正二十四烷烃石蜡/膨胀石墨（EG）复合相变材料分子动力学模拟

相变材料(PCMs)在相变时的恒温、高能量密度等特性,经常应用于设备的热管理,但是PCMs导热系数低的缺点影响了其使用范围.本文采用分子动力学方法,模拟了在正二十四烷烃石蜡PCMs中添加不同结构(层状、交叉状)的膨胀石墨(EG)之后构成的复合PCMs的物性.文章通过径向分布函数(RDF)、声子态密度(PDOS)、比热容和导热系数这四个指标,分析了夹角为0°的层状结构,夹角为45°、90°的交叉状EG添加物对于石蜡热物性的影响. EG(0°、45°、90°)添加使得石蜡的原子分布在不同程度上变得更加均匀、紧密,使得石蜡的比热容有所增加.同时,两种类型的添加物提高了石蜡的PDOS,提高了导热系数.其中,EG(90°)添加物对于石蜡导热系数的提升最为明显,石蜡/EG(0°、45°、90°)模型中EG的含量分别为33.63 wt%、30.86 wt%和23.20 wt%,相比于的石蜡的导热系数分别提升了417.1%、345.7%和522.9%. EG的添加能够提高石蜡的导热系数,不同结构的EG对石蜡导热系数的影响有着较大的区别.     

Experimental study on enhanced heat transfer of nanocomposite phase change materials

Nanocomposite phase change materials (NCPCMs) containing different mass fractions (nanomaterial concentration) and different copper nanoparticle (CN)/multi-walled carbon nanotubes (MWCNT) mass ratios were prepared and experimentally studied. Latent heat and thermal conductivity of the NCPCMs were studied and calculated by using the T-history method. The results revealed that addition of CN or MWCNT to the phase change material (PCM) resulted in NCPCMs with enhanced thermal conductivity and charge rates, respectively. However, when both nanoparticle materials were added to the PCM simultaneously, the resulting NCPCMs improved their thermal performance below expectations as a result of agglomeration and sedimentation between the two additives. Thus, the NCPCMs containing only CN or MWCNT showed superior thermophysical properties than the pure PCM, while the NCPCM containing both CN and MWCNT did not improve the thermal characteristic of PCM significantly.     

纳米石墨烯片-正十八烷复合相变材料制备及热物性研究

本文分别制备了纳米石墨烯片质量分数为0%, 0.5%, 1%, 1.5%, 2%的纳米石墨烯片-正十八烷复合相变材料,并通过扫描电镜测试、红外光谱分析、差示扫描量热实验及导热分析等实验对其形貌结构及热物性进行表征和研究.实验表明本文制备的纳米石墨烯-正十八烷复合相变材料具有很好的相变稳定性;当纳米石墨烯片的质量分数达到2%时,复合相变材料的导热系数相对于纯十八烷高出了89.4%.     

Study on the properties of a novel shape-stable epoxy resin sealed expanded graphite/paraffin composite PCM and its application in buildings

In this study, a novel phase change material (PCM) of epoxy resin sealed expanded graphite/paraffin composite was developed as an independent attachment for building applications. A relatively high thermal energy storage density and a high thermal conductivity (2.141?W/(m·K)) were obtained in the composite PCM. The mass fraction of paraffin in the composite PCM could reach 94% without leakage of liquid paraffin when being heated at 50°C for more than 3 h, and the thermal cycle stability was good. Moreover, the thermal storage and release properties of this composite PCM with different thicknesses were studied by numerical simulation. The results showed that the thermal storage and release time are proportional to the thickness of the composite PCM, and there was almost no temperature gradient during the thermal storage and release process, which indicated that the thermal conductivity of this kind of composite PCM was high enough for building applications.     

A study on preparation and properties of carbon materials/myristic acid composite phase change thermal energy storage materials

Microscale graphite (Gr) and nanoscale multi-walled carbon nanotubes (MWCNTs) were chosen to modify the organic phase change material (PCM) of myristic acid (MA). The Gr/MA and MWCNTs/MA composite PCMs were prepared by adding the carbon materials at different mass fractions into MA. The experimental results indicated that both Gr and MWCNTs could enhance the thermal conductivity of MA. For the 3?wt% loading, the solid thermal conductivity of MA increased by 37.42% with Gr and 62.26% with MWCNTs. The FT-IR spectra showed that the reactions between carbon materials and MA were physical. The DSC results illustrated that the phase change latent heats of the composite PCMs decreased gradually with the additives increasing. Gr and MWCNTs strengthened the thermal stability of MA. The heat release rates of the composite PCMs accelerated. Three hundred thermal cycles of the chosen composite PCMs revealed that the prepared composite PCMs presented good thermal cycling stability.     

快速热响应复合材料的储/放热性能分析

以膨胀石墨为无机支撑材料、石蜡为有机相变材料,制备出高导热系数和储热密度的快速热响应复合材料,并构建传热性能测试系统对该复合材料的储/放热性能进行研究分析。研究结果表明,复合材料不仅能在熔融状态下保持形状稳定,而且显著提高了在储/放热过程的传热性能,从而有效地解决了传统相变材料高储热密度和低导热系数之间的矛盾。     

Preparation,characterization, and performance study of beeswax/expanded graphite composite as thermal storage material

In this study, beeswax as a new energy storage material and its composite with expanded graphite were prepared and characterized for their surface and thermal properties. Surface characterization showed no chemical interaction between beeswax and expanded graphite. The thermal conductivity of the composite was improved with 117% enhancement. The thermal performance of beeswax and its composite as a heat storage material was studied in a rectangular shell-and-tube thermal storage unit. The melting point of the composite remained almost same as that of beeswax; however, the melting time was reduced considerably, from 540 to 360 min with inlet water at 80°C and a 2-lpm flow rate.     

Investigation on characteristics of thermal conductivity enhancement of nanofluids

It has been shown that a nanofluid consisting of nanoparticles dispersed in base fluid has much higher effective thermal conductivity than pure fluid. In this study, four kinds of nanofluids such as multiwalled carbon nanotube (MWCNT) in water, CuO in water, SiO₂ in water, and CuO in ethylene glycol, are produced. Their thermal conductivities are measured by a transient hot-wire method. The thermal conductivity enhancement of water-based MWCNT nanofluid is increased up to 11.3% at a volume fraction of 0.01. The measured thermal conductivities of MWCNT nanofluids are higher than those calculated with Hamilton–Crosser model due to neglecting solid–liquid interaction at the interface. The results show that the thermal conductivity enhancement of nanofluids depends on the thermal conductivities of both particles and the base fluid.     

Nonmetallic crystals with high thermal conductivity

Nonmetallic crystals transport heat primarily by phonons at room temperature and below. There are only a few nonmetallic crystals which can be classed as high thermal conductivity solids, in the sense of having a thermal conductivity of > 1 W/cmK at 300K. Thermal conductivity measurements on natural and synthetic diamond, cubic BN, BP and AIN confirm that all of them are high thermal conductivity solids. Studies have been made of the effect on the thermal conductivity of nitrogen impurities in diamond, and oxygen impurities in AIN. The nitrogen impurities scatter phonons mostly from the strain field, the oxygen impurities scatter phonons mostly from the mass defects caused by aluminum vacancies. Pure A1N as well as pure SiC, BeO, BP and BeS conduct heat almost as well as does copper at room temperature, while pure natural and synthetic diamonds conduct heat five times better than copper.All of the nonmetallic solids that are known to possess high thermal conductivity have either the diamond-like, boron carbide, or graphite crystal structure. There are twelve different diamond-like crystals, a few boron carbide-type crystals, and two graphite structure crystals that have high thermal conductivity. Analyses of the rock-salt, fluorite, quartz, corundum and other structures show no candidates for this class. The four rules for finding crystals with high thermal conductivity are that the crystal should have (1) low atomic mass, (2) strong bonding, (3) simple crystal structure, and (4) low anharmonicity. The prime example of such a solid is diamond, which has the highest known thermal conductivity at 300K.     

Morphological,Thermal, and Electrical Characterization of Syndiotactic Polypropylene/Multiwalled Carbon Nanotube Composites

In this work, syndiotactic polypropylene/multiwalled carbon nanotubes (MWCNT) nanocomposites, in various concentrations, were produced using melt mixing. The influence of the addition of MWCNT on the morphology, crystalline form, and the thermal and electrical properties of the polymer matrix was studied. To that aim, scanning electron microscopy, Raman spectroscopy, X-ray diffraction, differential scanning calorimetry, and dielectric relaxation spectroscopy were employed. Significant alterations of both the crystallization behavior and the thermal properties of the matrix were found on addition of the carbon nanotubes: conversion of the disordered crystalline form I to the ordered one, increase of the crystallization temperature and the degree of crystallinity, and decrease of the glass transition temperature and the heat capacity jump. Finally, the electrical percolation threshold was found between 2.5–3.0 wt.% MWCNT. For comparison purposes, the results of the system studied here are also correlated with the findings from a previous work on the isotactic polypropylene/MWCNT system.     

Physical properties of carbon composite materials with low percolation threshold

Electrical and thermal properties of binary systems consisted of stearine and expanded graphite (EG) of different bulk densities (0.003 and 0.4 g/cm³), stearine and fine-crystalline graphite (CG) were examined. Heat capacity measurements display that phonon spectrum of graphite does not change after chemical and heat treatment in the temperature range from 300 to 700 K. It was shown that the value of samples’ percolation threshold depends on aspect ratio of using the electroconducting filler: EG as electroconducting filler is 20 times more effective than common crystal graphite.     

基于拉曼热测量技术的铜基复合物法兰GaN基晶体管的热阻分析

采用拉曼热测量技术结合有限元热仿真模型,分析比较新型铜/石墨复合物法兰封装与传统铜钼法兰封装的GaN器件的结温与热阻,发现前者的整体热阻比铜钼法兰器件的整体热阻低18.7%,器件内部各层材料的温度分布显示铜/石墨复合物法兰在器件中的热阻占比相比铜钼法兰在器件中的热阻占比低13%,这证明使用高热导率铜/石墨复合物法兰封装提高GaN器件热扩散性能的有效性.通过对两种GaN器件热阻占比的测量与分析,发现除了封装法兰以外,热阻占比最高的是GaN外延与衬底材料之间的界面热阻,降低界面热阻是进一步提高器件热性能的关键.同时,详细阐述了使用拉曼光热技术测量GaN器件结温和热阻的原理和过程,展示了拉曼光热技术作为一种GaN器件热特性表征方法的有效性.     

Study of thermal conductivity and thermal rectification in exponential mass graded lattices

Concept of exponential mass variation of oscillators along the chain length of N oscillators is proposed in the present Letter. The temperature profile and thermal conductivity of one-dimensional (1D) exponential mass graded harmonic and anharmonic lattices are studied on the basis of Fermi-Pasta-Ulam (FPU) β model. Present findings conclude that the exponential mass graded chain provide higher conductivity than that of linear mass graded chain. The exponential mass graded anharmonic chain generates the thermal rectification of 70-75% which is better than linear mass graded materials, so far. Thus instead of using linear mass graded material, the use of exponential mass graded material will be a better and genuine choice for controlling the heat flow at nano-scale.     

Experimental Investigations on Latent Heat Storage Unit using Paraffin Wax as Phase Change Material

Thermal performance of a latent heat storage unit is evaluated experimentally. The latent heat thermal energy storage system analyzed in this work is a shell-and-tube type of heat exchanger using paraffin wax (melting point between 58°C and 60°C) as the phase change material. The temperature distribution in the phase change material is measured with time. The influence of mass flow rate and inlet temperature of the heat transfer fluid on heat fraction is examined for both the melting and solidification processes. The mass flow rate of heat transfer fluid (water) is varied in the range of 0.0167 kg/s to 0.0833 kg/s (1 kg/min to 5 kg/min), and the fluid inlet temperature is varied between 75°C and 85°C. The experimental results indicate that the total melting time of the phase change material increases as the mass flow rate and inlet temperature of heat transfer fluid decrease. The fluid inlet temperature influences the heat fraction considerably as compared to the mass flow rate of heat transfer fluid during the melting process of the phase change material.     

Experimental study on the heat transfer characteristics of a new type flat micro heat pipe heat exchanger with latent heat thermal energy storage

An experimental energy storage system has been designed using a new type flat micro heat pipe heat exchanger that incorporates a moderate-temperature phase change material paraffin with a melting point of 58°C. The basic structure, working principles, and design concept are discussed. The heat transfer process during the charging and discharging of the heat exchanger under various operating conditions has been experimentally investigated. Results show that the performance of the new type flat micro heat pipe was steady and efficient during charging and discharging. The average thermal storage power and absorption efficiency have been determined to be approximately 537 W and 92.5%, respectively.     

Experimental thermal performance study of natural graphite as sensible heat storage material using different heat transfer fluids

In this paper thermal performance of graphite-based sensible heat storage system with embedded helical coil in rectangular shell was studied. Plain water at four flow rates (0.25 LPM–1.0 LPM) and four inlet temperatures (60°C–90°C) was passed through the graphite bed and charging time was measured. Expanded graphite/water suspension and Al₂O₃/water nanofluid were also used to study charging behavior of graphite. Results showed that charging time of packed bed was reduced with increase in flow rate and inlet temperature of heat transfer fluid. Charging time using expanded graphite/water solution and nanofluid was 14.2% and 21.2% lesser than water.

Abbreviations: h_i: internal heat transfer coefficient (W m^?2 K^?1); HTF: Heat transfer fluid; h_o: External heat transfer coefficient (W m^?2 K^?1); LPM: liter per minute; k: thermal conductivity (W m^?1 K^?1); TSU: Thermal storage unit     

Effect of dispersion state of carbon nanotube on the thermal conductivity of poly(dimethyl siloxane) composites

Herein, the effect of dispersion uniformity of multi-walled carbon nanotube (MWCNT) on the thermal conductivity of poly(dimethyl siloxane) (PDMS) composites was investigated by comparing experimentally obtained and calculated results based on simple models. Two different MWCNTs, i.e., raw and oxidized/masterbatched MWCNTs, were used and compared. For raw MWCNT, the dispersion in PDMS was poor, resulting in the significant reduction in the aspect ratio of MWCNT. However, for composites using masterbatched MWCNT, the thermal conductivity was always about 10% greater than those prepared with raw MWCNT and the aspect ratio calculated by the model equation was also 1.7 times greater. Above 1.5 phr masterbatched MWCNT concentrations, the aspect ratio of 430 was maintained. Finally, the results suggest that the thermal conductivity can be correlated with the degree of dispersion and aspect ratio obtained from the model equation used.     

Thermal energy storage behaviour of nanoparticle enhanced PCM during freezing and melting

The present research work aimed to investigate the melting and solidification characteristics of NPCM. The NPCM was prepared using paraffin as the PCM and high conductive MWCNT as the nanomaterial without using any dispersant. The NPCM was prepared by dispersing MWCNTs with volume fractions of 0.3%, 0.6% and 0.9% in PCM as the base PCM. SEM morphology showed the uniform dispersion of MWCNTs in the paraffin wax. The MWCNT nano-additives PCMs showed two peaks in the heating curve by DSC measurement. Lessening in melting and solidification time of 30% and 43% was attained in the case of NPCM with 0.3% and 0.9%, respectively. It is observed from the DSC analysis that the latent heat of pure paraffin during freezing and melting cycle was 139.2 J/g (at 56.61 °C) and 131.8 J/g (at 57.55 °C), respectively. Whereas, the latent heat of NPCM with 0.9% of nanofluid was 150.7 J/g (at 56.36 °C) and 148.3 J/g (58.35 °C). It is construed that a maximum change in latent heat of 7.6% and 11% was observed between pure PCM and NPCM during freezing and melting cycle. For the lesser nanoparticle concentration (0.3% and 0.6%), the percentage change in latent heat was lesser than 0.9%.     

Mechanical and Flame Retardant Properties and Microstructure of Expandable Graphite/Silicone Rubber Composites

Flame-retardant expandable graphite (EG)/silicone rubber (SR) composites were prepared using nano-CaCO₃ particles as reinforcement filler. In addition to mechanical measurements, limited oxygen index (LOI), UL-94 and cone calorimeter tests (CCT), the thermal properties were tested by thermogravimetric analysis (TGA). The results showed that the content and particle size of the EG both had large effects on the flammability and mechanical properties of the EG/SR blends. The composites that contained 25 phr EG (50–80 mu) had excellent LOI values, 47–48, and achieved the UL-94 V-0 level while the pure SR sample had the LOI value of 25 and achieved the UL-94 V-2 level. The data obtained from the CCT indicated that the addition of EG decreased remarkably the heat release rate, smoke emission, and mass loss rate of the composites. SEM microphotographs of the EG/SR composites before and after combustion demonstrated that EG underwent a large volume expansion, and the multiporous char structure blocked heat transfer and protected the substrate from fire.     

Grain-size-dependent thermal conductivity of nanocrystalline materials

In order to study the influence of grain size and lattice strain on the thermal conductivity of nanocrystalline (NC) materials, both experimental and theoretical studies were carried out on NC copper. The NC copper samples were prepared by hot isostatic pressing of nano-sized powder particles with mean grain size of 30 nm. The thermal behaviors of the samples were measured to be 175.63–233.37 W (m K)^?1 by using a laser method at 300 K, which is 45.6 and 60.6 % of the coarse-grained copper, respectively. The average grain size lies in the range of 56–187 nm, and the lattice strain is in the range of ?0.21 to ?0.45 % (in the direction of 111) and ?0.09 to 0.92 % (in the direction of 200). In addition, a modified Kapitza resistance model was developed to study the thermal transport in NC copper. The theoretical calculations based on the presented theoretical model were in good agreement with our experimental results, and it demonstrated that the thermal conductivity of NC materials show obvious size effect. It is also evident that the decrease in the thermal conductivity of NC material can be mainly attributed to the nano-size effect rather than the lattice strain effect.