Effect of composition and processing parameters on the characteristics of tannin-based rigid foams. Part II: Physical properties

The present work is the continuation of the previous one published in the same issue of this journal, but now focuses on some selected physical properties of tannin-based rigid foams and derived glasslike carbon foams. Such materials are new, lightweight, cellular solids, prepared from 95% natural precursors: bark extracts and furfuryl alcohol, as detailed in the companion paper. After a few structural characteristics are briefly recalled, physical properties like compressive strength, permeability to fluids, solvent absorption, and electrical conductivity are measured, discussed and modelled. The effects of changing a few experimental parameters that have been varied in the synthesis of the foam: amounts of blowing agent, strengthener and nanofillers, shape of the moulds and restricted foaming are discussed in relation with the pore structure observed in the companion paper. Slightly anisotropic properties are evidenced, in agreement with the orientation of the cells, as expected for foams grown vertically in cylindrical moulds.     

Si元素对碳纳米管增强铝基复合泡沫组织与性能的影响

针对金属基复合材料,添加合金元素是提升其综合性能的有效途径.本文通过高能球磨和填加造孔剂法,制备了添加Si元素的碳纳米管(CNTs)增强铝基(CNTs/Al-Si)复合泡沫,通过准静态压缩实验测试其压缩性能和吸能性能,进一步研究烧结温度和不同Si元素含量对CNTs/Al-Si复合泡沫微观组织、压缩性能和吸能性能的影响,...     

Experiments and modeling for thermal conductivity of graphite nanoplatelets/carbon composites

In this work, the graphite nanoplatelets/carbon composites were fabricated from graphite nanoplatelets and pitch powders by a hot-pressing technology followed by carbonization and graphitization. The XRD and pole figure results show that the incorporation of pitch induces the decrease of size (L_a) and orientation degree of graphitic crystallites, while the in-plane thermal conductivity of graphitized sample is increased with the increasing pitch content up to 6 wt.%, achieving a maximum value of 405 W/m K. The pitch binders are filled into the voids to bridge two or more graphite nanoplatelets particles together to form extra thermal paths, which makes a great contribution to the enhancement of thermal conductivity. A thermal conductivity model for the graphitized composites is constructed based on a bridging mechanism, and the predicted results fit well with the experimental results.     

Effect of processing technique on the transport and mechanical properties of graphite nanoplatelet/rubbery epoxy composites for thermal interface applications

Graphite nanoplatelet (GNP)/rubbery epoxy composites were fabricated by mechanical mixer (MM) and dual asymmetric centrifuge speed mixer (SM). The properties of the GNP/rubbery epoxy were compared with GNP/glassy epoxy composites. The thermal conductivity of GNP/rubbery epoxy composite (25 wt.% GNP, particle size 15 μm) reached 2.35 W m⁻¹ K⁻¹ compared to 0.1795 W m⁻¹ K⁻¹ for rubbery epoxy. Compared with GNP/rubbery epoxy composite, at 20 wt.%, GNP/glassy epoxy composite has a slightly lower thermal conductivity but an electrical conductivity that is 3 orders of magnitude higher. The viscosity of rubbery epoxy is 4 times lower than that of glassy epoxy and thus allows higher loading. The thermal and electrical conductivities of composites produced by MM are slightly higher than those produced by SM due to greater shearing of GNPs in MM, which results in better dispersed GNPs. Compression and hardness testing showed that GNPs increase the compressive strength of rubbery epoxy ∼2 times without significantly affecting the compressive strain and hardness. The GNP/glassy epoxy composites are 40 times stiffer than the GNP/rubbery epoxy composites. GNP/rubbery epoxy composites with their high thermal conductivity, low electrical conductivity, low viscosity before curing and high conformability are promising thermal interface materials.     

A comparison of composite metal foam's properties and other comparable metal foams

New closed cell composite metal foams are processed using casting and powder metallurgy (PM) techniques. The foam is comprised of steel hollow spheres packed into a random loose arrangement, with the interstitial spaces between spheres occupied with a solid metallic matrix. The characterization of composite metal foams was carried out using monotonic compression, compression-compression fatigue, loading-unloading compression, micro-hardness and nano-hardness testing. The microstructure of the composite metal foams was studied using optical, scanning electron microscopy imaging and electron dispersive spectroscopy. The composite metal foams displayed superior (5-20 times higher) compressive strengths, reported as 105 MPa for cast foams and 127 MPa for PM foams, and much higher energy absorbing capability as compared to other metal foams being produced with similar materials through other technologies.     

Cellulose nanofiber/nanodiamond composite films: Thermal conductivity enhancement achieved by a tuned nanostructure

Thermally conductive and electrically insulating composite materials are required for thermal management in advanced electronic industry. The present work aimed at creating a composite film of cellulose nanofiber (CNF) and nanodiamond (ND) with superior thermal conductivity. The thermal conductivity of the prepared nanocomposite film was ～2.7?Wm^?1?K^?1, which corresponds to triple of usual CNF/ND composites with similar composition. The distinct thermal conductivity is attributed to a unique nanostructure we made out in the nanocomposite film. The nanostructure can be characterized by CNF fibrils which are densely covered with ND particles.     

Thermal conductivity measurement of the epoxies and composite material for low temperature superconducting magnet design

The thermal conductivities of solid materials were measured by a G-M cryo-cooler based apparatus in the temperature range of 2.6–21.0 K. The performance of this apparatus was verified by measuring the thermal conductivity of 304-stainless steel, and good reproducibility as well as accuracy was shown when compared with the certified values. The thermal conductivities of EC1017 and Stycast2850FT and a composite material were measured. Similar behavior to amorphous materials was shown for EC1017 and Stycast2850FT in that there was an apparent plateau, which could be ascribed to a very lower crystallinity of epoxy. An equivalent model was proposed to predict the thermal conductivity of the composite material.     

Enhanced conductivity behavior of polydimethylsiloxane (PDMS) hybrid composites containing exfoliated graphite nanoplatelets and carbon nanotubes

Polydimethylsiloxane (PDMS) hybrid composites consisting of exfoliated graphite nanoplatelets (xGnPs) and multiwalled carbon nanotubes functionalized with hydroxyl groups (MWCNTs-OH) were fabricated, and the effects of the xGnP/MWCNT-OH ratio on the thermal, electrical, and mechanical properties of polydimethylsiloxane (PDMS) hybrid composites were investigated. With the total filler content fixed at 4 wt%, a hybrid composite consisting of 75% × GnP/25% MWCNT-OH showed the highest thermal conductivity (0.392 W/m K) and electrical conductivity (1.24 × 10⁻³ S/m), which significantly exceeded the values shown by either of the respective single filler composites. The increased thermal and electrical conductivity found when both fillers are used in combination is attributed to the synergistic effect between the fillers that forms an interconnected hybrid network. In contrast, the various different combinations of the fillers only showed a modest effect on the mechanical behavior, thermal stability, and thermal expansion of the PDMS composite.     

Manufacturing of Al/SiCp composite foams using calcium carbonate as foaming agent

Abstract

The aluminium composite foams reinforced by different volume fractions of SiC particles were manufactured with the direct foaming route of melt using different contents of CaCO₃ foaming agent. The density of produced foams changed from 0·43 to 0·76 g cm^?3. The microstructural features and compressive properties of the Al/SiC_p composite foams were investigated. Compressive stress–strain curve of Al/SiC_p composite foams is not smooth and exhibits some serrations. At the same relative density of composite foams, the plateau stress of the composite foams increases with increasing volume fraction of SiCp and decreasing weight percentage of CaCO₃. The relation between plateau stress, relative density, weight percentage of CaCO₃ and SiCp volume fraction of Al/SiCp composite foams with a given particle size was investigated.     

基于插层法协同提升碳纤维树脂基复合材料的导电性能与层间韧性

碳纤维增强树脂基复合材料(CFRP)因其低密度、高比强度等特点,在航空航天领域得到了广泛的应用,但其导电性和层间韧性的不足降低了CFRP作为飞机结构件的使用安全性.为了改善CFRP弱的导电性和层间断裂韧性,本文采用溶液浇铸法制备了多壁碳纳米管(MWCNTs)和石墨烯纳米片(GNPs)掺杂聚醚砜(pES)的导电热塑性薄膜...     

Thermal conductivity of polymer composites based on the length of multi-walled carbon nanotubes

The anisotropic development of thermal conductivity in polymer composites was evaluated by measuring the isotropic, in-plane and through-plane thermal conductivities of composites containing length-adjusted short and long multi-walled CNTs (MWCNTs). The thermal conductivities of the composites were relatively low irrespective of the MWCNT length due to their high contact resistance and high interfacial resistance to polymer resins, considering the high thermal conductivity of MWCNTs. The isotropic and in-plane thermal conductivities of long-MWCNT-based composites were higher than those of short-MWCNT-based ones and the trend can accurately be calculated using the modified Mori-Tanaka theory. The in-plane thermal conductivity of composites with 2 wt% long MWCNTs was increased to 1.27 W/m·K. The length of MWCNTs in polymer composites is an important physical factor in determining the anisotropic thermal conductivity and must be considered for theoretical simulations. The thermal conductivity of MWCNT polymer composites can be effectively controlled in the processing direction by adjusting the length of the MWCNT filler.     

Influence of cell aspect ratio on architecture and compressive strength of titanium foams

Titanium foams have been of interest in dental and orthopedic implants over the past few decades on account of their excellent mechanical properties, chemical stability, and biocompatibility. A powerful tool, X-ray computed microtomography was used to measure quantitatively the effect of pore morphology on foam architecture. Mechanical properties of titanium foams with varying pore structure were investigated. Aspect ratio of the pores was quantitatively demonstrated to affect strength, degree of anisotropy and strain-rate sensitivity of the produced titanium foams. Needle-like pored foams showed 30-55% lower strength when compared to the foams having lower aspect ratio pores. Lower aspect ratio pored foams were 3-11%, higher aspect ratio pored foams were 17-34% weaker in the direction parallel to the compaction direction when compared to the perpendicular one. High aspect ratio pores also resulted in more pronounced strain-rate sensitivity.     

Thermal conductivity and thermal contact conductance studies on Al2O3/Al–AlN metal matrix composite

Al₂O₃/Al–AlN is a metal matrix composite (MMC) used for making heat sink of electronic devices. This paper presents the detailed investigations carried out on thermal contact resistance across this MMC contact in vacuum at different contact pressures. The experimental results are compared with the theoretical models available in the literature for metallic contacts and they are found to be in good agreement with each other.     

Fabrication of a polymer composite with high thermal conductivity based on sintered silicon nitride foam

A novel route was developed to fabricate Si₃N₄/epoxy composite. In this route, the Si₃N₄ particles were constructed into the foamed shape by using protein foaming method, firstly. Then the Si₃N₄ foams were sintered to bond these Si₃N₄ particles together. Finally, the Si₃N₄/epoxy composite was fabricated by infiltrating the epoxy resin solution into the sintered Si₃N₄ foams. This route was proved to be an efficient way in enhancing the thermal conductivity of epoxy matrix at a low loading fraction. For example, the thermal conductivity of the as-prepared Si₃N₄/epoxy composite with a loading fraction of 22.2 vol% was up to 3.89 W m⁻¹ K⁻¹, which was about 17 times higher than that of neat epoxy.     

Thermal conductivity of AZ91 magnesium integral foams measured by the Transient Plane Source method

The thermal conductivity of a collection of magnesium integral foams has been measured by using the Transient Plane Source (TPS) method. The results have shown a power-trend dependency with bulk density as the existing models predict. Additionally, micro-computed tomography (μCT) studies have been carried out on selected samples whose thermal conductivity values slightly deviate from the fitted curve to inspect the density distribution. Differences have been explained in terms of the local average density obtained by μCT in the volume covered by the heat flux. These results have revealed the high accuracy of the TPS method when it is combined with micro-tomographic techniques.     

石墨表面化学镀Cu对天然鳞片石墨/Al复合材料热物理性能的影响

对天然鳞片石墨（GF）进行化学镀Cu的表面处理,对化学镀Cu石墨（Cu-GF）和Al粉采用真空热压的工艺制备出镀Cu石墨/Al（Cu-GF/Al）复合材料。研究了Cu-GF/Al复合材料的微观结构和微观界面,同时也研究了Cu-GF对Cu-GF/Al复合材料热导率和抗弯性能的影响。结果表明,GF上的Cu层能抑制界面脆弱相Al₄C₃的产生,使Cu-GF/Al复合材料的抗弯性能有了显著提升。当Cu-GF体积分数从50%增加到70%时,Cu-GF/Al复合材料的抗弯强度也从104 MPa降低到74 MPa。当GF体积分数为70%时,Cu-GF/Al复合材料的热导率达到最高值为522 W/（m·K）。     

碳纳米管增强铝基复合泡沫的阻尼性能

通过填加造孔剂方法制备了碳纳米管（CNTs）增强铝基复合泡沫,采用热机械分析仪研究了测试温度、频率、外加振幅、泡沫的孔隙率和CNTs含量对其阻尼性能的影响,并分析了相关阻尼机制。结果表明:复合泡沫铝的阻尼性能随孔隙率和振幅的增大而提高,随着频率的增加而下降。在环境测试温度25~200℃范围内,复合泡沫的损耗因子变化较小;当温度高于200℃后,损耗因子随温度升高有明显的提高。CNTs的加入可以显著提高泡沫铝的阻尼性能,常温下3.0% CNTs增强的铝基复合泡沫的损耗因子达0.27,为泡沫铝的3.71倍。复合泡沫的阻尼机制主要为位错阻尼、晶界阻尼、孔隙阻尼、CNTs的本征阻尼和CNTs-Al间界面阻尼,其中本征和界面阻尼发挥了重要的增强作用。      

Rheology and thermal conductivity of calcium grease containing multi-walled carbon nanotube

Recently, nanofluids attract considerable interest for enhanced rheological behavior and thermal performance. The aim of this research is to study the influence of additives Multi-Walled Carbon Nanotubes (MWCNTs) on the rheological behavior and its structure, thermal conductivity, and the influence of shear thinning rate on oil separation at different temperatures for calcium grease. Various concentrations of MWCNTs (0.5, 1, 2, 3, and 4%) have been added to the grease to obtain the best percentages that improve the properties of nanofluid. The microstructure of MWCNTs and nanofluid were examined by X-ray diffraction (XRD), Transmission Electron Microscope (TEM), and Scanning Electron Microscope (SEM). These experimental investigations were evaluated with a Brookfield programmable Rheometer DV-III ULTRA. The results indicated that the optimum concentration of MWCNTs was 3%, and the dropping point increasing about 11%. The rheological behaviors of the nanofluids show that the grease with various concentrations of MWCNTs demonstrates non Newtonian behaviors and the results indicated that the shear stress, apparent viscosity and thermal conductivity increase with the increase of volume concentration of MWCNTs to 65%, 52%, and% 56, respectively.     

Mechanical Behavior and Failure Mechanisms of Carbon Fiber Composite Pyramidal Core Sandwich Panel after Thermal Exposure

An attempt has been made here to evaluate the effect of thermal exposure on the mechanical behavior and failure mechanisms of carbon fiber composite sandwich panel with pyramidal truss core under axial compression.Analytical formulae for the collapse strength of composite sandwich panel after thermal exposure were derived.Axial compression tests of composite laminates and sandwich panels after thermal exposure were conducted at room temperature to assess the degradation caused by the thermal exposure.Experimental results showed that the failure of sandwich panel are not only temperature dependent,but are time dependent as well.The decrease in residual compressive strength is mainly attributed to the degradation of the matrix and the degradation of fibere-matrix interface,as well as the formation of cracks and pores when specimens are exposed to high temperature.The measured failure loads obtained in the experiments showed reasonable agreement with the analytical predictions.     

碳纳米管的表面修饰及其对碳纳米管/氟橡胶复合材料导电性能的影响

分别采用混酸和四氟化碳(CF4 ) 等离子体处理技术对碳纳米管(MWCNTs) 进行了表面修饰, 将处理前后的碳纳米管进行了XPS 和SEM 测试, 获得了处理后前的表面形貌和结构, 并采用溶液浇注的方式制备了MWCNTs/氟橡胶(FE) 复合材料, 探讨了不同碳纳米管状态(未处理、混酸处理、CF4等离子体处理) 的导电性能, 结果表明两种表面处理方式可以使MWCNTs 表面接上极性官能团。而且在相同的碳纳米管添加量下(质量分数分别为0. 1 %、0. 5 %、1. 0 %、2. 0 %) , 酸处理MWCNTs/ FE 的渗流阈值最小, 达0. 5 %。