Electromagnetic properties and microwave absorption properties of BaTiO3-carbonyl iron composite in S and C bands

BaTiO₃ powders are prepared by sol-gel method. The carbonyl iron powder is prepared via thermal decomposition of iron pentacarbonyl. Then BaTiO₃-carbonyl iron composite with different mixture ratios was prepared using the as-prepared material. The structure, morphology, and properties of the composites are characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning electron microscopy (SEM), and a network analyzer. The complex permittivity and reflection loss of the composites have been measured at different microwave frequencies in S- and C-bands employing vector network analyzer model PNA 3629D vector. The effect of the mass ratio of BaTiO₃/carbonyl iron on the microwave loss properties of the composites is investigated. A possible microwave absorbing mechanism of BaTiO₃-carbonyl iron composite has been proposed. The BaTiO₃-carbonyl iron composite can find applications in suppression of electromagnetic interference, and reduction of radar signature.     

混杂复合材料等效热传导性能预测的小波-机器学习混合方法

混杂复合材料是一种新型复合材料,其复杂的细观结构导致预测其等效热传导性能极富挑战性.本文结合渐近均匀化方法、小波变换方法和机器学习方法发展了一种新的可以有效预测混杂复合材料等效热传导性能的小波-机器学习混合方法.该方法主要包括离线多尺度建模和在线机器学习两部分.首先借助渐近均匀化方法通过离线多尺度建模建立了混杂复合材料...     

Interfaces in metal matrix composites

The interface region in a given composite has a great deal of importance in determining the ultimate properties of the composite. An interface is, by definition, a bidimensional region through which there occurs a discontinuity in one or more material parameters. In practice, there is always some volume associated with the interface region over which a gradual transition in material parameter(s) occurs. The importance of the interface region in composites stems from two main reasons: (i) the interface occupies a very large area in composites, and (ii) in general, the reinforcement and the metal matrix will form a system that is not in thermodynamic equilibrium. One can discuss the interface in a composite at various levels. An optimum one should be neither so simple that it covers only a few special cases nor so complex that it is not useful in designing composites from processing and applications points of view. In this paper, my objective is to give examples of interface microstructure in different metal matrix composite systems and suggest some ways of controlling the interface characteristics in order to control the properties of the composite. I shall give examples of the interface microstructure in different metal matrix composites (particle and fiber reinforced as well as laminates) and discuss some of the important implications on various aspects of metal matrix composites, from the processing stage to ultimate performance of the composite.     

A review on abaca fiber reinforced composites

This paper reviews literatures and information on Abaca fibers (Musa textilis Nee) as reinforcing material for aerospace composite materials. Characterization of Abaca as well properties of Abaca reinforced composites and its applications were discussed. Therefore, challenges and future works for Abaca Reinforced composites were explored. Studies reveal that Abaca fiber pre-treatment helps in improving the mechanical properties of the composite. In addition, there have been efforts in combining Abaca fibers to existing mixture of synthetic composites to improve its mechanical properties and environmental performance. The future of Abaca is seen as one of the potent sources of reinforcing fiber for various material construction including aerospace materials.     

Micromechanical Behavior of Carbon Nanotube-Covered SiC Fibers in Polymer Matrix Composites

The incorporation of nanotube-covered fibers in continuous fiber/epoxy composites has been shown to influence the mechanical, electrical, and thermal properties of the composite. Increased interlaminar shear stress, flexural strength and modulus have been reported in such composites over composites containing bare fibers. In this study, the microstructure and interfacial shear strength (ISS) of continuous silicon carbide fiber/epoxy composites with and without nanotubes grown from the SiC fiber surface were investigated with micro-Raman spectroscopy (MRS) and microscopy. The fibers with nanotubes grown from the surface were found to have a reduced ISS compared with the bare fibers. Electron microscopy showed good wetting of epoxy in the nanotube forests, but poor attachment of the nanotube forests to the fibers. These results suggest that the mechanism leading to improvements in bulk composite properties is not due to an improvement in the fiber/matrix ISS.     

Synthesis and characteristics of form-stable n-octadecane/expanded graphite composite phase change materials

N-octadecane/expanded graphite composite phase-change materials were prepared by absorbing liquid n-octadecane into the expanded graphite. The n-octadecane was used as the phase-change material for thermal energy storage, and the expanded graphite acted as the supporting material. Fourier transformation infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal diffusivity measurement were used to determine the chemical structure, crystalline phase, microstructure and thermal diffusivity of the composite phase-change materials, respectively. The thermal properties and thermal stability were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The DSC results indicated that the composite phase-change materials exhibited the same phase-transition characteristics as the n-octadecane and their latent heat increased with the n-octadecane content in composite phase-change materials. The SEM results showed that the n-octadecane was well absorbed in the porous network of the expanded graphite, and there was no leakage of the n-octadecane from the composites even when it was in the molten state.     

Piezoelectric cement-based 1-3 composites

This paper presents a new functional material for smart structure applications. Piezoelectric PZT/cement 1-3 composites that have good compatibility with civil engineering structural materials have been studied. The composites with different volume fractions of PZT ranging from 0.25 to 0.77 were fabricated by the dice-and-fill method. It was found that the 1-3 composites have good piezoelectric properties that agreed quite well with theoretical modeling. The thickness electromechanical coupling coefficient could reach 0.55 in the composite with a ceramic volume fraction of 0.25. Those composites have potential to be used as sensors in civil structure health monitoring systems. PACS 77.65.-j; 77.84.Lf     

The dielectric constant of a composite material—A problem in classical physics

The problem of calculating the effective dielectric constant of a composite material ε_e is analyzed by separating out the dependence of ε_e on the microscopic geometry. A set of characteristic geometric functions is defined, which depend in detail on the microscopic geometry of the material under discussion, but whose general analytical properties can be derived. The pole structure of these functions is shown to have important consequences. It is experimentally observable in certain optical and microwave experiments on metal-insulator composites. Furthermore, it can be used to obtain a considerable amount of quantitative information about ε_e in the form of rigorous upper and lower bounds. Finally, it allows a new approach to be taken in the discussion of the critical properties of composite systems near a percolation threshold.The characteristic functions defined and discussed in this article are applicable not only to ε_e, but also to other scalar material constants of the composite material, such as the magnetic permeability, electrical and thermal conductivity, and diffusivity.     

Capacitance properties of poly(3,4-ethylenedioxythiophene)/carbon nanotubes composites

The electrochemical properties of composites prepared from an electrically conducting polymer poly(3,4-ethylenedioxythiophene), i.e. PEDOT and multiwalled carbon nanotubes (CNTs) have been investigated for supercapacitor application. The novel composite material was prepared by chemical or electrochemical polymerization of EDOT directly on the nanotubes or from a homogenous mixture of PEDOT and CNTs. Acetylene black (AB) has been also used as a composite component in order to evaluate whether nanotubes are giving improved properties or not. Electrodes prepared from such composites were used in supercapacitors operating in acidic (1 M H₂SO₄), alkaline (6M KOH) and organic (1 M TEABF₄ in AN) electrolytic solutions. The capacitance values were estimated by galvanostatic, voltammetry and impedance spectroscopy techniques with two- or three-electrode cell configuration. Due to the open mesoporous network of nanotubes, the easily accessible electrode/electrolyte interface allows quick charge propagation in the composite material and an efficient reversible storage of energy in PEDOT during subsequent charging/discharging cycles. The composites with AB supply quite good capacitance results, however, nanotubes as electrode component gave definitively a more homogenous dispersion of PEDOT that should give a better charge propagation. The values of capacitance for PEDOT/carbon composites ranged from 60 to 160 F/g and such material has a good cycling performance with a high stability in all the electrolytes. Organic medium is especially interesting because of higher energy stored. Another quite important advantage of this composite is its significant volumetric energy because of the high density of PEDOT.     

Effect of Modified Intumescent Flame Retardant via Surfactant/Polyacrylate Latex on Properties of Intumescent Flame Retardant ABS Composites

In order to prepare intumescent flame retardant acrylonitrile-butadiene-styrene (ABS) composites with only a small decrease in their mechanical properties, we investigated the effect of adding an elastomeric polyacrylate latex and the surfactant TX-10 phosphate to modify the ammonium polyphosphate, melamine, and calcium 3-hydroxy-2, 2-bis(hydroxymethyl) propyl phosphate normally used, which resulted in an intumescent flame retardant composite (IFRC) powder with the aim of improving compatibility. These ABS/IFRC composites were compared with standard material containing unmodified intumescent flame retardant (NIFR) by investigating their thermal properties, melt characteristics, mechanical properties, and microstructure. The data showed that the glass transition temperature of the ABS/IFRC composites decreased slightly in all cases, the complex viscosity of the ABS/IFRC composites was remarkably reduced, and the mechanical properties improved in comparison with the material containing NIFR. A slight increase in impact strength retention, as well as a remarkable increase in tensile and flexural strength retention of ABS/IFRC, was achieved due to superior compatibility between ABS and IFRC in comparison with ABS/NIFR.     

Superconducting properties of MgO-whisker reinforced BPSCCO composite

Based on a recently introduced solid-state processing method, a new class of high-temperature BPSCCO (2223) ceramic matrix composites reinforced with MgO whiskers has been developed. The (MgO)_w/BPSCCO composites are shown to possess attractive superconducting properties with significantly improved mechanical properties. It is recognized that the superconducting properties of the (MgO)_w/BPSCCO HTS composite are related to proper material microstructures achievable by optimizing the processing variables involved, such as repeated hot pressing and annealing, hot-pressing temperature and pressure level, and the length of annealing. In this paper, relationships among superconducting properties and composite processing variables are established. Also, effects of the BPSCCO (2223) phase purity and the MgO whisker reinforcement on superconducting properties of the (MgO)_w/BPSCCO composites are discussed. The important issue of the field dependence of superconducting properties of both the monolithic and the composite HTS materials is addressed.     

Interphase sizing temperature effect of LaRC PETI-5 on the dynamic mechanical thermal properties of carbon fiber/BMI composites

The dynamic mechanical thermal properties of carbon fiber-reinforced bismaleimide (BMI) composites processed using polyacrylonitrile(PAN)-based carbon fibers unsized and sized with LaRC PETI-5 amic acid oligomer as interphase material at 150°C, 250°C, and 350°C were investigated by means of dynamic mechanical thermal analysis. It was found that the storage modulus, loss modulus, tan δ and the peak temperature significantly depend on the sizing temperature as well as on the presence and absence of LaRC PETI-5 sizing interphase. The result showed that the carbon fiber/BMI composite sized at 150°C had the highest storage modulus at a measuring temperature of 250°C. The storage modulus decreased with increasing sizing temperature from 150°C to 350°C, being influenced by interdiffusion and co-reaction between the LaRC PETI-5 interphase and the BMI matrix resin. The present result is quite consistent with the interfacial result reported earlier in term of interfacial shear strength and interlaminar shear strength of carbon fiber/BMI composites. It is addressed that in the present composite system the sizing temperature of LaRC PETI-5 interphase critically influences not only the interfacial properties but also the dynamic mechanical thermal properties and its control is also important.     

各向异性堆叠结构环氧树脂复合材料的热防护性能

基于导热-隔热原理,通过在环氧树脂(Epon)中添加质量分数为5%,15%,25%的六方氮化硼(h-BN)作为填料制备环氧基散热层,质量分数为1%的膨胀蛭石(E-ver)作为填料制备环氧基隔热层,设计了宏观交替堆叠的环氧复合材料,并进行了热防护性能的研究。研究结果表明：具有各向异性结构的复合材料,顶部中心温度较传统材料的温度下降13～16 ℃,热延迟时间大大提升,并随着h-BN含量的增加,热性能得到明显改善。理论分析了该堆叠结构下复合材料“横向散热、纵向抑热”的机理。     

Thermal conductivity of the pine-biocarbon-preform/copper composite

The thermal conductivity of composites of a new type prepared by infiltration under vacuum of melted copper into empty sap channels (aligned with the sample length) of high-porosity biocarbon preforms of white pine tree wood has been studied in the temperature range 5–300 K. The biocarbon preforms have been prepared by pyrolysis of tree wood in an argon flow at two carbonization temperatures of 1000 and 2400°C. From the experimental values of the composite thermal conductivities, the fraction due to the thermal conductivity of the embedded copper is isolated and found to be substantially lower than that of the original copper used in preparation of the composites. The decrease in the thermal conductivity of copper in the composite is assigned to defects in its structure, namely, breaks in the copper filling the sap channels, as well as the radial ones, also filled by copper. A possibility of decreasing the thermal conductivity of copper in a composite due to its doping by the impurities present in the carbon preform is discussed.     

Transverse tensile properties of spun-type carbon fabric/phenolic composites

The transverse tensile properties of phenolic composites reinforced with spun-type carbon fabrics (spun C/P composites) have been investigated in order to evaluate the adherent failure behavior of composites in the transverse (90°) direction due to tension. The transverse tensile strength of the spun C/P composite is about 3.4 times higher than that of the conventional composite reinforced with filament type carbon fabrics (filament C/P composites). It is found from stress–strain curve of composites that it exhibits above 4 times higher failure strain than the filament C/P composite. However, the transverse tensile modulus of the spun C/P composite is similar to that of the filament C/P composite. The results indicate that the protruded fibers of spun yarns between the interlaminar layers in the spun C/P composite play an important role in improving the transverse tensile properties by the effects of fiber bridging. Consequently, this result suggests that use of spun yarn type carbon fabrics as reinforcement in a phenolic composite may significantly contribute to improving the interfacial properties of carbon/phenolic composites.     

含铕类水滑石/聚苯胺导电性复合材料的荧光

采用超声法将定量的荧光性含铕类水滑石(Eu-HTLc)掺混到导电性聚苯胺(PAN-CSA)中,获得一类新型的含铕类水滑石/聚苯胺导电性荧光复合材料(Eu-HTLc/PAN-CSA)。采用电导率测试、荧光光谱、红外光谱、X射线衍射和热分析等手段对其进行表征。研究结果表明:该复合材料的电导率σ=7.97×10-3(S/cm);在紫外光下该复合材料可发出红色荧光(613nm),其荧光寿命为441.32μs;热分析结果显示,该复合材料比PAN-CSA具有更高热稳定性,有望作为一类新型荧光材料而获得应用。     

Thermal stabilization of the resistive states of superconducting composites: Quasilinear approximation

The conditions of the occurrence and development of thermal instabilities in the composite superconductor with a continuously increasing current-voltage characteristic, which is described by the power equation, have been studied. The conditions for thermal stabilization have been analyzed in the general form using dimensionless variables that keep their invariance when varying. For the local temperature disturbance, the critical energies and velocities of its irreversible propagation have been calculated. It has been proved that composites superconductors can have stable states, when the ultimate currents can be higher or lower of the conventionally preset critical current of the composite. Furthermore, superconductivity destruction at supercritical currents takes place not in the form of a stepwise transition from the superconducting to normal state, but due to the formation of thermal and electric switching waves that propagate along the composite superconductor with a constant speed. The condition for full thermal stabilization has been formulated for the superconducting composites with a power current–voltage characteristic. The results of the numerical experiments have proved that the existing theory of thermal stabilization, which assumes a stepwise superconducting–normal transition, leads to the considerable limitation of the range of the stable currents, at which a superconducting state can be kept.     

碳纳米管/聚二甲基硅氧烷复合薄膜的制备及力敏特性研究

针对传感器的敏感单元发展需求,提出了一种碳纳米管复合材料.该材料是以碳纳米管作为填充粒子,结合聚二甲基硅氧烷(PDMS)有机基体,采用超声共混方法制备的一种新型传感器敏感元件.详细分析研究了复合材料的制备工艺参数,以及在不同工艺参数下该复合材料的力敏特性.扫描电镜测试表明碳纳米管在PDMS中分散均匀且镶嵌良好.通过对不同体积分数的碳纳米管与PDMS复合材料进行电学性能测试,研究薄膜的"力-电阻"和"力-电容"耦合性能,测试了薄膜结构的力敏效应.计算得到复合薄膜材料的压阻灵敏度因子达到40,压电容灵敏度因子达到70.实验研究表明,通过改变碳纳米管与PDMS的比例,可以很好地调节其电子输运特性以及电阻和电容的应力敏感特性,可以为该类型的力敏材料在不同的力敏传感技术领域提供新的研究思路.     

Identification des propriétés thermiques de composites fibres de verre/résines thermodurcissables: Application à l'optimisation des procédés de moulage

Identification of glass fiber/thermosetting resins composites thermal properties. Application to the optimization of molding processes. A procedure has been developed to optimize molding processes of thermosetting composite materials. Three stages have been distinguished. In the first one, some thermal and kinetic properties have been measured by differential scanning calorimetry. Thermal conductivities have been identified afterwards in thick instrumented pieces, placed in a thermally regulated press. High dependences of thermal conductivities on temperature and transformation degree have been shown. Secondly, coupled heat transfers have been numerically simulated and results have been satisfactorily compared with experimental thermograms. Finally, optimization technics based on effective inverse methods have been used.These points have been illustrated with two examples : glass fiber/epoxy resin and glass fiber/polyester. Sufficient mechanical characteristics of the first one, which is cured in oven, and good surface aspect of the second, that is made by injection in heated and closed molds, had to be obtained. The results let foresee real improvement of the corresponding molding processes.     

Mechanical properties of flat braided composite with flexible interphase

Textile composites have been used extensively as industrial materials because of the excellent mechanical properties resulting from the continuously oriented fiber bundle. In a study of the mechanical properties, it is important to consider the fiber/matrix interface property as for other composite materials. In a recent study, the fiber/matrix interface is regarded as an interphase that has its own material constants and thickness; consequently, the mechanical properties of a composite can be controlled by specifically designing the interphase. In this study, we applied this concept to braided composites with flexible resin as interphase for the purpose of designing the interphase. In a static tensile test, though there were no improvements in Noncut specimens (normal braided composites), but a Cut specimen (each side of the Noncut specimen was cut) with flexible interphase was improved in fracture load and displacement. The observation of the specimen edge was carried out and it was confirmed that the progress of debonding at the fiber bundle intersection was interrupted by a flexible interphase, and a matrix crack did not occur in the Cut specimen with flexible interphase. In a fiber bundle pull-out test, it was confirmed that debonding progressed not into the fiber/resin interface but into the flexible interphase in the specimen with flexible interphase, and the interfacial property at the fiber bundle intersection was improved.