW composites have a higher thermal conductivity than their HDPE/SCW counterparts with the same filler loading. In addition, the heat resistance, Young’s modulus and yield strength of both HDPE/SCW and HDPE/mSCW composites are also improved compared with that of HDPE. mSCW can be homogenously dispersed in the HDPE matrix, which contributes to the formation of thermally conductive networks by the inter-connection of mSCWs.     

Stepwise and Pulse Transient Methods of Thermophysical Parameters Measurement

Stepwise transient and pulse transient methods are experimental techniques for measuring the thermal diffusivity and conductivity of solid materials. Theoretical models and experimental apparatus are presented, and the influence of the heat source capacity and the heat transfer coefficient is investigated using the experiment simulation. The specimens from low-density polyethylene (LDPE) and polymethylmethacrylate (PMMA) were measured by both methods. Coefficients of variation were better than 0.9 % for LDPE and 2.8 % for PMMA measurements. The time dependence of the temperature response to the input heat flux showed a small drop, which was caused by thermoelastic wave generated by thermal expansions of the heat source.     

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.