Molecular dynamics simulation of thermal conductivity of silicone rubber

Silicone rubber is widely used as a kind of thermal interface material(TIM)in electronic devices.However few studies have been carried out on the thermal conductivity mechanism of silicone rubber.This paper investigates the thermal conductivity mechanism by non-equilibrium molecular dynamics(NEMD)in three aspects:chain length,morphology,and temperature.It is found that the effect of chain length on thermal conductivity varies with morphologies.In crystalline state where the chains are aligned,the thermal conductivity increases apparently with the length of the silicone-oxygen chain,the thermal conductivity of 79 nm-long crystalline silicone rubber could reach 1.49 W/(m·K).The thermal conductivity of amorphous silicone rubber is less affected by the chain length.The temperature dependence of thermal conductivity of silicone rubbers with different morphologies is trivial.The phonon density of states(DOS)is calculated and analyzed.The results indicate that crystalline silicone rubber with aligned orientation has more low frequency phonons,longer phonon MFP,and shorter conducting path,which contribute to a larger thermal conductivity.     

The effect of particle size on the thermal conductivity of alumina nanofluids

We present new data for the thermal conductivity enhancement in seven nanofluids containing 8–282 nm diameter alumina nanoparticles in water or ethylene glycol. Our results show that the thermal conductivity enhancement in these nanofluids decreases as the particle size decreases below about 50 nm. This finding is consistent with a decrease in the thermal conductivity of alumina nanoparticles with decreasing particle size, which can be attributed to phonon scattering at the solid–liquid interface. The limiting value of the enhancement for nanofluids containing large particles is greater than that predicted by the Maxwell equation, but is predicted well by the volume fraction weighted geometric mean of the bulk thermal conductivities of the solid and liquid. This observation was used to develop a simple relationship for the thermal conductivity of alumina nanofluids in both water and ethylene glycol.     

Interface effect in the silicone rubber/mineral powder composites

Silicone rubber/mineral powder composites have been prepared by surface modification and ultrafinecrashing of mineral powder, mixing and vulcanizing with silicone rubber resin. The surface and interface energy for mineral filler and silicone rubber matrix were investigated. It was found that there is a correlation between W _a/σ_SL (interfacial adhesive work/interfacial tension) and the tensile strength of the corresponding composite, especially for unmodified ultrafine mineral filler. On the other hand, the chemical modification of the surface changes the surface group on the mineral filler and results in improvement of the interfacial interaction between silicone rubber matrix and mineral filler, consequently, altering the reinforcing effect of the mineral filler.     

Optimized AC conductivity correlated to structure,morphology and thermal properties of PVDF/PVA/Nafion composites

Polyvinylidene fluoride (PVDF) and polyvinyl alcohol (PVA) composites were prepared by controlled loading of Nafion (5 to 15 wt%) by solution casting using water and dimethylformamide (DMF) as a solvent. The surface morphology of composite analyzed by atomic force microscopy (AFM) reveals the presence of Nafion ionomers. The increase in interlayer spacing of modified PVDF/PVA polymer system as a function of Nafion was detected by X-ray diffraction (XRD). The major change in Fourier transform infrared (FTIR) spectroscopy confirms the chemical bond C=O stretching around 1,700 cm^?1 due to Nafion. Differential scanning calorimetry (DSC) demonstrates the thermal stability of polymer composites and the decrease in melting temperature (T _m). The optimized AC conductivity (σ) of the prepared composite was evaluated by using an impedance analyzer as a function of temperature (40 to 150 °C) at constant 30-MHz frequency. The highest conductivity of 1.3?×?10^?2 S m^?1 was observed at 80 °C for 10 wt% of Nafion and correlated with structure, morphology and thermal properties of modified PVDF/PVA/Nafion composites. The experimental results may be useful for sensors, fuel cells and battery application domains.     

Influence of the particle size distribution on the thermal conductivity of nanofluids

In a previous study, we have obtained an equation to predict the thermal conductivity of nanofluids containing nanoparticles with conductive interface. The model is maximal particle packing dependent. In this study, the maximal packing is obtained as a function of the particle size distribution, which is the Gamma distribution. The thermal conductivity enhancement depends on the averaged particle size. Discussion concerning the influence of the suspension pH on the particle packing is made. The proposed model is evaluated using number of sets from the published experimental data to the thermal conductivity enhancement for different nanofluids.     

Increasing the thermal conductivity of boron nitride and aluminum nitride particle epoxy-matrix composites by particle surface treatments

The thermal conductivity of boron nitride and aluminum nitride particle epoxy-matrix composites was increased by up to 97% by surface treatment of the particles prior to composite fabrication. The increase in thermal conductivity is due to decrease in the filler-matrix thermal contact resistance through the improvement of the interface between matrix and particles. Effective treatments for BN involved acetone, acids (nitric and sulfuric) and silane. The most effective treatment involved silane such that the coating resulted from the treatment amounted to 2.4% of the weight of the treated BN. The effectiveness of a treatment was higher for a larger BN volume fraction. At 57 vol.% BN, the thermal conductivity reached 10.3 W/ m·K. The treatments had little effect on the specific surface area of the BN particles. Silane treatments were also effective for AlN. At 60 vol.% AlN, the thermal conductivity reached 11.0 W/m·K.     

Influence of the electrical field applied during thermal cycling on the conductivity of LLDPE/CNT composites

The effect of the electrical field on the conductivity of linear low-density polyethylene/carbon nanotubes/ (LLDPE/CNT) composites during temperature cycling has been investigated. Under applied voltage a positive resistivity temperature coefficient was observed during heating already at low (2–3 wt%) CNT content, followed by a large negative temperature coefficient during cooling whose value depends on the applied voltage. The resistivity values after thermal cycling were markedly lower, while they slightly increased in the absence of an electrical field. The effects of thermal cycling on structural and physical properties of the composites have been evaluated by X-ray diffraction and differential scanning calorimetry.     

Flatness-dependent thermal conductivity of graphene-based composites

Since the graphene nanoplates (GNPs) are usually folded and wrinkled, we propose a factor, flatness ratio, to theoretical analyze the thermal conductivity of GNP composites. An analytical model for the thermal conductivity of GNP composites is presented, which shows an excellent agreement with the experimental data. Theoretical analysis reveals that flatness ratio acts as a dominant role in determining the influence of other factors. We further show that the two-dimensional geometry is the primary factor for GNP outperforming one-dimensional carbon nanotubes as thermal conductive filler, rather than the other factors of thickness, length and interfacial thermal resistance.     

纳米颗粒聚集形态对纳米流体导热系数的影响

纳米流体中悬浮的纳米颗粒可以增强其导热性能已经得到广泛认可,然而纳米流体颗粒增强传热的机理目前尚不清楚.研究表明,纳米颗粒的聚集是纳米流体导热系数增大的重要机制,而且纳米颗粒聚集的形态对纳米流体的导热系数有重要影响,但是目前的导热系数模型大多是建立在Maxwell有效介质理论的"静态"和"均匀分散"假设基础上.本文用平衡分子动力学模拟Cu-Ar纳米流体,采用Green-Kubo公式计算导热系数,采用Schmidt-Ott关系式计算不同聚集形态下的分形维数.对比导热系数与分形维数可以发现:在相同体积分数下,较低的分形维数会有更高的导热系数,分析了分形维数与导热系数的定量关系.此外,通过径向分布函数可以看出纳米颗粒紧密聚集与松散聚集的差异,基液分子在纳米颗粒附近的纳米薄层中处于动态平衡状态.研究结果有助于理解纳米颗粒聚集形态对导热系数的影响机理.     

Role of graphene waviness on the thermal conductivity of graphene composites

The waviness effect of graphene nanoplates (GNPs) on the thermal conductivity of GNP-based composites is investigated. Two types of wrinkled GNPs (w-GNPs) and flat GNPs (f-GNPs) are used to fabricate GNP/epoxy composites. Thermal conductivity enhancement is observed in both types of composites. However, under the same processing, f-GNPs exhibit a higher thermal conductivity enhancement than w-GNPs. We finally introduce a concept, the waviness factor, to theoretically analyze the thermal conductivity considering the waviness effect of GNPs. The theoretical predictions are found to show good agreement with experimental observations.     

二次渗滤现象对镍基导电硅橡胶屏蔽性能的影响

通过对镍粉填充RTV硅橡胶的电阻率的测量确定了镍基导电硅橡胶的渗滤值点. 在此基础上 添加不同含量的炭黑，发现了有别于单一导电填料填充两相体系的渗滤值规律，即二次渗滤 现象. 根据导电机理模型推测了二次渗滤现象产生的原因，并对实验结果进行了拟合. 同时 针对高导电硅橡胶的屏蔽性能，发现可用Schelkunoff理论较好地描述其屏蔽效果. 结合二 次渗滤现象，分析了其对屏蔽性能的影响. 关键词： 镍粉 渗滤阈值 二次渗滤现象 导电机理     

The influence of SiCp oxidized on the interface layer and thermal conductivity of SiCp/Al composites

In this work, oxidation of silicon carbide particles (SiCp) at elevated temperature and its influence on the interface layer and thermal conductivity of SiCp/ZL101 composites prepared using pressure infiltration process were investigated respectively. It is found that initial temperature for the oxidation of SiCp is about 850?°C, and that the oxidation increment of SiCp and the thickness of SiO₂ layer increase with the increase in pre-oxidation temperature and time, when the oxidized temperature exceeds 1100?°C, or the duration time exceeds 2?h at 1100?°C, a small amount of ablation will take place on the SiCp, as well as the oxidized layer has some loss. The formation of SiO₂ layer can provide certain interface reactions with interface layers (3.1–6.36?μm), and the higher the thickness of SiO₂ layer, the thicker the interface layer in SiCp/Al composites. However, the thickness of SiO₂ layer is more than 5.9?μm, which is not benefit for the formation of interface layer. With the increase in the thickness of interface layer, thermal conductivity declines, but is not linear.     

Effect of the primary particle morphology on the micromechanical properties of nanostructured alumina agglomerates

Depending on the application of nanoparticles, certain characteristics of the product quality such as size, morphology, abrasion resistance, specific surface, dispersibility and tendency to agglomeration are important. These characteristics are a function of the physicochemical properties, i.e. the micromechanical properties of the nanostructured material. The micromechanical properties of these nanostructured agglomerates such as the maximum indentation force, the plastic and elastic deformation energy and the strength give information on the product properties, e.g. the efficiency of a dispersion process of the agglomerates, and can be measured by nanoindentation. In this study a Berkovich indenter tip was used for the characterisation of model aggregates out of sol–gel produced silica and precipitated alumina agglomerates with different primary particle morphologies (dimension of 15–40 nm). In general, the effect of the primary particle morphology and the presence or absence of solid bonds can be characterised by the measurement of the micromechanical properties via nanoindentation. The micromechanical behaviour of aggregates containing solid bonds is strongly affected by the elastic–plastic deformation behaviour of the solid bonds and the breakage of solid bonds. Moreover, varying the primary particle morphology for similar particle material and approximately isotropic agglomerate behaviour the particle–particle interactions within the agglomerates can be described by the elementar breaking stress according to the formula of Rumpf.     

Influence of particle size on the electromagnetic and microwave absorption properties of FeSi/paraffin composites

The electromagnetic and microwave absorption properties of the composites employing FeSi alloy powders with different particle sizes as absorbent and paraffin as matrix were investigated. The results showed that the particle size had significant influence on the electromagnetic and microwave absorption properties of the composites in the 2-7 GHz frequency range. By decreasing the particle size of FeSi alloy powders, both the complex permittivity and permeability of the composites increased to a certain extent. In addition, the microwave absorption properties were improved, and the frequency of absorption peak shifted towards lower frequency range. In other words, the micron-grade FeSi alloy powders with smaller particle size were more suitable to be used as absorbent in measured frequency region.     

Influence of crystal-field on the thermal conductivity of PrNi5

The thermal conductivity of PrNi₅ in the temperature range 0·5 to 20 K is investigated. The analysis of crystalline electric field effect on the experimentally obtained temperature dependence of the thermal conductivity indicates that beside the main electronic component also other contributions (phonons, magnetic excitons) can take place in the heat transport in this intermetallic compound.The authors would like to thank Dr. G. Eska for lending the sample and Dr. . Jáno for his interest in the work and stimulating discussions.     

真空紫外辐照对MQ增强硅橡胶热性能及光学性能的影响

 采用空间综合辐照模拟设备研究了真空紫外辐照对MQ增强加成型硅橡胶的表面形貌、质量损失、热性能及光学性能的影响。试验结果表明：真空紫外辐照后,硅橡胶表面出现损伤裂纹,随辐照剂量的增加,裂纹的数量增多;真空紫外辐照后,硅橡胶的质量有所损失,其质损率随辐照剂量的增加而增加;真空紫外辐照后硅橡胶的耐热性随辐照剂量的增加先增加而后下降;真空紫外辐照对硅橡胶材料的体膨胀/收缩变形影响不大,但对材料的光学性能有较大影响,随着辐照剂量的增加,材料的光学透过率下降。     

LDPE phase composition in LDPE/Cu composites using thermal analysis and FTIR spectroscopy

Low-density polyethylene (LDPE) films with different copper contents were prepared from solu-tion. The TGA (thermogravimetric analysis) results show that the presence of copper particles can im-prove the thermal stability of the composite since a maximum increment of 14°C is obtained compared with the pure LDPE in this experiment. The results of DSC (differential scanning calorimetry) in stan-dard conditions show that the Cu content has little influence on the crystallinity, X _c , of LDPE. But a trace of DSC under non-standard conditions suggests that the presence of the copper microparticles has a greater effect on the network phase than on the crystalline long-range-order phase. FTIR spectroscopy was used to study the phase content of LDPE in LDPE/Cu non-oriented composite films prepared from solution with different copper contents by analysis of CH₂ rocking vibrations. A spectral simulation of transmission spectra performed using a two-phase model does not show any variation into the phase composition of the LDPE matrix for all copper contents. When a three-phase model was taken into account, the amount of the orthorhombic phase was found to be constant. However, the fraction of the amorphous and that of the network phase were found to increase and decrease respectively with increase in the copper particle load in the film.