Carbon-Nanofiber-Reinforced Syntactic Foams: Compressive Properties and Strain Rate Sensitivity

The current study is focused on exploring the possibility of reinforcing syntactic foams with carbon nanofibers (CNFs). Syntactic foams are hollow, particle-filled composites that are widely used in marine structures and are now finding applications in other modes of transportation due to their high stiffness-to-weight ratio. The compressive properties of syntactic foams reinforced with CNFs are characterized over the strain rate range of 10^?4 to 3,000 s^?1, which covers seven orders of magnitude. The results show that despite lower density with respect to neat epoxy, CNF/syntactic foams can have up to 7.3% and 15.5% higher quasi-static compressive strength and modulus, respectively, for the compositions that were characterized in the current study. In addition, these properties can be tailored over a wide range by means of hollow particle wall thickness and volume fraction, and CNF volume fraction. The compressive strength of CNF/syntactic foams is also shown to generally increase by up to a factor of 3.41 with increasing strain rate when quasi-static and high-strain-rate testing data are compared. Extensive microscopy of the CNF/syntactic foams is conducted to understand the failure and energy absorption mechanisms. Crack bridging by CNFs is observed in the specimens, which can delay final failure and increase the energy absorption capacity of the specimens. Deformation of CNFs is also noticed in the material microstructure. The deformation and failure mechanisms of nanofibers are related to the test strain rate and the structure of CNFs.     

Applications of Polymer Matrix Syntactic Foams

A collection of applications of polymer matrix syntactic foams is presented in this article. Syntactic foams are lightweight porous composites that found their early applications in marine structures due to their naturally buoyant behavior and low moisture absorption. Their light weight has been beneficial in weight sensitive aerospace structures. Syntactic foams have pushed the performance boundaries for composites and have enabled the development of vehicles for traveling to the deepest parts of the ocean and to other planets. The high volume fraction of porosity in syntactic foams also enabled their applications in thermal insulation of pipelines in oil and gas industry. The possibility of tailoring the mechanical and thermal properties of syntactic foams through a combination of material selection, hollow particle volume fraction, and hollow particle wall thickness has helped in rapidly growing these applications. The low coefficient of thermal expansion and dimensional stability at high temperatures are now leading their use in electronic packaging, composite tooling, and thermoforming plug assists. Methods have been developed to tailor the mechanical and thermal properties of syntactic foams independent of each other over a wide range, which is a significant advantage over other traditional particulate and fibrous composites.     

Effect of Dispersed TiC Content on the Microstructure and Thermal Expansion Behavior of Shrouded-Plasma-Sprayed FeAl/TiC Composite Coatings

FeAl intermetallic matrix composites reinforced by ceramic particles such as titanium carbide have attracted much attention in recent years. In this study, shrouded plasma spraying with nitrogen as a protective gas was employed to deposit FeAl/TiC composite coatings. Fe-35Al powder and Fe-35Al/TiC composite powders containing 35 and 45?vol.% TiC prepared by mechanical alloying were used as feedstock powders. The microstructures of the ball-milled powders and the as-sprayed coatings were characterized by scanning electron microscopy and x-ray diffraction. The mean coefficients of thermal expansion (CTEs) of FeAl and FeAl/TiC were measured. The results showed that dense FeAl and FeAl/TiC coatings with low oxide inclusions were deposited by shrouded plasma spraying. The mean CTEs measured in the present study were reasonably consistent with those calculated based on the formula. As a result, the mean CTE of FeAl-based composite coating can be properly controlled by adjusting TiC content in the composite coating to match with those of different substrate materials.     

Real-time monitoring of transverse thermal strain of carbon fiber reinforced composites under long-term space environment using fiber optic sensors

Composites are promising alternatives for space structures because of their versatile characteristics such as high specific stiffness and strength. When composite structures are exposed to the space environments (low Earth orbit, LEO), however, they are known to undergo considerable temperature change induced by the direct sunlight and the Earth's shadow in addition to ultraviolet, high vacuum, atomic oxygen and so forth. Therefore, for the successful completion of their missions, it is important for the structures to maintain the consistent dimensional stability in such a thermal cycling condition. The coefficient of thermal expansion (CTE) of the structures is suitable to express the dimensional stability, and it is needed to be monitored throughout the mission. For this purpose, fiber optic sensors, which have many advantages, were investigated to check their suitability in this paper.Two fiber Bragg grating (FBG) sensors have been adopted for the simultaneous measurement of thermal strain and temperature to get the CTE change of a carbon/epoxy composite laminate. LEO conditions with high vacuum, ultraviolet and thermal cycling environments were simulated in a thermal vacuum chamber. As a pre-test, a FBG temperature sensor was calibrated and a FBG strain sensor was evaluated through the comparison with the electric strain gauge (ESG) attached on an aluminum specimen in the same temperature range as the thermal cycling. The change of the transverse CTE in a composite laminate exposed to the space environment was measured for intervals of aging cycles in real time. As a whole, there was no abrupt change of the CTE after 1000 aging cycles. After aging, however, the CTE decreased a little all over the test temperature range. These changes are thought to have been caused by outgassing, moisture desorption, matrix cracking, etc. In this paper, embedding application of FBG sensors to composites operated under space environment and their successful real-time monitoring of thermal deformations over a long time was shown.     

热暴露对SiCf/Ti-6Al-4V复合材料热膨胀行为的影响

研究了热暴露对SiCf/Ti-6Al-4V复合材料热膨胀行为的影响。对基体合金、制备态复合材料、经900 ℃热暴露10, 25及75 h的复合材料的热膨胀行为进行了实验测量。结果表明,无论纵向和横向,热暴露的复合材料其热膨胀系数均高于制备态复合材料。且无论纵向和横向,经900 ℃热暴露10和75 h的复合材料热膨胀系数均高于经900 ℃热暴露25 h的复合材料。这是因为经900 ℃热暴露25 h的复合材料拥有较为合适的界面反应。另外,在对复合材料纵向热膨胀系数进行预测时,因为Schapery模型在整个加热过程中都考虑了热拉伸残余应力对复合材料热膨胀系数的影响,因此在CTE曲线的高温阶段,预测值低于实验值     

Thermal expansion and dimensional stability of unidirectional and orthogonal fabric M40/AZ91D composites

Unidirectional (60%, volume fraction) and orthogonal (50%, volume fraction) M40 graphite fibre reinforced AZ91D magnesium alloy matrix composites were fabricated by pressure infiltration method. The coefficients of thermal expansion (in the temperature range of 20-350 ℃) and dimensional stability (in the temperature range of 20-150 ℃) of the composites and the corresponding AZ91D magnesium alloy matrix were measured. The results show that coefficients of thermal expansion of the composites in longitudinal direction decrease with elevating temperature. The coefficients of thermal expansion (CTE) for unidirectional M40/AZ91D composites and orthogonal M40/AZ91D composites are 1.24×10-6 ℃-1 and 5.71×10-6 ℃-1 at 20 ℃, and 0.85×10-6 ℃-1 and 2.75×10-6 ℃-1 at 350 ℃, respectively, much lower than those of the AZ91D alloy matrix. Thermal cycling testing demonstrates that the thermal stress plays an important role on residual deformation. Thus, a better dimensional stability is obtained for the AZ91D magnesium alloy matrix composites. More extreme strain hysteresis and residual plastic deformation are observed in orthogonally fabric M40 reinforced AZ91D composite, but its net residual strain after each cycle is similar to that of the unidirectional M40/AZ91D composite.     

A novel approach to composite preparation by direct synthesis of carbon nanomaterial on matrix or filler particles

Carbon nanotubes (CNTs), nanofibers (CNFs) and graphene are promising components for next-generation high-performance structural and multifunctional composite materials. One of the largest obstacles to creating strong, electrically or thermally conductive CNT/CNF or graphene composites is the difficulty of achieving a good dispersion of the carbon nanomaterials in a matrix. Typically, time-consuming steps of carbon nanomaterial purification, ultrasound treatment and functionalization are required. We utilized a novel approach to fabricate composite materials by growing CNTs/CNFs directly on the surface of matrix, matrix precursor or filler particles. As the precursor matrix and fillers we utilized cement (clinker), copper powder, fly ash particles, calcinated soil and sand. Carbon nanomaterials were successfully grown on these materials without additional catalyst. Investigations of the physical properties of the composite materials based on these carbon-modified particles revealed enhanced mechanical and electrical properties. The improvement in the mechanical properties of the C/Cu-based composite materials is attributed the crystallite or grain formation of the matrix material.     

Thermal Properties of Mg–Al/AlN Composites Fabricated by Powder Metallurgy

Magnesium matrix composites reinforced with AlN particles were fabricated by the powder metallurgy technique.Different mixing methods were used in this study to control the distribution of Al N particles.The microstructure,thermal diffusivity and thermal expansion of the Mg–Al/Al N composites using different mixing methods were investigated.The results showed that the intergranular and intragranular distributions of Al N particles were obtained,respectively,by controlling the mixing methods.The composite with intragranular particles exhibited lower thermal diffusivity because of the existences of more interfaces,defects and grain boundaries,which acted as scattering centers and reduced the mean free path of electrons and phonons.The existence of Al N particles lowered the coefficient of thermal expansion(CTE)and enhanced the dimensional stability of the composites.And the use of the improved mixing method further reduced the CTE of Mg–Al/Al N composites.     

EFFECTS OF HEAT TREATMENT ON THE THERMAL EXPANSION BEHAVIOR OF SiC WHISKER REINFORCED ALUMINUM COMPOSITE

1. IntroductionHigh specific stiffness and strength, good fatigue properties and tribological propertiesmake SiC whisker reinforcement aluminum composite as attractive class of candidates formany applications such as aerospace and electronic industries[1--2]. As structural componellts with temperature variable the materials used must have good performance includingnot only high mechalilcal properties, but also lower and stable coefficients of thermal expansion. Therefore, it is very important…     

热膨胀性能对炭/炭复合材料在制动摩擦过程中热应力场的影响

测量以纯树脂炭、粗糙层热解炭和光滑层热解炭为基体的3种炭/炭复合材料的热膨胀系数,并采用有限元分析软件,模拟这3种炭,炭复合材料在飞机正常着陆能量条件下的热应力场,研究热膨胀系数对炭,炭复合材料热应力场及其摩擦性能的影响.结果表明:3种炭/炭复合材料在z方向上的热膨胀系数大于在X和y方向的,且热膨胀系数均随着温度的升高而逐渐增大,其中,基体为粗糙层热解炭的炭/炭复合材料的热膨胀系数最小,纯树脂炭试样的次之,光滑层热解炭试样的最大;在制动过程中,炭/炭复合材料摩擦表面产生的热应力与材料的热膨胀系数相关,材料的热膨胀系数越大,产生的热应力越大;过大的热应力使纯树脂炭试样具有相对稳定的摩擦曲线,在较大热应力的作用下,光滑层热解炭试样的摩擦曲线不稳定,影响其摩擦性能.     

Comparative analysis of methods of oxidative modification of carbon nanofibers

Oxidative modification of carbon nanofibers (CNFs) is performed using the following methods: ball milling followed by oxidation in air, treatment with a nitric acid solution, and electrochemical treatment with a solution of sulfuric acid. The changes in the properties of CNFs are monitored by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS). The effect of the method by which carbon nanofibers are treated on the thermal-oxidative stability of epoxy composites containing the initial or modified CNFs is established.     

Interface and thermal expansion of carbon fiber reinforced aluminum matrix composites

Two kinds of unidirectional PAN M40 carbon fiber(55%,volume fraction) reinforced 6061Al and 5A06Al composites were fabricated by the squeeze-casting technology and their interface structure and thermal expansion properties were investigated.Results showed that the combination between aluminum alloy and fibers was well in two composites and interface reaction in M40/5A06Al composite was weaker than that in M40/6061Al composite.Coefficients of thermal expansion(CTE) of M40/Al composites varied approximately from(1.45-2.68)×10-6 K-1 to(0.35-1.44)×10-6 K-1 between 20 °C and 450 °C,and decreased slowly with the increase of temperature.In addition,the CTE of M40/6061Al composite was lower than that of M40/5A06Al composite.It was observed that fibers were protruded significantly from the matrix after thermal expansion,which demonstrated the existence of interface sliding between fiber and matrix during the thermal expansion.It was believed that weak interfacial reaction resulted in a higher CTE.It was found that the experimental CTEs were closer to the predicted values by Schapery model.     

Anisotropy of the thermal expansion in Mg fibre composites

The dilatation characteristics of Mg–10 vol.% Saffil fibre composites were measured in the temperature range 20–380 °C. The planes of a 2D-random fibre array are either parallel or transverse (perpendicular) to the longitudinal axis of the specimen (LD and TD, respectively). During heating, the coefficient of thermal expansion (CTE) of the LD composite first increases up to 250 °C and then decreases, while the CTE of the TD composite increases over the whole temperature range.     

界面改性对SiCf/Cu基复合材料热膨胀性能的影响

研究了SiCf/Cu基复合材料分别在有无Ti6Al4V界面改性涂层两种情况下的纵向热膨胀行为,并采用扫描电镜对热循环后的试样进行显微形貌观察。结果表明,界面结合强度对纤维增强金属基复合材料的纵向热膨胀行为有很大影响。对于没有Ti6Al4V涂层的复合材料,其热膨胀行为不稳定,在经历连续两次热循环后,其纵向均表现为正的残余应变,原因是基体发生了严重的界面脱粘、滑移和膨胀;而对于有Ti6Al4V涂层的复合材料,其纵向热膨胀系数明显减小,两次热循环后其尺寸保持稳定,纤维/基体界面结合也保持稳定。     

The synthesis,compressive properties,and applications of metal matrix syntactic foams

Metal matrix syntactic foams are composites that incorporate hollow particles in a matrix, where enclosing porosity inside the thin shell of the particle leads to low density without large decreases in mechanical properties. Studies on Al, Mg, Pb, and Zn alloy matrix syntactic foams are available in the published literature. A large stress plateau region appears in the compressive stress-strain graphs of metal matrix syntactic foams. The height and length of stress plateau can be tailored by means of particle wall thickness, volume fraction, and size, and the total compressive energy absorption can be controlled. Metal matrix syntactic foams seem promising in various energy absorbing applications including automobile parts since their energy absorption capability per unit weight is better than other foams and lightweight materials.     

Experimental study of the compression behaviour of syntactic foams by in situ X-ray tomography

Syntactic foams (hollow glass microspheres embedded in a polymeric matrix) are being used increasingly for thermal insulation purpose in ultra-deep water. A better understanding of the damage mechanisms of these materials under such a hydrostatic loading condition would be useful in determining actual material limits, improving phenomenological modeling and developing novel formulations in the future. To achieve this goal, a study based on X-ray microtomography was performed on various syntactic foam materials (epoxy/PP/PU matrix). Spatial resolution of (0.7 μm)³ and in situ non-destructive scanning allowed unique qualitative and quantitative analysis of the composite microstructure during stepwise confined compression testing in the dry state. It is shown that in the three materials, the thickness of the spheres is rather constant while their diameter is strongly distributed. The adherence is strong between PU and glass and rather weak between both PP and epoxy and glass. In the two foams where the matrix is more compliant (PU and PP), damage is homogeneously distributed and affects mainly the larger spheres, while in the stiff and strong epoxy material, damage is localized in bands.     

Thermal Conductivity and Tensile Properties of Carbon Nanofiber-Reinforced Aluminum-Matrix Composites Fabricated via Powder Metallurgy: Effects of Ball Milling and Extrusion Conditions on Microstructures and Resultant Composite Properties

Carbon nanofiber(CNF)-reinforced aluminum-matrix composites were fabricated via ball milling and spark plasma sintering(SPS), SPS followed by hot extrusion and powder extrusion. Two mixing conditions of CNF and aluminum powder were adopted: milling at 90 rpm and milling at 200 rpm. After milling at 90 rpm, the mixed powder was sintered using SPS at 560 °C. The composite was then extruded at 500 °C at an extrusion ratio of 9. Composites were also fabricated via powder extrusion of powder milled at 200 rpm and 550 °C with an extrusion ratio of 9(R9) or 16(R16). The thermal conductivity and tensile properties of the resultant composites were evaluated. Anisotropic thermal conductivity was observed even in the sintered products. The anisotropy could be controlled via hot extrusion. The thermal conductivity of composites fabricated via powder extrusion was higher than those fabricated using other methods. However, in the case of specimens with a CNF volume fraction of 4.0%, the thermal conductivity of the composite fabricated via SPS and hot extrusion was the highest. The highest thermal conductivity of 4.0% CNF-reinforced composite is attributable to networking and percolation of CNFs. The effect of the fabrication route on the tensile strength and ductility was also investigated. Tensile strengths of the R9 composites were the highest. By contrast, the R16 composites prepared under long heating duration exhibited high ductility at CNF volume fractions of 2.0% and 5.0%. The microstructures of composites and fracture surfaces were observed in detail, and fracture process was elucidated. The results revealed that controlling the heating and plastic deformation during extrusion will yield strong and ductile composites.     

Enhancing Wear Resistance of A356 Alloy by Adding CNFs Based on Ultrasonic Vibration Casting

A356–carbon nanofibers(CNFs) composites with different contents of CNFs were fabricated by ultrasonic vibration casting to investigate the effect of CNFs in the matrix on the mechanical properties and wear resistance. The worn surfaces were investigated using scanning electron microscopy(SEM). As the CNFs content was increased, strength,hardness and wear resistance were significantly enhanced and the coefficient of friction was extremely reduced. The nanocomposite containing 1.2 wt% of CNFs exhibited more than 109 HV in hardness and less than 0.35 in the coefficient of friction. Compared with the as-cast matrix, the wear rate of the optimal composite was less than one-third of the matrix sample and the microhardness exhibited about 47% enhancement of the matrix. Meanwhile, steadier and lower friction coefficient was also achieved by the composite. CNFs were observed to be either partially or fully crushed forming a carbon film that covered the surface and acted as a solid lubricant, enhancing the wear behavior significantly.     

Morphological characterization of electrospun carbon nanofiber mats of polyacrylonitrile containing heteropolyacids

This work focuses on the development of electrically conducting carbon nanofiber (CNF) mats obtained by incorporating different amounts of heteropolyacids (HPAs) viz., silicotungstic acid (SiWA) and silicomolybdic acid (SiMoA) into polyacrylonitrile (PAN) precursor by electrospinning technique. Electron and transmission microscopic studies confirmed the cylindrical morphologies of CNFs with diameters ranging between 100 to 300 nm after carbonization at 1000 °C. Electrical conductivity of the CNF mats increased with increasing concentrations of HPAs. The optimal HPA concentration of 5 wt.% seems to be responsible for enhancements of morphological and physico-chemical properties of developed carbon nanofibers. X-ray diffraction proved the increase in intensity due to the presence of HPA in the matrix. TGA was used to understand the thermal behavior of the CNFs.     

Densification Behavior and Performances of C/C Composites Derived from Various Carbon Matrix Precursors

Three types of carbon/carbon (C/C) composites were manufactured by densifying the needled carbon fiber preform through resin and pitch impregnation/carbonization repeatedly, as well as propylene pyrolysis by chemical vapor infiltration plus carbonization after the resin impregnation/carbonization. The densification behavior and performances (involving electric, thermal, and mechanical properties, as well as impurity) of the C/C composites were investigated systematically. The results show that besides the processing and testing conditions, the electric resistivity, thermal conductivity (TC), coefficient of thermal expansion (CTE), strength, and fracture, as well as impurity content and composition of the C/C composites were closely related to the fiber orientation, interfacial bonding between carbon fiber and carbon matrix, material characteristics of the three precursors and the resulting matrix carbons. In particular, the resin-carbon matrix C/C (RC/C) composites had the highest electric resistivity, tensile, and flexural strength, as well as impurity content. Meanwhile, the pitch-carbon matrix C/C (PC/C) composites possessed the highest TC and CTE in the parallel and vertical direction. And most of the performances of pyro-carbon/resin carbon matrix C/C composites were between those of the RC/C and PC/C composites except the impurity content.